Fracture mineralization and dolomitization of Paleogene-Neogene carbonates, Al Ain area-UAE

Rus, Dammam and Asmari formations (Late Paleocene to Oligocene) are exposed in Hafit structure in eastern part of UAE- Al Ain area. The Hafit structure was formed as a result of Tarabat back thrust fault propagation folding which was initiated in mid Early Eocene till Miocene. These formations are composed of carbonate sequences which deposited in shallow marine environments. These carbonates were fractured mainly as a result of Zagros N-S maximum principal stresses. However, older E-W oriented fractures were also noticed in the area. These fractures are associated with EEN-WWS -EES-WWN and NNW-SSE - NNE-SSW conjugate shear fractures. Hydrothermal saddle dolomite and different generations of calcite cement were precipitated within these fractures and altered host rocks. Petrographic investigations combined with O, C, Sr isotope chemistry, major, trace and REE elements analysis, fluid inclusion microthermometery were performed on saddle dolomite, matrix dolomite and different generations of calcite cements to evaluate pore-fluid evolution and origin of these diagenetic phases. Petrographic examination of host dolostone revealed that there are two types of saddle dolomite; a replacive phase and a cement phase. The replacive dolomite, which replaces the precursor lime-packstone of Rus Formation and lime-wackestone of Dammam Formation has inner cores rich with inclusions and outer rims depleted with inclusions, undulosee extinction, non-planar to planar-e and average crystal size equal to 250microm with depleted delta 18OVPDB values (-6.02 to -2.91 ‰) for Rus Formation and (-8.34 to -6.59 ‰) for Dammam Formation, enriched delta 13CVPDB values (0.41 to 2.63 ‰) for Rus Formation and (1.18 to 1.77 ‰) for Dammam Formation, average Th of 86.3 ºC and average salinity 20.5 wt.% NaCl eq. Saddle dolomite cement has undulose extinction, curved surfaces and size ranging from 500 microm to 2mm with depleted delta18OVPDB values (-7.16 to -6.22 ‰) in Rus Formation and (-10.21 to -6.67 ‰) in Dammam Formation, enriched delta13CVPDB values (1.46 to 1.91 ‰) in Rus Formation and (0.34 to 1.78 ‰) in Dammam Formation, average Th of 97 ºC and average salinity of 22.0 wt.% NaCl eq. Both types of dolomite were precipitated from Mg-rich, hot, saline, radiogenic, delta 18OSMOW-enriched (0 to 7 ‰) basinal fluids with temperature higher than the temperature of ambient limestone. Hence, structurally-controlled hydrothermal dolomitization model is suggested in this study. Calcite filled veins in Rus, Dammam and Asmari formations have a mechanical twinning, bladed and dog-teeth, blocky and fibrous morphologies, wide range of depleted delta 18OVPDB values (-18.87 to -7.08 ‰), narrow range of depleted delta13CVPDB (-5.41 to -2.02 ‰), Th less than 50°C for Rus and Dammam calcite and 97.87°C for Asmari Formation and average salinity 0.5 wt.% NaCl eq. for Rus and Dammam calcites and 7.0 wt.% NaCl eq. for Asmari calcite. The Rus and Dammam calcite cement is probably originated from Ca-rich, less saline meteoric water with wide range of delta18OSMOW values ranging from -11.5 to -4 ‰, while the Asmari calcite cement may be originated from hot and saline basinal fluids mixed with meteoric fluids funneled from the surface during uplifting and they probably were interacted with gypsum units of Lower Fars Formation from surface with narrow range of delta18OSMOW (-6 to +1 ‰).