MAMMOTH MINE, UTAH: TYROLITE (ARSENATE)

One of the most famous mining districts in Utah is located about 50 miles south-southeast of Salt Lake City on the west central slope of the East Tintic Mountains.  The mountains are part of the Basin and Range Physiographic Province and connect the Oquirrh Mountains to the north (also Basin and Range mountains and the range visible directly to the west of Salt Lake City) and the Canyon Range to the south.  The highest point in the range is Boulter peak at 8,308 feet.  The "major" town is Eureka, now a much smaller community than the mining and financial center it was in the late 1800s.

Satellite image of Utah showing location of Tintic Mining District and Salt Lake City.  Image courtesy of Ray Sterner, Johns Hopkins University.

 

Google Earth image© of Tintic Mining District including Mammoth Mine.  Utah Lake is the large body of water in the northeast.

The Tintic Mining District was discovered in ~1869 and its geology/mineralogy was summarized by  Wilson (1995)  who noted the presence of ~175 species of minerals…”Much of the production of siliceous ore in the district [was] utilized by smelters in Tooele and Salt Lake City to mix with more iron-rich ores of Bingham.  Tintic has produced gold, silver, lead, copper, iron and zinc as its major commodities.”  Morris (1968) noted the primary ores consisted of “sulfides and sulfosalts of silver, lead, copper, iron, zinc and bismuth in association with jasperoid (silicified carbonate rock), barite, aggregates of quartz crystals, calcite, dolomite, and ankerite…gold is locally common…”  In addition, the Centennial Mine in the District is the type locality of eurekadumpite, perhaps my favorite mineral name!

Morris and Lovering (1979) described the general geology of the Tintic area as “… consisting of the eroded flank of a composite volcano, which, during the Oligocene, buried a preexisting mountain range that had been carved from folded and faulted Paleozoic sedimentary rocks.  During this volcanic episode many stocks, plugs, dikes and other intrusive bodies were injected into both the sedimentary and volcanic rocks.  As the Oligocene volcanic and intrusive episode subsided, great volumes of hydrothermal solutions coursed through the rocks, chiefly among faults and fissures eventually depositing large replacement ore bodies and veins that occur almost exclusively in the sedimentary rocks underneath the hydrothermally altered lavas.”

So, several large volcanoes erupted in the Tintic area (and much of western Utah) during the early Oligocene (Hintze and Kowallis, 2009) and covered Paleozoic rocks that were folded and faulted by an earlier mountain building event termed the Sevier Orogeny (Cretaceous).  During the later Oligocene these volcanoes begin collapsing and large calderas formed.  The hydrothermal solutions associated with the volcanics followed the cracks and faults in the Paleozoic rocks and helped dissolve portions of the limestones.  As these solutions cooled the minerals begin to crystallize forming the ore bodies in the limestone.

There are several mines in the Tintic District, including the Mammoth Mine.  At one time this mine supported the community of Mammoth, now essentially a ghost town.  The Mammoth produced silver and gold but also a host of unusual and rare minerals including tyrolite.  For many years the dumps at the Mammoth were available for collecting by rockhounds; however, I am uncertain about current collecting access, rules and regulations.

Tyrolite is an uncommon to rare calcium copper arsenate carbonate with the chemical formula of Ca₂Cu₉(AsO₄)₄(CO₃)(OH)₈-11H₂O.  Tyrolite is very soft (1.5-2.0 Mohs) and forms glassy to silky to pearly, blue to blue-green to turquoise to light green, feathery to botryoidal masses of radiating crystals.  In thin layers the crystals are translucent.  It does effervesce in hydrochloric acid indicating the presence of carbonate (CO₃).  Tangdanite is a similar mineral where the sulfate radical (SO₄) replaces the carbonate.

Specimen of leafy green tyrolite along with the azure blue azurite; broad view.  Width of view ~1.6 cm.

Photomicrograph of radiating tyrolite crystals; width of crystals ~2 mm.

Tyrolite is a secondary mineral (supergene) found in the oxidized zones of copper sulfide deposits where the ore contains both arsenic and copper and perhaps oxidized from something like the hypogene ores tennantite (Cu₁₂As₄S₁₃) or enargite (Cu₃AsS₄). Tyrolite often is associated with azurite, a copper carbonate.

Mostly, tyrolite is just one of those uncommon, colorful, and somewhat “strange” arsenates that I love to collect.  In addition, it is part of an entire suite of rather rare minerals coming from the Mammoth Mine and its associates in the Tintic Mining District. 

REFERENCES CITED

Hintze, L.F. and B.J. Kowallis, 2009, Geologic History of Utah: Brigham Young University Geology Studies, Special Publication 9.

Morris, H.T., 1968, the Main Tintic Mining District, Utah in Ore Deposits of the United States, 1933-1967: American Institute of Mining Engineers, Graton-Sales Volume, New York, v. 2. 

Morris, H.T., and Lovering, T.S. (1979), General Geology and Mines of the East Tintic Mining District, Utah and Juab Counties, Utah: USGS Professional Paper 1024.

Wilson, J.R., 1995, A Collector’s Guide to Rock, Mineral & fossil Localities of Utah: Utah Geological Survey, Misc. Publication 95-4.