Earthquake-induced clastic dyke and fluid inflow at the Miguel Burnier manganese-ore deposit, Quadrilátero Ferrífero of Minas Gerais, Brazil

A clastic dyke has been recognised within manganese (Mn)-rich Cenozoic sediments near a historical Mn-ore deposit in the Miguel Burnier district, Quadrilátero Ferrífero, in the southern São Francisco craton, Brazil. Here, we describe the clastic dyke, a subvertical fissure that is filled with friable arenaceous fragments, and characterise it as seismite. An overprint by stockwork-like Mn-oxide veinlets and Mn-oxide dissemination, mineralogically expressed as birnessite, lithiophorite and jianshuiite, and geochemically represented by metalliferous enrichments, particularly mercury (Hg), occurs in the clastic dyke and its immediate vicinity. Such an overprint also contains illite, which forms a mineral association with birnessite, lithiophorite and jianshuiite, and constrains the temperature of hydrothermal alteration to less than 300 °C. The recognition of seismite and its Hg enrichment indicate that the Cenozoic history of the Quadrilátero Ferrífero cratonic terrane has been affected by seismic episodes, which were not only conducive to local enrichment in Mn at Miguel Burnier, but also to the dam failure of the 5th of November 2015 (Agurto-Detzel et al. in Geophysical Research Letters 43: 4929–4936, 2016).


Introduction
Earthquakes are not only marked by sudden brittle failure of rock masses that occasionally causes devasting effects on human communities, but also by certain features that can be preserved in the geological record. As brittle fracturing is a recurrent process associated with cracking and sealing that involves fault-channelled fluid inflow, its preservation in the geological record can be manifested in the form of metalliferous concentrations of economic significance [1][2][3]. In addition, recognising earthquake-related features, deformation structures of seismic origin in rocks and soft sediments, is the fundamental course of action in palaeoseismological analysis [4,5].
Deformation structures that are genetically related to earthquakes, notwithstanding the caveats highlighted by Shanmugam [6], are known as seismites, a term that was originally proposed by Seilacher [7,8]. The identification of seismites is challenging. Where firmly recognised, however, a seismite does mainly indicate the seismotectonic character of a region [9][10][11][12]. Particularly challenging is the identification of seismite in tropical regions, where deep weathering obscures geological structures [13]. This is the case of lateritic covers on tectonically stable cratonic terranes.
The cratonic terrane of the Quadrilátero Ferrífero, located in the southernmost part of the São Francisco craton [14], is one of the most geologically investigated regions in the world (Fig. 1). While much attention has been dedicated to the Precambrian tectonics of the Quadrilátero Ferrífero [15][16][17][18][19][20][21][22], only a few authors have carried out tectonic studies involving small Cenozoic basins, a few km across, which are sparsely distributed in the Quadrilátero Ferrífero [23][24][25][26][27][28]. The opening of such sedimentary basins over the Precambrian terrane of the Quadrilátero Ferrífero indicates that the region was tectonically, and perhaps seismically, active in the Cenozoic. Evidence for hodiern seismic activity comes from smallmagnitude earthquakes that have been registered in the Quadrilátero Ferrífero. Agurto-Detzel et al. [29] related the dam failure of mine tailings and subsequent flood in Mariana, 80 km ENE of our study area at Miguel Burnier, on the 5th of November 2015, to several earthquakes of small magnitude, from 1.3 to 2.6. A 3.5-magnitude earthquake took place on the 25th of November 2019, the epicentre of which was 25 km W of Miguel Burnier [30].
Here, we report the finding of a clastic dyke that is incised into unmetamorphosed sediments of a Cenozoic basin in the Quadrilátero Ferrífero. The clastic dyke is located near a historical Mn-ore deposit in the Miguel Burnier district of Ouro Preto, the first and largest Mn-ore mining site in Brazil in the period from the late-nineteenth to the early-twentieth century [31][32][33]. We characterise the clastic dyke as seismite, its associated metalliferous enrichment, and discuss its implications.

