The characterisation of quartz and gold in the Beaconsfield and Lefroy areas, Northern Tasmania
Creators: name: Russell, DW
Publisher: name: University Of Tasmania
Year: 2023

description: This study consists of two sections; the first describes the local geology and ore mineralogy of the Beaconsfield and Lefroy goldfields in the Tamar region; the second section discusses ion beam (PIXE/PIGME) and Electron Paramagnetic Resonance (EPR) techniques used to characterise quartz from auriferous reefs in the Beaconsfield area. The quartz veins of the Tasmania Reef (Beaconsfield) are zoned and consist of auriferous quartz with a central core of carbonate. Sulphides are distributed throughout the two to six metre wide reef, which has average grades ranging from 14g/t to 38g/t gold. The vast majority of the gold occurs as either small free grains (up to 1.8 mm) in quartz or tiny inclusions (less than 0.5 mm) in pyrite. Ordovician quartzose sandstones/conglomerates host the reef. In the Lefroy region the host rocks for the reefs (grades up to 30g/t) are the siltstones/sandstones (early Ordovician to early Devonian). Approximately 250 analyses of the fineness of gold grains were conducted using an electron probe microanalyser. Analyses showed that Beaconsfield gold had an average fineness of 934, while Lefroy gold had an average fineness of 972. Both the high fineness values and the narrow fineness range indicate that both deposits are hypothermal. A study of the Tasmania Reef suggested that there were at least eight phases of mineral deposition, with the gold being deposited during phase 3 (quartz, pyrite and gold) and phase 4 (quartz and gold). Additionally, sulphides predominate in phases 2, 5 and 6. The first and penultimate phases consist of quartz, the last and central phase in the reef consists of ankerite. The pyrite associated with the mineralisation has a ˜í¬•\\(^{34}\\)S mean = +8.9‚ÄövÑ‚àû and a range of +6.5 to +12.7‚ÄövÑ‚àû. Pyrite not associated with the mineralisation has a ˜í¬•\\(^{34}\\)S mean = + 13.5‚ÄövÑ‚àû and a range of +7 to +26.7‚ÄövÑ‚àû. These values are indicative of mixing (sea water plus metamorphic waters) or a fractionation of parent fluids prior to the precipitation of the ore minerals. The inferred oxygen isotope values of the parent waters (˜í¬•\\(^{18}\\)0\\(_{water}\\)) of vein quartz range from +12.8‚ÄövÑ‚àû to +18.5‚ÄövÑ‚àû, indicating that they were of a metamorphic origin. A study of arsenopyrite provided an estimated temperature of deposition of about 400¬¨‚àûC. In order to characterise the quartz of the vein systems in the Beaconsfield area, trace element distributions in 348 samples of vein quartz were determined by Proton Induced X-ray Emission (PIXE) and Proton Induced Gamma Ray Emission (PIGME). These analyses from 100 mg of acid cleaned quartz powders, were compared with gold fire assays of one metre length diamond drill core splits. The most significant correlations were between AU(fire assay) and As, Ge and Li. Other highly significant correlations between one element and another element are those between Al/F, Al/Fe, Al/K, Al/Ti, Al/Li, As/Fe, Ge/Li and K/Rb. A system of numerical data evaluation of the chemical analyses was developed to aid in the identification of auriferous quartz. These methods are based on significant statistical correlation by Spearman Rank analysis, between pairs of elements for the mineralised and non-mineralised populations. Further statistical processing using a series of \screens\" enabled the non-mineralised samples to be removed retaining the majority of the mineralised population. Several successful screens were developed and one example follows: {mol(Al+F) mol(Ge+Li) mol(Na+Rb) mol(Ca+CI)}. Succesive applications of these pairs of elements separated auriferous quartz from barren quartz with a high degree of certainty. This technique for the characterisation of quartz and the subsequent identification of auriferous reefs provides a tool useful in exploration geochemistry. A study of the intensities of Electron Paramagnetic Resonance (EPR) spectra was useful in distinguishing between the auriferous and non-auriferous quartz samples. The intensities of the EPR signals correlated strongly with Cl Ti and Fe in the non-mineralised population and with Li Ge F Al and K in the mineralised population. A systematic sorting of elements vs EPR optimised the correlation coefficients into three groups within the quartz : Al (electron donor substitution atom); Fe Ti Ge (electron acceptors substitution atoms); K Li (compensating ions interstitial ions). Techniques for measuring EPR intensities were investigated using measurement at both a high power (100 mW) and a low power (0.05 mW). At each power setting the intensities of the spectra were measured using the appropriate peak height of the derivative curve as well as calculating the integral of the absorbtive spectra. This demonstrated a very strong correlation between the peak height and the integral of the spectra."