QUANTIFICATION OF SULFUR AND PHOSPHORUS WITHIN SECONDARY GOLD RIMS ONYUKON PLACER GOLD

An examination of Yukon placer gold nuggets using ultrathin-section (7 0 nm) transmission electron microscopy and energy-dispersive X-ray spe ctroscopy revealed three distinct morphological and compositional regi ons. The outer surface of the nuggets possessed silicate minerals, whi ch 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 parti cles ranged from 7.3 to 12.3 at% S and 0.53 to 1.5 at% P (4.4 to 5.4 a t% PO43-). The periphery of the nuggets also possessed crystalline gol d-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 compo sition 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 struc ture of the secondary gold around the periphery of the nuggets was hig hlighted by the occurrence of compositional banding caused by these li ghter, 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.