Gold deposits in highly metamorphosed terranes.

About 32% of global Au production has come from greenstone belts where numerous small to large, quartz vein-associated deposits are hosted in metamorphic rocks ranging from lower greenschist to granulite facies. Historical exploration has focused on rocks of greenschist to lower amphibolite facies as these seem to contain the most deposits, and these regions are now reaching mature brownfield status, particularly in developed nations, and finding new deposits is becoming progressively more difficult. Regions of similar tectonic setting but containing high-grade metamorphic rocks of mid-amphibolite to granulite facies have received little attention because the first explorers found fewer deposits there. These regions do however contain significant Au deposits such as Hemlo in Canada with about 684 t of Au, Renco in Zimbabwe where about 31 t of Au has been mined since 1980, the Challenger deposit in South Australia with 36 t of Au recovered + reserve + resource and the Tropicana deposit in Western Australia with about 182 t of Au reserve + resource. High-grade metamorphic terrains offer explorers an excellent opportunity to move onto relatively green fields within developed nations, which carry less risk associated with development. Based on consideration of the characteristics of Au deposits hosted in high-grade metamorphic rocks, two categories are defined, deposits which formed prior to peak metamorphism and deposits that formed well after peak metamorphism. No deposits have been convincingly been shown to have formed during peak metamorphism. Exploration should focus on old structures that have been metamorphosed and probably deformed by folding or reactivated. In these settings, Au occurs in unusual metamorphic rocks characterised by low-Fe silicate assemblages. Metamorphosed deposits are likely to be highly deformed, with complex orebody geometries. Where granulite facies terranes have been thrust over rocks over lower metamorphic grade, there is potential for f
About 32% of global Au production has come from greenstone belts where numerous small to large, quartz vein-associated deposits are hosted in metamorphic rocks ranging from lower greenschist to granulite facies. Historical exploration has focused on rocks of greenschist to lower amphibolite facies as these seem to contain the most deposits, and these regions are now reaching mature brownfield status, particularly in developed nations, and finding new deposits is becoming progressively more difficult. Regions of similar tectonic setting but containing high-grade metamorphic rocks of mid-amphibolite to granulite facies have received little attention because the first explorers found fewer deposits there. These regions do however contain significant Au deposits such as Hemlo in Canada with about 684 t of Au, Renco in Zimbabwe where about 31 t of Au has been mined since 1980, the Challenger deposit in South Australia with 36 t of Au recovered + reserve + resource and the Tropicana deposit in Western Australia with about 182 t of Au reserve + resource. High-grade metamorphic terrains offer explorers an excellent opportunity to move onto relatively green fields within developed nations, which carry less risk associated with development. Based on consideration of the characteristics of Au deposits hosted in high-grade metamorphic rocks, two categories are defined, deposits which formed prior to peak metamorphism and deposits that formed well after peak metamorphism. No deposits have been convincingly been shown to have formed during peak metamorphism. Exploration should focus on old structures that have been metamorphosed and probably deformed by folding or reactivated. In these settings, Au occurs in unusual metamorphic rocks characterised by low-Fe silicate assemblages. Metamorphosed deposits are likely to be highly deformed, with complex orebody geometries. Where granulite facies terranes have been thrust over rocks over lower metamorphic grade, there is potential for f