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Research 324 (2019) 220–235

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Precambrian Research

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Very distant Sudbury impact dykes revealed by drilling the geophysical anomaly T ⁎ Alexander Kawohla, , Hartwig E. Frimmela,b, Andrejs Bitec, Wesley Whymarkd, Vinciane Debaillee a Institute of Geography and , University of Würzburg, Am Hubland, 97074 Würzburg, b Department of Geological Sciences, University of Cape Town, Rondebosch 7700, South c Bite Geological Ltd., 144 Kuusisto Rd., Sudbury, P3E 4N1, 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 . 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 we report on a 2200 m deep drill core intersecting the area of maximum magnetic anomaly and provide Offset Dykes evidence of dykes therein being most likely related to the Sudbury . The fine-grained, strongly altered - diorite occurs below 2000 m, is intrusive into Archaean rocks and has the same major- and trace element as the SIC, which approximates the bulk composition of the local continental that was hit by the impact. A crustal affinity analogous to SIC impact-melt rocks is further 0 supported by whole- 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 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 . 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 , is one of the richest known ore provinces as it hosts sequent , 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 formation and that resemble 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, of the nearby Temagami 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 Lake Greenstone Belt. The TGB is dominated by intermediate to metavolcanic rocks (former flows, tuffaceous and pyroclastic deposits) of calc-alkaline affinity and siliciclastic me- tasedimentary rocks, including , wacke and conglomerate (Bennett, 1978; Jackson and Fyon, 1991). Minor tholeiitic occurs as well as a variety of syn-volcanic intrusive phases, such as the layered ultramafic Ajax (Kanichee) intrusion (James and Hawke, 1984), diorite-, - and 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 , and (sul de ) and overlies volcanic rocks. highs are shaded in red and purple, magnetic lows in blue and green (Canadian A second unit of banded chert, /haematite and chlorite ( 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 -facies meta- fi volcanic rocks of adjacent greenstone belts and surrounding Palaeo- morphism (Bennett, 1978). Evidence of sul de mineralization and ex- fl 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. This led Card Subsidence of the Huronian Basin began at the Archaean- fi et al. (1984) to propose a serpentinized ultramafic body as cause of the Proterozoic boundary and deposition of the basin in ll lasted until TA. The anomaly was apparently deformed in the same way as the SIC 2.22 Ga (Ketchum et al., 2013; Corfu and Andrews, 1986). The Hur- during Proterozoic orogenies by shortening along the NW-SE axis re- onian Supergroup comprises rift-related bimodal volcanic rocks at its sulting in the present oval shape of the initially circular SIC impact base (Bennett et al., 1991) and three sedimentary cycles of conglom- fl structure. Buchan and Ernst (1994) pointed out that both structures erate, siltstone and , that re ect glacial periods (Young et al., were once at the same level, and that the observed offset between the 2001; Long, 2004). A maximum thickness of 12 km is reached southeast ∼ – SIC and the TA is due to the NNW-SSE striking Onaping system. of Sudbury (Young et al., 2001) and a thickness of 1.5 3.4 km is as- Reflection seismic data were gained from N-S transects across the TA sumed within the area of the TA (Milkereit and Wu, 1996). Rocks of the and were interpreted by Milkereit and Wu (1996) as a large early Huronian Supergroup within the study area were subjected to very low- Huronian rift graben structure, covered by 3 to 4 km-thick Huronian grade sub-greenschist-facies and deformation related to sedimentary rocks, and bounded by northerly and southerly dipping the Penokean and Grenvillian orogenies (Easton 2000). There is evi- faults. A seismically transparent zone below this presumed trench dence of a metasomatic event sometime between 1.93 and 1.7 Ga, system, at depths below 6 km, is particularly striking. According to which is ascribed to contemporaneous alkaline in the Milkereit and Wu (1996) this zone of low reflectivity could be a larger Southern Province (Schandl et al., 1994; Fedo et al., 1997; McLennan intrusion within the basement, having a diameter of about 15 km. The et al., 2000). depth and spatial dimensions of this seismic anomaly is consistent with the gravimetric and aeromagnetic data as modelled by Card et al. 2.4. Other magmatic events (1984). This led to the conclusion that the seismic anomaly might be due to an (ultramafic) intrusion that also produced the Temagami Other magmatic events in the region encompass the emplacement of magnetic and gravimetric anomalies (Milkereit and Wu, 1996). the following: The TA is located some 50 km northeast of Sudbury City, east of fi Lake Wanapitei, and is situated near the boundary of three geological i) layered ma c intrusions of the 2.48 Ga East Bull Lake Suite, hosting fi provinces, namely, the -greenstone domains of the Neoarchaean sub-economic PGE-Ni-Cu sul de and Fe-Ti oxide occurrences, and Abitibi Subprovince of the Superior Province, the associated feeders now present as the Matachewan dyke swarm Southern Province, which is dominated by metasedimentary rocks of (Heaman, 1997; Ciborowski et al., 2015, and references therein); the Huronian Supergroup, and the metamorphic ii) granitoid plutons of roughly the same age (e.g. Lightfoot, 2016; Grenville Province (Fig. 1). Bleeker et al., 2015); iii) very widespread mafic sills of the 2.22 Ga Nipissing Suite (Corfu and Andrews, 1986; Noble and Lightfoot, 1992); 2.2. Greenstone belts iv) alkaline complexes, such as the 1881 Ma Spanish River (Rukholv and Bell, 2009) and the 1.2 Ga Nemag Lake Fenite Surrounded by Paleoproterozoic sedimentary rocks and gabbroic (Siemiatkowska and Martin, 1975) west and south of Sudbury; and intrusions, several smaller Neoarchaean greenstone belts are exposed in v) NW-trending alkaline dykes of the so called Sudbury erosive windows (Fig. 1). The largest of these is the Temagami Green- Dyke Swarm (e.g. Shellnutt and MacRae, 2012), dated at 1235 Ma stone Belt (TGB) which forms a southwesterly plunging syncline, to 1238 Ma (Dudas et al., 1994).

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The Nipissing Gabbro (also referred to as Nipissing Diabase) is of specific interest, as it is the most widespread intrusive unit in the study area (Fig. 1), covering almost 25% of the Huronian Basin, and having an estimated volume of some 105 km3 (Lightfoot et al., 1993). It occurs as up to hundreds of meters thick, funnel-shaped or undulating sills, rarely as dykes. Major rock type is a -bearing tholeiitic gabbro, but highly differentiated varieties in the form of , , granite and also exist. Pegmatoidal textures are common in the -Temagami-Wanapitei area (Lightfoot, 2016), and in most areas, the Nipissing Gabbro was affected by (very) low- grade metamorphism at (sub-)greenschist-facies conditions (Easton, 2000). The Nipissing Gabbro hosts, in places, sub-economic low-grade Ni-Cu-PGE-sulfide occurrences (Lightfoot, 2016).

