I have written about Gold Hill, Utah, in a couple of
previous posts. It remains an amazing
place to collect specimens of arsenate minerals, and in fact, ore was mined at
Gold Hill specifically to produce arsenic (used for controlling insects on cotton
plants). The arsenates are those
minerals where a metallic cation combines with the radical AsO4. This arsenate radical is about the same size
as the phosphate [PO4] and vanadate [VO4] radicals so
there is much interchanging, substitution and solid solution series among
minerals containing one of the three radicals.
Some of the minerals also contain water [H2O] or a hydroxyl
radical [OH]. I like to collect members of the arsenates since many are
colorful and are fairly small crystals---all the better for a collector with
limited display space. The color of the
arsenate minerals is determined by the cations, commonly copper with resulting
green or blue specimens.
Ore shoot at Gold Hill, Utah. Photo courtesy of Ron Peterson at
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Gold Hill is an old mining community located south
of the bi-state town of Wendover, Nevada/Utah, that was mined for gold, copper,
zinc, lead, arsenic and tungsten from the mid to late 1800s until the late
1940s. The peak activity was in the
early 1900s when a spur railroad reached the area in 1917. There was only sporadic mining after World
War I.
Gold Hill or the Clifton District, contains numerous
mines, including an open pit, and is located near the northwest end of the Deep
Creek Mountains, perhaps Utah’s most isolated and unknown mountain range. Peaks do
reach 12,000 feet—Ibapah Peak at 12,087 and Haystack at 12,020. The Deeps
are the major topographic feature in western Utah. The range has a
Precambrian core surrounded by Paleozoic sedimentary rocks with later Mesozoic intrusions—mostly
quartz monzonite and granite/granodiorite, and later Tertiary volcanics.
Mineralization began at Gold Hill approximately 152 Ma
as magma was intruded into the overlying Paleozoic rocks (Robinson, 1993). This
intrusion created three different types of ore deposits: skarn deposits,
replacement type bodies, and vein deposits. At Gold Hill skarns are small structures
but contain a rich suite of metals that includes copper, iron and tungsten
(Ege, 2005).
Replacement deposits occur along the contact between
the cooled magma (granodiorite) and limestone. The host rocks for the ore deposits
are Mississippian to Pennsylvanian (360 to 330 Ma) limestones (Ege, 2005; Robinson,
1993). These deposits are rich in silver,
gold, arsenic, copper, and lead.
Vein deposits are found within the intruding igneous
rocks. All of the veins are small and represent only a small portion of the ore
mined from the district (Ege, 2005).
A second stage of mineralization commenced in the
early to middle Tertiary (perhaps 38 Ma) when magma again intruded into
Mississippian to Pennsylvanian sedimentary rocks. These deposits are rich in silver, lead,
copper, gold, and other metals. In
addition, extrusive volcanic rocks, ~8 Ma, created some low-grade beryllium
mineralization in veins associated within the granodiorite (Ege, 2005; Robinson,
1993).
The primary minerals (hypogene) in and near the
cooked limestone include galena, sphalerite, chalcopyrite, pyrite, pyrrhotite,
tetrahedrite, and arsenopyrite. The
overlying oxidized supergene includes all of those beautiful arsenates:
adamite, austenite, arsenosiderite, beudanite, conichalcite, clinoclase,
mimetite, olivenite, pharmacosiderite, scorodite, and veszelyite (listed by El-Shoutoury and Whelan,
1970). Since that publication, there
have been other supergene minerals identified—see MinDat.com. The arsenic in the arsenates probably was
derived from the primary (hypogene) mineral arsenopyrite (FeAsS). Shoutoury and Whelan (1970) noted the
arsenates of zinc, lead, copper; iron and calcium were produced when primary
chalcopyrite, galena, sphalerite and pyrite were oxidized in close proximity to
arsenopyrite.
One of those generally not-so-common hydrous iron
arsenates, scorodite (FeAsO4-2H2O), is, at Gold Hill, the
most abundant of the secondary arsenates.
