Geology of the Cerro de Pasco Area.
At Cerro de Pasco, the magmatic center was emplaced directly west of a major high-angle, N 15° W-striking reverse fault (“the Longitudinal Fault”), mainly intruding weakly metamorphosed shales of the Middle Paleozoic Excelsior Group, the oldest lithological unit in the area, and polymictic conglomerates and sandstones of the Middle-Late Triassic Mitu Group (Spikings et al., 2016). East of the Longitudinal Fault there is a thick sequence (about 1,000 m) of massive carbonate rocks, mainly limestones with locally sandy intercalations, black bituminous limestones, and beds with chert nodules belonging to the Late Triassic Chambará Formation. The latter is part of the Pucará Group that overlies the Excelsior and Mitu groups (Rosas et al., 2007; Angeles, 1999). The sedimentary sequence was folded prior to the Mid-Miocene magmatism, thus creating the main structural feature in the area, the Cerro anticline with a north-south axis and plunging to the north (Baumgartner et al., 2008; Angeles, 1999).

The magmatic core of the Cerro de Pasco district consists of a large diatreme-dome complex, 2.5 km in diameter, which was formed by a succession of phreatomagmatic and magmatic events (Baumgartner et al., 2009; Rogers, 1983). An early phase of explosive activity produced a diatreme-breccia known locally as the Rumiallana agglomerate, which is the most common lithology in the magmatic complex and has been dated at 15.36 ± 0.03 Ma and was followed by emplacement at 15.40 ± 0.07 Ma of dacitic to rhyodacitic lava-dome complexes along the western margin of the diatreme (Baumgartner et al., 2009).

East-west–trending quartz-monzonite porphyry dykes cut the diatreme breccias and the magmatic domes. These dykes do not propagate into the Excelsior shales west of the diatremedome complex; to the east they crosscut locally the carbonate sequence. Two of these dykes have been dated at 15.35 ± 0.05 and 15.16 ± 0.04 Ma (Baumgartner et al., 2009). 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, E-W–trending, milled-matrix fluidized breccia dykes, occurring in various parts of the diatreme-dome complex.

Following this event, epithermal base metal mineralization took place, mainly in carbonate rocks along the eastern margin of the magmatic complex (Baumgartner et al., 2008; Einaudi, 1977). A striking feature of the Cerro de Pasco mineralization is the occurrence of an NS- trending, 1.5 km-long, 250 m-wide, and more than 550 m-deep, funnel-shaped massive pyrite- quartz body that replaced mainly carbonate rocks from the Pucará Group, as well as, subordinately, the diatreme dome complex itself (Baumgartner et al., 2008; Baumgartner, 2007). At least five main pipe-like, up to 150 m-wide, massive pyrrhotite-dominated bodies have been recognized. They grade outward into massive Fe- rich sphalerite (up to 80% in volume) and galena.

In 2015, ore production came from mine sectors having higher ore grades and a greater volume of reserves. Due to the characteristics of the terrain, conventional semi-mechanized mining techniques were employed, including wood bracing and shoring as well as cemented hydraulic backfill.

However, the underground mine is subject to ever-increasing deformation, which means that operations need to have greater operational safety. Therefore, mine operations are advancing more slowly and greater quantities of support features and materials are required. Also, higher cement ratios were used in the cemented hydraulic backfill mix, making it possible to access the nearby operational areas in a shorter time (twelve days) and recover higher percentages of mineral reserves.

Since 2016, the Paragsha mine has been temporarily suspended.

Operations at the Paragsha Plant were halted on December 23, 2012 as a result of lower production levels at the underground mine and as a consequence of the suspension of production at the Raul Rojas pit. In 2013, Paragsha’s crushing, milling and filtration circuits were used because of restrictions on these circuits at the San Expedito Plant.

Treatment capacity and yield at San Expedito plant were boosted by improvements in lead-silver recovery. These improvements include the installation of an SK-240 flash cell and an automatic lime dosing control system, as well as greater flotation time in the lead-zinc circuits with large cells transferred from the Paragsha concentrate plant.

During 2015, the crusher, milling and filtering circuits at the Paragsha concentrator plant continued to be operational, as were the zinc circuit at the Paragsha plant and the lead circuit at the San Expedito plant.