What can we learn from GIS-based prospectivity mapping in mature terranes?

This study used robust spatial statistics to systematically examine regional-scale targeting criteria that have been suggested for orogenic gold mineralization in the Yilgarn Craton. The spatial relationships between known orogenic gold mineralization and a series of targeting criteria were examined using both the number and endowment of the deposits. Both proximity and containment analyses were performed and a series of measures used to assess whether the geological feature being tested could be considered a good regional-scale targeting criteria. Spatial analysis indicated the best targeting criteria for orogenic gold mineralization in the Yilgarn Craton are: containment within greenstones, proximity to mafic group granite intrusions, proximity to regional-scale fault bends, and regional fault density. Using these targeting criteria, a fuzzy logic prospectivity map was generated. Fuzzy membership values were derived using the %endowment/%area statistic and the proximity and density threshold values set from the class at which the maximum %endowment/%area statistic was observed. The spatial analysis revealed some surprising results, with several targeting criteria that were thought to have a strong relationship with orogenic gold mineralization in the Yilgarn craton shown to have weak relationships when a robust statistical analysis was performed. Due to the analysis focussing on the relationship between endowment and the geological features, it raised the question as to how much influence the Golden Mile had on the results. Separate analysis of the super-giant and giant deposits compared to the remaining smaller deposits indicated that controls on the large deposits may differ to the controls on smaller deposits

Zimbabwe's coloured gemstone endowments - A regional geological overview

Zimbabwe hosts a varied array of coloured gemstones. With the exception of emerald deposits and several world-class pegmatites, few of the gemstone occurrences have received detailed attention from the scientific or mineral exploration communities. In the present contribution we summarize the status of knowledge of the gemstone deposits and occurrences in Zimbabwe, paying particular attention to the geological settings in which they were formed. Synthesis of this regional geological approach reveals that there may be significant exploration potential for further gemstone occurrences, particularly in the extensive pegmatite fields and in Al-enriched orogenic belts that have undergone greenschist to granulite facies metamorphism. Further socio-economic and developmental initiatives will aid in optimizing the value generation from this important sub-sector

Regional-scale targeting for gold in the Yilgarn Craton: part 1 of the Yilgarn Gold Exploration Targeting Atlas

The Yilgarn Craton of Western Australia is well known for its gold endowment (more than 324 Moz, or 9730 t), including the giant Golden Mile deposit at Kalgoorlie (67.3 Moz, or more than 2000 t). Despite considerable success in the identification of near-surface gold during the periods 1890–1910 and 1985–2005, the discovery rate for gold in the Yilgarn has since slowed, a situation that is commonly attributed to exhaustion of economic near-surface resources. This view has led to a greater focus on exploration for ‘blind’ orebodies with tonnages and grades in excess of modern nearsurface resources. Exploration for blind orebodies relies on identification of a geophysical expression of subsurface ore, a geochemical expression of the ore in overlying transported sediments or sedimentary rock, or a conceptual approach that predicts the presence of ore on the basis of the geological controls of known near-surface mineralization. Although each approach has both advantages and limitations, this Atlas focuses on the conceptual approach. Eighteen targeting criteria for gold exploration at regional scale are addressed in the Atlas, with individual criteria ranging from well-established geological controls such as intersecting faults to more recent suggestions (e.g. proximity to late-stage basins). Each criterion (and sub-criteria, in some cases) is examined in relation to gold distribution, at scales ranging from craton to domain, using a GIS platform. The results of the spatial analyses are summarized in the form of maps, and histograms (containment analyses) or curves (proximity analyses) showing ounces per square kilometre and deposits per square kilometre. Full results of the spatial analyses are presented in the Appendix. \ud \ud The results of this study suggest that the four most effective targeting criteria for gold in the Yilgarn Craton are proximity to intrusions of Mafic Group granites, elevated fault density, proximity to regional fault bends, and the well-known preference for gold to be found in greenstone belts rather than in intervening areas of granite. A fuzzy logic approach has been used to combine and weight the results of spatial analyses for these four criteria to produce spatial arrays of prospectivity values that have then been used to produce regional prospectivity maps for gold in the Yilgarn Craton and in the Eastern Goldfields Superterrane. Correlation coefficients relating gold endowment to prospectivity values range from 0.38 to 0.78, depending on the parameter used to measure endowment. The results for the Yilgarn Craton as a whole are better than those for the Eastern Goldfields Superterrane, and results expressed in terms of deposit density are better than those expressed as ounces per square kilometre. Although spatial analyses for domes (containment) and late-stage basins (proximity) suggest a relationship to gold mineralization, prospectivity maps produced after their addition to the previous fuzzy logic analysis compromised the correlation between gold endowment and prospectivity value. \ud \ud This report forms the first part of a three-part Atlas. Parts two and three deal with district-scale and deposit-scale targeting, respectively

What can we learn from GIS-based prospectivity mapping in mature terranes?

This study used robust spatial statistics to systematically examine regional-scale targeting criteria that have been suggested for orogenic gold mineralization in the Yilgarn Craton. The spatial relationships between known orogenic gold mineralization and a series of targeting criteria were examined using both the number and endowment of the deposits. Both proximity and containment analyses were performed and a series of measures used to assess whether the geological feature being tested could be considered a good regional-scale targeting criteria. \ud \ud Spatial analysis indicated the best targeting criteria for orogenic gold mineralization in the Yilgarn Craton are: containment within greenstones, proximity to mafic group granite intrusions, proximity to regional-scale fault bends, and regional fault density. \ud \ud Using these targeting criteria, a fuzzy logic prospectivity map was generated. Fuzzy membership values were derived using the %endowment/%area statistic and the proximity and density threshold values set from the class at which the maximum %endowment/%area statistic was observed.\ud \ud The spatial analysis revealed some surprising results, with several targeting criteria that were thought to have a strong relationship with orogenic gold mineralization in the Yilgarn craton shown to have weak relationships when a robust statistical analysis was performed. Due to the analysis focussing on the relationship between endowment and the geological features, it raised the question as to how much influence the Golden Mile had on the results. Separate analysis of the super-giant and giant deposits compared to the remaining smaller deposits indicated that controls on the large deposits may differ to the controls on smaller deposits