Iron-formation-hosted gold occurrences in the Ellice Hills area, Committee Bay belt, Nunavut

The supracrustal rocks of the Committee Bay belt are dominated by clastic sedimentary rocks, ultramafic flows and intrusions, iron-formation, and quartz arenite, with lesser amounts of mafic to felsic volcanic rocks. In the Ellice Hills area, three iron-formation–hosted gold prospects are recognized: the Inuk, Mist-Koffy, and Peanut showings. In these occurrences, gold mineralization is localized in areas where progressive D2 deformation has resulted in dilational settings, either at lithological contacts within high-strain zones or in the hinges of F2 folds. These dilational settings allowed for synkinematic alteration and gold introduction associated with sulphidation of iron-formation. These structural/stratigraphic settings are similar to those of gold deposits in the Hayes River area of the Committee Bay belt, and those of the Meadowbank deposits of the Woodburn Lake group. Initial interpretation of the deformation fabrics suggest that much of the iron-formation–hosted gold mineralization in the Committee Bay belt may be Paleoproterozoic in age. Page 1 1 Contribution to the Targeted Geoscience Initiative (TGI) 2000–2003. Resume : Les roches supracrustales de la ceinture de Committee Bay sont constituees en predominance de roches sedimentaires clastiques, de coulees et d’intrusions ultramafiques, de formations de fer et d’arenites quartzeuses. On y trouve aussi en moindres quantites des roches volcaniques de composition mafique a felsique. Dans la region des collines Ellice, trois gites auriferes dans des formations de fer ont ete identifies : les indices Inuk, Mist-Koffy et Peanut. Dans ces gites, la mineralisation aurifere se situe a des endroits ou la deformation progressive D2 a engendre des cadres de dilatation, soit aux contacts lithologiques dans des zones d’intense deformation, soit dans les charnieres de plis P2. Ces cadres de dilatation ont permis une alteration syncinematique et l’ajout d’or associes a la sulfuration des formations de fer. Ces cadres structuraux/ stratigraphiques sont similaires a ceux des gisements d’or de la region de la riviere Hayes, dans la ceinture de Committee Bay, et des gites de Meadowbank, dans le groupe de Woodburn Lake. L’interpretation initiale des fabriques de deformation laisse croire que la mineralisation aurifere dans les formations de fer de la ceinture de Committee Bay pourrait dater du Paleoproterozoique. INTRODUCTION The Committee Bay belt forms part of the north-central extension of an approximately 2000 km long belt of plutonic and supracrustal rocks that extends from northern Saskatchewan through the Committee Bay region to north-central Baffin Island (Fig. 1A, 1B). The supracrustal rocks in the Committee Bay region, collectively referred to as the Prince Albert group, are dominated by clastic sedimentary rocks, ultramafic flows and intrusions, iron-formation, and quartz arenite, with lesser amounts of mafic to felsic volcanic rocks. These rocks are prospective for different commodities and mineral-deposit types — most notably, gold associated with iron-formation, and nickel-copper (± platinum-group elements) associated with ultramafic intrusions and flows. The diamond potential of this area remains unquantified. The Committee Bay belt has been explored intermittently since 1970 (Sherlock and Deyell, 2002); more recently, exploration by Apex Geoscience Ltd. (conducted since the early 1990s) has identified several gold showings in the Laughland Lake (NTS 56 K), Hayes River (NTS 56 J), and Ellice Hills (NTS 56 P) areas. This contribution reports preliminary observations and interpretations of the geological setting and timing of gold mineralization in three auriferous iron-formations in the Ellice Hills area (NTS 56 P): the Inuk, Mist-Koffy, and Peanut prospects (Fig. 1C). The three Ellice Hills occurrences are compared with similar iron-formation–hosted gold occurrences in the Hayes River area (NTS 56 J), previously reported on by Hyde et al. (2002), and with the Meadowbank deposits of the Woodburn Lake group (Sherlock et al., 2001a, b). This work was completed during the 2002 field season, as part of the Committee Bay Targeted Geoscience Initiative project. REGIONAL GEOLOGY OF THE COMMITTEE BAY BELT The