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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:

  1. 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.
  2. 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.
  3. Deep weathering and leaching beneath palaeochannels. some of which are now relief inverted. Metal concentration within these leached zones are anomalously low.
  4. 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.

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