Skarn mineralogy and in-situ LA–ICP–MS U–Pb geochronology of wolframite for the Caojiaba tungsten deposit, southern China: Implications for a reduced tungsten skarn deposit
Zhiyuan Zhang, Guiqing Xie, J. M. Thompson
Abstract
The Caojiaba tungsten deposit (19.03 [email protected] 0.37 wt% WO3) is hosted in skarn within clastic and carbonate rocks in the Xiangzhong metallogenic province (XZMP), southern China. The deposit is characterized by an early prograde skarn overprinted by a retrograde assemblage, and then by quartz–scheelite–sulfide veins. The Caojiaba skarn is characterized by grossular garnet (Grs65–85Adr7.1–31) and hedenbergitic pyroxene (Hd63–86Di11–33), similar to the well–studied reduced tungsten skarns worldwide. In this paper, we present in-situ laser ablation inductively coupled plasma mass spectrometry (LA–ICP–MS) analysis of U–Pb isotopes and trace elements of wolframite grains coexisting with scheelite in the quartz–scheelite–sulfide veins. Uranium is correlated positively with Nb5+, tetravalent (Ti4+, Sn4+, Zr4+, Hf4+), and trivalent (Sc3+, V3+, Y3+, REE3+) cations in all wolframite samples, suggesting that the incorporation of U into wolframite is controlled by the coupled substitution mechanisms. Two wolframite samples (CJB–43 and CJB–44) yielded 206Pb/238U ages of 212.4 ± 1.0 Ma (2σ, n = 50, MSWD = 0.92) and 211.8 ± 1.1 Ma (2σ, n = 43, MSWD = 1.4), overlapping with the late Triassic granitic rocks (225.6–204.5 Ma) in the XZMP. Mineralogical and geochronological evidence collectively indicate that Caojiaba is a typical example of the late Triassic reduced tungsten skarn deposits in the XZMP. This study demonstrates the potential of U–Pb dating of wolframite and highlights its importance for directly dating hydrothermal ore–forming processes.