Hartmut Beurlen

The competing models for the origin and internal evolution of granitic pegmatites in the light of melt and fluid inclusion research

In this paper we discuss the main petrogenetic models for granitic pegmatites and how these models have evolved over time. We suggest that the present state of knowledge requires that some aspects of these models to be modified, or absorbed into newer ones. Pegmatite formation and internal evolution have long supposed the need for highly water- and flux-enriched magmas to explain the differences between pegmatites and other intrusives of similar major element composition. Compositions and textural characteristics of fluid and melt inclusions in pegmatite minerals provide strong evidence for such magmas. Furthermore, we show that melt inclusion research has increased the number of potential flux components, which may include H2O, OH−, CO2, HCO3−, CO32−, SO42−, PO43−, H3BO3, F , and Cl, as well as the elements Li, Na, K, Rb, Cs, and Be, herein described as melt structure modifiers. In this paper we emphasize that the combined effect which these components have on the properties of pegmatite melts is difficult to deduce from experimental studies using only a limited number of these components. The combination and the amount of the different magmatic species, together with differences in the source region, and variations in pressure and temperature cause the great diversity of the pegmatites observed. Some volatile species, such as CO32− and alkalis, have the capacity to increase the solubility of H2O in silicate melt to an extraordinary degree, to the extent that melt-melt-fluid immiscibility becomes inevitable. It is our view that the formation of pegmatites is connected with the complex interplay of many factors.

Borborema Pegmatitic Province: geological and geochemical characteristics

Abstract The geological features of the pegmatites from the Borborerna Pegmatitic Province (BPP) are described, combining data from the literature with new field and laboratory observation. A geochemical study was performed against this geological background to test the crystallization model of mineralized pegmatites against barren pegmatites and to compare the BPP with other provinces, fields or individual pegmatites throughout the world. The field evidence (mode of emplacement, textural relationships, zonation and mineralogy) as well as the geochemical characteristics support the pegmatite crystallization model proposed by London (1990). The geochemical and mineralogical evidence places these pegmatites as “medium Ta-mineralized” compared to other pegmatite provinces.

Fluid inclusion microthermometry in Be–Ta–(Li–Sn)-bearing pegmatites from the Borborema Province, Northeast Brazil

Abstract Fluid inclusions in Be–Ta–Li–Sn-bearing pegmatites have been reported in the literature as hypersaline aqueous brines or low-salinity aqueous or aqueous–carbonic fluids. The evolution paths of volatile fluids in equilibrium with the different crystallizing phases of rare-element pegmatites do not seem to be unique. The aim of this work is to contribute to this discussion, on the basis of fluid inclusion information from the Borborema Pegmatite Province (BPP) in Northeast Brazil. Some well-known heterogeneous Be–Ta–(Li–Sn) mineralized pegmatites of this province were systematically sampled along cross sections with the purpose of studying the fluid inclusions in the different pegmatite zones (I, muscovite-rich contact zone; II, homogeneous or graphic pegmatite; III, blocky feldspar zone; and IV, quartz core). The observed fluid inclusions (mainly in quartz) were almost always secondary or pseudosecondary, grouped along healed fractures with random orientation. Even some trails paralleling quartz grain limits were not undoubtedly primary. Almost all these fluid inclusions were filled with low-salinity aqueous or aqueous–carbonic fluids. Primary inclusions were recognized only in some well-formed crystals from druses with erratic distribution in the pegmatites. Microthermometric data from these samples confirm that the various crystallizing phases of the pegmatites were in equilibrium with low-salinity aqueous or aqueous–carbonic fluids (with insignificant N 2 or CH 4 contents) with frequent accidentally trapped solids. The various types of primary inclusions found in these crystals compare well with the types of secondary inclusions of the samples from the systematic cross sections. The relative age of the crystals hosting primary fluid inclusions, the relative age of primary inclusion types in successive crystal-growth zones, the relative age of secondary fluid inclusion trails and the corresponding microthermometric data allowed to establish the most probable evolution of the fluid characteristics during pegmatite crystallization. An early low-salinity aqueous–carbonic fluid with CO 2 contents of 20 mol% was present during the crystallization of early minerals, such as garnet and tourmaline of the border zone and graphic quartz, beryl, manganotantalite, and blue tourmaline of zones II and III. The end of zone III crystallization occurred in the presence of a low-salinity, aqueous fluid with very low CO 2 contents, as indicated by inclusions in the smoky core of quartz crystals at the limit between zones III and IV. Later aqueous fluids barren in CO 2 and with salinities increasing from 5% to 20% coexisted with the formation of the pegmatite nucleus and replacement bodies, as indicated by inclusions in the border of these crystals and in euclase. The estimated trapping conditions of 3.5 kbar and 580°C for the earliest inclusion types agree with the regional metamorphic conditions and with the known liquidus conditions of rare-element-bearing pegmatite crystallization. The trapping conditions of inclusions in euclase and coexisting quartz crystal rims in the pegmatitic core, at 3.5 kbar and 400°C are slightly below the conditions admitted for the solidus of rare-element pegmatites and indicate an isobaric cooling during pegmatite crystallization.

