No abstract

The world-class Roberto gold deposit is a major discovery in the James Bay region. Hosted by <2675Ma (Timiskaming) sedimentary rocks, the deposit is located a few kilometres south of the tectonometamorphic contact between the Opinaca (paragneiss to migmatite) and the La Grande subprovinces (tonalitic basement, volcano-sedimentary belts and syn- to late-tectonic intrusions). Multiple deformation events, long-lived metamorphic event(s), and magmatic activity, including proximity to the migmatitic Opinaca domain, constitute a geological context that is favourable for long-lived hydrothermal systems or multiple mineralizing and/or remobilization events. Detailed underground and surface mapping of the ore zones and host rocks of the Roberto gold deposit (Eleonore mine) provide new insights into host-rock variations, relationships between metamorphism and ore zones, syn- to post-mineralization structural events, and magmatic activity. These have led to a diversity of mineralization styles including i) stockwork of quartz, dravite (magnesian tourmaline) veinlets with microcline, phlogopite replacement zones with pyrrhotite, arsenopyrite, and lollingite (Roberto zone), ii) quartz, diopside, schorl (iron-rich tourmaline), arsenopyrite veins (East-Roberto zone), and iii) more atypical ore zones in migmatitic rocks or in biotite, amphibole schist (Lake zone). All of the mineralized zones are deformed (isoclinal folding, mylonitic shearing, boudinage) and some (e.g. Roberto zone) are locally strongly metamorphosed, suggesting that the bulk of the ore is pre-peak metamorphism (amphibolite facies). The timing of the mineralization relative to peak metamorphism and the complex geological setting are the dominant factors that influenced the nature of the Roberto deposit.