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Scott Member
Stratigraphic label: [narc]sct
Map symbol: nAsct

First published: 5 December 2018
Last modified:

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Translation of original French

 

 

Informal subdivision(s)
Numbering does not necessarily reflect the stratigraphic position.
 
nAsct6 Graphitic metasedimentary rocks, chert, exhalite
nAsct5 Quartz porphyritic rhyolite
nAsct4 Lac William Dacite
nAsct3 Aphyritic rhyolite, rhyolitic volcaniclastic rock
nAsct2 Rhyolite and dacite
nAsct1 Andesitic basalt and andesite
 
Author:Daigneault and Allard, 1990
Age:Neoarchean
Reference section:None
Type area:An area ~18 km northeast of Chapais (SE portion of NTS sheet 32G15)
Geological province:Superior Province
Geological subdivision:Abitibi Subprovince
Lithology:Felsic volcanic rocks
Type:Lithostratigraphic
Rank:Member
Status:Formal
Use:Active

 

Related unit(s)

 

 

Background

The felsic volcanic rocks outcropping north of Scott Lake (SE portion of sheet 32G15) were originally included by Daigneault and Allard (1990) in an informal unit of the Waconichi Formation, the “Lemoine type”, which is dominated by rhyolitic porphyries and rhyolite. Detailed work in the area (Brisson, 2007, 2008, 2009) allows for the characterization and the distinction of units in the Scott deposit area from those of the former Lemoine Mine. Leclerc et al. (2011) define the Scott Member to designate rocks previously assigned to the “Lemoine type” that are located north of Scott Lake.

Description

Scott Member 1 (nAsct1): Andesitic Basalt and Andesite

The basal unit of the Scott Member consists of andesitic basalt and andesite of transitional to calc-alkaline affinity. These rocks form the dominant facies. Basalts are massive, pillowed or brecciated and exhibit a dark green colour. They are aphanitic or very fine grained and contain plagioclase and pyroxene phenocrystals (Jeffery, 1988). They have avariolitic textures (2-5%) locally, containing 1-2% amygdules of chlorite or carbonate (1-3 mm) and quartz (0.5-3 mm). The rock contains fine-grained disseminated pyrite. The pillowed morphology south of Williams Lake suggests a normal stratigraphic polarity to the north.

Economically speaking, this unit contains the Selco-Scott zinc deposit which has an evaluated tonnage (Scott deposit) described by Daigneault and Allard (1990, p.135): “In 1976, Selco drilled a first hole in Scott Township. This led to the discovery of a lens of 680 000 tonnes at 0.55% Cu, 6.9% Zn and 13.3 g/t Ag (UTM NAD83, zone 18: 525620 mE, 5522778 mN). The discovery followed an airborne survey that defined a discrete but important electromagnetic conductor. This stratiform lens is associated with mafic and felsic flows, pyroclastics, and a mass of quartz porphyry (Anderson and Downie, 1980). South of the lens, rocks are mainly mafic and highly chloritized; multiple phases from the Chibougamau Pluton, located less than 200 m to the south, can be recognized.” Moderate to severe alteration to chlorite, silica and epidote has been recognized in the Selco-Scott deposit environment (deposit number 2; Figure 2; Jeffery 1988; Brisson, 2007, 2008, 2009).

Scott Member 2 (nAsct2): Rhyolite and Dacite

Rhyodacite and dacite of transitional to calc-alkaline affinity are greenish grey and aphanitic to very fine grained. These rocks locally contain plagioclase phenocrystals, up to 1% epidote amygdules, as well as traces of carbonate (amygdules <2 mm), quartz (1-5 mm) and pyrite (0.5-2 mm; Brisson, 2007). Pillows have a whitish core resulting from silicification. A rusty alteration is also common, indicating the presence of pyrite ± chalcopyrite in interstitial spaces at the cushion margin (Riverin, 1979). This unit is interbedded between the Gwillim and Scott shear zones. These volcanic rocks overlie porphyritic rhyolite of tholeiitic to transitional affinity. They correspond to a volcanic centre centred on a quartz-plagioclase porphyritic dome of tholeiitic to transitional affinity. 

Scott Member 3 (nAsct3): Aphyritic Rhyolite, Rhyolitic Volcaniclastic Rock

Aphyritic rhyolite and rhyolitic volcaniclastic rocks form beds generally <20 m thick, interbedded with pillowed and brecciated mafic volcanic rocks and gabbros (Jeffery, 1988; Brisson, 2007, 2008, 2009). These rocks are of calc-alkaline affinity.

