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.

The Cerro de Pasco Unit consists of two underground mines (Vinchos, Paragsha) and an open pit (Raúl Rojas). In 2012-2014 the open pit and underground mining operations wound down and processing shifted towards the treatment of old stockpiles which continues today.

In 2015, Volcan commissioned a precious metal leaching facility. The plant processes Cerro stockpiles with silver and gold content as well as in-situ oxides located at the edge of the south of the Raúl Rojas pit.

Vinchos Mine (beginning of Operation 1975).
Since 2016, Vinchos mine has been temporary suspended.

Paragsha underground mine (beginning of Operation 1902).
Since 2016, the Paragsha mine has been temporary suspended. The infrastructure for the pumping system remains operational and in good condition.

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.

Raúl Rojas open pit mine (beginning of Operation 1956).
In December 2015, the Raul Rojas mine has been temporary suspended.

Since 2016, processing of marginal ore from the stockpiles mined at the Raul Rojas open pit in previous years begins.

In 2019, marginal ore stockpiled from the Raúl Rojas Pit and sulfides ore from In-situ Oxides Pit was treated.

Sulphide Concentrators - Paragsha/San Expedito

Throughout the 2017 year, operational changes were made to increase production and improve the metallurgy of concentrates. In the crushing area, the classification of the marginal with high humidity was optimized, making it possible to continue operations. In the third quarter of the year, the construction of the sorting circuit was completed and the testing process for the treatment of the marginal began in 2018. This, together with the optimization projects for secondary / tertiary crushing, grinding and flotation, will mean an improvement of the quality of the grades, better recoveries and a greater volume of concentrates.

During 2018, the crushing, grinding and filtering circuits of the Paragsha plant continued to be used, the San Expedito Pb circuit was transferred to the Paragsha plant and the Zn circuit was expanded in San Expedito.

In 2019, the tonnage was progressively increased from 5,000 tpd (J ........