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Evidence for a Two-stage Process in the Genesis of Sediment-hosted Gold-Arsenic Deposits


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Large, RR (2010) Evidence for a Two-stage Process in the Genesis of Sediment-hosted Gold-Arsenic Deposits. In: Gold Metallogeny India and Beyond. Alpha Science Internation Ltd, pp. 30-47. ISBN 978-1-84265-646-4

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Currently accepted ore genesis models for orogenic and Carlin style gold deposits consider that the concentration of gold is a one stage process. Gold is commonly assumed to be sourced from deep below the deposits, in the lower crust or mantle, and transported upwards, along major crustal fractures or shear zones, during metamorphism and deformation, to become concentrated in second order structures during the peak, or post-peak, of metamorphism. However, recent evidence, based on the textural and chemical association of both gold and arsenic in pyrite in certain sedimentary rock facies, supports a two-stage process of gold ore genesis for some gold provinces. The first stage involves pre-concentration of gold and arsenic in black mudstone facies and/or banded iron formation facies of continental margin sedimentary basins. In the sediments, the gold is present as either, invisible gold trapped in diagenetic arsenian pyrite or, micro-nuggets (< 5 microns) of free gold associated with fine-grained clays and organic matter. These gold-bearing sediments are also enriched in a characteristic suite of elements, particularly V, As, Mo, Se (Te), Ni, Ag and Zn, giving rise to the term VAMSNAZ sediments. Gold contents of the VAMSNAZ sediments commonly range from 5 to 100 ppb, arsenic from 10 to 100 ppm, and organic carbon from 0.2 to 2 wt %. The second stage, of the two-stage process, occurs during late diagenesis and metamorphism of the sediments, associated with deformation, basin inversion and/or granite intrusion. Gold and arsenic are released from the arsenian pyrite and from clay-organic matter intergrowths, associated with diagenetic and low grade metamorphic reactions in the VAMSNAZ sediments. Conversion of gold-bearing diagenetic arsenian pyrite to pyrrhotite at higher grades of metamorphism (lower greenschist to upper greenschist facies) is a particularly effective way of releasing gold to the metamorphic fluid. The H2S-CO2-CH4-bearing metamorphic fluid (probably a mixture of seawater, meteoric water and diagenetic fluid trapped in the sediments), transports the gold and arsenic, through the permeable silt and sand facies sedimentary rocks, to become concentrated in zones of intense fluid flow and related pressure release, such as anticlinal fold axes, shear zones or dilatant quartz-vein reefs or stock-work zones. This two-stage model is the first to account for the ubiquitous association of gold and arsenic in sediment-hosted orogenic and Carlin style deposits. Keywords: Orogenic gold, pyrite, invisible gold, VAMSNAZ, black shales

Item Type: Book Section
Keywords: Orogenic gold, pyrite, invisible gold, VAMSNAZ, black shales
Publisher: Alpha Science Internation Ltd
Page Range: pp. 30-47
Additional Information: Copyright © 2010 Alpha Science International Ltd.
Date Deposited: 11 Aug 2011 05:12
Last Modified: 18 Nov 2014 04:21
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