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ABATRACTS OF THE 21ST ANNUAL FM-TGMS-MSA MINERALOGICAL SYMPOSIUM THE MINERALS of BRAZIL.

INTRODUCTION

The 21st Annual Tucson Mineralogical Symposium, sponsored by the Friends of Mineralogy, the Tucson Gem and Mineral Society, and the Mineralogical Society of America, was held in conjunction with the 46th Tucson Gem and Mineral Show on Saturday, February 12, 2000. Minerals of Brazil were the feature of the 2000 Tucson Show and were the subject of the 2000 mineral symposium.

Brazil is the fifth largest country in the world and the largest in South America, comprising nearly half of that continent's area. Perhaps it should not be surprising that Brazil has yielded considerable mineralogical wealth. What never fails to amaze, however, are the beauty and significance of many of the wonderful minerals and gems that it continues to produce.

Many of the papers presented at the symposium dealt with minerals from the pegmatites and related rocks in the state of Minas Gerais. Several others, however, dealt with deposits in other parts of the country, from amethyst deposits in the far south to pegmatites and skarns in the northeast.

This symposium was dedicated to Dr. Richard V. Gaines (1917-1999), whose love affair with minerals spanned more than seven decades. Richard's work on tantalum and beryllium deposits took him to pegmatites all over the world. He made 22 trips to Brazil, studying deposits and collecting minerals and friends. Several of the contributors to this year's symposium were good friends of Richard's, and other friends of his responded to the call for papers, saying that they wished they had something to offer. We all owe Richard a debt of gratitude for a life dedicated to enriching the professional and amateur mineralogical community in many ways.

Minas Gerais--Past and Present

Anthony R. Kampf

The name Minas Gerais means "General Mines" in Portuguese, and with good reason. By the mid-18th century, the area around Diamantina was the largest producer of diamonds in the world and that around Ouro Preto was Brazil's largest producer of gold. The wonderful baroque architecture of both of these cities dates to this period.

Though the first pegmatite gem discoveries date even earlier, the majority of these deposits were first exploited for strategic minerals such as muscovite, quartz and beryl during WWI and especially WWII. Gemstones and gem mineral specimens remained largely by-products of this mining for many years, but, thanks to adventur ous mineralogists and mineral dealers, including such Americans as Allan Caplan, Martin Ehrmann, Fred Pough and Dick Gaines, our knowledge of and access to Brazilian minerals has blossomed during the last six decades.

In recent years, the search for gems has reached into the extensive metamorphic terrain of Minas Gerais, netting rich deposits of emerald, alexandrite and more. Today, Minas Gerais stands out as one of the world's foremost producers of fine gems and minerals. For this, we are indebted to the Brazilian gem spirit epitomized by the industrious garimpeiro and guided by enlightened entrepreneurs such as Jules Sauer, Kahlil Elawar and Agenor Tavares.

Throughout the gem centers of Belo Horizonte, Ouro Preto, Governador Valadares, Teofilo Otoni and Aracuai, and in the backcountry, one senses a continuing excitement as the search for gem treasures goes on.

The Eastern Brazilian Pegmatite Province

C. Preinfalk

G. Morteani

A.H. Horn

The Eastern Brazilian Pegmatite Province (EBPP) encompasses the entire state of Minas Gerais and also the southern part of the state of Bahia, the western margin of the state of Espirito Santo and the northern part of the state of Rio de Janeiro (Paiva, 1946; Putzer, 1976; Correia Neves et al., 1986). In northeastern Brazil, the much smaller pegmatite province of Borborema is also known (Beurlen, 1995; Da Silva et al., 1995).

