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.