Abstract

In the Abitibi belt of Ontario and Quebec, sedimentary black argillites with carbon contents of up to 5% C contain up to 0.5 ppm gold. Sheared black argillites, now graphitic schists, reflect the values of the sedimentary precursors, although their appearance is very different. Gold values of 15 ppm and more are found in graphitic schists only where alteration, shown by quartz veins and increased carbonate content, gives independent evidence of a later hydrothermal event. Gold-rich graphitic schists are characterized by veining; raised gold values closely follow vein margins. As mass balancing shows gold leaching and redeposition to be unlikely, gold enrichment is interpreted to reflect introduction of gold and its preferential deposition in carbonaceous rocks by reduction at vein margins. Natural carbons in these rocks may have been surface activated by both shearing and hydrothermal fluids.

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