Tungsten mineralisation in the Zhazigou W–(Mo) deposit, Henan Province, China: Constraints from scheelite geochemistry and molybdenite geochronology

Abstract

Skarn-type scheelite deposits are one of the most strategically and economically significant resources in China. The Zhazigou deposit, which contains 27.27 Mt of ore reserves at an average grade of 0.64% WO3 and 0.02% Mo, is a large W–Mo deposit located within the East Qinling Orogenic Belt between the North China and Yangtze cratons. This study presents new scheelite mineralogical and molybdenite geochronological data for the deposit, and uses these data to explore the evolution of ore-forming fluids associated with the deposit and the processes that formed the mineralisation. The deposit is a redox-intermediate W–(Mo) skarn deposit, and records four stages of mineralisation, namely, skarn (I), quartz–scheelite (II), quartz–sulphide (III), and carbonate (IV) stages. The W mineralisation mainly developed during the quartz–scheelite stage (II), whereas the Mo mineralisation generally developed during the quartz–sulphide stage (III); trace molybdenite that coexists with scheelite formed during the quartz–scheelite stage (II). Re–Os geochronological analysis of molybdenite intergrown with scheelite yields a weighted mean age of 146.3 ± 0.9 Ma that is consistent with a 145.4 ± 2.6 Ma Re–Os isochron age and a 147 Ma zircon U–Pb age for the Yuku porphyritic granite, which is close to the Zhazigou deposit. The substitution of the rare earth elements (REEs) into scheelite is controlled predominantly by the reaction 3Ca2+ = 2REE3+ + □Ca, which indicates that the REE characteristics of the scheelite in the Zhazigou deposit were inherited from the mineralising fluids. The REE and Sr–Nd isotopic compositions of the scheelite within the deposit are similar to those of the Yuku porphyritic granite, indicating that the ore-forming fluid was mainly magmatic in origin. Scheelite samples from the skarn and quartz–scheelite stages (I–II) have high Mo contents (up to 38930 ppm), whereas notably decreasing Mo contents (727–2629 ppm) characterise the quartz–sulphide and carbonate stages (III–IV). The changing redox state had a key role in affecting Mo abundances in the Zhazigou scheelite. A relatively oxidising fluid in the skarn (I) and quartz–scheelite (II) stages transformed into a reducing fluid in the quartz–sulphide (III) and carbonate stages (IV). Changes in the mineral assemblages from ferric- (e.g., andradite) to ferrous- (e.g., pyrrhotite) dominated also record these redox changes. The scheelite was precipitated as a result of fluid–rock interaction that released Ca2+ into the mineralising fluids. Oxygen fugacity exerted an important control on molybdenite precipitation, and a decrease in oxygen fugacity might have promoted the formation of scheelite, as well as the change from high- to low-Mo scheelite.