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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 D Mihir and RJ Goldfarb (eds.), Gold Metallogeny India and Beyond , Alpha Science Internation Ltd, pp. 30-47.

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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
Authors/Creators:Large, RR
Keywords: Orogenic gold, pyrite, invisible gold, VAMSNAZ, black shales
Publisher: Alpha Science Internation Ltd
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Copyright © 2010 Alpha Science International Ltd.

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