by Eric Greene
The Capillitas Mine is the believed to be the world's largest mass of rhodochrosite - famous for its unique formation, occurring as stalactites and stalagmites. Rhodochrosite is a manganese carbonate mineral with the chemical composition MnCO3. The mine is located in the Andalgalá Department, Capillitas Province, in northwestern Argentina.
Mined since Incan times, the polymetallic sulphide veins at the Capillitas Mine were worked in the 17th century for silver, and later for lead and zinc. Today, sulphides extraction is no longer economical, but the mine is still being worked for and lapidary material for specimens and carving, producing 100-200 tons of material a year. The banded rhodochrosite is often sliced and polished into slabs for collectors.
Rhodochrosite from the Capillitas Mine was first described in the literature in 1873, when it was given the name, "Inca Rose" rhodochrosite. Indigenous people probably mined the nearby placers for gold in pre-Colombian times. From the 13th through the 15th centuries, rhodochrosite was mined here by the Incas. In the 17th and 18th centuries, the area was mined by the Jesuits for silver, gold and copper. From the 1850's onwards mining was carried out by Spanish, English and German companies. In 1940, the mine was taken over by the Argentine military to mine copper. More than 20 miles of adits have been driven at the mine, making it the largest mine in Argentina.
Because the mine is located at an altitude of 10,000 feet, mining is difficult. Until 1903, ore was moved by mules over 50 kilometers to the nearest smelting furnace, near Andalgalá. Production was limited to 2000-2500 tons per year. Starting in 1904 a 27.5-kilometer tram system with a continuous moving cable was built to transport the ore, but various difficulties lead to the system being abandoned in 1913.
Mining of rhodochrosite for jewelry commenced in 1937, when Dr. Franz Mansfeld, German geologist, extracted 10,000 pounds of rough. Since 1947 the Mansfeld occurrence has been mined by the Argentinian government.
The stalactites found at the mine have been cut and polished and sold worldwide, which made Capillitas rhodochrosite famous. In December 1986, a 15-foot-high and 6-foot-wide cavity was discovered in the 25 de Mayo vein, with individual stalactites up to 2 feet thick and 10 feet long. The walls were completely covered with thick stalactite formations, and large stalactites hung from the cave ceiling.
Rhodochrosite is a manganese carbonate mineral that ranges in color from light pink to bright red. At the Capillitas Mine, rhodochrosite occurs as large masses and spectacular stalactites. While small crystals to 5 mm have been found on occasion, the majority is massive, formed by seasonal water flows resulting in dripping and precipitation of manganese-rich rock, with banded concentric growth layers in shades of color from red, pink, tan and yellowish brown.
Some pieces have up to 30 fine strips, each a few millimeters thick, but they can be up to 5 cm in thickness. The strips are sometimes interlayered with white of pink crystalline manganocalcite, and rarely chalcedony. The chocolate brown strips are called capilliite, which is very rich in zinc. Probably the most impressive color is the vivid crimson red, which is the equal of the color of rhodochrosite from the Sweet Home Mine in Colorado and the N'Chwanning Mine in South Africa.
Unlike most sulphide deposits, most of the Capillitas veins consist almost entirely of rhodochrosite with only minor amounts of lead and zinc sulphides. These veins are the result of hydrothermal mineralization in diatreme volcanics and the adjacent Paleozoic granite. The veins are related to Miocene-Pliocene volcanism, and are said to be of tertiary age. The mine is part of the Farallón Negro Volcanic Complex, which is located at the interface between the Sierras Pampeanas and the Puna physiographic and tectonic provinces. The principal veins are Capillitas, Carmelitas, La Rosario, Ortiz, Restauradora, and 25 de Mayo.
In addition to rhodochrosite, gold, silver and copper, the mine has produced more than 120 different minerals, including world rarities like germanite, hessite, colusite and putzite. The most common primary minerals include pyrite, sphalerite, galena, chalcopyrite, marcasite, pyrrhotite, tennantite, enargite, bornite, and stannite.