Around 1500, rumors of very rich occurrences of precious stones, which included green tourmaline mistaken for emerald, induced the Portuguese bandeirantes (pioneers/adventurers) to follow the great rivers Rio Doce, io Sao Mateus, and Rio Jequitinhonha upstream to the center of what is now the state of Minas Gerais and the EBPP to search for the fabulous "Serra das Esmeraldas." The pegmatites of the EBPP are found (in many cases concordant) in Proterozoic mica-schists and amphibolites. The major pegmatite-forming event took place between 525 and 545 million years ago (Brasiliano event) and is coeval with the intrusion of important post-tectonic granites. Mining of the pegmatites in the EBPP is mostly done by garimpeiros (diggers) in search of precious stones (multicolored tourmaline, aquamarine, topaz, brasilianite, kunzite), and subordinately for cassiterite, columbitetantalite, spodumene and pollucite. Most of the gem material is traded in the cities of Teofilo Otoni and Governador Valadares. Pegmatite mini ng is strongly influenced not only by market prices, but also by the climate. In periods of drought and consequent bad crop production, dormant garimpos (diggings) are reactivated as a source of income. In some of the larger pegmatites, feldspar is produced by small mining enterprises in partly mechanized open pits and sold to the Brazilian ceramic and glass industry.

References

BEURLEN, H. (1995) The mineral resources of the Borborema Province in Northeastern Brazil and its sedimentary cover: A review. J.S. Am. Earth Sci., 8, 365--376.

CORREIA NEVES, J. M., PEDROSA SOARES, A. C., and MARCIANO, V. R. P. da R. O. (1986) A provincia pegmatitica oriental do Brasil a luz dos conhecimentos atuais. Rev. Bras. Geoc., 16, 106--118.

DA SILVA, M. R. R. (1995) Geology of the Borborema Province, Northeast Brazil. J. S. Am. Earth Sci., 8,17--38.

PAIVA, G. de (1946) Provincias pegmatiticas do Brasil. DNPM, DGM, Boletim 78, Rio de Janeiro, 13--21.

PUTZER, H. (1976) Metallogenetische Provinzen in Sudamerika. Schweizerbart'sche Verlagsbuchhandlung, Stuttgart, 316 pp.

The History of Brazilian Pegmatite Gem Mining

Keith Proctor

The northeastern portion of the state of Minas Gerais, Brazil, contains the world's greatest concentration of complex granitic pegmatites, which are especially noted for the production of gem beryl, chrysoberyl, topaz, tourmaline, garnet, kunzite and brazilianite . Because of the geologic environment of these deposits, this region of Brazil is the greatest laboratory workshop in the world for the study of pegmatite geology, crystallography and gemology.

Pegmatite gemstones were first found in this region over 400 years ago; in the last 100 years, Brazil has produced most of the world's supply of these superb gem crystals and cut gemstones. Early exploration was spurred by a quest for gold and precious stones, but the search for and exploitation of deposits was hampered by the coastal mountains and dense interior forests. A key factor in the modern development of gem mining was the colonization of the Teofilo Otoni area by German immigrants during the latter half of the 19th century and by Lebanese during the first part of the 20th century.

Although mining today continues to involve simple methods, larger operations often employ modern equipment. An appreciation for the geologic processes involved in pegmatite formation and subsequent weathering to form secondary deposits is important in considering the range of mining methods necessary for the harvesting of gem crystals.

References

PROCTOR, K. (1984) Gem pegmatites of Minas Gerais, Brazil: Exploration, occurrence, and aquamarine deposits. Gems & Gemology, 20, 78-100.

The Development and Present Geologic Environment of the Most Important Gem Crystal Mines of Minas Gerais, Brazil

Keith Proctor

Minas Gerais has produced the world's finest examples of gem tourmaline (red, blue, green and multicolored), gem beryl (aqua marine, green, heliodor and morganite), topaz (blue and imperial), kunzite, brazilianite, hydroxyl herderite and rose quartz.

The primary sources of these gems are pegmatites that were emplaced approximately 490 million years ago. Subsequent periods of uplift and erosion under largely tropical climatic conditions have exposed the primary pegmatites deposits and have produced several types and variations of secondary deposits referred to as eluvial, colluvial and alluvial.

Among the primary pegmatitic deposits, several of the most famous are the Cruzeiro, Xanda, Jonas, Corrego do Urucum, Golconda and Medina mines. The secondary deposits include the Barra de Salinas, Ouro Fino, Santa Rosa, Morro Redondo, Frade, and Hematita mines.

