Evolution of the magmatic-hydrothermal system at the Erdenetiin Ovoo porphyry Cu-Mo deposit, Mongolia: constraints on the relative timing of alteration and mineralization

Abstract

The evolution of the magmatic-hydrothermal system that formed the Triassic Erdenetiin Ovoo porphyry Cu-Mo deposit, northern Mongolia, is reconstructed through the study of the sequence of stockwork veining and the petrographic characteristics of the veins, as well as the cathodoluminescence and fluid inclusion characteristics of the vein quartz. Early A veins associated with potassic alteration formed at temperatures ≳ 500 °C under lithostatic pressures. The quartz in these veins shows a blue cathodoluminescence emission and is extensively recrystallized. Subhedral to euhedral quartz crystals contained in AB veins precipitated at the ductile to brittle transition at ~ 400–450 °C. The quartz crystals have cores showing a long-lived purple cathodoluminescence color with well-developed growth zoning, whereas the rims of the crystals lack growth banding and have a brownish-red cathodoluminescence emission. Fluid inclusions in the A and AB veins show elevated CO₂ concentrations but show no evidence for immiscibility, implying that Erdenetiin Ovoo formed at ≳ 5 km below paleosurface. Quartz in the A and AB veins is crosscut by molybdenite ribbons. The presence of encapsulated molybdenite and sericite grains along growth zones in the quartz rims of the subhedral to euhedral quartz crystals constrains the timing of Mo introduction and suggests an association with the sericite alteration of the host rocks, which affected 50–60% of the deposit. Elevated Cu grades at Erdenetiin Ovoo are linked to the occurrence of C veins, which consist primarily of chalcopyrite and pyrite. These veins surrounded by halos of sericite-chlorite alteration lack quartz as a gangue mineral, suggesting formation at conditions of retrograde quartz solubility at ~ 400 °C and hydrostatic pressures. Late D veins formed at ≲ 375 °C and hydrostatic conditions. The veins consist primarily of pyrite and are associated with texturally destructive sericite alteration of the host rocks. The study demonstrates that hypogene Cu mineralization at Erdenetiin Ovoo was predated and post-dated by sericite alteration, which has important implications to alteration vectoring in porphyry exploration worldwide.