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Brauna |
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Bahia, Brazil |
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The Braúna 03 pipe of the Braúna diamondiferous kimberlite field is located ~7 km south of the town of Nordestina, and ~330 km NW of Salvador, in the State of Bahia, Brazil (#Location: 10°; 54' 9"S, 39° 25' 23"W).
Twenty-two diamondiferous kimberlites have been discovered in the Braúna field, comprising three pipes, Braúna 03, 04 and 07, and 19 dyke systems (Leroux et al., 2010). According to CPRM (2017), at the end of 2016, there were 28 intrusions in total, of which 8 wer e diamondiferous.
Prior to the 1980s, artisanal gold and diamond mining had been undertaken in the vicinity, principally between 1927 and 1953. The bulk of the mining activity was along the Itapicuru River where more than 5000 garimperos were active, and are reported to have recovered ~5000 stones, the largest of which was 10.5 carats. Modern systematic exploration for diamonds was initiated in 1980 by De Beers Brasil Ltda, with the Itapicuru River catchment being one of their targets, based on the historic recovery of diamonds. An initial regional stream sediment sampling program proved negative. However, in 1989-90 a more detailed localised stream sediment sampling program identified a kimberlite indicator mineral anomaly in the form of garnets and Cr-spinels in samples from the Riacho Grande Creek in the Itapicuru River basin. Follow-up work, including systematic stream and soil sampling, led to the discovery of the Braúna 01 and 02 dykes and the Braúna 03 kimberlite. Mineral chemistry and microdiamond analysis results from these kimberlitic occurrences were deemed characteristic of diamondiferous kimberlite. Limited surface bulk sampling in the mid-1990s by De Beers demonstrated the presence of macrodiamonds from samples of <5 m3 with the largest stones recovered being 0.50 and 0.31 carats. Detailed soil sampling and ground magnetic surveys defined 12 additional dykes, but a regional airborne magnetic survey at a line spacing of 250 m, covering an area ~7600 km2 failed to locate any additional kimberlitic occurrences. It was concluded the kimberlites and district were too small and too low grade to be economic interest and no further work was undertaken, although the exploration titles were retained by De Beers until 2004. However, in 2000-01, extensive artisanal mining activity was conducted by a local land owner downslope of the Braúna 03 kimberlite. Some 250 diamonds with a total weight of 372.9 carats were recovered from colluvial gravels and soils, with individual diamonds reportedly ranging from 0.20 to 15.5 carats, with a significant portion in the 1 to 3 carat range, averaging 1.49 carats. The 1.5 to 2 m thick calcrete cap apparently prohibited mining of the kimberlite itself, although an 8 m deep pit through the calcrete reportedly recovered a 3.55 carat diamond from the kimberlite. In 2004, a Canadian company, Majescor Resources Inc., signed an option with De Beers to gain a 100% interest in the property. In 2005, another Canadian company, Vaaldiam Resources Ltd., acquired a 60% interest in the titles under a joint venture agreement and became the manager, undertaking various sampling and drilling programs to delineate the resource. In 2007, Vaaldiam acquired Majescor’s remaining 40% interest and in 2007 undertook a mini bulk sampling program of 9 of the kimberlites, including Braúna 03. This was followed with a diamond valuation and a Mineral Resource estimate. In 2009, Vaaldiam sold an 80% equity interest in the project to Lipari Mineração Ltda, a private Brazilian company. Production commenced at Braúna 03 in July 2016.
