Texture and trace element geochemistry of quartz in porphyry system: Perspective from the Gaogangshan Mo Deposit, NE China

Abstract

The Gaogangshan deposit in the northern Lesser Xing'an Range, NE China, consists of typical collision-type porphyry Mo mineralization related to Permo-Triassic granitic intrusions. Ore-forming process is recorded by multiple generations of quartz, including pre-ore stage quartz with unidirectional solidification texture (UST), early ore-stage quartz-molybdenite veins with K-feldspar alteration halos (Q1), late ore-stage quartz-sulfides veins with sericite alteration (Q2), and post-ore stage quartz veins (Q3) associated with calcite and fluorite. Cathodoluminescent textures, trace elements and fluid inclusions in quartz reveal physicochemical conditions, evolution of ore fluids and the Mo mineralization process. The UST quartz and Q1 veins are dominated by CL-bright homogenous and/or granular mosaic textures, containing more Ti concentrations (average = 35-42 ppm) than Q2 and Q3 veins. The molybdenite-bearing Q2 veins are dominated by CL-gray granular and zonal textures, displaying less CL intensity and lower Ti concentrations (average = 16 ppm) than early-stage quartz. The Q3 veins have the lowest CL-intensity and the lowest Ti concentrations (average = 2.5 ppm) among all quartz types. Three types of inclusions are identified in above quartz samples, including liquid-vapor aqueous inclusions, CO2-bearing liquid-vapor aqueous inclusions and halite-bearing multiphase aqueous inclusions. The ore-forming fluids in ore-stage Q1 and Q2 veins are dominated by large salinity variation (2.3-38.9 wt% NaCl equiv.), CO2-bearing (4.2-8.9 mol%) two-phase aqueous inclusions with vapor volumetric proportions of 30-65%. Intersections of fluid inclusion isochores with Ti-in-quartz isopleths yield quartz formation conditions of ~2.2 kbar at ~640°C for UST quartz, ~1.25 kbar at ~510°C for Q1, ~1.0 kbar at ~440°C for Q2, and ~0.37 kbar at ~220°C for Q3. The Gaogangshan porphyry Mo deposit formed at depths of 3.8 to 4.7 km. Fluid decompression and temperature decreasing resulted in molybdenite precipitation.