Subseafloor mineralization related to the shallow seawater-hydrothermal circulation system in the Longqi hydrothermal field, Southwest Indian Ridge (49.6°E): Evidence from in situ trace element and sulfur isotope compositions of pyrite varieties

Abstract

The shallow seawater-hydrothermal circulation system plays a crucial role in the subseafloor mineralization of the hydrothermal field. However, its key fluid processes and impacts on the metal mobilization and sulfur cycles in the stockwork mineralization are still poorly understood. We first present the systemic variations in micro-scale trace element and sulfur isotope compositions of pyrite varieties in a stockwork-like sulfide assemblage from the Longqi hydrothermal field to constrain the transport and deposition of metals and the origins and cycles of sulfur in the shallow seawater-hydrothermal circulation system. Pyrites considered as the dominant sulfides can be clarified into disseminated fine-grained (Py-I), euhedral (Py-II), and subhedral-euhedral (Py-III) varieties based on the textures. The wall-rock-derived elements Ti, Cu, and Ni and seawater-derived elements V, U, Mo, Mg, and Mn are concentrated in Py-I in the breccias, which is related to the fluid-rock interaction and fluid-seawater mixing in the shallow seawater-hydrothermal circulation system. Short-lived shallow seawater-hydrothermal circulation results in fluid fluctuation and oscillatory-zoned Py-II with depletion of Co, Ni, Cu, As, and Se in the mantles relative to those in the rims and cores. As the later hydrothermal activity was active, Py-III was overgrown from Py-II rich in hydrothermally inherited metals Se, Te, and Co, possibly implying the hydrothermal field is coming into the main mineralization. The sulfur isotope compositions of pyrites range from 4.30 to 9.98‰ (n = 37), with distinct δ34S variations in the individual Py-I crystals (>1.5 ‰ within a 20 × 20 µm2 region). This variation is attributed to changes in the relative proportion of sulfur sourced from (i) the shallow-origin reduced seawater via reduction by ferrous iron released from basalt, and (ii) the reduction of pre-existing anhydrite by later hydrothermal overprinting in the shallow subseafloor. These findings provide evidence for a model to better understand the effect of the shallow seawater-hydrothermal circulation system on the subseafloor stockwork mineralization of hydrothermal fields.