A comparison is made between Ta-Nb mineral assemblages of two Li-enriched but mineralogically distinct rare-element granitic pegmatites in the Black Hills, South Dakota. The Bob Ingersoll No. 1 pegmatite is lepidolite-bearing, whereas the Tin Mountain pegmatite is spodumene-bearing. Both pegmatites display a diverse assemblage of Ta-Nb minerals. A general paragenetic sequence of the Ta-Nb mineral species in these pegmatites is represented by: ferrocolumbite --> manganocolumbite --> manganotantalite --> microlite + tapiolite (+ wodginite). In both pegmatites, the Ta/(Ta + Nb) ratio of the columbite-tantalite (CT) increases systematically f rom the wall zone to the core. The systematic increase of Ta at the expense of Nb in the pegmatites correlates with a sequence of crystallization determined by other mineral indicators (i.e., tourmaline, K-feldspar, mica). The wall zone, which crystallized first, contains CT with the lowest Ta/(Ta + Nb) ratios. The intermediate zones crystallized simultaneously and show an overlap in Ta/(Ta + Nb) values. The core contains the highest levels of Ta and was the last unit to consolidate. In addition, the Mn/(Mn + Fe) ratio in the CT increases from wall zone to core. An early Mn-enrichment of the CT occurs in the wall zone of the lepidolite-bearing pegmatite, and may parallel an early F-enrichment of the crystallizing melt. The difference in F content between the two pegmatites also influenced the differences in Sn mineralization. Microlite and tapiolite in each pegmatite have the highest Ta/Nb of the Ta - Nb oxides. In the core of the Tin Mountain, microlite, tapiolite, and wodginite are associated with albite-mica replacement of spodumene + quartz. The microlite contains evidence of hydrothermal alteration following crystallization. An increase in f(O2) during crystallization may be indicated by Fe3+ in the wodginite and microlite.