References

PROCTOR, K. (1984) Gem pegmatites of Minas Gerais, Brazil: Exploration, occurrence, and aquamarine deposits. Gems & Gemology, 20, 78-100.

PROCTOR, K. (1985) Gem pegmatites of Minas Gerais, Brazil: The tourmalines of the Aracuai districts. Gems & Gemology, 21, 3-19.

PROCTOR, K. (1985) Gem pegmatites of Minas Gerais, Brazil: The tounnalines of the Governador Valadares district. Gems & Gemology, 21, 86-104.

The Famous Tourmaline, Aquamarine and Kunzite Mines of Minas Gerais, Brazil, and Their Greatest Gem Crystals

Keith Proctor

Thirteen of the more prominent gem mines in Minas Gerais have produced many of the greatest gem crystal specimens of tourmaline, aquamarine and kunzite. These include the primary deposits at Cruzeiro, Jonas, Xanda, Limoeiro, Golconda, Medina and Corrego do Urucum and the secondary deposits at Barra de Salinas, Ouro Fino, Santa Rosa, Morro Redondo, Frade and the Pioneer mine in the Tres Barros region near Marambaia.

Beyond an appreciation of the mineralogy and geology of these deposits, it is particularly rewarding to consider the exciting personal stories of the miners and mine owners who were the movers and shakers in the greatest gem crystal discoveries at these mines. For gem crystal lovers, these stories are part of the mystique and lore that make mineral collecting so exciting.

References

PROCTOR, K. (1984) Gem pegmatites of Minas Gerais, Brazil: Exploration, occurrence, and aquamarine deposits. Gems & Gemology, 20, 78-100.

PROCTOR, K. (1985) Gem pegmatites of Minas Gerais, Brazil: The tourmalines of the Aracuai districts. Gems & Gemology, 21, 3-19.

PROCTOR, K. (1985) Gem pegmatites of Minas Gerais, Brazil: The tourmalines of the Governador Valadares district. Gems & Gemology, 21, 86-104.

The Chemical Composition of Biotite from Granitic Pegmatites and from Metamorphic Emerald Deposits in the Eastern Brazilian Pegmatite Province

C. Preinfalk and G. Morteani

In the discussion of the genesis of "schist-type" emerald deposits, two theories are still under debate. The first explains the formation of emerald by exometasomatism, i.e. a reaction during the intrusion of Be-rich pegmatitic melts and fluids with chromium-rich rocks (Fersman, 1929). The second theory relates the crystallization of emerald to a regional, and in most cases multistage reaction between already-emplaced Be-rich and alkali-rich rocks, predominantly pegmatites, with chromium-rich ultrabasic rocks (Gundmann and Morteani, 1989; Nwe and Morteani, 1993). Emerald usually occurs in biotitites which formed at the border of pegmatite bodies. However, biotite-rich zones are also found in the border zone of pegmatites without emerald mineralization. Biotite in biotitites reaches a grain-size of up to 2 cm, whereas biotite crystals found in the border zone of pegmatites are up to 1.5 m in length.

In the Eastern Brazilian Pegmatite Province (Paiva, 1946; Correia Neves et al., 1986), many pegmatites with conspicuous biotite megacrysts in border zones lacking emerald mineralization as well as biotitites that do include emerald mineralization are found. A typical example of a schist-type emerald mineralization is the Capoeirana mine, Nova Era (Minas Gerais). Biotite from this emerald deposit shows markedly lower Fe/(Fe+Mg) ratios and typically higher F contents compared to biotite from emerald-barren pegmatites. These differences in composition indicate that the two biotite types crystallized under different physiochemical conditions. This suggests biotite and emerald formation occurred during a regional metamorphic event with the metasomatic formation of a blackwall-zoning. However, biotite in the border zone of the many emerald-barren pegmatites formed from the pegmatitic melts in a single stage at the very beginning of the pegmatite crystallization with limited, if any, contribution from the country r ocks.

