Figures 1-2. Left: a view of the extensive mine workings and spoil heaps of the eastern part of the Mineral de Pozos mining camp.
Right: colourful secondary copper salts investing rock near the large open cut of the Santa Brigida mine.

Figures 3-4. Left: hand specimens 3086 and (right) 3087, each showing white vein quartz cutting intensely altered wall rocks, the latter hosting almost all the abundant base-metal sulphides. This can be seen quite clearly in the sawn offcuts of rock and the corresponding polished thin sections. Right: photomicrograph shows the euhedral, six-sided cross section of a quartz prism growing into a void infilled by pale yellow iron oxyhydroxide, limonite. Long-axis field of view 0.9 mm, nominal magnification 100X, in plane-polarized transmitted light.

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"Rock of the Month #148, posted October 2013" ---

Mineral de Pozos (Meade, 2011, pp.199-206) is described as a kind of "recovering ghost town", founded on numerous abandoned mines, almost deserted for a time, now enjoying modest growth and tourism. The town is located northeast of San Miguel de Allende in eastern Guanajuato state. The Santa Brigida mine east of town (p.202) was first worked by native people before coming under Spanish colonial control. The site is said to feature wells but no shafts (hard to believe: a short visit on 02 December 2012 revealed an open working along veins and evidence of at least 4 shafts, one stone-lined, one of which was largely removed by later expansion of the elongate open pit - GCW). Further mines situated west of the town include the Cinco Senores mine (pp.202-203). The long history of mining peaked in the Porfiriato era of the late 19th century. Santa Brigida was the first mine opened in Pozos, by natives, then Jesuits, until the latter were expelled from Mexico in 1767. The local mines, especially those west of town, reopened in the late 19th century and over 300 active operations came into being in that era. The dictator renamed the town for himself, Ciudad Porfirio Diaz, but with the 1910 revolution the mining largely ceased. The flooding of workings and drop in metal prices killed the last mine in 1927. The population waned to 200 residents by the mid-20th century. In 1982 the president named Pozos a National Historic Monument Zone and today several thousand people live in the area. At Mina Santa Brigida, featured here this month, three large smokestacks remain, the roasting ovens used in metal refining, while a linked line of arched ovens are said to have been employed in the mercury amalgamation of silver-rich ores.

Panczner (1989) notes the occurrence of various ore minerals in the area, i.e.,

• Galena (pp.200-205);
• Chalcopyrite (pp.150-155) from many localities, including the Santa Bridgida (Santa Brigida), Trinidad and La Joya mines at Mineral de Pozos;
• Malachite (pp.259-262), widespread, including the Santa Brigida, Progreso, Mascota and Trinidad mines;
• Native silver (pp.340-347) from many mines, including mines of the Veta Madre at Guanajuato, La Luz, and widespread sites at Mineral de Pozos: Santa Brigida, San Rafael, Cinco Senores, Progreso, Angustias and Trinidad;
• Sphalerite (pp.350-356) at Mineral de Pozos is noted at the La Joya mine.

A rockhound highlight of a trip to the region is a visit to the Museo de Mineralogia at the Universidad de Guanajuato, an outing we made happily on 04 December 2012. The museum is situated high on a hillside campus on the northwestern side of the capital city of the eponymous state. Thousands of samples are on display in many cases on two levels. The cabinets are arranged "old-school", at their best a Mexican version of the systematic collections of the Natural History Museum in London in the 1970s. This is to say, a casual visitor will find the approach bewildering: a devotee will probably get more from the exhibits, the more time she or he devotes to them. Many samples are small, but doubtless a valuable record of material from many mines, some obscure and others famed Mexican mineral localities. Several small samples are from Mina Brigida (Mina Brigitta), such as:

• Brownish wulfenite crystals;
• Sample 672, labelled "raminina", descloizite, the Pb Zn V mineral, a vanadate;
• Samples 890-891 are "argentite" (acanthite) and native silver;
• Furthermore, sample 4540 is native silver is attributed to Mina Cinco Senores, and samples 4003, 4012 and 4020 are native silver collected by Ing. Aguilar from Cinco Senores.

