Mineralogical and chemical evolution of tantalum–(niobium–tin) mineralisation in pegmatites and granites. Part 2: Worldwide examples (excluding Africa) and an overview of global metallogenetic patterns

摘要

AbstractColumbite-group minerals (CGM) account for the majority of the production of tantalum, an important metal for high-technology applications. Along with other Ta–Nb oxides such as tapiolite, wodginite, ixiolite and pyrochlore supergroup minerals, CGM are recovered from rare-metal granites and granitic rare-element pegmatites. In this paper mineralogical and geochemical data with a focus on CGM, tapiolite, wodginite and ixiolite are presented for rare-element granites and pegmatites from worldwide occurrences except Africa that has been covered in a previous contribution (Melcher et al., 2015). Major and trace element data of the Ta–Nb oxides are presented and compared for a total of 25 granite/pegmatite provinces, and one carbonatite for comparison. Based on CGM compositions, the data allow to distinguish between various subgroups of Li–Cs–Ta (LCT)-family pegmatites, Nb–Y–F (NYF)-family pegmatites, mixed LCT–NYF pegmatites, and rare-element granites.Each period of Ta-ore formation in Earth history is characterised by peculiar mineralogical and geochemical features. Some of the largest and economically most important rare-element pegmatite bodies are located within Archean terrains and intruded ultramafic and mafic host rocks (e.g., Tanco/Canada, Wodgina and Greenbushes/Western Australia, Kolmozero/Kola). They are highly fractionated, of LCT affinity throughout and yield complex mineralogical compositions. The variety of minor and trace elements incorporated attests to a rather insignificant role of the immediate host rocks to their geochemical signature and rather points to the significance of the composition of the underlying crustal protoliths, internal fractionation and the processes of melt generation. Many of the Archean pegmatites carry significant Li mineralization as spodumene, petalite, and amblygonite, and all of them are also characterised by elevated Li in CGM. In addition, Sb and Bi are important trace elements, also reflected by the occasional presence of stibiotantalite and bismutotantalite. REENpatterns of CGM are dominated by the MREE or HREE, and range from very low to high total REE concentrations. Negative Eu anomalies are omnipresent. Scandium contents are also highly variable, from very high (Tanco) to very low concentrations (Wodgina, Kolmozero).A second period of worldwide pegmatite formation was in the Paleoproterozoic. All CGM analysed derive from LCT-family pegmatites except samples from the Amazonas region where Ta is mined from rare-metal granites at Pitinga. Pegmatites intruded highly variable lithologies including metasediments, metabasites, gneiss, granite and quartzite within a variety of structural and paleogeographic settings; however, most of them are syn- to post-orogenic with respect to major Paleoproterozoic orogenic events. Minor and trace element signatures are similar to CGM from Archean pegmatites. Some are characterised by considerable REE enrichment (S?o Jo?o del Rei/Brazil; Amapá/Brazil; Finnish Lapland/Finland), whereas others have normal to low total REE concentrations (Black Hills/USA, Bastar/India). Examples with high REE commonly are enriched in Sc and Y as well, and are often transitional to NYF-family pegmatites.The Mesoproterozoic period is comparatively poor in rare-element pegmatites and rare-metal granites. Mineralogical and chemical attributes of ixiolite–wodginite, tapiolite, CGM and rutile from placer material in Colombia point to an unusual pegmatite source of NYF affinity, yield