Rare Inclusions of Coexisting Silicate Glass and Cu-PGM Sulfides in Pt-Fe Nuggets, Northwest Ecuador: Fractionation, Decompression Exsolutions, and Partial Melting

Barron, B. Jane; Barron, Lawrence

doi:10.3390/min15121329

Open AccessArticle

Rare Inclusions of Coexisting Silicate Glass and Cu-PGM Sulfides in Pt-Fe Nuggets, Northwest Ecuador: Fractionation, Decompression Exsolutions, and Partial Melting

by

B. Jane Barron

^1,*

and

Lawrence Barron

²

¹

School of Biological, Earth and Environmental Sciences (BEES), University of New South Wales (UNSW Sydney), High Street, Kensington, NSW 2052, Australia

²

The Australian Museum, 1 William Street, Sydney, NSW 2010, Australia

^*

Author to whom correspondence should be addressed.

Minerals 2025, 15(12), 1329; https://doi.org/10.3390/min15121329

Submission received: 29 October 2025 / Revised: 10 December 2025 / Accepted: 14 December 2025 / Published: 18 December 2025

(This article belongs to the Section Crystallography and Physical Chemistry of Minerals & Nanominerals)

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Abstract

Pt-Fe alloys with abundant inclusions are from the Camumbi River placer deposit, Ecuador. They are derived from unknown Alaskan–Uralian-type intrusion(s) within the Late Cretaceous Naranjal accreted terrane. Compositions of our previously documented chilled silicate glass inclusions are increasingly fractioned from hydrous ferrobasalt to rhyolite in terms of TAS (total alkalis vs. silica). Their liquid lines of descent change from tholeiitic to the calc-alkaline magma series. Here, we document seven rare composite inclusion parageneses of Cu–PGM (platinum-group mineral) sulfides, each coexisting with and exsolved from related fractionated silicate glass (melt). Differentiation is dominated by fractional crystallization in PGM bulk compositions from tholeiitic silicate melts at the highest T (temperature): ~1018 °C. Silicate glass inclusions following the lower T calc-alkaline trend coexist with sulfide PGM parageneses that were likely differentiated, in terms of Pt-Rh-Pd and BMs (base metals), by incongruent melting due to decompression and S-degassing at ~983–830 °C. S-saturated sulfide melts become S-undersaturated below 845 °C. The calculated temperatures are for silicate glass. Pt-rich braggite shows increasing fractionation towards Pd-rich vysotskite within one inclusion paragenesis. A late braggite–vysotskite fractionation trend shows decreasing minor base metals (BMs). Thiospinels are dominated by cuprorhodsite. Minor thiospinels indicate Fe and then strong Ni enrichment at the lowest T. Decompression exsolutions, deflation, and the partial melting of some sulfide inclusion parageneses support rapid ascent to higher crustal levels within a deep-sourced cumulate intrusion.

Keywords:

Pt-Fe alloys; inclusions; silicate glass; Cu-PGM sulfides; decompression mineral exsolutions; melting

1. Introduction

Silicate glass inclusions in Alaskan–Uralian-type Pt-Fe nuggets from the Sabaleta placer deposit, Camumbi River, NW Ecuador (Figure 1), define a fractionated comagmatic series from primitive hydrous ferrobasalt to basaltic andesite and andesite; the most fractionated compositions are groundmass silicate glasses of dacite–rhyolite composition in feldspar-porphyritic inclusions [1]. Primitive tholeiitic ferrobasaltic melt inclusions are progressively fractionated towards calc-alkaline compositions, a trend that is comparable to the fractionation of experimental hydrous ferrobasalt reported by Botcharnikov et al. [2]. Trace elements of fractionated silicate melt inclusions [1] share a similar pattern with their Late Cretaceous accreted Naranjal island arc terrane [3] and suggest a possible back-arc setting. It was also proposed [1] that in a cooling Alaskan–Uralian-type intrusion, the fractionating “wet” residual silicate melts concentrate platinum group element (PGE) clusters and ligands, facilitating crystallization of PGM during mush/melt intrusion within a fault-located conduit/pipe. Compressed by the intruding cumulate, the fractionating (increasingly more felsic) interstitial “wet” melts are sequentially expelled, forming the typical cylindrical zoned layering of Alaskan-type intrusions [1].

Host Pt-Fe alloy nugget chemistry [6] shows almost equal proportions of native platinum and isoferroplatinum. The latter is comparatively depleted in Rh, Os, and Ru. Mineral inclusions of Os-Ir alloy and laurite (RuOsIr)S₂ are first crystallized at high T in native platinum, while isoferroplatinum is crystallized at lower T and hosts more fractionated crystal and residual melt inclusions [6,7]. This is supported by the finding that laurite and Os–Ir–Ru alloys are liquidus phases first crystallized at high temperatures and low sulfur fugacity [8].

Experimental studies confirm that a primitive Cu-bearing PGE–S-(As) melt is first exsolved from primitive basalt and that a subordinate exsolved Cu-depleted PGE–As-(S) melt exsolves at a lower T [6,9,10,11]. It is also shown experimentally that from an As-(S) melt, the immiscible melts Pt-As-(S) and Pd-As-(S) crystallize distinctive PGM [12,13,14].

We documented [6] similar natural Cu-depleted PGE sulfarsenide mineral inclusions hosted in two separate isoferroplatinum nuggets from the Camumbi River deposit. In the first, zoned sulfarsenides sperrylite and genkinite define a Pt-enriched sub-system (Pt > Rh, (Pd, Ir, Ru), As > S >> Sb, Bi). In the second, mineral inclusions of zoned arsenopalladinite, sperrylite, törnroosite, and gold define a lower-T, fractionated, Pd-enriched sub-system: Pd > Rh ≈ Pt > Ir > As > S > Te >> Sb, Bi > Au. In the latter, we define (in terms of Pt-Ir-Rh) the previously undocumented natural S-rich sperrylite (formerly “platarsite”) solid solution series and later-crystallized irarsite series. We compare the latter with experimental studies of Helmy et al. [15].

In the experimental system of Pt, Pd, S, As, Se, and Te, also at high T (~1250 °C), the highest concentrations of S, As, Se, and Te in basaltic melt are recorded by Helmy et al. [15] when the Fe²⁺ cation is the principal metal ligand. Comparable natural conditions of increasing FeO define initial tholeiitic basaltic fractionation. Importantly, it is also shown experimentally (in the absence of silicate melt) that the affinities of the chalcophile metals for an immiscible arsenide melt follow the order Pt > Pd > Ni > Fe > Cu [14,16]. Therefore, Cu is the least likely element to partition into a subordinate exsolved As-enriched melt, and the residual sulfide melt is further enriched in Cu. The latter is like the present Cu-enriched and Ni-bearing PGE sulfide natural system.

In a separate study [7], we documented complex inclusions of mineral (crystal) and melt parageneses within one host isoferroplatinum nugget, defining a complex, natural Cu-enriched and Ni-bearing PGM sulfide system, possibly exsolved from primitive basalt. At the highest T, cooperite crystal inclusions are first crystallised, followed by Cu-bearing sulfide PGMs [kingstonite ± Rh-rich cuprorhodsite–(malanite) and monosulfides], while in an adjacent domain, there are eight melt inclusions with Cu-PGM sulfide parageneses. The latter are dominated by a fractionated cuprorhodsite–(malanite) solid solution series coexisting with exsolved monosulfide minerals. Fractionated compositions of the cuprorhodsite–(malanite) series define four narrow, subparallel crystallization “fronts” of sequentially cooled residual decompression melts. Under conditions of decreasing T (and fS₂), the natural cuprorhodsite–(malanite) compositions show decreasing Rh and increasing Pt (±Ir), defining a relative T stability range [7]. Supporting our findings, the experiments of Li et al. [17] show that at 1000 and 1100 °C, under S-saturated conditions, Rh is compatible and fractionated into Mss (monosulfide solid solution), but Pt, Pd, and Cu are strongly incompatible and are fractionated into residual sulfide melts. Crystal inclusions are cooperite, kingstonite, cuprorhodsite–malanite, and PGE–monosulfides. Melt inclusions are fractionated Cu-PGE sulfide parageneses that indicate both S-saturated (higher T) and S-undersaturated (lower T and some decompressed) conditions that compare with the experiments of Li et al. [17].

Here, we consider the lower T fractionation of rare composite melt inclusions in seven separate host Pt-Fe nuggets that vary from 0.22 to 0.99 mm in diameter, hosting inclusions measuring ~20–120 µm across. Each inclusion type comprises quenched silicate glass and exsolved, coexisting PGM parageneses of Cu-PGE-bearing sulfides ± minor As, Te. The known compositions of the fractionated silicate glass vary from hydrous ferrobasalt, basaltic andesite, andesite, and finally, dacite–rhyolite [1], and here, we document the complex fractionation of their coexisting PGM parageneses.

2. Samples and Methods

Samples of heavy mineral concentrate are from the Rio Dorado Limited (now liquidated) Sabaleta Project area, Ecuador (Figure 1, above). Petrological/mineralogical examination confirms significant platinum and gold nuggets. Pt-Fe alloy nuggets selected for this study are labelled by Rio Dorado with prefix numbers (A2, Mag 5, A1 and B). Area numbers (4, 5, 7, and 9) refer to areas marked on polished thin sections to locate individual grains for EPMA (electron probe microanalysis) in inclusions within each host Pt-Fe alloy nugget.

2.1. Platinum-Group Sulfide Mineral Analysis

Prior to analyses, all samples were coated with 20 nm of carbon using an HHV Auto 306 carbon evaporator (HHV Ltd., Crawley, UK).

Compositional analyses were acquired on a JEOL JXA-8530F Plus (JEOL, Ltd., Tokyo, Japan) field emission electron microprobe at the Central Science Laboratory, University of Tasmania, equipped with five wavelength dispersive spectrometers and using an accelerating voltage of 20 kV, probe current of 20 nA, and beam diameter in the range from 0.1 to 1 µm depending on the size of the respective feature.

The data were acquired and processed with the “Probe For EPMA” software package (version 12.8.9) by Probe Software, Inc. (Eugene, OR, USA) and quantified with the Armstrong/Love-Scott matrix correction algorithm and FFAST mass absorption coefficients. For analytical details cf. [6].

Detailed settings for the different elements are listed below. For elements Ir, Pt, and Au, X-ray intensities were acquired for both Lα and Mα lines. Depending on individual mineral compositions, the Mα lines were used for quantification, except in the following cases:

Ir Lα was used instead of Mα in the case of major Os (>10 wt.%) due to strong Os interference on Ir Lα.
Pt Lα was used in the case of major Ir (>5 wt.%).
Au Lα was used in the case of major Pt (>15 wt.%).

Si and Ca were analyzed as indicators for analysis contributions from gangue minerals across grain boundaries or from inclusions.

Conventional two-point off-peak background measurements were performed and converted to multi-point background curves using the shared background technique [18]. Background fit curves for each element were optimized individually for different mineral compositions.

A Thermo Pathfinder Pinnacle energy-dispersive X-ray spectrometry (EDS) system (Thermo Fisher Scientific, Madison, WI, USA) with an UltraDry Extreme 30 mm² silicon drift detector (Thermo Fisher Scientific, Madison, WI, USA) and a JEOL backscattered electron (BSE) detector (JEOL, Ltd., Tokyo, Japan) on the same EPMA instrument were used to acquire BSE images, and semi-quantitative EDS analyses were carried out to aid the selection and documentation of microprobe analysis locations.

2.2. Element Mapping of Cu-PGM-Sulfide Exsolution Inclusion Assemblages

X-ray element maps and corresponding BSE images were acquired on a Hitachi SU-70 field emission scanning electron microscope (SEM; Hitachi High-Tech, Tokyo, Japan) fitted with an Oxford AZtec EDS system (Oxford Instruments, Oxford, UK; software version 4.3), which was further fitted with an XMax 80 silicon drift detector and Hitachi photo diode BSE detector (Thermo Fisher Scientific, Madison, WI, USA), at the Central Science Laboratory, University of Tasmania, using an accelerating voltage of 17 kV and beam current of 3 nA. In total, 90–100 individual frames were acquired and integrated per map at an individual pixel dwell time of 100–200 µs, leading to a total acquisition time per pixel of 10–20 ms, with pixel sizes ranging from 0.2 to 0.5 µm.

3. Samples: Silicate Inclusions with Coexisting Cu-PGM Sulfide Parageneses

Seven Pt-Fe alloy nuggets host melt inclusions of Cu-enriched, Rh- and Pt-bearing sulfide parageneses, each coexisting with (or closely associated with) their quenched parental silicate melt (mainly quenched glass). Here, we consider samples in the order of increasingly fractionated silicate glass compositions from primitive hydrous basalt to ferrobasalt, basaltic andesite, and andesite and, finally, those with dacite–rhyolite and rhyolite groundmass fractions, following Barron et al. [1].

3.1. Samples, Mineralogy, and Analyses

3.1.1. Sample A2 Area 7

(a): Host alloy and ferrobasalt silicate glass (SiO₂ 47.43 wt. %)

The host nugget is rounded (Figure 2a) but distinctly deformed (flattened) and measures ~987 µm in length and ~370 µm in width. The prominent, rounded silicate glass inclusion (~191 µm long and ~146 µm across) is also slightly flattened, with adjacent, small, elongated, triangular, and somewhat irregular strain shadows, possibly parallel to a foliation direction. Analysis 277 (Barron et al. [1]; their Tables 1A and 1B); indicate host native platinum (Pt_2.95Rh_0.09Pd_0.06 Ir_0.01Os_0.01)_3.12(Fe_0.80Cu_0.07Ni_0.01)_0.88, with the following minor elements in decreasing order of abundance: Rh, Cu, Pd, Ir, Os, and Ni.

A distinct “corona” is defined by an arc of small (~1–5 µm) inclusions with Cu-PGE sulfide parageneses located near the nugget margin and distant from their host basaltic glass inclusion (Figure 2a). These inclusions have irregular shapes and show elongate, narrow strain shadows in Figure 2b.

Analysis 176 (Barron et al. [1]; Table 2E) indicates that coexisting (host) silicate glass in this inclusion is ferrobasalt with SiO₂ 47.43, FeO 17.62, and MgO 6.13 (wt. %) and minor H₂O at ~0.31. This glass is the least siliceous of the present samples.

(b): PGM sulfide paragenesis

Cuprorhodsite–(malanite) (Figure 2b) is a small (~2.5 µm) subhedral crystal set in a strain-shadow microstructure. Analysis 327 (Table 1) indicates (Cu_0.85Fe_0.12Ni_0.03)_1.00(Rh_1.26Pt_0.62Pd_0.05Ru_0.05Ir_0.03Os_0.01)_1.96S_3.96. The strain-shadow mineral is too small for accurate analysis.

Vysotskite occurs as a subhedral crystal in a separate inclusion (Figure 2c). Analysis (328) (Table 1) indicates the following Pd-rich vysotskite formula: (Pd_0.64Pt_0.20Cu_0.04Fe_0.04Ni_0.03)_0.91S_1.04 (cf. Cabri and McDonald [19]; Figure 1 in their study). The fibrous, irregular matrix fraction is too small for analysis.

