Lithofacies geochemistry characteristics of alkali volcanic rocks and prospecting prediction in Tupiza copper deposit, Bolivia

In sediment-hosted copper deposits, altered volcanic rocks have special significance for diagenesis and mineralization. Based on the methods of tectonic lithofacies mapping, volcanic lithofacies classification, and electron microprobe analysis (EPMA), the authors studied lithofacies types of volcanic rocks, their geochemical characteristics, physical-chemical conditions of magmatic evolution and their relationship with copper (silver) enrichment. The following results show that mesogenetic intrusive facies, subvolcanic intrusive facies (sub-volcanic neck facies), volcanic overflow facies, pyroclastic facies and sink volcanic rocks are developed in the Tupiza copper deposit. The assemblage of rock types is diabase, gabbro, alkaline basalt, potash-trachybasalt, olivine basalt trachyandesite, and latite. In this area, alkaline basaltic magmatic emplacement has multiple stages and phases. In the Tupiza copper mining area, mineral geothermometer-geobarometer was used to do estimation. When the formation temperature and pressure of hornblende respectively are 630.97-748.43℃ and 55-251 MPa, the depth of diagenetic formation is estimated to be 2.04-9.27 km, revealing that the diagenesis evolution process under decreasing pressure-increasing temperature (decompression melting) and decreasing pressure-decreasing temperature had a high-temperature and high-oxidation diagenetic environment during magmatic decompression and emplacement, suggesting a multi-stage emplacement. Chlorite formation temperature is 112-305℃, lgf(O₂)=-45.03—-56.68, lgf(S₂)=-4.46—-18.07, suggesting a low temperature reduced diagenesis mineralization environment representing the main copper (silver) ore formation period. The Tupiza copper (silver) deposit was formed by the subvolcanic hydrothermal alteration diagenetic mineralization. Altered volcanic rock is a metallogenic material supply system for copper deposits. Copper (silver) orebody is concentrated in altered volcanic lithosphere and structural superposition, particularly concentrated in the intersection of NNE and NW-trending structures. In the pyrite glutenite at the top of the second lithologic section below the third lithologic alteration volcanic rock in the Upper Cretaceous Aroifilla Formation, the verifying drilling revealed a copper (cobalt) mineralized body, which was the sign of deep prospecting for hidden sedimentary rock type copper (cobalt) orebodies. In this paper, it is believed that, in the Tupiza copper deposit, the central phase of the sub-volcanic hydrothermal metallogenic system is distributed in the altered secondary volcanic neck phases, enriching the copper (silver) orebody. Peripheral veinlet vein-fractured-alteration zone is the transitional facies zone of the copper-lead-zinc metallogenic system, while the glutenite-type copper (cobalt) ore and Cu-Pb-Zn anomaly in the second lithologic zone of the Aroifilla Formation is the outer fringe facies zone of the copper (cobalt) lead-zinc metallogenic system. It has the prospecting potential for copper (silver), copper (cobalt) and copper-lead-zinc orebodies in the deep surrounding altered subvolcanic facies.