By: Jean Francois Ravenelle, with SRK Consulting (Canada) Inc.
The Roberto gold deposit represents one of the most significant gold discoveries of the past 10 years in Canada. The latest resource estimation (publicly released in February 2010) indicated a total of more than 7.2 million ounces of gold in inferred (4.17 Moz) and indicated resources (3.03 Moz). The geology of the Roberto deposit differs from most Archean gold deposits of the Superior Province and cannot clearly be identified as a typical quartz-carbonate gold deposit or as a gold deposit that formed at shallower depth. The key geological parameters involved in its genesis need to be identified in order to understand its formation and define exploration criteria that will help discover similar gold deposits in the James Bay territory and elsewhere in the Superior Province.
The Roberto deposit is located on the Éléonore property, which straddles the contact between the Archean metasedimentary/migmatite OpinacaSubprovince and the volcano-plutonic amphibolite facies La Grande Subprovince. Thebulk of the auriferous system primarily occurs in a series of sub-parallel decameter-scale auriferous zones globally confined to a km-scale steeply-plunging polyfolded geometry that has been traced to more than 1,500 meters below surface. Two principal auriferous zones, referred to as Roberto and East-Roberto, can be distinguished from one another based on their structural style, mineralogy, gold content, and stratigraphic position. The Roberto auriferous zone (typically averaging 12 g/t Au over 10 m) represents the main phase of gold mineralization and consists stockworks of K-bearing veins and veinlets and Ca-bearing quartz veins contained within K-altered thinly-bedded greywacke. The K-bearing veinlets are typically composed of quartz, microcline, dravite (Mg-rich tourmaline), biotite, arsenopyrite/löllingite, and pyrrhotite. Ca-bearing quartz veins of the Roberto zone are typically composed of quartz, amphibole, diopside, titanite, carbonate, pyrrhotite, arsenopyrite/löllingite, and locally visible gold. The East-Roberto auriferous zone (up to 8 g/t Au over6 m) is composed of several mineralogical assemblages that have different textural characteristics including hydrothermal breccias, veins, and zones of silica replacement.
The Roberto deposit is affected by polyphase folding but is not hosted within a shear zone. Field observations indicate that the main part of the gold mineralization has been overprinted by D2 and D3. F2 and F3 folds control the finite deposit-scale geometry of the main auriferous zones and the attitudes of ore shoots within those zones.
The mineralogy and the metamorphic textures of auriferous material suggest that the veins and disseminated mineralization have been generated before or during the peak of metamorphism. A study of sulphide minerals with a scanning electron microscope (SEM) confirms that gold mineralization either occurred before the peak of metamorphism in association with pre-peak arsenopyrite or precipitated with löllingite at near-peak conditions.
Geochronology constrains the absolute age of the main stage of gold mineralization to have occurred between ca. 2675 Ma (Ravenelle et al., 2010) and 2603 Ma (Dubé, pers. comm., 2010), the maximum age of Roberto’s host rock and the age of a pegmatite dyke that crosscuts the main auriferous zones, respectively. Some of the pegmatite dykes that crosscut the mineralized zones are auriferous, which is interpreted to result from a contamination process caused by emplacement of dykes through mineralized material. Other pegmatite dykes are in petrogenetic continuity with auriferous quartz-feldspar veins, and crosscutting relationships between two aplite dykes and an auriferous quartz vein indicate that such dykes were broadly coeval with a part of the gold mineralization that occurred between ca. 2615 Ma and 2607 Ma (Dubé, pers. comm., 2010). Pegmatite magmatism was therefore contemporaneous with a part of the gold mineralization or “remobilization” and/or represents a second stage of gold mineralization.
The contrasts between the various styles of gold mineralization, from early stockwork to late pegmatitic veins, indicate that Roberto was affected by a long-lasting tectono-metamorphic event during which gold mineralization was generated, deformed, metamorphosed, and remobilized. The tectonic setting of the area represents an important clue in understanding the genesis of the Roberto deposit, and even though the origin of the low-P high-T metamorphism is unknown, a “deep-earlier” type model explains the early timing of gold mineralization relative to deformation and metamorphism. Accordingly, the potential input of metamorphic fluids should not be ruled out on the basis that gold mineralization was introduced early relative to tectono-metamorphic events. The potential input from magmatic fluids in generating the main part of the gold mineralization should also not be discounted, as indicated by the presence of feldspar porphyry dykes dated at ca. <2680 Ma (Dubé, pers. comm., 2010) in the vicinity of the deposit.
Dr. Jean-François Ravenelle, holds a B.Sc. Honours degree (2002) and an M.Sc (2005) in geology from McGill University. In the past eight years, Jean-François studied and modelled numerous precious metal deposits hosted in various parts of the world including West Africa, Central America, United States, and Canada. In 2012, Jean-Francois completed his Ph.D. in economic geology with the Geological Survey of Canada at the INRS in Quebec City. His thesis focussed on the geology of the world-class Roberto gold deposit located in James Bay, Quebec. Currently, Jean-François is a senior structural geologist for SRK Consulting, based in Toronto, where he specializes in defining structural controls on ore deposition and generating 3D models of ore deposits.