The Ten Mile Mining District, Winnemucca, Nevada: Geology, Mineralogy and Supergene Gold Enrichment

The Ten Mile district, is a small gold producing area 15 km
west of Winnemucca, northern Nevada. The mines in the area
worked oxidized deposits with all workings stopping before or just
in sulfide ore. A possible relationship exists between gold fineness
and depth. Along with groundwater geochemistry and variations in
the concentration and distribution of gold, evidence exists of supergene
enrichment in the upper zones where some exceptionally rich
pockets of gold (up to 65 g/t Au (2 opt Au) in this study) were found
concentrated in favorable structural traps. Thus, crystalline native
gold is believed to be bi-modal in origin.
Primary gold is represented by electrum (600-650 gold fineness)
and minor petzite associated with partially oxidized pyrite and
secondary alteration products in a gangue of quartz and adularia.
The electrum forms crystalline leaves, reticulated dendrites, and
rough grains. The second form, believed to be supergene has a much
higher fineness and often forms small platelets, wires, and euhedral
crystals derived from altered protore and concentrated by residual
and supergene processes in the oxidized levels of the mines.
Numerous, small, bonanza style gold deposits occur in the region
and this paper describes how they may have formed by similar
mechanisms.