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Open File Report 118

Regional regolith mapping around Alice Springs, Northern Territory

Fraser, S.J., Skwarnecki, M.S. and Robertson, I.D.M.

This report is part of the final documentation of a collaborative research initiative between Gutnick Resources N.L. and CRC LEME. The CRC provided geochemical and regolith research into a major exploration program being undertaken by Gutnick Resources N.L. for Witwatersrand-style gold mineralization in the sediments of the Ngalia and Amadeus Basins.

As part of this initiative, the CRC produced a rapid, first-pass assessment of the regional landform and regolith over an area of twenty-five 1:250 000 map sheets. The specific objectives were to: -

  1. Establish broad spatial relationships between landforms, regolith and lithology;
  2. Propose effective geochemical exploration strategies for different regolith-landform settings.

Two maps were produced: -

  1. A Landform Interpretation Map (1:1 000 000 scale) showing the distribution of various landform units. These units were based on i) a re-assessment of previous land system mapping (1:1 000 000 scale; Perry, 1961), ii) interpretation of landforms from Landsat MSS imagery, and iii) five weeks of limited fieldwork. Because of inaccuracies in the land system mapping, it is not recommended that the final maps be viewed at scales greater than 1:1 000 000.
  2. A Regolith-Landform Status Map (1:1 000 000 scale) grouping the landform units into three broad terrain categories: Relict, Erosional and Depositional.

The relict terrains group is tentative, subjective and relatively small. In the erosional terrains, there has been little preservation of deeply weathered profiles on lithologies of the Arunta Province. Rates of erosion have kept pace with, or exceeded, those of chemical weathering. Rocks of the Amadeus Basin are either not deeply weathered or weathering is difficult to recognize; saprock is common.

Depositional terrains dominate the area. Two types can be identified; i) flat, low, regional plains; ii) 'perched' deposits that occur above the regional plains.

The regional plains are low and flat, formed by filling of a deeply eroded basement topography with proximal and distal sediments, now largely overlain by a veneer of wind-blown sand. Hidden beneath some of these are Tertiary basins up to 200 m deep. 'Perched' depositional deposits are small and are dominated by locally derived materials. They include fans, river terraces or valleyfills, which are being reworked. All these comprise a significant barrier to exploration. Drilling or use of techniques, that can 'see through' the cover will be needed to prospect them adequately.

Lateritic, ferruginous materials should be sampled within relict terrains. Within the erosional terrains, stream sediment, soil, lag and rock-chip sampling should be adequate but small anomalies are likely, unless iron-rich mottles and nodules can be located. Where detritus in the streams reflects local lithologies, significant stream sediment dispersion trains in all fractions are expected but, where the detritus is dominated by distal material, weak dispersions are likely only in the finest fraction (<75 um).

Low, regional plains are a significant barrier to exploration. They will need sub-surface sampling. Although substantial occurrences of calcrete were observed, initial indications are that at least some are of the groundwater type and so are unsuitable for Au search. Consideration should be given to groundwater geochemistry. The groundwaters are of low salinity, anomalies are expected to be weak but distinct.

Last updated: Friday, January 25, 2002 3:56 PM


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