Major ore types of the Paleoproterozoic Lalor auriferous volcanogenic massive sulphide deposit, Snow Lake, Manitoba

Abstract

The Lalor deposit is a newly discovered Paleoproterozoic volcanogenic massive sulphide (VMS) deposit located at the east end of the Flin Flon-Snow Lake belt, within the Snow Lake arc assemblage. Lalor is the largest of 11 VMS deposits in the Snow Lake arc assemblage, with combined resources and reserves of 25.3 Mt and average grades of 2.9 g/t Au, 25 g/t Ag, 5 wt% Zn, and 0.79 wt% Cu, including 8.8 Mt at 4.6 g/t Au. Lalor and all other deposits in the Snow Lake arc assemblage have been affected by intense polyphase deformation and amphibolite facies metamorphism. As a result, both the original hydrothermal alteration assemblages and the ore mineral assemblages have been completely recrystallized. However, a variety of different ore types have been preserved, allowing the partial reconstruction of the hydrothermal system, including massive Fe-Zn sulphide lenses, discordant Cu-Au stringer zones, and distinctive precious metal-rich Au- Ag-Pb-Cu zones. The different ore types occur in a series of stratigraphically and structurally mp;lt;"stackedmp;gt;" ore lenses that partly overlap but still largely preserve the original architecture of the deposit. The ore is distributed in 12 discrete lenses or zones of mineralization that are interpreted to be the result of several distinct and overlapping hydrothermal events. Type 1 Fe-Zn massive sulphide ore is the most common ore type in 6 ore lenses and consists of massive coarse-grained pyrite and sphalerite with trace galena in dominantly quartz-muscovite}kyanite-biotite schist (K-alteration association). Type 2 Cu-Au mineralization consists of semi-massive and stockwork chalcopyrite and pyrrhotite in garnetiferous quartzbiotite} staurolite-amphibole-cordierite gneisses (footwall Mg-Fe alteration association). Despite extensive recrystallization and local remobilization, these two ore types are interpreted to represent the (metamorphosed) low- and high-temperature ore assemblages, respectively, of a typical volcanogenic massive sulphide deposit. Type 3 Au-Ag-Pb-Cu-rich ore consists of stringer and disseminated sulphides and sulphosalts mainly hosted in chlorite-carbonate-actinolite schist (Mg-Ca and Ca alteration associations). Galena is an important indicator of Au mineralization and occurs in this ore type as fine-grained blebs in a matrix of chlorite, dolomite, calcite, anthophyllite, Ca-plagioclase, and calc-silicates (epidote, grossular, diopside, Caamphibole } scapolite). Where abundant, the galena is associated with chalcopyrite, pyrite, and pyrrhotite, and with minor to trace sphalerite, Ag-Sb-Pb sulphosalts, electrum, and native gold. Type 4 low-sulphide ore contains ?10 vol% disseminated pyrite in quartz-biotite-anthophyllite gneiss, with minor chlorite, staurolite, and coarse almandine garnet, and has variable Au grades. The sulphides and sulphosalts in ore types 3 and 4 are interpreted to be metamorphically remobilized from pre-existing disseminated mineralization. The hydrothermal system developed during two main episodes of seafloor volcanism. Type 1 massive sulphide in the 10 and 11 lenses and in the 20, 30, 31, and 40 lenses were formed at the paleoseafloor. These lenses are underlain by Type 2 Cu-Au stockwork mineralization (27 Lens) and originally conformable zones of Type 3 Au-Ag-Pb mineralization (21, 24, 25, 26, and 28 lenses). The large Cu-Au stockwork zone (27 Lens) may have been the main feeder of the deposit and is partly continuous with disseminated Au-Ag-Pb- Cu galena-sulphosalts-chalcopyrite mineralization below the 20 Lens. The Au-Ag-Pb-Cu mineralization is thought to have formed in the subseafloor from late-stage, lower temperature hydrothermal fluids (approximately <300°C). In this model, significant Au was introduced first by high-temperature (>300°C) fluids responsible for the Type 2 Cu-Au mineralization and then by lower temperature (possibly boiling) hydrothermal fluids responsible for Type 3 Ag-Au-Pb-Cu mineralization. Although all the ore types are extensively recrystallized and partly remobilized, their distribution strongly supports primary hydrothermal Au enrichment at Lalor. The lead isotopic compositions of the ore galena show no evidence of post-magmatic disturbance, which would be expected if Au had been introduced during deformation and metamorphism (e.g. as in the nearby New Britannia orogenic Au deposit), and the Au-rich assemblages are very similar to those that commonly occur in unmetamorphosed Au-rich volcanogenic massive sulphide deposits.