Ventures Sill
Stratigraphic label: [narc]ven
Map symbol: nAven

First published: 20 March 2018
Last modified:








Translation of original French




Informal subdivision(s)
Numbering does not necessarily reflect the stratigraphic position.
nAven3 Granophyric gabbro, leucogabbro
nAven2 Ophitic gabbro
nAven1 Peridotite, websterite and pyroxenite


Author: Duquette, 1970
Age: Neoarchean
Reference section: The Ventures Sill stratigraphy was established underground at the Opémiska property (McMillan, 1972; Watkins and Riverin, 1982). Since mines are no longer accessible, we propose an alternative section along Road 167, west of Cummings Lake (see Bédard et al., 2009).
Type area: West of Cummings Lake (NTS sheet 32G16)
Geological province: Superior Province
Geological subdivision: Abitibi Subprovince
Lithology: Mafic and ultramafic intrusive rocks
Type: Lithodemic
Rank: Lithodeme
Status: Formal
Use: Active




The Ventures Sill stratigraphy was established during underground work on the Opémiska property (Springer and Perry mines) and in the Chapais area (McMillan, 1972; Watkins and Riverin, 1982). Stratigraphic drillings east of Barlow Lake (Poitras, 1984) and the roadway section along Cummings Lake (Bédard et al., 2009) characterize the petrography and geochemistry of the Ventures Sill.






Ventures Sill 1 (nAven1): Peridotite, Websterite and Pyroxenite

Unit nAven1 is composed of basal websterite with orthopyroxene pseudomorphs (±40%, <2 mm) replaced by bastite and clinopyroxene pseudomorphs (±45%) replaced by a talc-chlorite assemblage. The rock becomes richer in olivine towards the top and includes some poekilitic crystals of orthopyroxene, clinopyroxene or hornblende. Intercumulate minerals are replaced by greenschist facies minerals (Bédard et al., 2009). Serpentine peridotite is fine to medium grained and includes ovoid olivine pseudomorphs (85%, 0.1-0.4 mm) and some larger phenocrystals. Euhedral clinopyroxene is present in smaller amounts (15%, 2-6 mm). Greenish to grey clinopyroxenite is medium grained (≤1 mm) and has an equigranular texture obliterated by alteration (Bédard et al., 2009). Unit nAven1 corresponds to the Lower Green Pyroxenite, Black Pyroxenite and Upper Green Pyroxenite subunits of Watkins and Riverin (1982) on the Opémiska property, in the Chapais area.



Ventures Sill 2 (nAven2): Ophitic Gabbro

Unit nAven2 consists of foliated gabbro and ophitic gabbro (Leclerc et al., 2017). According to Watkins and Riverin (1982, pp. 433-434), the foliated gabbro thickness is approximately 150 m. The latter is the base and is commonly composed of a 15 to 30 cm-thick layer of clinopyroxene, including 30 to 40% magnetite. The typical structure is rhythmic bedding caused by segregation of the main magmatic minerals including andesine, clinopyroxene and titanomagnetite. The rock is invariably characterized by strong foliation due to the alignment of mafic minerals and the feldspars’ long axis. Contact with the overlying unit is sharp and marked by an abrupt change in texture and size. As for the Ventures’ ophitic gabbro unit, it forms the upper portion of the sill over a thickness of 350 m. Its mineralogy is similar to that of foliated gabbro and includes up to 5% free quartz grains. The rock has a coarse and typical ophitic crystalline texture. 


Ventures Sill 3 (nAven3): Granophyric Gabbro, Leucogabbro

According to Watkins and Riverin (1982, p. 434), a narrow 6 m wide zone containing pyrite and chalcopyrite appears locally on the top of the Ventures gabbro, accompanied by zones where gabbro develops a granophyric texture. According to Duquette (1982, p. 27), granophyric gabbro “consists, for the most part, of an intergrowth of quartz and albite and some albite phenocrystals with myrmekitic margins up to 6 mm in size. It also contains some actinolite needles and rare sheets of zircon-bearing chlorite. Downward, that is, as you approach gabbro, the rock becomes increasingly coarser-grained and richer in mafic minerals and eventually becomes a normal Ventures gabbro.”


Thickness and Distribution

The Ventures Sill is 250 m to 1000 m thick (Poitras, 1984). It is known on both sides of the Chibougamau Anticline over a distance of more than 85 km (Duquette, 1982; Poitras, 1984). West of Chapais, the Ventures Sill is identifiable west of Maryse Lake, north of Road 113 (in the northeastern portion of sheet 32G14-200-0101) and south of Pacific Lake, on the flanks of a synclinal structure (in the northeastern portion of sheet 32G14-200-0102). 



Stratigraphic Relationship(s)

Rocks of the Ventures Sill appear between those of the Roberge and Bourbeau sills. They cut volcanic rocks of the Roy Group’s second volcano-sedimentary cycle (Duquette, 1970, 1982). 


Does not apply.


Author(s) Title Year of Publication Hyperlink (EXAMINE or Other)
BÉDARD, J.H. – LECLERC, F. – HARRIS, L.B. – GOULET, N. Intra-sill magmatic evolution in the Cummings Complex, Abitibi greenstone belt: Tholeiitic to calc-alkaline magmatism recorded in a subvolcanic conduit system. Lithos; volume 111, pages 47-71. 2009 Source
DUQUETTE, G. Stratigraphie de l’Archéen et relations métallogéniques dans la région de Chibougamau. Ministère des Richesses naturelles, Québec; ES-008, 25 pages, 1 plan. 1970 ES-008
DUQUETTE, G. Demie nord des cantons de McKenzie et de Roy et Quart nord-ouest du canton de McCorkill. Ministère de l’Énergie et des Ressources, Québec; DPV-837, 80 pages, 4 plans. 1982 DPV 837
LECLERC, F. – ROY, P. – HOULE, P. – PILOTE, P. – BÉDARD, J.H. – HARRIS, L.B. – McNICOLL, V.J., VAN BREEMEN, O. – DAVID, J. – GOULET, N. Géologie de la région de Chibougamau. Ministère de l’Énergie et des Ressources naturelles, Québec; RG 2015-03, 97 pages, 6 plans. 2017 RG 2015-03
McMILAN, R.H. Petrology, geochemistry and wallrock alteration at Opemiska – A vein copper deposit crosscutting a layered Archean ultramafic sill. University of Western Ontario; thèse de doctorat, 169 pages. 1972
POITRAS, A. Caractérisation géochimique du Complexe de Cummings, région de Chibougamau-Chapais, Québec. Université du Québec à Montréal; Masters Thesis, 159 pages. 1984
WATKINS, D.H. – RIVERIN, G. Geology of the Opemiska copper-gold deposits at Chapais Québec. In: Precambrian sulphide deposits (Hutchpor, R.W., Spence, E.D. and Franklin, J.M., editors). Geological Association of Canada; Special Paper 25, pages 427-446. 1982



8 mai 2019