Summary:
The mineralized Huachocolpa Uno deposit is on a sequence of Cenozoic volcanic rocks, belonging to the Huachocolpa Group, locally two units are recognized: Laymen and Mc Iver.
At Huachocolpa Uno, Mesozoic sedimentary rocks, Cenozoic igneous rocks, and Tertiary volcanosedimentary sequences, travertine and Quaternary deposits are identified.
Local structural context predominantly includes sub-vertical structures resulting from compressive stresses relating to the Andean Orogeny. Amongst these the Chonta fault, the Huachocolpa fault, and several other local minor lineaments are identified. Structural controls on mineralization result in well-developed arrays of ENE to NE trending mineralized vein-sets (such as the Bienaventurada vein-set structures). The structural pattern can repeat and extend over other nearby sets of differently oriented structures, resulting in adjoining sequences over-lapping so as to produce a form of `structural pairing´.
The Chonta Fault is a NW-SE bearing regional structure (fault trace >200 km), located to the SW of the Huachocolpa district, and interpreted as a vertical to subvertical reverse fault, with a reported sinistral component of movement.
Similarly aligned to the Chonta fault, the Huachocolpa Fault also has a predominantly NW-SE trend and is located towards the East of the Chonta Fault in association with the geological contact between the Pucara Group and the Sacsaquero volcanic units, near the town of Huachocolpa.
Huachocolpa Uno is a Polymetallic Epithermal Low Sulfidation deposit, with vetiform style of mineralization occurring as fracture filling by hydrothermal solutions. Geological characteristics (host rock, metallic contents, hydrothermal alterations) are similar to neighboring mineralized deposits such as Recuperada, San Genaro, Caudalosa Grande, El Palomo, Dorita, amongst others.
Mineralization at Huachocolpa shows general geologic, structural and mineralogical characteristics of low sulfidation epithermal deposits.
Mineralization includes pyrite, electrum, gold, silver, argentite, acanthite, silver sulphosalts, sphalerite, galena, chalcopyrite, and/or selenide minerals. In alkalic host rocks, tellurides, vanadium mica (roscoelite), and fluorite may be abundant, with lesser molybdenite. These mineralized systems have strong geochemical signatures in rocks, soils, and sediments and Au, Ag, Zn, Pb, Cu, As, Sb, Ba, F, Mn, Te, Hg, and Se may be used to vector to mineralization.
The largest mine production source is Bienaventurada mine, which includes 260 veins. The second
underground production area has been Chonta / Escopeta operations, which included up to 76 veins.
All veins are considered polymetallic sulfide minerals containing silver, lead, zinc and, in lower concentration, copper.
Bienaventurada Vein
The Bienaventurada Vein forms a part of the Bienaventurada mine, the most important structure of the Huachocolpa Uno deposit, with preferential bearing 060° NE-SW and dipping predominantly 60° - 75°S. The Bienaventurada vein represents a fracture-filling structure classified as a Polymetallic Epithermal Low Sulphidation style of mineralization with a longitudinal extension over 4.3 km long, while at depth it has been identified from 4,650 masl (by underground workings) to 4,030 masl (by drilling).
The economic mineralization occurs in irregular mineralization-shoots, mainly in the Bienaventurada vein, where principal mineralization-shoots towards the central sector reach lengths of between 300 to 500 m, and up to 230 m long towards the west sector. Economically mineralized mineralization-shoots plunge at approximately 30°W, have a Ag-Pb-Zn-(Cu) metallic content, and variable vein thickness ranging from 0.5 m wide towards the closing limits of the structures, out to widths of up to 6.0 m within the inner, central zones. The estimated average width is of about 1.7 m.
The Bienaventurada Vein represents a brecciated structure with mineralization occurring in the form of massive bands, patches, veins, blebs and disseminations composed principally of sphalerite, galena and tetrahedrite. The mineralization generally forms a matrix encompassing sub-angular breccia fragments composed of altered host volcanic rock.
The host rock is a porphyritic andesitic volcanic, with plagioclase phenocrysts, selectively altered to clay, in some sectors with phyllic alteration and weak to moderate argillic alteration observed adjacent to the phyllic zones. According to microscopic studies, it would be a porphyritic volcanic rock that has undergone a process of hydrothermal alteration. They have generally been altered by hot fluids passing through them, with which the polymetallic mineralization is associated. Argillic alteration, followed to a lesser extent by silicification, predominates on the surface.
Yen vein
The Yen vein is the second most important structure of the Huachocolpa Uno deposit, with polymetallic mineralization that includes a relict massive sulfide fill from the Rublo Vein. Additionally, parallel splays, tension veins, blebs and disseminations trend towards the roof and floor of the mineralized system. These occurrences form a structure with a tabular geometry that exceeds 350 m in strike length.
The Yen mineralization occur along an average direction of N 75° to 90° E. The western sector of the structure corresponds to a local change of strike towards the East with dips varying between 78° and 87° towards the S. On the other end of the Yen mineralization, the dip also inclines sub- vertically, but now between 70 ° and 78° towards the N.
The Rublo vein is the master vein in all of the Yen mineralized system. Relative to the rest of the veins, it moved dextrally normal, forming sigmoid or tensional veins like for example: Leticia vein, Fortuna vein, Fortuna Norte vein, Tapada and Tapada Norte vein.
Yen's polymetallic mineralogy is similar across its length with variations in concentration. The hydrothermal fill concentrates massive white quartz and druse quartz, with barite and calcite. The observed mineralization textures of sphalerite, galena, argentiferous galena, chalcopyrite, pyrite, realgar, and orpiment, are massive, banded, stockwork and implosion breccia-like.
Alteration
During the Tertiary Andean Orogeny Quechua III Stage (4th and final phase), volcanic centers formed, and late minor intrusions occurred, which favored the occurrence of mesothermal to epithermal mineralization, characterized by argillic alteration aureoles.
At Huachocolpa Uno, hydrothermal alteration has been identified adjacent to the mineralized structures from centimeter up to one-meter-wide scale. Argillization, occurs in hypogene and supergene types where hypogene argillization is characterized by the assemblage of feldspars pervasively substituted by clay minerals, especially within the matrix. Silicification, with quartz replacing feldspars, silicification occurs mainly in the Caudalosa 1, Caudalosa 2, Esperanza, Marisol, and Diana vein systems.
Phyllic with a quartz-sericite and potassic alteration assemblage, with variable intensities of strong, moderate, and weak and irregularly distributed throughout the mine area, mainly in association with the Bienaventurada vein. Propylitization, with an epidote-chlorite-pyrite alteration assemblage, mainly observed in the slightly altered areas of the host volcanic rocks.
The above represent the styles of hydrothermal alteration identified in the Bienaventurada and Yen zones, as well as in all the mineralized zones of Huachocolpa Uno.