The Radioactive Rare Metal Mineralization in the World-Class Sn-Nb-Ta-U-Th-REE-Deposit Madeira (Pitinga, Amazonas State, Brazil): With Special Reference to the Complex Alteration of Pyrochlore-Group Minerals

Abstract

This study focuses on the relationship between U and pyrochlore in the world-class Sn-Nb-Ta (U, Th, REE, Li) Madeira deposit within the Pitinga mining district of northern Brazil. The primary U mineralization is of intrusive-type and early magmatic origin, hosted in the peralkaline albite-enriched granite facies of the A-type Madeira granite (~1820 Ma). U-Pb-LREE-enriched pyrochlore is the only primary U ore and is widely and homogeneously dispersed in two albite-enriched granite subfacies: the albite-enriched granite core (AGC) and the albite-enriched granite border (AGB). In both zones, the pyrochlore crystals underwent strong hydrothermal alteration by F-rich, low-temperature aqueous fluids. During this hypogene alteration process, cations such as LREE, Nb, and F were selectively released, while others like Fe and Si were introduced. This led to the successive formation of various secondary pyrochlore varieties and a relative enrichment of U (up to 13.73 wt.% UO2). The alteration of pyrochlore eventually resulted in the breakdown of its structure, leading to the formation of U-bearing columbite pseudomorphs and the precipitation of U-rich silicates (up to 34.35 wt.% UO2), galena, and LREE-rich fluorides within pyrochlore vugs. In contrast to the homogeneous distribution of the primary ore mineralization, the secondary pyrochlore mineralization shows striking zonation, being most intense in the AGB and AGC proximal to a massive cryolite deposit. The U mineralization in the Madeira deposit exhibits grades of 328 ppm UO2, comparable to the main deposits of this type, with significant reserves of up to 52 kt U. However, it is different from those deposits in four key aspects: homogeneous dispersion of mineralization; pyrochlore as the exclusive primary ore mineral; U and Th mineralizations formed at different stages; and intense hydrothermal alteration. These characteristics are attributed to the special conditions imposed by the fluorine-rich nature of the peralkaline magma.