here - State of Arizona Department of Mines and Mineral Resources

GILMAN DISTRICT
The Gilman (Battle Mountain, Red Cliff) district,
in southeastern Eagle County on the northeast
flank of the Sawatch Range, is between Gilman and
Red Cliff, about 20 miles north of Leadville. Gold
has been extracted from pyritic gold ores and as a
byproduct of base-metal ores.
The initial discovery in 1879 of silver-lead ore in
limestone was followed in 1884 by finds of gold ore
in the underlying quartzite. The relative importance
of the metals mined varied periodically in the history
of the district. Before 1905 silver and gold
were the major commodities, whereas lead and copper
were of minor importance. From 1905 to 1930
zinc was the principal product and from 1931 to
1941 silver-copper ore containing considerable gold
was mined. Zinc again regained importance from
1942 through 1959. The total gold production of the
district through 1959 was roughly 348,000 ounces.
Precambrian granite, schist, and gneissic diorite
are exposed in the bottom of Eagle Canyon and are
overlain by thin lower Paleozoic formations that
include the Sawatch Quartzite of Cambrian age, the
Harding Sandstone of Ordovician age, the Chaffee
Formation of Devonian age, and the Leadville Limestone
of Mississippian age. Overlying these units is
a thick section of Pennsylvanian and Permian( ?)
sedimentary rocks. A sill of Cretaceous or Tertiary
quartz latite appears a few feet above the Leadville
Limestone throughout the district. The sedimentary
rocks dip about 12° NE. and are cut by beddingplane
faults and a few weak high-angle faults (Ogden
Tweto and T. S. Lovering, in Vanderwilt and
others, 1947, p. 379-381).
Ore bodies occur in (1) veins in Precambrian
rocks and in the Sawatch Quartzite and (2) in replacement
deposits in quartzite and limestone of
Devonian and Mississippian age. Most production
has come from replacement bodies in the Leadville
Limestone and the Dyer Dolomite Member of the
Chaffee Formation. The veins in the Precambrian
rocks contain pyritic gold and complex sulfide ores
in which sphalerite, chalcopyrite, galena, and pyrite
are the chief minerals. The pyrite-gold veins are
almost entirely of pyrite with a little gold. Most of
the veins terminate or become thin at the base of the
Sawatch Quartzite, but those that extend into the
quartzite contain scattered pockets of gold-silver tellurides,
chief among which are petzite and hessite.
A large part of the early output from the Gilman
district came from oxidized manto or bedding-vein
deposits in a breccia zone in the Sawatch Quartzite,
about 180 feet above the base. Two distinct stages
COLORADO 99
of mineralization are recognized in these-depo..its.
The first deposited largely pyrite, a very little chalcopyrite,
and inconsequential amounts of gold and
silver. During the second stage, manganoside,,-,ite,
f>yrite, chalcopyrite, galena, sphalerite, and pyrite
were deposited. Gold and silver are chiefly associated
with the chalcopyrite of the second stage.
The replacement deposits in the limestones consist
of chimneys of pyritic silver-copper ore and
mantos of base-metal sulfide ores. The chimr"!ys
are downward-tapering pipes that extend from the
ends of manto ore bodies near the top of the LP..adville
Limestone downward into the Parting Quartzite
Member of the Chaffee Formation. The chimr"!ys
are roughly circular or elliptical and are as mucl' as
300 feet in diameter at the top and taper downward.
There is no physical break between the two type:> of
ore bodies, but there is a pronounced mineralogic
difference. The chimney ore, the chief source of the
gold, consists of a core of pyrite containing minor
quantities of other minerals which make them valuable
for silver, copper, and gold. The chief COIJper
mineral is chalcopyrite. Silver and gold are ass')ciated
with chalcopyrite, galena, and with a grour of
late copper and silver minerals including tetrahedrite,
freibergite, polybasite, stromeyerite, bC"lrnonite,
and schapbachite. Late accessory mine)'als
include manganosiderite, dolomite, barite, apatite,
and quartz. The galena contains small inclusiom of
hessite and a little petzite, and the petzite contains
minute blebs and veinlets of free gold.
The manto ore bodies are valued mainly for zinc.
The minerals of these deposits are sphalerite, pyrite,
manganosiderite, minor galena, and accessl)ry
chalcopyrite, barite, dolomite, and quartz. Oxidized
parts of these deposits contain considerable lead, silver,
and gold. The mantos are 50 to .300 feet wide,
5 to 150 feet thick, and as much as 4,000 feet IO'1g.
All are in the Leadville Limestone (Ogden Tweto
and T. S. Lovering, in Vanderwilt and others, 19