Postmagmatic,hydrothermal origin of sulfide and arsenide mineralizations at Limassol Forest,Cyprus

The Ni-Co-Cu ores of Pevkos and Lakxia tou Mavrou, Limassol Forest, Cyprus, have been investigated microscopically and by electron microprobe analysis. At Pevkos, the mineral association consists of pyrrhotite, pentlandite, maucherite, chalcopyrite, cubanite, magnetite, chromite and valleriite with minor amounts of westerveldite, bornite, neodigenite, covellite and cobaltite. The mineralization at Lakxia tou Mavrou comprises pyrrhotite, pentlandite, löllingite, chalcopyrite, cubanite and chromite with traces of magnetite, pyrite, maucherite and valleriite. Paragenetic, compositional and textural features suggest a nonmagmatic origin for the sulfides and arsenides; they were deposited during serpentinization of the ultramafic host rocks. A conceptual model for mineralization linked to decreasing temperatures in a hydrothermal system is presented.     

First occurrence of Pd-bearing galena (Sedova Zaimka copper–nickel mineralization,Western Siberia)

The paper presents data on the chemical composition and mineral association of Pd-bearing galena, discovered in hydrothermal–metasomatic sulfide Cu–Ni ores of the Sedova Zaimka mineralization (Western Siberia). In the Sedova Zaimka mineralization, galena is an accessory mineral and occurs in association with pyrrhotite, pentlandite, chalcopyrite, sphalerite, argentopentlandite, tsumoite, and native bismuth. The Pd contents in galena are 0.5–0.9 wt %. Palladium occurs in galena in the form of isomorphic impurities and is not related to microinclusions of Pd-bearing minerals.     

Ore Mineralization in Ultramafic and Metasomatic Rocks of the Ospin–Kitoi Massif,Eastern Sayan

In the Ospin–Kitoi ultramafic massif of the Eastern Sayan, accessory and ore Cr-spinel are mainly represented by alumochromite and chromite. Copper–nickel mineralization hosted in serpentinized ultramafic rocks occurs as separate grains of pentlandite and pyrrhotite, as well as assemblages of (i) hexagonal pyrrhotite + pentlandite + chalcopyrite and (ii) monoclinal pyrrhotite + pentlandite + chalcopyrite. Copper mineralization in rodingite is presented by bornite, chalcopyrite, and covellite. Talc–breunnerite–quartz and muscovite–breunnerite–quartz listvenite contains abundant sulfide and sulfoarsenide mineralization: pyrite, gersdorffite, sphalerite, Ag–Bi and Bi-galena, millerite, and kuestelite. Noble metal mineralization is represented by Ru–Ir–Os alloy, sulfides, and sulfoarsenides of these metals, Au–Cu–Ag alloys in chromitite, laurite intergrowth, an unnamed mineral with a composition of Cu₃Pt, orcelite in carbonized serpentinite, and sperrylite and electrum in serpentinite. Sulfide mineralization formed at the late magmatic stage of the origination of intrusion and due to fluid–metamorphic and retrograde metasomatism of primary rocks.     

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.     

Chalcophile and platinum-group element (PGE) concentrations in the sulfide minerals from the McCreedy East deposit,Sudbury, Canada,and the origin of PGE in pyrite

Magmatic sulfide deposits consist of pyrrhotite, pentlandite, chalcopyrite (± pyrite), and platinum-group minerals (PGM). Understanding the distribution of the chalcophile and platinum-group element (PGE) concentrations among the base metal sulfide phases and PGM is important both for the petrogenetic models of the ores and for the efficient extraction of the PGE. Typically, pyrrhotite and pentlandite host much of the PGE, except Pt which forms Pt minerals. Chalcopyrite does not host PGE and the role of pyrite has not been closely investigated. The Ni–Cu–PGE ores from the South Range of Sudbury are unusual in that sulfarsenide PGM, rather than pyrrhotite and pentlandite, are the main carrier of PGE, probably as the result of arsenic contribution to the sulfide liquid by the As-bearing metasedimentary footwall rocks. In comparison, the North Range deposits of Sudbury, such as the McCreedy East deposit, have As-poor granites in the footwall, and the ores commonly contain pyrite. Our results show that in the pyrrhotite-rich ores of the McCreedy East deposit Os, Ir, Ru, Rh (IPGE), and Re are concentrated in pyrrhotite, pentlandite, and surprisingly in pyrite. This indicates that sulfarsenides, which are not present in the ores, were not important in concentrating PGE in the North Range of Sudbury. Palladium is present in pentlandite and, together with Pt, form PGM such as (PtPd)(TeBi)₂. Platinum is also found in pyrite. Two generations of pyrite are present. One pyrite is primary and locally exsolved from monosulfide solid solution (MSS) in small amounts (<2 wt.%) together with pyrrhotite and pentlandite. This pyrite is unexpectedly enriched in IPGE, As (± Pt) and the concentrations of these elements are oscillatory zoned. The other pyrite is secondary and formed by alteration of the MSS cumulates by late magmatic/hydrothermal fluids. This pyrite is unzoned and has inherited the low concentrations of IPGE and Re from the pyrrhotite and pentlandite that it has replaced.     

