Partitioning Of Platinum-group Elements Research Articles

Behavior of Platinum-Group Elements during Hydrous Metamorphism: Constraints from Awaruite (Ni3Fe) Mineralization

Abstract Natural Fe-Ni alloys are common in meteorites and, presumably, the Earth’s core, where they host significant platinum-group elements (PGE). However, little is known on PGE concentrations in hydrothermal or metamorphic Fe-Ni alloys (i.e., awaruite Ni3Fe) from terrestrial rocks. In this work, we examine the geochemistry of awaruite and related minerals from several placer deposits sourced from the suprasubduction ophiolitic (Kamchatsky Mys, Karaginsky Island, and Mamet) and Ural-Alaskan (Galmoenan) complexes of Kamchatka and the Koryak Highlands (Far East Russia) in order to assess the abundance of PGE in awaruite and constrain their mobility under metamorphic and hydrothermal conditions. Studied awaruite from ophiolitic and Ural-Alaskan type complexes formed via desulfurization of pentlandite during serpentinization. Three groups of platinum-group minerals (PGMs) are associated with awaruite from Kamchatsky Mys: (1) Pt-Fe alloys such as ferronickelplatinum (Pt2FeNi) or unnamed Ni2FePt alloys; (2) Os-Ir-Ru alloys of various composition; (3) Pd-Sb minerals which form together with serpentine during hydrothermal alteration. Despite the abundance of PGM inclusions, no significant PGE concentrations were measured in awaruite from the Kamchatsky Mys, Karaginsky Island, or Mamet ophiolites. In contrast, pentlandite relicts in awaruite from placers related to the Galmoenan Ural-Alaskan type complex contain exceptionally high, previously unreported, Os (up to 540 ppm). Awaruite that forms on behalf of this pentlandite does not show any significant Os enrichment. Rare Galmoenan awaruite analyses yield up to 3 ppm Pd. The new data are not in complete accordance with previous studies that reported relatively high (up to first 10 ppm) PGE content in awaruite. We attribute this to low PGE concentration in precursor sulfides and preferential partitioning of PGE into discrete secondary PGM within awaruite. Nevertheless, abundant inclusions of secondary PGM in awaruite provide evidence of PGE mobility during metamorphic and hydrothermal alteration of ultramafic rocks.

Origin of noble-metal nuggets in sulfide-saturated arc magmas: A case study of olivine-hosted sulfide melt inclusions from the Tolbachik volcano (Kamchatka, Russia)

AbstractMinerals that contain platinum-group elements (PGEs) and occur in some magmatic Cu-Ni sulfide deposits have been ascribed to crystallization from an originally PGE-rich sulfide liquid. The occurrence of PGE-bearing minerals (PGMs) in some sulfide-undersaturated primitive melts has been envisaged and recently reported, whereas direct crystallization of PGMs in sulfide-saturated silicate magmas is seemingly hindered by strong partitioning of PGE into immiscible sulfide melts. In this study, we discovered abundant nanoparticles containing noble metals in association with sulfide melt inclusions entrapped inside primitive olivine phenocrysts (Fo85–92) from the recent basaltic magma of the Tolbachik volcano (Kamchatka arc, Russia). These nuggets occur in swarms on the surface of the sulfide globules and are represented by native metals, sulfides, and alloys of Pd, Pt, Au, Pb, and Bi. The nuggets on different globules can be either Pd- or Pt-rich nuggets, and the compositions are highly variable, even among adjacent nuggets. We argue that the diffusive supply of Pd from the external nuggets can be responsible for significant uptake of Pd (up to 2 wt%) in the sulfide melt. We consider direct crystallization of PGMs in a primitive basaltic melt undergoing sulfide unmixing, and possibly sulfide breakdown due to oxidation, as another mechanism additional to their “classic” origin from the PGE-rich sulfide melt in response to solidification.

