The magnesites at Main Creek, a tributary of Savage River in northwestern Tasmania are associated with dolomite and talc-chlorite schist in a mildly metamorphosed belt running northeastward across the area. The nearly vertically dipping lens of magnesite is over 218 m thick, and although the extension to the south and north is very uncertain, reserves most likely exceed 40 million metric tons. As a result, mining is being considered. A further deposit of about equal size is found to the north at Arthur River. The carbonate mineralogy is well characterized by the use of a very sensitive backscattered electron detector which easily distinguishes magnesite and dolomite and also demonstrates the zoning of Mg and Fe in both the magnesite and dolomite. Apart from the thin bands of schist and dolomite, the magnesite ore contains about 5 to 10 percent quartz and 5 to 10 percent dolomite. In the upper dolomite horizons especially, replacement of dolomite by magnesite is evident. Mg metasomatism by dilute MgCl 2 solutions is considered more likely than a sedimentary origin. The amount of FeCO 3 in the magnesite falls steadily from about 3 percent at the surface to less than 1 percent FeCO 3 at 305 m. Experimental studies of Johannes (1970) suggest that a MgCl 2 solution with small amounts of FeCl 2 would be expected to form more Fe-rich magnesite at higher (cooler) horizons and more Mg-rich magnesite at depth. It is also suggested that the Savage River magnesites, which were once dolomites, may be contemporaneous with the Savage River dolomites which lie to the west, outside the Arthur lineament, and which are considered to be early Cambrian in age.