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Three-Dimensional Computer-Based Gold Prospectivity Mapping Using Conventional Geographic Information Systems, Three-Dimensional

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Three-Dimensional Computer-Based Gold Prospectivity Mapping Using Conventional Geographic Information Systems, Three-Dimensional Three-Dimensional Computer-Based Gold Prospectivity Mapping using Conventional Geographic Information systems, Three- Dimensional Mine Visualization Software and Custom Built Spatial Analysis Tools Eduardo Videla1 and Carl M. Knox-Robinson2 Centre for Teaching and Research in Strategic Mineral Deposits Department of Geology and Geophysics The University of Western Australia NEDLANDS WA 6907, Australia [email protected] [email protected] Presented at the second annual conference of GeoComputation ‘97 & SIRC ‘97, University of Otago, New Zealand, 26-29 August 1997 Effective mineral exploration requires a detailed knowl- the research aims to identify potential continuations of edge of the factors and processes which result in the for- known ore bodies, and to attempt to locate new prospec- mation of economic deposits. To apply this knowledge, a tive areas for gold mineralisation further to the south of sound three-dimensional understanding of the geology and the present goldfield. structure of a region is required. In most cases, surface The Wiluna goldfield comprises a region approximately geology can be mapped with a high degree of accuracy, 3km x 5km, and is situated in the northern part of the however, the geology at depth has to be inferred from Archaean Yilgarn Block of Western Australia, approximately geophysical methods, or through drilling programs, and is four kilometres south of the Wiluna townsite. The area therefore mapped at much lower spatial resolution. This has been mined for gold since the early 1900s, and pres- anisotropy in spatial-data quality, coupled with the scarcity ently comprises 11 open-cut and underground gold mines. of three dimensional geographic information systems (GIS), Geological information for the region includes detailed makes computer-based exploration at a camp- or district- surface mapping and over 6,000 unevenly distributed drill scale very difficult. holes, totalling in excess of 300km of core samples. The The process of computer-based three-dimensional min- drill-core has been assayed for gold and gold-related ele- eral exploration is being addressed at the Centre for Teach- ments, including arsenic, antinomy, and sulphur. Plans and ing and Research in Strategic Mineral Deposits within the sections from mine construction activities provide detailed Department of Geology and Geophysics at The University three-dimensional information for limited areas. These data of Western Australia. A research project is in progress which have been entered into a 3D mining package, and a model attempts to define a three-dimensional gold prospectivity of the surface and interpolated sub-surface geology and model for the Wiluna goldfield. The aim of this research is structure constructed. This model will be used as a base to gain a better understanding of the factors which spa- on which to conduct a gold prospectivity analysis tially control the location of the known ore bodies, and Several GIS-based methods have been developed to as- especially of high-grade zones within these bodies. Also, Proceedings of GeoComputation ‘97 & SIRC ‘97 15 sess and map prospectivity on a global to district scale 2- Data collection using datasets such as geological maps, aeromagnetic and Data available for this project includes: radiometric data, topography, and satellite imagery. a) a detailed geological surface map at scale 1:2 500 pro- Prospectivity mapping methodologies can be split into two duced in a previous research (S. Hagemann, 1992). This broad groups: knowledge driven and data driven. Knowl- map is available in digital format and was updated with edge driven methodologies involve the application of con- more recent information. It includes main ceptual models to appropriate spatial datasets, whereas lithostratigraphic units, first-, second- and third-order data-driven methodologies look for significant spatial rela- structures, and measurements of azimuth and dip of tionships between known sites of mineralisation and sur- faults. rounding geological features. Identified spatial relationships b) an extensive drillhole database including exploration are quantified as mappable criteria and are ultimately inte- and evaluation records. The information contained in grated into a single prospectivity map. Techniques applied the database includes- with varying degrees of success include Boolean logic, in- > 6000 drillholes (RC, diamond, evaluation) dex-overlay, Bayesian statistics, fuzzy logic and artificial > 375 000 meters of drill neural networks. > 200 000 Au assays The majority of assessments to date have been two-di- > 95 000 geochemical