Platinum Group Element Tenors Research Articles

PGE geochemical and Os-S-Sr-Nd isotopic constraints on the genesis of the Shitoukengde magmatic sulfide deposit in the East Kunlun Orogenic Belt, NW China

Numerous early Paleozoic mafic–ultramafic intrusions that host important Ni-Cu sulfide mineralization occur in the northern part of the Qinghai-Tibet Plateau, NW China. The sulfide-bearing Shitoukengde Ni-Cu deposit, which is hosted in a ∼424 Ma mafic–ultramafic intrusion, is one of the most represent magmatic sulfide deposits in the East Kunlun Orogenic Belt. The intrusion is composed of a large gabbroic body, cut by lherzolite, olivine websterite, websterite, and minor dunite. Disseminated and net-textured sulfide mineralization occurs as lenses in the center and towards the bottom of the ultramafic bodies. The sulfide mineralization has variable sulfur contents of 0.3 to 13.6 wt% with Ni grade of 0.41 wt% on average, and low platinum-group element (PGE) concentrations (<8.2 ppb Ru and <11.9 ppb Pd). Pd and Ru tenors of bulk sulfides in the deposit are characterized by a positive correlation, indicating that PGE tenor variations are mainly controlled by varying R-factors (magma/sulfide mass ratio). The estimated PGE concentrations in the Shitoukengde magma are about three orders of magnitude lower than those in continental tholeiites. The significant PGE depletions in the Shitoukengde deposit may be due to sulfide removal in the magma conduit at depth. Our new sulfur isotope data reported range in δ34S values from −5.0 to 4.9‰ for the sulfide minerals of the Shitoukengde deposit, clearly distinct from those of mantle-derived sulfur (0 ± 2‰) and immediate country rocks (1.1 to 2.0‰). This, together with the modeling for S isotope variation as a function of R-factor, suggest that the current δ34S range is attributed to the addition of external sulfides with anomalous δ34S into the magma at depth, followed by progressive modification to crustal signature by uncontaminated pulses of magma. The ultramafic rocks exhibit low εNd(t) values (−4.71 to 2.12) and high (87Sr/86Sr)i ratios (0.7073 to 0.7130), which are more enriched relative to the isotopic compositions of depleted mantle. Mixing calculations indicate that about 15 to 20 wt% of lower crustal contamination plus variable upper crustal contamination is required to explain the variations of Sr-Nd isotopes in the Shitoukengde intrusion. Significant crustal contamination is also supported by the mineralization at Shitoukengde having high (187Os/188Os)i (0.24 to 4.51) and γOs(t) values (0.24 to 2534) that are comparable to or higher than those of the important Ni-Cu deposits worldwide. Current results provide evidence that assimilation of crustal sulfur at deep crust and siliceous material with variable crustal contamination during magma ascent and emplacement played critical roles in the attainment of sulfide saturation in the parental magma and the formation of the Shitoukengde deposit.

Quantitative assessment of the relative roles of sulfide liquid collection, magmatic degassing and fluid-mediated concentration of PGE in low-sulfide ores of the Norilsk intrusions

Low-sulfide ores in the upper marginal series of the Norilsk intrusions are distinct from the main massive and disseminated sulfide orebodies in a high platinum-group element (PGE) tenor up to 1,300–1,700 ppm PGE + Au in 100 % sulfide, lower Cu/Pd of ∼100–300 and increased Pt/Pd of ∼0.2–2. These features have previously been considered either as a result of collection by sulfide liquid at extremely high R-factor (silicate to sulfide melt mass ratio) or postmagmatic sulfide removal, although the decoupling between PGE grade and PGE tenor may suggest an additional process of PGE concentration in addition to partitioning into immiscible sulfide liquid. In this study, the distribution of diverse platinum-group minerals (PGM) and their spatial associations with sulfide-silicate assemblages were investigated by automated mineralogy techniques to recognize the mechanism of the PGE concentration. Mineral assemblages were studied from 2D surfaces of polished sections and mineral separate blocks using the Mineral Liberation Analysis and TESCAN Integrated Mineral Analysis in combination with X-ray microtomography and lab- and synchrotron-based microbeam X-ray fluorescence mapping collected over slabs and thick sections, representative of the 3D sub-surficial layer.Palladium minerals (mostly stibnides, bismuthides, tellurides and arsenides) prevail over Pt minerals in all three intrusions (Norilsk 1, Talnakh and Kharaelakh) based on ∼6,600 identified PGM grain statistics from both thin sections and heavy mineral separates. Among Pt minerals, sperrylite and Pt-Fe alloys occur in an equal proportion in Norilsk 1 whereas the alloys are exceptionally rare in Talnakh-Kharaelakh low-sulfide ores. The PGM grains are predominantly locked within secondary silicates with a percentage of their mutual boundaries of ∼50 rel. % in consistency with a high abundance (35–90 area %) of secondary silicates in mineralized rocks. No correlation between the proportions of secondary silicates and PGMs as well as with the degree of their spatial affinity was found. The preferred confinement of PGM grains to the disseminated chromite schlieren is observed in the studied samples, although, among PGMs, only Pt sulfides demonstrate significant spatial affinity (15–34 rel. % mutual boundary) intergrowing with chromite. The spatial association between PGM and base-metal sulfides (∼20 rel. %) is notably higher than the proportion of sulfides in rocks (1.8–2.8 area %) that is interpreted as a quantitative proof of the cogenetic link between PGM and sulfides. However, the significant mm-scale detachment of the PGM grains from the sulfide blebs is persistent throughout the mineralized sections.Element mapping techniques reveal widespread textures of early magmatic degassing (such as amygdules, segregation vesicles, fluid caps and halos), the common tight intergrowths of PGM with volatile-rich minerals and the presence of PGM grains nucleating on the former gas bubble walls. These features are all distinct from the textures of postmagmatic hydrothermal-metasomatic alteration. We propose that an initial composition of sulfide liquid was modified due to S partitioning into a fluid phase during early magmatic degassing. Therefore, the high PGE tenor is not solely a result of sulfide-silicate exchange but was likely enhanced by early magmatic S loss during degassing and PGE fluid transport.

Redox-controlled chalcophile element geochemistry of the Polaris Alaskan-type mafic-ultramafic complex, British Columbia, Canada

