Origem e evolução de depósitos de urânio na porção noroeste da província uranífera de lagoa real (pulr), bahia

Resumen

The Lagoa Real uranium province (LRUP) is localized at the northern portion of Araçuaí orogen in the inverted portion of Paramirim aulacogen. The LRUP is represented by the rocks of ca. 1.75 Ga of Lagoa Real intrusive suite (LRIS), also known as São Timóteo Granite, which are interlayered with metric pods and lenses of albitite. The albitite rocks are the main ore host may be mineralized or barren. This study covers the main geological aspects of the northwestern portion of LRUP represented by Gameleira I (AN 35), Barreiro (AN 31) and Barrinha (AN 34) deposits, presenting new petrographical, mineral chemistry, geochronological and whole-rock geochemical data. The LRSI is characterized as metaluminous, alkaline, reduced and ferroan. In the diagrams of geotectonic discrimination LRSI is A-type, intraplate and A2. In the studied deposits, LRSI is represented by the facies hypersolvus alkali-feldspar granite, which predominates perthitic orthoclase, hedenbergite, hastingsite, biotite and titanite. The calculated crystallization parameters indicate minimum liquidus temperature of 900 ºC and late-magmatic temperature between 660-690 ºC. The LRIS rocks are converted into albitite through two main stages of alteration, which developed under predominantly ductil regime. The first occurred before the Brasiliano orogeny and corresponds to the sodic-calcic and iron-calcic-magnesian alterations, which resulted in the crystallization of albite, clinopyroxene of variable composition (hedenbergite–augite–aegirine-augite), titanite, andradite and magnetite over the magmatic assemblage of São Timóteo Granite. These alterations are also acompanied by desilicification. The first stage developed under variable oxygen fugacity conditions and high temperature (680–400 ºC), probably during the late-magmátic stage of LRSI. The second alteration stage covers the syn-to-pos collisional phase related to Brasiliano orogeny and resulted in the potassic and late calcic alterations. The potassic alteration is represented by the crystallization of a retrograde assemblage, mainly amphibole and biotite, which composition have been controlled by the clinopyroxene precursor. The geothermometers indicate temperature between 650 and 400 ºC, and pressure between 3 and 0.9 kbar. The late calcic alteration, in turn, was responsible by the epidote, allanite and calcite precipitation and developed under ductil-to-brittle regime and maximum temperature of 300 ºC. Uraninite is intergrown with titanite and zircon veinlets concordantly to foliation. This assemblage is associated with diopside-bearing albitite. Uraninite give precise age of 580 Ma, coincident with Brasiliano sin collisional stage. Uraninite chemistry (low U/Th ratio and high Y contents) attests for high temperature of crystallization (> 450 ºC). During the Brasiliano tardi-to-post collisional stages (530–450 Ma), mineralization was redistributed as disseminations, associated mainly to retrograde assemblage, and altered to U+6 species, i.e. uranophane. Geochronological data obtained in this work and from literature shows that titanite and zircon ages are older and variable than those from uraninite. This variability is not an artefact, as the ages are coincident with the multiple rift events occurred in the Paramirim aulacogen between ~1750 and 580 Ma. The results, therefore, show the multistage character of uranium mineralization, which had the onset during multiple geodynamic events, but ended in the Ediacaran during the Brasiliano orogeny. The ore geochemical fingerprint is characterized by high U, V, Nb, Ta, Zr/Hf, Mg and Na contents. The São Timóteo Granite is a presumable U source, but volcanic rocks associated to the rifting events should also be considered. Oxygen and hydrogen isotope data show that fluids were impoverished in 18Osmow, which shows the influence of meteoric water during the final stages of alteration.