This study describes the structural and stratigraphic setting of Devonian bedded barite and sedimentary exhalative lead-zinc-silver-barite deposits near MacMillan Pass in eastern Yukon. It shows that the deposits occur within MacMillan Fold Belt, an anomalous west-trending feature made up of three parallel elongate domains called the North, Central and South Blocks. Each is characterized by distinctive styles of deformation and Devonian strata. The North Block is characterized by a thick Early and Middle Devonian chert and shale sequence included in the Lower Earn Group and by an intricate array of southerly directed thrust faults. The Central Block includes unusually thick Early and Middle Devonian silty limestone intercalated with volcanic and volcaniclastic rocks within the upper part of the Road River Group. The Tom and Jason sedimentary exhalative zinc-lead-silver-barite deposits are associated with a thick sequence of coarse clastic rocks thought to belong to a submarine fan complex within the Lower Earn Group. Tight, upright folds, high angle reverse faults and irregularly oriented faults are characteristic. In the South Block, the Lower Earn Group comprises a thin Devonian shale sequence. Open upright folds and few faults are the structural style. The differences in Devonian strata and contrasting style of deformation between blocks may reflect older (Devonian?) fault control to depositional patterns, but structures of that age have not been identified. This report is accompanied by three 1:50 000-scale geological base maps of the MacMillan fold belt, east-central Yukon Territory (NTS map sheets 105 O/1,2,7,8 and parts of NTS map sheets 105 P/4,5 ).

The Macmillan Pass area is underlain by Hadrynian to Middle Devonian fine grained sedimentary strata and volcanic rocks of Selwyn Basin. Widespread occurrences of Upper Devonian debris flows and turbidites record uplift and erosion of older rocks to the west or northwest. Deposition of overlying, locally derived coarse clastic assemblages are related to subsequent formation of a graben-like, fault-bounded trough in the study area. Stratiform barite-lead-zinc mineralization on the Tom claims is contained in two tabular zones separated by a fault. Both zones occur at the transition between Upper Devonian locally derived coarse clastic rocks and overlying basinal shales. Ore textures vary from massive quantities of poorly bedded galena, sphalerite and pyrite to finely laminated barite and cherty argil lite with disseminated sulphide minerals. A mineralized and altered breccia body underlies the stratiform massive sulphide mineralization. A copy of this thesis is available at the EMR library – QE195 C38. This thesis is available online at http://hdl.handle.net/2429/21561.

Mineralized zones on the Jason property are stratifrom, sediment hosed Pb-Zn-barite deposits that occur in a Late Devonian age marine carbonaceous shale and turbidite sequence. The Jason Property is located 400 km northeast of Whitehorse near Macmillan Pass on the Canol Road. Since the discovery of mineralization in 1975, eighty-nine diamond drill holes have delineated three mineral deposits. In order of their discovery, they are known as the Main, South and End zones. Geological reserves indicated and inferred for the three zones total 14.1 million tonnes averaging 7.1% Pb, 6.6% Zn and 79.9 g/t Ag. The following questions are the focus of the study:: 1) What is the stratigraphic position and setting of the South zone? 2) What is the geological relationship of the South zone to the Main zone? 3) What is the geometry of the South zone? 4) How can the mineralization in the South zone be described in terms of mineralogical and textural facies? 5) What constraints on the processes of ore formation can be demonstrated by utilizing the above studies?

The Macmillan Pass area, in east-central Yukon Territory, contains what promises to be some of the most valuable mineral deposits yet discovered in the Territory. In addition to the TOM and JASON stratiform lead-zinc-silver-barite deposits, the area contains one of North America's largest tungsten deposits (Mactung deposit) as well as a relatively large tonnage stratiform barite deposit (TEA deposit) presently being exploited for use as oil well drilling mud. This report specifically deals with the TOM and JASON deposits, based on field investigations carried out by the writer during July and August. This report is accompanied by a 1:111 111-scale bedrock geology map of the TOM and JASON claims, Macmillan Pass area, east-central Yukon Territory (NTS 105 O/1 east half).

The MacMillan Pass district (map sheet 105O) located in east Yukon contains several sedimenthosted Ba ± Zn ± Pb deposits including the Tom and Jason deposits, as well as a multitude of 'barren' sediment-hosted barite occurrences. A classic sedimentary-exhalative (SEDEX) model has been postulated for these occurrences in which the barite horizons represent distal expressions of a hydrothermal vent system. Fieldwork was completed at the Walt and Tyrala barite occurrences that occur within the MacMillan Pass district in order to examine the deposit-scale geology and to sample undeformed barite horizons for subsequent geochemical analysis. Samples were also collected from drill core from the Hess barite occurrence. Barium mineralization occurs in both the Devonian Portrait Lake Formation (Lower Earn Group) and in underlying Ordovician-Silurian limestone of the Road River Group. A variety of textures were encountered that were indicative of both synsedimentary deposition of barite, as well as diagenetic to epigenetic barite mineralization. Base metal sulphides that are interpreted to post-date the barite mineralization were encountered at depth in drill core and are primarily hosted by Road River Group carbonates.

