Lithogeochemical localisation of disseminated gold in the White River area, Yukon, Canada

Gold mineralisation in the White River area, 80 km south of the highly productive Klondike alluvial goldfield, is hosted in amphibolite facies gneisses in the same Permian metamorphic pile as the basement for the Klondike goldfield. Hydrothermal fluid which introduced the gold was controlled by fracture systems associated with middle Cretaceous to early Tertiary extensional faults. Gold deposition occurred where highly fractured and chemically reactive rocks allowed intense water–rock interaction and hydrothermal alteration, with only minor development of quartz veins. Felsic gneisses were sericitised with recrystallisation of hematite and minor arsenic mobility, and extensively pyritised zones contain gold and minor arsenic (ca 10 ppm). Graphitic quartzites (up to 5 wt.% carbon) caused chemical reduction of mineralising fluids, with associated recrystallisation of metamorphic minerals (graphite, pyrrhotite, pyrite, chalcopyrite) in host rocks and veins, and introduction of arsenic (up to 1 wt.%) to form arsenopyrite in veins and disseminated through host rock. Veins have little or no hydrothermal quartz, and up to 19 wt.% carbon as graphite. Late-stage oxidation of arsenopyrite in some graphitic veins has formed pharmacosiderite. Gold is closely associated with disseminated and vein sulphides in these two rock types, with grades of up to 3 ppm on the metre scale. Other rock types in the White River basement rocks, including biotite gneiss, hornblende gneiss, pyroxenite, and serpentinite, have not developed through-going fracture systems because of their individual mineralogical and rheological characteristics, and hence have been little hydrothermally altered themselves, have little hydrothermal gold, and have restricted flow of fluids through the rock mass. Some small post-metamorphic quartz veins (metre scale) have been intensely fractured and contain abundant gold on fractures (up to 40 ppm), but these are volumetrically minor. The style of gold mineralisation in the White River area is younger than, and distinctly different from, that of the Klondike area. Some of the mineralised zones in the White River area resemble, mineralogically and geochemically, nearby coeval igneous-hosted gold deposits, but this resemblance is superficial only. The White River mineralisation is an entirely new style of Yukon gold deposit, in which host rocks control the mineralogy and geochemistry of disseminated gold, without quartz veins.     

A decade of sedimentation in ice-contact, proglacial lakes, Bering Glacier, AK

Bathymetric surveys during the 1991–2000 decade in two ice-contact, proglacial lakes on the eastern sector of Bering piedmont lobe captured the buildup effects of the 1993–1995 surge. Following ice-front advance of 1.0–1.5 km into Tsivat and Tsiu Lakes, the basins were significantly altered by surge-related sedimentation including the impact of a subglacial outburst into Tsivat Lake. The subsequent changes in basin shape, size, and morphology were monitored by six bathymetric surveys. Measured changes in water depth serve as a proxy for determining increments of sediment accumulation.

Upwelling, ice-front vents fed by subglacial tunnels transported suspended fine sediment directly into the lake system. The rate of suspension settling within both lakes varied from 0.6 to 1.2 m year⁻¹ prior to the surge. Suspended load during surge years increased sixfold from 1.7 to 13.9 g l⁻¹, accompanied by increased sediment accumulation of 2.2–3.1 m year⁻¹. Vent-related aggradation and subsequent filling of Tsivat Lake caused sediment bypassing to Tsiu Lake, where encroachment by delta growth contributed to a postsurge rate of bottomset accumulation of 3.0 m year⁻¹.

The total sediment influx from subglacial sources is represented by the sum of bathymetrically determined accumulation, plus an estimated volume of sediment that remained suspended, thus passing through the lake system. Total sediment flux along the eastern Bering piedmont lobe from 1991 to 2000 is approximately 227 million cubic meters.     

