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Sigma-Lamaque - Val d Or District |
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Quebec, Canada |
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Au
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Super Porphyry Cu and Au
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IOCG Deposits - 70 papers
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All papers now Open Access.
Available as Full Text for direct download or on request. |
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Sigma-Lamaque is the largest of a group of shear-zone related quartz-tourmaline vein deposits occurring north of the Larder Lake-Cadillac fault zone in the Val d'Or district of the Abitibi greenstone belt in Quebec, Canada. It is hosted by 2705 Ma andesitic volcaniclastic rocks intruded by an irregular comagmatic mass of sub-volcanic porphyritic diorite and comprises a large vein network of laminated veins within shear zones. This vein network extends over an area of 2 x 1.5 km at surface and to a depth of 1.8 km.
Three main lithological associations are known within the mine area, namely:
i). volcanic rocks, that include various tuffaceous rocks and associated pillowed and massive lava flows of andesitic composition which strike east-west and dip steeply to the north.
ii). porphyritic diorite, which is also known as the C porphyry, corresponds to a massive plagioclase phyric diorite of subvolcanic origin intruding the lavas. This diorite occurs as an irregular body located in the central part of the deposit and as other irregular and smaller isolated bodies, all of which swell at depth to merge into a single intrusive mass.
iii). feldspar porphyry dykes, also known as 'C Dykes', which intrude the previous described lithologies and strike approximately east-west while dipping steeply to the south. Individual dikes range in thickness from a few centimetres to about 10 m and averages 3 m. They occur as three distinct groups or swarms near the surface, where they are known as the Main, A-F and Mine no. 2. The individual dyke swarms are distibuted over widths from about 30 to 80 m and can be traced along strike for more than 1.5 km. The Main and A-F swarms are curviplanar and located respectively north and south of the central diorite body to define a lenticular-shaped feature at the deposit scale, which is also reflected in the geometry of the shear zones and their fault-fill veins, and is continuous vertically for more than 2000 m. The Mine no. 2 swarm, located south of the A-F swarm, is a more linear feature and strikes east-west.
The dominant structural features of the deposit are the numerous shear zone. Mineralised third-order shear zones up to 6 m wide, dipping moderately to steeply to the souths 50° to vertical) and striking generally east-west (from 70° to 110°). S fabric shapes of most indicate a predominantly reverse movement and steeply east plunging slip-related linear fabrics. Some are known from mining to be continuous along strike and dip for more than 500 m. These structures are known to a depth of 2000 m in an en echelon pattern within an east-west-trending, approximatley 500 m wide corridor broadly confined between two second-order shear zones, known as the North and the South shears which are up to 6 m wide, vertically dipping and may be traced for more than 1 km along their strike. The second- and third-order shear zones overprint the weak S2 regional schistosity which overprints all rock types locally.
The auriferous veins comprise quartz and tourmaline with lesser carbonates, pyrite, scheelite, chlorite and chalcopyrite. Four vein types have been recognised on the basis of their host rock associations and geometries namely: i). steeply to moderately dipping fault-fill veins within shear zones; ii). subhorizontal extensional veins; iii). arrays of subhorizontal extensional veins hosted within the feldspar porphyry dykes, known as dyke stringers; and iv). moderately north-dipping extensional-shear veins, known as the North Dipper veins.
The lavas and porphyritic diorite are dated at around 2704 ± 3 Ma, (U-Pb, Wong et al., 1991). This was followed by early, east-west trending tilting of the strata, the DI folding-tilting at approximately 2700 Ma. The feldspar porphyry and minor dioritic dykes were emplace into upright strata at 2694 ± 2 Ma (U-Pb: Wong et al., 1991). The regional S2 schistosity and greenschist facies metamorphism occurred between 2690 to 2675 Ma (Hanes et al., 1992), and was related to a major north-south directed D2 shortening event, while shear zone development took place during an advanced stage of D2 followed by the subsequent development of gold-bearing veins.
The deposit contained in total around 290 tonnes of gold at grades of around 6 g/t Au.
This summary is largely drawn from information presented in Gaboury et al., (2001). For detail see the reference(s) listed below.
The most recent source geological information used to prepare this decription was dated: 2001.
This description is a summary from published sources, the chief of which are listed below.
© Copyright Porter GeoConsultancy Pty Ltd. Unauthorised copying, reproduction, storage or dissemination prohibited.
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Cowan, E.J., 2020 - Deposit-scale structural architecture of the Sigma-Lamaque gold deposit, Canada - insights from a newly proposed 3D method for assessing structural controls from drill hole data: in Mineralium Deposita v.55, pp. 217-240.
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Gaboury D, Carrier A, Crevier M, Pelletier C, Sketchley D A 2001 - Predictive distribution of fault-fill and extensional veins: example from the Sigma Gold mine, Abitibi subprovince, Canada: in Econ. Geol. v96 pp 1397-1405
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Olivo, G.R., Chang, F. and Kyser, T.K., 2006 - Formation of the Auriferous and Barren North Dipper Veins in the Sigma Mine, Val d Or, Canada: Constraints from Structural, Mineralogical, Fluid Inclusion, and Isotopic Data: in Econ. Geol. v.101, pp. 607-631.
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Perrault G, Trudel P, Bedard P 1984 - Auriferous halos associated with the gold deposits at Lamaque Mine, Quebec: in Econ. Geol. v79 pp 227-238
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Robert F, Brown A C 1986 - Archean Gold-bearing Quartz veins at the Sigma Mine, Abitibi Greenstone Belt, Quebec: Part II. Vein paragenesis and hydrothermal alteration: in Econ. Geol. v81 pp 593-616
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Robert F, Brown A C 1986 - Archean Gold-bearing Quartz veins at the Sigma Mine, Abitibi Greenstone Belt, Quebec: Part I. Geologic relations and formation of the vein system: in Econ. Geol. v81 pp 578-592
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Robert F, Brown A C 1984 - Progressive alteration associated with Gold-Quartz-Tourmaline veins at the Sigma Mine, Abitibi Greenstone Belt, Quebec: in Econ. Geol. v79 pp 393-399
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Robert F, Kelly W C 1987 - Ore-forming fluids in Archean Gold-bearing Quartz veins at the Sigma Mine, Abitibi Greenstone belt, Quebec, Canada: in Econ. Geol. v82 pp 1464-1482
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Porter GeoConsultancy Pty Ltd (PorterGeo) provides access to this database at no charge. It is largely based on scientific papers and reports in the public domain, and was current when the sources consulted were published. While PorterGeo endeavour to ensure the information was accurate at the time of compilation and subsequent updating, PorterGeo, its employees and servants: i). do not warrant, or make any representation regarding the use, or results of the use of the information contained herein as to its correctness, accuracy, currency, or otherwise; and ii). expressly disclaim all liability or responsibility to any person using the information or conclusions contained herein.
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