Pulsed exsolution of magmatic ore-forming fluids in tin-tungsten systems: a SIMS cassiterite oxygen isotope record

Y Utilizing in situ oxygen isotope analysis, we demonstrate the potential of cassiterite as a robust recorder of fluid source and evolution. Cassiterite is an ore mineral, and its mineral-water oxygen isotope fractionation factor is only weakly temperature-dependent. Unlike most explored gangue minerals such as quartz, cassiterite can provide a direct and robust archive of ore-forming fluids, e.g., fluid oxygen isotope composition (delta O-18 values). Core and rim domains of a representative cassiterite crystal from the Piaotang tin-tungsten (Sn-W) deposit, China, are characterized by contrasting delta O-18 values. Cassiterite delta O-18 values are -2.14 +/- 0.41 parts per thousand for the core and 2.36 +/- 0.36 parts per thousand for the rim, which equate to fluid delta O-18 values of similar to 4.1 parts per thousand (core) and similar to 8.6 parts per thousand (rim). Additionally, the cassiterite rim is enriched in niobium (Nb) and tantalum (Ta) compared to the mineral core. The delta O-18, and Nb and Ta data are interpreted to reflect core to rim crystallization from distinct pulses of magmatic-hydrothermal fluids that possessed a discrete oxygen isotopic, and Nb and Ta composition. Such a pulsed process could be a common feature for Sn-W deposits, and is critical to the formation of giant deposits with high metal grades. Involvement of meteoric water associated with the first mineralization stage reaches similar to 33%, but is limited (similar to 7%) in the second (main) mineralization stage. Therefore, cooling induced by fluid mixing may not be necessary for tin deposition, and our new findings invite a reassessment of the role of meteoric water in other Sn-W deposits. Our petrologic modelling shows that fluids exsolved from a 10 - 15 km(3) parental granitic magma can yield the Sn-W endowments recorded at Piaotang. Further, during magma fractionation, Sn and W are preferentially transferred into fluids compared to Nb and Ta. As a consequence, fluid chemistry is the primary factor controlling metal endowment and zoning in W-Sn deposits, as observed at Piaotang, and explains the predominant magmatic origin of Nb-Ta deposits.