TECTONIC AND FLUID CONTROL OF THE CACHOEIRA ALBITITE-TYPE URANIUM DEPOSIT, LAGOA REAL PROVINCE, CaetitÉ, BAHIA, BRAZIL

Cachoeira Uranium mine is a part of the Precambrian Lagoa Real Albitite Province which encircles five sub-vertical ore deposits and about twenty potentially important U-anomalies, sprawling along the north-south direction. Uraniferous albitite of Cachoeira occupies a large, mylonitized shearing zone marking, a recently discovered, structural discontinuity between a tight sequence of 10m-dimension, reclined folds and sub-parallel strong, horsetail splay shear zone system. The U-mineralizations form cigar-shaped (prolate) and secondarily pancake-shaped (oblate) albitite bodies, with steep dips along the discontinuous structural zone. Two or three, cigar-shaped U-rich bodies are distributed along the albitite in consequence of the strong mylonitization and pressure solution, which segregated a metasomatically derived albite-hornblende-uranium ore and ultramylonitic, low-silica skarn-type rock from the enclosing biotite microcline plagioclase gneiss, locally known as “Zebra Gneiss”. K-metasomatism produced concordant biotite-calcite-quartz veins along the shear zones. Brown chalcedony is late in the veinlets. Plagioclase breakdown produced the albitite under CO₂-rich fluids: 8 Anortite-2 Albite + 6 CO₂ + H₂O = 2 Albite + Amphibole + 6 CaCO₃ + 8 SiO₂ which also formed the hydrothermal pockets. Protolith is a pyroxene-plagioclase-rich rock with local magnetite segregations. Metasomatism drove by CO₂-H₂O rich fluids generated the U-albitites during the regional, syndeformational, amphibolite facies metamorphism. Fine-grained, disseminated uraninite, concentrated in the mafic portions of the albitite, is the main U-mineral, followed by pechblende herein derived. Andradite, sphene, apatite and epidote are the product of the Ca-metasomatism. Late hydrothermal activity produced concordant calcite-hornblende±kyanite-quartz pods and pockets resulting from local dissolution-deposition process. Weathering derived uranophane veinlets partly fill dilatant, sub-horizontal fracture zones.