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In situ Sn–O isotopes
Cassiterite
Meteoric-water incursion
Large-scale Sn mineralization
Magmatic–hydrothermal system
Abstract:
Tin (Sn) is critical for advanced technologies, yet the fundamental mechanisms involved in generations of large-scale mineralization are poorly understood. Here we combine in situ Sn and oxygen (O) isotope analysis of cassiterite from the multiphase granites of the Cretaceous Mikengshan Sn district, South China, to better constrain the key factors in Sn ore formation. Petrological imaging and trace-element compositions of different types of cassiterite indicate that they crystallized from distinct pulses of exsolved magmatic fluids. Cassiterite O isotope compositions imply that these fluids had up to 50% meteoric water. Corresponding Sn isotopes define a trend in which δ124Sn decreases from early to late cassiterite, indicating a redox-controlled mechanism for cassiterite formation. Furthermore, the variable but relatively elevated δ124Sn values in cassiterite are explained through a combination of vapor- and redox-controlled isotope fractionation. These findings suggest that post-magmatic meteoric–water incursion during progressive cooling of shallow granitic intrusions leads to oxidation. This process plays a key role in the redistribution of Sn and thus the formation of large-scale deposits, suggesting that the timing of meteoric-water incursion is a key control on the scale of Sn mineralization. Combining traditional and non-traditional metal isotope systematics, measured in situ on the same sample material, proves invaluable for unraveling and quantifying unrecognized details in the evolution of magmatic–hydrothermal systems.
