Fluid sources and evolution of early Oligocene Be-W deposits in the Mahuaping ore district, Western Yunnan

The Cenozoic Sanjiang Tethyan metallogenic belt is renowned for large-scale porphyry-skarn Cu-Mo-Au mineralization systems. Recent studies have shown that this belt also host coeval Be and W mineralization, however, the genesis and evolution mechanisms of such coeval critical metal (e.g., W, Be) mineralization remain poorly constrained. This study focuses on the Yinchangping quartz-vein type Be-W deposit, which occurs within ophiolitic mélange in the Mahuaping ore district, western Yunnan. Through in situ muscovite Rb-Sr dating and scheelite geochemical analysis, the ore-forming age is precisely constrained at 29.5 ± 1.3 Ma (Early Oligocene). Scheelite display relatively uniform 87 Sr/ 86 Sr ratios (0.7056 – 0.7085), significant Y/Ho fractionation (19.1 – 59.8), and non-charge-and-radius-controlled (non-CHARAC) behavior, suggesting that the ubiquitous positive Eu anomalies and absence of Ce anomalies in scheelite collectively record a persistently reducing hydrothermal environment. The decoupling of in-situ Sr isotopes and Sr contents, accompanied by high-frequency ’sawtooth’ fluctuations and core-to-rim isotopic profile characteristics, robustly demonstrates that driven by a ’fault-valve’ mechanism, pristine magmatic fluids underwent multi-component mixing with highly radiogenic fluids that had intensively interacted with the basement, mid-ocean ridge basalt (MORB) components, or carbonate wall rocks. This intense water–rock interaction thoroughly disrupted the fluid chemical equilibrium, thereby triggering the large-scale co-precipitation of scheelite and beryl. More importantly, this study reveals that the Yinchangping and adjacent Mahuaping deposits are not isolated occurrences. Instead, they likely belong to the same magmatic‑hydrothermal metallogenic system. As a proximal end-member, the Yinchangping deposit directly records the early stage process of deep magmatic fluid ascent to shallow levels, undergoing water–rock interaction and initial mixing with metamorphic fluids. With the continuous upward evolution of fluids, the distal Mahuaping deposit experienced more intensive metamorphism, leading to attenuation of magmatic signals and completion of a geochemical transition to metamorphic fluid dominance. These two deposits constitute a temporally and spatially continuous Early Oligocene “magmatic-metamorphic transitional” Be-W metallogenic system within the Mahuaping ore district.