Issue 2(46), 2017

ISSN 2307-2091 (Print) 

ISSN 2500-2414 (Online)

DOI 10.21440/2307-2091-2017-2-7-12                                                                                     Publication Date 11.07.2017

Chemical and Os-isotope composition of platinum-group mineral assemblages from the Kimberley conglomerate formation (Witwatersrand basin, South Africa) pdf

I. Yu. Badanina, K. N. Malitch, A. V. Antonov, I. N. Kapitonov, V. V. Khiller, S. M. Tuganova, R. K. W. Merkle

Complex Os–Au–U paleoplacers of the Witwatersrand basin, known as the Witwatersrand reefs, have been mined for more than a hundred years. In order to clarify the sources of ore matter, the duration and conditions for formation of platinum-group element (PGE) mineralization this study presents the original data on chemical and Os-isotopic composition in platinum-group minerals (PGM) derived from the Kimberley gold-bearing conglomerate formation located in the upper section of the Central Rand (Turffontein Subgroup of The Central Rand Group). The study employed a number of analytical techniques including electron microprobe analysis, laser ablation and mass spectrometry with ionization in inductively coupled plasma. The 187Os/188Os value in individual PGM grains of various compositions (ruthenium, osmium, iridium, rutheniridosmine, laurite, erlichmanite, unnamed polycomponent solid solutions of the Ru–Os–Ir–Pt ± Fe system) was found to range from 0.10481 ± 0.00006 to 0.10895 ± 0.00006, and 187Re/188Os lower than 0.0004. The studied PGM assemblage is represented by two main groups with mean 187Os/188Os values of 0.10510±0.00026 (n = 11) and 0.10682±0.00046 (n = 23), and a subordinate 187Os /188Os cluster of 0.10871 ± 0.00034 (n = 2). The Os-isotope results identify similar 187Os/188Os values for coexisting Os-bearing PGM pairs including Os-rich alloy and erlichmanite (i. e., 0.10482 ± 0.00002 and 0.10483 ± 0.00002, respectively), Os–Ru–Ir–Pt alloy and laurite (0.10528 ± 0.00002 and 0.10533 ± 0.00003, respectively). 187Os/188Os ages of the PGM, calculated relative to a chondrite universal reservoir
(CHUR) model (Chen et al., 1998), vary between 2.73 and 3.33 Ga. Two main PGM groups have mean model TMA CHUR ages of 3.250 ± 0.035 and 3.018 ± 0.062 Ga. Subordinate PGM group is characterized by the third age cluster (2.762 ± 0.046 Ga). The obtained results are consistent with: (i) high-temperature formation of the studied Ru–Os–Ir–Pt alloys, polycomponent solid solutions of the Ru–Os–Ir–Pt (±Fe) system and Ru–Os sulfides, (ii) similarity of the initial Os-isotope composition in coexisting Os-rich alloys and Ru-Os sulfides, (iii) subhondritic Archaean source of platinum-group elements (PGE), and (iv) clastic origin of the studied PGM.

Keywords: Os–Ru–Ir–Pt alloys; Ru–Os sulfides; Os-isotopic composition; formation conditions; Kimberley conglomerate formation; Witwatersrand basin; South Africa.



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