Brugger, and 4 co-authors, reported in 2006 that 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 (410 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 authors offered in closing that the association of base metals with REE and Fe oxides at Paratoo is reminiscent of that found in the giant Olympic Dam Cu-U-Au-REE deposit, with the notable exception of uranium. However, the age of the mineralization (early Mesoproterozoic at Olympic Dam, but Palaeozoic at Paratoo) and the geological context (Mesoproterozoic granite of the Gawler Craton for Olympic Dam; Neoproterozoic sediments of the Adelaide Geosyncline for Paratoo) differ greatly. A similar association of Fe oxides, U, REE and minor Cu is found in the Palaeozoic magmatic and epithermal deposits in the Mt Painter Inlier. The inlier is also well known for Mesoproterozoic granites enriched in incompatible elements such as U, Th and REE. The Paratoo deposit can, therefore, be considered as a further example of a REE-B-Cu-B-U system in the province that was active from the early Proterozoic and into the Palaeozoic.
The wide distribution through time of the otherwise unusual REE-base metal association suggests that the occurrence of this elemental association in hydrothermal deposits of the province is not related to a particular tectonic or magmatic event, but rather reflects the unusual geochemistry of the Mesoproterozoic basement and of its REE-U rich granites. The mechanisms driving hydrothermal circulation appear to vary through time. Magmatic heat was probably important in the Mesoproterozoic (e.g. Olympic Dam) and possibly in the Palaeozoic, whereas basinal fluid flow linked with the tectonic evolution of the Adelaide Geosyncline occurred during the Neoproterozoic.
The Paratoo deposit is deeply weathered, with malachite and chrysocolla (B 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.
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