The origin of diamond-bearing quartzite breccias from the Sopa Brumadinho Formation, Southern Espinhaço Range, Minas Gerais, Brazil

Revista Espinhaço, 2025, 15 (1) ISSN 2317-0611 https://doi.org/10.5281/zenodo.17553543 The origin of diamond-bearing quartzite breccias from the Sopa Brumadinho Formation, Southern Espinhaço Range, Minas Gerais, Brazil A origem das brechas quartzíticas diamantíferas da Formação Sopa Brumadinho, Serra do Espinhaço Meridional, Minas Gerais, Brasil Pedro Angelo Almeida-Abreu UFVJM https://ORCID 0000-0001-6273-4809 [email protected] Renato Ferreira Miranda LEV Brasil renatogeove[email protected] Gislaine Amorés Battilani UFVJM https://ORCID 0000-0003-2554-5253 [email protected].edu.br Eduardo Fontana UFVJM https://ORCID 0000-0002-1963-7795 [email protected] Abstract The breccias of the Sopa-Brumadinho Formation, recognized as diamond-bearing since 1840, have had an igneous origin proposed since 1928. Their quartzite/metarenite clasts are millimetric to 50 cm, angular, pinkish to red in color, and similar to the substrate rocks. Their predominantly phyllitic matrix has a composition incompatible with that of igneous rocks, but REE data indicate a kimberlitic nature, with diamond xenocrysts, which can exceed 30 ct. The bodies (locally intrusive) have varying shapes— including a “champagne glass” shape—and reach up to 10 m thick and more than 50 m long. A proposed origin by cohesive debris flows from adjacent steep slopes appears unfeasible due to the consistent presence of diamonds in all deposits along a 50 km N–S extension. Since mudflows are not mineral concentrators and considering the absence of exotic material and the universal presence of diamonds, a proximal origin through the reworking of diamond-bearing tephra (tuff rings or pyroclastic surges) must be considered. Furthermore, the shape of the deposits and their contact relationships show that the breccias are confined within craters generated by phreatomagmatic volcanism, such as at Mina do Barro, where the breccias, circumscribed by quartzite walls, reach at least 60 m in depth. The physical characteristics of the gems reveal an absence of sedimentary reworking, and their mineral inclusions indicate a lithospheric origin for the diamonds. Regionally expressed lineaments appear to have acted as conduits for kimberlite magmas to reach the surface around ~1.7 Ga. Keywords: Diamondiferous breccia; Sopa-Brumadinho Formation; phreatomagmatic volcanism; Southern Espinhaço range. Revista Espinhaço, 2025, 15 (1) ISSN 2317-0611 https://doi.org/10.5281/zenodo.17553543 Resumo As brechas da Formação Sopa-Brumadinho, reconhecidas como diamantíferas em 1840, tem origem ígnea advogada desde 1928. Mostram clastos de quartzitos/metarenitos milimétricos a 50cm, angulosos, de coloração vermelha-rosada, semelhante às rochas do substrato. Sua matriz predominantemente filítica tem composição incompatível com rochas ígneas, mas as terras-raras mostram natureza kimberlítica e hospeda xenocristais de diamantes, que podem superar 30ct. Os corpos (localmente intrusivos), mostram formas variadas – inclusive em “taça-de-champagne” - e exibem espessuras de até 10m e extensão lateral maior que 50m. Uma suposta origem por fluxos de detritos coesivos oriundos de encostas íngremes mostra-se inviabilizada pela recorrência diamantífera em todos os depósitos numa extensão N-S de 50 km. E desde que mudflows não são concentradores de minerais e considerando a ausência de materiais exóticos e a presença universal de diamantes seria imperioso admitir origem proximal pelo retrabalhamento de tefra diamantífera (tuff-rings ou pyroclastic-surges). Ademais, a forma de jazimento e relações de contato evidenciam que as brechas estão acondicionadas em crateras geradas por vulcanismo freatomagmático como na Mina-do-Barro, onde as brechas, circunscritas por paredes quartzíticas, alcançam pelo menos 60m de profundidade. As feições físicas das gemas revelam ausência de retrabalhamento sedimentar e suas inclusões minerais indicam origem litosférica dos diamantes. Lineamentos de expressão regional emergem como estruturas condutoras dos magmas kimberlítico à superfície em ~1.7 Ga. Palavras-chave: Brecha diamantífera; Formação Sopa-Brumadinho; vulcanismo freatomagmático; Serra do Espinhaço Meridional. 1. Introduction Diamond exploration in the Southern Espinhaço Range (SER) dates to the early 18th century, when the Diamond District was established, becoming the world's largest diamond producer until the mid-19th century. The region's diamonds were initially mined from alluvial and colluvial deposits of the drainage network (mainly the Jequitinhonha Basin) and, later (from the 19th century onward), also from conglomerates and breccias of the Sopa Brumadinho Formation (SBf). Although the rocks of the Espinhaço Supergroup exposed within the study area are metamorphosed and deformed, they almost always preserve their sedimentary identity and, therefore, will be referred to according to their respective protolith, without the use of the prefix metaor metamorphic equivalents. The source areas and primary rocks of diamonds have been the subject of controversy since the 19th century, based on two main hypotheses: (1) kimberlites from a source area outside the Espinhaço Basin, i.e., the São Francisco Craton; (2) syn-sedimentary volcanism associated with the SBf within the Espinhaço Basin. The quartzite breccias of the SBf, according to the autochthonous origin hypothesis, would represent vent breccias of phreatomagmatic magmatism from the final phase of SBf deposition (Almeida Abreu, 1996). On the other hand, from the perspective of diamonds originating outside the basin, the breccias would be sedimentary in origin, i.e., gravitational deposits of the debris flow type (Chaves, 1997; Uhlein, 1991), with diamonds derived from the interior of the São Francisco Craton and transported by the river system into the Espinhaço Basin. Revista Espinhaço, 2025, 15 (1) ISSN 2317-0611 https://doi.org/10.5281/zenodo.17553543 Despite their economic importance and controversial origin, these breccias have received limited study regarding deposit geometry, petrography, and lithogeochemistry. Due to their friability and diamond content, most quartzite breccia deposits in the SER have already been almost completely mined, except for those in the João Boa and Córrego Novo mines, and the remaining deposits in the Barro Branco, Brumadinho, and Caldeirões mines. The Córrego Novo mine is currently the only one actively mining quartzite breccias in the SER. In this work, therefore, the geological characterization of breccias from the Córrego Novo and Brumadinho mines was conducted through detailed geological mapping, stratigraphic section surveys, and lithogeochemical studies - integrated with investigations of mineral inclusions in diamonds from the region and the analysis of geophysical data (aeromagnetometry) provided by CODEMIG. 