Botelho et al.,1994

Yanomamite, In(AsO4)2H2O, a new indium mineral from topaz-bearing greisen in the Goiás Tin Province, Brazil

Botelho,N.F.
Roger,G.
D'Yvoire,F.
Möelo,Y.
Volfinger,M.

European Journal of Mineralogy(1994), 6: 245-254.

Palavras chave: Arsenato, Granito, Greisen, Indio, Sulfeto

Abstract

Yanomamite occurs as green to yellow-green 0.2 mm dipyramidal crystals in aggregates associated with scorodite in quartz-topaz greisen in the tin deposit of Mangabeira, Goiás State, Brazil. Yanomamite crystals are always coated by a film of In-rich scorodite. Electron microprobe analysis gave In₂0₃ 45.80, Fe₂0₃ 1.11, Al₂0₃ 0.21, As₂0₅0.10, H₂0 (by difference) 12.78, sum 100 wt%, corresponding to (In_O.94Fe_0.04Al_0.01) As04.2.02H₂0. Yanomamite is isotypic with scorodite, and belongs to the variscite series. The mineral has vitreous luster, white streak, VHN₂₅= 631 (571-743) kg/mm², and is soluble in strong HCl or H₂S0₄. Dcalc = 3.876(3) glcm³ for Z = 8. Yanomamite is colorless in transmitted light, biaxial positive, and has 2 V = 55-76°, average n = 1.65, orientation X = a, Y = c, Z = b. Single-crystal X-ray study indicated orthorhombic symmetry, space group Pbca, a = 10.446(6), b = 9.085(4), c = 10.345(6) A as refined from the powder pattern; strongest lines (CoKal radiation) are 5.70(70, 111), 4.53(100,020,210), 4.163(50,021), 3.874(60,121), 3.250(60,122), and 3.110(50,311), in good agreement with data for synthetic InAs0₄.2H₂0.
The Mangabeira tin deposit contains several In-bearing minerals, including sphalerite with up to 11 wt% In. The maximum In content found in scorodite overgrowths is 6 wt% In₂0₃, corresponding to Fe_O.90In_0.l0; maximum Fe substitution in yanomamite is In_0.85Fe_0.l5, suggesting a possible miscibility gap. The origin of yanomamite is thought to be related to a hydrothermal event rather than later oxidation, formed by alteration of early arsenopyrite and In-rich sphalerite. The new name relates to the Yanomami Indians of the Amazon. Type material is in the museum of the Ecole Nationale Superieure des Mines de Paris, France, and in the Mineralogical Collection of the Instituto de Geociencias, Universidade de Brasília, Brazil.

Ferreira Filho et al.,1994

Abstract

The Niquelandia intrusion is among the largest layered intrusions in the world. It is part of a 350 km long belt of large mafic-ultramafic complexes in central Brazil. Both intrusion and country rocks were subjected to high-grade metamorphism ranging from amphibolite to granulite facies, and associated ductile deformation. The first U-Pb zircon and rutile ages for this region are presented. Zircon fractions from a meta-anorthosite from the upper gabbro-anorthositic unit are variably discordant (0-28% with regard to the lower intercept) along a mixing line from 777+ or -10 Ma lower intercept and 1583+ or -25 Ma upper intercept. Zircon fractions from a later quartz diorite within the Central Mafic Unit are variably discordant (0-14% with regard to the lower intercept) along a mixing line from 794 (super +4) (sub -6) lower intercept and 1565+ or -22 upper intercept. The upper intercepts for both rocks, interpreted to be the magmatic age, overlap and suggest a Mesoproterozoic age for the intrusion. The lower intercept is interpreted to be the high-grade metamorphic event. Zircons from two quartz-rich blastomylonites representing different metamorphic domains and cutting through the layered intrusions were analyzed. A single zircon from the amphibolite facies domain blastomylonite gives a concordant (-0.3%) ²⁰⁷Pb/²⁰⁶Pb age of 777+ or -3 Ma, and a single zircon from the granulite facies blastomylonite gives a ²⁰⁷Pb/²⁰⁶Pb age of 786+ or -5 Ma (-0.8% discordance). Both results are consistent with the lower intercept ages and set the 770-795 Ma interval for the high-grade metamorphic event. Rutile ages (approximately 650 Ma) are considerably younger than zircon metamorphic ages and suggest a milder regional tectonomagmatic episode or a complex and long-lasting cooling history for the region. The U-Pb data provide evidence for an important Mesoproterozoic extensional tectonic event in central Brazil. It also sets the 770-795 Ma interval for the high-grade amphibolite to granulite facies prograde metamorphism, interpreted to represent the timing of a continental collision event.