Investigation of microstructures controlls on alteration and iron mineralization in Khosrow Abad deposit, Northeastern of Sonqor

Metamorphism, magmatism and metasomatism were active in the Sonqor series and have a significant effect on the geological fate of the region (Mohajjel, 1997). Petrofabric analysis of metamorphic rocks is an effective method in recognizing mechanism of orogenic belt evolution and its’ relationship with plate tectonics (Twiss and Moores, 1992). Emplacement of intrusive bodies associated with tectonic event provide suitable conditions for skarnification, mineralization, alteration and metasomatism in the regions. Tectonic deformation caused the mylonitization of intrusive bodies and metamorphic rock fabric, which is related to orogenic evolution and plate tectonics. Since tectonic structures are considered as the main controllers of mineralization in such areas, it is vital to study tectonics, structures and deformation phases and their affiliation with mineralization for understanding the geometry, grade control and location of associated mineralization.

Microstructures in the Khosrow-abad region were studied during two field surveys mainly in the mineralized area. Fifty-three thin and thin polished sections of skarnified meta basaltic andesite rock units, slatic- and crystalline limestone (marble), quartz monzonitic and syenitic intrusive bodies were prepared and studied using Zeiss Axio-Plan2 research optical microscope in mineralogy laboratory of Kharazmi University in Tehran.

Iron ore mineralization in the Khosrow-Abad deposit can be divided into two types according to the geological and structural features and also based on micro-texture fabrics:1) Ductile type mineralization; the host rock of this type of mineralization is skarnified limestone. Ductile mineralization occurred in Khosrow Abad region in a zone with lens-shaped geometry. Ductile shear zones are also affected by sodic (albite) and magnesium (serpentine and talc) metasomatism. Results of the analyzed samples taken from trenches and boreholes in ductile shear zone indicate low grade iron mineralization associated with highly deformed sections with sodic and magnesium alteration. 2) Brittle type mineralization; the host rocks of the mineralization are alternate of volcanic (meta basaltic andesite) and skarnified rocks with strong deformation. The high-grade iron ore in Khosrow-Abad deposit is associated with brittle deformation. This type of mineralization, which is exposed in the inner parts of the mylonitic zones of the region, includes fractures, veins and oxide-sulfide veins-veinlets that are parallel and intersecting the mylonitic foliation. It seems that the normal faults’ related strain field has led to remobilization, migration, and re-concentration of iron ore along these faults. Deformations has provided suitable spaces and conduits for channeling hydrothermal fluids, causing leaching, alteration and mineralization, even sometimes re-concentration of iron ore. Therefore, the parallel and intersecting joints with mylonitic foliation, has caused a voluminous migration of mineralized fluids and ore deposition within these sub-structures. The highest iron ore grade is observed in the intensely fractured brittle deformation, associated with fragmentation and displacement of mylonitic units and bonds.

The occurrence of vein or replacement structures in the Khosrow-Abad skarn iron deposit is due to the flow of mineralization fluids along the joints and fractures created due to tectonic deformation in the host and intrusive rocks. The tectonic forces caused the occurrence of ductile and brittle shear zones in the region. Considering the association of intense alteration and metamorphism with high-grade iron ore in Khosrow-Abad deposit, it can be concluded that metasomatism and hydrothermal alteration and mineralization in the region, is controlled by deformations and their intensity. The brittle deformation along with the fractures, provide a suitable space and conduits for the influx of mineralized fluids and ultimately the ore precipitation as veinlets, veins, lensoids and deformation fabrics, that controls the magnetite mineralization and its’ grade.