2. Geological Context The Southern Espinhaço Range (SER), an orogenic belt that delimits the southeastern edge of the São Francisco Craton (Almeida, 1977) (Figure 1), extends northward from the Iron Quadrangle for approximately 300 km. It is predominantly composed of Paleoto Mesoproterozoic rocks of the Espinhaço Supergroup (Almeida Abreu and Renger, 2002), especially metarenites/quartzites, pelites (phyllites), conglomerates, and volcanic rocks metamorphosed under greenschist facies conditions (Almeida Abreu, 1993; Dossin et al., 1984; Martins Neto, 1998, 2000; Schöll and Fogaça, 1979). Revista Espinhaço, 2025, 15 (1) ISSN 2317-0611 https://doi.org/10.5281/zenodo.17553543 Figure 1. Simplified Geological Map of the Southern Espinhaço Mountain Range and adjacent regions (adapted from Renger et al., 2025). The rectangle locates figure 2. Towns and villages indicated on the map: CM – Conselheiro Mata; Da – Datas; DO – Desembargador Otoni; Dta – Diamantina; Ex – Extração; Gd – Guinda; PK – Presidente Kubitschek; Se – Serro; SJ – São João da Chapada. Revista Espinhaço, 2025, 15 (1) ISSN 2317-0611 https://doi.org/10.5281/zenodo.17553543 The stratigraphic succession of the Espinhaço Supergroup was divided by Pflug (1968) into eight lithostratigraphic units. Pflug's column was revised in subsequent works and subjected to nomenclature adjustments (Pflug and Renger, 1973, Schöll and Fogaça, 1979), as well as subdivisions into smaller units and the grouping of formations into the Guinda (Knauer, 1990) and Conselheiro Mata (Dossin et al., 1984) groups. Sedimentological studies carried out since the 1980s have led to new interpretations of the depositional systems and the basinal context of the Espinhaço Supergroup units. (Garcia and Uhlein 1987; Dossin et al. 1984; Almeida-Abreu 1993; Uhlein et al. 1998; Martins-Neto 1998, 2000, Almeida-Abreu and Renger, 2007). Figure 2. Geological map of the Córrego Novo mining area (original scale 1:8,000, Miranda, 2019). In section A-B, the structures observed under the site of the occurrence of quartzite breccias (subvertical black dashed lines) represent the conduits of phreatomagmatic injections. Revista Espinhaço, 2025, 15 (1) ISSN 2317-0611 https://doi.org/10.5281/zenodo.17553543 The SBf is of special relevance to this study, as it represents the only unit of the Espinhaço Supergroup mineralized in diamonds. It is composed essentially of siliciclastic sediments, including diamond-bearing conglomerates and, subordinately, basic and alkaline volcanic rocks, including vent breccias, i.e., the diamond-bearing quartzite breccias that make up the Campo Sampaio Member, the top unit of the SBf (Almeida Abreu, 1996). The Córrego Novo mine has quartzite breccias as the only rock mineralized in diamonds and is located in the diamond district of São João da Chapada – Campo Sampaio (Figure 2). Almeida Abreu and Renger (2007) presented the stratigraphic column of the successions that make up the Espinhaço System in the different domains of the SER, considering the respective basinal environments. Martins-Neto (1998, 2000) proposed that the Espinhaço Basin would be a sag basin, assuming that the iron formations on the eastern edge of the SER would be of early Proterozoic age, belonging to the Minas Supergroup of the Quadrilátero Ferrífero. However, evidence of late Paleoproterozoic ages of these iron formations (Silveira-Braga et al., 2015; Rolim et al., 2016) reinforces the original model of Pflug (1965), i.e., the Espinhaço Basin configures a passive continental margin (originally defined as a geosyncline), and therefore, the iron formations were deposited on the continental shelf of the basin. Other geological evolution models for the Espinhaço Supergroup deal with the occurrence of superimposed rifting at ~1700 Ma and ~1250 Ma, i.e., “sag cycles,” the latter remaining active during the Neoproterozoic until undergoing tectonic inversion in the Brasiliano Cycle (Chemale et al., 2012; Alkmim and Martins-Neto, 2012; Santos et al., 2013; Guadagnin and Chemale, 2015). On the other hand, Almeida Abreu and Renger (2002, 2007) argue that the orogenesis of the SER occurred before 1 Ga, given that the glaciogenic deposits of the Macaúbas Group, which form fringes on the western and northeastern edge of the mountain range, are discordantly based on different units of the Espinhaço Supergroup. This glaciation ("Jequitaí Glaciation") occurred around 1 Ga (D’Agrella Filho et al., 1990), and this age is reinforced by the absence of clasts of basic rocks of the Pedro Lessa Suite (906 ± 2 Ma, Machado et al., 1989) in its glaciogenic deposits. This suite constitutes a LIP (Chaves et al., 2019) that currently occupies approximately 2.5% of the SER surface (Hoppe, 1986), and many of its dikes occupy 1stand 2nd-order lineaments that cut tectonic structures shaped in rocks of the Espinhaço Supergroup. Data and information on the evolution of geological knowledge of the Espinhaço Mountain Range, including different models of geological evolution, can be found in Renger (1979), Almeida Abreu (1993), and Renger and Knauer (1995). Discussions on the discovery, Revista Espinhaço, 2025, 15 (1) ISSN 2317-0611 https://doi.org/10.5281/zenodo.17553543 exploration, and provenance of diamonds from the Espinhaço Mountain Range were presented by Chaves et al. (1998, 1999), Almeida Abreu and Renger (1999, 2001), and Renger et al. (2025). 