Electron probe analysis, x-ray diffraction analysis, physical properties, occurrence as inclusions and replacements in umangite, as stringers and veinlets in carbonate veins
Osmium-isotope ratios of individual PGM (platinum-group minerals) were determined lr sl/a using an ion microprobe. All samples except for one came from placers associated with Alpine- or Alaskan-type ultramafic intrusions. Most Alpine+ype samples are Os-Ir-Ru alloys, and most Alaskan-type samples are Pt-Fe alloys (isoferroplatinum or tetraferroplatinum), which contain Os-lr-Ru alloys and laurite/erlichmanite. Osmium-isotope measurements for the Pt-Fe alloys were carried out on those Os-bearing inclusions. The Os-isotope values are similar between cores and rims of PGM and among different inclusions of different phases within individual nuggets. No isotopic variations are observed in PGM that show profound chemical zoning. The-lack o^! isotopic heterogeneiry in individual nuggets precludes a low-temperature origin for rhe nuggets of PGM, 1876t71869, values of all samples show a narrow spread, ranging from 0.99 to 1.12, with most of the values between 1.00 and 1.06. These values fall within the range of l87Os/l86Os values of the mantle, suggesting essential derivation of platinum-group elements (PGE) from the mantle without a significant contribution of crustal Os. The grains of PGM were formed in intrusions' weathered, eroded, and concentrated in placers by mechanical processes. The Os-isotopic data are consistent with the occurrence of exsolulion lamellae of PGM, the inclusions of pristine unwealhered olivine, and the lack of foreign mineral inclusions, unrelated to ultramafic rocks, within the nuggets. Os-isotopic values for a placer nugget and PGM from chromitite in Urals also support the conclusion that the placer nugget was derived from the latter. The irregular shape and nonabraded surfaces of some nuggets may simply reflecr their durable nature and a short distance of transporl from the eroded ultramafic intrusions.
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.
STUDIES of magmatic deposits and the processes which produced concentrations of platinum-group elements continue to dominate the platinum-group element literature since the last special issue (Economic Geology, 1982, v. 77, no. 6). Development of ideas on the fluid dynamics of mafic magma chambers to explain the layering (McBirney and Noyes, 1979; Huppert and Sparks, 1980; Campbell et al., 1983; Irvine et al., 1983; Huppert et al., 1984; Sparks and Huppert, 1984) have significantly contributed to our current understanding of these deposits. The challenge facing us now is a detailed petrogenetic explanation of the significance of certain other phenomena, such as fluid inclusions and Cl-bearing minerals. Papers on the Bushveld Complex are an important component of this special issue, but they are not as predominant as in the last. Of particular interest are the first papers representing results of the current worldwide search for new Bushveld-type platinum-group element deposits. Another feature of this special issue is an increased focus on chromitites, both from the Bushveld and elsewhere.
ABSTRACT For gold deposits, varying combinations of gold grains, sulphides, platinum-group minerals (PGM), tellurides, scheelite and rutile, and some secondary minerals are useful indicator minerals depending on the deposit type, bedrock geology and weathering regime. Gold grain size, shape, and chemical composition for a variety of sediment types, including stream and glacial sediments, have been documented and the data used to determine potential source rocks and distance of transport. Useful indicator minerals for PGE deposits include those oxide and silicate minerals that indicate the host rocks and PGM, gold, sulphides, arsenides and antimonide minerals that indicate mineralization. Composition and morphology of PGM also have been well documented and this information is used to determine their genesis, potential source rocks and transport distance. Gold grains have been recovered from glacial and stream sediments for more than 100 years. PGM grains have a similar long history of recovery from streams, but only a few cases of recovery from glacial sediment have been reported. Research has focused on the development of microchemical characterization techniques for placer gold and PGM, while the focus for indicator minerals from glacial sediments has been the characterization of oxide and silicate suites.
AssrRActThe Pd-As-Sb system contaius many knowncompounds and to it several minerals have re-cently been ascribed. The poor r-ray powderdiffraction characteristics of these minerals aswell as the occurrence of structurally distinct, butcompositionally very close phases, makes theircharacterization very difficult.The folloqdng minerals were studied: stillwa-terite, P(Ase, a new mineral, hexagonal, witha - 7.399(4), c - 10.311(11A, space eroop itor P3; an unnamed PdrAq mineral, orthorhom-bic, with a = 11.261(4), b = 3.857(1), c =11.346(lA; palladoarsenide, PdAs; unknown(Pd, Ni, As) and (Pd, Cu, As) minerals; sperrylite,PtAs2; unnamed PdsSba, with_ hexagonal axes a- 7.565(1), c = 43.2O7(3)A; and mertieite II,Pds(Sb, AsL, with hexagonal axes s - 7.546Q't,c - 43.18(1)4.The results of some experiments in the Pd-As-Sb system are discussed with reference to min-erals studied or reported in the literature.Rfsuu€Le systdme Pd-As-Sb continent plusieurs compo-s6s connus et plusieurs mindraux viennent r6cem-ment de lui 6tre attribu6s. La caract6risation dq cesmin6raux est trds difficile du fait des pauvrescaract6ristiques de la diffraction des poudres parrayons-X et de la pr6sence de phases de compositiontrds serr6e et de structure diff6rente.*The mineral aud name were approved by theCommission on New Minerals and MineralNames, I.M.A.
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