Chemical composition of gahnite and degree of pegmatitic fractionation in the Borborema Pegmatitic Province, northeastern Brazil

Abstract

Gahnite electron probe microanalyses from three pegmatites (Mirador, Capoeira and Quintos) of the Borborema Pegmatitic Province in northeastern Brazil allowed to determine molar compositions ranging from 86.

Nb–Ta–(Ti–Sn) oxide mineral chemistry as tracer of rare-element granitic pegmatite fractionation in the Borborema Province, Northeastern Brazil

“…At least in two pegmatites, e.g. Quintos and Capoeira 2, other mineral chemistry data on feldspar, white mica, garnet, tourmaline and gahnite give additional support to this explanation (Soares 2004;Soares et al 2007).…”

Melt evolution in felsic dikes inferred from the composition of gahnite in two new occurrences, Pala Chief and Elizabeth R composite dikes, California

“…In granitic pegmatite dikes gahnite is commonly overlooked due to its small size, even though it may occasionally form emerald green semi-precious stones and occur spatially associated with economic rare-metal (Li, Ta) mineralization (e.g., Morris et al 1997;Soares et al 2007). In analyzing the chemical composition of pegmatite dikes and their minerals, Zn is often not included in the list of elements analyzed for as it is typically not of economic importance in this kind of rock nor is gahnite considered a classic petrogenetic Editorial handling: L. Danyushevsky indicator.…”

“…In analyzing the chemical composition of pegmatite dikes and their minerals, Zn is often not included in the list of elements analyzed for as it is typically not of economic importance in this kind of rock nor is gahnite considered a classic petrogenetic Editorial handling: L. Danyushevsky indicator. Therefore, the information currently available for gahnite in granitic pegmatite dikes is scarce (e.g., Černý and Hawthorne 1982;Batchelor and Kinnaird 1984;Neiva and Champness 1997;Szuszkiewicz and Łobos 2004;Soares et al 2007;Merino et al 2013;Neiva 2013) compared to other pegmatite minerals that are economically important or typically used as petrogenetic indicators (i.e., muscovite, feldspars, tourmaline, beryl, Li-bearing minerals, Ta-Nb oxides, etc. ).…”

“…Limited previous studies indicate that the chemical composition of gahnite can potentially provide important information about the evolution of a pegmatite melt (e.g., Batchelor and Kinnaird 1984;Soares et al 2007) and may also serve as an exploration guide in the search for economic rare-metal deposits in pegmatites (Morris et al 1997). In spite of this, there is only a very limited number of detailed chemical studies of gahnite in pegmatite-aplite dikes and even fewer have recognized intra-crystal chemical variations in gahnite that can reveal indications of melt evolution (Beurlen et al 2014).…”

“…In spite of this, there is only a very limited number of detailed chemical studies of gahnite in pegmatite-aplite dikes and even fewer have recognized intra-crystal chemical variations in gahnite that can reveal indications of melt evolution (Beurlen et al 2014). Because gahnite occurs in some highly evolved granitic pegmatites that contain economic rare-metal mineralization, in particular of Li and Ta (Morris et al 1997;Selway et al 2005;Soares et al 2007;Beurlen et al 2014), as well as gem-quality minerals (e.g., Soares et al 2007;Beurlen et al 2014), a better understanding of the variations in the chemical composition of gahnite in felsic dikes can potentially be useful as petrogenetic indicators for the evolution of melts that form pegmatites and their strategic rare-element mineral deposits. Further, even though Zn is considered incompatible during the evolution of magmatic melts (Taylor 1965), very scarce information on the behavior of Zn during magma evolution is available from the study of Zn-rich minerals in granitic pegmatite-aplite dikes and granitoids.…”