Gold Mineralization at Morro Do Ouro: A Late Proterozoic Orogenic Gold Deposit in the Espinhaço Range, Brazil

Gold mineralization in alluvial deposits of the Southern Espinhaço Mountains has been known since the 17^th century, but the recognition of primary gold mineralization in quartz veins embedded in supracrustal rocks dates back to the mid-19^th century and has been explored ever since. Morro do Ouro located in the central area of the Espinhaço Range is made up of hematitic phyllites from the basal unit of the Espinhaço Supergroup and on its frontal slope (west) it is delimited by an inverse fault that allowed the percolation of metasomatic fluids and the generation of gold-bearing quartz veins. The gold particles are embedded in the quartz of veins that cut through and, or are lodged in the schistosity of the hematitic phyllites. In the Serra do Espinhaço, dozens of gold deposits similar to those of Morro do Ouro are known, including the Mil Oitavas, Serra do Pasmar and Costa Sena mines. All this gold mineralization are epithermal deposits due to the low temperatures of the mineralizing fluids and their late insertion in relation to the orogenic structures, also characterizing them as lode deposits in the context of the regional orogenic gold deposits. The occurrence of similar gold mineralization in rocks of the Minas Supergroup (Paleoproterozoic) of the Iron Quadrangle indicates the regional character of the event and the mineralization processes. This late Neoproterozoic gold mineralization was the result of regional crustal heating caused by the subduction of the oceanic plate related to the tectonic inversion of the Brasiliano/Pan-African orogeny

Keywords:Gold; Morro do Ouro; Orogenic epithermal deposits; Espinhaço mountain range

The colonization of the Serra do Espinhaço Meridional (Southern Espinhaço Range – SER) dates back to the 17th century with the first settlements by adventurers in search of gold and precious stones. Gold mining in alluvial deposits and drainage basins in the region has been recorded since the end of the 17th century [1] and reached production of several tons with the exploration of the Jequitinhonha River and its tributaries during the 20th century by mining with bucket dredges from the Tejucana and Rio Novo companies [1]. The primary source of gold that mineralized the drainages and colluviums of the region was recognized in the 19th century [2-5], that is, the quartz veins that occur embedded in supracrustal rocks that build the SER. These primary sources are distributed throughout the entire length of the SER, given that all drainages in the orogenic belt have a greater or lesser concentration of gold in their alluvium, and although no large primary gold deposit is known within the SER, dozens of small mines have been or are being explored in different domains of the mountain range. The lack of large deposits, i.e., world-class deposits, therefore made it unlikely for large mining companies to set up gold mining in the SER, but, on the other hand, these small and medium-sized deposits cause less environmental impact for their exploration, in addition to providing more sustainable mining involving prospectors and miners from the region itself.

The main objective of the research carried out in this work focused on the characterization of the gold mineralization of Morro do Ouro and then seek to establish a relationship with other gold deposits in the SER, in order to understand the genesis and tectonic controls of the primary gold deposits in the Espinhaço Range region. Furthermore, knowledge of the origin and control of gold mineralization in the SER leads to the rational exploration of the region’s gold deposits, which may facilitate the prospecting and location of new gold deposits in the SER.

The Serra do Espinhaço (original name given by Eschwege [2] as Espinhaço Gebirge), an orogenic belt from the Mesoproterozoic, protrudes for about 1,200km in a N-S direction across central Brazil and is segmented into different orographic domains [6]. The southern portion, exposed in the north-central region of Minas Gerais (Figure 1), constitutes the Southern Espinhaço Range (SER), whose substrate is granitoids from the Basal Complex (BC) and successions from the Rio Paraúna Supergroup (RPSg) - schists, metavolcanites and quartzites. The SER building however, is mainly made up of Paleo-Mesoproterozoic rocks from the Espinhaço Supergroup (ESg). This supergroup, within the SER, is represented by three formations of the Guinda Group - São João da Chapada (SJf), Sopa Brumadinho (SBf) and Galho do Miguel - and by five formations of the Conselheiro Mata Group (Santa Rita, Córrego dos Borges, Córrego Bandeira, Córrego Pereira and Rio Pardo Grande), which form a narrow strip in the western part of the mountain range [6,7].