ABSTRACT The Early Jurassic Polaris Alaskan-type intrusion in the Quesnel accreted arc terrane of the North American Cordillera is a zoned, mafic-ultramafic intrusive body that contains two main styles of magmatic mineralization of petrologic and potential economic significance: (1) chromitite-associated platinum group element (PGE) mineralization hosted by dunite (±wehrlite); and (2) sulfide-associated Cu-PGE-Au mineralization hosted by olivine (±magnetite) clinopyroxenite, hornblendite, and gabbro-diorite. Dunite-hosted PGE mineralization is spatially associated with thin discontinuous layers and schlieren of chromitite and chromitiferous dunite and is characterized by marked enrichments in iridium-subgroup PGE (IPGE) relative to palladium-subgroup PGE (PPGE). Discrete grains of platinum group minerals (PGM) are exceedingly rare, and the bulk of the PGE are inferred to reside in solid solution within chromite±olivine. The absence of Pt-Fe alloys in dunite of the Polaris intrusion is atypical, as Pt-enrichment of dunite-hosted chromitite is widely regarded as a characteristic feature of Alaskan-type intrusions. This discrepancy appears to be consistent with the strong positive dependence of Pt solubility on the oxidation state of sulfide-undersaturated magmas. Through comparison with experimentally determined PGE solubilities, we infer that the earliest (highest temperature) olivine-chromite cumulates of the Polaris intrusion crystallized from a strongly oxidized ultramafic parental magma with an estimated log f(O2) &amp;gt; FMQ+2. Parental magmas with oxygen fugacities more typical of volcanic arc settings [log f(O2) ∼ FMQ to ∼ FMQ+2] are, in turn, considered more favorable for co-precipitation of Pt-Fe alloys with olivine and chromite. More evolved clinopyroxene- and hornblende-rich cumulates of the Polaris intrusion contain low abundances of disseminated magmatic sulfides, consisting of pyrrhotite and chalcopyrite with minor pentlandite, pyrite, and rare bornite (≤12 wt.% total sulfides), which occur interstitially or as polyphase inclusions in silicates and oxides. The sulfide-bearing rocks are characterized by strong primitive mantle-normalized depletions in IPGE and enrichments in Cu-PPGE-Au, patterns that resemble those of other Alaskan-type intrusions and primitive arc lavas. The absolute abundances and sulfur-normalized whole-rock concentrations (Ci/S, serving as proxy for sulfide metal tenor) of chalcophile elements, including Cu/S, in sulfide-bearing rocks are highest in olivine clinopyroxenite. Sulfide saturation in the relatively evolved magmas of the Polaris intrusion, and Alaskan-type intrusions in general, appears to be intimately tied to the appearance of magnetite. Fractional crystallization of magnetite during the formation of olivine clinopyroxenite at Polaris resulted in reduction of the residual magma to log f(O2) ≤ FMQ+2, leading to segregation of an immiscible sulfide melt with high Cu/Fe and Cu/S, and high PGE and Au tenors. Continued fractionation resulted in sulfide melts that were progressively more depleted in precious and base chalcophile metals. The two styles of PGE mineralization in the Polaris Alaskan-type intrusion are interpreted to reflect the evolution of strongly oxidized, hydrous ultramafic parental magma(s) through intrinsic magmatic fractionation processes that potentially promote sulfide saturation in the absence of wallrock assimilation.

UPGRADING OF MAGMATIC SULFIDES, REVISITED

Abstract There has been vigorous debate for several decades about whether the extreme enrichments of platinum group elements (PGEs) in some magmatic sulfide deposits could have resulted from simple equilibration of sulfide liquid with silicate melt. Key examples include the Ni-Cu-Pd mineralization in the Norilsk mining camp, the UG2 and Merensky reef Pt-Pd deposits in the Bushveld Complex, the Pd-rich J-M reef of the Stillwater Complex, and the Skaergaard Pd-Au mineralization. It was argued historically that the observed PGE tenors in these latter deposits are too high to be consistent with simple equilibration of sulfide and silicate melt. A commonly cited mechanism for increasing PGE tenor in magmatic sulfide is the upgrading of initially low tenor sulfide by allowing a small volume of sulfide to react with successive batches of fresh, previously undepleted silicate magma. Here we review several previous models for sulfide upgrading in light of recent changes in accepted values of the partition coefficients governing PGE exchange between sulfide and silicate, and we critically examine the physical scenarios implicit in each previous model. We show that, although sulfide upgrading may occur in natural settings such as fractional melting of the mantle, during the formation of sulfide accumulations from magmas it is unlikely to have effects that can be distinguished from simple one-stage batch equilibration. Even the most PGE-rich deposits currently known have compositions that can easily be accounted for by the simple one-stage batch process, with the possible exception of the Skaergaard Pd mineralization. It is generally not possible to use the measured composition of accumulations of magmatic sulfide to infer that sulfide upgrading has or has not occurred.

Low-Sulfide Platinum Group Element Ores of the Norilsk-Talnakh Camp

Abstract Low-sulfide platinum group element (PGE) mineralization of the Norilsk-type intrusions is located within the Upper Gabbroic Series, which comprises rocks heterogeneous in texture and composition. The highest grade of 10 to 50 g/t PGEs is confined primarily to chromitiferous taxitic gabbrodolerite, which forms irregular lens- and vein-like bodies that interfinger with contact gabbrodolerite, intrusion breccia, leucogabbro, and gabbrodolerite variably enriched in olivine, from olivine free up to picritic compositions. The abundant amygdules and pegmatoidal textures in Upper Gabbroic Series taxitic rocks, as well as the high enrichment of halogen in minerals (e.g., ≤4.6 wt % Cl in apatite), indicate a higher volatile content of the local magma compared to the magma that precipitated the Main Series. The observed diversity in spinel compositions, which evolve from chromite to Cr magnetite as well as toward hercynite, titanomagnetite, and ulvöspinel, is also indicative of crystallization from a fluid-saturated mush that subsequently reacted, to varying degrees, with contaminated trapped melt and immiscible fluid. The high PGE/S ratio is a primary feature of this mineralization style, albeit the ratio partly increased during sulfide replacement and resorption. The PGE tenor of bulk sulfides calculated as ΣPGE (g/t) in 100% sulfides exceeds 160 and may reach up to 1,400 to 2,500 in low-S ores (0.2–3 wt % S), whereas the value does not exceed 42 in the Talnakh disseminated ore and ranges from 35 to 120 in the Norilsk disseminated ore (1–10 wt % S). Several PGE peaks in the vertical sections correlate well with Cu, Ni, S, and Cr peaks, as well as with observed elevated proportion of amygdules. Low-sulfide ores are composed of two primary sulfide assemblages of pyrrhotite + pentlandite + chalcopyrite and pentlandite + pyrrhotite. The primary sulfides are depleted in the heavier 34S isotope relative to sulfides of the corresponded main orebodies (e.g., mean δ34S = 8.9‰ versus δ34S = 12.3‰, respectively, in the Kharaelakh intrusion). A secondary pyrite + millerite + chalcopyrite assemblage has isotope composition enriched in 34S by 2 to 6‰ δ34S with respect to primary sulfides. The directly measured PGE content in sulfides (e.g., 11–2,274 g/t Pd in pentlandite and 0.10–33.3 g/t Rh in pyrrhotite) is within the range of the typical Norilsk-type magmatic sulfide compositions. The textural setting and diversity of platinum group minerals (PGMs) favor the hypothesis of fluid-controlled crystallization. However, the distinct PGM assemblages in Norilsk 1 and Talnakh-Kharaelakh low-sulfide ores are comparable with those of the corresponding presumably magmatic disseminated and massive orebodies. The most remarkable characteristic is the widespread Pt-Fe alloys in Norilsk 1 and their absence in Talnakh-Kharaelakh, which is interpreted to reflect better preservation of the high-temperature PGMs in Norilsk 1 in contrast to their substantial replacement in more oxidized fluid-enriched environments in Talnakh-Kharaelakh.