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).     

Cu-Ni-PGE mineralization at Rometölväs,Koillismaa layered igneous complex,Finland

Sulphides, tellurides and sulpharsenides, with special reference to the platinum-group minerals (PGM), have been studied from a subeconomic Cu-Ni-PGE mineralization encountered within the Syöte section of the Lower Proterozoic (2.44 Ga) Koillismaa layered igneous complex (KLIC) in northern Finland using electron microprobe and ore-microscopical methods. The ore minerals occur partly as strata-bound patches and spots associated with spots of light-coloured secondary low-temperature silicates in the gabbronorite IV of the general igneous stratigraphic column of the complex and partly as a fine-grained impregnation in the penecontemporaneous basic sills and dykes. Among the PGM sperrylite, michenerite and a palladian bismuthian melonite have been encountered. The chemical composition is reported for these minerals as well as for the rest of the ore minerals (chalcopyrite, pentlandite, pyrrhotite, pyrite, sphalerite, cobaltite and hessite). It is concluded that volatile components played a significant role in the solution, transport and the final deposition of the sulphides and the PGM.     

The Crater Mountain Deposit,Papua New Guinea: Porphyry‐related Au–Te System

Ore mineralization and wall rock alteration of Crater Mountain gold deposit, Papua New Guinea, were investigated using ore and host rock samples from drill holes for ore and alteration mineralogical study. The host rocks of the deposit are quartz‐feldspar porphyry, feldspar‐hornblende porphyry, andesitic volcanics and pyroclastics, and basaltic‐andesitic tuff. The main ore minerals are pyrite, sphalerite, galena, chalcopyrite and moderate amounts of tetrahedrite, tennantite, pyrrhotite, bornite and enargite. Small amounts of enargite, tetradymite, altaite, heyrovskyite, bismuthinite, bornite, idaite, cubanite, native gold, CuPbS₂, an unidentified Bi‐Te‐S mineral and argentopyrite occur as inclusions mainly in pyrite veins and grains. Native gold occurs significantly in the As‐rich pyrite veins in volcanic units, and coexists with Bi‐Te‐S mineral species and rarely with chalcopyrite and cubanite relics. Four mineralization stages were recognized based on the observations of ore textures. Stage I is characterized by quartz‐sericite‐calcite alteration with trace pyrite and chalcopyrite in the monomict diatreme breccias; Stage II is defined by the crystallization of pyrite and by weak quartz‐chlorite‐sericite‐calcite alteration; Stage III is a major ore formation episode where sulfides deposited as disseminated grains and veins that host native gold, and is divided into three sub‐stages; Stage IV is characterized by predominant carbonitization. Gold mineralization occurred in the sub‐stages 2 and 3 in Stage III. The fS₂ is considered to have decreased from ～10^?2 to 10^?14 atm with decreasing temperature of fluid.     

Petrology and mineralisation of the southern Platreef: northern limb of the Bushveld Complex, South Africa

