Regolith geochemistry and EPR : characterization of gold mineralization in deeply weathered terrains, Australia
Creators: name: Xu, Li
Publisher: name: University Of Tasmania
Year: 2023

description: 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.