Sulfide melt inclusions associated with magmatic Ni-Cu-platinum-group element (PGE) mineralization in the Caribou Lake Gabbro, Blatchford Lake intrusive suite, Northwest Territories, Canada

Magmatic Fe-Ni-Cu-Co sulfide melt inclusions are preserved within cumulate and intercumulate phases (apatite, plagioclase, olivine, ilmenite) in the mineralized Caribou Lake Gabbro, a marginal subalkaline-alkaline mafic-ultramafic rock assemblage that forms part of the multiphase Blatchford Lake intrusive suite, Northwest Territories, Canada. Three distinct styles of sulfide mineralization occur in the Caribou Lake Gabbro: (i) disseminated sulfides, (ii) semi-massive to massive sulfides, and (iii) trace sulfide “platinum-group element (PGE)-enriched” mineralization. Within all mineralization styles, the major mineral assemblage consists of pyrrhotite + chalcopyrite + pentlandite + magnetite + ilmenite. Trace metal phases identified in the disseminated and semi-massive to massive mineralization comprise sphalerite, galena, glaucodot-gersdorffite solid solution [(Co,Fe)AsS-NiAsS], molybdenite, tellurobismuthite (Bi2Te3), altaite (PbTe), electrum, nickeline, melonite (NiTe2), empressite (AgTe), and unidentified Pb-Se-Te, Bi-Te, and Pb-Te phases. No platinum-group minerals (PGM) were observed in any mineralization styles. Within coarse-grained olivine gabbro sulfide melt inclusions of primary origin occur in olivine and ilmenite, and secondary inclusions occur in plagioclase. Secondary sulfide melt inclusions also occur in cumulus apatite in pegmatitic gabbro. Sulfide melt inclusions consist predominantly of pyrrhotite, with lesser amounts and variable proportions of chalcopyrite, and cobaltian pentlandite (∼31–38 wt% Co), and magnetite. In a minority of inclusions, sphalerite, magnetite, and glaucodot-gersdorffite were also observed however, no PGM were observed in the sulfide melt inclusions.The metal tenors of sulfide melt inclusions and sulfide mineralization are comparable with respect to Cu concentration, with sulfide melt inclusions containing between 0.06 and 2.7 wt%, and mineralization containing between 0.54 and 2.9 wt% (recalculated to 100% sulfide). Cobalt concentrations are also similar in sulfide melt inclusions (950–4400 ppm) and mineralization (1350–3500 ppm). In contrast to Cu and Co, Ni is enriched in mineralization (0.94–2.1 wt%) compared to sulfide melt inclusions (0.13–0.59 wt%). The difference in Ni concentration suggests that the inclusions trapped a residual sulfide melt that had been compositionally modified by olivine crystallization and associated “trapped liquid shift effect” prior to inclusion entrapment. Notably, PGE concentrations in sulfide melt inclusions are very low (i.e., below limits of detection of LA-ICPMS; < ∼4 ppm), consistent with the observation of low PGE grades in the associated mineralization styles, and 1–2 orders of magnitude lower than in sulfide melt inclusions reported in economically mineralized layered mafic-ultramafic intrusions.Using Cu and Pd bulk rock compositions for various mineralization styles, together with estimates of starting magma composition (S-undepleted tholeiite, and local, fine-grained, fresh gabbro), R-factors were estimated numerically. S-undepleted tholeiite (from published data) is too Cu- and Pd-rich to be a suitable starting magma composition to source disseminated and semi-massive to massive sulfide, and would require an initial (earlier) stage of metal depletion. Using the local gabbro as a starting magma composition estimate, the metal tenors of disseminated, and semi-massive to massive sulfide mineralization are estimated to have resulted from an R-factor of <100. On the other hand, trace sulfide “PGE-enriched” mineralization could have formed from a S-undepleted tholeiite at an R-factor of at least 1000. It does not appear likely that all mineralization styles in the Caribou Lake Gabbro formed from the same starting silicate melt composition.Low metal tenors of sulfide mineralization within the Caribou Lake Gabbro are likely the result of a magmatic system with a low R-factor, supported by low Ni in olivine and modelling of Cu and Pd systematics. In this case, sulfide melt inclusions predict the sub-economic PGE tenor of the sulfide mineralization as being a primary feature rather than the result of post-solidus downgrading by later metamorphic or hydrothermal fluids. More broadly, sulfide melt inclusions may serve as a proxy for sulfide metal tenor at different stages in the history of a magmatic system, providing a potentially valuable tool for exploration where post-entrapment changes may have modified bulk metal tenors, or where sulfide mineralization has not yet been located.

Formation of the Lubei magmatic Ni–Cu deposit in a post-subduction setting in East Tianshan, North West China

The Lubei Ni–Cu sulfide ore deposit with 7.8 Mt sulfide ores at an average grade of 0.88 wt% Ni and 0.5 wt% Cu, is associated with a mafic–ultramafic intrusion in the western part of the East Tianshan in Xinjiang Uygur Autonomous Region, NW China. The primary mineralization is hosted by ultramafic rocks which are part of a 2.1 km2 mafic–ultramafic intrusion located within the Kanggurtag fault zone. In this paper, we intend to probe the ore-formation processes for the Lubei Ni–Cu sulfide ore deposit and discuss key factors that control the ore formation processes in a post-subduction setting. The Lubei primary magmas were derived from 13 to 17 % partial melting of a hydrous mantle containing ∼0.29 wt% H2O estimated using the silicate-liquid model pMELTS. The Lubei sulfide ores are characterized by low platinum group elements (PGE) and high mantle-normalized Pd/Ir ratios, and disseminated sulfide ores have higher Cu, Ni and PGE tenors than massive and vein sulfide ores indicative of crystal fractionation of Fe-rich monosulfide solid solution. The PGE-depleted Lubei sulfides possibly resulted from early sulfide segregation from the Lubei primary magmas as indicated by high Cu/Pd ratios (up to 106), and the proportion of early sulfide losses was 0.02 wt% which has depleted the Lubei parental magmas and later fractionated sulfides in chalcophile elements. This depletion would been compensated at different levels by later sulfide–magma interaction when the parental magmas reached in situ sulfide saturation, with an R factor (mass ratio of silicate to sulfide liquids) of 100–1000. The formation of the observed Lubei disseminated sulfide ores was associated with 5–15 vol% magma crystallization with the main silicate minerals being spinel and olivine, as modeled using the silicate-liquid model MELTS. The δ34S values of the Lubei sulfides vary over a narrow range of −0.3 to +1.8‰, indicating a mantle origin for the sulfur. The key factor that controls the formation of magmatic Ni–Cu deposits in post-subduction settings is a metasomatized mantle accompanied by activation of translithospheric faults during post-collisional extension. This mechanism alternatively interprets the widespread magmatic Ni–Cu deposits in the southern Central Asian Orogenic Belt and implies translithospheric fault as a potential target for Ni–Cu exploration.

