GOLD HILL, UTAH, ARSENATES

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 

www.Ghosttowns.com.

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. 

Majestic Deep Creek Mountains of west-central Utah. Public Domain photo.

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.

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.

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.

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.

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.

Gemmy adamite (A) crystals along with limonite/goethite (L).  Width FOV ~7 mm.

Short stubby crystals of adamite.  Width FOV ~7 mm.

Cartoon of adamite crystals.  Compare with photo above.

Probably the most common arsenate mineral at Gold Hill, and elsewhere, is adamite, a zinc arsenate hydroxide [Zn₂AsO₄OH].  It is a secondary mineral in the supergene and occurs where there are primary (maybe secondary) zinc and arsenic minerals.  Arsenic is present in the ubiquitous arsenopyrite; however, I am uncertain about the source of zinc in both adamite and austinite! 

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.

Mixite is an interesting rare copper bismuth arsenate mineral: BiCu₆(AsO₄)₃(OH)₆-3H₂O.  Again, it is a secondary arsenate that needs access to solutions adjacent to arsenopyrite and bismuth-bearing ores.  Most often mixite occurs as radiating, acicular prisms either individually or as masses (sometimes encrusting). The crystals range from green to blue and shades in between (and colorless although I have not observed these).  Mixite is somewhere between 3-4 (Mohs) in hardness although the crystals are so tiny it seems impossible to get a good reading.  Crystals are translucent to transparent and have a vitreous luster.  If you can locate a full radiating group the crystal “clump” is really beautiful.  Unfortunately my specimen is a small broken fragment.  And, my identification of mixite may be suspect as MinDat notes there is an entire “group of chemically complex, visually indistinguishable arsenates and phosphates” in the Mixite Group.  My identification was based on the mineral list at Gold Hill provided by MinDat—the only other Mixite Group mineral list was zalesite, a really complex REE arsenate “often only unambiguously identifiable by quantitative electron microprobe analysis.”

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.

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.

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.