Reconnaissance of Iron Occurrences in Colorado Page: 9
v, 82 p. : ill., map. ; 26 cm.View a full description of this report.
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Magnetite
Magnetite (Fe304) is a hard, strongly magnetic, black to brownish-black
mineral with a metallic to dull luster. It has a hardness of 5.5 to 6.5 and
a specific gravity of about 5. It occurs as an accessory mineral in pegma-
tites, gneisses, schists, and igneous rocks; as magmatic segregations with
apatite, pyroxene, and often titanium; in contact-metamorphic replacement-
type deposits, in limestone associated with nearby dioritic intrusions, which
may contain titanium, manganese, chalcopyrite, pyrite, hematite, olivene,
diopside, and garnet; and in relatively high-temperature sulfide veins.
Weathering and oxidation alter it to limonite and hematite.
Siderite
Siderite (FeCO3), the iron carbonate, is widely distributed in bedded
sedimentary deposits associated with shale, clay, and coal seams; this implies
a biogenic origin in an oxygen-deficient environment. It also occurs as re-
placements of limestone by iron solutions of igneous origin,in many shades of
brown and gras and has a hardness of 3.5 to 4 and a specific gravity of about
3.8. Siderite may be massive, fine grained, compact, or concretionary. It
also is known as spathic iron, clay ironstone, kidney ore, and blackband ore.
GENERAL GEOLOGY
Pyrite
Extensive deposits of pyrite (FeS) occur in many parts of the State,
including Rico, Leadville, and the Gilman mining districts, as vein-, blan-
ket-, and replacement-type deposits.
Limonite
Limonite has formed in concentrations of economic significance through
the near-surface oxidation of replacement bodies of iron-bearing carbonate,
which originally was introduced into limestone by ascending solutions. This
is exemplified by the deposits at the Orient mine, Saguache County, where
limonite with some hematite, siderite, and ochre occurs in the lower Leadville
limestone of Mississippian age. The limestone is massive and closely jointed,
and ore has been found in it over a vertical range of 1,000 feet.
Limonite and ochre replaces Paleozoic, Ordovician(?) limestone in the
Indian Creek basin a few miles east of Sargents, in northwest Saguache County.
Limonite and ochre have been formed by the oxidation of pyrite and other
iron-bearing minerals, as shown by the gossans that cap innumerable nonferrous
metal deposits in western Colorado.
Limonite also occurs as an inorganic precipitate around springs and in
swamps or bogs. Ground water passing over oxidizing pyritic bodies leaches
the iron in the form of a sulfate that, on hydrolyzing, forms limonite. The
limonite, with some goethite and turgite, is found in fairly pure deposits at
the orifices of springs and immediately below them. Examples of this type of525831 0 - 59 -2
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Harrer, C. M. & Tesch, W. J., Jr. Reconnaissance of Iron Occurrences in Colorado, report, 1959; Washington D.C.. (https://digital.library.unt.edu/ark:/67531/metadc170710/m1/21/: accessed April 25, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.