CRC LEME
Open File Report 132
ABSTRACT
Regolith-landform characteristic, evolution and implications
for exploration over the Buckley River - Lady Loretta 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 radiometrics have been produced over the Buckley
River-Lady Loretta region (92 x 35 km) approximately 40 km north-northwest
of 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.
In addition, a geochemical sampling strategy map has been generated
which can be used as an aid in the interpretation of surface geochemistry
and drill-hole samples.
The maps reveal a complex history of landscape evolution. A combination
of a long weathering history and variable degrees of stripping has
resulted in a landscape of highly variable regolith. Rocks exposed
at the surface reflect weathering processes which operated from
the Jurassic to the present day. Regolith consists of duricrusts
(5% map sheet area) which may reflect both local and transported
derivations, saprolite (67% map sheet area, includes bedrock) and
sediments (28% map sheet area).
Duricrusts and saprolite
Duricrusts typically cap deeply weathered zoned profiles which
include ferruginous, mottled and bleached saprolite at depth. These
highly weathered materials are associated with relict parts of the
landscape, including palaeoplains, plateaux and mesas.
Three types of ferruginous duricrust are recognised including:
massive, fragmental, nodular and slabby duricrust. Ferruginous duricrusts
are commonly associated with Fe-rich lithologies (e.g., shales,
basalt and dolomitic siltstones). Siliceous materials include massive
microcrystalline silcrete, silicified sands and gravels and siliceous
saprolite (typically cementing the mottled and bleached zones).
Silcretes are associated with palaeo-lows in the landscape (e.g.,
river channels) and with siliceous bedrock lithologies (e.g., siltstones).
Veneers of sheet wash gravels, residual sand and clay overlying
mottled saprolite are common in areas of relatively low relief (rises,
erosional plains and pediments). Lithosols lying directly on bedrock
or saprock occur on steeper slopes are most common over the higher
relief and geomorphologically active eastern half of the study area.
Sediments
Alluvial gravel, sand and clay are associated with river channels,
terraces and alluvial plains. Colluvial sands, gravels, clays and
lags form sheet flow and footslope deposits. Extensive blankets
of sheet flow deposits occur over the central western part of the
study area where they overlie deeply weathered saprolite. Major
rivers at the southern end of the map sheet are superimposed over
the predominantly north-south structure of the underlying Proterozoic
rocks. In places these rivers have been captured and their flow
redirected to the north. Silica or Fe 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.
Implications for exploration:
Some regolith-landforms should be assessed carefully when interpreting
surface and drill hole geochemistry, these include:
- Exhumed landscapes which have largely removed Cambrian sediments
exposing ferruginous and mottled Proterozoic bedrock. In many
places not all the Cambrian has been completely removed, leaving
behind pockets or veneers of Cambrian sediments in the form of
cherty breccia or gravel lags. These patches of Cambrian can give
false geochemical anomalies.
- Regolith developed on Mesozoic and Cambrian lithologies are
unlikely to directly relate to mineralisation at depth. Re-worked
Proterozoic bedrock and metals precipitated from groundwaters
in the Mesozoic sediments may give false anomalies.
- Bedrock below palaeoplains are commonly deeply weathered and
leached. Metal concentration within these highly weathered zones
are typically low.
- Massive, fragmental and nodular duricrusts and ferruginous
saprolite have developed largely in situ. These materials can
be sampled to detect mineralisation at depth.
- Slabby iron duricrusts are thought to be largely formed from
lateral movement of iron and, as a result, may give false anomalies.
Nevertheless, they can be used to give broad geochemical indicators.
- Mottling and Fe granules derived from the mottles in silcretes
and silicified saprolite may be useful sampling media in highly
siliceous terrains.
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