Chemical constraints on the weathering of serpentinites containing nickel-iron sulphides

Abstract

A review of mineralogical observations and results of Eh-pH measurements made on weathered profiles over serpentinites containing Ni-Fe sulphides in several areas of Western Australia permits some conclusions to be drawn relating to the chemical conditions prevailing during, and as the result of, weathering. At depth, conditions of relatively low Eh and high pH prevail. Eh increases and pH decreases upward in response chiefly to a series of anodic oxidative changes in the sulphides. Sulphides decompose at the water table, resulting in a sharp drop in pH which, in turn, promotes the decomposition of serpentine and carbonates. Above the water table, where the mineralogy is dominated by goethite, the pH increases again, promoting the local deposition of carbonates. Near-surface silicification is attributed to deposition of silica from silicic acid generated by the decomposition of serpentine during an earlier phase of weathering.