Cortez to Eureka Gold Deposits: Ruby Hill/Archimedes Mine Area, Cortez Gold Mines

Welcome to the GSN Spring 2000 field trip to the central and southern portion of the Battle
Mountain-Cortez-Eureka mineral belt. This two-day tour will offer visits to a relatively new mine in
the historic Eureka district, and to three open-pits in the prolific Cortez-Gold Acres sector of the
belt. The presently- active Ruby Hill and Pipeline/South Pipeline operations are products of some
of the newer discoveries of Carlin-type deposits in Nevada, and are especially notable for their
favorable economic profiles and robust oxide ores.
The Ruby Hill mine exploits the most significant Carlin-style gold discovery made to date in the
Eureka district. The historic carbonate replacement Ag-Au-Pb-Zn deposits at Eureka (Ruby Hill)
yielded ~1.65 million ounces of gold and 39 million ounces of silver, much of which was produced
during the bonanza period of 1870-1890. These replacement ores were hosted primarily
by fault slices of Cambrian Eldorado Dolomite, and were found within structurally complex and
fractured portions of the Prospect Mountain horst block. These deposits are confidently linked to
middle Cretaceous (~106 Ma) granodiorite intrusives. The Archimedes deposit was discovered by
Homestake in 1991-92, as an outgrowth of a district re-evaluation for both carbonate replacement
and Carlin-style deposits. The West Archimedes deposit is a “pediment-style” Au-As-Hg system
hosted by Pogonip carbonate rocks of Early Ordovician age. The deposit is particularly notable
for its thorough post-mineral oxidation, for the abrupt margins of its ore blocks, and its location
on a major fault zone. Origin of this deposit is still vigorously debated, but a case will be made on
this tour for an Early Tertiary age for the gold system. Mining operations commenced in 1997
with published reserves of ~8 Mst grading about .099 oz Au/ton. Current reserves, as of January
2000, stand at 3.8 Mst at about .110 oz Au/ton.
The Cortez trend includes some of the earliest mined Carlin-type deposits in Nevada and is host
to the Pipeline deposits, in aggregate one of the most significant gold discoveries of the 1990s.
The Gold Acres operation has been intermittently active since 1942 and has been under Placer
Dome US control since 1969. Predominant hosts at Gold Acres are lower plate silty limestones of
the Roberts Mountains Formation (Srm), but some upper plate rocks are mineralized as well. The
Cortez deposit was discovered in 1966 and mined between 1969 and 1973. Placer added
reserves and reinitiated operations in 1988, which continued until 1993. Srm lithologies, again,
hosted much of the deposit. The structural setting for both Cortez and the ADA 52 extension was
created by low-angle thrust features (a duplex zone) and high-angle faults. The age of these
deposits is believed to be pre-Oligocene, and suspected ore-related products have been dated variously
at 72 and + 44 Ma. The Pipeline deposit was discovered by Cortez geologists in 1991, in
the course of a leach pad condemnation drilling program. South Pipeline was found the same
year, resulting from targeting lineaments and projected fault intersections similar to those present
at Pipeline. Drilling thus far has defined reserves of 188 Mst containing 11.6 M ounces of gold.
Gold production from the Pipeline operations, through 1999, stands at 3.84 million ounces from
30 million tons of ore. High-angle faults provided solution conduits for the hydrothermal systems
at both Pipeline deposits, and low-angle tabular (duplex) zones within the Srm are the hosts.
The tours to these active or recently active operations should provide fertile ground for discussion
of Carlin-type gold deposits. The relative importance or unimportance of Mesozoic-age plutonic
rocks to the gold systems will doubtless raise its head at all sites, and at Cortez we will view barren
Oligocene dikes that are now believed to be post-mineral intrusives. At Archimedes, we will
discuss ore-controlling structures visible in pit walls, and can point to Oligocene (?) ash beds
within the post-mineral overburden. Implications for future discoveries in covered terranes and in
the root zones of Carlin-type systems should make the discussions lively.