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Boquira - Sobrado, Pelado, Cruzeiro, Maranhao |
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Bahia, Brazil |
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Pb Zn Ag
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Super Porphyry Cu and Au
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The Boquira metamorphic hosted Pb, Zn deposits is located ~30 km north of Macaubas and ~500 km west of Salvador, in the Paramirim Valley, in central Bahia state, Brazil (#Location: 12° 48' 46"S, 42° 44' 9"W).
The discovery of mineralisation at Boquira was prompted in 1952, when local farmers wished to satisfy their curiosity as to composition of the heavy white mineral that occurred within the soil over the Pelada Hill that was barren of vegetation. Pelado means naked in Portuguese. They took a sample to the parish priest who in turn passed it on to a chemist who identified it as the lead carbonate cerussite. Significant ore production commenced in 1959, and to the end of 1985, total production amounted to 5.60 Mt @ 8.85% Pb, 1.43% Zn (Espourteille and Fleischer, 1984), at a rate of around 0.3 Mt per annum until 1987. Mining has been to a minimum width of 1 m, with a 5% Pb cutoff. Remaining reserves in 1985 were estimated to be 1.25 Mt @ 5% Pb, 1.7% Zn. This would equate to a production + reserve of 6.85 Mt @ 8.15% Pb, 1.48% Zn. Beeson (1990) quoted a total endowment of ~8 Mt @ 9% Pb, 2% Zn, 28 g/t Ag. The economic resource was exhausted in 1992 and the operation closed.
The deposit lies on the western side of the NW-SE trending, 40 km wide, Paramirim Valley which corresponds to the central horst within the Paramirim Block which is composed of recessive Palaeoarchaean basement gneisses in the core of the Palaeo- to Mesoproterozoic Paramirim Aulacogen. The latter is filled with the Late Palaeoproterozoic to Late Mesoproterozoic continental rift Espinhaço Supergroup, which includes abundant resistant quartzites that form 200 to 300 m high cliffs on either side of the valley. The NW-SE Paramirim Aulacogen cuts across and divides the arcuate São Francisco Craton into a northeastern and a southern segment.
The basement complex is composed of an orthogneiss suite of Na- and less frequently K-rich, granites and granodiorites, and migmatites, with minor meta-volcanic and meta-sedimentary rocks, all of which are extensively covered by lateritic soil and Cenozoic fluvial sands. Regionally this complex includes significant Neoarchaean gneisses, but is also intruded by more recent suites. Ages in the Boquira district include 3150 Ma (K/Ar on tremolite from an amphibolite), 3020 Ma (total rock on meta-mafic gneiss), 1860 Ma (K/Ar on tremolite from a tremolite-marble), 815 Ma (K/Ar on biotite from gneiss) as reported by Pedreira et al (1978) as well as Rb/Sr isochrones of 2600 Ma on gneisses, migmatites, granites and metavolcanics, and 1530 and 1080 Ma from non-foliated granites (Jardim de Sa et al.,1976), and 2680 ±56 Ma (Mascarenhas et a, 1982). These geochronological data are interpreted to be consistent with a Palaeo to Mesoarchaean basement that has been reworked to amphibolite facies metamorphism and migmatised at ~2.6 Ga, but also underwent later thermal events including the 1.3 to 1.0 Ga cycle that folded and metamorphosed the overlying Espinhaço Supergroup. In the Boquira district, the latter sequence has a basal polymictic conglomerate with associated thin mica bearing ferruginous quartzite overlain by 60 to 80 m of rhyolitic-trachytic tuffs and flows of comenditic filiation. These are succeeded by various cycles of fluvio-deltaic quartzites and shallow marine shales. Folding and metamorphism of this sequence increases from weak in the east, to lower amphibolite facies metamorphism and isoclinal folding in the west (Jardim de Sa et al.,1976). Distal to the Paramirim Valley, the Espinhaço Supergroup is overlain by weakly disturbed to flat-lying platformal sedimentary rocks of the Neoproterozoic Bambui group.
Mineralisation at Boquira is hosted by the Boquira Formation, which has been variously interpreted to belong to either the basement complex or to the Espinhaço Supergroup, as its contacts with these units is covered by ubiquitous coIluvial debris. However, Fleischer, Gutierrez and Espourteille (1999), have presented convincing evidence that it belongs to the basement complex, based on detailed mapping and stratigraphic relations. These include observation of pebbles of some of the critical and diagnostic lithologies of the Boquira Formation within the basal conglomerates of the Espinhaço Supergroup. However, the exact age is uncertain and may be either late Neoarchaean or Palaeoproterozoic, although it is seen to interfinger with gneisses interpreted to be Archaean, although some authors (e.g., Jardim de Sa et aI., 1976; Mascarenhas et aI., 1979) argued the gneisses are Palaeoproterozoic rocks that have been subjected to incomplete migmatisation during the 2.0 Ga Transamazonian cycle.
