Sulfide Rb-Sr, Re-Os and In Situ S Isotopic Constraints on Two Mineralization Events at the Large Hongnipo Cu Deposit, SW China

Abstract

The Hongnipo deposit, a newly discovered large copper deposit in the Kangdian copper belt, SW China, is hosted in the Paleoproterozoic Hekou Group. This deposit contains ~4200 Mt of Cu ores, with an average grade of 1.42 wt.% Cu. Orebodies occur mainly as stratiform, stratoid and lenticular forms. Mineralization consists predominantly of high grade vein-type ores and low grade laminated ores. Field relationships indicate vein-type mineralization crosscuts laminated mineralization and host rocks, indicating that there were at least two mineralization events during the formation of the deposit. Pyrite separates from the laminated ores yield a Rb-Sr isochron age of 1552 ± 80 Ma, with a highly radiogenic initial ⁸⁷Sr/⁸⁶Sr ratio of 0.71214 ± 0.00081, indicating a major contribution from the ore-hosting rocks. Sulfides from the laminated ores have δ³⁴S values ranging from −1.8‰ to 11.4‰, with the vast majority in the range of 5.3‰ to 11.4‰, suggesting the mixed derivation of sulfur from seawater sulfates and magmatic fluids. Chalcopyrite separates from the vein-type ores have a Re-Os isochron age of 794.8 ± 7.9 Ma. The initial ¹⁸⁷Os/¹⁸⁸Os (2.8 ± 1.2) and γ_Os (+2202) values are slightly lower than the average values of continental crust, indicating a major metal source of the Hekou Group with minor mantle input. Sulfides from the vein-type ores have δ³⁴S values that range from −10.3‰ to 4.0‰ and cluster between 0‰ to 2.2‰, which implies a significant contribution of magmatic-sourced sulfur with minor biogenic sulfur. Two major mineralization events have been identified. The Rb-Sr age of the laminated ores likely records a VMS mineralization event at ~1.6 Ga. The much younger Re-Os age is considered to represent the timing of an important mineralization event, which is likely related to the Neoproterozoic magmatism and/or metamorphism and represents a newly documented mineralization event to be targeted by exploration.

1. Introduction

The Kangdian copper belt, which extends for ~300 km in the southwest of the Yangtze Block [1,2,3,4], is one of the most important iron-copper-gold production areas in China. The Lala copper ore field (Figure 1) is one of the most important and typical ore fields within this belt. Two large deposits (Luodang and Hongnipo, with Cu resources of ~0.56 and ~0.64 Mt, respectively) and several medium-small deposits have been discovered in this ore field. Numerous studies of the Luodang deposit are available. These studies have reached an agreement that the host strata formed during the late Paleoproterozic, with zircon U-Pb ages of ~1.6 to 1.7 Ga [5,6,7,8,9,10].

The origin of the Luodang deposit has been discussed by many researchers, but it still remains controversial. Some authors argued that it is volcanogenic in origin, a model similar to VMS deposits, and mineralization is coeval with the host rock (~1.6–1.7 Ga [11,12]). Alternatively, the deposit was thought to be epigenetic in origin [2,13,14,15,16,17,18]. Li et al. [13], Sun et al. [14] and Greentree [15] reported the Re-Os model ages of molybdenite (928–1005 Ma), a Pb-Pb isochron age of sulfides (887 Ma), and Ar-Ar plateau ages of muscovite and biotite (848 Ma and 860 Ma), respectively, proposing that it has formed by Neoproterozoic retrograde metamorphism. However, proper details and descriptions of the samples are lacking. Chen and Zhou [2,16] reported a molybdenite Re-Os age of 1086 Ma and a primary allanite U-Pb age of 1067 Ma. Zhu and Sun [17] and Song [18] reported chalcopyrite Re-Os isochron ages of 1290 Ma and 1085 Ma, respectively. They suggested that the deposit formed in a single mineralization event related to regional Mesoproterozoic magmatism (~1.1 to 1.3 Ga [19,20,21]), but was overprinted by post-ore hydrothermal events related to regional Neoproterozoic magmatism or metamorphism [22]. More recent work proposes the deposit formed by multiple episodes of mineralization events. Zhu et al. [23] reported four molybdenite Re-Os ages, ranging from 835 to 1306 Ma, and suggested that the deposit was formed by two major mineralization events at ~1.3 and ~1.1 Ga. By contrast, Chen et al. [24] reported two in situ sulfide Pb-Pb errorchron ages of ~1.67 and ~1.1 Ga, and suggested that sulfides in this deposit have dominantly formed in these two periods. Additionally, abundant sulfur isotopic studies showed multi-source characteristics, including magmatic, seawater-derived sulfate and local bedrock strata [16,18,25].

The Hongnipo deposit, located in the Lala copper ore field, is a newly discovered large copper deposit, with an inferred ~4200 Mt of ores, with an average grade of 1.42 wt.% Cu [26]. Only a few studies have been conducted since its discovery. Zhu [27] summarized the geological setting, orebody characteristics and ore compositions of the deposit, and defined it as an IOCG (iron-oxide copper gold) deposit. Hu et al. [28] studied the metallogenic geological background, and indicated that complex tectonic evolution and frequent magma events provided favorable space and material conditions for the formation of the deposit. He described the geological characteristics of the deposit, and suggested that Cu mineralization is closely related to sodic volcanic rocks. Consequently, he proposed that the association of copper mineralization to sodic volcanics was an important prospecting tool. Zhang et al. [29] discussed the main ore-controlling factors, including lithology, structure, and alteration assemblages. They suggested that strata with sodic volcanic rocks, structural intersections, gabbros and relevant proximal wall-rock alterations (e.g., albitization, silicification, biotitization, chloritization) should be significant prospecting targets. Geochemical studies of ore-bearing volcanic rocks indicate that they have formed in a subduction-related extensional setting [30]. Sulfur isotope studies based on the types of ore-hosting rocks demonstrated that sulfur was dominantly derived from the mantle, and fluid inclusion microthermometry suggested a medium-high temperature and high salinity fluid was responsible for the formation of the deposit [30]. However, none of the studies described the mineralization styles or mineralization ages of this deposit. The sulfur isotopic compositions reported by Wang [30] were determined using conventional methodology, which may result in the potential mixing of polygenetic age populations of sulfides during mineral separation [31]. Precise age and sulfur isotope determinations of mineral deposits are vital for determining genetic type, constructing genetic and prospecting models, and guiding further mineral exploration [32].

