Platinum-group Minerals Grains Research Articles

PLATINUM-GROUP MINERALS AND OTHER DETRITAL COMPONENTS IN THE KAROO-AGE SOMABULA GRAVELS, GWERU, ZIMBABWE

Heavy-mineral concentrates were obtained from a recent exploration campaign for diamond in the Somabula gravels, a paleoplacer of Upper Karoo (Triassic, ~200 Ma) age near Gweru, Zimbabwe. The polymict spectrum of heavy minerals in the Somabula placers comprises staurolite, zircon, monazite, columbite–tantalite, cassiterite, chromite, native gold, platinum-grou p minerals (PGM), and other components, which suggests derivation from a variety of source lithologies. Age determinations revealed that most of the detrital grains of zircon, monazite and columbite–tantalite originate from Pan-African source rocks. Altogether, 622 detrital PGM grains were identified. Pt and Pt alloys, mainly with Fe, Rh, Pd and Ru, predominate (79%), followed by Os–Ir–Ru–Rh–Pt alloys (10%), rustenburgite [Pt 3Sn] (9%), and rare Pt-rich phases (1%). The compositions of the alloys suggest high-temperature crystallization from a complex Pt–Pd–Rh–Os–Ir–Ru–Fe solid solution in a sulfur-poor magmatic environment, probably mafic–ultramafic rocks. The suite of PGM markedly differs from that of detrital PGM originating from the Great Dyke and from the primary platinum-group element mineralization of the Main Sulfide Zone of the Great Dyke. The mineralogical evidence and Os isotope data obtained for the Os–Ir–Ru alloy rule out the possible derivation of the PGM from the Great Dyke. Instead, they are likely derived from late Archean greenstone belts of the Zimbabwe craton.

Possible platinum-group element (PGE) oxides in the PGE-mineralized chromitite from the Northern Oman Ophiolite.

A number of platinum-group mineral (PGM) grains with characteristic optical and geochemical properties are found in the recently discovered PGE-mineralized chromitite at Wadi Hilti in the deeper mantle section of the northern Oman ophiolite. They usually occur as polygonal inclusions within fresh chromian spinel. They have exclusively Ru-Os-Ir compositions with low totals on conventional microprobe analysis and show the same Ru-Os-Ir ratios as Os-rich laurite, (Ru, Os, Ir)S2, commonly found in the chromitite. Qualitative and quantitative analyses show that oxygen is one of major constituents of these PGM grains, and they are Ru-Os-Ir oxides. They are characteristically lower in Fe-Ni contents than Ru-Os-Ir oxides ever reported. The characteristic radial-fibrous textures as well as a hollow space around the polygons possibly mean their secondary origin with significant volume reduction. The similarity of element ratios and mode of occurrence between the PGE oxides and laurite indicates that the former have been produced by desulfurization/oxidation of the latter. Formation of the PGE oxides from Oman has been performed with an increase of Eh at a very early stage of serpentinization.

The Platinum Group Minerals in two parts of the Massive Sulphide Body of the Uitkomst Complex, Mpumalanga, South Africa

Platinum Group Elements (dominated by palladium) are present in the Ni-Cu-Co bearing massive sulphide ore of the Uitkomst Complex, Mpumalanga Province, South Africa. Mineralogical investigations were conducted in two different parts of the ore body and it was found that these elements are contained in fine-grained mineral phases that are largely attributed to five mineral types based on composition and optical properties. Most of these phases fall within the quaternary system Pd-Bi-Te-Sb. Mineral assemblages of this composition are generally regarded as indicative of relatively low temperature (probably less than 490°C) conditions for the formation of the associated massive sulphide ores. Although the mineral phases reported are similar, there are significant differences in the relative abundances of individual mineral phases between the two areas. The distribution of the individual phases recorded for the deeper part of the ore body indicates an increase in Type II (michenerite) grains towards the base of the zone sampled, an antipathetic relationship between Type I (merenskyite) and Type II (michenerite) phases, and a concentration of Type III (testibiopalladite) grains at the top and the base of the zone. The Platinum Group Mineral grains are preferentially associated with pyrrhotite and generally located along grain boundaries. The relative abundances of the Platinum Group Elements in the massive sulphide body are similar to that of other Ni-sulphide deposits such as Sudbury and Noril’sk, but differ significantly from that of the Merensky Reef, UG-2 Reef, and Platreef ore bodies of the Bushveld Complex.

