Disturbance of frozen ground, through placer mining activities, may lead to slope failure. A stockpile of frozen overburden has recently begun eroding from the formation of a runoff channel. A large cut, approximately 300 m long and up to 50 m deep has been created adjacent to Gold Run Creek, a heavily mined area southeast of Dawson City. This stockpiled overburden has been in place for several years and has revegetated with several species of mature willows (age dated to 15 years). Either due to heavy rainfall or climate change, melting has commenced in the stockpile resulting in the formation of a runoff channel. This has increased in size and slope failure continues to enlarge the cut. Laberge Environmental Services conducted a reconnaissance survey of the site in July 2003, to assess the site with the purpose of exploring ways of halting the slope failure and stabilizing the disturbed section of overburden. In the fall of 2005, several bioengineering structures were installed to assist in controlling the erosion. Retaining walls were installed on the side walls and the face of the cut, and a live willow flume was built to direct and reduce the velocity of the water entering the cut.

A large erosion cut, approximately 300 metres long and up to 50 metres deep, has occurred on a stockpile of frozen overburden on the east bank of Gold Run Creek, a placer mining area southeast of Dawson City. In September 2005 erosion control structures reinforced with live willow cuttings were installed at the site. These structures consisted of earth retaining walls built to stabilize the slope failure and a flume to convey the water past the retaining walls while the willows had time to grow and strengthen the structures. In July 2006 the site was revisited. It was found that, while the earth retaining walls were intact, the flume had malfunctioned and further erosion had occurred beyond the walls. It was concluded that the collapse of the flume was the primary cause of the failure of the erosion control project.

Previously unrecognized glacial erosional landforms (i.e. cirques, u-shaped troughs, truncated spurs and arêtes, in order of increasing doubt), and glacial depositional landforms (i.e. end moraine and possibly ground moraine) occur in the Fifty Mile Creek area, west of the pre-Reid Cordilleran glacial limit. The cirques and end moraine, representing the best evidence of glaciation, are similar to landforms in the adjacent Yukon-Tanana uplands of Alaska and formed during the Eagle glaciation (>40 ka, or Reid in age). Glaciation caused climate-controlled variations in runoff and cycles of aggradation and incision in the Fifty Mile Creek drainage. This resulted in the formation of upper- and lower-level terraces along Fifty Mile Creek and its tributaries. The terraces are composed of slightly muddy, sandy gravel of locally derived lithologies, and are fluvial in origin. Placer gold occurs along Fifty Mile Creek and several of its tributaries, as well as in the lower-level terraces. The upper-level terraces are potentially placer-gold bearing.

Grew Creek epithermal gold prospect, in south-central Yukon Territory, is adjacent to and southwest of the Robert Campbell Highway, halfway between the communities of Ross River and Faro. The prospect is within the Tintina Trench, which from Late Cretaceous to Tertiary time was a zone of major right lateral movement that juxtaposed the Cambrian and Ordovician slate and phyllites of the Pelly-Cassiar Platform (to the southwest) against rocks of the Anvil Allochthon (to the northeast). Grew Creek rocks are mid-Eocene based on K-Ar dates of basalt of 51.4 ±1.8 Ma and 50.7 ±1.8 Ma and pollen spores in volcaniclastic rock dated at 56 to 46 Ma. Felsic volcanic and volcaniclastic rocks were overlain by a sequence of interbedded coarse clastic sediments, basaltic flows, and basaltic volcaniclastic rock. Late Tertiary uplift and faulting resulted in graben formation and consequent preservation of Eocene rocks in a structurally complex graben bounded to the south by the Grew Creek fault and to the north by the Danger Creek fault. Mineralization at Grew Creek occurs at the tip of a westwardly pointing wedge of dominantly felsic, crystal lithic lapilli tuff. The zone of precious metal deposition is truncated to the northeast by steeply dipping clastic sediments and to the southwest by the Grew Creek Fault. Gold, electrum, pyrite, and silver selenide were identified in a high grade sample from the discovery outcrop. Alteration at Grew Creek is both surficial and hydrothermal. Surficial alteration is ubiquitous, pervasive, and characterized by mixed-layer clays and carbonates. Hydrothermal alteration, responsible for the gold-silver mineralization is closely associated with rhyolitic dykes and is of three types:: silicic, acid sulphate, and argillic acid sulphate. K-Ar dating of sericite indicates hydrothermal alteration is mid-Eocene (51.5 ±1.8 Ma and 47.0 ±1.7 Ma) and synchronous with deposition of the volcanics. Quartz associated with mineralization at Grew Creek is enriched in heavy oxygen isotopes. A deep magmatic source for the mineralized fluids is one explanation for this enrichment.

