This paper reports the results of a petrographic and geochemical study on a large talc-magnesite deposit near Timmins, Ontario. This deposit is characterized by four distinct mineral assemblages. The central section of the carbonate zone contains medium-grained magnesite, talc, quartz and accessory iron oxide minerals. One isolated outcrop at the east end of the carbonate deposit contains fine-grained magnesite and quartz with minor talc and hematite. A mineral assemblage consisting of fine-grained magnesite, talc, serpentine, and accessory iron oxides is also recognized in the eastern section and is in close proximity to serpentinite countryrock in which minor magnesite is present. Petrographic evidence suggests that these assemblages represent various stages in the progressive alteration of a serpentinite parent rock.Chemical and petrographic data from the various assemblages indicate that negligible amounts of magnesia were added to or removed from the system during alteration. Similarly, very little silica was added during alteration but it is possible that some SiO 2 was removed from the system during early stages of carbonatization. Volumetric changes due to carbonate alteration are minor, although a late-stage of silicification probably resulted in a significant increase in the volume of the system. Published data on the system MgO-SiO 2 -H 2 O-CO 2 indicate that during early stages of alteration, temperatures may have been in the range 150-300 degrees C but increased during the alteration which resulted in the magnesite-talc-quartz assemblage of the central section of the deposit.