Abstract

An examination of Yukon placer gold nuggets using ultrathin-section (70 nm) transmission electron microscopy and energy-dispersive X-ray spectroscopy revealed three distinct morphological and compositional regions. The outer surface of the nuggets possessed silicate minerals, which included electron dense particles containing gold (13.6 to 27.2 at%) and silver (6.7 to 13.2 at%). The organic composition of these particles ranged from 7.3 to 12.3 at% S and 0.53 to 1.5 at% P (4.4 to 5.4 at% PO43−). The periphery of the nuggets also possessed crystalline gold-bearing material that was enriched with gold (81 to 93.12 at%; 30Au:1Ag) compared to the particulate material (2Au:1Ag) present within the silicate minerals annealed to the nuggets' surface. The organic composition of this peripheral secondary gold ranged from 0.29 to 5.9 at% S and from 0.63 to 5.9 at% P (4.5 to 9.5 at% PO43−). The internal structure of the secondary gold around the periphery of the nuggets was highlighted by the occurrence of compositional banding caused by these lighter, electron-translucent compounds. Internal regions of the nuggets were composed of gold or gold and silver, indicating the presence of an original unweathered grain that served as a nucleating site. These findings suggest that sulfur and phosphorus, presumably of biological origin, are important to placer gold formation.

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