Orogenic gold in the Tibooburra area of northwestern NSW- a ~440 Ma ore system with comparison to the Victorian Goldfields

Abstract

In the Tibooburra area, within the northeastern Koonenberry Belt, geological mapping has delineated a sequence of shallow marine sediments, interbedded with distal airfall tuffs that have been dated at 503-496 Ma. These rocks were strongly deformed at approximately 441-424 Ma, producing tight east verging folds and reverse faults (D1), and a penetrative west-dipping cleavage (S1). The sequence was intruded by post-D1 monzodioritic sills and dykes (~423-416 Ma) and the geochemically-related I-type Tibooburra Granodiorite (412±10 Ma). Gold mineralization in the Tibooburra Goldfields is associated with syn-D1 replacement quartz veins which were injected parallel to S1 cleavage and formed long narrow stringer zones. Auriferous fluids were probably oxidized, low-sulphide, CO2-bearing fluids produced during peak M1 metamorphism. Two types of gold- associated alteration are present: narrow phengite- chlorite-pyrite-carbonate halos around quartz veins, and carbonate-sericite ?bleached? zones extending kilometres along strike. The style and timing of gold mineralization and the structural history of the Tibooburra Goldfields has strong similarities to the Victorian Goldfields within the Western Lachlan Orogen and Delamerian Orogen of Victoria. However, there are significant differences in the interpreted tectonic setting between the two areas which have implications for the development of the Gondwana margin and mineral exploration.