Abstract

Stratabound sulphur deposits of northern Iraq occur in folded Middle Miocene evaporites overlying Lower Miocene hydrocarbon reservoirs. Rocks hosting sulphur mainly comprise gypsum, dolomitic marl, carbonate mudstones and bioclastic dolomitized limestones, whereas the mineralized rocks are mainly composed of calcite and sulphur. Sulphur was formed by the oxidation of H₂S produced from gypsum dissolution followed by sulphate reduction in the presence of hydrocarbons. Sulphate reduction occurred in a partially closed to closed system as reflected by the sulphur δ³⁴S values of +7‰ to +20‰ (original gypsum +21‰). Host rock carbonates were calcitized by the dissolved carbonate formed by oxidation of the hydrocarbons, which produced low δ¹³C values ( – 14 to – 32‰). Periodic invasion by meteoric water and elevated temperatures produced ¹⁸O-depleted compositions for the newly-formed calcite (down to – 9‰) and dolomite (down to – 7‰), and oxidized the H₂S produced during sulphate reduction.

These sulphur deposits formed through the favourable stratigraphic juxtaposition of gypsum sulphate, brines and hydrocarbons. Tectonically produced fracture permeability in carbonates allowed rising brines from the underlying hydrocarbon reservoirs to mix with laterally migrating and descending, sulphate-bearing meteoric waters and produced the oscillating redox conditions needed for the bacterial reduction of sulphate and the oxidation of sulphide.