One of the least appreciated aspects of the 19th Century gold mining boom in North America was the necessary and parallel boom in quicksilver, or mercury. Numerous mercury bearing minerals are known, but by far the bulk of historical mercury production has come from cinnabar, or HgS. For clarity, cinnabar is distinct from vermillion which is a pigment derived from cinnabar. 

Recovery of gold can be performed by methods as simple as plucking nuggets from a pan or by gravity separation in the form of sluicing. Unfortunately, in many areas placer gold is quickly exhausted by eager miners. Where there is placer gold there is often a lode formation to be found. Gold in a lode can be much more problematic in its recovery.  

Gold from a lode may be found comingled with quartz in bulk form, partitioned in a vein, or dispersed at high dilution in a host rock at a large scale. Lode gold very often has to be extracted from a problematic matrix. In this circumstance, chemical means are necessary to extract and concentrate the value from the rock.

A chemical solution to gold isolation is limited to only a few economically viable possibilities. Beyond macroscopic placer gold there is amalgamation with mercury, borax, cyanidation with NaCN, and chlorination with Cl2 or NaClO. 

Amalgamation has been attractive historically because of its great simplicity. First, cinnabar is readily coerced to liberate mercury by simple roasting and condensation. Dispersed gold is contacted with mercury and selectively extracted. The resulting solution of Au-Hg is relatively easy to isolate by natural phase separation. Finally, gold is easily recovered from the amalgam by heating in a retort. Chemists would call this a simple distillation.

Some silver will also be amalgamated, but it is separated by roasting to silver oxide followed by amalgamation of the residuals. Unfortunately, gold tellurides are problematic for direct gold amalgamation. Gold tellurides must be roasted first to liberate volatile tellurium oxides and native gold residues. Energy becomes a major cost driver at this stage.

Cinnabar ore (Image from Mineral Information Institute)

US cinnabar ore deposits are found predominantly  in California and to a lesser extent in Nevada, Oregon, Arizona, Texas, and Arkansas. The geology of cinnabar ore bodies share a few general features. Cinnabar ore is found in zones historically associated with volcanic activity and alkaline hydrothermal flows.  Ascending flows of metal sulfide saturated water infiltrated faults and fractures and deposited HgS rich mineral.  This is a common ore forming mechanism and is responsible for diverse metalliferous deposits, including mercury. 

Figure 1. Franciscan Quicksilver Ore Body Structure (C.N. Schuette, The Geology of Quicksilver Ore Deposits, Report XXXIII of the State Mineralogist, January 1937.)

According to Schuette, a common feature to economically viable cinnabar occurrences was the presence of a cap rock formation over the ore body. The infiltration of cinnabar laden hydrothermal fluids into fissures and shrinkage cracks in basalt intrusions as well as deposition in brecciated rock in the fault zones lead to enrichment of the mineral.  An impermeable layer above caused a pooling accumulation of mineral and a barrier to oxidation. 

Figure 2. Diagram of Sulphur Bank Mine (C.N. Schuette, The Geology of Quicksilver Ore Deposits, Report XXXIII, of the State Mineralogist, January 1937.)

In these California formations cinnabar is regarded as a primary mineral, meaning that it is the direct result of transfer from deeper source rock. An example of secondary rock would be serpentine (Fig 1) which is formed as a result of aqueous alteration of another mineral. Serpentine is a group of minerals comprised of hydrated silicate which may contain some combination of  Mg, Fe, Al, Mn, Ni, Ca, Li, or Zn. According to Schuette, serpentine is often found associated with cinnabar formations. 

The Sulphur Bank Mine near Clearlake Oaks in Northern California offers an interesting example of cinnabar mineralization. Figure 2 shows a fault that provided a channel for fluid flow to upper level rock formations. Over time oxygen and water caused the oxidation of sulfur to sulfuric acid which aided the decomposition of cinnabar and the host rock. 

Note that the uppermost layer is said to be white silica which resulted from extensive demineralization of solubles from a silicate matrix. Further down, native sulfur was discovered in more reducing conditions and was actually recovered in early mining operations. Cinnabar was located below the layers of oxidized mineral. 

