Primary diagenetic copper carbonate at the Malbunka copper deposit, Amadeus Basin, Northern Territory, Australia

Abstract The Malbunka copper deposit, located about 220 km west of Alice Springs, in the Northern Territory of Australia, may be a rare example of primary formation of copper carbonate mineralization. This deposit consists of unusual azurite disks up to 25 cm diameter, and lesser amounts of secondary azurite crystals and malachite. Carbon isotope values of the copper carbonate minerals are consistent with formation from groundwater-dissolved inorganic carbon. Oxygen isotope thermometry formation temperature estimates are 5–16 °C above ambient temperatures, suggesting the copper carbonates formed at a depth between 0.3 and 1.6 km in the Amadeus Basin. Azurite fluid inclusion waters are rich in boron, chlorine, and other elements suggestive of dilute oil basin formation fluids. In addition, presence of euhedral tourmaline with strong chemical zonation suggest that this was a low temperature diagenetic setting. The strong correlation of structures associated with hydraulic fracturing and rich copper carbonate mineralization suggest a strongly compartmentalized overpressure environment. It is proposed that copper carbonates of the Malbunka deposit formed when deep, copper-rich formation fluids were released upward by overpressure-induced failure of basin sediments, permitting mixing with carbonate-rich fluids above. This work bears directly upon exploration for a new type of primary copper deposit, through understanding of the conditions of genesis.