Geochemistry and mineralogy of platinum group element in ores of the Kingash deposit,Eastern Sayan,Russia

The paper discusses the results of studying the contents of platinum group elements (PGE) and platinum group minerals (PGM) in ores of the Kingash deposit. The bulk of PGE has been established as concentrated in disseminated sulfide chalcopyrite–pyrrhotite–pentlandite ore and is represented by palladium bismuth–tellurides. During melt differentiation, the content and relationship of PGE are changed; the Pd/Pt value increases (up to 1.9 and 4.2 in dunite and wehrlite, respectively) with decreasing Mg number. The distribution of PGE, sulfur, and REE in various ore types suggests two formation mechanisms of high-grade ores: (1) the product of liquid immiscibility and gravity separation at the early magmatic stage and (2) involvement of the residual melt saturated in volatiles, which contributed to transportation and segregation of PGE at the late magmatic stage. The evolution of the ore system of the Kingash massif is characterized by sequential enrichment of PGM in Ni from high-Mg to low-Mg rocks similarly to sulfide minerals of disseminated ore. The criteria for ore content in utramafics of the Kansk block have been identified based on compared ore element and PGE concentrations in ultramafic rocks of the Kingash and Idar complexes.     

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.     

Platinoids in the Kaalamo Differentiated Massif in the Northern Ladoga Region,Karelia, Russia

The Kaalamo massif is located in the Northern Ladoga region, Karelia, on the extension of the Kotalahti Belt of Ni-bearing ultramafic intrusions in Finland. The massif, 1.89 Ga in age, is differentiated from pyroxenite to diorite. Nickel–copper sulfide mineralization with platinoids is related to the pyroxenite phase. The ore consists of two mineral types: (i) pentlandite–chalcopyrite–pyrrhotite and (ii) chalcopyrite, both enriched in PGE. Pd and Pt bismuthotellurides, as well as Pd and Pt tellurobismuthides, are represented by the following mineral species: kotulskite, sobolevskite, merenskyite, michenerite, moncheite, keithconnite, telluropalladinite; Pt and Pd sulfides comprise vysotskite, cooperite, braggite, palladium pentlandite, and some other rare phases. High-palladium minerals are contained in pentlandite–chalcopyrite–pyrrhotite ore. Native gold intergrown with kotulskite commonly contains microinclusions (1–3 μm) of Pd stannides: paolovite and atokite. Ore with 20–60% copper sulfides (0.2–6.0% Cu) contains 5.1–6.6 gpt PGE and up to 0.13–2.3 gpt Au. Pd minerals, arsenides and sulfoarsenides of Pt, Rh, Ir, Os, and Ru are identified as well. These are sperrylite, ruthenium platarsite, hollingworthite, and irarsite; silvery gold and paolovite have also been noted. All these minerals have been revealed in the massif for the first time. The paper also presents data on the compositions of 25 PGE minerals (PGM) from Kaalamo ores.     

Heavy concentrate halos as prospecting guides for PGE mineralization

Platinum-group minerals (PGM) in primary ores and placers are compared in order to substantiate prospecting guides for layered and differentiated intrusions containing sulfide Cu-Ni ores with platinum-group elements (PGE). It is shown that supergene placer mineral assemblages bear information on primary sources and their probable economic value. The mineralogical and geochemical data on the large Siberian intrusions that host Cu-Ni and low-sulfide PGM deposits (Noril’sk 1, Kingash, Chinei, and Yoko-Dovyren) are used to elaborate mineralogical prospecting guides based on the comparative study of PGM assemblages in primary ore, heavy concentrate halos, and hillside sediments. The mechanism of PGM redistribution under supergene conditions is exemplified in the Chinei deposit. The placer mineral assemblage with prevalence of Pt-Fe alloys, atokite-rustenburgite, sperrylite, and multicomponent Pd-Sn-Cu-Pb compounds can be used as a prospecting guide for Noril’sk-type primary PGM ore and related economic placers. The paolovite-sperrylite or sperrylite PGM assemblage in heavy concentrate halos indicates occurrence of Cu-Ni ore in the prospecting area. Sperrylite with isomorphic admixture of Ir and Os typical of the Kingash pluton could be a orospecting guide for Ni-bearing mafic-ultramafic intrusions.     