In this study, we identified two mineralization styles on the basis of field observations. In order to constrain the timing of the two mineralization styles and determine the sources of ore-forming minerals, pyrite Rb-Sr dating, chalcopyrite Re-Os dating and in situ sulfur isotopic analyses of sulfides from the Hongnipo deposit were carried out. We further discuss the implications of our study for the regional metallogeny of the Kangdian copper belt.

2. Regional Geology

South China has a complex tectonic history, and consists of two major Precambrian continental blocks involving the Yangtze Block to the northwest and the Cathaysia Block to the southeast, separated by the Sibao orogen (Figure 1a) [33,34]. The Yangtze Block predominantly comprises Proterozoic strata and Neoproterozoic igneous rocks [35]. Archean basement rocks, the ~2.90–2.95 Ga Kongling Complex, were only previously known to be exposed in the northern part of the Yangtze Block [36]. Recent studies of zircon xenocrysts in Paleozoic lamproites and detrital zircons in Proterozoic sedimentary rocks in the southwestern Yangtze Block indicate a higher proportion of Archean to Paleoproterozoic unexposed basement rocks beneath the Yangtze Block than previously thought [37,38,39,40,41]. The Archean rocks were metamorphosed to granulites at ~2.03–1.97 Ga, and then were intruded by the ~1.85 Ga A-type granites and mafic dykes [42,43,44,45,46,47]. The Yangtze Block collided with the Cathaysia Block at ~1.3–1.0 Ga, with the consolidation occurring at ~1.0–0.85 Ga, resulting in widespread deformation, uplift, and regional metamorphism in these two blocks [35,48,49].

The Kangdian copper belt, a major N–S-trending Cu metallogenic belt, is located in the southwestern part of the Yangtze Block (Figure 1a). The strata exposed here are predominantly Proterozoic volcanic-sedimentary successions that consist of arenaceous to argillaceous sedimentary rocks with minor volcanic rocks, and the unconformably overlying late Neoproterozoic (Sinian) to Cenozoic cover sequence, consisting of clastic, carbonate, and volcanic rocks [50]. The Proterozoic sequences, mainly controlled by the NS-trending Luzhijiang fault and a series of NNE-trending faults (Figure 1b), include the Paleoproterozoic Dongchuan, Hekou, and Dahongshan Groups with zircon U-Pb ages dominantly between ~1.7 Ga and ~1.6 Ga [6,7,8,10,39,51], and the Meso- to Neoproterozoic Huili, Kunyang, Julin, and Yanbian Groups, with zircon U-Pb age populations at ~1.4–0.9 Ga [19,20,52,53,54]. As a result of the Neoproterozoic collisional orogenesis, termed the Sibao orogeny (860–815 Ma) [23], the Paleoproterozoic volcanic-sedimentary sequences underwent upper greenschist-lower amphibolite facies metamorphism [55], whereas lower greenschist facies metamorphism overprinted the Meso- to Neoproterozoic sequences [55,56]. Numerous Fe-Cu deposits are hosted within these meta-volcanic and meta-sedimentary rocks of the Paleoproterozoic Dahongshan, Hekou, and Dongchuan Groups in the Kangdian copper belt (Figure 1b) [1,16,22,23,57,58,59].

Mafic and felsic intrusions of Late Paleoproterozoic to Mesoproterozoic age (~1.7–1.5 Ga) occur within the belt [39,59,60,61,62,63], show geochemical affinities of intra-plate magmatism, and are considered to have formed in a continental rift setting [7,39]. In addition, extensive tectonic-magmatic activity occurred at ~960–740 Ma, which was almost simultaneous with regional metamorphism, and produced a variety of intrusive rocks comprising diabase, gabbros, diorites and granites in the Kangdian copper belt [38,53,64,65,66,67,68,69]. These Neoproterozoic plutons have been interpreted as either subduction-related [53,64,66,67] or mantle plume-related [65,68,69], of which the two largest granitic bodies are the Eshan pluton (819 ± 8 Ma [65]) and the Yuanmou pluton (746 ± 13 Ma [64]).

3. Deposit Geology

The Hongnipo copper deposit, with central geographic coordinates of 101°57′00″E and 26°11′30″N, is situated in the southwestern part of Huili County (Figure 1b). The exposed strata are the late Paleoproterozoic Luodang and Changchong Formations, Mesoproterozoic Heishan Formation, Triassic Baiguowan Formation, and overlying Quaternary (Figure 2). The Luodang Formation is sparsely distributed in the eastern margin of the deposit, and is composed predominantly of garnet-biotite schist, carbonaceous slate, quartz albitite, biotite-quartz schist, muscovite-quartz schist, two-mica quartz schist, and metatuff. Lenses of marble occur locally in this formation. The Changchong Formation, in conformable contact with the Luodang Formation, is exposed to the west and northeast of the deposit, and its lithological associations are similar to those of the Luodang Formation. The Heishan Formation, exposed in the northwest of the deposit, comprises carbonaceous slate, sericite phyllite, and marble. This formation is in fault contact with the Changchong Formation to the SE. The Baiguowan Formation, locally distributed in the southwest corner of the deposit, unconformably overlies the Changchong Formation. It mainly contains basal conglomerate, gritstone, shale, feldspar-quartz sandstone and siltstone. The Quaternary eluvial, alluvial and aqueoglacial sediments cover the entire central part of the deposit. The EW-striking Hongnipo synclinorium and NNE-striking faults (F₁₃, F₂₇, F₃₀) (Figure 2) are the major structures at Hongnipo.