Empirical constraints on partitioning of platinum group elements between Cr-spinel and primitive terrestrial magmas

Recent experimental studies and in situ LA-ICP-MS analysis on natural Cr-spinel have shown that Rh and IPGEs (Ir-group platinum group elements: Ru, Ir, Os) are enriched in the lattice of Cr-spinel. However, the factors controlling the partitioning behaviour of these elements are not well constrained. In this study, we report the Rh, IPGE, and trace element contents in primitive Cr-spinel, measured by LA-ICP-MS, from nine volcanic suites covering various tectonic settings including island arc picrites, boninites, large igneous province picrites and mid-ocean ridge basalts. The aim is to understand the factors controlling the enrichment of Rh and IPGEs in Cr-spinels, to estimate empirical partition coefficients between Cr-spinel and silicate melts, and to investigate the role of Cr-spinel fractional crystallization on the PGE geochemistry of primitive magmas during the early stages of fractional crystallization.There are systematic differences in trace elements, Rh and IPGEs in Cr-spinels from arc-related magmas (Arc Group Cr-spinel), intraplate magmas (Intraplate Group Cr-spinel), and mid-ocean ridge magmas (MORB Group Cr-spinel). Arc Group Cr-spinels are systematically enriched in Sc, Co and Mn and depleted in Ni compared to the MORB Group Cr-spinels. Intraplate Group Cr-spinels are distinguished from the Arc Group Cr-spinels by their high Ni contents. Both the Arc and Intraplate Group Cr-spinels have total Rh and IPGE contents of 22–689ppb whereas the MORB Group Cr-spinels are depleted in Rh and IPGE (total<20ppb). Palladium and Pt contents are below detection limit for all of the studied Cr-spinels (<1–5ppb). The time-resolved spectra of LA-ICP-MS data for Cr-spinels mostly show constant count rates for trace element and Rh and IPGEs, suggesting homogeneous distribution of these elements in Cr-spinels. The PGE spikes observed in several Cr-spinels were interpreted to be PGE-bearing mineral inclusions and excluded from calculating the PGE contents of the Cr-spinels.On primitive mantle normalized diagrams the Arc Group Cr-spinels are characterized by a fractionated pattern with high Rh and low Os. The Intraplate Group Cr-spinels show flat patterns with positive Ru anomalies. Our results, together with the experimental and empirical data from previous studies, show that PGE patterns of Cr-spinel largely mimic that of the rock in which they are found, and that Rh, Ir and Os contents increase with increasing Fe3+ contents (i.e. magnetite component) in Cr-spinel, although Ru does not. These observations suggest that the enrichment of Rh and IPGEs in Cr-spinel is controlled by a combination of the Rh and IPGE contents in parental melts and the magnetite component of the spinel. Empirical partition coefficients (D) for Rh and IPGEs between Cr-spinels and silicate melts were calculated using the Rh and IPGE contents of the Cr-spinel and their host volcanic rocks after subtracting the accumulation effect of Cr-spinel. The D values for the Intraplate and MORB Group Cr-spinels increase with increasing magnetite component in Cr-spinel and range from 6 to 512, which is consistent with previously reported experimental and empirical values. In contrast, the Arc Group Cr-spinels have significantly higher D values (e.g. up to ∼3700 for Ru) than those of the Intraplate and MORB Group at the same magnetite concentration in the Cr-spinel, suggesting Rh and IPGEs dissolved in silicate melt have stronger affinity for Cr spinel under arc magma conditions than in intraplate magmas. This may be partly attributed to the low temperature of arc magmas relative to intraplate magmas, which leads to the Arc Group Cr-spinels having more octahedral sites at the same magnetite components than the Intraplate Group Cr-spinels. Because of significantly higher D values for the Arc Group Cr-spinels, compared with the Intraplate Group and MORB Group spinels, fractional crystallization of Cr-spinel will more efficiently fractionate Rh and IPGE from Pd and Pt in arc systems than in intraplate and MORB systems, which accounts for the highly fractionated PGE patterns in arc basalts.

