فهرست مطالب zohdi a

The late Devonian has been associated with major global climate changes, widespread anoxia events, and the Hangenberg Crisis. The main aim of this study is to explore the relationship between paleoenvironment and paleoredox condition during deposition of the lower Devonian Jeirud Formation in the Garmabdar section, northeast of Tehran. The Jeirud Formation in the Garmabdar section, with a thickness of about 280 m, is composed of siliciclastics, carbonates, carbonate-siliciclastic hybrids and phosphorite deposits. Sedimentary facies analysis led to recognition of three siliciclastic facies associations related to estuarine environments under the influence of waves, shoreface and offshore and two carbonate facies associations related to intertidal and subtidal environments. Moreover, the geochemical paleoredox proxies (such as U/Th and V/Cr) also suggest prevailing anoxic conditions during deposition of black shales and phosphorites in the studied section. In overall, these results suggest that there is a good agreement between the depth of the depositional environment and anoxic conditions during deposition of black shales of the Jeirud Formation.

Rock units in the Madabad celestite deposit are composed of medium to thick-bedded and massive limestone interlayered with marly limestone and marl units of the Qom Formation (lower Miocene). Mineralization occurs as lens-shaped orebody, hosted by limestone units of member of the Qom Formation usually crosscutting bedding of the host rocks. Three stages of mineralization occurred in the Madabad deposit. The first stage is characterized by calcite formation during syn-depositional to syn-diagenesis processes. The second stage is related to hydrothermal processes that are distinguished by formation of fine-grained and sugary crystals of massive stage-1 celestite, vein-veinlets of coarse-grained stage-2 celestite along with minor strontianite and barite, coarse-grained euhedral crystals of stage-3 celestite with vug infilling texture, and finally late-stage quartz and calcite vein-veinlets. Stage three includes supergene processes. Hydrothermal alteration includes dolomitization, calcitization and silicification. Celestite along with minor strontianite and barite are ore minerals, and calcite, dolomite, quartz and iron oxides-hydroxides are gangue minerals at Madabad. The ore minerals show vein-veinlets, vug infilling, brecciated and cataclastic textures. Microthermometric measurements of two-phase liquid-rich fluid inclusions hosted in celestite II indicate that salinities values range from 6 to 18 wt.% NaCl equiv. (avg. 10.6 wt.% NaCl equiv.). These inclusions have homogenization temperatures range from 248 to 365 °C, with an average of 278 °C. These data indicate a minimum trapping depth of 510 m for the Madabad deposit. Sr was originated from evaporate units within the marly parts of the Qom Formation and volcanic units of the Karaj Formation. Characteristics of the Madabad deposit are similar to epigenetic replacement celestite deposits.

The Qom Formation is the only hydrocarbon reservoir in the central Iran sedimentary basin. In most parts of the central Iran, the Qom Formation conformably overlies the Lower Red Formation with an erosional discontinuity and is in turn conformably overlain by the Upper Red Formation. In the Madabad celestite deposit (south of Zanjan), the Qom Formation is composed of 190 m of medium to thick-bedded and massive limestone, marly limestone and marl. Five main microfacies are identified in the limestone units of the Qom Formation in the Madabad area. These microfacies probably were deposited on a shelf carbonate platform. Petrographic studies suggest original calcite mineralogy for limestone units of the Qom Formation in the Madabad area. Geochemical studies (Ca, Mg, Na, Sr, Mn and Fe) also represent the original calcite mineralogy in a closed diagenetic system with low dissolution rates. These evidence show significant role of fractures rather than diagenetic processes such as dissolution for increasing the reservoir quality of the Qom Formation for the oil and gas fields (such as Serajeh and Alborz) of central Iran.