Rosebud is a low-sulfidation gold deposit located in Pershing County, NW
Nevada. It occurs in the Oligocene age Kamma Mountain volcanics, which overlie
metasedimentary rocks of the Jurassic-Triassic Auld Lang Syne Group. The Rosebud
underground mine produced nearly 400,000 oz of gold at a 0.18 oz/st cutoff. The shallowest
ore is more than 400 feet (123 m) from the surface, whereas the deepest ore at
Dozer Hill is about 800–1200 feet (246–369 m).
Sagebrush (A. tridentata and A. arbuscula) was sampled in a 1989 biogeochemical
survey under the auspices of LAC Minerals. The survey covered what was later
defined as the South Zone orebody.
Plots of Au, Ag, As, Sb, and Se concentrations in sagebrush are superimposed
over the 0.05 oz/st footprint to demonstrate parity between the occurrence of ore and
biogeochemical anomalies. The Au plot shows leakage up some of the high-angle fracture
and fault zones, revealing pervasive northeast regional trends. Mineralized sections
of the South Ridge Fault are identified, but a more complete trace of the
silicified fault scarp can be seen in the As data. Antimony is co-spatial with Ag, revealing
other faults and zones of variable Ag-Sb enrichment. Anomalous Se concentrations
are generally confined to the ore footprint.
The early exploration history at Rosebud included a rapid assimilation of geological,
rock chip, soil and geophysical data from 1988–1989. Initial drilling was targeted
on a multi-element soil anomaly, but the trend of mineralization from the discovery
hole was not clear. The results of the 1989 biogeochemical survey came late in the
sequence of events and did not have a significant impact on the drilling program.
However, in hindsight the biogeochemical data clearly reveals the trend of mineralization
and the location of ore that was mined.
This paper describes the occurrence of ore at Rosebud and presents compelling
evidence that the 1989 sagebrush survey accurately defines the location and structural
controls of the South Zone orebody.