Detrital tourmaline as an indicator of provenance: a chemical and sedimentological study of modern sands from the Black Hills, South Dakota

Detrital tourmaline has proven useful as a provenance indicator mineral of ancient sedimentary/metasedimentary units due to its presence in many rock types, chemical responsiveness to environments of formation, complex and variable chemical compositions, high resistance to chemical and mechanical weathering, and stability through diagenesis and metamorphism. This study further establishes detrital tourmaline as a provenance indicator mineral by examining the chemical and sedimentological relationships between modern detrital tourmalines in the sediments of the Black Hills, South Dakota, USA, and in situ tourmalines from southern Black Hills’ tourmaline-bearing metasedimentary rocks, granites, and rare-element enriched pegmatites. Results show that detrital tourmaline is chemically effective at indicating provenance down to the scale of individual drainage areas within a geographically complex region. The wide range of chemical variation in Black Hills detrital tourmalines demonstrates that a single source, consisting of multiple rock types, can produce a tremendous amount of chemically diverse detrital tourmalines. However, the effects of source rock lithology, grain size, and hydrodynamic behavior on detrital tourmaline can greatly influence the detrital tourmaline signature of a source area. Of the three dominant tourmaline-bearing lithologies in the Black Hills, only two, granites and metasedimentary rocks, contribute significant amounts of tourmaline detritus to the sediment in the basins studied. Rare-element pegmatites contribute little or no tourmaline detritus due to the limited volume of these pegmatites as well as the large size of tourmalines in these pegmatites. The ratio of granitic to metamorphic detrital tourmaline varies with sand size fraction (very coarse, coarse, medium, fine, very fine); granitic tourmalines dominate at larger fractions and metamorphic tourmalines dominate at smaller fractions. This trend can be attributed to differences in both tourmaline size and erosive potential between these lithologies. This trend is noteworthy for provenance analyses using tourmaline because, as this study demonstrates, heavy mineral analyses of small, narrow sand fractions will be biased towards smaller, metamorphic tourmalines and can imply incorrect proportions of lithologies in the source area. Without advance knowledge of distance of transport, using a wide size fraction range for tourmaline provenance analysis is recommended to obtain the most accurate results