Evidence of Paleoproterozoic phosphogenesis in the Salvador-Curaçá Orogen (Tanque Novo-Ipirá Complex), northeastern São Francisco Craton, Brazil

Abstract

This paper analyzes mineralogical, geochemical, and geochronological aspects, along with the effect of hydrothermal/metasomatic overprints, to identify the presence of primary phosphate as well as depositional and paleoenvironmental conditions in marble and calcsilicate sequences recrystallized under transitional amphibolite-granulite metamorphic conditions in the Tanque Novo-Ipirá Complex within the Salvador-Curaçá Orogen, northeastern São Francisco Craton, state of Bahia, Brazil. Petrographic studies have identified up to 10 vol.% disseminated apatite and whole-rock P 2 O 5 contents up to 3.2 wt.%. Post-depositional events affected the lithofacies to varying degrees. Late hydrothermalism did not modify the rare earth element and yttrium (REEY) patterns considerably. When normalized to Post-Archean Australian Shale (PAAS), these lithofacies are marked by flat pattern REEY, true negative Ce anomalies, and positive Y and Gd. The highly variable Eu anomalies were inherited from the source composition but may have been affected by interaction with fluids. U-Pb LA-ICP-MS (laser ablation multicollector inductively coupled plasma mass spectrometry) ages indicate a maximum depositional age of 2128 Ga, as well as Paleoproterozoic and Neoarchean sources. Samples with anomalous phosphorus show Y/Ho ratios >30 and Ce/Ce* anomalies between 0.53 and 1.0 with an average of 0.70, suggesting a sub-oxic environment for phosphate precipitation.