Age and geological setting of the Rankin Inlet greenstone belt and its relationship to the gold endowment of the Meliadine gold district, Nunavut, Canada
Introduction
The western Churchill Province (wCP) represents a vast and gold-rich region of Canada's North (Fig. 1). The largest gold deposits, including the world-class Meadowbank mine (e.g., Sherlock et al., 2004, Janvier et al., 2015), are hosted by Archean greenstone belts and associated meta-sedimentary successions that include turbidite and banded iron formation (BIF; Fig. 1). Deposit host rocks are metamorphosed from greenschist (e.g., Meadowbank; e.g., Pehrsson et al., 2013a) to upper amphibolite facies (e.g., Three Bluffs; Davies et al., 2010) and deformed during at least four temporally-distinct Paleoproterozoic orogenic episodes (e.g., Berman, 2010). Each episode was coupled with the development and/or reactivation of lithospheric-scale faults and smaller-scale, subsidiary structures (Jones et al., 2002, Spratt et al., 2014). Gold is linked to this Paleoproterozoic metamorphic and reworking history and, at the scale of the wCP, occurs where regional faults cut favourable Archean greenstone belts (Fig. 1).
The broad association between gold, greenstone belts, faults and metamorphism is typical of the orogenic gold deposit type and characterizes most Archean gold districts globally (Groves et al., 1998, Groves et al., 2003, Goldfarb et al., 2005, Robert et al., 2005, Dubé and Gosselin, 2007, Robert et al., 2007, references therein). However, gold deposits within the wCP are atypical as gold post-dates the timing of mafic volcanism and cratonization by up to one billion years (Miller et al., 1995, Sherlock et al., 2004, Carpenter et al., 2005, Davies et al., 2010, Lawley et al., 2015a). This age gap contrasts with classic Neoarchean orogenic gold districts, such as the Abitibi, which share a close spatial and temporal relationship between gold, basin development, basin inversion along basin-bounding faults and attendant metamorphism during orogenesis (e.g., Bleeker, 2012, Bleeker, 2015). The spatial link between Proterozoic gold and Archean greenstone belts may reflect an extremely effective gold depositional mechanism(s) that operates exclusively in these settings (e.g., Groves et al., 1998) and/or may suggest that greenstone belts and associated supracrustal successions represent the ultimate source for gold (e.g., Pitcairn et al., 2006, Large et al., 2009).
The Meliadine gold district (MGD) represents a large (2.8 Moz contained Au in reserves and total resources of 5.8 Moz Au, www.agniecoeagle.com) and important example of BIF- and greenstone-hosted mineralization within the wCP (Fig. 1; Miller et al., 1994, Miller et al., 1995, Carpenter et al., 2005, Lawley et al., 2015a, Lawley et al., 2015b). The largest deposits occur north and along the Pyke Fault, which cuts Neoarchean mafic to felsic volcanic rocks, turbidite and BIF successions comprising the Rankin Inlet greenstone belt (RIGB; Fig. 2). The Pyke Fault also represents the approximate surficial trace of one segment of a lithospheric-scale fault network that cuts the wCP and potentially demarcates the inferred cratonic boundary between the Hearne craton and combined Rae craton and Chesterfield block (Fig. 1; e.g., Jones et al., 2002, Berman et al., 2007, Spratt et al., 2014, Pehrsson et al., 2015). The RIGB thus occupies a critical position within the wCP, but its age and relationship to nearby greenstone belts are undocumented.
In this contribution we report new U–Pb detrital and igneous zircon ages that confirm Neoarchean RIGB volcanic and siliciclastic rocks (2.67–2.66 Ga) are structurally imbricated with multiple Paleoproterozoic (≤2.50 Ga, this study; ≤2.155 Ga, Davis et al., 2008) conglomerate units and dismembered volcano-sedimentary basins. New field data are also synthesized with lithogeochemistry and Nd-isotopic data in order to explore tectonic links between the RIGB and neighbouring greenstone belts. The significance of these findings and particularly their relationship to the gold endowment of the MGD are discussed.
Section snippets
Western Churchill Province geology
The wCP extends along the western Hudson Bay coast (Fig. 1; Hoffman, 1988) and is subdivided into at least three distinct crustal domains (Berman et al., 2007). From north to south, these include the Rae craton, Chesterfield block and Hearne craton (Pehrsson et al., 2013a). Each domain is briefly discussed below, whereas a more complete, time-space comparison is provided in Eglington et al. (2013).
