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Introduction In northeastern mainland Nova Scotia, a long-lived fault system (the Minas Fault Zone; MFZ) delineates a major structural feature that hosts many mineral occurrences classified as iron oxide-copper-gold (IOCG) deposits. Iron oxide-copper-gold deposits are structurally controlled magmatic-hydrothermal systems with economic concentrations of Cu and Au. They contain Fe-oxides and/or Fe-silicates, minor pyrite, and are typically coeval to, but not necessarily spatially related to, plutonic rocks (Groves et al., 2010; Ridley, 2013). An example of IOCG mineralization along the MFZ is the College Grant deposit in Antigonish County (Fig. 1). The deposit offers an opportunity to examine the spatiotemporal relationships of fault movement, deformation, metamorphism, plutonic activity, hydrothermal fluid flow, and IOCG mineralization.

Geological Context College Grant is located approximately 30 km south of Antigonish, Nova Scotia (Fig. 1). Most of the area is underlain by calcareous siltstone, mudstone, and shale of the Silurian Arisaig Group (Benson, 1974) that was intruded by gabbro (White, 2018). To the south of College Grant is the Chedabucto Fault (Fig. 1), which is part of the MFZ. Undeformed clastic sedimentary rocks of the Devonian-Carboniferous St. Marys Basin crop out to the south of the MFZ (Murphy and Rice, 1998), but IOCG mineralization is not recognized in this basin. In the late 1800s, copper mineralization was discovered at the College Grant locality, but the old adits have since been filled (Fig. 2a). Disseminated chalcopyrite, pyrite, sphalerite, and minor bornite are hosted in breccia and quartz-carbonate veins in both the Arisaig Group and the College Grant

pluton (Fig. 2b-f). Locally the Arisaig Group primarily comprises fine-grained clastic sedimentary rocks and minor limestone beds assigned to the Lower Devonian Knoydart Formation (Benson, 1974; Boucot et al., 1974). All rocks experienced greenschist facies metamorphism, but the timing is unknown. At the College Grant deposit, bulk grab and core samples have copper concentrations of ~ 1-3 wt % (Doiron, 1985; Mazerolle, 2009). Traditionally considered a porphyry system (Doiron, 1985), more recent studies indicate that the deposit is an IOCG deposit-type possibly related to other IOCG systems located along the MFZ. Examples along the MFZ include Copper Lake (Kontak et al., 2008; MacHattie and O'Reilly, 2009a; MacHattie and O’Reilly, 2009b), Polsons Brook, Erinville, and other smaller occurrences (O'Reilly et al., 2016). Only the Copper Lake deposit was studied in detail. It is a vein-controlled Cu-Au-Co system associated with carbonate and iron-oxide alteration (Kontak et al., 2008). Re-Os and 40Ar/39Ar data yielded ages of ca. 320 Ma (Kontak et al., 2008), indicating IOCG mineralization occurred during the later stages of movement along the MFZ (Waldron et al., 2015).

The College Grant Pluton So far our work has focused on the College Grant pluton, which is a porphyritic, dioritic to gabbroic intrusion consisting of mainly plagioclase and altered ferromagnesian minerals (Doiron, 1985; Kontak, 1976; MacKinnon, 2018). Thick till in the area limited our field sampling; therefore, most samples are from archived drill core (Fig. 1). The pluton is undated but is inferred to be Devonian-Carboniferous age based on contact relationships with the Knoydart Formation (Benson, 1974; White et al., 2011). Our preliminary assessment of new lithogeochemical data indicates the gabbro has flat rare-earth element (REE) slopes and only minor

Freeborne, S., and Archibald, D. B. 2020. in Geoscience and Mines Branch, Report of Activities 2019-2020; Nova Scotia Department of Energy and Mines, Report ME 2020-002, p. 25-29.

