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Open File Report 121

Regional regolith mapping around Alice Springs, Northern Territory

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

Epithermal quartz veins occur at Pajingo within relatively flat-lying andesites, tuffs, volcaniclastic sediments and sandstones. The host rocks are relatively fresh on the Mt Janet Range but are weathered and mottled on the surrounding pediment, where they are partly covered by Tertiary sediments (mottled Southern Cross and less weathered Campaspe formations) and by various more recent colluvia and alluvia.

The Tertiary sediments, exposed by mining of the Scott and Cindy lodes, were mapped and sampled. They consist of immature, clay-rich conglomerates and grits which have drawn their detritus from a range of levels in the regolith (fresh rock, saprolite and pisolitic material). These sediments were then further weathered (weathering of fresh rock fragments and mottling).

Geochemical backgrounds, over 1 km from known mineralisation, are slightly elevated (>30 ppb) compared to 5-10 ppb more distant. The Southern Cross Formation sediments, exposed by mining at Scott Lode, are all rich in Au (>100 ppb) as their detritus was largely derived from the Scott mineralisation and its environs. Local Au anomalies of >500 ppb occur near the base and well above the base of the profile. In contrast, at Cindy, the background in the Southern Cross Formation sediments is much lower, as their detritus was derived up-slope from Cindy. A localised Au anomaly (150-5OO ppb) occurs near the base of a palaeochannel which drained the eastern side of the Cindy mineralisation. All this suggests mechanical Au dispersion together with dispersions in W and Mo. However, partial extraction, using water, potassium iodide and potassium cyanide, indicates that a proportion (about 18%) of the Au is now relatively soluble and has been relocated slightly by weathering.

Data from the exploration drilling were sifted and maximum and arithmetic mean Au contents in each drill intersection in the Southern Cross Formation sediments were determined and plotted for each study area. Gold is dispersed into these sediments at several levels and anomalies of 100-300 m occur not only at Scott and Cindy but also related to zones of numerous auriferous quartz veins, unrelated to economic mineralisation. It is necessary to understand the palaeotopography of the basement to interpret these anomalies.


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