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Rimando, J.; Peace, A.; , . Geologic Background of Sixtymile Gold District, Yukon, Canada. Encyclopedia. Available online: https://encyclopedia.pub/entry/21420 (accessed on 05 August 2024).
Rimando J, Peace A,  . Geologic Background of Sixtymile Gold District, Yukon, Canada. Encyclopedia. Available at: https://encyclopedia.pub/entry/21420. Accessed August 05, 2024.
Rimando, Jeremy, Alexander Peace,  . "Geologic Background of Sixtymile Gold District, Yukon, Canada" Encyclopedia, https://encyclopedia.pub/entry/21420 (accessed August 05, 2024).
Rimando, J., Peace, A., & , . (2022, April 06). Geologic Background of Sixtymile Gold District, Yukon, Canada. In Encyclopedia. https://encyclopedia.pub/entry/21420
Rimando, Jeremy, et al. "Geologic Background of Sixtymile Gold District, Yukon, Canada." Encyclopedia. Web. 06 April, 2022.
Geologic Background of Sixtymile Gold District, Yukon, Canada
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This entry is adapted from the peer-reviewed paper 10.3390/min12030291

The Sixtymile gold district, Yukon, Canada has been mined for placer gold since the late 19th century. 

Sixtymile gold district Yukon Canada Gold

1. Introduction

The Sixtymile mining district in the Yukon Territory, Canada is located in the Sixtymile River watershed which is ~75 km west of Dawson City and ~10 km east of the border with Alaska, USA (Figure 1). Since the late 19th century, it has been known primarily for its placer gold deposits, and is proximal to the Klondike Mining district—the site of the famous Klondike Gold Rush. The main streams that were mined in the district are the Bedrock, Big Gold, Little Gold, Glacier, and Miller [1] (Figure 2). Combined, these creeks likely produced >400,000 ounces of gold from 1892–2005 [2]. Placer gold deposits in this mining district, and in the vicinity, occur both in low-level creek gravels (valley bottoms), and in low- and high-level terraces (resting on schist bedrock) that were mainly formed by aggradation and incision associated with phases of glaciation and deglaciation in the past ~40 ka [3][4].
Minerals 12 00291 g001 550
Figure 1. Regional structures in the Alaska-Yukon region (modified after Allan et al., 2013) [5]. Inset map shows the location of the Tintina Gold Belt (T.G.B.). The focused area (red box) is located close to the Alaska–Yukon border and lies within an area interpreted as a pull-apart basin resulting from left stepover on the left-lateral Sixtymile-Pika Fault.
Minerals 12 00291 g002 550
Figure 2. Local geology of the Sixtymile area which consists of a range of units which span the Mesozoic and Paleozoic (modified after maps by Mortensen, 1996, and the Yukon Geological Survey, 2020) [6][7]. Abbreviations of geological formations are from the Yukon Geological Survey (2020). Contour interval is 20 m.
The interpreted bedrock sources for gold in the region are Jurassic orogenic (mesothermal) gold deposits hosted in uplifted and exhumed greenschist facies rocks [8]. Prospectors have historically taken advantage of the proximity between exposed bedrock occurrences and placer deposits for their investigations on the location of orogenic veins [9][10].
However, almost 120 years since the Yukon placer gold rush, little is known about the structural setting of bedrock gold occurrence in the region [8][11][12]. A recent increase in demand for gold has prompted the exploration of new minable placer gold deposit sites and lode deposits [13]. Yet, there has only been small, incremental progress in bedrock mining due to the poor exposures, and consequently, insufficient knowledge of the bedrock geology [14]. In the main Klondike Gold district, placer gold has been suggested by geochemical signatures and microchemical characterizations to originate from a local bedrock source, and that these sources are associated with intrusions and/or hydrothermal/epithermal auriferous quartz veins within the underlying Klondike Schist [15][16][17].

