Geochemistry of shales of the Shemshak Formation of the Dash-Aghel mining district, Bukan, NW Iran: Provenance, source weathering, paleo-oxidation conditions, and tectonic setting

Introduction The Dash-Aghel area is located about 20 km east of Bukan town, south of West-Azarbaidjan Province, NW Iran. Shales of the Shemshak Formation (Lower Jaurassic) form cap rock of layers and lenses of the iron-rich laterite ores in this area. The propose of the present study is to indentify the source rock characteristics as well as the source-area weathering, tectonic setting, and paleo-oxidation conditions by using major, trace, and rare earth elements geochemistry of shales of the Shemshak Formation exposed in the Dash-Aghel area.

Materials and Methods For the identification of mineralogical phases in the shales, four samples from a selective section were chosen for X-ray diffraction (XRD) analyses. XRD analyses were carried out using diffractometer model D5000 SIEMENS in Geological Survey of Iran (Tehran). The chemical compositions of the shales (#10) were determined at the ACME Analytical Laboratories Ltd. , Vancouver, Canada. The values of major and trace elements were determined by inductively coupled plasma emission spectrometry (ICP-ES). Rare earth element (REEs) contents were determined by inductively coupled plasma mass spectrometry (ICP-MS). Loss on ignition (LOI) was determined by weight loss of 1 g sample after heating at 950 °C for 90 min. The REE contents were normalized relative to chondrite and PAAS (Taylor & McLennan, 1985). Additionally, the geochemical data were evaluated by correlation coefficient analysis, elemental ratios, and plot of binary and ternary diagrams.

Discussion The result of XRD analyses revealed that quartz, kaolinite, illite, montmorillonite, calcite, albite, plagioclase, and goethite are the main mineral constituents of the studied shlaes. The positive correlations of K2O, TiO2, and Na2O with Al2O3 indicate that these elements are associated entirely with detrital phases in the samples. The Dash-Aghel shales show lower TiO2 values than the PAAS, which suggests more evolved (felsic) material in the source rocks. The depletion of P2O5 may be explained by the lesser amount of accessory phases such as apatite and monazite compared to PAAS in the studied samples. The high Sr content in shales indicates that Sr may be associated with calcite minerals. Correlation coefficients of positive and strong between K with elements such as Cr, U, Th, Ba, Hf, Nb, Rb, V, Sc, Co, and Ni suggest that illite have played the important role in distribution and concentration of these trace elements in the studied shales. The distribution pattern of REEs normalized to chondrite shows differentiation and strong enrichment of LREEs relative to HREEs and Eu and Ce negative anomalies in the studied shales. The geochemical signatures of clastic sediments have been used to find out the provenance characteristics (Armstrong-Altrin et al. , 2004). Al2O3/TiO2 ratios of most clastic rocks are essentially used to infer the source rock compositions, because the Al2O3/TiO2 ratio increases from 3 to 8 for mafic igneous rocks, from 8 to 21 for intermediate rocks, and from 21 to 70 for felsic igneous rocks (Hayashi et al. , 1997). In the studied shales, the Al2O3/TiO2 ratio ranges from 23. 57 to 44. 62, which suggests felsic rocks must be the probable source rocks for the shales of the Shemshak Formation. This interpretation is further supported by the TiO2 vs. Ni and Al2O3 vs. TiO2 bivariate plots, which also indicate that these shales were mainly derived from felsic source rocks. Ratios such as La/Sc, Th/Sc, Th/Cr, and Cr/Th are significantly different in felsic and basic rocks and may allow constraints on the average provenance composition. La/Sc, Th/Sc, Th/Cr, and Cr/Th ratios of shales from this study are compared with those of sediments derived from felsic and basic rocks (fine fraction) as
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well as to upper continental crust (UCC) and PAAS values. This comparison suggests that these ratios are within the range of felsic rocks. The REEs pattern and europium anomaly in the sedimentary rocks will provide important clues regarding the source rock characteristics. Higher LREEs/HREEs ratios and negative Eu anomalies are generally found in felsic igneous rocks, whereas the mafic igneous rocks exhibit lower LREEs/HREEs ratios and no or small Eu anomalies (Cullers, 1994). The Dash-Aghel shales show enrichment of LREEs relative to HREEs pattern with strong negative Eu anomalies. This pattern suggests that these sedimentary rocks were mainly derived from the felsic source rocks. Geochemical parameters such as U (0. 1-2. 6), authigenic U (0. 00-0. 28), Mn* (0. 41-1. 11), Ni/Co (3. 04-4. 92), Cu/Zn (0. 12-0. 47), U/Th (0. 33-0. 43), and Ce/Ce* (0. 44-0. 86) suggest that Dash-Aghel shlaes were deposited under oxic environmental conditions (Rimer, 2004). Alteration of rocks during weathering results in depletion of alkalis and alkaline earth elements and preferential enrichment of Al2O3. Therefore, weathering effects can be evaluated in terms of the molecular percentage of the oxide components by using the geochemical indices such as chemical index of alteration (CIA= [Al2O3/(Al2O3 + CaO* + Na2O +K2O] × 100) (Nesbitt & Young, 1982), chemical index of weathering (CIW = [Al2O3/( Al2O3 + CaO* + Na2O] × 100) (Harnois, 1988) and plagioclase index of alteration (PIA = (Al2O3–K2O) /[ (Al2O3–K2O) + CaO* +Na2O)] ×100) (Fedo et al. , 1995). However, samples having highly variable CaO contents due to variation in calcite abundance (such as those included in this study), may produce misleading conclusions if the CIA, PIA, and CIW are used to infer the degree of weathering (Cullers, 2000). Therefore, in this study we used a modified chemical index of weathering (CIW´ = molecular [Al2O3/ (Al2O3 + Na2O] × 100, in which CaO is left out of the CIW; Cullers, 2000). The CIW´ value of the Dash-Aghel shale samples vary from 96. 18-97. 07 %, which refer to intense weathering of source material of the shales. Based upon geochemical data such as ternary diagrams of (K2O+Na2O) (TiO2+Fe2O3+Al2O3) (Kreonberg, 1994) and Th-Sc-Zr/10 (Bhatia & Crook, 1986), the potential tectonic setting for Dash-Aghel shales is the passive continental margin environment.