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Brejui, Boca de Lage |
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Rio Grande do Norte, Brazil |
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W Mo
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Super Porphyry Cu and Au
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IOCG Deposits - 70 papers
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All papers now Open Access.
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The Brejuí and Boca de Lage scheelite-molybdenite skarn deposits are located 170 km WSW of the city of Natal, and 8 km SW of Currais Novos in Rio Grande do Norte, NE Brazil. Brejuí is the main scheelite deposit of the Seridó Tungsten Province that lies within the Borborema tectonic province.
Geological Setting
The Seridó tungsten province comprises some 5600 known scheelite occurrences distributed over an area of 300 x 100 km. It forms a NNE-SSW oriented corridor along the eastern interior margin of the Seridó Mobile Belt in the northern part of the Borborema Tectonic Province (Almeida et al., 1981). The rocks within the Seridó tungsten province comprise a Meso- to predominantly Ediacaran Neoproterozoic metasedimentary and metavolcanic sequence, the Seridó Group, that overlies reworked Archaean to Palaeoproterozoic basement blocks. The basement blocks had been re-oriented during the ~600 Ma early Neoproterozoic Brasiliano Orogeny by a crustal-scale complex of NE-trending strike-slip shear zones (Brito Neves et al., 2000). The evolution of the Borborema province culminated in the intrusion of voluminous granitic magmatism during the late stages of the Brasiliano Orogeny which is manifested as the Acari Massif granitoids. These intrusions are surrounded by regional and contact metamorphic aureoles overprinting the Neoproterozoic country rocks.
The Seridó Mobile Belt (or Rio Piranhas-Seridó tectonic domain) and Seridó Group have been subjected to two metamorphic episodes, M1 and M2. M1 occurred under greenschist facies conditions and developed a schistosity S1, which was superimposed by M2, a medium to high-temperature low-pressure greenschist/amphibolite facies episode. This second event was responsible for the metamorphic aureole centered on the Acari Massif granitoids (Souza Neto et al., 2008) reaching an estimated temperature-pressure peak of 560 to 630°C at 2.7 to 4.5 kbar (Silva, 1995, 2000). This metamorphism produced an outward zonation from the central granitoids of cordierite-sillimanite → cordierite-andalusite with lenses of staurolite-andalusite → biotite-garnet → biotite-chlorite-muscovite. This metamorphosed sequence also enclosed inner enclaves of biotite-feldspar paragneiss and marbles adjacent to the granitoids. Post granitoid mylonitic garnet-sillimanite quartzite and mylonitic gneisses cut both the granitoids and the metamorphosed Neoproterozoic sequence. The Seridó Mobile Belt is bounded to the south, west and east by the Patos, Portalegre and Picuú-João Câmara shear zones respectively.
The Seridó Group that overlies the Archaean to Palaeoproterozoic basement in the immediate Brejui area is a Neoproterozoic supracrustal sequence that is composed of three units (after Jardim de Sá and Salim 1980), the:
• Jucurutu gneisses, quartzites, amphibolites, marbles and calc-silicate rocks;
• Equador muscovite quartzites; and
• Seridó aluminous schists.
The Brasiliano plutonic activity within the Seridó Belt took place between 600 and 525 Ma (Archanjo et al., 2013; Nascimento et al., 2015; Souza et al., 2016; Hollanda et al., 2017), and mainly comprised high-K, calc-alkaline monzogranites, quartz-monzonites and monzonites of the Acari Massif. These granitoids are usually accompanied by metaluminous, intermediate to locally mafic rocks of shoshonitic composition (Nascimento et al., 2015; Hollanda et al., 2017). The magmatic rocks reached a thermal peak at 630°C and 3.5 kbar between 579 and 555 Ma (U-Pb) and subsequent cooled from 400 → 300°C at ~481 Ma (Ar-Ar; Souza, 1996). The latest magmatic pulses are represented by dyke swarms and pegmatite sills.
Brejuí W-Mo skarn altered deposit
The Brejuí W-Mo deposit is hosted by metasedimentary rocks of the Jucurutu Formation of the Seridó Group, proximal to the Brasiliano age Acari pluton. This pluton is composed of equigranular, high-K calc-alkaline monzogranites of the Dona Inês Suite which is dated at 557 ±13 Ma (Rb-Sr whole-rock; Borges, 1996), and associated mafic rocks of the 579 ±7 Ma, São João do Sabugi Suite (U-Pb zircon; Leterrier et al., 1994). Both the igneous and metasedimentary host rocks are crosscut by a 514 ±1 to 509 ±2 Ma late Brasiliano pegmatite dyke swarm (U-Pb monazite; Baumgartner et al., 2006).
