مطالعه زمین شیمی عناصر اصلی، کمیاب و نادر خاکی در کانسار منگنز شهرستانک؛ رهیافتی در تعیین شرایط تشکیل کانسار

Introduction The Shahrestanak Mn deposit is located in southern Qom province, 12 km southwest of the city of Kahak. Based on geological-structural divisions of Iran, the deposit belongs to central volcanic belt or Urumieh-Dokhtar zone. The Venarch deposit is one the most important known manganese deposits in Iran. The Sharestanak and Venarch deposits are spatially and temporally related to each other, and have similar geology, mineral texture and structure, host rocks, relationships with faults, and depositional environment. So, their magmatism and deposition conditions can be related to each other. Since no systematic study on the Shahrestanak deposit had been performed before discussing its geological and geochemical characteristics, here it is being attempted to study the geology, petrography, geochemistry of major, minor and trace elements, and Rare Earth Elements (REE) of ore, to distinguish the depositional environments and genesis of this deposit and to compare REE of ore in this deposit with other deposits. Sampling and method of study Fourteen samples of manganese ore were selected for geochemical study and analyzing of major, minor, trace elements and REE by ICP-AES and ICP-MS and were sent to SGS Co., Toronto. Detection limits for major elements and trace elements are 0.01% and 0.05ppm, respectively. Result and discussion The deposit is characterized by various lithology and stratigraphy units, consist of: 1) Middle to -Upper Eocene volcano-sedimentary rocks, 2) Oligocene lower red conglomerate and sandstone, 3) Oligo-Miocene limestone and marl (Qom Formation), and 4) Eocene and Lower Miocene basic to intermediate dykes. The most abundant minerals of the deposit are braunite, hausmannite, pyrolusite, and manganite. Evidences such as high Mn/Fe (11.33) and Si/Al (4.86) ratios, low contents of trace elements specially Co (11.40 ppm), Ni (24 ppm), Cu (81.85 ppm), and Ce, with high amounts of SiO2, Mn, Fe, Ba, Zn, As and Sr, all represent hydrothermal processes. It seems that hydrogenous processes have not had significant role on the genesis of the Shahrestanak Mn deposit. During deposition of Fe and Mn from hydrothermal solution, they separated from each other and produced different Fe/Mn ratios in sedimentary exhalative deposit (SEDEX). The Fe/Mn ratios are 5.7 to 40.35 (ave., 11.33). Very high and very low ratios of Fe/Mn can be interpreted as fractionation and separation of these two elements from transportation during hydrothermal activities and mineralization. So, high Fe/Mn ratios here can be considered as in submarine hydrothermal deposits. Cann et al. (Cann et al., 1977) suggested that Fe/Mn ratios in volcano-sedimentary and hydrothermal deposits are so variable and characteristic. Hydrothermal deposits are in close relationships with ferruginous silica gel which itself formed from submarine hydrothermal outpouring and discharging of metals in marine sediments. So, Si wt. % versus Al wt. % is high in exhalative activities. The average Si/Al ratio is 4.86 in the Shahrestanak deposit which is in the range of hydrothermal deposits (SEDEX). Nicholson (Nicholson, 1992) suggested Na versus Mg content diagram for distinction between fresh water, shallow and deep marine environments. Bonatti et al. (Bonatti et al., 1992) introduced Fe-Mn-(Co+Cu+Ni)*10 ternary diagram for distinction between marine sedimentary and hydrothermal Fe-Mn deposit. According to this diagram, hydrothermal oxides depleted in Ni, Cu, Co and zinc relative to sedimentary-marine deposits. Nicholson (Nicholson, 1992) suggested that hydrothermal Mn deposit distinguished with Zn, V, Mo, Cd, Li, Sr, Sb, Pb, Cu, Ba, and As and sedimentary deposit with enrichment in Ni, Cu, Co, Sr, Mg, Ca, Na and K. Hydrogenetic ferromanganese deposit has higher enrichment of Ni, Cu and Co relative to hydrothermal (exhalative) deposit. Low contents of Cu, Co and Ni indicate low input of these elements from hydrothermal activities and derivation of Zn from hydrothermal source. As (Co/Zn)-(Co+Cu+Ni) diagram, the samples from Shahrestanak deposit show close similarities with hydrothermal deposits which in turn show common genesis. Using Pb versus Zn diagram, dubhite (deposits derived from previous mineralized sequence) can be distinguished from other Mn oxide (hydrothermal or supergene) deposits. The dubhite deposits have high Pb/Zn ratios and more than 1 percent Pb and Zn contents. Meanwhile, other types of deposits like shallow marine deposit, hot springs, SEDEX, weathered deposits have lower contents of Pb and Zn. The Shahrestanak deposit has more similarities with SEDEX and shallow marine deposits. Conclusion Geological and geochemical evidences show that deposition of ore occurred by submarine hydrothermal activities in Neotethys oceanic basin during Middle to Upper Eocene in calcareous tuff with intercalation of micrite and calcareous limestone. For the genesis of the deposit, it can be stated that the pillow basalt and andesite lavas were leached by hydrothermal activities and Mn, Fe, Si, Ba, Sr and As entered in sedimentary basin by exhalative – volcanic activities through faults, then by regression of the sea and forming oxidizing condition, primary oxide-hydroxide Mn-minerals are deposited. Acknowledgement We gratefully thank the Research and Technology Department of Bu-Ali Sina University for supporting the research. References Bonatti, E., Kraemer, T. and Rdell, H., 1972. Classification and genesis of submarine iron- manganese deposits of the ocean floor. In: D.R. Horn (Editor), Ferromanganese Deposits of the Ocean Floor. Aren House Harriman, pp. 149-166. Cann, J.R., Winter, C.K. and Pritchard R.G., 1977. A hydrothermal deposit from the floor of the Gulf of Aden. Mineralogical Magazine, 41(318): 193-199. Nicholson, K., 1992. Genetic types of manganese oxide deposits in Scotland: Indicators of paleo-ocean-spreading rate and a Devonian geochemical mobility boundary. Economic Geology 87(5): 1301-1309.