2.5. Sudbury Igneous Complex

The geology of the SIC has been discussed in numerous publications, most recently by Lightfoot (2016), and much of the information below is taken from this review. Formation of the 1.85 Ga Sudbury structure is attributed to the impact of a ∼10–12 km large or comet. The original rim-to-rim diameter of this impact structure is estimated at 150 to 250 km. Vast volumes (c. 15,000 km3) of granodioritic impact melt filled the central cavity of the resulting multi-ring basin and subse- quently differentiated into a ∼3 km thick sequence (called Main Mass) of, from bottom to top, mafic quartz-, felsic norite, - and oxide-rich quartz monzogabbro, and granophyre. The proportions of norite to gabbro to granophyre thickness are approximately 25:15:60. At the base of the Main Mass, a -bearing, in places heavily mineralized, ‘Contact Sublayer’ of noritic composition discontinuously overlies topographic depressions of the brecciated and thermally me- tamorphosed footwall country rocks. The SIC experienced tilting and NW-SE shortening during Proterozoic orogenies, at upper greenschist- to lower -facies metamorphic conditions. Today the SIC is exposed as an elliptical, 60 × 30 km large asymmetric synform, over- lain by impact-induced fall-back (Onaping Formation) and sedimentary rocks of the Whitewater Group. Pseudotachylite breccias, also referred to as Sudbury , occur at a distance of over 50 km, in Fig. 3. Schematic lithological profile through drill core AT-14-01 with strati- places even 150 km, away from the SIC, extending as far as Temagami graphic interpretation (colour code as in Fig. 1). to the east (Bennett, 1978; Lightfoot, 2016). The term ‘Offset Dykes’ is applied to narrow (10 to 100 m wide) and between dykes. Their composition corresponds to the bulk com- igneous bodies of quartz (monzo-)dioritic composition, directly related position of the SIC Main Mass and the composition of impact melt to the SIC. These Offset Dykes, in many places hosted by pseudo- preserved in the Onaping Formation. Ostermann et al., 1994, , disect the footwall rocks either radially to (e.g. Foy, Parkin, Ostermann et al., 1996) and Bleeker et al. (2015) provided - Trill, Cascaden), or concentrically around (e.g. Hess, Manchester), the lization ages (U-Pb on ) of 1849.4 Ma + 3.5/−2.6 Ma, SIC and can be traced for several kilometers along strike, extending as 1852 + 4/−3 Ma and 1848.5 ± 0.8 Ma, which is within error of the much as 30 km away from the SIC (see Grant and Bite, 1984 and age of the SIC (1849.53 ± 0.21 Ma to 1849.11 ± 0.19 Ma; Davis, Fig. 1a). Currently, 17 Offset Dykes and extensions are known 2008). A lack of inherited grains or their, if present, corroded (Lightfoot, 2016), some of which host highly economic amounts of appearance, led some authors to argue for a superheated parental sulfide ore, accounting for ∼50% of the total metal resource of the (> 1700 °C; e.g. Ostermann et al., 1996). All these findings Sudbury mining district. Recently, new Offset Dykes were discovered in indicate that the Offset Dykes were derived from one single, probably the north of the SIC (Smith et al., 2013). The Offset Dykes tend to be superheated melt, and were emplaced soon after the impact, prior to composite dykes with a marginal inclusion-free and sulfide-poor facies the differentiation and solidification of the Main Mass, maybe during in sharp or gradational contact with a xenolith-bearing, sulfide-miner- crater collapse (see Lightfoot, 2016; Prevec and Büttner, 2018 for re- alized facies that is typically, but not always, present in the dyke in- views). terior. Petrographically, the Offset Dykes can vary significantly: tex- tures range from aphantic, spherulitic or dendritic at their chilled margins and distal ends, to equigranular and granophyric; they consist 3. Drill core AT-14-01 mainly of mafic (c. 45–55 vol%), (30–45 vol%), quartz (5–15 vol%), with traces of granophyric intergrowths of quartz 3.1. Stratigraphy and K-feldspar (e.g. Keays and Lightfoot, 2004). Depending on the de- gree of metamorphism, primary might be present, or replaced Borehole AT-14-01 (Fig. 3) was sunk at Afton Township (Fig. 1), by amphibole and biotite. In addition, Smith et al. (2013) and Pilles coordinates 5198927 N / 549320 E, and reached a depth of 2200 m. It et al. (2018) reported also on granophyric, pyroxenitic and gabbroic intersected a gabbroic from 0 to 321 m, which bears all the hall- dykes, more akin to the Main Mass granophyre and gabbro, respec- marks of the Nipissing Gabbro as exposed on surface (Fig. 1b). This is tively. Despite petrographic variations, the geochemistry of the quartz followed by cross-bedded sandstone of the Lorrain Formation, siltstone dioritic Offset Dykes is remarkably uniform along strike of a given dyke and conglomerate of the Gowganda Formation (Cobalt Group, upper

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Fig. 4. Photographs of the main rock types in the studied drill core: a) medium-grained and b) pegmatoidal variety of the Nipissing Gabbro; c) Banded iron formation with alternating layers of magnetite and chert; d) Shale with pyrite framboids; e)-g) Porphyric metavolcanic rocks containing abundant altered of feldspar (e-g) plus embayed quartz (g); h) Drill core, and i) polished slab of diorite.

Huronian Supergroup) and from a depth of ∼600 m to 2200 m, a vol- 3.2. Petrography cano-sedimentary succession likely of Archaean age and correlative with the Temagami and Emerald Lake greenstone belts. These are felsic 3.2.1. Nipissing Gabbro to intermediate porphyric volcanic rocks, subordinate aphyric and The Nipissing Gabbro is medium- to coarse-grained (Fig. 4a), locally strongly altered tuffaceous, pyroclastic and volcaniclastic beds, banded pegmatoidal (grain size > 0.5 cm) (Fig. 4b), massive, and of dark iron formation of the oxide facies, pyritic shale, and wacke (Fig. 3). green colour. Where unaltered (Fig. 5a), it exhibits an intergranular Finally, two diorite sills or dykes, 25 and 52 m in thickness, were en- texture of euhedral, locally zoned, laths (50–60 vol%), in- countered within the feldspar and iron formation at depths tergrown with interstitial anhedral to subhedral clinopyroxene (c. below 2000 m. 40 vol%) and some anhedral orthopyroxene (up to 5 vol%). Accessory

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Fig. 5. Representative photomicrographs of the three igneous units; a)-c) Nipissing Gabbro (under crossed polars); e)-f) Metavolcanic rocks with different degrees of alteration (under crossed polars); c) Diorite (under plane and crossed polars); Pl – plagioclase; Cpx – clinopyroxene; Am – amphibole; Zo – (clino-)zoisite; Fsp – feldspar; Qtz – quartz; Ser – sericite; Bt – biotite; Mt – magnetite; Ep – . phases include biotite and opaque minerals. Secondary minerals (am- thickness from a few millimeters (‘microbands’)to2–3cm (‘meso- phibole, , chlorite) (Fig. 5b) occur in minor amounts (< 10 vol%). bands’), and dip, in places, steeply at 80° towards the core axis. Strata of Some parts of the Nipissing Gabbro (e.g. Fig. 5c) underwent strong Fe-rich chlorite, biotite, stilpnomelane and are common and alteration by which ferromagnesian minerals have been, in part or might represent metamorphosed pelagic sediments or pyroclastic beds. entirely, replaced by amphibole, chlorite and leucoxene, and plagio- Micro- to quartz shows signs of recrystallization and clase by clinozoisite. In places of extreme alteration, the primary ig- grain coarsening, and magnetite occurs dispersed within the chert neous texture of the original gabbro is no longer apparent and the ig- layers. No Fe(III)- (haematite, ) were observed; is neous assemblage appears completely converted to absent. Some deformation of the BIF resulted in fold structures and clinozoisite + amphibole ± chlorite ± quartz. boudins. Brittle deformation is evident in chert breccias that are ce- mented with magnetite and chlorite. Microfaults and fractures are filled with chlorite, quartz, carbonate, and in addition, pyrite and arseno- 3.2.2. Banded iron formation pyrite were identified in late quartz veins. Oxide-facies banded iron formation (BIF) occurs from 650 to 710 m, from 1388 to 1585 m, and from 1797 to 2068 m, interbedded with volcanic and volcaniclastic rocks, pyritic shale and wacke. The BIF is 3.2.3. Metavolcanic rocks made up of alternating layers of magnetite and chert (Fig. 4c) and, in The metavolcanic rocks are light- to dark grey (Fig. 4e.f), with places, of , pyrite and Fe-silicates. Individual layers range in strongly altered portions being either ochre (Fig. 4g) or green where

225 A. Kawohl, et al. Precambrian Research 324 (2019) 220–235 altered to chlorite. The majority of volcanic rocks are holocrystalline rock Sr, Pb and Nd isotopic compositions following the analytical and porphyric-seriate textured, although aphyric varieties exist as well. protocol as described in the Online Supplementary Material Appendix Phenocrysts, amounting 30–45 vol%, are embedded in a fine-grained, A. mosaic-like and homogenous groundmass of quartz, sericite, biotite and some carbonate (Fig. 5d-e). The dark colour of some sections of the 4.1. Geochemistry rock (e.g. Fig. 4e) is due to large amounts of biotite (> 25 vol%) in the groundmass, which formed at the expense of sericite and quartz. The Nipissing Gabbro is the most primitive of the drill