Table 1. Analyses sample A2 area 7.

wt. %	Mineral	S	Fe	Ni	Cu	Ru	Rh	Pd	Os	Ir	Pt	Total
1 (277)	Pt		6.95	0.05	0.68		1.50	1.01	0.17	0.23	89.38	99.96
3 (327) *	Crh	27.02	1.49	0.43	11.49	1.01	27.70	1.22	0.30	1.12	25.91	97.74
4 (328)	Vys	22.40	1.31	1.30	1.62	0.07	0.93	45.27			26.26	99.17
at. %	Mineral	S	Fe	Ni	Cu	Ru	Rh	Pd	Os	Ir	Pt
1 (277)	Pt		20.05	0.13	1.74		2.35	1.53	0.14	0.19	73.87	100.00
3 (327) **	Crh	56.58	1.79	0.49	12.14	0.67	18.08	0.77	0.11	0.39	8.92	100.00
4 (328)	Vys	52.16	1.75	1.65	1.91	0.05	0.67	31.76			10.05	100.00
apfu	Mineral	S	Fe	Ni	Cu	Ru	Rh	Pd	Os	Ir	Pt	Me
1 (277)	Pt		0.80	0.01	0.07		0.09	0.06		0.01	2.95	3.99
3 (327) ***	Crh	3.96	0.12	0.03	0.85	0.05	1.26	0.05	0.01	0.03	0.62	3.02
4 (328)	Vys	1.04	0.04	0.03				0.64			0.20	0.91

Note: * Co 0.07; ** Co 0.07; *** Co 0.01. Pt = Native platinum. Crh = Cuprorhodsite. Vys = vysotskite. Me = Metal elements.

3.1.2. Sample A2 Area 6

(a): Host alloy and ferrobasalt silicate glass (SiO₂ 47.83 wt. %).

This host Pt-Fe nugget (~0.7 mm diameter) is rounded with a partly broken margin (Figure 3a,b). Analysis 276 (Barron et al. [1]: their Tables 1A and 1B) indicates isoferroplatinum (Pt_2.95Rh_0.08Pd_0.05 Ir_0.01)_3.09(Fe_0.74Cu_0.17)_0.91 with minor Cu, Rh, and Pd.

Seven inclusions are present, four of which are considered here (Figure 3a,b). The largest is a rounded silicate glass inclusion (~28 µm diameter) with a small vesicle (possible gas cavity) about 7 µm across, located near the margin. The silicate glass has a narrow but irregular partial rim of exsolved PGM with Cu-sulfides, the boundary of which against the host nugget is exceptional. It shows well-preserved quench textures of cuspate crests and related small “droplets” within the host Pt-Fe alloy. A similar, very narrow rim of exsolved Cu-PGM occurs around a smaller silicate glass inclusion not considered here. Three smaller, rounded inclusions are Cu-PGM parageneses, like that of the exsolved PGM sulfide rim of the largest silicate glass inclusion (above). They show similar, well-preserved, irregular boundaries against the host nugget.

Analysis 172 of the silicate glass (Barron et al. [1] their Table 2E) indicates ferrobasalt with SiO₂ 47.83, FeO 15.68, MgO 5.52, and minor H₂O at~1.17 (wt. %).

(b): PGM and Cu-sulfide paragenesis

Sulfide parageneses and their textural features are similar in each of the five melt inclusions (Figure 3). Small subhedral crystals are braggite and cuprorhodsite, while zoned braggite occurs as subhedral to anhedral patches (up to 13 µm long). Bornite contains up to ~20% of PGE monosulfide as exsolved crystallites (see below). Some of the latter are subhedral (up to ~3.0 µm grain size), and many are skeletal, branching, and crystallographically controlled (reaching ~5 µm long). Bornite also hosts subordinate irregular patches of chalcopyrite (yellow, Figure 3a).

Braggite occurs as subhedral white crystals (Figure 3f). Analysis 8 (324), Table 2, shows Pt at 35.21 and Pd 5.56 with 4.17 at. % minor elements (Cu, Fe, Rh, Ni, and Os) and slightly high S. A second braggite analysis, analysis 9 (325) (Table 2, Figure 3c), shows Pt 19.64, Pd 19.55 (at. %), and minor PGE 6.19 (at. %) (cf. Figure 1, Cabri and McDonald [19]). This analysis is for an anhedral interstitial grain with marginal zoning. It occurs within the narrow rim of PGM sulfide parageneses adjacent to the largest silicate glass inclusion (Figure 3c).

Table 2. WDS analyses. Sample A2 area 6.

# wt. %	Mineral	S	Se	Fe	Ni	Co	Cu	Ru	Rh	Pd	Os	Ir	Pt	Total
1 (276)	Ifp			6.37			1.69		1.27	0.83		0.22	88.62	99.00
(320)	Ms	27.15	0.16	4.85	0.02	0.16	27.53	0.08	15.74			1.13	19.92	96.73
4 (321)	Crh	28.98	0.16	2.41	0.15	0.22	13.71	0.13	25.18		0.09	0.90	27.75	99.69
8 (324)	Bg	18.72		0.71	0.06		1.61		0.46	6.27	0.11		72.84	100.79
9 (325)	Bg	21.41	0.34	0.50	0.98		1.45		2.69	25.61		0.34	47.16	100.47
# at. %	Mineral	S	Se	Fe	Ni	Co	Cu	Ru	Rh	Pd	Os	Ir	Pt	Total
1 (276)	Ifp			18.51			4.31		2.00	1.27		0.19	73.73	100.00
(320)	Ms	51.83	0.13	5.31	0.02	0.16	26.53	0.05	9.36			0.36	6.25	100.00
4 (321)	Crh	57.77	0.13	2.76	0.17	0.24	13.79	0.09	15.64		0.03	0.30	9.09	100.00
8 (324)	Bg	55.06		1.20	0.09		2.40		0.42	5.56	0.06		35.21	100.00
9 (325)	Bg	54.26	0.35	0.73	1.35		1.85		2.12	19.55		0.14	19.64	100.00
# apfu	Mineral	S	Se	Fe	Ni	Co	Cu	Ru	Rh	Pd	Os	Ir	Pt	Me
1 (276)	Ifp			0.74			0.17		0.08	0.05		0.01	2.95	4.00
(320)	Ms	1.00		0.10			0.51		0.18			0.01	0.12	0.92
4 (321)	Crh	3.99	0.01	0.19	0.01	0.02	0.95	0.01	1.08			0.02	0.63	2.91
8 (324)	Bg	1.00		0.02			0.04		0.01	0.10			0.64	0.81
9 (325)	Bg	0.99	0.01	0.01	0.03		0.03		0.04	0.36			0.36	0.83

Note: Bg = Braggite; Crh = cuprorhodsite–(malanite); Ifp = isoferroplatinum; Ms = monosulfide mineral. Italics, semiquantitative analysis, small grain size < 3 µm, cf. Nesterenko et al. [20].

A third, semiquantitative [20] analysis (317) is for an irregular, narrow (~3 µm across) zoned grain (Figure 3d), indicating that Pd reaches 30.76, and Pt is 11.41 at. % (+ ~6.50 at. % PGE minor elements, and Se is also present). The formula for this composition (Fe_0.02Ni_0.05Cu_0.05Pd_0.59Pt_0.22S_1.00) defines vysotskite (cf. Cabri and McDonald [19], Figure 1).

Cuprorhodsite–(malanite) occurs as mid-grey subhedral crystals (Figure 3e) set in bornite. Analysis 4 (321), Table 2, gives the formula Cu_0.95(Rh_1.08Pt_0.63Fe_0.19Ir_0.02Co_0.02Ni_0.01Ru_0.01)_1.95(S_3.99Se_0.01)_4.00, with Rh at 15.64 at. % and Pt(+Ir) at 9.39. Bornite and chalcopyrite are confirmed as anhedral interstitial minerals. Semiquantitative [20] analyses 318 and 323 (Figure 3d,f) confirm bornite. Analysis 322 (Figure 3e) confirms chalcopyrite. The respective formulae are Cu_4.69Fe_0.90S_3.99Se_0.01 for bornite and Cu_1.00Fe_0.90S_2.00 for chalcopyrite.

A monosulfide mineral occurs as irregular and skeletal-shaped exsolutions set in host bornite [Figure 3c,e]. WDS analysis (320) (Table 2) of the latter [a small (<3 µm) grain] gives a slightly low total; thus, it is semiquantitative (cf. Nesterenko et al. [20]). However, a distinctive formula indicates a Cu-rich, Ni-poor PGE monosulfide mineral (Cu_0.55Fe_0.11)_∑0.66(Rh_0.19Pt_0.13Ir_0.01)_∑0.33S_1.00 with dominant Cu(+Fe); subordinate PGE; and minor detectable Co, Ru, and Ni (cf. Tolstykh & Krivenko [21]).

3.1.3. Sample Mag 5 Area 4

(a): Host alloy and basaltic andesite silicate glass (SiO₂ 52.30 wt. %)

The host nugget for this sample is oval-shaped (~0.22 mm long dimension) and hosts a composite rounded inclusion (~0.09 mm diameter) located near one margin (Figure 4a).

Analysis 129 (Tables 1A and 1B of Barron et al. [1]) indicates isoferroplatinum (Pt_2.89Ir_0.06Pd_0.02Os_0.01)_2.98(Fe_0.88Cu_0.06Rh_0.08)_1.02 with minor Rh, Cu, Pd, Os, and S. The inclusion comprises a remarkable PGM paragenesis coexisting with a small, rounded to partly irregular “pool” of exsolved silicate glass partly infilled with late Pt-Fe alloys and subhedral chalcopyrite (Figure 4a,b).

The small “pool” of host silicate glass shows a minutely scalloped boundary with the variably thick rim of exsolved PGM. The boundary of the latter against the host nugget is partly rounded and partly finely scalloped. A small void along part of the curved margin of the inclusion suggests minor compressional deformation (possibly in the alluvial pile) accompanied by distinct parallel brittle fractures in the PGM paragenesis.

Analyses of the silicate glass inclusion indicate tholeiitic basaltic andesite with SiO₂ 52.30, FeO 12.96, and H₂O at ~0.45 wt. % (average of analyses Sp24 and Sp25, Barron et al. [1] their Table 2C). Silicate glass accounts for ~5.7% of the total composite inclusion.

(b): PGM sulfide paragenesis

Phase mapping, based on EDS analysis of this inclusion (Figure 5a), gives an approximate fraction % based on relative pixel counts for each phase. There are five significant minerals detected, and their approximate fraction % (minus 5.7% silicate glass) is as follows: Green shows the Pt-Fe alloy 3.18; PtSRhFe (composite cuprorhodsite + alloy = monosulfide) 24.92; PtSPd (braggite) 30.22; SCuFePt (chalcopyrite) 33.40; and FeS (pyrrhotite) 8.27. A minor mineral (analysis FeSCu) is also detected.

An approximate calculated bulk chemistry (Fe > Cu > Pt >>> Pd > Rh >> Ir > Ni > Co > Os at. %) based on EDS element mapping of the PGM paragenesis is shown in Table 3.

A composite monosulfide mineral, PtSRhFe identified by phase mapping (Table 4), is shown in tan with orange exsolutions (Figure 5a). It comprises small (from ~2 µm up to ~8 µm long and ~3–4 µm across) subprismatic to equant crystals that are evenly disseminated throughout. This mineral accounts for ~23.5% of the inclusion area in the plane of the present section (see above). In some domains, the crystals are sub-parallel, but elsewhere, they are unoriented. Thin exsolved lenses are subparallel and crystallographically controlled in many larger crystals, but some smaller crystals (minus alloy lenses) are homogeneous in the present section (Figure 5a,b).

Figure 5. Sample Mag 5 area 4. BSE images. Analysis points listed in Table 5. (a) EDS phase-mapped false colors of host isoferroplatinum (green); native platinum (green); silicate glass (black); and Cu-PGM paragenesis: braggite (pink), chalcopyrite (dark brown), pyrrhotite (dark blue), cuprorhodsite (tan with orange exsolutions), and minor isocubanite (FeSCu, dark purple). (b) Part of inclusion showing WDS analysis points: Isoferroplatinum, analysis 130; pyrrhotite, 131; cuprorhodsite without exsolutions, 132; braggite, 133; and chalcopyrite, 134. Silicate glass (black) [1].

The lenses in the PtSRhFe host mineral, as shown in the EDS phase map analysis (Figure 5a), are too fine-grained for accurate EMPA. However, as a composite PGM, both lenses and their host mineral are analyzed together (analysis 3PtSRhFe, Table 4, normalized minus minor O). This composite mineral analysis, including the host mineral and narrow lenses, results in a Me (metal)-deficient formula indicating the monosulfide mineral (Fe_0.23Pt_0.21Cu_0.16Rh_0.15Pd_0.04Ir_0.02Os_0.01Co_0.01)_0.83S_1.00. However, WDS analysis 3 (132) (Table 4 and Table 5; Figure 5b) of the host mineral lacking exsolutions shows Fe-, Pt-, and Pd-poor cuprorhodsite–(malanite) with formula (Cu_0.79Fe²⁺_0.19Co_0.02)_1.00(Rh_1.14Pt_0.40Fe³⁺_0.25Ir_0.15Os_0.04)_1.98S_4.02. This suggests that a composite monosulfide precursor mineral has exsolved Fe-Pt-(Pd) alloy lenses in Fe-, Pt-, and Pd-poor cuprorhodsite.

A calculated approximate composition for the exsolved alloy lenses is ~Fe 46.48, Pt 41.07, and Pd 12.44 at. % (Table 4), indicating Pd-bearing tetraferroplatinum near PtFe (cf. Cabri et al.; Jung et al. [22,23]).

Table 4. Analyses of sample Mag 5 area 4. Composite monosulfide mineral and cuprorhodsite-lacking alloy. Calculation of exsolved tetraferroplatinum exsolutions.

3PtSRhFe ^Δ	S	Se	Fe	Co	Ni	Cu	Ru	Rh	Pd	Os	Ir	Pt	Total
wt. % Ms	26.85		10.90	0.24	0.15	8.27		12.60	2.86	0.84	3.13	34.18	100.00
at. % ~Ms	55.30		12.89	0.26	0.17	8.59		8.09	1.77	0.29	1.07	11.57	100.00
apfu ~Ms	1.00		0.23	0.01		0.16		0.15	0.04	0.01	0.02	0.21	0.83
3(132) ^	S	Se	Fe	Co		Cu	Ru	Rh	Pd	Os	Ir	Pt	Total
wt. % Crh	29.19	0.03	5.58	0.29		11.39	0.24	26.48		1.84	6.73	17.78	99.55
at. % Crh	57.25	0.02	6.28	0.31		11.27	0.15	16.18		0.61	2.20	5.73	100.00
apfu Crh	4.02		0.44	0.02		0.79	0.01	1.14		0.04	0.15	0.40	Me 2.99
Alloy *			6.61						1.77			5.84	14.22
~Tfpt at. %			46.48						12.44			41.07	100.00

Note: ^Δ Phase map EDS 3PtSRhFe (minus minor O, includes alloy lenses). ^ WDS 3 (132). Ms = Monosulfide. Crh = Cuprorhodsite–(malanite). Tfpt = Tetraferroplatinum. * Calculated exsolved alloy (Fe, Pd, and Pt); PtSRhFe minus Fe, Pd, and Pt of Crh 3 (132).

Pt-Fe alloy, green (Figure 5a), accounts for only ~3.0% of the inclusion area and occurs as anhedral patches with variable grain size (up to ~5 µm across). Analysis 1 (130) (Table 5 and Figure 5b) shows that this alloy is also isoferroplatinum, distinct from the host isoferroplatinum, with unusually high concentrations of minor elements (8.35 at. %): Cu (2.91 at. %), Pd (2.65 at. %), Au (1.82 at. %), Rh, Ag, and Os (in order of abundance).

Table 5. WDS analyses of sample Mag 5 area 4.

