Two Periods of Skarn Mineralization in the B aizhangyan W – Mo Deposit, Southern A nhui Province, Southeast C hina: Evidence from Zircon U – Pb and Molybdenite Re – Os and Sulfur Isotope Data

The recently discovered B aizhangyan skarn‐porphyry type W – Mo deposit in southern A nhui Province in SE C hina occurs near the M iddle– L ower Y angtze V alley polymetallic metallogenic belt. The deposit is closely temporally‐spatially associated with the M esozoic Q ingyang granitic complex composed of g ranodiorite, monzonitic g ranite, and alkaline g ranite. Orebodies of the deposit occur as horizons, veins, and lenses within the limestones of S inian L antian F ormation contacting with buried fine‐grained granite, and diorite dykes. There are two types of W mineralization: major skarn W – Mo mineralization and minor granite‐hosted disseminated Mo mineralization. Among skarn mineralization, mineral assemblages and cross‐cutting relationships within both skarn ores and intrusions reveal two distinct periods of mineralization, i.e. the first W – Au period related to the intrusion of diorite dykes, and the subsequent W – Mo period related to the intrusion of the fine‐grained granite. In this paper, we report new zircon U – Pb and molybdenite Re – Os ages with the aim of constraining the relationships among the monzonitic granite, fine‐grained granite, diorite dykes, and W mineralization. Zircons of the monzonitic granite, the fine‐grained granite, and diorite dykes yield weighted mean U – Pb ages of 129.0 ± 1.2 Ma, 135.34 ± 0.92 Ma and 145.3 ± 1.7 Ma, respectively. Ten molybdenite Re – Os age determinations yield an isochron age of 136.9 ± 4.5 Ma and a weighted mean age of 135.0 ± 1.2 Ma. The molybdenites have δ 34 S values of 3.6‰–6.6‰ and their Re contents ranging from 7.23 ppm to 15.23 ppm. A second group of two molybdenite samples yield ages of 143.8 ± 2.1 and 146.3 ± 2.0 Ma, containing Re concentrations of 50.5–50.9 ppm, and with δ 34 S values of 1.6‰–4.8‰. The molybdenites from these two distinct groups of samples contain moderate concentrations of Re (7.23–50.48 ppm), suggesting that metals within the deposit have a mixed crust–mantle provenance. Field observation and new age and isotope data obtained in this study indicate that the first diorite dyke‐related skarn W – Au mineralization took place in the E arly C retaceous peaking at 143.0–146.3 Ma, and was associated with a mixed crust–mantle system. The second fine‐grained granite‐related skarn W – Mo mineralization took place a little later at 135.0–136.9 Ma, and was crust‐dominated. The fine‐grained granite was not formed by fractionation of the Q ingyang monzonitic granite. This finding suggests that the first period of skarn W – Au mineralization in the B aizhangyan deposit resulted from interaction between basaltic magmas derived from the upper lithospheric mantle and crustal material at 143.0–146.3 and the subsequent period of W – Mo mineralization derived from the crust at 135.0–136.9 Ma.