Petrogenetic significance of LA-ICP-MS trace-element data on quartz from the Borborema Pegmatite Province, northeast Brazil

Abstract Quartz from different zones within five granitic pegmatites of the rare-element class from the Borborema Pegmatite Province in northeast Brazil were analysed for fourteen trace elements using laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS). The concentrations of Li (6-150 ppm), B (1-9 ppm) and Ge (1-23 ppm) in quartz show a positive correlation with Al (30-770 ppm). The concentrations of these elements increase from the border zone to the quartz core of pegmatites of the spodumene or lepidolite subtypes. The Ge concentrations in the quartz core are the highest so far reported in igneous quartz. In the less evolved pegmatites of the beryl-columbite subtype, the Al, Li, B, and Ge concentrations in quartz from all zones remain at the same level as the border and wall zones. The Ti concentrations in quartz from the core of the more evolved pegmatites are below 3 ppm (with Al >250 ppm), contrasting with 7-25 ppm (with Al <280 ppm) in samples from the border and wall zones of the less evolved and more evolved pegmatites. The concentrations of Al, Li, B, Ge, and Ti in quartz are therefore confirmed as good indicators of the degree of magma fractionation and analyses of pegmatite quartz cores can be used for exploration purposes to distinguish pegmatites with high metallogenic potential. Atoms of Li and Al are incorporated into quartz such that Li/Al ranges between 0.75 and 1.0. This suggests a coupled substitution of the form Si4+ ↔ (Li+ + Al3+). The other elements analysed either showed an erratic distribution (e.g. Be and P) or were below the respective limits of detection (Na, K, Rb, Ca, Sr, Mn, Fe) in most samples.

The mineral resources of the Borborema Province in Northeastern Brazil and its sedimentary cover: a review

Abstract The geotectonic Borborema Province in Northeast Brazil includes several NE-trending Proterozoic metamorphic belts which amalgamate some small older massifs. It has been known since the Second World War for the skarn-hosted scheelite mineralization and the TaLiBeSn-bearing pegmatites of the Serido Region. With the drastic drop of the international tungsten prices, three mines were closed in the last decade and only the state-owned Bodo Mine remains working. Since 1961, when Brazilian universities begun to graduate geologists, several dozen mineral deposits were discovered and older prospects were reevaluated. These include the copper deposits of Aurora-State of Ceara (CE) and Serrote da Lage-State of Alagoas (AL), the nickel deposits of S. Joao do Piaui-State of Piaui (PI), the uranium deposits of Itataia-CE, a dozen gold prospects, the ilmenite deposit of Floresta-State of Pernambuco (PE), the vermiculite mine of Paulistana-PI, and the export quality granite at Bom Jardim-PE and Sume-Congo-State of Paraiba (PB). In the mostly Cretaceous or Cainozoic sedimentary cover, the discovery includes two important oil and gas fields ((Carmopolis-State of Sergipe (SE) and Mossoro-Macau-State of Rio Grande do Norte (RN)); deposits of sulphur (Castanhal-SE), of KNaMg, salts (Carmopolis-SE, Sto. Antonio-AL), of phosphate (Olinda-PE and Joao Pessoa-PB), of gypsum (Araripe-PE/CE), of bentonite (Boa Vista-PB) and the Ti placers at Mataraca-PB. In the following pages the current knowledge about these deposits is summarized.

Chemical composition of gahnite and degree of pegmatitic fractionation in the Borborema Pegmatitic Province, northeastern Brazil

Gahnite electron probe microanalyses from three pegmatites (Mirador, Capoeira and Quintos) of the Borborema Pegmatitic Province in northeastern Brazil allowed to determine molar compositions ranging from 86.22 to 95.41 mol% gahnite (ZnAl2O4), 1.51 to 6.62 mol% hercynite (Fe 2+ Al2O4), 0.11 to 5.89 mol% spinel (MgAl2O4), 1.48 to 2.74 mol% galaxite (MnAl2O4), and Zn/FeTotal atomic ratios ranging from 11.53 to 60.87. These data compared with gahnite compositions from albite subtype pegmatites from Cap de Creus, Spain (Zn/FeTotal between 10 and 60) and from the source granite of the Separation Rapids pegmatite field, Ontario, Canada (Zn/FeTotal =3.5) indicate a high degree of fractionation of the studied pegmatites. This interpretation agrees with conclusions based on compositional data of tourmaline (vacancy in X-site 0.42 to 0.49), white mica (K/Rb between 8 and 33), feldspar (K/Rb between 8 and 109), garnet (Mn/(Mn+Fe 2+ ) between 0.93 and 0.97) and in Nb-Ta oxides (Mn/(Mn+Fe 2+ ) > 0.5) in the same pegmatites.

