Characteristics of Gold Mining Placers -CANADA

In Yukon Territory and at Yellowknife, Northwest Territories some of the supergene gold is of great fineness (990), whereas the primary gold is in the range 900 to 950. Another feature of placer gold, in the Klondike, is that the outer parts of nuggets commonly have a higher fineness compared with the inner parts. The rim of greater fineness is rarely more than 0.03 mm (30 microns). A rim of relatively pure gold on nuggets seems to be rather general.

The same is true in the gold of the Cariboo of British Columbia; in the Morobe goldfield, New Guinea; in much of the placer gold of Russia, in Rheingold; in much of the placer gold of Yukon; and in placer gold from various parts of the United States. However, the rind of purer gold on nuggets, flakes and dust is not always present. The rim effect has generally been interpreted as the result of solution of silver, but it also may represent the precipitation of gold (and some silver) on gold nuclei. An argument in favor of the latter process is given in a later section on the origin of placers.

It should be noted that the rim effect in placer gold evidently begins during oxidation of the primary deposits, because it is found in gold in the oxidized zones of auriferous deposits and in eluvial deposits. Gold with enriched rims has been observed by a number of investigators in near-surface shallow deposits (mainly Tertiary) in a number of places, and the phenomenon is noticeable in some kuroko ores in Japan. Enriched rims are rare in deep-seated gold deposits. In the oxidized ores and placer deposits the rims are generally much wider and better developed than in the primary deposits.

Placer gold commonly contains many inclusions usually small in size (0.0054 - 0.50 mm). The inclusions as seen in polished sections and under the microprobe include various sulphides, particularly pyrite, galena and chalcopyrite; also arsenopyrite, various sulphosalts, tellurides and quartz, sericite, rutile, etc., most of the minerals normally found in gold deposits. Man y of these minerals are probably original constituents of the gold nuggets since they are commonly near the core of the gold particles. Some often appear as though they were the nuclei about which gold precipitated in the oxidized zones of the deposits. These inclusions should receive detailed study during geochemical prospecting surveys since one might be able to divine the exact source of the gold, especially if material from a few primary sources in the district under investigation is available for control purposes.

Other internal features of placer gold have been extensively studied. A long term study of the placer gold of Russia, also note the nearly universal occurrence of high grade rims on gold nuggets and flakes and founds that these have a fine-grained polyhedral texture rather than a laminated one as thought by the earlier investigators. They also noted that much of the placer gold retains the internal granular structure and other microscopic features of the primary deposits of gold, but found that much placer gold, especially that held in placers for long periods of time, exhibits marked internal deformation and recrystallization textures. They also observed the presence of intergranular stringers of gold in nuggets and flakes. These were found in the placer gold and in the gold from oxidized zones of auriferous deposits but not in the gold from the primary ores.