Scale dependent electrical properties of sulphide deposits
Creators: name: Fitzpatrick, AD
Publisher: name: University Of Tasmania
Year: 2023

description: The electrical properties of sulphide rocks are observed to be influenced by the scale of investigation. To test this scale-dependence an empirical study of in-situ and laboratory electrical properties was undertaken at three diverse economic sulphide ore deposits: Golden Grove - Scuddles, McArthur River- HYC and North Parkes - E27. Apparent resistivity and chargeability data were collected from decimetre to metre scales at a number of sites within homogenous sections of each deposit. Equivalent laboratory measurements were performed on prismatic samples collected from each site to assess the suitability of laboratory measurements for estimation of bulk-rock properties. Chargeability can vary significantly but does not display a consistent pattern of scale variability from site to site or between deposits. However, in-situ and laboratory data for all deposits show a consistent trend of decreasing apparent resistivity with increasing scale of investigation. Apparent resistivity variations of up to three orders of magnitude were observed at some sites between small scale measurements (0.1 hl electrode spacing) and bulk rock measurements (2m electrode spacing). Laboratory data generally correlates well with in-situ measurements at small electrode spacing but in some cases displays a bimodal distribution representative of resistive and conductive end members. In these cases the most conductive laboratory measurements lie close to the bulk-rock apparent resistivity. Although apparent resistivity consistently decreases with increasing observation scale, the degree and detailed pattern of this variation is often difficult to predict even for related sites within a single deposit. No simple generic scaling rule is universally applicable for predicting bulk properties from laboratory or small-scale measurements. However, for all sites surveyed, a pragmatic best estimate of the bulk apparent resistivity from a set of laboratory measurements would be obtained by adopting the minimum sample resistivity value rather than the arithmetic mean, geometric mean or median of the sample population. Two new laboratory techniques, continuity mapping and current mapping, have been developed and refined to assess the influence of ore texture on sample resistivity. These methods provide detailed maps of electrical continuity on the sample surface and directly map the current path through the sample. Both techniques have been successfully applied to samples from Golden Grove and McArthur River and have highlighted the influence of small proportions of well connected conductive phases on the bulk electrical properties of the rock mass. The three main factors that appear to control the degree of scale variation of apparent resistivity are the resistivity contrast between mineral phases, the proportion of the most conductive phase and the ore texture. These factors, originally inferred from the empirical measurements, are supported directly by the results of three dimensional numerical modelling that predicts the most pronounced scale-variability for textures that include planar conductive components such as conductive sulphide veins. Scale variation in apparent resistivity will be significant where there is a large resistivity contrast between mineral phases and the conductive component is well connected even if it is present only in very small proportions.