Pyrochlore composition and Sm[sbnd]Nd isotope signature as indicators of magmatic-hydrothermal processes: The case of Ririwai complex, north-Central Nigeria

Abstract

This study presents a geochemical and SmNd isotopic study on pyrochlore from the Ririwai alkaline complex in north-central Nigeria, aiming to reconstruct the magmatic-hydrothermal history and provide insights into the formation of niobium deposit hosted in alkaline igneous rocks. Three texturally and compositionally distinct types of pyrochlore (Pcl I, II, and III) are identified. Primary Pcl I grains are enriched in Nb2O5 (mean of 60.5 wt%) and other high field strength elements (HFSE), rare earth elements (mean of 17.2 wt% REE2O3), Ca, Na, and F, suggesting that they crystallized in a Nb-rich peralkaline magma. The occurrence of patchy zoning as well as the development of microfractures and pores within these pyrochlore grains indicate two stages of hydrothermal alteration responsible for the formation of Pcl II and Pcl III. The first stage of alteration from Pcl I to Pcl II was induced by high-temperature, alkali-rich, and F-moderate fluids, leading to the removal of Na, Ca, and F while filling the vacant A- and Y-sites with K, Rb, Y, and Zn. The Nb and REE contents in Pcl III (mean of 41.3 wt% and 9.4 wt%, respectively) are lower than those in Pcl I and Pcl II (mean of 58.1 wt% and 17.3 wt%, respectively). Thereby, the second stage of alteration from Pcl II to Pcl III was related to low-temperature, Si- and Al-rich alkaline fluids, which resulted in the leaching of Nb, Ti, Th, U, and REE (mainly LREE) from the A- and B-sites, compensated by the incorporation of Si, Pb, K, Al, Fe, and Mn. The leached elements either precipitated into Nb-bearing minerals such as rutile, thorite, and zircon in alteration assemblages as an in-situ replacement of Pcl II or remobilized locally to form hydrothermal veins of Si, Al, Nb, and CO2-bearing oxides. The comparable εNd(t) values between hydrothermal Pcl II and III (−1.9 to −2.8 and − 1.5 to −3.2, respectively) and magmatic Pcl I (−2.0 to −2.6) indicate that there was only a minimal external contribution to Nd (and by analogy Nb as well) from fluids. Two-stage hydrothermal fluids in the Ririwai alkaline complex have limited effects on Nb mobilization, without significantly enhancing or reducing the Nb grade.