Fluor-elbaite, lepidolite and Ta–Nb oxides from a pegmatite of the 3000 Ma Sinceni Pluton, Swaziland: evidence for lithium–cesium–tantalum (LCT) pegmatites in the Mesoarchean

Abstract

Mineral evolution is concerned with the timing of mineral occurrences, such as the earliest reported occurrences in the geologic record. Minerals containing essential Li have not been reported from rocks older than ca. 3000 Ma, thus the lithian tourmaline (fluor-elbaite) and mica (lepidolite) assemblage from a pegmatite near Zishineni associated with the ca. 3000Ma Sinceni Pluton presents unusual interest. Fluor-elbaite (0.75–0.98 F per formula unit) forms green crystals up to 50mm long. Spindle stage measurements give ω = 1.652(1), ε = 1.627(1) (589.3nm). Optical absorption spectroscopy shows Fe and Mn are divalent; infra-red spectroscopy demonstrates the presence of Li and indicates the presence of (OH) at both the (OH) sites. Electron microprobe analysis of 330 points on several prisms, the largest of which is zoned in Fe and Ca, gives the following average and standard deviations in wt%: SiO₂ 37.29 (0.26), TiO₂ 0.05 (0.05), Al₂O₃ 38.14 (0.35), Cr₂O₃ 0 (0.02), MgO 0.02 (0.01), MnO 3.57 (0.25), FeO 2.48 (0.60), Na₂O 2.48 (0.09), K₂O 0.03 (0.12), CaO 0.77 (0.21), F 1.80 (0.11) wt%; Cl 0 (0.01). Nuclear reaction analyses gave Li₂O 0.91 (0.04) and B₂O₃ 10.55 (0.45). The empirical formula of fluor-elbaite was determined by integrating crystal-chemical data from electron microprobe analysis, nuclear reaction analysis, crystal structure refinement using X-ray diffraction, infra-red and optical absorption spectroscopy: ^X(□_0.09Na_0.77K_0.01Ca_0.13)_Σ1.00^Y(□_0.35Li_0.59Mn²⁺_0.49Fe²⁺_0.33Al_1.23Ti_0.01)_Σ3.00Al₆(Si₆O₁₈)(BO₃)₃^O3(OH)₃^O1[F_0.92(OH)_0.08]_Σ1.00. The crystal structure of fluor-elbaite was refined to statistical indices R1 for 1454 reflections ∼2% using MoKα X-ray intensity data. Structural data confirm the presence of significant vacancies at the Y site. Micas include lepidolite in flakes several millimeters across that are veined and overgrown by fine-grainedmuscovite. Silica and (FeO+MnO) increase, and Al decreases with F, all giving tight linear fits for both micas taken together, suggesting both micas can be regarded as interstratified muscovite and lithium mica consisting of 35.2 wt% masutomilite containing nearly equal amounts of Mn and Fe, 52.8 wt% polylithionite and 11.9 wt% trilithionite. Muscovite and lepidolite contain Ta, hübnerite(?) and a possible Pb-bearing microlite (Ta > Nb). The oxides, together with the muscovite, are interpreted to be related to later hydrothermal reworking of the primary lepidolite–fluor-elbaite assemblage. Given the 2990 ± 43MaRb–Sr isochron and 3074 ± 4Maevaporation Pb–Pb ages reported for the Sinceni Pluton and Rb/Sr mineral ages ranging from 2906 ± 31Ma to 3072 ± 33Ma reported for the pegmatites, the fluor-elbaite–cesian lepidolite–fluorcalciomicrolite-bearing pegmatite is the first reported occurrence of a lithian tourmaline and lepidolite in the geologic record, as well as one of the two earliest known examples of the lithium–cesium–tantalum (LCT) family of pegmatites. The Sinceni magma is most plausibly derived from a metasedimentary source by intrusion of hot mantle melts into the crust from below, thereby indicating that a “mature” continental crust existed in the Kaapvaal craton at ca. 3000 Ma.