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Romaria |
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Minas Gerais, Brazil |
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Diamonds
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Super Porphyry Cu and Au
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The historic Romaria mine produced diamonds from lithified Cretaceous conglomerates, and is located in a tributary of the diamondiferous Bagagem River, ~400 km NW of Belo Horizonte in western Minas Gerais, Brazil, (#Location: 18° 52' 41"S, 47° 34' 59"W).
Diamonds were first discovered in the Bagagem River in 1722 by the explorer João Leiteda Silva Hortis. Subsequently, in 1867, they were also found on the banks of a small tributary of that river, resulting on the establishment of the Romaria mine and the village of Água Suja, later to become the town of Romaria. Mining continued at Romario over an area of ~1.2 km2 from soon after discovery until 1984, over a century later. The last operator of the mine was Extratífera de Diamantes do Brazil (EXDIBRA), from 1969 until its closure. In the final years, it treated ~600 m3 of ore per day through a washing plant, with the final separation of diamonds performed by hand (Svisero et al., 1981). It has produced numerous large diamonds, including the 105.5 carat Desconhecido in 1867 and 179.5 carat Estrela de Minas in 1909.
Romaria lies on the southwestern limb of the NW-SE aligned Alto Paranaíba Arch, and on the northeastern margin of the Paraná Basin. It also lies on the southwestern margin of the Coromandel diamond district, which embraces >580 'kimberlitic' intrusions and abundant alluvial diamond workings that have produced an estimated minimum of 40 Million carats of diamonds. This district covers an area of 160 x 150 km, and comprises the northern third of a ~400 x 150 km zone that follows the longitudinal core of the Alto Paranaíba Arch. Like the Coromandel diamond district this larger zone is defined by abundant alluvial diamond workings and a considerable number, but less densely distributed 'kimberlitic' intrusions. The 'kimberlitic' intrusions in the Coromandel diamond district and the broader zone includes 100 to 80 Ma kimberlites, 90 to 75 Ma kamafugites (an ultrapotassic alkaline ultramafic rock) and 91 to 71 Ma alkaline-carbonatitic complexes, collectively known as the Alto Paranaíba Igneous Province. Of these intrusives, only a little over ten individual kimberlites or clusters, are diamondiferous. The extrusive equivalent of these intrusions are alkaline, mafic volcanic and pyroclastic rocks and epiclastic sediments of the Mata da Corda Group deposited over the arch and to its north, covering an area of ~8000 km2, and extending into the cratonic Sanfranciscana Basin. This group is composed of the <20 m thick basal Patos Formation pyroclastic breccias, tuffs and lacustrine silitites, overlain by the ~65 m thick Capacete Formation. The latter includes basal, up to 20 m thick, pyroclastic breccia and polymictic conglomerates, both of which are diamondiferous, overlain by a thin cross-bedded conglomeratic sandstone and a succession of clay and lithic tuffs that are ~45 m thick (Karfunkel et al., (2015). The Mata da Corda Group, is the stratigraphic equivalent of the Bauru Group to the SW in the Paraná Basin, which hosts Romaria deposit, as described below.
The diamonds of the Romaria deposit, which is on the southern limb of the Alto Paranaíba Arch, are hosted within a polymictic conglomerate deposited on, or in channels cutting, the Cretaceous aeolian Botucatu Sandstone, which, in turn, unconformably overlies the basement Neoproterozoic Araxá Group mica-schists and intrusive granites. The host is locally known as the Tauá Conglomerate. It contains blocks that are generally angular, and are up to 80 cm across, as well as small rounded pebbles of mica-schist, phyllite, quartzite, metabasite, Botucatu sandstone and basalt. This unit is overlain by another polymictic conglomerate known as Secondina, which contains less diamonds, smaller pebbles, and has a maximum thickness of 3 m. It is considered to be the result of erosion and re-deposition of the Tauá Conglomerate. These two conglomerates are covered by a homogeneous succession composed of fine‑ and coarse‑grained greenish sandstones, associated in this region of Minas Gerais with the Late Cretaceous volcanic activity of the Mata da Corda Group. Finally, the entire area is covered with unconsolidated Tertiary material composed of laterite, sandstone pebbles, lateritised conglomerate blocks and clays (Pereira 2007). The Tauá Conglomerate and Secondina conglomerates belong to the Uberaba Formation, in the basal Bauru Group on the northeastern margin of the Paraná Basin, correlated with the similarly diamondiferous Capacete Formation of the Mata da Corda Group in the São Sanfranciscana Basin deposited over the São Francisco Craton to the NE, as described above. As well as the conglomerates, the Uberaba Formation includes sandstones and mudstone (Svisero, et al., 2017; Pereira et al., 2017).
