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

摘要

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/TiO 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 GeDaHe in the internal zone to GeDaHe, in the external zone. The same variation is observed in an albitized siderophyllite vein, from GeDaHe in a slightly albitized zone to GeDaHe 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 fs and f, in the fluids. Genthelvite is stable under alkaline and oxidizing conditions.