A preliminary geological map (1:25,000 scale) of Mount Skukum volcanic complex, southern Yukon (NTS 105D/2,3,4,5), including one cross section.

Several centres of continental volcanism are situated in southern Yukon. Two of these, the Skukum Volcanic Complex and the Bennett Lake Cauldron Complex BLCC are closely associated in space and gross geological characteristics, including their important role as hosts for epithermal precious metal mineralization. The BLCC was studied extensively by Lambert and the Skukum complex is currently under investigation by the author. The Skukum complex was previously thought to represent a similar structural and volcanic setting to the BLCC. Study of the Skukum complex began in the summer of 1982 with the idea that it represented a cauldron subsidence feature and that an improved understanding of its geology would assist in developing potential targets for mineral exploration. The geology in the area is complex due to the discontinuous nature and incomplete exposure of volcanic units and is further complicated by severe faulting. The author found that the Skukum complex differs from the BLCC in three respects:: 1) the presence of a bimodal suite of extrusive rocks (which suggests the tapping of two magma chambers), 2) its structural complexity and the absence of major cauldron subsidence and 3) the variability of depositional environments within the complex. Contrary to past speculation, the two complexes, which have been considered to be closely related in time and space, each represent a distinctive structural and volcanic setting.

The Eocene Skukum volcanic complex, 60 km south-southwest of Whitehorse, is elliptical in plan, covers an area of about 140 km², and unconformably overlies Cretaceous granitic rocks and Precambrian metasedimentary rocks. The complex is fault-bounded and in places has been intruded by felsic dykes and stocks. A major north-trending fault divides the area into two parts: a western part which includes a lower interlayered sedimentary-volcanic sequence and an upper unit, approximately 500 m thick, characterized by andesite lava flows, pyroclastic flows and sedimentary units; and an eastern part which comprises about 800 m of altered felsic pyroclastic flows and brecciated, flow layered and spherulitic felsic lava flows. Study of the interlayered sedimentary-volcanic formation provides a control on the paleotopography of the Skukum area, and the depositional environment and provenance of the formation..

The Mount M'Clintock map area, northeast of Whitehorse, is dominated by Middle Triassic to Jurassic sedimentary and volcanic strata of Stikinia with small portions of the Cache Creek and Yukon-Tanana Terranes. These assemblages were deformed prior to the mid-Cretaceous intrusion of three plutonic suites and the deposition of two suites of volcanic rocks. Sedimentary rocks previously mapped as undifferentiated Lewes River and Laberge Group strata are separated into their respectve groups and further sub-divided into several members. Lewes River Group rocks form three units that are Carnian and older, Norian, and Upper Norian in age and are represented by siliceous siltstone and calcareous sandstone, conglomerate and limestone respectively. Laberge Group strata are divided according to lithology and dominated by siltstone-sandstone couplets and massive siltstone with lesser conglomerate and volcanogenic sandstone. Volcanic rocks previously mapped as Hutshi Group are divided into the dominantly mafic and submarine, MiddleTriassic Joe Mountain volcanic complex (JMVC) and the dominantly felsic and sub-aerial Mount Byng volcanic complex (BCVC). The volcanic and sedimentary rocks generally increase in age from Middle Triassic to Jurassic from west to east across the map area. Plutons cutting these strata belong to the M'Clintock Lakes (120 Ma), the Whitehorse (115 Ma), and the Mount McIntyre (109 Ma) plutonic suites. The predominantly felsic fragmental rocks of the BCVC are genetically associated with the Byng Creek pluton of the Mount McIntyre plutonic suite. The BCVC is nested into the country rocks and its eastern margin and is preserved as the rim of a tilted caldera. Strata of the Laberge and Lewes River groups are folded throughout. Wavelengths are on the order of approximately 1-2 km, but are much tigter in black siltstone units and adjacent to northwest-trending faults. Faults are ubiquitous throughout the map area and form three sets. North-trending faults are the most common. They are spaced a few kilometres from each other and dictate the physiography and drainage of the region. Northwest-trending faults, in the northeastern corner of the map area control the Teslin River valley and juxtapose Yukon-Tanana rocks with Stikinia. Older faults are dominantly east-trending but are terminated or reactivated by younger faults. Copper (gold-molybdenum-tungsten) skarns and gold-bearing quartz veins are the two mineral deposit types most likely to be discovered in the map area. The source of the placer gold in Sheldon Creek is unknown but may be related to gold veins in the JMVC, BCVC, Sheldon Creek volcanics or the surrounding sedimentary rocks. Hydrothermal activity in the JMVC is characterized by orange weathering alteration, breccias, and carbonate veins. Cache Creek rocks in the southern part of the map area are targets for listwaenite associated gold veins.

The Skukum area is located 58 km south-southwest of Whitehorse. It is an elliptical area of volcanic rocks, Tertiary in age, and surrounded by hypabyssal rhyolite intrusives. Field and petrographic evidence, fluorite and tourmaline stockwork, breccia pipes, roof pendants, miarolitic cavities and spherulites in the nine Skukum rhyolites suggest that they were emplaced at a high level. The intrusives vary in composition from rhyolite to dacite. The variation in texture within and between the intrusives can be explained by different rates of crystallization, temperature differences and compositional variability. Chemical data are in accord with the expected trends in a cogenetic suite of igneous rocks. Relatively low CaO and MgO, high SiO2 and anomalously low Sr concentrations indicate that the rhyolites were formed from a highly differentiated magma. Sr and Ba versus Al2O3 plots show that both k-feldspar and plagioclase were important fractionating phases. Rare earth element data further support this conclusion and also suggest that some accessory phase(s), such as monazite, allanite or fluorite help control the rare earth element behaviour. Partial melting of an already depleted source rock with residual plagioclase can also explain the patterns. The Bennett Lake ring and associated dykes are petrographically and chemically similar to the Skukum intrusives. However, Zr and TiO2 are present in higher concentrations in the Bennett Lake complex, indicating that they were derived by a slightly different fractionation process.

Geological map (1:50,000 scale) of Mount M'Clintock map area, southern Yukon (NTS 105D/16) including geological cross sections and mineral occurrences.

Preliminary geological map (1:50,000 scale) of Mount M'Clintock area, southern Yukon (NTS 105D/16) including geological cross sections and mineral occurrences.

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