Open File Report 125
Regolith-landscape characteristics, evolution and regional synthesis
of the Mt Isa Region, Progress Report
R.R. Anand, C. Phang, J. Wilford, J.E. Wildman, Li Shu, I.D.M.
Robertson and T.J. Munday
The Little Eva Cu Prospect is located 12 km north of the Dugald
River Zn-Pb orebody and is situated in scapolitic granofelses of
the Corella Formation and is associated with feldspar porphyry and
magnetite-rich rocks. Although, in the vicinity of the Little Eva
shaft, these rocks are exposed or occur under a very thin soil,
the prospective rocks to the south are masked by colluvium. This
prospect had been investigated intensively by CRA Exploration Pty
Ltd., (CRAE), using the geochemistry of samples drilled from bedrock.
The geomorphology and regolith units were investigated on a district
and on a local scale. This was to provide the setting for an orientation
survey to test the effectiveness of soil sampling in areas where
there is a thin layer of transported cover. South of the Little
Eva Cu Prospect, detritus, from low quartzite hills and quartz veins,
has been shed onto pediments gently inclined, towards Cabbage Tree
Creek, to form a thin colluvial mantle of acid red earths with a
quartz- and quartzite-rich lag. Near Cabbage Tree Creek, erosion
has been active, etching into and through the colluvium, exposing
the basement. This is largely covered by a thin, carbonate-rich
lithosol, characterised by a lag rich in quartz and magnetite clasts.
The magnetite is primary, although it has been partly weathered
at the surface or near-surface. To the north, the area is dominated
by alluvium and colluvium on which black clay soils are developed.
Part of the colluvium, south of the Prospect, has been almost completely
dismantled, leaving an area of linear 'gilgai', occupied by dark
brown, smectitic, cracking clay soils.
Soil sampling on an approximately triangular 200 m grid, shows
that Cu and Au are the only indicator elements for Little Eva style
mineralisation. Muted anomalies in both elements indicate the trend
of the bedrock Cu anomaly, as determined by CRAE, even through the
thin colluvium. The fine fraction (<75 µm) is more effective
than the coarse (710-2000 µm) fraction. Iron, Co and V in
the soil indicate concentrations of magnetite which may have some
Bioturbation by ants and termites and/or hydromorphic dispersion
could have contributed to moving the Cu and Au geochemical signal
in the fine fraction upward from the basement and through the colluvium.
However, the low tenor of the anomalies requires that careful attention
be paid to setting the correct thresholds and the use of appropriate
display methods (e.g., logarithmic scales).