Lithium and tantalum mineralization in rare-element pegmatites from southern Africa

Abstract

Abstract provided by author:

Manganotantalite and Ta-rich microlite are the dominant Ta-minerals in the rare-element Li-rich pegmatites of Namaqualand, Tantalite Valley and Karibib. In contrast, columbite (Nb-rich) is prevalent in the Li-poor, less differentiated pegmatites in the eastern Pegmatite Belt near Kenhardt. Microlite replaces manganotantalite in Li-rich rare-element pegmatites in all three field areas. A uranmicrolite from Karibib, Namibia contains 14. 35 percent UO2, 1. 03 percent PbO, 56. 12 percent Ta2O5, 13. 18 percent Nb2O5, 0. 58 percent Fe2O3, 6. 87 percent CaO, 0. 54 percent SrO, 0. 59 percent MnO, 0. 86 percent Na2O and 0. 47 percent F. U-plumbomicrolite or Pb-uranmicrolite is intergrown with manganotantalite from the same pegmatite. Throughout one aggregate of microlite PbO varied from 1. 57 to 21. 98 percent and UO2 from 12. 89 to 16. 20 percent. Pb appears to be concentrated around the periphery of the crystal. Backscattered electron images reveal metamict textures in radioactive microlites and distinctive subspheroidal features. A uranoan microlite from Tantalite Valley, Namibia, revealed two essentially different compositions; a more hydrated rim area of 200 um radius containing 7 percent higher Ta2O5, 10 percent lower CaO and 1. 3 percent lower F than a main central area of slightly variable composition. Crystals of uranoan microlite from Steinkopf, Namaqualand contain remnants of a bismuth phase. Bismuth intergrowths with quartz reveal the presence of a rare-mineral, mottramite [PbCu (VO4) OH], intergrown with pyromorphite, new data is given for these minerals. Ferrotantalite occurs at Rubicon mine, Karibib

A schematic diagram is produced for the paragenetic sequence of mineral assemblage in each of the pegmatite areas: Karibib, Tantalite Valley and Steinkopf, in relation to the T and P of formation, and the magma and fluids effecting the crystallization sequence

Finally different fractionation trends of Ta-Nb, Mn-Fe, Rb-K and Cs-K in columbite-tantalites and lithian mica have highlighted variable paths of differentiation in contrasting rare-element pegmatites which may reflect different sources of original parental magma