Ontario Mineral Inventory

Ontario Geological Survey
Permanent Link to this Record: MDI52A15SW00007

Record: MDI52A15SW00007

General

Commodities

Primary Commodities: Uranium

Location

Township or Area: Greenwich Lake Area

Latitude: 48° 46' 59.79"    Longitude: -88° 51' 50.32"

UTM Zone: 16    Easting: 363076.48   Northing: 5405039.7    UTM Datum: NAD83

Resident Geologist District: Thunder Bay South

NTS Grid: 52A15SW

Point Location Description: General

Location Method: Conversion from MDI

Access Description: Greenwich Lake is located approximately 56 km northeast of Thunder Bay. Access is by air.

Exploration History

1949: Tom Christianson made the discovery of radioactive rocks in the area. 1955-56: Pan Canadian Development Company Ltd. Conducted trenching, sampling, and drilled 16 DDH totalling 402 m. 1958: Checklin & Othmer conducted trenching and geological mapping. 1969: Oja Ltd. conducted an airborne radiometric survey. 1969: Univex Exploration and Development Corporation Ltd. drilled 4 DDH. 1975: Copper Lake Exploratoin Ltd. conducted prospecting, radiometric survey, trenching and drilling. Consolidated Monarch MW Resources Ltd. drilled 6 DDH totalling 406 m. 1976-77: Rio Tinto conducted trenching, geological mapping, radiometric survey, and drilled 3 DDH. 1978: Greenwich Lake Exploration drilled 20 DDH totalling 1226.5 m. 1979: Norcen conducted geological mapping, mag survey, radon gas, and geochem. 2006: Mega Uranium Ltd. flew an airborne magnetic and radiometric survey. 2009: Mega Uranium flew an EM and Mag survey and conducted prospecting and sampling. 2010: Mega Uranium drilled 8 DDH totalling 1803 m. 2011: Mega Uranium drilled 6 DDH totalling 963 m. 2012: Magma Metals Ltd. optioned the property from Mega Uranium.

Assessment Work on File

Geology

Province: Superior

Subprovince: Quetico

Geological Age: Archean  

Lithology

Lithology Comments

Jun 21, 2017 (Therese Pettigrew) - Sibley Group rock fragments ^have been identified in the Christiansen Showing breccia at Greenwich Lake, implying that the unconformity, although now eroded away, was present not too far above the current erosion surface (Scott, 1997). The Greenwich Lake area is underlain by rocks of the Quetico Gneiss Belt, and consists of arenaceous to argillaceous metasedimentary rocks, sills and lensoid stocks of quartz monzonite, and albititic pegmatite dykes. Late Precambrian diabase dykes intrude this rock assemblage. Medium grained, quartz-biotite-plagioclase gneisses, biotite schists and pegmatitic, white-weathering granitoids are the oldest rock units in the Greenwich Lake area and represent migmatized sedimentary rocks that have undergone regional metamorphism of almandine-amphibolite rank. The gneisses are black to dark grey in color and have a strong laminar appearance. The rocks are highly folded in areas; ptygmatic structures are common. Non-migmatized metasedimentary rocks crop out near the south end of the lake and exhibit graded bedding, small scale crossbedding and clast rip-up features, all indicating a north-facing sequence. Metamorphic grade increases northward from greenschist to almandine-amphibolite rank. The gneissic rocks are intruded by tonalite and quartz monzonite. Tonalite is the dominant granitic and pegmatitic phase within the migmatites. It is equigranular, and composed of sodic plagioclase, quartz, biotite and muscovite. It can occur as thin bands within the biotite schists or as pegmatite dykes several metres in width. The tonalite is strongly sheared and usually parallels the regional gneissosity. Xenolithic blocks of country rock in various stages of assimilation occur in the tonalite. The quartz monzonite intrudes all rock types except the diabase dykes. The quartz monzonite .intrusions are elongate and have steeply dipping contacts. The rock is massive, coarse to medium grained, and is composed of K-feldspar, quartz, biotite and muscovite. Contact phases of whitish granodiorite may border the intrusion. Pegmatitic phases do occur and their contacts are sharp and parallel the regional genissosity. Xenolithic blocks of biotite gneiss do not show rotation or any degree of assimilation by the quartz monzonite. The quartz monzonite bodies vary in width from 60 m to in excess of 2,500 m. The quartz monzonite is cut by numerous north-trending fractures. The diabase dykes follow north-trending linears and form gullies that are steep-walled, flat-bottomed and have generally uniform widths. The diabase is less resistant to erosion than the rocks it intrudes and, therefore, weathers out. The dykes can be up to 10 m wide. They are considered to be Late Precambrian in age and cut all other rock units in the area (Scott, 1987).