Zonation of Merensky-Style Platinum-Group Element Mineralization in Turfspruit Thick Reef Facies (Northern Limb of the Bushveld Complex)*

Research Article| September 01, 2017 Zonation of Merensky-Style Platinum-Group Element Mineralization in Turfspruit Thick Reef Facies (Northern Limb of the Bushveld Complex)* Marina A. Yudovskaya; Marina A. Yudovskaya † 1Economic Geology Research Institute, School of Geosciences, University of Witwatersrand, Wits 2050, South Africa2Institute of Geology of Ore Deposits, Mineralogy, Petrography and Geochemistry RAS, Staromonetny 35, Moscow 119017, Russia †Corresponding author: e-mail, marina.yudovskaya@wits.ac.za. Search for other works by this author on: GSW Google Scholar Judith A. Kinnaird; Judith A. Kinnaird 1Economic Geology Research Institute, School of Geosciences, University of Witwatersrand, Wits 2050, South Africa Search for other works by this author on: GSW Google Scholar Danie F. Grobler; Danie F. Grobler 3Exploration & Geology, Ivanplats (Pty.) Ltd., Mokopane, South Africa Search for other works by this author on: GSW Google Scholar Gelu Costin; Gelu Costin 4Department of Earth Science, Rice University, 6100 Main Street, Houston, Texas 77005 Search for other works by this author on: GSW Google Scholar Vera D. Abramova; Vera D. Abramova 2Institute of Geology of Ore Deposits, Mineralogy, Petrography and Geochemistry RAS, Staromonetny 35, Moscow 119017, Russia Search for other works by this author on: GSW Google Scholar Tim Dunnett; Tim Dunnett 3Exploration & Geology, Ivanplats (Pty.) Ltd., Mokopane, South Africa Search for other works by this author on: GSW Google Scholar Sarah-Jane Barnes Sarah-Jane Barnes 5Sciences de la Terre, Université du Québec à Chicoutimi, Québec G7H 2B1, Canada Search for other works by this author on: GSW Google Scholar Author and Article Information Marina A. Yudovskaya † 1Economic Geology Research Institute, School of Geosciences, University of Witwatersrand, Wits 2050, South Africa2Institute of Geology of Ore Deposits, Mineralogy, Petrography and Geochemistry RAS, Staromonetny 35, Moscow 119017, Russia Judith A. Kinnaird 1Economic Geology Research Institute, School of Geosciences, University of Witwatersrand, Wits 2050, South Africa Danie F. Grobler 3Exploration & Geology, Ivanplats (Pty.) Ltd., Mokopane, South Africa Gelu Costin 4Department of Earth Science, Rice University, 6100 Main Street, Houston, Texas 77005 Vera D. Abramova 2Institute of Geology of Ore Deposits, Mineralogy, Petrography and Geochemistry RAS, Staromonetny 35, Moscow 119017, Russia Tim Dunnett 3Exploration & Geology, Ivanplats (Pty.) Ltd., Mokopane, South Africa Sarah-Jane Barnes 5Sciences de la Terre, Université du Québec à Chicoutimi, Québec G7H 2B1, Canada †Corresponding author: e-mail, marina.yudovskaya@wits.ac.za. Publisher: Society of Economic Geologists Accepted: 20 Apr 2017 First Online: 25 Aug 2017 Online Issn: 1554-0774 Print Issn: 0361-0128 © 2017 Society of Economic Geologists.Society of Economic Geologists Economic Geology (2017) 112 (6): 1333–1365. https://doi.org/10.5382/econgeo.2017.4512 Article history Accepted: 20 Apr 2017 First Online: 25 Aug 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Marina A. Yudovskaya, Judith A. Kinnaird, Danie F. Grobler, Gelu Costin, Vera D. Abramova, Tim Dunnett, Sarah-Jane Barnes; Zonation of Merensky-Style Platinum-Group Element Mineralization in Turfspruit Thick Reef Facies (Northern Limb of the Bushveld Complex). Economic Geology 2017;; 112 (6): 1333–1365. doi: https://doi.org/10.5382/econgeo.2017.4512 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyEconomic Geology Search Advanced Search Abstract The stratigraphy, whole-rock composition, and mineralogy from a thick undisturbed section of the Platreef at depth on Turfspruit in the northern limb of the Bushveld Complex have been examined in detail. This section has much more in common with the Critical zone than with the Platreef structure updip although the high-grade platinum-group element (PGE) reefs, located close to the boundary with the Main zone, are much thicker than any known Merensky reef facies. A key feature that marks the conversion of the Platreefstyle mineralization into the Merensky style is the regular appearance of chromite seams, which become more persistent with decreasing contamination westward and downdip. The similar zonal distribution of platinum-group minerals (PGM) in two consecutive thick PGE reefs correlates with changes in silicate compositions and vertical zonation of PGE concentrations in whole rocks and base metal sulfides. This supports orthomagmatic in situ crystallization of ore minerals during mixing of consecutive magmatic influxes. The typical Merensky-style ore assemblages of predominant Pt sulfides and alloys are developed within ~1-m-thick intervals, which correspond to the highest temperature mixing zones between two magmas, whereas the rest of the reefs host Pt-Pd bismuthotellurides and arsenides. PGE content and tenor, as well as Cu/Pd in major base metal sulfides in the Turfspruit reefs, are within the range of the typical high-grade Merensky values, suggesting the same enrichment process. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Genesis of the Huangshannan high-Ni tenor magmatic sulfide deposit in the Eastern Tianshan, northwest China: Constraints from PGE geochemistry and Os–S isotopes

The Huangshannan magmatic Ni-Cu sulfide deposit is one of a group of Permian magmatic Ni-Cu deposits located in the southern Central Asian Orogenic belt in the Eastern Tianshan, northwest China. It is characterized by elevated Ni tenor (concentrations in recalculated 100% sulfide) in sulfide within ultramafic rocks (9–19wt%), with values much higher than other deposits in the region. Sulfides of the Huangshannan deposit are composed of pentlandite, chalcopyrite, and pyrrhotite and the host rock is relatively fresh, indicating that the high-Ni tenor is a primary magmatic feature rather than formed by alteration processes. It is shown that sulfides with high-Ni tenor can be generated by sulfide-olivine equilibrium at an oxygen fugacity of QFM +0.5, for magmas containing 450ppm Ni and 20% olivine. Ores with >10wt% sulfur have relatively low PGE and Ni tenors compared to other ores, R factor (mass ratio of silicate to sulfide liquid) modeling of Ni indicates that they formed at moderate R values (150–600). Based on this constraint on R values, ores with <10wt% sulfides in the Huangshannan deposit can be segregated from a similar parental magma with 0.05ppb Os, 0.023ppb Ir, and 0.5ppb Pd at R values between 600 and 3000. This, coupled with the supra-cotectic proportions of sulfide liquid to cumulus silicates in the Huangshannan ores imply mechanical transport and deposition of sulfide liquid in a magma pathway or conduit, in which sulfides must have interacted with large volumes of silicate magma. Platinum and Pd depletion relative to other platinum group elements (PGEs) are observed in fresh and sulfide-rich samples (S>4.5wt%). As sulfide-rich samples are also depleted in Cu, and as interstitial sulfides in those samples are physically interconnected at a scale of several cms, the low Pt and Pd anomalies are attributed to solid Pt and Pd phases crystallization and retention with the monosulfide solid solution (MSS) and Cu-rich sulfide liquid percolation during MSS fractionation. This finding indicates that Pt anomalies in sulfide-rich rocks from magmatic Ni-Cu deposits in the Eastern Tianshan are the result of sulfide fractionation rather than a hydrothermal effect. 187Os/188Os(278Ma) values of the lherzolite samples vary from 0.27 to 0.37 and γOs(278Ma) values vary from 110 to 189, indicating significant magma interaction with crustal sulfides, rich in radiogenic Os. Well constrained γOs values and δ34S values (−0.4 to 0.8‰) indicate that crustal contamination occurred at depth before the arrival of the magma in the Huangshannan chamber. Regionally, deposits with high-Ni tenor have not been reported other than the Huangshannan deposit; however, many intrusions with high-Ni contents in olivine are present in NW China, such as the Erhongwa, Poyi and Poshi intrusions. Those intrusions are capable of forming high-Ni tenor sulfides due to olivine-sulfide-silicate equilibrium and relative high-Ni content in parent magma, making them attractive exploration targets.