Extensive gabbro sills and dikes intrude the strata, but no other intrusive rocks have been identified. Intrusive breccias genetically related to the gabbro occur sporadically along boundaries between the strata and the intrusions (Figure 2). Additionally, abundant gabbro dikes and intrusive breccias also occur at various depths below the deposit (Figure 3). It is worth noting that fractures adjacent to the gabbro dikes, especially those in the hanging wall, commonly contain some mineralization.

Five orebodies occur as stratiform, stratoid and lenticular, and are hosted in the Changchong and Luodang Formations (Figure 3). These orebodies have an average copper grade of 1.42%. The majority are strata bound, dipping 10°–20° to the southwest [26]. There are two ore varieties, namely, laminated and vein-type ores (Figure 4a–d), the latter of which is vastly more important in terms of metal reserves of the deposit. In the laminated ores, sulfides always occur parallel to the foliation (Figure 4a), indicating that they may have formed contemporaneously with the host rocks. Typical of the vein-type ores are sulfide-calcite±quartz veins that fill the fractures of the host rocks (Figure 4b) or overprint early laminated ores along the foliation (Figure 4c). It is common that vein-type mineralization crosscuts the laminated mineralization (Figure 4d), suggesting that economic Cu mineralization postdated the low-grade laminated mineralization.

Ore minerals in the laminated ores generally consist of abundant subhedral to anhedral pyrite and minor chalcopyrite (Figure 5a). Arsenopyrite, pyrrhotite and sphalerite are scarce (Figure 5b,c). Metasomatic texture and sieve texture of two stage pyrite are occasionally observed (Figure 5d). Ore mineral assemblages of vein-type ores are comprised of voluminous chalcopyrite and pyrite (Figure 5e–i), with minor bornite, pyrrhotite, hematite (Figure 5h,i), and magnetite. Gangue minerals of the two types of ores commonly comprise calcite, quartz, albite, chlorite, biotite, muscovite, and scarce titanite. In both types of ores, granular and metasomatic textures are most common (Figure 5). Exsolution texture of chalcopyrite within sphalerite is only observed in laminated ores (Figure 5c). Zoned pyrite is common in the vein-type ores (Figure 5g).

4. Sampling and Analytical Methods

4.1. Sampling

For the present study, 21 ore samples were taken from the drill cores provided by Geological Team 403 of Sichuan Geological and Mineral Exploration and Development Bureau. The sampling locations are shown in Figure 2 and

Table 1. Locations of samples used in this study.

Sample	Drill Hole	Depth (m)	Ore Type	Sample	Drill Hole	Depth (m)	Ore Type
Hnp017	ZK−301	441	Laminated	Hnp483	ZK−459	256	Vein-type
Hnp019		443		Hnp484		260
Hnp022		449		Hnp487		266
Hnp024		450		Hnp488		267
Hnp026		453		Hnp490		278
Hnp031		458		Hnp493		285
Hnp053		765	Vein-type	Hnp495		289
Hnp178	ZK−1006	394	Laminated	Hnp499	ZK−801	519	Vein-type
Hnp181		399		Hnp503		529
Hnp223	ZK−553	136	Vein-type	Hnp504		542
Hnp287	ZK−903	549	Laminated

. The ore type of these samples are also given in Table 1. Six relatively low-grade laminated ores (Hnp019, Hnp024, Hnp026, Hnp031, Hnp178 and Hnp181) were selected from the No.1 orebody for pyrite Rb-Sr dating. They are hosted in quartz albitite or muscovite-quartz schist. Among these samples, no obvious post-ore hydrothermal alteration was identified with the naked eye. The sulfides occur parallel to the foliation, showing a crude stratiform mineralogical partitioning. Petrographic observation revealed that mineral assemblages consist of abundant subhedral to anhedral pyrite, with minor chalcopyrite, quartz and calcite. In addition, it is worth mentioning that two stages of pyrite were observed in samples Hnp024 and Hnp026. In these samples, the early stage pyrite is locally enclosed by the late stage pyrite and/or show signs of metasomatism (Figure 5d). Six vein-type ores (Hnp484, Hnp487, Hnp488, Hnp490, Hnp493 and Hnp495) with high-grade Cu were selected from the No.1 orebody for chalcopyrite Re-Os geochronology. Irregular sulfide veins crosscut the wall rocks of marble, and are characterized by the mineral assemblage comprising dominant chalcopyrite, with minor pyrite, quartz and calcite. No later hydrothermal overprint was found in hand specimens and corresponding polished thin sections. Pure pyrite and chalcopyrite separates were handpicked under a binocular microscope after crushing, followed by magnetic and heavy liquid separation. These separates were then ground to about 200 mesh in an agate mortar.

For sulfur isotope work, three representative laminated ores (Hnp017, Hnp022 and Hnp287) with minor laminated sulfides and six typical veined ores (Hnp053, Hnp223, Hnp483, Hnp499, Hnp503 and Hnp504) were made into polished thin sections. They share similar structural features, mineral associations and host rocks with those of samples used for Rb-Sr and Re-Os analyses.

