Colloidal and physical transport textures exhibited by electrum and naumannite in bonanza epithermal veins from western United States, and their significance

It is reasonably clear that disequilibrium or “far-from equilibrium” conditions
lead to the formation of silica colloids and their deposition inmany epithermal deposits.
This implies ore-forming solutions had elevated concentrations of dissolved silica,
well in excess of amorphous silica saturation.We have previously demonstrated that
such colloidal silica particles were deposited in epithermal veins as silica gels and opal,
which may later progress along a path to crystallize into more thermodynamically favored
(less-soluble) silica phases such as quartz and chalcedony. Also, in some deposits,
amorphous silica is co-deposited with precious-metalminerals, such as electrum in
the banded super-bonanza ores of the Sleeper deposit (NV). Ore-mineral textures
from some western USA bonanza epithermal ores indicate that two precious-metal
phases (electrum and naumannite, Ag2Se) form colloidal particles that are transported
by ore-forming fluids and are deposited either by aggregation (by sticking to
other precious metal-particles) to make dendrites, or are deposited on the “lee” side of
protrusion along vein walls (or perhaps by both processes).We can infer by analogy to
silica that this also implies that ore-forming solutions contained elevated (supersaturated)
dissolved concentrations of both gold and silver that formed colloidal particles
under disequilibrium (often chaotic) conditions. Thus physical transport and deposition
textures seem to indicate the presence of strongly precious-metal-enriched ore
forming fluids, which led to (not surprisingly) the bonanza grades of these remarkable
ores. What causes such a precious-metal-rich solution is debatable, but that is the subject
of our continued investigations.