Calcium isotopes in a proglacial weathering environment: Damma glacier, Switzerland

The biogeochemical cycling and isotopic fractionation of calcium during the initial stages of weathering were investigated in an alpine soil chronosequence (Damma glacier, Switzerland). This site has a homogeneous silicate lithology and minimal biological impacts due to sparse vegetation cover. Calcium isotopic compositions, obtained by TIMS using a ⁴³Ca-⁴⁶Ca double spike, were measured in the main Ca pools. During this very early stage of weathering, the young soils which have formed (δ^44/42Ca=+0.44‰) were indistinguishable to the rocks from which they were derived (δ^44/42Ca=+0.44‰) and stream water (δ^44/42Ca=+0.48‰) was also within error of the average rock. This lack of variation indicates that the dissolution of the bulk silicate rock does not strongly fractionate Ca isotopes. The only Ca pool which was strongly fractionated from bulk rock was vegetation, which exhibited an enrichment of light Ca isotopes. Significant Ca isotope fractionation between bulk rock and the dissolved flux of Ca is likely to only occur where the Ca biogeochemical cycle is dominated by secondary processes such as biological cycling, adsorption and secondary mineral precipitation.     

Sediment production and transport in a proglacial stream: Hilda Glacier, Alberta, Canada

Hilda Glacier, a small cirque glacier in the Canadian Rocky Mountains, yields two principal types of sediment: ablation till, deficient in fine material and produced by rockfalls and avalanches falling on to the glacier surface, and basal lodgement till, rich in fines and formed mainly by subglacial erosion. Recent recession from its Neoglacial maximum has exposed large areas of basal till with thin veneers of ablation till which, when combined with present subglacial and supraglacial debris, provide abundant material for erosion and transport by the mcltwatcr stream. Sediment transport measurements over two summers (1977–1978) showed that bed load and suspended load occur in approximately equal proportions and that dissolved loads are minor. Local source variations, especially bank slumps, are a major cause of scatter in sediment rating curves. Suspended-sediment concentrations are greater early in the melt season due to availability of loose sediment produced by freezing and thawing. Other contributors to scatter in suspended-sediment rating curves include rain showers and diurnal hysteretic effects. Although the distinction between bed load and suspended load is never sharp, available data suggest that the sand/ gravel grain-size boundary (-1ø) approximates the suspendcd-load/bed-load division for characteristic Hilda flows transporting gravel. This approximation, combined with till grain-size analyses, suspended-sediment measurements, and spatial distributions of till types, leads to the following computations of fluvial sediment sources: for suspended load - 6% supraglacial, 47% subglacial, 47% channel banks; for bed load - 46% supraglacial, 27% each subglacial and channel banks. Supraglacial debris provides only about one-fourth of all fluvial sediment, but nearly half of the bed load.     

Holocene glacial and tree-line variations in the White River Valley and Skolai Pass,Alaska and Yukon Territory

Complex glacier and tree-line fluctuations in the White River valley on the northern flank of the St. Elias and Wrangell Mountains in southern Alaska and Yukon Territory are recognized by detailed moraine maps and drift stratigraphy, and are dated by dendrochronology, lichenometry, ¹⁴C ages, and stratigraphic relations of drift to the eastern (1230 ¹⁴C yr BP) and northern (1980 ¹⁴C yr BP) lobes of the White River Ash. The results show two major intervals of expansion, one concurrent with the well-known and widespread Little Ice Age and the other dated between 2900 and 2100 ¹⁴C yr BP, with a culmination about 2600 and 2800 ¹⁴C yr BP. Here, the ages of Little Ice Age moraines suggest fluctuating glacier expansion between ad 1500 and the early 20th century. Much of the 20th century has experienced glacier recession, but probably it would be premature to declare the Little Ice Age over. The complex moraine systems of the older expansion interval lie immediately downvalley from Little Ice Age moraines, suggesting that the two expansion intervals represent similar events in the Holocene, and hence that the Little Ice Age is not unique. Another very short-lived advance occurred about 1230 to 1050 ¹⁴C yr BP. Spruce immigrated into the valley to a minimum altitude of 3500 ft (1067 m), about 600 ft (183 m) below the current spruce tree line of 4100 ft (1250 m), at least by 8020 ¹⁴C yr BP. Subsequent intervals of high tree line were in accord with glacier recession; in fact, several spruce-wood deposits above current tree line occur bedded between Holocene tills. High deposits of fossil wood range up to 76 m above present tree line and are dated at about 5250, 3600 to 3000, and 2100 to 1230 ¹⁴C yr BP. St. Elias glacial and tree-line fluctuations, which probably are controlled predominantly by summer temperature and by length of the growing and ablation seasons, correlate closely with a detailed Holocene tree-ring curve from California, suggesting a degree of synchronism of Holocene summer-temperature changes between the two areas. This synchronism is strengthened by comparison with the glacier record from British Columbia and Mt. Rainier, Likewise, broad synchronism of Holocene events exists across the Arctic between the St. Elias Mountains and Swedish Lappland. Finally, two sequences from the Southern Hemisphere show similar records, in so far as dating allows. Hence, we believe that a preliminary case can be made for broad synchronism of Holocene climatic fluctuations in several regions, although further data are needed and several areas, particularly Colorado and Baffin Island, show major differences in the regional pattern.     