Figure 1:Simplified geological map of the Southern Espinhaço Range (SER) and adjacent regions (adapted from Renger et al. [1]). The rectangle indicates the area shown in Figure 2. Towns and villages labeled 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.

In tectonic windows of the median-central belt of the SER rocks from the BC and schists and quartzites from the RPSg outcrop, as well as metabreccias, hematitic phyllites and metarenites from the SJf and ferruginous metarenites and phyllites from the SBf of the ESg. In the area of the Morro do Ouro Mine (UTM 641070 E; 7971640 N) the rocks are structured according to the N-S trend (consistent with the regional tectonic pattern [8]), with kinematic indicators of mass transport from east to west, highlighting thrusts with basal detachment associated with reverse faults of low to high angle (Figure 2). Late brittle structures trending W-E and NW-SE segment the sedimentary pile and its basement rocks into different blocks, and these structures are almost always filled with intrusive rocks of the Pedro Lessa Metaigneous Suite (PLMS - Large Igneous Province of post-tectonic basaltic magmatism from about 940 - 906Ma [9,10]). The lower ESg units that outcrop in the central area of the SER exhibit greenschist facies metamorphism (~450 °C and 4-5Kb [8]), which indicates a tectonic pile of at least 8km when the orogenic edifice was formed [11]. Readers interested in learning about the evolution of knowledge on the geology of the SER, as well as the different models of geological evolution of the orogenic belt, can find information in Renger [5], Renger & Knauer [12] and Almeida-Abreu & Renger [13].

Figure 2:Geological Map of the Morro do Ouro Area, Datas/MG (central area of the SER). The original map scale is 1:10,000. Section A-B has 5x vertical exaggeration.

The studies of this work were aimed at characterizing the gold mineralization of Morro do Ouro and its relationship with other gold deposits of the SER in order to understand the genesis of these deposits. The research began with a bibliographic review of the geology of the SER, especially of the central region of the orogenic belt, as well as a bibliographic review of the works that addressed the gold geology of the SER, and also a review of the updated literature on the geology of gold. During and after bibliographic studies field work was carried out in the Mil Oitavas and Pasmar mines and, above all, geological mapping work on a scale of 1:10,000 of the Morro do Ouro area (Figure 2). Once the geological mapping was completed, detailed studies of the physical characterization of the gold mineralization of Morro do Ouro were carried out in the exposures of the old cavities and in the exploration pits, with dimensioning of the gold bearing quartz veins, recognition of the associated minerals and collection of samples for lithogeochemical and petrographic analyses.

Samples from different veins exposed throughout the pit area were collected (from eleven veins that cut the schistosity and from veins accommodated in the schistosity). The collected samples were pulverized for separation of gold grains by manual pan concentration. The larger grains obtained (between 3 and 12mm) were separated for textural characterization and subsequent chemical analysis. During the fieldwork, broadband magnetotelluric surveys were carried out using a Metronix ADU-07 device from the National Observatory (Ministério da Ciência, Tecnologia e Inovação - Government of Brazil). Six data collection stations were established for a 24-hour period and, of the six stations, four presented data with good signals for the interpretation of subsurface geology. The results of the laboratory analyses were visualized in light of the field data and correlated with the results of gold mineralization studies in similar environments of the SER. The results of the studies were then integrated into the regional geology of the SER and compared with the results of similar studies in other gold mines of the region, in order to enable the interpretations and conclusions of this work.

Morro do Ouro is a protruding body of hematitic phyllites measuring approximately 300m in the N-S direction and 500m in the W-E direction, bounded by slopes higher than 50m (Figure 3-5). Together with metabreccias and metarenites it forms the base of the SCf in the area, and on the frontal (western) slope it overlies SCf metarenites, being bounded to the E by a thrust surface that rests RPSg schists on the hematitic phyllites (Figure 2). The proposals regarding the controversial origin of the hematitic phyllites were summarized by Knauer & Schrank [14], when they proposed their origin from paleo-laterite/-bauxite by the Proterozoic weathering of volcanic rocks, later metamorphosed during the Espinhaço Orogeny. In the Morro do Ouro area this proposal appears to be coherent, since the gradual passage of alternating greenschists and hematitic phyllites (UTM 640423E; 7971925 N, Figure 3) can be observed and, furthermore, the concentration of disseminated specularite at millimetric to centimetric levels at the top of Morro do Ouro (as in other outcrops of hematitic phyllites in different locations of the SER) suggests that it is an “iron hat” related to Proterozoic weathering that, after metamorphism, recrystallized in the form of hematite/specularite.

