Distribution of Gold in an Archean Greenstone Belt as Exemplified by the Kakagi Lake area, Northwestern Ontario

Abstract

<p>Aiming to explore into the background gold content of various rock types of a typical Archean greenstone assemblage, an area of about 180 sq. mi. (460 km²) centered on Kakagi Lake in the western tip of the Wabigoon plutonic-volcanic belt has been carefully sample. 166 analyses by neutron activation on the various rock types exposed in the area show that none of the major rock groups, namely, the mafic volcanics, felsic volcanics, the metasediments, mafic-ultramafic intrusives and felsic plutonic stocks, is characterized by anomalous gold. The gold distribution in each of them, however, is highly skewed. Little correlation of gold with bulk rock composition has been detected for the major rock groups but there are indications of some possible correlation of gold with water in basaltic rocks reflecting remobilization of the metal during high rank metamorphism. A carbonate zone of 30-100 feet wide underlying a well-differentiated mafic-ultramafic still approximately in the centre of the area is the only rock type which is obviously richer in gold than all the other rock types of the area. Field evidences and the mineralogy of the carbonate rocks suggest that it is the shearing occurring in the contact zones of the overlying ultramafics and the underlying felic pyroclastics that acts as a key factor allowing the percolation of aqueous, CO2 rich fluids which presumably has caused the formation of carbonates and which has also transported the gold. Detailed studies of gold across quartz veins, shear zones and felsic dikes/sills and lithologic associations of known occurrences in the area further illustrate the overwhelming importance of process-related factors e.g. the availability of suitable structures and physiochemical conditions, the precipitation mechanisms of gold, etc. towards the formation of potential ore bodies. Apparently, for the numerous, small gold occurrences scattered throughout the area, volcanism is like to have provided the appropriate conditions for the initial localization of gold. Subsequent concentration(s) may have been effected from remobilization over short distances under the thermal effect of intrusions or by regional metamorphism.</p>