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Rocinha, Patos de Minas, Lagamar |
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Minas Gerais, Brazil |
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P
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Super Porphyry Cu and Au
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The Rocinha or Patos de Minas sediment hosted phosphorite deposit is located 55 km NW of the town of Patos de Minas in the Brasilia Fold Belt of Minas Gerais State, Brazil, some 350 km SSE of Brasilia and 450 km NW of Belo Horizonte. The low tonnage, high grade Lagamar deposit is ~40 km NNE of Rocinha.
In central Brazil, in the states of Minas Gerais and Goias, the western margin of the São Francisco craton is delineated by the Brasilia fold belt that underwent orogenesis during the ~600 Ma Brasiliano orogeny. Rocks of the fold belt are thrust to the east and display increasing deformation and metamorphism westward. The São Francisco craton is composed of an Archaean to Palaeoproterozoic crystalline basement exposed in the Quadrilatero Ferrifero, overlain by the flat-lying Mesoproterozoic Espinhaço Supergroup which is only exposed over limited areas where the upper units of sandstones and siltstones outcrop, and the ~2000 m thick transgressive Neoproterozoic São Francisco Supergroup. The São Francisco Supergroup comprises the basal lenticular Jequitaí Group, composed of tillites lying on glacial striated surfaces, followed by the Bambuí Group which commences with a local lenticular conglomeratic unit, the Carrancas Formation, overlain by four formations characterised by alternating finely detrital sediments and stromatolitic carbonate beds. This suite is overlain by the partially continental upper unit, the Três Marias Formation.
Both the Espinhaço and São Francisco Supergroup are found in the Brasília fold belt. To the west the Araxá Group composed of basic and ultrabasic magmatic rocks which are succeeded by gneisses, mica-schists, schists and quartzite that are equivalents of the Espinhaço Supergroup. Eastward they pass into the Canastra Group, a sequence of quartzites and schists. The Araxá and Canastra groups were deposited in a rift basin (aulacogen) to the west of the São Francisco craton. The Canastra Group passes upwards into the Paranoã Group which can be correlated with the upper part of the Espinhaço Supergroup on the craton to the east. The Araxá, Canastra and Paranoã groups were most likely metamorphosed during the Uruaçu orogeny at ~1100 Ma, the intensity of which diminishes rapidly to the east. The overlying Unai Formation found in the Brasilia fold belt, with its basal tilloid Ibia Formation can be correlated with the similar Jequitaí Group tillites and overlying Bambuí Group of the São Francisco Supergroup on the craton. At around 600 Ma all the metasedimentary rocks and basement were refolded by the Brasiliano orogeny obliterating earlier structural textures and thrusting the Araxá, Canastra and Paranoã group rocks eastward onto the São Francisco craton.
Almost all the phosphate deposits are found in the Brasilia fold belt, either along its boundary with the craton or in external structural units show polycyclic deformation and greenschist metamorphism. They occur in the metasedimentary rocks of the Mesoproterozoic Paranoã Group of the Espinhaço Supergroup and in the Neoproterozoic Unai Formation of the São Francisco Supergroup and its equivalents.
The Mesoproterozoic phosphorites of the Paranoã Group are the more economically important, and are spatially and temporally associated with the 250 km long, north-south oriented Vazante stromatolite reef barrier, where black pyritic metasedimentary pelitic rocks are intercalated with phosphatic metasedimentary carbonate layers and breccias, overlying metasedimentary reef dolomites. The phosphate is present either as grains, pellets and intraclasts or, less frequently as homogeneous but discontinuous centimetre-thick layers. Limited tonnages of high grade phosphate (e.g., the Lagamar deposit, ~40 km NNE of Rocinha, that in 1976 contained 5 Mt @ 35 to 38% P2O5) are recorded from these occcurrences, with some having accompanying larger tonnages of lower grades, as at Rocinha where a high grade core of 25 to 30% P2O5 has an overall resource as quoted below.
The Vazante reef barrier rests on a segment of sub-stratum upwarped by normal faulting, with the phosphorites located on the eastern side of the barrier, in a low energy platformal environment. In contrast to the other examples associated with the reef, Rocinha-Patos de Minas, the only presently economically exploited deposit, is located on the southern extremity of the reef barrier, developed on a shallow to deep marine basin ramp, open to the south and SW, located between a wide internal basin with reduced oxygen conditions and limited detritus, and a more oxygenated marine environment. The Paranoã Group sequence in this section of the Brasilia fold belt were deposited on a large westward dipping platform resting on basement that was to become the São Francisco craton to the east, and were composed of very fine and well-sorted detrital sediments derived from this basement. The Vazante reef barrier characterises the western border of this platform.
