Magmatic Platinum-group Element Research Articles

Petrographic and mineral chemistry evidence for the origin of sulfide mineralization in the Main Sulfide Zone, Sebakwe Subchamber of the Great Dike, Zimbabwe

The Great Dike, one of the largest repositories of platinum-group elements (PGEs) and largest layered mafic intrusions in the world, is currently being exploited for its PGEs. We undertook a petrographic and mineral compositional investigation in the Sebakwe Subchamber to place some constraints on the genesis of its PGEs. Sulfides, which host some of the PGEs, occur either as inclusions within medium-grained euhedral orthopyroxenes or in association with biotite and intercumulus plagioclase. Sulfides which occur as interstitial phases or as inclusions in silicates are consistent with a magmatic origin of PGE sulfide mineralization. However, some sulfides which occur in association with biotite, are consistent with concentration of PGEs by magmatic fluids. Average sample Mg# range from 83.5–80.1 for orthopyroxenes, 93.1–84.4 clinopyroxenes, and 79.2–76.7 for phlogopite, whereas average sample An# for feldspars range from 81.3 to 5.9. Pyroxene thermometry yields temperatures of 837–986 °C that are late magmatic, with biotite thermometry yielding temperatures of 478–645 °C that are interpreted to be consistent with recrystallization of earlier primary biotite during hydrothermal alteration by late- to post-magmatic fluids. Observations from petrographic and backscattered electron images are consistent with these interpretations. Primary, magmatic PGE mineralization is inferred to have been cogenetic with sulfides which occur either as inclusions within, or as intercumulus phases between, orthopyroxenes. Sulfide minerals coexisting with biotite likely represent primary, magmatic sulfides that were remobilized by late- to post-magmatic fluids. The inferred depth of crystallization and hydrothermal alteration of the Sebakwe Subchamber of ∼10 km, is broadly comparable to other Subchambers of the Great Dike, lending support to the view that similar processes operated in the uppermost part of Great Dike ultramafic rocks in all its Subchambers. The involvement of fluid migration in the possible remobilization and concentration of primary magmatic PGE-mineralization in most layered mafic intrusions where it has been documented, must be a more significant process in concentrating PGEs in layered intrusions than currently thought.

Genesis of the Jinbaoshan PGE-(Cu)-(Ni) deposit: Distribution of chalcophile elements and platinum-group minerals

ABSTRACT The Jinbaoshan platinum group element-(Cu)-(Ni) deposit in southwest China is a sulfide-poor magmatic platinum-group element deposit that experienced multiple phases of post-magmatic modification. The sulfide assemblages of most magmatic Ni-Cu-platinum-group element deposits in China and elsewhere in the world are dominated by pentlandite-pyrrhotite-chalcopyrite with lesser magnetite and minor platinum-group minerals. However, Jinbaoshan is characterized by (1) hypogene violarite-pyrite 1-millerite-chalcopyrite and (2) supergene violarite-(polydymite)-pyrite 2-chalcopyrite assemblages. The platinum-group minerals are small (0.5–10 μm diameter) and include moncheite Pt(Te,Bi)2, mertieite-I Pd11(Sb,As)4, the atokite Pd3Sn – rustenburgite Pt3Sn solid solution, irarsite IrAsS, and sperrylite PtAs2 hosted mainly by violarite, silicates (primarily serpentine), and millerite. The platinum-group minerals occur in two sulfide assemblages: (1) mertieite-I-dominant (with irarsite, palladium, and Pd-alloy) in the hypogene assemblage and (2) moncheite-dominant (with irarsite, sperrylite, and atokite) in the supergene assemblage. Palladium and intermediate platinum-group elements (Os, Ir, Ru) are concentrated mainly in violarite, polydymite, and pyrite 2. Platinum is seldom hosted by base metal sulfides and occurs mainly as discrete platinum-group minerals, such as moncheite, sperrylite, and merenskyite. Violarite and polydymite in the Jinbaoshan deposit contain more Pb-Ag than pentlandite and pyrrhotite in the Great Dyke and Lac des Iles deposit. The formation of the sulfide assemblages in Jinbaoshan can be interpreted to have occurred in three stages: (1) a magmatic Fe-Ni-Cu sulfide melt crystallized Fe-Ni monosulfide and Cu-rich intermediate solid solutions, which inverted to a primary pyrrhotite-pentlandite-chalcopyrite-magnetite assemblage; (2) an early-secondary hypogene voilarite-millterite-pyrite 1-chalcopyrite assemblage formed by interaction with a lower-temperature magmatic-hydrothermal deuteric fluid; and (3) a late-secondary supergene violarite-polydymite-pyrite 2-chalcopyrite assemblage formed during weathering. Late-magmatic-hydrothermal fluids enriched the mineralization in Pb-Ag-Cd-Zn, which are incompatible in monosulfide solid solution, added Co-Pt into violarite, and expelled Pd to the margins of hypogene violarite and millerite, which caused Pd depletion in the hypogene violarite and the formation of mertieite-I. Supergene violarite inherited Pd and intermediate platinum-group elements from primary pentlandite. Thus, the unusual sulfide assemblages in the Jinbaoshan platinum-group element-(Cu)-(Ni) deposit results from multiple overprinted post-magmatic processes, but they did not significantly change the chalcophile element contents of the mineralization, which is interpreted to have formed at high magma:sulfide ratios (R factors) through interaction of crustally derived sulfide and a hybrid picritic-ferropicritic magma derived from subduction-metasomatized pyroxenitic mantle during impingement of the Emeishan plume on the Paleo-Tethyan oceanic subduction system.