Limnology of the Klutlan moraines,Yukon Territory,Canada

Lakes of the Klutlan moraines originate by down-melting of stagnant ice under a mantle of rock debris and vegetation ranging from scattered herbs and shrubs on the younger moraines to multiple-generation closed spruce forest on the oldest moraines, which are 600–1200 yr old. Lakes on the youngest moraines are temporary, turbid with glacial silt, and marked by unstable ice-cored slopes. On older moraines most lakes have clear water and stable slopes. On the oldest moraines many lakes have brown water caused by dissolved humic materials derived from the thick forest floor, but even here some slopes are unstable because of continued melting of buried ice. Morainic lakes contain bicarbonate waters of moderate alkalinity and conductivity and low levels of nutrients. The highly diverse phytoplankton is dominated by chrysophytes and cryptomonads, with few diatoms. Extremely low values for phytoplankton biomass place most of the lakes in an “ultraoligotrophic” category. Zooplankton is dominated by copepods, which were found even in ice ponds only a few years old, and by the cladoceran Daphnia pulex. Surface-sediment samples contained a total of 16 species of chydorid Cladocera. Of these, Alonella excisa and Alona barbulata are apparently the pioneer species in the youngest lakes. Chydorus sphaericus only appears in lakes of the oldest moraines. A successional pattern is not conspicuous, however, partly because some of the lakes on the older moraines originated by recent collapse over buried ice. Lakes on the upland outside the dead-ice moraines yielded 39 species in the zooplankton. The distinctive assemblage on upland lakes may relate more to different water chemistry than to age.     

The Mount Nansen gold-silver deposit,Yukon territory,Canada

The Mount Nansen ore deposit consists of a system of narrow steeply dipping veins in metamorphic rocks of Precambrian to Paleozoic age, andesitic volcanics and granodioritic to granitic intrusives of Mesozoic age, and Cretaceous to early Tertiary dacitic porphyries.Pyrite, arsenopyrite, sphalerite, galena and freibergite are the main constituents of the sulphide ore which has been mined for its gold and silver content. In parts of the mine the sulphide minerals are altered to a complex association of secondary minerals. No supergene enrichment zones have formed, suggesting very limited transport during alteration as a possible result of the existing permafrost conditions. Statistical evaluations of gold and silver assay data indicate a down-dip decrease of the silver content and a more uniform gold distribution.Two possible metallogenetic explanations are given: 1. a hydrothermal origin of the deposit as a final phase of the Cretaceous-early Tertiary magmatic episode 2. a source horizon concept with the metal content of the ore deposit having been derived from the surrounding country rocks.

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Zusammenfassung Die Mount Nansen Lagerstätte setzt sich aus einem System geringmächtiger, steil einfallender Gänge zusammen. Diese durchsetzten präkambrische bis paläozoische Gesteine, mesozoische, andesitische Extrusiva und granodioritische bis granitische Intrusiva sowie kretazische bis frühtertiäre dazitische Porphyre.Pyrit, Arsenkies, Zinkblende, Bleiglanz und Freibergit sind die Hauptbestandteile des sulfidischen Erzes, welches seines Gold- und Silbergehaltes wegen abgebaut wurde. Die Sulfidmineralien wurden in gewissen Abschnitten der Mine zu einer komplexen Vergesellschaftung sekundärer Erze umgewandelt. Die Abwesenheit einer supergenen Anreicherungszone kann auf die herrschenden Permafrostbedingungen zurückgeführt werden, welche nur einen limitierten Stofftransport während der Verwitterung zuließen. Die statistische Auswertung der zahlreichen Gold- und Silberanalysenresultate lassen eine Abnahme des Silbergehaltes nach der Teufe und eine eher gleichmäßige Verteilung des Goldgehaltes erkennen.Zwei metallogenetische Deutungen werden für das Lager diskutiert. Erstens eine hydrothermale Herkunft als abschließende Phase des kretazischen bis frühtertiären Magmatismus, und zweitens eine pseudohydrothermale Herkunft (source horizon concept), wobei das umliegende Nebengestein als Quelle für die Metallgehalte des Lagers betrachtet wird.