Geological setting
The structural framework of the Quadrilátero Ferrífero resulted from two main deformational events, the Transamazonian and the Brasiliano [17,34]. The former was an extensional event of Palaeoproterozoic age, between 2.1 Fig. 1 Simplified geological map of the Quadrilátero Ferrífero [35,79], with emphasis on the distribution of canga plateaux and Cenozoic basins. The study area, Miguel Burnier, is located in the western extreme of the Dom Bosco syncline, the southern border of which is marked by the Engenho strike-slip fault and 1.7 Ga, which is thought to have led to the emplacement of Archaean granite-gneiss domes and the nucleation of regional synclines and fault systems, comprising supracrustal rocks of the Rio das Velhas Supergroup and the Minas Supergroup [35]. Relevant to this study are the Dom Bosco syncline and the Engenho strike-slip fault [31,36]. The second event was compressive and marked the closure of the Pan-African/Brasiliano proto-ocean (0.65-0.50 Ga). The Brasiliano event is mainly recorded in the eastern border of the Quadrilátero Ferrífero as a westverging, fold-and-thrust belt [17,34]. The aforementioned structural development is, nevertheless, not consensual. For different views, see for instance Hippertt and Davis [37] and Endo et al. [38].
Cenozoic basins are sparsely distributed in the Quadrilátero Ferrífero [35,39,40]. They are small grabenlike basins, reaching a few km across, in which sediments accumulated over Precambrian rocks, from the Late Eocene to the Early Miocene [40,41]. Many Cenozoic basins are located in Precambrian rocks of the Itabira Group of the Minas Supergroup [35], previously known as the Itabira iron formation [42]. The Itabira Group consists of itabirite-i.e., a metamorphosed, banded Fe-oxide-rich rock, which grades upwards to dolomitic rocks.
One of such Cenozoic basins is the Fonseca basin, located in the eastern border of the Quadrilátero Ferrífero ( Fig. 1). There, normal faults were observed in Oligocene-Miocene sediments [28,43]. Importantly, clastic dykes were recorded in Quaternary sediments covering the Fonseca basin-i.e., a plateau of conglomeratic deposits of itabirite-derived ironstone pebbles, cemented by Fe oxyhydroxide [28].

Methods
Samples collected during fieldwork were dried in a furnace at 60 °C, crushed and ground in an agate mill. Their powders were then investigated by X-ray diffraction (XRD), using a RIGAKU X-ray diffractometer, model D\MAX ULTIMA IV, housed at CDTN (Centro de Desenvolvimento da Tecnologia Nuclear). The instrument operated from 4° to 80° (2θ angle) at 40 kV and 30 mA, and goniometer speed of 4°2θ/min. Mineral identification was carried out following the database of the International Centre for Diffraction Data and the Joint Committee on Powder Diffraction Standards-JCPDS. Diffractograms and XRD tables are presented in the Electronic Supplementary Material. Samples were also measured for As, Ag, Cd, Sb, Te, Hg, Pt, Au, Pb and Pd at Bureau Veritas Commodities Canada Ltd., Vancouver, by inductively coupled plasma-mass spectrometry (ICP-MS), following aqua regia digestion.

