Discovery and Geology of the Toki Porphyry Copper Deposit, Chuquicamata District, Northern Chile
Sergio L. Rivera, Rubén Pardo, 2005. "Discovery and Geology of the Toki Porphyry Copper Deposit, Chuquicamata District, Northern Chile", Andean Metallogeny: New Discoveries, Concepts, and Updates, Richard H. Sillitoe, José Perelló, César E. Vidal
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The Toki porphyry copper deposit is located in the Chuquicamata district, northern Chile, approximately 15 km southwest of the Chuquicamata mine. It is the newest giant copper deposit discovered in Chile and one of the most significant base metal exploration discoveries worldwide in the last 10 years. Inferred resources at Toki are >2,400 million metric tons (Mt) at ~0.5 percent Cu, one-third of it oxide and mixed oxide-sulfide mineralization.
The discovery was made by Codelco-Chile in 2000, as a result of a systematic exploration program begun in 1998 in the Chuquicamata district. During that year, all available geologic information was analyzed, a simple district geologic model was assembled, and exploration targets were conceptually evaluated and prioritized. The program was guided by the belief that remaining potential in the district was restricted to covered zones, and the West Fissure paradigm needed to be changed. One of the highest ranked targets was the Pampa Genoveva covered area, where inspection of core from holes drilled by the Chuquicamata division at the beginning of the 1990s, some 3 km north of the eventual Toki discovery, showed subtle but encouraging copper geochemistry and alteration features. An exploration model was generated and quickly tested by drilling reverse- circulation scout holes on a 1-km grid. These holes revealed evidence of porphyry-style alteration-mineralization and high copper contents in two sectors. They were followed-up by more closely spaced holes, resulting in the discovery of the Genoveva deposit by the end of 1999 and the Toki deposit at the end of 2000. Together with Opache, discovered in 1996, the deposits comprise the Toki cluster.
The geologic setting of the Toki deposit consists of Paleozoic schist and Permian to Triassic volcanic rocks covered by Jurassic to Early Cretaceous marine and terrestrial sedimentary rocks, intruded by Eocene stocks. The older stratified units are folded and faulted, following a dominant north-northeast structural trend. The deposit is completely concealed by 120 to 200 m of Miocene gravel and lacustrine sediment. Plan dimensions of the deposit are 2.5 × 1 km. The tested mineralization has a vertical extent of ~800 m, one-half of which is oxide, mixed oxide-sulfide, and supergene enriched sulfide mineralization, with the other half being a hypogene copper-bearing sulfide zone. The alteration and mineralization events are related to emplacement of a multiphase tonalite porphyry complex hosted by a Permian to Triassic bimodal volcanic sequence and late Eocene equigranular intrusive rocks. Hypogene mineralization is zoned laterally from a core of bornite (-digenite) and chalcopyrite to a zone of chalcopyrite and pyrite. Alteration displays a centrally located, pervasive potassic assemblage related to moderate to intense quartz-K-feldspar veins, grading outward to propylitization. Biotite from the potassic zone yields a K-Ar age of 37.3 ±1.3 Ma. A series of conspicuous northwest-trending D-type veins associated with sericite alteration crosscuts the earlier hypogene sulfide zoning pattern and represents a late hydrothermal event. Sericite from a D vein halo yields a 40Ar-39Ar age of 34.52 ± 0.20 Ma. The supergene profile consists of an upper 100- to 160-m-thick oxide zone (malachite, chrysocolla, atacamite, wad), locally interrupted by leached D veins and parallel faults. This is followed below by irregular, structurally controlled, mixed oxide-sulfide (atacamite-chalcocite), and weakly to moderately enriched chalcocite-covellite zones, between 50 and 200 m thick. Postmineral, northwest-trending faults interrupt the continuity of the mineralized body, which is tilted to the south. At the district scale, the mineralizing events that generated the Toki deposit at ~37 Ma preceded emplacement of the giant Chuquicamata porphyry copper complex, dated between 35 and 31 Ma. Evidence from Toki confirms that this world-class copper district is characterized by several discrete pulses of intrusion and mineralization.
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A variety of metals and deposit types define the metallogeny of the Andes from Colombia through Ecuador, Peru, and Bolivia to Argentina and Chile, although porphyry copper and epithermal gold deposits undoubtedly predominate and will continue to do so. Discoveries over the last 30 yrs or so, predominantly in the central Andes and especially Chile, have been made using routine, field-based geologic and complementary geochemical methods, a situation that is considered unlikely to change radically in the foreseeable future. The only clearcut evolutionary change is the increased number of deposits being discovered beneath pre- and postmineral cover. The predictive capacity of conceptual geology has had minimal impact on the Andean discovery record but is thought to offer much promise for the future. This introductory article selects mineralization styles and relationships as well as some broader metallogenic parameters as simple examples of geologic concepts that may assist exploration. Emphasis is placed on porphyry copper ± molybdenum ± gold and high-, intermediate-, and lowsulfidation epithermal gold ± silver deposits, although reference is also made to several carbonate rock-hosted precious and base metal deposit types and styles as well as subvolcanic tin, volcanogenic massive sulfide, and slate-belt and intrusion-related gold deposits. Particular emphasis is placed on the potential for exceptionally high grade porphyry copper, porphyry gold, epithermal gold, and subvolcanic tin deposits. Deposits resulting from the oxidation, enrichment, and chemical transport of copper and zinc and mechanical transport of gold and silver during supergene weathering are also briefly highlighted.
Si bien la metalogenia de los Andes de Colombia, Ecuador, Perú, Bolivia y Chile se encuentra definida por una gama de metales y estilos de mineralización, son los depósitos tipo pórfido de cobre y epitermal de oro los que dominan en el presente y continuarán prevaleciendo en el futuro. Los descubrimientos de los últimos 30 años, predominantemente en los Andes centrales y especialmente en Chile, han sido realizados mediante métodos geológicos rutinarios de campo, generalmente complementados satisfactoriamente por métodos geoquímicos. Se estima que esta situación difícilmente experimentará variaciones radicales en un futuro cercano. El único cambio destacable en esta historia evolutiva está dado por el aumento apreciable de descubrimientos de depósitos cubiertos, bajo cobertura pre o postmineral. A nivel andino, la capacidad predictiva de la geología conceptual ha tenido un impacto mínimo en el número total de descubrimientos, aunque se piensa que su uso debiera garantizar buenas perspectivas futuras. El presente artículo