Introduction to the special issue on the Flatreef PGE-Ni-Cu deposit, northern limb of the Bushveld Igneous Complex

More than 30 years ago, Cox and Singer (1986) suggested that magmatic platinum-group element (PGE)-Ni-Cu deposits are amongst the best understood of ore deposits, yet the origin of PGE mineralization in the Bushveld Igneous Complex (BIC) remains controversial after a century of study. In the northern limb of the BIC, the unravelling of ore formation proved particularly difficult due to relatively poor outcrop, which is typically affected by contamination of the intruding magmas with the host rocks and expressed in the form of abundant xenoliths, footwall rafts and disturbance of magmatic stratigraphy. In this thematic issue, we present contributions on the Flatreef, a recently discovered world-class PGE-Ni-Cu deposit constituting a downdip extension of the mineralized unit of the Platreef of the northern limb. Two deep shafts are currently being sunk, making the Flatreef one of the most significant new mine development on the Bushveld in several decades.

Magmatic PGE Sulphide Mineralization in Clinopyroxenite from the Platreef, Bushveld Complex, South Africa

The Platreef, at the base of the northern limb of the Bushveld Complex in South Africa, hosts platinum-group element (PGE) mineralization in association with base-metal sulphides (BMS) and platinum-group minerals (PGM). However, whilst a magmatic origin of the stratiform mineralization of the upper Platreef has been widely confirmed, the processes responsible for the PGE and BMS mineralization and metasomatism of the host rocks in the Platreef are still under discussion. In order to contribute to the present discussion, we present an integrated petrographical, mineral-chemical, whole-rock trace- and major-element, sulphur- and neodymium-isotope, study of Platreef footwall clinopyroxenite drill core samples from Overysel, which is located in the northern sector of the northern Bushveld limb. A metasomatic transformation of magmatic pyroxenite units to non-magmatic clinopyroxenite is in accordance with the petrography and whole-rock chemical analysis. The whole-rock data display lower SiO2, FeO, Na2O and Cr (<1700 ppm), and higher CaO, concentrations in the here-studied footwall Platreef clinopyroxenite samples than primary magmatic Platreef pyroxenite and norite. The presence of capped globular sulphides in some samples, which display differentiation into pyrrhotite and pentlandite in the lower, and chalcopyrite in the upper part, is attributed to the fractional crystallization of a sulphide liquid, and a downward transport of the blebs. In situ sulphur (V-CDT) isotope BMS data show isotopic signatures (δ34S = 0.9 to 3.1 ‰; Δ33S = 0.09 to 0.32‰) close to or within the pristine magmatic range. Elevated (non-zero) Δ33S values are common for Bushveld magmas, indicating contamination by older, presumably crustal sulphur in an early stage chamber, whereas magmatic δ34S values suggest the absence of local crustal contamination during emplacement. This is in accordance with the εNd (2.06 Ga) (chondritic uniform reservoir (CHUR)) values, of −6.16 to −6.94, which are similar to those of the magmatic pyroxenite and norite of the Main Zone and the Platreef in the northern sector of the northern Bushveld limb. Base-metal sulphide textures and S–Se-ratios give evidence for a secondary S-loss during late- to post-magmatic hydrothermal alteration. The textural evidence, as well as the bulk S/Se ratios and sulphide S isotopes studies, suggest that the mineralization in both the less and the pervasively hydrothermally altered clinopyroxenite samples of Overysel are of magmatic origin. This is further supported by the PPGE (Rh, Pt, Pd) concentrations in the BMS and mass-balance calculations, in both of which large proportions of the whole-rock Pd and Rh are hosted by pentlandite, whereas Pt and the IPGE (Os, Ir, Ru) were interpreted to mainly occur in discrete PGM. However, the presence of pentlandite with variable PGE concentrations on the thin section scale may be related to variations in the S content, already at S-saturation during magmatic formation, and/or post-solidification mobilization and redistribution.