Partitioning of platinum-group elements (PGE) and chalcogens (Se, Te, As, Sb, Bi) between monosulfide-solid solution (MSS), intermediate solid solution (ISS) and sulfide liquid at controlled fO2–fS2 conditions

In order to better understand the behavior of highly siderophile elements (HSEs: Os, Ir, Ru, Rh, Pt, Pd, Au, Re), Ag, Pb and chalcogens (As, Se, Sb, Te and Bi) during the solidification of sulfide magmas, we have conducted a series of experiments to measure partition coefficients (D values) between monosulfide solid solution (MSS) and sulfide melt, as well as MSS and intermediate solid solution (ISS), at 0.1MPa and 860–926°C, log fS2 −3.0 to −2.2 (similar to the Pt–PtS buffer), with fO2 controlled at the fayalite–magnetite–quartz (FMQ) buffer. The IPGEs (Os, Ir, Ru), Rh and Re are found to be compatible in MSS relative to sulfide melt with D values ranging from ∼20 to ∼5, and DRe/DOs of ∼0.5. Pd, Pt, Au, Ag, Pb, as well as the chalcogens, are incompatible in MSS, with D values ranging from ∼0.1 to ∼1×10−3. For the same metal/sulfur ratio, D values for the IPGEs, Rh and Re are systematically larger than most past studies, correlating with higher oxygen content in the sulfide liquid, reflecting the significant effect of oxygen on increasing the activity coefficients for these elements in the melt phase. MSS/ISS partitioning experiments reveal that Ru, Os, Ir, Rh and Re are partitioned into MSS by a factor of >50, whereas Pd, Pt, Ag, Au and the chalcogens partition from weakly (Se, As) to strongly (Ag, Au) into ISS. Uniformly low MSS- and ISS- melt partition coefficients for the chalcogens, Pt, Pd, Ag and Au will lead to enrichment in the residual sulfide liquid, but D values are generally too large to reach early saturation in Pt–Pd-chalcogen-rich accessory minerals, based on current solubility estimates. Instead, these phases likely precipitate at the last dregs of crystallization. Modeled evolution curves for the PGEs and chalcogens are in reasonably good agreement with whole-rock sulfide compositions for the McCreedy East deposit (Sudbury, Ontario), consistent with an origin by crystallization of MSS, then MSS+ISS from sulfide magma.

Partitioning of platinum-group elements and Au between sulfide liquid and basalt and the origins of mantle-crust fractionation of the chalcophile elements

The partitioning of platinum-group elements (PGE; Os, Ir, Ru, Rh, Pt, and Pd) and Au between sulfide melt and silicate melt (i.e., DPGEsul) exerts a critical control on the PGE composition of the Earth’s crust and mantle, but previous estimates have been plagued by experimental uncertainties and vary through several orders of magnitude. Here we present direct experimental measurements of DPGEsul, based on in situ microanalysis of the sulfide and silicate melt, with values ranging from ∼4×105 (Ru) to ∼2–3×106 (Ir, Pt). Our measurements of DPGEsul are >100 times larger than previous results but smaller than anticipated based on comparison of alloy solubilities in sulfide melts and S-free silicate melts. The presence of S in the silicate melt greatly increases alloy solubility. We use our new set of partition coefficients to develop a fully constrained model of PGE behavior during melting which accurately predicts the abundances of PGE in mantle-derived magmas and their restites, including mid-ocean ridge basalts, continental picrites, and the parental magmas of the Bushveld Complex of South Africa. Our model constrains mid-ocean ridge basalt (MORB) to be the products of pooled low and high degree fractional melts. Within-plate picrites are pooled products of larger degrees of fractional melting in columnar melting regimes. A significant control on PGE fractionation in mantle-derived magmas is exerted by residual alloy or platinum group minerals in their source. At low pressures (e.g., MORB genesis) the mantle residual to partial melting retains primitive mantle inter-element ratios and abundances of PGE until sulfide has been completely dissolved but then evolves to extremely high Pt/Pd and low Pd/Ir because Pt and Ir alloys form in the restite. During melting at high pressure to form picrites or komatiites Ir alloy appears as a restite phase but Pt alloy is not stable due to the large effect of pressure on fS2, and of temperature on fO2 along an internal oxygen buffer, which causes large increases in alloy solubility. The magmas parental to the Bushveld Complex of South Africa appear, at least in part, to be partial melts of mantle that has previously been melted to the point of total sulfide exhaustion at low pressure, closely resembling mantle xenoliths of the Kaapvaal craton. Using the new extremely large DPGEsul the world-class Merensky Reef and UG2 Pt deposits of the Bushveld Complex can readily be modeled as the result of sulfide saturation due to mixing of magmas with unremarkable PGE contents, obviating the need to postulate anomalously PGE-rich parent magmas or hydrothermal inputs to the deposits.