The Platinum Group Minerals in two parts of the Massive Sulphide Body of the Uitkomst Complex, Mpumalanga, South Africa

Research Article| December 01, 2001 The Platinum Group Minerals in two parts of the Massive Sulphide Body of the Uitkomst Complex, Mpumalanga, South Africa H.F.J. Theart; H.F.J. Theart Department of Earth Sciences, University of Pretoria, Pretoria, South Africa, E-mail: htheart@postino.up.ac.za Search for other works by this author on: GSW Google Scholar C.D. de Nooy C.D. de Nooy Anglovaal Mining Limited, P O Box 1212, Florida, 1710, South Africa, E-mail: dorritn@avmin.co.za Search for other works by this author on: GSW Google Scholar South African Journal of Geology (2001) 104 (4): 287–300. https://doi.org/10.2113/gssajg.104.4.287 Article history first online: 07 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share MailTo Twitter LinkedIn Tools Icon Tools Get Permissions Search Site Citation H.F.J. Theart, C.D. de Nooy; The Platinum Group Minerals in two parts of the Massive Sulphide Body of the Uitkomst Complex, Mpumalanga, South Africa. South African Journal of Geology 2001;; 104 (4): 287–300. doi: https://doi.org/10.2113/gssajg.104.4.287 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietySouth African Journal of Geology Search Advanced Search Abstract Platinum Group Elements (dominated by palladium) are present in the Ni-Cu-Co bearing massive sulphide ore of the Uitkomst Complex, Mpumalanga Province, South Africa. Mineralogical investigations were conducted in two different parts of the ore body and it was found that these elements are contained in fine-grained mineral phases that are largely attributed to five mineral types based on composition and optical properties. Most of these phases fall within the quaternary system Pd-Bi-Te-Sb. Mineral assemblages of this composition are generally regarded as indicative of relatively low temperature (probably less than 490°C) conditions for the formation of the associated massive sulphide ores. Although the mineral phases reported are similar, there are significant differences in the relative abundances of individual mineral phases between the two areas. The distribution of the individual phases recorded for the deeper part of the ore body indicates an increase in Type II (michenerite) grains towards the base of the zone sampled, an antipathetic relationship between Type I (merenskyite) and Type II (michenerite) phases, and a concentration of Type III (testibiopalladite) grains at the top and the base of the zone. The Platinum Group Mineral grains are preferentially associated with pyrrhotite and generally located along grain boundaries. The relative abundances of the Platinum Group Elements in the massive sulphide body are similar to that of other Ni-sulphide deposits such as Sudbury and Noril’sk, but differ significantly from that of the Merensky Reef, UG-2 Reef, and Platreef ore bodies of the Bushveld Complex. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Petrology of the Mechanic Settlement Pluton and Related Platinum-Group Element Mineralization

The Mechanic Settlement pluton is a small body of mafic and ultramafic rocks of tholeiitic affinity, occurring in the Late Neoproterozoic volcanosedimentary rocks of the Avalon terrane of the Appalachian Orogen. The lenticular-shaped intrusion is layered and consists of ultramafic and mafic lithologies in at least six cyclic units that are relatively thin or short-lived. The lowermost exposed portion of the intrusion is characterized by the cumulate sequences of olivine and olivine-clinopyroxene and display relatively flat rare earth element profiles. In the overlying lithologies, plagioclase is the dominant cumulus phase following olivine in the crystallization sequence. Olivine ranges in composition from Fo 73 to Fo 85 and plagioclase from An 62 to An 89. Variations in the compositions of orthopyroxene and clinopyroxene are within Wo 0–4 En 52–87 Fs 11–45 and Wo 39–50 En 23–50 Fs 3–19 . These mineral compositional variations in part reflect cryptic patterns and correspond to normal fractionation trends in cyclic units. Elevated Pt and Pd concentrations of up to 2.6 ppm occur in ten zones within the central cyclic series in association with both the peridotitic and gabbronoritic rock types. The mineralized zones are about several meters thick and appear to have lateral continuities over several hundred meters, suggesting that they are stratiform. Platinum-group elements (PGE) occur as discrete platinum-group minerals (PGM) including vysotskite, merenskyite, michenerite, sperrylite, stillwaterite, and hollingworthite that are associated with magmatic sulfides composed of pentlandite, pyrrhotite, and chalcopyrite. PGM grains range in size from a few micrometers to well above 20 μm. Sulfur isotope ratios and the Se/S ratios of the mineralized rocks are typically higher than the mantle values. Sulfide saturation probably occurred as a result of assimilation of crustal rocks and magma mixing during emplacement of subsequent pulses of magma. Repetition of the PGE mineralization in rather thin cyclic units suggests that the replenishing magma probably had high PGE levels and was saturated with respect to sulfide during emplacement. Ingression of the replenishing magma into the chamber in a turbulent plume and its mixing with the resident magma was probably responsible for the formation of the PGE mineralizations in the Mechanic Settlement pluton.

