Stable isotope and geochemical evidence for genesis of secondary copper deposits at Girilambone, New South Wales, Australia

Abstract

AbstractThe Girilambone copper deposit of New South Wales, Australia, serves as an end-member model for development of a classic oxidation-zone profile in an environment virtually free of both the effects of active tectonics and significant fluctuation in the local water table. The oxidation zone of other copper deposits may be interpreted for history of recently active tectonics, palaeoclimate, and water-table stability by comparison to the Girilambone deposit. Unlike the oxidation profiles of porphyry copper deposits of western North America, which have been overprinted by many water-table fluctuations produced by active tectonics, the Girilambone deposit appears to have little modification to the original oxidation profile. Oxidation of primary sulfides at Girilambone was an exothermic process facilitated by chemolithotrophic bacteria, recorded by malachite oxygen isotope thermometry estimates of up to 52°C, and very light malachite carbon isotope values. The bacteria generated CO2 which migrated upwards to react with copper rich meteoric fluids of the vadose zone to precipitate malachite. Unlike porphyry copper deposits of western North America which experienced recent tectonic activity, the secondary minerals (clays, iron oxides and copper carbonates) at Girilambone were not repeatedly fractured and offset during oxidation to re-establish permeability. This reduced permeability of the oxidation zone and slowed the release of CO2 from the system, producing significantly elevated partial pressure of CO2, sufficient for azurite formation to dominate. Azurite oxygen and carbon isotope values indicate formation at lower temperatures up to 32°C, and with less bacterial activity than for malachite. The sulfide-digesting bacteria at Girilambone were relatively free of seasonal swings in population, as the deposit does not have the interbanded azurite and malachite typical of episodic bacterial populations triggered by seasonal precipitation. Thus the absence of significant Cu-carbonate banding at Girilambone serves as a palaeoclimate indicator. Deeper in the Girilambone oxidation zone, native copper and cuprite dominate, whereas chalcocite formed an enrichment blanket just above and at the modern water table. Oxygen and carbon isotope values for pseudomorphs of malachite after azurite indicate that these were generated as a retrograde reaction when CO2 production from bacterial digestion of sulfides waned and temperatures in the oxidation zone were near ambient. In the post mining environment, chloride-rich groundwater seeps actively precipitate atacamite, while exposed remnants of sulfide masses form an outer rind of porous malachite. Exceptions to this oxidation zone sequence occur due to localised fluid channelisation and perched water-table lenses that generated mineralogical overprints.