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.

Pb-Zn-Ag-Cu Mineralization at the Cerro the Pasco Mine.
Epithermal base metal mineralization at Cerro de Pasco occurred principally in carbonate rocks along the eastern margin of the magmatic complex (Baumgartner et al., 2008; Einaudi, 1977). An important structure related to the Cerro de Pasco mineralization is a NS-trending, funnel-shaped massive pyrite-quartz body that replaced mainly carbonate rocks from the Pucará Group, and in less measure, the diatreme dome complex (Baumgartner, 2007; Einaudi, 1977; Ward, 1961). Five main pipe-like massive pyrrhotite-dominated bodies reaching up to 150 m in width have been identified. They are hosted by the pyrite-quartz body and the Pucará carbonate rocks and locally crosscut the diatreme breccia. The pipe-like pyrrhotite bodies are characterized by low sulfidation state assemblages (pyrrhotite + Fe-rich sphalerite + arsenopyrite, as well as pyrite + Fe-rich sphalerite). The pipes core zone, only observed at deep levels, is composed of the assemblage pyrrhotite-quartz-wolframite. The intermediate-level assemblage consists of pyrrhotite-sphalerite-chalcopyrite-stannite (Einaudi, 1977). The outer zone, which is present over the entire vertical extent of the pyrrhotite bodies and which includes the Zn-Pb ore, consists of the association pyrrhotite + Fe-rich sphalerite + arsenopyrite with minor marcasite, tennantite, chalcopyrite, chlorite, muscovite, siderite, and calcite. According to Baumgartner et al. (2008, 2009), high- sulfidation mineralization took place prior to the formation of the pyrite-quartz body. The mineralization consists of EW-trending Cu-Ag- Au-Zn-Pb) enargite pyrite veins hosted by the diatreme breccia and includes at least eight zoned Zn-Pb-(Ag) and Ag-Cu-Bi replacement orebodies in the eastern part of the deposit. These orebodies replaced carbonates and overprinted Fe-rich sphalerite and galena rims from the pyrrhotite pipes. The replacement bodies follow sub vertical faults trending N35° E, N120° E, and N170° E and locally favorable Pucará beds, mainly dolo-arenite layers.

During 2015, the processing of mineral reached 399,000 tons, with ore grades of 3.94% Zn, 1.52% Pb, 0.05% Cu and 3.89 oz Ag/MT, including the ore from the underground mine, marginal ore from the Raúl Rojas open pit, and the minerals from Vinchos and Islay. In December 2015, underground mine operations were suspended, as was the processing of marginal ore from the open pit.

During 2015, there was no production of ore at the Vinchos mine. Operations are temporarily suspended at this mine.

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.

In 2015, 94% of the mineral extracted at the Chungar unit was processed at the Animón plant, while 6% was processed at the San Expedito plant, part of the Cerro de Pasco unit.

During 2014 the Company continued using crushing, milling and filtration circuits at the Paragsha plant to increase the treatment capacity of the San Expedito plant. In addition, recovery improved after installation of a flash SK-240 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 plant. However metallurgical testing was continued in order to improve recovery and zinc grade, given the higher copper head grade of the ore.