Formation temperature and ages of the True North ‘orogenic’ gold deposit in Manitoba, Canada

The True North orogenic gold deposit is situated in the 2.99–2.70 Ga Rice Lake greenstone belt near Bissett (Manitoba, Canada). This belt is the western equivalent of northern Ontario's Abitibi gold-producing region. The lithology and alteration, structural control, geochronological framework, and ore geology of the True North orogenic gold deposit have been addressed, but its formation temperature and age are poorly constrained. The deposit's gold-bearing veins are composed mostly of quartz (40 to 80%), ankerite (20 to 30%), and albite (5 to 25%), with minor muscovite, sericite, chlorite, and calcite. Sulfide minerals are dominated by pyrite and chalcopyrite and with minor pyrrhotite; native gold appears as inclusions in sulfides or as free gold. In general, most of the major minerals, the sulfides, and gold all precipitated during the main ore stage. The oxygen isotopic composition of co-existing quartz–albite and quartz–ankerite couples were used to calculate a formation temperature of approximately 230°C for these minerals, which is comparable to the nearby 007 gold deposit. These temperatures are on the low end of the temperature range reported for orogenic gold deposits. 40 Ar/ 39 Ar geochronology (single grain laser fusion of muscovite and sericite) revealed the presence of two age groups, at 2441.2 ± 8.8 Ma and 2483.1 ± 10.2 Ma, approximately 200 myr lower than the age of the main collision-related deformation within the Rice Lake belt. These ages indicate that the True North deposit was clearly post-orogenic, similar to vein gold deposits elsewhere, and was likely produced by the emplacement of the Huronian Supergroup and the thermal effects of the Matachewan large igneous province, which has the same ages. It is suggested that the True North deposit may belong to a descriptive category of post-orogenic low-temperature quartz–carbonate–albite vein gold deposit, more in line with recent developments in classification schemes.