Assemblages and genesis of platinum-group minerals in low-sulfide ores of the monchetundra deposit,Kola Peninsula,Russia

New data on the composition, assemblages, and formation conditions of platinum-group minerals (PGM) identified in platinum-group element (PGE) occurrences of the Monchetundra intrusion (2495 +- 13 to 2435 ± 11 Ma) are described. This intrusion is a part of the Paleoproterozoic pluton of the Monche-Chuna-Volch’i and Losevy tundras located in the Pechenga-Imandra-Varzuga Rift System. The rhythmically layered host rocks comprise multiple megarhythms juxtaposed to mylonite zones and magmatic breccia and injected by younger intrusive rocks in the process of intense and long magmatic and fluid activity in the Monchetundra Fault Zone. The primary PGM and later assemblages that formed as a result of replacement of the former have been identified in low-sulfide PGE occurrences. More than 50 minerals and unnamed PGE phases including alloys, Pt and Pd sulfides and bismuthotellurides, PGE sulfarsenides, and minerals of the Pd-As-Sb, Pd-Ni-As, and Pd-Ag-Te systems have been established. The unnamed PGE phases—Ni₆Pd₂As₃, Pd₆AgTe₄, Cu₃Pt, Pd₂NiTe₂, and (Pd, Cu)₉Pb(Te, S)₄—are described. The primary PGM were altered due to the effect of several mineral-forming processes that resulted in the formation of micro- and nanograins of Pt and Pd alloys, sulfides, and oxides, as well as in the complex distribution of PGE, Au, and Ag mineral assemblages. New types of complex Pt and Pd oxides with variable Cu and Fe contents were identified in the altered ores. Pt and Pd oxides as products of replacement of secondary Pt-Pd-Cu-Fe alloys occur as zonal and fibrous nanoscale Pt-Pd-Cu-Fe-(±S)-O aggregates.     

The mineralogy and mineral associations of platinum group elements and gold in the Platreef at Zwartfontein, Akanani Project, Northern Bushveld Complex, South Africa

The mineralogy of the platinum-group elements (PGE), and gold, in the Platreef of the Bushveld Complex, was investigated using an FEI Mineral Liberation Analyser. Polished sections were prepared from 171 samples collected from two boreholes, for the in-situ examination of platinum group minerals (PGM). PGM and gold minerals encountered include maslovite (PtBiTe, 32 area% of total PGM), kotulskite (Pd(BiTe), 17?%), isoferroplatinum (Pt₃Fe, 15?%), sperrylite (PtAs₂, 11?%), cooperite (PtS, 5?%), moncheite (PtTe₂; 5?%), electrum (AuAg; 5?%), michenerite (PdBiTe; 3?%), Pd alloys (Pd, Sb, Sn; 3?%), hollingworthite ((Rh,Pt)AsS; 2?%), as well as minor (all <1 area% of total PGM) merenskyite (PdBiTe₂), laurite (RuS₂), rustenburgite (Pt_0.4Pd_0.4Sn_0.2), froodite (PdBi₂), atokite (Pd_0.5Pt_0.3Sn_0.2), stumpflite (PtSb), plumbopalladinite (Pd₃Pb₂), and zvyagintsevite (Pd₃Pb). An observed association of all PGM with base metal sulfides (BMS), and a pronounced association of PGE tellurides, arsenides and Pd&Pt alloys with secondary silicates, is consistent with the remobilisation and recrystallisation of some of the PGM’s during hydrothermal alteration and serpentinisation subsequent to their initial (primary) crystallisation from BMS (e.g. Godel et al. J Petrol 48:1569–1604, 2007; Hutchinson and McDonald Appl Earth Sci (Trans Inst Min Metall B) 114:B208–224, 2008).     

PGE mineralization of dunite-wehrlite massifs at the Gutara-Uda interfluve, Eastern Sayan

The Pt-Pd and Au-Ag mineralization hosted in both wehrlite without visible links to sulfide mineralization (dispersed assemblage of the Tartai massif) and disseminated Cu-Ni sulfide ore (ore assemblage of the Ognit massif) was found in dunite-wehrlite massifs localized in the fold framework of the Siberian Craton. The Pt minerals in both assemblages comprise sperrylite (PtAs₂) and secondary Pt-Fe-Ni alloys in the Ognit massif and Pt-Fe-Cu and Pt-Cu alloys in the Tartai massif. The Pd minerals are widespread in the ore assemblages as compounds with Te, Sb, and Bi, whereas in the dispersed assemblage Pd is concentrated primarily in Pd-Cu-Sb compounds. Both assemblages are characterized by similar substitution of sperrylite with orcelite (Ni_{5 ? x}As₂) and then with secondary Pt-Fe-Ni or Pt-Fe-Cu and Pt-Cu alloys; the occurrence of Au-Ag alloys with prevalence of Ag over Au; and replacement of them with auricupride (Cu₃Au) at the late stage. Sperrylite in both assemblages contains Ir impurities, while the Pd minerals contain Cu and Ni admixtures, which are typical of mineral assemblages related to the ultramafic intrusions with nickel specialization. PGM were formed under a low sulfur fugacity and high As, Bi, and Sb activities. The postmagmatic fluids affected the primary mineral assemblages under reductive conditions, and this effect resulted in replacement of sperrylite with Ni arsenide (orcelite) and Pt-Fe-Ni and Pt-Fe-Cu alloys; Ni and Cu sulfides were replaced with awaruite and native copper.     

