Two Periods of Skarn Mineralization in theBaizhangyanW–MoDeposit, SouthernAnhui Province, SoutheastChina: Evidence from ZirconU–Pband MolybdeniteRe–Osand Sulfur Isotope Data

Abstract

AbstractThe recently discoveredBaizhangyan skarn‐porphyry typeW–Modeposit in southernAnhui Province inSE China occurs near theMiddle–LowerYangtzeValley polymetallic metallogenic belt. The deposit is closely temporally‐spatially associated with theMesozoicQingyang granitic complex composed ofgranodiorite, monzoniticgranite, and alkalinegranite. Orebodies of the deposit occur as horizons, veins, and lenses within the limestones ofSinianLantianFormation contacting with buried fine‐grained granite, and diorite dykes. There are two types ofWmineralization: major skarnW–Momineralization and minor granite‐hosted disseminatedMomineralization. Among skarn mineralization, mineral assemblages and cross‐cutting relationships within both skarn ores and intrusions reveal two distinct periods of mineralization, i.e. the firstW–Auperiod related to the intrusion of diorite dykes, and the subsequentW–Moperiod related to the intrusion of the fine‐grained granite. In this paper, we report new zirconU–Pband molybdeniteRe–Osages with the aim of constraining the relationships among the monzonitic granite, fine‐grained granite, diorite dykes, andWmineralization. Zircons of the monzonitic granite, the fine‐grained granite, and diorite dykes yield weighted meanU–Pbages of 129.0 ± 1.2 Ma, 135.34 ± 0.92 Ma and 145.3 ± 1.7 Ma, respectively. Ten molybdeniteRe–Osage 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 δ34Svalues of 3.6‰–6.6‰ and theirRecontents 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, containingReconcentrations of 50.5–50.9 ppm, and with δ34S values of 1.6‰–4.8‰. The molybdenites from these two distinct groups of samples contain moderate concentrations ofRe(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 skarnW–Aumineralization took place in theEarlyCretaceous peaking at 143.0–146.3 Ma, and was associated with a mixed crust–mantle system. The second fine‐grained granite‐related skarnW–Momineralization 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 theQingyang monzonitic granite. This finding suggests that the first period of skarnW–Aumineralization in theBaizhangyan 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 ofW–Momineralization derived from the crust at 135.0–136.9 Ma.