In the deposit area the Boquira Formation is composed of chlorite-garnet-biotite schists, quartzites, limestones, marbles, itabirites/banded iron formation and amphibolites and strikes subparallel to the eastern border of the Espinhaço Range. The amphibolites, which are the host to the mineralisation, are expressed as four prominent elongated hills. These are, from the north to south, the Pelado, Sobrado, Cruzeiro and Maranhao hills. The main lithologies of the Boquira Formation are lenticular and interfinger, e.g., the amphibolites grade into itabirites in one extreme and into limestones at the other, whilst itabirites may grade directly into limestones, and the latter directly into chlorite-garnet-biotite schists.
The host amphibolites are very variable in composition, such that Rocha (1985) had defined seventeen different types, grouped into three main facies, namely: i). quartz-magnetite, ii). silicate-magnetite and ii). carbonate-silicate bearing amphibolites. The principal silicates are cummingtonite, grunerite, pargasite, anthophillite, actinolite and tremolite, whilst the carbonates are predominantly calcite, dolomite and ferroan-dolomite. Both the amphibolites and itabirites, which are gradational, are often well banded due to alternating centimetre thick magnetite or carbonate and amphibole rich layers. This banding is interpreted to probably be a primary feature as it is tightly folded and deformed by the most penetrative schistosity. In addition toi these banded varieties, there are also massive and schistose varieties. The former are fine- to medium-grained with crystalline magnetite aggregates disseminated in the groundmass, whilst the latter are often associated with shear zones, although they usually represent the transition to chlorite-garnet-biotite schists.
The itabirites are banded quartz-magnetite or quartz-hematite equivalents of the amphibolites and contain amphiboles, principally actinolite, with accessory chlorite and biotite. As a result of deep weathering, there is no outcrop of limestones and dolomitic marbles in the Boquira area, although in underground workings it is apparent they are common within the Boquira Formation, where they are medium to coarse grained, containing accessory biotite, sericite and amphiboles. As the amphibole content increases, they become banded until they grade into banded amphibolites. In the deposit area, and semi-regionally within the Paramirim Valley, dolomitic marbles are frequently associated with sericitic and ferruginous quartzites forming very distinctive outcrops within the basement gneisses and migmatites. Where dolomitic marbles and calc-amphibolites are in contact with quartzites, they are replaced by epidotites. Within the deposit area, the chlorite-garnet-biotite schists are in contact with quartzites and dolomitic marbles, whilst more diatal, they occur as remnants within the basement gneisses and migmatites.
The Boquira Formation has been severely affected by folding, metamorphism and migmatisation, including at least two pulses of isoclinal folding and associated thrusting. This, in conjunction with the frequent lateral facies changes, the absence of reliable geopetal features, and poor outcrops, have hindered the definition of a vertical succession of lithologies or a coherent stratigraphic sequence. Underground mapping suggests the Boquira Formation is composed of: i). a lower, thick, coarse-grained quartzites overlain by mica-schists; ii). a middle suite of limestones, dolomitic marbles amphibolites and itabirites, and iii). An upper chlorite-garnet-biotite schists with thin intercalations of fine-grained quartzites. The two mica-schists are very similar in appearance and have not been separated in mapping in the deposit area.
Three significant ore zones have been exploited, the Sobrado, Pelado and Cruzeiro and the smaller Maranhão, with production tonnages between 1959 and 1985 respectively of 2.236, 1.316, 2.044 and 0.009 Mt tonnes of ore. Sobrado and Pelado form a north-south trend, whilst Cruzeiro lies to the east. Maranhão may represent the southern continuation of the Sobrado-Pelado trend. Mineralisation is sheet-like, and is concordant with the amphibolite banding. It is continuous over lengths of the order of several hundred metres, e.g., 1400 m at Cruzeiro and >1250 m at Sobrado, thicknesses that vary from centimetres to several metres and persisting to vertical depths of >450 m as at Sobrado. Each of these zone is composed of two or more parallel lens-like mineralised bodies. The individual lenses have the following dimensions:
• Sobrado: Main lens - 650 x 460 m; South lens - 320 x 180 m; North lens - 100 x 50 to 70 m.
• Pelado: Main lens - 1000 x 340 m; East lens - 200 x 110 m
• Cruzeiro: West lens - discontinuous lenses mined over a length of 1400 m and to a depth of 250 m.
• Maranhão: 110 x 340 m.
The individual parallel lenses that constitute these deposits are in some cases interpreted to be due to structural repetition of a mineralised horizon on the limbs of syn-metamorphic isoclinal folds. At Cruzeiro, the Main and West lenses merge on the 480 level in a synclinal hinge. At Pelado, large mineralised outcrops at the discovery site, are the result of three fold repetitions of the same mineralised horizon. Elsewhere the repetition does not appear to be an artefact of folding, although the exact reason is not immediately evident. At Sobrado, the Main and the sub-parallel A and B zones are an example. Where the three intersect, the B zone is sheared, interpreted to indicate a mineralised fault that truncated the A Zone. However, removed from that intersection, the B Zone mineralisation reverts to its 'normal' schistose texture, suggesting the B Zone is similar to the Main and A zones and has been folded, or transposed during folding, or alternatively corresponds to a parallel lens to the A ore body. Both have been sheared in their extremities by an oblique fault zone, thus creating a connection between them (Fleischer, Gutierrez and Espourteille, 1999). These mineralised zones are commonly interrupted and displaced by NW-SE trending faults and by east-dipping shear zones. These structures have also been intruded by late, coarse-grained porphyritic granites which have affected the mineralisation. The largest of these granite bodies is up to 100 m across and intersects the northern end of the Sobrado Zone, although contact metamorphism is restricted to an aureole that is only a few centimetres wide, and resulted in recrystallisation and chlorite alteration. Smaller granite intrusions are common in the Pelado and Maranhao zones, but are generally only a few metres in diameter.