Loypishnyun Low-Sulfide Pt–Pd Deposit of the Monchetundra Basic Massif,Kola Peninsula,Russia

The geology of the basal-structural Loypishnyun low-sulfide Pt–Pd deposit is characterized, including its mineral composition and the peculiarities of its PGE and chalcophile-element distribution in ore. The deposit is situated in the northeastern part of the Monchetundra basic massif and is localized in its lower norite–orthopyroxenite zone, intensely injected with late gabbroic rocks. Two ore zones are distinguished within the deposit. Ore zone 1 has been traced by drilling for about 1.5 km at a thickness from 10–15 to 120 m and incorporates from two to nine separate lenticular–sheetlike orebodies 0.5–25 m in thickness. Ore zone 2 has been traced for 550 m and is represented by one orebody 5–35 m thick. The internal structure of the orebodies is characterized by alternation of low-grade (Pt + Pd = 0.5–0.9 gpt), ordinary (Pt + Pd = 1.0–1.9 gpt), and high-grade (Pt + Pd > 2 gpt) interlayers of various thickness. The ores are spatially and genetically related to sulfide mineralization (pentlandite–chalcopyrite–pyrrhotite) in an amount of 1–5 vol %. The PGE distribution in ores normalized to primitive mantle is characterized by fractionation of easily fusible platinoids with a positive Pd anomaly. The spectra of chalcophile elements normalized to primitive mantle are notable for elevated Te, Bi, As, and Se contents with respect to Sn, Hg, and Pb, which reflects the significant contribution of Te, Bi, and As in the formation of platinum group minerals (PGM), whereas Se, which is devoid of proper mineral phases, most likely is an admixture in the composition of sulfides. The S/Se value in ore of the Loypishnyun deposit varies from 31 to 814. The platinum group elements (PGE) in ore are represented by 45 noble metal minerals. Ore zone 1 is characterized by lateral mineral zoning, which is expressed as replacement of a bismuthotelluride–sulfide PGM assemblage by an assemblage of copper–PGE compounds and alloys. In ore zone 2, a mineral assemblage of tellurides, copper–PGE compounds and alloys predominates, with native gold, silver, and palladium, as well as sulfides and bismuthotellurides, playing a subordinate role. The formation of PGM ore proceeded under variable sulfur fugacity conditions, beginning with the late magmatic stage at temperatures of 900–700°C and ending with hydrothermal transformation at a temperature of <500°C.     

Platinum-group elements in alpine-type ultramafic rocks and related chromite ores of the main ophiolite belt of the Urals

On the basis of a representative collection of ultramafic rocks and chromite ores and a series of technological samples from the largest (Central and Western) deposits in the Rai-Iz massif of the Polar Urals and the Almaz-Zhemchuzhina and Poiskovy deposits in the Kempirsai massif of the southern Urals, the distribution and speciation of platinum-group elements (PGE) in various type sections of mafic-ultramafic massifs of the Main ophiolite belt of the Urals have been studied. Spectral-chemical and spectrophotometric analyses were carried out to estimate PGE in 700 samples of ultramafic rocks and chromite ores; 400 analyses of minerals from rocks, ores, and concentrates and 100 analyses of PGE minerals (PGM) in chromite ores and concentrates were performed using an electron microprobe. Near-chondritic and nonchondritic PGE patterns in chromitebearing sections have been identified. PGE mineralization has been established to occur in chromite ore from all parts of the mafic-ultramafic massifs in the Main ophiolite belt of the Urals. The PGE deposits and occurrences discovered therein are attributed to four types (Kraka, Kempirsai, Nurali-Upper Neiva, and Shandasha), which are different in mode of geological occurrence, geochemical specialization, and placer-forming capability. Fluid-bearing minerals of the pargasite-edenite series have been identified for the first time in the matrix of chromite ore of the Kempirsai massif (the Almaz-Zhemchuzhina deposit) and Voikar-Syn’ya massif (the Kershor deposit). The PGE grade in various types of chromite ore ranges from 0.1–0.2 to 1–2 g/t or higher. According to technological sampling, the average PGE grade in the largest deposits of the southeastern ore field of the Kempirsai massif is 0.5–0.7 g/t. Due to the occurrence of most PGE as PGM 10–100 mm in size and the proved feasibility of their recovery into nickel alloys, chromites of the Kempirsai massif can be considered a complex ore with elevated and locally high Os, Ir, and Ru contents. The Nurali-Upper Neiva type of ore is characterized by small-sized primary deposits, which nevertheless are the main source of large Os-Ir placers in the Miass and Nev’yansk districts of the southern and central Urals, respectively.     

