Abundant Platinum-group Minerals Research Articles

The chromitites of the Herbeira massif (Cabo Ortegal Complex, Spain) revisited

The ultramafic rocks of the Herbeira Massif in the Cabo Ortegal Complex (NW Iberia) host chromitite bodies. The textural and compositional study of the host rocks and the chromitites classified them into: (1) Type-I chromitites, forming massive pods of intermediate-Cr chromite (Cr# = 0.60–0.66) within dunites; and (2) Type-II chromitites forming semi-massive horizons of high-Cr chromite (Cr# = 0.75–0.82) interlayered with dunites and pyroxenites. Minor and trace elements (Ga, Ti, Ni, Zn, Co, Mn, V and Sc) contents in the unaltered chromite cores from both types show patterns very similar to fore-arc chromitites, mimicked by the host dunites and pyroxenites. Calculated parental melt compositions suggest that Type-I chromitites crystallized from a melt akin to fore-arc basalt (FAB), while Type-II chromitites originated from a boninite-like parental melt. Both melts are characteristic of a fore-arc setting affected by extension during rollback subduction and have been related to the development of a Cambrian-Ordovician arc. These chromitites are extremely enriched in platinum-group elements (PGE), with bulk-rock PGE contents between 2,460 and 3,600 ppb. Also, the host dunites and pyroxenites exhibit high PGE contents (167 and 324 ppb, respectively), which are higher than those from the primitive mantle and global ophiolitic mantle peridotites. The PGE enrichment is expressed in positively-sloped chondrite-normalized PGE patterns, characterized by an enrichment in Pd-group PGE (PPGE: Rh, Pt and Pd) over the Ir-group PGE (IPGE: Os, Ir and Ru) and abundant platinum-group minerals (PGM) dominated by Rh-Pt-Pd phases (i.e. Rh-Ir-Pt-bearing arsenides and sulfarsenides, Pt-Ir-Pd-base-metal-bearing alloys, and Pt-Pd-bearing sulfides). The PGM assemblage is associated with base-metal sulfides (mostly pentlandite and chalcopyrite) and occurs at the edges of chromite or embedded within the interstitial (serpentinized) silicate groundmass. Their origin has been linked to direct crystallization from a S-As-rich melt(s), segregated by immiscibility from evolved volatile-rich small volume melts during subduction. At c. 380 Ma, retrograde amphibolite-facies metamorphism occurred during the exhumation of the HP-HT rocks of the Capelada Unit, which affected chromitites and their host rocks but preserved the primary composition of chromite cores of the chromitites. This event contributed to local remobilization of PGE as suggested by the negative slope between Pt and Pd and high Pt/Pd ratios in the studied chromitites, and host dunites and pyroxenites. In addition, it promoted the alteration of primary PGM assemblage and the formation of secondary PGM. Nanoscale observations made by focused ion beam high-resolution transmission electron microscopy (FIB/HRTEM) analysis of a composite grain of Rh-bearing arsenide with PGE-base-metal bearing alloys suggest the mobilization and accumulation of small nanoparticles of PGE and base-metals that precipitated from metamorphic fluids forming PGE-alloys. Finally, we offer a comparison of the Cabo Ortegal chromitites with other ophiolitic chromitites involved in the Variscan orogeny, from the Iberian Peninsula to the Polish Sudetes. The studied Cabo Ortegal chromitites are similar to the Variscan chromitites documented in the Bragança (northern Portugal) and Kraubath (Styria, Austria) ophiolitic massifs.

Inclusions in Placer Pt-Fe Alloy Nuggets, Northwest Ecuador: Evolution of a Ural-Alaskan Type PGE Mineralizing System (Cr-Spinel, IPGE, Sulfarsenides, and Cu-Depleted PGM)

Abstract A Cu- and Rh-enriched magmatic ore system is defined by abundant PGM (platinum group mineral) inclusions in forty-four Pt-Fe alloy nuggets from the Camumbi River gold placer, northwest Ecuador. Isoferroplatinum is depleted in Rh, Os, and Ru compared with native platinum, suggesting most crystallized after Os-(Ir) alloy, laurite, and some Rh-PGM. Two Pt-Fe alloy nuggets have zoned hydrothermal alteration rinds, and an UM (unnamed mineral) is (Rh,Pd)4As3. Our previous work shows that silicate glass inclusions define a fractionated co-magmatic compositional series related to primitive hydrous ferrobasalt, and trace element chemistry matches their Late Cretaceous accreted volcanic arc terrane. Here we report exceptional Cr-spinel (Ural-Alaskan type) inclusions coexisting with primitive ferrobasaltic glass crystallized at highest T. Laurite inclusions also indicate high T and S saturation of early melt. Os-(Ir) inclusions are Ru-depleted while two discrete Ir-enriched osmium crystals have remarkable, extreme Ru enrichment and depletion, confirming crystallization before and after laurite. Laurite and osmium inclusions in one Pt-Fe alloy reflect concomitant crystallization and fluctuating low fS2 melt conditions. In experimental primitive Cu-bearing Pt-Pd-S-(As) melt (cf. exsolved from primitive basalt), first Cu-PGM-sulfide crystallization generates a Cu-depleted, Pt-Pd-As-(S) residual melt. At lower T immiscible melts Pt-As-(S) and later Pd-As-(S) crystallize distinctive PGM. We report analogous natural multiphase PGM inclusion assemblages in separate isoferroplatinum nuggets: (1) zoned sulfarsenides, sperrylite, and genkinite, with rare resorbed cognate xenocrystic cooperite (captured from primary sulfide melt) define a high T, Pt-enriched sub-system [Pt > Rh(Pd,Ir,Ru)As,S ≫ Sb,Bi] and (2) zoned sulfarsenides, arsenopalladinite, sperrylite, törnroosite, and gold define a lower T, fractionated Pd-enriched sub-system [(Pd > Rh ≃ Pt > Ir > Au)As,S > Te ≫ Sb,Bi]. The previously undocumented natural S-rich sperrylite (formerly “platarsite”) solid solution series and later crystallized irarsite series are discriminated in terms of Pt-Ir-Rh. Both trends fractionate toward increasing Rh (hollingworthite). The discrete PGM assemblage, sperrylite-telluropalladinite (with exsolved palladium and electrum) defines an IPGE-depleted Pd > Pt(Au > Ag)As ≥ Te ≥ Sb sub-system and records extreme fractionation. Cu-bearing multiphase PGM inclusions (some coexisting with silicate glass) derived from the fraction of Cu-bearing exsolved Pt-Pd-S-(As) melt will be reported separately.

