An Overview of the Geology of Nunavut

David 1. Scott, Canada-Nunavut Geoscience Office

P0 Box 2319, Iqaluit, ND, XOA OHO, djscott@NRCan.gc.ca



        The geology of Nunavut records almost three-billion years of Earth history. Our understanding of this history ranges from excellent in selected areas that have undergone multi-disciplinary study in recent years, to rudimentary in numerous vast tracts that were initially (and most recently!) investigated in the Geological Survey of Canada's great helicopter reconnaissance operations of the 1950's and 1960's.

        For the purposes of the present summary, Nunavut's geological history can be subdivided into three main temporal blocks; the Archean, the Proterozoic and the Phanerozoic. Each era is characterized by a distinctive suite of rocks, tectonic events and resource endowment.

                ARCHEAN

        Rocks of Archean age are exposed throughout Nunavut, and undoubtedly underlie much of the territory that is covered by younger rocks. The Archean of Nunavut is characterized by granite-greenstone terranes, similar in most respects to Archean cratons globally. Nebulitic migmatitic gneisses, that range from granodiorite to quartz monzonite are among the oldest rocks in Nunavut and exposed in parts of northern Baffin Island, the Melville Peninsula, and southwest across the mainland. These rocks are complex and texturally diverse, ranging from massive to finely foliated and thinly banded. Schlieren, lenses and boudinaged bands of amphibolite, metasedimentary, granitoid and rare ultramafic rocks are present. Discordant mafic sheets that truncate early compositional layering in the rocks, but that are folded and locally boudinaged, may represent dykes that predate younger supracrustal rocks.

        Late Archean supracrustal rocks, consisting of ultramafic, mafic, intermediate and felsic volcanic rocks, siliciclastic rocks and ironstones, comprise the greenstone belts that host much of the gold and base metal endowment of Nunavut. In the northern Slave craton of western Nunavut, these rocks (Yellowknife Supergroup) host numerous deposits (e.g. Ulu, George Lake, Boston, etc.) and the iron-formation host rocks of the Lupin Au Mine. In southern central Nunavut (western Churchill Province), similar rocks host many deposits, including those at Victory Lake, Noomut, Heninga Lake, and the Meliadine Au deposits. In the northern part of the Churchill Province, ultramafic volcanic rocks, quartzite and iron formation characterize the Woodhurn and Prince Albert groups; while the former hosts the Meadowbank Au deposits, the latter package of rocks is less well known hut appears to have significant potential for Au mineralization. The Canada-Nunavut Geoscience Office will launch a major new regional mapping project in the southern Prince Albert Group (Committee Bay belt) this coming summer. Rocks of the Prince Albert Group continue northeastward across the Melville Peninsula and onto northern Baffin Island, where they are represented by the Mary River Group. Some of the richest iron deposits in the world, as well as elevated Au potential, are found in these rocks.

        Late plutonic activity is in large part synchronous with, to slightly younger than, volcanism in these greenstone belts, suggesting that there may be, in part, a genetic relationship between the extrusive rocks and some of the intrusions. Monzogranite to granodiorite plutonic rocks intrude both the greenstones and the older orthogneisses. A phase of high-grade metamorphism is locally recorded at 2.5 Ga across much of the Churchill Province.

            PROTEROZOIC

        Much of the Churchill Province was covered by extensive siliciclastic deposits in the earliest Proterozoic; this is recorded in the strata of the Hurwitz Group and related successions. Younger siliciclastic and carbonate rocks of the Penrhyn Group on the southern Melville Peninsula, and their along-strike correlatives of the Piling Group (central Baffin Island) represent a continental margin succession deposited on Archean crust. The stratigraphically lowest strata consist of quartzite and feldspathic quarzite, and is overlain by dolostone, marble and calcsilicate gneiss, with minor amounts of siliciclastic rocks and as well as rare rusty schist and minor sulphide-facies iron formation. Much of the upper part of the basin comprises a relatively homogeneous succession of grey-weathering psammitic rocks that are volumetrically the most important component of the Piling and Penrhyn groups. These rocks have elevated potential for base metal and Au mineralization; the Black Angel Mine, a past-producer of Zn and Pb, is hosted in correlative carbonate rocks of the Karrat Group on the coast of central West Greenland. On southern Baffin Island, similar contemporaneous stratigraphy (the Lake Harbour Group) contains ultramafic sills that have recently been investigated for their Ni potential. In southernmost Nunavut, basal clastic rocks and overlying volcanic and carbonate units are well exposed on the Belcher Islands in southern Hudson Bay. These rocks record the subsidence and rifting of the western margin of the Superior craton. Paleoproterozoic deformation, and a strong, attendant thermal overprint are recognized throughout the map area. These Proterozoic successions were deformed and metamorphosed during the Ca. 1.80 Ga Trans-Hudson Orogen, a period of global collisional and accretionary orogenesis. In collaboration with the GSC (Ottawa), the Canada-Nunavut Geoscience Office will launch a major new regional mapping project in the Piling Group (central Baffin Island) this coming summer.

