Historic placer mining areas in Yukon can be grouped into ten areas: Klondike; Sixtymile; Fortymile; Clear Creek; Moosehorn Range; Stewart River; Clear Creek; Mayo; Dawson Range; and Livingstone Creek. Each area has its own geomorphic setting and depositional history which is related to its glacial history. Several Quaternary glacial advances have been described in Yukon, and these are generally divided into three episodes, commonly known as the pre-Reid, Reid and McConnell, in order of oldest to most recent. Placer deposit in the unglaciated Klondike, Sixtymile, Fortymile and Moosehorn drainages occur in valley-bottoms, alluvial fans, in gulch gravels and as high level terraces. Placer deposits in glaciated areas occur in variably reworked and buried valley-bottom, bench and gulch settings, in auriferous glacial till and glaciofluvial gravels, and in non-glacial gravels which were deposited on top of glacial drift. Targets for new placer deposits in unglaciated areas include drainages such as Stewart, North Ladue and Yukon Rivers which lie outside of the pre-Reid glacial limit. Mineable placer deposits may also have formed on top of pre-Reid glacial drift and may be buried in valleys beneath Reid-age non-glacial alluvium. Prospective areas of this type are drainages which are near lode gold deposits in the Clear Creek area and in drainages near felsic volcanics in the Dawson Range. At the limits of both the Reid and McConnell glaciations, auriferous pre-glacial or interglacial gravel can often be buried by glacial and glaciofluvial deposits. Low-grade auriferous glaciofluvial gravel can also be derived form the reworking of pre-glacial gold-bearing gravel. Prospective areas for these types of placer deposits are the South McQuesten River valley and the creeks draining the Ruby Range on the east side of Kluane Lake. Within the McConnell glacial limits, placer deposits may be found in valleys oriented obliquely to the paleoflow direction of the glacial ice. Economic to sub-economic placers may also be found along meltwater channels within the McConnell ice limit. Prospective areas of this type of deposit are the drainages which lie to the north of Livingstone placer camp. The possibilities for new placer mining areas within glaciated areas must be investigated, and new placer gold reserves will undoubtedly be found within these areas. These potential gold deposits may be explored by techniques such as surficial mapping, airphoto interpretation and bulk sampling of potential gold-bearing units.

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.

Due to the depletion of traditional economic gold placer deposits in unglaciated areas of the Yukon, the study of the relationship of placer gravels to overlying glacial sediments in ice covered regions is important to future exploration activities. The livingstone Creek area in south central Yukon has supported placer mining operations for over 90 years. Present activity is centred on gold-bearing gravels buried by thick Pleistocene glacial deposits. The placer gravels are associated with coarse interglacial stream and gulch deposits and they are overlain by fine-grained, proximal, glaciolacustrine sediments. During the last glaciation, damming of gold-bearing, high gradient interglacial tributary stream channels by main valley ice caused rapid environmental changes. Depositional processes dominated over erosion in the ice-marginal lakes, and thick sequences of fine-grained suspension deposits, debris flow sediments, and deltaic sands and gravels accumulated. In addition, when glaciers expanded and overrode the area, these thick deposits protected the underlying placer gravels from subglacial erosion and dilution. Subglacial tills were probably deposited by lodgement, meltout and flow. During deglaciation, ice-marginal sedimentation again dominated. Post-glacial streams later cut through the glacial sediments and re-exposed the gold-bearing gravels. Stratigraphic, sedimentologic and geomorphic evidence from the Livingstone Creek area suggests that small tributary valleys, oriented transverse to the former direction of ice flow in the adjacent main valleys would make good exploration targets in regions of new placer interest.

Placer gold deposits are widespread throughout the largely unglaciated Stewart River and southern part of the Dawson map areas. These deposits include the world famous Klondike goldfields, the historic Fortymile and Sixty Mile goldfields, and well known placers along Black Hills, Scroggie, Thistle and Kirkman creeks. Although the deposits have been mined for over 100 years and have produced an estimate 311 tonnes of gold, they still account for about 85% of Yukon's annual placer gold production. The placer deposits are classified into three levels of gravel with four main units: high-level gravel, which usually forms prominent, continuous high-level terraces and is subdivided into the White Channel Gravel (which is locally subdivided into a lower White Gravel and an upper Yellow Gravel unit) and Klondike Gravel; intermediate-level gravel, which mostly forms relatively small, irregularly distributed intermediate to low-level terraces; and low-level gravel, which represents alluvium along present day creeks, gulches and rivers. The White Channel Gravel, is up to 46 m thick and characterized by a predominance of quartz clasts (which are generally more abundant in the White Gravel than in the Yellow Gravel). It is considered Early Pliocene to earliest Late Pliocene in age (~5 to 3 Ma). The Klondike Gravel, not considered an economical placer, is up to 53 m thick and is distinguished by chert clasts derived from the Ogilvie Mountains, located northeast of the map areas. It was deposited as glaciofluvial outwash during the end of the initial and most widespread of the pre-Reid glaciations, and is probably latest Early Pliocene to earliest Late Pliocene (~3 Ma). The intermediate-level gravel, the least important economically, is up to 9 m thick. The low-level gravel, historically the most important gold-bearing unit, is 5 m thick in creeks and up to 20 m thick in rivers. The intermediate-level and low-level gravel have similar amounts of quartz, igneous and metamorphic rock particles, although locally, the low-level gravel contains sedimentary rock particles. The intermediate-level gravel is thought to be Late Pliocene to Early Pleistocene (~3 Ma to 750 Ka) in age and the low-level gravel is considered Late Pleistocene to Holocene in age. Practically all of the placers are fluvial in origin and were deposited primarily in braided streams that flowed parallel to the present day streams along which the deposits occur. Gold recovered from the various levels of gravel is detrital in origin and was mainly derived from early Mesozoic auriferous quartz veins. The concentration of gold in the gravel is related to a hierarchy of physical scales: at the lithofacies scale (metres), bed roughness determined sites of gold deposition; at the element scale (tens of metres), gravel bars were preferentially enriched in gold; at the reach scale (hundreds of metres), stream gradient was an important factor; at the system scale (hundreds of kilometres), braided river environments transported large amounts of gold; and at the sequence scale (thousands of kilometres), economic placers formed initially in the high-level White Channel Gravel and later in the intermediate- and low-level gravel.

