Craton-scale distribution of Archean greenstone gold deposits; predictive capacity of the metamorphic model

Abstract

Genetic models should not only attempt to explain features of Archean gold mineralization on the scale of mines or districts but should also explain its marked spatial and temporal heterogeneity in greenstone belts. The application of genetic models developed with such a philosophy can have predictive capacity in gold exploration.The metamorphic model, one of a number of current genetic models for Archean gold deposits, involves generation of auriferous H 2 O-CO 2 fluids by devolatilization of the stratigraphically lowermost greenstones during metamorphism and broadly synchronous synkinematic granitoid intrusion. Crustal-scale fault-shear systems are required to induce channeled fluid flow and to focus ore fluids toward suitable host rocks and structural sites. Declining temperature below the amphibolite-greenschist transition, combined with sulfidation of Fe-rich host rocks, is a particularly efficient Au-depositing mechanism. The model suggests that the major controlling factors (in declining order of importance) are: (1) high temperature-low pressure, generally low-strain, metamorphic regimes, (2) crustal-scale fault systems, (3) unaltered Fe-rich rocks (tholeiites, banded iron-formation) at a suitable stratigraphic-metamorphic level, and (4) abundant Au-enriched komatiites and S-rich sediments in source regions.The low-pressure metamorphic conditions combined with the occurrence of elongate high strain zones in most, if not all, generally low strain greenstone belts may explain the ubiquitous occurrence of gold deposits in Archean terranes. All four favorable parameters appear to act in concert to produce the largest gold deposits and most abundant gold mineralization in 2.7 + or - 0.1-b.y.-old rift-phase greenstone belts, interpreted to have formed under conditions of high crustal extension and extensive crustal thinning (i.e., with predicted high geothermal gradients, high fault density, limited synvolcanic alteration, and abundant komatiites and sulfidic sediment horizons). More widespread, commonly older, platform-phase greenstone belts, which formed under conditions of lesser crustal extension and thinning, are less mineralized, as predicted by the metamorphic model.The ability of the model to explain the regional distribution of known Archean gold deposits lends credence to its predictive capacity. For example: (1) greenstone belts in any one craton can be broadly ranked in order of likely relative gold potential, (2) the likelihood of discovery of large deposits in poorly exposed or poorly explored belts of known type can be estimated, (3) areas with the highest potential can be defined in rift-phase greenstone belts by the coincidence of critical parameters such as specific host-rock sequences and metamorphic facies, (4) prospective structural sites for gold mineralization can be postulated in platform-phase greenstone belts affected by diapiric tectonics, and (5) the possible spacing of large deposits can be estimated. Further, the potential use of the model in exploration emphasizes serious gaps in our knowledge of parameters crucial to understanding greenstone belt metallogeny, including alteration patterns, metamorphic P-T paths, and regional-scale structural regimes.