5
The principal formations are patches of gneiss and gran-
ite overlain by a thick series of igneous rocks with feldspar
stringers along original lava flow lines (trachytes), andesite,
latite (the extrusive form of monazite), tuffs, rhyolite, and
basalt. Parts of this series are intruded by monzonitic, gra-
nitic, and rhyolitic porphyrys. The dominant ore-bearing
formation is the greenish Oatman andesite.
Ore veins occur within fissures along which faulting
has occurred before, during, and after vein formation.
Some of the veins are tabular, but the larger ones are string-
ers with complex structures. Few of the veins exceed widths
of 50 feet.
The gangue within the veins is mainly quartz and cal-
cite, either of which can dominate. Gypsum and kaolin are
locally abundant in the oxidized zone above the historic
water table.
The metallic minerals are limited, consisting of free
gold with rare pyrite and chalcopyrite. The gold is very fine-
grained and visible only in high-grade ore. Nuggets were
uncommon and efforts at placer mining were unproductive.
MINING METHODS
Ore veins throughout the district were irregular and unpre-
dictable. They could be 20–50 feet in width, pinch out with
the next round of blasting, then resume at original width if
the mine foreman had the right instincts. Therefore, min-
ing was opportunistic and selective. The most common
method was shrinkage stoping, which minimized dilution
while allowing high recoveries of gold mineralization.
Nearly all mines had competent ground with extensive
timbering needed only in the vicinity of faults. Dewatering
requirements usually increased as shafts approached the
water table, but perched aquifers occasionally dictated tem-
porary suspension of mining. For many years, extending as
late as the 1990s, the Tom Reed supplied potable water to
the town of Oatman.20
METALLURGY
Gold was almost exclusively very fine, defying efficient
gravity concentration and amalgamation. Discrete silver
minerals were not common, so the silver that was recov-
erable may have existed primarily as the gold/silver alloy,
electrum. Some bullion was 30 percent silver.
Consequently, recoveries of both metals were very low
until the advent of cyanidation.
20. Personal communication (1992) with Chris Sawyer,
President, Mojave Mining &Milling Company.
The cyanidation circuit at the Tom Reed was typical
and consisted of a primary jaw crusher, a Merrick-type
belt scale, and two stages of Allis-Chalmers grate-discharge
6-foot by 6-foot ball mills in closed-circuit with Dorr rake-
type classifiers.
The 2nd stage classifier overflow at typically 60–90 per-
cent minus 200-mesh was thickened and leached in a series
of three 40-foot high by 12-foot diameter Pachuca-type
agitated leach tanks. Following a 50-hour leach, the resid-
ual slurry was washed by countercurrent decantation in two
rows of four 30-foot diameter by 10-foot Dorr thickeners.
(Polymeric flocculants had not yet been invented, so the
thickeners seem large compared with current practice for
a 300 ton/day CCD circuit.) Thickener underflows were
advanced with Frenier pumps and overflows advanced by
gravity.
CCD overflow solution was settled in a 40-foot diam-
eter by 12-foot Dorr thickener, filtered, and treated with
zinc dust in a conventional Merrill-Crowe circuit. The pre-
cipitate was melted in a tilting furnace and cast into bullion
bars. Overall recoveries from ore to bullion were approxi-
mately 90–92 percent.
The Vivian mine was a significant producer, beginning
in the 1860s, but little information is available, except that
it was moderately productive in the early-1930s.21 The
owners built a mill but, whereas all other mills that were
built after 1900 used agitated cyanidation, there is a Daily
Mill Report form for the Vivian Mining Company for July
27, 1940, that contained blanks into which entries could
be made for flotation reagent and frother addition rates,22
clearly indicating that flotation, not cyanidation was being
used or considered.
1943–1985
There is no well-documented record of gold production
during this period. However, it is doubtful that there was
significant production, as cost inflation after World War II
was serious and the gold price was not allowed to float until
1968. Furthermore, it did not exceed $100 per troy ounce
until 1974.23
1985–PRESENT
After significant gold price appreciation that occurred
during 1975–1985, interest was gradually renewed in the
old Oatman-Goldroad District. It is possible that earlier
exploration and property transactions occurred, but the
21. Wilson, Ibid, see table titled Production, Oatman District,
facing p. 80.
