Journal of the Geological Survey of Brazil

Geochemical and isotopic constraints on the petrogenesis of the Cretaceous alkaline Morro Redondo intrusive complex, Rio de Janeiro, Brazil: implications for alkaline magma genesis

Carlos Eduardo Mota — 2024-03-20

The Morro Redondo Complex is composed of plutonic and volcanic alkaline rocks. This igneous complex is part of the Poços de Caldas - Cabo Frio Magmatic Alignment (PCCFA). This study aims to contribute to the understanding of the petrogenesis of the rocks in the Morro Redondo complex and, thereby, contribute to the debate on the genesis of the PCCFA. In this work, we conducted analyses of whole-rock geochemistry, ⁴⁰Ar/³⁹Ar thermochronology, U-Pb geochronology on zircon grains, and Rb-Sr, Sm-Nd, and Lu-Hf isotopic analyses. Four distinct lithotypes are present: nepheline syenites, alkali feldspar syenites, trachytes, and phonolites. The rocks of the Morro Redondo complex are miaskitic, have intermediate values of SiO₂, and show significant crustal assimilation, mainly observed in the relationship between the SSI and (Na+K)/Al indices, together with the Hf isotopic signature. A hornblende crystal obtained from a sample of nepheline syenite provided a plateau age of 74.35 ± 0.50 Ma. The initial ratios of ⁸⁷Sr/⁸⁶Sr range from 0.70565 to 0.71042, and the initial ratios of ¹⁴³Nd/¹⁴⁴Nd range from 0.51234 to 0.5124. Hf data suggest a hybrid origin for the Morro Redondo complex, with sources from both crustal and mantle origins. The data obtained indicate that at least three magmatic pulses contributed to the formation of the Morro Redondo complex, with the participation of both crustal and mantle sources, generating magmas of miaskitic affinity.

Geomorphological landscape analysis for the study of surface and groundwater in a carbonate rock environment in northern Minas Gerais, Brazil

Patricia Simões — 2024-05-03

The relief is formed by the conjunction of processes that can be influenced to a greater or lesser extent by the morphostructure or morphosculpture, both of which act in the formation of the landscape. This work aims to, through the analysis of relief formation, provide subsidies for the studies of the run-off and the groundwater in an area of carbonate rocks in the north of Minas Gerais. The geomorphological landscape of this area is strongly influenced by morphostructural factors. Such influence is observed in the rectilinear drainage pattern of some rivers, in the presence of structural valleys, in abrupt changes in the direction of drainage net, and in scarps aligned in the same direction of joints and faults present in the area. Such structural influence is quite clear in the southern and central portions of the study area, being less significant in the northern portion, which has a much higher concentration of sinkholes than what was observed in the other areas. Considering a systemic perspective of the processes and dynamics that act on the surface and underground, this work presents these results of the formation of the geomorphological landscape so that such analysis helps in understanding the dynamics of groundwater in this area, as well as the relationship between surface waters and underground.

Distribution of lithium in soil from selected states of Brazil: mineral potential and health relationships

Cassio Silva — 2024-06-21

Lithium (Li, atomic number 3), the lightest metal on Earth, is in growing demand due to its use in batteries to power various electrical devices and electric vehicles. Li is also used to stabilize mood and prevent suicide in patients with affective disorders. Geochemical mapping provides a means of visualizing spatial variations in the chemical composition of the surface of the Earth. The objective of the study was to determine the concentration and distribution of Li in soils to identify areas with potential for prospecting and look for relationships with human health. The research was carried out between 2003 and 2017 in 13 Brazilian states, covering an area of 3,313,750 km², in which 3688 soil samples were collected in a grid of 25x25km (625km²). The samples (2kg) collected at the top of the B horizon (up to 30 cm) were reduced at the laboratory with aqua regia and analyzed by ICP-MS. The data indicate that in the total of the 13 states, the concentration of Li in the soils (mg/kg) ranges between 0.5 and 167, with a median of 2. Higher levels stand out in the states of Paraíba (0.5-167.15), Ceará (0.7-63.7), Pernambuco (0.5-63.7), Minas Gerais (0.5-83.2), Alagoas (2.0-40.4) and Mato Grosso do Sul (0.5-61.2). In the Pernambuco and Minas Gerais states, the highest values overlap with the Borborema Province and District of Araçuaí, respectively, which already produce Li hosted in pegmatites and have the potential for new deposits. Anomalous Li values above 11.75 mg/kg (estimated as national reference) cover a prospective area of around 25,225 km², distributed for 140 municipalities, or approximately 0.76% of the studied area. On a regional scale, there was no causal correlation between state suicide rates and the levels of Li in the soil.