This mostly blue, but sometimes blue-green to green to yellow-brown to
colorless mineral may occur in small crystals (mostly pyramidal) but generally is
an earthy mass that often looks like a “smear” on the host rock. The crystals have a vitreous luster and are
transparent; however, the earthy masses are dull and non-crystalline. The hardness is somewhere around 3.5-4.0
(Mohs) so it can be scratched by a steel knife.
Scorodite is found in the supergene, secondary zone, and results
(generally) from the oxidation of arsenopyrite. At the Western Utah Mine, some ore bodies are
composed almost entirely of scorodite (Nolan, 1935).
Scorodite, both blue, colorless, and green
boytroids. FOV width ~1.0 cm.
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Another arsenate at Gold Hill, in fact this is the
type locality (Staples, 1935), is the rather rare calcium zinc mineral known as
austinite [CaZnAsO4(OH)]. In
contrast to the earthy habit of scorodite, austinite is often found as
prismatic (elongated along C-axis) orthorhombic terminated crystals; many are
colorless while others are green (variety termed cuprian austinite) to pale
white to clear with a hint of blue.
These crystals have vitreous luster, a hardness of 4.0-4.5 (Mohs), and
are quite brittle. Fibrous and nodular forms have a silky luster.
Austinite crystals, clear and gemmy. Orange-red crystal in center is unknown. Width FOV ~2 mm.
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Austinite crystals, clear and gemmy. Orange-red crystal in center is unknown. Width FOV ~6 mm. |
Cartoon of prismatic austinite crystal. Compare with photos above.
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As with other arsenates, austinite is a supergene
mineral associated with the oxidation of arsenopyrite. Austinite seems a late precipitating mineral
and is often developed on globular limonite/goethite. Austinite is the zinc end
member of a solid solution series with conichalcite, the copper end member.
Globules of conichalcite. FOV width ~6 mm.
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Conichalcite [CaCuAsO4(OH)] is also a secondary mineral in the supergene
but likely the result of the oxidation of enargite (Cu3AsS4)
near or with arsenopyrite. It is the
most widespread of the green copper arsenates in the oxidized ores at Gold Hill
and occurs chiefly as a mammillary coating on scorodite, limonite/goethite, or
as a replacement product of olivenite (Nolan, 1935). It is yellow to emerald green in color, ~4.5
(Mohs) in hardness, and mostly massive and fibrous in habit---numerous, tiny,
vitreous crystals forming green masses.
Gold Hill is one of the premier North American localities for the
occurrence this arsenate.
Besides the solid solution with austinite, conichalcite
is the copper end member in a solid solution with cobaltaustinate, the cobalt
end member; with duftite where lead replaces the calcium; and with tangeite
where the vanadate radical replaces the arsenate radical.
Radiating crystals of adamite. Width FOV ~7 mm.
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Gemmy adamite (A) crystals along with
limonite/goethite (L). Width FOV ~7 mm.
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Short stubby crystals of adamite. Width FOV ~7 mm.
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Cartoon of adamite crystals. Compare with photo above.
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Perhaps there are several possibilities! Sphalerite (ZnS) is moderately abundant as massive
veins or disseminated crystals at several localities in rocks at Gold Hill
(Nolan, 1935). Tetrahedrite and/or tennantite [(CuFe)12Sb4S13]
– [Cu6[Cu4(FeZn]AsO13], both present at Gold
Hill, can have up to ~15% zinc substituting for the copper. Rick, the Gold Hill guru at the premier rock shop
in Salt Lake City, Rockpick Legends (www.rocks4u.com),
told me that many collectors and researchers at Gold Hill believe the zinc came
from mineral(s) that are now represented as pseudomorphs in the limonitic/goethite
gossan. He suggests examining some of the small vugs and noting the
appearance of “smithsonite-like” crystals that have, in reality, pseudomorphed
into the limonite/goethite gossan. This would suggest that at one time a
large amount of smithsonite was present in the rocks at Gold Hill and was able
to provide the zinc for other minerals in the supergene. A more abstract possibility might be the
presence of trace amounts of zinc moving upwards in the hydrothermal solutions.
