Very Distant Sudbury Impact Dykes Revealed by Drilling the Temagami Geophysical Anomaly T ⁎ Alexander Kawohla, , Hartwig E
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Precambrian Research 324 (2019) 220–235 Contents lists available at ScienceDirect Precambrian Research journal homepage: www.elsevier.com/locate/precamres Very distant Sudbury impact dykes revealed by drilling the Temagami geophysical anomaly T ⁎ Alexander Kawohla, , Hartwig E. Frimmela,b, Andrejs Bitec, Wesley Whymarkd, Vinciane Debaillee a Institute of Geography and Geology, University of Würzburg, Am Hubland, 97074 Würzburg, Germany b Department of Geological Sciences, University of Cape Town, Rondebosch 7700, South Africa c Bite Geological Ltd., 144 Kuusisto Rd., Sudbury, Ontario P3E 4N1, Canada d Inventus Mining Corp., 83 Richmond Street East, Floor 1, Toronto, Ontario M5C 1P1, Canada e Laboratoire G-Time, DGES, Université Libre de Bruxelles, CP 160/02, Avenue Franklin Roosevelt 50, 1050 Brussels, Belgium ARTICLE INFO ABSTRACT Keywords: The Temagami Anomaly is one of the largest unexplained magnetic features in North America. It is similar in size Sudbury Complex and shape to the geophysical anomaly that marks the 1.85 Ga Sudbury Igneous Complex (SIC) in its immediate Impact melt vicinity but its geological cause and potential link to the Sudbury impact structure have remained elusive. Here Temagami magnetic anomaly we report on a 2200 m deep diamond drill core intersecting the area of maximum magnetic anomaly and provide Offset Dykes evidence of diorite dykes therein being most likely related to the Sudbury impact event. The fine-grained, strongly altered biotite-amphibole diorite occurs below 2000 m, is intrusive into Archaean basement rocks and has the same major- and trace element geochemistry as the SIC, which approximates the bulk composition of the local continental crust that was hit by the impact. A crustal affinity analogous to SIC impact-melt rocks is further 0 supported by whole-rock Nd and Pb isotopes. Low ɛNd between −27.6 and −18.7, as well as Nd model ages of 2.75 Ga are considered as inherited from the crustal precursor rocks that became largely homogenized in the course of impact melt formation. The 206Pb/204Pb ratios are between 15.77 and 19.38, 207Pb/204Pb between 15.22 and 15.59 (corresponding to initial 207Pb/204Pb at 1850 Ma between 15.14 and 15.22), yielding an “isochron age” of 1780 +320/−330 Ma, and the 208Pb/204Pb ranges from 35.47 to 41.15, all values that compare well with published data on SIC impact-related igneous rocks of mainly quartz dioritic composition, locally referred to as Offset Dykes, and that are distinctly different to those reported for other magmatic units in the wider region. Although several such dykes have been well known to occur both radially and concentrically around the SIC, none have been described so far from the area east of the SIC. The recognition of former intrusive impact melt at an even greater distance (50 km) from the SIC than has been known so far increases the extent of the impact structure but also the exploration potential of the area of the Temagami magnetic anomaly for Ni-Cu- PGE-sulfide deposits. The actual cause of the Temagami Anomaly remains open to debate. 1. Introduction structure as these form the principal hosts of the ore. Representing a former impact crater, the SIC was initially most The 1.85 Ga Sudbury Igneous Complex (SIC) around Greater likely a more or less circular feature. Its current ellipsoidal shape Sudbury, Ontario (Fig. 1a), the second largest preserved impact struc- (Fig. 1a) is the result of repeated deformation in the course of sub- ture on Earth, is one of the richest known ore provinces as it hosts sequent orogenies, including the 1.77–1.7 Ga Yavapai, the 1.7–1.6 Ga numerous world-class Ni-Cu-PGE-sulfide deposits. Consensus exists on a Mazatzalian-Labradorian, and the 1.5–1.4 Ga Chieflakian-Pinwarian genetic link between mineralization and impact-induced melts through events (Papapavlou et al., 2017, Papapavlou et al., 2018). Moreover, separation of sulfidic melt droplets and gravitational accumulation this deformed structure corresponds to a prominent positive magnetic thereof at the base of sloping sides of the impact crater (see review by anomaly. Interestingly, a further magnetic anomaly of similar size and Lightfoot, 2016). Evidence of this can be found near the bottom of the shape occurs immediately to the northeast of the SIC (Fig. 2). This is Main Mass of the SIC, in the so-called Sublayer, and in the so-called known as the Temagami Anomaly (TA), which is one of the largest Offset Dykes that occur radially and concentrically around the impact magnetic anomalies in North America, covering an area of 1200 km2 ⁎ Corresponding author. E-mail address: [email protected] (A. Kawohl). https://doi.org/10.1016/j.precamres.2019.02.014 Received 17 July 2018; Received in revised form 31 January 2019; Accepted 12 February 2019 Available online 13 