Mineralogical, Textural and Chemical Characteristics of Ophiolitic Chromitite and Platinum Group Minerals from Kabaena Island (Indonesia): Their Petrogenetic Nature and Geodynamic Setting

This contribution presents the first systematic mineralogical study of chromite composition, silicates and PGM (platinum group minerals) by electron microprobes in the podiform chromitite of Kabaena Island (Indonesia) mined in the past. The main target of this study is to understand the petrogenetic nature of the parental melt from which the chromitites of Kabaena Island precipitated and, indirectly, define the geodynamic tectonic setting of their emplacement. The evolution of PGM, from the magmatic stage to low-temperature processes, is also discussed. The variation of the Cr# = Cr/(Cr + Al), being comprised between 0.65 and 0.75, is similar to the podiform-type chromitite and indicates the absence of Al-rich chromitite. The calculated composition of the parental melt varies from arc to MORB (mid-ocean ridge basalts). Several grains of olivine and clinopyroxene have been found in the silicate matrix or included in fresh chromite. Olivine shows a composition typical of a hosted mantle, and clinopyroxene is similar to those analyzed in the forearc of an SSZ (supra-subduction zone). Small PGM, varying in size from 1 to 10 μm, occur in the chromitites. The most abundant PGM is laurite, which has been found included in fresh chromite or in contact with ferrian chromite along the cracks in the chromite. Laurite forms polygonal crystals, and it occurs as a single phase or in association with clinopyroxene and amphibole. Tiny blebs of Ir-Os alloy (less than 2 μm across) have been found associated with grains of awaruite in the serpentine gangue of the chromitites. The composition of the investigated chromitites suggests that they formed in the mantle of a forearc ophiolite. All the discovered grains of laurite are considered to be magmatic in origin, i.e., entrapped as solid phases during the crystallization of chromite at temperatures above 1000 °C and a sulfur fugacity below sulfur saturation. Iridium–osmium alloys are secondary in origin and represent a low-temperature, around 400 °C, exsolution product.

New mineralogical data on platinum-group minerals from the Río Santiago alluvial placer, Esmeraldas province, Ecuador

Mineralogical studies on platinum-group minerals found in placer deposits from the Río Santiago (Ecuador) are scarce. In this investigation, one sample collected from the Río Santiago alluvial placer was studied via a multi-disciplinary approach, including optical microscopy, scanning electron microscopy, electron microprobe, and Raman spectroscopy. Whole-rock geochemistry data of the sample confirms elevated Au and platinum-group elements contents and the chondrite-normalized pattern reveals pronounced positive Ir and Pt anomalies. Free grains of platinum-group minerals were separated via hydroseparation techniques and identified as: i) Pt-Fe alloy (Pt3Fe), ii) tulameenite (Pt2FeCu) and iii) hongshiite (PtCu). The most abundant platinum-group mineral is Pt-Fe alloy (85%) that occasionally hosts cuprorhodsite (CuRh2S4) inclusions. Although the primary source remains unknown, the geochemical and mineralogical data suggests that the source of platinum-group minerals in the Río Santiago alluvial placer is a mafic-ultramafic Ural-Alaska type complex. Possible primary sources are the mafic and ultramafic rocks found in the mafic basement of the coastal region and the Western Cordillera (Piñón, San Juan and Pallatanga units), which derive from the Late Cretaceous Caribbean Colombia Oceanic Plateau (CCOP).

Orthopyroxenite hosted chromitite veins anomalously enriched in platinum-group minerals from the Havana-Matanzas Ophiolite, Cuba

The Havana–Matanzas Ophiolite contains one of the few examples of ophiolitic platinum group minerals (PGM)-rich chromitites associated with orthopyroxenites in the mantle section of ophiolitic complexes. The chromitites occur as veins hosted by orthopyroxenite bands within mantle peridotites. The peridotites are mostly harzburgites and their accessory chromite shows high-Al compositions (Cr# [Cr/(Cr+Al), atomic ratio] = 0.39–0.50), which are typical of spinels in abyssal peridotites. Conversely, chromite from the chromitite veins and their host orthopyroxenite are high-Cr (Cr# = 0.72–0.73 and 0.62–0.69, respectively), with lower Mg# [Mg/(Mg+Fe2+), atomic ratio]. This suggests that both the chromitite and the orthopyroxenite formed from melts with boninitic affinity. The abundant PGM inclusions found in the chromitites are mainly Os-rich laurite grains, which is also characteristic of chromitites formed from magmas with boninitic affinity. Therefore, we propose that the chromitite veins and the orthopyroxenite bands probably formed contemporaneously in the fore-arc setting of an intra-oceanic arc during subduction. The chromitite-orthopyroxenite pair of the Havana-Matanzas Ophiolite could form after the reaction of a Si-rich melt with boninitic affinity and mantle harzburgite, with the orthopyroxenite bands preserving fingerprints of the infiltration of boninitic-affinity melts within the mantle. The small volume of forming chromitite could maximize the efficiency for the mechanical collection of the PGM forming in the parental melt of these rocks, resulting in the anomalous enrichment of primary PGM in the chromitites.

