Eluvial placers (Some geologists, particularly in Russia, classify these placers as eluvial, deluvial and proluvial. By this classification eluvial placers are those whose outlines coincide more or less with those of the primary deposits. Deluvial (scree or talus) placers are those whose upper limit is at or near the primary source and whose downhill front lies at the foot of a slope. Proluvial placers form in the disintegrated debris at the foot of hills or mountains.) are formed in the weathered residuum over or in the immediate vicinity of primary gold deposits of all types. Commonly these placers develop downhill from the outcrop of the primary deposits. Some are of the nature of talus deposits or fanglomerate facies on mountain slopes or in the debris at the foot of hills and mountains. An unusual type is formed in the depressions of deeply and irregularly weathered terrain (karst topography). Eluvial placers are closely associated with alluvial placers in many districts, the former giving birth to the latter in many places.
The principal mechanism for the concentration of heavy minerals in eluvial placers is the winnowing action of gravity and downhill creep, the latter being essentially dependent on the angle of slope or gradient where the placers are formed on the sides of hills or mountains. Secondary factors include the thickness of the slope materials (scree, talus, residuum), the size and specific gravity of the weathered particles in the residuum, the coefficients of thermal expansion and contraction of residual particles, the coefficient of friction, the movement of ice and snow (glaciers) and the annual and daily variation of temperature. The last governs the freezing and thawing of the residuum in temperate zones, and is particularly important in permafrost regions where solifluction is widespread. The continuous downhill creep appears to be the main way by which gold reaches gulches and creeks. Moving water plays only a small part in the concentration of heavy minerals in eluvial placers, and it is common to find that these placers are largely independent of watercourses either in the detritus or on the surface.
Auriferous eluvial placers commonly exhibit considerable gradation and variation in the content and nature of their gold and associated heavy and light minerals. Near the primary deposits the particles of gold are invariably larger and commonly of less fineness than those farther down the dispersion fans and trains on the slopes. During the downward creep of minerals and rock fragments on slopes the lighter fractions tend to gravitate to the top layers and move relatively faster downhill than the heavier fractions. This is why one commonly finds a greater concentration of light minerals and rock fragments such as quartz floats at the foot of hills and mountains rather than farther uphill and at the outcrop of auriferous quartz veins. Finally, nonuniformity and erratic behavior commonly prevail in the continuity and value of the pay streak in most eluvial placers, a feature that dictates that most of these deposits must be carefully pitted, trenched, drilled and evaluated before exploitation.
Eluvial gold placers were widely worked in many parts of the world prior to the turn of the century. Most were residual blanket-like deposits developed on a relatively flat terrain, or fanlike accumulations formed on gentle slopes. The materials of these deposits varied greatly depending principally on the types of bedrocks and deposits and the type and degree of weathering. Most had high iron (lateritic) content although some were relatively highly enriched in aluminum (bauxite or clay). Most consisted of a residuum of quartz, sand, clay, iron oxides, aluminum oxides and manganese oxides in which the particles of gold, commonly with a number of other chemically resistant heavy minerals, were concentrated on or near the bedrock. Some of these deposits are highly cemented with clay, limonite or caliche (carbonates), more rarely by silica. Most eluvial placers are low grade in gold, but some are of enormous volume (tonnage). Much of the gold in eluvial placers is rough and irregular in form, and the fineness is commonly only slightly higher than that in the primary deposits. Eluvial placers are known for their large nuggets or masses of gold, some weighing several hundred to a thousand ounces or more; alluvial placers only rarely contain large nuggets (>100 oz). Some eluvial placers yield silver, lead, cassiterite, cinnabar, diamonds and platinoids in addition to gold. A few descriptions of typical Examples of eluvial deposits follow.
Extensive eluvial placers are unknown in Canada, but there are a number of small Examples mainly in Yukon that are of interest from an academic viewpoint. In the Dublin Gulch area, Yukon, small eluvial placers are developed on the sides of the hills above the economic gold placer in the gulch. The eluvial placers contain mainly scheelite with small amounts of wolframite and gold, these minerals being derived from a complex of scheelite bearing skarn, quartz-wolframite veins and northeast striking quartz-arsenopyrite-pyrite-scorodite-gold veins, all lying slightly uphill from the eluvial placers. The constitution of the eluvial material is varied depending on the type of bedrock.
Courtesy: Mine Links