Tellurides, selenides and Bi-mineral assemblages from the Río Narcea Gold Belt, Asturias, Spain: genetic implications in Cu–Au and Au skarns

Summary Gold ores in skarns from the Río Narcea Gold Belt are associated with Bi–Te(–Se)-bearing minerals. These mineral assemblages have been used to compare two different skarns from this belt, a Cu–Au skarn (calcic and magnesian) from the El Valle deposit, and a Au-reduced calcic skarn from the Ortosa deposit. In the former, gold mineralization occurs associated with Cu–(Fe)-sulfides (chalcopyrite, bornite, chalcocite-digenite), commonly in the presence of magnetite. Gold occurs mainly as native gold and electrum. Au-tellurides (petzite, sylvanite, calaverite) are locally present; other tellurides are hessite, clausthalite and coloradoite. The Bi-bearing minerals related to gold are Bi-sulfosalts (wittichenite, emplectite, aikinite, bismuthinite), native bismuth, and Bi-tellurides and selenides (tetradymite, kawazulite, tsumoite). The speciation of Bi-tellurides with Bi/Te(Se + S) ≤ 1, the presence of magnetite and the abundance of precious metal tellurides and clausthalite indicate fO₂ conditions within the magnetite stability field that locally overlap the magnetite-hematite buffer. In Ortosa deposit, gold essentially occurs as native gold and maldonite and is commonly related to pyrrhotite and to the replacement of l?llingite by arsenopyrite, indicating lower fO₂ conditions for gold mineralization than those for El Valle deposit. This fact is confirmed by the speciation of Bi-tellurides and selenides (hedleyite, joséite-B, joséite-A, ikunolite-laitakarite) with Bi/Te(+ Se + S) ≥ 1.     

The distribution of platinum group elements (PGE) and other chalcophile elements among sulfides from the Creighton Ni–Cu–PGE sulfide deposit, Sudbury, Canada, and the origin of palladium in pentlandite

Concentrations of platinum group elements (PGE), Ag, As, Au, Bi, Cd, Co, Mo, Pb, Re, Sb, Se, Sn, Te, and Zn, have been determined in base metal sulfide (BMS) minerals from the western branch (402 Trough orebodies) of the Creighton Ni–Cu–PGE sulfide deposit, Sudbury, Canada. The sulfide assemblage is dominated by pyrrhotite, with minor pentlandite, chalcopyrite, and pyrite, and they represent monosulfide solid solution (MSS) cumulates. The aim of this study was to establish the distribution of the PGE among the BMS and platinum group minerals (PGM) in order to understand better the petrogenesis of the deposit. Mass balance calculations show that the BMS host all of the Co and Se, a significant proportion (40–90%) of Os, Pd, Ru, Cd, Sn, and Zn, but very little (<35%) of the Ag, Au, Bi, Ir, Mo, Pb, Pt, Rh, Re, Sb, and Te. Osmium and Ru are concentrated in equal proportions in pyrrhotite, pentlandite, and pyrite. Cobalt and Pd (∼1 ppm) are concentrated in pentlandite. Silver, Cd, Sn, Zn, and in rare cases Au and Te, are concentrated in chalcopyrite. Selenium is present in equal proportions in all three BMS. Iridium, Rh, and Pt are present in euhedrally zoned PGE sulfarsenides, which comprise irarsite (IrAsS), hollingworthite (RhAsS), PGE-Ni-rich cobaltite (CoAsS), and subordinate sperrylite (PtAs₂), all of which are hosted predominantly in pyrrhotite and pentlandite. Silver, Au, Bi, Mo, Pb, Re, Sb, and Te are found predominantly in discrete accessory minerals such as electrum (Au–Ag alloy), hessite (Ag₂Te), michenerite (PdBiTe), and rhenium sulfides. The enrichment of Os, Ru, Ni, and Co in pyrrhotite, pentlandite, and pyrite and Ag, Au, Cd, Sn, Te, and Zn in chalcopyrite can be explained by fractional crystallization of MSS from a sulfide liquid followed by exsolution of the sulfides. The early crystallization of the PGE sulfarsenides from the sulfide melt depleted the MSS in Ir and Rh. The bulk of Pd in pentlandite cannot be explained by sulfide fractionation alone because Pd should have partitioned into the residual Cu-rich liquid and be in chalcopyrite or in PGM around chalcopyrite. The variation of Pd among different pentlandite textures provides evidence that Pd diffuses into pentlandite during its exsolution from MSS. The source of Pd was from the small quantity of Pd that partitioned originally into the MSS and a larger quantity of Pd in the nearby Cu-rich portion (intermediate solid solution and/or Pd-bearing PGM). The source of Pd became depleted during the diffusion process, thus later-forming pentlandite (rims of coarse-granular, veinlets, and exsolution flames) contains less Pd than early-forming pentlandite (cores of coarse-granular).     

