Geology, Mineralization, and Genesis of the La Quinua Gold Deposit, Yanacocha District, Northern Peru
Patrick M. Mallette, Raul E. Rojas, Alberto R. Gutierrez, 2005. "Geology, Mineralization, and Genesis of the La Quinua Gold Deposit, Yanacocha District, Northern Peru", Andean Metallogeny: New Discoveries, Concepts, and Updates, Richard H. Sillitoe, José Perelló, César E. Vidal
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With premining reserves plus resources of 10.2 million oz (Moz) of gold, 424 million metric tons (Mt) at 0.75 g/t, La Quinua is one of several world-class gold deposits that comprise the Yanacocha mining district in the Cajamarca province, northern Peru. Unlike volcanic-hosted high-sulfidation gold deposits that characterize much of the Yanacocha district, La Quinua is hosted by unconsolidated gravel. In 1996, Newmont and Minera Yanacocha geologists discovered gold-bearing gravel while drilling an alluvial basin to test “blind” basement targets. Exploration and development drilling proceeded rapidly; 1 yr after initial discovery the reserve plus resource stood at 7 Moz. Production began in mid-2001.
At La Quinua, there is infilling of coarse clastic sediments in two structural basins situated along the western flank of the Yanacocha Sur and Yanacocha Oeste gold deposits. Gravel fans reach a maximum thickness of 350 m on the downthrown side of the basin-bounding La Quinua fault. Sediments fine down gradient from chaotic boulder gravel in proximal facies to gravelly silt and sand in distal facies. Bedding becomes more pronounced down gradient with a decrease in bed thickness. Five deposit-scale stratigraphic units are recognized. These include, from bottom to top, (1) regolith directly overlying basement rocks, (2) high-to low-energy deposits of clay-and alunite-bearing sand and gravel, (3) low-energy deposits of organic-rich mud, peat, and bog iron, (4) ferruginous gravel, and (5) high-energy deposits of pebble-cobble-boulder gravel grading distally to fine-grained sandey silt.
La Quinua gold was derived from erosion, transportation, and deposition of gold particles and mineralized clasts from the Yanacocha Sur and Yanacocha Oeste deposits. However, a portion of the gold may have resulted from chemical mobilization and reprecipitation. Gold particles are mostly micron sized, liberated within mud matrix, and disseminated within mineralized clasts, although liberated particles to 0.2 mm have been observed. Gold is disseminated throughout the deposit with only gradual lateral and vertical grade transitions. Placer “paystreaks” have not been encountered. The La Quinua gold-trapping system was efficient; grade dilution from source deposits is <25 percent. The Ag/Au ratio is 6/1, compared to >10/1 for the Yanacocha Sur and Yanacocha Oeste deposits. Copper and iron are locally enriched in specific stratigraphic horizons. Copper values up to a few percent are associated with detrital and authigenic minerals. Copper mineralization is erratic and not considered economic and may present future ore-processing challenges. Authigenic iron is present as bog-iron lenses, ferricrete cement, and gravel matrix impregnation.
La Quinua formed in response to dynamic interaction of climate and tectonics. A subsiding tectonic basin preserved gold-bearing sediments during periods of intense mechanical weathering in the adjacent highlands. Cold alpine climatic conditions resulted in two pulses of rapid sedimentation and basin infill. Temperate conditions resulted in diastems marked by organic accumulation and surficial iron deposits. La Quinua is characterized by a paucity of channel deposits, lack of coarse-grained placer gold, and preservation of fine-grained gold. Structural offset and warping of sedimentary units indicates that basinal tectonism continued after deposition of the gravel sequence.
Gold production from La Quinua will exceed 1 Moz/yr during 5 yrs of an 8-yr mine life, with a peak of nearly 2.5 Moz predicted in 2006. Light blasting improves mining efficiency. Oxide ore is belt agglomerated prior to placement on the leach pad. The majority of gold is recovered in carbon columns with a smaller portion recovered in a Merrill-Crowe facility. Mine equipment includes Hitachi 5500EX shovels, Cat 992 loaders, and Cat 785 and 793 haul trucks.
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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