Supergene mobilization and redistribution of platinum-group elements in the Merensky Reef, eastern Bushveld Complex, South Africa

2021 ◽  
Vol 59 (6) ◽  
pp. 1381-1396
Author(s):  
Maximilian Korges ◽  
Malte Junge ◽  
Gregor Borg ◽  
Thomas Oberthür
Keyword(s):  
South Africa ◽  
Platinum Group Element ◽  
Bushveld Complex ◽  
Platinum Group Elements ◽  
Element Distribution ◽  
Group Element ◽  
Recovery Rates ◽  
Merensky Reef ◽  
Platinum Group Minerals ◽  
Platinum Group

ABSTRACT Near-surface supergene ores of the Merensky Reef in the Bushveld Complex, South Africa, contain economic grades of platinum-group elements, however, these are currently uneconomic due to low recovery rates. This is the first study that investigates the variation in platinum-group elements in pristine and supergene samples of the Merensky Reef from five drill cores from the eastern Bushveld. The samples from the Richmond and Twickenham farms show different degrees of weathering. The whole-rock platinum-group element distribution was studied by inductively coupled plasma-mass spectrometry and the platinum-group minerals were investigated by reflected-light microscopy, scanning electron microscopy, and electron microprobe analysis. In pristine (“fresh”) Merensky Reef samples, platinum-group elements occur mainly as discrete platinum-group minerals, such as platinum-group element-sulfides (cooperite–braggite) and laurite as well as subordinate platinum-group element-bismuthotellurides and platinum-group element-arsenides, and also in solid solution in sulfides (especially Pd in pentlandite). During weathering, Pd and S were removed, resulting in a platinum-group mineral mineralogy in the supergene Merensky Reef that mainly consists of relict platinum-group minerals, Pt-Fe alloys, and Pt-oxides/hydroxides. Additional proportions of platinum-group elements are hosted by Fe-hydroxides and secondary hydrosilicates (e.g., serpentine group minerals and chlorite). In supergene ores, only low recovery rates (ca. 40%) are achieved due to the polymodal and complex platinum-group element distribution. To achieve higher recovery rates for the platinum-group elements, hydrometallurgical or pyrometallurgical processing of the bulk ore would be required, which is not economically viable with existing technology.

PGE distribution in Merensky wide-reef facies of the Bushveld Complex, South Africa: Evidence for localized hydromagmatic control

2021 ◽  
Vol 59 (6) ◽  
pp. 1305-1338
Author(s):  
Stephen A. Prevec ◽  
Savvas Anthony Largatzis ◽  
William Brownscombe ◽  
Tobias Salge
Keyword(s):  
Platinum Group Element ◽  
Bushveld Complex ◽  
Platinum Group Elements ◽  
Group Element ◽  
Primitive Mantle ◽  
Reef Facies ◽  
Element Abundances ◽  
Merensky Reef ◽  
Element Concentrations ◽  
Platinum Group

ABSTRACT The wide-reef facies of the Merensky Reef in the eastern part of the western lobe of the Bushveld Complex was sampled in order to better resolve otherwise spatially constrained variation in highly siderophile elements across this geological unit. The platinum group element mineralogy and whole-rock highly siderophile element concentrations were measured across two vertical sections in close proximity. In one section, the Merensky Reef unit was bound by top and bottom platinum group elements-enriched horizons (reefs) with a well-developed pegmatoidal phase in the top third of the intrareef pyroxenite, but with neither a top nor a bottom chromitite present. The other drill core section featured a thin (<1 cm thick) chromitite layer associated with the highest platinum group element concentrations of any rock in this study as the bottom reef, but with a chromitite-absent top reef, and very poor development of the pegmatoid. Primitive mantle-normalized profiles of the main lithological units show relatively flat, primitive mantle-like highly siderophile element abundances (Cr, V, Co, Ni, platinum group elements, Au and Cu) in the Merensky pyroxenite, with modest depletion in Ir-affiliated platinum group elements. The platinum group element-rich top and bottom reefs, and the pegmatoidal upper pyroxenites, display characteristic enrichment in the Pt-affiliated platinum group elements and undepleted Ir-affiliated platinum group elements. The leuconoritic hanging wall and footwall rocks show comparable highly siderophile element profiles, distinguished from one another by relative depletion in the Pt-affiliated platinum group elements of the footwall samples. The vertical variation in highly siderophile element abundances through both sections is characterized by low platinum group element abundances through the lower reef pyroxenite, with platinum group element, Au, and Cu ± Ni concentrations increasing through the upper pegmatoidal pyroxenite, and main enrichment peaks at the top and bottom reefs. Significant localized (centimeter-scale) zones of chalcophile metal depletion are present immediately above the top reef and below the bottom reef. In addition, a wider zone of Pt-affiliated platinum group elements (with Pd more depleted than Pt)-depletion was identified within the pegmatoidal pyroxenite around one meter below the top reef. The platinum group element mineralogy of the bottom reef consists mainly of platinum group element sulfides, with minor arsenides and antimonides. In contrast, the platinum group element mineralogy of the top reef, and the small amount of data from the intrareef pyroxenite, mainly consist of Pt-affiliated platinum group elements-Bi-tellurides. The Pt-sulfides are mainly equant, relatively coarse crystals (many grains between 50 to 100 μm2 area), contrasting with the Pt-affiliated platinum group elements-Sb-As and -Bi-Te minerals that tend be high aspect-ratio grains, occurring in veinlets or as rims on earlier-forming platinum group element phases. These Te-As-Bi-Sb compounds are closely associated with chlorite, actinolite, quartz, and chalcopyrite, consistent with secondary deposition at lower temperatures and association with aqueous fluids. A model is proposed involving the emplacement of the Merensky unit as a magma pulse into at least semi-crystallized host rock, followed by aqueous fluid saturation and local migration, combined with concentration of late magmatic fluids around the top and bottom contacts of the magma pulse. Late remobilization of Pt-affiliated platinum group elements from the zones immediately (centimeter-scale) above the top reef, and from the underlying meter or two of pyroxenite, and from the centimeters underlying the bottom reef, have added additional platinum group elements to the reefs as late platinum group elements-Te-As-Bi-Sb minerals, independent of whether or not chromite is present in the reef initially.

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

2021 ◽  
Vol 59 (6) ◽  
pp. 1397-1435
Author(s):  
Thomas Oberthür ◽  
Frank Melcher ◽  
Simon Goldmann ◽  
Fabian Fröhlich
Keyword(s):  
Platinum Group Element ◽  
Bushveld Complex ◽  
Platinum Group Elements ◽  
Critical Zone ◽  
Group Element ◽  
Group Mineral ◽  
Incompatible Elements ◽  
Platinum Group Minerals ◽  
Fe Alloys ◽  
Platinum Group

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.

Variations in Composition, Texture, and Platinum Group Element Mineralization in the Lower Group and Middle Group Chromitites of the Northwestern Bushveld Complex, South Africa

2019 ◽  
Vol 114 (3) ◽  
pp. 569-590 ◽  
Author(s):  
Felix E.D. Kaufmann ◽  
Marie C. Hoffmann ◽  
Kai Bachmann ◽  
Ilya V. Veksler ◽  
Robert B. Trumbull ◽  
...  
Keyword(s):  
South Africa ◽  
Platinum Group Element ◽  
Bushveld Complex ◽  
Group Element ◽  
Middle Group ◽  
Platinum Group ◽  
Lower Group
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