assays mensional in nature and normally conducted at a scale > 95 000 magnetic susceptibility of host rock meas- where deposits can be adequately represented as point urements features. This research has the additional complications in > 15 000 rock descriptions that the third dimension must be addressed, and that the detailed geological maps of pit and underground works scale of observation is such that ore-bodies have a definite and interpreted geological cross sections. volume, and cannot be regarded as simple point objects. Although present mining packages are good at visualising 3- Data integration and 3D modelling three-dimensional bodies, and are capable of measuring To achieve the best graphic visualization of the complex lengths, areas and volumes, most packages lack an in-built environment of the geological subsurface, the available in- macro language which would allow a quantitative exami- formation was integrated using a mining visualisation soft- nation of gold prospectivity. Consequently, dedicated data ware. A 3D model was constructed correlating the sur- handling programs are being developed to extract the spa- face map with the drillhole information, underground min- tial information from the mining software and to conduct ing maps and interpreted geological sections. quantitative spatial analysis techniques to identify and quan- A number of problems need to be addressed in the proc- tify significant spatial relationships between high-grade ore ess of interpretation and integration of data. These prob- zones and the surrounding geology. lems are technological as well as inherent to the data. 1- Objectives Present mining visualisation software require high speciali- sation and the process of updating the model according to Through the integration of two-dimensional surface geo- new information is difficult and extremely time consum- logical maps and-three-dimensional subsurface information ing. In terms of the data, good correlation is achieved in of the Wiluna goldfield, the aim of this research is to con- dense sampled areas but an increasing degree of interpo- struct a three-dimensional geological model of the area lation and uncertainty is introduced in poorly sampled ar- and to quantitatively analyse controls on gold mineralisa- eas combined with the inherent anisotropy and complex- tion. Use of these controls to define methodology for re- ity of the geologic subsurface. gional GIS-based gold prospectivity analysis. 16 Proceedings of GeoComputation ‘97 & SIRC ‘97 4- Controls of the mineralisation with fluid-wallrock interaction and physico-chemical con- It is accepted that the gold mineralisation is late in the ditions in the time of the mineralisation. Extensional veins, tectonic evolution of the Yilgarn craton (Groves, 1993). dilational jogs, shear veins, divergent bends, etc, are all terms The fault system, along with relevant lithological contacts, related with the geometry of the structures formed after is the principal control of the mineralisation. As the faults the application of a directed regional stress to the rockmass. play a critical role in the siting of the ore bodies, an accu- A measure of this deformation is the displacement along a rate spatial representation of this structures is required. fault, the azimuth and dip, the angle formed between faults, veins, joints, the orientation of the schistosity, etc. The traces of the faults on the surface map, their projec- tion in the underground mining maps, and the drillhole In order to find relationships between gold mineralisation intersection of the fault in subsurface, provide the lines and the hosting structures, it is necessary a spatial and points used to create these entities in space. An em- discretisation of the faults into basic components, at a scale pirical spatial resolution of 15 m was adopted for the basic relatively similar to that of the gold assays, and to generate cells that represent three dimensional geological solids. new variables relating the relative spatial position of gold Lines and points were spatially gridded at this resolution, and structure. and a best polynomial algorithm fitted a TIN (triangulated By construction, the fault surface is made from a variable irregular network) to these points. In all cases, control number of ordered triangular facets connected by the sides, points were left without gridding to validate the interpola- they represent the topology of the physical surface. The tion accuracy. The final surfaces look smooth and realistic computation of the centroid (properly called hypocenter) and serve as a basis for further spatial analysis. of every triangular facet generates the points necessaries for the analysis. 5- Data extraction At the same time, operating on the normal vector to every In this study the kind of datasets required for analysis are facet it is possible to calculate its spatial orientation, in dependent primarily on
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