First report of platinum-group minerals from a hornblende gabbro dyke in the vicinity of the Southeast Extension Zone of the Voisey’s Bay Ni-Cu-Co deposit, Labrador

Summary This study reports the first documented occurrence of platinum group-minerals (PGM) in the vicinity of the Voisey’s Bay magmatic sulfide ore deposit. The PGM are present in a sulfide poor, hornblende gabbro dyke in the Southeast Extension Zone of the massive sulfide Ovoid deposit. The dyke has somewhat elevated concentrations of platinum-group elements (PGE) and gold (up to 1.95 g/t Pt, 1.41 g/t Pd, and 6.59 g/t Au), as well as Cu, Pb, Ag, Sn, Te, Bi and Sb. The PGM formed by magmatic processes and were little disturbed by subsequent infiltration of an externally-supplied hydrothermal fluid. To date, no similar PGM occurrences have been discovered in the Ovoid deposit itself. Whole rock REE patterns indicate that the dyke is geochemically related to the main conduit troctolites, which carry the bulk of the massive sulfide mineralization at Voisey’s Bay. The PGE mineralization is Pt- and Pd-rich, where the Pt and Pd occur predominantly as discrete PGM with minor Pd in solid solution in galena (average=1.8 ppm) and pentlandite (average=2 ppm). The discrete PGM are predominantly hosted by disseminated base-metal sulfides (bornite, chalcopyrite, and galena) (56 vol%) and are associated with other precious metal minerals (13 vol%) with only ∼3 vol% of the PGM hosted by silicate minerals. In whole rock samples, the PPGE (Pt, Pd, and Rh) correlate with abundances of chalcopyrite, bornite, galena, and other precious metal minerals (PMM), whereas the IPGE (Ir, Ru, and Rh) correlate with pyrrhotite and pentlandite. There are no correlations of the PGE with chlorine. Lead isotope compositions of galena associated with the PGE mineralization in the Southeast Extension Zone are broadly similar to those for galena in the Ovoid. The lead isotope compositions are much different from those in the Voisey’s Bay Syenite, which is a potential external hydrothermal fluid source. The observed Cu-rich, Pb-rich sulfide compositions and associated Pt-Pd-Au-Ag-Sn-Te-Bi-Sb assemblage in the dyke can be produced magmatically as late ISS differentiates (e.g., Prichard et al., 2004). Melting temperatures of the PGM are also consistent with a magmatic origin. Following crystallization of PGM from magmatic sulfide, an external REE-enriched hydrothermal fluid was introduced to the system, producing secondary amphibole and locally remobilizing the Pb and Sn from the sulfides hosting the PGM. Author’s address: M. A. E. Huminicki, Department of Earth Sciences, Memorial University of Newfoundland, St. John’s, NL, Canada A1B 3X5     

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.     

