Precambrian geology of the Atikokan area, northwestern Ontario

Abstract

This bulletin reviews Precambrian geology, including lithology, geochemistry, petrogenesis, structure, and metamorphism, in the vicinity of Atikokan, northwestern Ontario. The area spans approximately 1100 km2 of the volcano-plutonic Wabigoon Subprovince and the metasedimentary Quetico Subprovince - two large linear lithostructural elements of the Superior craton. Nineteen major rock units are classified into pre- to synvolcanic intrusions, supracrustal rocks, and syn- to postvolcanic intrusions. The early intrusive suite comprises four varieties of tonalite and tonalite gneiss, the oldest of which is dated at 3001 Ma (U/Ph zircon). Tonalite is concentrated mainly in the Marmion batholith whereas gneisses occupy belts and a large complex in the north. These rocks probably originated by melting of a pyroxene granulite of tholeiitic composition. The Marmion batholith is unconformably overlain by supracrustal rocks of the Steep Rock Group consisting of discontinuous metaconglomerate and 111etasandstone (Wagita Formation) , limestone beds (Mosher Carbonate Formation), ironsto11es (Jolliffe Ore Zone Formation), and an ultramafic pyroclastic rock (Dismal Ashrock Forma1ion). The unconformity typically comprises a 2 to 10 m transition zone from tonalite through altered.friable to11alite and quartz-sericite "grit" 10 bedded metasediments. The Jolliffe Ore Zone Formation was mined from 1944to1979 and yielded 100 million tonnes of mainly goethite ore. The upper contact of the Dismal Ash rock Formation is tectoni:ed and poorly exposed and precludes clear definition of stratigraphic relations of the Steep Rock Group to contiguous metavolcanic rocks. Jn addition to the ashrock, metavolcanic rocks include mafic pillow flows and intermediate to felsic flows, tuffs, and breccias. The ashrock is unusual amo11g Archean komatiites in its dominantly pyroclastic mode of occurrence and enriched incompatible trace element composition. The ultramafic magma may have formed by melting of anomalously enriched mantle or by interaction with sialic crust. The mafic lavas are typical Archean tholeiites. They probably evolved by fractio11ation of more primitive high-magnesium basalt. The intermediate to felsic metavolcanic rocks have relatively high contents of compatible trace elements and other geochemical characteristics, which are i11consiste11t with an origin by fractionation of the mafic lavas. Processes of magma mixing or assimilation are required to explain the petrogenesis of these rocks. Units of elastic metasediments i11 the metavolcanic belts are made up of agglomerate, sandstone, conglomerate, and greywackelargillite. Volumi11ous turbidites were derived mainly from an intermediate to felsic volcanic source with a lesser component of tonalite and mafic vo/r·c,11,",- detritus and were deposited in the Quetico basin about 2695 Ma ago. Although some metagabbro dykes predate deposition of the Steep Rock Group, the majority are grouped with syn- to postvolcanic intrusions. Metagabbro dykes and sills are partirnlarly common in the Steep Rock belt and compose up to 30 per ce11t of some felsic intrusions such as the Marmion batholith. Dioritelhornblendite occupies a stock central in the area and thi11 units marginal to metavolcanic and gneiss belts. The syn- to postvolcanic felsic intrusions range in size and form from small dykes in gneisses to plutons. U!Pb zircon dates range from 2936 Ma for tonalite to 2665 Ma for granite of the Eye-Dashwa pluton. The monzodiorite rim of this youngest pluton could have been derived by partial melting of a lithophile-element-enriched mantle source. The regional linear disposition of Quetico metasediments adjacent to Wabigoon metavolcanic rocks has been attributed to subduction-driven accretion of trench-fill turbidites (Quetico) against the Wabigoon volcanic arc about 2695 Ma ago. Accretion was followed hy strong dexrral transpression and imposed a pervasive east-west strike to foliations, bedding, and isoclinal folds in Quetico metasediments. Metamorphic grade increases south toward late granites at the central axis of the Quetico Suhprovince. The Wabigoon Subprovince is dominated hy large felsic oval structures that typically have a felsic plutonic core, an outer gneissic envelope, and concentric foliations. Belts of folded supracrustal rocks and mafic gneisses separate oval structures and can be metamorphically zoned with greenschist cores and amphiholite rims. This structural and metamorphic pattern may result from emplacement and spreading of hot, low-density felsic plutons and gneisses in oval structures that shortened and contact metamorphosed supracrustal rocks in adjacent belts. Dip-slip faults occur at the southwestern margin of the Dashwa oval structllre and within the Steep Rock belt; the Marmion batho!ith is cut by a complex, northeast-trending network of faults. The Eye-Dashwa pluton was pervasively faulted and fractured during a brittle deformation event at about 2300 Ma. Mild fracturing occurred at about 1100 Ma. Fractures are open and transmit groundwater mainly within 200 m of surface.