Origin of rare-element-mineralized Damara Belt pegmatites: A geochemical and light stable isotope study

Abstract

Neoproterozoic Li–Cs–Ta (LCT) pegmatites and pegmatitic sheeted leucogranites (SLGs) of the Damara Belt, Namibia, although coeval, are variably mineralized and have intruded multiple tectono-stratigraphic units. Major and trace element data together with oxygen and hydrogen isotopes were used to identify possible sources for these pegmatite melts. Two groups of pegmatites can be identified on the basis of O-isotopes. Group A pegmatites, which include U-enriched pegmatitic SLGs with a Nb–Y–F (NYF) pegmatite affinity (Kinnaird and Nex, 2007), and Li–Be-rich LCT pegmatites, with δ18O = 11.2–12.9‰, are metaluminous to peraluminous, suggesting that they evolved from a partial melt of a mixed metasedimentary and igneous source. Group B pegmatites, which contain Sn, Li–Sn and tourmaline mineralization, have higher δ18O values (14–16‰), consistent with their derivation from a purely S-type source, with possible incorporation of, or exchange with, carbonates. Crystallization temperatures calculated using Ti-in-quartz and O-isotope thermometry are in agreement with each other, and range from 420 to 575 °C for LCT pegmatites and 650 °C for pegmatitic SLGs. The juxtaposition of coeval, post-tectonic LCT pegmatites and pegmatitic SLGs with an NYF affinity in the Damara Belt precludes tectonic setting as the only source of compositional variability in LCT and NYF pegmatite melts.