Among the phenocrysts, euhedral to subhedral feldspar (predominantly core, having a basic composition of 50.7–51.7 wt% SiO2, 8.0–8.4 wt% plagioclase) is the most abundant. Ranging in size from 20 µm to 5 mm MgO and a high Mg# (i.e. Mg/(Mg + Fe)) of 40–47. Low concentra- it appears strongly altered. Sericitization and saussuritization were so tions of (LOI between 0.8 and 2.6 wt%) are in good agreement extreme that only the form relics of the original plagioclase can be with petrographic observations and correlate with the degree of al- discerned in (Figs. 4g,5e). Locally, up to 3 cm large and teration, i.e., the abundance of secondary hydrous silicates (amphibole, zoned megacrysts of alkali feldspar were observed (Fig. 4e). Rounded clinozoisite). A mafic composition of the Nipissing Gabbro samples is and deeply embayed phenocrysts of blueish quartz (2–4 mm in dia- also supported by high concentrations of Cr (162–496 ppm) and Ni meter) constitute up to 5 vol% (Fig. 4g), and in places, phenocrysts of (115–263 ppm), negatively correlated with concentrations of in- biotite or are also present. , , pyrite, zircon, compatible elements (e.g. 0.3–0.6 wt% K2O, 36–49 ppm Zr, 10–23 ppm apatite and allanite occur in accessory amounts. The porphyric volcanic Rb, 40–64 ppm REE + Y, 0.3–0.5 ppm U, 1.0–1.4 ppm Th). rocks underwent intense hydrothermal alteration as evident from om- The porphyric volcanic rocks are characterized by a large compo- nipresent sericitization of , accompanied by silicification. The sitional range and a high degree of differentiation with respect to SiO2 latter process led to the formation of cherty flood quartz at (59.4–68.9 wt%), Na2O (0.1–4.6 wt%) and K2O (2.1–7.9 wt%), accom- 932.0–935.4 m depth, where a massive, 3 m-thick exhalative pyrite unit panied by low MgO (1.2–5.2 wt%) and moderate Fe2O3 (2.3–8.8 wt%, occurs within the volcanic pile. A poorly developed lineation is in- representing total iron). Concentrations of Zr (ppm 123–321 ppm) are dicated by a preferred orientation of sericite, and some portions of this high as well, but those of Th (2.2–7.3 ppm) and REE + Y (74–195 ppm) lithology have been converted to almost monomineralic sericite rock are lower than for the diorite dykes, although still higher than for the (up to 90 vol% sericite) and relics of quartz (10 vol%) (Fig. 5f). Due to Nipissing Gabbro. Concentrations of Ni and Cr are overall low the intense alteration, it is virtually impossible to classify the protolith. (19–122 ppm and 52–221 ppm, respectively), except for brecciated and The fine-grained texture and the weak lamination could point to a tuff/ chloritized sample AT-199 (122 ppm Ni and 119 ppm Cr). A large ash-fall deposit or a clastic sediment. Chloritization was also common, variability in the concentrations of alkali metals (Ba, Rb, K) and alka- resulting in almost monomineralic chlorite fels, speckled with pyrite- line earth metals (Mg, Sr, Ca), indicates that significant post-magmatic and carbonate-porphyroblasts. Biotite occurs also as veinlets and thus element mobility has affected these rocks. This is especially true for seems to have been remobilized on a local scale. samples altered to chlorite, in which Rb, Ba, K, and Na were completely leached out. The aphyric volcanic (pyroclastic?) rocks have a restricted 3.2.4. Diorite range in composition and are slightly more primitive than the porphyric

The diorite is greyish-green in drill core (Fig. 4h) and dark grey on variety, viz. 47.8–62.9 wt% SiO2. Concentrations of MgO and Fe2O3, for polished surfaces (Fig. 4i), massive, fine-grained and texturally homo- example, are between 3.0 and 7.3 wt% and 6.4–19.6 wt%, respectively. geneous. It has a relatively high density and magnetic susceptibility. Again, a significant enrichment in K2O (up to 11.7 wt%) is noted. Trace The diorite appears chilled against the , becoming slightly element concentrations are indistinguishable from those in the por- coarser grained away from the contact. Due to strong alteration, pos- phyric volcanic rocks. sible quench textures that might have been present originally are not The diorite has a relatively uniform and , preserved; contacts to the country rocks are sheared and altered to with 53.4–56.2 wt% SiO2, 11.2–12.6 wt% Fe2O3 (representing total chlorite and . The rock has a subophitic to interstitial texture of iron) and 3.5–3.9 wt% MgO. The Mg# is 22–25, and the LOI is variable euhedral, twinned and zoned plagioclase laths (0.5 to 1 mm in length, between 1.0 and 6.6 wt%. The rock is overall poor in Cr (7–88 ppm) and up to 70 vol%) (Fig. 5g), in places strongly or entirely replaced by Ni (43–91 ppm), except for three outliers with up to 308 ppm Cr and epidote. Olive-green biotite and pale green needles and bundles of ac- 203 ppm Ni. Despite being the second-most primitive lithology in this tinolite occur as interstitial phases in amounts of 20–25 vol% and 5 vol drill core – after the Nipissing Gabbro – the diorite is anomalously

%, respectively (Fig. 5h). Accessory minerals include magnetite with enriched in incompatible elements, for example K2O (1.3–3.2 wt%), U exsolution lamellae of Fe-Ti-oxide, anhedral quartz, titanite/leucoxene (1.1–1.3 ppm), Th (6.5–7.2 ppm), REE + Y (169–186 ppm) and Zr and pyrite. Secondary minerals are epidote, chlorite (after biotite) and (139–155 ppm). calcite, all of which occur as thin veins (Fig. 5i ) or dispersed within the groundmass (Fig. 5h). An inclusion-bearing variety has been recognized 4.2. classification between 2081 m and 2086 m, in the center of the upper diorite body. It contains angular to subrounded fragments of BIF, diorite and chlorite According to the TAS-diagram (total alkali vs. silica, Fig. 6a) after fels, a few millimeters and several decimeters in size, within a dioritic Le Bas et al. (1986), the diorite’s composition corresponds to groundmass (for images see Appendix C in the Online Supplementary (diorite), (monzodiorite) or basaltic andesite, and that of Material). the Nipissing Gabbro to basalt (gabbro). However, as Na and K were most likely mobile during alteration, classification plots using immobile 4. Results elements (Si, Ti, Nb, Y, Zr) are preferred. In these diagrams (Winchester and Floyd, 1977, not shown; Pearce, 1996, Fig. 6b), the diorite samples A total of 62 samples (17 diorite, 3 Nipissing Gabbro, 42 meta- plot within the field of basalt, stretching to the field of . Again, volcanic rocks) were chosen for whole rock geochemical analyses using the samples taken from the Nipissing Gabbro correspond to basalt conventional XRF (for major element concentrations) and ICPMS (gabbro), which is in perfect agreement with its mineralogical compo- techniques (for trace element concentrations). For analytical details see sition. The metavolcanic rocks of the drill core are classified as basaltic Appendix A in the Online Supplementary Material. The geochemical andesite or trachyandesite using the plot of Pearce (1996), with some data for the diorite are listed in Table 1, and the complete dataset for all altered outliers plotting in the basaltic field. Based on petrographic 62 analyses is given in the Online Supplementary Material (Appendix observations, this classification seems appropriate. Secondary element

B). In addition, the 17 diorite samples were analyzed for their whole mobility has clearly caused a shift in the concentrations of Na2O and

226 .Kwh,e al. et Kawohl, A.

Table 1 Geochemical data for diorite in drill core AT-14-01.