# wt. %	Mineral	S	Fe	Co	Ni	Cu	Rh	Pd	Ag	Os	Ir	Pt	Total
(129)	Ifp	0.05	7.82			0.37	0.69	0.31		0.28	0.45	90.21	100.17
1 (130) *	Ifp	0.07	10.27			1.26	0.46	1.92	0.16	0.12		85.53	102.31
2 (131) ^†	Po	37.78	58.52	0.04	0.07	0.22	0.21	0.10				0.71	97.70
3 (132) ^††	Crh	29.19	5.58	0.29		11.39	26.48			1.84	6.73	17.78	99.55
4 (133)	Br	17.79	2.17	0.02	0.11	3.01	0.90	4.50		0.19	0.22	69.80	98.69
8FeSCu ^	Icb	34.56	40.42	0.57	1.05	22.34						1.05	100.00
Note: * Te: 0.06; Au: 2.45. ^† Ca: 0.04; ^†† Se: 0.03; Ru: 0.24.
# at. %	Mineral	S	Fe	Co	Ni	Cu	Rh	Pd	Ag	Os	Ir	Pt	Total
(129)	Ifp host	0.23	22.47			0.94	1.08	0.46		0.24	0.37	74.21	100.00
1 (130) *	Ifp	0.34	26.96			2.91	0.66	2.65	0.22	0.09		64.27	100.00
2(131) ^†	Po	52.62	46.80	0.03	0.05	0.15	0.09	0.04				0.16	100.00
3 (132) ^††	Crh	57.25	6.28	0.31		11.27	16.18			0.61	2.20	5.73	100.00
4 (133)	Br	52.63	3.68	0.03	0.18	4.49	0.83	4.01		0.09	0.11	33.95	100.00
8 FeSCu ^	Icb	49.30	33.09	0.44	0.82	16.08						0.25	100.00
Note: * Te: 0.07; Au: 1.82. ^† Ca: 0.04. ^†† Se: 0.02; Ru: 0.15.
# apfu	Mineral	S	Fe	Co	Ni	Cu	Rh	Pd	Ag	Os	Ir	Pt	Me
(129)	Ifp	0.01	0.90			0.04	0.04	0.02		0.01	0.01	2.97	4.00
1(130) *	Ifp	0.01	1.08			0.12	0.03	0.11	0.01			2.57	3.99
2(131) ^†	Po	1.00	0.89										0.90
3(132)	Crh	4.00	0.44	0.02		0.79	1.13			0.04	0.15	0.40	2.97
4(133)	Br	1.00	0.07			0.09	0.02	0.08				0.65	0.91
8 FeSCu ^	Icb	2.97	2.00		0.06	0.97							3.03
Note: * Au: 0.07; ^† (Co 0.001 + Ni 0.001 + Cu 0.003 + Rh 0.002 + Pd 0.001 + Pt 0.003 = 0.01).

Note: ^ EDS, minus minor O, Si, and Ca. Ifp = Isoferroplatinum; Po = pyrrhotite; Crh = cuprorhodsite–(malanite) minus alloy lenses; Br = braggite. Icb = Isocubanite.

Braggite, shown in pink in Figure 5a, accounts for ~28.5% of the inclusion area and forms elongate (possibly deformed) anhedral patches enclosing subhedral prisms of the composite monosulfide mineral with alloy exsolutions (above). Analysis 4 (133) (Figure 5b, Table 5) shows that this mineral, with Pt 69.80 wt.% and Pd 4.50 wt. %, contains significant (6.62 at. %) minor elements (Cu, Rh, Ni, Ir, Os, Co, and Ir).

Chalcopyrite, shown in yellow in Figure 5a, accounts for ~31.5% of the inclusion area. It is a prominent anhedral, interstitial mineral in three quadrants of the thick rim of PGM sulfides and forms an unevenly distributed mineral throughout the fourth quadrant, where it is intergrown with pyrrhotite (below). A semiquantitative analysis 5 (134) (Figure 5b) confirms chalcopyrite Cu_0.50Fe_0.50S_1.02 with minor detectable <0.02 at. % elements Se, Pt, Zn, and Ca.

Pyrrhotite, shown in blue in Figure 5a, accounts for ~7.8% of the inclusion area. It is also anhedral and interstitial and intergrown with minor patchy chalcopyrite. Analysis 2 (131) (Table 5) shows that seven minor elements are detected: Pt at 0.71 wt. % and <0.22 wt. %, each of Cu, Rh, Ni, Ca, Pd, and Co (in decreasing order of abundance).

Isocubanite, identified by phase mapping (Figure 5a), accounts for only ~1.1% of the inclusion area. It occurs as irregular, narrow interstitial patches up to ~5.8 µm long but is < 3 µm across, so the crystals are too narrow for accurate EPMA. An EDS analysis (8FeSCu Table 5) indicates isocubanite (Fe_0.66Cu_0.32Ni_0.02)_1.00S_0.98, which was first defined by Caye et al. [24].

3.1.4. Sample A1 Area 9

(a): Host alloy and basaltic andesite silicate glass (SiO₂ 52.75 wt. %)

The host nugget for this sample is ~0.29 mm across. It is rounded but is partly chipped and broken (Figure 6a). Analysis (267) (Barron et al. [1], their Tables 1A and 1B) indicates isoferroplatinum (Pt_2.89Ir_0.04Pd_0.02Rh_0.02Os_0.01Ru_0.01)_2.99(Fe_0.88Cu_0.06Rh_0.06)_1.00 with minor Rh, Cu, Ir, Pd, Os, and Ru.

Three inclusions are present (Figure 6a). The first inclusion is slightly fractured, homogeneous silicate glass (~42.9 µm across) lacking coexisting exsolved PGMs. Two separate inclusions (~75–90 µm across and ~20 µm across) have subrounded shapes with somewhat irregular margins and comprise melt inclusions with similar Cu-PGM sulfide parageneses. Three separate Cu-PGM sulfide “droplet”-shaped inclusions are minute (<4 µm across).

Analysis 154 of the largest of two silicate glass inclusions indicates alkali-enriched basaltic andesite with SiO₂ 52.75, FeO 10.54, and H₂O ~1.71 (wt. %) (Barron et al. [1]; Table 2E and Figure 6).

(b): Cu-PGM sulfide paragenesis

The texture of the largest PGM-bearing sulfide inclusion shows that subhedral crystals (up to ~10 µm) are set in anhedral interstitial minerals (Figure 6b).

The Pt-Fe alloy forms irregularly disseminated, sparse, and small (up to ~6 µm) subhedral crystals, but this mineral is absent in the smaller inclusion in the plane of the present section. Analysis 4 (314) (Table 6) indicates isoferroplatinum (Pt_2.75Pd_0.08Rh_0.05Os_0.01Cu_0.12Ni_0.01)_3.02Fe_0.99, with significantly high minor Pd, Cu, Fe, and Ni and low concentrations of Rh and detectable S. This is distinct from the host alloy in analysis 1 (267).

Table 6. WDS analyses of sample A1 area 9.

# wt. %	Mineral	S	Se	Fe	Co	Ni	Cu	Ru	Rh	Pd	Os	Ir	Pt	Total
1 (267)	Ifp host			7.67		0.04	0.62		1.29	0.29	0.28	1.72	88.54	100.45
2 (312) *	Vs	13.24	0.11	0.36		0.12	11.39		0.10	74.19			0.29	99.85
3 (313)	Crh	30.39		3.28	0.03	0.79	10.50	0.25	28.82		0.37	2.64	25.18	102.25
4 (314)	Ifp incl	0.06		9.07		0.13	1.25		0.78	1.37			88.32	100.98
5 (315) ^†	Cpy	35.67		27.21		0.16	31.61		0.39	0.23		0.10	0.62	96.03
6 (316) ^{^}	Pn	31.64		30.16	0.46	19.14	3.80		2.91	8.06		0.08	0.26	96.55
# at. %	Mineral	S	Se	Fe	Co	Ni	Cu	Ru	Rh	Pd	Os	Ir	Pt	Total
1(267)	Ifp host			21.90		0.11	1.56		1.99	0.44	0.23	1.43	72.34	100.00
2 (312) **	Vs	31.71	0.10	0.50		0.15	13.76		0.08	53.55			0.11	100.00
3 (313)	Crh	58.76		3.64	0.03	0.83	10.24	0.15	17.36		0.12	0.85	8.00	100.00
4 (314)	Ifp incl	0.28		24.64		0.34	2.98		1.16	1.95			68.66	100.00
5 (315) ^††	Cpy	52.73		23.09		0.13	23.57		0.18	0.10		0.02	0.15	100.00
6 (316) ^{^^}	Pn	48.69		26.62	0.39	16.09	2.95		1.40	3.74		0.02	0.07	100.00
# apfu	Mineral	S	Se	Fe	Co	Ni	Cu	Ru	Rh	Pd	Os	Ir	Pt	Me
1 (267)	Ifp host			0.88			0.06		0.08	0.02	0.01	0.06	2.89	4.00
2 (312) ***	Vs	6.97	0.02	0.11		0.03	3.03		0.02	11.99			0.02	15.20
3 (313)	Crh	4.00		0.25		0.06	0.73	0.01	1.24		0.01	0.06	0.54	2.93
4 (314)	Ifp incl			0.99		0.01	0.12		0.05	0.08	0.01		2.75	4.01
5 (315) ^††	Cpy	2.00		0.88			0.89		0.01				0.01	1.79
6 (316) ^{^^}	Pn	8.00		4.37	0.06	2.64	0.48		0.23	0.61			0.01	8.42

Note: * Te: 0.06; ** Te: 0.03; *** Te: 0.01. ^† Zn: 0.03; ^†† Zn: 0.02. ^{^} Zn: 0.03; ^{^^} Zn: 0.02. Cpy = Chalcoppyrite; Ifp = isoferroplatinum; Pn = pentlandite; Vs= vasilite. Italics, semiquantitative analyses, small grain size ~3 µm [20].

Cupororhodsite–(malanite) also forms subhedral crystals (some reaching >7 µm grain size) and accounts for ~40% of the inclusion area (Figure 6b). Analysis 3 (313) (Table 6) gives the formula (Cu_0.69Fe_0.25Ni_0.06)_1.00(Rh_1.18Pt_0.54Ir_0.06Os_0.01Ru_0.01)_1.80S_4.00. This analysis is distinctly Me-deficient and contains an unusually high number (9) of detectable metal elements.

Vasilite (~20% of the inclusion area) forms anhedral to subhedral crystals (up to ~6 µm) interstitial to the cupororhodsite–(malanite) above (Figure 6b). Analysis 2 (312) (Table 6) gives a slightly Me-deficient formula: (Pd_11.77Cu_3.03Fe_0.11Ni_0.03Pt_0.02Rh_0.02)_14.99(S_6.97Se_0.02Te_0.01)_6.99.

Chalcopyrite is anhedral forming irregular interstitial patches (up to ~7 µm across). It accounts for ~20% of the relevant section area (Figure 6b). Analysis 5 (315) (Table 6) indicates 0.45 at. % PGE (Rh, Pt, Pd, and Ir), and the calculated formula (Cu_0.89Fe_0.88Zn _0.02Pt_0.01Rh_0.01)_1.81S_2.00 suggests that it is Me-deficient.

Pentlandite forms sparse anhedral patches intergrown with interstitial chalcopyrite (Figure 6b). Analysis 6 (316) indicates the formula (Fe_4.37Ni_2.64Pd_0.61Cu_0.48Rh_0.23Co_0.06Zn_0.02Pt_0.01)_8.42S_8.00 [(Me)_9-xS₈, x = 0.58]. It incorporates significant minor elements, particularly Pd, Cu, and Rh.

3.1.5. Sample B Area 5

(a): Host alloy and andesite silicate glass (SiO₂ 59.89 wt. %)

The rounded host nugget for this sample reaches ~0.4 mm across. It is isoferroplatinum (Pt_2.88Pd_0.01Rh_0.08Os_0.01Ir_0.02)_3.00(Fe_0.95Cu_0.05S_0.01)_1.01, with minor Rh, Cu, Pd, Ir, Os, and Ni, as shown in analysis 061 (Barron et al. [1]; their Tables 1A and 1B). A prominent round composite inclusion measuring ~175 µm (~0.18 mm) across is not centrally located in the host nugget. It comprises a dominant rim of PGM enclosing a small rounded “pool” (~ 50 µm across) of quenched silicate glass located near one margin of the PGM paragenesis (Figure 7a,b).

Analyses of the silicate glass in this composite inclusion indicate andesite with SiO₂ 59.89, FeO 3.25, and H₂O ~1.06 (wt. %) (average of analyses Sp38, Sp39, and Sp40 (Barron et al. [1], Table 2D)).

(b): PGM sulfide paragenesis

Phase mapping of the inclusion using EDS analyses and relative pixel counts for each phase indicates that silicate glass is 6.72 in fraction %. Eight minerals are identified in the PGM paragenesis (Figure 8), and the normalized fraction % for each is as follows (fraction % is calculated minus unassigned pixels): Pt-Fe alloy: 4.98; Rh-Cu-S cuprorhodsite: 80.25; Pd-Cu-S vasilite: 4.98; Rh-Cu-Fe-S thiospinel: 3.98; Pt-Fe-Pd alloy: 2.59; Rh-Cu-Ni-S thiospinel: 2.00; Rh-Ni-Cu-S thiospinel: 1.00; Pd-Te keithconnite: 0.20; hematite: 0.20.

An approximate calculated bulk chemistry based on the EDS element mapping of the PGM paragenesis is given in Table 7. The order of major elements present is as follows: Rh >> Cu > Fe >> Pd > Ni.

Cuprorhodsite (Figure 7 and Rh-Cu-S in Figure 8b) is the dominant mineral (80.25 fraction %) in this inclusion. It forms a spongy framework of skeletal to subhedral and elongate prisms, showing partial radial structure oriented away from the silicate “pool”. Some curved narrow cuprorhodsite crystals are intergrown with ~20 fraction % of all other finer-grained patchy minerals. Analysis 4-2 (067) (Table 8) indicates (Cu_0.52Fe_0.42Ni_0.02)_0.96(Rh_1.94Ir_0.02Pt_0.03)_1.99(S_4.02Se_0.01)_4.03, and phase-mapped analysis 3 (22), as shown in Table 9, also indicates cuprorhodsite (Cu_0.56Fe_0.44)_1.02(Rh_1.91Ni_0.03Fe_0.02)_1.96(S_3.99Se_0.01).

Table 8. WDS analyses. Sample B Area 5.