The production of acid rock drainage (ARD) from mine tailings is a significant environmental concern at various abandoned mine sites in Yukon. Leachate with low pH and high dissolved metals concentrations deriving from mine tailings impoundment areas can negatively impact various groundwater and surface water resources. The presence of both oxygen and water is required for ARD to develop. Therefore, the reduction of the oxygen source and the water source from the mine tailings through the use of a low permeability cover (which acts as an oxygen/infiltration barrier) will limit ARD production. This is the basis of the design and recent reclamation (1998-99) of the Arctic Gold and Silver Tailings Site in Carcross, Yukon that EBA has been involved with in association with PWGSC.

The Grew Creek epithermal gold-silver deposit in southeast Yukon (MINFILE 105 K 009) is hosted by Eocene volcanic and sedimentary rocks deposited in a pull-apart basin within the Tintina Fault Zone. Flow rhyolites forming a dome in the Tarn Zone area, 1.5 km east of Grew Creek, pass westward into a succession of rhyolitic ignimbrites and air fall tuffs, exposed along Grew Creek and in the Main Zone, 500 m west of Grew Creek. These rhyolitic rocks are faulted against fluvial sediments to the north, along the W-E Fault, and basaltic rocks to the west. In the Main Zone, the volcanics, sediments, and the W-E Fault all dip steeply to the north. The gold-silver mineralization forms an elongate tabular zone within the rhyolitic tuffs. The zone strikes parallel to the W-E Fault and dips vertically or steeply to the north. The eastern end of the mineralized zone is defined by a decrease in grade, whereas the western end is faulted off against the basaltic rocks. Within the zone, stockwork veins and hydrothermal breccias contain assemblages which include quartz, adularia, carbonates, quartz peudomorphous after calcite, pyrite, marcasite, and traces of arsenopyrite, chalcopyrite, acanthite, electrum, silver selenides, galena, and sphalerite. There is good correlation between gold and silver in drill core assays, with a gold::silver ratio of around 1::4 for the ore grade mineralization. The mineralization is strongly anomalous in arsenic and mercury, but there is only a weak correlation of mercury with gold and silver, with most high values for mercury lying above the gold-silver zone and associated with the W-E Fault. Arsenic conccentrations are elevated over much of the area but there is no statistical correlation with the locally high concentrations of gold or silver. Outcropping rhyolitic rocks are hydrothermally altered to intermediate argillic and advanced argillic assemblages, whereas subsurface rhyolitic rocks are altered to quartz-adularia or illite-quartz assemblages adjacent to veins, and to illite-quartz-adularia ± carbonate elsewhere. Advanced illite-quartz-adularia ± carbonate alteration is accompanied by an increase in Na2O and decreases in TiO2, CaO and Al2O3. Basalts are altered to carbonate-chlorite (propylitic) assemblages, accompanied at an advanced stage by a slight increase in CaO and decreases in K2O, Na2O, SiO2, and Al2O3. Mineralization postdated tilting of the host pyroclastic and sedimentary rocks. Episodic fault movements in the Tintina Fault Zone structurally focused the hydrothermal fluids by providing locally high secondary permeability, whereas the high primary permeability of the rhyolitic tuffs promoted the development of stockwork veins and breccias. The absence of significant alteration and mineralization in the sediments suggests that a partly welded and intensely altered tuff unit, along the footwall of the W-E Fault, acted as an aquiclude, confining the hydrothermal fluid within the rhyolitic tuffs. Intense pyritic alteration of this unit and high concentrations of mercury in the vicinity of the W-E Fault form pyrite and mercury zones north of the mineralization.

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.

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Two tills and related deposits are the products of at least two ice advances over the Tintina Trench near Ross River, Yukon Territory. These ice advances are termed the McConnell and pre-McConnell glaciations. Erosional remnants provide evidence of the pre-McConnell glaciation and indicate that the ice was flowing to the west or northwest. The onset of McConnell glaciation was marked by an early ice advance out of the Lapie River valley, which was followed by a general ice flow toward the west or northwest along the Tintina Trench. During the retreat of the McConnell glacier, an ice tongue advanced up the Lapie River valley, blocking the drainage and forming a glacial lake. To develop and apply drift prospecting techniques in the Tintina Trench, 204 till samples were collected over the study area. The silt plus clay size fraction was analysed for Au and the clay fraction was analysed for 30 elements. Only Au, Ag, Hg and Sb results are discussed in this paper. The geochemical data for till down-ice from the Grew Creek Au-Ag mineralization (MINFILE 105K 009) show a dispersal train for gold, but not for pathfinder elements such as Ag, As, Hg and Sb. A possible relationship between Au and Tertiary volcanic rocks is illustrated. However, closer-spaced samples would have to be taken to verify this hypothesis, since the length of the Au dispersal train is about 500 m, much smaller than the sampling interval.

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