TRACKING THE ESKAY RIFT THROUGH NORTHERN BRITISH COLUMBIA - GEOLOGY AND MINERAL OCCURRENCES OF THE UPPER ISKUT RIVER AREA (TELEGRAPH CREEK NTS 104G/1, 2, 7, 8, 9, 10) By D.J. Alldrick1, M.L. Stewart1, J.L. Nelson1 and K.A. Simpson2 KEYWORDS: Targeted Geoscience Initiative-II (TGI-II), The two-year study will cover 6,250 km2, extending Bedrock mapping, Eskay Creek, Eskay Rift, Hazelton 125 km north from the Eskay Creek mine to the Spectrum Group, Stuhini Group, Mineral deposits porphyry copper-gold deposit (Figure 2). The paved Stewart-Cassiar Highway (Highway 37) runs northward through the eastern part of the map area. In 2003, the first INTRODUCTION field season, an eight-person team mapped 70 km along the rift sequence between Kinaskan Lake and More The Eskay Creek gold-silver mine, located in Creek, west of the highway (Figure 2). northwest British Columbia, is an unusually high-grade The project area straddles the eastern edge of the ore deposit. The mining industry continues to spend more Coast Mountains and the broad valley of the upper Iskut than $2 million each year on exploration for similar River. This area lies within the Tahltan First Nation deposits in the area. The geologic setting at the minesite is traditional area and they participated directly in this well studied, but large tracts in north-central British project. Topography varys from rounded glacial valleys Columbia require more detailed surveys to determine if along the upper Iskut River, to the extensive Spatsizi favourable sites exist for formation and preservation of Plateau, to high serrated ridges and peaks that are being additional deposits. The British Columbia Geological actively glaciated. Elevations range from 250 metres Survey and the Geological Survey of Canada have above sea level at the confluence of Iskut River and launched a two-year mapping program to delineate the Forrest Kerr Creek, up to 2,662 metres at the summit of critical ore horizon through the region north of the mine Hankin Peak in the west-central region of the field area. and to assess potential for additional Eskay Creek type Mount Edziza can be seen rising to 2780 meters near the deposits. This horizon lies within Lower to Middle northern boundary of the study area. Vegetation Jurassic, arc-related, rift sequence rocks along the comprises boreal spruce-pine-fir forest at low-elevation. northwest perimeter of the Bowser Basin, a large (48,000 Timberline is at 1400 metres elevation with subalpine fir 2 km ), Middle to Upper Jurassic sedimentary basin (Figure and meadow areas above. 1). Regional-scale geology maps and reports for this area include: Operation Stikine (1957), Souther (1972), Read (1989), Evenchick (1991), Logan et al. (1990, 1992, 1993, 1997, 2000), Gunning (1996) and Ash et al. (1995, 1996, 1997a, 1997b) (Figure 2). Detailed geological maps I N T E are available in theses by Schmitt (1977) and Kaip (1997) R M and in many company assessment reports cited in ARIS O BB N T and MINFILE. The most recent and most comprehensive A N study of the Eskay Creek orebodies is the Ph.D. thesis by E Tina Roth (October, 2002) which offers an extensive bibliography of all previous reports on the deposit, including many progress reports and final reports that were part of the Iskut Metallogeny Project of the Mineral Deposits Research Unit at the University of British Columbia (Macdonald et al., 1996). Anderson (1993) interpreted the present study area as Project the northern extension of a large fault-bounded belt or rift. Area Sections of this area have been mapped at 1:50,000 scale Figure 1. Project location map and Bowser Basin (BB). 1 Modified from Logan (2000). British Columbia Ministry of Energy and Mines 2Geological Survey of Canada, Vancouver 1 This report Tulsequah Two-year TGI project x 104H Evenchick and Thorkelson (2004) Ash et al. (1997) Golden Logan et al. (1997) Bear x Logan and Koyanagi (1989, 1994) Bradford and Brown (1993) Red Spectrum Chris Stikine Project - Geoscience x x x Maps 1993-3 to 1993-6 GJ Souther (1992) Schaft x Creek Alldrick et al. (1987, 1988, 1989, 1990, 1991, 1992) xGalore Creek Read et al. (1989) Read (1983, 1984) 104J Gabrielse (1980) Snip Eskay Creek x x Johnny x 104G Souther (1972) Mtn. Sulphurets x 104K Souther (1971) 104F Souther (1959) Premierx Kerr (1948) Mine or Developed prospect Figure 2. Previous geological mapping and current project outline. Modified from Logan (2000). by Read (1991), Logan et al. (1990, 1992, 1993) and Ash deformation and erosion, termed the Inklinian and et al. (1997b). The current project will complete Nassian orogenies, respectively (Souther, 1972). 