The Braúna kimberlite field is located in the centre of the Serrinha Block, within the northeastern section of the São Francisco Craton. The Serrinha Block forms a >21 000 km2 ellipsoidal
structure that has remained relatively rigid during and following the Palaeoproterozoic Transamazonian collisional/reworking event with the Gavião and Jequié blocks to the west and SW respectively. The Serrinha block is composed of: i). a Mesoarchaean basement of north-south foliated migmatitic gneisses and calc-alkaline to tonalite-trondhjemite-granodiorite (TTG) plutons, predominantly granodiorite; ii). the infolded, 150 x 50 km, NNW-SSE aligned, Palaeoproterozoic volcano-sedimentary sequences of the Rio Itapicuru Greenstone Belt and its more restricted neighbour to the east, the Rio Capim Greenstone Belt. Both are composed of ~2209 Ma tholeiitic basalt, andesite, an ~2170 Ma calc-alkaline felsic volcanic unit and a chemical and clastic to epiclastic sedimentary suite; and iii). Palaeoproterozoic granitic intrusions (Silva et al., 2001; Oliveira et al., 2004; Mello et al., 2006), emplaced in two major episodes. The older is dominated by trondhjemitic plutons aged between 2152 Ma and 2163 Ma, including the 2155 ±9 Ma (Mello et al., 2006) Nordestina Granodiorite, an ~40 x 20 km syntectonic intrusion cutting the Rio Itapicuru greenstone belt and hosting the Braúna kimberlite intrusions in its southern half. The Nordestina Batholith straddles the boundary between the Archaean basement and the Rio Itapicuru Greenstone Belt. It is a foliated and inequigranular zoned intrusion, with gneissic borders containing schlieren and pegmatites, and a porphyritic core. The younger intrusive suite mainly comprises 2130 to 2080 Ma calc-alkaline plutons, lamproites and syenites. The latter make up a distinct but minor assemblage in the western Serrinha Block, postdating the major volcanic-plutonic cycles and much of the early deformation (Oliveira et al., 2004; Rios et al., 2005, 2007; Mello et al., 2006).
The distribution of the kimberlites in the Braúna field is controlled by a generally NW-SE oriented fracture system of joints and faults with an overall 330° trend. They are distributed in a number of sub-parallel trends over a NW-SE length of ~12.5 km and width of ~2.5 km. The form of the kimberlite occurrences varies from elongated dykes, to larger 'blows' that are emplaced along elongated NW-SE fissures, to pipes. The dykes form segments that are up to 1 km long and 1 to 5 m wide, whilst at depth they frequently split and branch, and vary from 0.2 to 1.3 m in width. The larger dykes and blows may have surface areas of up to 2 ha, although substantial internal dilution of country rock material may be included in the interpreted bodies, and it is often unclear whether the included country rock is in situ or xenolithic. In addition, the dykes and blows do not have smooth regular walls, but may have numerous morphological irregularities such as embayments, outwardly dipping contacts, overhangs and irregular splitting or appendages.
Pipes generally have smoother, more regular contacts, and plan shapes at surface that vary from circular to elliptical, elongated parallel to the controlling fracture system. They often occur as a combination of semi-separated lobes and are composed of multiple irregular petrographic facies that have intruded the Nordestina Batholith. The Braúna 03, 04 and 07 pipes cover areas of 2, 0.5 and 2.5 ha respectively. Braúna 04 and 07 are variously described as 'blows', whilst the southern lobe of Braúna 03 appears to be the only well defined pipe that persists to depth.
The Braúna kimberlites are typically massive or brecciated and normally carry kimberlitic autoliths, as well as mantle and crustal xenoliths that can range up to 8 cm in size, but are typically <1 cm and comprise <1 vol.% of the kimberlite. The presence of autoliths is taken to suggest the kimberlite magma was emplaced in a number of pulses over a period of time. These kimberlites are considered to be the root zones of shallower kimberlite pipes that have been deeply eroded, and are classified as hypabyssal macrocrystic phlogopite kimberlites. They contain varying amounts of macrocrysts that include phlogopite, diopside, garnet, Cr-spinel and minor ilmenite, in a groundmass of abundant phlogopite ±apatite, ilmenite and perovskite. The groundmass varies from segregation-textured to uniform and is composed of calcite and serpentine, with minor chlorite (Pisani et al., 2001; Donatti Filho et al., 2008). The kimberlite composition is iron-rich with an Mg# of 85.6 to 89.1, and a geochemical character that is transitional between micaceous kimberlites and olivine lamproite (Donatti Filho et al., 2008).
