Origin of the secondary REE-minerals at the Paratoo copper deposit near Yunta, South Australia

Abstract

The Paratoo copper deposit, located in the Neoproterozoic to Cambrian Adelaide Geosyncline, South Australia, produced around 360 tons of Cu between 1888 and 1967 from oxidized ores. The deposit is located in the core of a breached, doubly plunging anticline, near a zone of disruption containing brecciated Adelaidean sedimentary rocks and dolerite ('Paratoo Diapir'), and hosted in dolomitic shales of the Neoproterozoic Burra Formation. Near the surface, the mineralization resides mainly in deeply weathered quartz-magnetite-sulphide (pyrite, chalcopyrite) veins (≥ 10 cm wide). At depth, drill cores reveal disseminated magnetite, pyrite, chalcopyrite, copper sulphide and native copper associated with extensive potassic alteration. K-Na-rich fluids also affected the dolerite in the 'Paratoo diapir', resulting in the precipitation of K-feldspar, dravite and K-bearing chabazite-Na. The most likely scenario for the genesis of the Paratoo deposit involves circulation of basinal fluids, focusing into the 'Paratoo Diapir', and ore precipitation through neutralization by fluid-rock interaction with the dolomitic shales hosting the mineralization. The Paratoo deposit is deeply weathered, with malachite and chrysocolla (± tenorite and cuprite) containing the bulk of the copper recovered from the shallow workings. A diverse assemblage of secondary REE-bearing carbonate minerals, including the new species decrespignyite-(Y) and paratooite-(La), is associated with the weathered base metal and magnetite ores. Whole-rock geochemical analyses of fresh and mineralized host rock and of vein material reveals that the mineralization is associated with a strong, albeit highly variable, enrichment in light rare earth elements (LREE). This association indicates that REE and base metals were introduced by the same hydrothermal fluid. The strong negative Ce anomaly found in secondary REE minerals and mineralized rock samples suggests an upgrade of the REE contents in the weathering zone, insoluble Ce4+ being left behind. The Fe-oxide-REE-base metal association at Paratoo is also characteristic of the giant Mesoproterozoic Fe oxide copper gold deposit of Olympic Dam, located 350 km to the NW. A similar association is found in the Palaeozoic deposits of the Mt Painter Inlier, 300 km to the NNE. The widespread occurrence of this elemental association in the Province probably reflects the geochemistry of the basement, which contains numerous Mesoproterozoic granites enriched in REE and U.