Depth (m) 2109.00 2105.00 2097.50 2096.20 2091.00 2089.20 2166.00 2165.00 2165.00 2159.50 2157.00 2156.90 2156.30 2155.00 2153.50 2149.50 2148.20 Sample AT-36 AT-38 AT-39 AT-64 AT-40 AT-41 AT-14 AT-15 AT-58 AT-17 AT-19 AT-59 AT-20 AT-21 AT-22 AT-24 AT-25

Major element oxides (wt%) SiO2 53.4 53.7 53.4 53.6 53.5 53.9 54.5 54.5 54.7 55.8 55.3 55.6 55.5 55.8 56.2 53.6 54.3 TiO2 1.08 1.09 1.08 1.09 1.07 1.08 1.09 1.12 1.11 1.15 1.16 1.17 1.16 1.15 1.18 1.10 1.11 Al2O3 13.7 13.8 13.7 13.8 13.7 13.7 13.8 14.0 14.0 14.3 14.3 14.4 14.2 14.4 14.3 13.8 14.0 Fe2O3 11.3 11.7 11.3 11.5 11.5 11.2 11.5 12.1 11.7 12.2 12.3 12.4 12.2 12.2 12.6 11.3 11.7 MgO 3.8 3.9 3.7 3.7 3.7 3.5 3.5 3.7 3.5 3.6 3.6 3.6 3.7 3.9 3.8 3.5 3.7 CaO 5.4 4.8 5.3 4.9 6.2 5.8 6.8 6.2 6.6 5.6 5.7 5.6 5.5 5.4 4.6 5.2 5.2 MnO 0.15 0.15 0.14 0.12 0.12 0.11 0.14 0.14 0.14 0.15 0.16 0.15 0.16 0.16 0.16 0.14 0.14 Na2O 4.0 4.0 3.9 4.0 4.0 4.2 2.9 3.0 2.9 3.3 3.3 3.6 3.4 3.7 4.0 4.1 4.2 K2O 2.72 3.24 2.65 2.76 1.45 1.33 1.87 2.24 2.26 2.50 2.42 2.39 2.50 2.30 2.05 2.91 2.27 P2O5 0.09 0.07 0.08 0.09 0.10 0.10 0.11 0.09 0.11 0.08 0.09 0.09 0.08 0.07 0.06 0.04 0.08 LOI 5.5 4.8 5.6 5.0 6.6 6.3 4.7 4.0 3.8 1.1 1.0 1.2 1.1 1.1 1.5 4.6 5.3 Total 101.10 101.22 100.85 100.60 101.86 101.30 100.86 101.03 100.76 99.79 99.39 100.07 99.44 100.04 100.42 100.32 101.93 Trace elements (ppm) Cr 11.2 21 7.5 12.2 81 11.8 296 51 18.6 32 308 65 244 88 45 13.5 11.5 Ni 45 52 43 45 86 48 195 67 51 58 203 74 170 91 67 46 47 Cu 111 103 117 119 101 114 119 115 121 106 90 107 101 73 61 118 120 Zn 96 103 99 105 89 89 129 122 120 104 103 100 115 111 116 102 121 227 Rb 149 189 136 139 62 49 72 82 87 46 38 39 42 37 38 127 103 Sr 491 561 462 423 542 527 744 732 843 1019 1155 1127 1203 1126 875 514 370 Zr 147 147 145 147 146 150 145 139 144 148 149 142 149 146 155 148 150 Nb 7.5 7.5 7.2 7.4 9.8 7.7 16.9 8.4 8.1 8.6 18.6 9.3 15.3 10.2 9.1 7.7 7.4 Ba 458 550 517 562 331 332 560 637 642 947 1000 961 1020 982 819 669 535 La 33.6 32.9 33.3 33.8 33.1 34.1 35.1 33.8 34.6 34.9 35.0 34.6 35.1 32.8 32.3 31.3 33.9 Ce 65 66 65 66 65 68 68 66 67 69 69 68 69 65 65 64 66 Pr 7.9 7.6 7.5 7.6 7.7 8.1 8.0 7.9 7.9 7.9 8.1 8.1 8.2 7.8 7.6 7.4 7.6 Nd 30.8 29.8 30.0 30.4 29.7 31.8 30.5 30.2 29.9 31.4 32.1 30.7 31.9 29.8 29.8 29.0 29.8 Sm 5.5 5.7 5.5 5.8 5.5 5.8 5.9 6.0 5.7 6.1 6.1 6.0 6.1 6.0 5.6 5.5 5.5 Eu 1.68 1.53 1.58 1.68 1.59 1.60 1.63 1.60 1.66 1.63 1.64 1.46 1.64 1.38 1.34 1.44 1.57 Gd 4.9 4.9 5.0 4.9 4.9 4.9 5.3 5.1 5.5 5.4 5.5 5.5 5.4 5.3 5.3 5.1 5.4 Tb 0.57 0.60 0.59 0.58 0.57 0.57 0.65 0.67 0.65 0.65 0.70 0.68 0.69 0.63 0.66 0.63 0.65 Dy 3.1 2.9 3.2 3.1 3.0 3.1 3.8 3.8 3.8 3.9 3.9 4.1 4.0 3.7 3.8 3.8 3.7 Ho 0.53 0.53 0.58 0.56 0.56 0.56 0.71 0.67 0.69 0.70 0.71 0.68 0.71 0.65 0.69 0.71 0.67 Er 1.46 1.42 1.50 1.51 1.56 1.48 1.85 1.79 1.90 1.92 2.06 1.93 1.94 1.92 1.88 1.92 1.89 Tm 0.22 0.20 0.23 0.21 0.20 0.21 0.26 0.25 0.24 0.26 0.26 0.25 0.27 0.28 0.26 0.26 0.26 Precambrian Research324(2019)220–235 Yb 1.43 1.38 1.36 1.44 1.38 1.43 1.64 1.66 1.56 1.71 1.61 1.66 1.66 1.63 1.69 1.70 1.70 Lu 0.23 0.21 0.21 0.23 0.21 0.23 0.25 0.23 0.24 0.25 0.25 0.29 0.27 0.27 0.24 0.25 0.25 Y 14.3 14.0 14.4 13.9 14.7 14.5 18.7 17.7 18.5 19.0 19.2 18.2 18.8 18.6 18.9 17.8 17.8 Hf 3.8 3.7 3.9 3.8 3.7 3.9 3.7 3.7 3.5 3.8 3.7 3.7 3.9 3.6 4.1 3.9 3.9 Ta 0.53 0.49 0.48 0.50 0.52 0.48 0.56 0.54 0.53 0.55 0.59 0.53 0.55 0.53 0.52 0.53 0.52 Pb 6.2 11.0 11.3 9.7 11.4 4.8 12.3 11.2 12.2 15.4 14.6 14.1 14.8 14.6 20.8 27.9 43.4 Th 6.5 6.8 6.5 6.7 6.7 6.7 6.9 6.8 6.7 7.2 7.1 7.1 7.2 7.0 7.2 7.0 6.8 U 1.2 1.1 1.1 1.1 1.1 1.1 1.2 1.2 1.2 1.2 1.2 1.2 1.2 1.2 1.3 1.2 1.2 A. Kawohl, et al. Precambrian Research 324 (2019) 220–235

Fig. 6. Geochemical classification diagrams for the Nipissing Gabbro, meta- volcanic rocks and diorite; a) Total alkali versus silica diagram after Le Bas et al. (1986); b) Nb/Y versus Zr/Ti plot after Pearce (1996).

K2O and thus a large spread of data points in the TAS plot. Whereas the Nipissing Gabbro has a tholeiitic affinity, the magmatic affinity of the diorite samples is somewhat ambiguous. The diorite sills/ dykes are classified as calc-alkaline using the Zr-Y plot by Barrett and MacLean (1994) (not shown) and the Zr/Y-Th/Yb diagram of Ross and Bedard (2009) (Fig. 7c). In the AFM ternary plot (Irvine and Baragar, 1971)(Fig. 7b), however, the diorite samples straddle the boundary between the tholeiitic and calc-alkaline differentiation trends, and a tholeiitic affinity is suggested in the plot after Miyashiro (1974) (Fig. 7a), a shoshonitic (high-Th, high-K) affinity according to the ffi classification of Hastie et al. (2007) (not shown). In all trace element Fig. 7. Diagrams showing the magmatic a nity of the three igneous units; a) SiO versus FeO*/MgO plot after Miyashiro (1974) with total iron as ferrous diagrams, the metavolcanic rocks are of calc-alkaline affinity, except for 2 iron; b) AFM plot after Irvine and Baragar (1971); c) Th/Yb versus Zr/Y plot the extremely altered (chloritized) samples, whose hypothetical classi- after Ross and Bedard (2009). fication as tholeiitic (Fig. 7a and b) is geologically meaningless.

