Volcanic rocks in the Dawson map-area occur as isolated lenses within the early Paleozoic Selwyn Basin assemblage and as sets of flows and complexes within Proterozoic carbonate rocks near the edge of the Mackenzie Platform. Mount Harper complex, an example of the latter group, has been studied in greater detail than the others because it is more completely exposed, and contains two mafic to felsic volcanic cycles. Unlike the sedimentary rocks, whose correlation with established units in the Wernecke Mountains and Alaska is now well advanced, understanding of the stratigraphic position and internal relations of the volcanic piles is less definite. This report, based on two field mapping seasons, has two parts. Characteristics of the two volcanic groups and correlative occurrences are discussed in the first part; the second presents interpretations from the stratigraphy of the Mount Harper complex as an illustration of the style of volcanism in the region.

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.

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.

The Rackla River area is underlain by normal faulted and gently folded sedimentary strata of the Paleoproterozoic Wernecke Supergroup, Mesoproterozoic Pinguicula Group, Neoproterozoic Hematite Creek Group and Windermere Supergroup, and Paleozoic Bouvette Formation. Gabbro dikes and sills that are likely age equivalent to the ca. 1380 Ma Hart River Sills cut the Wernecke Supergroup rocks. The presence of a mafic volcaniclastic horizon within the Bouvette allows its informal subdivision into a lower and upper member. These volcaniclastic rocks may the distal equivalent to volcanic rocks near the Tiger deposit, located ~20 km to the southwest. Three major angular unconformities are documented in the map area: at the base of the Rapitan Group, the base of the lower Bouvette, and the base of the upper Bouvette Formation.

Pentlandite-bearing serpentinized ultramafic flows with a komatiitic composition have been identified within volcano-sedimentary stratigraphy in the Nadaleen Range. Associated listwaenites or silica-carbonate-fuchsite-altered serpentinites carry locally significant gold, copper, nickel and cobalt values. The occurrence of laterally extensive ultramafi c units at the northern edge of the Selwyn Basin remains difficult to explain within the current scope of geological knowledge in the area. However, it represents a new style of exploration target for copper-nickel-bearing massive sulphide deposits, as well as listwaenite-associated gold.

The Mount Vermilion area is located at the southeast end of the Pelly Mountains volcanic belt, about 90 km southeast of Ross River, and includes the Wolf volcanogenic massive sulphide deposit. This well exposed area was mapped at a scale of 1:25 000; several stratigraphic sections were measured across the belt. Results from this study show that the southeast end of the belt is made up of dominantly felsic volcaniclastic strata. The base of the succession consists of dominantly brown-pink lapilli tuff interbedded with argillite and lesser trachyte sills/dikes. The middle of the succession is made up primarily of heterolithic lapilli tuff with distinct argillite clasts, maroon matrix tuff with green lapilli-sized fragments and trachyte flows/sills/dikes; the upper part consists of chlorite-altered volcaniclastic rocks containing intermediate dykes and flows. The Wolf deposit is hosted within the middle portion of the volcanic succession proximal to a syenite intrusion. To the west, towards the centre of the volcanic belt the felsic volcaniclastic component decreases as the number of sills, flows and dikes becomes more numerous, and the amount of intermediate volcanic material increases.

The 1.71 Ga Bonnet Plume River intrusions (BPRI) and related volcanics are preserved only as clasts in the 1.60 Ga Wernecke breccias of Yukon that host iron-oxide copper gold (IOCG) occurrences. Field work conducted in 2009 confirmed that they did not intrude the surrounding <1.64 Ga Wernecke Supergroup. Petrography shows that they are extensively altered and/or metasomatized, although relicts of primary igneous minerals remain. The major oxides are of little use in classification. Trace element geochemistry however, reveals a mafic to intermediate, calc-alkaline volcanic arc signature. Geochemical modelling has demonstrated that crystal fractionation was dominated by pyroxenes, plagioclase and olivine. The BPRI and related volcanic rocks are thought to have originated in a calc-alkaline volcanic arc that was obducted onto the Wernecke Supergroup, subsequently partially brecciated, and finally sank within the Wernecke breccias to the level of the Wernecke Supergroup.

Bimodal Tertiary volcanic rocks are exposed along the Tintina Trench in central Yukon in the Glenlyon, Grew Creek, and Ketza areas. Basalt and rhyolite are interbedded with coarse sedimentary rocks and preserved in grabens. The basalt forms both subaqueous and subaerial flows, hydrovolcaniclastic, pyroclastic, and autoclastic deposits. Rhyolite forms intrusions, lava flows, and volcaniclastic deposits. The volcanic and sedimentary rocks probably formed in a series of extensional basins whose original size and shape is yet to be determined. At Grew Creek, gold-bearing chalcedony veinlets and associated argillic alteration are near rhyolite dykes. In the Glenlyon Area, quartz, chalcedony, and fluorite veins and associated silicification are in and near rhyolite dykes.

Northwestern Lansing map area, 120 km east of Mayo, lacks known mineral occurrences yet contains upper Paleozoic stratigraphic units similar to those at volcanic-hosted and sedimentary exhalative deposits such as Marg and Macmillan Pass. Pyritic and locally baritic Earn Group and (previously unmapped) younger strata underlie most of the area. The sedimentary rocks are cleaved and folded but less strained than equivalent rocks in adjacent Mayo map area, and most contain abundant iron sulphide laminae, nodules, and replacements. A muscovite granite intrudes the grey-green phyllite. The Robert Service Thrust at the southern boundary of the Upper Paleozoic units, appears offset by steep, northwest-trending faults.

The Paleoproterozoic Wernecke Supergroup is a >13 km-thick metasedimentary succession exposed in the Wernecke, Ogilvie and Richardson mountains of central and northern Yukon. A program of field and laboratory investigations was initiated in 2007 in order to constrain the provenance, age and environment of deposition of the Wernecke Supergroup, as well as to better constrain the age of subsequent Proterozoic deformation (Racklan orogeny). Clastic and carbonate samples were collected from the Wernecke Supergroup for analysis of detrital and metamorphic minerals, as well as whole rocks, using a range of isotopic methods. Preliminary results from U-Pb analysis of detrital zircons from quartz sandstone beds, using ion probe mass spectrometry, are provided in this report. Patterns of the detrital zircon ages are broadly comparable to other Paleo- to Mesoproterozoic basins in Canada, suggesting a common Laurentian source. The maximum age of the Supergroup of 1.61 ± 0.03 Ga is provided by the age of the youngest detrital grain, which is ~0.1 Ga younger than expected.