The polymetallic sulphide veins of the Capillitas Mine in northeastern Argentina have been mined since the times of the Incas. Later mining activity continued under the Spaniards, the English and even the Germans. All to all more than 30 kilometer of adits were driven into the mountains, which makes Capillitas the largest mine in Argentina. It is also the one with the most diversified mineralogy and about hundred minerals are known from this locality, including world rarities like germanite, hessite, colusite and putzite.
Principal orebodies of the Capillitas mine are the Capillitas, Carmelitas, La Rosario, Ortiz, Restauradora, and 25 de Mayo veins. The sulphide mineralisation is clearly hydrothermal in origin and of tertiary age, i.e. the whole deposit as such is only a few millions years old. The mine is located quite high at 3000 - 3300 m altitude, so regular mining is a difficult venture here. Consequently sulphide extraction proved uneconomic at some stage and was abandoned decades ago.
There is however one specific geological curiosity, which makes Capillitas very special and well known even nowadays. Generally the gangue accompanying any sulphide vein consists out of quartz and calcite with minor amounts of baryte, dolomite, fluorite and some other minerals. In Capillitas rhodochrosite = manganese carbonate is the predominant, if not exclusive gangue mineral. In fact, most of the Capillitas vein consists almost entirely out of rhodochrosite with only minor amounts of sulphide minerals.
The rhodochrosite shows all sorts of banding and all shades of colours from almost white to rich pink to a yellowish brown. Often the bandings shows a distinct chocolate brown colour, which is called capilliite and is a very zinc rich variety of rhodochrosite. The most impressive and highly esteemed colour however is a deep crimson red, equalling the colour of the very best rhodochrosite specimen from world renowned localities such as the Home Sweet Home mine / Colorado or the Northern Kalahari Manganese Fields in South Africa. Unfortunately this very best colour is almost always hidden in the middle of stalactites and stalagmites and exhibits its outstanding beauty only when cut and polished.
The individual crystal size of the Capillitas rhodochrosite is generally small and only seldom reaches 1 cm, though the crystals easily aggregate to large masses and spectacular stalactites. In 1986 a large cavity was discovered within one of the veins with individual stalactites up to 0,5 m thick and 5 m long. Unfortunately only little documentation work was done on this cave with the photo shown above believed to be the only one in existence. This cave was plundered very rapidly and the stalactites were cut and polished and sold worldwide, founding the fame of the Capillitas rhodochrosite. Mining for rhodochrosite continues into the present and about 100 - 200 tons of ornamental rhodochrosite are produced annually in these days. Possibly there is another great cave to be discovered shortly and - hopefully ! - this one will be documented in more detail and preserved for posterity.
Update : November, 21th, 2006
Interestingly enough, there is another argentinian locality in the Nesquen province, where large blue celestite stalagtites are found, which closely resemble the Capillitas rhodochrosite stalagtites, but are of fine blue colour !
Update : January, 24th, 2007
German Mineralienatlas hosts an excellent description about the discovery of the Rhodochrosite stalagtites, which you can find here.
Update : May, 6th, 2007
A new publication online about the geology of Capillitas, please look here.
