Relationships between lead-zinc sulfide precipitation and clay mineral alteration around the Pb-Zn Gays River deposit of mainland Nova Scotia are presented and discussed. The study focuses on the Visean Gays River Formation (Lower Windsor Group) and covers the Shubenacadie and Musquodoboit sub-basins at the southern margin of the Maritimes basin. X-ray diffraction and scanning electronic microscopy (SEM) have revealed the presence of detrital chlorite and illite and epigenetic smectite and kaolinite. Variations in illite composition are recorded by 002/001 peak intensity ratios and the crystallinity index. At Gays River the mineralizing event only lightly affected the detrital illite composition. A 5-km-wide halo of epigenetic smectite is centered on the deposit and is accompanied by a significant decrease in chlorite. Near the deposit, alteration of the interlayer hydroxide sheet in chlorite is indicated by a decrease of its 002/001 X-ray peak intensity ratio. This alteration consists of an exchange of iron for magnesium or a partial leaching of iron cations from that interlayer hydroxide sheet. At the Gays River deposit a low pH fluid was responsible for chlorite alteration and kaolinite precipitation. Neutralization of this acidic fluid in the carbonate environment favored the precipitation of smectite. The variations in clay mineral assemblages around the Gays River deposit constitute a potentially useful exploration tool for the region because of the genetic linkage with mineralization.