2. Geologic Background

The Sixtymile mining district lies in the Tintina Gold Belt that spans more than 1000 km along the North American Cordillera (Figure 1). The Tintina Gold belt hosts diverse types of gold deposits (for example, orogenic, intrusion-related epithermal, porphyry, skarns and so on) that developed as a result of Late Jurassic to Late Cretaceous arc magmatism. The initiation of plutonism and fluid migration in the region supposedly correlates well with the onset of oblique subduction, and movement of the major right-lateral strike-slip Denali, Tintina, and associated fault systems [5][18][19]. The ~150 km-long Sixtymile-Pika Fault (Figure 1), a Late Cretaceous, NE–SW-striking, sinistral strike-slip fault system, with possibly up to ~30 km of displacement, is a major ‘orogen-perpendicular’ structure that runs through the Sixtymile mining district [5][6][20][21]. Previous regional analyses suggest this fault possibly exerted a structural control on Late Cretaceous localization of intrusions and volcanic deposits, as well as porphyry, skarn, and epithermal style deposits and occurrences in the region, which include high-grade gold (up to ~20 g/t) [22][23].
The geology of the Sixtymile district is characterized by a range of Mesozoic to Paleozoic units (Figure 2), that were originally documented in detail and given abbreviations, by MacKenzie et al. (2008) [8]. The area is underlain by a basement composed of stacked thrust slices including the Upper Devonian to Mississippian Finlayson Group (DMF1, 3 and 4), Mississippian Simpson Range Suite (MgSR), Middle to Late Permian Klondike Schist (PK2), and Carboniferous to Permian Slide Mountain Terrane (CPSM4), which are overlain by the Lower Cretaceous Indian River Formation (lKIR) and the Upper Cretaceous Carmacks Group (uKC1) [7][8].
The Upper Devonian to Mississippian Finlayson Group exposed in the area is composed of intermediate to mafic volcanic and volcaniclastic rocks, and mafic metamorphic rocks (DMF1); carbonaceous metasedimentary rocks, which includes the Nasina Schist (DMF3); and fine-grained siliciclastic and metavolcaniclastic rocks (DMF4) (Yukon Geological Survey, 2020). The Mississippian Simpson Range Suite (MgSR) consists of foliated to strongly foliated, fine to medium-grained granodiorite to tonalite rocks [7]. The Middle to Late Permian Klondike Schist includes felsic, intermediate and, mafic metavolcanics intercalated with metaclastic rocks. While the Klondike Schist in the area was mapped by the Yukon Geological Survey (2020) [7] as mostly muscovite and/or chlorite quartz phyllite and quartzites (PK2), greenschist facies chloritic, quartzofeldspathic, and micaceous schists, similar to those described by Mortensen (1990) [24] in the Klondike field, can also be found in the area. The Carboniferous to Permian Slide Mountain Terrane (CPSM4) is composed mostly of serpentinized ultramafic rocks [7]. The Lower Cretaceous Indian River Formation (lKIR) is a sedimentary sequence dominated by clast-supported, pebble-to-cobble conglomerate with clasts composed of vein quartz, foliated quartzite, sandstone, and tuff [25]. The lKIR is unconformably overlain by the Upper Cretaceous Carmacks Group (uKC1) is a volcanic succession of mostly mafic (for example, basalt), and to a lesser extent intermediate (for example, andesite), and locally felsic (for example, trachyte) layers, usually with a base that exhibits a downward transition from coarse clastic sedimentary rocks (for example, conglomerate) into an autochthonous basement breccia [26].
In the nearby Klondike district, Mackenzie et al. (2008) [8] proposed a structural evolution that consists of five deformation stages, which due to proximity, possibly also governs the structural setting of the Sixtymile mining district: an initial stage of compression in the Permian that resulted in foliation (D1); a second stage of compression, also in the Permian, causing pervasive foliation (D2), which involved parallel sub-parallel superimposition of remnants of D1 foliation to D2 foliation; a third stage of compression, which involved thrust stacking (D3) during the Jurassic (?); a fourth stage of compression, which involved fault-fold deformation (D4) also during the Jurassic (?); and finally, a stage of regional extension in the Cretaceous, involving normal faulting (D5), through reactivation of D4 faults.

