38. Preliminary Results from Detailed Geological Mapping

Summary of Field Work and Other Activities 2017, Ontario Geological Survey, Open File Report 6333, p.38-1 to 38-8. © Queen’s Printer for Ontario, 2017

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38. Preliminary Results from Detailed Geological Mapping and Core Logging of Volcanic and Sedimentary Rocks in the Footprint of the Kerr–Addison Mine, Virginiatown, Ontario

N. St-Jean1, J. Blackwell2, R.L. Sherlock1 and B. Lafrance1

1Metal Earth, Mineral Exploration Research Centre, Laurentian University, Sudbury, Ontario P3E 2C6 2Long Point Geologic Ltd., Kamloops, British Columbia V2C 1N4

INTRODUCTION This paper summarizes the preliminary results of the first summer of field work related to the senior

author’s MSc thesis. Detailed geological and structural mapping was undertaken in select locations in Virginiatown and on the Kerr–Addison Mine property, located in northeastern Ontario, approximately 35 km east of Kirkland Lake, along Highway 66. The study area lies within the southern Abitibi Subprovince of the Superior Province, along the Larder–Cadillac deformation zone (LCDZ; Figure 38.1). This work is part of the multiyear Metal Earth project carried out by MERC (Mineral Exploration Research Centre, Laurentian University, Sudbury).

The Larder Lake group (equivalent to the Piché Group in Quebec) consists of complex intercalations of ultramafic and mafic volcanic rocks, and associated sedimentary rocks, and is the main host of gold deposits along this segment of the LCDZ, including the world-class Kerr–Addison–Chesterville deposit (~11 million ounces at 9 g/t Au production; Smith et al. 1990). At the Kerr–Addison Mine, a thick package of Larder Lake group is preserved, making it an ideal location to study the stratigraphic and structural framework of the rocks, which was the focus of the 2017 field work. The senior author also conducted detailed core logging of 2 sections of drill core, from holes recently completed by Gold Candle Ltd that intersected the Larder Lake group stratigraphy. Additionally, a new highway bypass in Virginiatown provided exposures of the contact between the Timiskaming sedimentary rocks and the Larder Lake group; this contact is considered to be the LCDZ. Detailed mapping and sampling along the new bypass was completed in order to characterize the nature of this contact.

The main objectives of this study are to 1) characterize the lithostratigraphy of the Larder Lake group volcanic and associated sedimentary rocks; 2) identify and characterize key structural relationships between units within the Larder Lake group and place the gold mineralizing event within this context; and 3) determine the nature of the contact between the Larder Lake and Timiskaming sedimentary rocks and place it in a regional context.

REGIONAL GEOLOGY The Abitibi greenstone belt is predominantly composed of felsic to mafic metavolcanic rocks with

localized metasedimentary packages that are separated from the volcanic rocks by major unconformities. Geochronological data from previous studies identified 6 major mafic to felsic volcanic successions, from oldest to youngest: Pacaud (2750–2735 Ma), Deloro (2730–2724 Ma), Stoughton–Roquemaure (2723–

Metal Earth (38) N. St-Jean et al.

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2720 Ma), Kidd–Munro (2719–2711 Ma), Tisdale (2710–2704 Ma), and Blake River (2704–2696 Ma) (Ayer et al. 2005). A series of calc-alkalic to alkalic intrusions accompanied the deposition of the Timiskaming metasediments, with ages ranging from 2681 to 2672 Ma (Frarey and Krogh 1986; Corfu et al. 1989; Corfu and Noble 1992; Ayer et al. 2005). The Timiskaming sediments are of particular importance because of their spatial association with major structural boundaries. Along the LCDZ, the lithological break juxtaposes younger Timiskaming sedimentary rocks with underlying older metavolcanic units. This break is associated with intense carbonatization of host rocks and has a strong spatial association with gold deposits (Thomson 1941a, 1941b, 1943).

LOCAL GEOLOGY The study area is located in the south-central portion of McGarry Township, between the

communities of Virginiatown and Kearns. Directly north of Highway 66 are marine-facies Timiskaming sedimentary rocks and associated interlayered Timiskaming volcanic rocks, which lie unconformably above the Blake River Group mafic volcanic rocks (Thomson 1941a, 1941b, 1943). The Timiskaming assemblage in this area yields an age of 2674 Ma, calculated from a greywacke sampled by Ayer et al. (2005). These sediments are undeniably Timiskaming in age; however, there remains substantial confusion related to the age of the metasedimentary packages south of the LCDZ, and the timing of deposition of these units remains unresolved. Following the initial classification of these units as Timiskaming by Hewitt (1949), Jensen (1985) mapped them as part of the Larder Lake group. However, zircons from samples of a sandstone from the Martin Bird property, within Hearst Township, yielded a maximum age of 2679 Ma (Corfu, Jackson and Sutcliffe 1991). Therefore, at least some of these metasedimentary packages are contemporaneous with the Timiskaming assemblage.