应用偏光显微镜和电子探针技术研究安徽铜官山矽卡岩型铜铁矿床伴生元素金银铂钯铀的赋存状态

安徽铜官山矽卡岩型铜铁矿床富含多种稀有贵金属金银铂钯和铀,本文应用偏光显微镜与电子探针技术对该地区贵金属和铀矿物的含量、矿物种类、赋存状态及其嵌布特征进行研究,并利用电子探针Th-U-Pb定年技术推测铀矿物的形成时期。研究表明:金主要以银金矿独立矿物存在,成色均值约为638,与铜的硫化物密切依存,金矿物形成于成矿中晚期的中低温环境;银的独立矿物有银金矿、碲银矿、辉银矿,还与铜铋铅等以类质同象形式结合形成不同种类的矿物组合,且含量在74.15%~0.12%不等;铂钯矿物以含铂碲钯矿为主;铀以晶质铀矿独立矿物存在且与磁铁矿密切依存,晶质铀矿的形成年龄约为124±14 Ma,晚于岩体形成年龄(约139 Ma),早于黄铜矿和含金银铂钯等矿物,而与磁铁矿同在燕山中晚期形成。结合镜下观察,认为铜官山矽卡岩型铜铁矿床主要矿物生成顺序依次是:石榴子石-磁铁矿、晶质铀矿,含金银铂钯矿物,黄铜矿。本研究为贵金属选矿提供了线索,同时利用晶质铀矿的年龄数据界定了伴生贵金属的形成年代。     

Melonite group minerals and other tellurides from three Cu–Ni–PGE prospects, Eastern Desert, Egypt

Melonite group minerals and other tellurides are described from three Cu–Ni–PGE prospects in the Eastern Desert of Egypt: Gabbro Akarem, Genina Gharbia and Abu Swayel. The prospects are hosted in late Precambrian mafic–ultramafic rocks and have different geologic histories. The Gabbro Akarem prospect is hosted in dunite pipes where net-textured and massive sulfides are associated with spinel and Cr-magnetite. Michenerite, merenskyite, Pd–Bi melonite and hessite occur mainly as inclusions in sulfides. Typical magmatic textures indicate a limited role of late- and post-magmatic hydrothermal processes. At Genina Gharbia, ore forms either disseminations in peridotite or massive patches in hornblende-gabbro in the vicinity of metasedimentary rocks. Actinolitic hornblende, epidote, chlorite and quartz are common secondary silicates. Sulfide textures and host rock petrography suggest a prolonged late-magmatic hydrothermal event. Michenerite, merenskyite, Pd–Bi melonite, altaite, hessite, tsumoite, sylvanite and native Te are mainly present in secondary silicates. The Abu Swayel prospect occurs in conformable, lens-like mafic–ultramafic rocks in metasedimentary rocks and along syn-metamorphic shear zone. The sulfide ore and host rocks are metamorphosed (amphibolite facies; 550 to 650 °C, 4 to 5 kbar) and syn-metamorphically sheared. Melonite group minerals are represented by merenskyite and Pd–Bi melonite. Other tellurides comprise hessite, altaite and joséite-B. Melonite group minerals and tellurides occur as inclusions in mobilized sulfides and along cracks in metamorphic garnet and plagioclase.The different geological history of the three prospects permits an examination of the role played by magmatic, late-magmatic and metamorphic processes on the mineralogy of melonite group minerals and diversity of tellurides. The contents of PGE and Te in the ore and temperature of crystallization control the mineralogy and compositional trends of the melonite group minerals. Crystallization of the melonite group minerals over a wide range of temperatures in a Te-rich environment enhances the elemental substitutions. Merenskyite dominates the mineralogy of the group at low Te activity, while Pd–Bi melonite is the common phase at high Te activity.     

Laser ablation ICP-MS study of platinum-group elements in sulphides from the Platreef at Turfspruit,northern limb of the Bushveld Complex,South Africa

The Platreef unit of the northern Bushveld Complex comprises a diverse package of pyroxenites, peridotites and mafic lithologies with associated Ni–Cu–platinum-group element (PGE) mineralisation. Base metal sulphides (BMS) are generally more abundant in the Platreef than in other Bushveld PGE deposits, such as the Merensky Reef and the UG2 chromitite, but the Platreef, though thicker, has lower overall PGE grades. Despite a commonly held belief that PGEs are closely associated with sulphide mineralisation, a detailed study by laser ablation ICP-MS (LA-ICP-MS) on a core through the Platreef at Turfspruit suggests that this is not strictly the case. While a significant proportion of the Pd, Os and Ir were found to be hosted by BMS, Pt, irrespective of its whole-rock concentration, was not. Only at the top of the Platreef is Pt directly associated with sulphide minerals where Pt–Pd–(±Sb)–Te–Bi-bearing inclusions were detected in the chalcopyrite portions of large composite sulphides. In contrast, Pd, Os, and Ir occur in solid solution and as discrete inclusions within the BMS throughout the core. For Os and Ir, this is usually in the form of Os–Ir alloys, whereas Pd forms a range of Pd–Te–Bi–(Sb) phases. Scanning electron microscope observations on samples from the top of the core revealed the presence of ≤0.2-mm-long (PtPd)₂(Sb,Te,Bi)₂ michenerite–maslovite laths within the chalcopyrite portions of large composite sulphides. Additional Pt-bearing minerals, including sperrylite and geversite, and a number of Pd(–Te–Bi–Sb) minerals were observed in, or close to, the alteration rims of these sulphides. This textural association was observed throughout the core. Similar platinum-group minerals (PGMs) were observed within the felsic assemblages composed of quartz, plagioclase, alkali feldspar and clinopyroxene produced by late-stage felsic melts that permeated the Platreef. Many of these PGMs occur a significant distance away from any sulphide minerals. We believe these features can all be linked to the introduction of As, Sb, Te and Bi into the magmatic system through assimilation of sedimentary footwall rocks and xenoliths. Where the degree of contamination was high, all of the Pt and some of the Pd formed As- and Sb-bearing PGM that were expelled to the edges of the sulphide droplets. Many of these were redistributed where they came into contact with late-stage felsic melts. Where no felsic melt interactions occurred, the expelled Pt- and Pd-arsenides and antimonides remained along the margins of the sulphides. At the top of the Platreef, where the effects of contamination were relatively low, some of the Pt remained within the sulphide liquids. On cooling, this formed the micro-inclusions and blade-like laths of Pt–Pd–(Sb)–Bi–Te in the chalcopyrite.     