The Southwest McQuesten-northern Carmacks area is primarily underlain by rocks of the Yukon-Tanana terrane which is divided into two distinct belts separated by the Willow Creek fault: 1) a central belt of polydeformed, upper greenschist-amphibolite facies metasedimentary and metaplutonic rocks of Permian and older ages; and 2) a northeastern belt of generally undeformed and unmetamorphosed volcano-plutonic rocks of the Early Mississippian Reid Lakes complex. The southern part of the area is underlain mainly by rocks of Quesnellia and Stikinia, including: 1) Paleozoic retrogressed metamorphic rocks of the Boswell assemblage; 2) Upper Triassic augite-phyric volcanic rocks; and 3) Early Jurassic granitoids of the Aishihik plutonic suite. These rocks are dissected by a series of dextral strike-slip faults, probably related to the Teslin fault system. Post-accretion rocks include: 1) mid-Cretaceous biotite monzogranite plutons; 2) dacite and minor basalt of the Upper Cretaceous Carmacks Group; and 3) Quaternary basalt of the Selkirk volcanics. The southwest McQuesten-northern Carmacks area is under-explored, but shares many geological attributes with nearby, highly prospective districts such as the Dawson Range mineral belt, the recently discovered White Gold area and the producing Minto Mine.

The shale-hosted massive sulphide Zn-Pb deposits of Howard’s Pass were deposited during the Silurian and subsequently deformed during the Cretaceous Cordilleran orogeny. A recent model proposes that the deposits are hosted within a regional thrust duplex with strong transposition of bedding. This study aims to test this model and is focused on the XY group of deposits. Lithostratigraphic mapping and structural observations indicate one main phase of folding, F1, and the XY group of deposits is located on the southern limb of a macroscopic F1 syncline. F1 folds are steeply inclined and gently plunging to the WNW–NW. A regionally developed, steep, NE dipping, cleavage, S1, is axial planar to the F1 folds across Howard’s Pass. S1 manifests as a slaty cleavage comprising pervasively developed dissolution seams. WNW and NNE striking extensional faults overprint F1 folds. No shear fabrics or evidence for transposition of bedding were identified.

During 1977, Archer, Cathro and Associates Limited noted a number of conformable or paraconformable sills, at least 30 cm thick, of locally vesicular porphyritic augite-hornblende-biotite lamprophyre. Potassium argon analyses of biotite and hornblende indicate a concordant Upper Eocene age. Rubidium strontium isotopic data indicates a 87 Sr/86 Sr ratio of 0.7066. The calculated initial ratio, using a 40.3 Ma age, is 0.7065. This initial ratio is not unreasonable for alkalic igneous rocks in anorogenic settings.

The Tantalus Formation developed within confined intermontane river valleys during the late stages of collision and amalgamation of Stikinia and associated arc systems with the North American plate in Upper Jurassic and early Lower Cretaceous times. While most of the extensive chert pebble conglomerate in the Tantalus Formation can be interpreted as shallow braided gravel-bed river deposits, some may represent the products of deposition from deeper, braided and meandering gravel-bed rivers. Floodplain and lake deposits are restricted to poorly exposed slope forming intervals. Coal deposits developed locally on abandoned segments of floodplains in confined river valleys, in places associated with high constructive river deposits. The age profiles of detrital zircons indicate major contributions from reworking of older strata, combined with continued un-roofing of the Stikine terrane. In addition more distal sources were tapped in the Yukon-Tanana and adjacent terranes to the north and west of the Whitehorse trough. Much of the chert in the Tantalus Formation appears petrographically similar to chert in the Cache Creek terrane, now preserved only to the south of the Whitehorse trough. As both decrease in maximum grain size, and paleocurrents, are generally south to southwesterly trending, this source is considered unlikely. Chert may have been derived from now eroded supracrustal rocks that once formed the top of the Yukon-Tanana terrane, or more likely from an obducted block of Cache Creek terrane once present to the north and west of the Whitehorse trough. The latter may have been thrust over metamorphosed rocks of the Yukon-Tanana terrane beginning in the early Bajocian, and has subsequently been removed by erosion. A proximal North American cratonic source is excluded, as there are no Archean zircon grains in the Tantalus Formation. The possibility that strata of the Tantalus Formation may host significant conventional reserves of oil or gas is very low, due to lack of trapping mechanisms. There may be some undiscovered coal reserves, and limited potential for coal-bed methane production.

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