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Open File Report 132

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.


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:

  1. 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.
  2. 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.
  3. Bedrock below palaeoplains are commonly deeply weathered and leached. Metal concentration within these highly weathered zones are typically low.
  4. Massive, fragmental and nodular duricrusts and ferruginous saprolite have developed largely in situ. These materials can be sampled to detect mineralisation at depth.
  5. 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.
  6. 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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