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. 

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