Formation of Cu‐Bearing and Mo‐Bearing Miarolitic Cavities During the Magmatic–Hydrothermal Transition in Porphyry Metallogenic Systems

ABSTRACT

Miarolitic cavities (MCs) serve as key indicators of volatile phase exsolution during magmatic differentiation, providing critical insights into the transition between igneous and hydrothermal regimes. Despite their metallogenic significance, the mechanisms controlling the selective extraction of metal‐bearing fluids remain poorly understood. This study investigates the microstructural and geochemical characteristics of chalcopyrite‐ and molybdenite‐dominated MCs in the Hongyuan porphyry Mo–Cu deposit (NW China). The cavities exhibit sulfide cores surrounded by graphic quartz‐feldspar, aplitic, and K‐alteration halos. Mass balance calculations reveal enrichments in Cu, Fe, Na, and Si during the magmatic–hydrothermal transition. The term “sulfide‐silicate bulb” refers to silica‐saturated fluid pockets or bubbles that develop during hypogene copper mineralization in porphyry deposits. The central sulfide‐silicate bulbs represent hypersolidus mineral assemblages, with differential Cu–Mo precipitation linked to quartz growth rates. Late‐stage meteoric water alteration further influences metal precipitation. We propose a genetic model wherein sulfide‐silicate bulbs act as the primary vehicles for the extraction, transport, and focused precipitation of Cu‐ and Mo‐rich fluids. Our results highlight the critical role of quartz growth rates and meteoric fluid influx in controlling metal segregation, providing a new framework for understanding the spatial zonation in porphyry Cu–Mo systems.