Formation of gold deposits: Review and evaluation of the continuum model

Abstract

The continuum model has been one of the more widely advocated genetic models for ‘gold-only’ deposits such as those found in Archaean greenstone belts. It postulates that hydrothermal gold deposits were formed throughout a 20–25 km vertical crustal profile, from temperatures above 700 °C to below 180 °C, and that the deposition occurred synchronous with the peak of metamorphism. The continuum model is reviewed at this stage because we believe that it does not successfully account for many aspects of gold deposit formation. The most obvious shortcoming is in considering ore deposits found in rocks of uppermost amphibolite and granulite facies domains where temperatures were appropriate for partial melting. The main chemical condition that favours melting is access to H 2O from either aqueous fluid or the breakdown of hydrous minerals. A major gold-forming hydrothermal event at the peak of high-grade metamorphism (as implied by the continuum model) is incompatible with partial melting of wallrocks: instead of forming a hydrothermal gold deposit, an aqueous fluid introduced during partial melting would be consumed to produce further melt. Five gold deposits are documented from high metamorphic grade domains within four separate Archaean cratons; one is the type example used in the continuum model, the other four have been significant producers. Partial melting has been recorded in the wallrocks adjacent to gold mineralisation at Big Bell, Hemlo, Challenger and Renco gold deposits, in the sulphide-rich ore itself at Challenger and Hemlo deposits, and as cross-cutting dykes and migmatites at Griffins Find. For these five deposits, and indeed in general, the continuum model does not easily account for aspects of the fluid source, fluid composition, gold transport, and metal deposition. The evidence that has been used to support deposit formation at the peak of high-grade metamorphism, as required by the continuum model, is generally compatible with one or more alternative genetic models. Textural and structural siting of the gold, high-variance mineral assemblages, proportion of various gold host rocks, and relative element enrichments at high metamorphic grade can be accounted for by gold formation at greenschist facies conditions followed by progress of the deposit to high metamorphic grade (metamorphic model). Moreover the type examples in the continuum model of deposits in sub-greenschist facies domains appear to be incorrectly ascribed, and are likely to be in greenschist facies domains. The continuum model does not adequately explain Archaean gold deposits.