Uchucchacua: A Major Silver Producer in South America
Ulrich Petersen, Oscar Mayta, Luis Gamarra, César E. Vidal, Angel Sabastizagal, 2005. "Uchucchacua: A Major Silver Producer in South America", Andean Metallogeny: New Discoveries, Concepts, and Updates, Richard H. Sillitoe, José Perelló, César E. Vidal
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Although known since at least 1897, Uchucchacua was first explored on a major scale by Compaúía de Minas Buenaventura since 1960. Narrow vein mining started in 1975, but orebodies discovered at depth enabled expansion to today’s 2,000-t/d operation, transforming “Chacua” into the largest primary silver producer in South America.
The ores occur in fractures and faults, as well as in pipes, irregular replacement bodies, and mantos hosted by Late Cretaceous limestone. Porphyritic dacite bodies are probably pre-, syn-, and postore. Most of the ore occurs in distal manganiferous exoskarn and limestone and is mineralogically diverse, consisting mostly of the following.
The grade of the ore mined varies between 16 and 20 oz/t Ag combined with about 10 percent Mn, 1.5 percent Zn, and 0.9 percent Pb. Between 75 and 80 percent of the reserves are high in silver and manganese, whereas about 7 percent contain high zinc and lead grades with only moderate silver and low manganese.
Logarithmic-grade graphs show very good positive linear correlations for zinc versus lead, moderate correlations for silver versus manganese, and arcuate correlation bands for silver or manganese versus zinc or lead. These relationships indicate that the outward zoning sequence is from lead-zinc to silver-manganese or vice versa. The corresponding longitudinal vein sections can generally be contoured unambiguously, showing that the bands of highest grades of lead and zinc coincide very well. The highest silver grades can be contoured convincingly as a band that is zoned outward and/or at a higher elevation than the lead and zinc bands. However, the manganese grades often require two high-grade bands: a main band that mostly coincides with the highest silver grades and a thinner upper band that may represent near-surface manganese enrichment.
Ore intervals in individual veins, pipes, and replacement bodies are up to 200 m in vertical extent. However, the elevations of these intervals change progressively, reflecting the overall geometry of the hydrothermal cell (or cells) responsible for the mineralization. In addition, postore faulting has displaced the ore intervals. As a result, ore has been found to date over a vertical interval of 600 m, between 4,730 and 4,040 m.
At surface, manganese oxide stains in the host limestone and limonite in fractures and faults indicate proximity to ore. Underground, multiple calcite veinlets constitute a guide to nearby orebodies. Geochemical anomalies of 60 to 80 ppm Ag have been documented up to 15 m from an orebody. By extrapolation, 10 ppm Ag anomalies may extend 25 m from ore, and 1 ppm Ag anomalies may attain 40 to 45 m. Ore continues to be found at depth as well as laterally and between known ore zones.
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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