        Renewed rifting of the assembled continental mass is recorded in northern Baffin and Bylot islands, where the Bylot Supergroup comprises > 6 km of non-metamorphosed basal volcanic, siliciclastic and carbonate rocks. A system of northwest-trending normal faults, some of which played a role in physically controlling the deposition of the Bylot Supergroup, divides the Borden Peninsula into a series of horsts, grabens, and half-grabens interpreted to record two phases of ca. 1.27 Ga rifting and regional subsidence. Zinc and lead ore of the Nanisivik Mine is hosted in carbonate strata of the Bylot Supergroup. Up to 6000 m of correlative siliciclastic and volcanic rocks (Fury and Hecla Group) unconformably overlie Archean and Paleoproterozoic crystalline basement along the shores of Fury and Hecla Strait It has been estimated that the Bylot Supergroup/ Fury and Hecla Group may have been deposited over a period of ca. 75 million years. These rocks were gently folded and faulted prior to deposition of the overlying Phanerozoic strata.

        Diabase dykes, tens to hundreds of metres wide, and tens of kilometres in length, comprise the NW-trending Franklin swarm. These unmetamorphosed tholeiitic dykes are ca. 0.72 Ga.

            PHANEROZOIC

        Paleozoic rocks cover approximately one-third of Nunavut. Strata lying to the west and northwest of Fury and Hecla Strait are part of the Arctic Platform that continues northward onto Ellesmere Island, whereas those to the southeast underlie the Foxe Basin and represent the northern continuation of the Hudson Platform.

        The earliest Cambrian strata, characterized by a thick clastic sequence, record the initial subsidence and rifling of the craton. subsequent marine inundations of the stable craton. From the Cambrian to the Silurian, thick carbonate successions record stable platform conditions; these rocks have significant potential for oil and gas reserves. From the Late Silurian to Early Devonian, much of the Arctic Platform was affected by

        Caledonian orogeny; uplift and erosion led to the deposition of a thick orogenic clastic wedge, with elevated potential for red-bed type Cu deposits. In the late Devonian, east-west compression (Ellesmerian orogeny) may have been the driving force behind the Mississippi-Valley type mineralizing event(s) that gave rise to the Zn-Pb deposits of the Polaris district in the central Arctic archipelago. In collaboration with the GSC (Calgary), the Canada-Nunavut Geoscience Office will initiate a thematic investigation of the regional dynamics of the Polaris Zn-Pb system(s) in the summer of 2000.

        From the Carboniferous to the Cretaceous, renewed rifting led to the formation of the Sverdrup Basin in northernmost Nunavut characterized by the deposition of a thick clastic and carbonate succession. These strata host significant reserves of gas and oil, including the past-producing Bent Horn light crude field. In the late Cretaceous to early Tertiary, renewed rifting led to alkaline volcanism and the deposition of a succession of siliciclastic rocks in northern Baffin Island, southwestern Bylot Island, as well as northern Ellesmere Island. These rocks are remnants of a succession that covered much of the Canadian Arctic archipelago, deposited as a consequence of local uplift driven by a complex series of plate adjustments related to the opening of Baffin Bay and the Labrador Sea. In northernmost Nunavut the Eureakan orogeny is characterized by compression and strike slip faulting, a consequence of the rotation of Greenland.

        In eastern Nunavut, a cluster of kimberlite pipes is centered on Somerset Island, with exposures on the northern Brodeur Peninsula and northwestern Baffin Island. Several of the Somerset Island pipes are known to be diamondiferous; they were emplaced ca. 100 Ma. In western Nunavut, diamondiferous pipes such as Jericho occur in the northern continuation of the Lac des Gras field in the northern Slave Province. Numerous other pipes have been identified on Victoria Island.

        In conclusion, the geology of Nunavut spans much of Earth history, and contains a wide spectrum of economic commodity types. While Nunavut presently trails other parts of Canada in the development of these resources, ongoing and new investigations will advance our understanding of the territory.