# wt. %	Mineral	S	Fe	Ni	Cu	Se	Rh	Pd	Te	Os	Ir	Pt	Total
6 (061)	Ifp Host ^◊	0.04	8.31	0.03	0.50		1.22	0.17		0.18	0.64	88.29	99.40
1 (063)	PdCu Vs	13.53	0.12	0.04	13.60	0.09	0.06	72.56				0.52	100.53
2 (064)	PdTe Kei *	0.12	0.21	0.04	0.43		0.09	70.97	29.31	0.11			102.26
4-2 (067)	Crh ^□	33.07	5.96	0.26	8.54	0.19	51.12				1.17	1.68	102.01
5-2 (069)	Ifp Rim ^†	0.04	9.75	0.27	1.36		0.65	11.24		0.13		75.94	99.54
7 (070)	PtFe Incl.	0.08	9.17	0.10	1.27		2.87	2.57		0.14		84.68	100.88
9 (071)	PdCu Vs	13.66	0.22	0.35	13.53	0.21		73.33	0.10			0.45	101.84
13 ^	Ms ^	27.05	12.40	11.00	5.74		20.60			0.12	7.89	12.79	101.07
Note: ^◊ Co: 0.02. * Sn: 0.11; Sb: 0.06; Pb: 0.45; Bi: 0.87; Ag: 0.10. ^□ Co: 0.02. ^† Au: 0.16. ^ As: 0.1; Ru: 0.07.
# at. %	Mineral	S	Fe	Ni	Cu	Se	Rh	Pd	Te	Os	Ir	Pt	Total
6 (061)	Ifp host ^◊	0.18	23.65	0.07	1.24		1.89	0.26		0.15	0.53	71.96	100.00
1 (063)	PdCu Vs	31.83	0.17	0.06	16.15	0.09	0.04	51.46				0.20	100.00
2 (064)	PdTe Kei **	0.42	0.40	0.08	0.73		0.10	72.35	24.91	0.06			100.00
4-2 (067)	Crh ^□	57.55	5.94	0.26	7.51	0.14	27.67				0.33	0.59	100.00
5-2 (069)	Ifp Rim ^††	0.19	24.78	0.66	3.03		0.90	14.99		0.10		55.23	100.00
7 (070)	Ifp Incl	0.39	24.31	0.24	2.97		4.13	3.57		0.11		64.28	100.00
9 (071)	PdCu Vs	31.71	0.29	0.44	15.84	0.20		51.29	0.06			0.17	100.00
13	Ms ^	51.05	13.43	11.34	5.46		12.11			0.04	2.48	3.97	100.00
Note: ^◊ Co: 0.06. ** Sn: 0.10; Sb: 0.06; Pb: 0.23; Bi: 0.45; Ag: 0.11. ^□ Co: 0.02. ^†† Au: 0.12. ^ As: 0.08; Ru: 0.04.
# apfu	Mineral	S	Fe	Ni	Cu	Se	Rh	Pd	Te	Os	Ir	Pt	∑Me
6 (061)	Ifp host	0.01	0.95		0.05		0.08	0.01		0.01	0.02	2.88	4.01
1 (063)	PdCu ~Vs	7.47	0.04	0.01	3.79	0.02	0.01	12.09				0.05	15.99
2 (064)	PdTe Kei ***	0.11		0.02	0.20		0.03	19.60	6.75	0.02			19.93
4-2 (067)	Crh	4.02	0.42	0.02	0.52	0.01	1.94				0.02	0.03	2.98
5-2 (069)	Ifp Rim ^†††		1.00	0.03	0.12		0.04	0.60				2.21	4.00
7 (070)	Ifp Incl		0.98		0.12		0.16	0.16				2.56	3.98
9 (071)	PdCu ~Vs	7.44	0.07	0.10	3.72	0.05		12.03	0.01			0.04	16.01
13	Ms	1.00	0.26	0.22	0.11		0.23				0.05	0.08	0.95
Note: *** Pb: 0.06; Sn: 0.03; Sb: 0.02; Bi: 0.12; Ag: 0.03. ^††† Au: 0.01. Ifp = Isoferroplatinum; Crh = cuprorhodsite; Kei = keithconnite; Vs = vasilite; Ms = monosulfide mineral. Italics, semiquantitative analysis, small grain size, cf. Nesterenko [20].

Table 9. Sample B Area 5. PGM analyses (EDS based on pixel counts).

# wt. %	Mineral	Elements	S	Fe	Ni	Cu	Rh	Pd	Pt	O	Total
3 (22)	Crh ^	Rh-Cu-S	33.17	6.53	0.41	9.19	50.49				100
6 (25)	PtFePdCu	Pt-Fe-Pd		10.27		1.56		10.91	77.25		100
7 (26)	Crh-Pld	Rh-Cu-Ni-S	33.48	1.83	4.68	9.96	50.05				100
10 (29)	Crh-Pld	Rh-Ni-Cu-S	35.13	4.68	13.32	7.57	39.30				100
11 (30)	Ms	Rh-Cu-Pt-S	25.00	6.86	0.75	6.49	34.06		26.83		100
14 (33)	* Crh-(Fe)	Rh-Cu-Fe-S	32.81	7.95	0.81	11.94	46.48				100
16 (35)	⁺ Hem	Fe-O	0.61	68.07	0.88					29.56	100
#	at. %		S	Fe	Ni	Cu	Rh	Pd	Pt	O	Me
3 (22)	Crh ^^	Rh-Cu-S	57.02	6.51	0.39	8.05	27.31				42.41
6 (25)	PtFePdCu	Pt-Fe-Pd		26.01		3.48		14.51	56.00		100
7 (26)	Crh-(Pld)	Rh-Cu-Ni-S	58.02	1.82	4.43	8.71	27.02				41.98
10 (29)	Crh-(Pld)	Rh-Ni-Cu-S	57.44	4.40	11.90	6.25	20.02				42.57
11 (30)	Ms	Rh-Cu-Pt-S	52.47	8.27	0.86	6.87	22.27		9.25		47.52
14 (33)	* Crh-(Fe)	Rh-Cu-Fe-S	56.24	7.83	0.76	10.33	24.83				43.75
16 (35)	⁺⁺ Hem	Fe-O	0.61	38.91	0.48					58.98	98.37

Note: * Recalculated minus O. ⁺ Al: 0.35; Si: 0.54. ⁺⁺ Al: 0.42; Si: 0.61. ^ Se = 0.22; ^^ Se = 0.15. Crh = Cuprorhodsite. Pld = Polydymite. Ms = Monosulfide mineral. Hem = Hematite.

Three additional Rh-thiospinel minerals (notably Pt-deficient) are identified by chemical differences in element abundances in phase maps (using pixel counts):

(1): Rh-Cu-Fe-S thiospinel [red in Figure 8d, analysis 14 (33), Table 9] fills some sub-parallel and curved grain boundaries and marks the silicate/PGM boundary. It has an Fe-enriched composition (Cu_0.73Fe_0.27Ni_0.05)_1.05(Rh_1.76Fe_0.29)_2.05S_3.99. In this analysis, after Rh (24.83 at. %), Cu and Fe have the highest concentrations (Cu 10.33 and Fe 7.83 at. %), while Ni (0.76 at. %) is a minor metal element. Thus, this minor thiospinel is ferrorhodsite (cf. Cabri et al. [19]). In Figure 8d, this mineral is fine-grained with a patchy distribution and defines some sub-parallel curving brittle fractures.
(2): Rh-Cu-Ni-S thiospinel is red in Figure 8f, and analysis 7 (26), as shown in Table 9, indicates that it is cuprorhodsite–(polydymite): (Cu_0.60Ni_0.17Fe_0.13)_0.90(Rh_1.86Ni_0.14)_2.00S_4.00. This mineral occurs along part of the rounded inclusion boundary with the host Pt-Fe alloy. It also forms small blebs and partly defines minor fractures along the silicate glass boundary with the PGM sulfide fraction.
(3): Rh-Ni-Cu-S thiospinel is a well-defined small anhedral pink mineral in Figure 8g. It forms an irregular aggregate (~23 µm long) in the PGM paragenesis adjacent to the rounded silicate glass fraction. Analysis 10 (29), as shown in Table 9, indicates polydymite–(cuprorhodsite) (Cu_0.44Fe_0.30Ni²⁺_0.26)_1.00(Rh_1.40Ni³⁺_0.56)_1.96S_4.00, with significantly higher concentrations of Ni than (2) above.

Vasilite [Pd-Cu-S Figure 8c] also occurs as fine-grained patches but has an uneven distribution. Analysis 1 (063) (Table 8) indicates (Pd_12.09Cu_3.80Pt_0.05Fe_0.04Ni_0.01Rh_0.01)_16.00(S_7.48Se_0.02)_7.50.

The minor monosulfide mineral (red specks in Rh-Cu-Pt-S Figure 8h) occurs as small grains crystallized along mineral boundaries and fractures. Analysis 11 (30), as shown in Table 9, indicates (Rh_0.42Pt_0.18Fe_0.16Cu_0.13Ni_0.02)_0.91S_1.00. The latter is Me-deficient.

Keithconnite and hematite each account for only 0.2 fraction % of the PGM paragenesis. Keithconnite forms minor, very small anhedral patches similar to vasilite. Analysis 2 (064) (Table 8) indicates keithconnite with formula (Pd_19.44Cu_0.20Fe_0.11Pb_0.06Rh_0.03Ag_0.03Os_0.02Ni_0.02)_19.94(Te_6.72S_0.11Bi_0.12Sb_0.02)_6.97. This has a remarkably high Me:S = 2.86. Hematite occurs as wispy grains marking the boundary between the silicate and PGM fractions. Analysis 16 (35), as shown in Table 9, indicates (Fe_1.95Ni_0.02Al_0.02Si_0.03)_2.02O_3.00S_0.03.

The minor Pt-Fe alloy is approximately evenly distributed as fine-grained (<~5 µm across) anhedral patches (white in Figure 7a,b and red Pt-Fe in Figure 8a). Analysis 7 (070) Table 8 indicates isoferroplatinum (Pt_2.56Pd_0.16Rh_0.16)_2.98(Fe_0.96Cu_0.12)_1.08 with significant Pd 3.57, Rh 4.13, and Cu 2.97 (at. %) and detectable Ni. A second minor Pt-Fe-Pd alloy (also red in Figure 8e) forms a wispy discontinuous boundary with the host nugget. Analysis 6 (25) (Table 9) indicates Pd (and Cu)-enriched isoferroplatinum [(Pt_2.24Pd_0.58Cu_0.14)_2.96Fe_1.04]_4.00.

3.1.6. Sample B Area 4

(a): Host alloy and porphyritic dacitic/rhyolitic groundmass silicate glass (SiO₂ 70.67 wt. %)

The host nugget for this sample is round (~0.48 mm diameter) but slightly irregular (Figure 9). Analysis 051 (Tables 1A and 1B of Barron et al. [1]) indicates isoferroplatinum (Pt_2.90Pd_0.04Ir_0.02Os_0.01)_3.05(Fe_0.88Rh_0.08Cu_0.05Ni_0.01S_0.01)_1.03 with FeO 22.09 (at. %) and minor Rh, Cu, Pd, and Ir. It hosts a remarkable composite inclusion up to ~120 µm across, with an unusual irregular distinctly wavy outline (Figure 9). The inclusion is located almost centrally in the host nugget. The central silicate fraction (~65 µm across) is rounded, except for an inward bulge. This fraction is micro-porphyritic with subhedral amphibole and diopsidic clinopyroxene micro-phenocrysts set in chilled silicate glass of the dacite/rhyolite composition [1].

Analyses of the groundmass silicate glass indicate dacite–rhyolite with SiO₂ 70.67, very low FeO 0.24, and H₂O ~2.17 (wt. %) (average of analyses (004) and (005); Table 3 of Barron et al. [1]).

(b): PGM sulfide paragenesis

The PGM paragenesis forms a “rim” of variable thickness (about ~5 µm up to ~50 µm) around the almost centrally located silicate “pool”, as shown in Figure 9a,b. This assemblage comprises an irregularly intergrown aggregate of anhedral grains measuring > 60 µm across.

Phase mapping (Figure 10) gives an approximate fraction % for each mineral present (based on relative pixel counts using EDS analysis and BSE imaging minus unassigned pixels). The PGM fraction accounts for 77.8 fraction % of the inclusion, while the exsolved silicate fraction is 22.2 fraction % of the inclusion area. An approximate normalized modal mineralogy for the PGM fraction is as follows (fraction %): (a) Pt-Fe alloy: ~1.5; (b) cuprorhodsite: 65.3; (c) braggite (and subordinate Ni-vysotskite): 25.6; (d) bowieite: 3.6; (e) vasilite: 3.6; UM (unnamed mineral): 0.42. PdTeSRh is a sulfide-telluride analog of palladoarsenide (Pd₂As).

An approximate calculated bulk chemistry (Table 10) is based on the EDS element mapping of the PGM paragenesis.

Table 10. Sample B area 4. Calculated bulk chemistry PGM (minus silicates).

S	Fe	Ni	Cu	Rh	Pd	Os	Ir	Pt	Te	As	Total, wt. %
18.10	7.04	1.90	3.48	12.20	37.50	0.07	0.72	14.42	4.54	0.04	100.00
											Total, at. %
41.42	9.25	2.37	4.02	8.70	25.86	0.03	0.27	5.42	2.61	0.04	100.00

Note: Me:S (+ligands) = 55.92:44.07 = 1.27 at. %. S-undersaturated.

The Pt-Fe alloy forms small clusters of purple grains (Pt, Figure 10a) in the PGM sulfide paragenesis fraction. The Pt-Fe alloy set in cuprorhodsite (Figure 9b) is Rh-enriched isoferroplatinum (Pt_2.94Rh_0.06)_3.00(Fe_0.85Rh_0.16)_1.01 (EDS analysis 9Ifp, Table 11). In contrast, the Pt-Fe alloy set in vysotskite is Pd-rich isoferroplatinum (Pt_2.52Pd_0.38Rh_0.03Ru_0.01)_2.94 (Fe_0.97Cu_0.06Ni_0.04)_1.07 (analysis 8 (060), Table 11, Figure 9b).

Cuprorhodsite (Rh-Cu-Fe-S, green in Figure 10b) is the dominant mineral in this inclusion and occurs as two anhedral domains; the largest is ~0.15 mm across. Analysis 3 (056) (Table 11) indicates cuprorhodsite–(malanite) (Cu_0.72Fe_0.18Ni_0.05Co_0.01)_0.96(Rh_1.53Ir_0.07Pt_0.35)_1.92S_4.00 (Me-deficient).

Vysotskite (1) (Vys, turquoise in Figure 10c) occurs as irregular large patches that are pale grey in Figure 9b. Analysis 5 (058), as shown in Table 11, gives Pd 24.15 and Pt 18.58 with 3.36 (at. %) minor elements (Cu, Fe, Rh, Ni, and Os). It has slightly high S. Analysis 5 (059) is similar. Minor vysotskite (2) is slightly darker grey in Figure 10. Analyses 10 (053) and 11 (054), as shown in Table 11, are more Pd- and Ni-enriched with lower Pt.

Table 11. WDS analyses sample B area 4.