The Origin of Platinum Group Minerals from the FreetownIntrusion, Sierra Leone, Inferred from Osmium Isotope Systematics

The origin of certain alluvial platinum-group mineral (PGM) grains is controversial, with two major proposed models. One involves primary formation within ultramafic intrusions, followed by weathering into laterites with no further alteration prior to alluvial concentration. The other involves in addition, solution of the platinum-group elements (PGE) during weathering of the ultramafic source rocks, probably by organic reactions, followed by the supergene growth of the macroscopic grains within laterites. In order to provide further evidence concerning the formation of macroscopic (~ μ m in size) platinum-group mineral grains, the authors have studied the osmium isotope ratios in PGM grains from the Freetown peninsula in Sierra Leone using an ion microprobe. Indirect isotopic evidence and modeling are required to infer their origin as no PGM grains larger than ~10 mm have ever been discovered in situ within the source rocks. Unlike virtually all other alluvial PGM grains that have been studied, the Freetown ones are distinguished by 187 Os/ 188 Os ratios that are much higher than canonical mantle values and vary considerably between different grains (0.12 187 Os/ 188 Os < 0.28). The 187 Os/ 188 Os ratios are, with one exception, the same for individual osmium-rich inclusions within the platinum-iron alloy host of each grain, although different grains have significantly different 187 Os/ 188 Os ratios. In the one significant exception, there are two Os-Ru-Ir alloy inclusions within a single platinum-iron alloy host which have 187 Os/ 188 Os ratios that are different from each other. Various models are considered to explain these observations: (1) the elevated osmium isotope ratios could be primary and derived from a mantle inhomogeneity such as a mantle plume; (2) the intruded material with a canonical mantle signature may have reacted with the host rocks at the time of the intrusion or at a later stage during postulated metamorphic or hydrothermal activity, or (3) the supergene remobilization could have modified the osmium isotope signature of the PGM. Interpretation of the osmium isotopes is by no means unambiguous. The range of 187 Os/ 188 Os ratios is far higher than known mantle sources which, combined with the grain to grain variation, seems to rule out the primary formation model. Sr, Nd, and O isotope evidence indicates no reaction with the host rocks during intrusion or by metamorphic or hydrothermal activity and mixing calculations show that enhancement of the radiogenic Os during intrusion could only have been small. The evidence points to some extremely localized process which could, however, have acted in two ways: either a preexisting high concentration of rhenium and PGE (a xenolith) was incorporated into the magma chamber, remained essentially unmixed with the majority of the material in the chamber, and after extrusion of the gabbro, PGE nuggets were weathered out into laterites, or alternatively, concentration processes within the magma chamber produced a localized concentration of rhenium—possibly in copper sulfides or late magmatic iron oxides in acid intrusions—which, when chemically dissolved in the supergene environment, precipitated PGE nuggets with extremely radiogenic 187 Os/ 188 Os signatures. The data presented in this paper in conjunction with work elsewhere have been used to demonstrate that the processes producing highly radiogenic 187 Os/ 188 Os signatures cannot have resulted from large-scale processes and have been used to dismiss a number of previously proposed models. However, a resolution between the two alternative models proposed is not possible using the present data.