3. Material and methods The work was developed in different stages. In the pre-field stage, the lineaments marked by anomalies in the magnetometric images (CODEMIG, 2001, scales 1:80,000 and 1:30,000) were traced and classified into three groups based on the intersection relationships between them. The representation of morphostructural lineaments was performed on Google Earth satellite images at a scale of 1:30,000, while shaded relief maps (illumination direction of 315° and inclination of 45°) were generated from SRTM images provided by USGS Earth Explorer. By superimposing the lineament maps on the detailed geological map of the Córrego Novo Mine area, we sought to establish correlations between the lineaments and structures with the regional distribution of quartzite breccias. During the pre-field phases a bibliographic survey was also carried out, accompanied by a synthesis of regional geology with a special focus on the bibliography dealing with the geology of diamond districts. Furthermore, bibliographic research was carried out on diamond geology, as per the summary provided in the chapter of this article. The subsequent stage was dedicated to the survey of detailed geological sections in the Córrego Novo and Brumadinho mines, accompanied by the description of 64 outcrops and the collection of 26 rock samples. Of the collected samples, nine (three hematitic phyllites and six diamondiferous quartzite breccias) were selected for petrographic studies from thin sections prepared in the rock lamination laboratory of CeGeo and described in a Zeiss Axio-LabA1 petrographic microscope of the Microscopy Laboratory of CeGeo/UFVJM. The mineral phases not recognized by the microscope were analyzed in a scanning electron microscope (SEM), using a HITACHI TM 3000 equipment with EDS attached from the multiuser laboratory of UFVJM. Two samples of reddish phyllites, two samples of kaolinized phyllites from two decimetric dikes embedded in quartzites of the bedrock under breccia body and two samples of quartzite breccia matrix were selected for lithogeochemical studies, analyzed by the SGS - GEOSOL Laboratory to obtain the content of major and minor elements using ICP-OES (inductively coupled plasma optical emission spectrometry) and trace elements (Ba, Sr, Zn, Zr, V, Co, Cs, Cu, Ga, Hf, Mo, Nb, Ni, Rb, Sn, Ta, Tb, Th, Tl, U, W, Y) and rare earths using ICP-MS (inductively coupled plasma mass spectrometry). Revista Espinhaço, 2025, 15 (1) ISSN 2317-0611 https://doi.org/10.5281/zenodo.17553543 Additional gravimetric studies were performed using a Sintrex CG-05 gravimeter, provided by the National Observatory within the PegBR program. Data acquisition was obtained through three 30-second readings, with four profiles being executed in the Córrego Novo mine area, with an approximate spacing of 30 meters between points. The data were processed using the GTraj software and interpolated using the minimum curvature technique, resulting in the Bouguer anomaly map (Figure 3). 4. The Córrego Novo Mining (MCN) 4.1. Location The MCN is located in an elevated area that is part of the plateau (altitudes slightly above 1400 asl) of the São João da Chapada – Campo Sampaio Diamond District. In this district, diamond mining in rocks has always been carried out in quartzite breccias of the SBf, that is, in the mines of Barro, Pagão, Campo Sampaio Velho and Novo, Água Fria, João Boa and Córrego Novo. The MCN occupies the elevated western flank of the Córrego Novo (a 1st order tributary of the right bank of the upper Rio Pardo Grande) approximately 10 km to the NW of São João da Chapada, which is approximately 30 km from Diamantina. The location of the mining front where part of the sections and sampling were carried out has the coordinates UTM – WGS84 – 23K – 625266m E; 8006242m S. Mineral exploration at MCN, then as semi-mechanized mining, has its first records of activity in the early 1970s. However, it was referred to by Thompson (1928) and Correns (1932) as a locus of "faiscagem" (rudimentary method of manual mining) since the first quarter of the 20th century). From 2015 onwards, it was taken over by Córrego Novo Mining with the mechanized exploitation of quartzite breccias and, in places, colluvial deposits in the vicinity of the mine pits. The open pits are implemented in the area where quartzite breccias occur and the dismantling operation is carried out with hydraulic jets and backhoes, with the material being transported by trucks to the pre-concentration site where, under aqueous flow, the clayey material is separated from the denser residues, and these are transported to the jigs of the concentration plant. MCN management information reports production of 3,131.84 ct between 2017 and 2024, an average of 448.27 ct/year, generally 3/1 diamonds (i.e., 3 diamonds to add up to 1 ct). The largest diamonds extracted from the Córrego Novo Mine were 6.30; 7.85; 8.27; 9.30; 11.45; and 33.48 ct, the latter extracted in December 2022. 4.2. Summary of the Geology of the MCN Area Revista Espinhaço, 2025, 15 (1) ISSN 2317-0611 https://doi.org/10.5281/zenodo.17553543 The three formations that make up the Guinda Group outcrop in the MCN area (Figure 2). The lower unit, the São João da Chapada Formation, occupies the eastern part of the map, dominating the eastern flank of the Córrego Novo. It features rugged relief with predominantly exposed bare rock interspersed with restricted areas of shallow, sandy soil. It consists exclusively of fineto medium-grained sandstones, locally coarse, occasionally with the presence of sparse, subangular to rounded granules or pebbles of milky quartz. Plane-parallel lamination is defined in decimetric to metric beds and banks, and tabular cross-stratifications with thicknesses of 40 cm to over one meter are common and sometimes abundant. Contact with the overlying unit does not outcrop in the map domain and its lower contact is marked by an inverse fault that juxtaposes it with rocks of the SBf throughout its western limit (Figure 2). The occurrence of hematitic phyllites along the margin of this fault throughout its extension (Figures 2 and 3) and considering the synsedimentary origin of these phyllites, suggests that this is a past normal fault, reactivated during the tectonic inversion. This fault, in addition, is conjugated to the first-order crustal lineaments (Figure 8). The SBf occupies the entire central part of the map, forming a NNW-SSE direction band with a width of 350 to 1000 meters. The SBf is limited to the east by a semi-linear reverse fault (N20W) that juxtaposes the São João da Chapada Formation along the entire length of the contact between these two formations and