     

U-Series Chronology of Lacustrine Deposits in Death Valley, California

Uranium-series dating on a 186-m core (DV93-1) drilled from Badwater Basin in Death Valley, California, and on calcareous tufas from nearby strandlines shows that Lake Manly, the lake that periodically flooded Death Valley during the late Pleistocene, experienced large fluctuations in depth and chemistry over the last 200,000 yr. Death Valley has been occupied by a long-standing deep lake, perennial shallow saline lakes, and a desiccated salt pan similar to the modern valley floor. The average sedimentation rate of about 1 mm/yr for core DV93-1 was punctuated by episodes of more-rapid accumulation of halite. Arid conditions similar to the modern conditions prevailed during the entire Holocene and between 120,000 and 60,000 yr B.P. From 35,000 yr B.P. to the beginning of the Holocene, a perennial saline lake existed, over 70 m at its deepest. A much deeper and longer lasting perennial Lake Manly existed from about 185,000 to 128,000 yr B.P., with water depths reaching about 175 m, if not 330 m. This lake had two significant “dry” excursions of 10²–10³yr duration about 166,000 and 146,000 yr B.P., and it began to shrink to the point of halite precipitation between 128,000 and 120,000 yr B.P. The two perennial lake periods correspond to marine oxygen isotopic stages (OIS) 2 and 6. Based on the shoreline tufa ages, we do not rule out the possible existence 200,000 yr ago of yet a third perennial lake comparable in size to the OIS 6 lake. The²³⁴U/²³⁸U data suggest that U in tufa owes its origin mainly to Ca-rich springs fed by groundwater that emanated along lake shorelines in southern Death Valley, and that an increase of this spring-water input relative to the river-water input apparently occurred during OIS 6.     

Asian dustfall in the St. Elias Mountains, Yukon, Canada

In April 2001, a major atmospheric dustfall event occurred in the St. Elias Mountains, Yukon Territory, Canada. Field samples were collected and analyzed for particle size, mineralogy, chemical composition and Sr, Nd and Pb isotopes. Dusts found above ∼3000 m had their source in the Gobi desert region of northern China and Inner Mongolia, and were transported to the Yukon following a series of major dust storms that took place in early April. Dusts found below 3000 m had local (Yukon) or mixed source(s). The Asian dusts had a modal volume diameter of ∼4 μm typical of far-traveled mineral aerosols. However larger (>10 μm) particles were also found at ∼5000 m, suggesting a very rapid trans-Pacific transport in the mid-troposphere. We estimate that the April 2001 event deposited from 5500 to 6335 tons of dust over an area of 21,000 km² in the southwestern Yukon, most of which probably fell within a week. Our findings are consistent with instrumental observations and model simulations of the April 2001 event. While the dust cloud was reportedly mixed with volatile pollutants from Asia, we found no evidence of metal pollution associated with the dustfall in the Yukon. Our findings contribute to clarify the dynamics and the geochemical impact of Asian dust long-range transport events, and to better estimate eolian fluxes of dustborne elements (e.g., Fe) to the Ocean associated with such events. They may also assist in identifying past Asian dust events in ice cores drilled from the St. Elias Mountains icefields, to develop a long-term record of their frequency, magnitude and source(s).     

