Bahia, Brazil

Main commodities: Au U
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The Jacobina conglomerate hosted Au, U deposit is located some 330 km north-west of Salvador, in Bahia State, central Brazil.

The deposit has yielded >30 tonnes of Au from 4 mines at 8 to 10 g/t Au, and is hosted by the Lower Proterozoic Jacobina Group (deposited between 2200 and 2500 Ma), which is now preserved as a fault block surrounded by Archaean crystalline basement.

It occurs within the north-south oriented, Paleoproterozoic (2080 to 1900 Ma) Jacobina foreland basin which is composed of stacked detrital and schist formations developed along a sinistral wrench-overthrust fault system of the Transamazonian orogeny. At least five successive stages are identified in the westward progradation of the basin, wherein bedding-parallel strike-slip thrusts and prograde metamorphism indicate progressive pulses of the Transamazonian orogeny.

Basic to ultrabasic dykes and granite stocks cross-cut the lithologies of the basin. Conglomerates and chaotic breccia layers occur within several formations and mark intervals of tectonic instability that were accompanied by erosion of the earlier stages of basin development. The economic gold-bearing clastic sediments are conglomerates and minor quartzites which were deposited during the fourth stage of basin development.

The mineralised sequence comprises a lower unit the Serro do Corrego Formation, an 1800 to 2000 m thickness of interbedded quartzites and lesser auriferous conglomerates; overlain by the Rio do Ouro Formation, a 2000 to 2300 m thick unit of well bedded white to pale green fine to medium grained quartzites and basal conglomerates; and the upper Cruz das Almas Formation which is >2100 m thick and is made up of pelitic schists with conspicuous porphyroblasts of andalusite and garnet, well bedded micaceous quartzites and minor conglomerate.

Four zones of conglomerate development are present in the Jacobina Group, namely a basal conglomerate at the base of the Serro do Corrego Formation, another 150 m thick zone in the same formation,100 m above the base; a middle 320 m thick zone also in the upper parts of the same formation, 300 to 500 m above the basement; and the top 20 m thick zone at the bottom of the Rio do Ouro Formation, 900 m above the basement.

The best mineralisation is found within the Main Reef in the second conglomerate zone 150 m above the base, and in two intervals, the Joao Belo/Morro do Vento/Cuz-Cuz reefs and the Canavieras Reef, both of which are in the lower part of the middle zone. Virtually all of the conglomerates in the four zones carry very fine Au throughout, averaging around 0.5 g/t from 1000 samples. The uranium content is proportional to the Au grade.

The basal conglomerate contains the Basal Reef which is around 4 m thick, has a strike extent of 300 m and contains up to 4.5 g/t Au.

The Main Reef has a good continuity over a strike length of more than 3 km. It occurs within a 150 m thick package that comprises a massive, pyritic but barren, well rounded, well packed quartz-pebble conglomerate, a succeeding quartzite containing mineralised conglomerates of the Main Reef and another barren coarse pebble conglomerate at the top. The Main Reef itself has a thickness of 2 to 3 m and an average grade of 5.7 g/t Au.

The middle zone in the upper parts of the Serro do Corrego Formation contains hundreds of conglomerate layers. Two important quartzite split the zone into three distinct conglomeratic units. The lower and middle units contain economically mineralised, tongue like lenses of conglomerates, while the upper is barren. The Joao Belo Reef in the lower conglomerate unit has a combined thickness of 6 to 10 m and a strike length of 700 m containing 2.8 g/t Au. The other deposits in this same conglomerate unit, the Morro do Vento and Cuz-Cuz reefs are 1 to 2 m thick and grade from 4 to 6 g/t Au, with local enrichments in the upper 0.5 m. The Canavieras Reef in the middle conglomerate unit is a 0.5 to 2 m thick pyritic conglomerate with quartzite interbeds and grades of around 8 g/t Au.

Reserves and resources at 31 December, 2010 (Yamana Gold website) were:
    Proven + probable reserve - 20.96 Mt @ 2.48 g/t Au for 52.0 t Au; plus
    Measured + indicated resource - 17.14 Mt @ 3.02 g/t Au for 51.7 t Au; plus
    Inferred resource - 13.56 Mt @ 2.97 g/t Au for 40.2 t Au

The most recent source geological information used to prepare this summary was dated: 2001.    
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:
Bateman J D,  1958 - Uranium-bearing auriferous reefs at Jacobina, Brazil : in    Econ. Geol.   v.53 pp. 417-425
Gross W H,  1968 - Evidence for a modified placer origin for auriferous conglomerates, Canavieiras Mine, Jacobina, Brazil : in    Econ. Geol.   v.63 pp. 271-276
Milesi J P, Ledru P, Marcoux E, Mougeot R, Johan V, Lerouge C, Sabate P, Bailly L, Respaut J P and Skipwith P  2002 - The Jacobina Paleoproterozoic gold-bearing conglomerates, Bahia, Brazil: A hydrothermal shear-reservoir model : in    Ore Geology Reviews   v19 pp 95-136
Teixeira J B G, Souza J A B, Silva M G, Leite C M M, Barbosa J S F, Coelho C E S, Abram M B, Conceicao Filho V M and Iyer S S  2001 - Gold mineralization in the Serra de Jacobina region, Bahia Brazil: tectonic framework and metallogenesis: in    Mineralium Deposita   v36 pp 332-344

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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