Stratiform platinum-group element mineralization in the layered Northern Ultramafic Center of the Lac des Iles Intrusive Complex, Ontario, Canada

The Northern Ultramafic Centre (NUC) of the Lac des Iles Complex, Northwest Ontario hosts several platinum group element (PGE) occurrences, including the Sutcliffe Zone, which consists of four subparallel, stratiform PGE-enriched intervals exposed within the cyclically layered eastern flank of the NUC. Field relationships, mineral paragenesis and lithogeochemistry allowed for the identification of 14 cyclic cumulate sequences of two distinct types – Cyclic unit type A (CUA) and Cyclic unit type B (CUB). CUA-type and CUB-type units are interpreted to have formed from a Si-enriched and Si-poor parent magmas, respectively. PGE-enriched intervals occur in four of the CUA-type cyclic units (CUA-5, -6, -8 and -11). PGE enriched intervals are commonly associated with websterite, olivine websterite and gabbronorite containing primary disseminated sulfide (0.2–2vol%) which are dominated by pyrrhotite, chalcopyrite, and pentlandite with minor cubanite, and troilite. In hydrothermally altered rocks enriched in PGE, primary sulfides are locally partially replaced by secondary chalcopyrite, sphalerite, heazlewoodite, and chalcocite. Palladium occurs either in solid solution with primary pentlandite or is associated with platinum group minerals (PGM) such as Pd-plumbide, Pd-telluride, and Pt-bismuthotelluride. PGMs commonly occur within primary sulfides, at contacts between primary sulfide–silicate minerals, or in association with secondary serpentine and actinolite. Gold and silver typically occur as electrum that exhibits similar textural characteristics and mineralogical associations as the PGMs.Two different chemostratigraphic patterns of PGE, Cu and S enrichment can be recognized among the mineralized CUA cycles: The first (top-loaded) occurs near the top of CUA cycles (CUA-6, -8 and -11) in websterite and/or gabbronorite, just below the levels at which CUB magmas were emplaced. The second (middle-loaded), occurs midway through the lower cycle (CUA-5) in the olivine websterite, which is overlain by CUA-6. Within the four mineralized intervals, PGE tenors average 643ppm Pd+Pt (in 100% sulfide), Pd/Pt and Pd/Ir ratios range from 0.9 to 3.5 and 35 to 537, respectively, and S/Se ratios range between 500 and 6000. The highest PGE tenors (4377ppm Pd+Pt) are found in the lowermost interval in serpentinized olivine websterite and have an average Pd/Pt ratio of 3.5 and a S/Se ratio of approximately 2000.It is proposed that orthomagmatic processes of fractional crystallization and dynamic magma recharge were the dominant mineralization processes triggering sulfide-saturation and PGE concentration at the Sutcliffe Zone. Textural relationships between PGM, sulfide minerals, and primary and secondary hydrous silicates suggest that late magmatic to postcumulus hydrothermal fluid infiltration occurred locally during and after sulfide mineralization of the PGE-enriched intervals. However, these fluids had a minimal effect on the distribution of PGE in the Sutcliffe Zone. The Sutcliffe Zone shares many similarities with classic stratiform PGE deposits in terms of Pd/Pt ratio, high PGE tenors, low abundance of sulfide, and PGM assemblages. However, it is distinguished from most stratiform PGE deposits by its tectonic environment and lithostratigraphic position and by the intimate spatial association of the two parental magmas that are interpreted to have been responsible for the observed chemostratigraphy and PGE enrichment.

Geochronological, Petrological, and Geochemical Constraints on Ni-Cu Sulfide Mineralization in the Poyi Ultramafic-Troctolitic Intrusion in the Northeast Rim of the Tarim Craton, Western China

The Poyi magmatic Ni-Cu sulfide deposit is situated in the Beishan fold belt in the northeastern rim of the Tarim craton. Many Permian magmatic Ni-Cu sulfide deposits, such as those in East Tianshan, are present in the adjacent Central Asian orogenic belt to the north. The Poyi deposit is hosted in a small dike-like ultramafic-troctolitic body that was emplaced into a much larger gabbroic intrusion. Our new zircon U-Pb isotope data reveal that these two intrusions formed ~6 Ma apart. The ultramafic-troctolitic intrusion was emplaced at 269.9 ± 1.7 Ma, whereas the gabbroic intrusion was emplaced at 276.1 ± 1.9 Ma. The results show that the Poyi deposit is the youngest among the major magmatic Ni-Cu sulfide deposits (≥0.2 Mt Ni) of Permian ages in the Beishan-Tianshan region. Sulfide mineralization in the Poyi deposit occurs as steeply dipping disseminated sulfide lenses mostly associated with wehrlites in the center of the dike. Olivine from the Poyi ultramafic rocks has Fo content up to 91 mol %, which is similar to the lower limit of mantle olivine and the most primitive within the Permian Beishan-Tianshan nickel belt. Like other magmatic sulfide deposits in this belt, olivine from the Poyi deposit is depleted in Ca (<1,000 ppm). The estimated parental magma for the Poyi most primitive ultramafic rocks contains 15 ± 2 wt % MgO. Cotectic olivine-sulfide segregation from the Poyi magma is inferred from systematic variation of Fo-Ni contents in olivine from some sulfide-barren ultramafic rock samples, and supported by the occurrence of sulfide droplets as small inclusions in olivine (Fo90 mol %) in these rocks. The involvement of multiple pulses of sulfide-charged magma with different compositions is indicated by the abrupt change of olivine Fo content with depth and the presence of olivine with similar Fo contents but dramatically different Ni abundances in the different parts of the deposit. The (87Sr/86Sr)i ratios and ɛ Nd(total) of the Poyi ultramafic rocks and troctolites range from 0.7042 to 0.7052 and from 4.9 to 6.0, respectively, which are close to the isotope compositions of depleted mantle and within the ranges of major magmatic Ni-Cu sulfide deposits of Permian ages in East Tianshan. No more than 5 wt % of bulk crustal contamination is required to explain the variations of Sr-Nd isotopes in the Poyi ultramafic-troctolitic intrusion. The abundances of incompatible trace elements in whole rocks and clinopyroxene crystals indicate very weak light REE enrichments coupled by significant negative Nb-Ta anomalies in the parental melts. Bulk sulfides in the Poyi deposit are characterized by positive correlations between any pair of platinum-group elements (PGE), indicating that PGE tenor variations in the deposit are mainly controlled by variable R-factors (magma/sulfide mass ratios). The estimated initial concentrations of PGE in the parental magma for the Poyi deposit are almost two orders of magnitude lower than the abundances of PGE in some continental picrites. Given that the parental magma for the Poyi deposit is as primitive as a primary mantle-derived magma, the depletion of PGE in the Poyi deposit is most likely due to previous sulfide segregation at depth. Based on these observations, we conclude that sulfide saturation in the Poyi PGE-depleted, Mg-rich magma was triggered by addition of crustal sulfur during magma ascent and that the Poyi deposit was a dynamic conduit used by multiple pulses of olivine- and sulfide-charged magma.

