Evidence of hydrothermal fluid migration pathways in pores and microfractures based on nuclear magnetic resonance and electron microscopic analyses from the Southwest Indian Ridge

Abstract

The formation of massive sulfide deposits on the seafloor is closely related to sub-seafloor hydrothermal circulation. Hydrothermal circulation depends on the connected pores and fractures. Most previous studies have focused on hydrothermal circulation at the macroscopic scale; however, the transport pathways of hydrothermal fluids have rarely been studied at the microscopic scale. This study showed that multiple measurement techniques could characterize the pores of rocks and the mineral fraction surrounding these pores. Nuclear magnetic resonance (NMR) was used to measure the porosity of fresh basalt, altered basalt, massive sulfide, and sulfide chimneys obtained from the Southwest Indian Ridge. Meanwhile, the distribution of pore radius and pore throats in these samples was obtained using the NMR transverse relaxation time distribution. The scanning electron microscopy results and polarizing light microscope images showed that the pores of altered basalts are more developed than those of fresh basalts, which is consistent with the NMR measurement results that altered basalts have a large pore size distribution. In addition, metallic minerals such as pyrite were found to be precipitated in these pores, and microfractures were observed in altered basalts. This demonstrates that the pores and microfractures in these altered basalts provide pathways for fluid transport and are enriched with metal minerals. Altered basalt is closely related to the formation of massive sulfide deposits and is an important ore-prospecting indicator. Therefore, the study of rock pore structures and components can provide a better understanding of hydrothermal circulation and contribute to the exploration of mineral resources.