The College Grant Cu-Fe-Au Deposit, Antigonish County, Nova Scotia

1

S. Freeborne2 and D. B. Archibald2

1This project was supported by a grant from the Nova Scotia Mineral Resources Development Fund (2019-2020) 2Department of Earth Sciences, St. Francis Xavier University, Antigonish, NS, B2G 2W5

26 Geoscience and Mines Branch

enrichment in light REEs relative to heavy REEs (Fig. 3a). The trace element characteristics are comparable to an enriched mid-ocean ridge basalt composition (Fig. 3b-d). Field relationships indicate that it is unlikely this pluton is related to the mineralization because the mineralization is

hosted in fractures that crosscut the pluton (Fig. 2c, d). However, this does not preclude the possibility that fluids derived from deeper molten sources overprinted shallower, solidified parts of the pluton. Additional data including mineral chemistry, radiogenic isotopes, and geochronological data will

Figure 1. Simplified geological map of the West Lochaber area showing mineral occurrences and drillhole locations modified after White (2018), O'Neill and Poole (2016), and O'Reilly et al. (2016). Rock outcrop are scarce in the study area. Samples are mainly grab samples from an old adit (SF19-CG-001-005) and samples of archived drill core (red crosses). Abbreviations: NSDEM, Nova Scotia Department of Energy and Mines.

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further constrain the petrogenesis of the pluton and spatially related mineralization.

Future Work

This project is still in its infancy. Future work will include dating of the ore mineralization and dating

and characterization of the ore fluids, as well as the dating of metamorphism and fault movement to develop a tectonic model for the formation of the College Grant deposit. These data will elucidate the local geological environment and geological processes at the time of ore mineralization with the broader goal of understanding IOCG mineralization along the MFZ in Nova Scotia.

Figure 2. Selected photographs: (a) field photo of a filled mine adit at the College Grant deposit; (b) cut slab of the College Grant gabbro; (c) hand sample containing chalcopyrite and malachite hosted in a quartz-carbonate vein that intruded the gabbro; (d) photograph of a quartz vein containing chalcopyrite, pyrite, and specular hematite; (e) reflected light photomicrograph of a mineralized sample collected from the waste pile at the College Grant mine adit showing the textural relationships of the ore minerals; (f) photomicrograph of slab cut from the same sample as (e). Abbreviations in (e) and (f): cal, calcite; cpy, chalcopyrite; he, hematite; py, pyrite; and qtz, quartz.

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Acknowledgments A Nova Scotia Department of Energy and Mines Mineral Resources Development Fund grant to D.B.A. financially supported this work (MRDF-2019-RS-032). We thank J.J. Hanley for a thorough review of the report. S.M. Barr is acknowledged for providing thin sections of the College Grant pluton. M. O’Neill (NSDEM) is thanked for his assistance at the Stellarton Core Library. Scott Grant is thanked for sharing maps, data and his personal knowledge of College Grant.

References Benson, D.G., 1974. Geology of the Antigonish Highlands, Nova Scotia; Geological Survey of Canada, Memoir 376, 92 p.

Boucot, A.J., Dewey, J.F., Dineley, D.L., Fletcher, R., Fyson, W.K., Griffin, J.G., Hickox, C.F., McKerrow, W.S., and Ziegler, A.M., 1974. Geology of the Arisaig area, Antigonish County, Nova Scotia; Geological Society of America, Special Papers, v. 139, 191 p. Doiron, M.L., 1985. A petrographic study of the College Grant Copper Deposit, Antigonish County, Nova Scotia; BSc thesis, St. Francis Xavier University, Antigonish, Nova Scotia. 38 p. Groves, D.I., Bierlein, F.P., Meinert, L.D., and Hitzman, M. W., 2010. Iron oxide copper-gold (IOCG) deposits through Earth history: Implications for origin, lithospheric setting, and distinction from other epigenetic iron oxide deposits; Economic Geology, v. 105, no. 3, p. 641–654.

Figure 3. (a) Chondrite-normalized rare-earth element diagram; (b) E-MORB (enriched mid-ocean ridge basalt) normalized multi-element spider diagram. Normalizing values for (a) and (b) are from Sun and McDonough (1989). (c) AFM (Na2O+K2O-FeOT-MgO) diagram (Irvine and Baragar, 1971); (d) Hf-Th-Ta classification diagram for mafic-intermediate rocks (Wood, 1980).

