Geological and geochemical constraints on the Farahabad vent-proximal sub-seafloor replacement SEDEX-type deposit, Southern Yazd basin, Iran
Résumé
Farahabad is a Lower Cretaceous-age sandy dolomite, sandstone and silty limestone hosted zinc-lead deposit in the southern Yazd basin, Iran. This deposit occurs in the lower part of Taft Formation. The Farahabad deposit is a stratiform and stratabound accumulation of hydrothermal sulfides. The mineralized zone at Farahabad is lens shaped and has a relatively flat top. It also ranges greatly in thickness, from a few meters to > 30 m. Three styles of ore facies have been differentiated at the deposit. Feeder zone, bedded ore facies, whereas the bulk of ore is contained within sphalerite and galena breccias that record a prolonged and texturally complex history of ore replacement (massive-replacement ore facies). The shape and size of the mineralized body suggest that dolo-mites replaced with sulfide minerals in a submarine channel. Dolomitization and sulfide mineralization's are the major hydrothermal products in the Farahabad deposit, and they occur in all host rocks, but only in the immediate vicinity of the syn-sedimentary normal fault that served as the main conduit for the mineralizing fluids. Based on microscopic studies, sulfide mineralization in the Farahabad deposit emplaced during two stages: fine-grained sulfide bands (stage I) are intricately interlayered with organic matter-rich beds and thin turbidite beds. They exhibit classic sedimentary textures, such as laminae, cross bedding, graded bedding and bedding, indicative of a synsedimentary origin. Main-stage (stage II) mineralization involved the progressive replacement of preexisting sulfides and the dolomite breccias, initially by replacement of the breccia matrix and ultimately by replacement of clasts. Dolomite minerals from the feeder zone and massive-replacement ore facies have 115-234°C trapping temperature with salinities ranging from 0.99 to 16.70 eq. wt% NaCl eq. δ 34 S values of sulfide minerals range from −19.47 to +3.9‰, suggesting that sulfur in the hydrothermal fluids was derived predominantly from reduction of seawater sulfate by bacteriogenic sulfate reduction. The δ 13 C PDB and δ 18 O SMOW values of host limestone's and hydrothermal dolomites plot the near marine carbonate rocks field in a plot of δ 13 C PDB vs. δ 18 O SMOW diagram. It suggests that CO 2 in the hydrothermal fluids was mainly originated from marine carbonate rocks. Abrupt lateral changes in facies and thickness, along with the existence of synsedimentary breccia's and debris flows within the ore sequence, suggest the proximity of synsedimentary faults and tectonic activity contemporaneous with the sedimentation in the Lower Cretaceous, favorable to the formation of deposit. So this deposit formed by combination of sub-seafloor replacement and on the seafloor processes. Characteristics of the Farahabad deposit are compatible with a vent-proximal sub-seafloor replacement SEDEX-type classification.