Volcanogenic exhalative massive sulphide deposits at Matagami are associated with a bimodal suite of tholeiitic basalt and rhyolite. However, these rocks now exhibit pseudo calc-alkaline alteration trends resulting from a hydrothermal alteration process in which footwall basalt was spilitized, and the host rhyolite was chloritized. These two alteration processes were contemporaneous events, which shared a common aqueous fluid, developed within cooling submarine volcanic rocks shortly after the extrusion of rhyolite, and terminated prior to extrusion of succeeding units.The geochemistry of the Garon Lake rhyolite and underlying basalt are examined in detail. Massive quantities of Fe, Mg and Ti, and over 10 ppm Zn and 5 ppm Cu were leached from basalt during spilitization, providing 30 000 t Zn and 15 000 t Cu, or approximately 1 Mt of ore grade material per km3 of basalt. Chlorite alteration zones and massive sulphide deposits in the overlying rhyolite were enriched in all those elements depleted from basalt. This evidence suggests a geothermal model for massive sulphide genesis, where Fe, Mg, Ti, Cu and Zn were leached from footwall rocks, flushed through a geothermal system, and then precipitated at the discharge point to form chlorite alteration zones in the overlying rhyolite and exhalite deposits at the sediment–seawater interface.