Etude de deux massifs de la province granitique stannifère de l'état de Goias (Brésil) et des formations métasomatiques associées aux minéralisations en Sn et Be
Résumé
The tin- bearing granites in the State of Goias, Brazil, form two groups of different ages: 1770 My for the Sucuri and Soledade massifs, 1658 - 1614 My. for the Serra Dourada, Serra da Mesa and Serra Branca massifs. The geology of the province suggests that these granites were emplaced in a rift context. The Sucuri and Soledade massifs are emplaced in gneisses of Archean age, the others massifs are intrusive in the Serra da Mesa Group, a sedimentary formation composed of pelites and greywackes. This Group was metamorphosed in the amphibolite facies (kyanite- almandine- biotite paragenesis). According to several thermobarometric tools, the temperature was around 540°C and the pressure between 5 and 7 kBar. The age of the sediments is supposed to be lower Proterozoic, in accordance with the age proposed for the Serra Dourada massif (1653 My.. The Serra Dourada massif is composed of four rock types. The petrographic, mineralogic observations and geochemical data reveal a main trend, composed of an amphibole- bearing facies, a biotite- bearing facies, and a fine grained porphyritic facies. . Garnet- bearing granites do not show any relations with the main trend. The contents of these granites in Nb, Y, Zr, Ce, La, Ga, and their MgO/TiO2 ratio (around 1) are similar to those of the anorogenic granites of South Western United States and Gabo suite of Australia. However, niobium shows a compatible behaviour in Serra Dourada, in contrast to its incompatible behaviour in many anorogenic granites... // ...The compositional variations of the beryllium minerals observed in skarns may be explained by the following processes : -either a late arrival of silica- rich solutions (high Si/Al ratio, presence of phenacite or helvite) within early formed alumina- rich parageneses (musgravite, chrysoberyl), - or a limited mobility of Al relatively to Si, which would induce a limited extension of the aluminous parageneses (musgravite, chrysoberyl). The activity of aluminum plays an essential role in the stability of the beryllium minerals : phenacite and/or helvite are stable when it is low, and beryl, chrysoberyl or musgravite when it is high. However, the increase of activity of other elements (Ca, Mn, Fe, Zn, alkalis) that would induce the formation of minerals incorporating alumina (garnet or felspars) would be unfavourable to the formation of beryl. Helvite is thus typically stable in calcic skarns, and it is associated with phenacite in the rocks submitted to alkaline metasomatism. Beryl, on the contrary, normally appears when acidic fluids separates from highly evolved granitic magmas (aplopegmatitic stage). The zonation of helvite crystals in albitites (genthelvite core, danalite border) reflects the decrease of alkalinity of the fluids. This zonation on the crystal scale is also observed on the outcrop scale: helvite composition varies from Ge64Da10He26 in the internal zone to Ge23Da50He26, in the external zone. The same variation is observed in an albitized siderophyllite vein, from Ge25Da42He33 in a slightly albitized zone to Ge55Da39He22 in a highly albitized one. The formation of genthelvite is favoured, as compared with the other members of the helvite group, by alkaline conditions. The compositional variations of helvite in the Sucuri massif would result from variations in fs2 and f02, in the fluids. Genthelvite is stable under alkaline and oxidizing conditions.
Les granites stannifères de l'état de Goias forment deux groupes d'âge different. Les massifs de Sucuri et de Soledade ont un âge de 1770 Ma et les massifs de Serra Dourada, Serra da Mesa et Serra Branca ont respectivement 1653, 1614 et 1658 Ma. La géologie de cette province conduit à situer la mise en place de ces granites dans la zone de Rift. Les éléments traces Nb, Y, Zr, Ce, La et Ga et le rapport MgO/TiO2 sont comparables à ceux des granites anorogéniques du sud-ouest des Etats-Unis et de la suite de Gabo en Australie. Les valeurs faibles du rapport Zr/Hf (23 à 28) des massifs étudiés permettent d'exclure une origine directe par fusion partielle, le rapport crustal par fusion est compris entre 30 et 40. La zonalité minéralogique (bordure à biotite et coeur à pyroxène) des lentilles à pyroxène du gisement de Mata Azul (massif de Serra Dourada) traduit le caractère métasomatique de ces roches et rappelle celles des endoskarns. Les études pétrographiques, minéralogiques et géochimiques ont mis en évidence l'importance du rôle du calcium et de la fugacité d'oxygène. A l'exception des albitites, la majorité des zones métasomatiques et les veines minéralisées sont localisées dans la partie apicale du granite fin au nord du massif de Sucuri. Les minéraux du béryllium (béryl, phénacite et helvite) sont présents dans la veine à quartz-muscovite, dans la veine à sidérophyllite. dans l'albitite, seules la phénacite et l'helvite sont présentes. Les variations de composition des minéraux de l'helvite dans le massif de Sucuri résulteraient des variations de la fugacité de soufre et de celle de l'oxygène et de l'alcalinité à l'intérieur du fluide.