VOLATILE RELEASE FROM GRANITES CRYSTALLISED AT DEPTH IN THE CRUST: EVIDENCE FOR LINKS WITH GOLD-ONLY HYDROTHERMAL ORES

Granitoids that crystallise in the middle or lower crust are one widely proposed model fluid source for the ‘orogenic-gold’ or ‘gold-only’ hydrothermal ores that are abundant in many low-grade metamorphic belts, particularly Archean greenstone belts. We have tested this model for the ore fluid source through examination of evidence for the abundance, flow pathways and the chemistry of volatiles released from granites that crystallised at suitable depth and are of suitable age to be a source of gold ore fluids. The area of study was in the Archean Yilgarn Craton of Western Australia. A common style of pathway of magmatic volatile release in the most widespread suite of evolved granodiorites and granites of suitable age in the craton appears as sub-vertical, quartz-dominated, infilled miarolitic pipes. The fluid inclusion record of quartz in these pipes shows primary fluids with densities consistent with entrapment at the granite solidus at the depth of granite crystallisation. Chemically, these are low- to moderate-salinity, mixed aqueous-carbonic fluids, and hence similar to ore fluids of gold-only hydrothermal systems. Nuclear microprobe (PIXE and PIGE) multi-element microanalyses show that the fluids have many chemical similarities with known compositions of gold ore fluids, for instance, the common sequence of relative abundance of the major cations, Na >> K ≈ Ca >> Fe ≈ Mg. The chemical characteristics of the magmatic volatiles indicate that the gold-only ore fluids could be derived from granitoids that crystallise at mid-crustal depths.