February 2019 0301-9268/ © 2019 Elsevier B.V. All rights reserved. A. Kawohl, et al. Precambrian Research 324 (2019) 220–235 Fig. 1. Geological map of the study area; a) Regional map based on data from the Ontario Geological Survey (2011) and Lightfoot (2016); b) Detailed geological map of the drilling location (Afton Township), after Ayer et al. (2006). and having an amplitude of over 11,000 nT (Card et al., 1984; which has major implications for the prospectivity of the area with Pilkington, 1997). Although the TA was discovered by airborne surveys regards to discovering new SIC-typical ore deposits. in the late 1940s by Norman Bell Keevil and should be of outstanding economic interest due to its proximity to the well-endowed SIC, its geological cause remains unexplained, a genetic link to the SIC spec- 2. Geological framework of the Temagami Anomaly ulative. Lack of outcrops and, until recently, of bore holes, prevented a 2.1. Temagami Anomaly proper understanding of this outstanding geophysical feature. In 2014, fi fi in an attempt to test the cause of the TA, an exploration borehole (AT- The Temagami Anomaly was rst mentioned in a scienti c context 14-01) was drilled by Canadian Continental vertically to a depth of by Coles et al. (1981), a more detailed discussion of its geophysical 2200 m in the Afton Township at the position where the TA reaches its features was given by Card et al. (1984) and Pilkington (1997). Ac- maximum. In this paper, we present first petrological, geochemical and cording to these authors, the magnetic anomaly consists of a component isotope data from this drill core, which intersected at its bottom rocks of shorter wavelength corresponding to banded iron formation and that resemble quartz diorite described from SIC Offset Dykes. We another component of hitherto unknown origin. While Archaean therefore compare these and discuss possible genetic links to the SIC, banded iron formation of the nearby Temagami Greenstone Belt is clearly visible on the aeromagnetic map as a curvy structure and can 221 A. Kawohl, et al. Precambrian Research 324 (2019) 220–235 covering an area of approximately 13 × 29 km; smaller outcrops occur ∼2 km east of the location of the studied deep drill hole (AT-14- 01), known as Emerald Lake Greenstone Belt. The TGB is dominated by intermediate to felsic metavolcanic rocks (former lava flows, tuffaceous and pyroclastic deposits) of calc-alkaline affinity and siliciclastic me- tasedimentary rocks, including shale, wacke and conglomerate (Bennett, 1978; Jackson and Fyon, 1991). Minor tholeiitic basalt occurs as well as a variety of syn-volcanic intrusive phases, such as the layered ultramafic Ajax (Kanichee) intrusion (James and Hawke, 1984), diorite-, pyroxenite- and lamprophyre dykes (e.g. Bennett, 1978). Coeval with the volcanic activity, dated at ∼2.7 Ga (Bowins and Heaman, 1991; Ayer et al., 2007), was the emplacement of three granitoid batholites. The TGB is well-known for its abundance of Al- goma-type banded iron formation (BIF). At least two units of BIF occur on both limbs of the TGB syncline and can be traced for tens of kilo- Fig. 2. Aeromagnetic map showing the Temagami Anomaly (TA) and the meters along strike: A lower unit is composed of 25 m-thick banded fi magnetic anomaly that marks the Sudbury Igneous Complex (SIC). Magnetic chert, pyrite and pyrrhotite (sul de facies) and overlies volcanic rocks. highs are shaded in red and purple, magnetic lows in blue and green (Canadian A second unit of banded chert, magnetite/haematite and chlorite (oxide Continental Exploration Corp., unpublished data). facies) is 100 m thick and occurs embedded in pyritic shale, tuff and ultramafic fragmental rocks (Bennett, 1978; Fyon and Crocket, 1986; easily be traced to the center of the Temagami Anomaly (Fig. 2), there Jackson and Fyon, 1991). Rocks of the Temagami and adjacent is no explanation for the longer wavelength component. As Archaean greenstone belts were subjected to regional greenschist-facies meta- fi volcanic rocks of adjacent greenstone belts and surrounding Palaeo- morphism (Bennett, 1978). Evidence of sul de mineralization and ex- fl proterozoic sedimentary cover rocks are characterized by a low mag- tensive Neoarchaean sea oor alteration exists and resembles features of fi netic susceptibility, Coles et al. (1981) and Card et al. (1984) excluded volcanic massive sul de (VMS) systems (Colvine, 1974; Fyon and them a priori as possible causes of the TA but speculated that an in- Crocket, 1986; Schwartz, 1995; Mark D. Hannington, pers. comm.), trusive, magnetite-rich body (with modelled modal magnetite propor- although no economic VMS deposits are currently known in the TGB. tion of > 6 vol%) within the basement, at depths below 2 km, could be the cause of the TA. The eastern portion of the TA coincides with a 2.3. Huronian Supergroup small-scale positive gravity high (see Appendix E in the Online Supplementary Material), indicating dense rocks at depth.