Newly discovered veinlet-disseminated type scheelite mineralization in the deep-seated biotite monzogranite from the giant Zhuxi deposit: Implications for source and evolution of ore-forming fluid
Yanshen Yang, Xiaofei Pan, Baoshun Liu, Jiaxiang Dong
Abstract
• Scheelite-bearing veinlet was found in deep-seated biotite monzogranite. • Ore-forming fluid originated from magma and Neoproterozoic metasedimentary rocks. • Fluid underwent apatite fractionation before scheelite precipitation. • Scheelites in Zhuxi share a similar source but are various in fluid process. The coexistence of multiple types of tungsten (W) mineralization is a hallmark of giant W deposits. The Zhuxi deposit in South China, the largest W deposit globally, contains 4.22 Mt of WO 3 . Here, we investigate newly identified scheelite-bearing biotite monzogranite from the Zhuxi deposit, analyzing the elemental compositions and Sr isotopic signatures of scheelite and apatite coexisting within veinlets hosted in the biotite monzogranite. Micropetrographic relationships and rare earth element (REE) compositions in minerals indicate that apatite crystallized earlier than scheelite. Using the Rayleigh distillation model, our simulations indicate that the ore-forming fluids underwent approximately 10 % apatite-dominated fractional crystallization prior to scheelite crystallization. The apatites exhibit homogeneous 87 Sr/ 86 Sr ratios ranging from 0.71743 to 0.71921, suggesting that the ore-forming fluids inherited their Sr isotopic signatures from both the parental magma and Neoproterozoic metasedimentary rocks. The 87 Sr/ 86 Sr ratios of scheelite, ranging from 0.71689 to 0.71820, are consistent with those of scheelite from other occurrences within the Zhuxi deposit. This similarity indicates that ore-forming fluids across the Zhuxi deposit were derived from common sources. This study also compiled compositional data for scheelite from various occurrences in the Zhuxi deposit reported in the literature, dividing them into four types: (type-I) scheelite overprinting massive prograde skarn, (type-II) scheelite overprinting prograde skarn veinlets, (type-III) scheelite hosted in quartz–muscovite veins within marble, and (type-IV, analyzed in this study) scheelite in veinlets within biotite monzogranite. The type-IV scheelite exhibits REE contents (mean: 83 ppm) and Y/Ho ratios (mean: 33) comparable to those of the ore-forming granites in the Zhuxi deposit. In contrast, the other three types display significantly lower REE contents and higher Y/Ho ratios, suggesting that the fluids responsible for type-I, −II, and −III scheelite crystallization underwent varying degrees of fluid evolution and interaction. Our systematic investigation of various occurrences of scheelite within the Zhuxi deposit reveals that the significant variations in composition of scheelites may be influenced by factors such as the assemblage of coexisting minerals, the host rock properties, and the intensity of fluid-rock interactions.