Accumulation and distribution characteristics of platinum group elements in roadside dusts in Beijing, China

The concentrations, distribution, and accumulation of platinum group elements (PGEs) were investigated in roadside dusts collected in four different foundational areas in Beijing during February to May 2010. The results showed that PGE levels in all samples were above the average upper crust values, with mean concentrations of 57.5 ng · g(-1) Pd, 28.2 ng · g(-1) Pt, and 9.8 ng · g(-1) Rh, respectively. Palladium concentration has increased rapidly in recent years. The rank of PGE levels in four different functional regions for roadside dusts was: heavy density traffic area > residential area > educational area > tourism area. Palladium, Pt, and Rh concentrations in dusts showed strong positive correlations, indicating a common traffic-related source of these metals. Meanwhile, PGEs in these samples were not correlated with other traffic-related metals except for Cr. The average PGE ratios of road dusts from Beijing were consistent with those in Germany and Western Australia, but lower than those in the United States and Mexico, indicating that various catalyst productions were used in different countries. In addition, grain-size partitioning of PGEs in dusts indicated that concentrations of PGEs differed from one particle size to another. The coarse fraction had higher PGE concentrations than the fine fraction in roadside dusts. These results showed that autocatalyst PGE contamination estimates in the environment would be significantly underestimated if only a fine-grain size fraction (<0.063 mm) is analyzed.

Critical evaluation of Re–Os and Pt–Os isotopic evidence on the origin of intraplate volcanism

Evaluation of Re–Os and Pt–Os isotope systematics indicates that the mantle plume model does not provide a unique explanation of the 187Os/ 188Os– 186Os/ 188Os isotopic variation in intraplate volcanic rocks. The low Os contents of ocean floor basalt and sediment compositions results in the amount of recycled crust (up to 55%) required in a plume to explain the range in 187Os/ 188Os, being unrealistically high unless non-equilibrium melting conditions are invoked. The low Pt content of MORB also limits the Pt/Os ratio of recycled crust, requiring plume models to resort to interaction with the outer core to explain suprachondritic 186Os/ 188Os ratios. Generation of the sources of intraplate volcanism by mixing of sediment into the mantle similarly requires high amounts (30–50%) of crustal materials. Non-equilibrium melting regimes can be avoided when the sources for intraplate volcanism are formed by metasomatic processes at shallow level in the mantle. Wetspot compositions formed by fluid metasomatism at convergent margins could produce the range of 186Os/ 188Os– 187Os/ 188Os signatures in intraplate volcanism over 2 Gyrs if the fluid was derived principally from the slab sediment component and the Pt/Os and Re/Os ratios of the hydrous peridotite remained unmodified. However, the most feasible method of generating suprachondritic 186Os/ 188Os– 187Os/ 188Os is from partitioning of platinum group elements into pyroxenites precipitated from Mg-rich melts. Isotopic compositions of 186Os/ 188Os=0.119850, γOs=+10 could be generated from depleted mantle compositions in 150 Myrs in an oceanic perisphere domain containing 60–70% pyroxenite veins, or in a continental mantle section containing as little as 15% pyroxenite over 2 Gyrs. Osmium isotope systematics therefore do not prove the mantle plume over alternative models, as illustrated for the Columbia River Basalt Group, where 187Os/ 188Os ratios of up to 0.144 in the least crustally contaminated basalts can be explained with a source containing 15–35% Mesozoic pyroxenite.

