Variations of Major and Minor Elements in Pt – Fe Alloy Minerals : A Review and New Observations

Compositional variations of major and minor elements were examined in Pt–Fe alloys from various geological settings and types of deposits, both lode and placer occurrences. They included representatives of layered intrusions, Alaskan-Uralian-(Aldan)-type and alkaline gabbroic complexes, ophiolitic chromitites, and numerous placers from Canada, USA, Russia, and other localities worldwide. Pt–Fe alloy grains in detrital occurrences are notably larger in size, and these are considered to be the result of a special conditions during crystallization such as temperature, pressure, geochemistry or time. In addition, the number of available statistical observations is much greater for the placer occurrences, since they represent the end-product of, in some cases, the weathering of many millions of tonnes of sparsely mineralized bedrock. Typically, platinum-group elements (PGE) present in admixtures (Ir, Rh, and Pd) and minor Cu, Ni are incorporated into a compositional series (Pt, PGE)2–3(Fe, Cu, Ni) in the lode occurrences. Relative Cu enrichment in alloys poor in Pt implies crystallization from relatively fractionated melts at a lower temperature. In contrast to the lode deposits, the distribution of Ir, Rh, and Pd is fairly chaotic in placer Pt–Fe grains. There is no relationship between levels of Ir, Rh, and Pd with the ratio Σ(Pt + PGE):(Fe + Cu + Ni). The compositional series (Pt, PGE)2–3(Fe, Cu, Ni) is not as common in the placer occurrences; nevertheless, minor Cu and Ni show their maximums in members of this series in the placer grains. Global-scale datasets yield a bimodal pattern of distribution in the Pt–Fe diagram, which is likely a reflection of the miscibility gap between the ordered Pt3Fe structure (isoferroplatinum) and the disordered structure of native or ferroan platinum. In the plot Pt versus Fe, there is a linear boundary due to ideal Pt↔ Fe substitution. Two solid solution series are based on the Ir-for-Pt and Pd-for-Pt substitutions. The incorporation of Ir is not restricted to Pt3Fe–Ir3Fe substitution (isoferroplatinum and chengdeite, plus their disordered modifications). Besides, Ir0 appears to replace Pt0 in the disordered variants of (Pt–Ir)–Fe alloys. There is a good potential for the discovery of a new species with a Pd-dominant composition, (Pd, Pt)3Fe, most likely in association with the alkaline mafic-ultramafic or gabbroic complexes, or the mafic units of layered intrusions. The “field of complicated substitutions” is recognized as a likely reflection of the crystallochemical differences of Pd and Ir, extending along the Ir-Pd axis of the Ir–Pd–Rh diagram. The inferred solid solution extends approximately along the line Ir–(Pd:Rh = 2:3). Minor Pd presumably enters the solid solution via a coupled substitution in combination with the Rh. An Ir-enrichment trend in Pt–Fe alloys typically occurs in the Alaskan-type complexes. The large size of the Pt–Fe nuggets associated with some of these complexes is considered to be related to an ultramafic-mafic pegmatite facies, whereas significant Pd-enrichment is characteristic of gabbroic source-rocks (e.g., Coldwell Complex), resulting in a markedly different trend for the Pt versus Fe (wt.%). However, based on our examination of a large dataset of Pt–Fe alloys from numerous origins, we conclude that they exhibit compositional overlaps that are too large to be useful as reliable index-minerals. Minerals 2019, 9, 25; doi:10.3390/min9010025 www.mdpi.com/journal/minerals Minerals 2019, 9, 25 2 of 15

The goals of this article are: (1) To evaluate the compositional ranges observed in Fe-bearing platinum (or ferroan platinum) and isoferroplatinum with respect to the main components and admixtures on the basis of compositions from a large number of Pt-Fe alloys reported worldwide in the literature for various geological settings and types of deposits (note that the prefix "ferroan" (used in ferroan platinum) does not imply the valence state of Fe); (2) To compare the compositional characteristics of Pt-Fe alloys that are present in lode deposits versus the ones present in placer deposits derived from source rocks; and (3) To evaluate whether Pt-Fe alloys may be used as useful indicator minerals to infer the type or character of the lode source for detrital grains or placer concentrations of PGM.

Materials and Methods
In the present review, we used a total of 2430 data-points accumulated from the literature, including our own data and from hitherto unpublished results in internal reports, e.g., [10].We believe this large dataset of electron-microprobe compositions (EMP) to be representative because it also reflects a large diversity of localities, geologic settings, and ore zones worldwide.The different types of lode and placer deposits reviewed for this study are listed in Table 1.

