Most rock and mineral shows that I attend have at least one dealer who displays colorful orange, red, yellow, and mixed-color specimens for sale. These vivid minerals often draw looks and attention and much reaching for their money stash, especially when children are asking for a purchase. Then an honest dealer will explain to the prospective buyer that the red to red-orange specimens are realgar (As4S4) while the orange yellow to yellow specimens are orpiment (As2S3); both are arsenic sulfides, and both are toxic. Therefore, care must be taken when handling these specimens, especially by children, and by adults who are not fond of scrubbing their hands after handling minerals.
These two arsenic sulfides are always associated with each other in nature. Both have nonmetallic lusters but may have adamantine (especially realgar) to resinous lusters (especially orpiment) and are quite soft at ~1.0—2.0 (Mohs). Orpiment is photosensitive and over time will degrade into a friable, white arsenic oxide. In past centuries orpiment was used as a lemon-yellow pigment for paintings by many of the masters. However, these oxides are quite soluble in moisture of any kind and their migration to the surface on the works of art caused a color change. Today, conservators seem constantly at work trying to protect these oil paintings by limiting exposure to strong light, controlling humidity, and discontinuing the use of water-based cleaners.
Realgar, AKA ruby sulfur, almost always occurs in the
same rocks as orpiment, and many times in the same individual specimen. Besides
the vibrant red color, realgar differs from orpiment is that the soft mineral
may be cut with a sharp knife (known as a sectile property) into thin strips
and pieces. In non-crystalline specimens’ realgar may be granular or powdered
or incrusting. It burns with a blue
flame and releases arsenic and sulfur fumes that smell like garlic (remember
toxic). In our basic mineralogy class, we were not allowed to subject realgar
to a blowpipe analysis due to the prospect of inhaling the toxic fumes. I doubt
if blowpipe analyses with charcoal, along with flame tests, are even noted in a
beginning mineralogy class today. It is much easier to stick samples in an
electronic gizmo and receive accurate results.
Like orpiment, realgar is very photosensitive and
degrades into orange yellow pararealgar (As4S4: same
elemental composition as realgar but different internal structure), or
arsenolite (arsenic trioxide As2O3), or orpiment.
There is a good chance that the arsenic sulfides noticed
at various shows were collected at the Getchell Mine in Humbolt County, Nevada,
about 35 miles northeast of Winnemucca (see Blog posting July10, 2023) on the
east side of the Osgood Range. The Osgood Range is a typical Basin and Range
group of mountains trending north-south defined by narrow faulted mountain
ranges with adjacent rather flat basins—a horst and graben topography with
normal faults. This landscape is largely due to extensional tectonics (pull-apart)
of the later Tertiary (Miocene, ~17 Ma and probably continuing) after a lull in
the previous extensive volcanism. However, later work by Berger and Taylor
(1980) identified a much earlier (~Late Cretaceous) complex fault system on the
east flank of the Osgood Mountains that was named the Getchell Fault System.
The Osgood Mountains have a thick Paleozoic section of
sedimentary rocks that formed on the shallow water Continental Shelf of the
North American (in current terminology) craton. However, the mountains are
cored by Cretaceous igneous plutons
(notably the Osgood Mountain Granodiorite Stock) that were exposed in the
horsts of the Basin and Range Orogeny. The entire section is then unconformably
overlain by late Tertiary volcanic rocks (Chevillon and others, 2000). Both the
Osgood Mountains Stock and the Getchell Fault System are critical to this story.
Early prospectors nosing around the Osgood Range were
initially interested in copper, silver and lead associated with skarn deposits
of the Osgood Stock. Instead they located skarn related tungsten and mining
started in 1916 and with starts and stops lasted until the late 1950s.
However, gold was the commodity most in demand and was
finally discovered ~1933 at what is now termed the Getchell Mine and brought
into production in 1938. Originally gold was produced from roasted sulfide and
oxide ores and the Getchell site produced nearly 800,000 ounces until
production was suspended after World War II. Someone also got the bright idea
to collect the arsenic produced from the roasting of the sulfides. Great idea
since around 1943 U.S. government restrictions shut down many/most non-essential
gold (and other) mines. However, arsenic was considered a strategic mineral and
Getchell continued operation. That led to another bright idea and in 1942
Getchell increased their production of tungsten from the mineral scheelite (CaWO4).
