Mode of occurrence, Texture and Geochemistry of Magnetite in Magnetite-Ilmenite and Magnetite-Apatite ore from Pathargora, Singhbhum Shear Zone, Eastern India

Abstract

The Singhbhum Shear Zone (SSZ) in the Eastern Indian Shield is known for its uranium and copper deposits. Multiple episodes of mineralization are reported from this Shear Zone (SSZ). Besides uranium and copper, several bodies of magnetite-apatite are reported from Singhbhum Shear Zone. However, there is no detailed work on textures and mode of occurrences of apatite-magnetite mineralization from this area Magnetite, a common mineral in many different types of ore deposit ranging from skarn to BIF or porphyry to IOA(Iron-Oxide-Apatite) and IOCG (Iron-Oxide-Copper-Gold), is extensively used as a petrogenetic indicator due to its formation in a wide arrange of physico-chemical environment(Including high-T magmatic conditions to low T hydrothermal conditions or metamorphic to sedimentary environment). Magnetite can incorporate different major or trace elements (Al, Ge, Ga, Mg, Ti, Zn, Co, V, Ni, Cr etc.) in its crystal structure depending on several intensive and extensive parameters like oxygen fugacity, temperature, partition co-efficient and compositions of the melt.. In Pathargora ,magnetite occurred mainly in two different modes. – 1.Magnetite (M1) associated with apatite-magnetite vein and 2. Massive magnetite body(M2). Both these type are hosted in feldspathic schist. Within massive magnetite body (M2), different kinds of ilmenite exsolution lamellae are observed . Major and trace element geochemistry clearly points to different formation conditions and processes are involved behind the origin of the two types of ore described here. Bivariate plots of V vs Ti Cr (ppm) vs Ti (ppm), Ga (ppm) vs V (ppm) etc. shows distinct chemical differences between massive magnetite body (M2) relative to magnetite of apatite-magnetite vein (M1). Some widely used discrimination plots like Ti (ppm) vs Ni/Cr ) and Ni/(Cr+Mn) vs (Ti+V) wt% indicates different genesis mechanism for these two different types of magnetite ore. Depending on trace element concentrations, element redistributions in between magnetite and ilmenite during sub-solidus oxidation are noted.