References

CORRELA NEVES, J. M., PEDROSA SOARES, A. C., and MARCIANO, V. R. P. da R. 0. (1986) A provincia pegmatitica oriental do Brasil a luz dos conhecimentos atuais. Rev. Bras. Geoc., 16, 106-118.

FERSMAN, A. E. (1929) Geochemische Migration der Elemente. Abh. Prakt. Geol. Bergwirtschaftslehre, 18, 74-116.

GRUNDMANN, G., and MORTEANI, G. (1989) Emerald mineralisation during regional metamorphism: The Habachtal (Austria) and Leydsdorp (Transvaal, South Africa) deposits. Econ. Geol., 84, 1835-1849.

NWE, Y Y, and MORTEANI, G. (1993) Fluid evolution in the [H.sub.2]O-[CH.sub.4]-[CO.sub.2]-NaCl system during emerald mineralisation at Gravelotte, Murchison Greenstone Belt, Northeast Transvaal, South Africa. Geochim. Cosmochim Acta, 57, 89-103.

PAIVA, G. de (1946) Provincias pegmatiticas do Brasil. DNPM, DGM, Boletim 78, Rio de Janeiro, 13-21.

Descriptive Mineralogy of Inclusions in Some Faceted Gem Topaz from Brazil

Bruce Geller

Seven faceted topaz specimens were analyzed using optical microscopy, X-ray fluorescence, and scanning electron microscopy to determine the identity and geologic significance of their mineral inclusions. Although these stones are from Brazil, their exact source is uncertain. The challenge of the study was to conduct these studies without damaging any of the lovely gemstones.

All seven gems contain inclusions visible to the naked eye; some have several varieties of mineral inclusions. An alphabetic list follows: ilmenorutile, limonite, muscovite, rutile, struverite, tourmaline, zinnwaldite, and three unknown phases. Scanning electron microscopy was performed on three of the stones that fortuitously have mineral inclusions which intersect topaz facets. This enables the confirmation of ilmenorutile, struverite, zinnwaldite, and indicated the need for further study in order to characterize the three unknown phases.

An original hypothesis that these stones originated from Virgem de Lapa was weakened because no recognizable lepidolite was observed in any of the specimens. However, a pegmatitic origin from elsewhere in Brazil is probable, as suggested by the suite of minerals represented as inclusions within the topaz and literature reports from Brazilian mineral occurrences containing these species (Adusumilli, 1991).

Recent Mineral Occurrences from Northeastern Brazil

Reinhard Wegner

Some significant mineral finds have recently been made in the states of Paraiba and Rio Grande do Norte, Brazil. Red and black manganotantalites, many of which are penetration twins weighing up to 600 gins, were found at the Alto do Giz 11 and Alto des Furnas pegmatites, Equador, Rio Grande do Norte. The Alto des Furnas pegmatite also produced manganotantalite pseudomorphs after simpsonite up to 5 cm across. Some of these pseudomorphs are quite sharp and are intergrown with red, needle-like manganotantalite of a different color. Apatite showing distinct color zoning from yellow cores to blue or green rims was found in crystals up to 12 cm in length from the Alto Feio pegmatite, Paraiba. A new species of the microlite group (fluornatromicrolite) was reported from the Alto Quixaba pegmatite, Quixaba, Frei Martinho Township, Paraiba, as bright green crystals up to 3 cm in size associated with manganotantalite, blue elbaite, lepidolite, and cleavelandite. Spectacular, perfectly crystallized cognac-colored herder ites up to 12 cm in length were found in a pocket at the Alto des Flechas pegmatite near Pedra Lavrada, Paraiba. Alto des Flechas has also produced fine gem-quality golden beryl in the past. Amethyst scepters with milky quartz bases up to 5 cm in length were found near Santana de Mangeiras, Paraiba, Most scepters are single crystals, but some matrix specimens were recovered.

Brazilian Hematite

Catherine J. Gaber

Brazil produces world-class, beautifully crystallized specimens of hematite occurring as reniform, specular, bladed iron "roses" and martite (hematite pseudomorph after magnetite). Less than one percent of Brazil's vast hematite resources produce crystals, but hematite, as an iron ore, is critically important to Brazil and to industry worldwide.

