Evolution of fluids in the Dasuji porphyry Mo deposit on the northern margin of the North China Craton: Constraints from Microthermometric and LA-ICP-MS analyses of fluid inclusions

The newly discovered Dasuji Mo deposit is located on the northern margin of the North China Craton, and the Mo mineralization is related to Triassic granite porphyry and quartz porphyry. Alteration associated with the deposit includes silicification, sericitization, chloritization, K-feldspathization, kaolinitization, carbonatization, and fluoritization. The silicification and sericitization (phyllic alteration) are closely related to the Mo mineralization. Based on mineral assemblages and cross-cutting relationships, the hydrothermal ore-forming process can be divided into four stages: stage 1 is characterized by K-feldspathization with minor molybdenum mineralization; stage 2 is the main stage of molybdenum mineralization; stage 3 is involved in lead-zinc mineralization; and stage 4 is characterized by no mineralization. Four types of fluid inclusions are recognized: liquid rich, gas-rich, CO2-H2O, and daughter-mineral-bearing. Microthermometric data show a decrease in homogenization temperatures and salinities from the early to late stages, so that stages 1-4 show homogenization temperatures of 468-375, 435-271, 318-206, and 222-142 degrees C, respectively, and the fluids of stages 1, 3, and 4 had salinities of 2.57-16.50 (along with a few high-salinity inclusions: 32.04-48.05), 0.88-11.19, and 0.88-7.45 wt% NaCl equiv., respectively. The salinities of stage 2 fluid inclusions have a bimodal distribution: 1.40-17.48 and 29.20-53.25 wt% NaCl equiv. Laser ablation inductive coupled plasma mass spectrometer (LA-ICP-MS) analyses show that the fluid compositions are dominated by Na, K, Fe, and Mn. In stage 2 fluid inclusions, the Mo concentrations vary over a range of more than one order of magnitude (more than for any other element) as a result of fluid-rock interactions and molybdenite precipitation. Cu concentrations remain always relatively high in the fluids during their evolution (26-7092 ppm) because there was little chalcopyrite precipitation. The stage 3 fluid inclusions are rich in Mn, Pb, and Zn, while Ag concentrations increased in stage 3 and 4 fluid due to the addition of meteoric water from the surrounding rock. Reactions between the wallrock and hydrothermal fluids affected the composition of the hydrothermal fluids; sericitic alteration in particular had a great effect on fluid compositions, manifesting the variety of the content of Sr, Ba, Ca, Pb. The ore-forming fluids were mainly magmatic-hydrothermal fluids that were mixed with meteoric water during stages 3 and 4. Fluid immiscibility, a rapid decrease in pressure, fluid-rock reactions and fluid cooling were collectively responsible for the sulfide precipitation in the Dasuji deposit.