The Earth's past: recovering the geological record from a Uranium deposit

The geosciences reveal Earth's past by analyzing the record from ancient terrains that document the planet's evolution. Ancient U-deposits, in particular, deserve careful study. The ore is a product of environmental conditions resulting from interactions among Earth's spheres (i.e., the lithosphere, hydrosphere, atmosphere, and biosphere) and from Earth's evolution over time. The U mobility and U-compound formation are determined by environmental conditions such as oxidation state, pH, and the nature of the geochemical reservoir. Both are the result of geological processes (e.g., magmatism, weathering, hydrothermalism), tectonic context (e.g., subduction zones, rifting, collisional episodes), and geochemical conditions (e.g., the availability of free O₂ in the atmosphere).

 

The main Brazilian target for uranium, the Lagoa Real Uranium Province (LRUP), for example, comprises rocks ranging from Archean to Neoproterozoic. It is typically viewed as a metasomatic, high-grade metamorphic deposit. Uraninite (UO₂) is the main ore mineral, associated with high-F biotite and evidence of disilicification, reflecting a reducing environment, low pH, and high F availability. These data indicate a low free O₂ paleoenvironment (e.g., at high depth and/or in an ancient stage before the Great Oxygenation Event - GOE), with contact with high HF concentrations during rifting episodes, followed by orogenic collisional events. However, this is not the only mineralization stage: uranil minerals indicate high free O₂, resulting from weathering under an oxidized atmosphere after the GOE.

Thus, the rocks preserve superposed processes resulting from distinct geochemical and tectonic contexts, some of which are partially or fully obliterated. Despite this, these rocks provided geochemical, petrographic, and field data that enabled the recovery of the ancient geological record. The LRUP has undergone the opening and inversion of overlapping rifts, as well as  A-type magmatism, metamorphism, and metasomatism. It has also witnessed some of Earth's major evolutionary events since the Archean, such as the incorporation of U into the crust from the mantle and atmospheric oxygenation. Currently, a previously unknown process has been identified at LRUP: partial melting linked to regional metamorphism that may be related to Pan-African Cycle collisional episodes (900 to 500 Ma).  The relevant record was preserved at the macroscale, microscale, and likely nanoscale. At the outcrop, leucosome appears parallel to shear zones or fills fold axes and low-pressure zones. Drill cores reveal melt segregating from the residual mafic lithologies and increasing LREE concentration with increasing melting in the residua. At the micro- to nanoscale, SEM studies show that mono- and polymineralic zircon inclusions have distinct compositions, which may reflect different formation stages. Core inclusions include K-feldspar, quartz, and fluorite. Towards the margin, inclusions shift to K-feldspar, albite, and quartz. At the very edge, inclusions are Co- and As-bearing calcite with REE minerals.

Therefore, in addition to the social and economic implications, there are other benefits from investigating ancient U-deposits. Despite their complexity, they may preserve a record of Earth’s long past, revealing interactions among Earth’s spheres, changes in the lithosphere, and helping geoscientists understand the main large-scale processes, or global cycles, that shape the planet.

Keywords: geological record, uranium deposits, partial melting.