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Abstract

Thick, commonly lateritic regoliths are widespread in inter-tropical regions of the
world, and present a challenges to current exploration techniques. The particular
problem in Australian is the widespread deep and varied regolith, frequently with a
complex geological history dating back to mid-Permian time. Within this long
period, several episodes of deep weathering and selective chemical solution, transport
and precipitation, differential erosion, and deposition of sediment, so that the primary
features of primary ore deposits are considerably modified and deeply concealed.
The lateritic and saprolitic Au associated with subsurface depletion is one of the
most common results of the deep weathering.
With these problems in mind, an integrated paramagnetic/lithogeochemical
technique has been developed in the exploration for gold in heavily weathered terrain.
This technique is based on the degree of impurity in quartz associated with ore
deposits and measured by electron paramagnetic resonance (EPR) powder
spectroscopy and chemical analysis of the quartz rich acid insoluble residue of the
regolith.
Electron paramagnetic resonance spectroscopy, which measures microwave absorption
by unpaired electrons in a magnetic field, is a new concept in mineral exploration as
quartz samples from mineralized zones often have strong EPR signals due to lattice
imperfections. Quartz associated with mineralization has stronger electron
paramagnetism than the barren quartz, whilst the EPR signals of residual quartz,
including secondary chalcedony with inclusion of primary quartz, are not strongly
affected by weathering.
The acid treatment consists of sequential treatment of regolith samples with hot
aqua regia, nitric acid and sulphuric acid, dissolving residual sulphides, iron,
manganese and aluminium oxides and hydroxides, carbonates, phosphates, sulphates
and clay minerals. The remaining acid insoluble residue consists of quartz with
some residue sericite, rutile and traces of other resistate minerals.
Three areas with a well described regolith profile and known bulk geochemistry were selected: the Mystery Zone, Mt Percy, at Kalgoorlie, the Rand Pit, Reedy Mine
near Meekatharra in Western Australia, and Jim's Find in the Tanami Desert of
Northern Territory. The depth of weathering in these areas varies between rock
types, but generally below surface 60-90 m. As strong acid digestion, most of the
weathering products have been removed from the reglith samples and this acid
insoluble residue reflects the primary signature.
In the acid insoluble residue, quartz associated with mineralization has an EPR spectra
typical of lattice imperfections. Case studies of the Mystery Zone, Mt. Percy, the
Rand Pit, Reedy Mine and the Jim's Find, Tanami have pronounced EPR signals at
the surface above the mineralized zones as well as in Au depletion zones and in the
wall rock alteration halos. The distribution of EPR 326.5±5 m T is in accordance
with the distribution of certain trace elements, such as Al, K, Rb, W, V, Ga and Ge
in all three working areas. In addition, wall rock alteration patterns are commonly
expressed by strong [Al04] 0 centers in the EPR spectra, accompanied by increased K
and Rb concentrations, and high Rb/Kand Rb/Al ratios. In contrast, the intensity
of EPR 326.5± 100 m T resembles the element distribution of Fe, Mn, Zn and Ni
indicating that EPR 326.5±5 mT weakly reflects the distribution of alteration.
In the acid insoluble residue, the chemical expression of sericitic wall rock is Al, K
Rb, V, Ga and W enrichment, high Rb/K ratios in surface material above
mineralization, and high Rb/Al ratios in wall rocks. Aluminium, K, Rb, Ga, V, and
W related to sericitic alteration in the acid insoluble residue, are commonly good
indicators of Au mineralization through the regolith. In the Mystery Zone, Rand Pit
and Jim's Find, Al, Cr, Ga, K, Rb, V, W, Rb/K and Rb/ Al consistently define the
location of Au mineralization. The composite halos Al x K x Rb, Ga+Ge and
As+ W +Mo also enhance the mineralization halo, with these cumulative indices more
useful than the single low-level elements of Ge, As and Mo. Sodium, Ca and Sr
depletion is caused by plagioclase destruction. Titanium, Zr, Nb, and possibly V,
have a strong correlation to lithological variation and are surface enriched in resistate
minerals. However, the acid insoluble residue of near surface material likely reflects
secondary dispersion patterns of Ca, Sr, Cr, Cl and S presumably due to extrapment
of submicroscopic inclusions.
Similar results may be obtained for these and other potential pathfinder elements,
like As, Sb, Bi and Cu, by total geochemical analysis of whole rock pulps. However,
not all Au deposits have an associated suite of pathfinder elements, whilst research
presented in this thesis suggests that there is always an EPR signal and high Al, K,
Rb, V, Ga and W chemical expression in the acid insoluble residue.

Item Type:	Thesis - PhD
Authors/Creators:	Xu, Li
Keywords:	Gold ores, Geochemical prospecting, Regolith
Copyright Holders:	The Author
Copyright Information:	Copyright 1999 the Author - The University is continuing to endeavour to trace the copyright owner(s) and in the meantime this item has been reproduced here in good faith. We would be pleased to hear from the copyright owner(s).
Additional Information:	Thesis (Ph.D.)--University of Tasmania, 1999. Includes bibliographical references
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