Physicochemical Factors Favoring the Formation of Greisen Tin Deposits: A New Look at Old Problems

By the example of the Tigrinoe tin-tungsten greisen deposit, Primorye (Russia), and based on the literature data on the composition of melt and fluid inclusions in minerals, conditions that are favorable for the formation of tin-bearing granites, mobilization of tin from them, and subsequent deposition of cassiterite are considered. It is shown that the factors favoring the formation of tin deposits related to granitoid magmatism include the following: (1) relatively low-temperature and shallow granites (720-770 degrees & Scy;/0.7-2 kbar, 3-6 km) formed under reducing conditions (fugacity of oxygen fO(2) below the fayalite-magnetite-quartz buffer, QFM), which is indicated by the absence of magnetite/presence of ilmenite, and by a reduced Ce anomaly in granite zircon; (2) low-salinity single-phase or two-phase fluid inclusions in magmatic quartz (vapor dominating the brine); and (3) fluid inclusions with the & Scy;& Ncy;(4)/& Scy;& Ocy;(2) ratio of 0.1-0.3 in the ore-vein minerals. A number of problems related to the formation of tin-tungsten deposits require further studies. First of all, this concerns the appraisal of the role of fluorine in magmatic accumulation and hydrothermal transport of tin. The partitioning of Sn and W between the granite melt and the fluids under strongly reducing conditions (at fO(2) below the QFM buffer) also has to be evaluated experimentally.