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Passagem de Mariana |
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Minas Gerais, Brazil |
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Au
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Super Porphyry Cu and Au
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The Passagem de Mariana gold deposit is located in the south-eastern section of the Quadrilátero Ferrífero Gold District in Minas Gerais, Brazil, 7 km east the city of Ouro Preto and 100 km south-east of Belo Horizonte.
The geology of the Quadrilátero Ferrífero is dominated by Archaean and Proterozoic volcano-sedimentary sequences and Precambrian granitic complexes. The oldest rocks in the district are the Palaeo- to Mesoarchaean gneiss-migmatite complex which is overlain by the Neoarchaean Rio das Velhas Supergroup.
This latter supergroup is sub-divided into the basal Quebra Osso Group, succession of komatiitic ultramafic and mafic rocks with BIF intercalations, overlain by the Nova Lima Group, a middle sequence of schists and phyllites that contain lenses of dolomitic and calcic white marble and by thin beds of iron formation, and finally the upper Maquine Group of quartzites, conglomerates and phyllites.
The Rio das Velhas Supergroup is discordantly overlain by the Paleoproterozoic Minas Supergroup quartzites, schists, phyllites, meta-conglomerates, carbonates and iron formations that produce the major iron deposits of the district. This supergroup commences with the clastic sediments (orthoquartzite, grit, conglomerate, phyllite, sericite quartzite, and phyllite) of the Caraca Group, which is overlain by the itabirite (banded iron formation), dolomite, limestone and phyllite of the Itabira Group, the rhythmic psammites, pelites, graphitic schists and carbonates of the Piracicaba Group and the thick schists, phyllites and conglomerates of the Sabara Group.
For a more detailed description of the geological setting, see the Quadrilátero Ferrífero District Gold - Geological Setting record.
Mineralisation at Passagem de Mariana and in adjacent mines in the Ouro Preto district is hosted over a 15 km interval of the Batatal Formation in the upper sections of the Minas Supergroup Caraca Group. The Batatal Formation contains three main facies, namely: i). grey graphitic phyllites, ii). biotite and sometimes garnetiferous ferruginous dolomite, and iii). meta-cherts. The richest orebodies are along the footwall contact with the overlying itabirites of the Itabira Group, and are hosted within the graphite-sericite phyllite. While many lithologic units are mineralised the graphite-sericite phyllite appears to have been most favourable for gold deposition.
The Ouro Preto district has been subjected to three phases of deformation, D1, D2 and D3, and upper-greenschist to lower-amphibolite regional metamorphism. Retrograde metamorphism is locally developed at Passagem de Mariana, characterised by chloritisation of biotite and chloritisation and biotitisation of garnet. The gold-bearing veins crosscut the main foliation and lithologic contacts at a low angle and occur within, or are in contact with, all lithotypes.
The mineralised interval is localised within a shear zone associated with a bedding-parallel thrust fault that juxtaposes the itabirite of the Paleoproterozoic Minas Supergroup over other units.
The gold ore occurs as veins composed of >60 vol.% white quartz,with accompanying carbonate (ankerite), tourmaline, sericite and sulphides. Dravite tourmaline, which comprises up to 10 vol.% of the vein, occurs as subhedral, coarse, commonly zoned crystals, and is concentrated along vein boundaries and on the edges of host rock inclusions within the veins. Tourmaline alteration is observed in all rock types in the mine, but is greater in sericitic phyllites, graphite-sericite phyllites and calcareous rocks.
Gold abundance is directly proportional to sulphide concentration. The most abundant sulphide is arsenopyrite, normally associated with pyrite and pyrrhotite, while minor amounts of chalcopyrite, galena, lollingite, berthierite and maldonite are present throughout the deposit. The sulphides are concentrated either at veins boundaries or are dispersed within the veins. Arsenopyrite is most commonly associated with calcareous rocks, and graphite-sericite phyllite, while pyrrhotite is usually found at the base of the itabirites. Hydrothermal alteration associated with the veins includes silicification, tourmalinisation, and sulphidation.
Gold deposition is interpreted to have occurred after the peak of metamorphism, within the late- to post-D2 period of deformation, correlated with a second set of structures of Transamazonian age, which in the Southern São Francisco Craton Region is between 2.124 and 2.04 Ga.
The Passagem de Mariana produced more than 60 tonne of gold, from the end of the 17th Century until 1954. The mine has not operated since 1985.
The most recent source geological information used to prepare this decription was dated: 2007.
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.
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Selected References:
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Trumbull, R.B., Garda, G.M., Xavier, R.P., Cavalcanti, J.A.D. and Codeco, M.S., 2019 - Tourmaline in the Passagem de Mariana gold deposit (Brazil) revisited: major-element, trace-element and B-isotope constraints on metallogenesis: in Mineralium Deposita v.54, pp. 395-414.
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Vial D S, Duarte B P, Fuzikawa K and Vieira M B H, 2007 - An epigenetic origin for the Passagem de Mariana gold deposit, Quadrilatero Ferrifero, Minas Gerais, Brazil: in Ore Geology Reviews v32 pp 596-613
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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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