Figure 38.1. Generalized geology of the Abitibi Subprovince, showing the location of major base and precious metal mineral deposits (modified after Poulsen, Robert and Dubé 2000).


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South of Highway 66, in Virginiatown, are the Larder Lake group ultramafic and mafic volcanic rocks, which are overlain to the southeast by Huronian Supergroup sedimentary rocks (Figure 38.2). The Larder Lake group is characterized by interlayered mafic and ultramafic komatiitic rocks with minor interflow sedimentary intervals. The Larder Lake group has an age of circa 2705 Ma (Corfu et al. 1989) and is considered to be part of the Tisdale assemblage (Ayer et al. 2005) based on age relationships and regional correlations. It is correlated with the Piché Group volcanic rocks in Quebec (Smith et al. 1990). Recent geochemical work indicates compositions of predominantly iron-tholeiitic basalt for the mafic volcanic units and komatiitic to komatiitic basalt for the ultramafic rocks (Lafrance 2015).

The contact between the Timiskaming sedimentary rocks and the Larder Lake group lies under Highway 66. It has recently been exposed in 2 locations where the new highway bypass is being built, and has been intersected in a number of new exploration holes drilled in 2017. Mapping for the present study focussed on the newly exposed outcrops of the contact and of tightly folded Timiskaming sedimentary rocks, which showed an increase in strain and alteration proximal to the LCDZ. Core logging also focussed on the contact between the 2 assemblages, as well as the overall structure and stratigraphy within the Larder Lake group.

Figure 38.2. Geology and location of the study area (modified from Thomson 1941a, 1941b), showing the location of the new highway bypass. Universal Transverse Mercator co-ordinates provided using North American Datum 1983 (NAD83) in Zone 17N.


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FIELD WORK AND OBSERVATIONS

Lithological Units Preliminary results from detailed mapping and core logging has revealed limited variation in the

ultramafic units of the Larder Lake group, which are relatively homogeneous komatiitic flows. Coarse-grained, well-preserved spinifex textures are present along a specific interval and may represent flows. Otherwise, the ultramafic units generally lack primary textures and have been overprinted by intense alteration and deformation.

Alteration of the ultramafic units is a useful vector toward mineralization. Gold mineralization is generally restricted to a zone of potassium alteration reflected by an assemblage of fuchsite, quartz, magnesite (green carbonate) to magnesian-chlorite, quartz, ferroan dolomite and magnesite (Lafrance 2015). Locally, the potassic alteration overprints the iron carbonate alteration. Distal talc-chlorite alteration assemblages are typically associated with barren ultramafic units. These units appear to have undergone higher degrees of strain compared to the green carbonate alteration assemblage. The detailed paragenesis of the alteration as it relates to deformation is still unknown.

The mafic volcanic units vary more in texture and facies, and include variolitic, porphyritic and massive flows, as well as hyaloclastites and interflow mafic sediments. Interflow sediments typically have a graphitic character. An unusual immature sedimentary unit within the mafic package is characterized by a fine-grained mafic matrix with minor rounded to subrounded quartz-vein fragments and polymict rounded to subrounded mafic and granitoid clasts (Photo 38.1A). This unit was previously mapped by Thomson (1941a, 1941b) as a “basic dyke containing boulders”. It is potentially a debris flow within the Larder Lake group and will be the focus of future work to determine its composition and potential for geochronological analysis. Pyrite and arsenopyrite mineralization within the mafic units is typically associated with quartz, ankerite alteration and localized albite alteration.

A series of felsic and mafic dikes intrude the entire package. Early mineralized mafic “albitite” dikes have been previously studied and characterized by Hamilton (1986) and Smith et al. (1990) and will not be extensively studied in this project. However, feldspar porphyry and quartz porphyry dikes that intrude the contact between the Timiskaming sediments and the Larder Lake group will be candidates for geochronological analysis.

Structural Geology A common texture recognized in the ultramafic units is a pseudo-breccia texture (Photo 38.1B).

This texture is interpreted as having been produced by volume loss during serpentinization. However, preliminary results indicate a more complex paragenesis, with potential hydrothermal origins. Samples were collected during the field season to identify the different stages of brecciation and associated alteration.

The ultramafic “breccia” is one of the earlier metasomatic or hydrothermal events, occurring during a stage of brittle deformation. It was subsequently overprinted by at least 2 stages of foliation development and, locally, completely overprinted by a penetrative fabric (Photo 38.1C). Zones of high strain that cross the ultramafic units have resulted in the complete flattening of the pseudo-breccia texture and development of a schistosity, resulting in finely laminated ultramafic schists. These zones of high strain are mappable, and preliminary results have indicated that these high-strain zones are symmetrically distributed around the lower-strain core of the Kerr–Addison deposit.