Number of Data-Points (N)
Coldwell trend of compositions, is uncommon.It is discordant to the Yubdo trend because of the strong Pd-for-Pt substitution occurring in these alloys (Figure 1).
Complex in northern Khabarovskiy krai, the Kachkanar and Nizhniy Tagil Complexes of the Urals, and the Gal'moenan Complex in the Koryak Upland, Kamchatka krai (Russia).The Coldwell Complex represents a giant Alkaline Gabbro-Syenite intrusion located in Ontario (Canada); Ophiolitic Chromitites are represented from the Uktus and Kytlym areas, central Urals (Russia).In addition, Pt-Fe alloys were studied in situ in a PGM-bearing saprolite developed after mineralized dunite at Yubdo (Ethiopia) (Table 1).Two compositional trends are generally observed in Figure 1.First, the Yubdo trend is extended and consistent with most of the data-points plotted.Compositions of the Pt-Fe alloys from the Alaskan-type complexes, i.e., Tulameen, Kondyor, Kachkanar, and Nizhniy Tagil, are similar and broadly overlap to form a small field close to the central portion of the Yubdo trend.The second, the Coldwell trend of compositions, is uncommon.It is discordant to the Yubdo trend because of the strong Pd-for-Pt substitution occurring in these alloys (Figure 1).Pt-Fe alloys from the layered intrusions (Bushveld and Kapalagulu) and from the Coldwell Alkaline Complex are consistently poor in iridium, containing from "not detected" (n.d.) to 0.3 wt.% Ir (see References quoted in Table 1).In contrast, the Pt-Fe alloys from Alaskan-type complexes are relatively enriched in Ir (Figure 2 Pt-Fe alloys from the layered intrusions (Bushveld and Kapalagulu) and from the Coldwell Alkaline Complex are consistently poor in iridium, containing from "not detected" (n.d.) to 0.3 wt.% Ir (see References quoted in Table 1).In contrast, the Pt-Fe alloys from Alaskan-type complexes are relatively enriched in Ir (Figure 2   The Rh admixtures (Figure 3) are below the level of detection (EMP) in the Pt-Fe alloys at Kapalagulu and Coldwell.The Bushveld Pt-Fe alloys contained variable amounts of Rh, from n.d. to 3.6 wt.% (mean 0.58) wt.% Rh.Pt-Fe alloys of the Alaskan-type complexes typically contain substantial levels of Rh: 0.52-0.75(0.62) wt.% Rh % at Tulameen; n.The Rh admixtures (Figure 3) are below the level of detection (EMP) in the Pt-Fe alloys at Kapalagulu and Coldwell.The Bushveld Pt-Fe alloys contain variable amounts of Rh, from n.d. to 3.6 wt.% (mean 0.58) wt.% Rh.Pt-Fe alloys of the Alaskan-type complexes typically contain substantial levels of Rh: 0.52-0.75(0.62) wt.% Rh at Tulameen; n.d.-1.66 (0.67) wt.% at Kondyor; 0.41-1.37(0.92) at Kachkanar; and 0.45-1.37(0.91) at Nizhniy Tagil, the values of which are close to 0.36-1.35(mean 0.87) wt.% Rh in the Pt-Fe alloys analyzed at Uktus and Kytlym.Pt-Fe alloys at Gal'moenan are reportedly poor in Rh: n.d.-0.43 (mean 0.04) wt.% Rh.
Maximum Pd contents are characteristic of the Pt-Fe alloys at Coldwell, with compositions ranging from n.d. to 29.7 (mean 10.98) wt.% Pd (Figure 4).Occasionally, the Pt-Fe alloys analyzed from the layered intrusions also display elevated levels of Pd: n.d.The symbols and sources used are the same as the ones shown in Figure 1 and listed in Table 1.The symbols and sources used are the same as the ones shown in Figure 1 and listed in Table 1.  1 and listed in Table 1.

Variations in the Compositions of the Pt-Fe Alloys from Placer Deposits
Variations in the contents of Pt versus Fe, shown in Figure 7, pertain to grains of the Pt-Fe alloys from the placer localities summarized in Table 1.Four sets of EMP data were grouped conditionally for the large territories (Canada and the USA; Russia; and "Various placers worldwide") to reduce the extensive overlaps existing amongst the compositional sets of regional scale.The symbols and sources used are the same as the ones shown in Figure 1 and listed in Table 1.

