Archean Karari gold deposit, Eastern Goldfields Province, Western Australia: a monzonite-associated disseminated gold deposit

摘要

The Karari gold deposit is situated in the Carosue sedimentary basin, 110km northeast of Kalgoorlie, in the Archean Eastern Goldfields Superterrane of Western Australia. The Carosue basin is a late-tectonic accumulation of volcaniclastic sedimentary rocks that unconformably overlies a deformed granite-greenstone association. The sedimentary basin is intruded by numerous plutons and dykes of monzonite, lamprophyre and syenite and is cut by a swarm of post-intrusion faults with north-south orientations. Gold mineralisation at Karari occurs in a fault-bound zone of volcaniclastic sedimentary rocks that are intruded by monzonite porphyry and lamprophyre dykes. The hangingwall of the central mineralised zone is formed by the eastern intrusive complex, a porphyritic monzonite unit intruded by numerous dykes of monzonite porphyry, syenite porphyry and lamprophyre. The eastern intrusive complex is characterised by widespread potassic alteration and contains minor low-grade copper mineralisation. In the Karari pit, gold is associated with W and As, whereas Ag, Bi, Cu, Mo, Pb, Te and Zn form spatially distinct anomalous zones in the eastern intrusive complex and associated bounding faults. The central mineralised zone is interpreted as a downfaulted, higher-level exposure of the magmatic system represented by the eastern intrusive complex. Gold lodes are steep tabular zones of sodic alteration within a more extensive area of potassic alteration. Sodic alteration zones contain numerous veins and veinlets, which contain a variety of assemblages, several of which are mutually overprinting. Hematite occurs as a dusting in fine-grained albite and carbonate in the sodic alteration zones but is interpreted as a later (post-gold) event. Modelling using Hch software suggests that potassic alteration and low-grade copper mineralisation were caused by a high-temperature, saline fluid, probably derived from magmas of the eastern intrusive complex. The sodic alteration assemblage at Karari could not be duplicated but the results of other workers show that sodic alteration could have formed by reaction of quartzo-feldspathic rocks with a mesothermal, low-salinity H2O-CO2 fluid. The data and observations described in this paper do not permit an unequivocal distinction between orogenic and orthomagmatic models for the gold mineralisation.