Geology, geochronology and geochemistry of the Dabate Cu-Mo deposit, northern Chinese Tien Shan: Implications for spatial separation of copper and molybdenum mineralization

The Dabate deposit is a porphyry Cu-Mo deposit located in the Sailimu Block, northern Chinese Tien Shan. The deposit shows a clear separation of Cu and Mo mineralization, with Cu mineralization occurring along the edge of the rhyolitic porphyry, and Mo mineralization in the inner part of the rhyolitic porphyry. Sm-Nd isotopic analyses of six fluorite samples from the Cu sulfides-fluorite veins yield an isochron age of 298.1 +/- 5.0 Ma, and Re-Os isotopic analyses of four molybdenite samples from the molybdenite-quartz veins yield an isochron age of 299.6 +/- 3.2 Ma. These geochronological data and accompanying geologic evidence indicate that the Cu and Mo mineralization belongs to the same hydrothermal system and are closely related to the rhyolitic porphyry, which has a youngest zircon Pb-206/U-238 age of 297.3 +/- 8.0 Ma. Rare earth element (REE) and fluid inclusion studies were carried out for fluorite in the Cu ores. The fluorite related to Cu mineralization have high Sigma REE (63.4-216.9 ppm) and show generally similar REE patterns with the rhyolitic porphyry, suggesting that the fluid of Cu mineralization is closely related to the rhyolitic porphyry. The slight LREE-depletion and Ce-depletion of fluorite relative to the rhyolitic porphyry may be due to preferential partitioning of HREE into the fluid in neutral to weak alkaline conditions and to mixing with meteoric water which is characterized by Ce-depletion. Three types of fluid inclusions were recognized in the fluorite from the Cu sulfides-fluorite veins: liquid-vapor (LV-type), CO2-bearing (C-type) and solid mineral-bearing (S-type). The LV-type inclusions have homogenization (vapor to liquid) temperatures from 175 to 328 degrees C and salinities of 0.4-14.9 wt% NaCl eqv. (mode at 0 similar to 8 wt% NaCl eqv.), and is of H2O + NaCl composition. The C-type inclusions have homogenization (vapor to liquid) temperatures from 206 to 279 degrees C and salinities of 1.6 similar to 8.5 wt% NaCl eqv., and is of H2O + CO2 + NaCl + CH4 composition. The S-type inclusions have partial homogenization (vapor to liquid) temperatures of 173-233 degrees C and salinities of 1.2-7.9 wt% NaCl eqv., and is of H2O + NaCl composition. The solid particle in S-type inclusions did not change during heating up to 550 degrees C, indicating that the solid is not daughter mineral and the S-type inclusions are actually the same as the LV-type inclusions. The LV-type inclusions have microthermometric attributes similar to those typically found in the late stages of porphyry deposits, whereas the C type inclusions have similar compositions as the inclusions in the Mo ores as documented in previous studies at Dabate. The presence of both LV-type and C-type inclusions suggests that the Cu mineralization may be related to mixing between meteoric water and magmatic fluids, as is also indicated by the REE of fluorite. Comparing with data from Mo ores reported in previous studies, it is suggested that the Mo-stage fluids were more reduced and acidic and Mo mineralization may be mainly related to fluid pressure decrease and immiscibility, whereas the Cu-stage fluids were more oxidized, neutral to weakly alkaline and Cu mineralization may be mainly controlled by mixing and cooling. The vapor phase produced by fluid immiscibility in the deep area may have carried Cu, As, Ag, HF, CO2 and migrated to the outer part of the porphyry, precipitated Cu sulfides, arsenopyrite and fluorite by mixing with meteoric water from the country rocks. Whereas the liquid phase produced by fluid immiscibility at depth may have carried Mo, NaCl and migrated to the middle-west part of the rhyolitic porphyry, become immiscible and precipitated molybdenite.