Tungsten mineralization in the Caojiaba tungsten deposit, southern China: Constraints from scheelite in-situ trace elements and Sr isotope evidence

Abstract

The Caojiaba tungsten deposit (19.03 Mt@ 0.37 wt% WO3) is hosted in skarn within clastic and carbonate rocks in the Xiangzhong metallogenic province (XZMP), southern China. The genesis of the Caojiaba tungsten deposit remains to be determined, and the role of trace element enrichment and precipitation in scheelite has received little study to date. The deposit is characterized by early prograde skarn overprinted by retrograde skarn assemblage, and then by quartz–scheelite–sulfide veins. Scheelite (CaWO4) across the mineralization stages can be divided into four generations, i.e., scheelite type–1 (Sch1) in the prograde skarn, early scheelite type–2 (Sch2) and late scheelite type–3 (Sch3) grains in the retrograde skarn, and scheelite type–4 (Sch4) in the quartz–scheelite–sulfide vein. In the cathodoluminescence (CL) images, Sch1, Sch2, and Sch3 have core–to–rim textures displaying CL–dark core and CL–bright rims, whereas Sch4 is homogeneous in the CL response across the grains. Sch1 contains high REE + Y and Ta concentrations, and has negative Eu anomalies relative to the other three types of scheelite grains, consistent with the contribution from magmatic fluids related to the tungsten granites. The increasing fluid–rock interaction intensity, precipitation of REE–rich minerals, and pH change in ore–forming fluids increase positive Eu anomalies and LREE/HREE fractionation of scheelite from Sch2 to Sch4. The in-situ initial 87Sr/86Sr ratios of Sch2 range from 0.71727 to 0.72142, which are within or slightly higher than those of the exposed Late Triassic tungsten granites in the XZMP. The broadly increasing initial 87Sr/86Sr ratios of Sch3 (0.72070 to 0.72764) suggest a source derived from magmatic fluids and contribution of the Neoproterozoic Banxi Group slate. This study highlights that the variations in trace element and Sr isotope compositions of scheelite can be used to reveal the source of ore–forming fluids and metal, and constrain the ore–forming processes.