Genesis of metasomatic gold mineralization in the Pahardiha-Rungikocha gold deposit, eastern India: Constraints from trace element signatures in chromite-cored magnetite and bulk rock geochemistry

Abstract

The Pahardiha- Rungikocha gold deposit is located south of the Dalma volcano-sedimentary belt within the North Singhbhum Mobile Belt (NSMB), in the eastern part of India. The rocks of this area have undergone three phases of deformation, viz. D1, D2, D3. While the D1 and D2 events signify progressive regional metamorphism, the D3 stage was a consequence of continued N-S compression resulting in E-W trending displacement fractures. This D3 deformation event was responsible for the mobilization of auriferous fluids within the host quartz-chlorite-schist and subsequent gold precipitation. Here, gold occurs as native grains either within the cracks or along grain boundaries of euhedral pyrite. Detailed mineral paragenesis reveals that euhedral pyrite associated with gold is coeval with abundant chromite-cored magnetite. The chromite-cored magnetite shows a complex paragenetic history, i.e., an initial chromite alteration event, followed by epitaxial growth of magnetite rims on the chromite cores, as revealed by LA-ICP-MS trace element data and EPMA X-ray compositional maps. The paragenetic stages of the chromite-cored magnetite is synchronous with the successive deformation events of the host rock. The final phase of deformation is responsible for the formation of the chromite-cored magnetite rims which is coeval with the gold mineralizing event. These magnetite rims formed as a result of metasomatism brought about by the interaction of hydrothermal fluids with the wall rock. The chromite-cored magnetite trace element signatures resemble the high temperature Ca-Fe alteration and high temperature K-Fe alteration facies, reported from various IOCG and IOA type deposits. There is a distinct enrichment of mafic elements such as Ni, Cr, V, Mg, V, Cu within magnetite rims. A similar pattern of enrichment in mafic elements is also seen in the bulk rock geochemical data. Major element discrimination diagrams reveal that the chlorite schists evolved from a mixed provenance of felsic and mafic/ultramafic rocks. Consequently, the D3 induced metasomatism most likely resulted in preferential stripping of the mafic elements, which is well reflected in the analyzed magnetite rims. Thus, we propound that the bulk rock chemical composition of the host chlorite-schist had a first order control on the chemistry of the chromite-cored magnetite, coeval with the gold mineralizing event. Given the fact that there are no causative igneous intrusions underlying the host chlorite schists, we infer that the gold mineralizing fluids were intrabasinal in nature and were mobilized due to reactivation of pre-existing faults and fractures during the D3 deformation event.