PLATINUM-BEARING PLACERS: MINERAL ASSOCIATIONS AND THEIR 190Pt-4He AND Re-Os AGES, AND POTENTIAL LINKS WITH LARGE IGNEOUS PROVINCES IN THE SIBERIAN CRATON

AbstractPlacer deposits in the Siberian platform are closely associated with well-known platiniferous intrusions (Kondyor, Inagli, Noril’sk, Chiney, and Guli) but there are also abundant platinum-group mineral (PGM) placers of large areal extent in the river basins of Siberia (Vilyui, Lena, Aldan, Anabar, and Olenek) for which the primary sources are unknown. The 190Pt-4He dating method is introduced, compared with existing Re-Os isotope dating of PGMs, and applied to Pt-bearing placers of Siberia. Correlations between the 190Pt-4He and Re-Os isotope ages of the PGMs and with ages of major magmatic events in the Siberian craton show that the potential sources for some of the placers are likely buried Precambrian mafic-ultramafic intrusions of Large Igneous Provinces (LIP). The approach of this paper can be applied to other cratonic areas with alluvial PGMs of unknown origin.

Platinum-Group Minerals and Other Accessory Phases in Chromite Deposits of the Alapaevsk Ophiolite, Central Urals, Russia

An electron microprobe study has been carried out on platinum-group minerals, accessory phases, and chromite in several chromite deposits of the Alapaevsk ophiolite (Central Urals, Russia) namely the Bakanov Kluch, Kurmanovskoe, Lesnoe, 3-d Podyony Rudnik, Bol’shaya Kruglyshka, and Krest deposits. These deposits occur in partially to totally serpentinized peridotites. The microprobe data shows that the chromite composition varies from Cr-rich to Al-rich. Tiny platinum-group minerals (PGM), 1–10 µm in size, have been found in the chromitites. The most abundant PGM is laurite, accompanied by minor cuproiridsite and alloys in the system Os–Ir–Ru. A small grain (about 20 μm) was found in the interstitial serpentine of the Bakanov Kluch chromitite, and its calculated stoichiometry corresponds to (Ni,Fe)5P. Olivine, occurring in the silicate matrix or included in fresh chromite, has a mantle-compatible composition in terms of major and minor elements. Several inclusions of amphibole, Na-rich phlogopite, and clinopyroxene have been identified. The bimodal Cr–Al composition of chromite probably corresponds to a vertical distribution in the ophiolite sequence, implying formation of Cr-rich chromitites in the deep mantle, and Al-rich chromitites close to the Moho-transition zone, in a supra-subduction setting. The presence of abundant hydrous silicate inclusions, such as amphibole and phlogopite, suggests that the Alapaevsk chromitites crystallized as a result of the interaction between a melt enriched in fluids and peridotites. Laurite and cuproiridsite are considered to be magmatic in origin, i.e., entrapped as solid phases during the crystallization of chromite at high temperatures. The sulfur fugacity was relatively high to allow the precipitation of Ir-bearing sulfides, but below the Os–OsS2 buffer. The alloys in the system Os–Ir–Ru are classified as secondary PGM, i.e., formed at low temperature during the serpentinization process. The (Ni,Fe)5P phase is the first occurrence of a Ni-phosphide in terrestrial samples. Its composition indicates that it may be a new mineral. However, the small size has, so far, prevented a crystallographic study to support this conclusion.

Gold and platinum group minerals in placers of the South Urals: Composition, microinclusions of ore minerals and primary sources

