Abstract:

The Tyranite gold deposit is situated in a north trending narrow shear zone that crosscuts Archean metabasalts and felsic intrusive rocks in southern Abitibi greenstone belt, of northern Ontario, Canada. Most of the gold is present as microscopic inclusions in pyrite with minor amounts present as native gold within quartz-carbonate veins. Pyrite mineralization is present as lenses situated within the shear zone. The alteration grades from a background metamorphic chlorite subzone inward to a chlorite-sericite subzone and finally to a central chlorite-sericite-carbonate subzone. Minor quartz and quartz-carbonate veinlets becomes more frequent in the vicinity of the shear zone. The deposit also exhibits K⁺ redistribution which is responsible for formation of the ubiquitous sericite. Sulphidation of pre existing iron oxides and iron-silicates have resulted in Fe-depleted chlorites and formation of pyrite within the chlorite-sericite-carbonate subzone. XRD analysis shows that calcite is the predominant carbonate present. Theoretical calculations suggest that gold introduction took place at approximately 350C from a fluid with X_CO2 between 0.01 and 0.04. Gold was carried as a bisulphide complex and precipitated due to simultaneous decreases in temperature, pH and f_S2