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ELUVIAL

Eluvial Deposits

Over schists the materials are mainly mixtures of sand, clay, limonite and disintegrated schist; over granodiorite the materials are principally a limonitic clay mixture with highly disintegrated feldspar and quartz. The principal heavy minerals in the eluvium are gold, scheelite, wolframite, extrinsic hematitic nodules of iron-formation, garnet, cassiterite, magnetite, bismuth, pyrite, arsenopyrite, jamesonite, siderite, nodules of oxidized galena, limonite, wad and scorodite. Eluvial material containing gold and scheelite is greatly enriched in arsenic, antimony and tungsten a feature that can be used in geochemical prospecting for this type of deposit.

In the western United States eluvial placers have been worked in the past in California, Oregon, Nevada and Montana, but few rivaled the alluvial types in these states in their economic importance. In the Appalachians of the United
States eluvial placers were worked on a fairly large scale before the turn of the century, especially in Georgia. There, in the Dahlonega gold belt in Lumpkin County, the bedrock is a complex of mica schist, mica gneiss, quartzite, amphibolite, migmatites and granite. The schists contain abundant gold-bearing quartz veins and stringers of various ages. All rocks and deposits are deeply weathered forming at the surface what has been called a saprolite. Saprolite is in reality only a form of laterite; it is brownish in color, composed of soft, earthy, clay-rich materials derived from thoroughly decomposed schist and other metamorphic rocks.
Zones of disorganized and disintegrated quartz veining in the saprolitic schists were mined by deep open cuts on a large scale near Dahlonega, the gold being won partly by washing and partly by crushing of the quartz in stamp mills. The enrichment in the saprolite was evidently due to considerable leaching and precipitation of gold by the geo-chemical reactions (up to 2.9 ppm) in freshly precipitated limonite. The gold distribution in the fresh vein quartz is low and spotty, and the primary deposits were apparently not attractive mining ventures.

Certain gossans on stockworks or massive sulphide deposits and their attendant eluvial deposits have been mined or are being mined for gold. At the Greenhorn Mine in Shasta County, California an auriferous gossan was worked profitably for many years. The gossan was developed on a copper sulphide orebody and consisted of a mass of limonitic material of varying thickness containing localized zones of rhyolite. The principal values in the gossan were gold and silver with some flakes of native copper. The gold particles were extremely fine, almost microscopic. The combined gold and silver content averaged about 0.20 oz/ton.
Similar gossans have been mined in many countries, e.g., Spain (Rio Tinto), Australia (Mt. Morgan) and elsewhere.
Eluvial deposits have been worked in many places in the lateritic residuum in Mexico, Central America, Cuba, Haiti, Puerto Rico and the countries of South America.

In Mexico eluvial-alluvial placers in three large fans of gravels downslope from mineralized porphyritic granite stock and its limestone contact zone are found near Guadalcazar in the central part of the State of San Luis Potosi. The placers were derived from the weathering and erosion of the mineralized parts of the granitic body and its contact zone and contain tin as cassiterite, mercury as cinnabar and minor amounts of gold and silver.
The gold in the finer fraction of the gravels and sands is free; in the coarser fractions it is chiefly in sulphide minerals (pyrite, arsenopyrite, galena, sphalerite, stibnite, silver sulphosalts) and their oxidation products. Part of the silver is in sulphides and sulphosalts and a part is alloyed with the native gold. Average assays of some of the placer materials show the following: 84 ppm Sn, 15 ppm Hg, 9.44 ppm Ag and 0.058 ppm Au. Some 550 million m3 of gravel are present in the fans.

In South America most of the eluvial deposits were blankets and eluvial fans in the lateritic materials near zones of quartz veining in Archean greenstones and associated sediments. In actual fact gold in the laterites of the Guianas is widespread, ranging from 0.003 to 0.135 ppm, over hundreds of square miles in some places. Such areas seem to be centered on auriferous Precambrian greenstone and sedimentary belts intruded by granitic rocks and porphyries. During weathering, therefore, gold has apparently been concentrated in the laterites both from the rocks and from epigenetic gold-bearing deposits. Extensive natural winnowing of the laterites has evidently given rise to the important alluvial placers, which are a feature of the Guianas. One interesting deposit at Omai, Guyana (ex British Guiana) is a deeply weathered aplite dyke highly decomposed to a depth of more than 100 ft. Slightly auriferous quartz stringers, heavily pyritized, the pyrite being auriferous, shot through the primary dyke material. The decomposed material consisted essentially of limonite-stained sand and clay, in which well-crystallized free gold was abundant.