The Platreef, the putative local analogue of the Merensky Reef, forms the floor to the mafic succession in the northern limb of the Bushveld Complex. We define the Platreef as ‘the lithologically variable unit, dominated by pyroxenite, which is irregularly mineralised with PGE, Cu and Ni, between the Transvaal metasedimentary footwall or Archaean basement and the overlying Main Zone gabbronorite’. We define the mineralisation around calcsilicate xenoliths within the Main Zone in the far north of the limb as a ‘Platreef-style‘ mineralisation. The Platreef (ss) has a strike extent of ∼30 km, whereas Platreef-style mineralisation occurs over a strike length of 110 km. The Platreef varies from 400 m thick in the S to <50 m in the N. The overall strike is NW or N, with dips 40–45°W at surface, shallowing down dip, The overall geometry of the southern Platreef appears to have been controlled by irregular floor topography. The maximum thickness of the southern Platreef occurs in two sub-basins on the farms Macalacaskop and Turfspuit. Lithologically, the southern Platreef is heterogeneous and more variable than sectors further north and, although predominantly pyroxenitic, includes dunites, peridotites and norite cycles with anorthosite in the mid to upper portion. Zones of intense serpentinisation may occur throughout the package. Faults offset the strike of the Platreef: a N–S, steeply dipping set is predominant with secondary ENE and ESE sets dipping 50–70°S. The fault architecture was pre-Bushveld and also locally controlled thickening and thinning of the succession. Country rock xenoliths, <1500 m long, are common. On Macalacaskop, these are typically quartzites and hornfelsed banded ironstones, shales, mudstones and siltstones whereas on Turfspruit dolomitic or calcsilicate xenoliths also occur. Sulphides may reach >30 modal% in some intersections. These are dominated by pyrrhotite, with lesser pentlandite and chalcopyrite, minor pyrite and traces of a wide compositional range of sulphides. In the southern sector, mineralised zones have Cu grades of 0.1–0.25% and Ni 0.15–0.36%. Massive sulphides are localised, commonly, but not exclusively towards the contact with footwall metasedimentary rocks. Magmatic sulphides are disseminated or net-textured ranging from a few microns to 2 cm grains of pyrrhotite and pentlandite with chalcopyrite and minor pyrite. Much of the sulphide is associated with intergranular plagioclase, or quartz-feldspar symplectites, along the margins of rounded cumulus orthopyroxenes. The PGEs in the southern sector occur as tellurides, bismuthides, arsenides, antimonides, bismuthoantimonides and complex bismuthotellurides. PGM are rarely included in the sulphides but occur as micron-sized satellite grains around interstitial sulphides and within alteration assemblages in serpentinised zones. The Pt:Pd ratio ∼1 and PGE grade may be decoupled from S and base metal abundance.     

The opaque minerals of the ultramafic rocks of New Caledonia

The ultramafic rocks of New Caledonia contain a diversity of disseminated ore minerals in non-economic amounts. Eighteen opaque minerals are described herein with pentlandite, millerite and heazelwoodite most prominent. Several new or unusual mineralogical features are recorded. These include an eutectic intergrowth between pentlandite and primary chalcocite, reaction between pentlandite and chalcocite to form chalcopyrite and millerite, exsolution intergrowth between pentlandite and millerite, between pentlandite and mackinawite and between millerite and cubanite. In the formation of the garnieritic ores of New Caledonia some of the nickel would appear to have been derived from the breakdown of disseminated sulphides as well as from the nickel inherent in the silicate minerals of the ultramafic rocks.

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Résumé Les roches ultramafiques de la Nouvelle-Calédonie comportent une diversité de mineraux disséminés dans des quantités sous-économiques. Dix-huit minéraux opaques y sont décrites avec la proéminence de la pentlandite et la millerite. Quelques caractéristiques minéralogiques, nouvelles et exceptionelles, sont notées. Ceci comprend un enchevêtrement eutectique entre la pentlandite et la chalcopyrite primaire, une réaction entre la pentlandite et la chalcocite qui se changent en chalcopyrite et en millerite, un entredéveloppement par l'exsolution entre la pentlandite et la machinawite et entre la millérite et la cubanite. Il est établi que, pendant la formation du minérais garniéritique de la Nouvelle-Calédonie une part du nickel vient des sulphures disséminées de même que du nickel naissant dans les silicates des roches ultramafiques.

     

Petrography, mineralogy and geochemistry of the Zarshuran Carlin-like gold deposit, northwest Iran

Gold mineralisation at Zarshuran, northwestern Iran, is hosted by Precambrian carbonate and black shale formations which have been intruded by a weakly mineralised granitoid. Granitoid intrusion fractured the sedimentary rocks, thereby improving conditions for hydrothermal alteration and mineralisation. Silicification is the principal hydrothermal alteration along with decalcification and argillisation. Three hydrothermal sulphide mineral assemblages have been identified: an early assemblage of pyrrhotite, pyrite and chalcopyrite; then widespread base metal sulphides, lead-sulphosalts and zoned euhedral arsenical pyrite; and finally late network arsenical pyrite, massive and colloform arsenical pyrite, colloform sphalerite, coloradoite, and arsenic–antimony–mercury–thallium-bearing sulphides including orpiment, realgar, stibnite, getchellite, cinnabar, lorandite and a Tl-mineral, probably christite. Most of the gold at Zarshuran is detectable only by quantitative electron microprobe and bulk chemical analyses. Gold occurs mainly in arsenical pyrite and colloform sphalerite as solid solution or as nanometre-sized native gold. Metallic gold is found rarely in hydrothermal quartz and orpiment. Pure microcrystalline orpiment, carbon-rich shale, silicified shale with visible pyrite grains and arsenic minerals contain the highest concentrations of gold. In many ways Zarshuran appears to be similar to the classic Carlin-type sediment-hosted disseminated gold deposits. However, relatively high concentrations of tellurium at Zarshuran, evidenced by the occurrence of coloradoite (HgTe), imply a greater magmatic contribution in the mineralising hydrothermal solutions than is typical of Carlin-type gold deposits. Received: 13 May 1999 / Accepted: 2 February 2000     

