Major and trace element geochemistry of tourmaline in feldspathic schist & tourmalinite from pathargora & surda, singhbhum shear zone, Eastern India

Abstract

Tourmaline super group of minerals are often found as accessory minerals in the rocks of the earth crust. Tourmaline composition is used as petrogenetic indicator. Because of its sensitivity to secondary processes and negligible diffusion, it can infer the physic-chemical ambience of the host environment. Tourmaline from Pathargora area is mainly associated with feldspathic schist and tourmaline from Surda area is mainly associated with tourmaline bearing quartzite. Both the regions are located in eastern part of Singhbhum shear zone (SSZ) and suffered alteration. Singhbhum Shear Zone is one of the most important mineralized area of India and it hosts uranium, copper and sulphide mineralization. Tourmaline samples are classified into two broad category – altered and unaltered sample from both the region. Field observations, Petrographic study and microprobe data document a multi-stage formational history of tourmaline samples. In most of the cases brown or yellowish brown tourmaline is replaced by blue tourmaline along grain boundary or micro fracture. Pathargora altered tourmaline is characterized by large variation of Ti content whereas the unaltered tourmaline from Pathargora and both the altered and unaltered tourmaline from Surda contain a constant amount of Ti. Blue tourmaline grains are rich in Mg and poor in Al whereas brown tourmaline is rich in Fe, Al. Alteration pattern indicates the ingress of hydrothermal fluid. Tourmaline grains with brown core and blue rim and replacement texture shows that blue tourmalines formed through a late hydrothermal event. The compositional variations can be explained by a combination of coupled substitutions represented by Fe2+  Mg2+, XR1+ + R3+  XCa + R2+ and XR1+ + R2+  X + R3+ and Ti + R2+  2 R3+. The XR1+ + R3+  XCa + R2+, Ti + R2+  2 R3+ and XR1+ + R2+  X + R3+ substitution occurred in both this two region. Trace element data of tourmaline shows a strong positive Eu anomaly. This study incorporates field study, petrographic study and major and trace element composition which can potentially identify different generations of tourmaline.