A bimodal volcanic suite of rhyolite intrusions and flows and columnar olivine basalt, basaltic tuff and tuff breccia occurs along and immediately north of the Tintina Fault from near Faro to Tuchitua in east-central Yukon. The results of recent fieldwork and geochronometry over the northern part of this belt expands the known areal distribution of rhyolitic volcanics and confirms their age as late Paleocene to Eocene. Basalt formerly thought Quaternary is also of Eocene age. The concordance of dates between the mafic and felsic volcanics indicates a single Paleogene bimodal volcanic province. Such bimodal suites are commonly linked to crustal extension and normal faulting, which in this case are presumably related to transcurrent slip along Tintina Fault. The connection between Paleogene volcanism and the geographical extent of placer gold in this area is more than circumstantial. The recent lode gold discovery at Grew Creek appears to be intimately associated with Paleogene felsic volcanism. Hydrothermal systems associated with similar felsic volcanic centres elsewhere in the region, perhaps unmapped or buried by drift, may be the ultimate source of the placer gold.

Geological mapping of Yukon-Tanana Terrane in Little Salmon Range has outlined a coherent stratigraphic succession in rocks that were previously described as strongly foliated and lineated mylonitic tectonites. The widespread occurrence of primary sedimentary and volcanic textures and the lateral continuity of the units are incompatible with the previous interpretation of the area. A laterally continuous volcanic arc sequence occupies the core of a broad synclinorium and rests unconformably on disparate clastic units to the east and west. The volcanic sequence is structurally overlain by an allochthonous sheet of distal turbidites. The occurrence of massive sulphide and exhalite within the volcanic sequence attests to the high mineral potential of this largely unexplored region.

The Tummel fault zone, a northwest-trending belt of rocks of uncertain age and/or tectonic affinity, separates Paleozoic miogeoclinal strata of Cassiar Terrane from Yukon-Tanana Terrane metavolcanic and metasedimentary rocks. Northeast of the fault, Cassiar Terrane comprises pelitic and semipelitic rocks with rare amphibolite, which are correlated with the Kechika Group. These are overlain by carbonate correlated with the Askin Group. Southwest of the fault, in Yukon-Tanana Terrane, Devono-Mississippian siliciclastic rocks are overlain by Mississippian arc volcanic rocks. Granodiorite and diorite of the Telegraph Plutonic Suite (348-350 Ma) intrude the siliciclastic rocks. Foliated greenstone, leucogabbro intrusions, serpentinite and chert occur in the Tummel fault zone. The Early Cretaceous Glenlyon Batholith intrudes strata of Cassiar Terrane. Contact metamorphism recognized across the Tummel fault zone is interpreted to have been imposed by the Glenlyon Batholith. If correct, this interpretation requires that post-mid-Cretaceous displacement across the Tummel fault zone has been minimal (~5 km).

for a copy of this paper please contact the Yukon Geological Survey; geology@gov.yk.ca.

for a copy of this paper please contact the Yukon Geological Survey; geology@gov.yk.ca.

Mapping was carried out in the summer of 2018 by Rosie Cobbett and Halley Keevil for areas north of the Tintina fault; additional geological data provided by G. Jilson. Geology south of the Tintina fault is from YGS Open File 2002-9. Two areas north of the Tintina fault are extensively thermally and hydrothermally altered. The interpretation of fault traces through areas of poor rock exposure was guided by a Yukon wide geophysical survey of magnetic data.

The upper Sixtymile River area is located approximately 128 km west of Dawson City, Yukon. Lithology in this area consists of Precambrian to Paleozoic metamorphic rocks, Paleozoic ultramafic rocks, Middle Jurassic pegmatitic and aplitic dikes, Upper Cretaceous porphyritic dikes and volcanic rocks with intercalated sedimentary rocks, Quaternary alkaline basaltic dikes and Quaternary alluvial sediments. Precious metal occurrences in these volcanic rocks are divided into two types, based on differences in local distribution, petrology and wall rock alteration: a gold-bearing pyrite-arsenopyrite type and a silver-bearing galena-sphalerite type. Both types are characterized by four stages of mineralization.

