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.