Figure 3:Google Earth image (dated 07/29/2024) of the Morro do Ouro mine area. Ravines and cavities can be observed at the mining front in the central part of the image (see Figure 2 for comparison). The white arrow marks the road section where the gradual transition from greenschists to hematitic phyllites is exposed, illustrating the Proterozoic laterization process affecting the greenschists. Locations P1 to P5 indicate the Magnetotelluric (MT) stations corresponding to the profiles presented in Figure 5.

Figure 4:3D image of Morro do Ouro (visible as the prominent feature on the right) obtained via drone survey, operated by Prof. Glauco Umbelino from the Center for Studies in Geosciences (ICT–UFVJM). Note the juxtaposition of the hematitic phyllites of Morro do Ouro against the elevated terrain to the west (left side of the image), composed of level C metarenites of the SCf. This contact results from reverse displacement along the “Morro do Ouro Fault.”

Figure 5:Images of magnetotelluric signals (2-D models) of stations P1 and P3, and P4 and P5 (points located in Figure 3). Discussion and explanations about the images are included in the text.

Morro do Ouro is composed of light to dark gray hematitic phyllites (rock composed essentially of sericite and variable amounts of hematite - up to 15 wt%-, occasionally with the presence of small amounts of tourmaline and quartz [8,12]), with an apparent thickness of around 50 to 100m. It exhibits very fine conspicuous schistosity (denoting the character of volcanic ash) and a mottled texture, that is, diffuse, oval, submillimeter to subcentimeter white spots, with or without pink spots, sometimes in levels arranged according to the foliation. The presence of hematite (usually specularite) is common, disseminated or concentrated in millimeter to centimeter levels, while magnetite is less common in octahedral crystals of millimeter dimensions dispersed or concentrated in contact with quartz veins. On the western slope of Morro do Ouro the occurrence of quartz veins is particular (in quantity and dimensions, Figure 3) with a centimeter thickness over 25cm and lateral extension that can reach several meters. Milky quartz of white or reddish pink to yellowish color predominates and, rarely, hyaline quartz. They are distributed predominantly in the foliation, but isolated branches and veins with directions transverse to the foliation are common, with indications of at least two generations or pulses of siliceous fluids. The presence of randomly dispersed quartz concentrations on surfaces over surfaces sub orthogonal to the foliation of the hematitic phyllites is frequent. At the contact of veins, the presence of a yellowish mass with whitish spots of a terrigenous nature rich in clay, including kaolinite, is common, apparently generated by the alteration of the host rock upon contact with hot (metasomatic) fluids, i.e., solutions rich in SiO₂ and other elements. Also noteworthy on the surface of quartz veins is a bright yellow to “mustard or ochre” mica, forming a rough mass, which, according to RDX and EDX analysis [15], (Figure 6), must represent a solid solution of muscovite, biotite and phlogopite and, probably, with the presence of alunite, according to the presence of SO₃ in the chemical composition of the analyzed minerals.

Figure 6:RDX diagram of the rough mass of bright yellow to ochre micas occurring on the surfaces of quartz veins at the Morro do Ouro mine. Considering the hues of the micas and the EDX chemical analyses, this mass likely represents a solid solution of muscovite, biotite, and phlogopite (Vieira-Santos & Almeida-Abreu [15]). Notably, the presence of SO₃ in the mica composition suggests a secondary occurrence of alunite.

It is not uncommon to observe shapeless gold particles of submillimeter to millimeter dimensions (sometimes up to 1.2cm) attached to quartz, sometimes associated with specularite. Apparently, the gold is mainly associated with the layered veins in the foliation of the hematitic phyllites, especially when terrigenous masses occur as the envelope of these veins. The gold mineralization is restricted to the western portion of the hematitic phyllite layers that make up the “Morro do Ouro”, a fact clearly evidenced by the occurrence of old pits only in this area (Figure 3), which demonstrates that the mineralization is related to the highangle W-vergent reverse fault (Figure 2).

