Constraints of scheelite and wolframite on multiple magmatic fluid generations and W mineralization of the Lijia Sn deposit, South China

Abstract

Multistage W mineralization often raises the question of whether ore-forming fluids originate from a single magmatic pulse or multiple distinct pulses. The Lijia Sn deposit (South China) hosts greisen- and quartz vein-type W mineralization primarily within porphyritic biotite granite, and to a lesser extent, in later biotite granite and aplitic dikes. Two W mineralization stages are identified: stage-I, characterized by sparse scheelite disseminated in the biotite granite, and stage-II, comprising scheelite-bearing greisen and wolframite-scheelite-bearing quartz-tourmaline veins that crosscut both the biotite granite and aplitic dikes. Here, we present in-situ U-Pb ages and trace element data of wolframite and scheelite from both mineralization stages. Wolframite and scheelite from stage-II veins yield U-Pb ages of ca. 427 Ma, consistent with the emplacement ages of the host granitic intrusions. Given that the aplitic dikes are more differentiated than the biotite granite, we infer that stage-I fluids were sourced from the granitic magmas, whereas stage-II fluids were derived from the aplitic magmas. Increasing Mg, FeO and Ti contents of wolframite from deep to shallow levels are attributed to biotite breakdown, whereas high and upward-increasing Sr contents of stage-II scheelite reflect feldspar decomposition. These trends indicate that both wolframite and scheelite deposition were related to greisenization. Although stage-II scheelite has higher Nb and Ta but lower Nb/Ta, La/Yb and Mo than stage-I scheelite, mass balance calculation demonstrates that these differences do not represent initial fluid compositions. Instead, they result from fluid-rock interaction and co-precipitation of hydrothermal minerals rich in these elements. Therefore, previously proposed geochemical indicators of scheelite (Nb, Ta, Nb/Ta, La/Yb and Mo) are unreliable for tracing multiple fluid exsolution events from evolved magmas.