Metamorphic origin of large nuggets of platinum-group metals: evidence from multiphase inclusions in Os-Ir-Ru alloys from the Adamsfield placer, Tasmania

Kutyrev, Anton ORCID: https://orcid.org/0000-0001-5933-6455, Kamenetsky, Vadim S., Chayka, Ivan F., Nekrylov, Nikolai A., Kryuchkova, Lyudmila, Shilovskikh, Vladimir V., Kontonikas-Charos, Alkiviadis, Krasheninnikov, Stepan P., Sapegina, Anna and Perchuk, Alexei L. 2024. Metamorphic origin of large nuggets of platinum-group metals: evidence from multiphase inclusions in Os-Ir-Ru alloys from the Adamsfield placer, Tasmania. Journal of Petrology 65 (8) , egae086. 10.1093/petrology/egae086

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Abstract

Platinum-group elements (PGE) are most effectively concentrated via sulfide-silicate melt immiscibility; however, under sulfide-undersaturated conditions, PGE may exhibit highly siderophile behavior and occur as native metals and alloys. In this case, they can form micrometer-size inclusions within Cr-spinel, but also large nuggets (up to several kilograms), found in chromitites and related placer deposits. The exact formation mechanism of such large nuggets and accumulations of PGE unrelated to sulfide melts remains controversial due to mass balance issues. In this study of multiphase inclusions in Os-Ir-Ru nuggets from the Adamsfield placer district (Tasmania, Australia), we constrain their crystallization environment. Multiphase inclusions comprise variable proportions of hornblende, enstatite, quartz, anthophyllite, anorthite, chlorite and native iridium and coexist with single-phase olivine and Cr-spinel inclusions. The heterogeneity in phase and chemical composition of the inclusions indicates a complex origin from an inhomogeneous source media. This is corroborated by grain scale disequilibrium mineral assemblages, where forsterite and quartz are both included within a single osmium grain. Our proposed multi-stage origin of Adamsfield Os-Ir-Ru nuggets involved magmatic olivine-Cr-spinel-Os-Ir-Ru cumulates in peridotite bodies that were subsequently overprinted by various hydrothermal and metamorphic processes, including serpentinization or growth of Os-Ir-Ru from supercritical fluids. The final metamorphic stage resulted in the obliteration of Os-Ir-Ru zonation, culminating in the current assemblage of inclusions.

Item Type:	Article
Date Type:	Publication
Status:	Published
Schools:	Earth and Environmental Sciences
Publisher:	Oxford University Press
ISSN:	0022-3530
Date of First Compliant Deposit:	21 August 2024
Date of Acceptance:	29 July 2024
Last Modified:	18 Oct 2024 01:29
URI:	https://orca.cardiff.ac.uk/id/eprint/171521

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