496 Journal of Applied Geochemistry Vol. 14, No.4 (2012). pp 496-508 SHEAR CONTROLLED FE-MINERALIZATION FROM PARTS OF SOUTH PURULIA SHEAR ZONE Moumita Talukdar*, Nandini Chattopadhyay and Sanjoy Sanyal Department of Geological Sciences, Jadavpur University, Kolkata *E-mail- mou.geoju@gmail.com Abstract South Purulia Shear Zone (SPSZ) of the East Indian shield area separates two Palaeoproterozoic terrains-the high grade Chotanagpur Granite Gneiss Complex (CGGC) in the north and the low-grade (greenschist-amphibolite facies) North Singhbhum Fold Belt (NSFB) in the south. In the SPSZ, the major rock types include alkaline–carbonatite rocks (Carbonatite, apatite deposits, ultramafic rock-Nephiline syenite, and alkali granite), quartzite and acid tuffs with minor mafic dykes. These rocks are folded and sheared in both ductile (early phase) and brittle (late phase) regimes. Brittle shearing is manifested by brecciation of the ductile deformed quartzite and apatite deposits. Extensive alteration of carbonatite, ultramafic rocks and granite has produced the thick kaolinite deposits that occur as vertical sheets. Along the brecciated zones in apatite deposits and quartz veins, massive goethite forms veins and network around apatite and quartz clasts respectively. Within the massive goethite deposits, angular clasts of apatite and quartz are replaced by goethite. Interpreting the information from field and petrography it has been demonstrated that the massive goethite deposits were formed by a low temperature (below 200 o C), acidic, circulating hydrothermal fluid. Fluid migration was facilitated by brittle fractures. Keywords: SPSZ, Goethite deposit, Hydrothermal, Breccia, Meteoric water. 1. Introduction Occurrence of fracture controlled oxide and hydroxide of Fe ( +Mn) has been reported from a wide geological setting ranging from ocean floor (Bonatti et al., 1972, Thompson 2003, Pichler and Veizer 1999) to brittle shears developed in continental rocks (Markl et al. 2006, Tanaka et al., 2008) to impact-induced structures (Osinski et al., 2001). These Fe-mineralized zones provide a unique opportunity to study interplay of brittle deformation, hydrothermal alteration of wall rocks and consequent ore-formation (Yardley 2005, Markl et al., 2006, Tanaka et al., 2008). In the South Purulia Shear Zone (SPSZ) of East Indian shield, several brecciated zones veined by Fe-hydroxide have been observed. In this study we present the microtexture of the brittle zone and mode of occurrence and mineralogy of the Fe-hydroxide deposits. Integrating all the geological attributes an attempt has been made to present a holistic model for the genesis of the Fe-deposits. 2. Geological Background The E-W to ESE-WNW trending South Purulia Shear Zone (SPSZ) of eastern India is a part of the crustal scale Tamar-Porapahar lineament (Acharya et al., 2006, Fig.1a). The SPSZ separates two Palaeoproterozoic fold belts - the North Singhbhum Mobile Belt (NSMB) to the south and Chotanagpur Granite Gneiss complex to the north (Acharyya et. al., 2006; Gupta and Basu, 2000). The status of this lineanent has been widely debated and is variably described as a ‘boundary fault’ (Ball, 1881), ‘zone of trench faults and shears’ (Majumdar, 1988), ‘thrust contact’ (Banerjee, 1985), ‘Khatra- Tamar overthrust’ (Mahadevan, 1992), ‘rift Zone’ (Ghosh Roy and Sengupta, 1993). This lineament preserves a mylonitic fabric as well as breccia development in some restricted stretches and is mostly ductile to brittle-ductile in nature (Pyne, 1992; Bhattacharya, 1989, Acharyya and Ray, 2004, Gupta and Basu, 2000). It has affected the rocks bordering both the NSMB and CGGC and shows sporadic Cu-Fe-P-U mineralization.