Metal and Mineral Zoning at the Greater Midas Au-Cu-Ag Skarn Deposit (Battle Mountain District), Lander County, Nevada

ABSTRACT
Petrographic study, electron microprobe, and multi-element
ICP analyses of Au-Cu-Ag skarn ores from the Greater Midas
deposit of the Phoenix project have identified lateral mineral and
metal zoning away from the 38 Ma Copper Canyon stock. A total of
59,170 kg (1.9 million ounces) of Au with substantial Cu and Ag
credits are included in the year-end 1999 proven and probable
reserve for the Greater Midas deposit. The bulk of the skarn ore is
hosted by calcareous siltstone and calcareous and siliceous conglomerate
of the middle and lower units of the Pennsylvanian Battle
Formation.
Minerals are zoned from a proximal pyroxene>garnet skarn
zone with strong retrograde alteration to stilpnomelane±amphibole
±chlorite±quartz±biotite adjacent to the stock (<320 meters away), to a pyroxene>garnet zone with less retrograde alteration
(320-1220 meters away), to an actinolite-rich zone with only minor
pyroxene (1220-1890 meters away), and finally to a distal zone
(1890-2400 meters away) containing biotite, chlorite, and clay. All
of these zones host Au-Cu-Ag mineralization and contain an average
total sulfide content of 7 percent by volume dominated by pyrite and
pyrrhotite. Dark-purple biotite hornfels barren of economic Au mineralization
occurs peripheral to the gold-bearing skarn assemblages.
Compared to typical Au skarns, Greater Midas garnet (Ad71-99)
is more Fe-rich and pyroxene (Hd3-52, Jo0-6) is relatively Fe-poor,
possibly reflecting the relatively oxidized protolith. Hydrous minerals
tend to be more Fe- and Cl-rich in proximal zones and Mg- and
F-rich in distal zones, in keeping with the general Fe-F avoidance
rule.
Metals are also zoned relative to the stock with Cu, Ag, Zn, Mn,
Cd, As, Pb, and Mo enrichment in the proximal zone, Au+Bi±Cu±Cr
in the intermediate zones, and Au+Bi in the distal zone. Almost all
metals, including total sulfide content, decrease in abundance away
from the stock with the exception of Bi, Au, Cr, and Cu.
Both the Greater Midas and the Phoenix (including West and
Fortitude) Au-Cu-Ag skarn deposits contain a proximal zone adjacent
to the stock containing moderate Cu (>0.3%), high Ag (>30
g/t), and low Au (<1.3 g/t), and a distal zone enriched in Au (>2.0
g/t) and Bi (>50 ppm) with low Cu (<0.1%). Elevated Mn, Zn, Pb, Ag, As, and Sb contents are associated with Cu (Au) mineralization adjacent to the stock at Greater Midas and with Au-Ag mineralization distal to the stock at the Phoenix deposit. Different hydrothermal fluid/rock ratios along with differing host rock compositions between Greater Midas (oxidized conglomerate of the lower unit of the Battle Formation) and Phoenix (reduced limestone of the Antler Peak Formation) may have helped influence a different lateral distribution of trace-metals, Au composition, and calc-silicate minerals between the two deposits.