By A. H. KOSCHMANN and M. H. BERGENDAHL - USGS 1968
Lawrence County is in western South Dakota along the Wyoming State line. It includes much of the northern part of the Black Hills and most of the mineralized area, including the famous Homestake mine in the Lead district.
Through 1959, Lawrence County produced a total of 26,386,000 ounces of gold, mostly from the Homestake mine in the Lead district. Other gold districts that have yielded more than 10,000 ounces are the Deadwood-Two Bit, Garden, Bald Mountain, and Squaw Creek.
BALD MOUNTAIN DISTRICT
The Bald Mountain district, which includes the Portland area, is 3.5 miles southwest of the Lead district.
Claims were located in the Portland area in 1877, but early mining was handicapped by the highly refractory nature of the ore. By 1891 the milling and metallurgical difficulties were overcome by the chlorination process, and in 1892 cyanidation proved successful (Connolly and O'Harra, 1929, p. 143-147). The district entered a period of prosperous development that ended at the close of World War I owing to high costs. The increased price of gold in 1934 caused a pronounced reactivation which lasted until World War II. After the war, mining was resumed on a small scale, but increased operating costs again forced the owners to close in 1959.
About $3 million worth of bullion, mostly in gold, was produced by the Mogul Mining Co. up to 1900 (Allsman, 1940, p. 38). Other companies were also active during this early period. Total gold production of the district through 1959 was roughly 1,400,000 ounces.
Rocks of the district are chiefly southwest-tilted strata of the Cambrian Deadwood Formation, underlain by the Precambrian schist. Numerous sheets, dikes, and irregular bodies of Tertiary phonolite and rhyolite porphyry cut the older rocks (Irving and others, 1904, p. 144-145).
The gold deposits are in replacement bodies in the Deadwood Formation and are the most productive deposits of this type in the Black Hills. Ore bodies are in two zones known locally as the "upper contact" and "lower contact." Deposits in the "lower contact" are in lenticular dolomite beds interbedded with shale within a stratigraphic interval of from 6 to 30 feet above the basal quartzite unit of the Deadwood Formation. The "upper contact" is defined as a sequence of mineralized dolomite beds and shale 12 to 18 feet below the Scolithus sandstone, which is the uppermost umt of the Deadwood Formation (Irving and others, 1904, p. 122-123).
Ore deposits occur in vertical shoots, probably mineralized fractures through which the ore solutions moved and from which replacement was started. The distance to which replacement has extended away from the fractures varies with the character of the beds and intensity of the mineralizing action. The widths of the vertical shoots range from thin seams up to 10 feet, and rarely up to 40 feet. Ore bodies of considerable width may be due either to coalescence of several parallel shoots or to the existence of many intersecting fractures (Irving and others, 1904, p. 134-136). The vertical extent of the shoots depends upon the thickness of the dolomitic rocks through which they pass, although some shoots extend into both overlying and underlying sandstone or shale. The maximum length of the shoots is several hundred feet; the longer shoots probably overlap fractures arranged en echelon.
The ore is classified under two types: blue ore which is the primary ore, and red ore which is oxidized ore. The primary ore consists of pyrite and probably arsenopyrite, most of which is very fine grained to microscopic in size. In some ore the gold telluride sylvanite has been found, but most of the gold is believed to be associated with the finegrained pyrite (Connolly and O'Harra, 1929, p. 162). Small amounts of galena and sphalerite are seen in polished sections of the ore. The gangue is chiefly quartz and some fluorite, gypsum, and barite. Most of the ore mined is the red ore, which consists mainly of gold-bearing limonite and is amenable to cyaniding. Blue ore is highly refractory and is not extensively mined.