1:50,000-scale coverage between these earlier mapping Strata range in age from Devonian through to Holocene projects, with more detailed mapping of the strata of the (Figure 3). The major stratigraphic components of the upper Hazelton Group, and detailed stratigraphic project area are the Stikine Assemblage, Stuhini Group, investigations within the Eskay Rift (e.g. Simpson and Hazelton Group, Bowser Lake Group and Mount Edziza Nelson, 2004). The federal and provincial governments Complex. The Stikine Assemblage was defined by a have jointly funded this study as part of the "Bowser Geological Survey of Canada team (Operation Stikine, Basin Energy and Mineral Resource Potential Targeted 1957) and has most recently been described by Logan et Geoscience Initiative". al. (2000). The Stikine Assemblage consists of Early Devonian to mid-Permian volcanic and sedimentary strata, which culminate in a thick carbonate succession. REGIONAL GEOLOGIC SETTING The Upper Triassic Stuhini Group is characterized by pyroxene porphyritic basalt flows and breccias with The project area lies on the western edge of the intercalated clastic sedimentary rocks and minor Intermontane tectonic belt, within Stikine terrane, and is carbonate units. The Early to Middle Jurassic Hazelton bounded to the east by the Bowser sedimentary basin Group is an island arc succession consisting of a lower (Figure 1). It straddles the tectonic elements of the package of intermediate volcanic rocks and derived Bowser structural basin and the Stikine Arch to the clastic sedimentary units; a middle interval of thin, but northwest. widely distributed felsic volcanic rocks; and an upper unit of fine clastic sedimentary rocks with local bimodal Souther (1972) and Logan et al. (2000) describe the volcanic rocks dominated by basalt. Carbonate units are geological history of the area as a series of five mid- rare or absent in Hazelton Group strata. The Middle to Paleozoic to mid-Mesozoic volcanic arcs developed in Late Jurassic Bowser Lake Group is a thick, clastic sediment-poor and sediment-rich marine settings. Lulls in marine sedimentary succession. Miocene to Recent volcanism at the Triassic-Jurassic boundary and in the uppermost Lower Jurassic were marked by tectonic uplift, 2 Late Miocene to Holocene MOUNT EDZIZA VOLCANIC COMPLEX ST SD CGL Middle to Upper Jurassic CGL CGL BOWSER LAKE GROUP CGL ST SD ST SD UNIT UPPER BASALT R B D Lower to Middle Jurassic R B HAZELTON GROUP R B WILLOW RIDGE COMPLEX R SD SEDIMENTARY CGL ST R SD MIDDLE ( see Figure 7 for detail ) UNIT SD ST ST An R gu la B LOWER BASALT UNIT r U B nc on fo rm it y R B CGL B CGL Upper Triassic B CGL STUHINI GROUP SD ST LST LST LST SD ST LST LST SD ST B B B SD ST LST LST R RT SD ST RT T tuff R LEGEND RT SD ST LST RT LST breccia R SD ST A pillows B B CGL AT A R rhyolite A A CGL n SD ST ee D dacite D DT t S A andesite No ase B basalt D B LST limestone Early Devonian SD sandstone to Middle Permian ST siltstone, mudstone STIKINE ASSEMBLAGE CGL volcanic conglomerate (not found in map area) Figure 3. Schematic regional stratigraphy. 3 volcanic strata from the Mount Edziza Volcanic Complex GEOLOGY OF THE MAP AREA blanket the northwest section of the project area. Mapping in the 2003 field season covered Upper Regional-scale unconformities within the study area Triassic to Middle Jurassic strata at the northern end of include a Late Permian - Early Triassic disconformity, a the two-year project area (Figure 2). Several topographic Late Triassic - Early Jurassic angular unconformity and features in this year’s map area have been informally nonconformity, and an Early Jurassic angular named to simplify description of locations (Figure 5). unconformity. Simplified geology of the 2003 map area is presented in Logan et al. (2000) describes five plutonic episodes Figure 6. Age control is provided by fossil collections in the area. The three youngest plutonic episodes have from Souther (1972) and Evenchick et al. (2001), and by important mineral deposits associated with them. isotopic age dates tabulated in the new BCAge database To the south, mid-Cretaceous regional (Breitsprecher and Mortensen, 2004). metamorphism reached a maximum grade of lower greenschist facies (Alldrick, 1993). In the current field STRATIFIED ROCKS area chlorite is rare to absent, thus the regional metamorphic grade is interpreted as sub-greenschist, probably mid-prehnite-pumpellyite facies (Figure 4). STIKINE ASSEMBLAGE (MIDDLE TO UPPER PALEOZOIC) TEMPERATURE (º C) Carbonate-dominated Permian strata of the upper 1 00 2 00 3 00 4 00 5 00 6 00 7 00 8 00 9 00 Stikine Assemblage have not been mapped in this year’s 1 3.5 survey. ) 2 7.0 S D STUHINI GROUP (UPPER TRIASSIC) R E A 3 10.5 Is P B k T u H O A t L 4 r 14.0 I e a In the map area, the Stuhini Group consists of a lower ( K K ( I L 5 17.5 volcanic package with lesser intercalated sedimentary O E M R rocks, overlain by a thick upper sedimentary package with U 6 21.0 E S T lesser interlayered volcanic rocks.