-modified mantle source and/or assimilation of country 4.3. Trace element patterns rocks (cf. Lightfoot et al., 1993). Trace element patterns for the metavolcanic rocks (Fig. 8c-d) ex- The measured composition of the Nipissing Gabbro is in very good hibit the diagnostic features of (calc-alkaline) subduction-zone mag- agreement with its tholeiitic affinity and other Nipissing intrusive matism, namely the decoupling and enrichment of large-ion lithophile bodies elsewhere, as reported by Lightfoot et al. (1986) and Lightfoot elements (Rb, Ba, K, Sr, Pb) from and over high-field strength elements and Naldrett (1996). Its trace element pattern is smooth and flat (Nb, Ta, Ti, Zr, Hf, U, Th). This is demonstrated by positive anomalies in (Fig. 8a-b), compared to the other rocks of the core, except for positive Rb, Ba, K, Pb, and negative ones for Ti, Nb and Ta. The metavolcanic peaks of K, Pb and Sr, and weakly pronounced negative anomalies of Ti, rocks are rich in LREE, depleted in HREE, and have steep REE-patterns Nb, and Ta. These features can be ascribed to derivation from a (LaN/YbN up to 42), similar to the intermediate volcanic rocks of the

228 A. Kawohl, et al. Precambrian Research 324 (2019) 220–235

Fig. 8. Extended primitive mantle-normalized trace element diagrams and chondrite-normalized REE diagrams for the Nipissing Gabbro (a-b), metavolcanic rocks (c- d), and the diorite (e-f); Normalization values after Sun and McDonough (1989) and McDonough and Sun (1995).

1850 adjacent greenstone belts (Fyon and Cole, 1989). (1850 Ma). The diorite samples define a narrow range in ɛNd , from 0 Despite the overall similarities in the trace element patterns, the −8.4 to −5.4, corresponding to an ɛNd of −27.6 to −26.1. The only 1850 0 diorite can be clearly distinguished from all other igneous rocks in the exception is sample AT-41, with an ɛNd of −4 and ɛNd of −18.7. drill core on the base of several features. First, the diorite is the only Model ages (TDM) are the theoretical time elapsed since melt separated lithology that has a ThN/UN > 1 and a ThN/KN = 1 (where ‘N’ denotes from a depleted mantle reservoir (e.g. Pattchet, 1992), and the calcu- the primitive mantle-normalized value). All other igneous rocks in the lated ages are between 2540 Ma and 2910 Ma, which is typical of the 87 86 core have a ThN/UN ≤ 1 and ThN/KN < 1. Thorium concentrations in Superior Province (Fig. 9). The present-day Sr/ Sr isotope ratio for the diorite are the highest of the entire drill core, whereas K con- the diorite is very radiogenic and varies between 0.710 and 0.734 centrations are the lowest. Second, the diorite has the highest con- (Table 2). centrations of rare earth elements among all rocks of the drill core, Whole rock Pb-Pb data are presented in Table 3. The 206Pb/204Pb higher than any other igneous rock with a similar degree of differ- ratio ranges from 15.77 to 19.38, 207Pb/204Pb from 15.22 to 15.59, and 208 204 entiation (54 wt% SiO2). The total REE content exceeds even that of the Pb/ Pb from 35.67 to 41.15. The uranogenic Pb ratios define a surrounding (66–69 wt% SiO2). Third, the diorite analyses linear array with an “isochron age” of 1780 + 320/−330 Ma (2 σ), display without exception a positive Sr anomaly, a Eu/Eu* < 1, and a calculated by using the Isoplot software package of Ludwig (2003) pronounced positive Pb anomaly. (Fig. 10a). In order to compare these Pb isotope ratios with those re- ported from the SIC, it is useful to calculate the initial 207Pb/204Pb at the time of formation, that is, 1850 Ma. While it is possible to model the 4.4. Nd, Sr and Pb isotopes initial isotope ratios by using the measured U, Th and Pb concentra- tions, not all literature data report on these concentrations. Thus, a Neodymium isotope data and the element concentrations of Sm and different approach, as described by Darling et al. (2010b) and ɛ ɛ Nd were used to calculate the Nd values (the notation being the de- McNamara et al. (2017), is applied here: The present-day 207Pb/204Pb viation of the measured Nd isotope composition compared to the ratio is projected back parallel to the 1850 Ma reference isochron (slope chondritic value) for the diorite at the assumed time of crystallization, 0.113) to a set 206Pb/204Pb of 15.464, which is the value of the silicate as derived from the U-Pb age of the Sudbury Igneous Complex

229 A. Kawohl, et al. Precambrian Research 324 (2019) 220–235

Ca, Sr and Pb. Alteration indices of the Nipissing Gabbro are close to the optimum values for unaltered rocks (see Appendix D). The isocon method (Gresens, 1967; Grant, 1986) is another way to test for element mobility during alteration. To this effect chemical compositions of rock pairs (altered-unaltered) are compared. Applica- tion of this approach to the diorite (Appendix D) demonstrates that concentrations of most elements, even Na and K, remained constant during fluid-rock interaction, while Ca, Ba, Rb, Sr and Pb show some mobility. Little element mobility is also supported by smooth and uni- form trace element patterns observed for this lithology, by alteration indices, and constant inter-element ratios. The measured Nd and Pb isotope ratios do not show any correlation with the degree of alteration (approximated by the volatile content) and are therefore considered to be representative of the igneous protolith. The Sr isotope ratio, how- ever, does exhibit a positive correlation with the LOI indicating inter- 87 86 Fig. 9. Nd evolution diagram for the diorite in drill core AT-14-01, with ɛNd action with crustal fluids; Sr/ Sr ranges from 0.710 even in the least plotted versus time; Depleted mantle isotope evolution line (DM) is after altered sample, to 0.734 in one of the most altered endmembers, and DePaolo (1988). Rb/Sr ratios have a large span from 0.03 to 4.5. Given the disturbance of the Rb-Sr system, Sr isotopes do not provide any meaningful petro- Earth evolution line according to Stacey and Kramers (1975) at genetic information and are omitted from further discussion. 207 204 The metavolcanic rocks of the drill core have undergone a different 1850 Ma. The modelled initial ratio is referred to as Pb/ Pb1850 and ranges from 15.14 to 15.22. type and degree of alteration than the Nipissing Gabbro. Phyllic al- teration and carbonatization is omnipresent and affected the con- centrations of Ba, K, Rb, Na, Ca, Sr, Fe, Mn, Mg, Pb, U, Th and probably 5. Discussion Ni and Cr, whereas the HFSE proved to be relatively immobile in most instances. Significant element mobility was noticed for brecciated and 5.1. Effects of post-depositional alteration chloritized portions of the metavolcanic rocks, for which the isocon method revealed total leaching of alkali metals, a mobile behaviour of The macro- and microscopic appearance of the rocks intersected in Al, Si and/or Ti, and a possible fractionation of HREE over LREE. This the drill core has revealed that effectively all lithotypes experienced alteration reflects a high fluid-rock ratio (e.g. Bau, 1991), has simila- hydrothermal alteration to some extent and regional metamorphic rities with submarine hydrothermal systems, likely VMS (e.g. Finlow- overprint at least at greenschist-facies conditions. Hydrothermal al- Bates and Stumpfl, 1981; Campbell et al., 1984; MacLean and teration is best reflected by the LOI, which ranges from 0.8 wt% in the Kranidiotis, 1987; MacLean, 1988) and culminates in an exhalative Nipissing Gabbro to 12 wt% in chloritized volcanic rocks. Thus, before pyrite layer with extensive quartz flooding. The strong degree of mi- discussing the protoliths’ geochemistry and isotopic composition in neralogical and geochemical changes is also demonstrated by various more detail, secondary element mobility needs to be evaluated. Cann alteration indices (Appendix D). (1970) for example pointed out that for a single differentiated intrusion such as the Nipissing Gabbro, immobile elements should be strongly 5.2. Equivalents of the Temagami Greenstone Belt correlated with the concentration of Zr (the least fluid-mobile element under various conditions). This approach reveals an immobile beha- Banded iron formation of the magnetite-chert-facies (oxide facies) is viour of almost all elements for the Nipissing Gabbro, except for Rb, Ba, the most important marker unit of drill core AT-14-01. The presence of

Table 2 Sm-Nd and Rb-Sr whole rock isotope data for diorite in drill core AT-14–01.

