Textural studies of Magnetite from the rocks of Beldih, Purulia, West Bengal 

Abstract

Kiruna-type Fe-oxide-apatite (IOA) mineralization is a potential source of phosphorous, iron rare earth elements (REEs), and other elements (Ag, Cr, Co, and U) of economic importance. In such deposits, textures associated with ore minerals provide important insights that can aid in characterizing the primary/secondary phases vis-à-vis the process of mineralization. Accordingly, apatite-magnetite veins (associated with calcite-bearing ultramafic rocks) from the Beldih area (South Purulia Shear Zone) of Eastern India were studied to appraise the formation of Fe-oxides and sulfide minerals hosted within these rocks. The present work integrates preliminary Petrological studies (both under microscope and Backscattered electron: BSE imaging) to comprehend the texture/s of Fe-Ti oxides and associated sulfides. The major/dominant phases in the samples comprise magnetite and ilmenite (among the Fe-Ti oxides) and pyrrhotite, pyrite, chalcopyrite, sphalerite, cobaltite, and gersdorffite (as sulfide phases). These rocks in the study area preserve varied textural assemblages/associations of magnetite: (a) Magnetite (Mag1) in the samples is coarse-grained with euhedral/polygonal grain boundaries, in which larger grains preserve two different sets of fine lamellae of ilmenite (in addition to individual grains) exhibiting both Trellis intergrowth and Widmanstatten texture; and (b) medium-sized, discrete grains of magnetite (Mag2) and ilmenite at the boundary of large pyrrhotite grains (associated with pyrite and chalcopyrite). Both these textural types include euhedral grains of apatite embedded on clusters of calcite, associated with minor REEs (allanite and monazite) and silicate phases (amphibole, biotite, muscovite, and titanite). Pyrite grains (exhibiting sharp boundary) in the rock are found both as inclusions and at the grain boundary of large (anhedral) pyrrhotite grains, indicating possible alteration of pyrite to form pyrrhotite. Inclusions of gersdorffite are also seen within a few pyrrhotite grains. Thus, from the afore-discussed textural assemblage of magnetite (Mag1), it stands to reason that high-fO2 environment prevailed during its formation and the possible difference of pressure in the solid solution between magnetite and ilmenite. The formation of Fe-oxides, calcite, apatite, and silicates is consistent with the primary crystallization of these minerals possibly from a silicate melt (by liquid immiscibility process) or from a high-T magmatic-hydrothermal fluid. Subsequently, the formation of secondary discrete magnetite (Mag2) and associated ilmenite grains (in addition to various sulfide phases), indicates possible ingress of a hydrothermal fluid into the system.