Zircon U-Pb, molybdenite Re-Os and muscovite Ar-Ar isotopic dating of the Xitian W-Sn polymetallic deposit, eastern Hunan Province, South China and its geological significance

摘要

The Xitian tungsten-tin (W-Sn) polymetallic deposit, located in eastern Hunan Province, South China, is a recently explored region containing one of the largest W-Sn deposits in the Nanling W-Sn metallogenic province. The mineral zones in this deposit comprise skarn, greisen, structurally altered rock and quartz-vein types. The deposit is mainly hosted by Devonian dolomitic limestone at the contact with the Xitian granite complex. The Xitian granite complex consists of Indosinian (Late Triassic, 230-215 Ma) and Yanshanian (Late Jurassic-Early Cretaceous, 165-141 Ma) granites. Zircons from two samples of the Xitian granite dated using laser ablation inductively coupled mass spectrometer (LA-ICPMS) U-Pb analysis yielded two ages of 225.6 +/- 1.3 Ma and 151.8 +/- 1.4 Ma, representing the emplacement ages of two episodic intrusions of the Xitian granite complex. Molybdenites separated from ore-bearing quartz-veins yielded a Re-Os isochron age of 149.7 +/- 0.9 Ma, in excellent agreement with a weighted mean age of 150.3 +/- 0.5 Ma. Two samples of muscovites from ore-bearing greisens yielded Ar-40/Ar-39 plateau ages of 149.5 +/- 1.5 Ma and 149.4 +/- 1.5 Ma, respectively. These isotopic ages obtained from hydrothermal minerals are slightly younger than the zircon U-Pb age of 151.8 +/- 1.4 Ma of the Yanshanian granite in the Xitian area, indicating that the W-Sn mineralization is genetically related to the Late Jurassic magmatism. The Xitian deposit is a good example of the Early Yanshanian regional W-Sn ore-forming event (160-150 Ma) in the Nanling region. The relatively high Re contents (8.7 to 44.0 ppm, average of 30.5 ppm) in molybdenites suggest a mixture of mantle and crustal sources in the genesis of the ore-forming fluids and melts. Based upon previous geochemical studies of Early Yanshanian granite and regional geology, we argue that the Xitian W-Sn polymetallic deposit can be attributed to back-arc lithosphere extension in the region, which was probably triggered by the break-off of the flat-slab of the Palae-Pacific plate beneath the lithosphere. (C) 2016 Elsevier B.V. All rights reserved.