The Rae craton is the largest, but least understood of these three domains and comprises, in part,
Rankin Inlet greenstone belt lithostratigraphy
Rocks near the community of Rankin Inlet were originally defined as the Rankin Inlet Group (Bannatyne, 1958) and later referred to as the RIGB (Fig. 2; Aspler and Chiarenzelli, 1996). The region is the focus of sustained mineral exploration interest because of the past-producing Rankin Nickel Mine (Bannatyne, 1958) and on-going gold (Carpenter et al., 2005, this study) and diamond mineral exploration efforts (Seller, 1999). However, the geological setting of the RIGB and, particularly its
Gold deposit geology
Gold deposits within the MGD share similarities with typical orogenic greenstone- and BIF-hosted mineralization (Miller et al., 1995, Carpenter and Duke, 2004). High-grade gold intervals occur as a series of sub-parallel and semi-continuous ore zones that are known locally as lodes (Fig. 6). Most of these lodes correspond to folded, hydrothermally altered and veined BIF intervals (Fig. 6, Fig. 7, Fig. 8, Fig. 9), which are intercalated and folded with turbidite (e.g., Discovery) and volcanic
Geochemistry
Lithogeochemical data from the RIGB were compiled from Carpenter (2003; n = 60), Tella et al. (2005; n = 19), Lawley et al. (2015c; n = 344) and unpublished data from Sandeman (n = 45). Details on the analytical method, detection limits, quality control and estimates on the precision and accuracy of the results can all be found in those publications. The petrogenetic significance of these datasets to the RIGB has not previously been discussed. Samples with unpublished data were analyzed for major and
U–Pb zircon geochronology methodologies
All U–Pb zircon geochronology was completed at the Geological Survey of Canada, Ottawa. Zircon was concentrated from crushed and milled samples following a combination of Wilfley table, density (Methyl Iodide) and magnetic (Frantz™ isodynamic separator) techniques. Non-magnetic zircon grains were hand-picked under ethanol using a transmitted light microscope from the resulting mineral concentrate for further analysis. Zircon grains from four metasedimentary samples (CL1210, CL1211, CL1315, and
Rankin Inlet greenstone belt petrogenesis
In this section, lithogeochemistry of lavas and inferred lavas (mafic, fine-grained amphibolite to greenschist facies rocks) are used to reconstruct volcanic source(s) and setting(s). For simplicity, gabbroic and ultramafic dykes and sills are excluded as many cannot be demonstrably linked to the primary construction of the greenstone belt. The compositions of volcanic rocks are predominately basaltic with rare intermediate to felsic rocks that extend to dacitic compositions (Fig. 11b). All LVA
Conclusions
The RIGB comprises intercalated ca. 2.66 Ga predominately mafic volcanic and turbiditic successions deposited on an unexposed Meso- to Neoarchean substrate. Neoarchean volcano-sedimentary panels are structurally bound by Paleoproterozoic conglomerate and basalt that are, in turn, unconformably overlain by pillowed-basalt and a distinct carbonate-siliciclastic sequence that presumably represents the remnants of Proterozoic basins. Detrital zircon ages at ca. 2.50 Ga, together with the timing of
Acknowledgements
The MGD study was conducted under the auspices of Natural Resource Canada's Targeted Geoscience Initiative (TGI)-4. The authors would like to thank the cooperation and input from Agnico-Eagle Mines Ltd. throughout the study period. In particular, Robert Fraser, Jérôme Lavoie, Jean-Claude Blais, and Francine Fallara are thanked for facilitating field-work logistics, assisting on site and providing borehole logs, and geologic plans/sections for the deposits. We thank Robert Carpenter for his
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2018, Precambrian ResearchCitation Excerpt :The western Churchill Province (wCP) of the Canadian Shield encompasses the Rae and Hearne cratons (Hoffman, 1988; Eglington et al., 2013), and the Chesterfield block (Berman et al., 2007; Fig. 1A). These crustal entities are best known for their world class Au and U districts (Meliadine and Meadowbank Au: Armitage et al., 1996; Lawley et al., 2016; Sherlock et al., 2004; Athabasca and Kiggavik U; Jefferson et al., 2007 and references therein). Several Ni-Cu-PGE deposits/mineral occurrences are known in the wCP, most prominently the past-producing Rankin Inlet mine (Bannatyne, 1958), and the undeveloped Nickel King (Thomas, 1976; Buhlmann, 1989), Ferguson Lake (Martel et al., 2004; Campos-Álvarez et al., 2011; Acosta-Góngora et al., 2018) and Axis Lake (Coombe Geoconsultants Ltd., 1991) deposits.
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