Postglacial climates inferred from a lake at treeline,southwest Yukon Territory,Canada

Pollen, chironomid, and ostracode records from a lake located at alpine treeline provide regional paleoclimate reconstructions from the southwest Yukon Territory, Canada. The pollen spectra indicate herbaceous tundra existed on the landscape from 13.6–11 ka followed by birch shrub tundra until 10 ka. Although Picea pollen dominated the assemblages after 10 ka, low pollen accumulation rates and Picea percentages indicate minimal treeline movement through the Holocene. Chironomid accumulation rates provide evidence of millennial-scale climate variability, and the chironomid community responded to rapid climate changes. Ostracodes were found in the late glacial and early Holocene, but disappeared due to chemical changes of the lake associated with changes in vegetation on the landscape. Inferred mean July air temperature, total annual precipitation, and water depth indicate a long-term cooling with increasing moisture from the late glacial through the Holocene. During the Younger Dryas (12.9–11.2 ka), cold and dry conditions prevailed. The early and mid-Holocene were warm and dry, with cool, wet conditions after 4 ka, and warm, dry conditions since the end of the Little Ice Age.     

Wastage of the Klutlan ice-cored moraines,Yukon Territory,Canada

A series of ice-cored Neoglacial moraines at the terminus of the Klutlan Glacier covers an area of 90 km². Studies were made to determine empirically how long ice persisted in the Klutlan moraines and to develop models that can accurately predict wastage rates under current climatic conditions. A meltout curve based on climatological data reflects the sum of three melting processes: surficial melting, melting by lake water, and melting by geothermal heat. About 950 yr are required to melt 180 m of ice with a debris concentration of 1%, or about 1200 yr for a 1.5% debris load. Another meltout curve, based on seismic data, suggests total meltout in about 875 yr. When all geologic factors are considered, the empirical meltout curve is remarkably similar to that derived by considering the major heat-flux parameters. Meltout rates can be predicted if (1) the fundamental climatic parameters can be ascertained, and (2) the sediment concentration in the ice is known.     

Lacustrine sedimentation in a semiarid alpine setting: An example from Ladakh, northwestern Himalaya

The Lamayuru lacustrine strata in Ladakh typify many of the carbonate-rich Pleistocene alpine lakes found in the semiarid environment of the northern Himalaya. Created by a 200-m-thick landslide, the lake was in existence by at least 35,000 yr ago, and may have persisted until 500–1000 yr ago. Represented in the center by thin turbidites and laminated muds, the lacustrine sedimentation along the lake margins and low-relief deltas characteristically displays a marked contrast between (1) clastic lenses representing rapid, sporadic, matrix-poor debris flows and periglacial inputs from the alpine slopes and (2) abundant, diverse, shallow-water, biologically dominated carbonate strata, among which organism-rich, chalky beds and oncolithic and encrusted stem-rich strata predominate. Resemblances of the Lamayuru lacustrine strata and their setting to those of former lakes throughout areas north of the Greater Himalayan crest suggest that the alpine, semi-arid environment would favor diversified, spacially restricted carbonate sedimentation punctuated by occasional clastic influxes. Such a depositional regime contrasts strongly with that found immediately south of the Himalayan crest where more humid conditions promote a more continuous clastic influx into intramontane lakes.     

Inferring groundwater contributions and pathways to streamflow during snowmelt over multiple years in a discontinuous permafrost subarctic environment (Yukon, Canada)

Research on large northern rivers suggests that as permafrost thaws, deeper groundwater flowpaths become active, resulting in greater baseflow, increased concentrations of weathering ions and reduced concentrations of dissolved organic carbon in the streamflow. In contrast, at the headwater-catchment scale, where understanding of groundwater/surface-water interactions is developed, inter-annual variability in climate and hydrology result in complex hydrological and chemical responses to change. This paper reports on a 4-year runoff investigation in an alpine discontinuous permafrost environment in Yukon, Canada, using stable isotopes, major dissolved ions and hydrometric data, to provide enhanced insight into the inter-annual-variability runoff-generation processes. Stable isotope results suggest that pre-event (old) water stored within the catchment dominates the snowmelt hydrograph, and dissolved ion results reveal that groundwater pathways occur predominantly in the near-surface during freshet. Dissolved organic carbon varies inter-annually, reflecting changing melt patterns, whereas weathering ions generated from deeper flowpaths become diluted. The total snow-water equivalent does not have a major influence on the fraction of snowmelt water reaching the stream or the runoff ratio. Results from multiple years highlight the considerable variability over short time scales, limiting our ability to detect climate-change influences on groundwater at the headwater scale.     