The recovery of platinum-group elements from the UG-2 chromitite, Bushveld Complex - a mineralogical perspective

¶The behaviour of platinum-group elements (PGE) during milling and flotation of the UG-2 chromitite layer is still poorly understood. The PGE in the UG-2 chromitite occur primarily in discrete platinum-group mineral phases, varying from sulphide assemblages (predominantly cooperite, braggite, malanite and laurite) to assemblages comprising a significant component of alloys (such as Pt-Fe alloy) or various telluride, bismuthinide, bismuthotelluride, arsenide and sulpharsenide phases. The mode of occurrence of platinum-group minerals ranges from being predominantly associated with base-metal sulphides (either locked within, or at the grain boundaries of, sulphide minerals) to being predominantly associated with gangue minerals (locked in chromite or silicate, or at grain boundaries of chromite and silicate grains). During milling, more than 50% of the platinum-group mineral grains in the samples investigated were liberated from the ore matrix. During flotation tests, most of the platinum-group mineral grains that remained associated with gangue reported to the flotation tailings, and those associated with liberated base-metal sulphides to the flotation concentrates. Because of their small grain size (generally less than ∼10 μm), the response of the liberated platinum-group mineral grains was more difficult to predict. It was found that most of these grains did report to the flotation concentrates, albeit at a slower rate than the base-metal sulphides. The flotation behaviour of these grains was determined both by grain size and type of mineral, with coarser braggite, cooperite, and to a lesser extent malanite being the fastest floating species.

Osmium Isotope Heterogeneity in Os-Ir-Ru Inclusions within Platinum-Iron Alloy Placer Grains from the Freetown Layered Complex; Sierra Leone

The measurement of osmium isotope ratios has been used in previous studies to evaluate the origin and mode of formation of platinum-group element mineralisation. In particular, where platinum-group mineral (PGM) grains occur as placer deposits in areas of the Cordilleran mountain chains of the USA and Canada, in Kalimantan, and in Sierra Leone, osmium isotopes have provided useful insights into their possible origins. (Bowles et al., submitted., Lyon et al., 1997; Hattori and Cabri, 1992) In the present work, the genesis of PGM grains in placers associated with the Freetown igneous complex, Sierra Leone, is being investigated. The Freetown intrusion is a roughly circular body approximately 30 km in diameter. It consists of five main cycles (zones) of troctolite, olivine-gabbro, norite, gabbro and anorthosite, four of which are exposed on the Freetown Peninsula and the fifth seen only in a borehole. Approximately one third of the intrusion is exposed on land with the four outcropping zones forming four arcuate ranges of hills on the Freetown Peninsula. The PGM grains are found in soils, and in sediments in, or near, the rivers and streams draining the area. The grains are mostly 1-2 mm in diameter, with larger grains known to occur. The grains have been reported as exhibiting both euhedral and dendritic growth habits. (Bowles, 1995). The main platinum-group minerals found in this locality occur as: (1) pyritohedra of erlichmanite (OsS2) and (2) platinum-iron alloy grains (Pt3Fe) with inclusions of Os-Ir-Ru alloy, erlichmanite (OsS2), laurite (RuS2), tulameenite (CuFePt2) and cuprorhodsite (CuRh2S4). The previously reported osmium isotope ratios measured on the Os-lr-Ru phases within Pt3Fe grains from Sierra Leone are within the range 0.15 0.25, when converted to equivalent 187Os/188Os ratios (Hattori, et al., 1991). This variation was determined between inclusions from separate grains, not between inclusions within a single grain, and has been variously interpreted as resulting from magrnatic, metasomatic or supergene processes. The present study also concentrates on the Os-IrRu alloys within Pt3Fe grains, the emphasis being on collection of data from Pt3Fe grains containing more than one Os-Ir-Ru inclusion to establish whether or not there is any detectable variation of the 187Os/188Os ratios within and between individual inclusions.