Study area and results
A dyke-like structure occurs 1 km W of the historical Mnore mining area in the Miguel Burnier district ( Fig. 1; UTM coordinates: 23K 628307; 7739935). The historical mining focussed on Mn ore that was believed to be a residual deposit, derived from the weathering of Precambrian dolomite and manganiferous itabirite (Fig. 2) [33]. However, our field observations in the remains of the historical mining area have indicated that the Mn ore was hosted in Cenozoic sediments. The dyke-like structure is subvertical, incised into Mn-rich mudstone of a Cenozoic basin (Fig. 3a). The 1.3-m-wide dyke is made up of pebble-toboulder-sized fragments of friable sandstone, which consists essentially of medium-to-coarse-grained quartz sand, with some illite, rutile and hematite. The dyke walls are approximately planar, trending 080° and slightly funnelling downward. Its exposure is roughly cuneiform within a collapsed Mn-rich mudstone wall rock (Fig. 3b).
The clastic dyke truncates Mn-rich mudstone underneath a lateritic cover of canga-i.e., a ferruginous duricrust. The Mn-rich mudstone is a fine-grained rock consisting of clay-and silt-sized particles of hydrous Mn oxide, hematite, kaolinite and quartz. Sandstone lenses occur within the Mn-rich mudstone. An interlocking network of Mn-oxide veinlets, resembling stockwork, occurs in the clastic dyke and its immediate vicinity. Stockwork-like Mn-oxide veinlets are spatially associated with brecciated arenaceous fragments within the dyke (Fig. 3c). The Mnoxide veinlets are made up of birnessite, lithiophorite, illite and jianshuiite, and minor quartz and hematite. The central part of the clastic dyke has an approximately 20-cmthick Mn-oxide zone, in which fine-grained Mn oxide is  [33]. Manganese-ore mining targeted on a layer of variable thickness, the average of which was 3 m. It was mainly composed of hard metallic and soft, hydrated Mn ore (4). The metallic ore made up, on average, 80% of the layer. It occurred as lenses or irregular bodies. The ore seam was underlain by earthy and impure Fe ore and Mn ore, up to 24 m in thickness (3), and overlain by a body of friable hematite, known as jacutinga, and itabirite (6). Above the itabirite there was grey limestone (7), and below the earthy material, white limestone (2). Iron-ore conglomerate and duricrust (5) partially covered the previously described rocks, excepting the ferruginous schist (1) Fig. 3 a Schematic geological cross section of the clastic dyke, emphasising its diagnostic attributes. b Photograph, looking 176°, of a subvertical fissure that is filled with fragments of friable sandstone, incised into Mn-rich mudstone. c Zoomed-in photograph towards 030° of the site indicated in b, showing a fragment of friable sandstone that is cross-cut by Mn-oxide veins (v). The latter cement sandstone fragments within the clastic dyke. Stockwork-like Mn-oxide veinlets occur in the immediate vicinity of a brecciated sandstone fragment (upper right). d Photographic detail, looking 113°, of the site marked in b, displaying an approximately 20-cm-thick zone of Mn-oxide dissemination as a dark-brown matrix, with whitish veinlets of illite and quartz. e Photographic close-up towards 176° of the site labelled in b to illustrate a whitish zone rich in illite disseminated, yielding a dark-brown matrix with whitish veinlets (Fig. 3d). The matrix of the Mn-oxide dissemination has birnessite, jianshuiite, quartz, rutile and minor hematite, whereas the whitish veinlets comprise illite and quartz. Besides the Mn-oxide dissemination, the dyke infill displays a 20-cm-thick zone of whitish-grey shades (Fig. 3e). This whitish-grey zone has illite and quartz as the main constituents, and minor amounts of lithiophorite and birnessite. Slickenside striae on the SE-dipping wall of the dyke are oriented at 120°/33° and 065°/30° (Fig. 4). Table 1 displays the results of reconnaissance wholerock chemical analyses for Sb, Hg, As, Cd, Te, Pb and precious metals within the clastic dyke and its vicinity. Compared to the upper continental crust (UCC, [44,45]), the clastic dyke and its stockwork-like Mn-oxide veinlets and Mn-oxide dissemination have enrichments in Au, Hg, Te and Sb, which reach between 10 and 100 times, whereas Cd attains its maximum between 100 and 1000 times (Fig. 5).

