Mineralogy and geochemistry of Al-phosphate and Al-borosilicate-bearing metaquartzites of the northern Serra do Espinhaço (State of Bahia, Brazil)

The investigated Al-phosphate- and Al-borosilicate-bearing metaquartzite horizon belongs to the Middle Proterozoic Serra de Vereda member of the Sõ Marcos Formation of the northern Espinhaço fold belt. Outcrops are located in the NNW-SSE trending hills of the Serra de Canas/Cana Brava (Northern Serra do Espinhaço, State of Bahia, NE Brazil) about 10km west of the town of Boquira. Sky blue lazulite and deep blue to lilac dumortierite are enriched in layers which sometimes show relic cross bedding. The occurrence of hematite patches and layers is typical. In rocks of type A, phosphorus contents range from 1.89 wt.% to 10.73 wt.% P₂O₅ and boron contents are mostly below 10 ppm whereas in the rocks of type B phosphorus contents are below 1% and boron contents reach 2600 ppm with a mean value of 1139 ppm. REE distribution patterns of rocks are predominantly controlled by the amount of xenotime, monazite and zircon. Oxygen thermometry on quartz and hematite and the stability of kyanite, augelite, trolleite and berlinite allow to estimate the minimum metamorphic peak P-T conditions at 475°C and 3.8 kbar i.e. within the stability field of scorzalite-lazulite and dumortierite. The association of Al-phosphates with hematite layers provides evidence for the deposition of the metaquartzite protolith in a seawater-fed sabkha-like sand flat fringing the Espinhaço rift. It is proposed that apatite, Al-phosphates and Al-borosilicates were chemically precipitated together with iron hydroxides from pore fluids. The antithetic behaviour of phosphorus and boron contents in the metaquartzites is the consequence of an influx of toxic boron-rich water that drastically affected algae populations and consequently reduced phosphorus precipitation.