FORMATION OF ALLUVIAL GOLD NUGGETS BY BIOCHEMICAL WEATHERING IN THE SOUTHERN APPALACHIAN PIEDMONT

During 1799-1803, 17 and 28 lb gold “nuggets” were discovered on a farm in Cabarrus County, North Carolina, which eventually led to the first significant gold rush in the USA. Because bedrock sources of gold in the Southern Appalachian Piedmont are typically very fine grained (~.01 to 1mm), it is highly unlikely large nuggets could have been formed by simple release of such large masses by weathering of host rocks. This fine-grained gold is typically associated with pyrite, arsenian pyrite, and arsenopyrite in the Piedmont. Thus it is proposed here that the biochemical weathering of gold and associated sulfides below but near the water-table leads to gold release to shallow groundwaters of the Piedmont by a bacteria-mediated reaction like this:

Fe(AsS)₂ +Au^o + 2H₂O + 2.5O₂ -->Au(S₂O₃)^- + H₃AsO₃ + Fe²⁺ +OH^-

Thiosulfate (S₂O₃) is a common but metastable weathering product of sulfides under mildly reducing, circum-neutral pH conditions, and after cyanide, forms the strongest aqueous complex of gold known. Shallow groundwaters of the Piedmont typically flow toward stream valleys and discharge as springs in the streams. These groundwaters, which are ~50 years old based on tritium age dating, are oxidized (again, apparently mediated by bacteria) upon discharge, leading to ferric oxyhydroxide and native gold precipitation. Bacterial oxidation of the gold-thiosulfate complex has been demonstrated in the lab, and precipitates resemble natural gold morphologies, suggesting that this process can form nuggets. In addition, we have panned delicate feathery crystalline gold from Piedmont streams, which could not have undergone much transport in the streams, and this also argues for their in-situ formation. Although alluvial gold nuggets can form from their direct release from hypogene sources, it is clear this is not how nuggets form in the Southern Appalachian Piedmont.