Komatiites, "boninites" and tholeiitic picrites in the central La Ronge metavolcanic belt, Saskatchewan and Manitoba, and their possible economic significance

CIM Bulletin, Vol. 74, No. 831, 1981

J.S. FOX Saskatchewan Mining Development Corporation, W.G.Q. JOHNSTON Saskatchewan Department of Mineral Resources

A semi-continuous belt of Aphebian magnesian extrusive and related subvolcank rocks has been delineated by the authors over a distance of 150 kilometres in the central La Ronge lithostructural domain in Saskatchewan. They range in composition from peridotite to basalt and attain true thicknesses of up to 800 metres. For the most part, they are confined to a basal position in the La Ronge/Wasekwan metavolcanic sequence, and appear to have been extruded onto sialic crust. However, at Lynn Lake, Manitoba, in the eastern continuation of the domain, similar rocks seem to be found only at a relatively high stratigraphic level.Although metamorphism seems to have obliterated any quench textures that may have existed in these rocks, ultramafic pyroclastics, pillows and amygdules as well as intercalated, fine-grained clastic sediment attest to the volcanic nature of these rocks. Bulk chemical analyses indicate that these rocks are heterogeneous, with komatiites, tholeiitic picrites and even rocks with some of the characteristics of boninite all being present. The close temporal and spatial relationship of "boninites" and komatiites in the belt may suggest a high degree of Aphebian mantle inhomogeneity in the domain.Nickel-copper mineralization has been found in differentiated tholeiitic stocks within the La Ronge domain (e.g., at Lynn Lake), but none has yet been found in the ultramafic volcanics. However, the tholeiitic picrites do host one economic gold deposit — the Agassiz Mine at Lynn Lake.Because of the depleted nature of their mantle source, the "boninites" of the domain are not considered to be likely hosts for mantle-derived nickel sulphide mineralization. However, the opposite is true of the komatiites, and this points to the need for whole-rock geochemistry in setting priorities for nickel exploration in the belt. In contrast, all of the magnesian rocks of the domain probably have the same gold potential. Inasmuch as the geophysical response to gold mineralization in the domain is likely to be unpredictable, lake-sediment geochemistry, followed by base-of-till and/or alteration-oriented lithogeochemistry, should be emphasized during gold exploration.

Keywords: Volcanics, Magnesian volcanics, Waddy Lake, Reindeer Lake, Agassiz Mine, Exploration, Nickel exploration, Gold exploration, La Ronge area, Saskatchewan, Manitoba.