Sedimentology and stratigraphy of Duncan Creek placer deposits, Mayo, central Yukon

Placer deposits in the Mayo area occur in a wide variety of geomorphic settings, including alluvial fans, gulch gravels, valley-bottoms (alluvial plains), and bedrock terrace (bench gravel) settings which have been variably buried and reworked by glaciofluvial processes. Placer gold is also known to occur in glacial till and glaciofluvial gravels especially where these sediment types are close to bedrock.

Three major Quaternary glaciations (the pre-Reid, Reid and McConnell, in order of oldest to most recent) and their associated interglacials have modified the drainage and topography of this area, and these events have affected the formation, preservation and proportionate size of the District's placer gold deposits.

Duncan Creek, a tributary of the Mayo river which drains Mayo Lake, is one of the most actively-mined drainages in the Mayo District. Placer mining began in the Duncan Creek area in the early 1900's and has continued almost continuously to the present day. Gold production from Duncan Creek in the last 15 years has been nearly 20,000 crude ounces, with historical production estimated to be at least twice that for the last 95 years.

Although the McConnell ice limit only reached into the first few kilometres of the Duncan Creek valley at its mouth and its headwaters, associated glaciolacustrine and glaciofluvial sediments have inundated the valley to depths of up to 40 or more metres. Gold-bearing gravels are currently being mined beneath this thick cover of barren overburden.

Preliminary sedimentological and stratigraphic data indicate three main lithostratigraphic assemblages:: 1) Crudely stratified, imbricate boulder-cobble gravel and muddy boulder-cobble diamict; 2) Stratified cobble-pebble gravel, stratified sand and laminated silt; and 3) Stratified silt, massive boulder-cobble gravel and silty boulder diamict.

Lithostratigraphic assemblage 1 is gold-bearing and is interpreted as Reid-age subglacial or proximal glaciofluvial outwash and Reid-age proximal alpine glacial till. Gold in these sediments is concentrated either as a function of hydraulic interaction with bedrock topography, or as a result of incorporation and dispersion of a pre-existing placer deposit formed during a previous interglacial period. Lithostratigraphic assemblage 2 is interpreted as an interglacial wandering gravel bed river indicated by several fining upward sequences and wood radiocarbon dated at 32 320a. ±1270 B.P. (Beta-86851). Lithostratigraphic assemblage 3 is interpreted to be McConnell age glaciolacustrine silt, glaciofluvial outwash and glacial till.

The Keno Hill Silver District (United Keno Hill Mines) lies a few kilometres upstream of the placers and native silver nuggets have been recovered during present and historic placer mining. Lode sources of gold are also known to occur on nearby Mt. Hinton, which indicates the gold in the placers is likely from local bedrock sources. The ubiquitous and extensive nature of facies assemblage 2 combined with the possible existence of other hardrock sources of gold indicates that a significant potential exists for more placer gold reserves in the same drainage.

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