Platinum-Group Minerals from the Durance River Alluvium,France

Summary Platinum-group minerals were discovered, during gold recovery, in the Durance river alluvium, near Peyrolles (Bouches-du-Rhône). The PGM grains (average size 130 microns) are strongly flattened (average thickness 64 microns). The PGM concentrate consists primarily of (Pt, Fe) alloys (92%), (Os, Ir, Ru) alloys (3.5%), and native gold and (Au, Cu, Ag) alloys (4.5%). The following minerals were observed: isoferroplatinum, ferroan platinum, native osmium, native iridium, iridosmine, rutheniridosmine, osmiridium, ruthenian osmium, osmian ruthenium, cuprorhodsite, guanglinite, shandite, tetrauricupride, native gold, bornite, heazlewoodite, (Pt, Pd)₂Cu₃, Pt(Cu, Au), (Ni, Pt)Sn, (Cu, Fe)_1–x (Pd, Rh, Pt)_2+xS₂, (Pt, Pd)_4–xCu₂As_1–x. Isoferroplatinum contains numerous inclusions of alloys, sulphides, arsenides, Pd-tellurides, and partly devitrified silicate glass droplets. Most of the non-silicate inclusions also exhibit a drop-like shape indicating their original entrapment in a liquid state.Cuprorhodsite crystals (up to 20 microns) are associated with bornite included in Pt₃Fe. Rarely, Pd- and Cu-sulphides, and Pd-tellurides appear in this association. Complex droplet-like arsenide inclusions in isoferroplatinum are composed of Pt bearing guanglinite and (Pt,Pd)_4+xCu₂As_1–x. Native iridium shows exsolutions of Ir-bearing isoferroplatinum and (Pt,Pd)₂Cu₃. In places, concentrations of Sn (up to 3 wt.%) were observed in (Au, Cu) alloys. Shandite and (Ni, Pt)Sn inclusions occur in (Au, Cu, Ag) alloys. Silicate-glass inclusions are TiO₂-poor and occasionally K-rich (plotting in the shoshonitic field). Taking into account mineralogical and chemical pecularities of the PGM association occurring in the studied concentrate, it seems highly probable that its primary source should be an Alaskan-type intrusion.

---

Platingruppen Minerale aus dem Alluvium der Durance, Frankreich

Zusammenfassung Minerale der Platingruppe wurden im Zuge von Goldgewinnung im Alluvium der Durance in der Nähe von Peyrolles (Bouches-du-Rhône) entdeckt. Die PGM Körner (durchschnittliche Korngröße 130m) sind flach gepreßt (durchschnittliche Dicke 64m). Die PGM Konzentrate bestehen vorwiegend aus (Pt, Fe) Legierungen (92%); (Os, Ir, Ru) Legierungen (3,5%), sowie gediegen Gold und (Au, Cu, Ag) Legierungen (4,5%). Folgende Minerale wurden beobachtet:Isoferro-Platin, Fe-Platin, gediegen Osmium, gediegen Iridium, Iridosmium, Rutheniridosmium, Osmiridium, Ru-Osmium, Os-Ruthenium, Cuprorhodsit, Guanglinit, Shandit, Tetrauricuprit, gediegen Gold, Bornit, HeazIewoodit, (Pt, Pd)₂ Cu₃, Pt(Cu, Au), (Ni, Pt)Sn, (Cu, Fe), (Pd, Rh, Pt)_2+xS₂, (Pt, Pd)_4+xCu₂As_1–x.Isoferro-Platin enthält zahlreiche Einschlüsse von Legierungen, Sulfiden, Arseniden, Pd-Telluriden und teilweise devitrifzierte Silikatglaströpfchen. Die meisten nichtsili katischen Einschlüsse sind ebenfalls tröpfchenförmig. Dies weist darauf hin, daß sie in flüssigem Zustand eingeschlossen wurden.Cuprorhodsitkristalle (bis zu 20m) sind gemeinsam mit Bornit in Pt₃ Fe einge schlossen. Selten sind Pd- und Cu-Sulfide, sowie Pd-Telluride mit diesen vergesellschaftet. Bei den komplexen tröpfehenförmigen Arsenideinschlüssen im Isoferro-Platin handelt es sich um Pt-führenden Guanglinit und (Pt, Pd)_4+xCu₂ As_1–x. Gediegen Iridium zeigt Entmischung von Ir-führendem Isoferro-Platin und (Pt, Pd)₂Cu₃. Stellenweise wurden Konzentrationen von Sn (bis zu 3%) in den (Au, Cu) Legierungen beobachtet. Shandit und (Ni, Pt) Sn Einschlüsse kommen in (Au, Cu, Ag) Legierungen vor. Silikatische Glaseinschlüsse sind TiO₂-arm und manchmal K-reich (im Shoshonitfeld liegend).Auf Grund der mineralogischen und chemischen Eigenheiten der untersuchten PGM Konzentrate ist eine Intrusion des Alaska-Typs als primäre Quelle sehr wahrscheinlich.

