Ore Genesis of the Lunwei Granite-Related Scheelite Deposit in the Wuyi Metallogenic Belt, Southeast China: Constraints from Geochronology, Fluid Inclusions, and H-O-S Isotopes

摘要

A granite-related scheelite deposit has been recently discovered in the Wuyi metallogenic belt of southeast China. The veinlet-disseminated scheelite occurs mainly in the inner and outer contact zones of the porphyritic biotite granite, spatially associated with potassic feldspathization and silicification. Re-Os dating of molybdenite intergrowths with scheelite yield a well-constrained isochron age of 170.4 +/- 1.2Ma, coeval with the LA-MC-ICP-MS concordant zircon age of porphyritic biotite granite (167.6 +/- 2.2Ma), indicating that the Lunwei W deposit was formed in the Middle Jurassic (similar to 170Ma). We identify three stages of ore formation (from early to late): (I) the quartz-K-feldspar-scheelite stage; (II) the quartz-polymetallic sulfide stage; and (III) the quartz-carbonate stage. Based on petrographic observations and microthermometric criteria, the fluid inclusions in the scheelite and quartz are determined to be mainly aqueous two-phase (liquid-rich and gas-rich) fluid inclusions, with minor gas-pure and CO2-bearing fluid inclusions. Ore-forming fluids in the Lunwei W deposit show a successive decrease in temperature and salinity from Stage I to Stage III. The homogenization temperature decreases from an average of 299 degrees C in Stage I, through 251 degrees C in Stage II, to 212 degrees C in Stage III, with a corresponding change in salinity from an average of 5.8wt.%, through 5.2wt.%, to 3.4wt.%. The ore-forming fluids have intermediate to low temperatures and low salinities, belonging to the H2O-NaCl +/- CO2 system. The O-18(H2O) values vary from 1.8 parts per thousand to 3.3 parts per thousand, and the DV-SMOW values vary from -66 parts per thousand to -76 parts per thousand, suggesting that the ore-forming fluid was primarily of magmatic water mixed with various amounts of meteoric water. Sulfur isotope compositions of sulfides (S-34 ranging from -1.1 parts per thousand to +2.4 parts per thousand) and Re contents in molybdenite (1.45-19.25 mu g/g, mean of 8.97 mu g/g) indicate that the ore-forming materials originated mainly in the crust. The primary mechanism for mineral deposition in the Lunwei W deposit was a decrease in temperature and the mixing of magmatic and meteoric water. The Lunwei deposit can be classified as a porphyry-type scheelite deposit and is a product of widespread tungsten mineralization in South China. We summarize the geological characteristics of typical W deposits (the Xingluokeng, Shangfang, and Lunwei deposits) in the Wuyi metallogenic belt and suggest that porphyry and skarn scheelite deposits should be considered the principal exploration targets in this area.