Geochemical and Clay Mineralogical Characteristics of the Black Shale and Constrains on Diagenesis and Maturation, Chia Gara Formation, Iraqi Kurdistan Region, Iraq

Abstract

Black shale of Tithonian (Late Jurassic)-Berriasian (Early Cretaceous) Chia Gara Formation have been investigated by clay minerals and geochemistry (total organic carbon, major and trace elements) to determine the chemical associations and the relation between their diagenesis and maturity. The X-ray diffraction data show that kaolinite and illite are predominant clay minerals. The samples show low illite crystallinity index (0.56ºΔ2θ), and most of the illite-smectite mixed layers are altered to the illite mineral. Analyzed black shales are recognized by high total organic carbon (TOC) content (1.47- 5.87wt%) and rich in SiO2 (33.19%), followed by CaO (20.54%) and Al2O3 (12.08%). Comparison between the obtained data and the Post Archean Australian Shale values indicate that CaO, Sr, U, V, Ni, Zn and Mo were present in higher concentrations; P2O5, Nb and Hf were nearly similar, while Na2O, MnO, Rb and Ba was much lower. Also, the behavior of the trace elements can be inferred from the enrichment factor. The studied black shales are enriched in Zn, U, V, Ni and Mo. Correlation between elements predict their association and origin. U, V, Co, Ni, Cu, Zn and Mo are related to the phosphate minerals, also the organic matter played a part in the enrichment of V and Ni elements. Al2O3 significantly correlates with Fe2O3, MnO, TiO2, Sc, Hf, Nb, Zr, Th, Ba, Rb in addition to REE indicating their associations with clay minerals. Dependent upon the predominance of illite clay mineral and illite crystallinity index, in addition to the Tmax (426 and 442ºC), the plurality of the studied shales is over mature and anchi-metamorphic. The shales are related to the deep diagenetic zone with a paleo-temperature between 150 and 200°C.