Structural controls on the distribution of gold-bearing quartz veins in the Arltunga gold field, Northern Territory, Australia

Abstract

The Arltunga gold field in central Australia is a Carboniferous epigenetic vein-type gold deposit which is hosted by quartzite, quartzofeldspathic gneiss, calc-silicate, and mica schist. It is situated within a large ductile shear zone that forms the floor thrust for a series of nappes: the Arltunga Nappe Complex. Gold and other minerals were deposited within quartz veins by fluids that were channeled along the shear zone during the waning stages of greenschist facies metamorphism. A geometric and genetic relationship existed between the evolving structure of the cooling shear zone and progressively emplaced quartz veins with associated gold and sulfide mineralization.Southward thrusting was inhibited late in the history of the shear zone and subsequent strain was partitioned along north-verging kink zones. The kinked anisotropy occurred at high angles to the kink boundary, within the shortening field of the regionally distributed strain pattern. This resulted in the development of simple kink geometries associated with minor buckling, which rotated the laminated rock along the kink zones into the extensional field. The accompanying dilatancy caused the deposition of extensional gold-barren quartz veins along the kink zones that can be divided into two types: en echelon arrays of dilational veins that formed as tension gashes in relatively incompetent mica schists, and tensional veins parallel to the kink zone boundaries. With progressive deformation and waning metamorphic temperatures, strain was further localized within the kink zones as an anastomosing system of narrow, steep south-dipping reverse faults. These faults reflect the transition from ductile to brittle structures and mainly occur in the more competent rock types such as the quartzite, the mica-poor quartzofeldspathic gneiss, and the extensional quartz veins. Where the fracture system transects these lithologies, extensive brecciation caused dilatant zones that host deposits of gold together with quartz, pyrite, chalcopyrite, calcite, and minor silver. Many of the earlier extensional veins that contained vugs filled in with comb quartz became mineralized.The preferred occurrence of gold in the monomineralic quartzites, and the conspicuous lack of wall-rock alteration in all other lithologies, suggests only limited control of the wall rock on gold deposition. General retrograde metamorphic conditions, in particular the fall in ambient temperatures, probably forced the gold out of solution in the same temperature range in which brittle structures were initiated. As a result of the irregular distribution of the anastomosing fault pattern and competent units within the preexisting ductile shear, gold mineralization is highly irregular and discontinuous, making exploration of the Arltunga gold field difficult.