Open File Report 134
Geochemical dispersion around the Maronan Cu-Au Prospect, N.E.
I.D.M. Robertson, Li Shu and J. E. Wildman
The area around Maronan homestead is on the margin of the Eromanga
Basin where the Proterozoic metamorphic rocks of the Mt Isa Inlier
have been partly covered with Mesozoic, Tertiary and Quaternary
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.
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 at the headwaters 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 mineralisation
in the Proterozoic basement.
The ancestral Fullarton River later deposited 5 to 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 fIuvial sediments were slightly
ferruginised and mottled and brown soil was developed on them.
There were no indications of mechanical dispersion of mineralised
material from the Maronan mineralisation into the alluvium, the
only geochemical indications were restricted to the saprolite in
the single drillhole investigated. Consequently there were no geochemical
indications in the soil over the prospect.
Thick Tertiary alluvial cover at Eloise presents an effective barrier
to geochemical exploration. Dispersions within the saprolite would
seem to be the best geochemical target, these are likely to be limited.