Platinum-group minerals from the Jinbaoshan Pd–Pt deposit,SW China: evidence for magmatic origin and hydrothermal alteration

The Jinbaoshan Pt–Pd deposit in Yunnan, SW China, is hosted in a wehrlite body, which is a member of the Permian (∼260 Ma) Emeishan Large Igneous Province (ELIP). The deposit is reported to contain one million tonnes of Pt–Pd ore grading 0.21% Ni and 0.16% Cu with 3.0 g/t (Pd + Pt). Platinum-group minerals (PGM) mostly are ∼10 μm in diameter, and are commonly Te-, Sn- and As-bearing, including moncheite (PtTe₂), atokite (Pd₃Sn), kotulskite (PdTe), sperrylite (PtAs₂), irarsite (IrAsS), cooperite (PtS), sudburyite (PdSb), and Pt–Fe alloy. Primary rock-forming minerals are olivine and clinopyroxene, with clinopyroxene forming anhedral poikilitic crystals surrounding olivine. Primary chromite occurs either as euhedral grains enclosed within olivine or as an interstitial phase to the olivine. However, the intrusion has undergone extensive hydrothermal alteration. Most olivine grains have been altered to serpentine, and interstitial clinopyroxene is often altered to actinolite/tremolite and locally biotite. Interstitial chromite grains are either partially or totally replaced by secondary magnetite. Base-metal sulfides (BMS), such as pentlandite and chalcopyrite, are usually interstitial to the altered olivine. PGM are located with the BMS and are therefore also interstitial to the serpentinized olivine grains, occurring within altered interstitial clinopyroxene and chromite, or along the edges of these minerals, which predominantly altered to actinolite/tremolite, serpentine and magnetite. Hydrothermal fluids were responsible for the release of the platinum-group elements (PGE) from the BMS to precipitate the PGM at low temperature during pervasive alteration. A sequence of alteration of the PGM has been recognized. Initially moncheite and atokite have been corroded and recrystallized during the formation of actinolite/tremolite, and then, cooperite and moncheite were altered to Pt–Fe alloy where they are in contact with serpentine. Sudburyite occurs in veins indicating late Pd mobility. However, textural evidence shows that the PGM are still in close proximity to the BMS. They occur in PGE-rich layers located at specific igneous horizons in the intrusion, suggesting that PGE were originally magmatic concentrations that, within a PGE-rich horizon, crystallized with BMS late in the olivine/clinopyroxene crystallization sequence and have not been significantly transported during serpentinization and alteration.     

Chemical composition of platinum-group minerals from the Bor-Uryakh massif (Maimecha-Kotui Province,Russia)

This contribution firstly presents particularities of mineral chemistry of platinum-group elements (PGE) mineralization from placer deposits linked to the Bor-Uryakh massif of the Maimecha-Kotui Province, northern part of the Siberian Craton. The chemical composition of PGE mineralization has been studied by electron microprobe analysis. At Bor-Uryakh, main platinum-group minerals (PGM) comprise Os-Ir and Pt-Fe alloys represented by individual crystals, and polyphase PGM assemblages. The majority (e.g., 12 out of 19) of the Os-rich nuggets are iridian osmium, with subordinate amounts of native osmium (Os) and chengdeite (Ir₃Fe). Pt-Fe alloys have a stoichiometric composition close to Pt₂Fe. According to the nomen-clature by L. Cabri and C. Feather [1975] these minerals correspond to ferroan platinum. Based on geological position and geochemical features of investigated PGE mineralization the particular rock sources have been established. This study has demonstrated the similarity of chemical characteristics of Os-Ir and Pt-Fe alloys of the Bor-Uryakh massif to those of PGM from the Guli massif (Maimecha-Kotui Province), platiniferous zoned-type ultramafic massifs (e.g., Kondyor, Inagli and Chad) of the Aldan Province and Platinum belt of the Urals (Nizhny Tagil, Kytlym, etc.).     