Phlogopite from Braúna 03 returned an Rb-Sr age of 682 ±20 Ma and an Ar-Ar date in garnet from Braúna 07 of 736 ±14 Ma (Pisani et al., 2001). However, Teixeira et al. (2010) argue that, despite the Cryogenian cooling age of Pisani et al. (2004; 2001), the Braúna kimberlite intrusions are controlled by a NW-SE trending strike-slip fault system that they interpret to be of Cambrian age. To explain the apparent
temporal discrepancy, they suggest that the phlogopite flakes may preserve early Rb-Sr ages from when they crystallised at high temperature in the mantle, before being brought to a higher level in the crust.
Braúna 03 is the largest of the kimberlitic bodies in the field. It is composed of a North and South lobe, to the NW and SE respectively, with a separate interconnecting dyke-like central lobe. Both are similar in gross surface area in plan view, although the North lobe contains significantly more country rock, both as internal dilution and as in situ wall rock (Maunula, 2006). At surface, in plan, the North lobe is 'peanut-shaped', elongated NW-SE, with surface dimensions of ~200 x 60 m, with a limited vertical extent, passing down into a SE plunging dyke that connects it to the South lobe. The South Lobe is more 'almond-shaped' in plan, and ~150 x 100 m at surface, underlain by a gently tapering, steeply SE-plunging, pipe like body. The kimberlite is generally overlain by <5 m of overburden, which comprise soil, calcrete, saprolite and broken sap-rock after kimberlite and granite, although the saprolitic layer may extend for >15 m depths. Below the saprolite layer, unweathered kimberlite is weakly altered, with no primary olivine remaining in the kimberlite matrix which is predominantly composed of phlogopite. Never-the-less, the kimberlite matrix remains very competent, with bulk densities of ~2.65 to 2.7 g/cm3 versus <1.4 to 1.9 g/cm3 in the saprolite.
As detailed above, the Braúna kimberlites are described as macrocrystic, hypabyssal, phlogopite-bearing varieties that are described as 'breccias' when crustal xenoliths, which commonly include granitic country rock, exceeds 15 vol.% of the rock. The country rock xenoliths are usually altered to varying degrees whereas the smaller fragments may be entirely 'kimberlitised'. Angular kimberlitic autolithic fragments only comprise a minor percentage of the total, and are commonly coarser grained than their host. Flow textures are observed on the margins of the Braúna 03 kimberlite, whereby olivine macrocrysts are aligned and sorted. The individual kimberlitic lithological units that make up the pipe are often crosscut by carbonate veins, lesser serpentine, and a rare blue amphibole. Macrocrystic olivine and lesser megacrystic olivine constitute between 10 and 20% of the Braúna 03 and 07 kimberlites. However, whilst the macrocrystic olivine is typically completely altered, serpentinised olivine pseudomorphs clearly preserve the original macrocrystic texture. Alteration of olivine is a characteristic of hypabyssal kimberlites where gases are confined, and hence, react with the kimberlite. Segregation textures are typical within the kimberlites, particularly in the South lobe of Braúna 03 where they include segregations or coarse crystals of phlogopite and carbonate crystals, with lesser serpentinite.
Within the Braúna 03 pipe, the abundance and grain size of mantle xenoliths are apparently closely correlated with the abundance and grain size of the olivine macrocrysts and megacrysts present. These xenoliths are typically partially to completely altered and are often evidenced by the presence of altered garnet, the most common mantle xenocryst. The garnet xenocrysts include orange eclogitic garnet, various-coloured peridotitic (pink, red and burgundy) garnet, and lesser purplish-coloured harzburgitic garnets. Other mantle xenocrysts/macrocrysts include large phlogopite crystals that are up to several centimetres across, black spinel and magnetite/chromite. Ilmenite is absent, whilst rare apple-green chrome diopside only occurs as inclusions in olivine megacrysts, and discrete grains have not been observed.
