Geology of the metamorphosed Roberto gold deposit (Éléonore Mine), James Bay region, Quebec: diversity of mineralization styles in a polyphase tectonometamorphic setting

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.

Quantifying Hydrothermal Alteration: A Review of Methods

“…The mineralisation is spatially associated with calc-silicate-and tourmaline-bearing quartz veins, and metasomatic replacement zones (Ca-and K-gains) [40]. The area records a long succession of metamorphic and magmatic events, and the deposit was deformed and metamorphosed to the amphibolite facies [41,42].…”

“…The CONSONORM_HG method has then been applied to 32 greywacke samples from the Roberto data set. This method is calculated for the 3AMP675 model (representing 675 • C and 3 kbar, which is close to the peak metamorphic conditions [42]). FeO and Fe 2 O 3 analyses are unavailable, and a Fe 2 O 3 /Fe 2 O 3 T ratio of 0.2 is used because of the generally reduced nature of orogenic gold systems [56].…”

“…FeO and Fe 2 O 3 analyses are unavailable, and a Fe 2 O 3 /Fe 2 O 3 T ratio of 0.2 is used because of the generally reduced nature of orogenic gold systems [56]. Normative minerals are displayed in a circle diagram (Figure 3), where the samples distribute between K-and Ca-bearing minerals in accordance with the documented K-and Ca-metasomatisms [40][41][42]. Alignment of some samples also suggests feldspar destruction and/or Fe-Mg-gains ( Figure 3).…”

Chemical composition of tourmaline in orogenic gold deposits

“…Subhedral orange tourmaline is found within ferro-magnesian greywacke and with disseminated gold mineralization. Anhedral brown tourmaline occurs in the selvage of mineralized quartz-diopside veins or in hydrothermal breccias in greywacke (Fontaine et al 2015 Ives) or pyrrhotite (New Consort; Fig. 1).…”

“…Several studies document the major and minor element composition, but only a few report trace element compositions of tourmaline from orogenic gold deposits (Jiang et al 2002;Deksissa and Koeberl 2004;Hazarika et al 2015;Hazarika et al 2016;Manéglia 2017;Kalliomäki et al 2017). The trace element composition of tourmaline from various environments and deposit types are also available, including porphyry Cu-Mo (Koval et al 1991;Yavuz et al 1999;Iveson et al 2016), VMS (Hellingwerf et al 1994;Griffin et al 1996;Slack et al 1999), Sn skarn (Jiang et al 2004), SEDEX (Griffin et al 1996;Klemme et al 2011), W-Sn (Harlaux 2016;Duchoslav et al 2017;Hong et al 2017); Sn (Harlaux et al 2018;Harlaux et al 2019); polymetallic Pb-Zn-Cu±U (Yavuz et al 2011); polymetallic Sn-Ag-Pb-Zn (Jiang et al 1999); U unconformity-related (Joyce 2016) deposits, pegmatites (Jolliff et al 1987; Roda et al 1995;Roda-Robles et al 2004;Roda-Robles et al 2011;Bačik et al 2012;Gadas et al 2012;pegmatitic dikes (Ravenelle et al 2010;Dubé et al 2011;Ravenelle 2013;Fontaine et al 2015). Additional samples include tourmaline from the Lincoln Hill gold deposit in the Humboldt Range (Nevada, USA) that has epithermal characteristics (R. Taylor, pers comm, 2015).…”