Typomorphic features of platinum group minerals in the Kundus Placer gold (Kaakhem Ophiolite Belt,Tuva)

Platinum group minerals (PGM) are rather widespread as admixture in gold placer deposits in Tuva. The present paper reports new data on PGM in the Kundus gold placer confined to the Kaakhem ophiolite belt. The minerals are mainly represented by solid solutions of the Os-Ir-Ru system. They make up rims of sulfoarsenides, sulfides, and arsenides of the platinum group elements (PGE) developed after primary minerals. PGMs of this placer always contain traces of Pd (0.33–1.58 wt %), Cu (0.29–0.50 wt %), and As (0.03–2.17 wt %), as well as Ni and Sb (within the detection limit). Typomorphic features of minerals along with the set of main elements and isomorphic trace-elements in the major and secondary mineral species, suggest that sources for the studied placer was represented by the Alpine-type ultramafics and associated chromitites. We cannot also rule out that PGM mineralization was influenced later intrusions that promoted the formation of rims of sulfoarsenides, sulfides, and arsenides of PGE. The PGM rims are marked by the S and As isomorphism, which characterizes the composition of mixtures rather than independent mineral types (end members of isomorphic series). In one case, minerals are represented by the isomorphous mixture of sulfoarsenides with a limited role of sulfides; in another case, by arsenides with a limited role of sulfoarsenides.     

Geochemical and Isotopic Geochemical Characteristics of PGE Mineralization in the Lukkulaisvaara Layered Massif,Karelia, Russia

The study of the Lukkulaisvaara layered massif from the Olang group of intrusions in northern Karelia corroborates the important role of supplementary intrusive phases of PGE mineralization. Injection and crystallization of new magma portions result in (1) the formation of potholelike depressions within intrusion and (2) a return to high-temperature olivine-bearing mineral assemblages in the mafic part of section. PGM formation is accompanied by crystallization of secondary minerals in a wide range of temperatures and pressures. To provide insight into these problems, a geochemical study of Nadezhda area has been performed and new data obtained for the distribution of isotopes in the Rb–Sr, Pb–Pb, and Sm–Nd systems.     

Ore petrology of chromite-PGE mineralization in the Kempirsai ophiolite complex

Summary The platinum group minerals (PGM) in chromite ores of the Kempirsai ophiolite massif, located south of the Ural Mountains, are extremely varied in composition and represented predominantly by alloys, sulfides, arsenides, and sulfosalts of the iridium-group PGE (IPGE). The earlier Ir-Os-Ru alloys prevail over the later Cu-Os-Ru, Cu-Ir, Ni-Ir, Ni-Os-Ir-Ru, and Ni-Ru-Os-Fe alloys rich in base metals (BM). The earlier Ru-Os disulfides crystallize coevally with Ir-Os-Ru alloys, whereas the later sulfides are represented by compounds with a variable stoichiometry and a wide miscibility of Ni, Cu, Ir, Rh, Os, and Fe. Phase relations of PGE alloys with PGE-BM alloys, sulfides and sulfoarsenides confirm that deposition of these minerals was defined by a general evolution of PGE fractionation in the mineral-forming system but not by a super-imposed process. The leading mechanism of PGM crystallization is thought to be their dendritic growth during gas-transport reactions from low-density gaseous fluid enriched in PGE. The representative technological sampling of 0.5 million tons of an ore showed that the average PGE content in chromite ore is 0.71 ppm which leads to an evaluation of the PGE resources to be no less than 250 tons. Hence, the Kempirsai deposit is not only a giant chromium deposit, but also a giant deposit of IPGE: Ir, Ru, and Os. The size parameters of PGM and their aggregates suggests that the PGE may be recoverable in separate concentrates. Author’s address: Vadim Vadimovich Distler, Institute of Geology of Ore Deposits, Mineralogy, Petrography and Geochemistry Russian Academy of Sciences (IGEM RAS), Staromonetny 35, 119017 Moscow, Russia     

Alteration of platinum-group minerals and dispersion of platinum-group elements during progressive weathering of the Aguablanca Ni–Cu deposit, SW Spain