# wt. %	Mineral	S	Se	Fe	Ni	Cu	Rh	Pd	Ag	Os	Ir	Pt	Total
(051) ^□	Ifp host	0.04		7.72	0.05	0.47	1.32	0.65		0.22	0.62	88.55	99.77
1 (052) ^†	Vs	13.45	0.14	0.06	0.03	12.70	0.08	74.29	0.17			0.31	101.39
2 (055)	Vs	13.49	0.06	0.13	0.04	12.67		74.26	0.08			0.51	101.23
3 (056) *	Crh	31.22		2.38	0.75	10.83	37.47	0.06		0.15	3.08	16.15	102.17
5 (058)	Vys (1)	21.34		0.10	2.10	0.12	0.23	31.91				44.82	100.61
7 (059)	Vys (1)	22.54		0.05	3.10		0.15	38.51	0.07			36.78	101.20
8 (060) ^◊	Ifp	0.07		9.14	0.35	0.62	0.43	6.77		0.10		83.25	100.83
10 (053) ^	Vys (2)	23.96	0.32	0.11	4.17	0.09	0.07	52.77				19.54	101.07
11 (054)	Vys (2)	23.92	0.27	0.08	4.14		0.08	55.25				17.40	101.13
Note: ^□ Pb: 0.14. ^† Te: 0.06; Bi: 0.09. * Co: 0.08. ^◊ Ru: 0.10. ^ Te: 0.05.
# at. %	Mineral	S	Se	Fe	Ni	Cu	Rh	Pd	Ag	Os	Ir	Pt	Total
(051) ^□	Ifp host	0.21		22.09	0.15	1.18	2.05	0.98		0.19	0.51	72.56	100.00
1 (052) ^††	Vs	31.65	0.14	0.08	0.04	15.07	0.06	52.66	0.12			0.12	100.00
2 (055)	Vs	31.75	0.06	0.17	0.05	15.04		52.67	0.05			0.20	100.00
3 (056) **	Crh	58.47		2.56	0.77	10.23	21.87	0.04		0.05	0.96	4.97	100.00
5 (058)	Vys (1)	53.81		0.14	2.89	0.15	0.18	24.15				18.58	100.00
7 (059)	Vys (1)	53.69		0.07	4.04		0.11	27.65	0.05			14.40	100.00
8 (060) ^◊	Ifp	0.32		24.15	0.88	1.45	0.62	9.38		0.08		62.98	100.00
10 (053) ^^	Vys (2)	52.52	0.28	0.14	4.99	0.10	0.05	34.86				7.04	100.00
11 (054)	Vys (2)	52.15	0.24	0.10	4.93		0.05	36.30				6.23	100.00
Note: ^□ Pb: 0.11. ^†† Te: 0.04; Bi: 0.03. ** Co: 0.08. ^◊ Ru: 0.14. ^^ Te: 0.03.
# apfu	Mineral	S	Se	Fe	Ni	Cu	Rh	Pd	Ag	Os	Ir	Pt	Me
(051)	Ifp host	0.01		0.88	0.01	0.05	0.08	0.04		0.01	0.02	2.90	4.00
1 (052) ^†††	Vs	6.97	0.03	0.02	0.01	3.32	0.01	11.60	0.03			0.03	15.03
2 (055)	Vs	6.99	0.01	0.04	0.01	3.31		11.60	0.01			0.05	15.02
3 (056) ***	Crh	4.00		0.18	0.05	0.70	1.50				0.07	0.34	2.85
5 (058)	Vys (1)	1.00			0.06			0.40				0.31	0.77
7 (059)	Vys (1)	1.00			0.08			0.52				0.27	0.87
8 (060) ^◊	Ifp			0.97	0.04	0.06	0.03	0.38				2.52	4.02
10 (053)	Vys (2)	0.99	0.01		0.09			0.66				0.13	0.90
11 (054)	Vys (2)	1.00			0.09			0.67				0.12	0.88
Note: ^††† Te: 0.01. *** Co: 0.01. ^◊ Ru: 0.01. Crh = Cuprorhodsite; Ifp = isoferroplatinum; Vs = vasilite Vys = vysotskite.

Bowieite (RhS, green in Figure 10d) occurs as irregular small patches at the margins of cuprorhodsite. EDS analysis 5RhS (Table 12) gives the following empirical formula: (Rh_1.73Pd_0.13Ir_0.05Pt_0.04Os_0.01Cu_0.05Ni_0.02Fe_0.02)_2.05S_3.00.

Table 12. Sample B (Area 4). EDS analyses, additional PGM identified by phase map analysis.

wt. %	S	Fe	Ni	Cu	Rh	Pd	Te	As	Os	Ir	Pt	O	Total
5 Bow ^	30.81	0.29	0.33	0.95	56.85	4.39			0.65	3.26	2.46		100.00
9 Ifp		7.37			3.45						89.18		100.00
17 Hem	3.23	54.80	1.38			9.26					3.45	26.81	100.00
18 UM ^	5.98		1.33	0.54		63.64	20.82	0.32			7.38		100.00
at. %	S	Fe	Ni	Cu	Rh	Pd	Te	As	Os	Ir	Pt	O	Total
5 RhS ^	59.56	0.31	0.34	0.92	34.28	2.56			0.21	1.07	0.78		100.00
9 Ifp		21.20			5.38						73.42		100.00
17 Hem	3.47	33.80	0.81			2.99	0.18				0.61	57.68	100.00
18 UM ^	18.25		2.21	0.82		58.58	15.97	0.42			3.70		100.00

Note: ^ Recalculated minus minor O, Si. Bow = Bowieite; Hem = hematite; Ifp = isoferroplatinum; UM = Pd sulfide-telluride.

Vasilite (PdSCu in Figure 10e) forms sparse anhedral patches along the margins of the irregular inclusion. The average of three analyses [1 (052), 2 (055), and 9 (053); Table 11] produces similar formulae. Analysis 2 (055) gives (Pd_11.60Cu_3.31Pt_0.05 Ag_0.01Fe_0.04Ni_0.01)_15.02(S_6.99Se_0.01)_7.00. This is slightly Me-deficient, and Me:S = ~2.15.

Hematite (FeO in Figure 10f) occurs as a small (10 µm) cluster of anhedral grains as inclusions in vysotskite (2). EDS analysis of 17FeO (Table 12) shows ~Fe₂O_3.

An UM (unnamed mineral) (PdTe) occurs as eight very small (up to ~3 µm) grains located mainly along the boundary of the PGM sulfide fraction with the silicate fraction. EDS analysis of 18PdTe (Table 12) indicates (Pd_1.79Pt_0.11Ni_0.07Cu_0.03)_2.00(S_0.56Te_0.48As_0.01)_1.05. It is a Pd-dominant sulfide-telluride analog of palladoarsenide (Pd₂As) and possibly naldrettite (Pd₂Sb).

The PGM paragenesis in this inclusion is dominated by cuprorhodsite, with significant intergrown irregular patches of vysotskite (1) and subordinate vysotskite (>Ni) (2), minor bowieite, and minor vasilite. The Pt-Fe alloy is unevenly distributed as fine-grained (<~5 µm across), anhedral (exsolved) patches in both cuprorhodsite and vysotskite (1), but it is lacking in vysotskite (2) (Figure 9b and Figure 10a). “Sprays” of Pt-Fe alloy inclusions define exsolutions in cuprorhodsite in Figure 9b. The Pt-Fe-Pd alloy also forms a wispy discontinuous boundary with the host nugget. Vasilite occurs as similar fine-grained patches but has an uneven distribution.

3.1.7. Sample A1 Area 7

(a): Host alloy and porphyritic rhyolitic groundmass silicate glass (SiO₂ 70.31 wt. %)

The small (~0.26 mm across) host nugget is partly rounded with some subhedral-shaped indents filled with alteration products, suggesting previous coexisting crystal sites (Figure 11a). Analysis 1 (264) (Tables 1A and 1B of Barron et al. [1]) indicates Pd- and Rh-enriched native platinum (Pt_2.83Pd_0.12Rh_0.12Ir_0.10Os_0.02Ru_0.01Fe_0.68 Cu_0.13)_4.01.

An exceptional oval-shaped silicate inclusion (about 65 µm across) with a finely scalloped margin is located towards one side of the host alloy. Plagioclase prisms, skeletal mafic (possibly amphibole) microlites, and minor wispy oxides are set in chilled silicate glass with SiO₂ at 70.31 and significant H₂O at ~4.23 wt. % (Barron et al. [1], analyses 145 and 147, Table 4). Significant quench textures are small, gas/volatile vesicles at the silicate droplet margin, near the point of ejection into the host alloy. A prismatic plagioclase phenocryst is normally compositionally zoned, with a calcic core of bytownite suggesting a picrobasaltic or picritic primitive parent melt and sustained fractionation [1].

(b): PGM sulfide paragenesis

The PGM sulfide paragenesis comprises 18 small inclusions (mainly <7 µm across) forming a “halo” within the host Pt-Fe alloy adjacent to the silicate inclusion: up to ~14 µm from the scalloped silicate inclusion margin, Figure 11b. They are not in contact with the silicate fraction (Figure 11a). Each PGM paragenesis is simple, comprising a euhedral crystal of cuprorhodsite set in vasilite (Figure 12a,b).

Cuprorhodsite crystals reach ~4.5 µm long and ~2.7 µm across. Analysis 2 (308) of Rh-Pt-Cu-S (Table 13) gives the formula (Cu_0.80Fe_0.08Ni_0.03)_0.91(Rh_1.42Pt_0.39Ir_0.09Ru_0.01Os_0.01)_1.92S_4.00 (Me-deficient, Me:S = 0.71), indicating cuprorhodsite–(malanite) with minor Fe and Ni. Analysis 4 (310) in a separate inclusion gives a similar formula: (Cu_0.80Fe_0.08Ni_0.03)_0.91(Rh_1.40Pt_0.39Ir_0.08Ru_0.01Os_0.01)_1.89S_4.00 (also Me-deficient Me:S = 0.70).

Vasilite is anhedral and forms a matrix for the cuprorhodsite crystals in each inclusion above. Analysis 3 (309) of Pd-Cu-S (Table 13) indicates (Pd_11.25Rh_0.13Pt_0.12Ag_0.05Cu_3.82Fe_0.05Ni_0.01)_15.42(S_6.96As_0.03Se_0.01)_7.00 (Me:S = 2.20), with significant Rh and Pt. Additional ligands are detectable As and Se.

Table 13. WDS analyses. Sample A1 area 7.

# wt. %	Mineral	S	As	Se	Fe	Ni	Cu	Ru	Rh	Pd	Os	Ir	Pt	Total
1 (264)	Pt-Fe				5.99		1.26	0.13	1.88	2.02	0.51	3.12	86.67	101.58
2 (308) ^†	Crh	30.13			1.09	0.34	11.97	0.25	34.29		0.27	4.15	17.59	100.20
3 (309) ^††	Vs	12.88	0.13	0.07	0.16	0.02	14.01		0.74	69.13			1.32	98.79
4 (310) *	Crh	30.58		0.04	1.00	0.36	12.10	0.24	34.28		0.26	3.90	18.23	101.02
Note: ^† Co: 0.03. ^†† Te: 0.05; Ag: 0.29. * Co: 0.04.
at. %	Mineral	S	As	Se	Fe	Ni	Cu	Ru	Rh	Pd	Os	Ir	Pt	Total
1 (264)	Pt-Fe				17.06		3.14	0.21	2.91	3.02	0.42	2.59	70.66	100.00
2 (308) ^†	Crh	58.62			1.22	0.36	11.74	0.15	20.79		0.19	1.35	5.65	100.00
3 (309) ^††	Vs	31.03	0.13	0.06	0.22	0.03	17.03		0.56	50.18			0.52	100.00
4 (310) *	Crh	58.88		0.03	1.10	0.37	11.75	0.14	20.56		0.08	1.25	5.77	100.00
Note: ^† Co: 0.03. ^†† Te: 0.03; Ag: 0.21. * Co: 0.04.
apfu	Mineral	S	As	Se	Fe	Ni	Cu	Ru	Rh	Pd	Os	Ir	Pt	Me
1 (264)	Pt-Fe				0.68		0.13	0.01	0.12	0.12	0.02	0.10	2.83	4.01
2 (308) ^†	Crh	4.00			0.08	0.02	0.80	0.01	1.42		0.01	0.09	0.39	6.82
3 (309) ^††	Vs	6.96	0.03	0.01	0.05	0.01	3.82		0.13	11.25			0.12	22.42
4 (310) *	Crh	4.00			0.07	0.03	0.80	0.01	1.40		0.01	0.09	0.39	6.80
Note: ^† Co: 0.03. ^†† Te: 0.03. * Co: 0.04; Ag: 0.05. Crh = Cuprorhodsite–(malanite). Vs = Vasilite. (Mla) = Malanite.

We calculate the approximate inclusion bulk chemistry (Table 14) for the Cu-PGM sulfide fraction in the present sample using the simple ore mineralogy of cuprorhodsite and almost equally abundant vasilite (Figure 12a). The order of major elements present is as follows: Rh >> Cu > Fe >> Pd > Ni.

Table 14. Sample A1 (area 7). Calculated bulk chemistry (assuming equal Crh and Vs).

S	Fe	Ni	Cu	Rh	Pd	Os	Ir	Pt	Total, wt. %
20.01	0.54	0.15	13.34	14.34	41.91	0.11	1.68	7.92	100.00
									Total, at. %
43.67	0.67	0.18	14.69	9.75	27.55	0.04	0.61	2.84	100.00

Note: Me:S = 43.67:56.33 = 1.29 at. %. S-undersaturated.

4. Discussion

4.1. Host Pt-Fe Alloys and Composite Inclusions

Composite melt inclusions of chilled silicate glass coexisting with exsolved Cu-PGM sulfides are hosted in seven Pt-Fe alloy nuggets dominated by isoferroplatinum (five nuggets) and native platinum (two nuggets) [1]. In one example (Figure 9 and Figure 10), a thick exsolved rim of ore paragenesis is distinctly deflated (degassed). Another inclusion comprises silicate glass surrounded by an adjacent corona of small exsolved PGM sulfide “droplet” inclusions set in the host alloy (Figure 11). In a third composite inclusion, the silicate melt fraction has a thin partial rim of exsolved Cu-PGM sulfides, while separate small inclusions are exsolved PGM sulfides lacking coexisting silicate melt (Figure 3). One host Pt-Fe nugget is distinctly flattened and deformed. It hosts a once rounded but now similarly deformed silicate inclusion, with a distant partial arc defined by related PGM droplet inclusions located near the nugget margin (Figure 2). These examples represent the latest stages of Cu-PGM sulfide melt exsolution and expulsion from host silicate melt into molten (or plastic) host alloy.

The excellent preservation of the composite inclusions is likely due to rapid quenching after fractionation and then partial decompression ± partial melting during deep-sourced cumulate intrusion to a high crustal level [1,6,7]. Also, these samples are likely to be geologically “young” and “belong” to the Late Cretaceous Narajal accreted arc terrane [1,3]. Therefore, they have undergone little degradation from weathering/alteration compared with more ancient and deformed Alaskan–Uralian-type intrusions. The youngest recorded Alaskan–Uralian-type intrusion (20 Ma) is Condoto in Colombia, with its related primitive Viravira lavas [25,26,27]. This intrusion and other zoned Alaskan–Uralian-type complexes are located along a Tertiary (Santa Cecilia arc) and an adjacent Late Cretaceous (Cañasgordas arc) trend extending southwards into northwestern Ecuador and towards the Camumbi River alluvial deposit. The nearby Santiago River alluvial deposit, from which similar nuggets are reported, is also in this region [28,29].

4.1.1. Fractionation and T of Host Silicate Melts Coexisting with Cu-PGM Sulfides

Analyses of the silicate glass in seven (melt) inclusions coexisting with Cu-PGM sulfide parageneses indicate that they are increasingly fractionated [1], from hydrous ferrobasalt to basaltic andesite, andesite, and groundmass dacite–rhyolite, in terms of the total alkali-silica (TAS) diagram (Figure 13a). This shows the differentiation trend (or liquid line of descent, dot-dash line) of the quenched silicate glass fractions in terms of alkalies (Na + K oxides)-FeO-MgO (AFM), each of which, in this study, hosts a distinctive, coexisting (exsolved) Cu-bearing PGM sulfide ore paragenesis.

Here, we calculate the approximate temperatures of quenching for coexisting silicate glass fractions using the Yuan et al. [32] geothermometer based on SiO₂ wt. % (anhydrous). This shows that the silicate glass inclusion with the most primitive basalt composition coexisting with small Ti-bearing chromite crystals, sample B area 1 (Barron et al. [1] Figure 13a,b), is quenched at 1021.2 °C. This glass is hydrous ferrobasalt (SiO₂ 48.52 and FeO 10.93 wt. % with estimated 2.24 wt. % H₂O). It compares with the synthetic primitive hydrous ferrobasalt composition for the Skaergaard intrusion (Greenland, Botcharnikov et al. [2]). They show experimentally that, at 200 MPa and 940–1200 °C, “the addition of H₂O decreases liquidus temperatures and changes significantly the proportions, temperature range and sequence of crystallizing mineral phases”. They also found that their “experimental data are in agreement with the experimental results of Sisson & Grove [33] who showed a change in the trend of liquid lines of descent from tholeiitic to calc-alkaline with increasing aH₂O and fO₂”, confirming the results of Berndt et al. and Koepke et al. [34,35].