4.2. Analytical Methods

The pyrite Rb-Sr isotope analyses were carried out at the Analytical Laboratory of the Beijing Research Institute of Uranium Geology, Beijing, China, following the procedures and methods described in the literature [71]. Pyrite grains were washed ultrasonically in analysis-grade alcohol and ultrapure water. Subsequently, accurately weighed pyrite powder (0.03−0.1 g) was loaded into Teflon vial (15 mL), and then wetted with a little high purity water. Proper amounts of ⁸⁷Rb and ⁸⁴Sr spikes were added into the vial, and then they were shaken to be uniform. After adding 3 mL hydrofluoric acid and 4 drops of perchloric acid and shaking the bottle, it was capped. Later, the bottle containing solution was heated on a hot plate with temperature around 120 °C for 24 h, until the sample dissolved completely. Hydrofluoric acid was expelled by heating it to 180 °C, following the operations of removing the cap and evaporating the sample. The internal wall of the tubular bottle was washed twice with 2 mL of 6 mol/L hydrochloric acid and then dried down at about 120 °C. Then, 1 mL of 0.5 mol/L hydrochloric acid was added into the tubular bottle to redissolve the sample, after which, the sample was transferred to plastic centrifuge tube. Finally, the supernatant used for the separation of rubidium and strontium was obtained through centrifugation for 6−10 min. The supernatant was then poured into the cation exchange column. A small amount of Milli-Q water was used to wash the corners of the column. Afterwards, 16 mL of 2 mol/L hydrochloric acid was used to elute and remove impurities such as potassium, sodium and iron. Rubidium was desorbed into a quartz beaker (10 mL) by the use of 5 mL of 2 mol/L hydrochloric acid. Moreover, 12 mL of 2 mol/L hydrochloric acid together with 2 mL of 3 mol/L hydrochloric acid was used to elute and remove ions such as calcium, magnesium and iron. Lastly, strontium was desorbed into a quartz beaker (10 mL) using 6 mL of 3 mol/L hydrochloric acid. Pure rubidium and strontium were loaded on single rhenium filaments with TaF₅ as activator. Rubidium and strontium isotopic compositions were then determined on a Phoenix TIMS instrument, in static multicollector mode. The internal precision of ⁸⁷Sr/⁸⁶Sr ratio is better than 0.002%. Mass fractionation corrections in Sr isotopic analyses were done with ⁸⁶Sr/⁸⁸Sr value of 0.1194. During this study, the ⁸⁷Sr/⁸⁶Sr ratio for NBS 987 Sr standard was 0.710250 ± 7 (2σ). Both of the blanks for Rb and Sr are 0.2 ng within the total procedure. Data regression for Rb-Sr isochron age was done with the Isoplot software [72], using errors of 2% for ⁸⁷Rb/⁸⁶Sr ratios and the within-run measurement precision for ⁸⁷Sr/⁸⁶Sr values (2σ).

The chalcopyrite Re-Os isotope analyses were performed at the Re-Os Laboratory of the National Research Center of Geoanalysis, Chinese Academy of Geological Sciences, Beijing, China, utilizing isotope dilution-negative thermal ionization mass spectrometry (ID-NTIMS). Detailed analytical procedures used in this study are found in Du et al. [73,74]. A precisely weighed sample, together with a predetermined amount of mixed ¹⁹⁰Os and ¹⁸⁵Re spike was loaded into a Carius tube, and then digested by reverse aqua regia. The tube was sealed and then heated in an oven at 200 °C for 24 h. After cooling, scratching, rewarming, and breaking off the tube’s neck, the contents were poured into a distillation flask, and the residue was washed out into the flask with 40 mL of Milli-Q water. Osmium was separated and purified using distillation and microdistillation [75]. Rhenium was extracted from the residue by acetone in an 8 mol/L NaOH solution. The acetone phase was then transferred into a 100 mL beaker with 2 mL of Milli-Q water. The beaker was heated on a hot plate at 50 °C to expel acetone. When the acetone was exhausted, the heating temperature was increased to evaporate the solution to dryness. Afterwards, 0.8 mol/L HNO₃ was added to neutralize and dissolve the residue. An appropriate amount of solution obtained from the previous step was added to an anion exchange column, which had been equilibrated with 0.8 mol/L HNO₃. Then, 3 mL of 0.8 mol/L HNO₃, 2 mL of 1mol/L HCl were added to elute impurities, and 1 mL of Milli-Q water was used to remove the HCl. Lastly, 3 mL of 4 mol/L HNO₃ was used to elute Re into a 7 mL Teflon vial. Re and Os concentrations and isotope compositions were determined using a Thermo Fisher Scientific Triton plus mass spectrometer, operating in negative ion detection mode [76]. Purified Re and Os were loaded onto platinum filaments with Ba(OH)₂ as an ion emitter and measured as ${\mathrm{ReO}}_4^{-}$ and ${\mathrm{OsO}}_3^{-}$, respectively. The signals of Re and Os were subsequently received by Faraday cups and secondary electron multiplier, respectively. Oxygen isotope interference was removed by means of a gradual spectral stripping method, according to the natural abundance of oxygen isotopes and statistical equal probability model [77]. The instrumental mass fractionation of Os was calibrated by normalizing the measured ¹⁹²Os/¹⁸⁸Os ratio to 3.08271 [78]. The mass fractionation of Re was corrected using normal Re (¹⁸⁵Re/¹⁸⁷Re = 0.59738) as an external standard determined at the same temperature as the samples. Moreover, Re isotopic fractionation correction uncertainties of 0.1% were included in the calculations, even though the mass bias for ${\mathrm{ReO}}_4^{-}$ is small using NTIMS. On the basis of blank runs analyzed together with samples, the total procedural blanks for Re and Os were about 3 pg and 0.5 pg, respectively. Uncertainties for Re-Os isotopic dating were determined by error propagation of uncertainties in Re and Os mass spectrometer measurement and mass fractionation, bank abundances and isotopic compositions, spike calibrations, and weighing uncertainties of samples and spike. In addition, an in-house sulfide Re-Os isotope references JCBY from Jinchuan Cu-Ni sulfide deposit was used for quality control of the whole procedure. Its results are identical with the recommended value within analytical uncertainty for Os isotopic compositions, as well as Re and Os concentrations.