Therefore, the indication of hydrothermalism with goldbearing solutions accumulated within cracks and fissures at low temperatures (according to the minerals associated with goldbearing quartz veins) indicates that these are epithermal deposits of the Lode type. The 2D models of broadband magnetotelluric signals obtained from MT stations with Metronix ADU-07 devices at stations P1 and P3 and P4 and P5 (Figure 3&5) in the Morro do Ouro area provided information on the subsurface geology, denoted by color gradations in the lateral and vertical distribution, which indicate the presence of at least four main lithological units. The crystalline basement (greenish and darker yellow tones, occupying depths greater than 700 meters, becomes shallower to the west), while the greater thicknesses of the supracrustal rocks in the eastern portions of the profiles must be due to thrusts and reverse faults involving, mainly, the hematitic phyllites and schists of the Barão do Guaicuí Formation, compatible with that observed in the surface geology (Figure 2). Station P4 presents a strong conduct signal (in black), indicating the presence of hematitic phyllites (total thickness less than 200 meters), which are circumscribed laterally and in depth by the blue signal (also of high conductivity), which must represent the schists of the Barão do Guaicuí Formation. The weaker conductivity (in light green tones) in the western part represents the metarenite and phyllite cover of the ESg and, as there is no tectonic thickening in the domain, the underlying crystalline basement appears at shallower depths. The profile of stations P1 and P3 presents a somewhat similar arrangement, however, the transition to the non-thickened set is abrupt due to the presence of the W-E normal fault that aligns in the northern vicinity of the profile which is filled by basaltic rocks of the PLMS.

Therefore, based on surface and subsurface data, the minimum volume of hematitic phyllites in Morro do Ouro is estimated at around 7,000,000m³.

Generality

Within the SER dozens of gold deposits somewhat similar to that of Morro do Ouro are known, which enriched the drainages of the region with this precious metal. In fact, human colonization of the region began with the exploration of gold in drainages and placers before the discovery of diamonds in these same drainages.

The Mil Oitavas mine

In the area of the city of Diamantina some gold mines were active during the 19th and 20th centuries with special emphasis on the Mil Oitavas mine, where a nugget weighing one thousand octaves was extracted in the first half of the 20th century (1/8 of an ounce=3.58 grams therefore, the nugget weighed approximately 3.5kg). The gold mineralization in the surroundings of the city of Diamantina were studied by Abreu [16], when he observed that gold is confined to quartz veins with kaolinite and breccias of kaolinite matrix embedded in basal portions of hematitic phyllites. It was concluded that the veins and breccias were generated by hydraulic fracturing and that they are late in relation to the shearing responsible for the generation of schistosity and regional stretching lineation imprinted on the host rocks. The fluid inclusions studied by the author indicated mineralizing fluids of aqua-carbonic nature with salinity of approximately 10% NaCl eq., under minimum temperatures of 250 °C generated by devolatilization due to the inversion of isotherms.

The Serra do Pasmar mine

The gold mineralization of Serra do Pasmar (UTM 635585 E; 7974447N), although associated with quartz veins, occurs embedded in the bedding and schistosity or cutting the metarenites of the SBf, as well as concordant with the schistosity of the RPSg schists’ [17], and therefore has no relation to hematitic phyllites as in Morro do Ouro and in the Diamantina area. They show tabular and lenticular shapes and, at times, form boudins, generally with millimetric and centimetric thickness, sometimes of metric size. The tabular veins are preferentially embedded in the metarenites of the SBf and exhibit millimetric to metric thickness, while the veins that are hosted in schist of the RPSg present millimetric to centimetric thicknesses in discontinuous and lenticular bodies. The quartz veins exhibit preferential orientation 272/75 and 240/55, therefore discordant in relation to the S0 and Sn of the host rocks, although secondarily veins concordant with the S0 or schistosity outcrop, as well as in other discordant directions. Concentrates from pan samples of gold-bearing quartz veins revealed the presence of hematite, rutile, zircon, monazite and gold [17]. The gold grains show their characteristic golden color, metallic luster, in euhedral to anhedral forms with dimensions normally less than 2mm aggregated or not with milky quartz [17].

