Por: Rolando Ligarda, Edwin Arias, Ronald Callupe, Juan Carlos Salazar, José Luis Aquino y María del Pilar Ventura, Compañía de Minas Buenaventura.

Abstract

The Uchucchacua mining district is located in the Western Andean Cordillera of Central Peru, at an altitude between 4,300 and 5,000 m.a.s.l. Geologically, it is located in the faulted and folded Marañón belt. The Uchucchacua mine, containing more than 410 M oz Ag (historical production + reserves-resources), is the most important underground silver mine in Peru. Another 40 M oz Ag, recently discovered at the Yumpag Project, 6 km northeast of Uchucchacua, have been added to these reserves and resources. The ore bodies are hosted in limestones of the Jumasha formation, particularly in pure limestones, with bioclastic packstone to grainstone facies of the Mid Jumasha at Uchucchacua, and similar facies at the base of the Upper Jumasha (βeta and Gastropod horizons) at Yumpag. 

In all cases, the presence of impermeable levels has been crucial to trapping the mineral in favorable rocks. At Uchucchacua, veins, multi-directional bodies or sub-vertical mantles occur in the short limb of the core of the N-S striking anticline. However, the mineralization was controlled by the Colquicocha (NS) and Cachipampa (NE-SW) faults, of cortical reach, considered as “thick skin faults”. 

Dacitic porphyritic stocks are emplaced where these faults meet, showing Pb-Zn-(Cu) skarn halos, grading to Ag-Mn mineralization in its distal periphery. At Yumpag, the Camila vein which is a mineralized Reidel structure related to the movement of the Cachipampa fault, when crossing the βeta or sub-horizontal Gastropod horizons, has allowed the formation of high-grade silver mineralized bodies, which also grade from a distal halo of Ag-Mn towards a proximal mineralization enriched in Pb-Zn-(Cu), following the plunge to the southwest. The source of the mineralization remains unknown, but is presumed to come from below the coverage of the Casapalca formation.