whose lineament hosts the Córrego Novo thalweg. To the W, the SBf is concordantly covered by quartz-arenites of the Galho do Miguel Formation. The SBf, within the scope of the MCN, shows a predominance of fine to coarsegrained sandstones, moderately selected, highlighting beds in centimetric to decimetric layers, in places defining metric-scale banks. Cross-stratifications are abundant, predominantly of the trough type with high width/height ratios, although tubular and tangential types are also frequent, whose dimensions vary from decimetric to metric and with paleocurrents indicating flow in the dominant W to E direction. The maturity of the sediments, their sedimentary structures and directional flow of the paleocurrents suggest that they are fluvial deposits of a braided channel system. The eastern strip of the SBf exposure area, i.e., the strip adjacent to the reverse fault that juxtaposes the São da Chapada Formation (Figure 2), exhibits particular tectonic deformation, i.e., a ductile-brittle deformation zone marked by conspicuous foliation (S1) trending N5 to 15W and dipping to the ENE (45-55°), highlighted mainly in hematitic phyllites rocks. Tight anticlines of submetric to metric order are molded in quartzites of this zone, occasionally associated with reverse faults of centimetric to metric rejects displacement seen on their Wvergent ramps. Revista Espinhaço, 2025, 15 (1) ISSN 2317-0611 https://doi.org/10.5281/zenodo.17553543 A petrographic study of the breccia matrix was performed after impregnation with resin to produce thin sections. Under the microscope, extensive presence of quartzite lithic fragments and quartz grains (50-55%) immersed in the sericitic matrix (40-45%) is observed, with frequent presence of opaque minerals (iron oxides), titanium oxides (according to SEM–EDS analyses, probably titanomagnetite and rutile, up to 2%) and rare zircons (<1%) (Figure 6a). The quartz grains are angular to subrounded, generally with undulatory extinction, frequently fractured and exhibiting corrosion at the edges (Figure 6b). Sericitized euhedral grains appear to represent “ghosts” of feldspars or feldspathoids (Figure 6c). Quartzite breccias from other mines, both in the São João da Chapada-Campo Sampaio and in the Sopa-Guinda districts, show similar characteristics, i.e., angular quartzite clasts, generally reddish in color, predominantly pelitic matrix and invariably containing diamonds. Quartzite breccias occur invariably in the upper part of the SBf, always associated with a succession of pelites containing layers of sandstone and siltstone, as seen in sections from the Brumadinho Mine (Guinda District - Figure 7). 5. Structural Lineaments The analysis and interpretation of aeromagnetometric images enabled the identification of structural lineaments revealed by the magnetic intensity signals. Three main groups of lineaments were recognized, which were active in different tectonic events. In the cratonic domains the largest lineaments stand out, which can be referred to as 1st order structures. However, these and other lower order lineaments (in terms of linear extension of the segments and not the number of segments) generally have recurrent movements as a result of tectonic cycles and events that have involved the cratons since Archean times. The analysis of the lineaments imprinted in cratonic areas covered by sedimentary successions and/or orogenic belts almost always show records of synsedimentary activities related to extensional tectonics, as well as to the subsequent reactivations promoted by the Precambrian orogenic cycles (Figure 8). The recognized lineaments present, according to their orientations, different dimensions and frequencies, in addition to showing age relationships between them. Quantitatively, the WNWESE trending lineaments are predominant, especially in the northern half of the SER and Serra do Cabral (Figure 8a,b), and, although segmented, the segments are often perfectly aligned for extensions of many tens of kilometers. Chaves (2013), considering that these lineaments are filled by rocks from the Januária mafic dike swarm of Mesoproterozoic age, related them to the intracratonic rifting responsible for the formation of the Espinhaço Superior basin. Revista Espinhaço, 2025, 15 (1) ISSN 2317-0611 https://doi.org/10.5281/zenodo.17553543 The second most abundant lineaments are the NE-SW trending lineaments, which generally preserve extensions slightly larger than the WNW-ESE pattern. However, the lineaments with the largest extension (although infrequent) exhibit NNW-SSE orientation, standing out as the youngest family (and, or of late reactivations), since they overlap the others (Figure 8b). Their first-order character is marked by significant extension and continuity in intermittent segments, and they commonly intercept all lineaments of other directions, therefore, of more recent kinematic activity. Its preferred northwest direction (N20-30°W) inflects to N40°W in the São Francisco Craton domain (Figure 8a) from where it extends continuously in an echelon pattern for more than 350 km, entering the SER through its NW domain, and being marked by strong positive magnetic anomalies. The longest of these NNW-SSE trending lineaments, maintaining its en echelon pattern, traverses the São João da Chapada - Campo Sampaio diamond district, continuing to the SopaGuinda diamond district and then to the São Gonçalo do Rio das Pedras region, until it intersects the SER in its central-eastern domain (Figure 8a). Along its course, the lineament crosses the diamond-bearing districts at a distance of less than 10 km from their respective mines. Caldas et al. (2019) interpret that this lineament constitutes a large reactivated normal fault and that it would have acted in the opening of the Espinhaço Basin and, associated with smaller structural lineaments, would have been responsible for the compartmentalization of the basin. Revista Espinhaço, 2025, 15 (1) ISSN 2317-0611 https://doi.org/10.5281/zenodo.17553543 Figure 6. Photomicrographs of MCN breccia matrix (images on the left side under natural light and those on the right side under crossed Nicols). (a) and (b) The breccia matrix shows quartzite and quartz grains, and opaque minerals embedded in a sericitic (partially kaolinized) matrix. (c) and (d) Matrix image of the