The study of fluid inclusions in hydrothermal quartz veins of Serra do Pasmar demonstrated the origin of aqua-carbonic and aqua-saline fluids with thermobarometric evolution marked by the circulation of primary, aqueous fluids, with CO₂ and medium to low salinity (between 8.9% and 14.6% NaCl eq.). The homogenization temperatures were estimated between 148 °C and 354 °C, while the fO₂ of the aqueous-carbonic fluid indicates hematite-magnetite buffer conditions and can be estimated due to the absence of N₂, CH₄ and H₂S in the fluids of the studied veins [17]. These results suggest a gold deposition mechanism similar to that already described for the gold deposits of the Mil Oitavas mine [16] and Costa Sena area [18], since the study of fluid inclusions in veins of the Costa Sena district (Conceição do Mato Dentro/MG) [18], as well as in veins of the Mil Oitavas Mine and mines in adjacent areas [16], shows that aqua-carbonic fluids (with compositions of H₂O, CO₂ and NaCl) and aqueous fluids were present during the mineralization phase.

The Costa Sena mine

Studies of fluid inclusions in quartz from gold veins in the Costa Sena area revealed salinity values between 6 and 15.1% NaCl eq. [18]. Primary aqua-carbonic fluid inclusions showed homogenization temperatures between 150 °C and 300 °C, while aqua-saline inclusions have homogenization temperatures between 70 °C and 200 °C [16,18]. The fO₂ of these fluids was estimated close to the conditions of the hematite-magnetite buffer with the gold being deposited as a result of the destabilization of the AuCl₂ – complex [18].

The results of the studies of fluid inclusions, which revealed the absence of CH₄, H₂S or N₂ and the presence of hematite (and absence of pyrite) in the quartz veins, indicate fluid fugacity conditions close to the Fe₃O₄/Fe₂O₃ buffer. Ronchi et al. [18] listed as factors that may have acted in the hydrothermal system and affected the physical-chemical parameters of the mineralizing fluids and the consequent deposition of gold, (1) decrease in fO₂ (oxygen fugacity); (2) decrease in Cl- activity; (3) increase in pH; (4) decrease in temperature; (5) changes in fluid chemistry.

Genetic and geochronological correlations between SER gold mines

As described above, the paragenetic and textural similarities of quartz, as well as fluid inclusions from the Mil Oitavas [16], Serra do Pasmar [17] and Costa Sena [18] mines are evident. Therefore, despite the small mineralogical variations associated with the gold veins of these mines, a similar mechanism of gold transport and deposition is indicated. The results of the geochronology of minerals associated with different gold-bearing quartz veins in the central part of the SER revealed ages of 650±130Ma [16], 490 to 440Ma [19] and between 515±55Ma and 524±16Ma [20], that is, the gold mineralization of the SER was related to the Brasiliano/ Pan-African orogeny of the late Neoproterozoic.

Among the various designations of the types of gold deposits, the classification related to the depth of the deposit is one of the most common, which includes the epithermal deposit, i.e., shallow depth (1 to 2km depth from the surface) and temperatures in the range of <150 °C to ~300 °C [21]. In low sulphidation systems the fluid has a near-neutral and low pH, and the separated steam with CO₂ and H₂S condenses in the vadose zone to form steam-heated water, acidified by the oxidation of H₂S, with deposition and accumulation predominantly in open space veins [21]. Still referring to low sulfidation deposits, the wide variety of textures is highlighted, including quartz and chalcedony veins in bands and crustiforms, cavities lined by druses and breccias with veins from multiple episodes of mineral deposition and hydraulic fracturing, followed by explosive release of pressure, which may be associated with hydrothermal eruptions at the surface [21]. Typical is the presence of low-temperature and acid-stable minerals, such as kaolinite and alunite from alterations caused by steam-heated waters near the surface. Despite minor differences, the SER gold deposits described above can therefore be classified as epithermal deposits in lowsulfidization systems, although the depth of accumulation of the deposits may have been somewhat greater than 2km. Groves et al. [22] indicate epizonal gold deposits at depths of up to 6 km and temperatures of 150 °C-300 °C.

