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

IF 3.5 2区 地球科学 Q1 GEOCHEMISTRY & GEOPHYSICS
Anton Kutyrev, Vadim S Kamenetsky, Ivan F Chayka, Nikolai A Nekrylov, Lyudmila Kryuchkova, Vladimir V Shilovskikh, Alkiviadis Kontonikas-Charos, Stepan P Krasheninnikov, Anna Sapegina, Alexei L Perchuk
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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 behaviour and occur as native metals and alloys. In this case, they can form μm-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.
大块铂族金属的变质起源:来自塔斯马尼亚州亚当斯菲尔德矿床的奥-铁-鲁合金中多相夹杂物的证据
铂族元素(PGE)通过硫化物-硅酸盐熔体的不溶性得到最有效的富集;然而,在硫化物不饱和的条件下,铂族元素可能会表现出高度亲硒的行为,并以原生金属和合金的形式出现。在这种情况下,它们可以在铬尖晶石中形成微米大小的包裹体,也可以在铬铁矿和相关的块状矿床中形成大型金块(重达几公斤)。由于质量平衡问题,这种与硫化物熔体无关的大型金块和 PGE 聚集的确切形成机制仍存在争议。在这项对亚当斯菲尔德(澳大利亚塔斯马尼亚州)块矿区的 Os-Ir-Ru 金块中多相包裹体的研究中,我们对其结晶环境进行了分析。多相包裹体由不同比例的角闪石、英安岩、石英、直闪石、阳起石、绿泥石和原生铱组成,并与单相橄榄石和铬尖晶石包裹体共存。包裹体在相位和化学成分上的异质性表明其来源复杂,来自不均匀的源介质。晶粒尺度的不平衡矿物组合也证实了这一点,在一个锇晶粒中同时包含了紫锂辉石和石英。我们提出的亚当斯菲尔德 Os-Ir-Ru 金块的多阶段成因涉及橄榄岩体中的岩浆橄榄石-Cr-尖晶石-Os-Ir-Ru 堆积物,这些堆积物随后被各种热液和变质过程覆盖,包括蛇纹石化或超临界流体中 Os-Ir-Ru 的生长。最后的变质阶段导致了Os-Ir-Ru分带的消失,最终形成了目前的包裹体组合。
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来源期刊
Journal of Petrology
Journal of Petrology 地学-地球化学与地球物理
CiteScore
6.90
自引率
12.80%
发文量
117
审稿时长
12 months
期刊介绍: The Journal of Petrology provides an international forum for the publication of high quality research in the broad field of igneous and metamorphic petrology and petrogenesis. Papers published cover a vast range of topics in areas such as major element, trace element and isotope geochemistry and geochronology applied to petrogenesis; experimental petrology; processes of magma generation, differentiation and emplacement; quantitative studies of rock-forming minerals and their paragenesis; regional studies of igneous and meta morphic rocks which contribute to the solution of fundamental petrological problems; theoretical modelling of petrogenetic processes.
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