The Pb-Zn ores of eight of the nine deposits are characterized by one or the other of the phyllosilicate assemblages chlorite + talc + phlogopite-biotite or chlorite + muscovite. The other contains chlorite + phlogopitic trioctahedral mica but no talc. Phyllosilicate mineral chemistry is highly systematic and is related to ore type and ore stratigraphy. In all cases chemical equilibrium between the phyllosilicates has been approached on a small scale. Equilibria were established substantially at the sedimentary-early diagenetic stage. The phyllosilicates were deposited as chemical precipitates in the hydrothermal aprons at the time of sea-floor exhalative activity. Subsequent metamorphism has done little more than perfect crystal structures and coarsen grain size.--Modified journal abstract.