Brumado, Bahia, is known for its spectacular, sharp, hexagonal flat plates or tabular crystals. Botryoidal or reniform hematite is best known from Cumbria, England, though comparable specimens are also found in Minas Gerais. Bladed aggregates of hematite crystals dubbed iron "roses" from Ouro Preto, Minas Gerais, are equal to those from the Alpine clefts of St. Gotthard and Binnatal, Switzerland.

Twin peaks, Millard County, Utah, is one of the most frequently mentioned localities for hematite, variety martite, though the octahedral "crystals" found in Ouro Preto, Minas Gerais, are equally noteworthy. One hematite assemblage that is probably better known from Brazil than from other localities is oriented epitaxial rutile crystals on hematite. Novo Horizonte, Bahia, is the premier locality for this stunning combination, which is often enhanced by being encased in quartz crystals. Similar associations of hematite and rutile are also found in Switzerland.

Metamorphosed, foliated, specular hematite is very common in Brazil, especially in Minas Gerais. The same type of material can be found at Forge Hill, Hawley, Massachusetts. "Rainbow" hematite from Minas Gerais seems to have no counterpart in the rest of the world. The thin film of aluminum phosphate on foliated, specular hematite is quite different from the iridescent hematite, variety turgite, found at Graves Mountain, Georgia, and Covington, Virginia.

Itabira, one of the world's richest iron ore deposits, contains an estimated 15 billion tons of high-grade ore. Over 2 million tons of iron ore are produced each year. Of Brazilian mineral exports, iron ore has been the most continuously and conspicuously successful, making Brazil the second largest producer of iron ore in the world.

Minerals of the Brumado Magnesite Deposits, Serra das Eguas, Bahia, Brazil

Alexander U. Falster

William B. Simmons

Karen L. Webber

James W. Nizamoff

Carlos P. Barbosa

Richard V. Gaines [*]

The magnesite occurrence at Brumado in the Serra das Eguas district is one of the largest deposits of magnesite in the world and is host to a surprisingly species-rich mineral assemblage. The mineralized areas at Brumado are located in a syncline composed of a lower unit of dolomite marble and an upper unit of quartzite. These rocks are underlain by Precambrian gneisses, schists and amphibolites. The magnesite and associated minerals appear to be related to the intrusion of nearby igneous dikes. The igneous activity produced hydrothermal fluids that interacted with the preexisting dolomite to form magnesite. Fracture fillings of coarse-grained magnesite, quartz, alumino-silicates and other late-stage minerals also appear to be related to the same episode of hydrothermal mineralization (Bodenlos, 1954; Cassedanne and Cassedanne, 1978).

In addition to superb crystals of magnesite, dolomite and quartz, many well-crystallized accessory minerals occur at Brumado. Uvite and dravite are found as sharp crystals up to 5 cm in length with colors that range from black to green to red. Topaz occurs as fine yellow, orange, pink or purple crystals up to several cm in length. Sellaite, considered to be the world's finest, is found as crystals that exceed 5 cm in size. Hematite has been found in large, spectacular crystals in excess of 10 cm. The finest zeunerite/metazeunerite are found in crystals up to 3.5 cm on an edge. Associated with these are druses of sklodowskite that equal any previously known examples. Beryl (varieties aquamarine and emerald) has been found in crystals to several centimeters in length and was one of the minerals mined from the area before the discovery of the magnesite lode. Other minerals encountered at Brumado include: talc, goyazite, svanbergite, chernotive-(Y), wakefieldite-(Y), florencite-(Ce), anhydrite, celestite, barite and numerous other species. Brumado certainly ranks among one of the most fascinating mineral-producing areas in the world and continues to supply collectors with superb specimens.

(*.) Deceased

References

BODENLOS, A. J. (1954) Magnesite deposits in the Serra das Eguas, Brumado, Bahia, Brazil. U.S. Geological Survey Bulletin 975-C, 167 p.