---

---

With 4 Figures and 2 Plates     

Platinum-group minerals in fluid inclusions from the Marathon deposit,Coldwell Complex,Canada

Summary Platinum-group minerals (PGM) project into fluid inclusions that occur in chalcopyrite and cubanite from the Marathon deposit, Two Duck Lake gabbro, Coldwell Complex, Ontario. Semi-quantitative analyses of the micron-sized PGM were made by SEM-EDS; they reveal Ag-bearing intermetallic compounds of Pd₃Sn-Pd₃Pb-Pd₃Te (i.e., atokitezvyagintsevite-keithconnite) and telargpalite (Pd₂AgTe) on broken, irregular surfaces of the Cu-Fe-S minerals. Halite daughter minerals, and quenched brine occur in and around some opened fluid inclusions. These data confirm the hypothesis based on petrography and mineral compositions that saline fluids remobilized PGE, Cu, and other elements and precipitated them well after crystallization of sulfide and silicate magmas in the Marathon deposit.

---

Platin-Gruppenminerale in Flüssigkeitseinschlüssen aus der Marathon Lagerstdtte, Coldwall Komplex, Kanada

Zusammenfassung Platin-Gruppenminerale (PGM) ragen in Flüzssigkeiteinschliisse, die im Kupferkies und Cubanit der Marathon Lagerstätte, Two Duke Lake Gabbro, Coldwell Komplex, Ontario, vorkommen, hinein. Semiqantitative Analysen der mikrongroßen PGM wurden mit einem SEM-EDS durchgeführt. Sie ergaben Ag-führende, intermetallische Verbindungen von Pd₃Sn-Pd₃Pb-Pd₃Te (Atokit-Zvyagintsevit-Keithconnit) und Telargpalit (Pd₂AgTe) auf zerbrochenen, irregulären Oberflächen der Cu-Fe-SMinerale. Steinsalz-Tochterkristalle und abgeschreckte Salzlösungen treten innerhalb und in der unmittelbaren Umgebung geöffneter Flüssigkeitseinschlüsse auf. Diese Daten bestätigen die Hypothese, basierend auf der Petrographie und der Mineralzusammensetzung, daß saline Fluida PGE, Cu und andere Elemente remobilisieren können, und daß d iese nach der Kristallisation der Sulfide und des Silikatmagmas in der Marathon Lagerstätte zur Ablagerung gekommen sind.

---

---

With 3 Figures     

Au-Cu-Ag mineralization in rodingites and nephritoids of the Agardag ultramafic massif (southern Tuva,Russia)

Gold-bearing albite-amphibole-pyroxene rodingites of the Agardag ultramafic massif (southern Tuva, Russia) are confined to the E-W striking serpentinite crush zone. A zone of gold-bearing nephritoids is localized at the contact of rodingites with serpentinites. Optical and scanning electron microscopy, electron probe microanalysis, and fluorescent, chemical, ICP MS, and X-ray phase analyses were applied to study Au-Cu-Ag mineralization in the serpentinites, rodingites, and nephritoids. Copper sulfides, chalcocite and digenite, are present in the serpentinites, whereas gold and silver minerals are absent. Copper impurity is found in antigorite, Cr-spinel, and magnetite (up to 0.1-0.3 wt.%) as well as parkerite (up to 1.2 wt.%) and millerite (up to 7.9 wt.%). A wide variety of native gold and copper minerals has been identified in the rodingites: (1) cuproauride and tetra-auricupride free of or containing silver impurities (0.1 to 1.2 wt.%); (2) electrum of composition Ag0.50_-0.49Au0.50_-0.5₁ (650-660%c) intergrown with AuCu, sometimes as exsolution structures; (3) electrum of composition Ag₀._70-0.6₄Au₀._30-0.₃6 (440-510%c), with inclusions of AuCu and copper sulfides (geerite and yarrowite); (4) high-fineness gold (750-990%c) as veinlets in electrum; and (5) native copper. The composition of copper sulfides varies from chalcocite to covellite. Submicron inclusions of hessite Ag2Te were found in chalcocite. The amount of copper, gold, and silver minerals in the nephritoids is much less than that in the rodingites. The nephritoids contain chalcocite, electrum of composition Ag0.6_4-0.63Au0.36_-0.3₇ (530-540%c), cuproauride, and tetra-auricupride. The detected hypergene minerals are auricuzite, apachite, brochantite, high-fineness gold, native copper, and cuprite. The sequence of mineral formation in the Agardag ore occurrence has been established on the basis of mineral structures and mineral relations in the rodingites and nephritoids. It is proved that Au-Cu-Ag mineralization formed with the participation of Au- and Ag-bearing chloride-free low-sulfur carbon dioxide alkaline fluids in reducing conditions.     

Gabbro Akarem, Eastern Desert, Egypt: Cu–Ni–PGE mineralization in a concentrically zoned mafic–ultramafic complex

