Gold Distribution in the Millonaria Vein, San Juan de Chorunga, Peru: Insights from Paragenesis and Fluid Inclusion Microthermometry

Gold-vein deposits of the Nazca–Ocoña belt constitute an important economic resource for small- and medium-scale mining in Peru. However, the mineralogical and physicochemical controls governing gold distribution in these veins remain poorly understood. This study presents an integrated analysis of the Millonaria vein, San Juan de Chorunga, aimed at establishing the paragenetic sequence, characterizing the mode of occurrence of gold, and reconstructing the evolution of the hydrothermal fluids. Petrographic, ore microscopy, scanning electron microscopy, fluid inclusion microthermometry, and Au fire-assay data were integrated. The results indicate a main mineralizing stage, in which gold precipitated mainly in fractures and interstices of pyrite I and, subordinately, in microcavities within quartz I, as anhedral aggregates of native gold and electrum (Au ≈ 80%, Ag ≈ 20%). Specularite was also identified in fractures and interstices of pyrite I. A post-mineral substage I, characterized by milky quartz, carbonates, and subordinate sulfides, was also recognized, without significant gold precipitation. Fluid inclusions record homogenization temperatures of ~180–350 °C and low- to intermediate-salinity fluids (0.8–7.86 wt.% NaCl eq.). The data suggest at least two thermal contexts for gold mineralization, possibly influenced by dilution/mixing, local cooling, and redox changes.