The Lost City hydrothermal field (LCHF) is considered an analogue of Archean alkaline hydrothermal vents where life on Earth may have spawned. Although Lost City was discovered more than 20 years ago, it remains unclear to what extent microbes are involved in the precipitation of the various minerals constituting the hydrothermal chimneys. Most chimneys preserve flow textures comprised of mineral walls bounding paleo-channels, which are preserved in inactive vent structures to a varying degree. Brucite lines the internal part of these channels, while aragonite dominates the exterior. Calcite is also present locally, mostly associated with brucite. Based on a combination of microscopic and geochemical analyses, we interpret brucite, calcite, and aragonite as primary minerals that precipitate abiotically from mixing seawater and hydrothermal fluids. We observe local brucite precipitation on microbial filaments and, in some cases, microbial filaments may affect the growth direction of brucite crystals. The link between brucite and organic compounds is further confirmed by fluorescence microscopy: brucite shows a strong fluorescence not observed in calcite and aragonite. Our results point to brucite as an important link to microbial life in alkaline fluids and is potentially a key aspect for future research on the origin of life.