Open File Report 131
Regolith-landform characteristics, evolution and implications
for exploration over the Selwyn Region, Mt Isa
J. R. Wilford
A regolith-landform map and a series of thematic maps based on
fieldwork, 1:25 000 colour air photography, enhanced Landsat TM
imagery and airborne radiometries have been produced over the Selwyn
region (40 x 40 km) approximately 140 km south-southeast from Mt
Isa. The maps show the distribution of regolith and landform types,
relationships between regolith materials and Landsat TM imagery,
associations between known mineral deposits, regolith materials
and landform features such as palaeochannels and erosional scarps.
An interpretative geochemical sampling map has been generated which
can be used as an aid in the interpretation of surface geochemistry
and drill samples.
The maps show a complex history of landscape evolution. A combination
of a long weathering history and variable degrees of stripping and
exhumation has resulted in a landscape of variable regolith. Rocks
exposed at the surface reflect weathering processes which operated
from the Jurassic to the present day. Regolith consists of Duricrusts
(7% map sheet area) which may reflect both local and transported
derivations, saprolite (62% map sheet area includes bedrock) and
sediments (32% map sheet area).
Duricrusts and saprolite
Duricrusts typically form the top of deeply weathered zoned profiles
which include ferruginous mottled and bleached saprolite at depth.
These highly weathered materials are associated with exhumed landforms
and relict parts of the landscape including plateaux and mesas.
Duricrusts reflect a long history of weathering from sub-aerial
exposure in the Late Cretaceous to the present day. Some of the
most intensely weathered bedrock is associated with palaeochannels
which were probably established during the Jurassic or perhaps earlier.
Three types of ferruginous duricrust are recognised including;
massive, brecciated and slabby duricrust. In places, silcrete and
silicified saprolite develop instead of iron duricrusts - particularly
where the underlying bedrock is siliceous. Veneers of sheet wash
gravels, residual sand and clay overlying mottled saprolite are
common in areas of relatively low relief (include; rises, erosional
and pediments). Lithosols lying directly on bedrock or saprock occur
on steeper slopes. These areas of high relief and geomophic activity
include; mountains, hills and low hills.
Alluvium consisting of variable proportions of gravel, sand and
clay is associated with channel, flood plain and sheet flow deposits.
In many places. rivers are superimposed over the predominantly north-south
structure of the underlying Proterozoic rocks. Silica or iron commonly
cements alluvium to form 'creek rock' or alluvial hardpan along
river floors. Small areas of colluvium occur as coarse footslope
deposits below steeper hill slopes.
Implication for exploration
Massive and brecciated duricrusts and ferruginous saprolite have
developed largely in situ and can be sampled to detect mineralisation
at depth. Valley silcretes (some of which are now relief inverted)
and slabby iron Duricrusts are thought to be largely formed from
lateral movement of silica and iron. Nevertheless, they can be used
to give broad geochemical indicators.
Several regolith-landforms should be assessed carefully when interpreting
surface and drill hole geochemistry, these include:
- Exhumed components of the landscape which expose in places
very highly weathered kaolinised Proterozoic bedrock due to possible
pre-Cretaceous weathering. Metal concentrations in these kaolinised
zones are low relative to surrounding bedrock.
- Regolith developed on Mesozoic and Cambrian lithologies are
unlikely to directly relate to mineralisation at depth. Reworked
Proterozoic bedrock and metals precipitated from groundnaters
in the sedimentary sequence can give false anomalies.
- Deep weathering and leaching beneath palaeochannels. some of
which are now relief inverted. Metal concentration within these
leached zones are anomalously low.
- Slabby ferruginous duricrusts may give false anomalies due
to lateral movement of iron oxides. However, slabby duricrusts
can be used to give broad geochemical indicators.