The altered rocks of the Mattagami Lake Mine, a stratabound, volcanogenic, massive sulphide deposit, were examined using whole rock analyses and electron microprobe studies of the constituent minerals. The significant chemical and mineralogical transformations involved in the progressive alteration of the footwall, vitroclastic tuff of rhyodacitic composition, are: (1) the removal of alkalies (sodium followed by potassium), and the addition of magnesium and iron during initial chloritization; (2) substantial removal of silica by the solution of quartz, to produce a chlorite-rich rock, and (3) gradual removal of aluminum and the transformation of chlorite (Mg2.5 Fe2.5Al2Si3O10(OH)8) to talc (MG2.5FE0.5Si4O10(OH)2 to produce units of talc–actinolite schist. The reaction: 3 chlorite + 10H2S(g) + 2.5 O2 + 11,H4SiO4 + 5Mg2+ = 5talc + 5pyrite + + 19H2O + 16H+ (log K, 25 °C = +145; log K, 100 °C = +102; log K, 250 °C = +68.2) is favoured by high temperature and Mg2+ activity, and low activity of aluminum.The alteration pipe zone immediately beneath the orebody is assumed to have been the discharge site of a thermally induced, convective flow system. The upper part of the system would have been characterized by the movement of seawater, of comparatively short residence time in the rocks, to areas of discharge. Under conditions of high permeability and high water flux in these zones, bulk seawater composition of comparatively high magnesium and low aluminum concentrations would ultimately control the composition of the volcanic material by the formation of alteration products in equilibrium with it, rather than the volcanic material significantly affecting the seawater chemistry. This would ensure the early development of magnesian chlorite in the vitric tuff. The transformation chlorite to talc took place at discharge sites, the locations of highest surface temperatures, under hydrologic conditions such that the flux rate was sufficiently high to remove the comparatively immobile aluminum.The massive sulphide units were emplaced in association with the development of talc. Layered pyrite–sphalerite, overlying and extending beyond talc units are chemical sediments.