 

Scott Member 4 (nAsct4): Lac William Dacite

The Lac Williams Dacite was recognized in the northeastern portion of the area. This rock of tholeiitic to transitional affinity is dark green to brown in altered surface and has a weakly vitreous appearance in fresh exposure, suggesting silicification occurred (Riverin, 1979). This unit is <100 m thick.

Scott Member 5 (nAsct5): Quartz Porphyritic Rhyolite

Porphyritic rhyolite identified in the Selco-Scott deposit area (Jeffery, 1988) is distinguished from rhyolite of unit Asct3 by its flow-banding structure and tholeiitic to transitional affinity. It varies in colour from medium grey to greenish or yellowish depending on the intensity of chloritization and sericitization. The rock is aphanitic or fine grained, containing 0.5-3% quartz phenocrystals (0.5-2 mm in diameter; locally up to 14 mm) (Brisson, 2007, 2008, 2009).

Scott Member 6 (nAsct6): Graphitic Metasedimentary Rocks, Chert, Exhalite

The conductive unit that characterizes the top of the Scott Member corresponds to alternating medium-grey graphitic metasedimentary rocks, chert and exhalite characterized by a rusty alteration, suggesting the presence of disseminated sulphides (Jeffery, 1988). Exhalite consists of alternating chert and beds rich in chlorite, fine-grained pyrite (10%) and disseminated or laminated sphalerite (Brisson, 2007).

Thickness and Distribution

The actual maximum thickness of the Scott Member is ~500 m, but it may be twice as thick due to the presence of numerous longitudinal shear zones. The Scott Member is recognized from the area south of Fleury Lake (NW of sheet 32G15) to the north of David Lake (NW portion of sheet 32G16) over a distance of ~15 km. 

Dating

Zircons sampled from a rhyolite in unit nAsct3 yielded a crystallization age of 2728.2 ±0.8 Ma (Leclerc et al., 2011). 

Isotopic SystemMineralCrystallization Age (Ma)(+)(-)Reference(s)
U-PbZircon2728.20.80.8Leclerc et al., 2011

Stratigraphic Relationship(s)

The base of the Scott Member is cut by an intrusion of quartz-rich granitoid from the granophyric zone (Lac Doré Intrusive Suite), as well as by diorite and tonalite of the Chibougamau Pluton. The summit of the Scott Member is limited by unit nAsct6, which is characterized by a positive linear magnetic anomaly. This unit is overlain by basalt and andesitic basalt of the Allard Member (Leclerc and Houle, 2013).

Paleontology

Does not apply.

References

Author(s)TitleYear of PublicationHyperlink (EXAMINE or Other)
ANDERSON, W.J. – DOWNIE, I.F.The Scott Township deposit, Quebec; a case history. Preprint (#40) Canadian Institute of Mining and Metallurgy Annual Meeting, Toronto; 19 pages.1980
BRISSON, T.Rapport sur les travaux 2006, projet Scott Lake, Assessment work submitted to the Government of Québec; GM 63253, 466 pages, 12 plans.2007GM 63253
BRISSON, T.Travaux 2008, projet Scott Lake, Assessment work submitted to the Government of Québec; GM 63639, 475 pages, 17 plans.2008GM 63639
BRISSON, T.Travaux 2008-2009, projet Scott Lake, Assessment work submitted to the Government of Québec; GM 64720, 619 pages, 11 plans.2009GM 64720
DAIGNEAULT, R. – ALLARD, G.O.Le Complexe du Lac Doré et son environnement géologique (région de Chibougamau – Sous-province de l’Abitibi). Ministère de l’Énergie, des Mines et des Ressources, Québec; MM 89-03, 275 pages.1990MM 89-03
JEFFERY, B.D.Summary report, geological and geochemical surveys, Scott property (PN4135) and Scott extension property (PN4140), Chibougamau, Québec, Assessment work submitted to the Government of Québec; GM 48353, 29 pages, 5 plans.1988GM 48353
LECLERC, F. – HOULE, P.Géologie de la région du lac Simon. Ministère des Ressources naturelles, Québec; RP 2013-02, 24 pages, 1 plan.2013RP 2013-02
LECLERC, F. – BÉDARD, J.H. – HARRIS, L.B. – McNICOLL, V. – GOULET, N. – ROY, P. – HOULE, P.Tholeiitic to calc-alkaline cyclic volcanism in the Roy Group, Chibougamau area, Abitibi Greenstone Belt—Revised stratigraphy and implications for VHMS exploration. Canadian Journal of Earth Sciences; volume 48, pages 661-694.2011Source
RIVERIN, G.Rapport géologique et géochimique sur la propriété Lac Williams, Assessment work submitted to the Government of Québec; GM 36180, 42 pages, 7 plans.1979GM 36180

 

 

 

 

12 septembre 2019