New Mexico Mineral Symposium — Abstracts

The city of Kongsberg is located approximately 70 km west southwest of Oslo at 171 m above sea level. The area of ore bearing rock is about 15 km wide trending along a north-south line for about 30 km. Silver ore was discovered in the area in the summer of 1623 and mining began later that year.

The oldest bedrock in the Kongsberg area is ~1.6 Ga. Two events of deformation and metamorphism affected the area, the first at ~1.5-1.6 Ga and the second at 1.1-1.2 Ga. The history of these rocks can be summarized into four basic stages. The oldest rocks began as volcanics with geochemistry similar to island arcs as well as some sediments. These rocks were intruded by gabbros and diorites followed shortly thereafter by the first event of deformation and metamorphism. This amphibolite to granulite facies metamorphic event resulted in quartzo-feldspathic gneisses, dioritic gneisses, and amphibolites. Gabbro and dolerite later intruded the rocks. Lastly, the Meheia and Helgevannet granites were emplaced penecontemporaneous to the second event of deformation and metamorphism to amphibolite facies at 1.1–1.2 Ga. Ultimately, these events created bedrock consisting of quartz-plagioclase-biotite gneiss, mica and chlorite schist, amphibolite, and granite gneiss.

The ore deposits at Kongsberg are a five-element-type (Co-Ni-As-Ag-Bi) vein system. The age of the hydrothermal system that formed the veins has been dated at 265±3 Ma and is genetically related to the Oslo Rift. Studies have shown that the elevated heat flow associated with rifts can mobilize formational brines to form five-element-type deposits. The silver in the deposits is derived from the black shales of the Oslo region, which have been calculated to contain more than enough silver to account for the Kongsberg deposits. The ore formed about 3 to 4 km deep from fluids 200 to 300° C with salinities as high as 35% NaCl equivalent. The bedrock of the Kongsberg area includes sulphide-rich zones locally known as fahlbands. When the hydrothermal fluids encountered the fahlbands, they chemically reacted and formed the silver deposits. Minerals found in the hydrothermal veins include: quartz, pyrite, calcite, barite, fluorite, galena, sphalerite, chalcopyrite, silver sulphosalts, argentite, native silver, and pyrrhotite. “Coalblende,” a bitumen compound likely derived from the Oslo Rift shales, is also found in the veins.

After the discovery of silver, King Christian IV of Denmark-Norway established the mines in 1623. The following year the town of Kongsberg was founded on a waterfall of the River Lågen to provide power for the stamp mill and smelter. At the time, the mining industry in Norway was not well-developed, so miners, engineers, and mining officers were imported from Germany to develop the silver mines. Firesetting was used during much of the history of the mining district as the main way to soften and break rock. The first use of black powder for blasting occurred in 1659, but it wasn’t until the mid-1700s that blasting was commonly used. Firesetting, however, continued to be used to create horizontal works because of its low cost. The problem of ventilation was solved with the use of an “adit loft,” which was created by dividing the adit by wood or brick into a lower level where the miners worked and an upper level for the smoke. After the use of dynamite was introduced in 1872, firesetting was abandoned, its last recorded use in 1890. The mines were initially dewatered using hand pumps, but waterwheels were soon installed to operate pumps. Many canals and aqueducts that were used to bring water to power the wheels can still be seen. Steam power and electricity were introduced to the mines in the 1880s.

For much of the Kongsberg Mining District’s history, it was the largest mining operation in Norway. As early as the 1600s, the mines offered workers desirable benefits such as sick pay, free medical care, pensions, and primary and secondary schools for children. In 1757, the Norwegian Mining Academy was established in Kongsberg to train mining engineers. The high point of mining in the district occurred in 1770, when 78 mines employed about 4,000 workers. By 1805, however, much of the best ore had been extracted and most of the mines closed. In 1814, the Mining Academy was also closed. Fortunately, promising ore zones were discovered in 1816 and many mines reopened. Peak yearly production was achieved from 1915 to 1916 when 13 tons of silver were produced. Despite declining production, mining continued into the 1950s. The last silver from the Kongsberg Mines was smelted in 1958, ending 335 years of operation that extracted around 1,350 tons of silver.

The Norsk Bergverksmuseum (Norwegian Mining Museum) in Kongsberg has preserved many artifacts from the mining operations, including them in displays explaining the history of the mining district. In addition, the museum’s vault contains hundreds of spectacular wire silver specimens on display. The museum also maintains surface facilities of the Kongens Gruve (King’s Mine) and offers an underground tour of the mine via the Christian VII adit.