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

<p>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&#176;C for UST quartz, ~1.25 kbar at ~510&#176;C for Q₁, ~1.0 kbar at ~440&#176;C for Q₂, and ~0.37 kbar at ~220&#176;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.</p>