Sulfides’ in situ sulfur isotopic composition measurements were conducted on a Nu Plasma II MC-ICP-MS instruments at the State Key Laboratory of Geological Process and Mineral Resources (GPMR), China University of Geosciences (Wuhan). The detailed analytical procedure and conditions are given in Liu et al. [79]. In brief, the sulfides on polished thin sections were ablated by a Resonetics-S155 laser ablation system, with a laser energy fluence of 3 J/cm², a beam diameter of 33 μm, a repetition rate of 10 Hz and an ablation time of 40 s. Ablated sulfide aerosols were carried by high purity helium and mixed with argon and minor nitrogen, before entering the plasma. Using the ³⁴S/³²S ratios of samples and the internal laboratory standard pyrite (WS-1) (δ³⁴S: + 0.9‰), the δ³⁴S values of sulfides from ore samples were obtained by using the sample-standard bracketing approach.

5. Results

5.1. Rb-Sr Isotopes

The Rb-Sr isotope data of six pyrite samples are listed in

Table 2. Rb-Sr isotope data for pyrite from laminated ores in this study.

Sample	Rb (ppm)	Sr (ppm)	87Rb/86Sr	87Sr/86Sr	2σ	(87Sr/86Sr)ia
Hnp019	0.07	1.41	0.1447	0.715554	0.000013	0.712330
Hnp024	0.022	0.269	0.2374	0.715583	0.000019
Hnp026	0.074	0.353	0.6095	0.716814	0.000019
Hnp031	0.17	1.74	0.2819	0.718292	0.000021	0.712010
Hnp178	0.287	1.04	0.6771	0.727045	0.000019	0.711957
Hnp181	0.446	1.09	1.1854	0.738650	0.000011	0.712236

^a (⁸⁷Sr/⁸⁶Sr)_i calculated using the isochron age of 1552 Ma (Figure 6) and ⁸⁷Rb decay constant of 1.42 × 10⁻¹¹ a⁻¹.

. They have variable contents of Rb (0.022 to 0.446 ppm) and Sr (0.269 to 1.74 ppm), with ⁸⁷Rb/⁸⁶Sr ratios from 0.1447 to 1.1854. The scatter observed probably suggests the heterogeneity of the initial Sr isotopic compositions of the pyrite [80] or inclusions [81,82,83]. Both Rb and Sr are lithophile elements and are incompatible within pyrite [1]. Combined with the typical sieve texture (Figure 5f), the minerals within pyrite may be responsible for the variation.

Regression of all six Rb-Sr isotopic data gives a highly imprecise model 3 [72] date of 1530 ± 880 Ma (MSWD = 289; Figure 6). Most of the scatter is attributable to Hnp024 and Hnp026. The isochron dating method requires that the samples have identical initial isotopic composition and the isotopic system remained closed after mineral crystallization [84]. In view of these requirements and our petrographic evidence (Figure 5d), samples Hnp024 and Hnp026 should be precluded for isochron age calculation. A best-fit line through the remaining four analyses defines a much more precise model 1 age of 1552 ± 80 Ma (Figure 6), with an initial ⁸⁷Sr/⁸⁶Sr ratio of 0.71214 ± 0.00081 (MSWD = 3.2). Using the age of 1552 Ma, the calculated initial ⁸⁷Sr/⁸⁶Sr ratios for the remaining samples are 0.712330, 0.712010, 0.711957, and 0.712236 (Table 1), with an average value of 0.712133.

5.2. Re-Os Isotopes

Re and Os data of six chalcopyrite samples in which the blank corrections have been made are shown in

Table 3. Re-Os isotope data for chalcopyrite from vein-type ores in this study.

Sample	Re (ppb)	2σ		Total Os (ppb)		2σ		Common Os (ppb)		2σ		187Re (ppb)		2σ		187Os (ppb)	2σ
Hnp484	3.579	0.026		0.0374		0.0001		0.00607		0.00005		2.249		0.017		0.0315	0.0002
Hnp487	5.869	0.043		0.0674		0.0002		0.00329		0.00003		3.689		0.027		0.0642	0.0005
Hnp488	17.23	0.13		0.1544		0.0005		0.00742		0.00006		10.83		0.080		0.1471	0.0011
Hnp490	3.373	0.025		0.0342		0.0001		0.00495		0.00004		2.119		0.016		0.0294	0.0002
Hnp493	5.156	0.038		0.0403		0.0001		0.00455		0.00004		3.241		0.024		0.0358	0.0003
Hnp495	0.7317	0.0054		0.0123		0.0001		0.00404		0.00004		0.4598		0.0034		0.0083	0.0001
Sample	187Re/188Os		2σ		187Os/188Os		2σ		187Osr a (ppb)		2σ b		%187Os c		Model Age (Ma) d		2σ e
Hnp484	2845		30		39.8		0.10		0.0292		0.00074		92.82		774		17
Hnp487	8612		91		149.7		0.5
Hnp488	11225		118		152.5		0.36		0.1444		0.00008		98.12		795		22
Hnp490	3288		35		45.58		0.17		0.0275		0.00058		93.73		775		19
Hnp493	5473		59		60.58		0.24
Hnp495	873.8		11.3		15.82		0.13		0.0068		0.00060		81.95

^a Radiogenic ¹⁸⁷Os was determined using the initial ¹⁸⁷Os/¹⁸⁸Os ratio, derived from the well-constrained 4-point isochron plot (Figure 7a). ^b Uncertainty of radiogenic ¹⁸⁷Os was calculated on the basis of the errors of Common Os content and initial ¹⁸⁷Os/¹⁸⁸Os ratio. ^c Percentage of all measured ¹⁸⁷Os was determined from the radioactive decay of ¹⁸⁷Re. ^d Model ages were calculated only for samples with ¹⁸⁷Re/¹⁸⁸Os ratios greater than 2800, utilizing the ¹⁸⁷Re decay constant of 1.666×10⁻¹¹a⁻¹ with a relative uncertainty of ±0.031% [91]. ^e Age uncertainty was determined based on error propagation of Re content, Os content, and ¹⁸⁷Re decay constant errors.

