Georgina Gordon, Jim Kendall, Jack Cole, and Chris Brown
The need for a new technology to provide evaluation of drill core and grab samples during exploration programs, and provide real-time analysis of gold values has driven the development of the “Gold Sniffer”. This device discriminates native gold and gold alloys in geological samples using spectral characteristics, in the visible wavelength range, for individual pixels from high-resolution digital photography. The results can assist with rapid estimation of gold grade to inform decision-making during exploration and mining. This paper describes a preliminary study using the technology on drill core samples from the Tunkillia gold deposit, with known gold assay values, and compares the results.
The Tunkillia gold deposit is located in the central Gawler Craton in South Australia, approximately 660 km NW of Adelaide (figure 1). Deeply weathered bedrock, averaging 50m thickness and largely depleted of Au, is covered by up to 10m of transported aeolian sand, with patchy calcrete development (Fabris & Keeling, 2010). Regional geochemical surveys in 1996, using calcrete sample media (Martin, 1997), identified anomalous zones, which led to the discovery of gold mineralisation in what is interpreted as a broad zone of hydrothermal alteration within the regional-scale Yarlbrinda Shear Zone. Extensive drilling has established a total resource of >0.5 million ounces Au in 26.3 million tonnes, at an average grade 1.04 g/t Au (WPG Resources Ltd, 2016).
The basement geology consists of deformed medium to coarse grained granite to granodiorite of the Tunkillia Suite (Ferris & Schwarz, 2004; Ferris & Wilson, 2004), along the northern part of the Yarlbrinda Shear Zone (YSZ). The gold occurs in narrow, steeply dripping quartz veins in association with pyrite, with trace chalcopyrite and galena, in shared and sericite – chlorite-altered Paleoproterozioc Tunkillia Suite granitoid.
Drill core samples intersecting Au mineralization at Tunkillia were selected from 5 drill holes to model the spectral characteristics of the mineralisation and to analyse Au content for high and low grade proportions of the mineralisation (Table 1). The samples were sawn split core from open file drill holes held in the Department of State Development Drill Core Reference Library at Tonsley, Adelaide.
The Gold Sniffer (Figure 2) is a portable device that analyses a high resolutiondigital image of the surface of an unpolished sample to detect gold particles > 1.6 microns across. The equipment consists of a digital camera, a macro lens, a light source that illuminates uniformly at micron dimensions across the visible spectrum, a stage to position the measurement sample, and software for processing the digital imagery to identify gold and mineral particles, and display the results. The spectral characteristics of individual pixels are analysed and matched to the colour of native gold or gold alloy present in a particular deposit. a spatial algorithm is used to detect the edges of the gold particles. The fractional coverage of gold in the imaged area is used to estimate the local gold grade. Averaging the local gold grades for 10 to 30 measurements provides an estimate of the gold grade of a sample. Each measurement requires 90 seconds to perform.
The instrument can also be used to detect other coloured minerals in the same fashion as gold, by analyzing the characteristic visible spectrum of the mineral, and using the spatial algorithm to detect the edges of the mineral particles. at Tunkillia, fine grained pyrite and slightly coarser chalcopyrite were identified using this method, and the concentrations of there minerals were estimated also from the fractional coverage in images, as for gold analyses. Gold at Tunkillia is mostly associated with pyrite; both minerals are typically very fine grained (Figure 3), and distributed throughout the quartz rich matrix, but occur also in coarser aggregates.
Results of analysis of gold detected on selected areas of drill core are summarized in Table 1 and displayed in Figure 4. Estimated values for gold content from the image analysis are compared with values obtained by SFA (screen fore assay) analysis on 1m intervals determined previously on crushed split core, for the same region of mineralisation.
The gold particle size distribution, and the shape distribution of gold particles were calculated and plotted as histograms (Figure 5). This is a standard output of the Gold Sniffer software, where histograms are expressed as the number of photosites, which is the minimum spatial resolution of individual analyses on the minimum spatial resolution of individual analyses on the mineral surface. Each photosite consists of four pixels and images an area on the mineral surface with dimensions 1.6 x 1.6 microns. The resulting plots are an indication of the gold particle size and shape expressed as bulkiness and flakiness, which quantify whether the gold particles are large and round or small and thin respectively. This is useful data for evaluating process options and maximize gold recovery.
A minimum of twenty measurements, each covering an area of 5.29mm2, were taken on every sample, and parameters for the detection of gold, pyrite and chalcopyrite were defined, specific to characteristics of these minerals in the Tunkillia deposit. The estimated gold grade was calculated from the area of gold detected in the regions where sulphide minerals were present, as a proportion of the imaged surface area of the drill core sample.
Results of preliminary analysis of intervals of gold ore from the Tunkillia deposit show that gold grade estimates from the new technology “Gold Sniffer” analyses, are comparable with results obtained by laboratory analysis. The technique was successfully employed to analyse over one hundred measurements that were taken from eight drill cores with a range of known gold grades, and has subsequently proven to be a rapid, effective method for the detection of gold. This technique was also used to detect the particle size and shapes associated with gold and its affiliates, confirming that the detected gold in fine grained, and is distributed throughout the quartz rich matrix in association with pyrite and chalcopyrite.
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