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ISÓTOPOS DE OXIGÊNIO E TEMPERATURA DE FORMAÇÃO DOS VEIOS MINERALIZADOS COM WOLFRAMITA DA JAZIDA PEDRA PRETA, SUL DO PARÁ

1998; Sociedade Brasileira de Geologia; Volume: 28; Issue: 3 Linguagem: Inglês

10.5327/rbg.v28i3.628

2317-4889

Francisco Javier Ríos, Raimundo Netuno Villas, Kazuo Fuzikawa, Alcídes N. Sial, Gorki Mariano,

Geological and Geochemical Analysis

The Pedra Preta wolframite ore deposit contains the main known tungsten reserves of the Amazon. The wolframite occurs in a vein network that cuts both the Musa granite (1.88 Ga) and the Andorinhas greenstone belt host rocks (2.9 Ga). Oxygen isotopic studies were carried out on quartz of the Musa granite as well as on quartz, wolframite and white mica of the post-intrusive hydrothermal veins (PHV). In the lower levels of the ore deposit (at depths below 160m from the present-day erosion surface) similar isotopic values (δ 18 O quartz = 7-8o /oo have been found for quartz regardless of its mode of occurrence. The calculated δ 18 O fluid in equilibrium with quartz fell in the range of -3.16 to -0.10 o/oo from 240 to 300°C, suggesting that 18 O-depleted fluids had an important role in the PHV formation. These fluids correspond to those of aqueous-carbonic nature that were convectively driven to the Pedra Preta system and might have acquired a lighter oxygen isotope composition due to their long residence time in the surrounding metamorphic rocks. The PHV quartz from the upper levels show δ 18 O in the 9.04 - 9.67 o /oo range with the corresponding equilibrium δ 18 O fluid varying from 1.53 to -1.79 o/oo in that same thermal range. The δ 18 O increase in both quartz and fluid is believed to have resulted from the CO 2 -enrichment of the ascending fluids that had been previously oxidized at the granite apical zones. Wolframite and white mica (phengite) also sampled in the PHV presented δ 18 O values of 0.85 and 5.44 o /oo respectively. Oxygen isotope geothermometry of quartz-wolframite yielded temperatures of 321 °C (Shieh & Zhang 1991) and 356°C (Zhang et al. 1994) which are in fairly good agreement with the higher values determined by fluid inclusion microthermometry (230-350°C). Using the quartz-muscovite pair (Shieh & Zhang 1991), temperatures in the order of 410°C were obtained, but they seem to be unrealistic since the white mica (phengite) was one of the latest minerals to precipitate in the PHV.