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Photo 38.1. A) Debris flow containing polymict rounded granitoid and mafic clasts with sandy matrix, within the mafic package of the Larder Lake group. B) Ultramafic pseudo-breccia texture, showing low strain. C) Highly strained ultramafic pseudo-breccia. D) Contact between Timiskaming sedimentary rocks and Larder Lake group ultramafic rocks with 2 m wide transition zone. Location: 604824E 5332042N (UTM co-ordinates are in NAD83, Zone 17N). Hammer, for scale, is 40 cm long. E) Ultramafic phyllonite with rotated quartz-carbonate boudins. F) Shallowly dipping S2 foliation crenulated with steeply dipping, axial planar S3 foliation. Compass, for scale, indicates north. Diameter of drill core in Photos 38.1A, 38.1B, 38.1C and 38.1E is HQ.


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A series of late brittle faults, such as the Kerr fault, cross the Kerr–Addison property. Earlier brittle-ductile faults and associated silicified cataclasite also occur as discrete zones in the footwall to the ore zone. Locally, foliations wrap around or are deflected by these faults. One of the objectives of this study will be to characterize the nature and timing of brittle and brittle-ductile faults and their relationship to mineralization or potential unit offset.

Initial observations from mapping noted the presence of 2 dominant foliations in the study area: a shallowly dipping fuchsite-chlorite cleavage foliation (Photo 38.1F), interpreted to be S2; and a steeply dipping S3 characterized by a fuchsite-chlorite cleavage foliation and associated with dextral shear indicators. The S3 foliation crosscuts and locally offsets S2 foliation. Both foliations overprint ultramafic pseudo-breccia textures. These events are broadly correlated to D2 and D3 compressional events (Wilkinson, Cruden and Krogh 1999); however, this interpretation will be further refined.

Larder–Cadillac Deformation Zone A notable feature, which was consistently mapped and logged in drill core on the western side of the

Kerr–Addison property, is the presence of a transition zone between the Timiskaming sandstone-mudstones and ultramafic rocks of the Larder Lake group. This zone is 2 to 4 m thick and is characterized by a mixed zone of ultramafic lenses and quartz-feldspathic-matrix clastic sedimentary rocks (Photo 38.1D). The varying degrees of strain have overprinted all primary textures and the challenge lies in determining whether this contact is a result of structural interleaving of 2 units or whether this was a depositional contact that was later overprinted by ductile deformation. Detailed sampling was conducted along this contact in both drill core and outcrop, to attempt to characterize the transition zone and shed light on the nature of the contact. In other locations, particularly east of the ore zone towards Kearns, the transition zone is absent and the contact between the Timiskaming sedimentary rocks and Larder Lake group is faulted or intruded by dikes.

Within the footwall of the contact between the Timiskaming rocks and the Larder Lake group, approximately 10 to 11 m south of the contact as exposed in a roadcut, is a zone of strong deformation that consists of a talc-chlorite-iron-carbonate assemblage with abundant rotated boudinaged quartz-carbonate veins (Photo 38.1E), termed “phyllonite” (K.H. Poulsen, personal communication, 2017). This unit or structure is traceable across the entire study area, in outcrop and drill core, and appears to maintain its position in the footwall of the contact. Its structural role and timing have yet to be determined, but what is notable is its position well within the footwall of the Timiskaming–Larder Lake contact.

FUTURE WORK A compilation of all mapping and core logging data will be completed during 2017–2018, with the

goal of correlating surface and subsurface structural and lithological data within the Larder Lake stratigraphy. An existing company database of multi-element geochemical data will be used to assist in the characterization of stratigraphic units, alongside samples collected for thin section.

Oriented samples were collected across the entire Timiskaming–Larder Lake contact and will be analyzed for multi-element geochemistry to complement a detailed thin section study. The first objective will be to characterize the nature of the contact, the alteration and the structural geology, on a microscopic as well as macroscopic scale.


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RELEVANCE In Virginiatown, the LCDZ represents a major stratigraphic break and hosts one of the largest gold

deposits found to date along this deformation zone. Despite being historically mapped as the northern contact between the younger Timiskaming sedimentary rocks and the older Larder Lake group, and recognized as a major structural boundary, a number of key questions related to the nature of the structure and contact remain. The geological relationship between these 2 assemblages will be critical to understanding how the LCDZ formed, as well as the sequential development of the structural geology within each assemblage.