In fact, classifications of deposit types and sources of gold have been the subject of debate for decades, which led Groves et al. [22] to propose the term orogenic gold deposit to replace a wide variety of terms that referred to gold deposits of only the vein and disseminated type, but which share many similar characteristics of deposit scale and tectonic time. Furthermore, it was pointed out that the gold-bearing quartz veins of hydrothermal ore deposits are located at depths of 15 to 20km down to the near-surface environment and, therefore, the term “mesothermal” does not apply to this type of deposit as a whole. This unique temporal and spatial association of this type of deposit with orogeny means that the vein systems are best termed orogenic gold deposits and, based on their depth, orogenic deposits are subdivided into epizonal (<6km, T=150-300 °C), mesozonal (6 to 12km, T=300- 475 °C) and hypozonal (>12km, T>475 °C) classes [22]. In the proposed classification, gold deposits, distributed throughout all collisional orogenic belts of the world, can be considered of syn- and post-orogenic origin, since the ore host rocks may have already undergone uplift and cooling (hence, “post-orogenic”), or the ore-forming fluids may be generated by simultaneous thermal processes at depth (hence, “syn-orogenic”) [22].

Obviously, the SER gold deposits can and should be considered orogenic, since they are inserted within an orogenic belt [6,11,13]. However, in the conceptualization of Groves et al. [22,23] and Wang et al. [24], crustal metamorphism as a universally viable process for generation of orogenic gold deposits was refuted. In other words, by integrating C-O and He-Ar isotope compositions of ore-related carbonates and pyrites, respectively, from several gold deposits around the world, the deposits indicate a fluid source with both mantle and crustal components, and timing incompatible with regional metamorphism, thus suggesting a deep subcrustal source of the mineralizing fluids. A didactic example (but not the only one, according [22-25]) of the lack of a direct relationship between the orogenesis of a mountain range and its gold mineralization comes from the gigantic Jiaodong gold province, which was formed more than 1,700 million years after regional metamorphism in the host terrain, i.e., the age of mineralization of ca. 130-120Ma is nearly 1,700 million years younger than the timing of metamorphism of the host rocks. These gold districts comprise mesozonal gold deposits that formed at 8-10km, the depth at which the high-grade metamorphic wall rocks were uplifted after their formation 1,700 million years before.

The gold mineralization of the SER (as well as some of the Paleoproterozoic Minas Supergroup of the Quadrilátero Ferrífero) show late Neoproterozoic ages [16,19,20,26-29], therefore, formed over 600 and 1400 million years after regional metamorphism in the host terrane, respectively. The precise age of the metamorphism of the Espinhaço Supergroup successions is not yet well defined (estimated between 1.3 and 1.2Ga [29]). However, stratigraphic markers indicate an age of deformation and metamorphism of the ESg successions prior to 1.0Ga, since glaciogenic deposits that form fringes on the slopes surrounding the SER reveal the reworking of metamorphosed rocks of the orogenic belt [6,13]. The age of the “Jequitaí Glaciation” at approximately 1.0-1.08Ga was established by paleomagnetism studies [30] and had the SER as the center of glacial dispersion [29]. This late Mesoproteozoic age of glaciation is corroborated by the absence of clasts of basic rocks of the PLMS (940- 906Ma [9,10]) in the glaciogenic deposits, despite the pervasive presence of these rocks within the SER (~2.5% of the surface of the mountain range). The weak or incipient metamorphism of the basic rocks of the PLMS, i.e., saussuritization of plagioclase and uralitization of pyroxenes, cannot be considered as indicative of a later metamorphic event, since the current level of exposure of these rocks is the same as that of the rocks that enclose them (rocks of the ESg), that is, when the magmas of this LIP intruded in the first 100 million years of the Neoproterozoic, the diabases and gabbros now outcropping crystallized under a thick pile of rocks that made up the orogenic edifice. Furthermore, the rock dikes of this suite cut the tectonic structures molded in rocks of the ESg, which also reveals the minimum age of the orogenic event. The structural lineaments that accommodate the rocks of this suite, as well as the rocks of the suite themselves, are not folded and/or transposed by subsequent deformation within the entire area of the SER [6], which makes it clear and materialized, therefore, that the “Braziliano Cycle Tectonics” did not affect the bulk of the rocks that build this mountain range, although it reactivated tectonic structures of the most mobile zones of the orogen [6,29]. The tectonic reactivation of orogenic belts resulting from subsequent tectonic cycles, even if hundreds of kilometers away from the new collision/subduction focus, especially of their larger structures, i.e., faults and shear zones, is known and described in different continents and from different geological eras [31-35].

