The Colquijirca epithermal gold(-silver) and base metal district is located 310 km NE of Lima and 8 km south of Cerro de Pasco, near Tinyahuarco, in central Peru. Individual deposits include Colquijirca, Marcapunta, Bohorquez and San Gregorio which are distributed over a 9 km, NNE aligned, trend length.
The regional geology in the Colquijirca district comprises folded Permo-Triassic red-beds of the Mitu Group, overlain by the Triassic to Jurassic Pucará Group limestone and dolomite, which are succeeded by carbonate breccia, conglomerate and fresh-water limestone of the Eocene Calera Formation. This succession is intruded and overlain by dacitic domes-diatremes and pyroclastic rocks of the 12.4 to 12.7 Ma Miocene Marcapunta volcanic centre. The major north-trending Longitudinal fault which passes through both the Cerro de Pasco and Marcapunta volcanic centres, controlled basin morphology during both the Pucará and Calera Formation sedimentation.
The mineralisation of the Colquijirca district occurs within and adjacent to the Marcapunta diatreme-dome complex,and is composed of two distinct, spatially related mineralisation types, as follows:
i). Disseminated and limestone replacement, enargite-Au-(Ag) with associated intense advanced argillic alteration (occurring as quartz-alunite ledges with argillic halos) and local vuggy silica that is typical of high-sulphidation epithermal ores - this style is hosted exclusively within the volcanic center at Marcapunta. The alteration assemblage zonation outward from the core is: vuggy quartz to quartz-alunite ±dickite, illite-kaolinite ±montmorillonite and external chlorite-calcitc envelopes. Copper mineralisation surrounds the subsurface diatreme vent, flaring outward along the base of the dacitic domes of the Marcapunta volcanic center. Semimassive to massive quartz-pyrite bodies preferentially replace limestone breccia and conglomerate of the Calera Formation, sandwiched between the underlying Mitu Group sandstone and the overlying lava domes. Alunite samples related to the Au-(Ag) epithermal ores have been dated at 11.3 to 11.6 Ma. On a broader scale, the Marcapunta and related enargite Cu-As-Au deposits are zoned symmetrically northward into the Zn-Pb-Ag ores of Colquijirca and south-westward into the Zn-Pb-Ag San Gregorio deposit.
ii). An economically more significant mineralisation type, referred to as "Cordilleran base metal lode and replacement deposits", hosted by Mesozoic and Cenozoic carbonate rich rocks that surround the diatreme-dome complex. This mineralisation" is zoned outward through the following assemblages - pyrite-enargite-quartz-alunite to pyrite-chalcopyrite-dickite-kaolinite to pyrite-sphalerite-galena-kaolinite-siderite. Mineralisation has been dated at 10.6 to 10.8 Ma and is broadly stratabound in lodes that are in general <2 m in thickness, separated by shaly bands.
The time gap (around 0.5 My) between the two mineralisation styles in the district suggests they were formed by different hydrothermal events within the same magmatic cycle. The estimated time gap between the younger mineralising event (base metal mineralisation) at approx. 10.6 Ma and the ages of approx. 12.5 Ma obtained on biotites from barren dacitic domes flanking the vicinity of the diatreme vent, suggest a minimum duration of the magmatic-hydrothermal cycle of around 2 Ma.
The estimated resource at the Colquijirca open pit base metal mine in 1996 was 70 Mt @ 7% Zn, 2% Pb. Historical production has been approximately 13 Mt
(Source: Porter GeoConsultancy Pty Ltd, www.portergeo.com.au, 2009)