The skarn alteration of the Brejuí deposit is developed from marbles of the Jucurutu Formation, close to the lithologic contact with gneisses from the same formation, producing banded calc-silicate rocks, formed by overprinting of prograde and retrograde skarn mineral assemblages (Salim, 1993). The prograde assemblage produced a banded rock with massive coarse-grained textures, composed of diopside-hedenbergite- and grossular-rich layers. Where found in contact with silicate gneisses, the W-Mo skarns tend to be enriched in metasomatic pyroxene and/or hornblende, whilst as the marbles are approached, garnet predominates over pyroxene (Salim, 1993). The same author also describes a hornblende-rich zone in the skarns in contact with gneisses.
The retrograde mineral assemblage was developed by successive pulses of hydrothermal alteration overprinting the prograde skarns. Three main hydrothermal stages have been recognised (Salim, 1993):
i). scapolite-vesuvianite formed at temperatures of from 450 to 380°C and fluid pressure of 2 kbar;
ii). epidote-prehnite developed at temperatures from 380 to 270° and fluid pressure of 2 kbar, and;
iii). zeolite which accompanied development of the main ore zone at temperatures of from 300 to 200°C and fluid pressure of 1 to 0.6 kbar.
The scapolite-vesuvianite stage resulted from high-temperature metasomatism caused by the circulation of magmatic-hydrothermal fluids through the skarn system, whilst the epidote-prehnite and zeolite stages reflect fluid mixing (Salim, 1993).
According to Salim (1993), the precipitation of scheelite and sulphides is interpreted to have been principally related to the epidote-prehnite and zeolite stages, representing early and late stage mineralisation, respectively. The zeolite stage resulted in a ~40% mass loss of host rock, and an increase in orebody volume, as evidenced by highly porous/vuggy and brecciated skarn alteration zones. These rocks host the main W-Mo mineralisation at the Brejuí mine, the mineralogy of which comprises scheelite, molybdenite, pyrite, chalcopyrite, bornite, ferberite, powellite, wolframite, magnetite, bismuth and bismuthinite (Souza Neto et al., 2008). Magnetite and pyrite are in equilibrium with scheelite and molybdenite, whilst Salim (1993) describes grossular as the main garnet formed in the prograde assemblage of these skarns. For this reason, Souza Neto et al. (2008) suggested participation of oxidised hydrothermal fluids in the formation of the Brejuí skarn mineral system. Hollanda et al. (2017) determined an age of 554 ±2 Ma (Re-Os in molybdenite) for the Brejuí W-Mo mineralisation, which correlates with the latest Brasiliano pulses of granite and pegmatite emplacement.
The Brejuí W-Mo deposit, which was still in operation in 2024, contains the largest current Brazilian scheelite ore reserve of 25.4 Mt @ 0.04 to 2.4% WO3 (Departamento Nacional de Produção Mineral, 2014, quoted by Costa et al., 2024).
The information in this Brejuí summary is largely drawn from Costa et al. (2024) cited below.
When visited in December 1977 (Porter 1978), Brejuí was the northernmost of three operating mines immediately to the south of the town of Currais Novos. The other two were the central Barra Verde and Boca de Lage in the south. At that stage, the total cumulative production to 1977 for Brejuí and Barra Verde was 15 000 and 13 000 tonnes of scheelite concentrates respectively.
The current Brejui mine is operated by Mineracao Tomaz Salustino S.A. in 2024.
Boca de Lage scheelite deposit
The following description of the orebodies at Boca de Lage, as known at the time, is based on notes taken during a visit in 1977. The occurrence of mineralisation, based on discussions with mine geologists and as observed from an underground traverse at Boca de Lage, is also described.
The ore horizon at Boca de Lage and the neighbouring Barra Verde and Brejui Mines is mineralised as a series of skarn lenses averaging 100 m in length by 50 m wide and 1 to 3 m in thickness. Locally these pods may reach thicknesses of 4 m. They are developed over a strike length of 4 km and a width of 400 m on seven levels within the host carbonates. The ore zone is restricted to an interval of parasitic folds within a broad south plunging syncline. This structure has three parasitic synclines and intervening anticlines with, in the mineralised area, wavelengths of around 100 to 150 m. Dips reach 60° on the flanks of these folds, which have a 15° plunge, parallel to the regional synclinal axis.
The immediate mineralised sequence comprises:
Biotite Schist.
Lower Marble - 15m thick.
Intermediate Gneiss - a well foliated biotite-quartz-feldspar gneiss, grading to schist which is some 6 m in thickness.
Upper Marble - 15m thick.
Granite Gneiss - quartz, feldspar, biotite gneiss ~150 m thick.
The marbles vary from grey to white to orange in colour, and are coarsely crystalline and massive, grading in some areas to banded white and grey varieties. To the west, the marbles thicken, accompanied by a thinning and lensing out of the intervening Intermediate Gneiss. The mineralised horizons are at the top and bottom margins of each of the marble units and at positions within them. The ore pods within the carbonate units tend to be best developed in either crest or keel zones of the parasitic folds. Those on the upper margins of the marbles are, in general, found at the crests of anticlinal structures, while those at the bottom of each are concentrated at the keels of synclines. The long axes of the mineralised pods parallel the fold axes. Mineralisation is largely restricted to these skarn pods. Three skarn assemblages are found within the mine. These are:
Garnet-diopside - which range from a finely, to a coarsely banded rock with individual garnet and diopside bands and lesser epidote and actinolite. Individual bands range from 1 to 100 mm in thickness.