Magma Mixing as a Trigger for Sulphide Saturation in the UG2 Chromitite (Bushveld): Evidence from the Silicate and Sulphide Melt Inclusions in Chromite

It is of great importance to understand the origin of UG2 chromitite reefs and reasons why some chromitite reefs contain relatively high contents of platinum group elements(PGEs: Os, Ir, Ru, Rh,Pt, Pd) or highly siderophile elements(HSEs: Au, Re, PGE). This paper documents sulphide-silicate assemblages enclosed in chromite grains from the UG2 chromitite. These are formed as a result of crystallisation of sulphide and silicate melts that are trapped during chromite crystallisation. The inclusions display negative crystal shapes ranging from several micrometres to 100 μm in size.Interstitial sulphide assemblages lack pyrrhotite and consist of chalcopyrite, pentlandite and some pyrite. The electron microprobe data of these sulphides show that the pentlandite grains present in some of the sulphide inclusions have a significantly higher iron(Fe) and lower nickel(Ni) content than the pentlandite in the rock matrix. Pyrite and chalcopyrite show no difference. The contrast in composition between inter-cumulus plagioclase(An_(68)) and plagioclase enclosed in chromite(An_(13)), as well as the presence of quartz, is consistent with the existence of a felsic melt at the time of chromite saturation.Detailed studies of HSE distribution in the sulphides and chromite were conducted by LA-ICP-MS(laser ablation-inductively coupled plasma-mass spectrometry), which showed the following.(Ⅰ) Chromite contained no detectable HSE in solid solution.(Ⅱ) HSE distribution in sulphide assemblages interstitial to chromite was variable. In general, Pd, Rh, Ru and Ir occurred dominantly in pentlandite, whereas Os,Pt and Au were detected only in matrix sulphide grains and were clearly associated with Bi and Te.(Ⅲ)In the sulphide inclusions,(a) pyrrhotite did not contain any significant amount of HSE,(b) chalcopyrite contained only some Rh compared to the other sulphides,(c) pentlandite was the main host for Pd,(d)pyrite contained most of the Ru, Os, Ir and Re,(e) Pt and Rh were closely associated with Bi forming a continuous rim between pyrite and pentlandite and(f) no Au was detected. These results show that the use of ArF excimer laser to produce high-resolution trace element maps provides information that cannot be obtained by conventional(spot) LA-ICP-MS analysis or trace element maps that use relatively large beam diameters.     

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.     

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     

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     

Geochemistry and mineralogy of Platinum-group elements in the Ransko gabbro–peridotite massif,Bohemian Massif (Czech Republic)

The Ransko gabbro-peridotite massif in Eastern Bohemia is a strongly differentiated intrusive complex of Lower Cambrian age. The complex hosts low grade Ni-Cu ores mainly developed close to the contact of olivine-rich rocks with gabbros, in troctolites and, to a much lesser extent, in both pyroxene and olivine gabbros and plagioclase-rich peridotites. The ore zone is characterized by strong serpentinization and uralitization. The total Ni + Cu locally reaches up to 4 wt%. Anomalous concentrations of platinum-group elements (PGE's) (maximum 532 ppb Pd, 182 ppb Pt, 53 ppb Rh, 15 ppb Ru, 41 ppb Ir) were detected in samples of Cu-Ni and Ni-Cu ores (maximum 2.63 wt% Ni and 2.31 wt% Cu) from the Jezírka orebody. The main ore paragenesis includes pyrrhotite, pentlandite, chalcopyrite, cubanite, pyrite, magnetite, mackinawite, valleriite, ilmenite and sphalerite. During this work, michenerite, froodite, sperrylite, gold, native bismuth, altaite, tsumoite, hessite, an unnamed Bi-Ni telluride, cobaltite-gersdorffite and galena were newly identified. The host rocks originated through partial melting of a slightly depleted mantle source with noble metals scavenged from this primitive magma prior to the development of these rocks.     

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.     

