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Book Chapter

Discriminating between Regional Metamorphic Remobilization and Syntectonic Emplacement in the Genesis of Massive Sulfide Ores

By
Brian Marshall
Brian Marshall
Department of Applied Geology, University of Technology–Sydney, PO Box 123 Broadway, New South Wales 2007, Australia
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Paul G. Spry
Paul G. Spry
Department of Geological and Atmospheric Sciences, Iowa State University, Ames, Iowa 50011-3212
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Published:
January 01, 1998

Abstract

Metamorphosed deposits predate metamorphism and deformation and are characteristically remobilized. Metamorphogenic deposits form through scavenging in a metamorphosing source volume, liquid-state transfer and concentration at the sink site; the scavenging fluid phase derives from the source volume. Metamorphogenesis is a subcategory of syntectonic-synmetamorphic emplacement, but the latter can be effected by externally derived nonmetamorphic fluids.

Distinguishing between remobilized and syntectonically emplaced deposits is made difficult by: overly simplistic notions of deposit models and geometry; an evolving appreciation of the kinematic interpretation of structural observations; a better understanding of the merits and limitations of many laboratory-based techniques; the potentially overlapping nature of a high degree of liquid-state remobilization and syntectonic emplacement; lack of preparedness to accept polymodal genesis; and inadequate scales of observation coupled with an insufficient range of investigatory techniques. Each of these is discussed and illustrated. The distinction controversy is intensified by the adversarial nature of research within the current research-funding climate.

Successful discrimination between remobilized and syntectonically emplaced deposits requires the integration and refinement of data from multiple lines of investigation. This principle is widely recognized, but its practice is fraught with uncertainty. As a basis for practical discrimination, fifteen types of guidelines are identified and evaluated. Some of the more significant of these involve evidence from solid-state remobilization, preserved primary textures, the gross relationships of ore and alteration to unaltered host rocks, and comparison of deformation event sequences, silicate assemblages, and Pb isotope systematics from ore, alteration, and unaltered host rocks. Because discrete guidelines are rarely definitive and should not be used in isolation, interpretation should adopt a broadly based probabilistic approach.

Application of the probabilistic approach to the Broken Hill (New South Wales) and Cannington (Queensland) deposits, the Mount Isa-Hilton system (Queensland), Dugald River (Queensland), the Zn-Pb deposits of the Mt. Read volcanic rocks (Tasmania), and the deposits of the Kanmantoo Group (South Australia), suggests that they are pretectonic emplacements, substantially modified by regional deformation, metamorphism, and variously related hydrothermal events. Nevertheless, the cases for Cannington, Dugald River and, perhaps, Rosebery and the Kanmantoo Cu deposit, are less well founded. Further resolution of the genetic history of all of these deposits, including the possibility of polymodal genesis, requires refinement of the Pb isotope models for the various regions, better constraint of the pressure, temperature, and chemistry of fluids associated with various parts of the pre-, syn-, and posttectonic evolution of the various regions, and evaluation of the capacity of the various fluids to differentially dissolve, transport, and precipitate the metals of interest.

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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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