Characteristics and evolution of quartz-calcite-sulfide veins in the Nazca-Ocoña belt, Peru

Abstract

The Nazca-Ocoña metallogenic belt in southern Peru host about 70 recognized Au-Ag and base metal (Pb-Cu-Zn) deposits such as Orión, Caravelí, Ishihuica, Calpa, Arirahua, Mollehuaca, and San Juan de Chorunga. These deposits are spatially associated with Cretaceous intrusive rocks of the coastal Batholith in a zone that extends more than 350 km along the Central to Southern part of Peru. Gold and silver are structurally controlled in mineralized NW-SE, N-S and W-E-trending fault-fill veins that are less than one-meter wide, crosscutting granodiorite and diorite bodies. The origin of the veins is still debated, and they may be classified as intrusion-related gold deposits (IRGD), orogenic or mesothermal deposits. In this study, we focus on two representative deposits in the Nazca-Ocoña metallogenic belt: Mollehuaca and San Juan de Chorunga. At both sites, the mineralization is composed of quartz-calcite sulfide veins hosted in Cretaceous Coastal Batholith intrusions. Petrographic and chemical analyses on pyrite were performed with field-emission scanning electron microscopy (FE-SEM), electron microprobe analysis (EMPA), nano-scale secondary ion mass spectrometry (NanoSIMS), and laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS).Native gold and Au-rich silver are disseminated throughout the veins as micro to sub-micron inclusions in pyrite and chalcopyrite, and in pyrite as cavity and fracture fill. The are three pyrite generations and these have complex internal zonation and are host to precious (Au, Ag) and base metals (Co, Ni, Cu) metals, and also metalloids (As, Sb) as lattice-bound impurities and micron inclusions. The San Juan de Chorunga pyrite has δ34S values ranging from 0.4 to 1.4 ‰, and the Mollehuaca pyrite δ34S from 5.8 to 7.4 ‰. In both San Juan de Chorunga and Mollehuaca, the highest Au concentrations occur in pyrite zones with the heaviest δ34S values. Fluid inclusion petrography of these veins shows recrystallized, wispy quartz with implosion and explosion fluid inclusion textures, indicative of pressure variations during quartz vein formation. The presence of CO2 in these inclusions requires mid-crustal pressure conditions. However, the main stage of gold mineralization occurs with carbonate, chlorite, and euhedral quartz crystals lacking post-entrapment recrystallization textures or CO2. Our observations suggest that most of the gold in these veins formed in association with a shallower hydrothermal event, potentially in association with the emplacement of batholiths and shallower intrusions in the Late Cretaceous.