Geology and geochemistry of paleoproterozoic magnesite deposits (∼1.8Ga), State of Ceará, Northeastern Brazil

Abstract

Five important magnesite mines and small occurrences in the state of Ceara form a sequence of lenses that extend, discontinuously, for over 140km. The magnesite rocks are hosted by metadolomites with lutecite, sulphate nodules pseudomorphosed by fiber-radiated quartz, scapolite and dissolution breccias. This metacarbonate sequence is more voluminous and more calcic in the southwestern extremity of the belt, but less voluminous and more magnesian towards the northeastern portion of the belt, the highest Mg contents being observed in the Alencar-Oros region. Magnesite deposits pass gradually into metadolomites and then to almost pure calcitic marbles westward the belt. This group is hosted in a greenschists to amphibolite-facies metavolcanic-sedimentary sequence crosscut by basic sills and granitic intrusions of variable size, form, composition and age (middle to late Proterozoic ~550Ma).Two types of magnesite marble may be distinguished: (I) medium-grained (I to 9mrn) and (2) sparry -magnesite marble (I to 15cm). The latter displays wide textural variation (porphyric, rosette, layered and palisadic types) and exhibits remnants of the original sedimentary features. The sparry crystals, in spite oftheir deformation, are hypidiomorphic and pinolitic. The colors vary from white to light or dark gray, or even red. The dark banded term presents traces of microfossils and stromatolites structures. The medium-grained magnesite marble, on the other hand, shows more homogeneous texture and color and anhedral crystals are more abundant.The sparry-magnesite marble is less enriched in Si0 2 , Fep), Alp) and CaO and richer in MgO than the medium-grained ones. The magnesitic marbles display positive Ce, and negative Eu anomalies. This suggests a shallow marine platform environment with continental, reduced influence (Iagunar). These magnesitic marbles also show positive Ce and Eu anomalies in relation to the present seawater, suggesting compositional differences, materialized mainly by an enrichement of these elements in the past seawater.Fluid inclusions in the medium-grained magnesites of the Riacho Fundo deposit are aqueous inclusions, while in the sparry magnesite deposits, particularly in Cabeca de Negro, aquo-carbonic inclusions dominate. Hydrocarbons, however, are found in both cases.The calcitic marble, deposited on open-marine environment, displays positive l3C (+0.5 to 2.1 %0 PDB), while dolomitic to dolostone marbles, deposited on less open marine environment, display negative shift (0.7 to -3.2%0 PDB). This l3C fluctuation is within the range for carbonates deposited around 1.8Ga.The depositional environment ofthese carbonates is close to a paralic system, lagoon, with strong evaporitic conditions. The depressions were of variable depth and lengths and could be isolated, in particular by stromatolitic barriers, dried and flooded by seawater with continental inflows. The largest and deepest ones correspond to the sparry magnesite deposits. These magnesites are located in a lateral way to almost pure calcitic marbles, ...

Timing of the HT/LP transpression in the Neoproterozoic Seridó Belt (Borborema Province, Brazil): Constraints from UPb (SHRIMP) geochronology and implications for the connections between NE Brazil and West Africa

“…However, there are important differences between the geology of the Orós and the Neoproterozoic Seridó belt. Firstly, the Orós volcanism occurred in the Late Paleoproterozoic (Sá et al, 1995) and its rift deposits include magnesite layers and possibly gypsum, which is suggestive of an evaporitic environment (Parente et al, 2004). Orós-type sequences have not been described until now in southern Nigeria and, evidently, the search for similar rock associations is critical to establish fine-scale geological correlations across the Atlantic.…”

Genetic and metamorphic conditions constrained by fluid inclusions from Paleoproterozoic (c. 1.8Ga) magnesite ore deposits, NE Brazil

“…1). Carbon and oxygen isotope data from calcite and dolomite marbles reported by Parente et al (2004), despite some modification during metamorphism, suggest that the deposition took place in a confined or shallow marine environment. The volcano-sedimentary sequence was deformed into narrow isoclinal folds, with vertical axial plane foliation and subhorizontal stretching lineations, compatible with the Brasiliano orogeny transpressive regime (Sá, 1991;Parente and Arthaud, 1995).…”

“…Magnesite-bearing marbles are composed of medium-grained (1-9 mm) or sparry (1-15 cm) magnesite. According to Parente et al (2004), the magnesite marbles are of sedimentary origin, having undergone important diagenetic evolution before the metamorphic event that took place during the Neoproterozoic Brasiliano orogenic cycle.…”

CO2 Absorption and Magnesium Carbonate Precipitation in MgCl2–NH3–NH4Cl Solutions: Implications for Carbon Capture and Storage

“…Nesquehonite is a common magnesium carbonate precipitated under experimental conditions at room temperature and CO 2 pressure due to its kinetic advancement [47]. However, the natural occurrence of nesquehonite in large deposition is rather rare compared with other magnesium carbonates, such as hydromagnesite [48], magnesite [49,50], etc. Fresh nesquehonite can be seen under low temperature in a natural environment (e.g., [51][52][53]).…”

“…Fresh nesquehonite can be seen under low temperature in a natural environment (e.g., [51][52][53]). Although it is not abundant, field and experimental evidence shows that nesquehonite could act as a precursor of more abundant hydromagnesite or magnesite [43,50,54], and dypingite is a transitory phase during this transformation [36,55]. Temperature, fluid pH, and CO 2 partial pressure are important parameters of phase transformation [55,56].…”