Geochronology, geochemistry and fluid inclusions of the Yechangping giant porphyry-skarn Mo-W deposit, East Qinling, China

The Yechangping giant porphyry-skarn Mo-W deposit is located in the East Qinling molybdenum belt, Henan Province. Molybdenum mineralization occurs mainly as veins and veinlets or disseminated within dolomite and granite porphyry. The mineralized granite porphyry, emplaced at 148.1 +/- 1.0 Ma, has petrological, mineralogical, and geochemical characteristics of I-type granite. The rock belongs to the high-K calc-alkaline to shoshonitic series and is characterized by enrichments in large ion lithophile elements and significant negative Eu, Ba, Sr, P, and Ti anomalies, attributed to strong fractional crystallization. The granite has negative wholerock epsilon Nd(t) values (-13.2 to -12.4) and zircon epsilon Hf(t) values (-14.2 to -6.8), corresponding to old wholerock Nd and zircon Hf model ages, which indicate that the primary magma was derived from partial melting of the ancient lower crust of the North China Craton. Four mineralization stages are identified in the Yechangping deposit. Stage I is a magmatic-hydrothermal transitional stage, which is respresented by the 1-5 mm-wide unidirectional solidification texture (UST), formed from fluids with high temperatures (375-504 degrees C), high/low salinities (46.3-51.0 vs. 7.81-13.75 wt% NaCl equiv.), and a high oxygen fugacity in an H2O-NaCl-CO2 fluid system. Stage II quartz-K-feldspar stockworks or quartz-K-feldspar-pyrite veins, with minor molybdenite, which are characterized by the K-feldspar and quartz with potassic-quartz alteration; they were formed from fluids with medium-high temperatures (320-464 degrees C), high/low salinities (38.8-43.4 vs. 5.41-12.85 wt% NaCl equiv.), and a relatively high oxygen fugacity. Stage III quartz-molybdenite stockworks and quartz-molybdenite-polymetallic veins, associated with quartz-sericite alteration, were formed from fluids with medium-low temperatures (273-396 degrees C), and variable salinities (3.38-40.0 wt% NaCl equiv.). Stage IV quartz-carbonate and fluorite veins, associated with fluid-related argillic alteration were characterized by low temperatures of 171-245 degrees C and low salinities of 3.23-8.00 wt% NaCl equiv.. The prolonged fractional crystallization, the high oxidization state of the magmatic-hydrothermal system and the fluid boiling process were the key factors that have contributed to the formation of the Yechangping giant Mo-W deposit.