Sodic Alteration in Magmatic-hydrothermal System

A magmatic volatile phase (MVP) exolves from a magmatic intrusion as a high salinity brine, a low salinity vapor, or a super-critical fluid due to a decrease in pressure and/or crystallization of anhydrous phases. The MVP can transport metals and other elements from the magma to the surrounding country rocks resulting in metallic ore deposits and mineral alteration. Potassic alteration zones (areas with K-feldspar as the final alteration product) are commonly associated with ore zones and exist directly above the magmatic intrusion. Sodic alteration zones (areas with albite as the final alteration product) exist on the flanks of the magmatic intrusion or further away from the intrusion. The potassic alteration zone has been studied extensively, whereas there is a dearth of experimental studies on the sodic alteration zone. This study examined the sodic alteration zones by performing experiments in the albite-andalusite-quartz brine system (65 wt. % NaCl equivalent) as well as albite-andalusite-quartz-vapor system (2 wt. % NaCl equivalent) from 600 to 700 °C and 80 MPa. The stable mineral phase was ascertained at each temperature as a function of the NaCl/HCl of the fluid. The equilibrium boundaries were found to be 87.5+/-12.5, 67.5+/-7.5, and 40+/-10 NaCl/HCl at 600, 650, and 700 °C, respectively. A thermodynamic model was developed for the albite-andalusite-quartz-brine system and can be applied to ore-systems such as the porphyry copper deposit at Yerington, Nevada.