New Mexico Mineral Symposium

formed material (toward the interior of the rock cavity) is more cloudy. The chalcedony fluo- resces bright green under short-wave UV light, typical of hydrous silica containing trace amounts of the uranyl (UO2r*) ion. The last- NewMexico Mineral Symposium formed layers of chalcedony tend to be the most brightly fluorescent, but some of the earliest- formed layers (adjacent to the host rhyolite) The Sixteenth Annual New Mexico also fluoresce brightly. The most cloudy/milky Mineral Symposium was held November chalcedony usually shows little fluorescence. II and12,7995, at New Mexico Institute of Fluorescent banding in the layers helps to trace Mining and Technology, Socorro. Following growth patterns in the chalcedony. are abstracts from talks given at the s)./mpo- Cross-sectioning reveals that the early- sium that concern New Mexico. The num- formed chalcedony in the interior of the nod- bers in parentheses refer to locations on the ules has grown radially from nucleation sites on map' the rock cavity walls, but later chalcedony growth appears to have progressed outward toward the cavity opening, parallel to the cavity walls. Thin, green-fluorescent chalcedony- MrNsnels AND MINING HISToRy oF THE filled fractures in the surrounding rhyolite Flsnno-He.lrovER DrsrRrcr, GneNr CouNrv, appear to mark pathways by which silica-bear- Nrw Mrxco, by Robert W. Eueleth, New ing water was carried into the cavities. Toward Mexico Bureau of Mines and Mineral the interior of the cavities, some of the last- Resources, Socorro, NM 87801-4796 (1) The origin of agate and chalcedony noduies formed fibrous chalcedony grades into a layer has always been something of an enigma to composed of minute, euhedral drusy qtartz, The Fierro-Hanover minins district in Grant mineralogists, as well as to the general public. with crystals about 0.5 mm in diameter. Also in County, southwestern New Merico, is one of Although the general concept of their formation places, the outermost layer of chalcedony and the state's more productive mining areas, hav- has been fairly well understood, the precise drusy quartz is overlain by a surface coating of ing seen nearly continuous production of base details involved in the processof their forma- iron-stained, fibrous, length-slow chalcedony(?) metals,precious metals,and iron for more than tion at each particular locality have tended to be about 0.5 mm thick. Recent studies have shown a century. more difficult to explain. that much length-siow chalcedony contains The entire district is contained within the A particularly intriguing occurrence of chal- moganite (also known in the literature as roughly trianguiar-shaped features known as cedony nodules has recently been recognized in lucetite). which is a monoclinic SiO, polymorph the Santa Rita horst, which is sharplv bounded mid-Tertiary silicic volcanic rocks near the distinct from quartz (Heaney and Post, L992); on the northwest by the Barringer'fiult, on ihe Arizona-New Mexico border in southwestern XRD confirmation for the presence of moganite northeast by the Mimbres fault, and less dis- Hidalgo County, New Mexico, and adjacent in the Hidalgo County chalcedony has not yet tinctly on the south by the Groundhog and Cochise County, Arizona. The occurrence is in been obtained. In some nodules, small patches Nancy faults. The intruslon of the Fier- the vicinity of Clanton Draw and Geronimo of glassy. blebby opal (optically isotropit) with ro-Hanover granodiorite stock uplifted and Pass, in the Coronado National Forest. The nod- a very bright green fluorescence were the last fractured the sedimentary rocks in the district. ules formed within a late Oligocene, high-silica material deoosited. These fractures provided channels through rhyolite lava flow (Rhyolite of Clanton Draw) The origins of agate, chaicedony, and geodes which the mineralizinq solutions invaded the along the north margin of the Geronimo Trail have been the subiect of increased interest and limestones and formed the extensive contact- cauldron (Mclntyre, 1988; Erb, 1979; and study in recent years. A major controversy, not metamorphic and skarn deposits in the Combi- Charles Bryan, personal communication, 7995). yet resolved, has been whether chalcedony nation, Princess, Kearney, Empire, and Pewabic The chalcedony nodules were recently forms by precipitation from dilute aqueous mines in the southern and central parts of the noticed by Charles Bryan (University of New solutions that flow through cavities, or by crys- district and the Union Hill irotr depositsand the Mexico) in the course of field work. Ramon S. tallization of a dense, viscous silica gel that fills Continental mine at Fierro in the northern r:art. DeMark and Brian lluntsman learned about the a cavity as a closed system. Building on the These and other smaller mines have pro- occurrence from Bryan. The noduies range in speculations and observations contained in duced a total of more than a billion pounds size from about an inch to l0-12 inchesin diam- such classic publications