Osmium isotope ratios of PGM grains associated with the Freetown Layered Complex, Sierra Leone, and their origin

Osmium isotope ratios for two types of plati- num group mineral (PGM) nuggets of eluvial (residual) origin, associated with the Freetown Layered Gabbro Complex, were determined in-situ using an ion micro- probe. The values for erlichmanite nuggets are ~ 1.08. Those for PGM inclusions in Pt-Fe alloy nuggets are higher, ranging from 1.2 to 2.1. Ratios of 187Os/186Os vary between the nuggets, but they are consistent within individual nuggets. The data suggest early formation of the erlichmanite nuggets, prior to a postulated substan- tial contribution of crustal Os. The Pt- Fe alloy nuggets, on the other hand, were formed later in a residual melt which was contaminated by crustal Os due to the assimi- lation (< 10%) or the gaseous/fluid transport of Os from Archaean host rocks into the magma. The lack of sys- tematic mineralogical and chemical changes of the Com- plex and extensive granulitization in the adjacent host rocks and xenoliths may favor the latter process. The lack of high Ia7Os/ZS6Os ratios, consistent ~87Os/ 186Os values within individual nuggets and their textures and mineralogy suggest that the studied PGM nuggets were not formed during lateritization or in low-tempera- ture depositional environments.

The platinum-group chemistry and mineralogy of the UG-2 chromitite layer of the Bushveld Complex

Individual platinum-group elements, as well as Au, Cu, and Ni, were determined in borehole cores of the UG-2 chromitite layer from different localities in the Bushveld Complex. The average noble metal content of the layer is about 7 ppm; Pt is most abundant, followed by Pd, Ru, Rh, Ir, and Au, in that order. Noble metal contents, recalculated to concentration in 100 percent sulfide, are extremely high, which raises questions regarding the distribution coefficients of noble metals in sulfide and silicate melts, and the initial concentrations of the platinum-group elements in Bushveld magmas.More than 6,000 grains of platinum-group minerals were identified qualitatively in polished sections of the ore, the most abundant species being laurite (RuS 2 ), cooperite (PtS), an unnamed Pt-Ir-Rh-Cu sulfide, braggite ([Pt, Pd, Ni]S), an unnamed Pt-Pb-Cu sulfide, vysotskite (PdS), gold, and electrum (Au, Ag) as well as intermetalloids such as Pt-Fe, Pd-Cu, Pd-Pb, and Pd-Hg. Associated base metal sulfides are mainly pentlandite, chalcopyrite, pyrrhotite, and pyrite. Variation in the platinum-group mineral assemblages can be explained by variations in the fugacity of sulfur between the various localities studied.Compositions of chromite grains show an increase of Fe (super +2) , Fe (super +3) , and Cr upward. Variations in Fe (super +3) /(Fe (super +2) + Fe (super +3) ) (laterally and vertically) may reflect variations in the prevailing fugacity of oxygen.

Mineralogy of platinum-group minerals in the Witwatersrand, South Africa

Approximately 80 percent of the platinum-group mineral grains in the Precambrian Witwatersrand conglomerate reefs are (Ir, Os, Ru) alloys. Osmium, iridosmine, osmiridium, and iridium are most abundant; rutheniridosmine, ruthenosmiridium, and ruthenian iridium are less abundant. Zoned grains are common, with Os-rich cores enclosed by (Ir, Pt)-rich outer zones filled with exsolution lamellae of (Pt, Fe) alloy. Discrete grains of this alloy proved to be isoferroplatinum of a composition close to Pt 3 Fe. In addition, grains ranging in composition between Pt 3 Fe and platiniridium are present.Sperrylite commonly occurs as an outer coating on all the above grain types. Sperrylite and isoferroplatinum make up 15 to 20 percent of the platinum-group mineral grains and account for nearly all the Pt present. Hollingworthite, iridarsenite, and ruthenarsenite occur in variable quantities (1-5%) as coatings on alloy grains, which in turn may be coated with sperrylite.Euhedral and subhedral inclusions in the common platinum-group mineral grains are michenerite, moncheite, laurite, geversite, and two unnamed minerals: rhodium sulfide and (Pt, Rh, Ru) alloy. Free grains of geversite, moncheite, sudburyite, and stibiopalladinite are very rare. The last two may be intergrown.Occasionally, partial or complete alteration of (Ir, Os, Ru) alloys has resulted in mixtures of erlichmanite, irarsite, osarsite, and phases that could not be identified.The evidence suggests that the (Ir, Os, Ru) and (platinum-group elements, Fe) alloys and their inclusions underwent little or no alteration during weathering, transportation, and burial, or even during later metamorphism of the reefs when sperrylite, holling-worthite, and related phases probably formed from less stable platinum-group minerals, possibly tellurides and antimonides.