The Gabbro Akarem (Late Precambrian) intrusion is concentrically zoned with a dunite core surrounded by lherzolite–clinopyroxenite enveloped by olivine–plagioclase hornblendite and plagioclase hornblendite. Cu–Ni–PGE mineralization is closely associated with peridotite, especially in the inner, olivine-rich core (dunite pipes) where net-textured and massive sulfides (pyrrhotite, pentlandite, chalcopyrite) are found in association with Al–Mg-rich spinel and Cr-magnetite. Primary magmatic textures are well preserved; however, deformation and mobilization due to shearing are locally observed. Platinum-group minerals (PGM) documented from the deposit are: merenskyite (PdTe₂) and michenerite (PdTeBi), as well as palladian bismuthian melonite (Ni,Pd) (Te,Bi)₂. These minerals occur in intimate association with hessite (Ag₂Te) and electrum (Au_0.65Ag_0.31Bi_0.04) in two distinct textural positions: (1) as inclusions in pyrrhotite, pentlandite, and rarely chalcopyrite and (2) at sulfide–silicate grain boundaries and on microfractures in base-metal sulfides (BMS) and olivine associated with serpentine and secondary magnetite. Textural features suggest that PGM were exsolved from monosulfide solid solution over a wide range of temperatures. Late-stage, low-temperature hydrothermal solutions led to redistribution of PGE. Mineralized samples show Ni/Cu ratios ranging from 0.2 to 2 with an average of 1.0. The (Pt + Pd + Rh)/(Os + Ir + Ru) ratio is generally >6 in most samples, and Os, Ru, and Ir are below the detection limit (2 ppb). The PGE contents show positive correlation with S only at low sulfur contents. The PGE patterns of Gabbro Akarem are similar to those of Alaskan-type deposits. Compared with stratiform deposits, Gabbro Akarem is depleted in PGE. The consistently low PGE contents of the mineralization and their uniform distribution in the ultramafic rocks despite the high sulfur content of the rock is attributed to rapid crystallization of sulfides in a highly dynamic environment. Received: 3 November 1999 / Accepted: 29 July 2000     

Insights into the extreme PGE enrichment of the W Horizon,Marathon Cu-Pd deposit,Coldwell Alkaline Complex,Canada: Platinum-group mineralogy,compositions and genetic implications

The W Horizon, Marathon Cu-Pd deposit in the Mesoproterozoic Midcontinent rift is one of the highest grade PGE repositories in magmatic ore deposits world-wide. The textural relationships and compositions of diverse platinum-group mineral (PGM) and sulfide assemblages in the extremely enriched ores (>100 ppm Pd-Pt-Au over 2 m) of the W Horizon have been investigated in mineral concentrates with ∼10,000 PGM grains and in situ using scanning electron microprobe and microprobe analyses.Here we show, from ore samples with concentrations up to 23.1 Pd ppm, 8.9 Pt ppm, 1.4 Au ppm and 0.73 Rh ppm, the diversity of minerals (n = 52) including several significant unknown minerals and three new mineral species marathonite (Pd₂₅Ge₉; McDonald et al., 2016), palladogermanide (Pd₂Ge; IMA 2016-086, McDonald et al., 2017), kravtsovite (PdAg₂S, IMA No 2016-092, Vymazalová et al., 2017). The PGM are distributed as PG-, sulfides (52 vol%), -arsenides (34 vol%), -intermetallics of Au-Ag-Pd-Cu and Pd-Ge(10 vol%) and -bismuthides and tellurides (4 vol%). The discovery of abundant (>330 grains) large unknown sulfide minerals with Rh allows us to present analyses three significant potentially new minerals (WUK-1, WUK-2, WUK-3) that are all clearly enriched in Rh (averaging 4.2, 8.5 and 28.21 wt% Rh respectively). Several examples of paragenetic sequences and mineral chemical changes for enrichment of Cu, Pd and Rh with time are revealed in the PGM and base-metal sulfides. We suggest this enhanced metal enrichment formed in response to increasing fO₂ causing the oxidation of Fe²⁺ to Fe³⁺ and to a lesser extent, S.Phase relations in the Ag-Pd-S, Rh-Ni-Fe-S, Pd-Ge, Au-Pd-Cu-Ag, Pd-Ag-Te systems help constrain the crystallization temperatures of the majority of ore minerals in the W Horizon at ∼500 °C or moderate to high subsolidus temperatures (400–600 °C). Local transport by aqueous fluids becomes evident as minerals recrystallize down to <300 °C. The PGE-enriched W Horizon ores exhibit a complex post-magmatic history dominated by the effects of oxidation during cooling of a Cu-PGE enriched magma source from a deep reservoir.     

Distribution of noble metals in ore mineral assemblages of the Volkovsky gabbroic pluton, central Urals

Relationships between noble-metal and oxide-sulfide mineralization during the origin of the Volkovsky gabbroic pluton are discussed on the basis of geochemical data and thermodynamic calculations. The basaltic magma initially enriched in noble metals (NM) relative to their average contents in mafic rocks, except for Pt, is considered to be a source of Pd, Pt, Au, and Ag in the gabbroic rocks of the Volkovsky pluton. The ores were formed with a progressive gain of NM in the minerals during the fractionation of the basaltic magma. The active segregation of NM in the form of individual minerals (palladium tellurides and native gold) hosted in titanomagnetite and copper sulfide ore occurred during the final stage of gabbro crystallization, when the residual fluid-bearing melt acquired high concentrations of Cu, Fe, Ti, and V, along with volatile P and S. Copper sulfides—bornite and chalcopyrite—are the major minerals concentrating NM; they contain as much as 22.65–25.20 ppm Pd and 0.74–1.56 ppm Pt; 4.39–8.0 ppm Au, and 127.2–142.6 ppm Ag, respectively. The copper ore and associated NM mineralization were formed at a relatively low sulfur fugacity, which was a few orders of magnitude (attaining 5 log units) lower than that of the pyrite-pyrrhotite equilibrium. The low sulfur fugacity and the close chemical affinity of Pd and Pt to Te precluded the formation of pyrrhotite, pyrite, and PGE disulfides. The major ore minerals and NM mineralization were formed within a wide temperature range (800–570°C), under nearly equilibrium conditions. Foreign elements (Ni, Co, and Fe) affected the thermodynamic stability of Pd and Pt compounds owing to the difference in their affinity to Te and to elements of the sulfur group (S, Se, and As). The replacement of Pd with Ni and Co and, to a lesser extent, with Pt and the replacement of Te with S, As, and Se diminish the stability field of palladium telluride. Comparison of Pd tellurides from copper sulfide ores at the Volkovsky and Baronsky deposits showed the enrichment of the former in Au, Sb, and Bi, while the latter are enriched in Pt, Ni, and Ag. The enrichment of Pd tellurides at the Baronsky deposit in Ni is correlated with the analogous enrichment of the host gabbroic rocks.     

