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Book Chapter

Metamorphism of Komatiite-Hosted Nickel Sulfide Deposits

By
Stephen J. Barnes
Stephen J. Barnes
CSIRO Exploration and Mining, Private Bag, Wembley, Western Australia 6014, Australia
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Robin E.T. Hill
Robin E.T. Hill
CSIRO Exploration and Mining, Private Bag, Wembley, Western Australia 6014, Australia
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Published:
January 01, 1998

Abstract

Komatiite-hosted magmatic nickel sulfide deposits fall into two major categories: sulfide-rich deposits at basal contacts of komatiite flow units (type 1, exemplified by the Kambalda deposits), and diffuse sulfidepoor disseminations within large extrusive bodies of olivine cumulate that are formed in major lava-flow pathways (type 2, exemplified by the giant Mt. Keith deposit in the Agnew-Wiluna belt of Western Australia).

Metamorphism of komatiites produces a very wide range of nonsulfide assemblages, despite the relatively restricted compositional range of the rocks; a crucial variable is the forumla of the metamorphic fluid. The sulfide mineralogy of the komatiite-hosted deposits is influenced by the temperature and composition of the metamorphic fluid. Typical greenschist-facies hydration gives rise to serpentinites, hosting assemblages very rich in pentlandite and in some cases heazlewoodite. Reduction reactions associated with the serpentinization front give rise to Ni-Fe alloy-bearing assemblages. Oxidized fluids associated with low-temperature talc-carbonate alteration give rise to vaesite, polydymite, and millerite coexisting with pyrite and hematite.

Despite the foregoing commonality, the role of metamorphism in modifying the two types of deposit is very different. In type 1 deposits, the relative modal proportion of sulfide to silicate minerals is generally high, and the bulk of the nickel in the ore resides within sulfide minerals. In this situation, metamorphic reconstitution of the rock has relatively little impact on the overall nickel tenor of the sulfide fraction. However, metamorphism may still have important consequences on mineral grain size and the nature of silicate-sulfide intergrowths, a common consequence being the development of bladed or triangular-textured intergrowths of sulfide with metamorphic olivine. Associated tectonism may result in mobilization of massive sulfide bodies into low strain zones, but there is no evidence for any significant upgrading of sulfide ores by this mechanism.

In type 2 deposits, both the sulfide and silicate components of the ore may contain substantial proportions of the total nickel budget. Partitioning of nickel between silicate and sulfide fractions is a sensitive function of the metamorphic mineral assemblage. Under favorable circumstances, metamorphism can give rise to almost complete residence of nickel in sulfide minerals. The economic viability of type 2 deposits rests on the high nickel tenor of the disseminated sulfide component of the ore.

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Contents

Reviews in Economic Geology

Metamorphic and Metamorphogenic Ore Deposits

Frank M. Vokes
Frank M. Vokes
Volume Editor
Department of Geology and Mineral Resource Engineering Norwegian University of Science and Technology Trondheim, Norway
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Brian Marshall
Brian Marshall
Volume Editor
Department of Applied Geology University of Technology Sydney NSW 2007 Australia
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Paul G. Spry
Paul G. Spry
Volume Editor
Department of Geological and Atmospheric Sciences Iowa State University Ames, Iowa 50011 USA
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Society of Economic Geologists
Volume
11
ISBN electronic:
9781629490182
Publication date:
January 01, 1998

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