breccia showing intensely corroded quartz grains. (e) and (f) Image of breccia matrix showing sericitized rectangular euhedral grain, which may represent a feldspar or feldspathoid “ghost”. This proposal of Caldas et al. (2019) is not supported by observations in the rocks of the São João da Chapada and Sopa Brumadinho formations, given that the paleocurrent record of fluvial sediments from these formations invariably shows a W-to-E sedimentary flow. The quasiunidirectional sedimentary flow was obviously determined by the paleo-relief and N-S synsedimentary faults that controlled the direction and propagation of the basin, which is consistent with the general orientation of the Espinhaço basin. The basin's Revista Espinhaço, 2025, 15 (1) ISSN 2317-0611 https://doi.org/10.5281/zenodo.17553543 compartmentalization was modulated primarily by W-E or WNW-ESE trending transfer faults (Almeida Abreu, 1995). Figure 7. Section in the south-central segment of the Brumadinho Mine. Note the different geometry of the breccia bodies and their association with pelitic rocks. A few dozen meters to the east of the upper-eastern part of the section, sandstones of the Galho do Miguel Formation outcrop overlying the SBf pelites, demonstrating that the breccias are included in the upper part of the lithostratigraphic unit. UTM location at the ends of the section: WNW Point (639612 – 7882795) and ESE Point (639767 – 7982729). Furthermore, a pair of lineaments that are part of this NNW-SSE family individualize the SER of the northern segment of the orogenic belt, i.e., the northern Serra do Espinhaço (Figure 8a, b). These lineaments can be interpreted as dextral strike-slip faults that would have been responsible, therefore, for the segmentation of the Serra do Espinhaço before the end of its orogenesis, since the termination of both segments is covered by fringes of glaciogenic rocks from the Jequitaí Glaciation (~1. Ga, D´Agrella Filho et al., 1990). The lineaments with the largest population within the SER, that is, those with a WNW-ESE direction, are revealed by a notable magnetic anomaly, as they are almost always filled with basic rocks of the Pedro Lessa Suite. Almeida-Abreu (1993, 1995) admits that these lineaments were very active during the crustal extension of the Espinhaço Basin, effectively influencing the paleogeographic arrangement of the basin. Revista Espinhaço, 2025, 15 (1) ISSN 2317-0611 https://doi.org/10.5281/zenodo.17553543 The major lineaments in the domains outside the SER, that is, those with a NW-SE orientation, are also filled with basaltic rocks which, dated by the Ar-Ar method (Chaves and Neves, 2005), revealed a crystallization age of 135.72 +/- 4 Ma. According to Chaves (2003), these mafic dikes are part of the Transminas dike swarm, and their formation is associated with intracontinental rifting related to the opening of the South Atlantic. The lineaments with a preferential northeast direction (N10-30° E) are mostly marked by positive magnetic anomalies and a bundle of these lineaments are prominent in the diamond district of São João da Chapada, also being present in the diamond districts of Sopa-Guinda, Extração, Datas and Presidente Kubitschek. According to Chaves (2013), these are associated with the Formiga lineaments (900-100 Ma) and are also commonly filled by the Pedro Lessa Suite dike swarm. The satellite and SRTM images present morphostructural lineaments and highlight all the lineaments identified in the aeromagnetometric anomaly images (Figure 8d, e), also evidencing the greater frequency of lineaments with WNW-ESE directions. It is possible to observe that, in the diamond district of São João da Chapada, the magnetic lineaments with NE-SW and WNW-ESE orientation intersect very close to the diamond mines, especially in the MCN area. The mine area is marked by a significant morphostructural lineament with an approximate NNW direction, which intersects the NE-SW and WNW-ESE magnetic lineaments (Figure 8c). 6. Lithogeochemistry The chemical composition of the matrix of quartzite breccias (M-B), phyllite dikes (D) and reddish phyllites (FV) from the Córrego Novo mine can be seen in Table 1. The matrix of quartzite breccias exhibits high levels of SiO2 (64 - 75%) and Al2O3 (15 - 24%) and low to moderate levels of K2O (2 - 3%), Fe2O3 (1%), MgO (0.6 - 1%), TiO2 (0.4 - 0.7%) and, to a lesser extent, CaO (0.02%), Na2O (0.03%) and P2O5 (0.05%). However, the influence of their clasts and the host quartzite rocks on the composition of the matrix is evidenced by the pervasive contamination of quartz, also observed in thin sections of the rock (Figure 6 a, b). Furthermore, the sediments and rocks of the continental successions of the Espinhaço Supergroup were subjected to severe weathering during and after sedimentation, which promoted leaching and profound alteration of the rocks and sediments especially in the rocks and sediments that contained labile minerals (Almeida Abreu, 1993). These conditions of severe weathering are also indicated by the absence of clasts from the basement and diamond source rock (including their satellite minerals) in the conglomerates and breccias of the SBf. On the Revista Espinhaço, 2025, 15 (1) ISSN 2317-0611 https://doi.org/10.5281/zenodo.17553543 other hand, the hematitic phyllites are records of the hot and humid climate during the deposition of the São João da Chapada and Sopa Brumadinho formations, as they are recognized as paleo-laterite/-bauxite deposits from the weathering alteration of volcanic rocks (Knauer and Schrank, 1993). The current exposure of the SBf successions (probably since the Pleistocene, Amaral-Santos et al. 2019) represents another weathering cycle imposed on the rocks of the unit, which contributes to the restrictions on the geochemical characterization (especially in relation to the major elements) of all rocks of igneous origin. With regard to minor elements and rare earths, indications of the nature of the rocks, that is, of the reddish phyllites, the matrix of the breccias and the synsedimentary “phyllite” dikes (Table 1) are seen when comparing the samples with diagrams of the primitive mantle (Figure 9). It can be seen that the quartzite breccias, “phyllite” dikes and reddish phyllites have very similar behavior in the negative anomalies of Sr, Nb and Ti and positive anomalies of Th, K and Zr. Revista Espinhaço, 2025, 15 (1) ISSN 2317-0611 https://doi.org/10.5281/zenodo.17553543 Figure 8. (a) and (b) Structural lineaments of the SER (outlined by the strong black line) recognized by anomalies from aeromagnetometric surveys, and from SRTM (morphostructural) and satellite images. The black points S, G, and E in image (b) locate the villages of Sopa, Guinda and Extração, respectively. (c), (d) and (e) Detail of the structural lineaments in the diamond district of São João da Chapada – Campo Sampaio, including the MCN area (identified by the crossed hammers). 