High resolution observations of sulphide alteration textures

Abstract

The usually small sulphide phase in serpentinized peridotites is of interest when trying to evaluate reaction conditions during alteration, such as fO2, aH2 and aH2S, with one important indication being the occurrence of native metals and metal alloys. Using high resolution imaging and element mapping on samples from an active, continental serpentinization area in southwestern Turkey, sulphides, their composition and mineralogy, elemental distribution, and textures on a scale down to 30nm are described in this thesis. At least two stages of fluid-rock-interaction are visible in the samples. With phase relations in the Fe-Ni-O-S and Fe-Ni-Co-S systems, precipitation temperatures for the sulphides (200-300°C) and activities of H2 (>10-2) and H2S (<10-4) during later, ongoing lower-temperature (50-100°C) alteration of the grains are estimated. In the microtextures, which display a fibre-like intergrowth of awaruite and silicate in reaction zones within pentlandite, part of the process which replaces pentlandite with awaruite can be observed. Some sulphides are enriched in Cu, possibly indicating another, Cu-bearing fluid interacting with the rocks.