as those by Liesegang each of copper and zinc, 50 mitiion pounds of eter. They are typically hollow, and only par- (1910) and Ross (1941),some very good treatis- lead, 5 million ounces of silver, and 50,000 tially fill somewhat angular, irregular-shaped es on the subject have appeared within the past ounces of gold. The district is the state's leading cavities in the host rhyolite. The chalcedony lin- dozen years; for example, Landmesser (1984), producer of iron ore totalling some 8rz million ing making up the nodules is typically from 0.5 Macpherson (1989), and Pabian and Zarins short tons. Concurrent with ihe production has to 1 cm thick. Some nodules are flattened or (1994). In addition, an increasing number of come a long and colorful mining history begin- elongated parallel to the direction of flow band- papers have applied modem techniques of ning in ca 1858 with German metallurgist Sofio ing in the rhyolite; others are subspherical. mineralogy and geochemistry to the study of Hinckle at the old Hanover mine neaipresent- Most nodules are found loose,as "float" pieces, agate and chalcedony; for example, Fallick and day Fierro.The richestcopper ores,some grad- usually still partially attached to pieces of the others (1985); Graetsch and others (1985); Wang ing 58% Cu, were produced here until the Civil host rhyoljte. and Merino (1990); Heaney (1993); and Lueth War. A few occur as loose nodules totallv free of and Goodell (in prep.). The Hanover and other mines have produced rhyolite matrix, and some are r""r-r e-b"dd"d Symposium participants are invited to share an abundance of col.lectiblemineril species in rhyolite boulders or outcrops. An unusual their own ideas and insights about the origin of including quartz. calcite, selenite, magnetite, feature of the nodules is the way the chal- agate and chalcedony in the discussion follow- native copper, and turquoise, as well as wide cedony appears to have "oozed" out of the inte- ing this presentation, so that together we may variety of copper oxides and sulphides. Excei- rior surface of the cavity in rhyolite; lobes of try to reach some increased level of under- lent specimensare still produced occasionally chalcedony surround, but usually do not com- standing of what these nodules are telling us at the currently active Continental mine, the pletely close off, the opening of the host cavity. about the deposition of silica within cavities in Union Hill iron pits, and until recently, the Rarely, completely closed, hollow noduies have rhyolite. I would like to particularly acknowl- mines along the Barringer fault north of Fierro. weathered out of the rhyolite; some such nod- edge the advice and unpublished information ules have thin enough walls and contain a large provided by Charles Bryan, Ramon S. DeMark, enough void to float on water. and Virgil E. Lueth in preparing this paper. The chalcedony is fibrous with a radiating OzucrN op cHALCEDoNy NoDULES rN RHyoLITE microstructure. Fibers are length-fast (elongat- References FROM THE PELONCILLOMOUXTAWS, HTOETCO ed perpendicular to the c-axis), as has been Erb, E. E.,1r.,1979,Petrologic and structural evolution CouNrv, Nrw Mrxrco, by Peter Modreski, l. reported to be the case for most agate and chal- of ash-flow tuff cauldronsmd noncauldron-related U.S. Geological Survey, MS 905, Box 25046, cedony. The color ranges from pale pink to volcanic rocks in the Animas and southern Federal Center, Denver, CO 80225-0046 (2) milky-white to colorless; typically, the later- PeloncilloMountains, Hidalgo Comty, New Mexico: February 1996 New Mexico Geology quartz Unpublished Ph.D. thesis, University of New related, main - monzonite intrusive vidual,0.5 mm, olive-green crystals perched on Mexico, Albuquerque NM,287 pp. (Williams, 1978). crusts of conichalcite. Aurichalcite and rosasite Fallick, A. E., Jocelyn, J., Donnelly, T., Guy, M., md The main workings consist of several shafts [(Cu,Zn)r(CO)(OH)r] are both found in attrac- Behm, C., 1985, Origin of agates in volcmic rocks and a number of adits, all of which were appar- tive specimens in a number of areas throughout from Scotland: Nature, v 313, pp. 672474. ently interconnected at one time. Workings that the mines, always in association with coiorless, Graetsch, H., Florke, O. W., and Miehe, G., 1985, The are currently accessible form a Byzantine series bladed, hemimorphite [ZnoSirOr(OH)rHrOl nature of water in chalcedonv and opal-C from crystals. Carminite was found in a single speci- Brazilian agate geodes: Physici and Chemistry of of drifts, declines, stopes, and "dog holes," Minerals, v. 12, pp. 300-306. Mining by "following the ore" is probably men on the dump of an adit high on the northwest side of the Granite Gap hill. This adit and Heaney, P J., 7993, A proposed mechanism for the responsible for this extreme irregularity of growth of chalcedony: Contributions to Mineralogy openings and suggests that the ore shoots dump were the source of the greatest volume and Petrology, v 71.5,pp.

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