petrography, geochemistry and proposed genesis of ordovician oolitic iron formation members of the lashkarak formation, eastern alborz

introduction oolitic iron formations are sedimentary rocks with >5 vol.% oolites and >15 wt.% iron, corresponding to 21.4 wt.% fe2o3 (young, 1989; petranek and van houten, 1997; mucke and farshad, 2005). in iran, new iron oolite-bearing members have been identified in the lashkarak formation (lower-middle ordovician) in the abarsej, dehmola and simehkuh sections, eastern alborz (ghobadi pour et al., 2011). at present, the mineralogy and geochemistry of these members are not known. consequently, research reported here was conducted to reveal the mineralogical and geochemical characteristics of ordovician oolitic iron formationmembers and to discuss their genesis and economic importance. materials and analyses field geology and sampling was carried out to collect 25 samples from the ooliticiron formation members in the abarsej, dehmola and simehkuh section in eastern alborz. samples were prepared for polished-thin sections (n=10), xrd analysis (n=15). whole-rock chemical analysis (n=15) by xrf for major elements and by icp-es for trace elements was performed by laboratories at the sarcheshmeh copper mine complex, kerman, iran. one sample was analyzed by sem at the wales museum, uk. results microscopic studies show that the oolitic iron formation members are hosted by carbonate argillite rocks. they are mainly composed of oolites rather than pisoliths (small bodies somewhat larger and more irregular than oolites), whereas oolites have mainly ellipsoidal forms and locally spherical shapes. most (6) oolites show banding with a central core. simple oolites without a core are scarce. mineralogically, oolites are mainly chamositic and hematitic in composition; goethite, pyrite and glauconite occur in traces and siderite is absent. quartz, calcite and zircon are accessory minerals which are present in the groundmass. geochemically, tfeo % of the oolitic iron formation horizons ranges from 8 to 48 % with an average of 21%. the cao content ranges from 2 to 37% and sio2 from 11 to 37 %. based on tfeo % content, oolitic iron formation horizons are divided into two geochemical groups: 1: low-grade iron formations ( the abarsej section) (8) with tfeo< 23%, high mgo (0.82-0.96 %) and high cao (32.99-37.22 %) and low tio2 (0.45-0.62 %), and 2: high-grade iron formations (the dehmola and simehkuh sections) with 23%