Abstract The new mineral driekopite, ideally PtBi, was found in a concentrate from the Driekop mine, one of three zoned Pt pipes (mined from 1925 to 1930) that crosscut the layered mafic and ultramafic sequences of the eastern Bushveld Complex, Republic of South Africa. The holotype grain of driekopite (∼ 22 × 13 μm) occurs in a complex, rounded aggregate ∼120 μm in diameter, in association with isoferroplatinum (Pt3Fe), hollingworthite (RhAsS), geversite (PtSb2), insizwaite (PtBi2), andrieslombaardite (RhSbS), stibiopalladinite (Pd5Sb2), sobolevskite (PdBi), possible tatyanaite (Pt9Cu3Sn4), osmium-bearing tulameenite (Pt2FeCu), and Pt-Fe alloy (∼Pt2Fe). Driekopite appears slightly orange under reflected light compared to Pt-Fe alloys. It shows moderate to strong bireflectance, varying from light yellow to brownish yellow, no pleochroism or internal reflections, and moderate to strong anisotropism. The empirical formula, calculated from the average of six wavelength-dispersive spectrometry analyses made on five grains, on the basis of two atoms, is (Pt0.68Pd0.31Fe0.01)Σ1.00(Bi0.53Sb0.43As0.02Sn0.02S0.01)Σ1.01. The mineral is hexagonal, space group P63/mmc (#194) with the refined unit-cell dimensions a = 4.1993(5), c = 5.6194(6) Å, V = 85.82 Å3, Z = 2, and Dcalc = 12.91 g/cm3. Driekopite is isostructural with NiAs, with mixed compositions of Pt and Pd at the 2a site (0.55:0.45, respectively) and Bi and Sb at the 2c site (0.63:0.37, respectively). Its crystal structure was refined to wR = 6.3% using 13 unique Laue reflections obtained using synchrotron radiation. The six strongest lines for the powder X-ray diffraction pattern calculated from the crystal structure refined from synchrotron data is [d in Å (I) (hkl)]: 3.0531 (92) (101), 2.2234 (100) (102), 2.0997 (77) , 1.5266 (28) (202), 1.2347 (24) , 1.1676 (18) . The holotype grain of driekopite is observed to be paragenetically later than isoferroplatinum and hollingworthite and is considered to be synformational with Bi-bearing geversite, insizwaite, andrieslombaardite, and sobolevskite. The entire aggregate containing these platinum-group minerals is overgrown by a rim of tulameenite and Pt-Fe alloy (∼Pt2Fe), indicating they are paragenetically the last minerals to form. Experiments designed to synthesize PtBi over the range of 200 to 500 °C were all successful. Synthetic PtBi melts congruently at 765 °C, suggesting that driekopite likely crystallized at sub-magmatic temperatures.
Research Article| December 01, 2014 ALLUVIAL AND ELUVIAL PLATINUM-GROUP MINERALS FROM THE BUSHVELD COMPLEX, SOUTH AFRICA T. OBERTHÜR; T. OBERTHÜR Bundesanstalt für Geowissenschaften und Rohstoffe (BGR), Stilleweg 2, D-30655 Hannover, Germany, e-mail: thomas.oberthuer@bgr.de Search for other works by this author on: GSW Google Scholar T.W. WEISER; T.W. WEISER Rischkamp 63, D-30659 Hannover, Germany., e-mail: weiser@wandix.de Search for other works by this author on: GSW Google Scholar F. MELCHER F. MELCHER Institute of Geology and Economic Geology, University of Leoben, Peter-Tunnerstraße 5, A-8700 Leoben, Austria., e-mail: frank.melcher@unileoben.ac.at Search for other works by this author on: GSW Google Scholar South African Journal of Geology (2014) 117 (2): 255–274. https://doi.org/10.2113/gssajg.117.2.255 Article history first online: 09 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share MailTo Twitter LinkedIn Tools Icon Tools Get Permissions Search Site Citation T. OBERTHÜR, T.W. WEISER, F. MELCHER; ALLUVIAL AND ELUVIAL PLATINUM-GROUP MINERALS FROM THE BUSHVELD COMPLEX, SOUTH AFRICA. South African Journal of Geology 2014;; 117 (2): 255–274. doi: https://doi.org/10.2113/gssajg.117.2.255 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 The present work provides an initial description of detrital platinum-group minerals (PGM) collected from alluvial sediments of rivers draining the Bushveld Complex, and from eluvial concentrations at the Onverwacht platiniferous pipe. During a field campaign, sediments were sampled at nine localities around the Bushveld Complex, and heavy mineral assemblages were investigated using optical and scanning electron microscopy (SEM) as well as microprobe analysis (EPMA) of PGM. All concentrates from