Geochemistry and the origin of the Mamouniyeh iron ore-terra rossa deposit, Markazi Province - Iran

Abstract

Introduction Iron is among the metals whose ore deposits are not confined to a specific geologic period of crustal formation and they have formed in various geologic environments during previous periods (Ghorbani, 2007). About 95% of iron ore deposits have sedimentary origin and have formed due to chemical deposition from ancient sea water. The remaining percent is the result of alteration and magmatic activities (Gutzmer and Beukes, 2009). In sedimentary environments, a large amount of sedimentary iron minerals have formed resulting in different iron facies. Iron oxide facies are of the most important facies (James, 1954). The most important Iranian iron deposits are located in Central Iran, Sanandaj- Sirjan and East Iran zones, and the Kordestan area (Ghorbani, 2007). In the Orumiyeh-Dokhtar Zone, many iron ore deposits have been formed in conjunction with granitic and granodioritic plutons related to Oligocene-Miocene plutonic and volcanic activities (Hoshmandzadeh, 1995). The Mamouniyeh iron ore-terra rossa deposit is located in the Orumiyeh-Dokhtar volcanic zone. Iron mineralization have occurred in trachytic-trachyandesitic lavas and pyroclastic rocks of Pliocene age. Materials and methods A total of 28 rock samples were picked up from ore and host rocks during field observations. Petrographical and mineralogical studies were performed on 15 thin sections of ore and host rocks. XRD studies were performed on 3 ore samples. In order to investigate the geochemistry of the ore, 10 samples were analyzed for major, trace and rare earth elements (REEs) using the ICP-MS method. Result Field and mineralogical studies reveal that the ore is composed of hematite along with crypto-crystalline silica as alternating layers of various thickness and color. The existence of alternating layers of hematite and quartz implies that the ore is similar to banded iron formations, but on a smaller scale, related to submarine hydrothermal activities. Silica is found as chert and minor jasper. Some secondary dolomite and calcite, filling the fractures and open spaces are found. Clay minerals are also minor constituents of the ore. The remaining fossils of green-blue algae indicate the conditions of iron deposition and effective biological processes in oxidizing Fe+2 and creation of new oxide minerals in a sedimentary basin. XRD studies show that tetraferriannite, hisingerite, barite, dolomite and calcite are present in addition to dominant hematite and quartz minerals. Hisingerite is formed in sedimentary iron deposits during hydrothermal alteration (Whelan and Goldich., 1961). Tetraferriannite occurs in low grade iron formations (Miyano, 1982). Structurally, the mineralization is controlled by a tectonic zone in which abundant breccias and faults are well found. The amount of Fe2O3 ranges between 11.62% and 65.73%, with an average value of 31% Fe2O3. The amounts of Cr (3-95 ppm) and Zr (<5-29 ppm) are low; while, the deposit contains a moderate amount of V (26-189 ppm) and high concentrations of Zn (28-218 ppm), Sr (66-1462 ppm) and Ba (62-5511 ppm). The concentration of REEs shows that total amount of these elements is variable and it falls in the range of 2.34-12.74 ppm. The amount of LREEs falls in the range of 1.66-11.94 ppm and that of HREEs falls in 0.21-2.22 ppm. These values clearly indicate the enrichment of ore in LREEs relative to HREEs. The Eu anomaly (Eu/Eu*) lies in the range of 1.32-10.2, indicating positive Eu anomalies. The Ce anomalies (Ce/Ce*) fall in the range of 0.076-0.52, suggesting negative anomaly. Discussion The low concentration of Cr and Zr, and high values of V, Zn and Sr in the ore suggest that mineralization is related to submarine volcanic activities. Geochemical data, including chondrite-normalized REE patterns, indicate that seafloor hydrothermal fluids are the most probable source for mineralizing solutions. The ∑(Cu+Co+Ni) vs. ∑REE diagram also indicates the role of deep sea hydrothermal fluids in the deposition of the ore. Chondrite-normalized REE patterns, LREE enrichment relative to HREE, positive Eu anomalies and negative Ce anomalies suggest that iron is derived from oceanic crust. Iron and SiO2-rich hydrothermal fluids that vented through seafloor conduits under reduced conditions came to contact with oxidizing, cold seawater, resulting in physicochemical changes in hydrothermal fluids. As a result of these physicochemical changes, iron is deposited in shallow seawater as hematite along with silica, making alternating layers. References Ghorbani, M., 2007. Economic geology of mineral and natural resources of Iran. Arian zamin, Tehran, 492 pp. Gutzmer, J. and Beukes, N.J., 2009. Iron and Manganese Ore Deposits: Mineralogy, Geochemistry and Economic Geology. In: B. De Vivo, B. Grasemann and K. Stüwe (Editors), Encyclopaedia of Life Support Systems. UNESCO, Paris, pp. 43-69. Hoshmandzadeh, A.R., 1995. Iran geology, Iron deposit of Iran. Geological Survey of Iran, Tehran, 145 pp. James, H.L., 1954. Sedimentary facies of iron formation. Economic Geology, 49(1-3): 235-293. Miyano, T., 1982. Ferri-annite from the Dales Gorge Member iron-formations, Wittenoom area, Western Australia. American Mineralogist, 67(1-2): 1179-1194. Whelan, J.A. and Goldich, S.S., 1961. New data for Hisingerite and neotocite. American Mineralogist, 46: 1412-1423.

Keywords

• Hematite
• Silica
• Rare Earth Elements
• Hydrothermal fluid