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.

The Haggart Creek study area contains unconsolidated sediments, which represent six main geomorphic settings: 1) Reid-age glacial and glaciofluvial deposits; 2) McConnell-age proximal braided stream deposits to periglacial alluvial fans; 3) McConnell-age medial to distal braided stream deposits; 4) post-McConnell wandering gravel bed river; 5) Holocene gulch deposits; and 6) Holocene colluvium. Placer gold within the study area is known to originate from at least two main local lode sources: 1) gold in sheeted quartz veins within a Late Cretaceous-age granodiorite stock; and 2) gold in isolated quartz-sulphide fissure veins found in bedrock along Haggart Creek. Placer gold is believed to have formed as a result of reworking and re-concentration of gold during interglacial times, as well as reworking of older auriferous interglacial gravel by McConnell-age periglacial alluvium. The primary setting for placer gold deposits is Late McConnell to Early Holocene alluvium, on or near bedrock. Sub-crystalline to crystalline placer gold grains recovered from sediments found far removed from currently known local bedrock sources suggests that other lode sources may be present within the study area. A copy of this thesis is available at the EMR library – QE195.W47 1999. This thesis is available online at http://hdl.handle.net/1880/25379.

The central Yukon Territory has a number of favourable placer deposit settings due to its unique history of multiple glaciations, active stream sedimentation in association with proglacial outwash settings and terrain which has remained unglaciated. Placer gold was found along the Stewart River on point bars in 1884 prior to the discovery of gold in the Klondike area. This was the first indication that the Yukon Territory contained important economic concentrations of placer gold. This study is concerned with the late Tertiary and Quaternary geology in the Lower Stewart River and adjacent Yukon River above Dawson. Previous systematic surficial geological mapping and testing for placer gold on the high-level terraces along these rivers has been limited. This report describes the sedimentology and stratigraphy of key gravelly exposures in this area because similar high-level terraces in the Fortymile River drainage in Alaska had been mined for gold for many years. Work of this type also provides information on the physical characteristics of gravelly deposits (e.g., grain size distribution) which may assist regulatory decisions on placer mining in the lower Stewart and Yukon drainages. Accompanying this report are two 1:50 000-scale surficial geology maps including marginal notes (Garner Creek, NTS 115O/13 and Matson Creek and Ogilvie NTS 115N/9 (east half) and 115O/12), as well as one 1:250 000-scale topographic map (Stewart River - NTS 115N/O) including field study site locations, heavy mineral sample sites and hardrock mineral occurrences.

The study area is underlain by four stratigraphic successions ranging in age from Middle Proterozoic to Early Paleozoic. From oldest to youngest, they are: Middle Proterozoic Wernecke Supergroup; Middle to Upper Proterozoic Pinguicula Group; Upper Proterozoic Windermere Supergroup; and Uppermost Proterozoic to Lower Paleozoic sandstone and carbonate. Together, they represent about a billion years of intermittent sedimentation punctuated by processes such as deformation, uplift, erosion, magmatism and mineralization. Rocks in the study area record eight phases of contractional and extensional deformation, some of which may be related to strike-slip faulting. Two phases of southwest-verging folds and thrust faults may be related to dextral transpression on the Snake River Fault. Mineral enrichments occur in two general forms:: breccia-related (Middle Proterozoic), and veins (Mesozoic to Tertiary). The breccia-related occurrences have enrichments of Cu ± U, Co, Au and Ag, as dissemminations and veinlets in and near intrusive breccia zones (Wernecke breccia). The vein occurrences comprise Zn-Pb-Ag ± Cu and Au, in veins and related lenses and irregular replacements of carbonate.

Yukon has over a century of placer mining history, predominately in unglaciated regions. However, as these targets are exploited, focus turns to more complex landscapes where glaciation has buried, eroded and incorporated placer gold. This study examines how Early Pleistocene glaciation in the Mount Nansen area, central Yukon, has affected placer gold deposits. Detailed stratigraphic analysis and sample collection has focused on Back Creek, where placer mining has exposed a 22 m section with several gold bearing units. In the section, sediment from two glacial advances cap sporadically preserved pre-glacial gravel. The section is variably dissected by younger placer gold bearing fluvial gravel with enrichment related to intersection of inter-glacial or pre-glacial placer gold deposits. Analysis at Back Creek reveals the potential for deeply buried placer gold deposits in other glaciated regions of Yukon.