Proterozoic Ultramafic-Mafic Intrusions of the Nipigon Plate, Beardmore, Ontario T.R. Hart1, M. terMeer1, C. Jolette1, and B.M. Duggan2 1Precambrian Geoscience Section, Ontario Geological Survey 2Department of Geological Sciences, Queens University e-mail: [email protected] A Proterozoic peridotite overlain by one of metasediment broken along bedding planes within a the Nipigon diabase sills is located in Eva and Kitto biotite-rich gabbro matrix indicates extensive townships, west of the town of Beardmore and interaction between the peridotite and country about 190 km northeast of Thunder Bay, Ontario. rocks. A tremolite rich hornfels zone, composed of The peridotite intrudes Archean clastic magnetite-rich and magnetite-poor lapilli to block metasedimentary rocks and mafic metavolcanic sized fragments, is also developed in the mafic rocks situated on the southern edge of the metavolcanic rocks and iron formation along the Beardmore-Geraldton belt (BGB), along the contact contact with the olivine gabbro. The shape of the between the eastern Wabigoon and Quetico intrusion is not known due to the lack of definitive subprovinces. A swarm of narrow, north-trending contact relationships, and could range from a diabase dykes cut the supracrustal rocks, and are steeply dipping cone sheet dyke to a shallow west- predominately 2170 Ma Marathon age dykes dipping sill. Controls on the location of the (Osmani, 1991) with lesser dykes representing intrusions may include a fault along the Archean branches of the Nipigon sills. The peridotite, metasedimentary and metavolcanic contact and/or mapped during the 2001 field season (Hart et al., the Wabigoon–Quetico subprovince boundary. 2001), has been interpreted to be time equivalent to A series of undulating, generally flat-lying the ultramafic (picrite) intrusions of the Leckie and to shallow-dipping, massive, medium- to fine- Disreali lakes area (Sutcliffe, 1991) that are grained diabase sills of the Nipigon Sill Complex currently being explored for platinum group intrude all rock units. Sills are generally 100 m element (PGE) mineralization (East West Resource thick, and vary by 50 to 60 m in elevation over Corporation, 2001). distances of 400 to 500 m. But a 185 m thick sill The peridotite forms a roughly ring-shaped was intersected in the workings of two local gold intrusion that is crudely zoned and dominated by a mines. The diabase contains trace to 5% fine- lherzolite inner zone with a wehrlite outer zone. grained olivine in distinct contrast to the Logan sills The lherzolite is composed of massive, medium- located closer to Lake Superior (J. Franklin and R. grained, cumulus olivine, orthopyroxene, and lesser Middleton, personal communication, 2001). Sill clinopyroxene. Irregular areas containing stratigraphy is varied and coarse-grained approximately 5% coarse-grained, subhedral granophyric zones were observed toward the base orthopyroxene phenocrysts and variable amounts of of the sill, and also near the sill top underlying a plagioclase are more common towards the inner magnetite porphyritic phase. Contacts are contact. The wehrlite lacks the irregular areas rich commonly polygonal jointed, aphanitic chills or in orthopyroxene phenocrysts, but occasionally xenolith-rich hybrid zone along the contact with contains very coarse-grained poikilitic adjacent metasedimentary rocks. The hybrid orthopyroxene. Both units contain 1% fine-grained contacts consist of a matrix of fine-grained, black biotite or hornblende in contrast to the Nipigon biotite diabase hosting blocks or pebble-size diabase. Outcrop exposure is sparse, and the only xenoliths of metasediment and subrounded to exposed peridotite contact has fine-grained to rounded pebble sized xenoliths of coarse-grained chilled diabase overlying the peridotite. A quartz and white to pinkish feldspar. These hybrid granophyric olivine gabbro located along the inner