The Gold Hill mining district northwest of Boulder, Colorado, is dotted with many signs of mining activity from an earlier time. This district is adjacent to the towns of Ward and Nederland and situated in the northeastern extreme of the Colorado Mineral Belt (CMB). The first significant gold lode discovery of the 1859 Colorado Gold Rush occurred in this area. Gold Hill is at the northern end of a particularly rich band of gold lode occurrences within the CMB stretching southward in parallel with the Front Range through Central City, Idaho Springs, and further south to Cripple Creek.
The town of Gold Hill (actually a CDP) is connected to Left Hand Canyon road via Lick Skillet road, reportedly the steepest maintained county road in the USA. I can verify the steepness of this gravel road and would heartily suggest shifting into first gear while driving down this mile-long toboggan run.
While mining has long since halted at the great majority of mines in this district, the Cash Mine east of town is still in operation.
To the south of Gold Hill is a CDP settlement called Wall Street. This was the location of a mine and a mill. Or they called it a mill. It should probably be called a smelter since roasting was used in the process.
The Wall Street Mill today sits on private property and access is not available to the motoring public. The site sits along the road on Four Mile Canyon Drive, a mile from the intersection with Gold Run Road and south of Gold Hill.
The imposing structure along the road is actually a cooling bin for the storage of freshly roasted ore. A sign posted along the road says that the mill process used roasting, chlorination, and cyanidation to recover the gold values. The sign also indicates that the mine closed after only a few years of operation due to poor management. Poor operating practices were not uncommon.
Roasting was a common step in the metallurgy of sulfur-rich gold and silver ore. The purpose of roasting is to change the chemical composition of the ore by oxidation of metal sulfides to produce metal oxides. The gold and / or silver in the ore was difficult to isolate without this process. The matrix of metal sulfides in the ore interfered with the extraction of distributed and native gold by amalgamation. And without chemical processing, silver was all but impossible to extract from the ore with 19th century technology.
But roasting was only a prelude to further processing. Roasted ore could be crushed in a stamp mill to produce a greater surface area for extractive metallurgy or could release particles of native gold. Reduced, native, gold could then be isolated with shaker tables to partition the dense gold particles into a slurry stream for isolation and further refinement. Alternatively, the pulverized ore could be passed over copper amalgamation tables for dissolution into mercury. A trip to the retort would distill away the mercury (mostly) and afford a button of isolated gold. Gold and silver can be extracted with mercury.
Gold that was highly distributed in microscopic particles could be extracted chemically using several options in the late 19th century. Selectivity was always a problem and processing trains became relatively complicated in an effort to provide the purest gold and silver possible.
Roasted gold and silver ores could be subjected to chlorination (or chloridation) processes that produced chemically extractable gold or silver. Roasting ore with sodium chloride in a reverberatory furnace, for instance was commonly done to produce gold and silver chlorides that were accessable via aqueous extraction methods. The origin of metallurgical chlorination traces back to von Patera in the Bohemian silver mining town of Joachimsthal.
Chlorination by generation of Cl2(g) was not uncommon. Wetted ore was exposed to freshly generated chlorine, producing metal chlorides in the roasted ore pulp. A process similar to the method of Scheele in 1774 (MnO2 + 4 HCl-> MnCl2 + 2 H2O + Cl2) was used to generate the chlorine on the spot for chlorination. Manganese dioxide, sometimes referred to as the peroxide of manganese, was treated with sodium chloride and sulfuric acid. Gold chloride could be extracted by water and then reduced to gold powder with zinc, iron scrap, or green vitriol (iron (II) sulfate, a 1-electron reducing agent).
Cyanidation was also used according to the information at the site. I’ll leave this method for another post.
An assay office sits adjacent the mill. The upper level of the assay office served as a residence and pool hall. At its peak, this site was home to 300 people. The Assay Office is now maintained by Boulder County.