Abstract
Platinum-group mineral (PGM) from alpine-type chromitites and placer deposits in southeast Kalimantan were studied by reflected-light microscopy, electron microprobe and a few SEM examinations. The PGM found in five chromitites of the Meratus-Bobaris ophiolite are small (< μm). They occur as Ru-rich laurites, Os and Ir-rich laurites and, rarely, as Os-lr alloys. Euhedral PGM inclusions in chromite are interpreted to have formed before chromite crystallization at T > 900 °C and log fS2 above -2·3. For the subhedral and anhedral PGM at chromite-serpentirze contacts and within serpentine interstitial to chromite a decrease in temperature and sulphur activity is proposed. Os- and Ir-enrichment in some of these grains are assumed to have occurred during serpentinization.
The PGM obtained from the placers are Ag- and Cu-bearing Pt-Fe alloys (sizes up to > 1 mm), rutheniridosmine containing Au and Cu, and Cu-bearing osmiridium. The Pt-Fe alloy grains vary in morphology from rounded or angular to embayed or ‘spongy’, the Ru-lr-Os alloys are subangular to angular.
The Ru-lr-Os phases in the placers could have come from the Meratus-Bobaris ophiolite chromitites by erosion alone, but this explanation is not conformable with the large size of the placer grains (>200 μm) compared to the small PGM found in the chromitites. An origin from non-ophiolitic ultramafic-mafic intrusions is not supported by the vague and disputed existence of such (Pre-Cretaceous) complexes in East Kalimantan. But there is evidence that the Ru-Ir-Os alloys have formed via reworking and reconcentration of PGE during transport from Meratus-Bobaris ophiolite source rocks after serpentinization and partial lateritization. Furthermore, the close association of the Pt-Fe PGM with the Ru-Ir-Os alloys in the placers suggests a common source and similar formation of both varieties of PGM. The individual placers are obviously characterized by a rather limited compositional variation of the Pt-Fe alloys and a separated grouping in the ternary diagram Pt-Σ Fe + Ni + Cu -Σ Pd + Rh + Ru + Ir + Os.
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Burgath, KP. (1988). Platinum-Group Minerals in Ophiolitic Chromitites and Alluvial Placer Deposits, Meratus-Bobaris Area, Southeast Kalimantan. In: Prichard, H.M., Potts, P.J., Bowles, J.F.W., Cribb, S.J. (eds) Geo-Platinum 87. Springer, Dordrecht. https://doi.org/10.1007/978-94-009-1353-0_40
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DOI: https://doi.org/10.1007/978-94-009-1353-0_40
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