The distribution, mineralogy and mobility of the platinum-group elements (PGE) in the surface environment are poorly understood. This study of the lower, less altered and upper, more altered gossan, overlying the Aguablanca Ni–Cu-(PGE) magmatic deposit (Spain), has shown that the platinum-group minerals (PGM) are progressively oxidised and dispersed into iron oxides that form the gossan. A combination of the characterization of PGE in host PGM, using a scanning electron microscope, and measurement of PGE at lower concentrations in host iron oxides, using laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS), has for the first time allowed the total distribution of the PGE within a gossan to be documented. This study has revealed a complete in situ alteration and dispersion sequence of the PGM including (1) breakdown of both the more stable Pt-arsenides, Pt/Pd-tellurides and the less stable bismuthotellurides, (2) formation of partially oxidised PGM, (3) development of a wide range of oxidised Pt- and Pd-bearing phases, (4) subsequent formation of Fe–PGE-oxides and PGE-hydroxides, (5) incorporation of PGE into ferruginous supergene products and lastly (6) concentration of PGE at the edges of veins and iron oxides. Dispersion of Pd is greater than for the other PGE with Pd being widely distributed throughout the iron oxides. This oxidising environment produced PGE-oxides rather than PGE-alloys, also commonly found in the surface environment, especially in placers. These results provide critical evidence for the stages of mineralogical change from PGE host mineralogy in magmatic ores to surface weathering producing PGE-oxides.     

Platinum-group element distribution in the oxidized Main Sulfide Zone, Great Dyke, Zimbabwe

In the Great Dyke mafic/ultramafic layered intrusion of Zimbabwe, economic concentrations of platinum-group elements (PGE) are restricted to sulfide disseminations in pyroxenites of the Main Sulfide Zone (MSZ). Oxidized ores near the surface constitute a resource of ca. 400 Mt. Mining of this ore type has so far been hampered due to insufficient recovery rates. During the oxidation/weathering of the pristine ores, most notably, S and Pd are depleted, whereas Cu and Au are enriched. The concentrations of most other elements (including the other PGE) remain quite constant. In the oxidized MSZ, PGE occur in different modes: (1) as relict primary PGM (mainly sperrylite, cooperite, and braggite), (2) in solid solution in relict sulfides (dominantly Pd in pentlandite, up to 6,500 ppm Pd and 450 ppm Pt), (3) as secondary PGM neoformations (i.e., Pt–Fe alloy and zvyagintsevite), (4) as PGE oxides/hydroxides that replace primary PGM as the result of oxidation, (5) hosted in weathering products, i.e., iron oxides/hydroxides (up to 3,600 ppm Pt and 3,100 ppm Pd), manganese oxides/hydroxides (up to 1.6 wt.% Pt and 1,150 ppm Pd), and in secondary phyllosilicates (up to a few hundred ppm Pt and Pd). In the oxidized MSZ, most of the Pt and Pd are hosted by relict primary and secondary PGM; subordinate amounts are found in iron and manganese oxides/hydroxides. The amount of PGE hosted in solid solution in sulfides is negligible. Considerable local variations in the distribution of PGE in the oxidized ores complicate a mineralogical balance. Experiments to evaluate the PGE recovery from oxidized MSZ ore show that using physical concentration techniques (i.e., electric pulse disaggregation, hydroseparation, and magnetic separation), the PGE are preferentially concentrated into smaller grain size fractions by a factor of 2. Highest PGE concentrations occur in the volumetrically insignificant magnetic fraction. This indicates that a physical preconcentration of PGE is not feasible and that chemical, bulk-leaching methods need to be developed in order to successfully recover PGE from oxidized MSZ ore.     

