Late Archean Rare Metal (Nb-Ta-Li-Be) bearing Granitic Pegmatite (LCT) in Nagamangala Schist Belt, Dharwar Craton, India: Insights from Uraninite and Zircon mineral chemistry

Abstract

The Late Archean rare metal (Nb-Ta-Li-Be) bearing granitic pegmatites of the Nagamangala Schist Belt, Dharwar Craton, India, represent an important geological setting for understanding the mineralogical and geochemical evolution of Lithium-Cesium-Tantalum (LCT) pegmatites. This study provides detailed insights into the mineral chemistry of uraninite and zircon, using Back Scattered Electron (BSE) imaging and quantitative Electron Probe Micro Analysis (EPMA), to characterize their composition, formation history, and evolutionary trends. Subsurface samples of pegmatites from the Marlagalla area reveal the presence of euhedral to anhedral Th-rich uraninite along with unusually high hafnium (Hf) content in the associated zircon-an aspect reported first time from these pegmatites. Uraninite grains are found partially or fully enclosed by phases such as garnet, microlite [(Ca,Na)₂Ta₂O₆(O,OH,F)], tapiolite [(Fe,Mn)(Ta,Nb)₂O], zircon and apatite in the studied samples. Uraninite grains exhibit significant compositional variations, with UO₂ ranging from 64.74 to 70.92 wt %, high ThO₂ concentrations (up to 10.85 wt %), and PbO content between 23.01 and 25.37 wt %. The rare earth element (REE) content is notably low (RE₂O₃: 0.15–0.89 wt %), and SiO₂ is nearly absent (<0.01 to 0.29 wt %). Chemical formula calculations indicate a U⁴⁺ range of 0.227–0.345 and U⁶⁺ between 0.301–0.340, suggesting significant auto-oxidation-a hallmark of ancient uraninites. Furthermore, EPMA-based U-Th-Pb chemical age dating of the uraninite suggests a minimum formation age of approximately 2335 ± 54 Ma, making it the oldest reported uraninite in pegmatite from Indian subcontinent.

The associated zircon grains display exceptionally high HfO₂ content (9.94–20.49 %), coupled with ZrO₂ (45.56–58.89 %) and SiO₂ (27.59–30.56 %), categorizing them as hafnian zircons. Their elevated radioactive element content and close association with uranothorite, alongside the high Th content in uraninite, indicate progressive magmatic fractionation. This is further supported by geochemical trends such as the Zr/Hf vs. Hf in zircon and UO₂/ThO₂ vs. ThO₂ in uraninite. The U⁶⁺/U⁴⁺ ratio close to unity suggests auto-oxidation as the dominant mechanism, reinforced by the high PbO content in these uraninites.

These findings provide crucial insights into the Late Archean Algoman orogeny, a global event marked by extensive granitic plutonism. The Marlagalla pegmatites share key mineralogical and geochemical similarities with other Algoman-related pegmatites, suggesting a possible genetic link. Furthermore, the evolved nature of these pegmatites, in relation to the adjacent Allapatna granite, underscores their complex magmatic history and potential economic significance in rare metal exploration. The identification of ancient uraninite and hafnian zircon in these pegmatites not only enhances our knowledge of Late Archean magmatism but also holds economic significance for future rare metal exploration efforts in the region.