Summary:
The mineral deposits of the Colquijirca Mining District, known as Cordilleran deposits, are porphyry copper related deposits.
The Colquijirca Mining District is situated on a stratigraphic sequence comprising rocks from the Excelsior Group phyllites, sandstones, and red conglomerates of the Mitu Group.
Over the years, the progression of mining has exposed the Colquijirca deposit, facilitating geological identification of the Tertiary basin. The area is characterized by asymmetric anticlines and synclines composed of carbonate and detrital rocks, attributed to the Eocene-Oligocene Calera Formation, which serves as the primary host of mineralization. The deposit also features volcano-clastic intercalations, such as ash tuffs, providing evidence of volcanic activity contemporaneous with sedimentation.
The Marcapunta diatreme-dome complex, located at the centre of the Colquijirca Mining District (Sillitoe 2000; Bendezú et al. 2003; Sarmiento 2004), is one of several Miocene volcanic edifices, including Cerro de Pasco and Yanamate. This complex consists of multiple dacitic lava-dome intrusions, accompanied by injections and explosion breccias and pyroclastic layers typical of diatreme conduits. These features are widely observed at depth.
Peripheral areas are marked by inward-dipping normal faults, suggesting that the entire volcanic edifice collapsed, likely before the main mineralization episodes (Bendezú et al. 2003).
Alteration
The generalized alteration of the diatreme-dome complex consists of quartz-alunite-dickitekaolinite ± pyrophyllite-zunyite-illite assemblages in mineralized areas, transitioning to kaoliniteillite ± (smectite)-sericite-chlorite-calcite assemblages outside of mineralized zones.
In the Marcapunta Volcanic Complex, advanced alteration has resulted in the formation of residual quartz cores, which are locally vuggy, surrounded by advanced argillic alteration halos dominated by quartz-alunite and kaolinite assemblages. Gold and silver, present mainly in veins and as oxide coatings, are predominantly contained within these vuggy quartz cores, which extend into the surrounding country rock.
The vuggy silica can be subdivided into quartz-alunite and argillic alteration zones that affect much of the Marcapunta area volcanic rocks. Quartz-alunite alteration is observed to post-date Au-(Ag)-bearing veins in several areas, suggesting that repeated episodes of silica-quartzalunite vuggy alteration and Au-(Ag) deposition occurred at Marcapunta.
Mineralization
The Colquijirca Mining District hosts two primary types of epithermal mineralization:
• Disseminated high-sulphidation Au-(Ag) mineralization hosted in volcanic rocks of the Marcapunta Complex.
• Sulphide-rich Cordilleran type polymetallic mineralization hosted in the carbonate rocks of the Pucará Group and Pocobamba Formation.
High Sulphidation Au-(Ag) Epithermal
High-sulphidation mineralization consists of oxide veinlets and disseminations hosted in vuggy silica. Typical gold and silver concentrations within vuggy silica range from 0.2 g/t to 3.0 g/t and 10 g/t to 70 g/t, respectively, with Ag/Au ratios varying from 10 to 30 (Vidal et al. 1997).
In the deeper portions of the vuggy silica, unoxidized Au-(Ag) mineralization is associated with less than 5% disseminated sulphides by volume. These sulphides form veins predominantly composed of pyrite-enargite, chalcocite, covellite, and sphalerite, accompanied by clays such as kaolinite, smectite, and/or illite.
Vuggy silica and surrounding quartz-alunite zones without veinlets contain minor amounts of Au-(Ag), indicating that most of the precious metals precipitated during veinlet formation.
Cordilleran Epithermal
Cordilleran epithermal deposits are characterized by high total sulphide content, comprising 30% to 50% of the volume on average. Pyrite is the most abundant mineral, forming during an early silica-pyrite stage, followed by enargite-pyrite, and concluding with late-stage chalcocite (Bendezú 2007).
Strongly oxidized zones, initially composed of enargite-pyrite, exhibit Ag/Au ratios ranging from 80 to 120, significantly higher than the 10 to 20 ratios observed in the disseminated Au-(Ag) minerals at Marcapunta.
Cordilleran-type mineralization in the Colquijirca Mining District exhibits distinct mineralogical zoning:
• Core zone: Cu-(Au-Ag), dominated by enargite, often associated with alunite assemblages.
• Intermediate zone: Cu-(Zn-Pb-Ag-Bi), dominated by chalcopyrite, sphalerite, and galena.
• External envelope: Zn-Pb-(Ag), composed primarily of sphalerite and galena.
Cordilleran type veins systematically cut the precious metal veins in the easternmost part of the Marcapunta Oeste project. Quartz-alunite zones formed during the high-sulphidation epithermal event contain Au-(Ag) veins, which were subsequently intersected by pyrite-rich veinlets (enargite) generated during the Cordilleran event.
The Colquijirca deposit comprises three distinct mineralized zones.
• The deepest part of the southwest sector of the North Pit exhibits a core with a tubular shape, primarily composed of enargite, along with varying amounts of pyrite and quartz.
• Surrounding the core is an envelope containing chalcopyrite and variable amounts of tennantite, as well as sphalerite and galena.
• This envelope is, in turn, surrounded by an extensive zone predominantly composed of sphalerite and galena. The latter zone, which extends predominantly to the north of the district, constitutes the bulk of the Colquijirca deposit (North Pit) currently under exploitation.
To the south of the North Pit, the enargite core extends for over 2 km, thickening and widening as it approaches the Marcapunta Volcanic Complex.
The Marcapunta Norte sector, located immediately south of the North Pit, represents an extension of the Colquijirca deposit. This sector is characterized by two internal zones:
• An enargite-dominated zone.
• A polymetallic zone, consisting of chalcopyrite, tennantite, sphalerite, and galena.
Unlike areas further south, the Marcapunta Norte sector has undergone supergene enrichment. This process has led to the formation of chalcocite bodies that have been superimposed on the enargite zone and, to a lesser extent, on the polymetallic zone. This enrichment has created a sector of relative mineralogical complexity, particularly in terms of intergrowth.
The Central Upper stratabound mineralized structure is hosted within the carbonate rocks of the Middle Member of the Calera Formation. Main features of this unit are:
• A sub-horizontal stratiform geometry with a strike of N160° and a dip of 6°N. It has an approximate length of 520 m, a width of 270 m, and an average thickness of 21 m. Breccia bodies and veins that intersect bedding are uncommon.
• Primarily consists of enargite with variable amounts of pyrite. Minor phases include luzonite, colusite, and small occurrences of chalcocite, tennantite, ferberite, and bismuthinite. The enargite-luzonite (Cu3AsS4) grades range between 1% and 3% Cu and 0.3% and 1% As. Silver grades range from 15 g/t to 30 g/t, while some internal sectors contain gold values between 0.3 g/t and 0.7 g/t. Gangue minerals include quartz, alunite, zunyite, and clays such as kaolinite, dickite, illite, and smectite.