GOLD-SILVER MINERALIZATION AND ASSOCIATED TRACE ELEMENT GEOCHEMISTRY OF DEPOSITS IN THE NAZCA-OCOÑA CRETACEOUS METALLOGENIC BELT, AREQUIPA, PERÚ

The Mollehuaca and San Juan de Chorunga deposits are hosted in the poorly explored gold and copper trends of the Nazca-Ocoña metallogenic belt in Arequipa-Peru, which extends from Trujillo (9ºS) to Nazca-Ocoña (14ºS). In the Nazca-Ocoña zone, mineralized structures are controlled by faults NW-SE, N-S and W-E trending faults, which are part of the Nazca-Ocoña corridor. Although they are hydrothermal vein-type deposits, their scale, distribution, and specific mineralization style remain enigmatic. Our interest in these studies is the occurrence and distribution of Au and related elements (e.g., Ag, Hg, Pb, Cu, Zn, and Bi) in the vein system, which is of fundamental importance to the development of more effective strategies for exploration, extraction, and environmental mitigation efforts. Here, we present detailed geological mapping, geochemical whole rock ICP-MS analysis, petrographic observations coupled with field-emission scanning electron microscope imaging, quantitative SEM-based automated mineralogy (TIMA – TESCAN Integrated Mineral Analyzer), and electron microprobe analysis to determine the major, minor and trace mineral chemistry.

The present study was carried out in the small-scale mining localities of Mollehuaca and San Juan de Chorunga. The ore deposits are hosted in the coastal batholith, and U-Pb LA-ICP-MS dating of zircons indicate that the intrusions are of Lower Cretaceous age (129.2 ± 1.0 Ma). The Au-Pb-Zn-Cu deposits are characterized by epithermal-style veins, which are mostly composed of quartz, gold, electrum (AuAg), sulfides (pyrite, galena, sphalerite, chalcopyrite and pyrrhotite), calcite and chlorite.

We have newly identified the presence of significant silver in this metallogenic belt and show that the most important Au-bearing mineral in the vein system is electrum (AuAg) containing 4 to 37 wt% Ag . Gold concentration in the vein systems is extremely variable with contents ranging from 21 to 136,000 ppb. The elements Bi, Hg, As, Pb, and Zn occur as potential economic commodities and environmental contaminants within the Au- and Ag- bearing veins. Although small-scale miners add mercury during gold processing, our results indicate that not all the mercury in the associated environmental contamination is anthropogenic; some is natural. Our analyses demonstrate that the ore is hosted in carbonate-bearing alteration haloes, which buffers the effects of sulfide oxidation and may mitigate the environmental impacts of acid mine drainage in the region.