CASSEDANNE, J. P., and CASSEDANNE, J. O. (1978) Famous mineral localities: The Brumado district, Bahia, Brazil. Mineralogical Record, 9,196--205.

Composition and Color of Uvite-Dravite Tourmaline from Brumado, Bahia, Brazil

Eugene E. Foord [*]

Peter J. Modreski

Carlos P. Barbosa

Rua Cel. Roberto Soares Ferreira, 586

CEP 35030, Governador Valadares, Minas Gerais, Brazil

Deep red and green tourmalines of several crystal habits are some of the most striking minerals found in the Brumado magnesite deposits. Microprobe and other analyses reported by Modreski and others (1997) show the relation of chemistry to color in these uvite to dravite tourmalines.

The most iron-rich tourmaline, containing 4 to 7 weight % Fe as FeO, is fibrous to prismatic, dark brown to reddish brown dravite. The proportion of ferric iron has not been measured but the presence of some [Fe.sup.3+] probably accounts for the dark red and brown colors. Deep red, platy to tabular crystals, very characteristic of Brumado, are sodic uvite which contains 3 to 4 weight % FeO. Tabular, green to brown, discoidal crystals flattened on c are uvite to calcic dravite, with the uvite tending to be darker and brownish colored and higher in iron (0.2-1.3 weight % FeO versus [less than or equal to]0.1 weight % FeO in pale green dravite). Prismatic, dominantly green but color-zoned crystals range from uvite through dravite to Na-deficient, Al-enriched dravite. Colorless tourmaline at the acicular terminations of some prismatic clusters is low-iron 'Mgfoitite" in which the X-site is dominantly vacant.

Manganese content is negligible, 0.0 to 0.02 weight % MnO. Chromium content is low, 0.00 to 0.14 weight % [Cr.sub.2][O.sub.3], [Cr.sup.3+] could play a minor role in the green color of some specimens. Titanium content is variable, 0.00 to 0.78 weight % [TiO.sub.2], but shows no consistent relation to color. Vanadium ([V.sub.3+]) appears to be responsible for emerald-green zones in some tourmalines. Most analyzed specimens contain between 0.00 to 0.66 weight % [V.sub.2][O.sub.3] but one unusually dark greenish black tourmaline has an uvite core containing approximately 7 weight % [V.sub.2][O.sub.3] (SEM-EDS analysis) surrounded by a paler green ([similar to]1 weight % [v.sub.2][O.sub.3]) dravite rim.

(*.) Deceased 1998

Reference

MODRESKI, P. J., FOORD, E. E., and BARBOSA, C. P. (1997) Crystal chemistry of uvite-dravite from the Brumado magnesite deposits, Bahia, Brazil. Tourmaline 1997. International Symposium on Tourmaline, Nove Misto na Morave, Czech Republic, Abstracts. pp. 59--60.

Cuprian Elbaite from the Bocheiron Zinho Pegmatite, Paraiba, Brazil

Alexander U. Falster

William B. Simmons

James W. Nizamoff

Karen L. Webber

"Paraiba tourmaline" is a name used in the gem trade to describe elbaite tourmalines from the state of Paraiba in northeastern Brazil that occur in unusually vivid shades of blue and green. The first major production of this uniquely colored elbaite came from Mina da Batalha near the village of Sao Jose da Batalha in 1988 (Bank et al., 1990; Fritsch et al., 1990). Several mines in the area have since produced the distinctive blue-green elbaite, including the Mina Bocheiron Zinho. At this location, tourmalines exhibit prominent color zonation: some possess pink cores and deep blue rims whereas others have blue cores with several outer zones of purple, green, blue, or gray.

Electron microprobe analyses and X-ray mapping show the relationship between chemical composition and color. Analyzed tourmalines contain as much as 1,4 weight % CuO in the blue zones, 0.2 weight % MnO in the pale pink cores, and 1.2 weight % FeO in the deep green rims. Deep green tourmalines from the footwall portion of the pegmatite hold as much as 3.0 weight % FeO and many contain thin, platy inclusions of native copper.

