Portable Infrared Spectroscopy as a Tool for the Exploration of Gold Deposits in Tropical Terrains: A Case Study at the Damang Deposit, Ghana
Alistair J. R. White, Vicki M. Robb, Laurence J. Robb, David J. Waters, 2010. "Portable Infrared Spectroscopy as a Tool for the Exploration of Gold Deposits in Tropical Terrains: A Case Study at the Damang Deposit, Ghana", The Challenge of Finding New Mineral Resources: Global Metallogeny, Innovative Exploration, and New Discoveries, Richard J. Goldfarb, Erin E. Marsh, Thomas Monecke
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Gold mineralization at the Damang deposit is unique among known deposits in Ghana, comprising two distinct styles of mineralization. These include a stratigraphically controlled auriferous quartz-pebble metaconglomerate that is overprinted by later gold contained in a complex fault-fracture vein array with surrounding hydrothermal alteration. A systematic study using portable, field-based infrared reflectance spectroscopy has proven to be a valuable exploration tool at Damang. Spectral parameters such as the ferrous-iron response, the AlOH/H2O absorption depth ratio, and automated mineral identification successfully distinguish metasedimentary and metadoleritic lithologic units at Damang. Systematic variations in these parameters, together with the water/OH absorption depth, both downhole and in three-dimensional models, provide vectors to gold mineralization. The spectral parameters AlOH wavelength and MgOH wavelength are used to define the regolith profile at Damang, throughout which the ferrous-iron response parameter provides a reliable indicator of gold mineralization. All recorded changes in spectral parameters can be linked to sample petrography and are supported by mineral-chemical data. These results show that portable infrared spectroscopy can be used in a variety of roles, including regolith mapping, geologic mapping and logging, and recognition of hydrothermal alteration patterns, as each lithology and alteration style exhibit distinct and identifiable spectral characteristics. These spectrally derived alteration proxies indentify a broader zone of potential gold mineralization than gold grades alone, providing a larger target for exploration. The rapidity of data collection and ease of analysis of spectral data make infrared reflectance spectroscopy a useful methodology that can be readily incorporated into both preexisting and established exploration programs in other tropical terrains.
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The Challenge of Finding New Mineral Resources: Global Metallogeny, Innovative Exploration, and New Discoveries
There seems to be general consensus throughout much of the global mining industry that the supply of base and precious metals and some other commodities (e.g., ferrous metals, uranium) is reasonably well assured into the oreseeable future because increases in total resources continue to keep pace with or outstrip global consumption. The basic assumption is that market forces and technological advances will combine to promote and perpetuate this trend (e.g., Tilton, 2003; Crowson, 2008). Others disagree, however, andpredict that shortages are inevitable if metal consumption continues to escalate (Beaty, 2010).
It is already becoming clear that many known resources seem unlikely to be mined, irrespective of commodity prices, because of their low grade and/or quality. Hence, many mineral resources that were uneconomic in the early 2000s are likely to remain so, both today and into the foreseeable future because of increases in both the direct (e.g., energy, labor) and indirect (e.g., environmental, social) production costs. This situation is being further exacerbated by the perceived decrease, over at least the past decade, in the discovery rate of base and precious metal resources measured in terms of both the number of major discoveries made and the exploration dollars spent per discovery (e.g., Dummett, 2000; Horn, 2002; Schodde, 2004). There is also a suggestion that the discoveries made are, on average, becoming both smaller and lower grade. Therefore, it seems reasonable to ask whether current exploration practices and success rates are going to be adequate to provide for the massive increases in metal consumption that world population growth, rising living standards, and rapid industrialization and urbanization in China, India, and other emerging markets appear to portend. For example, Rio Tinto's projections suggest that "by 2030 the additional supplyrequired will be equivalent to replicating the iron ore output of the Pilbara region of Australia every five years, adding another aluminium production complex the size of Canada's Saguenay every nine months, and developing another copper mine the size of Escondida in Chile each year. Future energrequirements are such that an entire Hunter Valley coal supply chain needs to be created each year plus a uranium mine the size of Ranger every four years" (Albanese, 2010, p. 7). Clearly, the exploration business has to become increasingly effective if it is to rise to the challenge of finding mineral resources of the right caliber to assure that this burgeoning demand can be adequately satisfied.