Paleogene Penticton Group, Boundary area, Southern British Columbia (Parts of NTS 082E): Geochronology and Implications for Precious Metal Mineralization T. Höy, Geological consultant, Sooke, British Columbia, [email protected] R. Friedman, Pacific Centre for Isotopic and Geochemical Research, The University of British Columbia, Vancouver, British Columbia J. Gabites, Pacific Centre for Isotopic and Geochemical Research, The University of British Columbia, Vancouver, British Columbia Höy, T., Friedman, R. and Gabites, J. (2021): Paleogene Penticton Group, Boundary area, southern British Columbia (parts of NTS 082E): geochronology and implications for precious metal mineralization; in Geoscience BC Summary of Activities 2020: Minerals, Geoscience BC, Report 2021-01, p. 55–66. Introduction rocks and minor sedimentary units of the Marron Forma- tion. Farther west in the White Lake basin area, the Marron Paleogene volcanic and sedimentary rocks of the Penticton Formation is unconformably overlain by basal conglomer- Group are exposed in numerous localities throughout ate, felsic lava and tuff of the Marama Formation, and southern British Columbia (BC), commonly preserved in interlayered sedimentary and volcanic rocks of the White north-trending structural basins. Regional mapping in the Lake and Skaha formations (Church, 1973). Boundary area in the eastern half of the Penticton map area (NTS 082E), in conjunction with Ar/Ar and U-Pb zircon Paleozoic basement rocks in the Boundary area have been dating, has helped to constrain the age of the Penticton studied extensively by Massey (2006). In the Greenwood Group, its relation to extensional tectonics and its control of map area (NTS 082E/02; Figure 1), they include poorly precious-metal mineralization. This project is a continua- dated metavolcanic and metasedimentary rocks, minor tion and enhancement of systematic 1:50 000 geological gabbro, serpentinite and locally paragneiss of the Carbonif- mapping that has resulted in the publication of six maps erous to Permian Knob Hill complex (Little, 1983; Church, (Figure 1): Grand Forks (NTS 082E/01), Greenwood (NTS 1986; Fyles, 1990; Massey, 2007). The Attwood Formation 082E/02), Almond Mountain (NTS 082E/07), Deer Park comprises mainly Mississippian to Permian metasediment- (NTS 082E/08), Burrell Creek (NTS 082E/09) and Chris- ary rocks; their relationship to the Knob Hill complex is not tian Valley (NTS 082E/10). The Boundary area has had an known as there are no recognized stratigraphic contacts be- extensive history of exploration and mining, particularly in tween the units. Farther north and to the east in the Grand the Greenwood, Franklin and Beaverdell mining camps, Forks map area (NTS 082E/01; Figure 1), Attwood Forma- where intermittent production of base and precious metals tion rocks are correlated with the lithologically similar continued from the late 1880s to the early 1990s. Explora- Wallace group or Mount Roberts Formation. tion continues to be active throughout the area, still largely focused on precious-metal targets. Middle to late Triassic chert breccia, limestone and volca- nic ‘greenstone’ of the Brooklyn Formation unconform- Geology ably overlie the Attwood Formation or Knob Hill Group in the Greenwood area (Little, 1983; Fyles, 1990). Volcanic The Penticton Group in the Boundary area of the Penticton rocks correlated with the Jurassic Rossland Group are in map area (NTS 082E) includes a basal succession of domi- thrust contact with serpentinites and gabbro, assumed to be nantly conglomerate, sandstone, reworked tuff and minor part of the Knob Hill complex, in the southern part of the volcanic flows of the Kettle River Formation that uncon- Grand Forks map area (Höy and Jackaman, 2005). formably overlies Paleozoic and Triassic metavolcanic rocks and large suites of intrusive rocks of varying ages A large part of the Penticton map area is underlain by gra- (Figure 2). The Kettle River Formation is conformably nitic and alkalic intrusive rocks that have variously been as- overlain by a thick sequence of dominantly alkalic volcanic signed to granodiorite of the ‘Nelson’ plutonic complex, granite of the Okanagan batholith or Coryell syenite (Tempelman-Kluit, 1989; Höy and Jackaman, 2019; Höy et This publication is also available, free of charge, as colour digital al., 2020). Locally, the Penticton Group unconformably files in Adobe Acrobat® PDF format from the Geoscience BC web- overlies Triassic to Jurassic Nelson intrusions and in the site: http://geosciencebc.com/updates/summary-of-activities/. Almond Mountain map area (NTS 082E/07; Figure 1), a Geoscience BC Report 2021-01 55 Figure 1. Regional terrane map of the Boundary area of southern British Columbia, showing location of major deposit camps, mineral