The tourmaline chemistry indicates that Fe concentration was initially low, followed by fluctuations in Mn and Cu, with a late Fe and Mn enrichment that produced the dark green rims. The enrichment of copper in the pegmatites of this region is exceptional; massive chalcocite occurs as large pods up to 20 cm across in the footwall of the Bocheiron Zinho. The enrichment may be the result of contamination of the pegmatite-generating magma by copper-rich country rock. The presence of native copper, [Fe.sup.2+], and chalcocite suggests that the conditions of formation were moderately reducing in a low to moderate pH environment.

References

BANK, H., HENN, U., BANK, F. H., v. PLATEN, H., and HOFMEISTER, W. (1990) Leuchtendblaue Cu-fuhrende Turmaline aus Paraiba, Brasilien. Zeitschrift der Deutschen Gemmologischen Gesellschaft, 39-1, 3-11.

FRITSCH, E., SHIGLEY, J. E., ROSSMAN, G. R., MERCER, M. E., MULMEISTER, S. M., and MOON, M. (1990) Gem-quality cuprian elbaite tourmalines from Sao Jose de Batalha, Paraiba, Brazil. Gems and Gemology, 26, 189-205.

REE Contents of Calcite and Gypsum from Amethyst Geodes in the Lava Flows of the Serra Geral Formation (Alto Uruguai, Southern Brazil)

C. Preinfalk

G. Morteani

A. Strieder

Amethyst geodes which occur in the regions surrounding Irai (Alto Uruguai) are the most important source for "Brazilian amethyst." The geodes are principally found in tholeiitic basalt flows in the Lower Cretaceous Serra Geral Formation. Most geodes show a characteristic zoning, with an outer rim of agate, including rare carbonate crystals overgrown on amethyst crystals. A second generation of euhedral calcite crystals (up to 10 cm) may be found on the amethyst crystals. In the "Garimpo da Testa" mine geodes are filled by anhedral, very coarse-grained gypsum instead of calcite. The geode contact with the country rock is delimited by a very fine-grained celadonite rim. Celadonite Rb/Sr ages (107[plus or minus]1 to 81 [plus or minus]1 Ma) are younger than the basalts (127 to 137 Ma) (Innocent et al., 1997).

The different calcite generations show three types of REE patterns (1) patterns with a continuous decrease in the normalized REE contents from the LREE to the HREE, with a marked to strong negative Eu anomaly and no Ce anomaly; (2) patterns with a continuous decrease in the normalized REE contents from the LREE to the HREE without an Eu anomaly but with a negative Ce anomaly; and (3) patterns with or without any anomaly but with slightly to markedly increased HREE contents in comparison to the LREE contents.

The negative Ce anomaly, the reddening of the "basalto portador" (country rock), and the presence of gypsum suggest that calcite crystallized under oxidizing conditions. The negative Eu anomaly is most likely inherited from the basaltic country rocks. Calcite with increased HREE contents probably crystallized from fluids which dissolved an older calcite generation. Based on age determinations for celadonite along with the low crystallization temperature of amethyst and calcite (30-50 [degrees]C; Juchem et al., 1999), it appears likely that the geodes were filled with amethyst, calcite and gypsum well after the crystallization and complete cooling of the lavas by fluids of meteoric origin.

References

INNOCENT, C., PARRON, C., and HAMELIN, B. (1997) Rb/Sr chronology and crystal chemistry of celadonites from the Parana continental tholeiites, Brazil. Geochim. Cosmochim. Acta, 62, 3753-3761.

JUCHEM, P. L., FALLICK, A. E., BEETTENCOURT, J. S., and SVISERO, D. P. (1999) Geoquimica isotopica de oxigenio em geodos mineralizados a ametista da regiao do Alto Uruguai, RS--um estudio preliminar. Boletim de Resumos, 1[degrees] simposio sobre vulcanismo e ambientes associados, 13-18 junho 1999, Gramado.

