Integration of advanced remote sensing and geospatial methodologies to enhance mineral exploration: An example from the southern Gawler Ranges, South Australia

Abstract

The world’s demand for metals is increasing and there is a growing need for mineral explorers to locate new ore deposits. Globally, discovery of economic mineral deposits is becoming more challenging due to the increasing depths where exploration is being conducted to discover mineral deposits. Most surficial deposits have been discovered, driving exploration into terrains with substantial weathered regolith cover, and requiring new exploration methods. Current traditional exploration methods including geophysics, high density soil sampling and geochemical analysis can be expensive, time consuming and limited in geographic extent. Although remote sensing methods have been applied to regional-scale mineral exploration, there is potential for them to be used more fully in regions where regolith is a continuing challenge. The overarching aim of this multidisciplinary thesis is to develop methods that integrate forms of remote sensing and geospatial information to reduce the risk and cost of exploration in weathered terrains by identifying and mapping surface alteration related to buried mineralisation. The study area used to develop and test these methodologies was the southern Gawler Ranges, South Australia, a region prospective for gold, porphyry-copper and epithermal-silver mineralisation. This semi-arid environment is moderately vegetated with limited geological exposures. Most basement rocks are overlain by approximately 100 m of weathered cover materials presenting challenges for both exploration and remote sensing methods. The broad research aim was addressed through three more specific objectives: 1. Development of an objective regolith-landform map using geospatial data and a repeatable methodology that can be used to guide the early stages of exploration potential assessment; 2. Characterisation of surface expressions of alteration mineralogy and interpretation of landscape processes using airborne hyperspectral imagery and mineralogical data; and 3. Integration of surface geochemistry, mineralogy and regolith-landform mapping to understand and map surface signatures of potential buried mineralisation. An unsupervised classification was applied to geospatial data layers including a Digital Elevation Model, Topographic Position Index and potassium, thorium and uranium gamma-ray radiometrics. This was clustered to generate an objective regolith-landform map representing the main regolith-landform types. This map captured many of the features typically mapped by traditional regolith-landform mapping as assessed by a statistical goodness of fit measure. While not a replacement for the resource-intensive traditional regolith maps derived from extensive field work, this method used freely available geospatial data an objective, repeatable methodology to produce a map that has potential to increase understanding of the landscape and assist targeting of areas of alteration and mineralisation for more detailed exploration. Airborne hyperspectral imagery was analysed by Spectral Feature Fitting, matching image spectra to reference spectra to identify alteration mineralogy. X-ray diffraction was used to independently validate mineralogy present in the landscape providing insight into unclear spatial distributions of some minerals and confirming the presence of key alteration minerals. Landscape processes were interpreted by integrating the spatial distribution of minerals with the objective regolith-landform map. Advanced argillic and argillic alteration were identified in the study area, focused around an exposed alunite breccia at Nankivel Hill. The results placed the central topographic feature, Nankivel Hill, proximal to potential porphyry mineralisation, with Peterlumbo Hill distal to mineralisation as possible chloritic alteration expressed at the surface in this region. Definition of lithologies from major element geochemistry identified ten rock and cover sequence types within the study area. A region-specific pathfinder element suite was defined using interpretation and thresholds of the Nankivel and Peterlumbo Hill rock exposures. The mineral hosts of these pathfinder elements were proposed from interpretation of semi-quantitative X-ray diffraction to determine the influence of weathering on dispersion of pathfinder elements from rock exposure to cover sequence materials. This suggested that most pathfinder elements were hosted in a variety of minerals including alunite, jarosite, microcline, muscovite, orthoclase and hematite in rock exposures and a broader range of feldspars, clays, micas, carbonates and iron oxides associated with cover sequence materials. Definitions of proximal and distal geochemical and mineralogical footprints of a porphyry deposit were delineated using the surface geochemistry, X-ray diffraction and hyperspectral mineralogical data. The landscape position of pathfinder elements was interpreted to recommend sample media with the most potential for identification of pathfinders at higher concentrations. The outcomes of this research demonstrate several encouraging approaches for use of land surface remote sensing and geospatial analysis in the context of mineral exploration in highly weathered and covered terrains. These methods can be integrated easily with more traditional methods and data to improve mineral exploration outcomes for the industry. The increasing need to explore terrains with extensive depths of cover in order to discover new ore deposits suggests that the industry would benefit from integrating these tools to enhance future exploration.