Untersuchung geochemischer Anomalien (Niob, Tantal, SEE) in Bachsedimenten des Südböhmischen Batholiths

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Abstract

As part of a master thesis within the context of the FFG project “Coltan in Österreich” stream sediments and granite eluvial material (“Granitgrus”) were sampled and mineralogically and chemically analysed. The main aims of this study were to clarify in which mineral phases niobium, tantalum and associated elements (REE, etc.) are bound and to figure out their provenance. It was of special interest to clarify if these anomalies are idicative for Nb-Ta mineralisation or not. On the basis of known geochemical anomalies (Geochemical Atlas of Austria and follow-up projects) detailed investigations were performed in the northeastern Mühlviertel (Sandl, Upper Austria) and in the western Waldviertel (Weitra, Lower Austria). Two different media were sampled, i.e. fine fraction and heavy mineral concentrates of stream sediments (SM). Additionally three „Granitgrus“ (GG) samples were investigated. The two study areas are located in the Southbohemian Batholith, which is dominated by Weinsberg granite. The local variety of the Weinsberg granite, however, differs petrographically and geochemically from other granites of the same type. In addition to the main granite type granite-cordierite-biotite-rocks, („Pseudo-Kinzingite“) are present. Also more differentiated, more felsic and more peraluminous granites (Plochwald granite) and dark fine-grained amphibole-bearing biotite granites with intercalations of intermediate plutonic rocks (diorite lenses) are minor lithologies present in the study area. Microscopic investigation and Mineral Liberation Analysis (MLA) did not reveal any distinct niobium or tantalum minerals. MLA analyses of mineral concentrates revealed 27 to 84 wt% ilmenite (average 71 wt%) and an average of 14 wt% for GG. The average wt% of apatite was 6 resp. 63 for SM resp. GG and 7 resp. 0,5 wt% for garnet. Using ICP-ES and ICP-MS the geochemical anomalies could be confirmed with 11 – 49 ppm Nb and 0,9 – 4,3 ppm Ta for the fine fraction-samples and 420 – 760 ppm Nb and 40 – 105 ppm Ta for the heavy mineral concentrate samples. Investigations on titanium minerals (ilmenite, TiO2 minerals) with the electron microprobe (EPMA) revealed mean values per sample of 740 - 1535 ppm Nb2O5 and 690 - 1205 ppm Ta2O5. These mineral phases are the exclusive carriers of Nb and Ta, ilmenite being by far the most abundant one. Moreover, considerable within-grain variations of Nb and Ta concentrations in Ti-minerals were determined as well as low Nb/Ta ratios (about 0.5 – 2). Total REE concentrations vary for the fine fraction from 355 – 3780 ppm and for the heavy mineral concentrates from 5200 – 15350 ppm (Sandl) resp. 6790 – 27500 ppm (Weitra) and for Granitgrus from 11450 – 22290 ppm. The main carriers of REE are xenotime, monazite and apatite. The enrichment of HREE in concentrates from Sandl is due to the presence of higher amounts of xenotime. An important conclusion of this study is that the studied geochemical anomalies do not indicate distinct Nb-Ta-mineralisations but represent higher regional geological concentrations of niobium,tantalum and SEE.

Details

Translated title of the contributionInvestigations of geochemical anomalies (niobium, tantalum, REE) in stream sediments of the Southbohemian Batholith
Original languageGerman
QualificationDipl.-Ing.
Supervisors/Advisors
Award date19 Dec 2014
Publication statusPublished - 2014