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.