Relevant discussion about the concentration and deposition of gold systems concerns the fluid migration processes and the controls on gold deposition in the orogenic environment. The fertility of gold-bearing orogenic systems is inexorably linked to subduction, so that both geodynamics and fertility parameters are related to convergent margin tectonics [23], with goldbearing fertile fluids being directly related to the devolatilization of the subducted plate and the overlying sedimentary wedge, or indirectly related to the reactivated mantle lithosphere, previously metasomatized by such fertile fluids related to subduction. The migrating fluids are confined by caps or seals, i.e., impermeable cap rocks. The rapid ascent of the fluids reduces the lithostatic pressure in interconnected faults, intensifying hydraulic fracturing, inducing extreme pressure fluctuations, and leading to the effective deposition of gold through fluid un-mixing episodes [23]. Firstorder faults are most highly endowed where they are intersected by high-angle accommodation structures. These generally propagate along lines of weakness, such as preexisting reactivated thrust faults or contacts between lithologic units with high competency contrasts [23].

The conditioning of the SER gold deposits does not differ from that observed in other orogenic belts, as described above. That is, the mineralized veins and breccias were generated by hydraulic fracturing and are late in age relative to the shearing responsible for the generation of the schistosity and regional stretching lineation imprinted in the host rocks [16]. Furthermore, the SER gold deposits show association with high-angle reverse faults, which facilitated the circulation of ascending fluids that were trapped at the interface between quartzite rocks and impermeable rocks (schists and phyllites), i.e., fluids were sealed by impermeable rocks.

Figure 7:Diagram and table of the chemical analysis of two gold particles from quartz veins of the Morro do Ouro (analysis provided by the owner of the Dumbá mining).

Regarding the chemical composition of the gold from the Morro do Ouro mine, the analysis of two nuggets (Figure 7), according to the percentages of Ag and other chemical elements analyzed, is compatible with the mineralization style of Orogenic Precambrian, Low-Temperature, oxidizing, chloride systems and Low-to-intermediate-sulfidation epithermal mineralization [36]. But according to Chapman et al. [36], to characterize the Ag profile of a sample population it is necessary to analyze approximately 30 particles, however the analytical result of these two samples was very similar (Figure 7), which points to a tendency of compositional regularity of the studied gold deposit.

The geological data on the gold mineralization of the SER demonstrates the primary source of gold hydrothermal fluids originating from the crustal and/or sub-crustal environment. From the perspective of recent studies on the geology of gold from different deposits on the planet, that is, the critical fertility factor is related to the subduction of an oceanic slab and overlying sediment wedge that can provide fluid, sulfur, gold and other ore metals to the orogenic gold system related to convergent margin tectonics. Fertile gold fluids are either directly related to devolatilization of the subducted slab and overlying sediment wedge or indirectly related to reactivated mantle lithosphere previously metasomatized by such fertile subduction-related fluids [22-25].

Lithospheric-scale shear zones and faults are conduits of fluids and ore metals derived episodically from the devolatilization of a subducted plate and/or from metasomatized and fertilized mantle. Fluids deposit gold mineralization in rock sequences adjacent to shear zones and in subsidiary faults at crustal depths of >20km to <5km, with hydraulic fracturing and fluid phase separation potentially the most important depositional processes for vein deposits and the dominant fluid-rock reaction in disseminated deposits [24]. The SER gold deposits present characteristics similar to those described above, that is, association with highangle reverse faults that facilitated the circulation of ascending fluids trapped mainly at the interface between quartzite rocks and impermeable rocks (schists and phyllites).