Variations in the Compositions of the Pt-Fe Alloys from Placer Deposits
Variations in the contents of Pt versus Fe, shown in Figure 7, pertain to grains of the Pt-Fe alloys from the placer localities summarized in Table 1.Four sets of EMP data were grouped conditionally for the large territories (Canada and the USA; Russia; and "Various placers worldwide") to reduce the extensive overlaps existing amongst the compositional sets of regional scale.
Figure eight-shaped compositional fields are observed, which is indicative of a bimodal distribution pattern and consistent with the four plotted data sets that are mutually overlapped (Figure 7).Note that the multiple points of the 8-shaped clouds are clustered approximately around the central compositions Pt 74 Fe 26 (very close to ideal Pt 3 Fe, i.e., isoferroplatinum) and Pt 83 Fe 17 (native or ferroan platinum); thus, these fields may reflect the existing miscibility gap.
As displayed in Figure 8, most of the Pt-Fe alloys contain <5-10 wt.% Ir.The observed enrichment in Ir does not necessarily occur in (Pt, Ir) 3 Fe-type compositions related to a solid solution of isoferroplatinum and chengdeite, which is the Ir-dominant analogue of isoferroplatinum [44].Thus, the incorporation of Ir is not controlled by the Σ(Pt + PGE):(Fe + Cu + Ni) ratio, and presumably, can occur in both types of structures: ordered and disordered.Interestingly, the Pt-Fe alloys from the lode deposits (Figure 2) display an Ir-enrichment trend that extends along a narrow range of compositions (Pt, PGE) 2-3 (Fe, Cu, Ni).
The observed distribution of admixtures of Rh (Figure 9) displays a similar character as Ir.There is no clear relationship between the levels of Rh and the values of the atomic Σ(Pt + PGE):(Fe + Cu + Ni) ratio in these minerals of placer alloys.In contrast, and uniform to Ir, the Rh-enrichment is related to the compositional series (Pt, PGE) 2-3 (Fe + Cu + Ni) in the lode deposits (Figure 3).However, the behavior of the Ir and Rh is not coherent in the Pt-Fe alloys.In the Rh-Ir plot, these elements formed separate trends of enrichment and were not involved in the mutual schemes of substitutions.
The Pd distribution (Figure 10) seems to be rather chaotic with respect to the ratio Σ(Pt + PGE):(Fe + Cu + Ni) in compositions of the placer grains.The observed extent of the Pd enrichment is modest in comparison to the high-Pd compositions of the Pt-Fe alloy from the Coldwell alkaline gabbro-syenite complex (Figure 4).The latter type of lode source was unlikely to have been involved as a contributor to the studied zones of the placer PGM.In contrast, similar to Ir-Rh, the trend of Pd enrichment corresponds to the series (Pt, PGE) 2 (Fe + Cu + Ni) in the lode deposits (Figure 4).As displayed in Figure 8, most of the Pt-Fe alloys contain <5-10 wt.% Ir.The observed enrichment in Ir does not necessarily occur in (Pt, Ir)3Fe-type compositions related to a solid solution of isoferroplatinum and chengdeite, which is the Ir-dominant analogue of isoferroplatinum [44].Thus, the incorporation of Ir is not controlled by the Σ(Pt + PGE):(Fe + Cu + Ni) ratio, and presumably, can occur in both types of structures: ordered and disordered.Interestingly, the Pt-Fe alloys from the lode deposits (Figure 2) display an Ir-enrichment trend that extends along a narrow range of compositions (Pt, PGE)2-3(Fe, Cu, Ni).     7 and listed in Table 1.The symbols and sources used are the same as the ones shown in Figure 7 and listed in Table 1.The observed distribution of admixtures of Rh (Figure 9) displays a similar character as Ir.There is no clear relationship between the levels of Rh and the values of the atomic Σ(Pt + PGE):(Fe + Cu + Ni) ratio in these minerals of placer alloys.In contrast, and uniform to Ir, the Rh-enrichment is related to the compositional series (Pt, PGE)2-3(Fe + Cu + Ni) in the lode deposits (Figure 3).However, the behavior of the Ir and Rh is not coherent in the Pt-Fe alloys.In the Rh-Ir plot, these elements formed separate trends of enrichment and were not involved in the mutual schemes of substitutions.7 and listed Table 1.
The Pd distribution (Figure 10) seems to be rather chaotic with respect to the ratio Σ(Pt + PGE):(Fe + Cu + Ni) in compositions of the placer grains.The observed extent of the Pd enrichment was is modest in comparison to the high-Pd compositions of the Pt-Fe alloy from the Coldwell alkaline gabbro-syenite complex (Figure 4).The latter type of lode source was unlikely to have been involved as a contributor to the studied zones of the placer PGM.In contrast, similar to Ir-Rh, the trend of Pd enrichment corresponds to the series (Pt, PGE)2(Fe + Cu + Ni) in the lode deposits (Figure 4).The symbols and sources used are the same as the ones shown in Figure 7 and listed Table 1.The observed distribution of admixtures of Rh (Figure 9) displays a similar character as Ir.There is no clear relationship between the levels of Rh and the values of the atomic Σ(Pt + PGE):(Fe + Cu + Ni) ratio in these minerals of placer alloys.In contrast, and uniform to Ir, the Rh-enrichment is related to the compositional series (Pt, PGE)2-3(Fe + Cu + Ni) in the lode deposits (Figure 3).However, the behavior of the Ir and Rh is not coherent in the Pt-Fe alloys.In the Rh-Ir plot, these elements formed separate trends of enrichment and were not involved in the mutual schemes of substitutions.7 and listed Table 1.
The Pd distribution (Figure 10) seems to be rather chaotic with respect to the ratio Σ(Pt + PGE):(Fe + Cu + Ni) in compositions of the placer grains.The observed extent of the Pd enrichment was is modest in comparison to the high-Pd compositions of the Pt-Fe alloy from the Coldwell alkaline gabbro-syenite complex (Figure 4).The latter type of lode source was unlikely to have been involved as a contributor to the studied zones of the placer PGM.In contrast, similar to Ir-Rh, the trend of Pd enrichment corresponds to the series (Pt, PGE)2(Fe + Cu + Ni) in the lode deposits (Figure 4).The symbols and sources used are the same as the ones shown in Figure 7 and listed in Table 1.
Similar to the Pt-Fe alloys from lode sources (Figure 5), the placer Pt-Fe grains display a trend of Cu enrichment that extends along the compositional series (Pt, PGE) 2-3 (Fe + Cu + Ni) (Figure 11).However, the majority of the plotted data-points are distributed more or less chaotically in the range n.d.-4 wt.% Cu (Figure 11).The symbols and sources used are the same as the ones shown in Figure 7 and listed in Table 1.
Similar to the Pt-Fe alloys from lode sources (Figure 5), the placer Pt-Fe grains display a trend of Cu enrichment that extends along the compositional series (Pt, PGE)2-3(Fe + Cu + Ni) (Figure 11).However, the majority of the plotted data-points are distributed more or less chaotically in the range n.d.-4 wt.% Cu (Figure 11).The symbols and sources used are the same as the ones shown in Figure 7 and listed in Table 1.
Unlike the other elements, the distribution of minor Ni is most consistent in the Pt-Fe alloys from the lode deposits (Figure 6) and from the placers (Figure 12  The symbols and sources used are the same as the ones shown in Figure 7 and listed in Table 1.The symbols and sources used are the same as the ones shown in Figure 7 and listed in Table 1. Unlike the other elements, the distribution of minor Ni is most consistent in the Pt-Fe alloys from the lode deposits (Figure 6) and from the placers (Figure 12), showing similar trends of Ni-enrichment in the compositional series (Pt, PGE) 2-3 (Fe + Cu + Ni).Additionally, the maximum contents of Ni are close in the Pt-Fe alloys in the lode and placer occurrences (≤4 wt.% Ni).The symbols and sources used are the same as the ones shown in Figure 7 and listed in Table 1.
Similar to the Pt-Fe alloys from lode sources (Figure 5), the placer Pt-Fe grains display a trend of Cu enrichment that extends along the compositional series (Pt, PGE)2-3(Fe + Cu + Ni) (Figure 11).However, the majority of the plotted data-points are distributed more or less chaotically in the range n.d.-4 wt.% Cu (Figure 11).The symbols and sources used are the same as the ones shown in Figure 7 and listed in Table 1.
Unlike the other elements, the distribution of minor Ni is most consistent in the Pt-Fe alloys from the lode deposits (Figure 6) and from the placers (Figure 12), showing similar trends of Ni-enrichment in the compositional series (Pt, PGE)2-3(Fe + Cu + Ni).Additionally, the maximum contents of Ni are close in the Pt-Fe alloys in the lode and placer occurrences (≤4 wt.% Ni).The symbols and sources used are the same as the ones shown in Figure 7 and listed in Table 1.The symbols and sources used are the same as the ones shown in Figure 7 and listed in Table 1.