Gold and platinum group minerals from the gold placers of the South Urals are studied in order to identify the metal sources. In placers from the Main Uralian fault zone (MUF), the primary gold contains Ag (up to 29wt.%), Cu (up to 2wt.%) and Hg (up to 4wt.%) and its fineness ranges from 538 to 997‰. Tetra-auricupride and cupriferous gold (up to 20wt.% Cu) are common for the Nizhny Karabash placer of the MUF zone. In the eastern part of the South Urals, the placer gold is mainly characterized by high fineness of 900–1000‰ and low Cu contents (max 1.38wt.%). Most of the placer gold grains consist of the primary domains, which are rimmed by secondary high-fineness gold with diffuse and clear boundaries. The secondary gold also develops along the shear dislocations of primary gold. Gold contains microinclusions of geerite, balkanite, chalcopyrite, Se-bearing galena, sphalerite, pyrite, pyrrhotite, arsenopyrite and hematite.Twenty four (including five unnamed) platinum group minerals (PGMs) were found in 28 placers; those from the Kialim and Maly Iremel placers of the Miass placer zone were studied in details. In the Kialim placer, ruthenium is most abundant PGM, which hosts microinclusions of isoferroplatinum, ferroan platinum, laurite, cupriferous gold, a mineral similar in composition to tolovkite, heazlewoodite and unnamed RhSbS phase. The osmium contains microinclusions of erlichmanite and laurite. The iridium grains hosts various sulfides and arsenides of platinum group elements (PGEs). The inclusion-free PGMs form Ru compositional trend in contrast to Os–Ru trend of the Ir-depleted inclusion-hosted PGMs. The isoferroplatinum from the Maly Iremel placer hosts laurite, rhodarsenite, bowieite, a mineral similar in composition to miassite and unnamed sulfide of Pt (Pt1.11S2.00) and antimonide of Pd ((Pd2.41Rh0.43Fe0.17)3.01(Sb0.91Te0.09)1.00). Ruthenium is a host to isoferroplatinum, PGE sulfides and arsenides, and heazlewoodite. Osmium contains microinclusions of ferroan platinum; iridium is a host to a mineral similar in composition to hongshiite. Three types of PGM intergrowths were identified in the Maly Iremel samples: (1) the intergrowths of platy grains of ruthenium with isoferroplatinum and a mineral similar in composition to tulameenite; (2) the open-latticework intergrowths of platy crystals of ruthenium with interstitial aggregates made up of gold, isoferroplatinum and a mineral similar in composition to xingzhongite and (3) the intergrowths of osmium and irarsite and iridarsenite, which are developed along cleavage of the osmium grains. Nickel sulfides associated with some PGMs contain Ru (11.32wt.%) and Rh (2.21wt.%) in millerite and Ir (31.00wt.%), Ru (5.81wt.%) and Rh (2.87wt.%) in vaesite.The primary metal sources were determined on the basis of the mineral assemblages and composition of minerals, taking into account the nearby mineral deposits and directions of rivers. The rodingite-associated gold, gold-bearing massive sulfide and chromite deposits are major sources of gold and PGMs in placers of the Miass placer zone confined to the MUF structure of the South Urals. In the southern part of this structure, gold was mainly originated from orogenic gold–sulfide deposits associated with volcanic/volcaniclastic rocks and listvenite-associated gold deposits. The placer PGMs were derived from the adjacent ultramafic massifs of ophiolitic origin. The distance between the placers and primary deposits varies from 2 to 5km (up to 20km in the extended valley of the Miass River). Usage of ore microinclusions and associated PGMs in study of placer gold is far more advanced than an ordinary consideration of gold composition alone. This approach allowed us to identify the concrete sources for individual placers and to predict some mineralogical findings in already known primary occurrences.

Chromite Composition and Accessory Minerals in Chromitites from Sulawesi, Indonesia: Their Genetic Significance

Several chromite deposits located in the in the South and Southeast Arms of Sulawesi, Indonesia, have been investigated by electron microprobe. According to the variation of the Cr# = Cr/(Cr + Fe3+), the chromite composition varies from Cr-rich to Al-rich. Small platinum-group minerals (PGM), 1–10 μm in size, occur in the chromitites. The most abundant PGM is laurite, which has been found included in fresh chromite or in contact with chlorite along cracks in the chromite. Laurite forms polygonal crystals, and it occurs as a single phase or in association with amphibole, chlorite, Co-pentlandite and apatite. Small blebs of irarsite (less than 2 μm across) have been found associated with grains of awaruite and Co-pentlandite in the chlorite gangue of the chromitites. Grains of olivine, occurring in the silicate matrix or included in fresh chromite, have been analyzed. They show a composition typical of mantle-hosted olivine. The bimodal composition and the slight enrichment in TiO2 observed in some chromitites suggest a vertical zonation due to the fractionation of a single batch magma with an initial boninitic composition during its ascent, in a supra-subduction zone. This observation implies the accumulation of Cr-rich chromitites at deep mantle levels and the formation of the Al-rich chromitites close or above the Moho-transition zone. All of the laurites are considered to be magmatic in origin, i.e., entrapped as solid phases during the crystallization of chromite at temperature of around 1200 °C and a sulfur fugacity below the sulfur saturation. Irarsite possibly represents a low temperature, less than 400 °C, exsolution product.

Al-and Cr-rich chromitites from the Eastern Havana-Matanzas ophiolites (Western Cuba)

Chromite deposits in the Havana-Matanzas ophiolites of Western Cuba include both Al- ([Cr/(Cr+Al)=0.44-0.46]) and Cr-rich ([Cr/(Cr+Al) = 0.65-0.74] chromitites that commonly occur in the upper part of mantle ectonites near the mantle-crust transition zone. These chromitites generally form small lenticular bodies spatially associated with strongly serpentinized dunite. Platinum group minerals (PGM) are found only in the Cr-rich chromitites, as euhedral or subeuhedral, very small (less than 10 μm long) crystals. The most abundant PGM is laurite, accompanied by minor osmium, cuproiridsite, iridium and an unnamed phase composed of Ir-Ni-Fe-S, all of which are magmatic in origin. These magmatic PGMs and amphiboles crystallized at high temperatures (~1300 °C) and at relatively high sulphur fugacities from fluid-rich magmas. The chromite mineral chemistry suggests that melts that were in equilibrium with Cr-rich and Al-rich chromitites had island-arc tholeiite - boninite and backarc basin basalt (BABB) affinities, respectively.

