Modelo geológico del proyecto aurífero tipo: High sulfidation de Arnacocha, distrito minero de Yanacocha - Cajamarca

Abstract

The Arnacocha Project corresponds to a group of high sulfidation epithermal deposits in the Yanacocha District aligned within the Maqui Maqui sub-district and located within the regional mineralized trend known as the “Yanacocha-Chicama Structural Corridor”. In this study, the area was divided in three zones of interest with the objective to better explain their geologic characteristics. Arnacocha is located at the eastern border of the Miocene calc-alkaline volcanic complex of the Yanacocha District. Radiometric age dates of the volcanic complex range from 20Ma to 8Ma. The volcanic strata at Arnacocha are probably related to the distal part of San Jose volcanic center or the Colorado Diatreme because the dominant rocks are medium grained to fine crystal tuffs with rare coarse pyroclastic layers in the upper part of the stratigraphy column. This sequence is intercalated with effusions from volcanic domes and is cut by numerous intrusions. The magmatic events were followed by hydrothermal circulation and the formation of narrow hydrothermal breccias related mainly to successive phases of the “Tectonica Quechuanas”. Alteration and mineralization were preferentially emplaced in the tuff units and in minor proportions within hydrothermal and phreatomagmatic breccias. The ore body is oriented nearly east-west and has two levels; the lower level wedges downward thus indicating the probable source of mineralization. It is clear that E-W and N-S pre-mineral structures provided subsequent controls to the mineralization. At deeper structural intersections along the Arnacocha Fault, mineralization as oxides and mixed sulfides remains open. The main gold ore body (Zone of Interest I) is related to massive silica, while the Zone of Interest II is mainly related to advance argillic alteration. In the lower parts of Zones I and II there is a thin (5-10meter) horizon of sulfides dominated by pyrite with minor quantities of calcosite, although microscopic studies reveal remnants of chalcopyrite and tetrahedrite in oxidized hydrothermal breccias. Following a 34-hole drilling program (totaling 7,723 meters), reserves calculated in Arnacocha are 41,500 ounces gold at a grade of 1.33 g/T Au in oxide, based on a gold price of US $325 / ounce. Bottle and column leach tests yielded gold recoveries of 78.1% and 80.7% respectively from oxide material. The location of the Arnacocha deposit near the Maqui Maqui facilities will make themining project feasible.