HYDROCARBON‐INDUCED DIAGENETIC DOLOMITE AND PYRITE FORMATION ASSOCIATED WITH THE HORMOZ ISLAND SALT DOME, OFFSHORE IRAN

Abstract

Hormoz Island, a salt diapir in the eastern Persian Gulf, is dominated by the Infracambrian Hormoz Complex comprising both evaporites (Hormoz Salt) and carbonates, siliciclastics and volcanic rocks. Minerals include black, white and grey dolomites, pyrite, gypsum, anhydrite, apatite and iron oxides. Formation of some of the dolomite crystals is interpreted to be linked to the oxidation of hydrocarbons.The δ13C values of black dolomite crystals range from −0.8 to −2.07‰ VPDB, indicating that little if any of their carbon is derived from hydrocarbon oxidation but that sea water has provided carbon and Mg for dolomite precipitation. The δ18O values for these dolomites range from −9.2 to −15.3‰ VPDB, reflecting a temperature effect on isotopic fractionation. By contrast, δ13C values for white to grey dolomites range from −17.81 to −35.68‰ VPDB, indicating that the carbon may be derived from the oxidation of hydrocarbons. Based on the δ18Odolomite and temperatures obtained from fluid inclusion studies (215°C), the calculated δ18Owater in equilibrium with these dolomites (+2 < δ18Ofluid < +12‰) indicates the involvement of brines evolved via the interaction of seawater with the Hormoz Salt and associated sedimentary rocks. Some of the dolomite may have precipitated from post‐Cambrian seawaters at lower temperatures (ca.100 °C). Thus, the dolomites may have begun to form during deep burial but have also formed during salt diapirism at more shallow depths.Pyrite and native sulphur are interpreted to have formed in reducing conditions where the source of sulphur was H2S produced by the thermochemical reduction of sulphate in the Hormoz Salt evaporites. Heavy δ34S values for the anhydrites (ranging from 28.7 to 30.8‰) and for sulphides (ranging from 17.2 to 23.4‰) preclude a major contribution of sulphur from volcanic sources or from Early Cambrian shales.Pyrites, apatites and dolomites formed at depth within the diapir. It is envisaged that hydrocarbons leaked along the flanks of the Hormoz Island salt dome, resulting in reducing conditions which promoted the formation of diagenetic minerals.