Sulfur isotopes geochemistry of the bituminous fatha evaporates in fatha formation, Hit–Abu Jir area, western Iraq

Abstract

Fatha Formation (Middle Miocene) in Hit area, western Iraq was targeted for this study. Pure gypsum, bituminous gypsum and native sulfur were investigated for δ34 S, FTIR and SEM to define the historical scenario of the paleo-environment and the relationship between sulfur, bitumen and evaporates (gypsum and anhydrite). SEM along with FTIR clarified that the bitumen were injected into the gypsum under high stress generated from the basin hydrodynamic pressure. The pure gypsum samples are enriched in heavy sulfur isotope and characterized by narrow range of δ34 S values (22.5 – 23.52 ‰) indicating a primary gypsum formed by direct precipitation from saline without bacterial activity. The lighter sulfur values (14.13 – 14.42 ‰) in the sulfur samples of the Abu-Jir area indicate a bacterial reduction of gypsum into native sulfur. The difference between δ34 S values in sulfur deposits in the Abu-Jir area (14.13 – 14.42 ‰) and in the Mishraq Mine (9.51 ‰) is clearly attributed to the remnant of gypsum as impurities within sulfur deposits in the Abu-Jir area. The bitumen sample has depleted in δ34S value (-2.57 ‰), whilst the bituminous gypsum samples have enriched (22.07 – 24.11 ‰). This high variation reflects the highly isotopic fractionation between oxic and anoxic environments.