News from Mexico: Peter Megaw

Peter Megaw  pmegaw@imdex.com        
MINERAL NEWS Vol. 16, No. 3, March 2000

Amethyst lot fresh out of the hole from La Valenciana Mine, Amatitlan, Guerrero

    Things have been pretty quiet in southern Mexico: Taxco, Guanajuato, Las Vigas, and Amatitlan haven't been producing much. Fresnillo seems to have been concentrating on producing excellent pyrargyrites and polybasites -arguably the best ever found, but not in quantity. It is worth remembering that this mine alone produces 8% of the world's annual silver production and they've found 4 new Santo-Niño type veins in the last 3 years! Peñoles is heading towards putting them into production by 2002, so we may just have seen the beginning for superb silver minerals from here!

  
    Smithsonite
    Santa Eulalia: The San Antonio Mine in the East Camp has produced significant amounts of gray to gray-green undistinguished smithsonite for years. Smithsonite occurs dominantly on the 8th level of the mine, at the water table. Here, zinc leached from oxidizing ores above, reached a chemical barrier and was redeposited as a supergene blanket. Some of the zinc precipitated as smithsonite coatings of sphalerite or arsenopyrite crystals and some pseudomorphed scalenohedral calcite, but the bulk came down as gray stalactitic cavern fillings in oxidation-created voids. Little production work takes place on the 8th level because the mixed oxide-sulfide ores are difficult to beneficiate. However, the mine periodically encounters water-filled structures that gush up to 90,000 gallons/minute and rapidly flood the lower levels of the mine. When this happens, attention perforce turns to the 8th level and mining of mixed oxide/sulfide ores again occurs. The last time this happened was in 1982 and a significant number of bright yellow and lime-green smithsonite specimens (colored by greenockite inclusions) hit the market then (See Panczner, 1987 for nice photos of these).
    Once the deeper levels were pumped out and rehabilitated, the 8th Level mining was abandoned. Specimen collectors periodically tried to work the zone, but the ground is hard and company vigilance is high since the principal underground repair shop is located very near the entrance to the smithsonite zones. About two years ago, a few rice-grain size and shape bright blue smithsonites associated with hemimorphite began to appear. Very pretty micros, but in need of enlargement before anyone but a locality or species collector would get excited (I have several!). Things began to change quickly last spring, when the mine was again partially flooded and production was temporarily resumed on the 8th Level. A small lot of superb brilliant green translucent smithsonite knobs and stalactites studded with hemimorphite appeared at this time.

       
       Smithsonite                  The best pieces were miniature to cabinet sized and they sold quickly at Denver through Graeber and Himes Minerals. Mining returned to the deep levels but specimen collecting in the zone continued, with the collusion of members of the mine staff. However, only gray-green to yellow-green balls and heart-shaped distorted twinned rhombs were found until late November when blue botryoidal and stalactitic masses from TN to museum-size were hit. Color ranged from a greenish to grayish-blue to an electric blue rivaling the classic Kelly material. Many had associated hemimorphite, some had sharp doubly-terminated anglesites to 5 mm, and a few had 7 mm cerussites perched on them. Some showed patches of white hydrozincite. A small lot of 1 cm aurichalcite balls with hemimorphite was also found in the zone; material again very reminiscent of the Kelly. There was only one good pocket of the massive blue smithsonite-totaling perhaps 25 good pieces, with 5 excellent ones (the best won the Romero Trophy at Tucson). Word of these got out and dealers descended on Chihuahua eager to acquire more. Naturally, prices skyrocketed for anything even vaguely blue and the zone was attacked furiously. Many additional flats of green and yellow smithsonite and numerous pieces with isolated 3-8 mm blue rice-grain-shaped crystals perched on colorless 5-10 mm hemimorphite blades were mined. (Some of these have sharp orange barite micro crystals as well.) A few blue heart-shaped distorted rhombs also appeared, the best making excellent TNs and small miniatures. Dave Bunk Minerals and Dennis Beals (Xtal) obtained the bulk of the best material and may have some good pieces left after their Tucson sales. Work continues in the zone, but so far nothing comparable to the best has emerged. Prices are still ridiculous and will probably remain so for a good while-especially if more good blue is found-which is far from certain. I have seen four large lots in the last week, of which perhaps 5% are worthy of collector interest-only a few of the recent smithsonites have good saturated green-blue color, there's very good aurichalcite, both balls and sprays, more anglesite, cerussite, and barite, and a few smithsonites associated with malachite pseudomorphs after azurite. The asking prices for the lots were astronomical-but at least three lots are gone!
    During early work in the smithsonite area, in late 1998, a parallel structure was found that yielded the first creedite to be found in the East Camp. Most are 1-4 mm sharp, elongate, pale to brilliant purple crystals on a tannish smithsonite matrix. Many form 1 cm balls, in some instances atop sphalerite and/or arsenopyrite, and rarely on 1 cm smithsonite pseudomorphs after scalenohedral calcite. Probably 20 flats were produced in total, much being micro-material, but some were plates to 12 inches (30 cm) across. However, the best pieces were miniatures boasting 3/8-3/4 inc (1-2 cm) purple crystals, strongly reminiscent of the material found in the West Camp of Santa Eulalia in the mid '80s. The best of these overgrew 1cm fluorite cubes. A few scraps have appeared recently, but with attention focused on the nearby blue smithsonite zone, I don't expect to see more of these for a while.

