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Fluid Inclusions in Metamorphosed and Synmetamorphic (including Metamorphogenic) Base and Precious Metal Deposits: Indicators of Ore-Forming Conditions and/or Ore-Modifying Histories?

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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Alan D. Giles
Alan D. Giles
Department of Applied Geology, University of Technology-Sydney, PO Box 123 Broadway, New South Wales 2007, Australia
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Steffen G. Hagemann
Steffen G. Hagemann
Department of Geology and Geophysics, Centre for Strategic Mineral Deposits, University of Western Australia, Nedlands, Western Australia 6907, Australia
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Published:
January 01, 1998

Abstract

Fluid inclusion studies have been applied to genetic investigations of metamorphosed, synmetamorphic (including metamorphogenic), and postmetamorphic ore deposits. They aim to identify the initial ore-forming conditions, no matter whether these precede, accompany, or follow peak metamorphism; the ore modifying conditions, no matter whether these accompany or follow the metamorphic peak; and the nature of synmetamorphic and postmetamorphic fluid-phase evolution. This implies that premetamorphic inclusions in metamorphosed ore are able to survive metamorphism and deformation, and that synmetamorphic inclusions preserve remobilizing fluids and/or were associated with the synmetamorphic concentration of metals. We focus on syngenetic base and precious metal deposits, metamorphogenic base and precious metal deposits, and synmetamorphic high P-T precious and base metal deposits.

The methodology for such studies mainly involves establishing parageneses for structural fabrics, and for metamorphic-alteration and vein-forming events; using fluid inclusion petrography to identify and refine the chronology of inclusion populations in host minerals from different parts of the parageneses; summarizing the integrated data in a table of mineral growth and inclusion chronology versus deformational and metamorphic events; and detailing the P-T-X-t evolution of fluid phases by microthermometric and laser raman work.

Four temperature-dominated fluid inclusion regimes are recognized. Other than in the lowest temperature regime, metamorphism and deformation destroy and reset pretectonic fluid inclusion populations. Thus, with few exceptions, fluid inclusions in metamorphosed deposits do not represent, and cannot provide, fluid phase data on pretectonic ore formation. Instead, at best, the populations of predominantly secondary synmetamorphic inclusions represent peak or near-peak metamorphic conditions; at worst, they experience ductile (crystal-plastic, dissolution-precipitation and diffusive) to brittle (decrepitation and/or leakage) reequilibrative processes that, particularly for brittle behavior, result in peak-near peak inclusions becoming repeatedly reset as evolving populations of secondary inclusions. In addition, late-tectonic extension may induce overprinting populations of postmetamorphic liquid + vapor low-salinity secondary inclusions.

Regardless of whether inclusions preserve peak-near peak fluids and conditions that accompany remobilization of pretectonic ore, or synmetamorphic emplacement of ore, they are most unlikely to discriminate between these models. This even more emphatically applies to the evolving and overprinting inclusion populations that variably reflect the fluid conditions of retrogressive reequilibration and postmetamorphic tectonism. Where supposedly early-formed inclusions yield data on the conditions of remobilization or synmetamorphic emplacement of ore, the possibility of postentrapment modification must be critically evaluated using independent constraints on the P-T-X conditions of mineralization; selection of the genetic model must similarly be constrained by other data. Late- to postmetamorphic inclusions yield data that are clearly unrelated to peak metamorphic ore modification-formation in medium- and high-grade metamorphic environments, but potential exists, in low-grade environments, for misinterpretation of mature, well-healed members of these secondary populations as peak metamorphic or premetamorphic primary inclusions. Such misuse can inevitably lead to claims that the inclusions support a favored genetic model.

Case studies illustrating much of the above are drawn from the low-temperature and intermediate- to high-temperature regimes. Regionally metamorphosed and metamorphogenic deposits from the low-temperature regime are used to particularly show the destructive capacity of brittle reequilibration and the potential misinterpretation of late-stage inclusions. Synmetamorphic gold deposits from the intermediate- to high-temperature regime illustrate the apparent preservation of peak-near peak inclusions and the variable consequences of postentrapment ductile reequilibration.

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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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