the alluvial samples contain discrete PGM grains with grain sizes in the range from ~50 to 150 μm (maximum 600 μm). The overall PGM proportions are: native Pt, Pt-Pd and Pt-Fe alloys (together 54%), sperrylite (33%), cooperite/braggite (11%), and stibiopalladinite (2%). This PGM assemblage distinctly contrasts to the suite of PGM in the pristine, sulfide-bearing mineralization in the Merensky, UG-2 and Platreef, the assumed sources of the detrital PGM. Specifically, PGE-bismuthotellurides and -sulfarsenides, common in the primary ores, are missing in the assemblage of detrital PGM in the fluvial environment. Nearly all detrital PGM (98%) are Pt minerals, corroborating earlier findings that Pd-dominated PGM are unstable and are dissolved in the supergene environment, and that PGE-bismuthotellurides and -sulfarsenides, common in the PGM assemblages of the pristine ores are unstable during weathering and mechanical transport.The eluvial material collected at Onverwacht contained ca. 150 PGM grains with sizes mainly in the range 100 to 300 μm range (maximum 1.87 mm). The PGM assemblage comprises grains of Pt-Fe alloys (66%), sperrylite (14%), and many rarer PGM including stibiopalladinite, hollingworthite, laurite, PGE arsenides and PGE sulfides. The suite of eluvial PGM observed is similar to the PGM assemblage described previously from the Onverwacht pipe proper, including the type locality minerals genkinite, irarsite and platarsite, as well as some additional and possibly new PGM. Most of the relatively rare PGM detected in the suite of eluvial grains from Onverwacht were also reported in the detrital PGM assemblage from the Moopetsi river, farm Maandagshoek (Oberthür et al., 2004), indicating that many of the latter grains originate from platiniferous pipes and not from the Merensky or UG-2 reefs.Detrital PGM can be expected to be present in rivers draining PGE-bearing layered intrusions, and economic placers may form under particular sedimentological conditions. Therefore, this work also highlights the fact that the nowadays somewhat neglected field methods and basic techniques have their merits and value in mineral exploration, especially if they are combined with modern micro-analytical methods. The systematic recovery of PGM from stream sediments, soils and till should regain wider application in mineral exploration as these tools can provide useful indicators of platinum mineralization. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.
High-resolution (partly in-situ) sulfur isotope measurements on sulfides in 17 samples from 5 different Pb-Zn deposits of the Drau range have been carried out. High-resolution sulfur isotope investigations provide knowledge on small-scale variations in the sulfur isotopic composition of coexisting sulfides and can help to interpret the mineralization processes. The sulfur isotopic composition provides evidence for different sulfur reservoirs and fluids involved in the mineralization process. Small-scale isotope heterogeneities can be related to textural aspects and this enables to recognize different sulfur reservoirs, one providing bacteriogene reduced sulfur (BSR) and another one responsible for thermochemical reduced sulfur (TSR). Both reservoirs were involved into ore formation and were able to contribute reduced sulfur during the whole mineralization process.
In addition to isotopic measurements, major, minor and trace element analyses by electron microprobe (EMP) have been carried out on sphalerites in order to investigate if any relation between sulfur isotopic and chemical composition exists.
Bor and Cukaru Peki are world-class porphyry deposits spatially and genetically associated with the Cretaceous Timok magmatic complex. This research was conducted to determine the age and geochemical affinity of the magmatic rocks that formed these ore deposits. Our new geochemical analyses of magmatic rocks from Bor and Cukaru Peki deposits imply they comprise adakite-like compositions that have undergone the amphibole fractionation and sulphide saturation processes. The zircon ages indicate that the Bor system was formed in the age span between 84.5?82 Ma, while the Cukaru Peki system was created in the age span between 86.5?85 Ma.