1 - João Boa Mine; 2 - Campo Sampaio Mine; 3 - Água Fria Mine; 4 - Pagão Mine; 5 - Barro Mine. The total rare earth elements of the breccias are enriched by 10 to 200 times more than those of the chondrites. The distribution curve indicates a preferential fractionation of light rare earths with ratios of La/Yb ranging from 13 to 28 with an average of 20.51, while the negative anomaly of Europium (Eu/Eu*) is 0.18 - 0.2. Revista Espinhaço, 2025, 15 (1) ISSN 2317-0611 https://doi.org/10.5281/zenodo.17553543 Table 1. Chemical composition of the matrix of diamond breccias (M-B1, MB-2), phyllite dikes (D1, D2) and reddish phyllites (F-A1, F-A2) of MCN. Major elements expressed as weight percentage and trace elements and rare earths in ppm. Revista Espinhaço, 2025, 15 (1) ISSN 2317-0611 https://doi.org/10.5281/zenodo.17553543 Figure 9. Representation of the analyzed samples in the primitive mantle composition diagram of Sun and McDonough (1989) and of the REE in the chondrite diagram of Anders and Grevesse (1989). MB – diamond bearing quartzite breccias; FV - reddish phyllites. Revista Espinhaço, 2025, 15 (1) ISSN 2317-0611 https://doi.org/10.5281/zenodo.17553543 The reddish phyllites are basic rocks composed essentially of SiO2 (45%), Al2O3 (35%), Fe2O3 (4.8-5.5%) and subordinately TiO2 (1-1.2%), K2O (0.5-0.6%), MgO (0.15%). The total rare earth elements of these reddish phyllites are enriched by 60 to 200 times more than those of the chondrites, and the distribution curve indicates a slight preferential fractionation of the light rare earths, with ratios of La/Yb ranging from 5 to 7, and the negative anomaly of Europium (Eu/Eu*) is 0.21. The REE patterns of the breccias and reddish phyllites show a strong correlation of 0.808 using the Pearson correlation coefficient. In relation to the reddish phyllites, the breccias present a similar LREE pattern and are more depleted in HREE. Comparing the fractionation ratio of the REE using the La/Yb ratio, it is possible to verify that the breccias present a much higher average pattern (20.51) than the reddish phyllites (6.48). Figure 10 – Comparison of the REE patterns of breccias (MB), reddish phyllites (FV), and phyllite hematitic (average patterns of the Bezerra Neto, 2016; and Cabral et al. 2012); olivine lamproites (Jacques et al., 1989); phlogopite lamproites (Mitchell and Bergman, 1991), and micaceous kimberlites (Mitchell and Brunfelt, 1975). Note that the breccias present a negative gap relative to the REE of hematitic phyllites and reddish phyllites but present a LREE fractionation similar to that of hematitic phyllites, with the correlation index between breccias and hematitic phyllites being 0.95. The REE patterns obtained for quartzite breccias and reddish phyllites were compared with the average patterns of hematitic phyllites (Bezerra Neto, 2016; Cabral et al., 2012), olivine lamproites, phlogopite lamproites (Mitchell and Bergman, 1991) and micaceous kimberlites (Mitchell and Brunfelt, 1975) (Figure 10). The breccias present a negative gap in relation to the HREE when compared to hematitic phyllites and reddish phyllites, presenting a LREE fractionation similar to hematitic phyllites. The correlation index of the breccias with the Revista Espinhaço, 2025, 15 (1) ISSN 2317-0611 https://doi.org/10.5281/zenodo.17553543 explosions, the entire tephra column at the diatreme may be affected and partially or completely fluidized at temperatures between 200 and 450 °C, and the heated groundwater will alter the existing minerals (e.g. olivine to serpentine) and may also dissolve certain minerals (e.g. carbonate) and fayalite is altered to magnetite and dissolved H2 and SiO2. The breccias of the São João da Chapada – Campo Sampaio Diamond District are located on a plateau at approximately 1,400 meters asl, and are located, along with the other lithologies that accompany them, in relatively shallow depressions with a N-S to NNW-SSE orientation, ranging in length from 350 to 1,300 meters and widths between 200 and 600 meters (these crater-depressions are certainly flattened by compressional tectonics, with narrowing of their E-W sections). The succession of sandstones of the São João da Chapada Formation that surrounds the breccias and associated lithologies, according to the escarpments that surround the plateau, has an outcrop thickness of around 250 meters. Therefore, this is the minimum thickness of sedimentary rocks that were traversed by the conduits of phreatomagmatic volcanism for the formation of the craters and subsequent “deposition” of the diamond-bearing breccias in these troughs. The total thickness of the sedimentary succession beneath the craters may be greater than 250 meters, since a thick sill of basic rocks of the Pedro Lessa Suite (posttectonic related to the SER orogenesis) is located beneath the outcropping succession of the São João da Chapada Formation around the plateau. However, it is likely that this sill is located at the interface of the sedimentary cover with the crystalline substrate, i.e., the pre-Espinhaço basement. As mentioned, Kurszlaukis and Lorenz (2008) propose that magma rises through rocks to shallow levels and in a first explosion close to the surface it can create a relatively wide and shallow crater and only through successive explosions does a diatreme develop which, in a continuous process, deepens and widens the crater with the formation of classic carrot-shaped diatremes. As magma rises to shallow levels, the level of neutral buoyancy can be reached due to the loss of volatile when the ascent speed decreases and sills can form and, in this case, with the cooling of the pressurization, the processes typical of phreatomagmatic volcanism cease. Apparently, in the mines of the