Magmatic-hydrothermal tin deposits form in response to efficient tin extraction upon magma degassing

Abstract

Most of the global Sn resources are from granite-related ore deposits, which form in response to cassiterite precipitation from hydrothermal fluids. However, the physical and chemical controls on the efficiency of Sn extraction from upper crustal plutons by exsolving magmatic fluids are still unclear. In this study, we determine the partition coefficient of Sn between aqueous fluids and granitic melts (DSnfluid/melt) at 800 °C, 150 MPa and the fO2 of the Ni-NiO buffer. To obtain equilibrium partition coefficients, a new experimental method has been used relying on local equilibrium between silicate melt and microscopic-sized fluid bubbles. The latter formed synthetic fluid inclusions in the quenched glasses, which in turn were analyzed by laser ablation inductively coupled plasma mass spectrometry along with the enclosing glass. The results show that at constant aluminum saturation index (ASI = 1.05–1.08) of the silicate melt, DSnfluid/melt increases from 1.9 to 35.0 as the total Cl concentration (mCltotal) in fluid increases from 1.0 to 16.6 mol/kg H2O. At a fixed mCltotal = 2 mol/kg H2O, DSnfluid/melt increases from 4.3 to 10.6 as the HCl concentration in the solution increases from 0.15 to 0.79 mol/kg H2O, which in turn is a function of the ASI of the melt (ASI = 1.06–1.29). Numerical modeling suggests that Sn is extracted by magmatic fluids from upper crustal plutons most efficiently at the late stage of crystallization and degassing. At a similar degree of crystallization, granitic magma with lower initial water concentration and higher ASI will separate a fluid phase with higher Sn concentration and thus has higher Sn mineralization potential. Due to the relatively high DSnfluid/melt value, fluids exsolved from highly evolved magmas can sequester enough Sn to form Sn deposits and the sub-solidus remobilization of Sn from granite bodies is not a pre-requisite for ore genesis.