here - State of Arizona Department of Mines and Mineral Resources

194 PRINCIPAL GOLD-PRODUCING DISTRICTS OF THE UNITED STATES
were still being worked, on a fairly large scale, in
the early 1950's by the Round Mountain Gold
Dredging Corp. Their operations were suspended
during 1953-57, but were resumed in 1958; in
1958-59 this company was the largest gold producer
in Nevada.
Ferguson and Cathcart (1954) estimated the total
gold production of the district to be worth $8 million,
of which about 15 or 20 percent was from
placers and the remainder was from lodes. This
would amount to about 329,000 ounces of lode gold
and 58,200 ounces of placer gold. Most of the district's
production during 1950-59 was combined
with reports from other districts, but it was probably
about 150,000 ounces. Total gold production
for the district through 1959 was about 537,000
ounces.
The oldest rocks in the area are lower Paleozoic
limestone, jasper, and dark slaty schist (Ferguson,
1922, p. 386-398). These rocks are intensely folded
and were intruded during Cretaceous time by bodies
of granitic magma. Tertiary rocks, chiefly porphyritic
rhyolite (Oddie Rhyolite) and lake beds of the
Siebert Fonnation, overlie the folded Paleozoic
rocks.
There were two periods of mineralization. The
first occurred just after the granitic intrusions and
is characterized by huebnerite-bearing veins in the
granite. The 8eaond period of mineralization is of
Tertiary age and resulted in the fonnation of epithermal
gold-bearing quartz veins in the rhyolite.
The mineralogy of these veins is relatively simple
and consists of gold, auriferous pyrite, and sparse
realgar in a gangue of drusy and comb quartz,
adularia, and alunite. After the primary gold mineralization,
additional fissures were fonned in which
iron and manganese oxides and gold were deposited
by supergene solutions. The gold in these later
fractures was probably derived from the primary
auriferous pyrite.
TONOPAH DISTRICT
This district is in the southern San Antonio
Mountains near Tonopah, the county seat of Nye
County and the largest town in the county.
Tonopah was predominantly a silver district, but
it also yielded large amounts of gold. The first
claims were staked in 1900, and by 1901 there was
vigorous activity which lasted until the late 1940's;
thereafter, production declined. The principal companies
were the Tonopah Mining, the Tonopah Belmont
Development, and the Tonopah Extension
(Kral, 1951, p. 171).
Lincoln (1923, p. 186) listed the gold production
of Tonopah from 1901 through 1921 as $30,360,903
(about 1,473,830 ounces). From 1901 throug"t 1959,
a total of 1,880,000 ounces of gold was pnduced.
The rocks exposed in the Tonopah district are
all of Tertiary age and consist of a series of lava
flows, volcanic breccias, tuffs, and intrusives that
have been somewhat displaced from their original
attitudes by extensive faulting. Seven fon'1ations
recognized in the district (Nolan, 1935b, p. 13) are,
from oldest to youngest: the Tonopah Formation
(volcanic tuffs, breccias, flows), Sandgrass Andesite
(dark lavas interlayered with the Tonopah
Fonnation), Mizpah Trachyte (2,000-foot-thick
flows and breccias overlying the Tonopah), Extension
Breccia (tabular intrusive mass in west half
of district), West End Rhyolite (sills as much as
600 feet thick that intrude the older formr,tions),
Fraction Breccia Member of Esmeralda Formation
(volcanic breccia unconformably overlying ore
bodies), and postore rhyolite (dikes and lenticular
bodies that intrude the older rocks). Numerous
faults cut the Tertiary rocks, and Nolan (1935b,
p. 28-39) divided them into three general groups:
the Halifax fault zone which strikes generally
northward; the Tonopah, a compound fault that in
cross section has a trace that is convex upward;
and the youngest group which includes a fault
which strikes northwest, another which strikes
northeast, and a third which strikes north and dips
west.
The ore bodies are replacement veins in faults or
fractures (Nolan, 1935b, p. 40-49). The Tonopah
fault is believed to have exerted a major control
on the movement of mineralizing solutions. Ore
bodies are found in all three groups of faults, but
the Tonopah and Halifax seem to be more heavily
mineralized. Hypogene ore contains electrum. argentite,
polybasite, and pyrargyrite in a gar?:,ue of
quartz, pink carbonate, barite, and altere'l wallrocks.
There has been some supergene enrichment
locally, but the hypogene ore has been m'lst important.
TYBO DISTRICT
The Tybo (Hot Creek, Keystone, Empire) district
is in northeastern Nye County, in the Hot; Creek
Range, 65 miles northeast of Tonopah and about
100 miles southwest of Ely (lat 38°23' :roT., long
116°23' E.). In the district, gold is a byproduct of
ores mined primarily for their lead and silYer content.
The first mineral discoveries were made in
1865, and the town of Hot Creek was the population
center during the early days (Kral, 1951, p. 189).
The district prospered from the rich near-surface