Diamond District of São João da Chapada – Campo Sampaio, only the first explosion occurred in the context of the phreatomagmatic volcanic evolution that would lead to the development of classic diatremes. Why? Could these magmas be somewhat deficient in volatiles? From this perspective, according to the presence of flat-bottomed trigons in the diamonds of the district, there would have been interactions with magmas rich in H₂O during the final stages of the ascent (Müller, 2021), which may have led to the loss of volatiles through chemical Revista Espinhaço, 2025, 15 (1) ISSN 2317-0611 https://doi.org/10.5281/zenodo.17553543 reactions. A low water table saturation of the sedimentary succession seems unlikely according to the indications of a humid climate, but in the case of sandy sedimentary rocks, therefore very porous (especially when poorly consolidated), it could lose significant volumes of water vapor due to the heating effect of the kimberlitic fluids (temperatures of up to 450°C inside the diatreme are reported by Kurszlaukis and Lorenz, 2008). It can also be assumed that the high porosity of the host sedimentary succession could dissipate a significant part of the volatiles during the ascent of the magma, as the pores are emptied by the loss of water vapor. 8. When, Where and Why Diamonds Were Formed The transition from the Archean to the Proterozoic at ~2.5 Ga has been considered as the apogee of the formation of cratonic cores, which represent most of the surface of the presentday Earth's crust. However, Mazumder et al. (2019, 2022) report two major changes in the Neoarchean: the cooling of the mantle and the stabilization of major cratons at 2.7 to 2.5 Ga, highlighting even, based on the occurrence of a 3.2 Ga paleosol and alluvial deposits on the Singhbhum craton (India), higher continental freeboard already prevailed during the Neoarchean. Retallack et al. (2023) also reported the presence of paleosols on an Archean craton (3.3 Ga) in western Australia. On the other hand, Hawkesworth et al. (2020, and references therein) proposed a reduction in crustal growth rate at ~3 Ga, suggesting a change in the processes of generation and/or preservation of the crust due to the advent of plate tectonics as the dominant process of crust generation. They also referred to the compilation of isotopic and bulk chemical data of silicate and sulfide inclusions in diamond with peridotitic composition before 3.2 Ga, whereas after 3.0 Ga eclogitic inclusions became prevalent. The eclogitic inclusions are considered to arise from the subduction of mafic material, and hence to the onset of plate tectonics, further suggesting that by 3 Ga the volume of continental crust was between 65% and 70% of its present-day volume. Smit et al. (2022, and references therein) referred as an updated definition of a craton, “regions of the Earth's continental crust that are underlain by 150–200 km thick continental lithospheric mantle providing long-term stability since at least 1 Ga, therefore, around 63% of exposed continental crust and 18% of Earth's surface can be considered as cratonic”. Pintér et al. (2022) pointed out that the thick mantle lithosphere beneath the cratons — the ancient cores of the continents — has cold, stable roots reaching ~250 km depth, which consists of strongly reduced rocks that have reacted with oxidized incipient melts with low-silica, and Revista Espinhaço, 2025, 15 (1) ISSN 2317-0611 https://doi.org/10.5281/zenodo.17553543 rich in CO2 and H2O. The reaction of the melts with the surrounding rocks forms an enriched zone at the base of the lithosphere, which is the source region for many diamonds. Diamonds reside in the lithospheric mantle beneath cratons, and the intersection of peridotite derived conductive geotherms with the diamond stability field, led to the suggestion that diamonds have formed through nearly all of Earth’s history in distinct episodes that can often be linked to larger-scale tectonic processes (Smit et al. 2022). Most diamond inclusion minerals originate in the sub-continental cratonic mantle lithosphere and only a small percentage of mineral inclusions originate beneath the lithosphere (~1%, Stachel and Harris 2008). Inclusions in diamonds are classified as protogenetic (preceding diamond formation), syngenetic (cocrystallizing with diamond) or epigenetic (crystallizing after diamond formation). Typically, a syngenetic origin for inclusions has been inferred if, regardless of their crystal system, the inclusions show a cuboidal-octahedral morphology that is imposed by their diamond hosts, which is most commonly the case (Walter et al. 2022). The fluid inclusions trapped in fibrous diamonds from the lithosphere report that the parental fluids exhibit a range in composition, including highand low-Mg carbonatitic, chlorine-rich and silica-rich aqueous fluids (Walter et al. 2022). Repeated episodes of crystallization and/or additional diamond growth have occurred in mantle domains of diamond stability throughout much of Earth's history, primarily in the lithospheric upper mantle beneath primitive continental cores, but also in the asthenosphere and in tectonically activated crustal rocks, phenomena associated with subduction-related melt generation, migration of metasomatic fluids, and reaction with preexisting peridotite, eclogite, and websterite in the mantle (Gurney et al., 2010). The major Archean crystallization of smooth crystalline diamonds in the Kaapvaal, Slave and Siberian cratons in metasomatized harzburgite provides evidence that by 3 Ga stable, buoyant, chemically depleted and reduced continental craton cores extended to depths at temperatures at which diamonds can be stable and stored for long periods of geological time. The subsequent formation of smooth crystalline diamonds in the subcontinental lithospheric mantle is closely associated with the establishment of these early craton cores. Fibrous cuboidal diamonds and fibrous layers in smooth crystalline diamonds are also of metasomatic origin, growing as xenocrysts in the same lithospheric mantle, but with much shorter mantle residence times, typically <5 million years. Their formation, shortly before sampling and magmatic transport to the crust, suggests that they represent a final burst of diamond formation as metasomatic influx of volatile components