Skarn Minerals from Rio Grande Do Norte State, Brazil

A. Bhaskara Rao

Maria S. Adusumilli

Claudio de Castro

The Ca-skarns (tactites) in Rio Grande do Norte State host scheelite orebodies that have been exploited intermittently since World War II. Recently, renewed mining activity has resulted in discoveries of additional ore material as well as fine mineral specimens. The old mine Malhada dos Angicos near Parelhas, a deep underground mine, is an example of this revitalization.

The Ca-skarns are found as beds, layers and lenses in Mg-rich crystalline limestones that form saddle reefs. They are part of a late Proterozoid metasedimentary sequence with underlying paragneisses and overlying biotite schists intercalated in feldspathized (K-feldspar porphyroblasts) paragneisses. The origin of the skarn mineralization has been attributed to metasomatism with later high-temperature hydrothermal effects, succeeded by secondary alterations at lower temperature.

Actinolite, garnet (andradite, grossularite and almandine), calcite (fine scalenohedral crystals in vugs), diopside (salite), epidote, hematite, hornblende, magnetite, quartz, biotite, muscovite, phlogopite, microcline, tremolite, vesuvianite and zoisite occur as major minerals in the skarn bodies. In addition to the W and Mo ore minerals scheelite (occurring in white, gray, black, and yellow crystals) and molybdenite, the skarns also host a number of interesting accessory minerals including: allanite, anhydrite, chalcocite, chalcopyrite, clinohumite, covellite, ilmenite, bismuthmite, marcasite, pyrite, pyrrhotite and scapolite. Late-stage minerals and alteration products include: heulandite, analcime, aragonite, azurite (rare), bismoclite(?), bismutite, chabazite, chrysocolla, cuprite, goethite, gypsum, huntite, malachite, molybdite, nontronite, stilbite and thulite.

Among these mineral species, a large number afford fine collector specimens.

Gem Pegmatites of the Ural Mountains, Russia

Peter Lyckberg

Viktor Ye. Zagorsky

The most famous gem pegmatites of Russia are those of the central Ural Mountains. The northern region is comprised of miarolitic pegmatites carrying topaz, beryl and tourmaline in the western endo and exocontact of the Mursinka-Adui intrusions and the desilicated pegmatites surrounded by emerald-alexandrite-phenakite mineralization.

The Alabashka pegmatite field is known for fine blue topaz, associated with smoky quartz and albite, from the Mokrusha mine. Heliodor crystals come from the Golodnij, Tyazhitsnitza, Mokrusha, Staraja Mylnitsa, Starzewa Jama and the Kazionnitsa mines. One of the finest specimens ever recovered from the Staraja Mylnitsa mine (found in 1991) consisted of two parallel-growth crystals (8 x 1.5 cm) on an albite ball. From 1990-1993 the Kazionnitsa mine produced topaz and world-class green beryl and gem heliodor. The finest specimens include a flawless electric-green crystal measuring 7 x 3 cm and a color-changing transparent, blue-green crystal 15.6 x 5 cm perched on two flat smoky quartz crystals.

Gem pegmatites northwest of the village of Mursinka are enclosed in serpentinite and contain miaroles with colored tourmaline. The pegmatites to the west and southwest contain green and yellow beryl and yellow or light blue topaz. The Sarapulka and Shaitanka pegmatites are irregular thin veins containing rubellite and rhodizite in the west and green and blue aquamarines in the east. The Adui field has produced two major pockets of large green aquamarine.

The Takovaja district schists have produced large quantities of large emerald, alexandrite and phenakite specimens since 1830. In 1995 mining of an open pit at the site of the old Sretensky mine resumed, but production was low and mining has now ceased. Rare recent specimens are combinations of phenakite/emerald and alexandrite/emerald.

The southern region around Miass in the Ilmen Mountains was heavily mined during the 19th century and produced primarily clear topaz crystals (3-4 kg) of complex forms and rare green beryls (to 25 cm) associated with amazonite in miarolitic pegmatites.
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Author:Kampf, Anthony R.; Simmons, William B.
Publication:The Mineralogical Record
Date:Mar 1, 2000
Words:4929
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