Weathering extends to a depth of ~20 m, with mineralisation being mainly oxidised to cerussite, smithsonite, limonite and anglesite, with minor pyromorphite, hemimorphite, hydrozincite, chrysocolla, bomite, covellite, malachite and azurite. Traces of huntite, gahnite and plumbo-jarosite have also been reported (Cassedanne, 1972).
Fresh mineralisation occurs as massive lenses of galena, sphalerite and pyrite with virtually no gangue minerals. Galena constitutes ~90% of the sulphides, whilst pyrite predominates on the extremities of the mineralised lenses. The sulphide lenses are concordant with the banding of the amphibolite, and vary from a few cm to 2 m, and exceptionally 5 m in thickness, with a sharp transition to the country rocks over a few centimetres width of disseminated sulphides. Sulphides occur as coarse, up to 2 to 3 cm crystals with a schistose texture, interpreted to be the result crystallisation under the tectono-metamorphic conditions.
Where the sulphide lenses are cut by faults there may be intense remobilisation forming recrystallised milky quartz, calcite and coarse-grained, up to 2 cm long, euhedral galena crystals with minor sphalerite. Where the fault zone is filled with milky quartz and calcite over its entire length, galena and sphalerite occurs adjacent to the fault, fading out over widths that range from a few centimetres to >20 m. In some intervals of tight folding, cataclastic sulphide filled breccias are formed, with a loss of schistosity.
Partial chloritisation of amphiboles, weak pyrite and chalcopyrite disseminations along milky quartz veins, the presence of serpentine and tourmaline in some quartz veins and quartz nodules, as well as the frequent occurrence of euhedral garnet crystals along shear zones have been attributed to hydrothermal alteration. However, these characteristics are not restricted to the vicinity of the mineralisation, but are a widespread feature of the Boquira Formation and elsewhere in the basement, and most frequently are not obviously related to mineralisation.
The Pb/Zn ratio in the mineralised lenses is very high, while there is a high correlation between Zn, Cd and In. Cadmium is present as Cd replacing Zn in the sphalerite crystal lattice. Indium is associated with sphalerite whereas silver is associated with galena. However, there is a negative correlation between the Pb, Zn, Cd and In associations and Cu, Mo and Au. The latter three elements are concentrated in a quartz filled fractures that cut the main sulphide lenses. Chalcopyrite and pyrite can be seen to have been introduced during a late crystallisation event. Age dating of galena from the deposit has yielded results of between 2.66 and 2.55 Ma (Misi, et al., 1995).
The information in this summary is largely drawn from Fleischer, Gutierrez and Espourteille (1999), as cited below.
The most recent source geological information used to prepare this decription was dated: 1999.
Record last updated: 15/6/2022
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.
Boquira
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Selected References:
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Carvalho, I.G., Iyer, S.S., Tassinari, C.C. and Misi, A., 1997 - Lead-and sulfur-isotope investigations of the Boquira sediment-hosted sulfide deposit, Brazil: in International Geology Review v.39, pp. 97-106.
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Espourteille, F.S. and Fleischer, R., 1988 - Mina de Chumbo de Boquira, Bahia: in Schobbenhaus, C. and Coelho, C.E.S., (Eds.), 1988 Principais Depositos Minerais do Brasil Republica Federatova do Brasil, Ministerio das Minas e Energia, DNPM and CVRD, v.3, pp. 91-99.
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Fleischer, R., Gutierrez, A. and Espourteille, F.S., 1999 - The Boquira lead-zinc mine in central Bahia, Brazil: in Silva, M.G., Misi, A. (Ed.), 1999 Base Metal Deposits of Brazil CPRM, Brazil pp. 44-53.
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Misi A, Iyer S S, Tassinari C G, Coelho C E S, Kyle J R, Franca-Rocha W J S, Gomes A S R, Cunha I A, Carvalho I G, Filho V M C 1999 - Integrated studies and metallogenic evolution of the Proterozoic sediment-hosted Pb-Zn-Ag sulfide deposits of the Sao Francisco Craton, Brazil: in Silva M G, Misi A (Ed.), 1999 Base Metal Deposits of Brazil CPRM, Brazil pp 84-91
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Teixeir, J.B.G., Silva, M.G., Misi, A., Cruz, S.C.P. and Sa, J.H.S., 2010 - Geotectonic setting and metallogeny of the northern Sao Francisco craton, Bahia, Brazil: in J. of South American Earth Sciences v.30, pp. 71-83.
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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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