THE FOSTERVILLE (CENTRAL VICTORIA, AUSTRALIA) AND GLOBE-PROGRESS (REEFTON, SOUTH ISLAND, NEW ZEALAND) DEPOSITS: EXAMPLES OF SHEAR ZONE-RELATED DISSEMINATED-STYLE GOLD SYSTEMS IN LOW-GRADE METAMORPHIC TERRAINS

Orogenic gold deposits in central Victoria and Reefton formed during the evolution of a Paleozoic accretionary system along the Pacific margin of Gondwana. The majority of deposits in both camps are characterised by coarse-grained gold that is hosted in laminated to massive quartz veins. These ‘lode’ structures are developed in isoclinally folded turbidites that have been metamorphosed to lower greenschist facies. At Fosterville, diffuse zones of mineralization occur along a high-angle reverse fault zone consisting of en-échelon, strike-parallel segments with oblique slip and strike slip movement. The Globe-Progress deposit is associated with an arcuate shear that flattens with depth forming a listric ramp flat. In contrast to ‘classic’ lode-style deposits, mineralization at Fosterville and Globe-Progress is predominantly hosted by massive sandstone beds, in quartz/carbonate vein stockworks, and in clay-rich fault breccias of quartz vein and sulfidic wall rock fragments. The porous sandstones, which are intercalated with carbonaceous slates, have a bleached appearance, are invariably sericitized and carbonatized, and are 'impregnated' with a disseminated arsenopyrite-pyrite(±stibnite) assemblage. Gold occurs as sub-micron inclusions within the sulfides and rarely as free grains with a diameter of 1-10 microns. Highest-grade disseminated mineralization occurs within acicular arsenopyrite crystals. Fluid inclusions from Fosterville range in composition from high salinity and relatively high CO2, to low salinity and H2O-predominance, suggesting precipitation of at least a portion of the veins under epizonal conditions.

Based on features of alteration, physico-chemical characteristics of the ore-bearing fluids and strong structural control, disseminated-style and lode gold deposits are considered end-members of a crustal continuum of orogenic gold emplacement in Phanerozoic metamorphic terrains, with disseminated mineralization more likely to develop at shallower levels and within more permeable units during the waning stages of uplift and exhumation. The resulting style of mineralization is largely controlled by the structural geometry, rheological properties, permeability and chemical receptiveness of the host rock, structural level of emplacement and pressure conditions.