Understanding the evolution of magmatic-hydrothermal systems based on microtextural relationships, fluid inclusion petrography, and quartz solubility constraints: insights into the formation of the Yulong Cu-Mo porphyry deposit, eastern Tibetan Plateau, China

Abstract

The evolution of the magmatic-hydrothermal system that formed the Eocene Yulong Cu-Mo porphyry deposit of the eastern Tibetan Plateau, China, is reconstructed based on the study of the sequence of stockwork vein sets showing systematic crosscutting relationships in hand specimen. Early A veins associated with potassic alteration of the porphyritic host are composed of vitreous quartz that is extensively recrystallized and has a dark blue cathodoluminescence. Fluid inclusions contained in the quartz have been affected by post-entrapment modification. Some of the A veins were reopened and contain ribbons of microscopic molybdenite flakes. Formation of the molybdenite was associated with recrystallization of the earlier formed quartz and concomitant with the development of a dark purple to brownish cathodoluminescence signature. Reopening of the early veins also resulted in the formation of AB veins containing euhedral quartz crystals that contain encapsulated molybdenite flakes and are intergrown with coarse laths of molybdenite. These euhedral quartz crystals showing a dark purple to brownish cathodoluminescence host vapor-rich, intermediate-density fluid inclusions as well as hypersaline liquid and vapor-rich fluid inclusions, indicating that fluid immiscibility at hydrostatic pressure conditions occurred during crystal growth. Later, C veins that crosscut and commonly reopen the earlier A and AB veins lack quartz as a gangue mineral and consist entirely of chalcopyrite and pyrite with minor molybdenite. Sulfide formation was accompanied by chlorite or sericite alteration of the wall rocks. Late D veins containing quartz and pyrite with associated sericitic alteration of the porphyritic wall rocks crosscut all earlier vein types. Only minor amounts of molybdenite and chalcopyrite are present. Euhedral quartz crystals in the D veins contain liquid-rich fluid inclusions that have not been affected by post-entrapment modification. The results of this study indicate that Mo and Cu mineralization at Yulong was introduced by single-phase, intermediate-density magmatic-hydrothermal fluids as the porphyry system was transitioning from ductile to brittle conditions. The bulk of the Mo endowment predated Cu mineralization and formed during the change from lithostatic to hydrostatic conditions and associated immiscibility of the magmatic-hydrothermal fluids. Chalcopyrite precipitation followed as the single-phase, intermediate-density magmatic-hydrothermal fluids was escaping into the hydrostatic-pressured regime. Cooling of the magmatic-hydrothermal fluids appears to have been the principal control on metal precipitation.