The principal kimberlite units of the Braúna 03 pipe include:
• Macrocrystic, hypabyssal, phlogopite, kimberlite, which is the dominant lithotype. It has an inequigranular texture and is composed of 15 to 25% rounded olivine pseudomorphs and irregular phlogopite grains. Olivine is typically serpentinised, although some fresh olivine is evident locally. Variations on this lithotype are the Megacrystic hypabyssal phlogopite kimberlite which commonly contains more mantle material than the other kimberlite units, and Macrocrystic hypabyssal kimberlite breccia that includes >15 vol.% xenoliths, particularly granitic country rocks, which are commonly rounded and altered.
• Macrocrystic hypabyssal kimberlite, which also has an inequigranular texture and is composed of 15 to 25% rounded olivine pseudomorphs and irregular phlogopite grains. Olivine is typically serpentinised, although some fresh olivine occurs locally. This variety may also be a phlogopite kimberlite with a finer groundmass than the previous lithotype above.
• Hypabyssal phlogopite kimberlite, which is differentiated from 'Macrocrystic hypabyssal kimberlite' or 'Macrocrystic hypabyssal kimberlite' by its aphanitic texture, with <5% olivine macrocrysts.
• Segregation-textured hypabyssal kimberlite, which is heterogeneous with only 10 to 15% olivine, and a segregation-textured matrix. It contains sub-spherical and tabular carbonate laths typical of a late crystallising phase, and are macroscopically evident within the oxide-poor segregations.
Mineral Resources in 2010 (Leroux, Roy and Masun, 2010)
North Lobe of Braúna 03
Indicated Mineral Resource - 79 000 m3 = 0.205 Mt @ 4.72 cpht for 10 000 carats;
Inferred Mineral Resource - 553 000 m3 = 1.516 Mt @ 4.72 cpht for 72 000 carats;
South Lobe of Braúna 03
Indicated Mineral Resource - 102 000 m3 = 0.257 Mt @ 24.58 cpht for 63 000 carats;
Inferred Mineral Resource - 1 520 000 m3 = 4.180 Mt @ 24.58 cpht for 1 099 000 carats;
TOTAL contained diamonds - 1.244 million carats.
NOTE: cpht = carats per hundred tonnes.
Mineral Resources as at 31 December 2016 were (SRK Consulting 2014, less depletion, Lipari Mineração Ltda presentation 2017):
North Lobe + Central dyke of Braúna 03
Measured + Indicated + Inferred Mineral Resource - 2.187248 Mt @ 10.1 cpht for 220 330 carats;
South Lobe of Braúna 03
Measured + Indicated + Inferred Mineral Resource - 5.67884 Mt @ 57.0 cpht for 3 237 026 carats;
TOTAL Mineral Resources + Stockpiles - 7.866087 Mt @ 44.0 cpht for 3 457 356 carats.
NOTE: Resource above are to 300 m depth. Mine plan contains 2.5 million carats to 260 m depth of planned open pit.
Recovered grade in 2016 = 23.9 cpht; Predicted grade 2016 = 21.9 cpht.
Tonnes processed 2016 = 0.489 Mt of ore. Average sales value = USD 194 per carat.
Remaining Mineral Resources at Braúna, as at 31 December, 2021 (Royalty stream recipient Osisko Mining Inc. website, viewed June 2022)
Measured + Indicated Mineral Resource - 1.8 milliom carats of diamond
Inferred Mineral Resource - 1.7 million carats of diamonds.
The information in this summary is largely drawn from: "Leroux, D.C., Roy, W.D. and Masun, K., 2010 - Technical report and Mineral Resource estimate on the Braúna 3 kimberlite, Braúna Kimberlite Property, Bahia State, Brazil; an NI 43-101 technical report prepared by A.C.A. Howe International Limited., Toronto for Vaaldiam Mining Limited, 135p."
The most recent source geological information used to prepare this decription was dated: 2010.
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.
Brauna
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Selected References:
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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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