Sample No Assumed Sm (ppm)* Nd (ppm)* 143Nd/144Nd ±2s 147Sm/144Nd ɛNd ɛNd at TDM Rb (ppm)* Sr (ppm)* 87Sr/86Sr ±2s age t (Ma) Measured Calculated present 1850 Ma (Ma)** Measured day

AT-14 1850 5.90 30.50 0.511298 0.000013 0.117 −26.13 −7.19 2760 72 744 0.715811 0.000008 AT-15 1850 6.00 30.20 0.511276 0.000013 0.120 −26.56 −8.38 2897 82 732 0.716820 0.000008 AT-17 1850 6.10 31.40 0.511252 0.000012 0.117 −27.04 −8.22 2852 46 1019 0.711776 0.000008 AT-19 1850 6.10 32.10 0.511244 0.000017 0.115 −27.20 −7.77 2788 38 1155 0.710665 0.000010 AT-20 1850 6.10 31.90 0.511280 0.000009 0.115 −26.50 −7.24 2751 42 1203 0.710862 0.000009 AT-21 1850 6.00 29.80 0.511298 0.000011 0.122 −26.14 −8.34 2913 37 1126 0.710429 0.000010 AT-22 1850 5.80 29.40 0.511263 0.000011 0.119 −26.83 −8.44 2892 38 875 0.710872 0.000012 AT-24 1850 5.50 29.00 0.511255 0.000012 0.115 −26.97 −7.49 2763 127 514 0.726431 0.000011 AT-25 1850 5.50 29.80 0.511287 0.000014 0.111 −26.35 −6.13 2626 103 370 0.728671 0.000012 AT-36 1850 5.50 30.80 0.511282 0.000012 0.108 −26.46 −5.38 2541 149 491 0.732724 0.000011 AT-38 1850 5.70 29.80 0.511248 0.000017 0.115 −27.12 −7.87 2804 189 561 0.733651 0.000010 AT-39 1850 5.50 30.00 0.511254 0.000012 0.111 −26.99 −6.60 2657 136 462 0.731453 0.000009 AT-40 1850 5.50 29.70 0.511255 0.000013 0.112 −26.97 −6.85 2686 62 542 0.720708 0.000013 AT-41 1850 5.80 31.80 0.511223 0.000010 0.110 −27.61 −7.09 2691 49 527 0.719547 0.000012 AT-58 1850 5.70 29.90 0.511282 0.000010 0.115 −26.45 −7.11 2737 87 843 0.715774 0.000011 AT-59 1850 6.00 30.70 0.511305 0.000012 0.135 −18.73 −4.07 2654 39 1127 0.710667 0.000012 AT-64 1850 5.80 30.40 0.511276 0.000010 0.115 −26.56 −7.24 2748 139 423 0.733426 0.000012 Average 0.511296 0.116 −26.27 −7.14 2750 0.720017

* Sm, Nd, Rb and Sr concentrations were determined by ICP MS Nd ** one-stage Nd model ages (TDM ) were calculated by intersecting the quadratic depleted mantle curve of DePaolo (1988) with the linear εNd(t) evolution of the depleted mantle

230 A. Kawohl, et al. Precambrian Research 324 (2019) 220–235

Table 3 U-Th-Pb whole rock isotope data for diorite in drill core AT-14-01.

Sample No U (ppm)* Th (ppm)* Pb (ppm)* 208Pb/204Pb Measured ± 2s 207Pb/204Pb Measured ± 2s 206Pb/204Pb Measured ± 2s 207Pb/204Pb 1850**

AT-14 1.2 6.9 6.2 38.6961 0.0032 15.4805 0.0009 17.7970 0.0009 15.22 AT-15 1.2 6.8 11.0 38.8245 0.0027 15.4724 0.0009 17.7485 0.0008 15.21 AT-17 1.2 7.2 11.3 37.5967 0.0025 15.3429 0.0008 16.6996 0.0007 15.20 AT-19 1.2 7.1 9.7 37.7978 0.0025 15.3470 0.0008 16.7932 0.0007 15.20 AT-20 1.2 7.2 11.4 37.6443 0.0031 15.3652 0.0010 16.8109 0.0009 15.21 AT-21 1.2 7.0 4.8 37.6221 0.0025 15.3272 0.0008 16.7120 0.0007 15.19 AT-22 1.3 7.2 12.3 36.7148 0.0029 15.2713 0.0009 16.2197 0.0008 15.19 AT-24 1.2 7.0 11.2 36.1687 0.0029 15.2509 0.0010 16.0588 0.0008 15.18 AT-25 1.2 6.8 12.2 35.6667 0.0024 15.2173 0.0008 15.7691 0.0007 15.18 AT-36 1.2 6.5 15.4 41.1497 0.0026 15.5381 0.0008 18.8262 0.0008 15.16 AT-38 1.1 6.8 14.6 37.8948 0.0028 15.3335 0.0010 16.9720 0.0009 15.16 AT-39 1.1 6.5 14.1 38.3395 0.0026 15.3609 0.0009 17.2746 0.0008 15.16 AT-40 1.1 6.7 14.8 38.6533 0.0031 15.3785 0.0010 17.4073 0.0014 15.16 AT-41 1.1 6.7 14.6 40.2583 0.0028 15.5865 0.0009 19.3769 0.0009 15.14 AT-58 1.2 6.7 20.8 38.5325 0.0028 15.4418 0.0008 17.4739 0.0008 15.21 AT-59 1.2 7.1 27.9 37.9210 0.0025 15.3568 0.0008 16.8618 0.0008 15.20 AT-64 1.1 6.7 43.4 38.7832 0.0036 15.3845 0.0011 17.4853 0.0010 15.16 Average 38.1332 15.3739 17.1993 15.18

* U, Th and Pb concentrations were determined by ICP MS. ** Calculated by projecting the measured 207Pb/204Pb ratio back parallel to the 1850 Ma reference isochron (slope 0.113) to a set 206Pb/204Pb of 15.464, which is the corresponding value of the Stacey and Kramers (1975) evolution line at 1850 Ma.

BIF within the TA has already been predicted by geophysical models (Bennett, 1978). A further conspicuous unit in the studied drill core is (Card et al., 1984). On aeromagnetic maps (e.g. Fig. 2), the BIF of the black shale with nodular pyrite. An analogous rock has been described Temagami Greenstone Belt can be traced to the drill location, which only from the Temagami Greenstone Belt by Bennett (1978), Schwartz leaves little doubt that the intersected BIF is the same unit as in the (1995) and Bowins and Crocket (1994). Temagami and Emerald Lake greenstone belts. The BIF in the drill core The volcanic rocks of the studied drill core seem to represent lava dips steeply at angles of up to 80° towards the core axis, so does the BIF flows sandwiched between metasedimentary rocks (wacke, shale and of the Temagami Greenstone Belt on both limbs of the Temagami BIF) analogous to the Temagami and Emerald Lake greenstone belts. In Syncline. In addition, the true thicknesses of both formations are very both belts, intermediate to felsic volcanic rocks and pyroclastic similar (e.g. Bennett, 1978; Hurley, 1985). In the Emerald Lake equivalents are the most widespread rock types, and pophyric textures Greenstone Belt, the BIF is hosted by massive intermediate volcanic and prevail. The petrography of the porphyric volcanic rocks of the core pyroclastic deposits (Ayer et al., 2006), which is also the case for the closely resembles the description given by Bennett (1978), Hurley BIF in the drill core. The BIF of the core differs, in terms of mineralogy (1985) and Schwartz (1995) for the Temagami Greenstone Belt. Apart and texture, only little from its equivalent in the Temagami Greenstone from equivalent primary mineralogy and texture, the rocks underwent Belt. Chert breccias cemented with magnetite, as they were previously the same type of hydrothermal alteration as reported for the Temagami described by Kawohl et al. (2017), are found at both localities (Bennett, Greenstone Belt (cf. Colvine, 1974; Bennett, 1978; Hurley, 1985; Fyon 1978), and are more likely of sedimentary or tectonic rather than of and Crocket, 1986; Schwartz, 1995). Thus, although no comprehensive hydrothermal origin. Layers of Fe-silicate (chlorite, stilpnomelane) are geochemical data for the volcanic rocks of the Temagami Greenstone common at both localities, as well as abundant carbonate (e.g. Bennett, Belt are available, we suggest that the intersected volcanic rocks cor- 1978; Bowins and Crocket, 2010; Ginley, 2016). In contrast to the Te- relate with those of the Temagami and Emerald Lake greenstone belts. magami BIF, the BIF is almost free of haematite, goethite and jasper. Wacke was intersected in bore hole AT-14-01 – a lithology that is also exposed in the Temagami and Emerald Lake greenstone belts, where it 5.3. A new Offset Dyke of the Sudbury Complex is tens of meters thick and positioned below siltstone and shale The 25 and 52 m-thick diorite intersected below 2000 m stands out