Platinum-group element and sulphide mineralogy in ultramafic complexes at western Andriamena, Madagascar

Small (<2 km diameter), ultramafic intrusive complexes in the Andriamena region of Madagascar carry high concentrations of platinum-group elements (PGE). The PGE mineralisation at Andriamena is unusual in that it is not obviously associated with abundant sulphide or chromitite layers or pods, as in many layered complexes and Alaskan-type intrusions. Mineralogical investigation of rocks from one complex, designated UM2, has revealed a complex array of Pt and Pd-bearing platinum-group minerals (PGM) that tend to be concentrated interstitially between olivine crystals in dunite units. These PGM are dominated by PGE arsenides and stibioarsenides, accompanied by a small population of Pd oxides that occur in unit close to the surface and appear to have formed by supergene alteration. The PGM population displays a notable contrast between Pt-bearing and Pd-bearing minerals. Palladium-bearing PGM are always found entirely within or at least partially enclosed by Fe–Ni–Cu sulphides. In contrast, Pt-bearing PGM are either found on the margins of Fe–Ni–Cu sulphides or more commonly, they are entirely isolated from sulphides in hydrosilicates or as inclusions in olivine. The discovery of these sperrylite (PtAs2), irarsite (IrAsS) and hollingworthite (RhAsS) inclusions in olivine implies that development of these minerals took place at magmatic temperatures, coincident with the growth of olivine, not in late-stage interstitial fluids. This leads to a new magmatic model for the development of the mineralisation. In this model, PGE and As are initially concentrated into a sulphide liquid in a deeper magma chamber and intruded as a slurry of silicate liquid–olivine–sulphide liquid into the dyke-like Lavatrafo intrusions. The high As content of the sulphide promotes early crystallisation of Pt, Ir and Rh arsenide and sulpharsenide PGM, some of which became detached from the sulphide and became incorporated into growing olivine crystals. Palladium arseno-antimonide PGM follow at a later stage. Early crystallisation of Pt and Pd-bearing PGM leaves the sulphide liquid depleted in these elements. Migration of the fractionated sulphide liquid inwards and downwards towards the centre of the intrusion produces a zone of Cu-rich sulphides that is PGE-poor. Such a strong spatial separation PGE from base metal sulphides is uncommon in magmatic PGE deposits and this is suggested to arise from the As-rich nature of the early sulphide liquid which promoted the formation of PGM earlier than would have occurred in an equivalent As-poor system.

