Engineering & Mining Journal

JUN 2013

Engineering and Mining Journal - Whether the market is copper, gold, nickel, iron ore, lead/zinc, PGM, diamonds or other commodities, E&MJ takes the lead in projecting trends, following development and reporting on the most efficient operating pr

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E X P LO R AT I O N Figure 2—Histogram from the Neftex Minerals Database showing the distribution of mineral deposits through time. allows for detailed examination of magmatic and tectonic events across a region throughout geological time. Within the earth model, Neftex has developed a geologically focused minerals database aimed at confronting exploration demands. So far, some 3,500 deposits (porphyry copper-gold, epithermal and orogenic gold and volcanogenic massive sulphides) are incorporated, with data confirming accurate locations, precise mineralization ages, bibliographic information and an interpretation within a global geodynamic framework that allows for greater insight into the spatial, temporal and geological contexts of ore deposits. The range of mineral deposit ages contained within this database is shown in Figure 2. The peaks represent the number of mineral occurrences recorded against time with specific clusters at around 35, 70, 350, 490, 1,800 and 2,700 Ma. The ability to geodynamically reconstruct and better understand these important Phanerozoic mineralization periods, and in the future the Precambrian, will no doubt lead to new formation models and will greatly assist mineral exploration, Dr. Nicoll believes. Correlations Confirm the Concept The validation of any model relies on its ability to identify known mineralization and to predict targets defined by the established criteria. For example, an exploration model reconstructing the Upper Devonian of Eurasia (Figure 3) shows the Siberian and LaurentianBaltica cratons separated by the Uralian (Urals) Ocean. Regions of volcanic arc magmatism associated with subduction of the oceanic plate are shown in yellow (representing a belt 80 to 250 km inboard of the subduction trench). This belt represents areas that have a higher chance of developing styles of subduction-related mineralization such as porphyry copper, epithermal gold and potentially even back-arc basin volcanogenic massive sulphides. Also shown are the predicted former positions of mid-oceanic ridges (thin red lines), with the areas where these are subducted being highlighted in solid black. These intersections potentially represent excellent targets for areas of mineral enrichment. The ability to understand the detailed construction of mineral belts in their palaeo-geographic context throughout geological time can highlight areas that are forming along trends to known mineral deposits, and therefore also high-grade areas with a higher exploration potential, and can reduce geological and financial risks. Dr. Nicoll pointed out that the earth model is dynamic, in that the geology at any one location will change over time due to plate tectonics. For example, if all Phanerozoic volcanic arcs are palaeo-geographically delineated and reconstructed to their present-day locations, then some of them (from different geological time periods) will overlap in areas where subduction was pro- longed, or where subduction-related magmatism repeatedly affected the same area of continental crust. Such areas are likely to have been prone to significant fluid flow and mineral enrichment, enhancing the potential for large-scale mineral deposits, he said. According to Neftex, the results of this work, mapping out volcanic arcs through time, shows a remarkable agreement with around 85% of the known Phanerozoic porphyry copper, epithermal gold and volcanogenic massive sulphide deposits worldwide, thereby providing a predictive framework and global road map for future exploration. In addition to the obvious use of such models directly in the mineral exploration sector, derivative products could also be of use to the financial sector to help guide, understand and mitigate geological risk, the company suggests. Figure 3—Geodynamic reconstruction of the Upper Devonian Period showing the closure of the Uralian Ocean. Double red lines represent active mid-ocean ridges and purple lines are subduction zones with saw tooth edges indicating direction of plate motion. The yellow polygons represent areas that are likely to have experienced subduction-related magmatism and black sections indicate where mid-ocean ridges are being subducted during this time frame. Insert shows the correlation to known Ural mineral deposits (green squares = VMS, orange triangles = porphyry copper/gold). JUNE 2013 • E&MJ 81

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