increased in the lithosphere prior to the kimberlite eruption (Gurney et al., 2010). Revista Espinhaço, 2025, 15 (1) ISSN 2317-0611 https://doi.org/10.5281/zenodo.17553543 Despite the occurrence (sometimes abundant) of microdiamonds and macrodiamonds (the latter occurring locally and always smaller than 3 ct) in rocks from the Archean and Lower to Middle Paleoproterozoic (Gurney et al., 2010), diamonds larger than 5 ct appear only in the late Paleoproterozoic (Statherian Period), precisely in cratonic areas of Brazil (Roraima and Espinhaço Range), which, therefore, reports the Diamond Province of the SER as the oldest in the history of the Earth. On the other hand, diamonds larger than 150 ct have their advent with the emergence of “CLIPPIR” (Cullinan-like, Large, Inclusion-Poor, Pure, Irregular, and Resorbed) around 1200 Ma (Stenian Period of the Mesoproterozoic), specifically in the Premier Kimberlite of South Africa (Gurney et al., 2010). Diamonds of 400 ct and larger have been found in diamond provinces of the Paleozoic (Siberia), Mesozoic (in different countries of Africa and in western Minas Gerais) and Cenozoic (Canada) (Kjarsgaard et al., 2022). Diamonds in detrital rocks aged between 3.3 and 3.5 Ga found in the Slave craton (Canada) show a combination of high δ13C and δ15N values, demonstrating that marine crustal carbonate was present in the source, brought to depths >170 km, possibly through subduction processes (Timmerman et al., 2022). This leads to the assumption that diamonds are formed primarily by the supply of organic carbon from the crust through recurrent subduction processes, thus allowing us to conjecture that the increase in the quantity and size of diamonds throughout Earth's history was due to the increasing incorporation of segments of oceanic crust into the mantle environment. In this case, the advent of robust diamond deposits with crystals measuring centimeters to decimeters from the end of the Paleoproterozoic (Zhang et al., 2024), which then includes the Diamond Province of the SER, and especially of the Mesoproterozoic era (Premier kimberlite and Argyle lamproite) and the subsequent growth in the quantity and volume of diamond deposits during the Phanerozoic, would not have been fortuitous. In a tight synthesis, it can be concluded that diamonds from the subcontinental lithosphere are formed under the metasomatism of carbonatitic or carbonated silicate melt (Babu et al. 2023). A somewhat similar conclusion was reached by Pintér et al. (2022) through experimental work, when they observed that the thick mantle lithosphere beneath cratons consists of strongly reduced rocks that have reacted with oxidized melts. The low-silica incipient melts are rich in CO2 and H2O and react with surrounding rocks forming an enriched zone at the base of the lithosphere, which is the source region for many diamonds. Also referred to as an alternative mechanism to subducted or delaminated crust, or underplating by voluminous silicate magmas for the formation of pyroxenitic garnet and clinopyroxene inclusions in some lithospheric diamonds. Revista Espinhaço, 2025, 15 (1) ISSN 2317-0611 https://doi.org/10.5281/zenodo.17553543 9. Conclusions In the SER and adjacent domains, diamonds are not known in Precambrian rocks or successions prior to (or subsequent to), the deposition of the SBf, a fact that allows us to conclude: (1) kimberlitic volcanism occurred at approximately 1.7 Ga, considering the ages obtained in synsedimentary volcanic rocks intercalated in the SBf (Dossin et al., 1993; Hagedorn, 2004; Bezerra-Neto, 2016); (2) this age therefore reveals that the diamond province of the SER is the oldest in Earth's history, since those of Argyle and Premier are from the Mesoproterozoic (Smit et al., 2022); (3) the intrinsic crustal instabilities of the SBf, according to its rudite deposits related to tectonic movements throughout the edge of the sedimentary basin, establish a cause and effect relationship with kimberlitic magmatism. The SBf breccias are located in successions at the top of the unit and show notable characteristics of in situ formation or very proximal sources, while the conglomerates that make up the lower successions of the unit have, in most of their clasts, characteristics of somewhat distant provenance, that is, tens of kilometers to the W (Renger et al., 2025), therefore, at least two generations of kimberlitic magmatism occurred in the SER. The intersection of structural lineaments of significant regional extent, i.e., those with a NNW-SSE orientation with the lineament bundles with a NE-SE and WNW-ESE orientation (Figure 8) within the São João da Chapada – Campo Sampaio Diamond District, as well as in the Diamond Extraction District, stand out as the crustal-lithospheric structures that provided the kimberlite magmatism of the diamond province of the SER. Despite composing deposits of a suis generis character when compared to the occurrences of breccias and associated rocks of kimberlite extrusions in other regions of the planet, the diamond-bearing breccias of the SER, especially those of the MCN, evoke the geological environment of maar craters typical of kimberlite volcanism, because: (1) all their occurrences within the SER are diamond-bearing (including gems of up to 38 ct) and mineral inclusions of their diamonds show affinity with peridotite rocks; (2) the breccias and associated deposits are housed in depressions shaped in quartzite rocks that surround them, configuring narrow and elongated troughs (craters); (3) the rocks that generated the clasts, i.e., quartzites (often reddish), compose the substrate of the breccia mines; (4) the pelitic and areno-pelitic successions that cover the breccias show sedimentary arrangements and structures characteristic of aggradational deposition in a lacustrine environment; (5) hundreds of narrow “phyllite” dikes permeate the quartzites of the substrate that support the breccias, and must represent alteration products (hydrothermal and weathering) of fluids ejected by Revista Espinhaço, 2025, 15 (1) ISSN 2317-0611 https://doi.org/10.5281/zenodo.17553543 phreatomagmatic volcanic processes; (6) although rare, breccia dikes embedded in arenites/pelites exposed in the Caldeirões and Córrego Novo mines have been recognized. 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