Geochronology, geochemistry and oxidation state of the Dongyuan biotite granite in the Jiangnan tungsten ore belt: Implications for the I-type granite-related W mineralization

Abstract

The Dongyuan deposit, located in the southern Anhui Province, China, is a large porphyry W–(Mo) deposit in the Jiangnan tungsten ore belt. Ore minerals are dominated by scheelite and molybdenite, which occur as disseminated or veined in biotite granite. Zircon U–Pb dating of biotite granite is 143.5 ± 0.7 Ma, which is consistent with formation age of regional tungsten mineralization. Chemical compositions of biotites from biotite granite plot near or below Ni–NiO buffer on a Fe3+–Fe2+–Mg2+diagram, suggesting that the Dongyuan biotite granite has a relatively low oxygen fugacity. The biotite granite samples are characterized by high SiO2 (68.78–70.98 wt%), Al2O3 (14.60–15.32 wt%), K2O (3.45–4.82 wt%) contents and K2O/Na2O ratios (0.98–1.59), with A/CNK ranging from 0.97 to 1.02, belonging to I-type granite. They show adakite-like geochemical features, with high Sr/Y (35.98–57.90) and (La/Yb)N (21.41–38.78) ratios but low Y (5.54–6.77 ppm) and Yb (0.44–0.55 ppm) contents, which have low MgO (0.60–0.77 wt%), Cr (15.39–28.52 ppm) and Ni (3.68–5.66 ppm) contents and Mg# (31–35) values. These samples have high initial 87Sr/86Sr ratios (0.7144–0.7160) and negative ƐNd(t) values (−9.1 to −5.5), with two-stage Nd model ages (TDM2) of 1379–1695 Ma, while the zircon ƐHf(t) values and two-stage Hf model ages (TC DM) are −6.4 to −0.7 and 1243 to 1601 Ma, respectively. These data suggest that the Dongyuan biotite granite was derived from partial melting of a thickened crustal source which had been previously produced by Neoproterozoic magmatic underplating. Combined with previously published data, we propose that the W mineralization of the Dongyuan deposit was controlled by metal-rich source region, fractional crystallization, low oxygen fugacity condition, and enrichment of fluorine volatiles, which also influenced other I-type granite-related W deposits in the Jiangnan tungsten ore belt. Furthermore, the W-related granitic rocks in this tungsten ore belt developed in a compression–extension transitional setting that was triggered by the subducted Paleo-Pacific slab roll-back.