Drilling indicates the host conglomerate was deposited on a relatively rugged relief, with palaeo-channels, and block faulting which resulted in small 'basin-like' depressions being formed in both the Botucatu Sandstone and the mica schists of the Araxá Group. These palaeo-structures were the focus of mud flow accumulations producing diamond rich mudstone layers or lenses within the conglomerate. The average diamond content within these mud layers averaged 0.33 to 0.69 carats/m3 (Pereira et al., 2017).
In the deposit area, the underlying Araxá Group mica-schist contains staurolite, garnet, rutile and tourmaline, and is cut in all directions by pegmatite veins and intrusions of mafic rock metamorphosed to amphibolite facies. As described by Draper (1911), the host conglomerate, or breccia, in the immediate deposit area, had a maximum thickness of 13 m, averaging ~6 m, and rested discordantly on the basement and local intrusions, as well as on the Botucatu Sandstone. It was composed of angular to poorly‑rounded clasts of mafic igneous rock, granite, quartz, shale and sandstone, cemented by red clay. Layers and masses of opal were observed within the conglomerate. Diamond, pyrope, perovskite, magnetite, ilmenite and olivine were found in the clayey matrix that cemented the fragments. Draper (1911) regarded the conglomerate layer or breccia, as well as the constituent minerals to be of local origin. Svisero et al. (2017) state that Coelho (pers. comm., 2008) has reported a diamondiferous kimberlite to the NE of the Romaria Mine, supporting the latter conclusion.
The conglomerate ar Romaria is interpreted to contain a high content of kimberlite fragments and includes thin layers, which are regarded to have structures characteristic of epiclastic kimberlite (Pereira et al., 2017). The conglomerate contains abundant ilmenite, garnet and/or spinel, also interpreted to be of kimberlitic origin (Svisero, 1979; Svisero and Meyer, 1981; Coelho, 2010), as reflected by concentrates obtained from washing the comminuted conglomerate that contain kimberlite indicator minerals. Based on the treatment plant recovery, Svisero and Meyer (1989) reported that the diamond content of the conglomerate ranged between 0.04 and 0.12 carats/m3. Pereira et al. (2017) quote a batch of 5317 stones weighed 450 carats, corresponding to an average weight of 0.084 carats per stone. The dominant shape observed was rhombo-dodecahedron, followed by cubic, octahedron and octahedron‑rhombo-dodecahedron combination. Another batch of 39 diamonds studied by De Beers weighed 3508 carats, a 0.09‑carat average weight, dominantly composed of rounded, grey, dodecahedral and octahedral shapes, with 10 to 15% being cubic stones (Robinson 1991; Robinson et al., 1995). No abrasion was evident, taken to suggest either a distal source of diamonds transported under low energy conditions (Robinson 1991) or, as indicated by other evidence, a young, proximal and primary source of diamonds (Pereira et al., 2017). Systematic sampling and concentrates analysed from the Romaria mine indicate garnets formed under pressure conditions of ~48 Kbar, whilst spinel grains are typical of inclusions in diamonds (Pereira 2007).
The most recent source geological information used to prepare this decription was dated: 2017.
This description is a summary from published sources, the chief of which are listed below.
© Copyright Porter GeoConsultancy Pty Ltd. Unauthorised copying, reproduction, storage or dissemination prohibited.
Romaria
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Selected References:
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Karfunkel, J., Hoover, D., Fernandes, A.F., Sgarbi, G.N.C., Kambrock, K. and Oliveira, G.D., 2014 - Diamonds from the Coromandel Area, West Minas Gerais State, Brazil: an update and new data on surface sources and origin: in Brazilian Journal of Geology v.44, pp. 325-338. doi: 10.5327/Z2317-4889201400020011.
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Pereira, R.S., Fuck, R.A., Franca, O.S. and Leite, A.A., 2017 - Evidence of young, proximal and primary (YPP) diamond source occurring in alluviums in the Santo Antonio do Bonito, Santo Inacio and Douradinho rivers in Coromandel region, Minas Gerais: in Brazilian Journal of Geology v.47, pp. 383-401. doi: 10.1590/2317‑4889201720170047
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Svisero, D.P., Shigley, J.E. and Weldon, R., 2017 - Brazilian diamonds: a historical and recent perspective: in Gems and Gemology v.53, pp. 2-33. dx.doi.org/10.5741/GEMS.53.1.2.
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Porter GeoConsultancy Pty Ltd (PorterGeo) provides access to this database at no charge. It is largely based on scientific papers and reports in the public domain, and was current when the sources consulted were published. While PorterGeo endeavour to ensure the information was accurate at the time of compilation and subsequent updating, PorterGeo, its employees and servants: i). do not warrant, or make any representation regarding the use, or results of the use of the information contained herein as to its correctness, accuracy, currency, or otherwise; and ii). expressly disclaim all liability or responsibility to any person using the information or conclusions contained herein.
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