Crixas-Serra Grande

Goias, Brazil

Main commodities: Au
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The Crixas mining district is located in the state of Goias in Brazil, some 250 km NNW and NW of the cities of Goiana and Brasilia.

It a historic producer of gold, having yielded over 175 t of Au from 4 major mines in the belt over the last more than 100 years. The current mine has been in production since 1989.

Mineralisation is hosted by the ~2.7 Ga Archaean Crixas greenstone belt on the western margin of the São Francisco Craton in the Cental Brazil Shield. The greenstones are surrounded by Archaean tonalitic gneisses and granodiorite.

The ore deposit is hosted by an overturned sequence of foliated mafic meta-volcanics and meta-sediments that dip north at 25 to 40°. The Crixas Greenstone Belt has 3 units, namely:

i). a western Basal Unit which is 600 m thick and is composed predominantly of serpentinites and magnesian schists with spinifex textures, and lesser ferruginous chert, quartz-chlorite schist and tremolite-albite-quartz-carbonate schist;
ii). the 800 m thick Middle Unit, which is dominantly a tremolite-actinolite-albite-chlorite-carbonate schist with minor pillow basalts, and more significant graphitic schist, quartz-chlorite schist and talc schist; and
iii). the Upper Unit is areally restricted but is predominantly graphitic biotite-chlorite schist and meta-greywacke some 1600 m thick, with bodies of dolomite, massive quartz and magnetite-garnet-chlorite-chloritoid schist at its base, commonly containing appreciable gold., including the Crixas gold deposit.

At Crixas, two main broadly concordant auriferous zones have been defined, aligned in a WNW direction, coincidental with a prominent lineation. The ore shoots plunge to the NW at 6 and 35° respectively, and have long axes that parallel isoclinal axes observed in the ore zone. The Upper Zone comprises ferroan dolomite schist and sericite-chlorite schist in which high gold concentrations are associated with massive arsenopyrite, pyrrhotite and chalcopyrite. The Lower Zone is approximately 120 m below the Upper and comprises a quartz cemented, graphitic pelite breccia in which gold is associated with arsenopyrite, pyrrhotite and chalcopyrite within and on the margins of graphitic pelite fragments. The breccia has 20 to 40% quartz cement. Gold is also found in association with sericite schists containing minor disseminated arsenopyrite and chalcopyrite.

Pre-mining drilling of the currently mined deposits defined 5.2 Mt @ 12.7 g/t Au. The production rate in 1999 was 620 000 tonnes of ore per annum at a grade of 8.2 g/t Au, with a recovery rate of 95% and a operating cost per ounce of $US 184.

The reserve at the end of 1999 was 0.56 Mt @ 9.5 g/t in the Mina III, and 0.50 Mt @ 4.7 g/t in Mina Nova.

In 2006, published proven + probable reserves (Kinross Gold Corp.) were: 2.652 Mt @ 5.08 g/t Au for 13.5 tonnes of contained gold.   Measured + indicated resources were 0.114 Mt @ 3.55 g/t Au for 0.4 tonnes of gold. Production in 2006 amounted to 0.79 Mt of ore @ 3.98 g/t Au, for 3.017 t of Au produced with a 95.3% recovery.

The mine is operated by Mineracao Serra Grande, which in 2006 was a 50:50 joint venture between Anglogold and Kinross Gold Corporation.

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:
Anonymous  2001 - Serra Grande: in   extract from AngloGold web site http://www.anglogold.com/informationforinvestors/annualreport99/20f/i1-4susa.htm    2p
Bettencourt J S, Costa R R, de Souza L H, de Matos T T  1991 - Controls of gold mineralization in the Corrego Geral sector, north central section of the Crixas greenstone belt: A preliminary report: in Ladeira E A (Ed.),  Brazil Gold 91 Balkema, Rotterdam    pp 699-703.
Jost H, Fortes P T F O  2001 - Gold deposits and occurrences of the Crixas Goldfield, Central Brazil: in    Mineralium Deposita   v36 pp 358-376
Oliveira Fortes P T, Pimentel M M, Santos R V, Junges S L  2003 - Sm-Nd studies at Mina III gold deposit, Crixas greenstone belt, central Brazil: implications for the depositional age of the upper metasedimentary rocks and associated Au mineralization: in    J. of South American Earth Sciences   v16 pp 503-512
Thomson M l   1986 - Petrology of the Crixas gold deposit, Brazil: Evidence for gold associated with hydrothermal alteration, subsequent to metamorphism: in   Proceedings of GOLD 86 Symposium, Toronto, 1986 Balkema, Amsterdam    pp 284-296
Thomson M l, Fyfe W S  1990 - The Crixas gold deposit, Brazil: thrust-related, postpeak metamorphic gold mineralization of possible Brasiliano Cycle age: in    Econ. Geol.   v85 pp 928-942

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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