Sn–Cu coexisting mechanism of Huaaobaote deposit, Inner Mongolia: Constraints from geochronology, mineralogy and geochemistry

Sn is predominantly associated with highly fractionated, reduced ilmenite-series granites, whereas Cu is commonly linked to less fractionated, oxidized magnetite-series granites. The sharply contrasting geochemical behaviors make their paragenesis theoretically improbable. Hence the global occurrence of numerous Sn–Cu coexisting deposits presents a geological paradox. The Huaaobaote deposit, located in the Southern Great Xing’an Range in China, is a representative Sn–Cu coexisting deposit. The Huaaobaote intrusion records a continuous magmatic evolution series, ranging from porphyritic monzogranite (PMG) through fine-grained syenogranite (FGG) to granite porphyry (GP), with mafic microgranular enclaves (MMEs) developed in the PMG. There are explicitly causal relationships between magmatism and mineralization, as well as the characteristics of the mantle, which contribute to investigating the Sn–Cu coexisting mechanism. LA–ICP–MS zircon U–Pb dating of the MMEs, PMG, FGG, and GP of the Huaaobaote intrusion yielded indistinguishable crystallization ages within analytical uncertainty, constraining their emplacement to the Early Cretaceous. Geochemically, the MMEs are characterized by relatively low SiO 2 contents, affiliation with the high–K calc-alkaline series, metaluminous features, and pronounced negative Eu anomalies. In contrast, the felsic rocks (PMG, FGG, and GP) are uniformly enriched in silica, alkalis, and aluminum, and also exhibit significant negative Eu anomalies. They are all enriched in Th, U, Zr, Hf, and Nd, but depleted in Ba, Sr, P, and Ti. A coherent magmatic evolutionary trend is evident from the PMG to the GP, marked by an increasing degree of fractionation and a compositional shift from weakly peraluminous to distinctly peraluminous. This trend is accompanied by a more pronounced negative Eu anomaly and stronger depletions in Ba, Sr, P, and Ti in the most evolved GP. Electron probe microanalysis of amphiboles from the MMEs, PMG, and FGG yields melt water contents of 4.29–6.82 wt%, 5.40–10.1 wt%, and 5.12–6.43 wt%, respectively. The corresponding oxygen fugacities (ΔFMQ) of the magmas are + 0.59 to + 1.29, +0.24 to + 1.11, and –0.20 to + 0.59. Thus, we conclude that the magmatic environment, characterized by high water content and moderate oxygen fugacity, was key to enabling the Sn–Cu coexisting mechanism of the Huaaobaote deposit.