Patterns of Late Pleistocene proglacial fluvial sedimentation in the SE Lithuanian Plain

We investigated proglacial fluvial sedimentation processes in the SE Lithuanian Plain by means of lithofacies analysis. The main parameters on which interpretations were based, were depositional structures of sandy sediments, paleohydraulic parameters and grain-size distribution. The development of the SE Lithuanian Plain illustrates the phenomenal shift of proglacial fluvial sedimentation from outwash plain to ice-marginal river during the ice retreat of the last glaciation. Three facies assemblages of braidplain deposits and two facies assemblages of palaeovalley deposits were distinguished in the sandy plain by means of qualitative and quantitative sedimentological research. This raises the question how the commonly accepted development of a SE orientated outwash plain could turn into an almost perpendicular (SW running) ice-marginal river. The vertical and lateral transitions of outwash plain facies present a classical example of braidplain evolution on slightly inclined lowlands, with well expressed proximal, middle and distal parts. Mid- and side-channel (point-bar) deposits of the proglacial valley point, however, which is unusual for proglacial fluvial systems, to a meandering character of the ice-marginal river. River types of various scale show a change from braided into meandering in the proglacial subenvironment. Lithofacies analysis and paleohydraulic parameters show distinct differences of the hydrodynamic regime during the first stage of the sandy plain development: from sedimentation on an outwash plain in a proglacial valley to sedimentation on a braidplain in a wide ice-marginal valley. The outwash system is characterized by a distinct downstream decrease in energy, whereas the ice-marginal river maintained most of its power and velocity.     

Holocene carbonate sedimentation in Lake Manitoba, Canada

The carbonate mineral suite of the modern offshore bottom sediment of the South Basin of Lake Manitoba consists mainly of high magnesian calcite and dolomite with minor amounts of low-Mg calcite and aragonite. The high-Mg calcite is derived from inorganic precipitation within the water column in response to supersaturation brought about by high levels of organic productivity in the basin. Both dolomite and pure calcite are detrital in origin, derived from erosion of the surrounding carbonate-rich glacial deposits. Aragonite, present only in trace amounts in the offshore sediments, is bioclastic in origin. The upward increase in the amount of magnesian calcite in the post-glacial sediment record is attributed to increasing photosynthetic utilization of CO₂ in the lake. Stratigraphic variation in the amount of magnesium incorporated into the calcite lattice is interpreted as reflecting a variable magnesium input to the lake from ground water and surface runoff, and possibly variable calcium removal in the precipitating lake water. The effects of long-term chemical weathering at the source and size segregation explain the changes in dolomite content throughout the section.     

Post-glacial loess deposition in a montane environment: South Thompson River valley,British Columbia,Canada

In the late Wisconsinan, the South Thompson River valley, British Columbia, was occupied by an ice-dammed lake. After the lake drained, the exposed lacustrine silt became the source material for a Holocene loess. The purpose of this paper is to establish the stratigraphic, depositional and geomorphic framework of loess occurring along the South Thompson River valley immediately east of Kamloops, British Columbia. This montane environment of loess deposition was characterised by active slope and fluvial processes depositing sediments contemporaneously with the accumulation of loess. The loess reaches an average of 4 m in thickness in the central part of the valley and thins towards the valley sides. Two tephras—Mount St Helens Y (3.4 ka) and Mount Mazama (6.8 ka)— occur in the loess and are invaluable stratigraphic markers. Most of the loess was probably deposited between 8.2 ka and 3.4 ka, a period coinciding with mid-Holocene increased summer temperatures and decreased precipitation in south-central British Columbia. Debris flows and small streams, originating on the valley sides, flowed out on to the loess depositing sand and gravel beds. These deposits form a definite proximal—distal relation across valley with the slope-derived sediments decreasing and the loess increasing in thickness towards the centre of the valley. The lactustrine silt particles were mobilised by diurnal mountain and valley, gravity, and canalised winds flowing within the South Thompson valley. An analysis of contemporary wind-flow data was undertaken to provide a possible analogue for valley wind flows in the mid-Holocene.     

