Enargite-Gold Deposits at Marcapunta, Colquijirca Mining District, Central Peru: Mineralogic and Geochemical Zoning in Subvolcanic, Limestone-Replacement Deposits of High-Sulfidation Epithermal Type
César E. Vidal, Rolando Ligarda, 2005. "Enargite-Gold Deposits at Marcapunta, Colquijirca Mining District, Central Peru: Mineralogic and Geochemical Zoning in Subvolcanic, Limestone-Replacement Deposits of High-Sulfidation Epithermal Type", Andean Metallogeny: New Discoveries, Concepts, and Updates, Richard H. Sillitoe, José Perelló, César E. Vidal
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The Marcapunta enargite-Au deposits are located in the center of the Colquijirca mining district, 310 km northeast of Lima and 10 km south of the Cerro de Pasco mine. The regional geology comprises folded Permo-Triassic red-bed deposits of the Mitu Group succeeded by Pucará Group limestone and dolomite of Triassic to Jurassic age, which are overlain by carbonate breccia, conglomerate, and fresh-water limestone of the Eocene Calera Formation. These units are intruded and overlain by dacitic domes and pyroclastic rocks of the Marcapunta volcanic center. The north-trending Longitudinal fault controlled basin morphology during both the Pucará and Calera Formation sedimentation as well as emplacement of the Cerro de Pasco and Marcapunta Miocene volcanoes.
The Marcapunta Cu-As-Au deposits are zoned symmetrically northward into the Zn-Pb-Ag ores of Colquijirca and southwestward into the Zn-Pb-Ag San Gregorio deposit. The Miocene volcanic center at Marcapunta is intensely altered to advanced argillic alteration assemblages in the form of quartz-alunite ledges with argillic halos. Alunite mineral separates have been dated at 11.6 ± 0.1 Ma by K-Ar and 10.6 ± 0.1 Ma by 40Ar/39Ar methods. The main silicification and quartz-alunite alteration are controlled by several prominent east-west fractures and attain thicknesses from a few centimeters to ~50 m. Mineral assemblages are zoned outward from a central zone of vuggy quartz to quartz-alunite ± dickite, illite-kaolinite ± montmorillonite and external chlorite-calcite envelopes. Copper mineralization surrounds an interpreted subsurface diatreme vent and flares outward along the base of the dacitic domes comprising the Marcapunta volcanic center. Semimassive to massive quartz-pyrite bodies preferentially replaced limestone breccia and conglomerate of the Calera Formation and are sandwiched between the underlying Mitu Group sandstone and the overlying lava domes.
The northern, western, and southwestern flanks of the Marcapunta volcanic center are characterized by a recently determined and drill-tested, crescent-shaped gravimetric high. Ore zones attain thicknesses as much as 100 m adjacent to the steep diatreme walls and thin laterally into discrete strata-bound manto and breccia horizons. Ore mineralogy is dominated by enargite, covellite, native gold, and several precious metal-bearing telluride phases. Hypogene chalcocite and digenite occur in a discrete lower manto beneath the enargite zone of western Marcapunta. Gold appears to be concentrated in the southwestern parts of the replacement bodies. Cu/As ratios increase from a homogeneous value of 3/1 in the Smelter area, immediately north of the volcano, to variable values of 4 to 40/1 in the western Marcapunta enargite and digenite-chalcocite mantos.
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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