The Merensky Cyclic Unit and its impact on footwall cumulates below Normal and Regional Pothole reef types in the Western Bushveld Complex

The Merensky Reef of the Bushveld Complex occurs in its highest stratigraphic position as a heterogeneous, pegmatitic, feldspathic melanorite bounded by two narrow chromitite stringers at the base of the Merensky Cyclic Unit (MCU). In the Swartklip Facies of the Rustenburg Layered Suite, the occurrence of widespread thermal and mechanical erosion termed “potholing” has led to the subdivision of the Merensky Reef into Normal Reef and Regional Pothole Reef sub-facies. The transition between the two sub-facies occurs where the MCU transgresses the lower chromitite stringer of the Normal Merensky Reef and cuts down into the underlying cumulate lithologies. In the Regional Pothole Reef at the Northam Platinum Mine, several economic reef types are identified, where the Merensky Reef becomes conformable to cumulate layering, in particular, to the footwall marker (NP2 reef type) and the upper pseudoReef (P2 reef type). The Normal Merensky Reef, as well as the P2 and NP2 Reefs, contains economic platinum group element (PGE) grades and includes the lower portion of the MCU melanorite and the Merensky Chromitite. Whole rock geochemistry indicates that this package is compositionally identical in Normal, P2, and NP2 Reefs, suggesting that the base of the MCU is a relatively homogeneous drape over both Normal and Regional Pothole Reef regions. However, the lower sections of the three Reefs are variables depending on the depth of transgression of the MCU. In the Normal and P2 reef types, transgression by the MCU was arrested within harzburgites, melanorites, and norites, resulting in coarse, pegmatitic textures in the immediate footwall units. For the NP2 Reef, transgression by the MCU was arrested within leucocratic rocks and resulted in the formation of troctolites below the Merensky Chromitite. These troctolites are characterised by a coupled relationship between olivine and sulphides and by changes in major element chemistry and PGE contents relative to equivalent units in the footwall of the Normal Reef. Along with micro-textural relationships, these features suggest that troctolization of leucocratic cumulates in the NP2 Reef beneath the Merensky chromitite was a result of a reactive infiltration of a chromite-saturated melt and an immiscible sulphide liquid from the overlying MCU, rather than a significant fluid flux from below. In all reef types, the concentration of S defines symmetrical peaks centred on the Merensky Chromitite (and chromitites from pre-existing cyclic units in Normal and P2 Reefs), whereas PGE concentrations define asymmetrical peaks with higher PGE contents in reconstituted footwall rocks relative to the MCU melanorite. This signature is attributable to a magmatic model of PGE collection followed by deposition towards the base of the MCU and within reconstituted footwall rocks. The continuity of the asymmetrical magmatic PGE signature between the Normal Reef and Regional Pothole Reef sub-facies indicates that PGE mineralization inherent to the Merensky magma occurred as a drape over a variably eroded and subsequent texturally and geochemically reworked or reconstituted footwall.

Self-Destructive Sulfide Segregation Systems and the Formation ofHigh-Grade Magmatic Ore Deposits

The metal concentrations of sulfide liquids depend upon the mass of silicate magma that they are able to process for metals. Some high-grade magmatic Ni-Cu (±PGE) deposits and many magmatic platinum group element (PGE) (±Ni-Cu) deposits demand very high magma/sulfide ratios that seem improbable in the light of physical and kinetic constraints. Recent models for some high-grade deposits invoke multistage upgrading processes, in which an early-formed sulfide liquid reacts with multiple later batches of silicate magma. Quantitative models of this open-system process demonstrate that it is indeed more efficient than a closed system. However, it is likely that most later magmas in such a system will be sulfur undersaturated and will thus partly redissolve preexisting sulfide liquids, further increasing metal concentrations in the remaining sulfide liquids. This combined process is termed multistage-dissolution upgrading, and quantitative models show that it could reduce the mass of silicate magma that must be processed by as much as two orders of magnitude. Furthermore, if sulfide liquids are extensively dissolved during enrichment, their base metal concentrations will generally stabilize at a limiting value, whereas their PGE concentrations will generally increase without limit. This divergence could account for unusually high PGE concentrations and high PGE/base metal ratios in many PGE-dominated deposits. The multistage-dissolution upgrading model is tested, using reasonable degrees of dissolution, in the context of natural deposits in the Noril’sk area and the Bushveld intrusion. The models reproduce the observed sulfide compositions significantly better than previous models and are also consistent with other aspects of the geology of these deposits. Thus, dynamic sulfide-forming magmatic systems may be intrinsically self destructive, but this apparently undesirable attribute could play an important role in forming high-grade deposits. However, dissolution could lead to the complete destruction of sulfide liquids and the return of all metals to later magma batches. In this case, no sulfide deposit would remain, but metal-depletion signatures indicating sulfide liquid segregation would be preserved in rocks that crystallized from early magmas. Caution is therefore advised in the use of magmatic depletion signatures to infer overall mineral potential or to estimate the possible sizes of undiscovered magmatic sulfide deposits.