Overall Variations in the Compositions of the Pt-Fe Alloys on a Global Scale
In Figure 13, the overall variations are shown in compositions of the Pt-Fe alloys in all types of occurrences, involving the examined lode and placer deposits (Table 1).In the Pt versus Fe diagram, the observed field of solid solutions displays a linear boundary due to the ideal scheme of Pt ↔ Fe substitution, with the observed equation of linear regression y = −x + 100 (Figure 13a).
In addition, Ru, Os, and Sb occur as traces or minor elements in the Pt-Fe alloys.The bulk of the compositions of alloy grains gave ≤0.5 wt.% Ru, with a maximum ~5 wt.% and mean 0.13 wt.% Ru (per n = 2430).The observed background of Os was notably higher.The majority of the Pt-Fe grains contained ≤0.5-2 wt.% Os, with episodic maximums ~6-8 wt.% and mean 0.53 wt.% Os (n = 2430).Typically, Sb appears to only occur in trace amounts (n.d. by EMP methods).Nevertheless, some grains of the Pt-Fe alloys yielded essential levels varying from ~0.4 to 1.4 wt.% Sb.  1).1).
The two main series of solid-solutions (Figures 13b,c and 14a) are based on the incorporation of high levels of Ir (≤20 at.%), and an even greater content of Pd (≤35 at.%), which are both negatively correlated with Pt.In contrast, Cu (≤20 at.%) did not correlate with Pt, where the bulk of the EMP compositions had fairly low amounts of ≤6 at.%Cu (Figure 13d).1).The total number of point analyses is 2430 (a) and 2325 (b).FCS is the inferred "field of complicated substitutions"; see text for discussion.

Discussion and Conclusions
In summary, we note the following observations: (1) PGE admixtures (Ir, Rh, and Pd) are typically incorporated into the alloys of composition (Pt, PGE)2-3(Fe, Cu, Ni) in a variety of lode occurrences associated with ore zones and complexes of different types and geologic settings worldwide (Figures 2-4).These compositional series extend  1).The total number of point analyses is 2430 (a) and 2325 (b).FCS is the inferred "field of complicated substitutions"; see text for discussion.
Thus, two series of extensive solid-solutions exist: Pt 3−x Fe (isoferroplatinum and ferroan platinum) ↔ Ir 3−x Fe (chengdeite, and possibly, its disordered modification), and a series extending toward Pd 3 Fe (unnamed), which was documented at Coldwell (Figure 14a).There is a good potential for the discovery of a new species of PGM with a Pd-dominant composition, (Pd, Pt) 3 Fe, in relation to the latter or another complex.The ordered and disordered variants of Pd 3 Fe exist in the Pd-Fe system, e.g., [45].
In the compositional Ir-Pd-Rh space, the presence of a "field of complicated substitutions" is proposed, which generally extends along the Ir-Pd axis (Figure 14b).In addition, there is a slightly rarefied area along the Ir-Rh axis, which could be ascribed to the lack of sufficient statistics.On the other hand, the observed solid-solution appears to extend, approximately, along the line Ir-(Pd:Rh = 2:3) in the Ir-Pd-Rh space, as inferred on the basis of a total of 2325 point analyses (Figure 14b).
In addition, Ru, Os, and Sb occur as traces or minor elements in the Pt-Fe alloys.The bulk of the compositions of alloy grains gave ≤0.5 wt.% Ru, with a maximum ~5 wt.% and mean 0.13 wt.% Ru (per n = 2430).The observed background of Os was notably higher.The majority of the Pt-Fe grains contained ≤0.5-2 wt.% Os, with episodic maximums ~6-8 wt.% and mean 0.53 wt.% Os (n = 2430).Typically, Sb appears to only occur in trace amounts (n.d. by EMP methods).Nevertheless, some grains of the Pt-Fe alloys yielded essential levels varying from ~0.4 to 1.4 wt.% Sb.

