Summary:
The Cerro de Pasco deposit is a complex epithermal polymetallic deposit with base and precious metal mineralization, mainly silver, characterized by vein, breccia-hosted, and carbonate-replacement mineralization. This deposit type has also been referred to as a “Cordilleran base- metal deposit” type (Baumgartner et al., 2008). The term “Cordilleran” was first applied to base metal-rich polymetallic vein deposits (Sawkins, 1972; Einaudi, 1982; Guilbert and Park, 1985; Bartos, 1987; Fontboté and Bendezú, 2009; Catchpole et al., 2015). Cordilleran deposits have also been referred to as Butte-type vein deposits (Meyer et al., 1968), polymetallic veins, and zoned base metal veins (Einaudi et al., 2003). Because the mineralization in many districts is dominantly mantos and not veins, and they commonly contain gold and silverin addition to base metals, Bendezú et al. (2008) and Fontboté and Bendezú (2009) prefer the more general term Cordilleran polymetallic deposits.
Rottier et al. (2018b) describe three successive mineralization stages at Cerro de Pasco resulting in epithermal low- to high-sulphidation mineral associations emplaced at a paleodepth from <500 m to 1,500 m in the shallow part of a porphyry system:
1) Pyrrhotite pipes grading outward to sphalerite and galena replacement bodies (Stage A).
2) Quartz-pyrite veins (Stage B1) and a funnel-shaped massive replacement body of pyrite-quartz (Stage B2) with quartz-sericite ± kaolinite alteration.
3) Well-zoned zinc-lead-(bismuth-silver-copper) carbonate-replacement (Stage C1) and east-west trending copper-silver-(gold-zinc-lead) enargite-pyrite veins (Stage C2) accompanied by advanced argillic alteration.
Rottier et al. (2018b) suggest that fluids associated with mineralization stages A, B1, B2, and C1 are the result of mixing between a moderate-salinity metal-rich magmatic fluid and a low-salinity fluid at the site of mineral deposition. The moderate-salinity metal-rich magmatic fluid results from mixing at depth between metal-rich hypersaline fluids and low-salinity magmatic fluids exsolved late in the lifetime of the magmatic-hydrothermal system. The moderate-salinity metal-rich magmatic fluid resulting from this deep mixing rose to the epithermal environment, where it in turn mixed with low-salinity fluids that were stored below the paleowater table and had similar temperatures to the moderate-salinity fluid. The similarity between fluid compositions and evolution during stages A, B1, B2, and C1 contrasts with their significantly different mineral assemblages that are controlled by changing fO2, pH, fS2, and temperature (Rottier et al., 2018b).
In contrast, enargite-pyrite veins of Stage C2 were formed by the ascent of CO2-bearing, vapor-like fluids that mixed with cold meteoric water. No interaction with the moderate-salinity, metal-rich magmatic fluids was noted (Rottier et al., 2018b).
The following description of the Cerro de Pasco deposit geology has largely been sourced from Baumgartner et al. (2008). The Cerro de Pasco Geology Department provided information to Baumgartner for her dissertation at the University of Geneva.
The weakly metamorphosed shale, phyllite, and quartzite of the Devonian Excelsior Group forms a north-south striking and north-plunging anticline, named the Cerro anticline on the western side of the Cerro de Pasco diatreme-dome complex. Permo-Triassic Mitu Group sandstone and conglomerate with pebbles of quartz and Excelsior-type argillaceous clasts (McLaughlin, 1924; Jenks, 1951) is observed at the south end of the Santa Rosa open pit.
A thick sequence (up to 1,000 m) of carbonate rocks of the Late Triassic Chambará Formation, part of the Pucará Group, includes mainly massive limestone with locally sandy intercalations, dolostone, black bituminous limestone, and beds with chert nodules. In the east wall of the open pit, the unit is principally composed of thickbedded, dark-coloured limestone and dolostone with local shale interbeds and siliceous concretions. A regional north-south fault (the Longitudinal Fault) juxtaposes the Excelsior Group metamorphic rocks against the Pucará Group sedimentary rocks. In the Cerro de Pasco mine area, the Longitudinal Fault is interpreted to be represented by high-angle, N 15° W-striking reverse faults.
West of the fault, a 2.5 km diameter Middle Miocene diatreme-intrusive dome complex was built up by a succession of magmatic, phreatomagmatic, and phreatic events. An early phase of explosive activity produced a diatreme-breccia known locally as Rumiallana agglomerate, which is the most common lithology in the magmatic complex. The Lourdes Fragmental unit to the southeast of the diatreme breccia at the Lourdes Shaft is considered as the first volcanic event.
The phreatomagmatic activity was followed by emplacement of dacitic to rhyodacitic lava-dome complexes along the western margin of the diatreme. East-west trending quartz-monzonite porphyry dikes cut the diatreme breccia and the magmatic domes. The dacitic porphyritic domes and quartz- monzonite porphyry dikes were emplaced between 15.4 Ma and 15.1 Ma (Baumgartner, 2007). The dikes do not propagate into the Excelsior shales west of the diatreme-dome complex; to the east, they locally crosscut the carbonate sequence.
Vertical breccia bodies, including the Cayac Norurga breccia and San Alberto breccia, cut the sedimentary sequence and contain angular clasts of Pucará carbonate rocks several centimetres in size and carbonate rock flour matrix. These breccia bodies follow a northeast-southwest trending corridor in the San Alberto area and can also be recognized in the north-south trending large pyrite-quartz body.
The end of the phreatomagmatic and magmatic activity at Cerro de Pasco is marked by the emplacement of numerous, 20 cm to 3 m wide, east-west trending, milled-matrix fluidized breccia dikes, occurring in various parts of the diatreme-dome complex.
Erosion removed part of the diatreme-dome complex, as well as the overlying rocks, as shown by the presence of collapse blocks of Mitu and Pucará Group rocks inside the diatreme and the absence of these rocks outside the diatreme. The total erosion from the middle Miocene to the present is estimated to be on the order of 500 m, as indicated by the fact that the pre-diatreme erosion surface in the Santa Rosa area is preserved below ~100 m of outflow deposits and by the diatreme size).
The major north-south trending Longitudinal Fault was probably already active during the deposition of the Pucará Group, which thickness is c. 3,000 m east of the fault and 300 m in the west. The Upper Cretaceous to Eocene Shuco member of the Pocobamba Formation occurs as breccia and conglomerate with Pucará clasts along the Longitudinal Fault, providing additional evidence for protracted fault movement.
A complex set of faults is prominent in the Pucará carbonate rocks in the Raúl Rojas open pit. The first set strikes N 120° E, dips 70° to 80° S and is present in the eastern part of the open pit. The second set strikes N 170° E, dips vertically, and is mainly present in the southern part of the deposit. The third fault set strikes N 35° E, dips 80° E, and is present in the northern open pit. The three fault sets are dextral and/or sinistral strike-slip faults and formed by compression in the later stages of folding.