Solubility of Quartz in Hypersaline Brine ñ Implications for Fracture Permeability at the Brittle-Ductile Transition

Abstract

Pressure variations within magma-ambient environments, such as the diorite intrusion in the Ngatamariki hydrothermal system, are a natural consequence of shallow (2-4 km) pluton emplacement. In this study we have calculated the solubility of quartz in 10-40 wt% NaCl solutions at temperatures between 400 and 600 OC. The solubility of quartz in these highly saline fluids is strongly dependent on pressure. This dependence is primarily due to changes in the bulk density and the relative proportions of high and low density fluid phases which affect the total quartz solubility. A pressure drop from 400 to 300 bar for a fluid similar to those analysed in the Ngatamariki diorite (ca. 30 wt% NaCl) would lead to precipitation of one half of the dissolved silica as quartz from the bulk fluid, a drop from 2 to 1 g kg-1 dissolved silica. This process explains, at least in part, the large number of quartz veins, veinlets and segregations observed throughout the diorite and enclosing microdiorite.