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

Abstract

The Sigma-Lamaque mine (>100 metric tons (t) of gold) is an example of an Archean greenstone belt gold deposit formed by processes related to multistage hydrothermal fluid circulation. The earliest recognized vein systems are steeply to moderately dipping fault-fill veins within shear zones and contemporaneous subhorizontal veins, which were overprinted by the late North Dipper vein system. The latter strikes east-west, dips moderately to the north, and exhibits complex internal geometry with features similar to fault-fill and extensional veins, and therefore is interpreted as extensional shear veins. This late vein system was considered one of the major sources of gold in the shallow underground and open-pit ore reserve estimations.These late North Dipper veins were studied in detail in the Sigma pit where they contain ore-grade gold and are hosted by the calc-alkaline tuffs of the Val d'Or Formation and in the North zone where they are barren and hosted by the tholeiitic volcanic rocks of the Jacola Formation. In the Sigma pit, these veins have apparent thicknesses of 0.8 to 3 m and very irregular contacts with the vein walls. They comprise irregular zones of milky quartz filling, disrupted blocks of layered quartz and tourmaline and slabs of foliated pyriterich wall rock. Layers with tourmaline and quartz crystals oriented perpendicular to the vein walls are observed close to the vein walls. The veins contain minor amounts of calcite, pyrite, rutile, and traces of chlorite, muscovite, pyrrhotite, chalcopyrite, galena, bismuth tellurides, and gold. Locally, the sulfide content can attain 10 modal percent of the vein mineralogy. In the veins, native gold (with 6 wt % Ag) occurs rarely associated with the main vein-filling minerals, commonly as inclusions in pyrite (main vein-filling stage), and most commonly filling fractures in pyrite, tourmaline, and quartz, where it is alloyed with 8 to 17 wt percent Ag and is associated with calcite and Bi tellurides (late auriferous stage). The proximal wall-rock alteration related to the auriferous North Dipper veins overprints the metamorphic assemblage and is characterized by abundant fine-grained plagioclase, calcite, quartz and minor chlorite, tourmaline, and pyrite in the tuffs and more abundant muscovite in the intrusive feldspar porphyry. In the wall rock, gold with 6 to 28 wt percent Ag is found locally filling fractures in pyrite, where it is associated with a late generation of calcite. The barren North Dipper vein also comprises mainly quartz and tourmaline but lacks calcite and gold, has a very low content of sulfides (<1 modal %, mainly pyrite and chalcopyrite), rutile, and Bi telluride, and does not exhibit visible proximal wall-rock alteration.Aqueous-carbonic fluid inclusions with salinities less than 8 wt percent (NaCl equiv) and homogenization temperatures between 228°and 440°C, and carbonic fluid inclusions with less than 0.2 mol percent CH4 coexisting in the same healed fractures, are common in the auriferous North Dipper veins but very rare in the barr...

New Insight into the Genetic Mechanism of Shear Zone Type Gold Deposits from Muping-Rushan Metallogenic Belt (Jiaodong Peninsula of Eastern China)

“…The occurrence of high-pressure fluids facilitates the development of brittle fractures at different structural levels, including ductile, ductile-brittle and brittle shear zones. In this study, gold deposits in the MR (China) are probably hosted both in the brittle fractures at middle crust level (ductile deformation) and brittle deformation part at shallow crust level, while some deposits are distributed in ductile part (e.g., the Sigma Mine, Val d'Or, Canada, see [11]). Finally, rapid pressure drop occur in these fractures and gold begin to precipitate during the stress-chemical process.…”

“…Moreover, the significant factor influencing gold precipitation causes widespread consideration. A fluid inclusion study shows that deposits from different structural environments are commonly formed in middle-temperature conditions (200-350 °C) [11,29,31,38]. Experimental researches show that gold solubility does not change much in this range [91].…”

“…As weak zones in regions, shear zones are always concentrated for intense magma activities, fluid actions and metamorphism, hence provide favorable transport channels and precipitation spaces for ore-forming elements, e.g., Au, Ag, Cu, Pb, Zn, W, Sn and Mo [2][3][4][5][6][7]. Many gold deposits are host in, or spatially associated with first-and second-order shear zones [8][9][10][11][12][13][14][15], which we call the shear zone type gold deposits (SZTGD). In the SZTGD, the formation, transport and precipitation of gold are strictly controlled by shearing activities [16,17].…”

Fluid inclusions and H–O–C–S isotope constraints on fluid evolution and ore genesis of the Wangjiadashan Cu–Au deposit in Suizao area of the Tongbai‐Dabie orogenic belt, central China

“…The fluid inclusions of many orogenic deposits can have temperatures as high as 400°C with some S‐type fluid inclusions, and we list a few good examples here: the Sigma deposit (285–440°C with S‐type fluid inclusions, salinity up to 23.5 wt% NaCl equiv. ; Neumayr & Hagemann, ; Olivo, Chang, & Kyser, ), the Donlin deposit in Alaska (250–350°C with salinity of 20–30 wt% NaCl equiv. ; Roedder, ), the Mesoarchean deposits (200–340°C with S‐type fluid inclusions; Roedder, ), Ipitinga deposit in Brazil (350–475°C; Klein & Fuzikawa, ), Lac Fouture deposit (301–362°C with S‐type fluid inclusions; Gao, ), and Macassa deposit (315–349°C with S‐type fluid inclusions; Klein & Fuzikawa, ), Gaojiapu deposit (108–399°C with salinity of 1.6–38.4 wt% NaCl equiv.…”

“…the Sigma deposit (285-440°C with S-type fluid inclusions, salinity FIGURE 14 Plot of δ 13 C values of fluid inclusions from the Wangjiadashan Cu-Au deposit, compared with the major carbon sources (modified after Ohmoto, 1972;Taylor, 1986;Veizer et al, 1980) [Colour figure can be viewed at wileyonlinelibrary.com] up to 23.5 wt% NaCl equiv. ; Neumayr & Hagemann, 2002;Olivo, Chang, & Kyser, 2006), the Donlin deposit in Alaska (250-350°C with salinity of 20-30 wt% NaCl equiv. ; Roedder, 1984), the Mesoarchean deposits (200-340°C with S-type fluid inclusions; Roedder, 1984), Ipitinga deposit in Brazil (350-475°C; Klein & Fuzikawa, 2010), Lac…”