22. Personal collection.
23. USGS, Ibid, “Annual Commodity Prices”
The principal formations are patches of gneiss and gran-
ite overlain by a thick series of igneous rocks with feldspar
stringers along original lava flow lines (trachytes), andesite,
latite (the extrusive form of monazite), tuffs, rhyolite, and
basalt. Parts of this series are intruded by monzonitic, gra-
nitic, and rhyolitic porphyrys. The dominant ore-bearing
formation is the greenish Oatman andesite.
Ore veins occur within fissures along which faulting
has occurred before, during, and after vein formation.
Some of the veins are tabular, but the larger ones are string-
ers with complex structures. Few of the veins exceed widths
of 50 feet.
The gangue within the veins is mainly quartz and cal-
cite, either of which can dominate. Gypsum and kaolin are
locally abundant in the oxidized zone above the historic
water table.
The metallic minerals are limited, consisting of free
gold with rare pyrite and chalcopyrite. The gold is very fine-
grained and visible only in high-grade ore. Nuggets were
uncommon and efforts at placer mining were unproductive.
MINING METHODS
Ore veins throughout the district were irregular and unpre-
dictable. They could be 20–50 feet in width, pinch out with
the next round of blasting, then resume at original width if
the mine foreman had the right instincts. Therefore, min-
ing was opportunistic and selective. The most common
method was shrinkage stoping, which minimized dilution
while allowing high recoveries of gold mineralization.
Nearly all mines had competent ground with extensive
timbering needed only in the vicinity of faults. Dewatering
requirements usually increased as shafts approached the
water table, but perched aquifers occasionally dictated tem-
porary suspension of mining. For many years, extending as
late as the 1990s, the Tom Reed supplied potable water to
the town of Oatman.20
METALLURGY
Gold was almost exclusively very fine, defying efficient
gravity concentration and amalgamation. Discrete silver
minerals were not common, so the silver that was recov-
erable may have existed primarily as the gold/silver alloy,
electrum. Some bullion was 30 percent silver.
Consequently, recoveries of both metals were very low
until the advent of cyanidation.
20. Personal communication (1992) with Chris Sawyer,
President, Mojave Mining &Milling Company.
The cyanidation circuit at the Tom Reed was typical
and consisted of a primary jaw crusher, a Merrick-type
belt scale, and two stages of Allis-Chalmers grate-discharge
6-foot by 6-foot ball mills in closed-circuit with Dorr rake-
type classifiers.
The 2nd stage classifier overflow at typically 60–90 per-
cent minus 200-mesh was thickened and leached in a series
of three 40-foot high by 12-foot diameter Pachuca-type
agitated leach tanks. Following a 50-hour leach, the resid-
ual slurry was washed by countercurrent decantation in two
rows of four 30-foot diameter by 10-foot Dorr thickeners.
(Polymeric flocculants had not yet been invented, so the
thickeners seem large compared with current practice for
a 300 ton/day CCD circuit.) Thickener underflows were
advanced with Frenier pumps and overflows advanced by
gravity.
CCD overflow solution was settled in a 40-foot diam-
eter by 12-foot Dorr thickener, filtered, and treated with
zinc dust in a conventional Merrill-Crowe circuit. The pre-
cipitate was melted in a tilting furnace and cast into bullion
bars. Overall recoveries from ore to bullion were approxi-
mately 90–92 percent.
The Vivian mine was a significant producer, beginning
in the 1860s, but little information is available, except that
it was moderately productive in the early-1930s.21 The
owners built a mill but, whereas all other mills that were
built after 1900 used agitated cyanidation, there is a Daily
Mill Report form for the Vivian Mining Company for July
27, 1940, that contained blanks into which entries could
be made for flotation reagent and frother addition rates,22
clearly indicating that flotation, not cyanidation was being
used or considered.
1943–1985
There is no well-documented record of gold production
during this period. However, it is doubtful that there was
significant production, as cost inflation after World War II
was serious and the gold price was not allowed to float until
1968. Furthermore, it did not exceed $100 per troy ounce
until 1974.23
1985–PRESENT
After significant gold price appreciation that occurred
during 1975–1985, interest was gradually renewed in the
old Oatman-Goldroad District. It is possible that earlier
exploration and property transactions occurred, but the
21. Wilson, Ibid, see table titled Production, Oatman District,
facing p. 80.
22. Personal collection.
23. USGS, Ibid, “Annual Commodity Prices”