164 DIGITAL MAPPING TECHNIQUES ʻ04 165 Geology, Stewart River Area (Parts of 115 N/1,2,7,8 and 115-O/2-12), Yukon Territory By S. P. Gordey, J.J. Ryan, and R.B. Cocking Geological Survey of Canada 101-605 Robson Street Vancouver, B.C., Canada V6B 5J3 Telephone: (604) 666-2116 Fax: (604) 666-1124 e-mail: [email protected] INTRODUCTION by truck on placer mining roads, many of which extend south from Dawson. Fieldwork in 2000-2003 included Geological mapping in the Stewart River area (NTS foot traverses from small camps mobilized along these 115 N, O, in the Yukon along the Alaska border) began routes and from helicopter or fixed-wing supported camps as part of the Ancient Pacific Margin NATMAP Project. in more remote areas. All-terrain vehicles were used Initiated by the Geological Survey of Canada, Yukon Geo- on placer mining access roads along Thistle, Kirkman, logical Survey, and British Columbia Geological Survey Henderson, Black Hills and Maisy Mae creeks and the Branch, the NATMAP Project seeks to improve under- Sixty Mile River. Helicopter spot checks were used to fill stand of the composition, relationships, and metallogeny in widely separated outcrops in the southwest part of the of terranes lying between the ancestral North American map area where foot traverses or fly camps were impracti- margin and those known with more certainty to be tec- cal. Bedrock mapping is hampered by a deep (~1 m) soil tonically accreted (Thompson and others, 2000; Colpron veneer, thick gravel, and loess deposits in valley bottoms, and others, 2001). The Stewart River component (Figure and by dense cover of forest, moss, and lichen. The de- 1) focuses on the Yukon-Tanana terrane (Mortensen, 1990 tailed aeromagnetic and gamma-ray surveys (Shives and and 1992), comprising complexly deformed, mostly (?) others, 2002) are an effective aid to bedrock mapping in Paleozoic meta-igneous and metasedimentary rocks. The this poorly exposed, unglaciated terrain. final two years of the Stewart River Project are funded under the Geological Survey of Canadaʼs Northern Re- GEOLOGICAL FRAMEWORK sources Development Program. The objective of the Stewart River Project is to in- The Stewart River area is underlain by twice-trans- vestigate the stratigraphic, structural, and tectonic history, posed, amphibolite-facies gneiss and schist of mostly (?) and the economic framework, of this large tract of Yukon- Paleozoic age. These are intruded by younger plutonic Tanana terrane by mapping about 2/3 of the area over a rocks and overlain by Upper Cretaceous volcanic rocks four year period. Geology is being interpreted in light of and local occurrences of Lower Cretaceous conglomer- new geophysical data collected in this area under the Tar- ate. The reader is directed to Ryan and Gordey (2001a, b; geted Geoscience Initiative (Figure 2; Shives and others, 2002a, b) and Ryan and others (2003) for a more compre- 2002). Concurrent surficial geological studies were aimed hensive description of the geology. at understanding the Quaternary history and setting of the Metasiliciclastic rocks are widespread, and domi- numerous placer gold deposits in the region (e.g. Jackson nated by psammite and quartzite, with lesser pelites and and others, 2001, 2002; Rotheisler and others, 2003). rare conglomerate. These were thought to be as old as In summer 2003, gaps in the previous mapping were late Proterozoic (e.g. Tempelman-Kluit, 1974); however, bridged and the mapping extended to cover about eleven preliminary detrital zircon geochronology and geochro- 1:50 000 scale map areas (see Figure 2). These new data nology for plutonic rocks suggest a middle Paleozoic age and previous work in surrounding areas (e.g. Bostock, (M. Villeneuve, Geological Survey of Canada, in prepa- 1942; Tempelman-Kluit, 1974; Mortensen, 1996) will ration). Intermediate to mafic composition amphibolite be synthesized into a new geological map of the Stewart interdigitates with, and lies stratigraphically above, the River map area (1:250,000 scale; 115N-O). siliciclastic rocks. Although intensely tectonized, het- Access into the heart of the Stewart River area is erogeneous compositional layering and local vestiges of afforded by boat along the Yukon and Stewart rivers and primary textures in the amphibolite, such as breccia clasts 165 166 DIGITAL MAPPING TECHNIQUES ʻ04 167 Figure 1. Regional tectonic setting of the Stewart River area. Figure 2. Cumulative progress of bedrock mapping in the Stewart River area (reference to previous Geological Survey of Canada open file reports are in parentheses). The grid outlines 1:50 000 NTS boundaries. Helicopter- borne detailed aeromagnetic and gamma-ray surveys (gray areas denoted as “Geophysics <date>”) were flown in 2000 and 2001. 