Discovery History of the La Colosa Gold Porphyry Deposit, Cajamarca, Colombia
Chris Lodder, Ruben Padilla, Robert Shaw, Timoleon Garzon, Edwin Palacio, Rudy Jahoda, 2010. "Discovery History of the La Colosa Gold Porphyry Deposit, Cajamarca, Colombia", 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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In May 2003, AngloGold Ashanti began greenfields exploration in Colombia with a team of four geologists. By 2007, the program employed 127 field geologists covering about 10.5 million hectares (Mha) with systematic reconnaissance exploration. To date, the result of this work is the discovery of several gold deposits, the most important being La Colosa, containing an initial resource of 381.4 million metric tons (Mt), grading 1.00 g/t Au or 381.4 t Au, using a 0.3 g/t cut-off grade.
The La Colosa deposit is a gold-only porphyry system related to a late Miocene multiphase porphyritic diorite-granodiorite complex. Gold grades exceeding 1 g/t are associated with early dioritic phases that are altered to potassic and sodic-calcic mineral assemblages. Potassic and sodic-calcic alteration also affects later diorite porphyries, but gold grades are, on average, <0.4 g/t. A late granodiorite porphyry is mostly barren, with only erratic anomalous gold grades, which are all <0.4 g/t, and weak to moderate propylitic and intermediate argillic alteration. The deposit contains >5 vol percent magnetite. Pyrite content varies between 3 and 5 vol percent. Gold is mainly contained within pyrite. Copper and molybdenum contents are generally at background values for diorite.
The La Colosa deposit is a grassroots discovery. It was made by AngloGold Ashanti geologists only 18 months after the initiation of a regional exploration program in the defined Mariquita target region. Discovery is the result of systematic regional data synthesis, conceptual target generation, and disciplined, multiphase, field-based, follow-up, which included stream sediment geochemistry, prospecting, rock chip sampling, and drilling.
Early target generation work by AngloGold Ashanti, undertaken at a northern Andean scale between 2000 and 2003, focused field activities into the most prospective regions of Colombia, based not only on geology and mineral potential but also upon factors that would lead to the discovery and eventual development and operation of a successful, socially, and environmentally responsible mining operation. The exploration strategy has maintained a systematic methodology that includes conceptualization, reconnaissance stream sediment surveys and related prospecting, target definition, target drilling and conceptual economic study, and finally prefeasibility and feasibility studies. Clear decision points were established at the end of each work phase, always keeping in mind the company’s minimum economic target criteria.
The key factors that led to the discovery of La Colosa included the execution of a well-planned business and exploration model, with recognition of the “first mover” advantage; the acquisition of a large land position with respect to legal exploration tenure, covering essentially all of the deemed prospective areas; implementation of exploration from the regional scale, working down to the prospect scale, instead of vice versa; maintaining a disciplined and systematic field-focused approach; use of a skilled exploration team; and maintaining a long-term (>5 yrs), adequately funded view to exploring in frontier mineral exploration regions.
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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.