Geology of the metamorphosed Roberto gold deposit and tonalite-hosted Cheechoo discovery, Superior Province, James Bay Region, Québec, Canada

English Abstract:

The world-class Roberto gold deposit, with resources of 4.03 Moz at 6.49 g/t Au and reserves of 4.10 Moz at 9.63 g/t Au, is a major discovery in the James Bay Region, located a few km south of the boundary between the La Grande and the Opinaca sub-provinces. Hosted by <2675 Ma sedimentary rocks, a structurally complex ≤ 70-80 m wide N-S trending mineralized envelope hosts various styles of mineralization. A large hydrothermal system is defined by distal calcium-rich metasomatism, proximal potassic alteration and Au-As-Sb-B signature. The main ore zones are characterized by quartz-dravite-microcline-phlogopite-arsenopyrite stockwork and replacement zones and by quartz-diopside-schorl-arsenopyrite veins while the high-grade zone is locally hosted by paragneiss. The bulk of the gold mineralization is confined within a steeply plunging ore shoot coaxial with an F₂ fold hinge.

Diversity of ore styles, mineralogy and geometry is in part related to i) lithological heterogeneity, ii) proximity to a granulitic metamorphic front, and iii) syn- to late magmatism. Paragneiss (<2685 Ma), which recorded a complex D₂ deformation, is exposed 1km east of the main ore zone illustrating the steep metamorphic gradient towards the Opinaca sub-province. Magmatism in the vicinity of the deposit includes granitic pegmatites (2620-2603 Ma), most of which have recorded the main phase of deformation (D₂) and postdate the bulk of the ore; however, some are undeformed, cut the main Roberto ore zone and contain clasts of foliated ore. Locally pegmatites host significant gold mineralization. The newly discovered CHEECHOO showing (up to 7.24 g/t Au over 7.9 m) is located about 10 km southeast of the Roberto deposit. The host tonalite yielded a preliminary U-Pb ID-TIMS zircon crystallization age of 2612 Ma. This age is similar to the pegmatites at Roberto, illustrating the complexity of the tectono-metamorphic setting and late-magmatic activity in the area, as well as the complex protracted or multi-stage nature of auriferous events.

Although clearly deformed and metamorphosed, the bulk of the gold mineralization is hosted by Timiskaming-age sediments, near the boundary between sub-provinces marked by a strong strain and metamorphic gradient, magmatism, local conglomerate, and therefore shares analogies with most Archean gold deposits