Distribution and mineralogy of platinum-group elements in altered chromitites of the Campo Formoso layered intrusion (Bahia State, Brazil): control by magmatic and hydrothermal processes

Summary Polyphase, penetrative hydrothermal metasomatism in chromitites of the Campo Formoso layered intrusion produced spectacular chromite – ferrian chromite zoning and transformed the primary intercumulus silicates into a chlorite – serpentine – carbonate – talc assemblage. Alteration did not substantially modify the composition of chromite cores and the distribution of platinum-group elements (PGE) through the sequence of chromitite layers, which still are consistent with magmatic fractionation processes. Texture and composition of laurite and Os–Ir–Ru alloys included in chromite cores indicate that these PGM were not altered, and are probably magmaticin origin. In contrast, the PGM located in the intergranular chlorite matrix (laurite, Ir–Ru–Rh sulfarsenides and Pt–Pd compounds with Sb, Bi and Te) display evidence of hydrothermal reworking. These PGM are intimately intergrown with low-temperature Ni-sulfides. The paragenesis suggests that the Ni-sulfides-PGM assemblage formed at the expenses of unknown PGM precursors, which must have been originally present in the intercumulus silicate matrix. Mechanism of formation involves a sequence of dissolution-precipitation events controlled by variation of redox conditions during chromite alteration. The presence of a secondary ore mineral assemblage consisting of galena, bismuthinite, native antimony, and various Pb–Sb compounds suggests a possible contribution of fluids derived from the adjacent granite.     

Palladium and gold mineralization in podiform chromitite at Kraubath, Austria

Summary ?We report, for the first time, the occurrence of five palladium-rich, one palladium bearing and two gold-silver minerals from podiform chromitites in the Eastern Alps. Minerals identified include braggite, keithconnite, stibiopalladinite, potarite, mertieite II, Pd-bearing Pt-Fe alloy, native gold and Ag-Au alloy. They occur in heavy mineral concentrates produced from two massive podiform chromitite samples (unaltered and highly altered) of the Kraubath ultramafic massif, Styria, Austria. Distribution patterns of platinum-group elements (PGE) in these chromitites show considerable differences in the behaviour of the less refractory PGE (PPGE-group: Rh, Pt, Pd) compared to the refractory PGE (IPGE-group: Os, Ir, Ru). PPGE are more enriched in chromitite showing pronounced alteration features. The unaltered chromitite displays a negatively sloped chondrite-normalised PGE pattern similar to typical ophiolitic-podiform chromitite. Except for the Pd- and Au-Ag minerals that are generally rare in ophiolites, about 20 other platinum-group minerals (PGM) have been discovered. They include PGE-sulphides (laurite, erlichmanite, kashinite, bowieite, cuproiridsite, cuprorhodsite, unnamed Ir-rich variety of ferrorhodsite, unnamed Ni-Fe-Cu-Rh- and Ni-Fe-Cu-Ir-Rh monosulphides), PGE alloys (Pt-Fe, Ir-Os, Os-Ir and Ru-Os-Ir), PGE-sulpharsenides (irarsite, hollingworthite, platarsite, ruarsite and a number of intermediate species), sperrylite and a Ru-rich oxide (?). Three PGM assemblages have been recognised and attributed to different processes ranging from magmatic to hydrothermal and weathering-related. Pd-rich minerals are characteristic of both chromitite types, although their chemistry and relative proportions vary considerably. Keithconnite, braggite and Pd-bearing ferroan platinum, together with a number of PGE-sulphides (mainly laurite-erlichmanite) and alloys, are typical only of the unaltered podiform chromitite (assemblage I). Euhedral mono- and polyphase PGM grains in the submicron to 100 μm range show features of primary magmatic assemblages. The diversity of PGM in these assemblages is unusual for ophiolitic environments. In assemblage II, laurite-erlichmanite is intergrown with and overgrown by PGE-sulpharsenides; other minerals of assemblage I are missing. Potarite, stibiopalladinite, mertieite II, native gold and Ag-Au alloys, as well as PGE-sulpharsenides, sperrylite and base metal arsenides and sulphides are characteristic for the highly altered chromitite (assemblage III). They occur either interstitial to chromite in association with metamorphic silicates, in chromite rims or along cracks, and are thus interpreted as having formed by remobilization of PGE by hydrothermal processes during polyphase regional metamorphism. Received August 3, 2000;/revised version accepted December 28, 2000     

Distribution of platinum-group elements in magmatic and altered ores in the Jinchuan intrusion, China: an example of selenium remobilization by postmagmatic fluids

