here - State of Arizona Department of Mines and Mineral Resources

68 PRINCIPAL GOLD-PRODUCING DISTRICTS OF THE UNITED STATES
Gold production of Mariposa County for 1880­
1959 was about 2,144,500 ounces: about 583,500
ounces came from placers and about 1,561,000
ounces came from lodes. Production before 1880 has
not been determined.
Gold mining began at an early date in the county.
Gravels along Agua Fria and Mariposa Creeks
were worked before 1849 and were thoroughly
mined out by the hordes of prospectors who overran
the area during the gold rush of 1849. By July
1849 a stamp mill was processing ore from the first
lode discovery in the county, the Mariposa mine on
the Mother Lode (Bowen and Gray, 1957, p. 39, 43).
Lode mining in Mariposa County was inhibited
by the controversial Las Mariposas land grant
which gave title to 14 of the 24 miles of the Mother
Lode in the county to Gen. John C. Fremont. This
grant was unsurveyed and was made before gold
was discovered. Long before Fremont attempted to
establish his right, the grant was overrun with
prospectors and miners, who understandably were
reluctant to give up what they considered just
claims. Mter years of conflict in and out of the
courts, Fremont's claim to the grant was formally
recognized. But by then the property was plagued
by mismanagement and inefficiency and the mines
never fulfilled the expectations of the authorities
who evaluated them (Julihn and Horton, 1940, p.
95-96).
Another large estate, the Cook estate, which encompassed
most of the mines along a 2-mile length
of the Mother Lode in the Coulterville area, further
complicated operations on the Mother Lode in Mariposa
County (Julihn and Horton, 1940, p. 96-97).
Despite the early frustrations, lode mining in
Mariposa County flourished and was especially successful
in the late 1930's and early 1940's before
most of the mines closed in compliance with War
Production Board Order L-208 issued in October
1942. After World War II gold mining declined,
and during 1950-59 the average annual gold output
was less than 1,000 ounces.
The western two-thirds of the county is underlain
by metasedimentary rocks and metavolcanics
of Paleozoic and Late Jurassic age, and the eastern
one-third is underlain chiefly by intrusives of Late
Jurassic or Early Cretaceous age (Bowen and Gray,
1957, p. 45). The intrusive rocks consist of various
types of granitic and peridotitic rocks, but biotitehornblende
granodiorite is predominant.
HORNITOS DISTRICT
The Hornitos district, in western Mariposa
County at lat 37°30' N. and long 120°14' W., is
noted for gold production from both plac"rs and
lodes.
In the early days the Quaternary grr.vels of
Hornitos Creek yielded considerable gold, b'1t these
were nearly exhausted before 1900. The lode mines
are all west of the Mother Lode, in the zone of veins
referred to as the West Belt. Total production from
the district is not known, but a minimum of 500,000
ounces seems to be a reasonable estimate.
The gold deposits of the West Belt are in veins
that cut several rock types, chiefly metasedimentary
rocks of the Mariposa Formation of Jurar"lic age.
The Mariposa is intruded locally by serpentinized
peridotite, pyroxenite, basic intrusives altered to
hornblende schists, and acid intrusives such as
granite and granodiorite (Julihn and Hort(J"l, 1940,
p. 116-117). Most of the gold deposits al'';! along
contacts of igneous rocks and metasedimentary
rocks.
MERCED RIVER PLACERS
Quaternary gravels along the Merced River west
of Bagby were a source of placer gold in the late
1860's and 1870's (Bowen and Gray, 1957, p. 187),
but little activity has been reported in recert years.
Production from these deposits is not known, but
probably was at least 50,000 ounces of gold.
MORMON BA.R DISTRICT
The Mormon Bar district is in T. 5 S., r.. 18 E.,
about 2lh miles south of the town of Mariposa.
In the early days of mining in Mariposa County,
the headwaters of Mariposa Creek near Mormon
Bar was the scene of considerable placer activity.
By 1870, however, the deposits, which were only
about 6 feet thick, were almost worked out, and
thereafter the placer output of the entire county
averaged only a few hundred ounces p"r year
(Julihn and Horton, 1940, p. 159, 162). In the late
1930's a slight revival took place, and the gravels
at Mormon Bar were worked industriously by dragline
(Julihn and Horton, 1940, p. 159). Total gold
production for the district is estimated r.t about
75,000 ounces.
MOTHER LODE A.ND EAST BELT DISTRI0TS
The Mother Lode, which has its southen terminus
in Mariposa County, is a northwest-t.rending
zone that is 3 or 4 miles wide and extends from the
town of Mormon Bar northward through Coulterville
into Tuolumne County. Roughly parallel veins
east of this zone are referred to as the East. Belt.
Though much has been said about the pr....duction
of the Mother Lode, the mines of the East Belt and