Adamite comes in a variety of colors from yellowish
tones (some trace iron compounds) to clear colorless to lime green and other
shades of green (due to trace copper). Trace
cobalt sometimes provides a delicate pink to violet color. Most adamite occurs
in tiny crystal (orthorhombic wedge-like prisms) masses, radiating clusters, or
vug fillings. It has a vitreous luster,
crystals are fairly soft (~3.5; Mohs) are quite brittle.
Adamite at Gold Hill often occurs with austinite and
I have some difficulty in distinguishing between the two arsenates! However, adamite has a nice yellow-green
fluorescence under both long-wave and short-wave excitement. There is also a mild phosphorescence (UV
color stays after lamp is shut off).
That is, if the copper content is not too high! Copper has a tendency to quench fluorescence. Adamite crystals are often short and stubby while
austinite has more elongate crystals that are more prismatic—it seems. Ah, to be a better mineralogist---a good New Year’s
resolution!
Spray of green mixite. Length of longest individual
~1 mm.
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OK, I suppose the arsenic came from arsenopyrite but
what about the bismuth? According to
Nolen (1935) “ores of …bismuth…are also widely distributed throughout the
quartz monzonite area… and locally bismuthinite [Bi2S3]
is fairly abundant [as a hypogene ore].” Aikinite [PbCuBiS3], another hypogene
sulfide, is “erratically distributed.” Bismutite
[(BiO)2CO3], a bismuth carbonate, is also a supergene
mineral at Gold Hill and is the direct result of oxidation of
bismuthinite. This leads me to believe
that mixite results from the oxidation of bismuthinite in proximity to
arsenopyrite (and/or enargite) and some copper mineral (maybe enargite).
Cornwallite is a
copper arsenate hydroxide [Cu5(AsO4)2(OH)4],
another secondary mineral found in the oxidized zones of copper sulfide
deposits where the ores contain both arsenic and copper and perhaps oxidized
from something like tennantite (Cu12As4S13) or
enargite ( Cu3AsS4), maybe in association with
arsenopyrite. Most cornwallite occurs as botryoidal to globular crusts of
microcrystalline radiating fibers. The
dominate color is essentially an emerald green; however, in order to have a
chance to see the very tiny individual crystals, the mineral must be observed
under high magnification.
Crystals of green cornwallite and blue azurite. Width FOV ~1.0 cm.
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Cornubite
(Triclinic System), also present at Gold Hill, is a dimorph of cornwallite (Monoclinic
System) ---same chemical formula but different crystal systems. And, if the phosphate radical PO4
replaces the AsO4, the mineral becomes pseudomalachite (not present
at Gold Hill).
An unknown. Gemmy
crystal in center is adamite. Could the
brown individuals limonite/goethite pseudomorphs after olivenite?? Width FOV ~ 2 mm.
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Well, there are numerous other minerals present at
Gold Hill and this small article simply represents some interesting arsenates
in my collection. In articles like this
I am usually “out of my league” in trying to describe similar looking minerals that
often are in solid solution with each other!
Most/many are microcrystals that might need an electron microprobe for
positive identification; however, I continue to muddle along and do my best.
If you really want to see someone’s fantastic, do
your best, paper, read the USGS contribution by Tom Nolan noted below (it is
available on the USGS web site). As you
read the paper also remember that field work was completed in the early 1930’s
and he did not have access to an electron microprobe!
You
can only do your best. That’s all you
can do. And if that isn’t good enough,
it isn’t good enough. Imelda
Staunton
REFERENCES
CITED
Ege, C.L., 2005, Selected mining districts in Utah:
Utah Geological Survey, Miscellaneous Publication 05-5.
El-Shatoury, H.M., and Whelan, J.A., 1970,
Mineralization in the Gold Hill mining district, Tooele County, Utah: Utah
Geological and Mineralogical Survey Bulletin 83.
Nolan, T.B., 1935, The Gold Hill Mining District
Utah: USGS Professional Paper 117.
Staples, L.W., 1935, Austinite, a new arsenate
mineral, from Gold Hill, Utah: American
Mineralogist, v. 20.
Robinson, J.P., 1993, Provisional geologic map of
the Gold Hill quadrangle, Tooele County, Utah: Utah Geological Survey Map 140, scale
1:24,000.
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