References

  1. Mortensen, J.K.; Chapman, R.; LeBarge, W.; Jackson, L.; Edmund, D.S.; Lewis, L.L.; Bradshaw, G.D. Application of placer and lode gold geochemistry to gold exploration in western Yukon. In Yukon Exploration and Geology, 2004; Exploration and Geological Services Division, Yukon, Indian and Northern Affairs Canada: Whitehorse, YT, Canada, 2004; pp. 205–212. Available online: https://emrlibrary.gov.yk.ca/ygs/yeg/2004/2004_p205-212.pdf (accessed on 1 September 2021).
  2. Yukon Geological Survey. Yukon Digital Bedrock Geology. Available online: https://data.geology.gov.yk.ca/Compilation/3 (accessed on 12 September 2021).
  3. Hamilton, T.D. Late Cenozoic glaciation of Alaska. In The Geology of North America. The Geology of Alaska; Plafker, G., Berg, H.C., Eds.; Geological Society of America: Boulder, CO, USA, 1994; Volume G-I, pp. 813–844.
  4. Lowey, G.W.; Emond, D.S.; Weston, L.H. Glaciation, gravel and gold in the Fifty Mile Creek area, west-central Yukon. In Yukon Exploration and Geology 1999; Emond, D.S., Weston, L.H., Eds.; Exploration and Geological Services Division, Yukon, Indian and Northern Affairs Canada: Whitehorse, YT, Canada, 2000; pp. 199–209. Available online: https://emrlibrary.gov.yk.ca/ygs/yeg/1999/1999_p199-210.pdf (accessed on 18 February 2021).
  5. Allan, M.M.; Mortensen, J.K.; Hart, C.J.R.; Bailey, L.A.; Sanchez, M.G.; Ciolkiewicz, W.; McKenzie, G.; Creaser, R. Magmatic and metallogenic framework of west-central Yukon and eastern Alaska. In Tectonics, Metallogeny, and Discovery: The North American Cordillera and Similar Accretionary Settings; Society of Economic Geologists Special Publication: Littleton, CO, USA, 2013; Volume 17, pp. 111–168.
  6. MacKenzie, D.J.; Craw, D.; Mortensen, J. Structural controls on orogenic gold mineralisation in the Klondike goldfield, Canada. Mineralium Deposita 2008, 43, 435–448.
  7. Boyle, R.W. The Geochemistry of gold and its deposits. In Bulletin, Geological Survey of Canada; Energy, Mines and Resources Canada: Ottawa, ON, Canada, 1979; Volume 280, p. 584. Available online: https://publications.gc.ca/site/eng/9.817728/publication.html (accessed on 17 February 2021).
  8. Henley, R.W.; Adams, J. On the evolution of giant gold placers. In Transactions of the Institution for Mining and Metallurgy; Institution of Mining and Metallurgy: London, UK, 1979; Volume 88, pp. B41–B51.
  9. MacLean, T.A. Lode Mining in Yukon: Investigation of Quartz Deposits in the Klondike Division (No. 222); Government Printing Bureau: Ottawa, ON, Canada, 1914. Available online: https://emrlibrary.gov.yk.ca/gsc/separate-reports/222-lode-mining-in-yukon-an-investigation-of-quartz-deposits-in-the-klondike-division-1914.pdf (accessed on 17 February 2021).
  10. Rushton, R.W.; Nesbitt, B.E.; Muehlenbachs, K.; Mortensen, J.K. A fluid inclusion and stable isotope study of Au quartz veins in the Klondike District, Yukon Territory, Canada; a section through a mesothermal vein system. Econ. Geol. 1993, 88, 647–678.
  11. Freeman, S. In Search of the Motherlode: Why Yukon is Experiencing a 21st Century Gold Rush. Financial Post. Available online: https://financialpost.com/commodities/mining/in-search-of-the-motherlode-why-yukon-is-experiencing-a-new-gold-rush (accessed on 1 August 2021).
  12. Naumov, V.; Barge, W.; Kovin, O. A new insight into origin of the Yukon placer gold. In Proceedings of the 45th International October Conference on Mining and Metallurgy, Bor, Serbia, 16–19 October 2013; Volume 4, p. 4.
  13. Chapman, R.J.; Mortensen, J.K.; LeBarge, W.P. Styles of lode gold mineralization contributing to the placers of the Indian River and Black Hills Creek, Yukon Territory, Canada as deduced from microchemical characterization of placer gold grains. Mineralium Deposita 2011, 46, 881–903.