. All samples contain low concentrations of Re and ¹⁸⁷Os, with large concentration ranges of 0.7317–17.23 ppb and 0.0083–0.1471 ppb, respectively. The abundances of common Os are low, but relatively uniform (0.00329–0.00742 ppb). The ¹⁸⁷Re/¹⁸⁸Os ratios vary between 873.8 and 11225, and the ¹⁸⁷Os/¹⁸⁸Os ratios range from 15.82 to 152.5. The regression of six ¹⁸⁷Re/¹⁸⁸Os-¹⁸⁷Os/¹⁸⁸Os data points with the corresponding uncertainty correlation value, yields a Re-Os model 3 [72] age of 874 ± 310 Ma. The scatter of these data around the regression line is extremely large (MSWD = 954; Figure 7a). A high MSWD value is indicative of the existence of geological scatter about the isochron in excess of the analytical uncertainty [85]. Most of the scatter is accounted for by samples Hnp487 and Hnp493, which deviate significantly from the regressive trend, compared with the remaining data (Figure 7a). Given the absence of petrographic evidence for post-ore hydrothermal overprint, the possible cause of deviation of the two samples is heterogeneity in their initial ¹⁸⁷Os/¹⁸⁸Os ratios, in comparison to the other four samples left. Considering the fundamental requirements for isochron dating [84], samples Hnp487 and Hnp493 are omitted. Regressing all the remaining data results in a model 1 (assumes that the scatter regarding to the linear regression only results from assigned uncertainty; [72]) Re-Os date of 794 ± 12 Ma (2σ, initial ¹⁸⁷Os/¹⁸⁸Os = 2.8 ± 1.2, MSWD = 2.9; Figure 7a). Using the calculation method given in Zhu and Sun [17], Selby et al. [86] and Li et al. [87], the abundance and uncertainty of radiogenic ¹⁸⁷Os (¹⁸⁷Os^r) of samples (Hnp484, Hnp488, Hnp490 and Hnp495) are derived from the corresponding total ¹⁸⁷Os and error (Table 3). The ¹⁸⁷Os budget is mainly composed of ¹⁸⁷Os^r (81.95%–98.12%; Table 3), which satisfies the characteristics of low-level highly radiogenic sulfide [87]. As a result, the ¹⁸⁷Re versus ¹⁸⁸Os^r data can be applied to determine a Re-Os date [88]. The ¹⁸⁷Re versus ¹⁸⁸Os^r data yield a model 1 four-point date of 794.8 ± 7.9 Ma (2σ, initial ¹⁸⁷Os = −0.0002 ± 0.00075 ppb, MSWD = 2.4; Figure 7b), which is within the uncertainty of the ¹⁸⁷Re/¹⁸⁸Os versus ¹⁸⁷Os/¹⁸⁸Os isochron age.

Using the ¹⁸⁷Re (sample with ¹⁸⁷Re/¹⁸⁸Os ratio exceeding 2800) and ¹⁸⁷Os^r data, calculated model ages are 774 ± 17 Ma, 795 ± 22Ma and 775 ± 19 Ma, respectively (Table 3), and the weighted mean age is 779 ± 26 Ma (MSWD = 1.3). The relatively large uncertainties for model ages are attributed to the introduction of large errors caused by the blank correction for the extremely low level of Re and ¹⁸⁷Os in chalcopyrite [88]. Nevertheless, they can still provide additional information for mineralization ages. The ¹⁸⁷Re versus ¹⁸⁷Os^r isochron age (Figure 7b) is within uncertainty and in accordance with both the four-point ¹⁸⁷Re/¹⁸⁸Os versus ¹⁸⁷Os/¹⁸⁸Os age (Figure 7a) and the weighted mean age.

Furthermore, based on the values of initial ¹⁸⁷Os/¹⁸⁸Os of 2.8 and ¹⁸⁷Re versus ¹⁸⁷Os isochron age of 794.8 Ma, an approximate γ_Os value (+2202) is calculated using the formulas shown in Jiang et al. [89] and Walker et al. [90].

5.3. Sulfur Isotopes

Sulfide isotopic data are listed in

Table 4. Sulfur isotope compositions (‰) of sulfides analyzed in this study.

Ore Type	Sample	Spot	Mineral	δ34S
Lamellar	Hnp017	1	Pyrite	−1.8
		2	Chalcopyrite	2.0
	Hnp022	1	Pyrite	6.5
		2	Chalcopyrite	5.3
		3	Chalcopyrite	5.4
	Hnp287	1	Pyrite	10.3
		2	Pyrite	10.4
		3	Pyrite	8.6
		4	Pyrite	11.4
Vein-type	Hnp053	1	Pyrite	4.0
		2	Chalcopyrite	1.1
	Hnp223	1	Pyrite	1.6
		2	Pyrite	1.6
		3	Chalcopyrite	2.2
		4	Chalcopyrite	0.8
	Hnp483	1	Chalcopyrite	1.3
		2	Chalcopyrite	1.8
	Hnp499	1	Pyrite	0.3
		2	Pyrite	0
		3	Chalcopyrite	-1.3
	Hnp503	1	Pyrite	−3.6
		2	Chalcopyrite	−4.3
	Hnp504	1	Chalcopyrite	−9.6
		2	Chalcopyrite	−9.4
		3	Chalcopyrite	−10.3

and illustrated in Figure 8. Sulfides from the laminated ores have δ³⁴S values ranging from −1.8‰ to 11.4‰. Except for spot Hnp017-1 with a negative δ³⁴S value of −1.8‰, both pyrite and chalcopyrite have high positive δ³⁴S values. The δ³⁴S values of sulfides from the vein-type ores range from −10.3‰ to 4.0‰, but mostly between 0‰ and 2.2‰. Additionally, there are two samples, especially sample Hnp504, which have δ³⁴S values characterized by strong negative values (down to −10.3‰). Overall, most of the δ³⁴S values of the sulfides from the laminated ores are much higher than those from vein-type ores (Figure 9).

6. Discussion

6.1. Ages of Two Mineralization Events

Field relationships show vein-type mineralization crosscut laminated mineralization (Figure 4d) at the Hongnipo deposit. Petrographic observation reveals that the percentage of chalcopyrite is significantly different in these two types of ores (Figure 5a–f and Figure 8). This evidence suggests these two ore type might have formed from different metallogenic events.

