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Open File Report 135

Surficial geology around the Eloise Cu-Au Mine and dispersion into Mesozoic cover from the Eloise mineralisation, N.E. Queensland

Li Shu and I.D.M. Robertson

The area around Eloise is on the margin of the Eromanga Basin where the Proterozoic metamorphic rocks of the Mt Isa Inlier have been partly covered with Mesozoic and Cainozoic sediments. This has presented a considerable challenge to geochemical exploration in the region. To date, exploration in the Eromanga and Carpentaria basins has been by investigation of geophysical targets by drilling. The intent of this study was to determine the geomorphic and linked sedimentary history of this region and, using this framework, to investigate a number of opportunities for using geochemistry in this difficult environment.

There has been a complex history of erosion and deposition during the Mesozoic and Tertiary. In the Late Jurassic and Early Cretaceous, fluvial and deltaic sediments of the Gilbert River Formation were deposited in broad valleys which later became mesas on the Proterozoic basement in the catchments of the Cloncurry, Bustard and Fullarton rivers to the southwest of the study area. Subsidence of the Eromanga Basin and marine transgression in the Cretaceous covered the Eloise area with mudstones and limestones 50-150 m thick, concealing the mineralisation in the Proterozoic basement.

The ancestral Fullarton River later deposited 5-8 m of Tertiary fluvial sediments on the Mesozoic. Since the Early Cretaceous, incision has created erosional terraces, plains, higher river terraces and lower river terraces. The Tertiary fluvial sediments were slightly ferruginised and mottled and brown soil was developed on them. Kaolinite of the brown soil has been partially converted to smectite, forming patches of black soil on the higher terrace, where the fine-grained sediments are water retentive. Black soil has developed on the lower river terrace, forming extensive black soil plains.

Thick Cretaceous cover at Eloise presents an effective barrier to geochemical exploration. Apart from the mineralisation, the most promising geochemical target at Eloise is the Proterozoic--Cretaceous unconformity. This consists of a thin and probably discontinuous layer of coarse sediments developed on and from erosion of the basement which might retain a mechanical or hydromorphic dispersion from the Eloise mineralisation. The palaeotopography of the unconformity was reconstructed from distant water bores and near mine drilling. Sampling of the decline and geotechnical and water bore drilling indicated no dispersion into the Mesozoic but there were indications of mechanical down-slope dispersion along the unconformity. Mechanical dispersion at the unconformity around Eloise may have extended about 100 m from the mineralisation or from mineralised faults. Drilling 3 km distant from the mine indicated some small anomalies, notably just above the unconformity. This site appears to have been located directly down the palaeo-slope from Eloise and early sediments infilling this area were, therefore, slightly anomalous.

Investigation of mechanical dispersions of Cu. Au, As and Sb in coarse sediments at the Proterozoic-Mesozoic unconformity seems to be a valid prospecting method in areas of unweathered or slightly weathered Mesozoic cover. The form of the palaeolandscape governs dispersion directions so this needs to be thoroughly understood.


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