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Book Chapter

Regional Metamorphism and Ore Formation: Evidence from Stable Isotopes and other Fluid Tracers

By
Christoph A. Heinrich
Christoph A. Heinrich
Institut für Isotopengeologie und Mineralische Rohstoffe, Departement Erdwissenschaften, ETH Zentrum - NO, 8092 Zürich, Switzerland
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Anita S. Andrew
Anita S. Andrew
CSIRO Division of Petroleum Resources, PO Box 136 North Ryde 2113, Australia
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Matthias D. Knill
Matthias D. Knill
Institut für Isotopengeologie und Mineralische Rohstoffe, Departement Erdwissenschaften, ETH Zentrum - NO, 8092 Zürich, Switzerland
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Published:
January 01, 1998

Abstract

How can stable isotopes and other fluid tracers be used to interpret the formation of ore deposits hosted by regional metamorphic terranes? The central question of metamorphogenic ore formation versus a regional metamorphic overprint must consider two basic constraints, as imposed by mass-balance and ore-metal solubility. First, regional metamorphism of an existing orebody generally involves rock-dominated fluid-rock interactions that rarely permit significant modification of preexisting elemental or isotopic compositions at a scale greater than that of an orebody. Second, the generation of a significant ore deposit during regional metamorphism requires, just as in any ore-forming hydrothermal system, a large amount of fluid from a km3-sized and generally rock-dominated source to be focused through a small and generally fluid-dominated depositional site.

Well-documented case studies are reviewed to illustrate these general principles. The small metamorphosed Pb-Zn-As deposit of Lengenbach (Alps) and the large metamorphogenic Cu deposit of Mount Isa (Queensland) illustrate why deposits hosted by metacarbonate rocks often yield unambiguous stable isotope evidence for a relationship between metamorphism and metal accumulation. In metamorphosed base metal deposits and metamorphogenic gold deposits hosted by common silicate metamorphic rocks (taking the base metal deposits at Ducktown, Tennessee and Cobar, NSW, and the lode-gold deposits at Bendigo-Ballarat, Victoria, as examples), isotopic and other fluid tracers alone do not yield conclusive evidence for any necessary relationship between hydrothermal metal enrichment and regional metamorphism. Structural and geological observations at a scale greater than that of an orebody are essential to interpret the isotopic data and the origin of such deposits. Metamorphogenic base metal mineralization is inherently rare because it requires rather atypical conditions of regional metamorphism, including a large supply of salt for metal complexing. The process is, however, difficult to identify isotopically, and may therefore be more common than reported. Mesothermal gold deposits are the only well-established group of major deposits formed by upward-focused flow of fluids with at least a component originating from prograde metamorphic devolatilization. Their common occurrence can be related to liberation of H2S, which forms extremely stable gold complexes, by prograde metamorphic reactions. Isotopic tracer data do not alone preclude an important precious metal addition from magmas or other deep sources, even in these clearly metamorphogenic deposits.

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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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