A laterite-hosted APS deposit in the Amazon region, Brazil: The physical-chemical regime and environment of formation

Several occurrences and deposits of aluminum-phosphate-sulfate (APS) minerals have been discovered during the last century, and most are closely related to lateritic weathering formations. These deposits have been found all over the world but are more common in the present intertropical climate zone. Only one such deposit is mined for phosphate. However, a second, the Bonito deposit, has recently started to be mined in the Amazon region. The deposit is located in one of the most important provinces for APS minerals in the world, the Gurupi region, which lies in the northeastern Amazon region and still remains relatively unexplored. The Bonito APS deposit comprises a complete mature laterite profile, and the APS minerals are exposed at the top of the profile in the form of an aluminum phosphatic-sulphatic crust and an aluminum phosphatic-sulphatic iron-oxyhydroxide crust. The phosphate-sulphatic ore is composed of crandallite-goyazite, woodhouseite-svanbergite, and wardite-millisite, as well as some quartz, kaolinite, hematite and goethite. The ore is high in phosphor (> 18% P2O5) and is rich in SrO, BaO and LREEs. The mineralogy and geochemistry of the whole profile show a complete lateritic evolution, with increasing enrichment in Fe, Al, Ti, P, S, Ca, Na, Ba, Sr, REEs, Th, U, Zr, Hf, Y, Sc, V, Ga, As, Se, etc., upwards in the profile. The mineralogy, geochemistry, and chemical element contents indicate a lateritic phosphate-sulphate sequence derived probably from Ca-phosphate-bearing sedimentary rocks from the Paleozoic Parnaíba Basin.