PGE distribution in sulfide ores from ultramafic massifs of the central East Sayan Mountains, Southern Siberia, Russia

Data on the composition of sulfide ores from ultramafic massifs in the central East Sayan Mountains and on the regularities of platinum group elements (PGE) in these ores are presented. It is found that the highest PGE contents are characteristic for net-textured and massive ores from the Zhelos massif: total PGE content there is up to 15 ppm, with Pd/Pt = 3–8, for Ni and Cu contents of 1.5–2.8 and 0.5–2.7 wt%, respectively. In the disseminated ores of the Zhelos massif, PGE contents vary from 1 to 7 ppm, at Ni and Cu contents varying in the ranges of 0.5–1.0 and 0.2–0.4 wt %, respectively. In the Tokty-Oi massif, disseminated ores are characterized by higher absolute PGE contents (1.6 to 3.3 ppm) at similar Ni content. PGE tenor of disseminated ores is higher compared to that of massive and net-textured ones. In the cross-sections of both massifs, net-textured and massive ores of an essentially pyrrhotine composition are found at the contact between ultramafic and host rocks. Total PGE in these ores is up to 12 ppm. The obtained data on sulfur isotopes indicate the common, well-homogenized sources, and close physical–chemical depositional conditions of all ore types.

High tenor Ni–PGE sulfide mineralization in the South Manasan ultramafic intrusion, Paleoproterozoic Thompson Nickel Belt, Manitoba, Canada

The South Manasan ultramafic intrusion, located in the Early Proterozoic Thompson Nickel Belt (TNB), contains Ni and platinum group element (PGE) mineralization in the form of disseminated sulfide. Whole-rock Ni values range from 0.3 to 1.7wt.% and total precious metals range from 0 to 1.3ppm Pt+Pd+Au. In 100% sulfide the concentration of Ni ranges from 11 to 39wt.%, and the concentration of Pt+Pd+Au ranges from 8 to 27ppm. The intrusion is a steeply dipping, boudinaged, sill-like body with a strike length of approximately 1200m, an average width of 125m, and a depth extent of >1000m. The intrusion is composed of ~25% fresh dunite, ~50% serpentine-altered dunite and ~25% tectonized and carbonate altered dunite. The most intense alteration is found near the margin of the intrusion where it is in contact with Pipe Formation metasedimentary rocks of the Ospwagan Group. The sulfide assemblage in fresh dunite is interstitial between cumulus olivine and is composed predominantly of pentlandite with accessory pyrite with symplectic intergrowths of the two same sulfides. The interstitial sulfide assemblage displays a primary magmatic position between olivine grains, but the sulfides are penetrated by platy intergrowths of chlorite and serpentine. In contrast, the most intensely carbonate altered dunite contains irregular patches of pyrrhotite and pentlandite.Representative drill core samples through the South Manasan Intrusion have been used to establish variations in whole rock geochemistry and mineralogy; these variations coupled with the stratigraphy of the host sedimentary rocks are consistent with magmatic differentiation in a sill-like body which was serpentinized and then tilted upright such that tops now face west. Intrusion of the original komatiitic magma and assimilation of sulfur from Ospwagan Group sedimentary rocks triggered sulfide saturation and formation of an immiscible sulfide melt. The early crystallization of olivine inhibited the sulfide melt from settling to the bottom of the magma column and, as a consequence, the sulfides now have a primary interstitial magmatic texture. Calculated Ni and PGE tenor values indicate an R factor of 500–2500. The current sulfide association dominated by pentlandite≫pyrite>chalcopyrite has a mineral paragenesis that is consistent with crystallization from a monosulfide solid solution which cooled down to pentlandite and pyrite. The subsequent processes of serpentinization, deformation and carbonate alteration modified the primary sulfide assemblages to pentlandite and pyrrhotite. The alteration of the dunite and serpentine to talc-carbonate occurred by open-system hydrothermal processes that produced a reduction in metal tenor through a dilution process accompanying the addition of carbonate and water.It is proposed that the enrichment of the sulfide assemblage at South Manasan in Ni and PGEs is a consequence of a primary magmatic process involving a high-R factor and that the effects of later overprinting processes (alteration and deformation) are not responsible for the high-tenor sulfide association. The study has important exploration implications, as it helps to establish the characteristics of a new type of Ni–PGE mineralization in the TNB, and thus provides a basis on which to better understand the formation of other low-grade ultramafic-hosted nickel deposits.

Controls on disseminated PGE–Cu–Ni sulfide mineralization within the Rietfontein deposit, Eastern Limb, Bushveld Complex, South Africa: Implications for the formation of contact-type magmatic sulfide deposits

The Rietfontein platinum group element (PGE)–Cu–Ni sulfide deposit of the Eastern Limb of the Bushveld Complex hosts disseminated contact-style mineralization that is similar to other economic magmatic sulfide deposits in marginal settings within the complex. The mineralization at Rietfontein consists of disseminated PGE-bearing base metal sulfides that are preferentially located at the contact between a distinct package of marginal norites overlain by a thick heterogeneous unit dominated by gabbronorites with lesser norites and ultramafic rocks. Down-hole composite data and metal scatterplots indicate that the PGE correlate well with Ni, Cu and S and that only minor metal remobilization has taken place within the basal norite sequence. Plots of (Nb/Th)PM vs. (Th/Yb)PM indicate that the melts that formed the Rietfontein intrusive sequence were strongly crustally contaminated prior to emplacement at Rietfontein, whereas inverse relationships between PGE tenors and S/Se ratios indicate that these magmas assimilated crustal S, causing S-saturation and the formation of immiscible sulfides under high R-factor conditions that generated high PGE tenor sulfides. Reverse zoning of cumulus minerals at Rietfontein suggests that fresh primitive melts were introduced to a partially fractionated staging chamber. The introduction of new magmas into the chamber caused overpressure and the forced evacuation of the contents of the chamber, leading to the emplacement of the existing magmas within the staging chamber at Rietfontein in two separate pulses. The first pulse of magma contained late-formed cumulus phases, including low Mg# orthopyroxene and plagioclase, was emplaced between footwall unreactive and S-poor Pretoria Group quartzites and a hangingwall sequence of Rooiberg Group felsites, and was rapidly chilled to form the basal norite sequence at Rietfontein. The second pulse of magma contained early formed cumulus phases, including olivine, chromite, and high Mg# orthopyroxene, and was emplaced above the chilled norite sequence as a crystal mush to form gabbronorites and ultramafic rocks. This second pulse of magma also contained PGE-bearing base metal sulfides that accumulated at the contact between this second batch of magma and the already chilled basal norite sequence. The formation of Platreef-type mineralization outside of the Northern Limb of the Bushveld Complex confirms there are a number of areas within the Bushveld Complex that are prospective for this style of mineralization.