Laboratory partitioning of platinum-group elements (PGE) and gold with application to magmatic sulfide–PGE deposits

Apparent diversity in laboratory partitioning of platinum-group elements (PGE)–Au between (Fe,Ni)-sulfide liquid and S-saturated basaltic melt is resolved by recognizing the marked control exerted by variation in metal/S ratio of the sulfide liquid. Partition coefficients ( D) increase markedly for Os, Ir and Pd and somewhat for Pt with increase in S content. In addition, the partitioning is markedly dependent on concentration of PGE–Au in the sulfide fraction, for all precious metals in metal-rich sulfide, and for Ru, Pt and Pd in relatively S-rich sulfide. Reversal of partitioning of PGE–Au is presently demonstrated for experiments with metal-rich sulfide liquids. Erratic partitioning behaviour, with anomalously high D(PGE–Au) values, is minimised by degassing starting materials. Summary D(PGE–Au) values for metal-rich sulfide liquids and near-natural PGE–Au abundances are: Os(2.2), Ir(1.8), Ru(2.4), Pt(1.4), Pd(2.9), Au(0.9) (×10 3) for the CCO oxygen buffer and molar Ni/(Fe+Ni)=0.02, and Os(3.7), Ir(3.2), Ru(4.4), Pt(4.6), Pd(5.0), Au(3.0) (×10 3) for the IQF buffer and Ni/(Fe+Ni)=0.12. Thus, PGE–Au would not be extensively fractionated for equilibrium between sulfide and deep mantle melts. For more S-rich sulfide liquids (WM oxygen buffer) and 100–1000 ppm in the sulfide, D(PGE–Au) values are: Os(30), Ir(26), Ru(6.4), Pt(10), Pd(17) (×10 3) at Ni/(Fe+Ni)=0.36, and Os(10), Ir(51), Ru(3.5), Pt(13), Pd(25), Au (1.2) (×10 3) at Ni/(Fe+Ni)=0.66. The laboratory partitioning is notably differential within the PGE–Au group, with relatively high Ds for Ir and Pd and low Ds for Ru and Au. Although separation of an early-magmatic sulfide liquid would result in significant concentration of some PGE (e.g., Ir and Pd), sulfide liquid immiscibility alone would not account for the high abundance of PGE–Au in the reef deposits of the Bushveld and Stillwater Complexes.

Partitioning of platinum-group elements (Os, Ir, Ru, Pt, Pd) and gold between sulfide liquid and basalt melt

New measurements on partitioning of platinum-group elements (PGE) Os, Ir, Ru, Pt, Pd, and Au between (Fe,Ni)-sulfide liquid and basaltic melt using low pressure, sealed silica glass tube, and controlled atmosphere methods at 1200–1250°C show an extremely wide variation, which spans much of the range reported in the literature. In spite of their geochemical coherence in natural high-temperature assemblages, PGE-Au apparently behave as seven different metals in individual laboratory experiments with silicate melts. Sulfide/silicate partition coefficients, D, generally increase with run time, in a manner consistent with either serial change in chemical state of the metals in the silicate melt or passivation of alloy phases. Ruthenium is the only metal presently investigated that exhibits reversible behaviour. Summary partition coefficients for the lowest PGE contents investigated under reducing conditions and low Ni contents remain unchanged, except for the addition of a value of (4.4 ± 2.4) × 10 3 for Ru. Absence of interference from the nugget effect and homogeneity and equilibration of silicate melt in the earlier experiments is demonstrated by new measurements on Ir in different grain-size fractions of glass. An additional set of partition coefficients for 37 mol% NiS and 100–1000 ppm PGE in the sulfide is: D(Os) = (30 ± 6) × 10 3, D(Ir) = (26 ± 11) × 10 3, D(Ru) = (6.4 ± 2.1) × 10 3, D(Pt) = (10 ± 4) × 10 3, and D(Pd) = (17 ± 7) × 10 3.