Platinum-group minerals in the Limoeiro Ni–Cu–(PGE) sulfide deposit, Brazil: the effect of magmatic and upper amphibolite to granulite metamorphic processes on PGM formation

The Limoeiro Ni–Cu–(platinum-group elements (PGE)) deposit is a recent discovery associated with an igneous tubular conduit system in northeastern Brazil. Representative ores from the deposit have been used for platinum-group minerals (PGM) identification and for PGE in base metal sulfides (BMS) quantification. Ninety-eighty percent of the PGM in the massive sulfide ores is homogeneous Pt–Ni–Bi-bearing merenskyite (PdTe2) enclosed primarily by pyrrhotite, suggesting that it is formed by exsolution from monosulfide solid solution (MSS). Merenskyite gradually but systematically becomes poorer in Pt and Ni with increasing fractionation, which is interpreted to reflect a transition to a more evolved sulfide liquid that segregated in the eastern parts of the intrusion. In massive sulfide ores, merenskyite forms unusually large (up to 5000 μm2) euhedral grains, commonly in contact with spherical silicate inclusions. BMS hosts 12–16 % of the Pd, with the remainder hosted by PGM, which is interpreted to indicate that merenskyite recrystallized from a PGE-bearing bismuthotelluride metamorphic melt formed during high-grade metamorphism. Sperrylite (PtAs2) is the second most abundant PGM (18 % of PGM in disseminated ore) and in contrast to merenskyite occurs mainly as very small (median of 25 μm2) inclusions in high-temperature silicates and oxides, interpreted to have crystallized at high temperatures directly from sulfide blebs that formed and were transported within the Limoeiro magma conduit.

Platinum-group minerals (PGM) nuggets from alluvial-eluvial placer deposits in the concentrically zoned mafic-ultramafic Uktus complex (Central Urals, Russia)

We report the results of a detailed mineralogical investigation of platinum-group minerals (PGM) and copper-gold nuggets from the Uktus Ural-Alaskan type complex in the Central Urals (Russia). The studied nuggets were sampled in alluvial-eluvial deposits from three small valleys, with temporary water flows, cutting across the Uktus massif. The volume of the washed samples varies from 0.03 to 0.08 m 3 and a few tens of PGM nuggets, ranging in size from about 100 μm to about 2 mm, were collected. According to their chemical composition, the most abundant PGM are native Ir-Os and alloys in the Pt-Fe-Cu-Ni system. The following less abundant PGM were also recognised: sulfarsenides of the irarsite-hollingworthite-platariste series, sulfides such as laurite, cuproiridsite, kashinite and the sulfantimonide tolovkite. One alloy corresponding to the formula Cu 3 Au 2 was found, and proved to be Cu-rich tetraauricupride. The nuggets of Uktus have, in some cases, a polygonal shape. However, most of them have an irregular morphology and are characterised by a porous rim and zoning. The investigated nuggets occur as single-phase crystals or as polyphase grains, composed of different PGM. One nugget displays a very complex texture, being composed of a Pt-Fe alloy associated with osmium and Cu-rich tetraauricupride. These minerals are in contact with quartz that contains minute inclusions of hollingworthite and platarsite. The mineralogical similarity with the PGM inclusions in the Uktus chromitites indicates these rocks as a possible source for the PGM nuggets. The presence of faceted morphology in some nuggets suggests that they were mechanically liberated and transported for a relatively short distance from their lode deposits. The nuggets characterized by a rounded shape and occurring in association with quartz and Cu-rich tetraauricupride have probably been reworked in the placer environment. Therefore, in the Uktus placers deposits, two types of PGM nuggets can coexist: (i) primary with a magmatic origin, i.e ., only mechanically liberated from their source rock, and (ii) secondary, i.e ., reworked and grown in the placers. The mineralogical assemblage of the Uktus PGM nuggets, the fact that the Uktus PGM placers have never been mined and the recent exponential increase in demand for noble metals make the placer deposits associated with the Uktus complex potentially important for the economic recovery of these rare metals, at least on a small scale.

Compositional variations of chromite and solid inclusions in ophiolitic chromitites from the southeastern Turkey: Implications for chromitite genesis

Chromitites, associated with upper mantle peridotites of ophiolites from southeastern Turkey, are lenticular bodies or veinlets made up of massive, banded and disseminated chromite. Chromite shows a wide range of Cr# [Cr/(Cr+Al)] varying between 0.42 and 0.81, and Mg# [Mg/(Mg+Fe2+)] of 0.45 to 0.75. Geochemically, they are divided into two compositional groups, such as high-Cr (Cr#≥0.7) and high-Al (Cr#<0.6). The contents of the platinum-group elements (PGE) in high-Cr chromitites range between 42 and 348ppb (average 158ppb), whereas high-Al ones are represented by a lower total PGE, ranging between 70 and 94ppb (average 84ppb). However, two high-Cr chromitite samples showed significant enrichments especially of Ir-subgroup of PGE (IPGE: Os, Ir, Ru) with total PGE contents reaching up to 2.7ppm. The chromitites mostly contain tiny inclusions (<5μm) of platinum-group minerals (PGM). The most abundant PGM is laurite which is rarely associated with irarsite±Os–Ir alloys; these minerals in particular are more abundant in high-Cr chromitites, especially in PGE enriched chromitites. The primary base-metal sulfide (BMS) inclusions consist of millerite and to a lesser extent pentlandite, and very rarely pyrite. Violarite, polydymite and heazlewoodite form the secondary BMS phases found mostly in altered silicate matrix. Both PGM and BMS are frequently associated with the silicates: amphibole, olivine and clinopyroxene. Textural relationships and the composition of PGM and BMS inclusions suggest that they crystallized at varying temperature (1300°C–1000°C) and sulfur fugacity conditions, before or during the event of crystallization of chromite. The high-Cr chromitites crystallized in equilibrium with boninite melt, probably originated in a supra-subduction zone (SSZ), while the high-Al chromitites crystallized in equilibrium with middle oceanic ridge basalt (MORB) type melts. The presence of amphiboles in high-Al chromitites suggests hydrous parental melts that could be produced, most probably, in back-arc tectonic setting.

