High grade ores of the Onverwacht platinum pipe, eastern Bushveld, South Africa

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.

Marathonite, Pd25Ge9, and palladogermanide, Pd2Ge, two new platinum-group minerals from the Marathon deposit, Coldwell Complex, Ontario, Canada: Descriptions, crystal-chemical considerations, and genetic implications

ABSTRACT Marathonite, Pd25Ge9, and palladogermanide, Pd2Ge, are two new platinum-group minerals discovered in the Marathon deposit, Coldwell Complex, Ontario, Canada. Marathonite is trigonal, space group P3, with a 7.391(1), c 10.477(2) Å, V 495.6(1) Å3, Z = 1. The six strongest lines of the X-ray powder-diffraction pattern [d in Å (I)(hkl)] are: 2.436(10)(014,104,120,210), 2.374(29)(023,203,121,211), 2.148(100)(114,030), 1.759(10)(025,205,131,311), 1.3605(13)(233,323,036,306), and 1.2395(14)(144,414,330). Associated minerals include: vysotskite, Au-Ag alloy, isoferroplatinum, Ge-bearing keithconnite, majakite, coldwellite, ferhodsite-series minerals (cuprorhodsite-ferhodsite), kotulskite and mertieite-II, the base-metal sulfides, chalcopyrite, bornite, millerite and Rh-bearing pentlandite, oberthürite and torryweiserite, and silicates including a clinoamphibole and a Fe-rich chlorite-group mineral. Rounded, elongated grains of marathonite are up to 33 × 48 μm. Marathonite is white, but pinkish brown compared to palladogermanide and bornite. No streak or microhardness could be measured. The mineral shows no discernible pleochroism, bireflectance, or anisotropy. The reflectance values (%) in air for the standard COM wavelengths are: 40.8 (470 nm), 44.1 (546 nm), 45.3 (589 nm), and 47.4 (650 nm). The calculated density is 10.933 g/cm3, determined using the empirical formula and the unit-cell parameters from the refined crystal structure. The average result (n = 19) using energy-dispersive spectrometry is: Si 0.11, S 0.39, Cu 2.32, Ge 18.46, Pd 77.83, Pt 1.10, total 100.22 wt.%, corresponding to the empirical formula (based on 34 apfu): (Pd23.82Cu1.19Pt0.18)Σ25.19(Ge8.28S0.40Si0.13)∑8.81 and the simplified formula is Pd25Ge9. The name is for the town of Marathon, Ontario, Canada, after which the Marathon deposit (Coldwell complex) is named. Results from electron backscattered diffraction show that palladogermanide is isostructural with synthetic Pd2Ge. Based on this, palladogermanide is considered to be hexagonal, space group , with a 6.712(1), c 3.408(1) Å, V 133.0(1), Z = 3. The seven strongest lines of the X-ray powder-diffraction pattern calculated for the synthetic analogue [d in Å (I)(hkl)] are: 2.392(100)(111), 2.211(58)(201), 2.197(43)(210), 1.937(34)(300), 1.846(16)(211), 1.7037(16)(002), and 1.2418(18)(321). Associated minerals are the same as for marathonite. Palladogermanide occurs as an angular, anhedral grain measuring 29 × 35 μm. It is white, but grayish-white when compared to marathonite, bornite, and chalcopyrite. Compared to zvyagintsevite, palladogermanide is a dull gray. No streak or microhardness could be measured. The mineral shows no discernible pleochroism, bireflectance, or anisotropy. The reflectance values (%) in air for the standard COM wavelengths for Ro and Ro' are: 46.8, 53.4 (470 nm), 49.5, 55.4 (546 nm), 50.1, 55.7 (589 nm), and 51.2, 56.5 (650 nm). The calculated density is 10.74 g/cm3, determined using the empirical formula and the unit-cell parameters from synthetic Pd2Ge. The average result (n = 14) using wavelength-dispersive spectrometry is: Si 0.04, Fe 0.14, Cu 0.06, Ge 25.21, Te 0.30, Pd 73.10, Pt 0.95, Pb 0.08, total 99.88 wt.%, corresponding (based on 3 apfu) to: (Pd1.97Pt0.01Fe0.01)Σ1.99(Ge1.00Te0.01)∑1.01 or ideally, Pd2Ge. The name is for its chemistry and relationship to palladosilicide. The crystal structure of marathonite was solved by single-crystal X-ray diffraction methods (R = 7.55, wR2 = 19.96 %). It is based on two basic modules, one ordered and one disordered, that alternate along [001]. The ordered module, ∼7.6 Å in thickness, is based on a simple Pd4Ge3 unit cross-linked by Pd atoms to form a six-membered trigonal ring that in turn gives rise to a layered module containing fully occupied Pd and Ge sites. This alternates along [001] with a highly disordered module, ∼3 Å in thickness, composed of a number of partially occupied Pd and Ge sites. The combination of sites in the ordered and disordered modules give the stoichiometric formula Pd25Ge9. The observed paragenetic sequence is: bornite → marathonite → palladogermanide. Phase equilibria studies in the Pd-Ge system show Pd25Ge9 (marathonite) to be stable over the range of 550–970 °C and that Pd2Ge (palladogermanide) is stable down to 200 °C. Both minerals are observed in an assemblage of clinoamphibole, a Fe-rich, chlorite-group mineral, and fragmented chalcopyrite, suggesting physical or chemical alteration, possibly both. Palladogermanide is also found associated with a magnetite of near end-member composition, potentially indicating a relative increase in fO2. Both minerals are considered to have developed at temperatures of 500–600 °C, under conditions of low fS2 and fO2, given the requirements needed to fractionate, concentrate, and form minerals with Ge-dominant chemistries.

Zirconium-bearing accessory minerals in UK Paleogene granites: textural, compositional, and paragenetic relationships

The mineral occurrences, parageneses, textures, and compositions of Zr-bearing accessory minerals in a suite of UK Paleogene granites from Scotland and Northern Ireland are described. Baddeleyite, zirconolite, and zircon, in that sequence, formed in hornblende + biotite granites (type 1) and hedenbergite–fayalite granites (type 2). The peralkaline microgranite (type 3) of Ailsa Craig contains zircon, dalyite, a eudialyte-group mineral, a fibrous phase which is possibly lemoynite, and Zr-bearing aegirine. Hydrothermal zircon is also present in all three granite types and documents the transition from a silicate-melt environment to an incompatible element-rich aqueous-dominated fluid. No textures indicative of inherited zircon were observed. The minerals crystallized in stages from magmatic through late-magmatic to hydrothermal. The zirconolite and eudialyte-group mineral are notably Y+REE-rich (REE signifies rare earth element). The crystallization sequence of the minerals may have been related to the activities of Si and Ca, to melt peralkalinity, and to local disequilibrium.