Element Mobility in Melts during Successive Intrusions of Crustal-derived Magmas and Sn-W Mineralization

Abstract

Continental collisions are the place where granitic plutons result from the melting of crustal components. Granitic plutons are built up by successive input of magma with a variable composition and hence temperature and chemistry. The intrusion of a new magma batch has consequences on the element mobility in the melt. Diffusion in already formed crystals is limited, due to the short time interval between magma input, and because of the low values of element diffusivity in solids. Because the new magma is generally hotter than the magma chamber, the temperature in the contact zone is modified. It acti- vates diffusion by and modifies its characteristic length for element mobility in the melt. A new intrusion also modifies the partition coefficients, decreasing compatibility and increasing the incompatibility. The change in temperature has also effects on fluid exsolution controlled by crystallization, or second boiling. The present paper examines the intrusion of magma (fel- sic or mafic) into a felsic magma chamber with a time interval of 30 ky. The intrusion of magma with similar composition, hence low (±100°C) temperature difference has few effects. The diffusion lengths for elements rarely exceed one order of magnitude. The fluids released by the cooling magma are balanced by their reincorporation into the warming magma. In con- trast, the intrusion of mafic magma into felsic magma chamber results in temperature difference that can reach ±300°C. It may change the diffusion length up to two orders of magnitude for elements having large activation energy. Partition coeffi- cients also vary by more than one order of magnitude. The effect is enhanced in the warming felsic magma, and damped in the mafic magma. In consequence elements like As, Sn, Sr, W, Zr are driven from the mafic magma toward the felsic magma. The release of H2O and CO2 are balanced between the two magma types. However the mafic magma releases an important amount of S that cannot re-dissolves into the felsic magma and remains in the fluid phase. This simple model also addresses processes acting during ore formation. In particular, it examines the behavior of ions with a four valences state, as Sn and W, which has implications on the incorporation of other elements sharing a similar structure. It points out the necessi- ty of external factors (S, halogens content and redox conditions) for controlling ore formation.