TY - JOUR

T1 - Mn-Fe-rich genthelvite from pegmatites associated with the Madeira Sn-Nb-Ta deposit, Pitinga, Brazil

T2 - new constraints on the magmatic-hydrothermal transition in the albite-enriched granite system

AU - Hadlich, I. W.

AU - Bastos Neto, A. C.

AU - Pereira, V. P.

AU - Botelho, N. F.

AU - Ronchi, L. H.

AU - Dill, H. G.

N1 - Publisher Copyright: © 2024 Cambridge University Press. All rights reserved.

PY - 2024/2/13

Y1 - 2024/2/13

N2 - 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.

AB - 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.

KW - albite-enriched granite

KW - beryllium

KW - Brazil

KW - genthelvite

KW - Pitinga

KW - solid solution

UR - http://www.scopus.com/inward/record.url?scp=85187244543&partnerID=8YFLogxK

U2 - 10.1180/mgm.2023.87

DO - 10.1180/mgm.2023.87

M3 - Article

AN - SCOPUS:85187244543

VL - 88

SP - 111

EP - 126

JO - Mineralogical magazine

JF - Mineralogical magazine

SN - 0026-461X

IS - 2

ER -