Precious and base metal geochemistry and mineralogy of the Grasvally Norite–Pyroxenite–Anorthosite (GNPA) member, northern Bushveld Complex, South Africa: implications for a multistage emplacement

The Grasvally Norite–Pyroxenite–Anorthosite (GNPA) member within the northern limb of the Bushveld Complex is a mineralized, layered package of mafic cumulates developed to the south of the town of Mokopane, at a similar stratigraphic position to the Platreef. The concentration of platinum-group elements (PGE) in base metal sulfides (BMS) has been determined by laser ablation inductively coupled plasma–mass spectrometry. These data, coupled with whole-rock PGE concentrations and a detailed account of the platinum-group mineralogy (PGM), provide an insight into the distribution of PGE and chalcophile elements within the GNPA member, during both primary magmatic and secondary hydrothermal alteration processes. Within the most unaltered sulfides (containing pyrrhotite, pentlandite, and chalcopyrite only), the majority of IPGE, Rh, and some Pd occur in solid solution within pyrrhotite and pentlandite, with an associated Pt–As and Pd–Bi–Te dominated PGM assemblage. These observations in conjunction with the presence of good correlations between all bulk PGE and base metals throughout the GNPA member indicate the presence and subsequent fractionation of a single PGE-rich sulfide liquid, which has not been significantly altered. In places, the primary sulfides have been replaced to varying degrees by a low-temperature assemblage of pyrite, millerite, and chalcopyrite. These sulfides are associated with a PGM assemblage characterized by the presence of Pd antimonides and Pd arsenides, which are indicative of hydrothermal assemblages. The presence of appreciable quantities of IPGE, Pd and Rh within pyrite, and, to a lesser, extent millerite suggests these phases directly inherited PGE contents from the pyrrhotite and pentlandite that they replaced. The replacement of both the sulfides and PGM occurred in situ, thus preserving the originally strong spatial association between PGM and BMS, but altering the mineralogy. Precious metal geochemistry indicates that fluid redistribution of PGE is minimal with only Pd, Au, and Cu being partially remobilized and decoupled from BMS. This is also indicated by the lower concentrations of Pd evident in both pyrite and millerite compared with the pentlandite being replaced. The observations that the GNPA member was mineralized prior to intrusion of the Main Zone and that there was no local footwall control over the development of sulfide mineralization are inconsistent with genetic models involving the in situ development of a sulfide liquid through either depletion of an overlying magma column or in situ contamination of crustal S. We therefore believe that our observations are more compatible with a multistage emplacement model, where preformed PGE-rich sulfides were emplaced into the GNPA member. Such a model explains the development and distribution of a single sulfide liquid throughout the entire 400–800 m thick succession. It is therefore envisaged that the GNPA member formed in a similar manner to its nearest analogue the Platreef. Notable differences however in PGE tenors indicate that the ore-forming process may have differed slightly within the staging chambers that supplied the Platreef and GNPA member.

Platinum-group elements in the oxide layers of the Hongge mafic–ultramafic intrusion, Emeishan Large Igneous Province, SW China

The Hongge layered intrusion (259.3±1.3Ma) is one of several mafic–ultramafic intrusions that host giant Fe–Ti–V oxide ore deposits in the ~260Ma Emeishan Large Igneous Province (ELIP), SW China. The Hongge intrusion consists of a lower olivine clinopyroxenite zone (LOZ), a middle clinopyroxenite zone (MCZ) and an upper gabbro zone (UGZ). Most of the 14 to 84m-thick and 300 to 1700m-long economic Fe–Ti–V oxide ore layers occur within the MCZ. This paper reports the concentrations of PGE in the oxide layers of the Hongge intrusion. Unlike in the economic PGE (platinum-group elements) mineralized (up to 1.2ppm Pt and 1.8ppm Pd) coeval Xinjie intrusion (259±3Ma), the oxide layers in the Hongge intrusion contain very low PGE (total PGE: 0.09–63.5ppb). Chromite-bearing horizons in the Hongge intrusion are enriched in IPGE (Ir: 0.46–0.65ppb; Ru: 2.25–3.29ppb) relative to PPGE (Pt: 0.54–1.28ppb; Pd: 0.30–0.90ppb). In contrast, the massive magnetite layers in the Hongge intrusion show no IPGE enrichments relative to PPGE. All our samples from the Hongge intrusion collectively show no correlation between PGE and S contents, and weak positive correlations between IPGE and Cr contents, indicating removal of Ir, Ru from magma with crystallization of chromite. Positive correlations exist between IPGE and PPGE in the samples, indicating that all of the PGE was controlled by sulfide liquid. Most of the samples have mantle-normalized PGE patterns with a slope similar to that of the Emeishan picritic basalts, which are less fractionated than the coeval high-Ti basalts derived from the same Fe-, Ti-, V-rich magma series. The Hongge oxide-rich samples are characterized by Cu/Pd ratios (1.47–202×104) significantly higher than those for primitive mantle and the coeval picrites (0.99×104). The PGE tenors in bulk sulfides (i.e., in recalculated 100% sulfides) in the sulfide-bearing oxide ores of the Hongge intrusion (<0.1–3ppm) are 2–3 orders of magnitude lower than the Xinjie intrusion (10–100ppm). This, together with extremely high Cu/Pd ratios in both sulfide-bearing and sulfide-barren oxide-rich samples, indicates that the parental magma of the Hongge intrusion was depleted in PGE. We suggest that PGE depletion in the Hongge parental magma was due to previous sulfide segregation at depth, mainly due to crustal contamination. The PGE-depleted sulfides in the Hongge intrusion are thought to have formed by second-stage sulfide saturation and segregation due to fractional crystallization involving abundant magnetite after magma emplacement at Hongge.

Platinum group element and nickel sulphide ore tenors of the Mount Keith nickel deposit, Yilgarn Craton, Australia

A set of platinum group element (PGE) analyses of about 120 samples from a 250-m continuous drill core through the Mount Keith komatiite-hosted nickel orebody, combined with Ni, Cu, Co, S, and major elements, reveals a complex trend of covariance between the original cumulus components of a thick sequence of nearly pure olivine–sulphide liquid adcumulates. The intersection is divided into informal chemostratigraphic zones, defined primarily by combinations of fine-scale cyclicity in original olivine composition, defined by Mg#, and sulphide composition, defined by Pt/S and Ni/S. Contents of Ni and PGE in 100% sulphides (tenors) were determined from linear regressions of the Ni–S and PGE–S covariance for each zone. Inferred olivine compositions range from about Fo92 to Fo94.6 and show a broad decrease from bottom to top of the sequence complicated by numerous reversals, revealing crystallisation in an open conduit system. Ni and PGE tenors of Mount Keith sulphide ores have typical values similar to the type I deposits of the Kambalda Dome. Mobility of S, at least on the scale of 2-m sample composites, is evidently relatively minor. Tenors for the various zones range 12–22% Ni, 370–1540 ppb Pt, 970–3670 ppb Pd, 100–460 ppb Ir, 170–460 ppb Rh, and 710–1260 ppb Ru. Pt, Pd, and Rh tenors are very strongly correlated, but the iridium group of platinum group elements (IPGEs; Ir and Ru) less so. Tenor variations are predominantly controlled by variations in magma/sulphide ratio R (100–350), with a minor component of variance from equilibrium crystallisation trends in the parent magma. PGE depletion in the silicate melt due to sulphide liquid extraction is limited by entrainment of sulphide liquid droplets and continuous equilibration with the transporting silicate magma. Ratios of the PGEs to one another are similar to those in the host komatiite magma, with the exception of Pt, which is systematically depleted in ores, relative to Rh and Pd and relative to host magma, by a consistent factor of about 2 to 2.5. This anomalous Pt depletion relative to PGE element ratios in unmineralized komatiitic rocks matches that observed in bulk compositions of many komatiite-hosted orebodies. The highly consistent nature of this depletion, and particularly the very strong correlation between Pt, Pd, and Rh in the Mount Keith deposit, argue that this depletion is a primary magmatic signal and not an artefact of alteration. Differential diffusion rates between Pt and the other PGEs, giving rise to a low effective partition coefficient for Pt into sulphide liquid, is advanced as a possible but not definitive explanation.

