Abstract

With a current resource of 13.4 Moz Au, plus past production of 5.1 Moz Au, the Canadian Malartic deposit represents the first bulk tonnage (measured and indicated resources of 372.9 Mt at 1.02 g/t Au) mine in the Superior province. Canadian Malartic is thus an important example of a large-tonnage, low-grade Archean gold deposit in which the mineralization is disseminated (or in fine veinlets) and hosted partly by felsic to intermediate intrusions. The deposit is located in the Abitibi greenstone belt, Quebec, within and immediately south of the Cadillac-Larder Lake tectonic zone, and occurs in porphyritic monzodiorite intrusions as well as clastic metasedimentary rocks of the Pontiac Group and mafic-ultramafic volcanics of the Piché Group. These rocks have undergone pervasive potassic alteration, carbonatization, pyritization, and local silicification. The main ore minerals are native gold and subordinate gold tellurides, accompanied by pyrite and minor chalcopyrite, galena, sphalerite, hematite, molybdenite, and Ag-Pb-Bi-bearing tellurides. Gold is concentrated in two generations of thin, discontinuous veins, and as finely disseminated grains in alteration envelopes around the main ore-stage veinlets. The main-stage veinlets consist of quartz-carbonate ± biotite ± albite surrounded by alteration haloes of K-feldspar-biotite ± pyrite ± calcite, whereas later veins are dominated by quartz-pyrite-calcite ± muscovite ± biotite ± chlorite. Alteration and gold mineralization were accompanied by large mass gains in K and S, extremely large mass gains in Ag, Te, and Au, and significant mass gains in Sb, W, Bi, and Pb; Cu underwent significant mass loss. The oxygen and hydrogen isotope composition of the mineralizing fluid determined from the corresponding compositions of quartz, biotite, and hematite (δ18Ofluid of 5.2 to 9.8‰, δDfluid of −52.0 to −45.0‰) is consistent with a predominantly magmatic source. A magmatic fluid source is supported by the composition of fluid inclusion leachates from quartz. Based on the isotopic composition of pyrite, sulfur was dominantly of magmatic origin but included a small contribution from a sedimentary source ( δ34SH2S from −4.5 to +3.3‰, with small, positive Δ33S values). The deposit is interpreted to have formed at a temperature of ~475°C, based on oxygen and sulfur isotope geothermometry. Using this temperature in conjunction with the titanium-in-quartz geothermobarometer, the deposit is interpreted to have been emplaced at a pressure of ~3 kbar or a depth of ~10 km.

Consistent with the constraints noted above, we propose a genetic model for the Canadian Malartic deposit in which felsic to intermediate, borderline alkaline to subalkaline magmas, emplaced at midcrustal levels, exsolved relatively oxidized (logfO2~19), CO2- and sulfur-rich (∑aS ≥ 0.1) auriferous fluids. These fluids rose to higher levels (~10 km; ~3 kbars), where they interacted with associated porphyritic monzodiorite intrusions, clastic Pontiac metasedimentary and Piché Group mafic to ultramafic rocks. The porphyries and metasedimentary rocks buffered the fluids to near-neutral pH, whereas the mafic-ultramafic rocks buffered the fluids to higher pH and lower fO2. Ore deposition resulted from pyritization of the host rocks and oxidation of the mineralizing fluid, which reduced aH2S and caused destabilization of gold bisulfide species, leading to precipitation of native gold.

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