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PHASE RELATIONS IN THE SYSTEM Au Cu Ag AT LOW TEMPERATURES, BASED ON NATURAL ASSEMBLAGES

2001; Mineralogical Association of Canada; Volume: 39; Issue: 3 Linguagem: Inglês

10.2113/gscanmin.39.3.889

1499-1276

J. Knight, C.H.B. Leitch,

Solidification and crystal growth phenomena

The composition of some 85 particles of native gold from the 15 Mile rodingite in serpentinite showing in the Coquihalla gold district and the Wheaton Creek placer in the Dease Lake district, British Columbia, falls mainly within the Cu-rich portion of the Au–Ag–Cu phase diagram. The particles display a variety of textures, including exsolution-induced textures. The textures and phase compositions are used to construct a low-temperature phase diagram for the system Au–Ag–Cu. This diagram represents a system that equilibrated for a longer period of time at a lower temperature than is accessible in synthetic systems. In this diagram, the composition AuCu is nearly stoichiometric, a three-phase region is found among AuCu (Au1.0Cu0.971Ag0.003), Au2Cu (Au2.0Cu0.962Ag0.045), and Au3.0Ag0.71Cu0.23, and there is a two-phase region on the Au side of the Au2Cu–Au3.0Ag0.71Cu0.23 join. There are insufficient data to determine the phase relationships that exist between the compositions Au and Au2Cu, which includes Au3Cu. Au2Cu may or may not be part of a solid solution centered around Au3Cu. The sequence of formation of extensive exsolution-induced textures and their relation to the other textures seen are explained by combining the phase diagram with a significant shift in the position of the solvi with temperature in this system. Low values of Ag in Au–Cu alloy and of Cu in Au–Ag alloy are reported. Grains of gold alloy from deposits where no Au–Cu alloy is found appear to be undersaturated with respect to Cu.