The Museum MineLast Updated: 5th Aug 2019
By Paulo Cesar Pereira das Neves; Darcson Vieira; Daniel Atencio
The Museum mine is located in Ametista do Sul, Rio Grande do Sul state, Brazil (27°21’38” S; 53°10’154” W), at a vertical height of sea level of 505 m.
Geologically it is composed of a sequence of tholeiitic basalts belonging to the Serra Geral Formation (Paraná Basin) - Serra Geral Group.
The mine peaked in productivity between 1970-1990, when it had one of the world’s largest amethyst productions. It is currently inactive serving as a model mine coupled directly to the Amethyst Park Museum, where in addition to the huge extracted there is also a mineralogy museum with over 1,000 species of minerals from around the world. The old mining keeps it galleries intact for visitation, where you have a general overview of what the mining activity was like.
The main mineral resources of this mine are the amethyst (lilac quartz variety), which occurs in geological structures called geodes, in which the amethyst is usually filling the concavity of chalcedony embedded in the host rock.
It has traditionally been found that these structures were generated by degasification of the lava (around 1150 ° C) by exsolutions of gases and coalescence of bubbles in supersaturated and volatile environments (GILG et al., 2002).
In contrast to the model previously described, the formation of cavities may have occurred in the process of lower temperatures (<150 ° C) from events of hydrothermal and epigenetic origin (DUARTE et al., 2014). These authors recognize the initial brittle origin of basalt in the early stage of the magmatic manifestations, in addition to the intense ascending hydrothermalism. Thus, the formation of the geodetic structures may have been due to the dissolution and/or the formation of the basalt in the ductile field, by the action of hot pressurized water, being possible the formation of the geodes at low temperatures. It is also worth nothing that the large geodes cloistereds in the center of lava flows have no connection with vesicular-amygdaloidal zone of the above spills nor with the base of the same ones. This was probably the genesis of mineralized geodes with amethyst from the Museum Mine.
In the Museum mine the following mineralogical association occurs: albite, anhydrite, apatite, augite, calcite, celadonite, gypsum, goethite, lepidocrocite, magnetite, “olivine”, pyrite, quartz and “tourmaline” (MOOSMANN et al., 2009; NEVES & ATENCIO, 2018).
DUARTE, L. da C.; HARTMANN, L. A.; MEDEIROS, J. T. do N.; JUCHEM, P. L. Origem epigenética-hidrotermal dos geodos de ametista e ágata na província vulcânica do Paraná. In: HARTMANN, L. A.; BAGGIO, S. B. (orgs.). Metalogenia e exploração mineral no grupo Serra Geral. Porto Alegre, UFRGS, 2014, P. 55-73.
GILG, A.; MORTEANI, G.; KOSTITSYN, Y.; PREINFALK, C; GATTER, I.; STRIEDER, A. Genesis of amethyst geodes in basaltic rocks of the Serra Geral Formation (Ametista do Sul, Rio Grande do Sul, Brazil: a fluid inclusion, REE, oxygen, carbon, and Sr isotope study of basalt, quartz, and calcite. In: CHEILLETZ, A. (ed). Springer Verlag, http//link.springer.de/link/service/journals/00 /26/content/02/0031, 2002.
MOSSMANN, EHRMANN, J. M.; BRUNING, R.; SEMPLE, L.; GROAT, L. A. Skunk calcite. Mineral paragenesis in an amethyst geode from Ametista, Rio Grande do Sul, Brazil. The Mineralogical Record, v. 40, n. 2, p. 121-125, 2009.
NEVES, P. C. P. das; ATENCIO, D. Enciclopédia dos minerais do Brasil – Tectossilicatos. Canoas/ULBRA, 360p., 2018.
Figure III - Geode on the basalt wall, with amethyst (lilac), quartz (white), chalcedony (gray), and celadonite (green).
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