Mn-Fe-rich genthelvite from pegmatites associated with the Madeira Sn-Nb-Ta deposit, Pitinga, Brazil

new constraints on the magmatic-hydrothermal transition in the albite-enriched granite system

verfasst von: I. W. Hadlich, A. C. Bastos Neto, V. P. Pereira, N. F. Botelho, L. H. Ronchi, H. G. Dill
Abstract: We studied the genthelvite from pegmatite veins hosted by the albite-enriched granite (ca.1.8 Ga) which corresponds to the Sn-Nb-Ta (F, REE, Li, Zr, U, Th) Madeira deposit (Amazonas, Brazil). Genthelvite, the exclusive Be-bearing mineral within the deposit, occurs as massive crystals of up to 4.7 cm. Compositions are homogeneous within individual crystals, but there is moderate variability in the overall composition. This reflects relatively limited substitutions within the helvine-genthelvite-danalite solid solution system, with relatively high Zn contents (36.96 to 49.45 wt.% ZnO), lower Mn contents (0.61 to 3.03 wt.% MnO), and variable Fe contents (2.10 to 10.94 wt.% FeO), filling an up-to-date compositional gap in this system. Genthelvite formed in an alkaline and subaluminous environment, under stable conditions within the late-evolved fluids, at relatively high temperature (>400°C), and in a reducing environment. The extremely high concentration of fluorine in the magma and the crystallisation of magmatic galena resulted in an effective reduction of H2S fugacity. This allowed for the stability of genthelvite during the transition from the late magmatic to early hydrothermal stages of the albite-enriched granite’s evolution. The variability in Fe content within genthelvite is primarily associated with localized variations in the mineral assemblage (e.g., riebeckite, polylithionite). Genthelvite was affected by low-temperature aqueous fluids rich in F. This alteration allowed for the incorporation of Fe, Mn, Mg, Pb, Ba, Na, K, U, and REE in the Zn²⁺ structural site, as well as the allocation of excess Si, Al, Ti, and P in the ^IVSi and ^IVBe structural sites. The substantial content of U and REE substituting for Zn, as well as Si substituting for Be, is charge balanced by the presence of vacancies at the A-site.
Organisationseinheit(en): Institut für Mineralogie
Externe Organisation(en): Universidade Federal do Rio Grande do Sul
Universidade de Brasilia
Universidade Federal de Pelotas
Typ: Artikel
Journal: Mineralogical magazine
Band: 88
Seiten: 111-126
Anzahl der Seiten: 16
ISSN: 0026-461X
Publikationsdatum: 13.02.2024
Publikationsstatus: Veröffentlicht
Peer-reviewed: Ja
ASJC Scopus Sachgebiete: Geochemie und Petrologie
Elektronische Version(en): https://doi.org/10.1180/mgm.2023.87 (Zugang: Geschlossen)

BibTeX

@article{d77b5e1fa0504938ae1b3f03d7ebeb94,
title = "Mn-Fe-rich genthelvite from pegmatites associated with the Madeira Sn-Nb-Ta deposit, Pitinga, Brazil: new constraints on the magmatic-hydrothermal transition in the albite-enriched granite system",
abstract = "We studied the genthelvite from pegmatite veins hosted by the albite-enriched granite (ca.1.8 Ga) which corresponds to the Sn-Nb-Ta (F, REE, Li, Zr, U, Th) Madeira deposit (Amazonas, Brazil). Genthelvite, the exclusive Be-bearing mineral within the deposit, occurs as massive crystals of up to 4.7 cm. Compositions are homogeneous within individual crystals, but there is moderate variability in the overall composition. This reflects relatively limited substitutions within the helvine-genthelvite-danalite solid solution system, with relatively high Zn contents (36.96 to 49.45 wt.% ZnO), lower Mn contents (0.61 to 3.03 wt.% MnO), and variable Fe contents (2.10 to 10.94 wt.% FeO), filling an up-to-date compositional gap in this system. Genthelvite formed in an alkaline and subaluminous environment, under stable conditions within the late-evolved fluids, at relatively high temperature (>400°C), and in a reducing environment. The extremely high concentration of fluorine in the magma and the crystallisation of magmatic galena resulted in an effective reduction of H2S fugacity. This allowed for the stability of genthelvite during the transition from the late magmatic to early hydrothermal stages of the albite-enriched granite{\textquoteright}s evolution. The variability in Fe content within genthelvite is primarily associated with localized variations in the mineral assemblage (e.g., riebeckite, polylithionite). Genthelvite was affected by low-temperature aqueous fluids rich in F. This alteration allowed for the incorporation of Fe, Mn, Mg, Pb, Ba, Na, K, U, and REE in the Zn2+ structural site, as well as the allocation of excess Si, Al, Ti, and P in the IVSi and IVBe structural sites. The substantial content of U and REE substituting for Zn, as well as Si substituting for Be, is charge balanced by the presence of vacancies at the A-site.",
keywords = "albite-enriched granite, beryllium, Brazil, genthelvite, Pitinga, solid solution",
author = "Hadlich, {I. W.} and {Bastos Neto}, {A. C.} and Pereira, {V. P.} and Botelho, {N. F.} and Ronchi, {L. H.} and Dill, {H. G.}",
note = "Publisher Copyright: {\textcopyright} 2024 Cambridge University Press. All rights reserved.",
year = "2024",
month = feb,
day = "13",
doi = "10.1180/mgm.2023.87",
language = "English",
volume = "88",
pages = "111--126",
journal = "Mineralogical magazine",
issn = "0026-461X",
publisher = "Mineralogical Society",
number = "2",
}