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Coupled substitutions in PGE-enriched cobaltite: new evidence from the Rio Jacaré layered complex, Bahia state, Brazil

Published online by Cambridge University Press:  02 January 2018

Andrei Y. Barkov*
Affiliation:
Research laboratory of industrial and ore mineralogy, Cherepovets State University, 5 Lunacharsky Avenue, 162600 Cherepovets, Russia
Yana Fedortchouk
Affiliation:
Department of Earth Sciences, Dalhousie University, Halifax, Nova Scotia B3H 4J1, Canada
Robert A. Campbell
Affiliation:
Lake Shore Gold Corp., P.O. Box 1067, Timmins, Ontario P4N 2G0, Canada
Tapio A.A. Halkoaho
Affiliation:
Geological Survey of Finland, “Bedrock and Raw materials”, Eastern Finland Office, 5 Neulaniementie Street, P.O. Box 1237, FIN-70211 Kuopio, Finland
*

Abstract

Microcrystals of platinum-group element (PGE)-bearing cobaltite occur in the Gulcari A deposit of vanadiferous titanomagnetite in the lower zone of the Rio Jacaré mafic-ultramafic layered intrusion, Brazil. Aggregates of cobaltite and sperrylite are cluster-like and developed generally along the boundary of Fe-Ti oxide grains with deuteric silicates. Our observations of cryptic zoning, compositional variability and interelement correlations are based on 37 analytical points (wavelength-dispersion spectrometry mode) of cobaltite, and indicate that Ir and Rh behave uniformly with Ni and antipathetically with Co which, in turn, correlates directly with S content. Iridium, Rh and Ni apparently substitute for Co in the As-enriched grain core, and the substitution mechanism invokes solid solution with a cattierite-type molecule: (Ni + Ir + Rh) + (AsS) = Co + (S2). The PGE-bearing cobaltite probably crystallized as a primary phase at 500 to 300°C, from microvolumes of a late fluid phase. The observed enrichment in S and decrease in the As:S ratio at the cobaltite grain margins is a reflection of the increase in sulfur fugacity (fS2) with decrease in temperature of crystallization.

Type
Research Article
Copyright
Copyright © The Mineralogical Society of Great Britain and Ireland 2015

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