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Irvine, T.N. and Baragar, W.R.A., 1971. A guide to the chemical classification of the common volcanic rocks; Canadian Journal of Earth Sciences, v. 8, no. 5, p. 523–548. Kontak, D.J., 1976. College Grant copper mines and its geologic significance; BSc thesis, St. Francis Xavier University, Antigonish, Nova Scotia, 43 p. Kontak, D.J., Archibald, D.A., Creaser, R.A., and Heaman, L.M., 2008. Dating hydrothermal alteration and IOCG mineralization along a terrane-bounding fault zone: the Copper Lake deposit, Nova Scotia; Atlantic Geology, v. 44, p. 146–166. MacHattie, T.G., and O'Reilly, G.A., 2009a. Field and geochemical evidence for contemporaneous mafic magmatism and iron-oxide-copper-gold (IOCG) mineralization and alteration along the Cobequid-Chedabucto Fault Zone; in Mineral Resources Branch, Report of Activities 2008; Nova Scotia Department of Natural Resources, Report ME 2009-1, p. 71–83. MacHattie, T.G., and O’Reilly, G.A., 2009b. Timing of iron oxide-copper-gold (IOCG) mineralization and alteration along the Cobequid Chedabucto Fault Zone; in Mineral Resources Branch, Report of Activities 2008; Nova Scotia Department of Natural Resources, Report ME 2009-1,, p. 63–69. MacKinnon, P., 2018. 2016 College Grant Prospect Guysborough County. Nova Scotia; Department of Energy and Mines, Assessment Report ME 1029913, 22 p. Mazerolle, G., 2009. Report of work for Lochaber Lake Claim Block, Antigonish Couty, Nova Scotia; Nova Scotia Department of Natural Resources, Assessment Report ME 2009-055, 61 p. Murphy, J.B., and Rice, R.J., 1998. Stratigraphy and depositional environment of the Horton Group in the St Marys Basin, central mainland Nova Scotia; Atlantic Geology, v. 34, no. 1, p. 1–25.

O'Neill, M., and Poole, J.C., 2016. DP ME 003, version 5, Nova Scotia drillhole database; Nova Scotia Department of Natural Resources, Digital Product ME 003, (version 5). O'Reilly, G.A., DeMont, G.J., Fisher, B.E., and Poole, J.C., 2016. DP ME 002, version 11, Nova Scotia mineral occurrence database; Nova Scotia Department of Natural Resources, Digital Product ME 002, (version 11). Ridley, J., 2013. Ore deposit geology, Cambridge University Press, Cambridge, 398 p. Sun, S.-S. and McDonough, W.F., 1989. Chemical and isotopic systematics of oceanic basalts: implications for mantle composition and processes; Geological Society, London, Special Publications, v. 42, no. 1, p. 313–345. Waldron, J.W.F., Barr, S.M., Park, A.F., White, C.E., and Hibbard, J., 2015. Late Paleozoic strike-slip faults in Maritime Canada and their role in the reconfiguration of the northern Appalachian orogen; Tectonics, v. 34, no. 8, p. 1661–1684. White, C.E., 2018. Bedrock geology map of the Antigonish Highlands area, Antigonish and Pictou counties, Nova Scotia; Nova Scotia Department of Natural Resources, Geosciences and Mines Branch, Open File Map ME 2018-001, scale 1:75 000. White, C.E., Archibald, D.B., MacHattie, T.G., and Escarraga, E.A., 2011. Preliminary geology of the southern Antigonish Highlands, northern mainland Nova Scotia; in Mineral Resources Branch, Report of Activities 2010; Nova Scotia Department of Natural Resources, Report ME 2011-1, p. 145–164. Wood, D.A., 1980. The application of a Th-Hf-Ta diagram to problems of tectonomagmatic classification and to establishing the nature of crustal contamination of basaltic lavas of the British Tertiary Volcanic Province; Earth and Planetary Science Letters, v. 50, no. 1, p. 11-30.