Residual enrichment and supergene transport of gold, Calhoun mine, Lumpkin County, Georgia

Gold is present in weathered wall rock and quartz veins of the opencut of the Calhoun mine, Lumpkin County, Georgia. Eleven samples of mica schist saprolite contain 0.02 to 0.1 parts per million (ppm) gold and three samples of weathered vein quartz, 0.04 to 0.6 ppm (limit of detection is 0.02 ppm). Gold was not found in any of the 18 samples of fresh mica schist and was found in only 3 of 16 samples of fresh vein quartz from underground workings 40-100 feet below the opencut.

Whole-rock chemical analyses of the unweathered schist and schist saprolite show large losses of minor elements and addition of water during weathering. The average specific gravity of the unweathered schist is 2.8 and of the schist saprolite, 1.65. The chemical analyses recalculated to weight per volume show a loss during weathering of large amounts of all major constituents including silica, alumina, and total iron. Of the minor elements, gold and arsenic show the most residual enrichment, but boron, beryllium, cobalt, lanthanum, niobium, nickel, lead, zinc, and zirconium show some enrichment in the saprolite as compared with the fresh rock. Most of the other minor elements have been at least partly removed, the greatest loss being shown by calcium, potassium, magnesium, sodium, and strontium. Fresh limonite coating the wall of the adit contains 2.9 ppm gold and appreciable amounts of these elements leached from the schist, particularly barium, calcium, and manganese.

Some layers of unweathered schist contain as much as 1 percent gold-bearing pyrite and may have a gold content of 0.01 and 0.02 ppm, or just at or below the limit of detection. Loss of 30 to 45 percent of the rock during weathering increases the gold content above the lower limit of detection. The gold in recently deposited limonite shows that some gold is also being transported with iron during weathering. If a concentration of limonite were found near the water table, it might be expected to show supergene enrichment in gold.