Placer gold is a notable resource in the western Yukon; however, identification of the lode sources feeding these placer deposits has been difficult. Previous studies have used electron microprobe (EMP) and manual morphological analyses of gold grains with some success to define source-mineralization-style areas, but have not been able to accurately predict lode locations. This study utilizes EMP in conjunction with a new method for morphological analysis based on semiautomated digital image analysis to re-examine this problem. Examination of a sample suite collected over the entire Klondike goldfields area demonstrates that there is significant complexity in Yukon placer gold deposits that has not previously been recognized. Confronting this complexity using a statistical approach based on this new shape analysis method, EMP and a planned future laser ablation mass spectroscopy study will hopefully produce a method for locating lode gold sources.

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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.

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 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.

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Placer gold in the Clear Creek drainage basin is found in a variety of gravel deposits, each of which is associated with a specific geological setting. The schematic profile of Clear Creek drainage basin illustrates the distribution of these gravel deposits, and indicates both known deposits and favourable sedimentologic conditions for placer mineral accumulation. Known placer gold deposits include creek and gulch placers, as well as preglacial fluvial gravel or buried channels. Favourable placer deposit settings include alluvial fans, gravelly sediments similar to the Pliocene (?) White Channel gravel of the Klondike area, and specific glacially derived sediments. This paper describes each of the above placer deposit settings, and outlines the associated stratigraphy and sedimentology of placer gold deposits.

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

Placer gold is widely distributed throughout the western Yukon; however, lode sources for most of these deposits remain unknown. Previous studies of gold compositions in this region using scanning electron microscope (SEM) and electron microprobe (EMP) methods showed 1) that there are consistent differences in average composition (although with considerable overlap) between gold from different styles of lode gold mineralization; and 2) the composition(s) of placer gold can be matched with specific lode sources, or the most likely style of lode source can be identified. In the current study we employ SEM and EMP methods together with laser ablation ICP-MS trace element analysis and study of the micro-inclusion suite(s) to more completely characterize the major, minor and trace element composition of the gold as well as the mineralogy of the lode sources themselves. We also report new data for placer and lode gold, mainly from the Klondike District.

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