THE FIRST REPORT OF PLATINUM-GROUP MINERALS IN MAGNETITE-BEARING GABBRO, FREETOWN LAYERED COMPLEX, SIERRA LEONE: OCCURRENCES AND GENESIS

Platinum-group minerals (PGM) have been located in magnetite-bearing gabbroic rocks of the Freetown Layered Complex, Sierra Leone for the first time. The PGM occurrences described here are from four stratiform layers characterized by a local, more primitive, composition of the associated silicate minerals and elevated platinum-group elements (PGE) and base metal assays. The uppermost layer (Horizon B) is the best exposed; the three lower layers are currently known only from single outcrops. Platinum-group minerals are irregularly distributed through these layers. Altered PGE arsenides and antimonides, possibly derived from sperrylite (PtAs2) and “mertieite” [Pd8(Sb,As)3 to Pd11(Sb,As)4], occur in the lowest known PGE-enriched layer (Horizon M). Cooperite (PtS) and Pt-Fe alloys were observed in a higher layer (Horizon D). Pd-Cu alloys including nielsenite (PdCu3) are unique to the next stratigraphically higher PGM-bearing layer (Horizon C). The uppermost layer (Horizon B) contains the most PGM. Pt-Fe alloy (usually Pt3Fe) is by far the most abundant PGM and cooperite is a significant component of the assemblage. Bowieite [(Pt,Ir,Rh)2S3], tulameenite (Pt2CuFe), and laurite (RuS2) have also been found in this horizon. Cooperite occurs in a late magmatic, high Ca-amphibole (cannilloite) which has formed interstitially to plagioclase and between plagioclase and olivine at a depth of about 20 km. The cooperite is often attached to rounded magnetite grains within the amphibole. Where cooperite is located at the edge of the amphibole or in interstitial locations it has been altered to Pt-Fe alloy. Platinum-group minerals sometimes occur near the margin of sulfides, mainly chalcopyrite and minor pentlandite. The entire PGM assemblage is found mainly interstitial to olivine, pyroxene, and plagioclase, often at the edge of amphibole or in interstitial sites associated with amphibole, chlorite, or quartz. Many of the PGM grains are less than 1 micron in size, with a few in the 2–4 micron range; the largest example is a grain of Pt-Fe alloy that measures 9 by 6 microns. The PGM described here from the rocks are three orders of magnitude smaller than the eluvial and alluvial PGM of the Freetown complex, which are predominantly Pt-Fe alloys, laurite, erlichmanite (OsS2), and Os-Ir alloys. The large eluvial and alluvial PGM might be derived from a horizon containing coarse-grained mineralization, but no vestige of such a horizon has yet been discovered. Previous studies of this area have suggested that secondary remobilization of the PGE has occurred, leading to the growth of large secondary PGM. The primary PGE-mineralization associated with magnetite and Cu-rich sulfide mineralization maybe due to late sulfur saturation in the magma after Ni has been removed by being incorporated into earlier olivine. The presence of digenite, native copper, and nielsenite indicates a low sulfur fugacity, possibly due to oxidation. The four distinctive PGM-bearing horizons are the result of separate injections of magma. The silicate compositions, the minor element content, the PGM mineral assemblage, and the Cu/Pd ratios of the rocks of the PGM-bearing horizons demonstrate that these four magma pulses are compositionally distinct.

MINERALOGY AND GEOCHEMISTRY OF PLATINUM-RICH CHROMITITES FROM THE MANTLE-CRUST TRANSITION ZONE AT OUEN ISLAND, NEW CALEDONIA OPHIOLITE

Micropods, irregular bodies and schlieren of chromitite occur within a unit of layered plagioclase-bearing wehrlite on Ouen Island, southern New Caledonia. The primary chromian spinel is Al-rich, with Cr# [atomic (Cr/Cr + Al)] between 0.52 and 0.62, and Mg# [Mg/(Mg + Fe 2+ )] between 0.57 and 0.72. Contents of minor and trace elements (Ga, Ni, Zn, Co, Mn, V, Sc) in primary spinel, obtained by LA–ICP–MS, are comparable to those reported in chromian spinel from podiform chromitites in other ophiolite complexes. The chromitites are strongly enriched in Pt and Pd (up to 11.5 ppm Pt + Pd), reflecting the presence of abundant platinum-group minerals (PGM), including alloys, sulfides, oxides and compounds containing Te, Sb or Hg. Also present are Ru–(Os–Ir) phases, in accessory amounts: sulfides (mainly laurite, two unidentified Os–S and Ir–S compounds) and sulfarsenides of Ir and Rh. Abundant base-metal minerals (BMM) occur together with the PGM; they include Ni–Cu–Fe sulfides (pentlandite, millerite, heazlewoodite, chalcopyrite and bornite), awaruite (Ni 3 Fe), and a phase with a composition close to NiAsS (probably gersdorffite). The geochemistry and mineralogy of the chromitite and the PGE mineralization show that the Ouen Island chromitites have a different origin than those found in the mantle and crust sequences of the New Caledonia ophiolite complex. The distribution, morphology and composition of the PGM and BMM in the chromitites define two distinct paragenetic assemblages: i) primary or magmatic, consisting of PGM and BMS grains enclosed in the unaltered core of chromian spinel grains and in primary anhydrous silicates (olivine and pyroxene) of the matrix; ii) secondary or postmagmatic, consisting of PGM and BMM grains found along fractures, enclosed or in contact with high-temperature secondary hydrous silicates (amphibole) and low-temperature silicates (serpentine, chlorite). Microtextural and compositional data for the secondary PGM and BMM mineral assemblages suggest that after the crystallization of chromian spinel, an excess of H 2 O generated a residual high-temperature hydrothermal fluid that modified the magmatic PGM mineralogy, producing small-scale remobilization of the PGE. Late serpentinization overprinted this hydrothermal alteration, and contributed to the degradation of the primary PGM and BMM. At this stage, PGE-bearing alloys and sulfides were replaced by new stable alloys and oxides of the PGE.

