Journal of the Geological Survey of Brazil

Stratigraphy of the Pitangui Synclinorium, northwest of the Quadrilátero Ferrífero mineral province - Brazil: magmatism and sedimentation from Archean to Neoproterozoic

Marcelo de Souza Marinho — 2023-03-06

The Pitangui Synclinorium is located in the northwestern portion of the Quadrilátero Ferrífero mineral province, São Francisco Craton, in southeast Brazil. It corresponds to a NW-SE-trending synformal structure composed of an Archean greenstone belt sequence, which is covered by Paleoproterozoic metasedimentary rocks to Neoproterozoic sedimentary rocks, and surrounded by Archean granite-gneiss complexes. This study reports the first detailed 1:25.000 scale geological mapping in the Pitangui Synclinorium and provides cartographic, stratigraphic and petrographic improvements, allowing the formalization of its lithostratigraphic units and unraveling the sedimentary and volcanic depositional environments. The Archean Pitangui greenstone belt occurs in most parts of the homonymous synclinorium, and is subdivided into the Pitangui Group and the Antimes Formation. The metavolcanosedimentary
sequence of the Pitangui Group comprises basal mafic/ultramafic metavolcanic rocks with subordinate felsic/intermediate metavolcanic and metavolcaniclastic rocks of the Rio Pará Formation, suggesting a bimodal volcanism. It is followed by volcanogenic and chemical metasedimentary rocks of the Rio São João Formation, and pelitic to sandy metasedimentary rocks of the Onça do Pitangui Formation deposited in subaqueous environments mainly corresponding to submarine fan systems. The occurrence of clastic rocks increases towards the top of the sequence. The Antimes Formation comprises quartzites and metaconglomerates related to fluvial and fan delta depositional environments. The Pitangui greenstone belt is overlapped, in the central and northwest regions of the study area, by the Fazenda Tapera Formation, a Paleoproterozoic siliciclastic package containing arkosic metasandstones, metagreywackes and metarhythmites deposited in a shallow marine environment. Subordinated tuff layers occur as ash/fall deposits from distal volcanism. Neoproterozoic clastic-chemical sedimentary rocks of the Bambuí Group cover the northern portions of the synclinorium, and are mainly composed of diamictites, limestones, marls and rhythmites from the Carrancas (lacustrine or restricted marine), Sete Lagoas (shallow marine) and Serra de Santa Helena formations (deep water grading to prodelta and delta front).

The bauxite-bearing lateritic profile of the Jequié Complex, São Francisco Craton, Brazil: potential for rare earth elements mineralization and insights on the mineral system

Daniel Augusto de Miranda — 2023-05-05

The Jequié Block is located at the northeastern portion of the São Francisco Craton and represents the Archean tectonic paleoplate that hosts the Jequié Complex. The rocks of the Jequié Complex are cogenetic, enderbitic-charnockitic plutons, which were intruded by gabbros, norites and anorthosites, and subsequently all these rocks were re-equilibrated in granulite facies. Three bauxite occurrences were mapped at the southeastern portion of the Jequié Block, named Itaji (ITJ), Casa de Farinha (CDF) and Valentim (VLT), which are hosted by the bedrock of the units Poço Preto and Santa Inês – Volta do Rio. The Poço Preto Unit is composed of granulitic granodiorites to tonalities, while Santa Inês – Volta do Rio is composed of granulitic granites to granodiorites. The stream sediment results highlighted good geographic correlation between the content of Al, Fe, REE+Y+Sc and Ce in the catchments at the mineralized area. On the ITJ, the vertical profile indicates a lateritic sequence composed of the bedrock (i), saprock (ii), saprolite (iii) and pedolith (iv), while the upper part is the lateritic duricrust (v). The results of petrography, X-ray powder diffraction and whole rock geochemistry, supports the lateritic evolution. This vertical profile was later redeposited through surficial process in a proximal place. The breakdown of K-feldspar and plagioclase into kaolinite started at the weathering front and was continuously intensified towards the saprolite, as well as the transformation of the biotite and amphibole into goethite. The halloysite was identified as an intermediate mineral between the K-feldspar and kaolinite. The kaolinite locally was represented by its polymorph dickite. The transformation of the kaolinite into gibbsite happened between the saprolite and the pedolith, the main reaction in terms of the mineralization was the pseudomorphic feldspar replacement by gibbsite. The presence of the nordstrandite at CDF could be related to local conditions of the weathering, and/or to the bedrock geochemical composition in the region.

Geophysical and geological framework of the southern edge of the Parnaíba Basin, NE Brazil

Nilo Costa Pedrosa Junior — 2023-07-21

The application of geophysical techniques to highlight and characterize magnetic and gravity anomalies was carried out along a research area at the southern edge of the Parnaíba Basin. This study was conducted through qualitative and quantitative interpretations of magnetic and gravity data, supported by surface geological information, reflection seismic data, stratigraphic well data, field data, and magnetic susceptibility and density measurements of rock samples collected in outcrops. The study used the 3D magnetization vector inversion method, and 4 regional transects were performed for 2.5D modeling of magnetic and gravity data. The investigation revealed the configuration of the structural framework of the southern edge of the Parnaíba Basin. The characterization of magnetic anomalies allowed the definition of four main domains. Three magnetic zones were fragmented by depth, in the intervals of 20 km to 30 km, 5 km to 8 km and 0.5 km to 1 km, which demonstrate the association of magnetic anomalies with shallow and deep crustal structures. The magnetic susceptibility data show good correlation with the main structural discontinuities, and in particular with the Transbrasilian Zone. The results show that the maximum thickness of the sedimentary cover in the basin is approximately 2 km. No features associated with grabens or rifts were identified along the transects, and the presence of gravity and pseudo-gravity lows are closely related to less dense lithological units of the upper crust, such as granitic masses and supracrustal sequences. The reactivations of the Transbrasilian Zone during the Mesozoic generated deep structures in the crust that promoted the rise of basaltic rocks and the intrusion of kimberlitic bodies.