Structural controls on hydrothermal circulation at a segment end: Insights from a silica chimney of the Daxi Vent Field, Carlsberg Ridge

• The silica chimney at the DVF is a first reported silica-dominated edifice at the NTO. • The first constraints on δ 34 S of sulfides in silica chimneys are provided. • Intense structural activity at the NTO reactivating pre-existing fluid pathways. • Silica sequestration may have contributed to the development of metal-rich SMS deposits. Silica chimneys at the ends of slow-spreading mid-ocean ridge segments provide a window into how structural features control hydrothermal fluid circulation, yet their formation and role remain poorly understood. Here we investigate the mineralogy, mineral chemistry, O–Si–S isotopic compositions, and Th/U geochronology characteristics of the first recorded silica chimney from the Daxi Vent Field (DVF), located at a non-transform offset (NTO) on the Carlsberg Ridge, Northwest Indian Ocean. The chimneys at the NE mound are dominated by amorphous silica with minor sulfide minerals, and are structurally controlled by a steep (∼70°) fault scarp. Microscopic observations reveal iron–silica filaments and hollow chert rods, suggesting that microbial activity played a critical role in generating iron oxide substrates that promoted silica nucleation. Sulfur isotopes for sphalerite from a silica chimney, previously unreported, exhibit uniformly elevated δ 34 S values (8.45–10.4‰), indicating sulfide precipitation under semi-closed conditions, with a possible contribution from microbial sulfur cycling. Oxygen isotopes of opal-A indicate average formation temperature of 86.5 ± 6.2 °C ( n = 29), increasing from the outer rim to the inner core. Silicon isotopes (δ 30 Si avg = –0.86 ± 0.04‰, n = 2) of opal-A reflect rapid mixing of seawater and fluid, with kinetic fractionation favoring incorporation of light Si. The silica chimneys formed through shallow hydrothermal circulation within the upper mafic rocks, with the NTO acting as a critical structural setting where intense structural activity reactivated pre-existing fluid infiltration pathways. Early hydrothermal episode (8295 ± 43 yr. B.P.) silica sequestration depleted the system in dissolved silica, potentially facilitating metal transport during subsequent (∼400 yr. B.P.) deep-seated, high-temperature hydrothermal circulation and contributing to the development of metal-rich seafloor massive sulfide deposits at the central mound. This study highlights the significance of structurally controlled silica chimneys for hydrothermal evolution and metal distribution within seafloor massive sulfide systems.