The Two Samples are vein quartz with disseminated grains of base-metal sulphides such as sphalerite and chalcopyrite, galena and arsenopyrite. Both are grab samples from the dumps, and a polished thin section was prepared from each one. The wallrocks, intensely silicified, are identified as black shale and altered volcanic rock, possibly with pale relict feldspar phenocrysts, but in fact very little survives of the original silicate mineralogy of the host rocks. Surficial oxidation products include orange goethite and blue azurite. Each sample consists of a few percent sulphides, in a matrix of >90% strained, irregular quartz grains. The quartz was deposited in perhaps three successive generations, 0.02-2 mm in grain size, the latest and coarsest quartz cut by late hairline fractures.

Figures 5-6. Left: Some of the coarsest strained quartz. Photomicrograph, long-axis field of view 1.7 mm, nominal magnification 50X, in cross-polarized transmitted light. Right: medium- to very fine-grained quartz, with isotropic sphalerite (top left) and opaque pyrite and lesser chalcopyrite (lower right). Photomicrograph, long-axis field of view 1.7 mm, nominal magnification 50X, in cross-polarized transmitted light.

Figures 7-8. Left: coarse sphalerite, with exsolved blebs of chalcopyrite, surrounded by coarser granular pyrite. Photomicrograph, long-axis field of view 1.7 mm, nominal magnification 50X, in plane-polarized reflected light. Right: photomicrograph of coarse sphalerite, rich reddish-brown in transmitted light, medium grey here, with exsolved blebs of chalcopyrite. The zinc sulphide is cut by a fracture infilled by later yellow chalcopyrite, which in turn is partially replaced by light grey galena. Granular pyrite lies in quartz at lower right. Long-axis field of view 1.7 mm, nominal magnification 50X, in plane-polarized reflected light.

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The most abundant ore mineral is granular pyrite, plus brown translucent sphalerite, grey galena and minor yellow chalcopyrite. The sphalerite exhibits "chalcopyrite disease", with exsolved, micron-sized blebs of chalcopyrite. There is a sense of the paragenesis (the time sequence of deposition of the various minerals) in the sphalerite, which is cut by later veinlets with partial replacement by chalcopyrite, which in turn shows some deposition of later galena. There is minor low-temperature (supergene) alteration of the margins of yellow chalcopyrite to bright blue covellite. Some euhedral quartz growing on vein margins is coated by yellow limonite, possibly pseudomorphous after the potassium-iron sulphate, jarosite.

With this basic appraisal of the thin sections, the mineral listing for Mina Santa Brigida totals 14 distinct species (not counting the oxyhydroxide mixture, limonite). In terms of systematic mineralogy, the MINDAT web site offers a further 10 species, though "Panczner (1987)" is the sole citation listed therein (web site visited 23 October 2013). The 10 additional species are bornite, cerussite, chalcocite, chrysocolla, cinnabar, native copper, native gold, hematite, litharge and massicot: a plausible selection of copper, lead, mercury, gold and iron phases. It is thought likely that detailed study of samples from Santa Brigida would reveal some or all of these minerals.

Acknowledgements: with special thanks to Irwin and Wanda who made this Mexican interlude possible, and great fun besides! Thin sections by Anne Hammond.

References

Meade,JD (2011) San Miguel de Allende, Guanajuato and the Bajio. Moon Handbooks, Avalon Travel, Perseus Books Group, Berkeley, CA, 281pp.

Panczner,WD (1989) Minerals of Mexico. Van Nostrand Reinhold Co., 459pp.

Universidad de Guanajuato (2012) Museo de Mineralogia. Museum visit (notes on displays open to the public on 04 December 2012).