  14. Knight, J.B.; Mortensen, J.K.; Morison, S.R. Lode and placer gold composition in the Klondike District, Yukon Territory, Canada; implications for the nature and genesis of Klondike placer and lode gold deposits. Econ. Geol. 1999, 94, 649–664.
  15. Mortensen, J.K.; Chapman, R.; LeBarge, W.; Crawford, E. Compositional studies of placer and lode gold from western Yukon: Implications for lode sources. In Yukon Exploration and Geology 2005; Emond, D.S., Bradshaw, G.D., Lewis, L.L., Weston, L.S., Eds.; Yukon Geological Survey: Whitehorse, YT, Canada, 2005; pp. 247–255. Available online: https://emrlibrary.gov.yk.ca/ygs/yeg/2005/2005_p247-256.pdf (accessed on 1 September 2021).
  16. Goldfarb, R.J.; Hart, C.; Miller, M.; Miller, L.; Farmer, G.L.; Groves, D.I. The Tintina gold belt—A global perspective. In The Tintina Gold Belt: Concepts, Exploration, and Discoveries; British Columbia and Yukon Chamber of Mines: Vancouver, BC, Canada, 2000; Volume 2, pp. 5–34.
  17. Bailey, L.A. Late Jurassic Fault-Hosted Gold Mineralization at the Golden Saddle Deposit, White Gold District, Yukon Territory. Master’s Thesis, The University of British Columbia, Vancouver, BC, Canada, 2013. Available online: http://hdl.handle.net/2429/44517 (accessed on 17 February 2021).
  18. Sánchez, M.G.; Allan, M.M.; Hart, C.J.; Mortensen, J.K. Extracting ore-deposit-controlling structures from aeromagnetic, gravimetric, topographic, and regional geologic data in western Yukon and eastern Alaska. Interpretation 2014, 2, SJ75–SJ102.
  19. Naibert, T.J. Structural geology observations in the northeast Tanacross map area. In Northeast Tanacross Geologic Mapping Project, Alaska; Wypych, A., Ed.; Alaska Division of Geological & Geophysical Surveys: Fairbanks, AK, USA, 2020; pp. 43–52.
  20. Sánchez, M.G.; Allan, M.M.; Hart, C.J.; Mortensen, J. Orogen-perpendicular magnetic segmentation of the western Yukon and eastern Alaska cordilleran hinterland: Implications for structural control of mineralization. In Yukon Exploration and Geology 2012; MacFarlane, K.E., Nordling, M.G., Sack, P.J., Eds.; Yukon Geological Survey: Whitehorse, YT, Canada, 2013; pp. 133–146. Available online: https://ygsftp.gov.yk.ca/publications/yeg/yeg12/YEG/9_Sanchez_etal.pdf (accessed on 17 February 2021).
  21. Hayward, N. Geophysical investigation and reconstruction of lithospheric structure and its control on geology, structure, and mineralization in the Cordillera of northern Canada and eastern Alaska. Tectonics 2015, 34, 2165–2189.
  22. Allan, M.M.; Mortensen, J.K.; Sánchez, M.G.; Hart, C.J.R. Structural and geochronological controls on Late Cretaceous porphyry andepithermal mineralization of the orogen-normal Sixtymile-Pika fault system, Yukon and Alaska. In preparation.
  23. Yukon Geological Survey. Yukon Digital Bedrock Geology. Available online: https://data.geology.gov.yk.ca/Compilation/3 (accessed on 12 September 2021).
  24. Mortensen, J.K. Geology and U–Pb geochronology of the Klondike District, west-central Yukon Territory. Can. J. Earth Sci. 1990, 27, 903–914.
  25. Lowey, G.W.; Hills, L.V. Lithofacies, petrography and environments of deposition, Tantalus Formation (Lower Cretaceous) Indian River area, west-central Yukon. Bull. Can. Petrol. Geol. 1988, 36, 296–310.
  26. Johnston, S.T.; Wynne, P.J.; Francis, D.; Hart, C.J.R.; Enkin, R.J.; Engebretson, D.C. Yellowstone in Yukon: The late Cretaceous Carmacks group. Geology 1996, 24, 997–1000.
  27. Mortensen, J.K.; Chapman, R.; LeBarge, W.; Jackson, L.; Edmund, D.S.; Lewis, L.L.; Bradshaw, G.D. Application of placer and lode gold geochemistry to gold exploration in western Yukon. In Yukon Exploration and Geology, 2004; Exploration and Geological Services Division, Yukon, Indian and Northern Affairs Canada: Whitehorse, YT, Canada, 2004; pp. 205–212. Available online: https://emrlibrary.gov.yk.ca/ygs/yeg/2004/2004_p205-212.pdf (accessed on 1 September 2021).
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