For all isochron dating methods, including Rb-Sr, the prerequisites are that all samples have the same initial isotopic composition and the isotopic system remains closed after mineral crystallization [84]. Owing to the Rb and Sr budgets of pyrite are predominantly from silicate inclusions, the Rb-Sr age of pyrite essentially represents the timing of the K-bearing minerals that cooled below the closure temperatures of the Rb-Sr isotope system therein [1]. Although previous studies have shown that the Lala area has suffered from upper greenschist facies metamorphism [5], the pyrite Rb-Sr isotope age of 1552 ± 80 Ma, with its large uncertainty, is consistent with the same stage pyrite Pb-Pb age (1669 ± 110 Ma [24]). This indicates that the Rb-Sr isotope systematics of mineral inclusions within pyrite still remained closed. Additionally, the pyrite samples, which defined the well-constrained four-point regression line, were separated from the same ore type collected from the same orebody. Consequently, we suggest that the Rb-Sr isochron age of 1552 ± 80 Ma likely represents the best estimate of the formation timing of the laminated ores.

In this study, we obtained a well-constrained four-point Re-Os chalcopyrite isochron age of 794.8 ± 7.9 Ma (Figure 7b) in this study. No deformation or hydrothermal overprint in chalcopyrite has been detected through field and microscopic observations. This suggests that the chalcopyrite Re-Os system has not been affected by post-ore tectonothermal events, and this age is considered to precisely record the timing of vein-type Cu mineralization. Apparently, it is significantly younger than the laminated mineralization. The field and textural relationships, coupled with our new geochemical data, indicate that at least two mineralizing events were involved in the formation of the Hongnipo deposit.

6.2. Sources of Ore-Forming Materials

6.2.1. Laminated Ores

The ⁸⁷Sr/⁸⁶Sr ratio is generally used to discriminate the sources of ore-forming fluids and materials. For example, Chen et al. [92] and Zhao et al. [93] managed to trace distinct fluids sources by comparing the Sr isotopic compositions of apatites with those of other reservoirs. The initial ⁸⁷Sr/⁸⁶Sr value (~0.712) at ~1.6 Ga is highly radiogenic, indicating a major contribution from crustal materials. The Paleoproterozoic host rocks with documented ⁸⁷Sr/⁸⁶Sr ratios from 0.711614 to 0.718937 at the time of the laminated mineralization [5], are likely candidates accounting for the radiogenic Sr of the laminated ores. This is in agreement with a previous Pb isotope study of pyrite from this ore type, which demonstrated that ore-forming materials were sourced from the Hekou Group [11].

The heavy sulfur isotope characteristics (up to 11.4‰) for the laminated sulfides are also observed in other deposits of the Kangdian copper belt; for instance, the Dongchuan, Etouchang and Tangdan deposits [59,94]. In addition, such sulfur isotope compositions are comparable to those of sulfides in Emmie Bluff (2.6‰–12.5‰, most values larger than 5.2‰ [95]) and Mount Gunson (7.1‰–11.7‰ [96]), both of which are located in the Olympic Dam District of South Australia. Bastrakov et al. [95] proposed evaporates, calcareous host rocks or seawater to be the source of sulfur in the Olympic Dam district. Zhao et al. [94] and Huang et al. [97] suggested that the marine evaporates contributed sulfur during precipitation of sulfides in the Tangdan deposit. Considering the volcanism, marine paleogeographic environment [98] and the δ³⁴S values of oceanic sulfates in the late Paleoproterozoic (18‰–25‰ [99]), we infer that sulfur sources of sulfides in the laminated ores are mixtures of seawater sulfates and magmatic fluids.

6.2.2. Vein-Type Ores

As a Cu-affinity element, Os shares similar geochemical behavior with Cu during magmatic evolution and ore-forming fluid migration. Consequently, Os can act as a tracer for the metal provenance, using the distinctive ¹⁸⁷Os/¹⁸⁸Os ratios and γ_Os values of the crust and mantle as effective discriminants of different sources.

The vein-type ores have an initial ¹⁸⁷Os/¹⁸⁸Os value of 2.8 ± 1.2 (Figure 7a). The large uncertainty is due to the measurement error caused by low levels of common Os [100]. Both it and the γ_Os value (+2202) are only slightly smaller than the mean values of continental crust (3.63 and +2758, respectively, [101]), which implies that the ore-forming materials mainly came from the continental crust [102,103,104]. With the Cu concentration of the Hekou Group being several times higher than its Clark Value [105] and the existence of older laminated ores (Figure 4d), the indications are that the copper of the veined ores was likely derived from the Hekou Group. In addition, the gabbro swarms, spatially and temporally associated with the deposit, originated from enriched mantle (EM I) [106], which has a ¹⁸⁷Os/¹⁸⁸Os value of 0.152 and a γ_Os value of +19.7 [107], may have contributed a portion of the metals to the vein-type mineralization.

Most δ³⁴S values of sulfides from the vein-type ores are in the range of 0‰ to 2.2‰ (Figure 9), suggesting magmatic-sourced sulfur [108,109,110]. The marked negative δ³⁴S values (from −10.3‰ to −3.6‰) of some veined sulfides largely correspond to the data from the Manxman prospect (−3.6‰ to −6.5‰) and from the Tangdan deposit (−5.2‰ to −14.7‰), given in Bastrakov et al. [95] and Huang et al. [97], respectively. Bastrakov et al. [95] suggested that such negative values resulted from involvement of sulfur bearing fluids with negative isotopic values. Huang et al. [97] also suggested that a reduced source, such as carbonaceous slate, provided the sulfur source. In view of the fact that carbonaceous slates often act as the floors of orebodies, the enrichment of light sulfur for these ores can be ascribed to the interaction between ore-forming fluids and organic matter. Thus, the sulfur of sources of the sulfides in the vein-type ores are mixtures of magmatic sulfur and biogenic sulfur.