contact textures are interpreted to indicate the contact between the peridotite and the supracrustal assimilation of adjacent metasedimentary rocks by rocks is interpreted to represent the upper contact of the diabase. the peridotite. A portion of the gabbro is coarse- The peridotite is compositionally similar grained with pinkish feldspar and small, irregular, to the ultramafic intrusions at Seagull and Disreali possibly mariolitic cavities interpreted to indicate lakes, south of Lake Nipigon, recently reclassified emplacement of the peridotite at a shallow crustal as dunites and lherzolites by Middleton (2001). level. A hybrid contact containing pebble to cobble The peridotite is also geochemically similar to the size, angular to subangular xenoliths of lower suite of the Osler Group volcanic rocks (Sutcliffe, 1987), with TiO2: 0.56 to 2.54%, Mg- Belt of the Pontiac Subprovince and the numbers: 0.71 to 0.80, La/Sm: 2.61 to 4.0, Gd/Yb: Use of Cu-Pd Ratios in Interpreting 1.76 to 3.73, and Th/Ta: 0.13 to 0.31. The olivine Platinum-Group Element Distributions. gabbro has similar ratios, but higher elemental Economic Geology, 88, p.1402-1418. abundances, than the peridotite indicating that it is East West Resources, 2001. East West Resources the fractionated and crustal contaminated upper Corporation website; Geology of the portion of the intrusion. However, this intrusion is Seagull and Disreali properties; older than the diabase sill and could be part of a www.eastwestres.com/prop_disraeli.htm magmatic event related to earlier rifting in the Hart, T.R., terMeer, M., Jolette,C. Duggan, B.M. Nipigon Plate. Nickel depletion within the 2001. Phoenix Bedrock Mapping Project: intrusion is relatively weak, but Cu/Pd ratios fall Geology of the Proterozoic Mafic and within the enriched range of Barnes et al. (1993) Ultramafic Intrusions in the Beardmore and overlaps with the Cu/Pd ratios reported for the Area. in Summary of Field Work and Disreali and Seagull lakes intrusions (Middleton, Other Activities, Ontario Geological 2001). There are a number of interflow chert- Survey, Open File Report 6070, p.20-1 to magnetite iron formations with minor sulphide 20-11. mineralization in the adjacent metavolcanic rocks Mackasey, W.O. 1975. Geology of Dorothea, that are potential sulphur sources for the formation Sandra and Irwin townships, District of of PGE mineralization in the peridotite Thunder Bay; Ontario Division of Mines, The diabase is geochemically similar to Report 122, 83p. the upper suite of the Osler Group (Sutcliffe, 1987), Middleton, R. 2001. Noril’sk Style Cu-Ni-PGE with TiO2: 0.66 to 2.49%, Mg-numbers: 0.29 to Mineralization in the Nipigon Plate-Lake 0.70, La/Sm: 1.59 to 2.93, Gd/Yb: 1.37 to 1.79, and Superior Region; abstract in Superior PGE Th/Ta: 0.11 to 0.33. Mineralization within the 2001, CIM Geological Society Field diabase consists of trace amounts of disseminated Conference, September 16-19, 2001. pyrrhotite and pyrite and chalcopyrite in joints Osmani, I.A. 1991. Proterozoic Mafic Dyke close to the upper or lower contacts. The Swarms in the Superior Province of assimilation of adjacent metasedimentary rocks Ontario; in Geology of Ontario, Ontario increases the potential for contact-style PGE Geological Survey, Special Volume 4, Part mineralization, with occurrences present near the 1, p.661-681. upper contact of a sill in southeast Kitto Township Sutcliffe, R.H. 1987. Petrology of Middle and the lower contact of a sill in southwestern Eva Proterozoic diabases and picrites from Township. Lake Nipigon, Canada. Contributions to Mineralogy and Petrology, 96, p.201-211. References Sutcliffe, R.H. 1991. Proterozoic geology of the Barnes, S.J., Couture, J.-F., Swayer, E.W., and Lake Superior area; in Geology of Ontario, Bouchaib, C. 1993. Nickel-Copper Ontario Geological Survey, Special Occurrences in the Belleterre-Angliers Volume 4, Part 1, p.627-658. .