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 pyrite-rich 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 ( 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 CH 4 coexisting in the same healed fractures, are common in the auriferous North Dipper veins but very rare in the barren vein where aqueous fluid inclusions have lower temperatures (dominantly 125°–225°C). Crush leaching and laser ablation time-of-flight inductively coupled mass spectrometry (LA-TOF-ICPMS) of fluids in the inclusions indicate elevated concentrations of Ni, Cu, Sb, Pb, and Ag in the fluids in the auriferous veins compared to the aqueous fluids in the barren veins. This suggests different sources for the metals in the fluids associated with the gold, possibly including both igneous and sedimentary rocks. Calculated oxygen isotope equilibration temperatures for quartz-tourmaline pairs from the auriferous main vein-filling stage are 323° to 370°C. This is within the range documented in fluid inclusions related to the late auriferous stage, indicating that the temperature of the auriferous hydrothermal fluids did not change appreciably during the evolution of the system. Calculated δ 18 O H 2 O values for the mineralizing fluids are 9.5 per mil, consistent with either low water/rock ratio systems associated with high-grade metamorphism or a magmatic origin for the fluids. Gold included in pyrite may have precipitated during the main vein-filling stage due to the decrease of H 2 Sconcentration in the hydrothermal fluid following the deposition of sulfides. However, gold in fractures in the main vein-filling minerals precipitated later, due probably to dilution of the auriferous aqueous-carbonic fluids by mixing with aqueous fluids. The absence of ore-grade gold in the barren North zone vein is probably due to the lack of circulation of auriferous aqueous-carbonic fluids in the North zone, located distal to the first-order Larder-Lake Cadillac fault, which may have served as the main regional conduit for the auriferous aqueous carbonic fluids.