Pyrochlore from the Bailundo Carbonatite Complex (Angola): Compositional variation and implications to mineral exploration

Abstract

Pyrochlore is a common accessory in carbonatite rocks and its composition can provide useful insights on petrogenetic and post-magmatic metal-enrichment processes, especially those which favour its occurrence and concentration.Comprehensive compositional and textural characterization of a large set of pyrochlores from the Bailundo Carbonatite Complex (SW Angola) and associated surface rocks was the basis to (i) evaluate the main effects of metasomatism and weathering as causes of metal leaching or concentration; and (ii) assess pyrochlore compositions as potential petrogenetic or metallogenetic tools, with particular emphasis on pyrochlore enrichment in economic components, such as Ta, REE, U, Th and Pb, during weathering processes.Unweathered fluor- and hydroxyl-calciopyrochlores from deep-seated carbonatitic rocks (provided by a 600 m-deep drill-core) often present high Ta/Nb ratios, as well as high U and Th contents, and comparatively low overall REE concentrations. Metasomatic effects are not easy to assess, given the extreme compositional variability of these pyrochlores.On the other hand, some systematic trends can be established in pyrochlores from weathered surface rocks: these pyrochlores usually show strong depletion in most A-site cations (e.g., Na, Ca, U), and clear enrichment in Nb and in large-ion metals (e.g., Ba, Sr, Pb) usually absent in unweathered pyrochlores. REE seem to be relatively immobile and to become concentrated during weathering.Along with some REE phosphates and oxides, pyrochlore is often present in several domains of the weathering profile, occurring in the outcropping weathered carbonatite as well as in the regolith immediately overlying the intrusion. Thus, both the Bailundo carbonatite intrusion and its weathering products, concentrated inside the ridge formed by differential erosion of the fenitic aureole, constitute good exploration targets for Nb (±Ta ± REE). However, future exploration work should also include a 3-D understanding of the chemical and geological processes at work in both geological environments.