Geochemical characteristics of scheelite and its implications for the ore-forming processes of the Relin Cu–Mo Deposit

Abstract

The Geza arc belt in Yunnan Province, located in the southern segment of the Yidun arc, is one of the important newly discovered Cu–Mo–W polymetallic metallogenic belts in the Sanjiang Tethyan tectonic belt of southwestern China in recent years. During the Late Yanshanian, porphyry-skarn-type Mo(Cu) deposits and hydrothermal-type W(Mo) deposits formed in association with collisional magmatic intrusions. Among them, the Relin Cu–Mo–W polymetallic deposit is a typical representative of Late Yanshanian diagenesis and mineralization in this region. Based on systematic field work, this study conducted comprehensive research on the deposit geology, mineralogy, and geochemical compositions of scheelite from the Relin deposit, and examined its diagenetic and metallogenic ages combined with zircon U–Pb dating. Laser ablation–inductively coupled plasma mass spectrometry (LA-ICP-MS) zircon U–Pb dating results show that the ore-hosting monzogranite was formed in the Late Cretaceous, with weighted mean ages of 80.2 ± 0.6 Ma and 80.7 ± 0.6 Ma. The Sr isotopic compositions of scheelite (0.70774–0.7091) are in excellent agreement with those of the ore-hosting monzogranite (0.70746–0.70930), indicating a close temporal coupling between scheelite mineralization and Late Yanshanian magmatic activity. Geochemical characteristics reveal that scheelite has significantly higher total rare earth element (ΣREE) contents (723.48–6991.74 ppm) than the host monzogranite, with strong fractionation between light and heavy rare earth elements (LREE/HREE = 1.59–53.00, average = 16.87) and strongly negative Eu anomalies (average δEu = 0.37). This suggests that REEs, as incompatible elements, were preferentially enriched in the residual ore-forming fluid phase during magmatic evolution. The substitution of REE³⁺ in scheelite follows a mixed mechanism: it is dominated by the vacancy-coupled substitution (3Ca²⁺ = 2REE³⁺ + □Ca, □Ca denotes a Ca-site vacancy), with minor contributions from Na⁺ charge compensation (2Ca²⁺ = REE³⁺ + Na⁺) and Nb⁵⁺ charge compensation (Ca²⁺ + W⁶⁺ = REE³⁺ + Nb⁵⁺) substitutions. Scheelite in the mining area is significantly enriched in Mo (4060.12–30,441.51 ppm, average = 16,821.14 ppm), and the Mo enrichment is mainly derived from exsolution of the monzogranite. The oxidation state of Eu, combined with Mo contents and δEu values, indicates that the ore-forming fluids were predominantly oxidized. The Y/Ho ratios of scheelite range from 22.73 to 30.35, consistent with the Y/Ho ratios of regional monzogranites (27–30). Synergistic evidence from Sr–Nd isotopes suggests that the ore-forming fluids were derived from anatexis of ancient continental crust, with no large-scale fluid mixing or significant water–rock interaction. The Y/Ho geothermometer estimates that the mineralization temperature of scheelite was approximately 440 °C. In summary, this study proposes that scheelite in the Relin deposit formed during the late stage of magmatic evolution. When volatiles in the magmatic melt reached saturation, fluid exsolution occurred, and tungsten was preferentially released into the fluid phase, forming tungsten-rich ore-forming fluids. During the outward migration of exsolved ore-forming fluids, variations in physicochemical conditions promote the combination of WO₄²⁻ in ore-bearing fluids with native Ca²⁺ from magma and minor Ca²⁺ released by plagioclase decomposition, generating scheelite (CaWO₄). This process ultimately completes the mineralization of various types of scheelite hosted within the Late Yanshanian granites of the Relin Cu–Mo deposit.