Mineralization

Mineralization Comments

Jun 21, 2017 (Therese Pettigrew) - At Greenwich Lake, the uraniferous veins occupy a set of northerly to northwesterly-trending fractures. The best documented of these is the Christiansen Showing. Syngenetic occurrences of uranium are associated with quartz monzonite stocks, and the quartz-feldspar mobilizate veins and dykes and albite pegmatites. Epigenetic deposits are characterized by pyrite-pitchblende veins. The albite pegmatites have been described as "tonalite" by Blair (1977). These pegmatites occur as semicontinuous dykes and lenses within the contact zone between the biotite schists and the quartz monzonite. In the north contact zone all radioactive pegmatites occur within 60 m of the quartz monzonite contact. The dykes consist of 70-80% albite, 10-20% quartz, 1-10% biotite, 1-5% muscovite and 0.1-0.5% apatite (Franklin 1978). The dykes are sheared and well foliated. The foliation is conformable to the trends of the enclosing migmatitic gneisses. The tonalite ranges in width from 1.5 m to 10 m, with 4.5 m to 6 m being average. Uranium is present as uraninite, in close association with biotite. Radioactive pegmatites in the Greenwich Lake area contain apatite and zircon as accessory minerals. Uranophane and other secondary uranium minerals occur as films and coating on fractures and joint planes. Uranium mineralization within the pegmatites is erratic; the best analysis obtained from a selected grab sample was 0.12% U3O8. While the radioactivity associated with the pegmatite zone is generally quite high, the uranium content appears to be negligible. Core analysis by Rio Tinto yielded a high value of 1.4 Ibs U3O8 per ton across 5 feet (Blair 1977). Most other sections assayed below 0.4 Ibs U3O8 per ton. The best results from the 1975 drill program included 0.06% U3O8 over 0.43 m and 0.07% U3O8 over 0.46 m. Uranium mineralization occurs in two principal modes in the Greenwich Lake area. The pyrite-marcasite-pitchblende veins of the Greenwich Lake area occupy north-trending fracture sets: The Christiansen fault trends NNW and the Univex Showings, located approximately 1,200 m to the NNW of the Christiansen Showing, are associated with a NNE lineament. These faults may represent a conjugate set. The largest vein found to date is the Christiansen Showing. Drilling by Pan Canadian Development Company Limited has outlined two "ore shoots": one is 5.9 feet by 150 feet, averaging 0.15% U3O8; and the other, 5.0 feet by 150 feet, averaging 0.29% U3O8. Extension to depth was not tested. The breccia zone that contains the vein is up to 3 m wide. The displacement along the fault is in the order of 35 m. The vein is up to 1 m wide and is composed of fine-grained quartz, pyrite, and marcasite together with brecciated host rock and Sibley Group fragments. Alteration of the wallrocks consists of disseminated pyrite and quartz and permeates approximately 1 m from the vein. Vein contacts are slickensided, indicating movement along the fault zone. At the original Christiansen Showing, an approximately 0.5 m width of pyrite is exposed for 10 m in a small ravine. While the pyrite is radioactive, the highest readings on the McPhar TV-1 gamma ray-spectrometer were obtained from a breccia zone in the hanging wall of the vein. The breccia is pale buff in color and fragment sizes do not exceed 2 cm across. A sample of the breccia collected by John Scott in 1982 assayed 1.1% U3O8 (Scott, 1987).

Mineral Record Details

References

Map - Nipigon-Schreiber, geological compilation series, Thunder Bay District

Publication Number: M2232 Scale: 1:253,440    Date: 1973

Author: Carter M.W., McIlwaine W.H., Wisbey P.A.

Publisher Name: Ontario Division of Mines

Mono - Uranium and thorium deposits of northern Ontario

Publication Number: MDC025 Scale:     Date: 1984

Author: Robertson J.A., Gould K.L.

Publisher Name: Ontario Geological Survey

Mono - Geology of the Greenwich Lake area, District of Thunder Bay

Publication Number: OFR5437 Scale:     Date: 1983

Author: Carter M.W.

Publisher Name: Ontario Geological Survey

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For detailed information regarding this mineral record please contact the Thunder Bay South Resident Geologist District Office