Geology, Skarn Alteration, and Au-Cu-Ag Mineralization of the Phoenix Project, (Battle Mountain Mining District), Lander County, Nevada

ABSTRACT
The Phoenix project represents a re-birth of milling and leaching
operations in the Copper Canyon area of the Battle Mountain
Mining District, Lander County, Nevada. The year-end 1999 proven
and probable reserve of 121.87 million tonnes of mill ore averaging
1.31 g/t Au, 9.88 g/t Ag and 0.16 percent Cu, and 14.85 million
tonnes of leach ore averaging 1.15 g/t Au and 8.38 g/t Ag contains
176,633 kg Au, 1,328,854 kg Ag, and 213,000 tons of Cu. The
Phoenix project will mine Au-Cu-Ag ore from five separate open
pits; Phoenix, Greater Midas, Reona, Iron Canyon, and Sunshine.
Gold, Ag, and Cu skarn mineralization is associated with the
38-Ma Copper Canyon granodiorite stock, and is hosted in the
Pennsylvanian and Permian Antler sequence, a fining upward package
of ferruginous conglomerate, calcareous sandstone and siltstone,
and limestone. The Cambrian(?) Harmony Formation, in the upper
plate of the Roberts Mountains thrust, and the Mississippian,
Pennsylvanian, and Permian Havallah sequence, in the upper plate
of the Golconda thrust, are the respective footwall and hangingwall
stratigraphic units. Gold mineralization is primarily hosted in
pyrrhotite- and pyrite-bearing skarn consisting of pyroxene, epidote,
and actinolite > garnet. Lesser amounts of Au mineralization is associated
with quartz-rich veins, fault breccia (± clay), and retrograde
skarn consisting of chlorite, quartz, actinolite, and epidote. Total sulfide
content of mill ore averages from 1 to 25 percent by volume.
Skarn mineralogy and Au-Cu-Ag mineralization are zoned about the
stock and are controlled by (1) carbonate-rich lithologies, (2) distance
from the stock, and (3) faults and lithologic contacts.
In the Phoenix deposit, skarn hosted by the Antler Peak
Limestone is zoned from a proximal assemblage of andraditic garnet
> diopsidic pyroxene adjacent to the stock (with Cu>Au), to an
intermediate assemblage of hedenbergitic pyroxene > grandite garnet
distal to the stock (with Au>>Cu). Skarn in the Greater Midas
deposit is zoned from a proximal assemblage of pyroxene>garnet
with strong retrograde alteration adjacent to the stock (with Cu>Au)
that grades outward to a similar skarn assemblage with lesser retrograde
alteration, and to distal assemblages containing actinolite with
only minor pyroxene, and chlorite + biotite + clay, respectively
(with Au>>Cu).