The most primitive silicate glass coexisting with Cu-PGM sulfides in this study is also tholeiitic ferrobasalt, quenched at 1018.5 °C (samples A2 area 7 and A2 area 6) with slightly lower SiO₂ than primitive sample B from area 1, but it has significantly higher FeO with low H₂O. This strongly Fe-enriched basalt silicate glass plots near the top of the Fe-enrichment trend in tholeiitic melts (Figure 13b), but it has little H₂O (0.31 wt. %). The more fractionated basaltic–andesite silicate glass (sample Mag 5 area 4), quenched at 1017.5 °C, also follows the tholeiitic trend, but with lower FeO and significantly higher H₂O (~2.55 wt. %), while the quenched glass in sample A1 area 9, also of basaltic–andesite composition, plots near the tholeiite/calc-alkaline boundary in Figure 13b. In the latter, FeO is still elevated but with moderate H₂O (~1.71 wt. %).

In contrast, a sharp decrease in Fe is shown during melt fractionation from tholeiitic basaltic andesite (above) to calc-alkaline andesite glass quenched at 983 °C (sample B area 5), with minor FeO at 3.25 wt. %, as shown in Figure 13b (cf. Barron et al. [1]; Figure 12).

The sequentially more siliceous, groundmass silicate glass compositions of microporphyritic dacite–rhyolite (quenched at 845 °C, sample B area 4) and rhyolite glass (quenched at 830 °C, sample A1 area 7), also coexisting with Cu-PGM sulfides, show substantially increasing H₂O, from ~2.32 to ~4.28 wt. %, respectively. They follow the calc-alkaline trend, showing continuous Fe and Mg depletion and concentration of remaining PGE, S, and minor Te. The data is from Barron et al. [1].

Ballhaus et al. [36] report that “sulfide droplets entrained from the mantle source may become dissolved as the basalt is decompressed on its passage to the surface, and that all the elements contained in sulfide, including the noble metals, are then released to the silicate melt”. If the silicate melt is PGE-saturated, as in the present silicate glass inclusions, the crystallization of PGE minerals further fractionates noble metals [37,38].

Supporting PGM mineralization in strongly differentiated silicate melts are two volcanic examples. Firstly, there is the unusual Early Cretaceous (~125 Ma) occurrence hosted in evolved explosive andesite breccia in the Poperechny iron–manganese deposit (Lesser Khingan Range, Far East Russia) [39]. Here, the PGMs are Fe-Pt alloy solid solutions (85%), other PGM (mostly Os-Ir-Ru) solid solutions, and sulfides and sulfarsenides (15%). They suggest formation by “high-temperature fractionation of a mantle-derived mafic parental melt (similar to Alaskan-type complexes) and that the PGM were entrained in the evolved andesitic melt during its emplacement in the crust”. Also, the “Early Cretaceous (~125 Ma) age of ferroplatinum in the explosive breccia suggests that PGM-bearing ultramafic material could have been sampled during regional slab window tectonics related to the Late Mesozoic subduction of Izanagi plate along southern margin of the North Asian continent.”

The second example is a young (~2 Ma) deposit of PGE sulfide mineral compositions, mainly Cu-poor monosulfide solid solutions (Mss), formed at distinct stages of magma evolution from deep to shallow crustal levels reported in dacitic rocks recently recovered from the Okinawa Trough [40]. They also note that such PGEs have a strong affinity for sulfur and tend to accumulate in deep continental crusts. The Okinawa Trough is an active back-arc basin located behind the Ryukyu Island Arc.

4.1.2. Bulk Compositions: Fractionation of Selected Coexisting Cu-PGM Sulfide Parageneses

Element and phase mapping was used (above) for inclusions in three samples (Mag 5, area 4; B, area 5, and B, area 4) to estimate the fraction % of each mineral present from pixel counts and calculate the bulk chemistry of each Cu-PGM sulfide paragenesis. We also calculated the inclusion bulk chemistry for the Cu-PGM sulfide fraction (almost equally abundant cuprorhodsite and vasilite) in sample A1, area 7. Coexisting silicate glass in these four samples spans a SiO₂ range of 52.30–70.67 (wt. %) and an approximate temperature range difference of 187.5 °C.

It is interesting to note that Borisov and Danushevsky [41] experimentally show, in the pseudobinary diopside-anorthite eutectic–silica system, that the effect of adding silica (up to 50 wt. %) results in a dramatic but systematic decrease in Pt and Rh solubilities and a maximum decrease in melts with 70 wt. % SiO₂ (cf. coexisting silicate glass in sample B, area 4, SiO₂ at 70.67 wt. %, and A1, area 7, SiO₂ at 70.31 wt. %). Their study, at 1450 °C (in air), is well above the Ts considered here. Nevertheless, their results indicate that SiO₂ affects Pt and Rh (and Pd) solubilities and “support a suggestion that magmatic fractionation from basaltic to silicic compositions under sulfur under-saturated conditions may result precipitation of Pt-Fe alloys”.

In Figure 14a, we plot the fractional crystallization path (black dashed lines) of the four calculated Cu-PGM sulfide bulk compositions in terms of Pt + Pd + Rh–Cu–Fe(+Ni + Co) at. %. Their stability is correlated with approximate quenching temperatures calculated above for coexisting silicate glass. This path, from the bulk composition of sample Mag 5, area 4 (highest T 1018 °C), shows that with falling T at constant Cu, ∑PGE (mainly Rh) increases at the expense of Fe(+Ni + Co), resulting in the extreme Rh-enriched bulk composition (at 983 °C) of Cu-PGM sulfides in sample B, area 4. With a further falling T and constant Fe(+Ni + Co), ∑PGE (mainly Pd) increases, resulting in the extreme Pd-enriched composition of sample B, area 4, at 845 °C. With a further decreasing T and constant ∑PGE, Cu is enriched at the expense of Fe(+Ni + Co), resulting in the bulk composition of PGM in sample A1, area 7 (at 830 °C).

In Figure 14b, bulk compositions of the four Cu-PGM sulfide fractions (in terms of Pt–Rh–Pd at. %) follow a strongly fluctuating path of fractional crystallization (black dashed lines) depending on the selective enrichment of each major PGE: first Pt 11.04 at. % in sample Mag 5, area 4, and Rh 24.12 at. % at the expense of Pt (0.68 at. % in sample B, area 5), and then Pd 25.86 at. % at the expense of Rh (8.70 at. % in sample B area 4) in successive residual melts with falling T within the range of 1018–830 °C. The inclusion bulk composition of sample A1, area 7, at 830 °C shows minor Pt depletion (with a significant increase in Cu) compared with that of sample B area 4 at 845 °C.

Instead of fractional crystallization (above), we suggest that incongruent melting has influenced bulk composition fractionation in three calc-alkaline Cu-PGM sulfide parageneses, as shown by red lines in Figure 14a,b. At mantle depth, the incongruent melting of Mss is likely to occur due to decompression, lowering the melt temperatures in Mss [42]. Thus, the incongruent melting of the S-saturated, Cu-PGM-sulfide bulk composition of sample Mag 5, area 4 (coexisting with silicate melt at 1017.5 °C) is likely to produce (at 983 °C and at ~830 °C, respectively) the two extreme Rh- and Pd-enriched S-undersaturated bulk inclusion compositions (samples B, area 5 and B area 4). The solubility of Pt, Rh, and Pd in these separate melts, as mentioned above, may be influenced by SiO₂ concentrations in bulk melt compositions [cf. 41].

Also supporting incongruent melting due to decompression, we identify a dominant Pt-poor cuprorhodsite (24.92 fraction %) with exsolved alloy lenses of Pd-tetraferroplatinum in sample Mag 5 area 4 (Figure 4 and Figure 5) and propose a possible PGE-enriched monosulfide mineral precursor prior to decompression. The incongruent melting model is also supported by the experimental work of Fonseca et al. [43], indicating that sulfides in subducted eclogite (and peridotite) can melt incongruently in the mantle, producing Cu–Ni-poor sulfide melt and the residue of Mss, into which highly siderophile PGEs are selectively partitioned (particularly Pt, Pd, Rh, Ru, Os, and Ir).

In this natural system, Cu is the only additional incompatible element to partition into the melt at 845 °C (possibly a partial melt of Cu-PGM sulfide bulk composition in sample B, area 4) and is strongly compatible in the Cu-rich PGM sulfide fraction of sample A1 area 7, coexisting with quenched silicate glass at 830 °C. Concentrations of Pd and Rh compare with those in the sulfide PGM inclusion bulk composition of sample B, area 4 (Table 10 above).

4.1.3. S-Saturation of Cu-PGM Bulk Compositions

The bulk compositions of the two inclusion Cu-PGM sulfide parageneses considered above are slightly S-saturated (samples Mag 5 area 4 and the extremely Rh-enriched sample B area 5). Their fractionation path represents a T decrease of only ~35 °C (red line in Figure 14a,b) and shows a precipitous increase in ∑PGE dominated by Rh, with decreasing Pt (+Pd) and concomitant decreasing Fe(+Ni + Co). In contrast, the fractionation path from the Cu-PGM sulfide bulk composition of sample Mag 5 area 4 towards the extremely Pd-enriched sample B area 4 (crystallized at 845 °C) represents a greater T decrease of ~173 °C and shows S(+Se + Te) undersaturation, confirming a significant increase in metal elements with moderate Cu depletion in Figure 14. The S-undersaturated bulk composition of PGM sulfide paragenesis in sample A1 area 7 at 830 °C is relatively Cu-enriched by a very small T decrease (15 °C) from that of sample B area 4 (Figure 14).

Thus, the bulk compositions for Cu–Fe(+Ni)–PGE natural parageneses coexisting with silicate melt in four samples show that Pt, Rh, and Pd are differentiated with cooling from 1018 °C to ~830 °C, firstly in slightly S-saturated sulfide melt (two samples) and, secondly, in melt that is S-undersaturated at 845 °C and 830 °C (two samples).

The experiments of Peregoedova and Ohnenstetter [44] at a lower T of 760 °C showed that “rhodium is characterized by a dual behavior. Like Pd, Rh preferentially partitions into base metal sulfides, especially monosulfide solid solution (up to 2.6 at. % Rh) but at conditions of very low or very high sulfur fugacity, it forms Rh minerals as does platinum”.

In the experimental Fe–Ni–Cu–PGE–S system, Ballhaus et al. [45] indicate that the higher the bulk metal/S ratio, the lower the liquidus and solidus temperatures, and the lower the temperature to which a sulfide melt may fractionate and be enriched in Cu. This is the case in the present system for the bulk compositions of the two S-undersaturated bulk compositions above.

The partition coefficients of Rh, Pt, and Pd also depend on the bulk S contents of the melt, and they are known experimentally to increase with increasing S contents in both Mss and liquid [17]. However, in the present natural Cu–Fe(+Ni)–PGE system, we show that Pt, Rh, and Pd are differentiated with cooling from 1018 °C to ~830 °C in slightly S-saturated sulfide melts that become S-undersaturated.

Further experiments of Peregoedova et al. [42] show that under conditions of declining sulfur fugacity and high T (~1200 °C), “the depletion of Mss in sulfur…. might lead to formation of individual PGE alloys and lead to Mss partial melting, producing a Cu–Ni-rich sulfide melt, and changing completely the original PGE distribution pattern”. This describes incongruent melting. They also found that sulfur fugacity plays an extremely important role in the formation of individual PGE minerals. Their sulfur-buffered experiments show that “variation in fS₂ directly affects the bulk metal/S ratio of sulfide assemblages and leads to a change in the stable assemblages of base–metal sulfides and PGE phases”.

4.2. Cu-PGM Sulfide Compositions and Stability

4.2.1. Braggite–Vysotskite

Braggite–vysotskite compositions in two samples indicate the strong fractionation of Pt and Pd within their confined inclusion boundaries. In the first sample (A2 area 6), we analyze braggite and vysotskite in three sulfide melt inclusions within one Pt-Fe host nugget (Figure 15; cf. Cabri and McDonald [19], Figure 1). One braggite composition plots close to the boundary with cooperite in Figure 15, suggesting equilibration at high T values (possibly >1000 °C). This sulfide paragenesis coexists with quenched silicate glass at ~1018 °C. Elevated minor elements (~4.17 at. %) other than Ni and Fe (1.29 at. %) preclude the use of the Verryn geothermometer [46].

Figure 15. Triangular plot in terms of (Pd,Rh)S-(Pt,Ir)S and (Ni,Fe,Cu,Co)S for nine EPMA analyses (numbered, mol. %, following Cabri et al. [19]; Figure 1 in their study). Dashed lines mark their boundaries for cooperite, braggite, and vysotskite. Three analyses from inclusions in one sample (filled circles) show a wide range of fractionated compositions from braggite (near cooperite) to vysotskite (dot dash lines). Arrows indicate the direction of fractionation. Four analyses in the second sample (filled diamonds) also show a large compositional range from vysotskite (near braggite) to the most (Pd, Rh)-enriched vysotskite (analysis 054), indicating a late trend towards decreasing base metal elements.

A second, more fractionated composition in this paragenesis is also braggite (Figure 15), which plots near the boundary with vysotskite. Here, Pt in braggite is considerably lower (19.64 at. %), with an almost equal concentration of Pd (19.55 at. %) and Ni + Fe (2.08 at. %). However, minor elements are high (4.11 at. %), also precluding T estimation [46].

A third irregularly zoned grain [semiquantitative analysis (317) above; Figure 3d] indicates Pd-rich vysotskite with high (~5.17 wt. %) PGE minor elements. The subhedral and partly zoned braggite and vysotskite crystals span a compositional range of Pt 35.21–11.41 at. %, with concomitant Pd from 4.17 to 30.76 at. % (Figure 15).

In the inclusion of the second sample (B area 4), four vysotskite analyses define two distinctly fractionated compositions. The first two analyses (058 and 059 Table 11, Figure 15) indicate dominant Pd (24.15–27.65 at. %) vysotskite (1). The second two analyses (053 and 054, Figure 16) represent strongly Pd-enriched vysotskite (2) compositions (near Pd 36.30 at. %). The analyses of Pd-rich vysotskite (2) also indicate Ni of 4.15 and Fe of 0.08 (at. %), with remarkably low minor elements (Rh 0.07 at. %). Therefore, since no elements other than Ni and Fe play a significant role in compositional variation, the calculation of an estimated experimental maximum temperature is possible using the Ni versus Pt (at. %) thermometer of Verryn and Merkle [46] (Figure 6 in their study). In the present analyses, Ni(+Fe) accounts for 4.23 at. %, which gives an approximate crystallization of T ~ 900 °C. The approximate calculated T from coexisting silicate glass is 845 °C.

Clusters of small Pd-rich isoferroplatinum grains (Table 11) set in vysotskite (1) likely represent decompression exsolutions. Their Pd-rich composition is distinctive but compares with equally Pt-rich isoferroplatinum grains hosted in the adjacent coexisting cuprorhodsite grains (see below). In contrast, the grains of vysotskite (2) lack alloy exsolutions and most likely crystallized at a higher crustal level.

In Figure 15, a single braggite composition, as shown in analysis 133 (sample Mag 5 area 4, Table 5) (~9.41 at. % minor elements), plots near cooperite. This is consistent with the high T (1018 °C) of coexisting silicate glass. In contrast, vysotskite (sample in A2 area 7, analysis 328 Table 1) suggests late and low-T crystallization in a negative crystal space within the host nugget, as shown in Figure 2c.