Chlorite-amphibolite - Very fine, massive, chlorite amphibole rock with, in places, alternating bands of calcitic material.
Vesuvianite - Fine vesuvianite, with lesser epidote and garnet.
Of these, the garnet-diopside and chlorite-amphibole skarns are mineralised, while the vesuvianite variety is usually barren. The skarn pods/lenses generally have sharp contacts with the enclosing marbles. The richest scheelite usually accompanies garnet rich bands.
Scheelite occurs as very fine disseminations to blebs, varying from 1 to 30 mm, to sheet like aggregates 1 mm thick and several cm long. In thick garnet rich bands, it occurs as fine disseminations throughout the band or as individual 2 to 3 mm bands of blebs separated by intervals of 2 to 10 mm. Coarse, clear quartz-garnet aggregates often occur on the rims of skarn pods.
A characteristic pod/lens was traversed, along its long axis in an underground opening. From the down dip extremity it comprised clear quartz-garnet aggregates that were 0.8 to 1 m thick, and 4 m long. These partially surround a lens shaped vesuvianite skarn pod that was 3 m long and 1 m thick at its maximum. Three metres further along trend, another vesuvianite lens is developed. Five metres further along, this interfingers with an amphibole-chlorite calc-silicate which persists for 50 m where it interfingers with a garnet skarn ranging from 2.5 to 4 m in thickness. Thirty metres or more of the garnet skarn were exposed. This interval carries high grade scheelite in the forms described above.
All of these skarn types have sharp or 'reasonably sharp' contacts with the enclosing marble. Traces of scheelite were occasionally encountered within the marble. Small massive sulphide pods are occasionally developed adjacent to some of the skarn pods. These are of the order of 2 m in thickness. The example sighted was 15 m wide with an undisclosed length. The sulphides comprise bornite, chalcopyrite and pyrite in a siliceous gangue. The rock type is strongly magnetic, apparently due to very finely disseminated magnetite. These pods carry 0.8 to 0.9% WO3, occurring as very fine scheelite disseminated through portions of the sulphide zone.
The biotite schists and granite gneisses in the region have background WO3 levels of from 10 to 20 ppm. We were informed that there is a halo of elevated WO3 levels in excess of 20 ppm in the hanging wall granite gneiss for an 800 m radius around the deposit. Within the ore zone, between the mineralised skarn pods, the WO3 grade of the unmineralised marble varies from zero to 0.2% WO3.
Because of the shallow plunge and basic flat lying nature of the orebody only two levels were developed at that stage within the Boca de Lage Mine, at 165 m and 195 m below the surface respectively. The monthly mining rate was ~7500 tonnes of ore, producing 30 to 40 tonnes of concentrate. The plant handled 250 tpd on a 3 x 8 hr shift basis. The concentrate produced at Boca de Lage was relatively pure, composed of 75.5% WO3; 0.01% Sn; 0.02% As; 0.76% Mo; 0.13% S; 0.2% P; 1.2% SiO2 (Pure scheelite is 80.52% WO3).
Proved reserves at Boca de Lage in 1977 were of the order of 1.8 Mt of ore. This reserve is in the form of sufficient proved reserves for a ten year period of mining only.
The total potential tonnage within the three mines in 1977 was estimated at 8 Mt @ 0.53% WO3.
In 1977, Boca de Lage was owned and operated by Tungstenio do Brasil Minerais e Metais Ltda, which was then an associate of Union Carbide. Scheelite mineralisation was discovered at Currais Novos in 1943, and worked during the second World War for use in the steel industry. Production declined during the 1980s due to low prices, and virtually ceased from the late 1990's, although it has resumed more recently. Brejui is also partly operated as a tourist mine.
The most recent source geological information used to prepare this decription was dated: 2024.
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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Costa, M.A.C., Naleto, J.L.C., Perrotta, M.M., Monteiro, L.V.S. and Souza, C.TR., 2024 - Visible-Near Infrared-Short-Wave Infrared Spectroscopy and Mineral Mapping of Hydrothermal Alteration Zones at the Brejui W-Mo Skarn Deposit, Serido Mobile Belt, Borborema Province, Brazil: in Econ. Geol. v.119, pp. 1171-1198. doi.org/10.5382/econgeo.5085.
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Neto, J.A.S., Legrand, J.M., Volfinger, M., Pascal, M.-L. and Sonnet, P., 2008 - W-Au skarns in the Neo-Proterozoic Serido Mobile Belt, Borborema Province in northeastern Brazil: an overview with emphasis on the Bonfim deposit: in Mineralium Deposita v.43, pp. 185-205.
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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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