The historic Cerro de Pasco deposit, located on the Andean Plateau in Central Peru, is one of the largest polymetallic resources in the world. Carbonate-hosted Zn-Pb-(Bi-Ag) ores presently being mined from the eastern part of the open pit contain Fe-poor sphalerite and display argillic to advanced argillic alteration, including aluminumphosphate-sulfate (APS) minerals.

The oldest exposed rocks in the Cerro de Pasco district are slightly metamorphosed Devonian shales and sandstones of the Excelsior Group. They are overlain by Permo-Triassic red beds (sandstones and conglomerates) of the Mitú Group. Over the half eastern part of the district, the Mitú Group is covered by a several hundred meter-thick Upper Triassic Lower Jurassic carbonate sequence belonging to the Pucará Group.

The earliest stage formed an extensive replacement body of quartzpyrite on the eastern margin of the vent. Only minor sphalerite was deposited during this stage. Probably as part of the quartz-pyrite stage, pyrrhotite replaced the central portions of the pyrite-quartz body in steep, pipe-like zones, grading out to ore consisting of Fe-rich sphalerite and galena. E W enargite-pyrite veins in the western area truncate this early mineralization stage.

Our working hypothesis is that sphalerite-3 orebodies, largely associated with advanced argillic alteration (characterized by an assemblage of kaolinite, quartz, dickite, hinsdalite, svanbergite) and the E-W trending high-sulfidation enargite-bearing veins that cut the silica-pyrite body were formed during the same stage. Alunite occurring within silica-pyrite bodies in the western face of the open pit nearby enargite veins also belong to this stage.

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.

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.

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.

Treatment capacity and yield 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.