The East Bull Lake Intrusive Suite: Remnants of a 2.48 GaLarge Igneous and Metallogenic Province in the Sudbury Area of the CanadianShield

The 2.49 to 2.475 Ga intrusions of the East Bull Lake intrusive suite occur in an east-northeast-trending discontinuous belt along the presently exposed boundary between the Archean Superior and the Proterozoic Southern provinces of the Canadian Shield near Sudbury, Ontario. The East Bull Lake intrusive suite is part of a regional Paleoproterozoic magmatic event, extending from 2.49 to 2.44 Ga, which also includes bimodal volcanic rocks, felsic plutons, and the regionally extensive Hearst and Matachewan dike swarms. This regional magmatic event is thought to be the result of a mantle plume-driven, intracontinental rifting event that led to the development of a major basin to the south that was subsequently filled by sedimentary rocks of the Huronian Supergroup. The East Bull Lake intrusive suite appears to have been emplaced along a major axial-rift fault related to this rifting event. Compelling field and geochemical evidence summarized here indicates that the three largest East Bull Lake suite intrusions, the East Bull Lake, Agnew Lake, and River Valley intrusions, crystallized from similar, low Ti, high Al tholeiitic parent magmas that originated in deeper, more basic chambers. It is proposed that the primary magmas for the East Bull Lake suite intrusions were second-stage melts derived from a sublithospheric depleted mantle source that had been modified by Neoarchean subduction and accretion events. Most of the petrological characteristics of the East Bull Lake suite intrusions reflect plagioclase-dominated fractional crystallization that generated a pronounced Fe enrichment trend in the residual magmas. Orthopyroxene is the high-temperature pyroxene and occurs as a cumulus phase through most of the stratigraphy of each intrusion of the suite. Local olivine-rich cumulates, present in all of the major East Bull Lake suite plutons, in some instances appear to be comagmatic with associated plagioclase-rich cumulates. A distinctive marginal facies, comprising brecciated and locally thermally recrystallized and/or partially melted footwall rocks, is a common feature of the East Bull Lake suite plutons and indicates that a high-energy flow regime existed during the initial stages of emplacement. This marginal unit typically grades into a heterolithic, inclusion-rich gabbronorite that contains erratically distributed mafic autoliths and footwall-derived xenoliths. Chemical contamination of the mineralized matrix of these inclusion- bearing rocks can be extreme at a local scale. The inclusion-rich zone is overlain by a thick interval of undifferentiated, plagioclase-rich cumulates that locally display spectacular glomerocrystic textures. Rhythmically and irregularly, modally layered leucogabbronorite and gabbronorite make up much of the middle parts of the stratigraphy. Massive to crudely layered ferrogabbro and ferrosyenite only occur in the uppermost part of the Agnew Lake intrusion. Varitextured gabbro and pegmatitic gabbro occur as irregular layers and local veins and pods throughout the stratigraphy. The East Bull Lake suite is currently being explored for contact-type platinum-group element (PGE)-Cu-Ni mineralization that has a clear spatial association with orthopyroxene-rich cumulates which are otherwise poorly represented in these intrusions. Disseminated PGE- and Cu-rich sulfide mineralization is concentrated within the inclusion-bearing zones but extends upward into overlying plagioclase cumulates and appears to have collected along the lower margins of these intrusions through the combined action of vigorous convection of the resident magma and downward percolation of dense, mafic residual liquid. The resultant narrow zones of sulfide mineralization were thus derived from a much larger volume of parent magma. Feeder dikes to the mineralized parts of the intrusions appear to have been saturated in sulfide upon emplacement and had relatively high background PGE contents compared to those for other known magmatic PGE deposits; however, the economic potential of the contact-type mineralization hinges on the efficacy of the physical concentration process, which upgraded the metal content and size of the original sulfide particles inside the magma chambers. Based on the proposed basal accumulation model, the best prospective mineralization is most likely to occur in embayments along the sidewall and floor of the intrusions.

