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Abstract

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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