This phenomenon of surface oxidation of an exposed ore body is observed in gold and silver mines as well. Miners often lamented that the nature of the lode changed as the mine operations got deeper. Of course, what was happening was that oxidized formations are encountered near the surface and as the mine gets deeper, progressively greater reducing conditions are found with a corresponding change in mineral species present. 

Air oxidation or infiltration of meteoric water with dissolved air and CO2 would cause the alteration of sulfide minerals to more water soluble H2S and sulfates, leaving native gold behind. But at greater depths, the composition of the ore changes to afford heavier sulfide loading and therefore a requirement for a different kind of milling. 

As it happens, the recovery of mercury from cinnabar is quite simple and has been done since Roman times. Typically, the ore was crushed and roasted in the combustion gases of a reverberatory furnace. This kind of furnace was constructed to isolate the fuel from the ore by a partition and rebound or reflect the hot gases off the ceiling of the furnace onto a heap of ore. Despite the name there is no acoustic aspect to the process. 

The hot gases would produce HgO and sulfides which would oxidize in the gas stream to volatile sulfur oxides. Thermal decomposition of HgO at ca 500 C produced mercury which was condensed out of the exhaust gas stream and collected as the liquid. 

Recently I came upon a copy of Geological Survey Circular 186, 1952, F.B. Moore and C.R. Butler, Pitchblende Deposits at the Wood and Calhoun Mines, Central City Mining District, Gilpin County, Colorado. Like many Geological Survey documents, it contains a pocket with neatly folded scale drawings of the mine workings. These drawings chart the location and elevation of the shafts and drifts and give a best estimate as to the extent of the formation.

Vein Structure in the Central City District. (From Geologic Survey Circular 186, 1952.)

What is interesting about the map above is not so much the minute detail of the locations, but rather the obvious trend of the veins (solid lines with dots).  They are all east-northeast trending.  The country rock is largely precambrian granite gneiss and quartz biotite schist according to the survey. It doesn’t take long to figure out that the mine locations correlate with the veins.

Geological Survey maps of 3 levels of the Wood Mine, Gilpin County, Colorado (Geological Survey Circular 186, 1952).

The second figure was generated by Moore and Butler in 1950 and shows the locations of pitchblende occurrences in three levels of the Wood Mine near Central City, Colorado. The red dots indicate the location of pitchblende along the three drifts at the 135, 197, and 275 foot levels of the mine.

Profile of the Wood Mine, Central City District, Illustration from Geological Survey Professional Paper 371, 1963.

 The Wood Mine was an early and prolific source of pitchblende, though presumably it began as a gold /silver operation. The workings reportedly reached a depth of 600 ft.  The Wood vein is in a fault fissure that shows extensive alteration from hydrothermal action. The width of the vein varies with the type of country rock in which it is found, but ranges from from 1 to 18 inches and has been followed for a lateral distance of nearly 1000 ft.

The productivity of uranium mines is commonly expressed in terms of equivalent weight of U3O8 rather than weight of ore, given the large variety of mineral forms uranium is found to occur in. The figure above is from Geological Survey Professional Paper 371, P.K. Sims and collaborators, Geology of Uranium and associated Ore Deposits, Central Part of the Front Range Mineral Belt, Colorado. Extensive stoping has been done in an attempt to vertically intercept the vein. This was a common practice in hard rock mining- let gravity bring the muck to you.

Pitchblende was discovered on the dump of the old Wood shaft in 1871. Circular 186 reports that by the end of 1872, 6,200 lbs of ore containing 3,720 lbs of U3O8 had been removed. By 1916, 102,600 lbs of ore bearing 30,040 lbs of U3O8 equivalent had been recovered.  The high grade pitchblende was hand sorted and that below ca 10 % was discarded or lost in gold and silver processing.

In Circular 371, Sims observes that “Pitchblende occurs as small, discontinuous lenses and streaks on the footwall of the Wood Vein, which are separated by nonuraniferous vein material.”

What is intriguing is that pitchblende was apparently an item of commerce in the early 1870’s. Radium, extracted from pitchblende, was not discovered until 1898 by the Curies.  A procedure for the preparation of sodium diuranate, Na2U2O7 6H2O, was reported as early as 1849 (Patera, J. pr Chem. 1849, [i] 46, 182. Early uses of uranium yellow were in paints and stains for glass and porcelain. This pigment has also been used for the production of fluorescent uranium glass.