4.2.2. Thiospinels

Cuprorhodsite–(malanite) occurs in all inclusion Cu-PGE sulfide parageneses. A range of Ir-poor compositions is represented in terms of Pt, Rh, and Ir (Figure 16), except for analysis 132 (sample Mag 5 area 4), which is weakly Ir-enriched (2.20 at. %). In terms of the binary Pt–Rh, this cuprorhodsite–(malanite) solid solution series varies from Pt:Rh= 21/79 at. % (sample B area 4) to Pt:Rh = 37/63 at. % (sample A2 area 6), with the extreme example of Pt:Rh= 2/98 at. % (sample B area 5). Excluding the latter, this range compares with the compositional range of cuprorhodsite–(malanite) compositions, Pt:Rh = ~20/80 at. % to Pt:Rh = ~47/53 at. %, in melt inclusion parageneses in one Pt-Fe alloy nugget (sample Mag 4 area 1) [7].

Figure 16. Pt–Rh–Ir ternary plot (at. %) showing the compositional range of cuprorhodsite–(malanite) in seven samples.

Cuprorhodsite–(malanite) analyses here can be subdivided into two groups based on Rh concentrations. In the first group, Rh in cuprorhodsite–(malanite) varies from 15.64 to 18.08 at. %, representing the four highest T (1020–1017 °C) samples that were fractionated along the tholeiitic trend. In the second group, Rh in cuprorhodsite–(malanite) varies from 21.79 to 27.67 at. % in the three (lower T, 983–830 °C) samples that were fractionated along the calc-alkaline trend [1].

The phase mapping of the Cu-PGM sulfide fraction in the inclusion of sample B area 5 reveals that, of seven sulfide minerals present, four are identified as thiospinels. Apart from dominant cuprorhodsite–(malanite), we identify three additional minor thiospinels; Rh-Cu-Fe-S is Fe-enriched ferrorhodsite (3.98 volume %), and Rh-Cu-Ni-S is cuprorhodsite–(polydymite) (2.00 volume %) and Rh-Ni-Cu-S polydymite–(cuprorhodsite), with significantly higher concentrations of Ni (1.00 volume %). Very small volumes of the latest-crystallized thiospinels suggest that a low-T residual melt was first enriched in Fe and then Ni, finally resulting in the most Ni-enriched thiospinel composition of polydymite–(cuprorhodsite). This unusual compositional range of the three minor thiospinels represents a sequentially fractionated low-T solid-solution series.

The most Ni-enriched Rh-Ni-Cu-S thiospinel shows concomitant decreases in Rh, Cu, and Fe. It is also the thiospinel that is the least abundant. Such compositions show that different thiospinels can form sequentially in minor, latest fractionating sulfide melts and that the Ni in these thiospinels increases as temperature falls.

4.2.3. Minor Cu-PGE Sulfide Minerals Coexisting with Thiospinel

Significant minor minerals are present in the parageneses of Cu-PGE sulfide melt inclusions in six samples. The minor minerals of sample A2 area 7 are too fine-grained for accurate analyses. As above, we discuss these in order of increasing SiO₂ fractionation (therefore decreasing the calculated T of the coexisting glass).

Minor chalcopyrite and bornite in sample A2 area 6 coexist with braggite–vysotskite and cuprorhodsite–(malanite). Bornite shows exsolutions and possible intergrowths of a distinctive monosulfide UM (unnamed mineral). This mineral compares with two analyses, (Sp31) and (Sp38), as shown in Table 7B of Barron’s study [7], and is possibly a Pt-enriched analog of the minerals generalized as Me_1−xS in Tolstykh & Krivenko’s study [21], showing lower Cu and lacking significant Pt compared with the present two analyses.

In contrast, minor minerals in sample Mag 5 area 4 are Me-deficient pyrrhotite (with a variety of minor elements) and unusual isocubanite. In this inclusion, cuprorhodsite–(malanite) with tetraferroplatinum exsolutions is dominant (see below).

Minor, equally dominant vasilite and chalcopyrite are present in sample A1 area 9, indicating increasing Pd and Cu, respectively, in the latest sulfide residual melt. Subordinate pentlandite, rich in minor elements, is Me-deficient [semiquantitative analysis 6 (316) Table 6], indicating low fS₂. This Cu-PGE sulfide melt paragenesis is also dominated by cuprorhodsite–(malanite) but lacks braggite or vysotskite.

In sample B area 5, minor alloys (Pt > Pd > Fe > Cu) and the associated keithconnite indicate low fS₂. These form interstitial patches that are evenly sized and distributed, suggesting exsolution. Minor vasilite is unevenly distributed. This paragenesis is dominated by cuprorhodsite–(malanite) and similarly lacks braggite or vysotskite. Three minor thiospinels representing late low-T deposits indicate increasing Fe and then Ni. A minor Me-deficient monosulfide mineral with Rh > Pt > Fe > Cu >> Ni occurs along mineral boundaries and fractures. Hematite, dominated by Fe and O, is possibly crystallized latest in sample B area 5 and indicates that S is completely depleted and cannot escape as a gas, so the round shape of the inclusion is preserved (Figure 7), unlike the deflated inclusion in sample B area 4.

In sample B area 4 (Figure 9), the minor minerals are Pd-rich, and isoferroplatinum is exsolved as blebs within dominant cuprorhodsite and vysotskite, indicating decompression; this possibly represents initial crystallization at mantle conditions and then rapid intrusion up to a higher crustal level. Also present are minor minerals bowieite; vasilite, a Pd-dominant sulfide-telluride UM analog of palladoarsenide Pd₂As and naldrettite Pd₂Sb; and hematite. These minerals indicate low fS₂, with significantly increasing Pd and Te at the latest stage of fractionation.

Small inclusions encircling their host silicate melt inclusion form a close corona in the adjacent host alloy of sample A1, area 7. They have a simple fractionated mineralogy dominated by subhedral cuprorhodsite–(malanite) crystals set in slightly subordinate vasilite, indicating latest enrichment in Pd and Cu with high Me:S = ~3.3, again confirming low fS₂.

4.3. Deformation and Decompression

4.3.1. Deformation

The distinct plastic deformation (flattening with decompression) of the host native platinum in sample A2 area 7 (Figure 2) may have influenced the expulsion of PGE melt inclusions from the host glass to the nugget margin. The round inclusion of basalt glass (1018.5 °C) is also moderately flattened. This suggests early intrusion and possibly the tectonic compression of mantle-derived basalt melt at the highest T.

4.3.2. Decompression Alloy Exsolutions in Cuprorhodsite and Vysotskite

Decompression exsolutions in cuprorhodsite occur in Cu-PGM sulfide fractions of two inclusions. In the first example, we show that tetraferroplatinum exsolutions are crystallographically controlled, subparallel, and narrow lenses in prismatic cuprorhodsite (sample Mag 5 area 4, Figure 5). Combining the compositions of lenses and cuprorhodsite, we define a precursor Pt-enriched monosulfide mineral (equivalent to Mss) that is likely stable at higher P and T conditions. Coexisting basaltic andesite glass was crystallized at ~1017.5 °C.

In the second example, an irregular (deflated) rim of exsolved PGM sulfide paragenesis encloses a central silicate host in the inclusion of sample B area 4, with groundmass glass of dacite–rhyolite composition crystallized at ~845 °C (Figure 10b). Deflation is interpreted to result from decompression-induced degassing. Also supporting decompression in this sample are remarkable clusters of alloy blebs (unlike the alloy in narrow subparallel lenses above), forming sprays and patches set in the host domains of cuprorhodsite. The exsolved blebs are very fine-grained, but semiquantitative EDS analysis 9Ifp (Table 12) indicates Rh-enriched isoferroplatinum (with Cu 2.54 wt. %). Rh enrichment reflects its Rh-enriched host thiospinel. This compares with the Pd-enrichment of alloy exsolutions in Pd-rich vysotskite (above).

The host cuprorhodsite is Me-deficient (Me 2.88: S 4.00 = 0.72), possibly due to the exsolved alloy exsolutions. Sulfur degassing is clearly associated with decreasing fS₂, and this is proposed here to facilitate the exsolution of alloys, possibly from an earlier-crystallized monosulfide mineral.

Similarly, in the paragenesis of sample B area 4, small alloy blebs are exsolved in coexisting vysotskite [analyses 7 (059) and 11 (054), Table 11], suggesting a precursor mineral with significant Pt enrichment at higher P (and T). Analysis 8 (060), as shown in Table 11, indicates that the exsolved alloy is significantly Pd-enriched (9.38 at. %) isoferroplatinum compared with the Pt-enriched exsolved alloy in the cuprorhodsite above.

Thus, a desulfurization process could potentially occur due to the depressurization of ascending magma with entrained mineralized droplets. Supporting this conclusion, the experimental study of Peregoedova et al. [42] found that PGE-bearing Mss undergoing S loss can produce Pt and Ir exsolution from the Mss matrix in the form of PGE-bearing alloys.

4.3.3. Partial Decompression Melting

Partial decompression melting in sample A1, area 7, is indicated by conspicuous negative indents in some Cu-PGM sulfide inclusions, indicating resorption. One inclusion is torn apart, with the host alloy filling a wide gap (Figure 13a). This shows that the pre-existing composite inclusion was fractured and then partly resorbed by a Pt-Fe alloy decompression partial melt at a lower T. We compare the latter with an example from the Durance River, France (Johan et al. [31], Plate 1F). Here, two fragments of an early quenched, round, brittle-fractured silicate glass inclusion are hosted in the Pt-Fe alloy.

Also, in sample A1 area 7, decompression partial melting together with hydrous (H₂O ~4.28 wt. %) silicate (rhyolitic) glass host composition, quenched at 830 °C, could have influenced the expulsion into the adjacent host Pt-Fe alloy of the corona of small Cu-PGM sulfide inclusions. This texture, including the scalloped outline of the silicate fraction, could result if the PGM paragenesis initially comprised an exsolved discontinuous thin rim (a small fraction of the whole inclusion) compared with the relatively thick exsolved Cu-PGM sulfide rims in other samples. A strongly zoned plagioclase phenocryst with bytownite centrally confirms the long fractionation history of this sample [1].

5. Conclusions

Rare inclusions of fractionated silicate glass melts coexist with related, exsolved Cu-bearing PGM parageneses hosted in seven Pt-Fe alloy nuggets from the Camumbi River, NW Ecuador. Two nuggets are high-T native platinum, and five are isoferroplatinum. Some Cu-PGM melt inclusion parageneses enclose “pools” of silicate glass, from which they are exsolved. In others, Cu-PGM melt inclusion “droplets” are expelled into the host Pt-Fe alloy. Barron et al. [1] show that trace element patterns in the silicate glass inclusions match those from arc volcanics in their accreted Cretaceous Naranjal terrane [3].
Primitive silicate glass in this inclusion suite [1] is hydrous ferrobasalt, similar to the experimental starting material of Botcharnikov et al. [2] from the Skaergaard intrusion, Greenland. Like their fractionation, the present suite with coexisting Cu-PGM first follows a tholeiitic path of Fe-enrichment (basaltic andesite) and then sharply shifts to follow the calc-alkaline trend (calc-alkaline andesite, dacite, and rhyolite), coinciding with Fe-depletion due to mafic/oxide crystallization and the presence of water [1,2,33,47]. The approximate calculated quenching temperatures of inclusion silicate glass vary from ~1018 °C to ~830 °C, based on the silicate geothermometer of Yuan et al. [32].
Four Cu-PGE sulfide inclusion parageneses coexist with basalt and basaltic andesite silicate melts within the high-T, high-Fe tholeiitic trend. Their Pt-enriched calculated bulk compositions are moderately to slightly S-saturated, and we suggest that differentiation is due to crystal fractionation at a high T (1020–1018 °C). In contrast, the calculated bulk compositions of the Cu-PGE sulfide inclusion fractions are extremely Rh-enriched (just S-saturated) and Pd–Cu-enriched (S-undersaturated). They coexist with andesite at 983 °C and with dacite–rhyolite groundmass glass at 845 °C, respectively, and they are crystallized along the lower Fe calc-alkaline trend at lower T. The latest fractionation is marked by strong Cu-enrichment in sample A1 area 7. This bulk composition is also S-undersaturated and coexists with rhyolitic silicate glass quenched at ~830 °C. Pt, Rh, and Pd are differentiated with cooling from >1018 °C to ~845 °C in S-saturated sulfide melts that become S-undersaturated.
The abrupt change from tholeiitic to the calc-alkaline trend for host silicate melt inclusions in the three samples above is correlated here with decompression exsolutions in coexisting braggite–vysotskite and cuprorhodsite–(malanite) and with partial (incongruent) melting. It is known [42] that incongruent (partial) melting of experimental Mss (Monosulfide solid solution) is due to decompression (lowering of melt Ts of Mss), resulting in extreme bulk compositions, such as those in the samples above.
A decrease in fS₂ with decreasing T is characteristic within the present Cu-PGM sulfide melt inclusions; this is confirmed by the increasing Me:S ratios of late-crystallized minerals. The further lowering of melt fS₂ by S-degassing is supported by the deflated shape of one composite inclusion, indicating decompression in a rapidly ascending Alaskan–Uralian-type melt.
Braggite–vysotskite compositions in two inclusions indicate the strong fractionation of Pt and Pd within their confined boundaries. Firstly, braggite is Pt-enriched and plots close to the boundary with cooperite, indicating equilibration at a high T of ~1000 °C. Other grains show increasing Pd towards Pd-enriched vysotskite. Coexisting silicate glass indicates a high T (1018 °C see above). Other grains show increasing Pd towards Pd-enriched vysotskite. Vysotskite analyses define two distinctly fractionated compositions. (1) is Pt-enriched, but Pd is still dominant and shows alloy exsolutions due to decompression, while Vysotskite (2) is more fractionated (Pd-enriched, near Pd 36.30 at. %) and lacks alloy exsolutions. Thus, the latter is likely crystallized at a higher intrusion level.
Cuprorhodsite–(malanite) occurs in all inclusion Cu-PGE sulfide parageneses. A range of Ir-poor compositions is represented in terms of Pt, Rh, and Ir. This solid solution series varies from Pt:Rh = 21/79 at. % to Pt:Rh = 37/63 at. %, with an extreme example: Pt:Rh = 2/98 at. %. Rh concentrations are lower in cuprorhodsite–(malanite) from four inclusions equilibrated with quenched silicate glass along the tholeiitic trend at the highest temperatures. Rh is the highest in cuprorhodsite–(malanite) from the three inclusions equilibrated along the calc-alkaline trend, partly due to the decompression exsolution of Pt.
Decompression exsolutions of tetraferroplatinum (lenses) in cuprorhodsite suggest a homogeneous monosulfide mineral precursor. They occur in a Cu-PGM sulfide paragenesis coexisting with silicate glass of tholeiitic basaltic andesite composition quenched at ~1018 °C. In contrast, the decompression exsolutions (blebs) of Rh-enriched isoferroplatinum in cuprorhodsite, reflect its Rh-enriched host thiospinel, unlike the Pd-enriched enrichment of Pt-Fe alloy exsolutions in coexisting vysotskite (1), as shown in Figure 9. Their Cu-PGM sulfide paragenesis coexists with minor groundmass glass of dacitic/rhyolitic compositions quenched at ~845 °C.
Decompression partial melting in sample A1 area 7 (Figure 11) is supported by negative indents in the shapes of Cu-PGM sulfide inclusions, indicating resorption. One inclusion is torn apart, with the host alloy filling a wide gap. Figure 13a shows that the pre-existing sulfide inclusion is fractured and partly resorbed by a late Pt-Fe alloy decompression melt. This is compared with a brittle-fractured glass inclusion with the infill of isoferroplatinum in a nugget from the Durance River, France ([31] their Plate 1F). We suggest that the latter possibly represents a lower T, partial decompression melt. Also, in sample A1 area 7, decompression partial melting could have influenced the separation of the Cu-PGM sulfide fraction (halo of small inclusions that surround the host silicate inclusion). Here, partial melting also could have been influenced by water from the hydrous (~4.23 wt. %) silicate groundmass (glass) fraction quenched at 830 °C.
Minor inclusion minerals in Cu–Fe(+Ni)–PGE parageneses depend on bulk compositions after the crystallization of braggite–(vysotskite) and cuprorhodsite–(malanite). Minor chalcopyrite and bornite mark strong, late Cu enrichment. Bornite shows rare exsolutions of a UM, possibly a Pt-enriched analog of the minerals generalized as Me_1-xS [21]. Minor pyrrhotite and isocubanite are also present. A series of Fe-enriched and later Ni-enriched thiospinels [ferrorhodsite, cuprorhodsite–(polydymite) and polydymite–(cuprorhodsite)] comprises minor minerals in one sample. They are crystallized last. Other minor minerals are bowieite and hematite, and a minor UM (unnamed mineral) (PdTe) is identified as an analog of palladoarsenide Pd₂As. Minor vasilite is crystallized last in two samples.
A strongly zoned plagioclase phenocryst with unusually calcic bytownite centrally is set in the rhyolitic groundmass silicate glass of one sample, confirming a long silicate fractionation history [1].