oc- currences discussed in this paper, and samples (numbered 1–10) collected for Ar/Ar and U-Pb dating. Paleocene granite that has been informally named the (NTS 082E/08; Figure 1; Tempelman-Kluit, 1989; Höy and ‘Taurus Creek stock’(Höy and Jackaman, 2016). Dikes and Jackaman, 2019). The widespread distribution of the small stocks of the Paleocene to Eocene Okanagan bath- Penticton Group rocks and correlations are described by olith and Eocene Coryell syenite locally intrude the Church (1973), who inferred that they represented the ero- Penticton Group, which helps constrain the age of these sional remnants of a continuous belt of dominantly volca- rocks. nic rocks that extended across southern BC and the north- ern State of Washington. Alternatively, the distribution of The Kallis Formation (Figure 2) is preserved in isolated basal Penticton Group rocks, particularly the Marron For- topographic highs throughout the area, unconformably mation, may be considerably more limited, largely re- overlying either the Penticton Group or intrusive rocks. It stricted to the fault-bounded, north-trending grabens. represents the remains of widespread Miocene–Pliocene plateau basalts. Kettle River Formation Penticton Group The Penticton Group is exposed mainly within the north- The Kettle River Formation is the dominantly sedimentary trending Rock Creek graben, shown on the western part of succession that overlies basement rocks, extending upward the Penticton east-half map area, in the northern extension into a generally conformable contact with alkalic volcanic of the Republic graben in the Greenwood area and farther rocks of the Marron Formation. It has been described in north in the western part of the Deer Park map area considerable detail by Monger (1968), Little (1983) and 56 Geoscience BC Summary of Activities 2020: Minerals Fyles (1990), and the following description is based largely on these works. The formation includes pale grey to buff feldspathic sandstone, grey to green-tinged volcanic sandstone and conglomerate, and locally felsic crystal-lithic tuff. The conglomerates occur at several stratigraphic lev- els but the thickest and coarsest are generally at the base of the formation. Clasts, locally to several tens of centimetres in diameter, are generally composed of lo- cal volcanic and chert fragments of the Paleozoic basement, granite and granodiorite of the Nelson plutonic complex and, in the Rock Creek graben, clasts and broken crystals of the immediately underly- ing Paleocene Taurus Lake granite. Volcanic rocks are a minor component of the Kettle River formation but include dacitic crystal-lithic tuffs comprising lithic fragments, euhedral feldspar crys- tals, pyroxene and quartz. Locally, white to pink por- phyritic dacite with phenocrysts of plagioclase, orthoclase and rounded quartz in a light green aphani- tic matrix also occur, generally near the upper contact with the Marron Formation. Correlation The Kettle River Formation correlates with the contig- Figure 2. Schematic stratigraphy of the Boundary area, and correlations uous O’Brien Formation in the Republic District, with the Republic District in the State of Washington (Cheney, 1994, Washington (Pearson and Obradovich, 1977), and 1998) and the White Lake basin (Church, 1973) area. Abbreviations: with the Springbrook Formation in the White Lake ba- Cret., Cretaceous; Pal., Paleocene. sin, 90 km northwest of Greenwood (Church, 1973, 1986). Age Numerous plant fossils and spores from the Kettle River Three samples of the Kettle River Formation were submit- Formation have been identified as ranging in age from ted to the Pacific Centre for Isotopic and Geochemical Re- Paleocene to Eocene (e.g., Rouse and Mathews, 1961). Po- search (PCIGR) for U-Pb zircon dating (Figure 1, Table 1). tassium-argon dating of dacite has returned a range of ages, Petrographic descriptions of these samples are not yet com- from 53.1 ±1.5 Ma (Pearson and Obradovich, 1977) to pleted and, hence, the following descriptions are based on 46.2 ±2 Ma (Mathews, 1963). field observations. Table 1. Summary of U-Pb zircon and Ar40/Ar39 mineral ages obtained for selected samples of intrusive and Penticton Group rocks collected during mapping in the Boundary area; more complete sets of the data have also been studied (Höy et al., 2019; T.Höy, J. Gabites and R. Fried- man, work in progress). Abbreviations: Bi, biotite; Ksp, K-feldspar; WR, whole rock; Zr, zircon. Geoscience BC Report 2021-01 57 Sample H20-60: Greenwood Sample H20-60 (no. 1 on Figure 1) is a vaguely layered, medium to fine-grained, light grey volcanic sandstone or reworked tuff from the Phoenix deposit pit near Green- wood (Figures 3, 4). It contains dominantly feldspar and quartz crystals and small, generally dark, lithic