In the Pnictogen Hall of Fame there is at least one p-block compound with a town named after it. The ghost town of Stibnite, Idaho, sits silently in the Yellow Pine mining district 40 or so miles NW of Cascade, Idaho.  The town of Stibnite is named after the sulfide of antimony- Sb2S3.  The chemical symbol, Sb, is related to this mineral name.

Idaho sits in the great North American cordillera.  A cordillera is a grouping of mountain ranges at the continental scale. In the case of the North American cordillera, it begins ca 103 west longitude and extends to the Pacific ocean. The Black Hills are found somewhat east of 103 degrees, but I’m generalizing again. In the US, the Rockies, Wasatch, Cascades, and the Sierra Nevada ranges are part of the cordillera formation.

North American Tungsten Belt. From Paul F. Kerr, Tungsten Mineralization in the United States, 1946, Waverly Press.

One characteristic of the cordillera is the broad occurrence of economically important metal deposits. In the illustration above, the occurrence of tungsten is associated with the mountainous regions of the west. An important feature found in economic metal bearing districts within the cordillera are vein deposits. Metals can be disseminated in rock or concentrated in veins. 

In Colorado, the Cripple Creek & Victor mine is situated in the throat of an ancient volcano. This ore body is an example of highly disseminated gold ore which is interlaced with vein structures containing higher concentrations of gold.

Rather than perform underground mining, the economics allow the large  scale removal and crushing of rock to pebble size followed by extraction with cyanide to isolate the value.  This mining technique was not possible until the advent of large scale mechanization. In the early days, the Cripple Creek district was limited to underground mining of vein formations that were more highly enriched in gold.

What is crucial to the placement of a metal ore body is some process that leads to concentration of valuable metals. Recall that the definition of an ore is based on economic considerations.  At some level of dilution all ore becomes just gangue or country rock. Concentration of value in the ore body near the surface can arise from several mechanisms.

A common process that concentrates desirable minerals is hydrothermal deposition. This is found widely in the cordillera. A natural consequence of mountain building is the generation of stresses within the upthrusting  rock. At some point stress gets relieved by fracturing which results in the formation of void spaces within the rock.

Underground water, which at depth is at high temperature and pressure, will dissolve components of rock in contact with the water. This water will naturally convect and flow towards the surface, carrying whatever solutes that were favored by higher solubility.

Deposition occurs as the water flows to the surface within whatever fracture network the waters find themselves in and may continue to deposit until the vein seals itself shut. Over the fullness of time the formations are thrust upwards and erosion wears down the rock to expose outcroppings of the desired mineral at the surface. 

Such processes have put vein lodes in place all over the world, including the American west. Deposits of gold, silver, antimony, iron, mercury, and tungsten are examples of metals that are concentrated in this manner. Ostensibly, this is happening in geothermal hotspots like Yellowstone or Iceland today.

These exposed outcroppings weather and oxidize, generating new mineral compositions. In the case of gold, its relative inertness leads it to form the native metal in these weathered formations and, under the influence of gravity and the hydraulic forces of snowmelt, gold will work its way downhill and into the alluvium.

Other elements besides gold are also mobilized, particularly the sulfides. In the deep crust, well below the depth to which oxygenated meteoric water can flow, is an environment rich in the anionic subunits oxide, sulfide, silicate, and aluminate. Metals and metalloids like Cu, Sb, Ag, As, Pb, Hg, etc., form complexes with the various anions and correspondingly, 3 dimensional networks of inorganic polymeric species. To the extent that a 3-D network of shared atoms, edges, and faces of tetrahedral crystalline subunits can be formed, the resulting bulk material may have a high melting point and high strength.

However, when connectivity is lowered by chain or network terminating constituents, the melting temperature and hardness of the material may be lowered.  An example is soda glass. When silica is diluted with chain or network terminating components like soda or lime, the high strength and high melting point of quartz, which is just pure polysilicate, is lost. The same thing can happen naturally in mineral formation processes.

Other kinds of ore are put in place by fractional melt crystallization and layer deposition by density within a magma chamber. The major ocurrences of platinum group metal (PGM) deposits are an example of such a process. Eventually, tectonic processes raise the frozen and extinct magma chambers to the surface where erosion exposes a narrow banded horizons referred to as a reef.  The Bushveld Igneous Complex in South Africa and the Stillwater Complex in Montana are examples of this mechanism.

The deposits found near Stibnite, Idaho, are comprised of antimony and tungsten as well as lesser amounts of gold and silver. In about 1900 gold, silver, and antimony were discovered in the area, leading to a gold boom at Thunder Mountain.  During the years from 1938 to 1944, the Yellow Pine (W, Sb) and Meadow Creek (Au, Ag, Sb) mines in this part of Idaho were the largest producers of tungsten and antimony in the United States.

The details of this mining district can be found in:

John R. Cooper,  Geology of the Tungsten, Antimony, and Gold Deposits Near Stibnite, Idaho; 1951, Geological Survey Bulletin 969-F.  Stibnite Idaho USGS

In the abstract, Cooper describes the W, Sb, Au, Ag deposits as being confined to an area about 1 mile by 3.5 miles in scope (as of 1951). The principal rock of the area is quartz monzonite which is extensively fractured and has been penetrated by dikes of basalt, quartz latite porphyry, trachyte, and rhyolite.

Cooper describes a deposit whose metallization has taken place in three stages with intervening episodes of fracturing. The first stage is described as extensive replacements by gold-bearing pyrite and arsenopyrite.  The second phase of deposition or replacement is less extensive and is by scheelite (CaWO4) within the gold ore bodies.

The third stage of growth or deposition is of stibnite and silver, largely within the same fracture systems as the scheelite. The ore bodies occur with the Meadow Creek fault and associated subsidiary faults in the quartz monzonite. The tungsten-antimony ore body within the formation took the shape of a

“flat upright funnel flaring to its widest diameter at the surface and tapering to a narrow neck, which extends below the bottom of the minable tungsten ore. The underside of the ore body is very irregular in detail.  The highest grade of tungsten ore was concentrated toward the center of the mass and was surrounded by an envelope of antimony ore containing only a little tungsten.”    – John R. Cooper

The Meadow Creek ore contained 0.23 oz gold per ton and 1.6 percent of antimony.  The Yellow Pine ore contained little gold but 4 percent of antimony and 2 percent of WO3. The Yellow Pine deposit was exhausted of tungsten in 1945, producing 831,829 units of WO3 equivalents in the concentrate. One unit of WO3 is 20 lbs of tungsten trioxide.

Much of the scheelite was found disseminated in brecciated gold ore.  Some scheelite was found in branching stringers and veinlets within the groundmass.

The stibnite occured as “disseminations, microveinlets, stockworks, massive lenses, small fssure-filling quartz stibnite veins, and euhedral crystals coating late fractures. ”  Oxidized antimony minerals such as kermesite (Sb2S2O) were reported as being very scarce.

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