Intensely silicified volcaniclastic mudstones that underlie the Brunswick No. 12 massive sulphide deposit in northern New Brunswick resemble silicified rocks described in the immediate footwall of many ancient and modern massive sulphide deposits. The white to grey, cryptocrystalline silica in the silicified rocks becomes more common with proximity to the vent, and is most abundant immediately below the massive sulphide zone. Mass-balance analysis of altered footwall sedimentary rocks on the 850 m level of the mine shows that SiO2 increases up to 300%. The high silica enrichment in the feeder zone is consistent with the presence of cherty silica in the massive sulphides and in associated exhalative iron formation. Coincident with silicification are enrichments in S, FeOt, MgO, MnO, CaO, P2O5, F, Cl, Y, Cu, Co, Cr, and Ni, as well as light rare earth elements and Eu. Oxygen isotope analyses of chloritized and silicified footwall sedimentary rocks suggest that the hydrothermal fluid had a δ18O composition of approximately 4 and probably was dominated by chemically modified sea water. Rapid oversaturation of the silica-bearing fluid likely explains the intensity and fine-grained nature of this silicification, although the actual mechanism for this oversaturation is uncertain.