Abstract

Vein-type gold mineralization in the Hill End gold field, New South Wales, is hosted by a tightly to isoclinally folded succession of Silurian to Devonian turbidites. Scattered occurrences of anomalous gold mineralization are known in veins throughout the gold field, but economic concentrations of gold are generally restricted to the northern portion of the doubly plunging Hill End anticline. The Hill End anticline is the highest amplitude fold and has the structurally highest crest of folds in the region. Most of the economic gold occurs in bedding-parallel veins on the east-dipping limb of the anticline, but smaller rich deposits have also been mined from the west-dipping limb and from adjacent anticlines. Rich ore shoots in the bedding- parallel veins pitch subhorizontally and are associated with the intersections of these veins with narrow, subhorizontal, extensional veins, or leaders. Vein textures indicate that both bedding-parallel and leader veins formed over a protracted interval, during regional deformation. Bedding-parallel veins started to form prior to folding and continued to form during regional deformation, acting as slip planes during late stages of flexural-slip folding. Leader veins formed as extensional fractures, due to transient supralithostatic fluid pressures developed in a low differential stress regime. Local zones of mineralized breccia and stockwork veining are also indicative of fluctuating fluid pressures and low differential stresses. During late stages of folding, as fold limbs attained steep dips, continued slip along them was permitted by transiently attained supralithostatic fluid pressures. Gold mineralization occurred during the middle to latest stages of deformation. Mineralization was via low- salinity, H 2 O-CO 2 -CH 4 fluids. Gold deposition occurred in the vicinity of vein intersections as a result of repeated fluid pressure fluctuations that may have allowed mixing of relatively reduced fluids, which had locally equilibrated with carbonaceous wall rocks, with more oxidized, deeply sourced, auriferous fluids.

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