Ones of tungsten’s major uses in WWII was the hardening of steel and it was
critical for the war effort. Production dropped off after the War but continued
sporadically until the late 1950s when the U.S. government terminated the
“tungsten purchase program” (Defense Production Act). The production of gold
after the War was off and on from both open pit and underground mines (Getchell,
Turquoise Ridge, North Zone, Twin Peaks, and others) as well as heap leaching
of the earlier accumulated dumps. Core drilling in the 1990s convinced mining
geologists that large reserves of gold were present in the area but tied up in
Carlin-type deposits (Cambrian/ ?Ordovician sedimentary rocks with sub-micron
sized gold found on arsenic-rich rims of pyrite and marcasite with the richest
deposits found along intersecting mid-to late Paleozoic fault zones. The
sources of the gold were hydrothermal fluids associated with the Osgood Pluton
and associated dikes with mineralization during two events: 1) ~83Ma [may
actually be older] during emplacement of the pluton (minor event); and 2) major
mineralization in the Eocene (Chevillon and others, 2000). That information was
followed in 2019 by the formation of Nevada Gold Mines LLC, a joint venture by
two giants of mining Carlin-type deposits: Barrick Gold Corporation (61.5% and
the operator) and Newmont Corporation (38.5%). Officially the Turquoise Ridge
Project but popularly known as Getchell, the complex is composed of Turquoise
Ridge Underground, Vista Underground, and the Turquoise Ridged Surface mines (Turquoise Ridge Complex
Technical Report NI 43-101 – March 25, 2020). I could not locate the current
production figures.
MinDat listed
93 valid minerals, including one Type (getchellite, AsSbS3), and an
impressive number of commodities (gold, silver, arsenic, tungsten, antimony,
mercury, barium-barite, molybdenum, fluorite, thallium, tellurium, bismuth,
tin, lead, zinc, and copper) from the Getchell Complex. Besides the commodities
and associated gangue minerals, the Complex is noted for the large number of colorful
mercury and arsenic minerals like common arsenopyrite, cinnabar, realgar, and
orpiment but also rare mercury minerals such as coloradoite (see Posting
January 10, 2021), getchellite, laffittite (see Posting February 26, 2919), and galkhaite (USGS,
retrieved November 2023).
As noted
earlier my interest in the Getchell centers on the arsenic and mercury
minerals, and for a “long time” I wondered why these minerals crystallized at
the Complex. My “knowledge” of geochemistry is a little weak, well actually
quite weak, and that question really bugged me. What I do know is that
hydrothermal fluids associated with the Osgood Pluton: 1) supplied the elements;
2) “As-W-Hg anomalies occur in rocks and soils over the arsenic-gold deposits
and that these anomalies are not broad haloes but are restricted to the
mineralized area” (Retrieved from MinDat November 2023 but original publication
unknown) and 3) it has long been speculated that the origin of the many heavy
metals such as Au, Hg, Sb, and Tl found in anomalous quantities in sediments in
Carlin-type systems were originally derived from biogenic concentration (2011 MinDat
paper authored by Phil Persson).
One of the more interesting mercury minerals from the Getchell Complex is galkhaite [Hg5Cu)CsAs4S12], a rare and complex sulfosalt (a sulfide with both metals (cesium, thallium, mercury, copper, and zinc) and semi-metals (arsenic and antimony) as cations. According to the Handbook of Mineralogy and Webmineral, it is the only known natural cesium-mercury and cesium-arsenic phase (Chen and others, 1981). Throw in the thallium and my sparse geochemical knowledge really lights up: Cs0.6Tl0.4Hg3.5Cu1.5ZnAs3.6Sb0.4S12. (Empirical formula from Webmineral). Although galkhaite was originally described from mercury deposits in Kirgizia, Russia, it is best known from the Getchell Complex and other Carlin-type rocks in Nevada.
Galkhaite,
like many other mercury minerals, is red in color, usually a dark cherry red or
dark orange. It belongs to the Isometric Crystal System (all three axes are
equal in length and meet at right angles to each other to form a cube). Galkhaite is soft (~3.0 Mohs) and the opaque
crystals have an adamantine luster, an orange-yellow streak, and an uneven
fracture. In other words, it would be easy for an ole plugger like me to
confuse galkhaite with other mercury-rich minerals.
I also noted a
2011 MinDat paper authored by Phil Persson of
Denver that “galkhanite is an important mineral for use in radiometric dating
of Carlin-type deposits. Galkhaite is a trace mineral in at least four Carlin-type
Nevada deposits and contains significant amounts of Rb and virtually no Sr,
making it an ideal candidate for radiometric dating. Galkhaite from the
Getchell Mine in the Potosi Mining District, Humboldt County, Nevada, was
analyzed using Rb-Sr isotope dating techniques, and was found to be ~39.5 Ma
[indicating the time of mineralization].”
RERENCES
CITED
Burger, B.R. and B.E. Taylor, 1980, Pre-Cenozoic
normal faulting in the Osgood Mountains, Humbolt County, Nevada: Geology,
Vol.8, No. 12.
Chevillon, V., E. Berentsen, M. Gingrich, B. Howard,
and E. Zbinden, 2000, Geologic overview of the Getchell gold geology,
exploration and ore deposits, Humbolt County, Nevada: Geological Society of
Nevada Symposium Geology and Ore Deposits: The Great Basin and Beyond.
Chen T. T. , J.T. Szymanski, 1981,
The structure and chemistry of galkhaite, a mercury sulfosalt containing,
Cs and Tl: The Canadian Mineralogist , Vol. 19.
USGS, 2023, https://mrdate.usgs.gov/mrds/show-mrds.php?dep_id=10310336