Alterações hidrotermais associadas às rochas máfico-carbonatíticas do depósito de fosfato Serra da Capivara, região de Vila Mandi (PA), extremo sul do Cráton Amazônico.

Abstract

Near the border of the states of Pará and Mato Grosso, in the Amazonian Craton, about 90 km west of the Vila Mandi district, Santana do Araguaia (PA) city, there is an unprecedented volcano–plutonism named Santana mafic-carbonatitic Complex. It is formed by a lower maficultramafic member with plutono–volcanic and other volcaniclastic lithofacies; besides an upper carbonatitic member with plutonic, effusive, and volcaniclastic lithofacies originated in a volcanic caldera environment with large areas of hydrothermal alterations and genetically related circular structures. The severe Amazon weathering partially affected this cluster, producing the Serra da Capivara Phosphate deposit supergenically. Although speculative, the Santana mafic-carbonatitic Complex is Paleoproterozoic in age, because it invades the Paleoproterozoic volcano-plutonic sequences Cinco Estrelas and Vila Mandi formations (1980–1880 Ma) and it is capped by sedimentary rocks from the same Era. The lower maficultramafic member has lithofacies with slabs of pyroxenite, and minor isolated metric blocks of ijolite and apatitite. They are medium-grained ceylonite-bearing (MgAl2O4) pyroxenites with augite (~ 90% vol.), magnesio-riebeckite, and olivine crystals replaced by clay minerals (saponite). The ijolite is composed of clinopyroxene and nepheline phenocrysts immersed in a fine-grained groundmass with nepheline, calcite, and interstitial magnetite. Apatitite blocks are composed of medium-grained apatite grains (~ 98% vol.) and calcite. The volcanic rocks of this lithofacies comprise isolated metric blocks of alkali basalt and rare associated outcrops of finegrained apatitite. This basalt rock presents plagioclase-rich groundmass and acicular augite phenocrysts as essential mineralogy. Aphyric samples have primary spherules filled with calcite and quartz, besides interstitial pyrite, iron oxides, apatite, barite, rutile, celestine, and monazite. This textural feature suggests silicate and carbonatitic melts immiscibility process. An explosive to autoclastic mafic volcaniclastic lithofacies encompasses poor sorting deposits of massive polymictic breccia, lapilli-tuff, crystal-rich tuff, and ash tuff. The autoclastic rocks reveal volcaniclastic texture comprising centimetric angular clasts sourced from autofragmentation of the mafic-plutonic plutono–volcanic lithofacies. Epiclastic sedimentary volcanogenic deposits usually cover all previous lithofacies. The upper carbonatitic member reveals coarse-grained carbonatite (sövite) lithofacies comprising reddish-yellow sövite (calcite carbonatite) composed of subhedral to euhedral calcite (85–90% vol.), with variations to magnesium-ferriferous calcite and dolomite. Primary accessories are magnetite, hematite, potassic feldspar, and pyrite. These lithotypes show hydrothermalized medium- to fine-grained carbonatite veins. Rare coarse-grained apatitite bodies occur associated with this lithofacies, which represents part of the proto-ore. An effusive carbonatite (alvikite) lithofacies reveals finegrained calcite-rich (80–85% vol.) to porphyritic alvikite, besides hematite, magnetite, potassic feldspar, and pyrite. Fragment-rich explosive carbonatitic volcaniclastic lithofacies encompassing poor sorting and texturally variable massive crystal-rich tuff, lapilli-tuff, and massive polymictic breccia formed by angular clasts sourced from host rocks and the complex. Syenitic stocks and dikes invade these rocks. The main hydrothermal magmatic alteration of the complex is represented by hydrothermalized carbonatitic rocks of reddish, brownish, and yellowish colors. The mineral paragenesis found was barite + fluorapatite + dolomite ± quartz ± rutile ± chalcopyrite ± pyrite ± monazite ± magnetite ± hematite. This alteration occurs in three distinctive ways; 1) in the deeper zones, where the minerals found were barite, fluorine apatite, and dolomite in pervasive to fracture-controlled alteration associated with deep fine carbonatites. 2) In the sövite, of weak interstitial form with mineralogy similar to the deep alterations. 3) in the alvikite with intense interstitial changes and formation of hydrothermal quartz associated with barite, fluorapatite, dolomite, monazite, celestine, and rutile. The mineral assemblage of the deeper alterations suggests initially sulphate-rich, magnesium, phosphorus, and CO2 fluids with possible transitional source between the late magmatic and the hydrothermal stages. In transition to more superficial phases of the volcanism, there was an assimilation of SiO2 from the country rocks evidenced by the formation of fine interstitial quartz crystals in alvikite. The interpreted environment of volcanic caldera occurs in the interception of regional NE-SW and NW-SE faults with up to 40 km of extension and that served as deep conduit of the precursor magma of the complex. The root of the system is represented by maficultramafic rocks and plutonic carbonatites. The pre-caldera phase involved intense degasification and hydrothermal activities as a function of magmatic evolution, and ascending by lithic faults and placing on the surface of large volume of carbonate lava (alvikites) that built the extinct volcanic building. The collapse of this structure and the topographic landslide coincided with explosive volcanism and formation of the volcanoclastic lithotypes, representing the intra-caldera filling. The late syenites may represent the post-caldera phase and sealing of these structures. The hydrothermal paragenesis identified in the Santana maficcarbonatitic Complex shows important metallogenetic potential for rare earth elements and phosphate and represents a prospective guide on Proterozoic terrains of the Amazonian Craton, like other areas of the planet.