Discussion and Conclusions
In summary, we note the following observations: (1) PGE admixtures (Ir, Rh, and Pd) are typically incorporated into the alloys of composition (Pt, PGE) 2-3 (Fe, Cu, Ni) in a variety of lode occurrences associated with ore zones and complexes of different types and geologic settings worldwide (Figures 2-4).These compositional series extend along and below somewhat the line Σ(Pt + PGE):(Fe + Cu + Ni) = 3, which corresponds to isoferroplatinum.
(2) Generally, minor Cu follows the same pattern of distribution in the Pt-Fe alloys in the lode occurrences (Figure 5), showing a clear tendency of decrease in the values of the ratio Σ(Pt + PGE):(Fe + Cu + Ni), while the Cu content increased.The trend of Cu enrichment in the minerals that are relatively poor in Pt implies their crystallization from relatively fractionated melts at lower temperatures.This trend agrees with observations noted below for the Pt-Fe alloys from placer associations.
(3) Similarly, the Ni admixture enters preferentially in the Pt-Fe alloy, nominally (Pt, PGE) 2 (Fe, Cu, Ni) in the lode deposits reviewed (Figure 6).One of the suggested possibilities is that Pt 2 Fe may in fact represent "invisible" mixtures of Pt 3 Fe and PtFe (≤1 µm in size) that are exsolved upon cooling within the miscibility gap [46].In contrast, examples of homogeneous Pt 2 Fe with a disordered fcc structure were documented in [33].
(4) The global-scale sets of EMP data generally display Figure 8-shaped compositional fields in the Pt-Fe diagram, which are based on a large number of compositions of the Pt-Fe minerals from a large variety of placer deposits worldwide (Figure 7).This distribution is likely a reflection of the miscibility gap existing between the ordered Pt 3 Fe structure (isoferroplatinum) and the disordered structure of native or ferroan platinum.
(5) The patterns of distribution of the admixtures of PGE (Ir, Rh, and Pd) appear to scatter chaotically in the plots of EMP compositions of placer grains of the Pt-Fe alloy minerals.There is no clear relationship observed between the amount of these elements and their values in the ratio Σ(Pt + PGE):(Fe + Cu + Ni).Minerals of the type (Pt, PGE) 2-3 (Fe, Cu, Ni) were not observed as the distinctive series (Figures 8-10).
(6) In contrast to the PGE, minor amounts of Cu and Ni admixtures yielded their maximums in alloys of the compositional series (Pt, PGE) 2-3 (Fe, Cu, Ni) in the placer occurrences (Figures 11 and 12).
(7) The global-scale set of EMP compositions plotted in the Pt-Fe diagram indicates that the linear boundary observed in the overall compositional field is due to an ideal scheme of Pt ↔ Fe substitution (Figure 13a).(9) The two series of solid solutions in the Pt-Fe alloys are based on the Ir-for-Pt and Pd-for-Pt substitutions.The incorporation of Ir is not restricted by the Pt 3 Fe-Ir 3 Fe substitution involving components of isoferroplatinum and chengdeite (also, likely their disordered modifications).Besides, Ir 0 appears to replace Pt 0 in disordered variants of the Pt-Fe alloys, as is implied by the observed pattern (Figure 8).There is a good potential for discovery of a new species of PGM having a Pd-dominant composition, (Pd, Pt) 3 Fe, most expectedly in association with the mineralized zones of mafic units of alkaline mafic-ultramafic or gabbroic complexes.(10) The presence of a "field of complicated substitutions" is proposed, which generally extends along the Ir-Pd axis in the Ir-Pd-Rh diagram (Figure 14b).The inferred solid solution extends approximately along the line Ir-(Pd:Rh = 2:3), suggesting that minor Pd enters the solid solution via a coupled scheme of substitution in the combination with Rh.
The "field of complicated substitutions, FCC" (Figure 14b) may likely be a reflection of the crystallochemical differences existing between Pd and Ir, which, at least under normal crystallization conditions, seem to avoid each other, and they do not participate in mutual substitution schemes in all of the PGM species presently known.Only minor substitutions are known, e.g., ~1 wt.% Ir in laflammeite Pd 3 Pb 2 S 2 [47].
The Pd-for-Rh substitution is not so common; nevertheless, it occurs in the series of solid solution of palladodymite (Pd, Rh) 2 As-rhodarsenide (Rh, Pd) 2 As [48,49].Among the PGE, Pd has the smallest size of the calculated atomic radius, which is a consequence of its outermost electron density deriving predominantly from d-levels, instead of s-levels [50].
(11) Pt-Fe alloys analyzed in situ in the lode deposits display some distinctive features of their compositions relative to the Pt-Fe grains examined in placer associations.As noted, the compositional series (Pt, PGE) 2-3 (Fe, Cu, Ni) clearly dominates in the lode occurrences of the Pt-Fe alloys from different sources (Figures 2-4).In contrast, this series was not as important in the placer occurrences (Figures 8 and 9).
In addition, placer grains of the Pt-Fe alloys generally have large grain-sizes due to crystallization under special conditions in their now-eroded source rocks, a subject extensively debated in the literature that is well-documented and discussed in [51], and references therein.
A large grain-size is known for some Pt-Fe alloy nuggets derived from placers related to Alaskan-type complexes, such as the ~1.5 kg specimens of native ferroan platinum at Kondyor [32] or the crystals exceeding 10 cm across at Nizhniy Tagil [52].These examples are consistent with the mineral-forming environments rich in volatiles and are likely related to ultramafic-mafic pegmatite facies.For example, at Kondyor, phlogopite-rich late zones rich in PGE are known in the Anomal'nyi area [53].(12) As an indication of the very large quantity of host Pt-bearing rocks needed to be weathered and eroded to produce tenors found in commercially-viable placers, two publications on the Goodnews Bay placers are relevant [54,55].Detailed study of the geology and geomorphology showed that Red Mountain is the source rock that has been eroded by 2000 vertical feet over a period of about 20 million years or longer.The amount of eroded rock in this case represents a volume of about 2.4 billion m 3 and underlines the huge quantities of weathered and eroded source rocks involved and why large grains are statistically nearly impossible to find microscopically.
(13) There are some compositional features that are generally characteristic of the Pt-Fe alloy minerals hosted by the different types of source rocks, e.g., the trend of Ir-enrichment typically occurs in Alaskan-type complexes, and the strong Pd-enrichment is related with the mineralized rocks of gabbroic compositions in the Coldwell complex.