The division of platinum-group elements (PGE) between those hosted in platinum-group minerals (PGM) versus those in solid solution in base metal sulfides (BMS) has been determined for ores from the PGE-bearing Ni-Cu-rich Jinchuan intrusion in northwest China. All the BMS are devoid of Pt and Ir, and magmatic BMS are also barren of Rh. These PGE may have been scavenged by arsenic to form PGM during magmatic crystallization of the BMS. Pd, Os, and Ru are recorded in BMS and Pd is predominantly in solid solution in pentlandite. Unlike the fresh magmatic ores, in altered or serpentinized ores, Pd-PGM are present. Froodite is hosted in magnetite, formed during alteration of BMS, accompanied by sulfur loss and liberation of Pd. Michenerite ([Pd,Pt]BiTe), sperrylite (PtAs₂), and Au-bearing PGM are located in altered silicates. Irarsite (IrAsS) occurs mainly enclosed in BMS. Padmaite (PdBiSe), identified at the junctions of magnetite and BMS, was the last PGM to form and locally partially replaces earlier non-Se-bearing PGM. We propose that padmaite formed under oxidizing conditions during late local remobilization of Se from the BMS. Se-bearing PGM are rare and our review shows they are frequently associated with carbonate, suggesting that Pd and Se can be mobilized great distances in low pH oxidizing fluids and may be precipitated on contact with carbonate. S/Se ratios are used by researchers of magmatic Ni-Cu-PGE ores to determine sulfur loss, assuming Se is immobile and representative of magmatic sulfur content. This study shows that Se as well as S is potentially mobile and this should be considered in the use of S/Se ratios.     

甘肃省金川超大型铜镍硫化物矿床富铂钯矿石成因研究

金川铜镍硫化物矿床的成因及侵位机制尤其是块状矿石和铂钯富集体成因一直存在较大争议,本文通过对金川矿石的空间关系、地球化学特征研究,指出金川矿床遭受构造及热液蚀变作用影响明显,块状矿石相对富集Os、Ir、Ru、Rh,铂钯富集体相对富集Pt、Pd、Au、Cu.研究认为,块状矿石是晚期纯硫化物矿浆上升贯入后经单硫化物固溶体结晶堆积而成,残余熔浆形成初始铂族矿物,后期矿体遭受热液蚀变及构造剪切-热液作用,使Pt、Pd、Cu、Au迸一步富集形成铂钯富集体,并在有利于成矿的空间聚集成矿.块状矿石与铂钯富集体关系密切,据此推测2 #矿体、24#矿体深边部裂隙中具有良好的勘探前景.     

Platinum–Group Minerals in Podiform Chromitite from the Kamuikotan Zone, Hokkaido, Northern Japan

Abstract: Ru–Os–Ir alloys have been found in two podiform chromitites located at the Chiroro and Bankei mines in the Sarugawa peridotite complex in the Kamuikotan zone, Hokkaido, Japan. This is the first report on the occurrence of PGM (= platinum-group minerals) from chromitites in Japan. The Ru–Os–Ir alloys most typically form polyhedra associated with other minerals (Ni–Fe alloys and heazlewoodite) in chromian spinel. The PGM are possibly pseudomorphs after some primary PGM such as laurite and are chemically highly inhomogeneous, indicating a low-temperature alteration origin. This is consistent with intense alteration (formation of serpentine, uvarovite and kämmererite) imposed on the Kamuikotan chromitites. High-temperature primary PGE (platinum–group elements)–bearing sulfides were possibly recrystallized at low temperatures into a new assemblage of PGM, Ni-Fe alloys and sulfides. Placer PGM around the peridotite complexes are chemically different from the PGM in dunite and chromitite possibly due to the, as yet, incomplete search for the rock-hosted PGM. The PGE content in chromitites is distinctly higher in those in the Kamuikotan zone than in those in the Sangun zone of Southwest Japan, consistent with the more refractory nature (Cr# of spinel, up to 0.8) of the former than the latter (Cr# of spinel, 0.5).