DUGWAY GEODES, ANTLERITE, SCARY PLACES

Google Earth© image of Dugway Range and vicinity.  The Deep Creeks are home to the Gold Hill mining area, subject of numerous blog postings.  The Thomas Range is home to collecting areas for topaz and other minerals (see blogs).  Interstate 80 and the Bonneville Salt Flats are far to the north.  The West Desert is impressive, massive and lonely.  Use extreme care in traveling to the area.

Mention the term “Dugway” in Utah, and other western states, and two pictures probably come to mind.  One is the vast piece of Federal land called the Dugway Proving Range, a US Army facility dedicated to the testing of biological and chemical weapons, and overflights by planes of the US Air Force dropping “things” and shooting up the target range.  I “believe” these activities are going on out there; however, the area is the home of intensively secret activities and the security is extremely “tight!” Their Mission Statement: "As the nation's Major Range and Test Facility Base (MRTFB) for Chemical, Biological, Radiological, Nuclear, and Explosives (CBRNE), Dugway Proving Ground facilitates testing, evaluation, and training for our nation's elite forces and first responders through our capable workforce, high desert venues, and facilities."  I love the government acronyms!

During my student time at the University of Utah (late 1960s) some of my fellow students decided to take a mineral collecting trip out to the West Desert.  They evidently strayed off public land and into the Proving Grounds.  As they later stated to me—“there were lots of guys with big guns and mean dogs pouring out of a helicopter and coming for us.”  As I understand, these erstwhile rockhounds were questioned and turned loose and told never to return.

 

Polished Dugway Geode.  Width ~ 15.5 cm.

The second remembrance of people is the Dugway geode so available to the rockhounds along the southern end of the Dugway Range and south of the Proving Grounds.  The geodes are somewhat famous and thousands have been collected off BLM land.  The original home of the geodes was in a Miocene (~6-8 Ma) rhyolite, an extrusive igneous rock (fine grain) that is potassium- and silica-rich.  However, the collecting of geodes takes place in sediments of Pleistocene Lake Bonneville located west of the Range.  It seems that wave action from this large Pleistocene lake eroded the geodes from the rhyolite and redeposited them in the very soft lake sediments, a fortuitous event for rockhounds---no digging and cracking the hard rhyolite.

The geodes range from nice hollow “balls” to forms that are completely filled with varieties of quartz (thunderegg) to forms that have two or more hollow areas.  The specimen I have left in my collection is nicely banded like an agate but is mostly (~85%) filled with quartz but also contains two major and one minor “hollows.” 

The Dugway Range also has a number of old and sometimes abandoned mines that mostly were concerned with copper, gold, silver, zinc and lead.  The two largest operations were the Four Metals Mine and the Rainbow Mine. One forgotten smaller mine is the Bertha Mine where copper was the primary target and lead a secondary target. 

 

This sketch shows a wagon train moving across the desert near the Dugway Range.  Granite Mountains in center distance. This sketch is Plate I in Lt. J.H. Simpson's 1859 Journal of Explorations in Utah.  Public Domain.

Stattz and Carr (1964)  described the geology of the region as: “The Bertha property is in dolomite of unknown age in the southeast end of a graben between two upfaulted blocks of Prospect Mountain quartzite [Cambrian in age]…The main ore ·deposit on the Bertha claim is a large irregular body of pyritic copper ore formed by replacement of the brown dolomite along ·fractures that in general parallel the two main boundary faults [although MinDat noted the Deer Peak Volcanics and a breccia pipe of porphyry and gneiss also served as host rocks for mineralization]…Near the surface the ore body has been oxidized to a brown limonitic gossan…and in places small patches of covellite and malachite [occur]. A polished section shows that the ore consists chiefly of pyrite and hematite in dark-green to black claylike material. Hematite occurs in flat plates and clots which may replace the pyrite… Chalcanthite was noted on the walls of the adit in several places… A chip sample taken around the edge of the stope at the southeast end of the adit contained 3.7. percent copper and 1.2 percent lead.”

As best that I can understand the Bertha was never a large producer of good ore and few people visit the area today.  MinDat listed a small suite of six valid minerals collected from the Bertha Mine (AKA Dugway Property): bornite [Cu₅FeS₄], chalcopyrite [CuFeS₂], covellite [CuS], galena [PbS], pyrite [FeS₂] and chalcanthite [CuSO₄-5H₂O].  However, at a small show in Denver last month I was sorting through minerals that were not displayed on the “top shelf” and located a really nice specimen of antlerite, a copper sulfate hydroxide [Cu₃(SO₄)(OH)₄].

Antlerite is a rare secondary mineral formed in the oxidized zone of copper deposits…[and was named for] the Antler mine in Mohave County, [Arizona]…the type locality (Anthony and others, 1995).  So, the primary copper minerals in the hypogene (sulfides [contain S] like bornite, chalcopyrite, covellite) weathered and formed copper sulfates [contain SO₄] including antlerite, Cu₃(SO₄)(OH)₄, brochantite, Cu₄(SO₄)(OH)₆, and posnjakite, Cu₄SO₄(OH)₆·H₂O.  Bronchanite, commonly confused with antlerite, precipitates in low temperatures, antlerite in more elevated temperatures (Zattleu and others, 2013).

Photomicrographs of masses of tiny antlerite crystals with individual sizes of less than 1 mm.  Some exhibit striations of faces.  Most appear to be of the “common” type as described bypalache (1939).  see figure below.

 

Photomicrograph twinned? crystal of antlerite.  Width of individual ~.4 mm.  

Crystal forms of antlerite as defined by Palache (1939).  At that time #1 was known from Bisbee, AZ while the remainder were from Chuquicmata, Chile.  All are Orthorhombic.

 Antlerite is some shade of green in color--emerald, light green, black green--as are many copper minerals, and has a light green streak.  Crystals are soft at ~3.0+ (Mohs), have a vitreous luster, and are usually transparent.  According to MinDat the crystal morphology is varies: commonly thick tabular [but] also equant or short prismatic [and]… as cross-fiber veinlets or friable interlaced aggregates of acicular or fibrous crystals; felt-like; granular.

People who visit the lonely Dugway Range might be a fan of the Allman Brothers who crooned one of my favorite traveling songs:

Lord, I was born a ramblin’ man

Tryin” to make a livin” and doin’ the best I can

And when its time for leavin”

I hope you’ll understand

That I was born a rambling” man.

REFERENCES CITED

Anthony, J.W., S.A. Williams, R.A Bideaux and R.W. Grant, 1995, Mineralogy of Arizona: The University of Arizona Press, Tucson.

Palache, C.I., 1939, Antlerite (Bisbee): American Mineralogist, v.24.

Staatz, M.H. and W.J. Carr, 1964, Geology and mineral deposits of the Thomas and Dugway Ranges, Juab and Tooele Counties, Utah: US Geological Survey Professional Paper 415.

Zitlau, A.H., Q. Shi, J. Boerio-Goates, B.F. Woodfield and J. Maizlan, 2013, Thermodynamics of the basic copper sulfates antlerite, posnjakite, and brochantite: Chemie der Erde—Geochemistry, v. 73, # 1.