MINERALOGICAL AND GEOCHEMICAL ZONING OF THE PEGMATITES

FROM THE ORIENTAL PEGMATITE PROVINCE (ARAÇUAÍ BELT),

MINAS GERAIS, BRAZIL

 

Fonseca, M.A.¹; Pires, F.R.M²; Ferreira. M.S.¹; Echeternach, M.²; Salum, M.J.³

 

1. Department of Geology, Federal University of  Ouro Preto, 35400-000, Ouro Preto – MG, Brazil - marco@degeo.ufop.br
2. Department of Geology, Federal University of Rio de Janeiro, 21949-900, Rio de Janeiro- RJ, Brazil - frpires@geologia.ufrj.br
3. Department of Mining Engineering, Federal University of Minas Gerais, 31.270-901, Belo Horizonte - MG, Brazil - gazzisalum@reitoria.ufmg.br

 

 

ABSTRACT
The pegmatites from the Oriental Province of Minas Gerais are characterized by their gem content (acquamarine, topaz, chrysoberyl, phosphate and lithium-bearing minerals, gigantic quartz crystals, murion and hyaline), besides the subordinate oxides (Sn, Nb, U) and sulfides (arsenopyrite, sphalerite). Apparently the pegmatites are related to the numerous proterozoic intrusive granites, granodiorites and tonalites in schists, gneisses, migmatites and charnockites. The granites may be grouped into (1) anorogenic (Caladão, Faísca, Itaipé, Caraí and Wolf), localized in the north-central part of the Province, and mineralized in acquamarine, topaz and chrysoberyl, and (2) syn- to late-kinematic (Galiléia, Urucum and São Vitor), with tourmaline, phosphate and lithium-bearing and metallic minerals. Mineralizations follow distinct trends according to the F-, B- and Li-contents in the fluids, Al/Si ratio and presence of Be. Pegmatite morphology varies, being canoe-shaped, as conformable bodies within the schists, and sub-horizontal sheets in the granites.

 

Keywords: Araçuaí Fold Belt, Oriental Pegmatite Province, pegmatites

 

INTRODUCTION

The Teófilo Otoni-Governador Valadares gem province, named as the Oriental Province of Pegmatite of Minas Gerais (Paiva, 1946), is known since the beginning of the 18^th century, and is famous by its acquamarine, tourmaline and collection specimens such as brazilianite, herderite, gormanite, axinite, kunzite, cleavelandite and others. More than thirty phosphate minerals have been described from the pegmatites (Lindberg & Pecora, 1958; Cassedanne & Cassedanne, 1982; Neves et al., 1980). During the 2^nd World War mica deposits have been intensively exploited (Pecora et al., 1950). Presently, it became important by the feldspar production for ceramics.

The Pegmatite Province is located within the limits of the Araçuai Belt, a Neoproterozoic fold and thrust belt that evolved in the eastern border of the São Francisco Craton.

The purpose of this paper is to address attention for the distribution of the pegmatites, its probable relationships with the granites, and its apparent mineralogical and geochemical zoning.

 

 

 

 

 

REGIONAL GEOLOGY

Geological formations, as well as mineralizations, appear to be controlled along a general north-south trend. The Espinhaço Belt, which corresponds to a sequence of Proterozoic quartzites and diamondiferous meta-conglomerates is bounded by strong westward (and cratonward!) thrust sheets. These thrusts brought up a complex lithological assemblage formed by gneiss, schist, BIF, gondite, greenstones and quartzite, along with intrusive granites. Parallel to the Espinhaço Belt there is another belt that can be named Amethyst Belt, distributed from Bahia State (Brejinho das Ametistas, Urandi) to Minas Gerais (Montezuma, Rio Pardo de Minas, Felício dos Santos, Ferros, Itabira and Rio Piracicaba). The next belt to the east corresponds to the granite zone.

The granites may be grouped into two main types, according to their compositions and time of emplacement: (1) North-central, post- to anorogenic, coarse- to medium grained, predominantly a microcline-orthoclase granite, locally xenolithic with round inclusion of charnockite and gneiss, distinct flow structure and weak magnetism. It is crosscut by pegmatites and quartz veins; (2) syn- to late kinematic granites, displaying varied textures and compositions from tonalite to granite (Nalini  Jr., 1997). Granites crosscut the São Tomé biotite schists, Archean metaultrabasites and gneisses, and its pegmatites may be grouped into a western, muscovite-rich zone (Mica Belt, Pecora et al., 1950) and an eastern, feldspar-rich zone (Feldspar Belt). Green, gem-type tourmaline and lithium minerals are more concentrated in the western zone, and schorlite dominates the eastern zone. The granite zone is bounded by a charnockite belt to the east (Figure 1).

 

PEGMATITES

Pegmatites from the North-central granites contain K-feldspar, quartz in graphic texture and as isolated, large and well formed crystals, biotite, garnet and subordinate albite and muscovite. Acquamarines found in these pegmatites are of the best quality, markedly at Ponto do Marambaia (Ferreira, 2003). Hyaline, colorless topaz occurs regularly, and blue topaz is only found at the Virgem da Lapa pegmatites (Proctor, 1985). Pegmatites from this zone evolved as an orthoclase-quartz graphic material exsolving to large orthoclase-microcline crystals and quartz, besides subordinate albite and long biotite flakes, by increasing the Al/Si ratio and the Fe-content in the fluids. After most of the biotite has crystallized the F-content in the fluid dictated the following path: under high F-content and increasing Al/Si ratios topaz would form, up to a dramatic decreasing in the F-content in the fluid, allowing crystallization of chrysoberyl. On the other hand, whether F-contents in the fluid are low, garnet will form with slight decrease in the Al/Si ratio. Persisting the Al/Si ratio decrease and with the increasing Be-concentration acquamarine would form. Up to this stage, brittle deformation is recorded in the minerals, such as micro-fissuring in topaz and acquamarine. Murion, hyaline, well formed quartz crystals, resulting from the ultimate Si- enrichment in the fluid, formed under no stress conditions (Figure 2).