Other interesting types of eluvial deposits occur in Brazil mainly in the Quadrilatero Ferrifero, Minas Geraes. All of these are associated with gold-bearing iron-formations (itabirites). One type of secondary deposit, not necessarily eluvial, but often grading into eluvial deposits is locally known as "jacutinga". The jacutinga occurs as thin (cm. to m.) lines or bands in itabirite and is decomposition, or more accurately a surficial chemical disintegration product of the iron-formation. It is composed essentially of powdery ferric oxide (limonite and hematite) manganese oxides, clay minerals and talc in which nuggets, plates and threads of native gold are present. Evidently the gold originated by chemical solution and reprecipitation of the metal, from low-grade gold-bearing iron-formation. Some of the jacutinga were rich, up to 16 g Au/ton or more, and some were mined to depths of over 200 m.. Some of the gold was rich in palladium.

The other type of deposit associated with iron-formations in Brazil is the gold-bearing 'Tapanhoancanga' or 'Canga' for short. This is an irregular layer or blanket up to 3 m. or more thick of limonite-cemented fragments of iron-formation (itabirite). It is commonly developed on all iron- formations throughout the world that are extensively oxidized. Much of the limonite appears to derive from the oxidation of iron silicates and pyrite in the iron-formation. Where the iron-formations are also gold-bearing the canga is commonly enriched in gold, the metal occurring in small flakes, wires, specks and also in a submicroscopic form associated in some manner or other with the limonite. In the past, deposits of this type were worked in a small way in many parts of the ferriferous quadrangle in Minas Geraes.

Eluvial placers have not been important sources of gold in Europe although the Romans, in Portugal, Spain, France, Switzerland, Germany and Romania, have worked them in the past. Important eluvial deposits were once worked in the Berezovsk, Kochkar and other districts in the Russia. In Africa a few eluvial placers have been worked mainly in Egypt, Sudan, Ethiopia, Zaire, Ghana, Mali and elsewhere in the western gold belt of Africa. Most were small but have mothered extensive alluvial placers in a number of countries. It is of interest to note that the great Rand deposits of South Africa gave rise neither to extensive eluvial nor to alluvial deposits, a feature that is difficult to explain. The reason for the paucity of placer gold associated with these enormous deposits probably has much to do with the size of the gold in the reefs (50 µ) and the presence of abundant pyrite, which weathers readily to give soluble iron salts and colloidal hydrous iron oxides. Both of these features would tend to render the gold soluble or mobile as an absorbed component, thus ensuring its broad dispersion.

Eluvial gold was relatively common in Australia before the turn of the century, but most of these placers are now exhausted. Fabulous nuggets were found in some districts in the eluvium or weathered rubble of the oxidized deposits. At Big Nugget Hill in the Hargraves goldfield of New South Wales a nugget weighing 106 lb was found in the eluvium by an aboriginal shepherd. At Lucknow, also in New South Wales, the eluvium was of great richness at and near the outcrop of the veins. In Victoria even larger nuggets were found in the eluvium and disintegrated country rock near the veins of Ballarat, Tarnagulla and other places, including the Welcome Stranger (2516 oz), Welcome (2195 oz), Blanch Barkley (1743 oz), Canadian (1319 oz), Dunolly (1364 oz) and Sarah Sands (755 oz). In Western Australia the eluvial placers were small, rich and soon worked out in the Pilbara, Kimberly, Coolgardie, Kalgoorlie and other Yilgarn fields.

In Asia eluvial gold has been won from soils and weathered debris near outcroppings of gold in Japan, China, Burma, India, Russia and elsewhere. Most of these deposits were small and have long been exhausted. One type of eluvial deposit in Russia is of considerable interest both economically and scientifically - the Kuranakh type.

[ Placer Deposits 1  2  3  Eluvial  5  6  Alluvial  8  9  10  Examples  12  13  14  15 ]

Maps of alluvial gold deposits in: California, Western Canada, Eastern Canada, Russia, World
Maps of primary gold deposits in: Precambrian, Paleozoic, Mesozoic, Cenozoic Rocks


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This document is in the public domain.

March, 2011