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.

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.

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 past-producing McKay Hill property on NTS map sheet 106D/6 (Nash Creek) has previously been described as polymetallic Ag-Pb-Zn ± Au-style mineralization. During the 2009 YMIP-funded exploration program the central claims on the property were mapped and numerous distinct differences from the proposed polymetallic model became apparent. Unlike Keno Hill, veins on the McKay Hill property lack siderite gangue and are not present as vein-faults. Propylitic alteration halos surrounding vertically zoned ore shoots (high-level Au-Cu and deeper level Ag-Cu-Pb) were observed within consistently north-northwest-striking, near vertical, siliciclastic and hypabyssal-volcanic rocks. The Ag-Pb-Zn veins in the Keno Hill Camp were emplaced in discrete dilational fault structures within polydeformed clastic metasediments and are not associated with extensive alteration. Host rock competency in both areas is vital in controlling mineralization. Re-evaluating the regional framework could potentially illustrate the area’s metallogenic potential for different types of mineral occurrences.

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This report includes a summary of the mineral exploration history of Yukon, an overview of major discoveries and a survey of exploration trends in the territory.

The Macmillan Pass in east-central Yukon (Figure 1) near the border with the Northwest Territories, is a highly mineralized area. The significant presence of metallic minerals in the MacMillan Pass area is reflected by the naturally elevated metal concentrations in both the soil and water (Soroka and Jack 1983, Kwong and Whitley 1993). A recent surge in metal prices has sparked renewed interest in mineral exploration and development at MacMillan Pass. Consequently, a broader understanding of the regional stream geochemistry is particularly important to help establish environmentally meaningful water quality objectives for the region.