Geochemistry and mineralogy of platinum-group elements (PGE) in chromites from Centralnoye I, Polar Urals, Russia

The Polar Urals region of northern Russia is well known for large chromium (Cr)-bearing massifs with major chromite orebodies, including the Centralnoye I deposit in the Ray-Iz ultramafic massif of the Ural ophiolite belt. New data on platinum (Pt)-group elements (PGE), geochemistry and mineralogy of the host dunite shows that the deposit has anomalous iridium (Ir) values. These values indicate the predominance of ruthenium–osmium–iridium (Ru–Os–Ir)-bearing phases among the platinum-group mineral (PGM) assemblage that is typical of mantle-hosted chromite ores. Low Pt values in chromites and increased Pt values in host dunites might reflect the presence of cumulus PGM grains. The most abundant PGM found in the chromite is erlichmanite (up to 15 μm). Less common are cuproiridsite (up to 5 μm), irarsite (up to 4–5 μm), and laurite (up to 4 μm). The predominant sulfide is heazlewoodite, in intergrowth with Ni–Fe alloys, sporadically with pentlandite, and rarely with pure nickel. Based on the average PGE values and estimated Cr-ore resources, the Centralnoye I deposit can be considered as an important resource of PGE.

Alteration of Platinum-Group and Base-Metal Mineral Assemblages in Ophiolite Chromitites from the Dobromirtsi Massif, Rhodope Mountains (Bulgaria)

The Dobromirtsi Ultramafic Massif, located in the Rhodope Mountains (SE Bulgaria), is a portion of a Paleozoic sub-oceanic mantle affected by polyphase regional metamorphism. This massif contains several small, podiform chromitite bodies which underwent the same metamorphic evolution as their host peridotites. Like other ophiolite chromitites, those found in Dobromirtsi carry abundant platinum-group minerals (PGM) and base-metal minerals (BMM). The PGM consist mainly of Ru-, Os-, and Ir-based PGM (laurite RuS2-erlichmanite OsS2, Os-Ru-Ir alloys, irarsite [IrAsS], Ru-rich pentlandite, and an unknown Ir-sulfide) but minor Rh- and Pd-based PGM (hollingworthite [RhAsS] and a series of unidentified stannides and sulfantimonides) are also present. In contrast, the BMM are dominated by pentlandite (Ni,Fe)9S8, followed by heazlewoodite (Ni3S2), breithauptite (NiSb), maucherite (Ni11As8), godlevskite (Ni7S6), gersdorffite (NiAsS), millerite (NiS), undetermined minerals containing Ni, As and Sb, orcelite (Ni5-XAs2), awaruite (Ni3Fe) and chalcopyrite (CuFeS2). The detailed study of the textural relationships, morphology and composition of the PGM and BMM inclusions indicate the existence of two different PGM-BMM assemblages: (i) a primary or magmatic; and (ii) a secondary related with postmagmatic alteration. The PGM and BMM inclusions in unaltered zones of chromite crystals (mainly laurite-erlichmanite and pentlandite) are considered to be primary magmatic minerals formed under variable temperature (1200–1100°C) and sulfur fugacity (between −2 and −0.5 log fS2). In contrast, PGM and BMM located along altered edges of chromite and serpentinised silicate matrix are considered to be secondary, formed from or re-equilibrated with altering fluids. Secondary PGM and BMM assemblages are considered as result of the combination of reducing and oxidising events related with regional metamorphism. Under low fO2 states, fS2 also drops giving place to the formation of S-poor Ni-rich sulfides and secondary Ru-alloys by desulfurisation of primary S-containing minerals. In contrast, predominance of platinum-group elements and/or base-metal arsenides and sulfarsenides associated with the altered edges of chromite (chromite strongly enriched in Fe2O3) is related with the fixing of remobilised PGE (mainly Ir, Rh and Pd) and base-metals (mainly Ni and Fe) when late oxidising fluids supplied As as well as Sb and Sn.

Footwall-type Cu-Ni-PGE Mineralization in the Broken Hammer Area, Wisner Township, North Range, Sudbury Structure

