Presentations

Here we present a case study of prospectivity modelling over a region with both data-rich and data-poor areas, in the Kumasi Basin, Ghana. Whilst a reasonable amount of geological, geochemical and geophysical data is available over much of the Asankrangwa Gold Belt, host of the large Nkran and Esaase gold deposits (measured, indicated and inferred resources >10 Moz Au), data availability over much of the remainder of the Kumasi Basin is generally poor and of much lower resolution. As part of a comprehensive prospectivity and targeting study undertaken by Corporate Geoscience Group for Asanko Gold, Kenex completed GIS-based prospectivity modelling using the weights of evidence (WoE) technique to delineate high priority targets for orogenic gold. WoE modelling provides a data-driven tool that combines relevant datasets, identifies anomalous thresholds in predictors of mineralisation and produces a map of geological potential.

Statistical methods ensure that when the final geological potential grid is created, areas with missing data coverage are not significantly down-weighted relative to anomalous areas. Areas of poor data coverage in the Kumasi Basin required creative examination to allow successful modelling. For example, Kumasi Basin orogenic deposits are often associated with broad zones of silicic alteration. Consequently, many deposits resist weathering and form topographic ridges, allowing analysis using detailed open-file DEM data. Ridges were extracted and attributed with scale, relative strength and orientation, all of which were tested for spatial correlation with known orogenic deposits. Another example involves limited coverage of available geophysical surveys. Scanned TMI image data was reclassified into a GIS and certain colour bands selected as most accurately representing TMI. Properties such as magnetic slope, a common predictor for orogenic mineralisation, could then be calculated. Many targets identified by the model were located in areas with high data density. By using data intelligently we have also identified targets in data-poor areas.

Auzex Exploration Limited owns a number of exploration tenements over the historically tin rich Khartoum area near Herberton, north Queensland, Australia and Kenex Ltd has completed both 2-dimensional prospectivity modelling and a 3-dimenional geological interpretation over this region. The initial 2-dimensional prospectivity model of intrusion related tin mineralisation is limited by the 2D nature of the data used, and regions of known Sn mineralisation were not identified, particularly in the contact zones of shallow dipping highly fractionated tin granites. To rectify this, a 3D geological model was created using Leapfrog Geo modelling software, and 3D spatial data has been projected to the surface topography and incorporated into an updated 2D prospectivity model of the region using ArcGIS software. The 2D and 3D models utilise newly compiled digital data including historical exploration data; geological data compiled from detailed geological mapping of north Queensland, academic literature and company exploration mapping; recent geophysical data collect by Fathom Geophysics Australia Pty Ltd; ASTER data analysed for alteration; and historical exploration geochemical data including rock-chip, stream sediment and soil sampling. The weights of evidence modelling technique was used to determine spatial correlations between known deposits and predictive maps in 2D, created from the available data, that represent each component of the currently accepted minerals systems model for intrusion related tin mineralisation defined for this project. The final updated 2D prospectivity model partially resolves the limitations of the initial 2D model, successfully identifying many of the areas originally missed.

Prospectivity modelling has been completed over the Lachlan Fold Belt, New South Wales, Australia, using the GIS based weights of evidence modelling technique to target porphyry Cu-Au, associated skarn Cu-Au, orogenic Au and VMS Au mineralisation. The Lachlan Fold Belt is a 700 km wide belt of Paleozoic accretionary terrains, stretching from Queensland to Tasmania. Porphyry and skarn mineralisation was associated with Ordovician shoshonitic magmatism, which was followed by Silurian regional metamorphism and deposition of orogenic gold deposits. Contemporaneous VMS-style mineralisation resulted in deposits in intra-arc rift basins of the Macquarie Arc. In preparation for the prospectivity modelling, lithological and structural data, extensive geophysical surveys and stream, drill-hole and rock chip geochemistry were used to create predictive maps that represent various parts of the mineral systems being modelled. Included in the models are maps that identify possible sources of heat and mineralised fluids, structures used for fluid migration, mineral trap zones, and outflow zones that may indicate a subsurface deposit. Prospectivity maps have been created for each mineralisation style and new areas of each deposit type located. The models have also independently identified areas of proven mineralisation, including Cadia, Northparkes, Woodlawn and other large producing mines. The prospectivity maps were reclassified to generate targets by delineating highly prospective areas from each model. Targets were compared and overlap examined among the four models, before further analysis of high priority targets. Single targets or clusters of targets were individually assessed by incorporating information such as tenure, geology, geochemistry and geophysical signature. Economic and risk factors were assessed and the targets ranked and mapped according to high and low exploration risk. Following this analysis, targets of interest can be highlighted as potential projects for acquisition, or used to prioritise new exploration data collection.

