Citation: Parrotti, D.D.; da Conceição, F.T.; Navarro, G.R.B. The Mineralogy, Geochemistry and Origin of the Supergene Manganese Occurrences in the Southern Minas Gerais, Brazil. Minerals 2023, 13, 1216. https://doi.org/10.3390/ min13091216 Academic Editor: Harald G. Dill Received: 6 July 2023 Revised: 4 August 2023 Accepted: 13 September 2023 Published: 15 September 2023 Copyright: © 2023 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). minerals Article The Mineralogy, Geochemistry and Origin of the Supergene Manganese Occurrences in the Southern Minas Gerais, Brazil Davi Diorio Parrotti, Fabiano Tomazini da Conceição* and Guillermo Rafael Beltran Navarro Instituto de Geociências e Ciências Exatas (IGCE)/Rio Claro (SP), UNESP—Universidade Estadual Paulista, Rio Claro 13506-900, Brazil; davi.parrotti@unesp.br (D.D.P.); guillermo.navarro@unesp.br (G.R.B.N.) * Correspondence: fabiano.tomazini@unesp.br Abstract: In equatorial and tropical regions, supergene mineral deposits created during water/rock interactions are found. Simply put, these supergene deposits are formed through the accumulation of low solubility ions or through the preservation of primary minerals. The supergene manganese (Mn) deposits are examples of the economic importance associated with the chemical weathering processes. In Brazil, the Southern Brasilia Orogen (SBO) was generated during the collision between the Paranapanema Craton and the passive margin of the São Francisco Craton. In the southern Minas Gerais (MG), several supergene Mn occurrences are hosted in the SBO, which were originated during the chemical weathering of gondites belonging to the Amparo Complex. Here, we studied the supergene Mn occurrences in the southern MG, more specifically in the municipalities of Ouro Fino and Careaçu. The MnO contents ranged from 25.50 to 28.40 wt% at Ouro Fino and from 16.80 to 21.20 wt% at Careaçu. These supergene Mn deposits have a diverse mineral assemblage, being composed of spessartine, quartz, Mn-oxides, goethite and kaolinite. The various Mn minerals formed due to spessartine incongruent dissolution were hollandites, cryptomelanes, romanechites, pyrolusites and lithiophorites. Both study areas are relevant for the possible opening of mines for the commercialization of Mn. Keywords: mining activities; supergene deposits; manganese ore; Southern Brasilia Orogen; chemical weathering; Brazil 1. Introduction Manganese (Mn) is an element that can occur in nature as oxides, hydroxides, silicates and carbonates, being found in various types of mineral classes [1,2]. Generally, Mn occurs in small proportions in soils as secondary minerals, originated during water/rock–soil interaction [3]. The Mn ore is one of the most essential raw materials for producing metallic alloys, especially combined with iron (Fe) for steel manufacturing. In addition, Mn can also be used in the production of other metal alloys associated with copper (Cu), zinc (Zn), aluminum (Al), titanium (Ti) and lead (Pb), in battery manufacturing and in the chemical industry [4,5]. Mn deposits are classified as hydrothermal, sedimentary or supergene deposits [6], which are the main Mn deposits found in tropical regions [7]. Supergene Mn deposits are associated with pedogenetic processes during the soil formation, reflecting the interactions between the bedrock and the local climate (mainly temperature and rainfall), as well as the vegetation and relief [8,9]. Mn oxides and hydroxides (here called MnOx) are the main secondary minerals forming during water/rock interactions, exhibiting a highly complex and diverse mineralogy [3]. MnOx are typical tunnel or layer structures resulting from the linkage of the MnO 6 octahedra, with high specific surface area and adsorption capacity, allowing the control of metal concentrations associated with soils and sediments in water [10–13]. In 2017, the largest global Mn reserves were described in Brazil (272 Mt—32.3%), South Africa (200 Mt—23.7%), Ukraine (140 Mt—16.6%), Australia (91 Mt—10.8%), India (52 Mt—6.2%), Minerals 2023, 13, 1216. https://doi.org/10.3390/min13091216 https://www.mdpi.com/journal/minerals