The Archean-Paleoproterozoic structural evolution of the Caraíba Cu-Deposit, northern São Francisco Craton, Brazil: A historical review of its understanding coevally with the development of a high-risk mining project

Luiz José Homem Del-Rey Silva — 2024-06-17

The belt of basic-ultrabasic rocks within the Curaçá River Valley Province of the Bahia State, northeastern Brazil, has been known for enclosing several Cu-orebodies that helped sustain the industrial development of Brazil in the last five decades. Caraíba, Vermelhos, Surubim, Terra do Sal, Angicos, Sussuarana are the most prominent of these deposits. The Caraíba Cu-Orebody, so far the most important of all, is part of a body of Archean hypersthenites and norites that underwent poly-deformation and granulitization in the roots of the Paleoproterozoic Itabuna-Salvador-Curaçá orogen, which records the continental collisions responsible for building the São Francisco-Congo paleocontinent in Rhyacian times, ca 2.0 Ga ago. The Caraíba Mine setup happened during the 1970 decade, together with the rush to free the country's economy from the costs of importations, exactly when the first international oil crisis (1974-1978) was suffocating the economy in many countries. Although that time coincided with the boom of geological knowledge associated with the worldwide acceptance of plate tectonics, the study of high-grade metamorphic terrains was still embryonic. In particular, the meaning of the poly-phased tectonic structures in areas recording a high degree of plasticity during deformation remained unraveled. Due to the international economic crisis, everything necessary for running the Caraíba Mine was done under pressure. Nonetheless, the coeval geological studies required for deciphering the area's structural evolution and the mineralization controls, vital for the booming mining planning and profitable exploitation of the Cu ore, had to keep the academic standards for surviving the hurry.

Part of this text is a historical novel as it describes in detail how the convenient geological knowledge of the Caraíba-Cu Orebody could be achieved progressively in the years 1980-1990, well in the middle of the rush affecting everything and everybody working in the Caraíba Mine Project.We emphasize thedetailed structural geology studies as the primary tool for guaranteeing timely technical accuracy during the ore's exploitation and profitability in the mine project.As such, it means a historical but profoundly scientific document necessary for those interested in academic and technical purposes related to large-scale mine projects everywhere. Moreover, it highlights the human drama lived by many fortunate professionals in that forgotten semi-desertic area of northeastern Brazil. The experience gained when studying several generations of tectonic structures and metamorphism at Caraíba transformed for the better and forever the lives of several geologists and other professionals equally in charge of operating the Caraíba Mine during those pioneer years.

A design of gold-bearing metallogenic provinces and districts in Brazil

Evandro Klein — 2024-07-01

There is a lack of consensus on how many and which are the Brazilian gold provinces and districts, and what are their limits or configurations. As an effort of the Geological Survey of Brazil to standardize this subject, we present a compilation of the Brazilian gold-bearing metallogenic/mineral provinces and districts, including those containing polymetallic gold-bearing deposits, and a digital file (shape file) with the design of their respective limits. Because many areas have classic definitions, and even though some of them do not meet the appropriate requirement for a strict definition, their classic names are maintained. However, we strongly suggest that the use of the term metallogenic (province, district), in a broader geotectonic sense, should be preferable in relation to mineral (province, district), which is more generic.