Paleozoic platformal and basinal strata of Cassiar Terrane are separated from rocks of Yukon-Tanana Terrane to the west by an unexposed fault in southeast Glenlyon map area. Quartzite, marble, phyllite, and amphibolite are grouped in Cassiar Terrane, and no rocks of Slide Mountain Terrane are recognized. The mid-Cretaceous Glenlyon batholith contains pendants of Cassiar Terrane and is intruded by at least five andesite dykes. West of the fault, the Yukon-Tanana Terrane includes: (1) mafic volcaniclastic rocks with preserved primary textures; (2) coarse-grained quartz-feldspar grit; and (3) metasiltstone and semi-pelitic schist. The grit is intruded by foliated hornblende granodiorite, likely of early Mississippian age. Small outcrops of tectonized serpentinite were tectonically emplaced into Yukon-Tanana Terrane, and a positive magnetic anomaly parallel to the fault suggests an unexposed extension to the southwest. Two mylonite localities and evidence of brittle cataclasis up to 1 km on either side of the presumed buried fault suggest a complex structural history along this terrane boundary.

Grew Creek epithermal gold prospect, in south-central Yukon Territory, is adjacent to and southwest of the Robert Campbell Highway, halfway between the communities of Ross River and Faro. The prospect is within the Tintina Trench, which from Late Cretaceous to Tertiary time was a zone of major right lateral movement that juxtaposed the Cambrian and Ordovician slate and phyllites of the Pelly-Cassiar Platform (to the southwest) against rocks of the Anvil Allochthon (to the northeast). Grew Creek rocks are mid-Eocene based on K-Ar dates of basalt of 51.4 ±1.8 Ma and 50.7 ±1.8 Ma and pollen spores in volcaniclastic rock dated at 56 to 46 Ma. Felsic volcanic and volcaniclastic rocks were overlain by a sequence of interbedded coarse clastic sediments, basaltic flows, and basaltic volcaniclastic rock. Late Tertiary uplift and faulting resulted in graben formation and consequent preservation of Eocene rocks in a structurally complex graben bounded to the south by the Grew Creek fault and to the north by the Danger Creek fault. Mineralization at Grew Creek occurs at the tip of a westwardly pointing wedge of dominantly felsic, crystal lithic lapilli tuff. The zone of precious metal deposition is truncated to the northeast by steeply dipping clastic sediments and to the southwest by the Grew Creek Fault. Gold, electrum, pyrite, and silver selenide were identified in a high grade sample from the discovery outcrop. Alteration at Grew Creek is both surficial and hydrothermal. Surficial alteration is ubiquitous, pervasive, and characterized by mixed-layer clays and carbonates. Hydrothermal alteration, responsible for the gold-silver mineralization is closely associated with rhyolitic dykes and is of three types:: silicic, acid sulphate, and argillic acid sulphate. K-Ar dating of sericite indicates hydrothermal alteration is mid-Eocene (51.5 ±1.8 Ma and 47.0 ±1.7 Ma) and synchronous with deposition of the volcanics. Quartz associated with mineralization at Grew Creek is enriched in heavy oxygen isotopes. A deep magmatic source for the mineralized fluids is one explanation for this enrichment.

Despite many years of exploration and relatively limited success, the Rusty Springs prospect retains considerable potential for a large-tonnage deposit. The property lies within the east-vergent Taiga-Nahoni fold belt, occurring in the core of a structural culmination exposing host dolostones of the Lower and Middle Devonian Ogilvie Formation. Mineralization occurs in stratabound and discordant zones along the contact with the overlying Devono-Mississippian unnamed shale. Various deposit models, ranging from Mississippi Valley-type to epithermal vein-type have been employed. Poor exposure and relatively deep weathering, resulting from the lack of Pleistocene glaciation, account for the lack of consensus with regard to genesis. Evidence points to the potential for a high-temperature, carbonate-hosted massive sulphide deposit (manto-chimney complex). The great extent of mineralized and altered rocks, together with their stratabound nature, significant thickness, local high grades, and potential for supergene enrichment, suggest that Rusty Springs remains an attractive drill-oriented exploration target.