Partitioning of platinum-group elements and Au in the Fe-Ni-Cu-S system: experiments on the fractional crystallization of sulfide melt - a discussion

Partitioning of platinum-group elements and Au in the Fe-Ni-Cu-S system: experiments on the fractional crystallization of sulfide melt - a discussion

Partitioning of platinum-group elements and Au in the Fe-Ni-Cu-S System: experiments on the fractional crystallization of sulfide melt - reply

Partitioning of platinum-group elements and Au in the Fe-Ni-Cu-S System: experiments on the fractional crystallization of sulfide melt - reply

Partitioning of platinum-group elements and Au in the Fe?Ni?Cu?S system: experiments on the fractional crystallization of sulfide melt

Partitioning of platinum-group elements (PGE) between sulfide liquid and monosulfide solid solution (mss) has been investigated by crystallizingmss from Fe−Ni−Cu sulfide liquid at 1,000–1,040° C, using bulk compositions and PGE contents typical of magmatic sulfides associated with mafic and ultramafic systems. Products were analyzedin situ for PGE and Au using SIMS. Sulfide liquid compositions were more Ni- and Cu-rich than coexistingmss. Liquid/mss partition coefficients are: Os-0.23±0.04, Ir-0.28±0.11, Ru-0.24±0.05, Rh-0.33±0.06, Pt-4.8±0.7, Pd-4.8±1.9, Au-11.4. Partitioning of PGE is independent of PGE concentration and Ni content in the composition range investigated. Additionally, Henry's law appears to be obeyed up to minor-element contents in the sulfide liquid andmss. Osmium, Ir, Ru, and Rh are compatible elements in the anhydrous Fe−Ni−Cu−S system, whereas Pt, Pd and Au are incompatible elements. These affinities correspond to the partitions of PGE between massive and Cu-rich magmatic sulfides. However, the detailed precious-metal compositions of the Cu-rich sulfides of mafic rock systems, disseminated ores of komatiites and Cu-rich assemblage of droplet ore from the Noril'sk-Talnakh deposits are not consistent with those expected for pristine fractionated sulfide liquids.

Partitioning of platinum group elements in the Fe‐O‐S System to 11 GPa and their fractionation in the mantle and meteorites

The partitioning of minor amounts of platinum group elements (PGE: Ru, Rh, Pd, Os, Ir, Pt) in the Fe‐O‐S system has been investigated to 11 GPa. The marked fractionation of Pt from Pd in the presence of coexisting alloy and sulfide liquid, reported earlier for the Fe‐Ni‐S system at low pressure, is extended to high pressure. The experiments were carried out at 1000°–1375°C and 4.5–11 GPa with graphite capsules using 18‐mm octahedral pressure cells in a uniaxial split‐sphere multianvil apparatus. Oxygen fugacities corresponded to wustite stability. Phases present included sulfide liquid (close to M3S2 stoichiometry), FeS (troilite, pyrrhotite, HPP), alloys (PtFe, PtFeIr, OsIr, FePtIrOs, OsFeIr), and wustite. Partition coefficients for OsIr alloy/Fe1−x S at 1200°C, 11 GPa are Os (1400), Ir (160), Pt (32), Ru (0.6), Rh (0.2), and Pd (0.04) and for FePtOs alloy/liquid at 1100°C, 8.5 GPa are Os (120), Ir (82), Pt (20), Ru (5.0), Rh (1.6), and Pd (0.1). Geochemical processes which involve separation of a metal‐rich phase from sulfide therefore would result in fractionation of PGE, with Os, Ir, and Pt concentrated in the former phase and Pd in the latter. These results are further support for the presence of a discrete metal‐sulfide liquid during the primary differentiation of planetary material and preclude an explanation for the fractionation of PGE in chromitites and mafic crustal rocks based on equilibrium partitioning between refractory alloys in residual assemblages and immiscible sulfide liquid in silicate melts.