Palladium–copper-rich platinum-group element mineralization in Legris Lake mafic–ultramafic complex, Western Superior Province, Canada

The Legris Lake Complex is a northeast-trending, 7.3 km × 3.5 km, mafic-ultramafic intrusive complex located in the western Wabigoon Subprovince of the Archaean Superior Province. It is part of a circular series of mafic-ultramafic complexes, the most notable of which is the Lac des Iles Complex, ca 7 km northwest of the Legris Lake Complex, which hosts the Lac des Iles palladium mine. Four phases of exploration have been conducted on the Legris Lake Complex property since autumn, 1999, defining mineralized zones on surface and at depth, with drill-core intersections of up to 2.04 g/t Pd, 0.41 g/t Pt, 0.71 g/t Au, 0.42% Cu and 0.13% Ni over 9.95 m.The Legris Lake Complex consists of mostly gabbroic rocks but also contains lithologies ranging from anorthosite to wehrlite and a variety of igneous breccias. The gabbroic rocks vary from melanogabbro to porphyritic leucogabbro. Medium-grained, massive, biotite-rich leucogabbro is the predominant exposed variety and probably caps the complex. The north-western margin of the complex (2 km × 600 m), which contains all the known platinum-group element (PGE) mineralization, is characterized by heterolithic breccia with abundant fragments of sedimentary rocks and numerous gabbroic dykes and sills.The PGE mineralization in the Legris Lake Complex has different characteristics from many PGE deposits, where the ore occurs in sulphide-rich bodies at the base of mafic and ultramafic rocks. In the Legris Lake Complex PGE enrichment occurs in sulphide-poor (1-10 vol%, mostly <5 vol%), medium- to coarse-grained, porphyritic leucogabbro (termed 'Main Showing-type') hosted within zones of intense magmatic brecciation. The mineralized leucogabbro is mineralogically highly evolved and exhibits a sill-like form near the stratigraphic top of the complex. The mineralization consists of disseminated to blebby sulphides rimmed by epidote and disseminated magnetite. The mineralized rocks typically contain Cu ranging from 0.2 to 0.4 wt% and Ni from 0.07 to 0.12 wt%, with low Pt/Pd ratios (ca 0.2) and high Cu/Ni ratios (ca 3). The PGE contents display a positive correlation with those of Cu and Ni. The origin of the mineralization is best explained by preferential partitioning of PGE into an immiscible sulphide melt in evolved silicate magma after fractional crystallization of olivine and clinopyroxene. The immiscible separation of sulphide melt may have been aided by incorporation of silica and sulphide from adjacent sedimentary rocks. Formation of magnetite and hydrous minerals in the mineralized zones suggests that the sulphide melt had high oxygen/(oxygen + sulphur) ratios and high contents of volatiles, most probably reflecting a high oxidation state and volatile-rich nature of the parental magmas. This primary magmatic PGE mineralization was followed by minor redistribution of PGE by deuteric hydrothermal fluids released from the parental magma.

PGE enrichment in chromitite layers and the Merensky Reef of the western Bushveld Complex; a Re–Os and Rb–Sr isotope study