In the Speik Complex (Eastern Alps, Austria), highly melt-depleted, metamorphosed harzburgites with abundant pods and layers of chromitite are interlayered with a suite of metamorphosed orthopyroxenites, clinopyroxenites and gabbros. Coarse-grained orthopyroxenites occur as centimetre- to metre-wide veinlets and pods, but also as intrusive plugs several tens of metres wide. Intimately associated metaclinopyroxenite and metagabbro are present as bodies up to several metres thick at a distinct stratigraphic level within the complex. In the ultramafic rocks, relict magmatic olivine, orthopyroxene, clinopyroxene and spinel have been overprinted by a metamorphic assemblage of forsterite, diopside, tremolite, anthophyllite, chlorite, serpentine, talc and Cr–Fe-rich spinel. Hornblende, epidote, zoisite and chlorite dominate the metamorphic paragenesis in metagabbros, in addition to rare relicts of clinopyroxene and two phases of Ca-rich garnet. The polymetamorphic evolution of the Speik Complex includes rarely preserved pre-Variscan (400 Ma) eclogite-facies conditions, Variscan (∼330 Ma) amphibolite-facies conditions (600–700°C, >5 kbar) and Eoalpine (∼100 Ma) greenschist- to amphibolite-facies conditions reaching 550°C and 7–10 kbar. Orthopyroxenites are characterized by high concentrations of SiO2, MgO and Cr, and by U-shaped chondrite-normalized rare earth element (REE) patterns similar to those of their harzburgite hosts. The REE patterns of the clinopyroxenites are flat to slightly enriched in light REE. Metagabbro compositions are variable, but generally characterized by low SiO2 and high mg-numbers (61–78). Their REE patterns all have GdN/YbN > 1; some samples have large positive Eu anomalies implying the original presence of cumulus plagioclase. In the orthopyroxenites, clinopyroxenites and some peridotites, Pt, Pd and Re are distinctly enriched compared with Os, Ir and Ru, whereas most harzburgites have unfractionated to slightly fractionated platinum-group element (PGE) patterns with respect to average upper mantle. The Re–Os isotope compositions of the pyroxenites define an errorchron at 550 ± 17 Ma and a supra-chondritic 187Os/188Os of 0·179 ± 0·003. An isochron age of 554 ± 37 Ma with εNd(i) +0·7 is indicated by the Sm–Nd isotope compositions of whole-rock pyroxenite and gabbro samples, whereas the harzburgites plot on an errorchron of 745 ± 45 Ma and εNd(i) +6. The pyroxenites and gabbros probably represent a cogenetic suite of magmatic dykes intruded into uppermost, highly depleted, suboceanic mantle below the crust–mantle transition zone in an oceanic basin close to the northwestern margin of Gondwana.
ABSTRACT The platiniferous dunite pipes are discordant orebodies in the Bushveld Complex. The Onverwacht pipe is a large body (>300 m in diameter) of magnesian dunite (Fo80–83) that crosscuts a sequence of cumulates in the Lower Critical Zone of the Bushveld Complex. In a pipe-in-pipe configuration, the main dunite pipe at Onverwacht hosts a carrot-shaped inner pipe of Fe-rich dunite pegmatite (Fo46–62) which comprises the platinum-bearing orebody. The latter was ca. 18 m in diameter and a mining depth of about 320 m was reached. In the present work, a variety of ore samples were studied by whole-rock geochemistry, including analyses of platinum group elements, ore microscopy, and electron probe microanalysis. Olivine of the ore zone displays considerable chemical variation (range 46–62 mol.% Fo) and may represent either a continuum, or different batches of magma, or vertical or horizontal zonation within the ore zone. Chromite is principally regarded to be a consanguineous component of the pipe magma that crystallized in situ and simultaneously with olivine. The Onverwacht mineralization is Pt-dominated (>95% of the platinum group elements) and the ore is virtually devoid of sulfides. Platinum-dominated platinum group minerals predominate, followed by Rh-, Pd-, and Ru-species. Pt-Fe alloys are most frequent, followed by Pt-Rh-Ru-arsenides and -sulfarsenides, platinum group element antimonides, and platinum group element sulfides. Our hypothesis on the genesis of the Onverwacht pipe and its mineralization is as follows: After near-consolidation of the layered series of the Critical Zone, the magnesian dunite pipe of Onverwacht was formed by upward penetration of magmas that replaced the existing cumulates initially by infiltration, followed by the development of a central channel where large volumes of magma flowed through. Fractional crystallization of olivine within the deeper magma chamber and/or during ascent of the melt resulted in the formation of a consanguineous, residual, more iron-rich melt. This melt also contained highly mobile, supercritical, water-bearing fluids and was continuously enriched in platinum group elements and other incompatible elements. In several closing pulses, the platinum group element-enriched residual melts crystallized and sealed the inner ore pipe. Crystallization of the melt resulted in the coeval formation of Fe-rich olivine, chromite, and platinum group minerals. The non-sulfide platinum group element mineralization was introduced in the form of nanoparticles and small droplets of platinum group minerals, which coagulated to form larger grains during evolution of the mineralizing system. The suspended platinum group minerals acted as collectors of other platinum group elements and incompatible elements during generation and ascent of the melt. With decreasing temperature, the platinum group mineral grains annealed and recrystallized, leading to the formation of composite platinum group mineral grains, complex intergrowths, or lamellar exsolution bodies. On further cooling, platinum group minerals overgrowing Pt-Fe alloys formed by reaction of leached elements and ligands like Sb, As, and S mobilized by supercritical magmatic/hydrothermal fluids. Redistribution of platinum group elements/platinum group minerals apparently only occurred on the scale of millimeters to centimeters. Finally, surface weathering led to the local formation of platinum group element oxides/hydroxides by oxidation of reactive precursor platinum group minerals.
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