Summary:
The Yauli unit region shows mineralization related to polymetallic epithermal systems, replacement bodies and polymetallic vein systems, as well as porphyry mineralization and skarns, thus indicating the great potential of the area.
The Yauli Dome is the foremost structural feature of the San Cristobal region. This dome measures thirty kilometers by twenty kilometers. The long dimension strikes about N 40° W. The center of the Yauli Dome is formed by the Chumpe Anticline, the main structural feature of the San Cristobal area.
The San Cristobal-Andaychagua mining district, located in the southern part of the Yauli Dome, is a classical high-temperature, carbonate-hosted, massive sulfide replacement ore deposit district. Such deposits have been traditionally considered in terms of a single evolving hydrothermal fluid traveling differing distances from an intrusive center.
Mineralization in the San Cristobal mineral district is of two different types, namely veins and mantos. Veins are emplaced transverse to the axis of the Chumpe Anticline, whereas mantos are located on the limestone side of the Paria limestone Mitu volcanic rocks contact.
The axial zone of the Chumpe Anticline is intruded by a quartz monzonite intrusion and two major alaskite dikes parallel to the axis of the anticline.
San Antonio Mantos
The San Antonio mantos are located in the southeast end of the two-kilometer Carahuacra-San Antonio area. The general strike of the San Antonio mantos is N 30° W and their dip is between 60° and 85° southwest. The mantos contain sphalerite and galena as ore minerals. Outcrop of the mantos weather to a black color resulting from the presence of manganese oxides.
Toldorrumi Manto
About one kilometer southeast of the San Cristobal mine lies the Toldorrumi manto. A seven hundred-meter long low-grade manto exists on the Mitu volcanic rocks-Paria limestone contact. The area has two weak veins, the Catalina and the Polonia veins.
The San Cristobal Vein system is made up of two main mineral-bearing structures, the San Cristobal Main Vein and the Siberia 1 Vein. The Oyama-Triunfo Vein, which is located on the northeast limb of the anticline, appears to be an extension of the San Cristobal Main Vein. Both veins are separated on the surface by Chumpe lake and no underground workings exist below the area that permit a clear correlation.
Mineralization of the San Cristobal Main Vein is of three different types: tungsten mineralization alone, combined metals with tungsten, and combined metals without, tungsten.
There is a well-defined horizontal zoning in the mine from east to west consisting of a Tungsten zone located in the Dike Section, a Copper zone located in the Phyllite and Contact sections, and a Zinc zone located in the Volcanic Section. Vertical zoning is less well-defined, probably because development has not gone deep enough to reveal it.
Paragenetic sequence in the San Cristobal Main Vein from oldest to youngest is: pyrite, wolframite, quartz, chalcopyrite, dark sphalerite, brown sphalerite, galena, carbonates, and marcasite.
The Prosperidad Vein, which is the structure dipping opposite to the San Cristobal Main Vein, is of particular interest for future exploration beyond the San Cristobal mine. Both structures tend to join in a zone below the Chumpe intrusive where the inferred source of ore-bearing solutions is located.
Alteration of wall rocks in the San Cristobal Main Vein is of three different types forming zones away from the vein which include the silicification, the sericitization-argillization, and the chloritization zones.
The source of mineralizing solutions or at least their main channel, is indicated by zoning relationships to be located 1,300 meters from the present surface in the area below the Chumpe intrusive. From this main channel, secondary channels invaded the whole structure in a dendritic pattern.
Lapping of late minerals over early minerals indicates a change in temperature of the ore-bearing solutions due to a decrease of activity of the intrusion which generated the ore fluids.
Mineral assemblages, ore textures and alteration of wall rocks place the San Cristobal Main Vein in the mesothermal classification.
Carahuacra
The Carahuacra deposit lies on the southwest flank of the Yauli anticline, formed by Tertiary folding. Manto ore, formed before the climax of the Andean orogeny, consists mainly of ferroan sphalerite and pyrite, with negligible chalcopyrite. These were later brecciated tectonically to form two zones of 'breccia ore'. The last phase of mineralization involved formation of epithermal cross veins with sphalerite, pyrite, galena, tetrahedrite, and jamesonite, and irregular cementation of fragments in the breccia ore by sphalerite-galena and pyrite-marcasite. Supergene alteration, particularly pronounced in the breccia zone, has formed ore much enriched in silver (native silver, argentite, pyrargyrite).
In Carahuacra, mineral resources with high grades of zinc, lead, silver and copper are consolidated, in the mantles of the Principal and Paula systems, as well as in the Eastern extension and the deepening of the Lourdes vein. In addition, continuity of economic mineralization is confirmed in the Jovencita mantle above level 720, with high grades of zinc and silver.
At the San Cristóbal Carahuacra mine, in the San Cristóbal sector, drilling programs have yielded economic mineral resources from lateral extensions and downthrusts of the Ramal Piso 722, 658, Split 658, Sheyla, and K veins, which are the main contributors of zinc and silver mineralization. Similarly, the discovery of new polymetallic bodies and seams is being consolidated. These bodies have developed at the intersection of vein systems with the same structural pattern as the main veins (NE-SW trend, Vein 658) and calcareous rocks of the Pucará Corridor. These veins are located immediately to the west and at high levels (above level 810) of the current operating area. Preliminary results show high zinc and silver grades, providing a better operating outlook in the medium and long term.
At the San Cristóbal Carahuacra mine, in the Carahuacra sector, high-grade mineral resources of zinc, lead, silver, and copper were consolidated and added in the Principal and Jovencita mantos above the 720 level. The deepening and eastward extension of the Lourdes vein and the Lourdes breccia also showed an increase in economic mineralization below the 1020 level, with high zinc and silver grades. In addition, new resources were identified in the Mary Piso, Diana, and Teresa veins, confirming the continuity of economic mineralization in these structures.
Drilling campaigns
The 2023 and 2024 drilling campaigns aimed to enhance the geological certainty of mineral resources. Additionally, exploratory drilling sought to identify new economic resources and assess areas with promising geological and economic potential, thereby extending the longevity of the assets in the medium to long term.
In San Cristóbal, drilling programs resulted in the incorporation of economic mineral resources in the lateral extensions and deepening of the Ramal Piso 722, 658, Split 658, Sheyla, and K veins, as the main contributors of zinc and silver mineralization. Additionally, the discovery of new polymetallic bodies and mantles, developed at the intersection of vein systems with the same structural pattern as the main veins (NE-SW orientation, Vein 658), and limestone rocks of the "Pucará Corridor," located immediately to the west and higher levels (above Level 810) of the current operating area, is being consolidated. Preliminary results show high grades of zinc and silver, generating a better operational outlook in the medium and long term.