Veined and Mineralized Andesite

from the Guanajuato mining camp, central Mexico

veined andesite [58 kb]

"Rock of the Month # 62, posted for August 2006" ---

Pyritic quartz-carbonate vein in porphyritic andesite, from the Cedros stope, Guanajuato, part of a famed stockwork gold-silver orebody in volcanic host rocks. Although parts of the camp are relatively gold-rich, this sample (798), donated by Contratista Tormex in 1986, is from the Torres Mining Complex which had a contemporary (1984) overall Ag:Au production ratio near 100:1. The pale pink feldspar-phyric andesite contains sparse quartz phenocrysts, and is traversed by an early fracture darkened with Fe and Mn oxides, cut in turn by late multi-stage quartz-calcite veining containing disseminated pyrite (maximum grain size near 1 mm). The quartz is darkened by fine-grained chlorite and ore minerals. A thin screen of andesite occurs at the border of the two stages of vein filling (earlier mineralized silica and late barren calcite). A small pyritic fragment in otherwise barren carbonate affirms that the calcite filling postdated the siliceous ore. The immediate host rock is very fine-grained and silicified. A polished thin section of the vein material contains 2% pyrite and traces of sphalerite, argentite, galena, native gold and chalcopyrite. Small grains of native gold occur in pyrite, sphalerite and galena, up to 70 microns in diameter.

The Guanajuato mining camp averages 20 km in length and 16 km in width. The town itself occupies a narrow valley almost at the centre of the district, at 21°01'N and 101°15'W. The first mineral discovery in the colonial era occurred on June 11, 1548 (Guiza et al., 1949). The crest of the major Veta Madre was found in what is now the Rayas mine in 1550 (Querol et al., 1991). Water has been a problem in all the main mines (Guiza et al., 1949). Interesting to note (ibid., pp.73-75), metal production was forecast to drop to insignificant levels a half-century ago, were new reserves not located. The Torres mining complex, composed of five Au-Ag mines (Torres, Bolanitas, Peregrina, Cebada and Cedros) has been the largest Au producer and one of the largest Ag producers in Mexico. The proven and probable metric reserves for Torres on 31 December 1989 were 3.7 MT grading 3.1 ppm Au and 220 ppm Ag (Ag/Au ratio = 71: Anderson, 1990). The longest vein system, the Veta Madre, has a strike length >25 km (Anon, 1980). Average Au Ag grades for the mines reach at peak at Peregrina, at 4.5 ppm Au and 350 ppm Ag (Ag/Au = 78: Querol et al., 1991).

Precious-metal minerals include native gold, native silver and various sulphides, sulphosalts and (notably) selenides, including auriferous naumannite, argentite, miargyrite, pyrargyrite, stephanite, polybasite, aguilarite, proustite and berzelianite. The local veins have a quartz matrix, with country rock xenoliths, some of them silicified. The veins also carry calcite, chalcopyrite, pyrite and trace galena (Echagaray, 1964). The host Cedros andesite is age dated at 30.0±0.7 Ma, overlain by two younger volcanic sequences (Cerca-Martinez et al., 1998). Veta Madre adularia K-feldspar is dated at 27.4±0.4 to 29.2±2 Ma (Querol et al., 1991).

References

Anderson,S (1990) Corona's becoming a Latin-American lover with Torres. Can.Min.J. 111 no.2, 45, February.

Anon (1980) Las Torres: capitalizing on a major discovery. Eng.Min.J. 181, 156-158, November.

Cerca-Martinez,M, Aguirre-Diaz,G and Lopez-Martinez,M (1998) Stratigraphy and geochronology of the limit between the Sierra Madre Occidental and the Mexican volcanic belt in the Sierra de Guanajuato, Mexico. GSA Abs.w.Progs. 30 no.7, 171, Toronto.

Echagaray,FA (1964) Monografia historica y minera sobre el distrito de Guanajuato. Consejo de Recursos Minerales No Renovables Publ. 17E, 588pp. (in Sp.).

Guiza,R, Rendon C,F and Baltierra B,JJ (1949) Estudio geologico del distrito minero de Guanajuato, Gto. (zona de la Veta Madre). Instituto Nacional para la Investigacion de Recursos Minerales Boletin 22, 75pp. (in Sp.).

Querol S,F, Lowther,GK and Navarro,E (1991) Mineral deposits of the Guanajuato mining district, Guanajuato. In `Economic Geology, Mexico' (Salas,GP editor), GSA DNAG volume P-3, 438pp., 403-414.

Graham Wilson, posted 04 September 2007.

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