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.

A vanload of members of the ACS, Colorado Section, were treated to an extensive tour of the Cripple Creek and Victor (CC&V) mine last Friday. The photo below shows a view of the mine from the abandoned American Eagle Mine above the CC&V pit operations. The mountains in the background are the Sangre de Cristo range.

View of Cripple Creek & Victor Mining Operations from the American Eagle Mine, May 2010.

A haul truck bed was converted to a scenic overlook platform (below).  A few section members take in the view.

CC&V Scenic overlook platform

From this high vantage point we could see blasting operations at work. Multiple sites may be prepared simultaneously. Blasting typically occurs at 1:00 pm. Holes are drilled on 20 ft centers for optimum coverage. Every blasting hole is sampled and analyzed by ICP to measure the gold value for that zone.  Zones with low value are hauled to the waste heap and the high value rock is taken to the crusher.

Preparation for blasting, CC&V, May 2010.

We visited the pit and watched haul trucks get loaded by a gigantic loader. For every one truckload of ore there are two truckloads of unproductive rock that have to be hauled to a separate location on site. The definition of unproductive rock depends entirely on the market price of gold.  In the photo below, drilling rigs for the blasting charges are in operation for the next round of blasting.

Loading area, CC&V Mine.

Later we watched the trucks unload into a crusher with a gyrating element that crushed the rock into football sized chunks. The resulting rock is conveyed to a screener and another crusher in order to further reduce the size to 0.75 inches. Every truckload of rock is treated with lime to help maintain a high pH for the aqueous cyanide leaching operation.  In the photo below, the white tank on the lower right contains the lime.

Heap leaching operations, CC&V May 2010.

The gold bearing rock is irrigated with a very dilute sodium cyanide solution which percolates to the bottom of the heap and is captured in a basin feature at the bottom of the heap. The “pregnant” solution of gold cyanide extract is pumped out from under the heap at 14,500 gal/min into a cascading series of charcoal filtration tanks in the extraction building (see photo).

Pregnant solution passing over charcoal filter bed. CC&V May 2010.

Once a specified loading of Au is sorbed onto the charcoal, the sorbent is treated with hot concentrated cyanide. This concentrate is then passed over steel wool where the gold precipitates. The precipitated gold is then smelted to produce bulk crude metal which is shipped off-site for further refinement.

The Colorado mineral belt (CMB) is a swath of metalliferous mineral veins and faults spanning 15 to 30 miles in width and running ~250 miles in length between Dolores and Jamestown, Colorado. This NE trending zone encloses most, but not all, of the significant occurrences of gold and silver deposits found to date in Colorado.

Significant finds like the Cripple Creek district have been found outside the CMB, but these are exceptions to the trend. The large gold/silver/tellurium lode in the Cripple Creek diatreme is the result of a volcanic past that stands somewhat apart from the vein deposition processes that produced the CMB lodes.

What is especially intriguing about the CMB is that it is coincident with a significant gravity anomaly. It turns out that a particularly deep negative gravity anomaly exists in the Colorado Rocky Mountains. A few papers on this effect can be found on the web. In particular, a paper (ref 1) by Mousumi Roy at the University of New Mexico offers some details on the  extent of the gravity anomaly and some possible reasons for the effect.

At first blush it might seem odd that a negative gravity anomaly should coincide with a region known for heavy metal deposits. After all, dense matter has greater mass per unit volume, and if there is a lot of volume, then one might expect the acceleration of gravity to be a tiny bit greater than some reference value.

While this line of reasoning has merit, it turns out that despite the presence of thin metalliferous veins in the region, the overall density of rock below the CMB formation is somewhat low. A density contrast exists in the CMB formation and the surrounding rock. A large, low density formation in the crust and/or upper mantle would cause the local acceleration of gravity to be slightly below that of the reference geoid value.  The structure of the density contrast is the subject of some scrutiny and has been addressed by Roy and others.

A large low density mass below the surface is expected to have some buoyancy. A buoyant mass is one that would exert an upward distortion on the crust. The Colorado Rocky Mountains are part of a region characterized by numerous past episodes of mountain building. Whether mountain building was the result of large scale tectonic interactions or more localized effects of density contrasts, the fact remains that a gravity anomaly exists coincident with the CMB.

The mechanical effect of the upthrust of the lower members of the crust to form the Colorado Rocky Mountains has been that a series of faults and fractures have formed. These void spaces have provided networks for the flow of mineral rich hydrothermal fluids over geological time.

High pressure, high temperature aqueous fluids are prone to cooling and depressuriation as they work their way upwards into cooler and less constricted formations. At some point these fluids throw down their solutes and suspensions in the form of solids that occupy the void network. Eventually the flows become self-sealing and circulation halts leaving veins filled with chemical species that were selectively extracted and transported from other formations.

The earths hydrothermal fluid system is continuously extracting soluble components and transporting them to distant locations where solubility properties force their deposition. But this process does not always produce solid, compacted veins. Void spaces can be left behind at all scales, from microscopic size to large chambers. These spaces are called “vugs”. Rock with a large fraction of void spaces is referred to as “vuggy”. It is possible to walk up to a mine dump in the CMB and find hand samples of vuggy rock. It is not unusual to find crystals of pyrite or other minerals lining the internal spaces of the vugs.

1.  McCoy, A., Roy, M., L. Trevino and R. Keller, Gravity models of the Colorado Mineral Belt, in The Rocky Mountain Region – An Evolving Lithosphere: Tectonics, Geochemistry, and Geophysics: American Geophysical Union Geophysical Monograph 154 (eds. Karlstrom, K.E. and Keller, G.R.), 2005.

[Note to the reader: Th’ Gaussling is just a chemist, not a geophysicist. But like many others, I have the ability to read and learn. When I learn something new and interesting, I like to write about it. It reinforces the learning.]