Genesis of the Zhunsujihua porphyry Mo deposit in the Erenhot-East Ujimqin metallogenic belt, Inner Mongolia, China: Evidence from geology, fluid inclusions, and isotope systematics

摘要

The newly discovered Zhunsujihua porphyry Mo deposit is located in the south-western part of the Great Hingan Range, north-east China. Molybdenum mineralization generally presents as veins or disseminations hosted in granitic intrusions, and multiple-stage hydrothermal activity has resulted in potassic, phyllic, and sericitic alteration. The mineralization process can be divided into four stages, that is, quartz-K-feldspar +/- pyrite veinlets in stage I, quartz-molybdenite veinlets in stage II, quartz-polymetallic sulfide veinlets in stage III, and barren quartz-calcite veins in stage IV. Four types of fluid inclusions can be distinguished by petrography in hydrothermal quartz, including liquid-rich two-phase, vapour-rich two-phase, pure gaseous, and pure liquid inclusions, but quartz in stage I contains all types of fluid inclusions. The homogenization temperatures in quartz of stages I, II, III, and IV vary from 300 degrees C to 520 degrees C, 230 degrees C to 410 degrees C, 170 degrees C to 260 degrees C, and 136 degrees C to 227 degrees C, with salinities of 4.18-13.30 wt.% NaCl equiv., 2.07-11.10 wt.% NaCl equiv., 1.74-8.95 wt.% NaCl equiv., and 0.70-5.71 wt.% NaCl equiv., respectively. The ore-forming fluids in this deposit are characterized by high temperature and intermediate to low salinity belonging to an H2O-NaCl system. delta O-18(quartz) values of quartz from stages I to IV range from +5.5 parts per thousand to +10.1 parts per thousand, with the calculated delta O-18(fluid) values ranging from -6.7 parts per thousand to +6.3 parts per thousand, and the delta D values of fluid inclusion waters in quartz ranging from -118 parts per thousand to -102 parts per thousand. The delta S-34 values of pyrite and molybdenite from different ore stages range from 3.8 parts per thousand to 6.8 parts per thousand with an average of 5.1 parts per thousand. Pyrite and molybdenite samples in the deposit have Pb-206/Pb-204 ratios of 18.394 to 18.766, Pb-207/Pb-204 ratios of 15.580 to 15.694, and Pb-208/Pb-204 ratios of 38.309 to 38.942. All these observations reveal that the hydrothermal fluids of the Zhunsujihua porphyry Mo deposit have a dominantly magmatic signature with an input of meteoric water during the carbonate stage, and the ore-forming components (metal and sulfur) were sourced from a greater magmatic source and some upper crustal materials.