Four poikilitic pyroxenites of the Bastard Unit, all being relatively low in osmium concentrations, define a Re-Os isochron age of 2043 = 11 Ma (with initial 187Os/ 188Os = 0.151), which is in accordance with the normally cited crystallization age of the Bushveld Complex. The isochron fit demonstrates an excellent radiogenic Os isotopic homogeneity of this portion of the complex at the time of crystallization and excludes any later disturbance of the Re-Os isotope system. Initial 187Os/ 188Os ratios of platinum-group element (PGE) enriched sulfides and whole rocks of the directly underlying Merensky Reef are highly radiogenic and variable (with values between 0.168 and 0.181). Separates of chromite grains and interstitial phases of Critical Zone chromitite layers show initial 187Os/ 188Os signatures ranging from the near-chondritic mantle values of lower group chromitites (i.e. ˜0.120) to 0.137 for middle group chromitites and to 0.150 for the upper group 2 chromitite layer. Osmium concentrations of interstitial phases exceed those of the associated chromite in all layers by up to 30 fold, showing the presence of highly PGE-enriched interstitial sulfides. Equal or very similar initial 187Os/ 188Os signatures of the interstitial minerals and the cumulus chromite grains together with their coupled increasing and decreasing trends in this ratio clearly demonstrate that interstitial sulfides and associated cumulus chromite of the different chromitite layers scavenged their osmium from the same or similar sources. Therefore, a PGE enrichment of the interstitial sulfides by a primary magmatic process, such as precipitation of immiscible sulfide droplets during magma mixing appears more likely than a scenario involving fluid or melt transport of PGE into these layers. Such a primary magmatic PGE enrichment process is also preferred for the formation of the Merensky Reef.

Anoxic sediments—an important environment for PGE; An overview

Anoxic sediments and metasediments have been recently recognized as important host environments for the platinum group elements (PGE). Economically significant PGE concentrations have been reported from marine, metal-rich black shales in China, Yukon (Canada) and Poland. These ore-bearing horizons are characterized by extremely restricted vertical, and sometimes larger horizontal extents. They are associated with the evolution of intracontinental rifting structures without any magmatic activity, suggesting a possible driving mechanism for mineralization processes. Complex PGE associations, including a significant relationship of Pd and Pt with Ni, Bi, Se, Y and also with Sb, Cu, Co, Au, U, and F are suggested using the results of a new statistical evaluation of available geochemical data. These occurrences show more common features with other hydrothermal PGE deposits. The rift-volcanic-related, marine metal-rich black shales of the Czech Republic and Finland, however, display lower PGE values. The mineralized layers are much thicker with a typically limited horizontal extent showing a close association with volcanic centres. Pd reveals a strong correlation with Ni, Zn, Au and V; Pt exhibits a highly significant link with the Cu and Mo phases. Low average Ni Cu ratios and PGE+Au chondrite-normalized patterns are similar to those of some magmatic PGE deposits hosted in mafic and ultramafic rocks. It is very likely that organic matter played an important role in the genesis of PGE accumulations in metal-rich black shales in both rift and rift-volcanic-related environments.

Thermodynamic calculations of the volatility of the platinum group elements (PGE): The PGE content of fluids at magmatic temperatures

The volatilities of the platinum-group elements as metals, oxides and chlorides were calculated at temperatures of 800–1600 K. Only Pd is significantly volatile as the metal. At logfH2O = 1 Kbar and 1200 K., the concentration (weight) of Pd in the vapor reaches 1 ppt and at 1600 K attains several ppb. The PGE oxides are extremely volatile at atmospheric oxygen fugacities. However, only Os and Ru have significant volatilities (≥ ppt) as oxides (OsO4, RuO3) at oxygen fugacities typical of magmatic PGE deposits (near QFM) and only at temperatures greater than 1400 K.Data on the volatility of PGE chlorides exist only for Pd and Ru, both of which are somewhat more volatile as chlorides than as oxides. At 1400 logfH2O = 1 bars, fHCl = 100 bars and at QFM, the calculated vapor concentrations of PdCl2 and RuCl3 are 500 ppt and 20 ppt, respectively (and less in the presence of sulfur). However, higher concentrations of PGE may be attained at higher temperatures, higher fO2, higher fHCl or lower fH2. Also, any interactions between water vapor and PGE vapor species (e.g. ionization, solvation) would tend to increase the vapor concentration of PGE. Volatility of Ir as IrF6 is insignificant at all conditions.Vapor transport of the more volatile PGE as chlorides may play some role in the transport of these metals in mafic igneous complexes such as the Stillwater or the Bushveld. However, under the conditions where the PGE are most volatile, the metals Fe, Ni and Cu are several factors often more volatile, so that enrichment of the PGE and Cu over Ni and Fe cannot be explained by chloride transport alone.