Bahia, Brazil

Main commodities: Pb Zn Ag
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The Boquira metamorphic hosted Pb, Zn deposits is located in central Bahia state, Brazil.

It originally comprised around 8 Mt @ 9% Pb, 2% Zn, 28 g/t Ag, (Beeson 1990).

Production to 1985 had been 5.6 Mt @ 8.85% Pb, 1.43% Zn (Espourteille & 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.

Mineralisation is hosted by a 500 to 1500 m wide unit comprising quartzite with intercalated lensoid mica schist, iron formation, 'dacitic rock', quartz-biotite-garnet schist and dolomite. It is bounded to the east by Archaean crystalline basement and to the west by a >11 000 m thick succession of quartzites with lesser mica schists (Johnson 1962), representing the Mesoproterozoic Espinhaco Supergroup (Espourteille & Fleischer, 1984). A major east dipping reverse fault separates the mineralised unit from the thick Espinhaco Supergroup quartzites to the west, while other thrusts are mapped in the orebody area (Johnson 1962). The host unit is a member of the lower volcano-sedimentary sub-division of the Lower Proterozoic within the Espinhaco Mobile Belt.

A map in Espourteille & Fleischer (1984) shows the three major lithological units to be amphibolite, quartzite and chlorite-biotite-garnet schist grading to migmatites. More than one thick band of each is represented in the mine area. Iron formation and carbonates, are less abundant. The iron formations grade laterally into ferruginous quartzite, while the marble units incorporates calcareous amphibolites. In the text of their paper Espourteille & Fleischer (1984), list the hosts to the ore lenses are chlorite-garnet-biotite-schists, quartzites, limestones, marbles, and iron formation within tremolite bearing amphibolite.

The mineralisation is associated with lensoid magnetite-tremolite banded amphibolite, quartz-mica schist and iron formation, and is overlain by calc-silicates and further BIF's (Johnson 1962). Espourteille & Fleischer (1984) show the immediate ore lenses within a unit of banded magnetite-amphibolite with fine intercalations of schists and local iron formation.

Galena, sphalerite and pyrite are the main ore minerals, with accessory chalcopyrite, pyrrhotite and molybdenite (Damasceno, 1982).

There are apparently four separate zones of orebodies along the trend on the mineralised zone which extends for around 5 km. Each zone includes up to 3 parallel lenses. The four zones are the Sobrado, Pelado, Cruzeiro and Maranhao. According to Espourteille & Fleischer (1984), the ore lenses are concordant with the banding in the enclosing amphibolite. Johnson (1962) describes the lodes as coarse grained galena rich orebodies with lesser sphalerite and rare pyrite, which are found within fault zones that are generally parallel to the main foliation and the larger reverse faults. Espourteille & Fleischer (1984) state that galena comprises around 90% of the sulphides.

The Cruzeiro zone is 1400 m long and has a down dip extent of 250 m, while the lens at Sobrado are developed over a 1250 m in length. Individual lenses in each zone have dimensions as follows:

* Sobrado: Principal lens - 650x460 m; South lens - 320x180 m; North lens - 100x50 to 70 m.
* Pelado: Principal lens - 1000x340 m; East lens - 200x110 m
* Cruzeiro: West lens - discontinuous lenses mined over a length of 1400 m and depth of 250 m.
* Maranho: 110x340 m

Individual lenses vary from a few centimetres to 2 m, and occasionally to 5 m in thickness. Oxidation is observed to depths of up to 20 m, with the development of cerussite, smithsonite, limonite and anglesite, and subordinate pyromorphite, hemimorphite, chrysocolla, bornite, covellite, malachite and azurite (Espourteille & Fleischer, 1984).

Age dating of galena from the deposit has yielded results of between 2.66 and 2.55 Ma (Misi, etal., 1995).

For detail consult the reference(s) listed below.

The most recent source geological information used to prepare this summary was dated: 1999.    
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.

  References & Additional Information
   Selected References:
Fleischer R, 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
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

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 takes no responsibility what-so-ever for inaccurate or out of date data, information or interpretations.

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