Figure 1 .
Figure 1.Compositional variations in the Pt-Fe alloy minerals from lode deposits associated with various complexes; contents of Pt plotted versus Fe are expressed in weight percent.

Figure 1 .
Figure 1.Compositional variations in the Pt-Fe alloy minerals from lode deposits associated with various complexes; contents of Pt plotted versus Fe are expressed in weight percent.

Figure 2 .
Figure 2. Plot of the values of the Σ(Pt + PGE):(Fe + Cu + Ni) ratio (in atomic %) versus the contents of Ir expressed in weight % in compositions of the Pt-Fe alloy minerals from various lode deposits.The symbols and sources used are the same as the ones shown in Figure 1 and listed Table 1.The dotted line displays the stoichiometry (Pt, PGE)3(Fe, Cu, Ni).

Figure 2 .
Figure 2. Plot of the values of the Σ(Pt + PGE):(Fe + Cu + Ni) ratio (in atomic %) versus the contents of Ir expressed in weight % in compositions of the Pt-Fe alloy minerals from various lode deposits.The symbols and sources used are the same as the ones shown in Figure1and listed Table1.The dotted line displays the stoichiometry (Pt, PGE) 3 (Fe, Cu, Ni).

Figure 2 .
Figure 2. Plot of the values of the Σ(Pt + PGE):(Fe + Cu + Ni) ratio (in atomic %) versus the contents of Ir expressed in weight % in compositions of the Pt-Fe alloy minerals from various lode deposits.The symbols and sources used are the same as the ones shown in Figure 1 and listed Table 1.The dotted line displays the stoichiometry (Pt, PGE)3(Fe, Cu, Ni).The Rh admixtures (Figure 3) are below the level of detection (EMP) in the Pt-Fe alloys at Kapalagulu and Coldwell.The Bushveld Pt-Fe alloys contained variable amounts of Rh, from n.d. to 3.6 wt.% (mean 0.58) wt.% Rh.Pt-Fe alloys of the Alaskan-type complexes typically contain substantial levels of Rh: 0.52-0.75(0.62) wt.% Rh % at Tulameen; n.d.-1.66 (0.67) wt.% at Kondyor; 0.41-1.37(0.92) at Kachkanar; and 0.45-1.37(0.91) at Nizhniy Tagil, the values of which are close to 0.36-1.35(mean 0.87) wt.% Rh in the Pt-Fe alloys analyzed at Uktus and Kytlym.Pt-Fe alloys at Gal'moenan are reportedly poor in Rh: n.d.-0.43 (mean 0.04) wt.% Rh.

Figure 3 .
Figure 3. Plot of the values of the Σ(Pt + PGE):(Fe + Cu + Ni) ratio (in atomic %) versus contents of Rh expressed in weight % in compositions of the Pt-Fe alloy minerals from various lode deposits.The symbols and sources used are the same as the ones shown in Figure1and listed in Table1.

Minerals 2019, 9 , 16 Figure 3 .
Figure 3. Plot of the values of the Σ(Pt + PGE):(Fe + Cu + Ni) ratio (in atomic %) versus contents of Rh expressed in weight % in compositions of the Pt-Fe alloy minerals from various lode deposits.The symbols and sources used are the same as the ones shown in Figure 1 and listed in Table 1.Maximum Pd contents are characteristic of the Pt-Fe alloys at Coldwell, with compositions ranging from n.d. to 29.7 (mean 10.98) wt.% Pd (Figure 4).Occasionally, the Pt-Fe alloys analyzed from the layered intrusions also display elevated levels of Pd: n.d.-3.8 (mean 0.57) wt.% Pd at Bushveld and n.d.-5.6 (mean 0.62) wt.% Pd at Kapalagulu.These mean values compare well with the values observed for the Pt-Fe alloys from some of the Alaskan-type complexes: n.d.-1.07 (mean 0.59) wt.% at Kachkanar; 0.2-0.5 (0.43) wt.% Pd at Tulameen; and n.d.-1.51 (mean 0.45) wt.% Pd at Kondyor.The corresponding levels are lower at Nizhniy Tagil (n.d.-0.64, mean 0.19 wt.% Pd) and at Uktus and Kytlym (n.d.-0.86, mean 0.26 wt.% Pd).