Prince Albert group occurs as coherent, northeast-striking strands of supracrustal rocks up to 15 km wide, and also as northeast-trending discontinuous rafts of supracrustal rocks included within plutonic rocks (Fig. 1C). Current constraints on the Prince Albert group (see also Sanborn-Barrie et al., 2002a, b, 2003; Sandeman et al., 2001a, b; Skulski et al., 2002, 2003) indicate that a lower, volcanic-dominated sequence of intercalated basalt and ca. 2.732 Ga felsic volcanic rocks is overlain by a (~300 m thick) komatiite sequence. An upper, sedimentary-dominated sequence of psammite, semipelite, and quartzite was deposited between ca. 2.72 and 2.711 Ga, the younger age being constrained by an overlying intermediate tuff. The uppermost part of the supracrustal sequence consists of minor komatiite, iron-formation, and younger (clastic rocks that locally contain granulite-facies metamorphic mineral assemblages. Iron-formation appears to have been deposited mainly in two stratigraphic intervals, interpreted as reflecting submarine hydrothermal systems active between and after ca. 2.73 Ga and ca. 2.71 Ga volcanism. The supracrustal sequence is intruded by foliated tonalite to granodiorite (biotite±hornblende±magnetite±titanite), abundant monzogranite pegmatitic veins, and potassium-feldspar–megacrystic to augen monzogranite. The plutonic rocks generally form two suites, at 2720 to 2700 Ma and 2610 to 2580 Ma (H. Sandeman, pers. comm., 2002). The metamorphic grade of the granite-greenstone domain is variable, but typically increases from lower amphibolite facies in the southwest (Laughland Lake region) to upper amphibolite facies in the northeast (Ellice Hills area). Lower–granulite-facies metasedimentary rocks are limited to the northern paragneiss domain in the Arrowsmith River region (NTS 56-O; Skulski et al., 2002). The Committee Bay area has been affected by two penetrative deformation events, by polymetamorphism, and by localized shortening (folding ± shearing; Sanborn Barrie et al., 2002b, 2003). D1 involved development of north-northwest–trending, likely west-verging folds and associated LS fabrics that affect both Prince Albert group strata and widespread ca. 2.6 Ga plutonic rocks (Sanborn-Barrie et al., 2002b, 2003). D2 structures dominate the structural fabric of the belt and are most easily recognized in the supracrustal rocks. These fabrics are by far the most common and have had the greatest impact on the geometry of the belt. All known gold-mineralization occurrences in the belt, including those described below, are interpreted to be synkinematic with D2 deformation. D2 fabrics include northeast-trending folds (F2), northeast-striking composite transposition foliation (S2±S1), and shallowly (<34°) plunging stretching lineations (L2). F2 folds are commonly upright to northwest-verging, and plunge both to the northeast and southwest. S2 (±S1) planes are mainly southeast-dipping, consistent with northwest-directed shortening during D2 (Sanborn-Barrie et al., 2002b, 2003). The final product of D2 deformation is expressed as two east-striking fault zones: the dextral, oblique-slip Amer fault zone in the southwestern part of the area, and the dextral strike-slip Walker Lake shear zone through the centre of the belt. D3 folds are sporadically developed, with approximately north-south–trending upright axial planes and poorly developed planar fabrics. ELLICE HILLS GOLD OCCURRENCES Mineral exploration in the Ellice Hills region prior to the 1990s was limited to reconnaissance work, mainly directed towards Cu and Ni mineralization, in the Kinngalugjuaq Mountain, Mist-Koffy, and Mitten Lake areas, by King Resources Company. In 1992, the Committee Bay Joint Venture (CBJV) began exploring for gold and was successful in identifying several new gold occurrences. In the Ellice Hills area, three main gold showings have been identified, all of which occur in or near the main Committee Bay supracrustal belt. These include the Inuk showing, the Mist-Koffy area, and the Peanut prospect. Detailed mapping at each of these showings was undertaken to constrain the geological setting and timing of gold mineralization relative to regional deformation events. Results of the mapping program and preliminary interpretations for each of the three showings are summarized below.