The São Francisco Supergroup Unai Formation contains phosphatic accumulations at two main stratigraphic levels within the region. The lower of these is in basal conglomerates transgressive onto the São Francisco craton basement. Thicknesses of <3 m of quartzites and phosphatic schists and conglomerates are deposited in narrow depressions within the basement. The phosphatic material occurs as nodules, grains and intraclasts, with a P2O5 content of <15%.
Higher in the Unai Formation, mineralisation occurs as phosphatic metapelite and siltstone beds, intercalated with schists, metasedimentary siltstones and metasedimentary dolomites ranging in thickness from a few tens of cms to 2 m carrying grades of 10 to 15% P2O5, but only a few percent in the adjacent host rocks. Grades of 14 to 34% P2O5 at some locations correspond to centimetre-thick beds of green clayey metasedimentary siltstone containing illite, chlorite and smectite, and in brown siliceous layers in which the P2O5 levels are greater. In others, grades of 3 to 18% P2O5 represent phosphatic pellets and nodules scattered in a carbonate matrix, alternating with black, pyritic siltstone overlying cherty limestones.
The Rocinha deposit, within the Mesoproterozoic Paranoã Group, is found in a similar structural position to the Paracatu gold, Vazante zinc and Morro Agudo zinc mines to the north, namely along a major regional, west dipping north-south thrust shear towards the eastern margin of the Brasilia Fold Belt. The mine sequence is present in four NNE-SSW oriented tectonic slices of schists, quartzoschists and phosphatic schists, separated by sub-horizontal shear planes defined by tectonic breccias with secondary apatite. The mine exposes a 50 to 75 m thick phosphatic unit, which is repeated by thrusting some 100 m lower over a strike length of ~10 km.
Within this structurally disrupted sequence, four lithofacies are distinguished, from east to west, namely the: (i) slate facies, which is slightly more than 100 m thick, and is characterised by rhythmic schists and feldspathic metasiltstone alternations, grading upwards into pyritic carbonate schist with minor dolomitites and dolomitic marble. The pyrite is associated with thin (10 µm) black organic films; (ii) transition facies, from slate to phosphatic rocks, composed of schists with thin phosphatic beds and phosphatic metasedimentary dolomites containing alternations of light to dark laminations. The facies is characterised by a large amount of clay minerals, pyrite microcrystals and a very fine black organic matter occurring as disseminated patches seperating rhombohedral crystals of dolomite, and as thin discontinuous films; (iii) phosphatic schist facies is 50 m thick, and is composed of mm- to cm-thick alternations of grey to yellow, locally pyritic, quartz schists and grey to black granular, often homogeneous phosphatic beds containing from 12 to 21%, locally reaching 30% P2O5. Apatite is accompanied by smectite, interlayered chlorite-smectite and phengite-like illite with lesser kaolinite and minor goethite and crandallite; and (iv) glauconitic schist facies, which is free of carbonates and is characterised by brown to dark yellow schists with rare mm-thick beds of green glauconite and apatite, with an average grade of 1% P2O5. Phosphate can be observed microscopically in the transition, phosphatic schist and glauconitic schist facies. The phosphate occurs as grains, angular fragments, lenses or centimetre thick beds in schists and carbonates or glauconitic schists.
These four contemporaneous facies are interpreted to reflect a lateral transition at the external margin of the platform from reduced terrigenous deposition to open sea marine conditions in the slate and glauconitic facies respectively. The absence of indicators of emergence or high energy sedimentary structures imply deposition in a large, deep bay. Phosphate is interpreted to have been deposited at the transition from a shallow ridge at the mouth of the bay (transition facies) to a relatively deep and quiet ramp environment (phosphatic schist and glauconitic facies).
The ore grade phosphate rock is a homogeneous crypto- to microcrystalline apatite in fairly continuous centimetre thick beds which were fragmented, stretched and boudinaged during the 600 Ma Brasiliano isoclinal folding and sub-horizontal shearing. Grades range from 10 to 15% P2O5.
Reserves at the Rocinha mine based on differing cut-off grades (da Rocha et al., 1992) are either:
415 Mt @ 5% P2O5, or
200 Mt @ 11 to 13% P2O5.
Reserves in 2006 (DNPM 2006 Mineral Annuary) were 304.6 Mt @ 12.36% P2O5, concentrated to grades of 24% P2O5.
The most recent source geological information used to prepare this decription was dated: 1992.
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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Carvalho, L.M.M., Pires, A.C.B. and Vidotti, R.M., 2015 - Phosphate prospection in the alkaline province of Alto Do Paranaiba, Brazil, based on airborne geophysical data: in Revista Brasileira de Geofisica (Brazilian Journal of Geophysics), v.33, pp. 237-249.
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Da Rocha A P R, Flicoteaux R, Parron C, Trompette 1992 - Phosphorites of Rocinha Mine - Patos de Minas (Minas Gerais, Brazil): genesis and evolution of a Middle Proterozoic deposit tectonized by the Brasiliano Orogeny: in Econ. Geol. v87 pp 332-351
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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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