   Smithsonite
    1999 smithsonite-zone work also resulted in the largest smithsonite pseudomorphs after scalenohedral calcite seen in many years. The recent pieces are cream to bone-colored smithsonite that replaced calcite scalenohedra up to 31/4 inches (8 cm) long. The biggest piece I've seen is 12 by 6 inches (30 cm by 15 cm) with at least six big pseudos on it. The pseudos are hollow with no vestige of remaining calcite. Some have sphalerite matrix. Jim McGlasson (Collector's Stope) had a good selection of miniatures of these at Tucson.
    The deeper sulfide levels have continued to produce a decent smattering of sphalerite plates, with increasing numbers of chalcopyrite crystals to 11/4 inches (3 cm). Curiously, calcite has been scarce. Excellent quartz crystals to 4 inches (10 cm) have also been forthcoming, both alone and with sulfides. Recently, I obtained a small lot of very nice pineapple or alligator-scale crystals ("elestials" in healie-feelie parlance) to 2 inches (5 cm) long, with pronounced scepters. The mine has also produced a few handfuls of spectacular micro-quartz that look like tiny Herkimer diamonds. The crystals apparently grew in palygorskite-filled pockets and are full-floater singles and groups or clusters of up to 15 linked crystals. Some have rhombs of mangano-calcite in them. These make superb micro-mounts and are available from LAM Minerals (see classified ads). Perhaps the most intriguing recent find from the San Antonio is a good sized lot of lustrous olive-green siderite from the 12th level. Crystals are 2-5 mm across and form full druzy plates from TN to cabinet size. Although attractive, their chief interest may lie in the fact that this style of siderite has been repeatedly found in close proximity to the pocket zones that produce the spectacular ludlamites and vivianites for which the San Antonio is famous. Given the quantity and quality of the siderite, the prospects look good for ludlamites sometime within the next year, depending on the mining schedule.

               
                Aragonite                         
    The West Camp has been fairly quiet recently, with only the usual enormous volumes of brown dog-tooth calcite and white aragonite being prominent. Considerable attention has been focused on the mimetite zone on the 13th Level of the Potosi Mine, and some nice rhombohedral calcites overgrowing butterscotch-colored "cauliflower" mimetite have appeared. Only a few flats of worthwhile material have been found-nothing like the huge hauls made in the late '70s and mid '80s. Notably, within this material was a small lot (25 pieces) of lustrous greenish-yellow mimetite balls to 5 mm on a dark ochre matrix. These are quite fetching-the best were acquired by Isaias Cassionas of IC Minerals.