Geology of Winu-Ngapakarra, Great Sandy Desert of Western Australia, a Recently Discovered Intrusion-Related Cu-Au Deposit

Abstract

The Winu-Ngapakarra Cu-Au deposit (Winu) was discovered in late 2017. Winu is hosted in metamorphosed massive sandstones, siltstones, and lesser mafic rocks that are possible distal and deep-water correlatives of the Malu Formation of the Yeneena basin, also host to the large Telfer Au-Cu deposit. Structure at Winu is dominated by an inclined dome formed through interference between NNW- and WNW-trending folds. Copper-gold mineralization occurs in en echelon left-stepping lodes with strike lengths between 350 and 750 m and vertical depths exceeding 750 m, northerly trends, and moderate easterly dips. These higher-grade lodes are surrounded by an extensive halo of low-grade mineralization. Two gold-rich lodes in the southeast part of the Winu deposit strike roughly easterly, toward a gold-rich satellite deposit (Ngapakarra) approximately 2 km east of Winu. Mineralization at Winu is hosted by numerous thin, brittle veins and breccias. At least four distinct vein sets associated with Cu-Au mineralization are recognized: V1 – early, weakly mineralized K-feldspar-white mica stockworks; V2 – transitional magmatic-hydrothermal K-feldspar–rich veins; V3 – quartz-sulfide–rich veins, breccias, and fractures and quartz-bismuth-gold veins with similar timing; and V4 – poorly mineralized quartz veins. Barren carbonate-, chlorite-, white mica-, and pyrite-rich fractures (V5), breccias, and faults as well as thin dolerite dikes cut mineralization at Winu. Re-Os dating of molybdenite intergrown with chalcopyrite yielded ages of 658 to 655 Ma for the mineralized veins at Winu, and Ar-Ar dating of biotite yielded 619 Ma for an unmineralized post-ore quartz-biotite vein. A well-developed supergene chalcocite blanket overlies the primary mineralization. Element associations and vein and alteration textures and mineralogy classify Winu as an intrusion-related Cu-Au deposit of Neoproterozoic age. Although it shares features with the reduced porphyry copper-gold group, other features, such as the dominance of pyrite over pyrrhotite and the presence of trace amounts of sulfate minerals in the mineralization, are more in accordance with oxidized intrusion-related systems. Mineralization at Winu was most likely triggered by rapidly changing stress fields during the Paterson orogeny and cooling of multiple fluid pulses above an intruding granite pluton that progressively decreased in temperature over time. Winu is unique in this region in that it preserves evidence for an early, reduced intrusion-related gold system overprinted by a more oxidized intrusion-related copper system.