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Placer deposits occur in unusual geological settings in the Mayo Mining District, where gold is mined from Reid-age glacial till and glaciofluvial gravel and from more recent (McConnell and post-McConnell-age) alluvial fans and fan-deltas. In other districts these types of deposits are not generally explored or prospected for placers. In the Mayo area, placer deposits are best preserved near the maximum limit of glacial ice where ice-scouring is minimal and depositional processes dominate. Pre-existing alluvial gold deposits were likely buried in this region where the ice limits of the Reid and McConnell glaciations exist in close proximity. Initial studies of the geomorphology of the region's known placer deposits show that they occur in three main types of landforms of different ages. Alluvial fans and fan-deltas contain placer deposits that are McConnell and younger in age while valley-bottomplacers are likely interglacial, glacial or glaciofluvial deposits of Reid age or older. A number of drainages have not been extensively explored or prospected; however, given similarities with known placer occurrences in the area, the potential for discovery of new placer deposits is good for many sites in the Mayo map area.

Yukon Gold-Silver File is an inventory of gold and silver-bearing mineral deposits and occurrences in Yukon. It follows the 1984 Open File entitled Gold-Silver Deposits and Occurrences in Yukon Territory by J.A. Morin and D.A. Downing. Other sources of information include: 1) Indian and Northern Affairs Canada Reports, including a) Yukon Exploration and Geology reports up to and including Yukon Exploration 1984-86 (noted as YEX, YEG and YGE), and b) Mineral Industry Reports (MIR); 2) preceding Mineral Industry Reports and other geological reports by the Geological Survey of Canada (GSC Papers, memoirs, bulletins and maps); 3) assessment reports (only non-confidential information is included) filed by mineral exploration companies with Indian and Northern Affairs Canada; 4) university theses; 5) personal communication with mineral explorationists and prospectors; and 6) data accumulated by property investigations by J.A. Morin and D.S. Emond (until August, 1987).

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Auriferous sheeted quartz veins and silicified shear zones occur along the margins and within adjacent hornfels zones of mid-Cretaceous Tombstone intrusions near the head of Clear Creek in the central Yukon. The lodes are the source for more than 120,000 ounces of downstream placer gold production. These lodes contain variable amounts pyrrhotite, pyrite, and arsenopyrite, with less abundant scheelite - alkali-feldspar, muscovite, biotite and tourmaline are common gangue phases. Grab samples of mineralization often contain gold grades in excess of 1 ounce per ton. Gold-to-silver ratios vary most commonly from 1:1 to 5:1. Gold-rich quartz veins cut all stocks, adjacent hornfels and associated lamprophyre dykes commonly contain greater than 1% arsenic. Bismuth, and less consistently tungsten and stibnite, characterize many of the most highly mineralized veins within and surrounding the stocks. Quartz veins along the intrusive-metasedimentary rock contact around the Pukelman stock are also enriched in lead and silver. R-mode factor analysis of multi-element geochemical data for 111 gold- and sulphide-bearing rock samples indicates that there are two geochemically distinct metal suites in the Clear Creek occurrences. The first is characterized by As-Au-Bi ± Sb, Te ore-related mineral association, which is typical of many intrusion-related deposits in the Tombstone gold belt. Less consistently, anomalous concentrations of Ag, Co, Cu, Fe, and Mo occur within these auriferous rocks. The second metal factor is defined by Ag-Bi-Pb ± As, Au and Te. It characterizes metalliferous vein samples that have uncommonly low Au: Ag ratios and may represent a second hydrothermal episode. Tungsten shows little consistent correlation with the metalliferous veins in either element suite.

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

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