Mineral chemistry of tantalate species new in the Borborema Pegmatitic Province, Northeast Brazil

Tantalate samples, supposedly of the columbite group, were collected in the Borborema Pegmatitic Province, aiming to test the Mn/(Mn+Fe) and Ta/(Ta+Nb) ratios as geochemical indicators of pegmatite fractionation. Surprisingly, preliminary microprobe data allowed recognizing some species, so far unknown in the Province, namely titanian ixiolite, fersmite, brannerite, struverite, natrobistantite, plumbo- and stibiomicrolite, plumboand uranpyrochlore. The identification of these exotic tantalates with unusual composition, in addition to its distribution in several pegmatites, far from the classical Alto do Giz and Seridozinho pegmatites, indicate that the elevated degree of fractionation is not restricted to these two occurrences but may be reached in other pegmatite areas of the Province. It indicates also that this degree of fractionation may be very variable between pegmatites in small areas. The zoning patterns observed in the titanian ixiolite, with Ti and Nb enrichment at the borders at expense of Ta enriched in the core, are also quite unusual and reverse in comparison with the normal trend of progressive Ta and Mn enrichment in tantalates with the degree of fractionation. A similar ‘‘reverse’’ trend was observed in titanian wodginite of petalite/pollucite bearing pegmatites of the Separation Rapids Province in Ontario, Canada.

High-Pressure Metamorphosed Fe-Ti Ore-Hosting Island-Arc Tholeiites at Itatuba (Paraíba) as an Indication of a Proterozoic Suture in the Pajeú-Paraíba Fold Belt, Borborema Province, Northeastern Brazil

Petrological investigations on metamafic rocks and three associated Ti-Fe occurrences from Itatuba in the state of Paraiba are reported. The rocks analyzed here are part of a Proterozoic gneissic-migmatitic complex that includes some marble intercalations. Microtextural relations support a complex metamorphic evolution of three stages. A first stage, reflecting eclogitic conditions, is indicated by the lack of primary plagioclase in relics of garnet pyroxenite, with symplectitic intergrowths of pyroxenes and/or amphibole and secondary piagioclase replacing garnets and primary pyroxene, ostensibly omphacite. A second event at granulitic conditions is represented by the paragenesis of garnet-clinopyroxene-plagioclase. Equilibrium textures between hornblende-plagioclase are typical for the third stage under amphibolite-facies conditions. Mineral chemistry of the clinopyroxene-garnet pairs suggests equilibrium conditions in the range of 726° to 1185°C and 4.8 to 8.2 kb, established during the granulitic and a...

Auriferous Quartz Veins from Northeastern Brazil: A Fluid-Inclusion Study

Auriferous quartz veins are known to exist in more than two dozen prospects, encompassing an area of 500 km2 northward from Serrita township (state of Pernambuco) in northeastern Brazil. Gold-bearing veins occur either with a strike of 70° to 110°, crosscutting muscovite schists of the Middle Proterozoic Salgueiro Group, or with a strike of 330° in granodiorite intrusions in the same schists. Small amounts of pyrite, galena, arsenopyrite, chalcopyrite, and sphalerite commonly are observed. Sericite, chlorite, and epidote are the most common wall-rock alteration products. Fluid inclusions were studied in samples of mineralized quartz veins from the Barra Verde III prospect in a small granodiorite body, and from the Inga, Sabura, and Riacho do Meio prospects in the Salgueiro schists. Some samples of barren quartz veins also were studied for comparison. Primary and pseudosecondary inclusions in the mineralized veins are triphasic or biphasic aqueous-carbonic at room temperature. The wide range of the CO2/H2O...

VARIEDADES GEMOLÓGICAS DE MINERAIS DA PROVÍNCIA PEGMATÍTICA DA BORBOREMA, NE DO BRASIL: UMA SÍNTESE

Pegmatitos graniticos sao fonte de numerosos minerais com elevado potencial gerador de variedades de boa qualidade gemologica, com destaque para berilo, varias especies do super-grupo da turmalina, topazio, espodumenio, quartzo, feldspatos, alem de outros minerais menos comuns {p. ex. granadas, euclasio, gahnita, tantalita-(Mn), microlitas}. Na Provincia Pegmatitica da Borborema (PPB), alvo deste estudo, destacam-se as turmalinas, incluindo a famosa “turmalina Paraiba, berilo, quartzo, espessartita, gahnita, euclasio, como minerais de bom potencial gemologico. A formacao destas gemas na PPB, assim como na maioria das Provincias Pegmatiticas mundiais, e localizada em cavidades situadas preferencialmente na transicao entre a zona intermediaria interna e o nucleo de quartzo de pegmatitos heterogeneos mais diferenciados. A descricao das principais ocorrencias de gemas da PPB sera objeto deste trabalho.