Archean rocks of the Separation Lake area consist of metavolcanic and metasedimentary rocks of the east trending Separation Lake greenstone belt, metasedimentary migmatites and plutonic rocks of the English River Subprovince and plutonic rocks of the Winnipeg River Subprovince. The 45 km by 5 km greenstone belt lies along the subprovincial boundary. It consists of a lower sequence of mafic metavolcanic rocks, with intercalated magnetite-bearing iron formations, a single discontinuous clastic metasedimentary unit, and overlying subordinate felsic metavolcanic rocks. Gabbro sills intrude the mafic metavolcanic sequence. A thin unit of polymictic conglomerate and sandstone lies along the northern margin of the belt. Metamorphic grade is amphibolite throughout the belt. Unaltered mafic metavolcanic rocks are high magnesian tholeiitic basalts. Trace element data show them to be of low potash ocean floor affinity, while rare earth element data indicate affinity with mid-ocean ridge basalts. Metagabbroic sills are chemically identical to metavolcanic rocks, indicating derivation from a common magma pool. Felsic metavolcanic rocks are fine-grained tuffaceous, calc-alkaline rhyolites and dacites. Limited rare earth element data for felsic metavolcanic rocks show variable chemistry, inconsistent with classification into FI, FII and FIII types. Granitic rocks of both subprovinces are fundamentally similar in composition, being granites sensu stricto. Rare earth element data indicate that, for all except one suite, plagioclase fractionation played a major role in their evolution. Hydrothermal alteration of some mafic metavolcanic rocks, indicated by abundant garnet, is confirmed by alkali element mobility, iron enrichment, and alumina, magnesium and calcium depletion. Rare earth element data support moderate alteration. Two contrasting structural zones are separated by an east trending lineament that lies along the north side of the greenstone belt. A major open east-plunging fold to the north of the lineament contrasts with a predominantly homoclinal, north facing sequence with local west-plunging folds to the south of the line. The lineament also marks the boundary between migmatised supracrustal rocks to the north and non-migmatised supracrustal rocks to the south. This domain boundary does not coincide precisely with the subprovincial boundary as proposed by previous workers. Sulphide zones in mafic metavolcanic rocks of the greenstone belt have to date yielded anomalous to low amounts of gold, copper and zinc. Gold varies from 10s of ppb to about 0.3 ounce Au per ton, while zinc and copper are common in the 1000 to 5000 ppm range, and rarely up to 5 to 10%. Three types of sulphide zones are indicated. Breccia-hosted, massive to stringer arsenopyrite-pyrrhotite zones host sphalerite, chalcopyrite and gold. Silicified, shear zone-hosted, massive to stringer pyrrhotite-pyrite zones host sphalerite and chalcopyrite. Iron formation and chert-hosted, massive to stringer veins of pyrite and pyrrhotite host gold, sphalerite and chalcopyrite. Examples include the Gauthier gold occurrence (breccia-hosted), Champion Bear Resources Ltd.'s Grid C base metal occurrences (silicified shear zone-hosted) and the Helder Lake gold occurrence (iron formation and chert-hosted). The iron formation units hold potential for discovery of other gold occurrences. Rare metals such as lithium, beryllium and tantalum are hosted in pegmatites generated by the 3 km (super 2) Separation Rapids granitic pluton. The presence of petalite as the lithium aluminosilicate mineral in a number of attendant dikes draws comparison between the Separation Rapids dike swarm and other petalite-bearing pegmatites elsewhere in the world that host rare-metal producing mines. Discovery of other beryl occurrences up to 10 km distant from the Separation Rapids pluton during the present survey suggests that the rare-metal bearing pegmatite field may be larger than currently known. Graphite occurrences in the migmatites of the English River Subprovince possibly resulted from pyrolization of organic material in precursor clastic sediments. Uranium-enriched granitic pegmatites are present both in the migmatites of the English River Subprovince and in the granitic rocks of the Winnipeg River Subprovince. The majority of the 87 samples taken for assay during the present survey were to test primarily for base metals from zones already tested by exploration companies. These in general confirmed previous results. However, samples taken from a number of other prospective sulphide zones and iron formations not previously explored returned anomalous to low base metal values. Further prospecting is warranted in these areas. Assay values for gold from all samples taken were uniformly low. Descriptions are provided of all documented exploration done in the area up to the time of the present survey, with the exception of a few uranium properties previously described by other workers. Dimension stone is another commodity worthy of investigation in the map area, particularly within the Winnipeg River Subprovince. Surficial sediment sampling for geochemical, gold grain and heavy mineral analysis, conducted as an adjunct to the present bedrock mapping program, has identified several sites of interest for gold, base metals and rare-metal pegmatites.
Eastern Canada; folds; Superior Province; tectonic elements; metal ores; North America; tectonics; construction materials; Canada; English River Belt; geologic maps; Canadian Shield; Precambrian; maps; geochemistry; uranium ores; resources; Separation Lake; metasedimentary rocks; metavolcanic rocks; base metals; Ontario; lithostratigraphy; graphite deposits; Archean; metamorphic rocks; Winnipeg River Belt; gold ores; northwestern Ontario; mineral exploration; metamorphic belts; lineaments; greenstone belts; dimension stone; (2000)