Discussion
The dyke-like structure found at Miguel Burnier resembles that of a clastic dyke, which represents infilling of open fissures, either by gravity or tectonic stretching. Clastic dykes, also known as sedimentary dykes, are categorised as: (a) neptunian dykes, which are fissures that are filled with superficial material [46], and (b) intrusion dykes, the infilling material of which comes from below. Both categories are seismically induced [47][48][49]. Clastic dykes have thus been characterised as seismites [49].
It is difficult to reconcile the straight contacts and the cuneiform cross section of the Miguel Burnier clastic dyke with a non-seismic slope process, such as creep. Field evidence suggests that sandstone fragments and arenaceous material collapsed from the surface into a vertical fissure that was tectonically opened, as indicated by slickenside grooves (Fig. 4). The latter are indicators of seismic slip [50]. Furthermore, the groove lineation of Fig. 4 plunges from 120°/33° to 065°/30°, pointing to an oblique strikeslip component of movement. Such a strike-slip component can tentatively be understood as reactivation of the Engenho strike-slip fault in the vicinity of Miguel Burnier (Fig. 1) [31,51], in a way similar to the reactivation of the Água Quente fault system, as suggested by the Fig. 4 Photograph of an oriented sample taken from the contact between the dyke and its wall rock. Arrows indicate slickenside lineation at the contact with the clastic dyke. The lineation plunge varies from 120°/33° to 065°/30° on the slickenside, the orientation of which is 127°/30°. Labels wr and d stand for wall rock and clastic dyke, respectively Table 1 Chemical analyses for Sb, Hg, As, Cd, Te, Pb and precious metals in the clastic dyke and its immediate vicinity deformational and hydrothermal overprint on Cenozoic sediments in the eastern Quadrilátero Ferrífero [28,52].
Additionally, reconnaissance whole-rock chemical analyses identified a remarkable enrichment in Hg. According to Yangfen et al. [65] and Zhang et al. [66], Hg becomes so distinctly elevated in groundwater during earthquakes that Hg can be used as an indicator of seismic activity. Furthermore, Hg and Sb are known to be typically enriched in hot-spring deposits. Their enrichment in the Mn-oxide veinlets is comparable to those found in modern hot springs [56,[67][68][69]. Such enrichments suggest that the clastic dyke was formed due to seismic rupturing followed by fluid inflow, hydrothermally overprinting the Cenozoic sediments within the dyke and its surroundings.
The recognition of hydrothermal overprint on Cenozoic sediments following seismic rupturing might provide an alternative interpretation to some mineral occurrences in the Quadrilátero Ferrífero, such as the cinnabar of Tripuí, Ouro Preto, 30 km E of the clastic dyke described here. There, cinnabar was observed in gravels cemented by limonite and pyrolusite [70]. Elsewhere in the Quadrilátero Ferrífero, considerable amounts of Hg have been recognised in Mn-oxide-rich rocks in weathered profiles with fault-related fabrics [53,54], and also in brittle fractures that are healed by Mn oxides containing Hg [71]. Both occurrences can be explained by deformational and hydrothermal activity during the Cenozoic.
The mudstone wall rock of the clastic dyke is analogous to that attributed to Cenozoic sediments elsewhere in the Quadrilátero Ferrífero [31,40,43,72]. Palynological studies of other Cenozoic sedimentary basins, such as the Fonseca and Gandarela basins (Fig. 1) [40,41], suggest that the sedimentation at Miguel Burnier is no older than the Late Eocene. It can thus be inferred that the clastic dyke is no older than ca. 38 Ma. Radiogenic dating of goethite that cements the lateritic cover of canga indicates (U-Th)/He ages from 48 to < 2 Ma [73]. It means that the seismically induced fissure responsible for the clastic dyke was coeval with the prolonged history of deep lateritic weathering in the Quadrilátero Ferrífero [73,74].
Deep lateritic weathering in the Quadrilátero Ferrífero is topographically expressed as canga plateaux. It is pertinent to point out that much of the topography of the Quadrilátero Ferrífero region, in particular its canga plateaux, has been attributed to extensional tectonics [28,75]. The regional topography of the Quadrilátero Ferrífero and its drainage systems are thought to have resulted from tectonically uplifted and depressed blocks [28,76]. Such lines of evidence further support the characterisation of the clastic dyke as seismite. Present-day seismicity has been recorded in the Quadrilátero Ferrífero and its surroundings [29,77,78].
Combined with hodiernal seismicity, which was conducive to the catastrophic dam failure on the 5th of November 2015 [29], the earthquake-induced dyke indicates that the Quadrilátero Ferrífero has seismically been active since the Late Eocene. It further indicates that hydrothermal  [44]; average UCC value for Te comes from Wedepohl [45] overprint of low temperature yielded local concentration of Mn at Miguel Burnier. Local concentration of Mn and other metals, notably Hg, has recently been documented elsewhere in the Quadrilátero Ferrífero and related to seismic rupturing and fluid inflow [53,54].

Conclusion
A subvertical fissure that is filled with sandstone fragments occurs at Miguel Burnier, a historical site of Mn-ore mining in the Quadrilátero Ferrífero. The fissure is a clastic dyke, formed in response to seismic rupturing. The clastic dyke and its vicinity have metalliferous enrichments (Mn, Hg, Sb, Cd and Te). Earthquake-induced fluid inflow could have played some role in the Mn mineralisation of Miguel Burnier and elsewhere in the Quadrilátero Ferrífero. Our observations indicate that the southernmost São Francisco craton has seismically been active since the Late Eocene. We are grateful to Professor Bernd Lehmann for financing the costs of reconnaissance whole-rock chemical analyses. We are indebted to the anonymous reviewers for their thoughtful comments that substantially improved the original manuscript. We are also grateful to Dr. Clifford Chuwah for his editorial handling and comments.

Compliance with ethical standards
Conflict of interest On behalf of all authors, the corresponding author states that there is no conflict of interest.
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