The use of modern day 3D GIS software packages such as GOCAD, GeoModeller and Leapfrog Geo has dramatically changed the way exploration targeting can be carried out compared to the last twenty years of using 2D Geographic Information System (GIS) for exploration. This is especially true in the last five years in which computer and GPS technology has developed to the stage where it is possible to digitally locate, accurately store, visualise and manipulate geological data in 3D at the scale of a mineral system, which is usually much greater than mine scale where most of the current 3D work is focussed. Most GIS can store, manage and manipulate data in 2D, with some able to visualise information in 3D. However, there are a number of packages that allow full 3D GIS functionality, including querying and modelling, allowing geologists to start exploration targeting in a 3D system. Auzex Exploration Limited owns a number of exploration tenements over the historically tin rich Khartoum area located near Herberton in North Queensland, Australia, exploring for Tin-Tungsten mineralisation. A 3D geological interpretation was created over a 60 km by 60 km region in Khartoum using Leapfrog Geo to improve targeting for tin systems adjacent and above buried granites and shallow dipping granite contacts, followed by 3D targeting using a Multi-Class index Overlay workflow of GoCAD Mining. The ranking of the 3D maps were based on a 2D prospectivity mapping exercise using the weights of evidence technique. By modelling geology and targeting in 3D, complex subsurface relationships and the correct vertical extents can be constrained. This will be invaluable for defining potential drill-hole targets.

New Zealand has an established history of gold production beginning with the gold rushes of the 19th century in the Coromandel, Nelson/Marlborough, West Coast and Otago regions. Despite this, the number of significant modern hard rock gold operations has been limited in recent years due to a lack of sustained exploration capital and not limited geological prospectivity. The New Zealand Government is actively encouraging explorers to invest in New Zealand through a series of targeted promotional visits, more importantly through the acquisition of precompetitive regional geophysical data. Data collection has been completed over the prospective Northland epithermal district and a large portion of the South Island’s west coast that is prospective for both orogenic gold and intrusive related gold. Analysis of new data has been an important component in aiding the generation of exploration targets from prospectivity modelling. Determining the prospectivity of an area involves reviewing all the available data and analysing it with respect to the most up-to-date mineral system model for the mineralisation style of interest. Using the weights of evidence modelling approach, the most prospective areas for epithermal gold-silver, orogenic gold and intrusion-related gold have been identified. The key exploration parameters relevant to each mineral system are first represented spatially and then statistically combined into a single prospectivity map. New potentially economic deposits could be found by focusing exploration on targets identified from these models. The prospectivity modelling approach can greatly reduce the risk involved in mineral exploration.

Substantial consideration has been given to the implications that the morphology of the Chatham Rise deposit will have on mining operations. The glacio-tectonic processes involved in the distribution of nodules on the rise have in several areas been quite significant. The recent cruises by Chatham Rock Phosphate Limited (CRPL) have collected data which has affirmed the assumptions previously made and catered for in historic resource estimations. The deformation and displacement of the phosphorite during glacial periods and the redistribution of the mobile sand during interglacial periods is interpreted to have produced a highly variable pattern of phosphorite concentration (kg phosphorite/m2) and coverage (% phosphorite/sample weight). The phosphorite resource probably has a significant spatial variability at a scale of tens of metres. Results of recent surveys show phosphorite-rich patches alternating with phosphorite-poor areas at distances of less than 20 m. The high spatial variability of the deposit has had a bearing on how historical information for the project has been regarded and integrated with the recent exploration approach and data collection process. This coupled with the proposed extraction tool has influenced the size, nature, extent and siting of the proposed mining blocks.

The use of computers in the mineral industry has dramatically changed the way exploration targeting is carried out over the last twenty years. This is especially true in the last five years where computer and GPS technology has developed to the stage where it is possible to digitally locate, accurately store, visualise and manipulate geological data in three dimensions (3D) at the scale of a mineral system. These tasks are commonly carried out using a Geographic Information System (GIS), which has become as an important tool to a geologist as his hammer. Most GIS store, manage and manipulate data in two dimensions (2D), with some having the ability to visualise information in 3D. However, there are now a number of packages that allow full GIS functionality including querying and modelling in 3Dgiving geologists a tool to carry out exploration targeting in 3D. A regional scale weights of evidence 2D prospectivity model was developed for the Taupo Volcanic Zone in New Zealand to assess the potential for epithermal Au mineralisation. A number of prospective areas have been identified including the known Ohakuri hydrothermal deposit. While this model has been successful at identifying mineralised areas the 2D data that is used gives little understanding of what is happening below the surface. Because geology does not just operate in 2D, trying to visualise 3D geometries in 2D can be challenging in exploration targeting. The development of 3D GIS such as GoCad and Geomodeller now give us the tools and techniques to use fuzzy logic and weights of evidence techniques for targeting in mineral exploration in 3D. A prospectivity modelling exercise using the weights of evidence modelling technique (developed by Bonham-Carter of the Canadian Geological Survey), was completed over the Ohakuri epithermal gold deposit in both 2D and 3D.