The gold mineralization of the different occurrences from the SER shows evidence of epithermal deposits, either due to the low temperatures of the mineralizing fluids [16-18], or due to the late emplacement of the mineralized quartz veins in relation to the orogenic structures (shears and associated structures, i.e., schistosity and stretching lineation). Furthermore, the regional metamorphism of the lower units of the Espinhaço Supergroup in the central belt of the SER, i.e., the rocks that enclose the goldbearing quartz veins, reached temperatures of the order of 450 °C and pressures of 4-5Kb [8], while the temperatures of the hydrothermal fluids associated with the gold mineralization did not exceed 300 °C [16-18]. Therefore, it is clear that the gold mineralization of the SER is not related to the metamorphism of the Espinhaço Orogen.

However, the origin of the fluids that mineralized the rocks of the ESg approximately 600 million years after the orogenesis and metamorphism of the mountain range cannot be considered enigmatic, given the nature, age of the mineralization, and their recurrence over a vast territorial extension. The regional character with vast territorial extension of the gold mineralization’s of the late Neoproterozoic (Brazilian Cycle) is revealed by similar deposits embedded in rocks of the Minas Supergroup (Paleoproterozoic) of the Quadrilátero Ferrífero (Minas Gerais) [26-28]. From the perspective of Almeida-Abreu [19], this regional crustal heating would have been motivated by the subduction of the oceanic plate related to the tectonic inversion of the West Congolese Belt under the lithosphere of its counterpart, the Araçuaí Fold Belt. Considering the vast extent of crustal heating and pervasive fluid generation from the suture zone, subduction must have occurred, probably, under flat slab conditions. Flat slab subduction is a relatively common process, such as the subduction of the Farallon Plate beneath North America [32] and the Nazca Plate beneath northwestern South America [37] and invariably causes magmatism and deformation over a wide region of the continental interior.

The voluminous and extensive magmatism of the Araçuaí Belt is indicated by different authors [38], who consider the repetitive and intensive interaction between the melting of the mantle and the crust and geochronological data confirm the peak of magma production around 500±15Ma, varying up to 525±3Ma. On the other hand, orthogneisses correlated with pre-collisional events and granitic plutons chrono-correlated with post-collisional events of the Araçuaí belt were described by Teixeira et al. [39], whose rocks revealed crystallization ages between 641±5Ma and 531±4Ma., and the geochemical data obtained in these rocks indicate contamination of the mantle in late Neoproterozoic magmatism, related to Brasiliano tectonics.

Fossen et al. [40], even without admitting a subduction process in the evolution of the Araçuaí orogen, reported it as a hot orogen with widespread magmatism and partial melting, which included enormous volumes of anatectic melts and flow towards the São Francisco foreland, with these melts crystallizing between 597 and 572Ma. Therefore, the peaks in crustal temperatures in the Araçuaí orogen were reached around 600Ma and followed by a very slow cooling. The intensity and extent of this ultra-regional heating of the crust within the SER and in its longitudinal extension to the eastern part of the Quadrilátero Ferrífero has been recognized by different authors (e.g., Machado et al. [9]). In the Quadrilátero Ferrífero domain, it reached the deepest part of the Archean crust by anatexis and magmatic mixing, possibly caused by magmatic underplating in an extensional environment in areas covered by successions of the Minas Supergroup related to the Abre-Campo shear zone, that is, a suture located to the east of the Quadrilátero Ferrífero responsible for the formation of a magmatic arc and its tonalitic magmatism around 600Ma [20].

The geochronology of magmatism derived from ultra-regional crustal heating that dominated the entire eastern part of Minas Gerais related to the Brasiliano/Pan-African orogeny at the end of the Neoproterozoic, that is, between 641Ma and 450Ma [26-28,38- 40], encompasses the ages obtained for the gold mineralization of the SER and the eastern of Quadrilátero Ferrífero (between 650 and 440Ma [16,19,20]), which establishes that the aforementioned gold mineralization is directly related to the crustal and subcrustal processes concerning the Brasiliano/Pan-African event.

We would like to thank Mr. Edgard Pigatti, owner of Dumbá Mining, for his logistical support and permission to access the mine, as well as for providing samples of gold particles for study and analysis. Special thanks also to Prof. Glauco Umbelino of the Center for Studies in Geosciences (CeGeo) of the Federal University of the Jequitinhonha and Mucuri Valleys for supervising the drone flight over the Morro do Ouro area and the subsequent processing to generate the image in Figure 4.

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

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