ACKNOWLEDGMENTS The senior author would like to thank Leslie Hunt for her assistance throughout the 2017 field

season. The author is grateful to the team at Gold Candle Ltd. for facilitating this study and for geological discussions. This is MERC-Metal Earth publication number MERC-ME-2017-004.

REFERENCES Ayer, J.A., Thurston, P.C., Bateman, R., Dubé, B., Gibson, H.L., Hamilton, M.A., Hathway, B., Hocker, S.M.,

Houlé, M.G., Hudak, G.J., Ispolatov, V.O., Lafrance, B., Lesher, C.M., Macdonald, P.J., Péloquin, A.S., Piercey, S.J., Reed, L.E. and Thompson, P.H. 2005. Overview of results from the Greenstone Architecture Project: Discover Abitibi Initiative; Ontario Geological Survey, Open File Report 6154, 125p.

Corfu, F., Jackson, S.L. and Sutcliffe, R.H. 1991. U–Pb ages and tectonic significance of late alkalic magmatism and nonmarine sedimentation: Timiskaming Group, southern Abitibi belt, Ontario; Canadian Journal of Earth Sciences, v.28, p.489-503.

Corfu, F., Krogh, T.E., Kwok, Y.Y. and Jensen, L.S. 1989. U-Pb zircon geochronology in the southwestern Abitibi greenstone belt, Superior Province; Canadian Journal of Earth Sciences, v.26, p.1747-1763.

Corfu, F. and Noble, S.R. 1992. Genesis of the southern Abitibi greenstone belt, Superior Province, Canada: Evidence from zircon Hf-isotope analyses using a single filament technique; Geochimica et Cosmochimica Acta, v.56, p.2081-2097.

Frarey, M.J. and Krogh, T.E. 1986. U-Pb zircon ages of late internal plutons of the Abitibi and eastern Wawa subprovinces, Ontario and Quebec; in Current Research Part A, Geological Survey of Canada, Paper 86-1A, p.43-48.

Hamilton, J.V. 1986. The structural and stratigraphic setting of gold mineralization in the vicinity of Larder Lake, south-central Abitibi greenstone belt, northeastern Ontario; MSc thesis, Queen’s University, Kingston, Ontario, 154p.

Hewitt, D.F. 1949. Township of Skead, District of Timiskaming, Ontario; Ontario Department of Mines, Annual Report Map 1949-3, scale 1:12 000.

——— 1951. Geology of Skead Township, Larder Lake area; Ontario Department of Mines, Annual Report 1949, v.58, pt.6, 43p.

Jensen, L.S. 1985. Stratigraphy and petrogenesis of Archean metavolcanic sequences, southwestern Abitibi Subprovince, Ontario; in Evolution of Archean supracrustal sequences, Geological Association of Canada, Special Paper 28, p.65-87.

Lafrance, B. 2015. Geology of the orogenic Cheminis gold deposit along the Larder Lake–Cadillac deformation zone, Ontario; Canadian Journal of Earth Sciences, v.52, p.1093-1108.


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Poulsen, K.H., Robert, F. and Dubé, B. 2000. Geological classification of Canadian gold deposits; Geological Survey of Canada, Bulletin 540, 106p.

Smith, J.P., Spooner, E.T.C, Broughton, D.W. and Ploeger, F.R. 1990. The Kerr Addison-Chesterville Archean gold-quartz vein system, Virginiatown: Time sequence and associated mafic “albitite” dike swarm; in Geoscience Research Grant Program, Summary of Research 1989–1990, Ontario Geological Survey, Miscellaneous Paper 150, p.175-199.

Thomson, J.E. 1941a. Township of McGarry, District of Timiskaming, Ontario; Ontario Department of Mines, Annual Report Map 50A, scale 1:12 000.

——— 1941b. Township of McVittie, District of Timiskaming, Ontario; Ontario Department of Mines, Annual Report Map 50B, scale 1:12 000.

——— 1943. Geology of McGarry and McVittie townships, Larder Lake area; Ontario Department of Mines, Annual Report 1941, v.50, pt.7, 99p.

Wilkinson, L., Cruden, A.R. and Krogh, T.E. 1999. Timing and kinematics of post-Timiskaming deformation within the Larder Lake–Cadillac deformation zone, southwest Abitibi greenstone belt, Ontario, Canada; Canadian Journal of Earth Sciences, v.36, p.627-647.

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St-Jean, N., Blackwell, J., Sherlock, R.L. and Lafrance, B. 2017. Preliminary results from detailed geological mapping and core logging of volcanic and sedimentary rocks in the footprint of the Kerr–Addison Mine, Virginiatown, Ontario; in Summary of Field Work and Other Activities, 2017, Ontario Geological Survey, Open File Report 6333, p.38-1 to 38-8.

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38. Preliminary Results from Detailed Geological Mapping