Hypozonal orogenic gold mineralization: a Precambrian mineral system

A review of hypozonal orogenic gold deposits indicates that they are restricted to Precambrian terranes. The hypozonal hydrothermal system is active at 500-700°C and 2-7 kbar in syn- to post-peak metamorphic shear zones. Hypozonal orogenic gold deposits formed at an apparent geothermal gradient of 40-80°C/km, which is similar to mesozonal deposits. The PT conditions of formation define a linear trend resembling terrane exhumation, largely below the granite wet solidus and in the 1-phase field for aqueous-carbonic ore fluid compositions. Metamorphosed orogenic gold deposits in high metamorphic terranes lie outside this trend. Hypozonal orogenic gold deposits are hosted in near-vertical extensional and compressional shear zones separating greenstone belts from granite-gneiss terranes, locally around metamorphiccore complexes. Such a setting is similar to Phanerozoic examples in accretionary orogens. Some Archaean and most Proterozoic hypozonal deposits are hosted in complex shear zone systems in collisional orogens. A possible explanation for the lack of Phanerozoic hypozonal orogenic gold deposits is a different thermal regime of the Precambrian crust, where higher metamorphic grades are reached at higher crustal levels. Hypozonal orogenic gold deposits formed in the centre or foreland of Precambrian accretionary and collisional orogens during the collision stage, when the terranes are uplifted.