Summary:
The Colquijirca mine consists of three important polymetallic deposits:
* Tajo Norte–Sur, which contains zinc, silver, lead, copper and gold;
* Marcapunta, which contains an auriferous mineralization in breccia oxides and an arsenic copper enargite mineralization as a continuation of the mineralized mantles of the Marcapunta mine;
* San Gregorio, which contains zinc.
The mineral deposits of the Colquijirca district belong to a member of the family of porphyry copper (Cu) related deposits known as Cordilleran deposits. These types of deposits, which are generally formed in the upper parts of a porphyry Cu, are fundamentally characterized by prominent zoning with internal parts that are dominated by Cu and external zones where Zn, Pb and Ag are the main economically-interesting elements. In the case of the Colquijirca district, and specifically the area between the Marcapunta Norte and Colquijirca sectors, such zoning generally consists of three zones, which mineralogically consist mainly of enargite in the internal parts; chalcopyrite in the intermediate parts; and sphalerite and galena in the external parts (El Brocal, 2021).
Due to the advance of stoping over the years, the Colquijirca deposit is currently exposed, which facilitates the geological identification of the Tertiary basin. Asymmetric anticlines and synclines composed of carbonate and detrital rocks, attributed to the Eocene-Oligocene Calera Formation, can be found and considered as the host of mineralization. The deposit also presents volcano-clastic intercalations (ash tuffs), which is evidence of volcanic activity that was contemporaneous to sedimentation. In addition, with the review of 5 drillholes, the Shuco conglomerate sequence of the upper Eocene has been identified in depth, which underlies Lower Calera and overlies in depositional contact with the Mitú sandstones of the Permian-Triassic (Megard, 1978). To the south, at Smelter and Marcapunta, the sequence is uplifted and intruded by domes and dykes of dacitic composition due to the diatreme, which shows strong advanced argillic alteration and is recognized as the focus of mineralization in the mining district.
The Marcapunta diatreme-dome complex, which is exposed in the center of the Colquijirca district (Sillitoe 2000; Bendezú et al. 2003; Sarmiento 2004), is one of a series of Miocene volcanic edifices, including Cerro de Pasco and Yanamate. It consists of multiple lava-dome intrusions of mainly dacitic composition. Injection and explosion breccias and pyroclastic layers, typical of diatreme conduits, are widely recognized at depth. The inward dipping normal fault, located in peripheral areas suggests that the entire edifice collapsed, probably before the main mineralization episodes (Bendezú et al. 2003).
Mineralization
The district hosts two main types of epithermal mineralization: (1) disseminated high-sulfidation Au–(Ag) mineralization, hosted by volcanic rocks from the Marcapunta complex, and (2) sulfide-rich Cordilleran polymetallic deposits hosted in the carbonate rocks of both the Pucará Group and the Pocobamba Formation.
High sulfidation Au-(Ag) epithermal.
Mineralization consists of oxide veinlets and disseminations hosted in vuggy silica. Typical gold and silver concentrations in vuggy silica are on the order of 0.2- 3 and 10-70 g/t, respectively (Vidal et al. 1997) and Ag/Au ratios vary from 10 to 30. The deep parts of the vuggy silica contain unoxidized Au- (Ag) minerals, which are composed of less than 5% of disseminated sulfides by volume, and sulfide veins composed mainly of pyrite-enargite, chalcocite, covellite and sphalerite with the presence of clays, mainly kaolinite, but also smectite and/or illite. The vuggy silica and surrounding quartz-alunite zones, which do not have veinlets, contain minor amounts of Au-(Ag), suggesting that most of the precious metals precipitated during veinlet formation.
Cordilleran Epithermal
A significant feature is the high total sulfide content, which fluctuates between an average 30 and 50% of the volume on average. The most abundant minerals are pyrite, which crystallized during an early silica-pyrite stage, followed by enargite-pyrite and, finally, late-stage chalcocite (Bendezú 2007). The strongly oxidized zones, originally composed of enargite-pyrite, show Ag/Au ratios ranging from 80 to 120, much higher than those found in the Au-(Ag) minerals disseminated throughout Oro Marcapunta (10 to 20). Another important characteristic of the Cordilleran type mineralization in the Colquijirca district is the mineralogical zoning:
1) A Cu- (Au - Ag) core dominated by enargite and generally accompanied by alunite assemblages.
2) An intermediate Cu- (Zn - Pb - Ag - Bi) zone dominated by chalcopyrite, sphalerite and galena; and
3) An external Zn - Pb- (Ag) envelope composed mainly of sphalerite and galena.
Cordilleran veins systematically cut the precious metal veins in the easternmost part of the Marcapunta Oeste project. The quartz-alunite zones developed during the high sulfidation epithermal event contain Au (Ag) veins, which were cut by pyrite-rich veinlets (enargite) generated during the Cordilleran event. In addition, most of the cavities within the vuggy silica contain intergranular enargite fillings from the Cordilleran stage, which in part destroy earlier Au-(Ag) veinlets with quartz-alunite assemblages.
The Colquijirca deposit exposes three zones. The deepest part of the southwest sector of North Pit shows a core of tubular shape, which is essentially constituted by enargite plus variable amounts of pyrite and quartz. This core has an envelope composed of chalcopyrite and variable amounts of tennantite, in addition to sphalerite and galena. In turn, this envelope is surrounded by a relatively extensive zone composed of sphalerite and galena. This last zone, whose largest extension is towards the north of the district, constitutes the bulk of the Colquijirca deposit (North Pit) currently in exploitation. To the south of North Pit, the enargite core extends for more than 2 km becoming thicker and wider as it approaches the Marcapunta volcanic complex.
The sector called Marcapunta Norte, located immediately south of North Pit, is the extension of the Colquijirca deposit. This sector is composed of two internal zones: The first is composed of enargite and that the second of polymetallic nature, i.e., of chalcopyrite, tenantite, sphalerite and galena. Unlike sectors located further south, the Marcapunta Norte sector is characterized by the fact that it has undergone a process of supergene enrichment. This process has generated chalcocite bodies, which have been superimposed to the enargite zone and to a lesser degree, to the polymetallic zone composed of chalcopyrite, tenantite, sphalerite and galena; this formed a sector of relative mineralogical complexity, especially in terms of intergrowths.
The mineralized structure of the Central Upper Mantle is hosted in carbonate rocks of the Middle Member of Calera Formation and has a sub-horizontal stratiform geometry of N160° strike and 06N dip. The structure has an approximate length of 520m, a width of 270 m and an average thickness of 21 m. The occurrence of structures secant to the bedding, such as breccia bodies and veins, is less common.
Mineralogically, the Central Upper Mantle consists essentially of enargite, accompanied by variable amounts of pyrite. Less important phases include luzonite, colusite and an even small quantity of occurrences of chalcocite, tenantite, ferberite and bismuthinite.
The Central Upper Mantle contains enargite-luzonite (Cu3AsS4) with grades varying between 1 and 3% Cu and 0.3 and 1% As. Ag contents vary between 15 and 30 g/t. Some internal sectors of the Central Upper Mantle show gold values between 0.3 and 0.7