166 DIGITAL MAPPING TECHNIQUES ʻ04 GEOLOGY, STEWART RIVER AREA, YUKON TERRITORY 167 and pillow selvages, indicate derivation from volcanic relationship helps distinguish F2 and F3 folds, which can and volcaniclastic rocks. Marble horizons occur within have very similar style. The latter are open, moderately the amphibolite, as well as the siliciclastic rocks. In inclined (but varying from shallow to steep), shallowly turn, dark carbonaceous quartzite, metapelite and minor plunging structures, that have weak axial-planar fabric marble of the Nasina assemblage, markedly sparse in vol- where developed in schistose layers, and have no associ- canic-derived material, lies structurally above and/or may ated extension lineation. The map area is also affected be partly equivalent to the aforementioned metaclastic by faults of varying significance. Most of these could not rocks. Abundant orthogneiss bodies with diorite, tonalite, be observed directly, but are interpreted from changes granodiorite, monzogranite and granite protoliths, intrude in rock type and/or structural grain; some are also well the above assemblages. Some are Devono-Mississip- delineated by prominent physiographic and aeromagnetic pian in age, whereas others are known to be Permian. lineaments. Locally, fault breccia and slickensides provide For many others, the age is probably one of these, yet direct evidence of fault contacts. remains unclear. The tectonic significance of ultramafic and gabbroic rocks that lie near the boundary between the ECONOMIC GEOLOGY siliciclastic and metavolcanic-metavolcaniclastic succes- sions is also unclear. One of the more significant findings is that parts of An extensive area of likely Permian, low to medium the area are dominated by a mid-Paleozoic volcano-plu- grade muscovite-quartz and chlorite-quartz schist in the tonic arc (?) complex with implied potential for VMS type western part of the map area, correlated by Tempelman- mineralization. In the Finlayson Lake area (Figure 1), Kluit (1974) with the Klondike Schist (McConnell, 1905) originally contiguous with the Stewart River area (allow- is derived from a combination of volcanic, volcaniclastic ing for 425 km of late Mesozoic-Tertiary dextral offset), and plutonic rocks. Southeast of the White River this suc- correlative mid-Paleozoic strata host massive sulphide cession may lie beneath a low-angle fault. To the north- mineralization in both felsic (e.g., Kudz Ze Kayah and west, contact relationships are uncertain. East of Ladue Wolverine Lake deposits; Murphy (1998, and references River these rocks are overlain by relatively unstrained, therein), Piercey and others, 2001) and mafic (Fyre Lake chlorite-altered intermediate to mafic volcanics, of un- deposit; Foreman (1998)) metavolcanic sequences. It known but possibly Permian age. should be noted that primary geochemical (e.g., al- In summary, the extensive metaplutonic and metavol- teration), structural, and lithological signatures may be canic rocks represent two periods of arc activity. The strongly modified by the amphibolite facies metamor- older arc, built upon a siliciclastic foundation, largely phism and high state of strain in the Stewart River area. comprises Devono-Mississippian amphibolite associated The Lucky Joe occurrence was explored in 2003 by with coeval widespread tonalitic orthogneiss that formed Kennecott Exploration. Two large strong parallel geo- its subvolcanic intrusive complex. A Permian arc, built chemical trends defined by high soil values of Cu and upon the previous, is represented by granitic orthogneiss Au, with associated Mo and Ag, have been identified (see and coeval metavolcanics. press release at http://www.copper-ridge.com). The origin of the occurrence is obscured by complex structure and STRUCTURE metamorphism. Cu-Au porphyry, Fe-oxide Cu-Au, or sediment-hosted Cu deposit models have all been sug- The Paleozoic rocks in the field area exhibit a re- gested. A metallogenic study now underway (Jan Peter gional foliation (ST), characterized by high-strain transpo- (GSC)) is aimed at identifying the deposit type and its sition of layering in the gneiss and schist, with abundant origin. The Lucky Joe represents a new type of potentially intrafolial isoclinal folds that are commonly rootless. The large occurrence within Yukon-Tanana terrane. intensity of strain within the regional foliation locally In Yukon and Alaska, mid-Cretaceous (105-90 grades to mylonite. Primary compositional layering (S0) Ma) and Late Cretaceous (70-65 Ma) plutons and their in metasedimentary rocks, unit contacts, and a pre-exist- country rock are prospective targets for intrusion-related ing foliation (S1) can be traced around closures of the gold deposits (e.g., Hart and others, 2000). Undeformed transposition folds, indicating