W-skarns from rubelita, Northern Minas Gerais State, Brazil: fluids related to lithological evolution

Abstract

In the southern Rubelita area, mid-Jequitinhonha Valley, scheelite mineralizations associated to calc-silicate rocks (with quartz, pyroxene, amphibole and grossular) and apatite bearing quartz veins occur. These rocks were intruded by pegmatitic granitoids and muscovitebiotite granites now cropping out in the area. Microthermometry and micro-Raman analyses of early (primary?) fluid inclusions in quartz, titanite and grossular from calc-silicate matrix and quartz veins indicate the presence of nitro-carbonic phases. They consist of variable proportions of CH 4 and N 2 (X CH4 = 0.35 - 1.0; X N2 = 0.0 - 0.65). At a later stage, percolation of aquo-carbonic fluid in microfractures originated secondary fluid inclusions. This low salinity fluid is CO 2 dominated (X CO2 =0.95 - 1.0) with N 2 and CH in trace amounts. These solutions have been studied in quartz veins of the calc-silicate rocks and presented fO 2 between 10 -36 and 10 -37 bars, formed under 1500 bar pressure and temperatures between 200 and 300°C. It is assumed that the O 2 increase in the fluids is a consequence of muscovite-biotite-granitoid intrusion. Although direct fluid inclusions study on scheelite crystals have not been feasible, the tungstate precipitation from these late aquo-carbonic solutions was estimated, based on paragenetic relations. These fluids may have originated from granitoids bodies and may probably have reacted with the host schists leaching Ca and Fe from them.