Figure 4 .
Figure 4. Plot of the values of the Σ(Pt + PGE):(Fe + Cu + Ni) ratio (in atomic %) versus contents of Pd expressed in weight % in compositions of the Pt-Fe alloy minerals from various lode deposits.The symbols and sources used are the same as the ones shown in Figure 1 and listed in Table 1.Compositional variations of Cu in the Pt-Fe alloys are presented in Figure 5.Note that the observed levels of Cu are generally increased with decreasing values of the atomic Σ(Pt + PGE):(Fe + Cu + Ni) ratio.

Figure 4 .
Figure 4. Plot of the values of the Σ(Pt + PGE):(Fe + Cu + Ni) ratio (in atomic %) versus contents of Pd expressed in weight % in compositions of the Pt-Fe alloy minerals from various lode deposits.The symbols and sources used are the same as the ones shown in Figure1and listed in Table1.Compositional variations of Cu in the Pt-Fe alloys are presented in Figure5.Note that the observed levels of Cu are generally increased with decreasing values of the atomic Σ(Pt + PGE):(Fe + Cu + Ni) ratio.

Figure 4 .
Figure 4. Plot of the values of the Σ(Pt + PGE):(Fe + Cu + Ni) ratio (in atomic %) versus contents of Pd expressed in weight % in compositions of the Pt-Fe alloy minerals from various lode deposits.The symbols and sources used are the same as the ones shown in Figure 1 and listed in Table 1.Compositional variations of Cu in the Pt-Fe alloys are presented in Figure 5.Note that the observed levels of Cu are generally increased with decreasing values of the atomic Σ(Pt + PGE):(Fe + Cu + Ni) ratio.

Figure 5 .
Figure 5. Plot of the values of the Σ(Pt + PGE):(Fe + Cu + Ni) ratio (in atomic %) versus contents of Cu expressed in weight % in compositions of the Pt-Fe alloy minerals from various lode deposits.The symbols and sources used are the same as the ones shown in Figure1and listed in Table1.

16 Figure 5 .
Figure 5. Plot of the values of the Σ(Pt + PGE):(Fe + Cu + Ni) ratio (in atomic %) versus contents of Cu expressed in weight % in compositions of the Pt-Fe alloy minerals from various lode deposits.The symbols and sources used are the same as the ones shown in Figure 1 and listed inTable1.

Figure 6 .
Figure 6.Plot of the values of the Σ(Pt + PGE):(Fe + Cu + Ni) ratio (in atomic %) versus contents of Ni expressed in weight % in compositions of the Pt-Fe alloy minerals from various lode deposits.The symbols and sources used are the same as the ones shown in Figure 1 and listed inTable1.
Figure eight-shaped compositional fields are observed, which is indicative of a bimodal distribution pattern and consistent with the four plotted data sets that are mutually overlapped

Figure 6 .
Figure 6.Plot of the values of the Σ(Pt + PGE):(Fe + Cu + Ni) ratio (in atomic %) versus contents of Ni expressed in weight % in compositions of the Pt-Fe alloy minerals from various lode deposits.The symbols and sources used are the same as the ones shown in Figure1and listed in Table1.

Figure 7 .
Figure 7. Pt versus Fe diagram (in weight %) showing the compositional distribution of the Pt-Fe alloy minerals from placer deposits worldwide, which are listed in Table1.

Figure 7 .
Figure 7. Pt versus Fe diagram (in weight %) showing the compositional distribution of the Pt-Fe alloy minerals from placer deposits worldwide, which are listed in Table1.

Figure 7 .
Figure 7. Pt versus Fe diagram (in weight %) showing the compositional distribution of the Pt-Fe alloy minerals from placer deposits worldwide, which are listed in Table1.

Figure 8 .
Figure 8. Plot of the values of the Σ(Pt + PGE):(Fe + Cu + Ni) ratio (in atomic %) versus contents of Ir expressed in weight % in compositions of the Pt-Fe alloy minerals from various placer deposits.The symbols and sources used are the same as the ones shown in Figure7and listed in Table1.

Figure 8 .
Figure 8. Plot of the values of the Σ(Pt + PGE):(Fe + Cu + Ni) ratio (in atomic %) versus contents of Ir expressed in weight % in compositions of the Pt-Fe alloy minerals from various placer deposits.The symbols and sources used are the same as the ones shown in Figure7and listed in Table1.

Figure 9 .
Figure 9. Plot of the values of the Σ(Pt + PGE):(Fe + Cu + Ni) ratio (in atomic %) versus contents of Rh expressed in weight % in compositions of the Pt-Fe alloy minerals from various placer deposits.The symbols and sources used are the same as the ones shown in Figure7and listed Table1.

Figure 9 .
Figure 9. Plot of the values of the Σ(Pt + PGE):(Fe + Cu + Ni) ratio (in atomic %) versus contents of Rh expressed in weight % in compositions of the Pt-Fe alloy minerals from various placer deposits.The symbols and sources used are the same as the ones shown in Figure7and listed Table1.