Fungal remains in Pleistocene ground squirrel dung from Yukon Territory, Canada

Fungi in dung of the Arctic ground squirrel (Spermophilus parryii) collected near Dominion Creek, Yukon Territory, Canada, have a radiocarbon age of 12,200 ± 100 yr B.P. Most of the fungal remains are assignable to modern taxa, and most of these are either widespread saprobes or nonspecific coprophiles. However, specimens identified as Chaetomium simile and Thecaphora deformans represent fungi that may be more characteristic of rodent dung than that of other animals, inviting consideration of dung fungi as a potential source of paleontological data.     

The origin and evolution of the Klondike goldfields, Yukon, Canada

The world famous Klondike goldfields are located in the unglaciated part of west-central Yukon, Canada. Since their discovery over 100 years ago, they have produced an estimated 311 tonnes of gold, primarily from bench and creek placers that are fluvial in origin and range from Pliocene to Holocene in age. Historically, the placers are classified into three levels of gravel with four main units. These include the high-level White Channel Gravel (Pliocene), presently the most important gold-bearing unit, which sits nonconformably on an erosional bedrock surface (i.e., the ‘White Channel strath’) and is overlain and interbedded with the glaciofluvial Klondike Gravel (Pliocene); the intermediate-level gravel (Pleistocene), the least important economically; and the low-level gravel (Pleistocene–Holocene), historically the most important gold-bearing unit, but it has been mined three or four times now. The goldfields originated from the weathering and erosion of early Cretaceous, discordant mesothermal quartz veins, and the light grey color of the matrix of the White Channel Gravel is due mainly to weathering and diagenetic alteration by groundwater flow. The concentration of placer gold 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 km), braided river environments transported large amounts of gold; and at the sequence scale (thousands of km), economic placers formed initially in the high-level White Channel Gravel and later in the intermediate-level and low-level gravel. The White Channel strath is interpreted as an erosional ‘tectonic’ terrace that formed during isostatic uplift and under conditions of dynamic equilibrium. The high-level White Channel Gravel and Klondike Gravel are interpreted as a depositional ‘climatic’ terrace that formed during a reversal in the tectonically induced downcutting, which is attributed to the initial and most extensive of the pre-Reid glaciations (3 Ma) in the Yukon. The intermediate-level gravel is interpreted as minor erosional ‘complex response’ terraces that formed during static equilibrium when there were pauses in valley-floor degradation, which are attributed to the subsequent and less extensive pre-Reid glaciations. The low-level gravel formed also during valley-floor degradation and may represent a return to dynamic equilibrium conditions. Hence, the dominant forcing mechanisms controlling the evolution of the goldfields were isostatically compensated exhumation and climatic change related to the repeated glaciation of the Yukon. In addition, the lowering of baselevel from high-level, to intermediate-level and finally to low-level gravel was accompanied by a decrease in accommodation space (as indicated by a decrease in gravel thickness), which resulted in an increase in the concentration of the placer gold.     

Late Holocene moisture balance variability in the southwest Yukon Territory,Canada

Analyses of sediment cores from Marcella Lake, a small, hydrologically closed lake in the semi-arid southwest Yukon, provides effective moisture information for the last ∼4500 years at century-scale resolution. Water chemistry and oxygen isotope analyses from lakes and precipitation in the region indicate that Marcella Lake is currently enriched in ¹⁸O by summer evaporation. Past lake water values are inferred from oxygen isotope analyses of sedimentary endogenic carbonate in the form of algal Charophyte stem encrustations. A record of the δ¹⁸O composition of mean annual precipitation at Jellybean Lake, a nearby evaporation-insensitive system, provides data of simultaneous δ¹⁸O variations related to decade-to-century scale shifts in Aleutian Low intensity/position. The difference between the two isotope records, Δδ, represents ¹⁸O-enrichment in Marcella Lake water caused by summer effective moisture conditions. Results indicate increased effective moisture between ∼3000 and 1200 cal BP and two marked shifts toward increased aridity at ∼1200 and between 300 and 200 cal BP. These prominent late Holocene changes in effective moisture occurred simultaneously with changes in Aleutian Low circulation patterns over the Gulf of Alaska indicated by Jellybean Lake. The reconstructed climate patterns are consistent with the topographically controlled climatic heterogeneity observed in the coastal mountains and interior valleys of the region today.