Komatiite Flows of the Kidd Creek Footwall, Abitibi Subprovince, Canada

Abstract Komatiites are the most abundant rock type in the stratigraphic footwall of the giant Kidd Creek volcanic-associated massive sulfide deposit. They comprise primitive, Al-undepleted flows with a preserved thickness of ∼0.8 km and correlate with ∼100 km3 of ultramafic rocks within 30 km. Individual flows in the immediate footwall have mesocumulate peridotitic bases, spinifex-textured and flow-breccia tops, and contaminated margins. They have estimated prestrain cross-sectional areas of ∼10 to 200 m in thickness and ∼250 to 700 m in width. As a package, they parallel the orebody and footwall rhyolite flow lobes to a depth of at least 3 km. Their textures and geometry suggest a channel flow facies within a tectoni-cally active linear topographic depression, consistent with a paleo-graben oriented subvertically and facing west. The komatiitic rocks are intimately intercalated with massive and epiclastic rhyolite beneath the ore and the ore-hosting mine rhyolite unit. Textural relationships indicate that komatiite flows postdated and partially melted earlier formed rhyolite. These include ultramafic dikes and apophyses that cut rhyolite, partial melt textures in rhyolite adjacent to komatiitic rocks, back-veining of siliceous partial melts into ultramafic rocks, possible thermally shocked quartz in siliceous partial melt at komatiite-rhyolite contacts, and olivine spinifex textures quenched at the contacts with rhyolite fragments. Liquid compositions from least contaminated, quench-textured komatiite flow tops and flow-top breccias have Mg/(Mg + Fe) of 79 to 82, MgO = 17.7 to 24.0 wt percent, Cr = 1,940 to 2,660 ppm, Al2O3 = 6.0 to 10.3 wt percent, TiO2 = 0.33 to 0.61 wt percent, and LaN/YbN = 0.7 to 1.1 (anhydrous, n = 4). Their average composition corresponds to a primitive mantle partial melt of ∼33 to 38 percent or to higher degrees of partial melting accompanied by olivine fractionation prior to emplacement. Other komatiites and related high Mg basalts can be explained by relatively limited fractionation of average peridotite cumulate, accompanied by slight contamination using the average footwall rhyolite as the contaminant. These rocks have Mg numbers of 55 to 76, MgO = 7.0 to 15.0 wt percent, Cr = 100 to 1,900 ppm, Al2O3 = 8.8 to 14.3 wt percent, TiO2 = 0.45 to 0.56 wt percent, and LaN/YbN of 1.4 to 2.2 (anhydrous, n = 4). More highly contaminated komatiites are also present, with many samples explained by ∼20 to 40 percent fractionation accompanied by ∼20 to 40 percent assimilation of the footwall rhyolite. The volcanology of the Kidd Creek komatiitic footwall rocks can be considered in the context of the Norseman-Wiluna greenstone belt of Western Australia, where komatiite volcanic facies have been defined. The footwall komatiites are comparable to the channel flow facies at Kambalda at the southern end of the Norseman-Wiluna belt, believed to be medial or distal with respect to source vents. A medial or distal facies would suggest coeval but spatially distinct source vents for the intercalated footwall komati-ites and proximal rhyolites. However, a source vent for komatiite flows in the Kidd Creek footwall is possible, given the presence of well-preserved ultramafic dikelets; the spatial coincidence with the vent area for a giant metal-precipitating hydrothermal system, with feeder dikes to the hanging-wall basalts and gabbros, and with synvolcanic faults. Contiguous ultramafic rocks within 30 km of Kidd Creek are 10 to 100 times less voluminous than those of the Norseman-Wiluna belt, and they lack appreciable dunitic rocks which are diagnostic of proximal, sheet, or channel flow facies. If the Kidd Creek footwall represents a locus of komatiite fissure eruptions, the komatiites were less voluminous, and less magnesian, those of the Norseman-Wiluna belt.