Mineral chemistry and genetic implications of garnet from the São João del Rei Pegmatitic Province, Minas Gerais, Brazil

Sarah Siqueira Da Cruz Guimarães Sousa,Felipe Emerson André Alves,Ciro Alexandre Ávila,Reiner Neumann,Victor Hugo Riboura Menezes Da Silva,Taís Proença Cidade,Fabiano Richard Leite Faulstich

doi:10.1590/2317-4889202120190136

Sarah Siqueira Da Cruz Guimarães Sousa, Felipe Emerson André Alves + Show 5 more

Open Access

https://doi.org/10.1590/2317-4889202120190136

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Abstract

The pegmatites of the São João del Rei Pegmatitic Province are related to the Siderian protoliths of the Cassiterita and Resende Costa orthogneisses and to the Ritápolis metagranitoid of Ryacian age. Chemical analysis of garnet from twelve pegmatites reveal two different types of grains, which were found in the same pegmatitic body in six of these samples. One garnet type has almandine-spessartine composition (Sps11-7-58.8 Alm36.8-86.5 Prp0.1-4.1Grs0.0-1.4Adr0.0-2.6), grains with orange and pink tones, and scarce mineral inclusions. These garnet grains may have been formed at the magmatic stage of pegmatite crystallization. The composition of these grains plot exclusively on the Alm-Sps axis at the Prp+Grs+Adr+Uvr × Alm × Sps diagram, as expected from garnet crystallized in pegmatites, and an expansion of the field associated to pegmatites is proposed. The second type has a distinct chemical composition (Sps26.9-84.8Alm3.6-40.0 Prp0.0-10.4 Grs9.3-45.6 Adr0.1-3.4), displaying enrichment in Ca. This Ca-enriched garnet has irregular shaped colourless grains and abundant mineral inclusions. These grains may have been formed by Ca-metasomatism during the late-stage crystallization of the pegmatites.

Highlights

The garnet supergroup includes all isostructural minerals represented by the general chemical formula {X3} [Y2] (Z3) Φ12, where X, Y, and Z stand for the 12-fold, 8-fold and 4-fold coordination sites, respectively, while Φ is filled with anions such as O2, OH- or F
Spessartine-rich garnet is mostly found in evolved pegmatites of the LCT family (Li-Cs-Ta), while almandine-rich component is common in granites and less evolved pegmatites (Baldwin and von Knorring 1983, London 2008)
Two distinct garnet types were identified in the São João del Rei Pegmatitic Province

Summary

Introduction

The garnet supergroup includes all isostructural minerals represented by the general chemical formula {X3} [Y2] (Z3) Φ12, where X, Y, and Z stand for the 12-fold, 8-fold and 4-fold coordination sites, respectively, while Φ is filled with anions such as O2-, OH- or F- This supergroup has been subdivided into five groups (henritermierite, bitikleite, schorlomite, berzeliite, and garnet) based on symmetry and total charge at the Z site (Grew et al 2013). The isometric garnet group, with a total charge at Z of 12 (occupied by Si4+), have the X-site occupied preferably by Fe2+, Mn2+, Mg2+, Ca2+ and the Y site by Fe3+, Al3+, and Cr3+ (Fig. 1) According to these occupancies, the endmembers pyrope (Mg2+, Al3+), grossular (Ca2+, Al3+), spessartine (Mn2+, Al3+), almandine (Fe2+, Al3+), uvarovite (Ca2+, Cr3+), and andradite (Ca2+, Fe3+) can be formed (Grew et al 2013). Being an unusual composition for pegmatites, garnet with enrichment in Ca have been reported worldwide (Novák et al 2013, Burival and Novák 2018, Tindle et al 2005, Pieczka et al 2019), but until now, garnet with this composition has not been described in the São João del Rei Pegmatitic Province

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