Sulfide mineralogy and sulfur isotope geochemistry of layered sills in the Deer Lake Complex,Minnesota

The Deer Lake Complex, located in north-central Minnesota, consists of a series of layered peridotite-pyroxenite-gabbro sills. Sulfide minerals occur as fine disseminations throughout pyroxenite and gabbro units, and occur more sporadically in peridotite and basal chilled margin units. Sulfide volume percentage rarely exceeds 0.5. A distinct zonation in sulfide mineralogy and sulfur isotopic composition characterizes the sills. Cobaltian pentlandite is the dominant sulfide mineral in peridotite (pd) units, with Ni-enrichment most likely linked to the serpentinization process. δ³⁴Spd values are variable, ranging from ?3.5 to +2.8‰. Sulfide assemblages in pyroxenite (px) and lower gabbro units consist of chalcopyrite, pyrrhotite, and minor pentlandite. δ³⁴S_px values range from ?1 to +1 ‰. Pyrite is the principal sulfide mineral in upper gabbro (μg) units. Its origin may be related to increased f0₂ conditions of the remaining melt and to reaction between a S-bearing volatile phase and mafic silicates. δ³⁴S_ug values range from 1 to 3.5 ‰. Sulfur isotopic values of chilled margin (2–9 ‰) and peridotite units, together with the erratic spatial distribution of sulfide minerals in these zones, suggests that the parent magma was not initially saturated with sulfur, and that local sulfide concentrations are the result of incorporation of sulfur derived from metasedimentary country rocks. Sulfide saturation was more uniformly reached during pyroxenite formation, with contained sulfur being of magmatic origin. Enrichment in ³⁴S of pyrite from upper gabbro may be explained by buildup of isotopically heavy sulfur following a Rayleigh process, coupled with possible involvement of a SO₂-rich fluid phase during hydrothermal alteration.     

Platinum-group element distribution in base-metal sulfides of the UG2 chromitite,Bushveld Complex,South Africa—a reconnaissance study

Two drill cores of the UG2 chromitite from the eastern and western Bushveld Complex were studied by whole-rock analysis, ore microscopy, SEM/Mineral Liberation Analysis (MLA), and laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) analysis. The top and base of the UG2 main seam have the highest bulk-rock Pd and Pt concentrations. Sulfides mostly occur as aggregates of pentlandite, chalcopyrite, and rare pyrrhotite and pyrite or as individual grains associated mostly with chromite grains. In situ LA-ICP-MS analyses reveal that pentlandite carries distinctly elevated platinum-group element (PGE) contents. In contrast, pyrrhotite and chalcopyrite contain very low PGE concentrations. Pentlandite shows average maximum values of 350–1,000 ppm Pd, 200 ppm Rh, 130–175 ppm Ru, 20 ppm Os, and 150 ppm Ir, and is the principal host of Pd and Rh in the studied ores of the UG2. Mass balance calculations were conducted for samples representing the UG2 main seam of the drill core DT46, eastern Bushveld. Pentlandite consistently hosts elevated contents of the whole-rock Pd (up to 55 %) and Rh (up to 46 %), and erratic contents of Os (up to 50 %), Ir (2 to 17 %), and Ru (1–39 %). Platinum-group mineral (PGM) investigations support these mass balance results; most of the PGM are Pt-dominant such as braggite/cooperite and Pt-Fe alloys or laurite (carrying elevated concentrations of Os and Ir). Palladium and Rh-bearing PGM are rare. Both PGE concentrations and their distribution in base-metal sulfides (BMS) in the UG2 largely resemble that of the Merensky Reef, as most of the Pd and Rh are incorporated in pentlandite, whereas pyrrhotite, chalcopyrite, and pyrite are almost devoid of PGE.     

Native bismuth,tsumoite, and Pb-bearing tsumoite from the Tarn’er copper-zinc massive sulfide deposit,northern Urals

Bismuth mineralization, including native bismuth, tsumoite (Bi_1.99–2.03Te_2.00), and Pb-bearing tsumoite (Bi_1.56–1.88Pb_0.45–0.14)_2.00–2.03Te_2.00, was identified in the Au-enriched disseminated ore at the Tarn’er massive sulfide deposit formed under the effect of a large diorite intrusion. Native bismuth associated with hessite forms idiomorphic inclusions in chalcopyrite. The assemblage of Pb-bearing tsumoite, hessite, and altaite occurs as angular allotriomorphic-granular inclusions in silicates or at the contact between silicate and sulfide aggregates. Tsumoite in allotriomorphic-granular aggregates with galena, hessite, and sphalerite is devoid of lead. Gold (Au_0.65Ag_0.35) was identified along with bismuth tellurides. The temperature of contact methamorphism (500–800°C) was estimated from the stability of andalusite, sillimanite, and cordierite. The morphology of the bismuth telluride aggregates in silicates and graphic intergrowth of tsumoite with galena suggest possible crystallization from anatectic melt. The positive correlation between Bi, Te, and Au confirms their probable joint transportation in the melt.