Hydrothermal Fluids of Magmatic Origin

Abstract

Hydrothermal fluids are natural heated water solutions wherein variety of elements, compounds and gases may be dissolved. They are generated by diverse crustal and mantle geological processes including basinal fluid interaction, magmatic differentiation and mantle degassing. Mixing of the fluids of two different origins is often possible, and their solidified product may be economic. As the magmatic hydrothermal fluid is formed during the course of magmatic evolution from low volatiles and CO2-CH4 rich at primary basaltic magmatic phase to the saline water rich during granite/pegmatite formation, large number of constituents including the common S, Cl, F, Na, K, N2, metals and even REE may be found in it. The enrichment of halogen in the magmatic hydrothermal fluids promotes the partitioning of economically useful elements like Cu, Pb, Zn, W, Mn, Li, Rb, Sr and Ba from melt to the fluid phase. Salinity of this hydrothermal fluid, which varies from near 0 to >50 wt.% eq. NaCl, is a function of pressure. A wide range of immiscibility in the magmatic fluids results compositional modification of the gradually developing phases from nearly anhydrous melt to last residual low temperature water solution. Wet magma with higher mol% of water flows easily and exsolves water at low pressure regime. Influx of water in the heated rock suit can lower the liquidus temperature triggering melting at a lower temperature than the anhydrous melting. Fluid inclusions have been widely used to understand behaviour of ore forming fluids and the magmatic immiscibility such as silicate melt, H2O-CO2, hydrosaline melt, dense CH4 and sulphide-metal melt. Ore deposition is generally linked with the late stage of magmatic hydrothermal fluid, and its repeated pulses may lead to the formation of large ore bodies. In addition to saline aqueous fluid, the volatiles like H2S, CO2, SO2, SO4, HCl, B and F, are found as significant ore-depositing agents in magmatic-hydrothermal fluids. A hydrothermal fluid may dissolve economically useful elements or simply act as carrier for them. PVTX conditions obtained from fluid inclusions are vital for defining hydrothermal system and resulting ore mineralization, though such interpretation is largely based on the knowledge of their phase equilibria. The fluid process related to the epithermal Au deposits, porphyry type deposits, Malanjkhand Cu deposit and Balda-Tosham tungsten province, India, have also been discussed briefly.