Pegmatites from the Western zone, subdivided into the Feldspar and Mica Belts are enclosed in the São Tomé biotite-schist, as concordant, steep dip bodies, and in granites as subhorizontal, tabular bodies. Both types are zoned, being more evident in that hosted by the granite. In the Conselheiro Pena, Galiléia, Linópolis area the Feldspar Belt displays a myriad of phosphate minerals, such as those at the Sapucaia Mine (Lindberg & Pecora, 1958), including the famous brazilianite of Córrego Frio (Pough, 1945). It could be noted that the phosphate minerals are arranged in a zonal

 

 

 

 
Figure 1.  Zonation of granites and mineralizations:  North-central anorogenic granites: Caladão granite; Faísca and Wolf granites;  Itaipé and Caraí granites; Feldspar Belt: X = Syn- to late-tectonic granites (Galiléia, Urucum, São Vitor); Mixed Zone: O= pegmatoidal granites;  Schists; GN = Gneisses; X= Borrachudos granite; Localities: PA=Pedra Azul, Sa= Salinas, Me=Medina, Ib=Itaobim, CM= Coronel. Murta, A= Araçuaí, C= Caraí, P= Ponto do Marambaia, MN= Minas Novas, Cb= Carbonita, nc= Novo Cruzeiro, Cp= Capelinha, Ma= Malacacheta, TO= Teófilo Otoni, Sf= São José da Safira, Se= Serro, Gu= Guanhães, Go= Golconda, Mn= Mantena, GV= Governador Valadares, G= Galiléia, CP= Cons.Pena, IT= Itabira, NE= NovaEra, Cr= Caratinga, RP= RioPiracicaba, Co= Colatina, B= Bertópolis, BT= Belmonte, Cv= Caravelas, UM= Mucuri, V= Vitória.

 

framework in the Linópolis district, forming a brazilianite-herderite zone to the north and a triphylite-lazulite zone to the south (Pires et al., 2003). Apatite, amblygonite, souzalite, scorzalite, gormanite, childrenite-eosforite, vivianite, wolfeite and duffrenite occur in both zones (Neves et al., 1986). Tourmaline in this belt is schorlitic and beryl is non-gem quality. The pegmatites from this zone evolved from a K-feldspar quartz in graphic texture towards a microcline-orthoclase, quartz, albite, biotite, cleavelandite, muscovite path, under increasing Al/Si ratio (Figure 3). The mica deposition corresponds to a critical stage in the fluid, whether to form tourmaline or garnet depending on its B-contents. After the deposition of tourmaline the Al/Si ratio in the fluid decreased allowing the beryl and the primary phosphate to crystallize. The still Si-rich fluids allowed

 

Figure 2.  Al-Si-Na+K diagram showing the evolution of the pegmatite and the mineralizations
at the North-central granite zone (atom %).

 

the formation of murion and hyaline quartz, under no stress conditions.

The Mica Belt is developed westward from the North-Central granites and the Feldspar Belt. Golconda, the famous pegmatite locality, Cruzeiro, and the pegmatite districts of Capelinha, Malacacheta and Minas Novas represent this zone. The metamorphic grade of the country rocks (schist and quartzite) is lower than that of the Feldspar Belt and granite plutons are scarce or absent. Kyanite and staurolite are the principal metamorphic minerals. Remnants of ultrabasic rocks, some transformed into anthophyllite are more common in this belt than in the Feldspar Belt. Muscovite in the pegmatites are relatively abundant and feldspars are mostly weathered. Green tourmaline, morganite, spodumene, lepidolite, kunzite, beryl and herderite are the main minerals, and it is noteworthy the scarceness of phosphate minerals. Pegmatites are zoned with “ruby” mica, quartz and K-feldspar and minor schorlite, biotite, garnet at the border zone (Pecora et al., 1950). Mica shoots formed in the hanging-wall zone and murion and clear quartz, concentric muscovite, microcline and spodumene occupy the cores. Fibrous tourmaline (Coelho, 1948) has been described at Cruzeiro and fluorite was found at Campo Grande pegmatite, near São José da Safira.

Figure 3 Al-Si-Na+K diagram showing the evolution of the pegmatites and the mineralizations
at the Feldspar belt zone (atom %).

 

The Araçuaí-Coronel Murta-Salinas district consists of numerous pegmatites with Li-minerals (spodumene, petalite, amblygonite and lepidolite), beryl, blue, green tourmalines, rubelite, blue topaz (Limoeiro and Xanda pegmatites), acquamarine (Frade pegmatite) hosted in a non-deformed, coarse-grained pegmatoidal granite and biotite schists of the Macaúbas Group (Sá, 1977; Soares, 1984). We have interpreted this complex area (mixed zone) as a region where partial melting of the schist and the syn- to late-tectonic granites occurred during the plutonic activity resulting in the North-Central granite, which explains the overlapping of the mineralizations.

 

ACKNOWLEDGMENTS

We acknowledge the Progemas Project and FAPEMIG (GRANT EDT 1430/03) for sponsoring field and laboratory work.

 

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