Geology of the Tonopah Copper Deposit, Nevada

ABSTRACT
The Tonopah Copper deposit, Nye County, Nevada, is a classic
example of supergene enrichment of low-grade copper mineralization
associated with a 77 ± 2 Ma composite quartz monzonite intrusion.
This intrusion, the Hall stock, is the likely source of heat and
fluids for the hydrothermal system that deposited proximal molybdenite
and distal chalcopyrite mineralization. Mineralization was
distributed by quartz-sulfide veining that occurs both within the
perimeter of the stock and the surrounding host, weakly altered
meta-volcanic sediments of the Permian Pablo Formation. Pyrite is
the most abundant sulfide mineral throughout the mineralized system.
Where veining is absent within the system, copper mineralization
is almost nonexistent. Sometime after the emplacement of the
hypogene mineralization, exposure to meteoric waters created an
environment suitable for the process of acid leaching. Much of the
copper present in chalcopyrite was released and re-precipitated at
the groundwater table as a tabular chalcocite orebody. Chalcocite at
the Tonopah Copper deposit typically coats pyrite in veins or occurs
as fine disseminated grains after pyrite or chalcopyrite. Argillic,
potassic, and chloritic alteration types are accompanied by an abundance
of iron oxides and silica. For production purposes, the copper
deposit has been divided into five metallurgical types, which include
the (1) Leached Cap, (2) Upper Transition, (3) Enriched, (4) Lower
Transition, and (5) Primary Sulfide. These subdivisions are based
upon field observations, assays, and drill data.
After emplacement of the supergene blanket, the northern San
Antonio Mountains, in which the deposit is located, were subjected
to several extensional tectonic episodes. As a result, the orebody is
extensively post-mineralization faulted, which leads to complexities
in resource determination and mining. The deposit is completely
bound by faulting on all sides, normal faulting on the east and west
sides and strike-slip (?) faulting to the north and south. Normal
faults have dropped the copper deposit down from its original position
and rotated the entire range approximately 70¡ about a N 30¡W
axis, giving the system a 20¡ plunge to the west-southwest. The timing
and effect of individual tectonic events is not known at this time,
though it is assumed that most of the rotation has occurred late in the
tectonic history of the range.