Petrology and geochemistry of the transamazonian granites of Campo Formoso and Carnaiba (Bahia, Brazil) and of the associated emerald-bearing phlogopites
Pétrologie et géochimie des granites transamazoniens de Campo Formoso et Carnaiba (Bahia, Brésil), et des phlogopites à émeraudes associées
Résumé
The granitic massifs of Campo Formoso and Carnaiba are located in the Serra de Jacobina (Bahia, Brazil); they are intrusive at 1,9 Ga in the archean basement of the São Francisco craton, and in the Lower Proterozoic cover. The granitic massifs are formed by multi-stage intrusions of two-micas granites, muscovite-garnet granites and aplopegmatites. Geochemical data show that the Campo Formoso massif is made of two non-comagmatic evolutive suites whereas the Carnaiba massif is made of only one comagmatic suite. Each suite evolves from two-micas granites to muscovite-garnet granites and aplopegmatites. For each suite, the evolution of the composition of the rocks and minerals (biotites, garnets and muscovites) can result from a process of fractional cristallization and a process of mixing between silicate melt and cumulate, generally close to a total cumulate. The occurence of drops in Be, Rb, K and Si contents is supposed to result from the demixion of a fluid phase from the magma, permiting the extraction of these elements from the melt and favouring the fracturing of the enclosing rocks. The geochemical differences between the two massifs concern the evolution of the contents in hygromagmaphile elements. The Carnaiba massif, which is surrounded by the most numerous emerald-ores, corresponds to a magma where these elements are more abundant than in Campo Formoso in the less evolved terms (Rb), and above all more enriched during differenciation (Rb, Nb and Be). The discovery in the northeastern part of the Campo Formoso massif, close to the emerald-ores of Socotó, of a granite not belonging to any suite of Campo Formoso and very similar to the Carnaiba granite, suggests the occurence of a Carnaiba-like intrusion. However, the Be contents of the magma remains relatively low, even in Carnaiba (average 8 ppm), when comparing with the so-called "specialized granites". Fluids of magmatic origin develop a metasomatic zoning on several meters around aplopegmatitic veins intrusive into the enclosing serpentinites of the granites: The replacement of the primary minerals of the serpentinites lead to the neoformation, in the altered zones, of a magnesian mineral association, where phlogopite is the most abundant phase (phlogopitites) ; the aplopegmatite is transformed in plagioclasite (albite to andesine) with phlogopite in various proportions and sometimes amphibole (magnesio-hornblende to edenitic-hornblende), epidote and chromite. The phlogopitites consist of five zones, from the central aplopegmatitic vein to the serpentinite: (*) zone 4B with phlogopite (± apatite) (*) zone 4A with phlogopite-spinel (*) zone 3 with phlogopite-spinel-amphibole (*) zone 2 with phlogopite-spinel-amphibole-talc (*) zone 1 with spinel-amphibole (or dolomite)-talc-serpentine-chlorite. From the inner zone to the outer zone: (¤) phlogopite composition evolves with drops by a decrease of Al and Fe and an increase of Si, Mg and K/Al, corresponding to the changes in the petrographic habitus between the successive microzones. (¤) amphibole evolves from an actinolite to a tremolite (to tremolitic hornblende) (¤) spinel evolves from an aluminous chromite to a chromiferous magnetite. Chemical mass balances indicate: (¤) a supply of many elements Al, K, F, Si, Fe, Rb, Be, Cs, Li, ... but also of Ti, Nb and REE (¤) a leaching of Mg, Ca, H20, Cr, ... but also of Th and HREE (¤) that some elements remain inert (Zr and Sc all along the metasomatic column, Ti and Fe all along the column except in the 4B-zone). (¤) that there is no significative variations in volume during metasomatism, at least from zone 1 to zone 4A. The secondary alteration phases (silicification and sericitization of the plagioclasites, muscovitization and chloritization of the phlogopitites) are developped probably from the same fluids which became acidic at lower temperature. The mineralization consists essentially of beryl and molybdenite, associated with the primary alteration phase and partly with the muscovitization phase. The occurence of chromiferous beryl (emerald) is linked to the availability of alumina (zone 4B and plagioclasite) and chromium solubilized during percolation. The occurence of emerald-mineralization is supposed to be rather due to the efficiency of the metasomatic trap than to a magmatic preenrichment in Be: The occurence of strong chemical gradients in the zones of preferential circulation of the solutions (zone 4B and plagioclasite) constitutes probably highly favourable conditions for the beryllium concentration.
Les massifs granitiques sont formés d'intrusions polyphasées constituées de granites à deux micas, granites à muscovite-grenat et aplopegmatites. Trois suites non comagmatiques évoluant des granites à deux micas aux granites à muscovite-grenat et aux aplopegmatites sont mises en évidence : Deux suites à Campo Formoso (une suite précoce en position périphérique et une suite tardive formant le coeur du massif) et une à Carnaiba. Dans chaque suite, l'évolution du chimisme des roches et des minéraux est compatible avec un modèle de cristallisation fractionnée combiné avec un mélange entre liquide silicate et cumulat, proche d'un cumulat total dans la plupart des cas. Le caractère peralumineux des granites est acquis au cours d'évolution. Les granites de la suite précoce de Campo Formaso ont subi un processus de contamination (Mg, Ni, Co, Cr et V).les phlogopites comportent cinq zones. Des zones internes vers les zones externes, la composition de la phlogopite évolue avec des sauts, à Al et Fe décroissants, Si, Mg et K/Al croissants, auxquels correspondent des fronts séparant des zones de phlogopitites à habitus pétrographiques différents. Parallèlement, l'amphibole évolue depuis une actinote jusqu'à une trémolite (à hornblende trémolitique) et le spinelle d'une chromite alumineuse à une magnétite chromifère. La minéralisation est essentiellement à béryl et molybdénite, associée à la phase d'altération primaire et en partie à la phase de muscovitisation. L'occurence de béryl chromifère (émeraude) est liée à la disponibilité de l'alumine (zone 4b et plagioclasite), et du chrome mis en solution lors de la percolation. Le préenrichissement magmatique en Be étant peu efficace, la présence de minéralisations en émeraudes est plutôt liée à l'efficacité du piège métasomatique, tant pour expliquer la quantité, somme toute peu élevée, de béryl que la qualité des gemmes : L'existence de forts gradients chimiques.