Distribution of gold and silver and its relation with hypogene ore minerals in the Esperanza porphyry deposit,Antofagasta Region,Chile

Modes of occurrence of Au‐ and Ag‐bearing phases and their relation with associated hypogene ore minerals were examined with the objective to elucidate Au‐Ag distribution at the Esperanza porphyry deposit in the Eocene Centinela copper belt, using ore‐microscope modal analysis, semi‐quantitative analyses by automated mineralogy, electron probe microanalysis, and secondary ion mass spectrometer. The Esperanza hypogene mineralization is characterized by early‐stage chalcopyrite‐rich veinlets in the potassic alteration zone and later polymetallic stage with tennantite and galena in the chlorite‐sericitic alteration zone. Only the early‐stage chalcopyrite contains fine‐grained electrum (Au₆₈Ag₃₂ ‐ Au₈₁Ag₁₉) and hessite (Ag₂Te), and thus yields positive correlations in Cu vs. Au and Cu vs. Ag grades that are clearly recognized in the hypogene sulfide zone. The early‐stage chalcopyrite grains frequently exhibit polysynthetic twinning suggestive of inversion from intermediate solid solution. These features suggest that the fine‐grained electrum and hessite are products exsolved in the cooling process with the intermediate solid solution to chalcopyrite inversion. In contrast, tennantite and galena of the later‐stage mineralization contain no detectable Ag, and it is thus proposed that the early‐stage inverted chalcopyrite is the principal storage of economically important precious metals.     

Platinum-Group Mineral Characterization in Concentrates from High-Grade PGE Al-rich Chromitites of Korydallos Area in the Pindos Ophiolite Complex (NW Greece)

The Pindos ophiolite complex, located in the north-western part of continental Greece, hosts various podiform chromite deposits generally characterized by low platinum-group element (PGE) grades. However, a few locally enriched in PPGE + Au (up to 29.3 ppm) chromitites of refractory type are also present, mainly in the area of Korydallos (south-eastern Pindos). The present data reveal that this enrichment is strongly dependant on chromian spinel chemistry and base metal sulfide and/or base metal alloy (BMS and BMA, respectively) content in chromitites. Consequently, we used super-panning to recover PGM from the Al-rich chromitites of the Korydallos area. The concentrate of the composite chromitite sample contained 159 PGM grains, including, in decreasing order of abundance, the following major PGM phases: Pd-Cu alloys (commonly non-stoichiometric, although a few Pd-Cu alloys respond to the chemical formula PdCu₄), Pd-bearing tetra-auricupride [(Au,Pd)Cu], nielsenite (PdCu₃), sperrylite (PtAs₂), skaergaardite (PdCu), Pd-bearing auricupride [(Au,Pd)Cu₃], Pt and Pd oxides, Pt-Fe-Ni alloys, hollingworthite (RhAsS) and Pt-Cu alloys. Isomertieite (Pd₁₁Sb₂As₂), zvyagintsevite (Pd₃Pb), native Au, keithconnite (Pd₂₀Te₇), naldrettite (Pd₂Sb) and Rh-bearing bismuthotelluride (RhBiTe, probably the Rh analogue of michenerite) constitute minor phases. The bulk of PGE-mineralization is dominated by PGM grains that range in size from 5 to 10 µm. The vast majority of the recovered PPGM are associated with secondary BMS and BMA, thus confirming that a sulphur-bearing melt played a very important role in scavenging the PGE + Au content of the silicate magma from which chromian spinel had already started to crystallize. The implemented technique has led to the recovery of more, as well as noble, PGM grains than the in situ mineralogical examination of single chromitite samples. Although, the majority of the PGM occur as free particles and in situ textural information is lost, single grain textural evidence is observed. In summary, this research provides information on the particles, grain size and associations of PGM, which are critical with respect to the petrogenesis and mineral processing.     

Genesis of the Jinchuan PGE deposit, China: evidence from fluid inclusions, mineralogy and geochemistry of precious elements

Summary The Jinchuan deposit is a platinum group element (PGE)-rich sulfide deposit in China. Drilling and surface sampling show that three categories of platinum group element (PGE) mineralization occur; type I formed at magmatic temperatures, type II occurs in hydrothermally altered zones of the intrusion, and type III in sheared dunite and lherzolite. All ore types were analyzed for Os, Ir, Ru, Rh, Pd, Pt and Au, as well as for Cu, Ni, Co and S. Type I ore has (Pt + Pd)/(Os + Ir + Ru + Rh) ratios of <7 and relatively flat chondrite-normalized noble metal patterns; the platinum group minerals (PGM) are dominated by sperrylite and moncheite associated with chalcopyrite, pyrrhotite and pentlandite. Type II has (Pt + Pd)/(Os + Ir + Ru + Rh) ratios from 40 to 330 and noble metal distribution patterns with a positive slope; the most common PGM are sperrylite and Pd bismuthotelluride phases concentrated mostly at the margins of base metal sulfides. Type III ores have the highest (Pt + Pd)/(Os + Ir + Ru + Rh) ratios from 240 to 710; the most abundant PGM are sperrylite and phases of the Pt–Pd–Te–Bi–As–Cl system. It is concluded that the Jinchuan deposit formed as a result of primary magmatic crystallization followed by hydrothermal remobilization, transport, and deposition of the PGE.     

