Magnesite abundance as a guide to gold mineralization associated with ultramafic flows, Timmins area

Altered komatiitic flows of the Tisdale Group locally contain secondary calcite, dolomite or magnesite, where alteration intensity and bulk-rock composition are favourable. Talc is present only in the least carbonatized samples and does not coexist with quartz and magnesite. Magnesite is present only in carbonatized ultramafic komatiitic flows (> 20 wt.% MgO) containing at least 18 wt.% loss on ignition (> 14 wt.% CO2). Magnesite abundance increases with increasing loss on ignition and MgO(MgO + CaO)) whole-rock ratio. Altered flows having magnesite to dolomite weight precent ratios exceeding 70 occur in proximity to mineable concentrations of gold and define surface-exploration targets 500 m in diameter.The spatial association of gold with the more magnesian ultramafic komatiitic flows arises in part because of the reactive nature of olivine-rich rocks in the presence of CO2-bearing aqueous hydrothermal fluids, and possibly because the most magnesian ultramafic flows occur in proximity to an eruptive vent area where the likelihood of rock-fluid interaction is greatest because of the local structural regime.Talc-magnesite—quartz assemblages are present in some ultramafic dykes which cut the older Deloro Group volcanic rocks; the apparent absence of gold in these rocks may be attributed to the higher formation temperature of the talc-bearing, relative to the talc-free, magnesite-quartz assemblages.