A synmetamorphic lateral fluid flow model for gold mineralization in the Archean southern Kalgoorlie and Norseman terranes, Western Australia

Gold deposits are widely distributed in the Archean southern Kalgoorlie and Norseman greenstone terranes, Eastern Goldfields superterrane, Western Australia, which have collectively produced more than 1,850 metric tolls (t) of gold. Gold deposits occur in a variety of host rocks and structural settings, but all are associated with quartz veins and/or shear zones that formed during the latest stages of regional compression. Available data indicate a similar H2O-CO2(-CH4) ore fluid at each deposit. Alteration assemblages vary with metamorphic grade in a manner which indicates that late tectonic regional metamorphic gradients were still in place when gold was deposited from hydrothermal fluids. These late tectonic thermal gradients were related to differential uplift of deep hot crust, at the margin of the greenstone belt, accompanied by intrusion of voluminous monzogranite magma. At this time, the western margin of the greenstone belt was a zone of high heat flow, as were intrusive centers in the northern part of the southern Kalgoorlie terrane. These spatial and temporal relationships, in combination with the apparent absence of a viable local source for the ore fluid at most deposits, suggest that gold was deposited from a small number of large, synmetamorphic hydrothermal systems. Published thermal models predict up-temperature, lateral fluid flow toward zones of high heat flow, under these circumstances. A model involving up-temperature, lateral flow of metamorphic fluids overcomes some of the problems associated with deeply sourced and conventional metamorphic models for the ore fluids and is consisent with most of the features of gold deposits in the southern Kalgoorlie and Norseman terranes. Potassium metasomatism and radiogenic isotope signatures associated with gold mineralization are ascribed to a metamorphic fluid that was modified by exchange with biotite-bearing felsic rocks (granitoids, felsic volcanic and volcaniclastic rocks) in the course of lateral flow toward zones of high heat flow.