Footwall-type Cu-Ni-PGE (Platinum-group element) deposits are a major focus of recent exploration in the Sudbury mining camp, owing to their higher Cu and precious metal contents compared to orebodies situated along the lower contact of the Sudbury Igneous Complex. The most important footwall-type orebodies occur in the Onaping-Levack area of the North Range, but economic deposits are found along the East Range of the structure as well. Recently, promising occurrences were also discovered in Wisner Township of the North Range. In this paper we report results from two main footwall-type Cu-Ni-PGE occurrences (the Broken Hammer zone and the South zone) of Wisner Township. Both discoveries are situated in intensely brecciated zones of the footwall (Sudbury Breccia), 1,300 and 500 m, respectively, north of the present Sudbury Igneous Complex-footwall contact. The mineralization consists of (1) massive sulfide veins, (2) disseminated/replacement sulfides, and (3) silicate-quartz–dominated veins. Although massive sulfide veins account for the major part of the resource, sulfide-poor disseminations may have similarly high precious metal contents. Ore assemblages are dominated by chalcopyrite, millerite, magnetite, and/or pyrite, quartz, and hydrous silicates. These are accompanied by platinum-group minerals (PGM) and other trace minerals, which commonly occur as composite grains within sulfides and hydrous silicates. These include minerals typical of footwall-type deposits (e.g., merenskyite, michenerite, hessite), but also minerals found only in a few Sudbury deposits (e.g., clausthalite, sopcheite, naumannite, bohdanowiczite). An unnamed CuPdBiS 3 mineral is one of the most abundant PGM at Broken Hammer and might be a Pd-equivalent of muckeite (CuNiBiS 3 ). Statistical investigation of metal distribution patterns using grab sample and drilling data sets from both localities indicate important differences between the two occurrences and the various mineralization styles (e.g., increasing Cu/(Cu+Ni) and decreasing Pt/(Pt+Pd) ratio with increasing sulfide content; metal concentrations recalculated to 100% sulfides decrease by up to two orders of magnitude for all metals from the disseminated to the massive sulfides). Fluid inclusion studies revealed three fluid generations which are very similar to those described from well known footwall-type deposits of the Onaping-Levack area. Primary polyphase fluid inclusions in Cu-Ni-PGE–bearing assemblages represent a highly saline (30–40 wt % NaCl equiv) magmatic-hydrothermal system with a high temperature (min 450°–500°C) and a lower temperature (min 300°–350°C) stage. A later low-temperature Ca-rich fluid (min 100°–200°C) is most probably part of a post-Sudbury Igneous Complex regional fluid flow in which temperature was controlled by the geothermal gradient and not related to ore-forming processes. The fluid inclusion data indicate that mobilization of highly saline and relatively high temperature (400°–500°C) Sudbury Igneous Complex-related magmatic-hydrothermal fluids similar to those described from the Onaping-Levack embayment also occurred in the Wisner footwall. Although our studies do not rule out the possibility of an initial magmatic sulfide melt emplacement, they prove that magmatic-hydrothermal fluids played a significant role in the transport and deposition of metals.

Platinum group minerals in podiform chromitites of the Bou Azzer ophiolite, Anti Atlas, Central Morocco

The Neoproterozoic Bou Azzer ophiolite complex hosts numerous, small lenticular bodies of massive and disseminated chromite. Metallurgical-grade high-Mg and high-Cr spinels (cores with 48–62 wt% Cr2O3) reveal complex alteration patterns of successive Cr and Mn enrichment and loss of Al towards the rims, while the Mg# ratios [(Mg/(Mg + Fe2+)] remain almost constant. Concentration patterns of platinum-group elements are typical for ophiolitic chromitite poor in sulfides, with predominance of the IPGE, variable Rh, and low Pt and Pd. The most abundant platinum-group mineral is Rh-bearing laurite that occurs either included in spinel or in silicate matrix, whereas Os-Ir-Ru alloy is always included in spinel. Laurite inclusions reveal complex intergrowth textures with Rh-Ru-Pt rich alloy, and with Rh-rich sulfide. Most laurites display trends to sulfur-poor compositions leading to local formation of very fine-grained Ru-Os-Ir alloy phases. Ni-Co-Fe sulfides, arsenides and sulfarsenides devoid of PGE are associated with the alteration of chromite. Textural position and chemical composition of the base metal inclusions, as well as comparison of alteration features between chromite and accessory chromian spinel in the Co-Ni-As ores of the Bou Azzer ophiolite indicate a close connection. It is suggested that hydrothermal fluids percolated through the marginal zones of the ophiolite belt during greenschist facies metamorphism and deposited Ni-Co-Fe arsenides, sulfarsenides and minor sulfides as accessories within altered chromitites, and also in structurally favourable zones as Ni-Co-As ores.

Mineralogy and geochemistry of platinum-group elements in the Aguablanca Ni-Cu deposit (SW Spain)

The Aguablanca Ni-Cu-(PGE) magmatic sulphide deposit is associated with a magmatic breccia located in the northern part of the Aguablanca gabbro (SW, Iberia). Three types of ores are present: semi-massive, disseminated, and chalcopyrite-rich veined ore. The principal ore minerals are pyrrhotite, pentlandite and chalcopyrite. A relatively abundant platinum-group mineral (PGM) assemblage is present and includes merenskyite, melonite, michenerite, moncheite and sperrylite. Moreover, concentrations of base and precious metals and micro-PIXE analyses were obtained for the three ore-types. The mineralogy and the mantle-normalised chalcophile element profiles strongly suggest that semi-massive ore represents mss crystallisation, whereas the disseminated ore represents an unfractionated sulphide liquid and the chalcopyrite-rich veined ore a Cu-rich sulphide liquid. Palladium-bearing minerals occur commonly enclosed within sulphides, indicating a magmatic origin rather than hydrothermal. The occurrences and the composition of these minerals suggest that Pd was initially dissolved in the sulphides and subsequently exsolved at low temperatures to form bismutotellurides. Negative Pt and Au anomalies in the mantle-normalised chalcophile element profiles, a lack of Cu-S correlation and textural observations (such as sperrylite losing its euhedral shape when in contact with altered minerals) suggest partial remobilisation of Pt, Au and Cu by postmagmatic hydrothermal fluids after the sulphide crystallisation.