The roles of magmatic and hydrothermal processes in PGE mineralization,Ferguson Lake deposit,Nunavut, Canada

The Ferguson Lake Ni–Cu–Co–platinum-group element (PGE) deposit in Nunavut, Canada, occurs near the structural hanging wall of a metamorphosed gabbroic sill that is concordant with the enclosing country rock gneisses and amphibolites. Massive to semi-massive sulfide occurs toward the structural hanging wall of the metagabbro, and a low-sulfide, high-PGE style of mineralization (sulfide veins and disseminations) locally occurs ~30–50 m below the main massive sulfide. Water–rock interaction in the Ferguson Lake Ni–Cu–Co–PGE deposit is manifested mostly as widespread, post-metamorphic, epidote–chlorite–calcite veins, and replacement assemblages that contain variable amounts of sulfides and platinum-group minerals (PGM). PGM occur as inclusions in magmatic pyrrhotite and chalcopyrite in both the massive sulfide and high-PGE zones, at the contact between sulfides and hornblende or magnetite inclusions in the massive sulfide, in undeformed sulfide veins and adjacent chlorite and/or epidote halos, in hornblende adjacent to hydrothermal veins, and in plagioclase–chlorite aggregates replacing garnet cemented by sulfide. The PGM are mostly represented by the kotulskite (PdTe)–sobolevskite (PdBi) solid solution but also include michenerite (PdBiTe), froodite (PdBi₂), merenskyite (PdTe₂), mertieite II (Pd₈[Sb,As]₃), and sperrylite (PtAs₂) and occur in variety of textural settings. Those that occur in massive and interstitial sulfides, interpreted to be of magmatic origin and formed through exsolution from base metal sulfides at temperatures <600°C, are dominantly Bi rich (i.e., Te-bearing sobolevskite), whereas those that occur in late-stage hydrothermal sulfide/silicate veins and their epidote–chlorite alteration halos tend to be more Te rich (i.e., Bi-bearing kotulskite). The chemistry and textural setting of the various PGM supports a genetic model that links the magmatic and hydrothermal end-members of the sulfide–PGM mineralization. The association of PGM with magmatic sulfides in the massive sulfide and high-PGE zones has been interpreted to indicate that PGE mineralization was initially formed through exsolution from base metal sulfides which formed by magmatic sulfide liquid segregation and crystallization. However, the occurrence of PGM in undeformed sulfide-bearing veins and in their chlorite–epidote halos and differences in PGM chemistry indicate that hydrothermal fluids were responsible for post-metamorphic redistribution and dispersion of PGE.     

Arsenic as an indicator element for gold exploration in the region of the Xiangxi Au–Sb–W deposit, NW Hunan, PR China

The Xiangxi Au–Sb–W deposit, the largest of its type in northwestern Hunan, China, is a sulfide-dominated ore body hosted by low grade metamorphic red slates of the Neoproterozoic Madiyi Formation. Three stages of mineralization, quartz–scheelite, quartz–gold–pyrite, quartz–gold–stibnite, and one metal-barren stage of veining, quartz–calcite, are recognized. Arsenopyrite occurs only as a minor mineral phase in the second stage. Analyses for 21 trace elements show that the enrichment factors of As in the metal deposit (EC [=element concentration of sample/average content of an element in the upper crust]: 190; 43 samples) in ore veins and in the Guanzhuang and Yuershan reference sections (3.7 km and 2.7 km away from the Xiangxi mine, EC: 3.5; 96 samples) are much smaller than those of Sb (52855 [in ore veins], 117 [in the sections]), W (5665, 7.5) and Au (2727, 5.3). The background concentrations of Au and As in the two sections were 1.4 ppb and 1.4 ppm, respectively. Arsenic (with an anomaly coefficient [AC = number of anomalous samples/total number of samples] of 76%) forms a larger geochemical halo than W (AC: 8%) and Au (AC: 32%). Gold and As in the deposit were transported mainly as metal complexes such as Au(HS)⁻₂, H_nAs₃S⁻⁽³⁻ⁿ⁾₆ (n=1, 2 or 3) and HAsS⁰₂. Au(HS)⁻₂ is rapidly precipitated by a geochemical oxidation barrier — the red slates of the Madiyi Formation. As–S complexes in the stratigraphic horizon can be transformed into As–O complexes (e.g., H₃AsO⁰₃) under oxidizing conditions, and are continuously transported. Therefore, they can be widely distributed in the red slate units, thus forming extensive geochemical haloes, so that As can be used as an indicator element for Au exploration in the Xiangxi region.     

Isoferroplatinum mineral assemblage in the northwestern Salair gold placer

Isoferroplatinum mineral assemblage has been first discovered in the Simonovsky Brook gold placer localized in the Egor'evskoe gold-bearing district (Salair Range). It includes isoferroplatinum (Pt₃Fe) grains with inclusions of cuprorhodsite CuRh₂S₄, braggite (Pt,Pd)S, vysotskite PdS, high-Pt rhodarsenide (Rh,Pd,Pt)₂As, keithconnite Pd₃Te, hollingworthite RhAsS, Cu and Fe sulfides, basic plagioclase, magnetite, epidote, quartz, kaolinite, and leucoxene. The assemblage might have been produced from small gabbroid bodies widespread in the northwestern Salair region, including those of the Uralian platinum-bearing type, or from gabbroids of other complexes.