Crustal contamination and PGE mineralization in the Platreef, Bushveld Complex, South Africa: evidence for multiple contamination events and transport of magmatic sulfides

Diamond drill core traverses across the Platreef were carried out at Tweefontein, Sandsloot, and Overysel in order to establish the relationship between crustal contamination and platinum group element (PGE) mineralization. The footwall rocks are significantly different at each of these sites and consist of banded iron formation and sulfidic shales at Tweefontein, of carbonates at Sandsloot, and of granites and granite gneisses at Overysel. As demonstrated in this study, Platreef rocks are characterized by two stages of crustal contamination. The first contamination event occurred prior to emplacement of the magma and is present in Platreef rocks at all three sites, as well as in the Merensky Reef. This event is readily identified on trace element spidergrams and trace element ratio scattergrams. The second contamination event was induced by interaction of the Platreef magma with the local footwall rocks. It is most easily identified at Tweefontein, where there is a large increase in the FeO content of the Platreef rocks, and at Sandsloot, where there is a large increase in their CaO and MgO contents, relative to Bushveld rocks that are uncontaminated by the local footwall rocks. At Overysel, the second contamination event did not result in pronounced changes in the major element composition of the Platreef rocks, but can be detected in their trace element chemistry. A strong inverse relationship between PGE tenors and S/Se ratios is interpreted to suggest that the PGE-rich sulfides were formed prior to emplacement of the Platreef magmas through assimilation of crustal S and became progressively enriched in the PGE during transport. Rather than promoting S-saturation, interaction of the Platreef magma with the footwall rocks diluted the metal tenors of the sulfides. Although both the Platreef and the Merensky Reef magmas were contaminated by the same crustal contaminant and were probably PGE-rich, they have radically different Pd/Pt ratios. Their Pd/Pt ratios suggest that whereas the Merensky Reef magma became PGE-rich due to dissolution of PGE-rich sulfides segregated from a pre-Merensky magma that had undergone relatively little fractionation prior to reaching S-saturation, the pre-Platreef magma had undergone greater fractionation prior to the sulfide saturation event, thereby increasing its Pd/Pt ratio. We suggest that the magmas that formed the Platreef and Merensky Reef may have simply been carrier magmas for sulfides that had formed elsewhere in the plumbing system of the Bushveld Complex by the interaction of earlier generations of magmas with the crustal rocks that underlie the Complex.

PGE Tenor and Metal Ratios within and below the Merensky Reef, Bushveld Complex: Implications for its Genesis

The Merensky Reef is the consequence of several major influxes of magma into the Bushveld magma chamber that gave rise to at least two cyclic units, including the Pre-Merensky and Merensky cyclic units. Analysis of the S, Ni, Cu, platinum group element (PGE) and Au contents of 1140 samples from 24 profiles through the rocks forming these units and two profiles extending downward from the Reef to close to the UG-2 horizon of the Western Bushveld Complex provide important petrogenetic constraints on the origin of the Reef. Particular attention is paid to variations in the calculated PGE tenors and Cu/Pd ratios of sulfides, both vertically and laterally. All of the profiles show a decrease in Pt tenor and concomitant increase in Cu/Pd ratio upward across the pyroxenite of the Merensky unit and laterally away from certain centres that are interpreted as feeder zones. The data also document that more mafic horizons within the Merensky to UG-2 interval of the southwestern Bushveld are characterized by much lower Cu/Pd (<100–400) than the ambient ratio (∼4000). This is interpreted as demonstrating that these horizons are the consequence of incursions of magma whose composition has been modified in a staging chamber by reaction with sulfide resident there. The favoured interpretation for the Merensky Reef is that the Bushveld chamber(s) was occupied by a resident magma crystallizing orthopyroxene and plagioclase. Pre-Merensky and, subsequently, Merensky magmas entered the chamber(s) intermittently from localized feeder structures, mixing or mingling with the resident magma and giving rise to the more mafic cumulates. At specific horizons, including the Merensky Reef, the mixing or mingling caused sulfide immiscibility. In these cases, as the new magma pulses flowed laterally away from the entry zones they deposited sulfide, becoming depleted in PGE. On coming to rest, the magma continued to segregate sulfide and crystallize pyroxene, so that the sulfides became progressively depleted in PGE and acquired a higher Cu/Pd upward through the pyroxenite. This model requires that the Merensky magmas could dissolve up to two orders of magnitude more PGE than current measurements on dry diopside–anorthite melts have indicated. It is suggested that very PGE-rich magmas can develop as a result of sulfide-unsaturated magma reacting with early formed sulfide in a staging chamber, dissolving FeS and enriching the remaining sulfide in PGE. Later magmas equilibrating with this enriched sulfide will themselves become highly enriched in PGE.

Assessing the potential involvement of an early magma staging chamber in the generation of the Platreef Ni–Cu–PGE deposit in the northern limb of the Bushveld Complex: a pilot study of the Lower Zone Complex at Zwartfontein

The Platreef of the northern limb of the Bushveld Complex is one of the world's most significant deposits of platinum-group elements (PGE) with associated Ni and Cu. The origin of the Platreef is controversial. Some workers suggest that it is a northern facies of the Merensky Reef or part of the Upper Critical Zone, while others have suggested that the Platreef formed by processes entirely contained within the northern limb, unrelated to mineralisation events elsewhere in the complex. The northern limb is separated from the rest of the complex by the Thabazimbi–Murchison Lineament (TML) and the effect that this structure had on the intrusion of Bushveld magmas is debated. The presence of chilled rocks and cross-cutting relationships between the Platreef and its hangingwall gabbronorites would seem to preclude the magma that formed the hangingwall also acting as a source of PGE to the Platreef. The base metal sulphides in the Platreef carry very high PGE tenors (comparable with the Merensky Reef) indicating that the PGE must have been concentrated from a large volume of magma, but the source of that magma has not been established. In order to solve this PGE mass balance paradox, McDonald and Holwell have suggested that the magmas that formed the (pre-Platreef) Lower Zone may have been the source of PGE. At present, other models do not involve any significant role for the Lower Zone magmas in forming the Platreef. The data presented in this pilot study of the Lower Zone intrusion at Zwartfontein test some of the predictions arising from the McDonald and Holwell model. They highlight some important first order differences between Lower Zone intrusions in the northern limb compared with the rest of the Bushveld Complex. The enrichment in Th and LREE that characterizes the Lower Zone rocks in the eastern and western Bushveld appears to be missing in the northern limb. The lithophile element signatures of the different types of Lower Zone are suggested to result from mafic magmas intruding north and south of the TML and being contaminated by these different types of crust. Most significantly, the study has also revealed strong depletion of chalcophile elements (Ni, Cu and PGE) in the Lower Zone intrusion at Zwartfontein. The results are consistent with the depleted products expected from the processing of pre-Platreef magma(s) by interactions with sulphides at a deeper level within the magmatic plumbing system. The results provide a positive first test of one of the predictions arising from the McDonald and Holwell Platreef model. The existence of a system capable of removing PGE and producing a large volume of depleted ultramafic cumulates, in close proximity to the most highly mineralised sector of the Platreef, is suggested to be highly significant.