Gold and Copper in the Superior Province: Current Situation, Future and Prospects

Gold and copper are strategic and critical metals of great economic importance to Canada. Despite growing combined annual production (2014 to 2023), the renewal and spatial distribution of these resources are very uneven. In the Superior Province, these metals are found in a wide variety of deposit types and styles (magmatic, magmatic-hydrothermal, orogenic sensu stricto and synvolcanic), and their discovery depends largely on the application of robust metallogenic models. The work undertaken as part of the GSC's Targeted Geoscience Initiative, in collaboration with the MNRF's DACG and academic and industry partners, aims to improve knowledge of the geological, structural and geochronological framework of many areas of proven or emerging potential. This work contributes to a better definition of the processes and age of formation of mineralization systems, which in many cases are highly complex and require a multidisciplinary and collaborative approach.

For a century, Abitibi has stood out with the remarkable average of one mine put into production each year. In a context of rising gold and copper prices, exploration has increasingly turned toward low-grade but high-tonnage deposits, with cut-off grades of about 0.5 g/t Au and 0.3% Cu—conditions typical of disseminated porphyry-type mineralization. Most of the major deposits recently developed in Abitibi are plutonic in nature, including Cu-Au (Mo) porphyries associated with TTGs, Au-Te-enriched sanukitoid porphyries, and deposits combining intrusions with deformation zones. Advances in understanding these systems highlight structural contrasts between northern and southern Abitibi: in the north, the thicker and older crust favored the development of shallow porphyries, whereas in the south, the thinner and younger crust promoted the coexistence of sulfide accumulations and smaller porphyry systems. It has also been shown that in sanukitoids, part of the metals are transported by ferromagnesian minerals. Their source may lie in the Opatica basement underlying the Abitibi terrain, whose subtle structural control could influence the localization of mineralization.

The latest version of the gold deposit compilation in progress at the Geological Survey of Canada (GSC) represents an update of the distribution, metal content and relevant geological information for gold deposits (≥ 100,000 oz Au) in the Superior Province. This is intended to support the various research activities carried out by the GSC as part of the Targeted Geoscience Initiative, and it is expected that this data will eventually be published. More than 60 deposits have been added to the previously published database (Gosselin and Dubé, 2005), while more than 135 deposits have been the subject of recent resource and/or reserve estimates (industry data dated February 2023). New entries include old mines brought back into production (e.g., Canadian Malartic), discoveries (e.g., Éléonore) and advanced exploration projects (e.g., Windfall). The revisions provide key information on the distribution, total content and gold content of the deposits. This data is enhanced by new knowledge in order to better classify mineralization types and styles based on diagnostic geological and genetic characteristics. The final product is intended to provide an analytical framework for: 1) thematic studies of the processes (source, transport, conduits and traps) leading to the formation and preservation of gold deposits, and 2) the development of exploration strategies.

Since 1935, nearly 950 gold and/or copper showings and deposits have been identified in the James Bay region, which extends over three geological subprovinces (La Grande, Opinaca and Opatica) covering an area of 225,000 km². These discoveries include 11 deposits. An analysis of the distribution of all these mineralizations in time, space and by method was carried out.

What do these past discoveries say about the potential for future discoveries? How can we assess the maturity of this region and optimize the exploration effort to achieve economic success? There are a number of approaches, complementary in some respects, that could be favoured by: 1) exploiting regional digital data (geochemistry, geophysics); 2) understanding the geological and gitologic contexts; and 3) incremental exploration of previously known showings and deposits.

Continued exploration near known deposits (Troilus, Cheechoo, Nisk) or showings (Elmer), as well as the discovery of new types of mineralization (antimony-gold sill at Wabamisk) demonstrate that the region's mineral potential is still at an early stage of development. Simply put, there is huge potential for growth. The preferred approach is to generate targets through advanced processing of regional geoscientific data, in close association with validation phases based on field prospecting. Many large-scale targets remain unexplored and could be converted into discoveries if the right conditions are maintained over the long term to facilitate exploration: access to land, presence of infrastructure, stability of the legal and fiscal framework.

The Abitibi Subprovince is characterized by a diversity of gold deposit types that formed during different periods at varying crustal depths. In the Urban-Barry greenstone belt, this diversity is represented by three main types of gold mineralization, each type differing in geometry, host rocks, styles of mineralization, and controlling geologic features. These deposits include: 1) synvolcanic tourmalinite-hosted Au ±Cu mineralization, 2) intrusion-related Au deposits, and 3) greenstone hosted quartz-carbonate vein deposits. Tourmalinite-hosted Au ±Cu mineralization (e.g., Black Dog) consists of a 100 m-thick stratigraphically controlled tourmalinite horizons that are cut by a sulphide breccia that formed during the ca. 2717-2715 Ma Macho formation. Roughly 15-20 M.yr. later, intrusion-related Au mineralization (e.g., Windfall) formed synchronous with a period of ca. 2698 Ma calc-alkaline quartz-feldspar porphyry intrusions. Gold mineralization consists of sericite-pyrite ± silica replacement zones and grey quartz-pyrite veins and stockworks. This is later followed by the main regional deformation event which is broadly coeval with the formation of quartz-carbonate vein deposits (e.g., Barry and Gladiator) which are hosted in mafic to intermediate volcanic rocks that are intruded by felsic porphyry intrusions and are commonly located in the structural hanging wall of regional scale ENE-trending shear zones.

Recognizing this diversity of gold mineralization and characterizing their geophysical, geochemical and geological features allows for the development of robust exploration models that can be applied to regional scale exploration programs and to develop tools for exploring for gold deposits under cover.

The Abitibi Subprovince, particularly its southern part, has been the focus of intense gold and copper exploration campaigns for over a century. However, in the last 20 years or so, major changes in the scientific and economic fields have called into question some certainties in this field. Initially, exploration focused on the discovery of high-grade, easily identifiable deposits in volcanic rocks, at a time when the price of gold was much lower and geochemical data less available. In more recent times, scientific work at the scale of deposits, mining camps and metallogenic provinces has demonstrated the existence of numerous types of deposit that move away from the simple duality between orogenic gold mineralization and VMS. This variety of prospective geological environments is opening up new avenues for exploration. The rediscovery of the Canadian Malartic deposit and its extensions illustrates the underestimated potential of sedimentary rocks. Despite this, exploration continues to focus on volcanic rocks, and knowledge of sedimentary basins remains limited, as shown by the recent discoveries of Timiskaming-type fluvial basins, which are considered to be important metallotects. The Abitibi's favourable geological context, combined with a more open view of prospective geological environments, should lead to the discovery of new deposits in this region, which is considered to be mature, provided that the exploration sector is open to change.

Exploration for gold and copper must be based on a good understanding of the formation processes in order to target areas with high potential and use the most appropriate methods. In Abitibi, the potential for orogenic gold deposits and Cu-Zn ± Au massive sulphides is well known, although important questions remain, such as the precise age of the mineralization episodes. The (re)discovery of Au-Cu deposits (Windfall, Upper Beaver) highlights the existence of new target types. In order to discover traces of future deposits, exploration methods need to be adapted either to probe at depth or to follow dispersal trains in surface sediments. In the surficial environment, the combination of multi-element analyses and machine-learning classification methods now makes it possible to identify the source of indicator minerals, vector them towards targets and even assess the metal content of possible mineralizations.