摘要：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 Discussion Mineralogical characteristics combined with geochemical data show that anomalous values of Fe in studied carbonate argillite formations with respect to common sedimentary rocks are related to the abundance of iron-bearing oolites as oxides such as hematite and goethite, and the clay mineral chamosite. Based on Fe, Mg and Ca concentrations, oolitic iron formations can be divided into low-grade and high-grade iron formations. The former is characterized by chamosite and calcite, whereas the latter consists ofhematite and calcite. This research, along with available paleo-geographic and sedimentological information suggests that the iron for the formation of iron oolites was available from normal sea water and Fe could be carried as clastic particles along with clays or coating of clay particles derived from weathering and erosion of shales from adjacent land. High contents of K and Si in oolitic iron horizons, the presence of detrital zircon, quartz and clay minerals within oolites and also in the matrix of these rocks confirm the proposed model and show the important role of Fe-bearing clay minerals in the genesis of the primary chamositic oolites in an environment with pH=5-9 and medium-weak redox conditions (Maynard, 1983; Maynard, 1986). The abundance of hematite relative to goethite in the Fe-oolites, dense and elliptical oolites as well as the frequent occurrence of calcite veinlets cutting oolite beds has been attributed to diagenetic processes and the modification of chamosite and goethite to hematite. Our findings indicate that the studied members can be classified as low-grade oolitic iron formation (average 21 wt.% Fe) which do not have economic importance at present. Acknowledgements This study is part of the senior author's M.Sc thesis at Golestan University, Gorgan, Iran. Logistical and financial support was provided by the Research Grant to senior author. We are grateful to SarCheshmeh Copper Complex for XRF analyses and IMPERC for XRD measurements. We gratefully acknowledge Dr. Ghobadipour for SEM analysis in National Museum of Wales, Great Britain. References Ghobadi Pour, M., Popov, L.E., Kebriaee-Zadeh, M.R. and Baars C.H., 2011. Middle Ordovician (Darriwilian) Brachiopods associated with the Neseuretus bio-facies, Eastern Alborz Mountains, Iran. Memoirs of the Association of Australasian Palaeontologists, 42(3): 263-283. Maynard, J.B., 1983. Geochemistry of sedimentary ore deposits. Springer-Verlag, NewYork, 382 pp. Maynard, J.B., 1986. Geochemistry of oolitic iron ores, an electron microprobe study. Economic Geology, 81(8): 1473-1483. Mucke, A.T. and Farshad, F., 2005. Whole-rock and mineralogical composition of Phanerozoic ooidal ironstones: Comparison and differentiation of types and subtypes. Ore Geology Reviews, 26(2): 227-262. Petranek, J. and Van Houton, F.B., 1997. Phanerozoic ooidal ironstone. Czech Geological Survey, Special Papers 7: 70 pp. Young, T.P., 1989. Eustatically controlled ooidal ironstone deposition: facies relationships of the Ordovician open-shelf ironstones of Western Europe. In: T.P. Young and W.E.G. Taylor (Editors), Phanerozoic Ironstones. Geological Society of London, Special Publication, 46(1): 51–64.