6.3. Implications for Regional Metallogeny

Our new geochronological data show that the Hongnipo deposit has formed by two mineralization events at ~1.6 and 0.8 Ga. By contrast, previous data of the Luodang deposit from the same ore field recorded four hydrothermal events, including three main mineralization events at ~1.67, ~1.3 and ~1.1 Ga and a local hydrothermal overprint events at ~0.82 Ga (Figure 10). Although the early mineralization event age (~1.6 Ga) in the Hongnipo deposit is slightly younger than that (~1.67 Ga) in the Luodang deposit, they are indistinguishable from the zircon U-Pb ages of their respective host rocks (~1.6–1.7 Ga [5,6,7,8,9,10]). This geochronological evidence, together with the rifting setting [6,7,51,59], mixed sulfur sources of seawater sulfates and magmas and the typical ore structures, indicates that they were likely to have formed through a volcanic-exhalative-sedimentary process. The early stage mineralization is a VMS-type mineralization in the Lala ore field. The late main mineralization event age (~0.8 Ga) in the Hongnipo deposit is much younger than the subsequent main mineralizaion ages (~1.3 and ~1.1–1.0 Ga) in the Luodang deposit. Neoproterozoic magmatism (ages: 850–796 Ma [53,64,65,67,111,112]) and/or metamorphism (ages: ~850 Ma [15,113,114]) is likely responsible for the youngest main mineralization event at Hongnipo.

Many other deposits (e.g., Yinachang, Etouchang and Dahongshan) in the Kangdian copper belt are hosted in comparable volcano-sedimentary sequences and share similar mineralization styles, indicating that they might have had a similar metallogenic history.

A number of geochronological data of other representative deposits in the Kangdian copper belt have been published so far. Ye et al. [115] and Hou et al. [116] reported chalcopyrite Re-Os isochron ages of 1690 ± 99 Ma and 1648 ± 14 Ma, respectively, for the formation timing of the Yinachang deposit. Zhao et al. [1] obtained two groups of Re-Os molybdenite ages, with average ages of 1654 ± 7 Ma and 1451 ± 6 Ma. They suggested that the Yinachang deposit was formed at 1654 ± 7 Ma, and overprinted by a less important and local hydrothermal event. Li and Zhou [117] reported a monazite U-Pb age of 842 ± 9 Ma and interpreted it as the timing of a post-ore hydrothermal event. Zhao et al. [1] also reported a chalcopyrite Re-Os isochron age and a pyrite Rb-Sr isochron age of 1487 ± 110 Ma and 1453 ± 28 Ma, respectively. They suggested that these ages likely recorded the timing of a secondary hydrothermal event. A zircon U-Pb age of 1653 ± 18 Ma for hydrothermal zircons from stockwork chalcopyrite-magnetite ore was proposed to represent the timing of major mineralization that formed the Dahongshan deposit [58]. Zhao et al. [58] also reported pyrite and molybdenite Re-Os ages of 1026 ± 22 Ma and 780 ± 36 Ma, and argued that they recorded the timing of post-ore hydrothermal overprint or remobilization. Song [18] and Jin et al. [57] reported two Re-Os chalcopyrite ages of 1083 ± 45 Ma and 1115 ± 28 Ma, respectively, to constrain the timing of the main mineralization event. Compiling all the geochronological data together (Figure 10), the Kangdian copper belt has experienced several mineralization and hydrothermal events as follows: (1) the earliest main mineralization at ~1.6–1.67 Ga, which is nearly synchronous with host rock (ages: ~1.6–1.7 Ga [6,7,8,10,39,51,59,118]); (2) a local hydrothermal overprint at ~1.45 Ga, which is related to early Mesoproterozoic magmatism (age: ~1.5 Ga [54,62,119]); (3) a second major mineralization at ~1.3 Ga, which may have been caused by volcanism (age: 1270 ± 95 Ma [20]); (4) the third major mineralization at ~1.1–1.0 Ga, which is closely associated with late Mesoproterozoic magmatism (ages: 1142–1208 Ma [19,20,21,52,120]); (5) the fourth major mineralization at ~0.8 Ga, which is a newly identified main mineralization event in the Kangdian copper belt, and likely related to the Neoproterozoic magmatism (ages: ~850 Ma [15,113,114]) and/or metamorphism (ages: ~850 Ma [15,113,114]).

7. Conclusions

The Hongnipo copper deposit, a newly discovered large deposit, is roughly strata bound, with orebodies consisting of laminated, vein-type and superimposed ores. Two mineralization events have been identified in this study. Pyrite separates from laminated ores yield a Rb-Sr isochron age of 1552 ± 80 Ma, representing an early mineralization event. In contrast, chalcopyrite separates from vein-type ores yield a much younger Re-Os isochron age of 794.8 ± 7.9 Ma, reflecting the main mineralization event.

The highly radiogenic initial ⁸⁷Sr/⁸⁶Sr and ¹⁸⁷Os/¹⁸⁸Os values of pyrites from the laminated ores and chalcopyrites from veined ores, respectively, suggest significant contributions of crustal materials. Sulfur isotopic compositions of the laminated and vein-type ores are quite different. The majority of the sulfides from the laminated ores have much higher δ³⁴S values (5‰ to 12‰) than those of the vein-type ores (−11‰ to 4‰). Sulfur of the laminated ores was likely sourced from seawater sulfates and magmatic fluids, while sulfur of the vein-type ores probably came predominantly from magmatic sulfur with minor biogenic sulfur.

The early laminated mineralization identified in the Hongnipo deposit is likely a VMS-type mineralization, and is widespread in the Kangdian copper belt. The late hydrothermal mineralization is likely related to extensive Neoproterozoic magmatism and/or metamorphic events and is responsible for the formation of economic vein-type mineralization. At present, there is no evidence of this later mineralization event at any of the other deposits of the Kangdian copper belt.