Fig. 10. Plots of Pb isotope data for the diorite in drill core AT-14-01; 206Pb/204Pb versus a) 207Pb/204Pb, and b) 208Pb/204Pb; error bars are smaller than the symbol size. Two stage Pb isotope evolution curves are shown from 0 to 2 Ga (Stacey and Kramers 1975).

231 A. Kawohl, et al. Precambrian Research 324 (2019) 220–235 in terms of petrography, geochemistry and isotopic composition, and rocks (mainly Superior Province and derived sedimentary rocks) that can neither be correlated with the Nipissing Gabbro, nor with any other made up the impact melt (e.g. Petrus et al., 2016). The diorite of drill igneous rock of the Temagami Greenstone Belt and the Huronian Basin. core AT-14-01 shares the same isotopic features as the SIC samples, viz. 1850 It shows, however, striking petrographic, geochemical and isotopic si- highly un-radiogenic and negative ɛNd values from −4.1 to −8.4 milarities to the Offset Dykes of the SIC (Fig. 1a) as outlined below. (Fig. 12a), and model ages of, on average, 2.75 Ga, which is again in The mineralogy of the diorite is very similar to other (more or less very good agreement with existing data for the SIC. Although these 1850 metamorphosed) Offset Dykes, namely biotite, amphibole, epidote, values overlap with the ɛNd reported for some of the Huronian plagioclase, quartz, sericite, Fe-Ti-minerals (e.g. Grant and Bite, 1984; rocks, the Matachewan dykes and Nipissing Gabbro, (cf. Prevec et al., Lightfoot, 2016, and references therein). The only aspect in which the 2000, and Fig. 12a), the Huronian, Matachewan and Nipissing rocks diorite of the drill core differs from most other known Offset Dykes is have slightly different TDM,different petrography, very distinct major- the absence of granophyric intergrowths between quartz and feldspar. and trace element patterns, and their overlap with the diorite/SIC in 1850 Their absence, however, is likely due to the intense hydrothermal al- terms of ɛNd is not accompanied by an overlap in Pb isotopes (see teration and metamorphism experienced by the rock. Anhedral quartz below). grains within the diorite might be relics of such granophyric inter- Extensive Pb isotope studies were carried out by Dickin et al. (1996, growths. Based on their petrography, a correlation with the amphibole- 1999), Darling et al. (2010a,b), Darling et al. (2012) and McNamara biotite quartz dioritic Offset Dykes of the SIC is suggested. An inclusion- et al. (2017). The Pb isotope data obtained for the diorite in drill core bearing variety of the diorite, as it occurs in some of the SIC’sOffset AT-14-01 are consistent with the SIC data reported by the above au- Dykes, also seems to be present. Magmatic sulfides, however, were not thors in terms of both present and initial uranogenic Pb (Fig. 12b) and observed in the diorite of the core. They were either never present or present-day thorogenic Pb (Fig. 12c), suggesting that the diorite and the became completely altered and remobilized. SIC share a common source. As shown in Fig. 12b and c, the diorite and Strong evidence of the diorite in drill hole AT-14-01 being equiva- the SIC, generally spoken, occupy an intermediate position between the lent to the impact generated dykes of the SIC comes from its whole-rock less radiogenic Archaean Superior Province rocks (, ) geochemistry. As shown in Fig. 11, the trace element distribution in the and the more radiogenic Proterozoic Southern Province rocks (Hur- diorite is in very good agreement with published data for the SIC Offset onian Supergroup and associated intrusions). This is explicable by the Dykes (e.g. Lightfoot, 2016), especially when comparing K, the light SIC (and the diorite) representing a mixture of all rock types that were REE, Y, Zr, Hf, Nb and Ta. Similar high concentrations of Th and U present in the target area at the time of the impact (cf. Darling et al. occur in each of them, and, although modified by fluid-rock interaction, 2010a; Petrus et al. 2016). A hypothetical Pb-Pb “isochron age” of average concentrations of Ba and Rb for the diorite equal those of other 1780 + 320/−330 Ma (Fig. 10a) is outside of the uncertainty of the Offset Dykes. Minor discrepancies are only noted with regard to Ti and age of other mafic intrusive suites in the area but overlaps with the P, but these are explicable simply by variable amounts of accessory 1850 Ma Sudbury impact event. Furthermore, the Pb isotopic compo- minerals (magnetite, ilmenite, titanite, apatite). Differences in Sr con- sition of the studied diorite is significantly different from all other tents are likely due to Sr addition during carbonate alteration. Overall, magmatic units known in the wider region. The Archaean basement the diorite’s chemistry best matches that of the Foy and Parkin Offset rocks have lower, the Huronian volcanic rocks higher 207Pb/204Pb and Dykes (cf. Fig. 11, and Lightfoot, 2016), i.e. those closest to the drilling 208Pb/204Pb ratios, the Nipissing Gabbro higher 207Pb/204Pb; the Ma- location and hosted by Archaean country rock. tachewan dykes and the East Bull Lake Suite have similar Pb isotope

The SIC has an unusual geochemical and isotopic composition ratios but distinctly higher ɛNd (Fig. 12). among all igneous complexes worldwide. Faggart et al. (1985), for Evidently, the diorite of the studied core and the SIC were produced example, demonstrated a generally negative (very un-radiogenic) ɛNd from the same source material with the same Sm/Nd and U/Th/Pb (at 1850 Ma) for the SIC, ranging from −6.1 to −8.9. This circum- ratios at the same time. Otherwise the strong similarity with regard to stance is due to the unique origin of the Sudbury ‘Igneous’ Complex, Nd and Pb isotope ratios would be difficult to explain. The analogous that is, an asteroid impact by which the in the target major and trace element patterns in combination with Nd and Pb iso- area was completely molten, only to crystallize to the SIC, and without topes provide a convincing argument that the diorite is the product of the contribution of mantle-derived melts. Model ages for the SIC are an impact-related melt, here suggested to be a hitherto unrecognized therefore around 2.7 Ga (Faggart et al., 1985), i.e. the age of the target Offset Dyke to the SIC. Nevertheless, some differences exist between the

Fig. 11. Extended primitive mantle-normalized trace element diagram for the diorite in drill core AT-14-01 and other quartz dioritic Offset Dykes of the Sudbury Igneous Complex (see Fig. 1a for their location); Normalization values after Sun and McDonough (1989).