Figure 9 .
Figure 9. Plot of the values of the Σ(Pt + PGE):(Fe + Cu + Ni) ratio (in atomic %) versus contents of Rh expressed in weight % in compositions of the Pt-Fe alloy minerals from various placer deposits.The symbols and sources used are the same as the ones shown in Figure7and listed Table1.

Figure 10 .
Figure 10.Plot of the values of the Σ(Pt + PGE):(Fe + Cu + Ni) ratio (in atomic %) versus contents of Pd expressed in weight % in compositions of the Pt-Fe alloy minerals from various placer deposits.The symbols and sources used are the same as the ones shown in Figure7and listed in Table1.

Minerals 2019, 9 , 16 Figure 10 .
Figure 10.Plot of the values of the Σ(Pt + PGE):(Fe + Cu + Ni) ratio (in atomic %) versus contents of Pd expressed in weight % in compositions of the Pt-Fe alloy minerals from various placer deposits.The symbols and sources used are the same as the ones shown in Figure7and listed in Table1.

Figure 11 .
Figure 11.Plot of the values of the Σ(Pt + PGE):(Fe + Cu + Ni) ratio (in atomic %) versus contents of Cu expressed in weight % in compositions of the Pt-Fe alloy minerals from various placer deposits.The symbols and sources used are the same as the ones shown in Figure7and listed in Table1.
), showing similar trends of Ni-enrichment in the compositional series (Pt, PGE)2-3(Fe + Cu + Ni).Additionally, the maximum contents of Ni are close in the Pt-Fe alloys in the lode and placer occurrences (≤4 wt.% Ni).

Figure 12 .
Figure 12.Plot of the values of the Σ(Pt + PGE):(Fe + Cu + Ni) ratio (in atomic %) versus contents of Ni expressed in weight % in compositions of the Pt-Fe alloy minerals from various placer deposits.The symbols and sources used are the same as the ones shown in Figure7and listed in Table1.

Figure 11 .
Figure 11.Plot of the values of the Σ(Pt + PGE):(Fe + Cu + Ni) ratio (in atomic %) versus contents of Cu expressed in weight % in compositions of the Pt-Fe alloy minerals from various placer deposits.The symbols and sources used are the same as the ones shown in Figure7and listed in Table1.

Minerals 2019, 9 , 16 Figure 10 .
Figure 10.Plot of the values of the Σ(Pt + PGE):(Fe + Cu + Ni) ratio (in atomic %) versus contents of Pd expressed in weight % in compositions of the Pt-Fe alloy minerals from various placer deposits.The symbols and sources used are the same as the ones shown in Figure7and listed in Table1.

Figure 11 .
Figure 11.Plot of the values of the Σ(Pt + PGE):(Fe + Cu + Ni) ratio (in atomic %) versus contents of Cu expressed in weight % in compositions of the Pt-Fe alloy minerals from various placer deposits.The symbols and sources used are the same as the ones shown in Figure7and listed in Table1.

Figure 12 .
Figure 12.Plot of the values of the Σ(Pt + PGE):(Fe + Cu + Ni) ratio (in atomic %) versus contents of Ni expressed in weight % in compositions of the Pt-Fe alloy minerals from various placer deposits.The symbols and sources used are the same as the ones shown in Figure7and listed in Table1.

Figure 12 .
Figure 12.Plot of the values of the Σ(Pt + PGE):(Fe + Cu + Ni) ratio (in atomic %) versus contents of Ni expressed in weight % in compositions of the Pt-Fe alloy minerals from various placer deposits.The symbols and sources used are the same as the ones shown in Figure7and listed in Table1.

Figure 13 .
Figure 13.Plot of the contents of the Pt.versus Fe (a), Pt. versus Ir (b), Pt versus Pd (c), and Pt versus Cu (d), all expressed in values of atomic %, in EMP compositions of Pt-Fe alloy minerals.A total of 2430 point analyses (n = 2430) are plotted, which pertain to a large variety of occurrences (Table1).

Figure 13 .
Figure 13.Plot of the contents of Pt. versus Fe (a), Pt. versus Ir (b), Pt versus Pd (c), and Pt versus Cu (d), all expressed in values of atomic %, in EMP compositions of Pt-Fe alloy minerals.A total of 2430 point analyses (n = 2430) are plotted, which pertain to a large variety of occurrences (Table1).

Minerals 2019, 9 , 16 Figure 14 .
Figure 14.Ternary diagrams of Pt-Pd-Ir (a) and Ir-Pd-Rh (b) showing atomic proportions of these elements in EMP compositions of the Pt-Fe alloy minerals from numerous localities (Table1).The total number of point analyses is 2430 (a) and 2325 (b).FCS is the inferred "field of complicated substitutions"; see text for discussion.

Figure 14 .
Figure 14.Ternary diagrams of Pt-Pd-Ir (a) and Ir-Pd-Rh (b) showing atomic proportions of these elements in EMP compositions of the Pt-Fe alloy minerals from numerous localities (Table1).The total number of point analyses is 2430 (a) and 2325 (b).FCS is the inferred "field of complicated substitutions"; see text for discussion.

( 8 )
Pairs of minor elements, Ir-Rh and Cu-Ni, despite an internal similarity in the observed character of distribution (Figures 2, 3, 8 and 9), displayed incoherent behavior and formed separate trends in terms of the plots of Rh versus Ir and Cu versus Ni.

Table 1 .
Worldwide occurrences of Pt-Fe alloy minerals involved in the present review.