Depósito de W-SN igarapé manteiga: geologia e metalogênese

Abstract

The Igarapé Maneiga W-Sn deposit is composed of a granitic intrusive stock in Paleoproterozoic basement rocks (Jamari Complex) and covered by alluvial-colluvial sediments. The stock granite is formed by two petrographic types: biotite-alkali feldspar granite and topaz-albite granite. The precursor stage magmatic mineralization is attributed to the topaz-albite granite, which displays geochemical signature peraluminous, subalkaline with medium K2O and REE distribution in concave pattern, with negative anomalies in Eu (Eu/EuN = 0.07) and with fractionation absence between LREE and HREE ([La /Yb]N = 0.8). The W-Sn ore (wolframite and cassiterite) is associated with greisens, breccia and veins-veinlets systems. In the greisens the wolframite and cassiterira crystals occur associated with quartz, topaz, Li-mica, fluorite, sphalerite, pyrite, marcasite, pyrrhotite, galena, chalcopyrite, molybdenite, bismuthinite, siderite, phengite, monazite, xenotime and hematite. The ore in the breccia occurs distributed so widespread and irregular, while in the veins-veinlets system can be identified at least two generations, usually surrounded by Li-mica crystals arrangement in comb and stocksheider structures, with the ore occupying both the edges as the central parts associated with pyrite, chalcopyrite, galena, molybdenite, bismuthinite and siderite. The preliminary study about fluid inclusions in greisens has recognized two fluid systems: H2O-NaCl and H2O-CO2-NaCl. The system H2O-NaCl is dominant, which has low density (0.8 to 0.9 g/cm3) and salinity between 0.1 to 6.7 wt% NaCl, while the system H2O-CO2-NaCl has salinity 9-9.8 wt% NaCl. Overall, the main temperature range of total homogenization of the fluid system is between 250 and 300º C. Analyses of oxygen (δ18O) and sulfur (δ34S) isotopes show that the ore hydrothermal fluid is likely magmatic, whose temperature isotopic to crystallization of the mineral pair wolframite-quartz is between 220 and 270° C, consistent with the preliminary data from fluid inclusions studies. The 40Ar/39Ar ages obtained for the Li-micas from greisens determine value of 988 ± 5 Ma, interpreted as the age-cooling of the system hydrothermal, which occurred around 350° C. The geological evolution proposed to the Igarapé Manteiga W-Sn deposit involved in the late-stage of crystallization of the topaz-albite granite magmatic phase, a segregation process of two stages: magmatic and hydrothermal. The hydrothermal stage driven by volatile adduced vast majority of incompatible elements, lodging at the interface granite and host-rocks. The tension in the hall host- rock provided a burst relieving of tension and generating fractures that have spread over the granite dome, producing hydrothermal breccia and a second generation of veins-veinlets systems, lowering the temperature in the hydrothermal fluid and provided the precipitation of ore-metallic content. This process took place under a temperature range 200-300º C, and confining pressure estimated at between 0.5-1.5 kbar and about of 988 ± 5 Ma.