Examples of alluvial deposits
Descriptions of typical auriferous alluvial and beach placers and districts
Alluvial placers were the first type of gold deposits worked by man. Ancient Chinese and Hindu writings mention these deposits, and we have the story in Greek mythology of Jason, commander of the good ship Argo, who sought the Golden Fleece at Colchis in Asia Minor on the far shore of the Euxine (Black) Sea. The metal gold also figures extensively in the Old Testament, and there are many allusions to its source in placers. Herodotus and Strabo, the Greek historian and geographer respectively, repeatedly mention the working of alluvial placers in Thrace, the Aegean Islands and Asia Minor. Pliny, writing at the beginning of our era, describes many of the Roman placer operations, which seem to have stretched from Asia Minor to Spain, France and Wales. In particular he mentions gold in the streambeds of the Tagus in Spain, the Po in Italy, the Hebrus in Thracia, the Pactolus in Asia Minor and the Ganges in India, the last probably from hearsay. Pliny mentions booming and sluicing and collection of gold on ulex, a rough, prickly plant that was burned and the gold washed out of the ashes. In the Dark and Middle Ages some gold was won from the old placers of the Mediterranean area and Western Europe, but by the time of the rediscovery of America by Columbus in 1492 nearly all of these placers were exhausted.
When Columbus made his first landfall in the New World he found the natives in possession of gold, and later exploring in Hispaniola (Haiti) in 1493 on the second voyage he observed crude placer operations in the streams and rivers of the interior. In 1494 Alonso de Qieda discovered the rich Cibao placers, and a year later Pablo Belvis arrived from Spain with a large quantity of mercury for amalgamation purposes. The first gold won was sent immediately to the King of Spain, who donated it to Pope Alexander VI in Rome where it was dedicated to the service of Christianity in the gilding of a cathedral dome. Thus began the auri sacra fames of the Spaniards in America that was to wreak such havoc on the natives and was also to introduce the Negro slave trade to the New World. The sordid story has been told many times and need not be repeated here; it is one of the black pages in the history of gold mining.
A more pleasant story can be written about the great gold rushes of the last century. Alluvial gold gravels were worked in the Altai of Siberia as early as 1820, but some 9 years later extensive alluvial deposits were found in the Lena Basin, probably the largest alluvial gold deposits known. These discoveries led ultimately to the colonization of southern Siberia. These and other deposits in eastern Siberia are still worked extensively today. The great gold rush of 1849 to California can be said to have opened up the American west.
The California gold rush was followed in 1851 by the Australian rushes to New South Wales and Victoria, an impetus to mining that has kept Australia in the forefront of world mineral production ever since. The golden gravels of the
Fraser River in British Columbia were known as far back as 1852, but it was not until 1858 that the great stampede up the Fraser began ultimately in 1861 reaching Williams and Lightning creeks in the Cariboo, the most celebrated of all gold creeks in the province. Gold placers in the Yukon Basin were worked as early as 1880, but it was not until 1896, and perhaps 2 years earlier according to some accounts, that gold was discovered on the tributaries of the Klondike River. The great rush took place in 1897-1898 to Dawson - one of the greatest gold rushes in history and certainly the most colorful, made immortal by Robert W. Service in his novel The Trail of '98, and in his poems, Songs of a Sourdough and Ballads of a Cheechako.
Alluvial gold placers in streams and rivers have been the richest and of most interest to the prospector. But some beach placers have also caught his eye and have been extensively worked. Gold was discovered in Alaska as far back as 1865-1866 and alluvial prospecting in the streams and rivers near Nome was in full swing in 1899, the year that gold was discovered on the beaches by a soldier and a prospector.
There followed a frenzied digging and grubbing along the coast for many miles, more than one million dollars having been won by hand rockers in less than 2 months.
The methods of the alluvial placer miner have changed much since Jason's time. Originally, early man probably plucked by hand the heavy shining nuggets from the gravels of the streams. This is the stage at which Columbus and his mining engineers found some of the natives working the placers of Haiti. Later the gravels of the streams were stirred up by the workers using crude booming (hushing) operations and sluiced over the fleeces of sheep and goat pelts, the gold remaining mainly trapped within the wool and goat's hair. This method is apparently still employed in some of the placer streams of Asia Minor, Afghanistan and Mongolia. Panning is an old technique certainly known to the ancients of the Old World and the natives of Africa who used the calabash (gourd) and the natives of Central and South America who employed the batea. The Greeks and Romans knew the rocker and the sluice, and they were adept at booming (hushing) operations. Those who have sought gold under desert conditions have long known the dry washer or dry blower. The water monitor-sluice, bulldozer-sluice and dragline-sluice operations and the great mechanical dredges are modern mechanical adaptations of age-old techniques of separating the gold from the dross.
Gulch and creek placers
Gulch and creek placers are common in nearly all placer districts of the world. They lie in the small valleys, guts and gulches that are tributary to the mainstream systems of a district. Literally thousands of these deposits have been worked, nearly all in a small way. The main characteristics of these types of placers are:
1. Most occur in moderately hilly country exhibiting the effects of protracted weathering and denudation. A few occur in regions of alpine topography, but these are generally marked by spotty pay streaks.
2. The gradients of the present (and past) stream systems are moderate.
3. The source of the gold and associated heavy minerals is generally close at hand, either at the heads of the gulches and creeks or along their valley sides. The source of the gold is commonly auriferous quartz veins or gold-bearing sulphide bodies; disseminated gold in quartz blows and stringers in the country rocks and in pyrite and other sulphides in graphitic shales and other rocks is the principal source in some instances.
4. The heavy mineral constituents accompanying the gold are mainly those found in the primary gold deposits, in closely associated deposits or in the enclosing country rocks.
5. The gold is usually coarse and commonly higher in fineness than that in the primary deposits. Large nuggets, wires and crystals are a feature, and nodules of vein quartz or sulphides with veinlets of gold or containing disseminated gold are common.
6. The pay streaks are rich and generally on the bedrock or in the top few feet of the bedrock. Most pay streaks are well defined and fairly regular. False bottoms may occur but are not a common feature.
7. The overburden covering the pay streaks is generally not deep except in depressed areas or where glacial deposits complicate the picture.
Gulch and creek placers are common in the Yukon and a number occur in the Keno Hill-Galena Hill area. One in particular, Dublin Gulch, is typical and will serve as an example of what one might expect to encounter with gulch and creek placers the world over.
The Dublin Gulch and Haggart Creek placers were discovered and first worked in 1898. Relatively low rounded hills and numerous small streams tributary to the main watercourses, all marks of a deeply dissected upland, characterize the topography. The bedrock of the area is folded and faulted quartzite, phyllite, graphitic schist, limestone and quartz-mica schist of the Yukon Group intruded by a body of granodiorite and granite probably of Cretaceous age. Skarn, carrying scheelite, is developed in places near the granitic rocks. In addition there are numerous quartz stringers carrying wolframite and scheelite in the granitic rocks and their contact zone, a cassiterite-tourmaline lode and several northeast striking gold-bearing quartz-arsenopyrite-pyrite-sulphosalt veins. Primary vein material in these veins averages about 6.4 g Au/ton and 48 g Ag/ton. The gold is largely submicroscopic to microscopic and is mainly in the arsenopyrite, pyrite and sulphosalts; only rarely is free gold seen in the quartz and sulphides.
Maps of alluvial gold deposits in: California, Western Canada, Eastern Canada, Russia, World
Maps of primary gold deposits in: Precambrian, Paleozoic, Mesozoic, Cenozoic Rocks
Rafal Swiecki, geological engineer email contact
This document is in the public domain.