Author Contributions

Conceptualization, B.J.B.; methodology, B.J.B.; investigation, B.J.B.; data curation, B.J.B. and L.B.; writing—original draft preparation, B.J.B.; references—original draft preparation, B.J.B. and L.B. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Data Availability Statement

Data are contained within the article.

Acknowledgments

Samples were supplied by RioDorado Limited (now liquidated) from their Sabaleta Project, Camumbi River, Ecuador. Many conclusions here depend on recent experimental studies and particularly the new silicate glass geothermometer of Yuan et al. [32]. We are very grateful to Karsten Goemann SEM & X-Ray Microanalysis, Central Science Laboratory, University of Tasmania, Hobart, Australia, for discussions and continued support for our ongoing research. We also thank Lin Sutherland for encouragement and the suggestion to publish in Minerals. We thank four anonymous reviewers, whose comments helped improve this manuscript. Invaluable ongoing access to the library facilities at UNSW is also appreciated.

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. Location of the Sabaleta Project area, Camumbi River, and accreted arc terranes, northwestern Ecuador. The Late Cretaceous Naranjal terrane comprises island arc lavas and intrusions and is cut and displaced by northeast-trending faults. It is partly covered by Cenozoic–recent sediments with small inliers north of the Santiago River and south of Pedernales according to geological maps [4,5].

Figure 2. Sample A2 area 7. BSE (backscatter electron images). (a) Native platinum nugget sample A2, area 7, showing slightly oval-shaped chilled basalt glass inclusion with distinctive adjacent, small strain-shadow microstructures. Small inclusions of Cu-PGM sulfide parageneses form an outlying arc-shaped “corona” near the nugget margin. (b) Cuprorhodsite–(malanite) crystal (analysis 327) set in an unanalyzed mineral that fills adjacent strain-shadow microstructures. (c) vysotskite crystal (analysis 328) set in a fibrous, irregular crystal site, with minerals too small for analysis.

Figure 3. Sample A2 area 6. Analysis in Table 2. Inclusions and analysis numbers. (a) Reflected light image. Cu-PGM sulfide melt inclusions. Note: PGMs are pale grey, bornite is shown in dark purple, and chalcopyrite is shown in yellow. Round inclusion (black) is silicate glass [1] with a coexisting rim of Cu-PGM sulfides. (b) BSE image. Partly broken round nugget enclosing two rounded silicate glass inclusions (black) with narrow rims of exsolved Cu-PGM sulfides showing scalloped margins. Other inclusions comprise only Cu-PGM sulfide parageneses in the plane of this polished section. Analysis 276 is host isoferroplatinum. (c) BSE image. Detail of rounded silicate glass inclusion (black) [1] coexisting with a narrow rim of multiphase sulfide PGM. Note the highly irregular (once molten) boundary of sulfide PGM paragenesis with host alloy. Minute PGM inclusions form a halo in the host alloy. Analysis 325 is braggite. (d) BSE image. Analysis 317 confirms braggite–(vysotskite); analysis 319 is subhedral braggite. Cu-PGE monosulfide mineral, analysis 320, forms skeletal crystallites in bornite (confirmed by 318), as shown in the (e) BSE image. Note the wispy, irregular melt penetration into the host alloy. Subhedral cuprorhodsite (321); chalcopyrite (322). (f) Wispy irregular melt penetration into host alloy and small sulfide droplets. Analysis (323) confirms bornite. Subhedral braggite (324) is close to cooperite in composition.

Figure 4. (a) Sample Mag 5 area 4. Reflected light. Oval-shaped host Pt-Fe alloy nugget (~220 µm long dimension) and composite inclusion (~90 mm diameter) of silicate glass (black) coexisting with thick rim of Cu-PGM sulfides. (b) Reflected light. Composite inclusion. Silicate glass (black); thiospinel and braggite (grey); chalcopyrite (yellow) and pyrrhotite (pink). Minor patchy Pt-Fe alloy is white.

Figure 6. Sample A1 area 9. BSE images. Analysis points, Table 6. (a) Rounded but partly chipped and broken host isoferroplatinum (analysis 267). Silicate glass (black) and two rounded Cu-PGM sulfide inclusions. (b) Largest Cu-PGM sulfide inclusion; isoferroplatinum is subhedral (analysis 314); cupororhodsite–(malanite) 313; vasilite 312; pentlandite 316; and chalcopyrite 315.

Figure 7. Sample B area 5. BSE images. (a) Platinum nugget with quenched round inclusion. The dominant PGM paragenesis is grey with abundant white (alloy) patches. Round black patch is exsolved silicate glass. Reflected light. (b) BSE image. Analysis points. Analysis 6 is host nugget ferroan platinum; analyses 1, 9, and 12 are vasilite; 2 and 11 are keithconnite; 3 and 4 are cuprorhodsite; 5 is a Pt-Fe-Pd-Cu alloy rim; 7 is a Pt-Fe-Rh-Pd-Cu alloy; 13 is an Fe-Rh-Ni monosulfide mineral.

Figure 8. Sample B area 5. The approximate normalized fraction % of eight minerals is based on relative pixel counts for each. (a) Pt-Fe = alloy; (b) Rh-Cu-S = cuprorhodsite; (c) Pd-Cu-S = vasilite; (d) Rh-Cu-Fe-S = cuprorhodsite–(ferrorhodsite); (e) Pt-Fe-Pd = alloy; (f) Rh-Cu-Ni-S = cuprorhodsite–(polydymite); (g) Rh-Ni-Cu-S = polydymite–(cuprorhodsite); (h) Rh-Cu-Pt-S = monosulfide mineral.

Figure 9. Sample B area 4. (a) Platinum nugget with irregular (deflated) rim of PGM inclusion paragenesis exsolved from central silicate host with mafic phenocrysts and dacite–rhyolite groundmass (black). Reflected light. (b) WDS analysis points; 1, 2: vasilite; 3: cuprorhodsite; 5: vysotskite (1); 6: Rh-Pt-Cu-S cuprorhodsite; 7: vysotskite (1); 8: Pd-isoferroplatinum; 9: Pt-Fe alloy (EDS analysis); 10: vysotskite (2); 11: vysotskite (2).

Figure 10. Phase maps using EDS analysis and BSE images for mineral identification and approximate normalized fraction % based on relative pixel counts. (a) Pt = Pt-Fe alloy; (b) RhSPtCu = cuprorhodsite; (c) PtPdS = vysotskite (1) and subordinate Ni–vysotskite (2); (d) RhS = bowieite; (e) PdSCu = vasilite; (f) FeO (hematite). The minor UM (unnamed mineral) Pd-dominant sulfide-telluride is not shown.

Figure 11. Sample A1 area 7. BSE images. (a) Subrounded to subhedral Pt-Fe alloy host nugget with coexisting altered subhedral grains at the nugget margin. Scalloped outline of porphyritic silicate inclusion (dark grey) with an irregular “halo” of small PGM inclusions (pale grey) located in the host Pt-Fe alloy (white). (b) Details showing prismatic outlines of subhedral silicate phenocrysts, wispy crystallites, and small, marginal vesicles (black) in silicate glass. Partial halo of rounded and partly resorbed, fractured, and displaced PGM inclusions.

Figure 12. BSE images. (a) PGM inclusion with subhedral prismatic cuprorhodsite–(malanite), analysis point (308), set in vasilite, analysis point (309). This inclusion has been torn apart, and rounded negative indents suggest resorption. The two portions are now displaced and separated by the host Fe-Pt alloy. The smaller portion also shows partial irregular resorption by the host Pt-Fe alloy (partial decompression melt, see discussion below). (b) Two inclusions with similar mineralogy to (a). The larger inclusion shows complex rounded indents in subhedral cuprorhodsite and vasilite (possibly a gas cavity). Negative indents in vasilite suggest resorption. In the smaller inclusion, euhedral cuprorhodsite–(malanite) extends beyond the rounded (possibly vasilite) matrix.

Figure 13. (a) Total alkali-silica (TAS) diagram (after Le Bas et al. [30]), with all analyses recalculated to 100% minus volatiles) showing the differentiation trend (or liquid line of descent) of silicate glass inclusions in the Pt-Fe alloy coexisting with Cu-PGE sulfide parageneses (dot-dash line, cf. Barron et al. [1] (Figure 11 in their study)). Filled circle: sample B area 1, silicate glass (1021.2 °C) coexists with Cr-spinel; open circle, synthetic ferrobasaltic glass (940–1200 °C), Skaergaard (Greenland) (Botcharnikov et al. [2]); open star, proposed approximate primitive picrobasalt melt (cf. Johan et al. [31] SiO₂ ~ 44 wt.%); this value may not be volatile-free. The following numbers refer to quenched glass in the present seven samples considered here. 1, thin star i (A2 area 7); 2, thin star h (A2 area 6); 3, filled plus (Mag 5 area 4); 4, thin star b (A1 area 9); 5, filled triangle (B area 5); 6, open square (B area 4); 7, filled diamond (A1 area 7). (b) AFM diagram showing liquid lines of descent for quenched silicate glass in the seven samples considered here, following first the tholeiitic and then the calc-alkaline magma series trends (dotted line). The dashed line marks the boundary between tholeiitic and calc-alkaline compositions. A = Alkali (Na + K oxides); F = Fe-oxide; M = Mg oxide. BT = Tholeiitic basalt; FB = ferrobasalt; ABT = tholeiitic basaltic andesite; AT = tholeiitic andesite; D = dacite; R = rhyolite; B = basalt; AB = basaltic andesite; A = andesite (cf. Barron et al. [1]; Figure 12).

Figure 14. The path of differentiation by fractional crystallization is shown by black dashed lines. The path of differentiation by incongruent melting is shown by solid red lines. (a) Inclusion bulk compositions of Cu-PGM sulfide fractions in four samples in terms of Pt + Pd + Rh–Cu–Fe(+Ni + Co) (at. %). (b) Inclusion bulk compositions of Cu-PGM sulfide fractions in four samples in terms of Pt–Rh–Pd (at. %). Temperatures are calculated using the SiO₂ concentrations in coexisting silicate glass [32].

Table 3. Sample Mag 5 area 4 calculated bulk chemistry of PGM (minus silicate glass).

S	Fe	Co	Ni	Cu	Rh	Pd	Os	Ir	Pt	Total, wt. %
26.08	16.50	0.16	0.18	12.00	4.36	5.74	0.22	0.78	34.03	100.00
										Total, at. %
51.50	18.70	0.17	0.19	11.96	2.68	3.42	0.07	0.26	11.04	100.00

Note: Me:S = 48.49:51.50 = 0.94 at. %. ~S saturated.

Table 7. Sample B area 5: calculated bulk chemistry droplet wt. % (minus silicates).

S	Fe	Co	Ni	Cu	Se	Rh	Pd	Te	Pt	Total, wt. %
29.22	8.55	0.02	1.09	11.70	0.17	43.10	3.83	0.04	2.29	100.00
										Total, at. %
52.48	8.82	0.02	1.07	10.60	0.12	24.12	2.07	0.02	0.68	100.00

Note: Me 47.52: S (+Se + Te) 52.62 = 0.90 at. % ~S-saturated.

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Barron, B.J.; Barron, L. Rare Inclusions of Coexisting Silicate Glass and Cu-PGM Sulfides in Pt-Fe Nuggets, Northwest Ecuador: Fractionation, Decompression Exsolutions, and Partial Melting. Minerals 2025, 15, 1329. https://doi.org/10.3390/min15121329

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Barron BJ, Barron L. Rare Inclusions of Coexisting Silicate Glass and Cu-PGM Sulfides in Pt-Fe Nuggets, Northwest Ecuador: Fractionation, Decompression Exsolutions, and Partial Melting. Minerals. 2025; 15(12):1329. https://doi.org/10.3390/min15121329

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Barron, B. Jane, and Lawrence Barron. 2025. "Rare Inclusions of Coexisting Silicate Glass and Cu-PGM Sulfides in Pt-Fe Nuggets, Northwest Ecuador: Fractionation, Decompression Exsolutions, and Partial Melting" Minerals 15, no. 12: 1329. https://doi.org/10.3390/min15121329

APA Style

Barron, B. J., & Barron, L. (2025). Rare Inclusions of Coexisting Silicate Glass and Cu-PGM Sulfides in Pt-Fe Nuggets, Northwest Ecuador: Fractionation, Decompression Exsolutions, and Partial Melting. Minerals, 15(12), 1329. https://doi.org/10.3390/min15121329

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Rare Inclusions of Coexisting Silicate Glass and Cu-PGM Sulfides in Pt-Fe Nuggets, Northwest Ecuador: Fractionation, Decompression Exsolutions, and Partial Melting

Abstract

1. Introduction

2. Samples and Methods

2.1. Platinum-Group Sulfide Mineral Analysis

2.2. Element Mapping of Cu-PGM-Sulfide Exsolution Inclusion Assemblages

3. Samples: Silicate Inclusions with Coexisting Cu-PGM Sulfide Parageneses

3.1. Samples, Mineralogy, and Analyses

3.1.1. Sample A2 Area 7

3.1.2. Sample A2 Area 6

3.1.3. Sample Mag 5 Area 4

3.1.4. Sample A1 Area 9

3.1.5. Sample B Area 5

3.1.6. Sample B Area 4

3.1.7. Sample A1 Area 7

4. Discussion

4.1. Host Pt-Fe Alloys and Composite Inclusions

4.1.1. Fractionation and T of Host Silicate Melts Coexisting with Cu-PGM Sulfides

4.1.2. Bulk Compositions: Fractionation of Selected Coexisting Cu-PGM Sulfide Parageneses

4.1.3. S-Saturation of Cu-PGM Bulk Compositions

4.2. Cu-PGM Sulfide Compositions and Stability

4.2.1. Braggite–Vysotskite

4.2.2. Thiospinels

4.2.3. Minor Cu-PGE Sulfide Minerals Coexisting with Thiospinel

4.3. Deformation and Decompression

4.3.1. Deformation

4.3.2. Decompression Alloy Exsolutions in Cuprorhodsite and Vysotskite

4.3.3. Partial Decompression Melting

5. Conclusions

Author Contributions

Funding

Data Availability Statement

Acknowledgments

Conflicts of Interest

References

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