Partitioning of platinum-group elements in the Fe-Ni-S system and their fractionation in nature

The partitioning of minor amounts of platinum-group elements (PGEs: Ru, Rh, Pd, Os, Ir, and Pt) has been investigated in the Fe-Ni-S system at 1000 to 1400°C and low pressure. Revising previous experimental results indicating lack of specificity of PGE, distinctly different partitioning behavior is demonstrated for heavy and light platinoids; Os, Ir, and Pt are preferentially concentrated in alloy phases, whereas Pd is depleted in alloys and relatively enriched in sulfide liquid. Partition coefficients for Fe alloy/sulfide liquid at 1000°C are 1 to 2 for Pd (comparable to values for Ni), 30 to 110 for Rh, and in excess of 1000 for Os, Ir, and Pt. Ruthenium and, to a lesser extent, rhodium are the only PGEs with significant solubility in troilite coexisting with sulfide liquid. The marked fractionation of Pt from Pd in the presence of coexisting alloy and sulfide liquid is contrasted with the minimal fractionation of PGEs in the primary differentiation of planetary material and of Pt from Pd in the generation of basic magmas in the upper mantle. The characteristic fractionation patterns for PGEs in upper mantle and crustal rocks are most likely related to the presence of a combination of host phases (alloys, spinel, diopside) for the refractory PGEs (Os, Ir, and Ru) during partial melting in the upper mantle.

Platinum-group element abundance and distribution in chromite deposits of the Acoje Block, Zambales Ophiolite Complex, Philippines

Platinum-group elements (PGE) occur in ore-grade concentration in some of the chromite deposits related to the ultramafic section of the Acoje Block of the Zambales Ophiolite Complex. The deposits are of three types: Type 1 - associated with cumulate peridotites at the base of the crust; Type 2 - in dunite pods from the top 1 km of mantle harzburgite; and Type 3 - like Type 2, but in deeper levels of the harzburgite. Most of the deposites have chromite compositions that are high in Cr with Cr/(Cr + Al) (expressed as chromium index, Cr#) > 0.6; high-Al (Cr# < 0.6) chromite deposits are relatively rare, occurring as interspersed orebodies in regions of Types 2 and 3. Sulfide-bearing Type 1 high-Cr deposits are PGE-enriched; the high-Al deposits have lowest Ir and Pd. Among the PGEs, Ir ranges from <20 to 550 ppb, Rh from <100 to 1100 ppb, Rh from 0.7 to 760 ppb, Pt from <1.0 to 5960 ppb and Pd from <0.5 to 8350 ppb. Chondrite-normalized (CN) PGE patterns are diverse, and include those with both negative slope (Ir > Pd, thought to be characteristic of PGE-barren deposits) and positive slope (Ir < Pd, characteristic of PGE-rich deposits). Iridium, Ru and Os commonly occur as micron-size laurite (sulfide) inclusions in unfractured chromite. Laurite and native Os are also found as inclusions in interstitial sulfides. Platinum and Pd occur as alloy inclusions (and possibly as solid solution) in interstitial Ni-Cu sulfides and as tellurobismuthides in serpentine and altered sulfides. Variability of PGE distribution may be explained by alteration, crystal fractionation or partial melting processes. Alteration and metamorphism were ruled out, because PGE contents do not correlate with degree of serpentinization or the abundance and type (hydroxyl versus non-hydroxyl) of silicate inclusions in chromite. Preliminary Os isotopic data do not support crustal contamination as a source of the PGEs in the Acoje deposits. The anomalous PGE concentrations in Type 1 high-Cr chromite deposits are attributed to two stages of enrichment: an early enrichment of their mantle source from previous melting events and a later stage of sulfide segregation accompanying chromite crystallization. High-Al chromite deposits which crystallized from basalts derived from relatively low degrees of melting owe their low PGE content to partitioning of PGEs in sulfides and alloys that remain in the mantle. High-Cr deposits crystallized from melts that were previously enriched with PGEs during early melting events of their mantle source; Pt and Pd ore concentrations (ppm levels) are attained by segregation of magmatic sulfides. The Acoje deposits indicate that ophiolites are a potential economic source of the PGEs.