232 A. Kawohl, et al. Precambrian Research 324 (2019) 220–235

5.4. Thoughts on the origin of the Temagami Anomaly

As the original aim of the drilling program was to provide in- formation that might unveil the cause of the Temagami Anomaly, the new data available now invite to reassess the various hypothesis brought forward to explain this outstanding geophysical anomaly and to assess a potential relationship to the SIC. The BIF recovered in drill hole AT-14-01 has a total thickness of 500 m and matches the assumed portion of 18 vol% magnetite used in the model of Card et al. (1984). It surely contributes to the TA and explains why the TA is more magnetic in the eastern than in the western part. However, it does not differ in thickness, structural position and mineralogy from the BIF in the TGB, where it is a distinct linear near- surface feature on aeromagnetic maps superimposed on the broader TA (Card et al. 1984; Pilkington 1997; Alan King pers. comm.) and thus cannot solely explain the TA. The Nipissing Gabbro is an unlikely cause of the TA because it is, in general and in the studied drill core, relatively weakly magnetic. Although some sections exhibit higher susceptibility due to large magnetite grains (1–5 vol%), the Nipissing Gabbro in bore hole AT-14-01 is too thin (300 m) to be a major contributor to the TA. Similarly, the Huronian sedimentary rocks cannot explain the TA. Al- though locally, magnetite can be concentrated in thin layers within the Gowganda Formation, they are volumetrically by far not sufficient to explain the TA, not to mention the overall thin thickness (c. 300 m) of intersected core. The diorite in the studied drill core is magnetic but the intersected thickness is insufficient to explain the TA. Furthermore, Lightfoot (2016) pointed out that most Offset Dykes of the SIC are too narrow (< 100 m), weakly magnetic, and not much denser than their surroundings to cause large-scale gravity and magnetic anomalies. Nevertheless, if the intersected diorite is the product of an SIC-related impact melt as suggested here, the possibility arises that a larger magmatic body related to the impact could be present at depths below 2200 m, which could then explain the TA. It should be born in mind, however, that a deep-seated mafic intrusion unrelated to the SIC could also be hidden at such depths. A possible candidate could be a layered mafic complex related to the early Proterozoic East Bull Lake Suite. Members of this suite are exposed all around the SIC, and the largest of these – the River Valley Intrusion – occurs only 25 km south of the drilling location. These mafic intrusive bodies are not only thick (> 1000 m), voluminous and dense but also contain abundant titano- magnetite that might explain the strong and broad-scale gravity and magnetic anomalies (compare Figs. 1 and 2).

6. Conclusions

Although drilling at hole AT-14-01 failed to resolve the question of what the ultimate cause of the Temagami magnetic and gravimetric Fig. 12. Radiogenic isotope composition of the diorite in drill core AT-14-01 anomaly might be, it provided unexpected insights of high economic compared to the Sudbury Igneous Complex and other geological units of the potential into the extent of the Sudbury impact structure. Most of the area, displayed as whisker plots for a) ɛNd at 1850 Ma, b) the 207Pb/204Pb at intersected rock types can be linked to known geological units of the 1850 Ma and c) the present-day 208Pb/204Pb. Whole rock Nd isotope data are area, that is, Neoarchaean basement rocks of the Temagami Geenstone taken from the compilation of Prevec et al. (2000), McLennan et al. (2000) and Belt, Palaeoproterozoic sedimentary rocks of the Huronian Supergroup Ciborowski et al. (2015). Pb-isotope data on whole rock and feldspar separates and the regionally widespread Nipissing Suite. Two diorite bodies were compiled by McNamara et al. (2017). (dykes or sills) near the bottom of the 2200 m deep hole do not match, however, with any of the known igneous rock types in the area except studied diorite and other Offset Dykes. First, the diorite studied by us is for those found in SIC-related Offset Dykes. A correlation between these more strongly altered, reflected by abundance of secondary minerals diorite bodies and Offset Dykes around the SIC is indicated by pet- and its geochemistry, for instance anomalously high concentrations of rology, whole rock geochemistry and Nd as well as Pb isotopic com- Sr, Fe, and volatiles. This might be due to as described positions. With a distance of some 50 km this new occurrence of SIC- by Schandl et al. (1994) elsewhere in the area. Second, it occurs at a related diorite is almost twice as far from the margin of the SIC struc- deeper level (< 2000 m), and third, with a distance of some 50 km ture as the most distal Offset Dyke so far known. This requires a re- further away than any other Offset Dyke known so far. assessment of the dynamics of impact melt distribution. Irrespective whether the newly discovered most likely SIC-related diorite is the proverbial tip of an iceberg, that is, of a much larger magmatic body at greater depth that could explain the Temagami Anomaly, the mere presence of a rock type typically associated with Ni-Cu-PGE

233 A. Kawohl, et al. Precambrian Research 324 (2019) 220–235 mineralization around the SIC creates a hitherto unknown potential for (Eds.), The Geology and Ore Deposits of the Sudbury Structure. Ontario Geological possibly discovering new SIC-type ore deposits in the area of the Survey, Special Volume 1, pp. 25–43. Ciborowski, T.J.R., Kerr, A.C., Ernst, R.E., McDonald, I., Minifie, M.J., Harlan, S.S., Millar, Temagami Anomaly. I.L., 2015. The early Proterozoic Matachewan : geochemistry, The mechanism by which impact melt was transferred at the given petrogenesis and implications for Earth Evolution. J. Petrol. 56, 1459–1494. crustal level into country rock over such great distance is not yet fully Coles, R.L., Haines, G.V., Hannaford, W., 1981. Broadscale magnetic anomalies over ff central and eastern Canada: a discussion. Can. J. Earth Sci. 18, 657–661. understood. The very distal position of the newly discovered O set Colvine, A.C., 1974. The , geochemistry and genesis of sulphide-related al- Dyke could suggest a former multi-ring impact structure with the Hess teration at the Temagami Mine, Ontario. Unpublished MSc Thesis. Western and Manchester Offset Dykes (Fig. 1a) representing the proximal ring University, Ontario, Canada, pp. 225. and the Offset Dyke reported here a further distal ring. Clarification of Corfu, F., Andrews, A., 1986. A U-Pb age for mineralized Nipissing diabase, Gowganda, Ontario. Can. J. Earth Sci. 23, 107–112. this is topic of an ongoing research project involving (re-)mapping and Darling, J.R., Hawkesworth, C.J., Lightfoot, P.C., Storey, C.D., 2010a. Shallow impact: reassessment of previously reported magmatic rock types in the area. insights into crustal contributions to the Sudbury impact melt sheet. Geochim. – To unveil the mystery around the geological cause of the Temagami Cosmochim. Acta 74, 5680 5696. Darling, J.R., Hawkesworth, C.J., Lightfoot, P.C., Storey, C.D., Tremblay, E., 2010b. Anomaly, further drilling to greater depths will be required, though. Isotopic heterogeneity in the Sudbury impact melt sheet. Earth Planet. Sci. Lett. 289, 347–356. Acknowledgements Darling, J.R., Storey, C.D., Hawkesworth, C.J., Lightfoot, P.C., 2012. In situ Pb isotope analysis of Fe-Ni-Cu sulfides by laser ablation multi-collector ICPMS: New insights into ore formation in the Sudbury impact melt sheet. Geochim. Cosmochim. Acta 99, This paper represents part of an ongoing PhD project by A. Kawohl, 1–17. funded by Canadian Continental Exploration Corp. and Inventus Mining Davis, D.W., 2008. Sub-million- age resolution of Precambrian igneous events by thermal extraction–thermal ionization mass spectrometer Pb dating of zircon: Corp. We thank Winston and Wayne Whymark for their logistic support, Application to crystallization of the Sudbury impact melt sheet. Geology 36, company and guidance throughout the field visits, Joerg Kleinboeck, 383–386. who initially logged the core, Uli Schüssler (Univ. Würzburg) for XRF DePaolo, D.J., 1988. Neodymium Isotope Geology. Springer, Berlin, pp. 187. Dickin, A.P., Artan, M.A., Crocket, J.H., 1996. Isotopic evidence for distinct crustal analyses and Petrus Le Roux (Univ. Cape Town) for the isotope ana- sources of the North and South Range , Sudbury Igneous Complex. Geochim. lyses. Brief discussions with Alan King, Peter Lightfoot, Richard Ernst, Cosmochim. Acta 60, 1605–1613. Pierre-Simon Ross and Mark Hannington are gratefully acknowledged. Dickin, A., Nguyen, T., Crocket, J., 1999. Isotopic evidence for a single impact melting VD thanks Sabrina Cauchies for technical support, and the FRS-FNRS origin of the Sudbury Igneous Complex. In: Dressler, B.O., Sharpton, V. (Eds.), Large impacts and planetary evolution II: Geological Society of America, Special and ERC StG “ISoSyC” for financial support. 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