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Crixas - Serra Grande |
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Goias, Brazil |
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Au
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Super Porphyry Cu and Au
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The Serra Grande mine in the Crixás Mining District is located in the state of Goias in Brazil, 5 km south of the town of Crixás, and some 250 km NNW and NW of the cities of Goiana and Brasilia respectively.
(#Location: 14° 34' 28"S, 49° 58' 6"W).
The Crixás Mining District is 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. Modern exploration was commenced in 1973 by INCO Gold with a phase of detailed mapping and diamond drilling, which continued until 1976. This led to the discovery of significant gold intersections below a group of historic excavations known as the Mina III Old Workings, leading to the focussing of exploration in that area. INCO sought a partner to help fund further exploration and development and, in April 1983, entered into a 50:50 joint venture with Kennecott Corporation to form the holding company Mineracao Serra Grande. However, in 1986, Kennecott Corporation sold its interest to Minorco, an Anglo American controlled company, whose interest passed eventually to AngloGold Ashanti. Underground development and exploration continued and a bankable feasibility study was completed in 1987. Mining operations began in 1986 at Mina III with the metallurgical plant start-up in 1989. In 1991, INCO Gold merged with TVX Mining Corp. to become TVX Gold. In 1999 Normandy Mining Limited acquired a 50% share of the TVX holding in the project. In late 2001, following a takeover of Normandy Mining, Newmont inherited that interest. By 2003, the TVX-Newmont share had been acquired by Kinross Gold Corporation. Serra Grande production peaked at 6 tonnes of gold per annum in 2006, supported by high grades. In 2009, the metallurgical plant was expanded to 1.3 Mtpa to compensate for a declining grade-profile. In 2012 Anglogold acquired the Kinross share to take full control. Exploration continued over the period from 1990 to the present, with a series of orebodies distributed over a NNW-SSE elongated area of ~5 x 2 km, with Palmeiras in the south to Mina III in the centre, Pequizão a further 800 m to the north, and Santos Reis/Corpo Baru on the northern extremity. A decline had been driven to access the Mina Nova orebody, located ~2 km north of Mina III, to allow definition drilling from 1995, with production commencing in 1996. During 1995, an exploration program discovered the Corpo Sul orebody south of Mina III, and a drive reached this orebody in January, 1996 (AngloGold, 2020; Notice of TVX Gold Inc. Shareholders Meeting, 2002).
In 2020, the Serra Grande operation comprised three underground mines, namely Mina III (including orebody IV, V and Ingá), Mina Nova (including the Pequizão orebody) and Mina Palmeiras. The two open pits mine the outcrop of Mina III Inferior and Structure IV zones, and the Pequizã orebody.
Mineralisation is hosted by the ~2.7 Ga Archaean Crixas Greenstone Belt on the western margin of the São Francisco Craton in the Tocantins Province of the Atlantic Shield. The Crixás Greenstone Belt is surrounded by Archaean tonalitic and granodiorite gneiss terrains of the 2820 ±6 to 2792 ±7 Ma Ribeirão das Antas and 2844 ±7 Ma Caiamar complexes to the west and east respectively, and is in faulted contact with metasedimentary rocks from the Santa Terezinha Group to the NE, which is part of the Neoproterozoic Goiás magmatic arc.
For detail of the regional geological and tectonic setting and context, go to the Atlantic Shield Tectonics, Geology, Metallogeny record, see the Figs. 1 and 2, and select the Goias Massif link at the top of the page.
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 Crixás gold deposits.
Two main deformational events have been identified at Crixás and surrounding district. The first, D1, is a west vergent thrust event, developed with irregular thrust ramp geometry. It was responsible for stacking and inverting the stratigraphic sequence. The second event, D2, involved the thrusting of the Santa Terezinha sequence over the Crixás Greenstone Belt, folding the rocks and generating the structural controls for gold mineralisation, generally parallel to the fold axis.
Mineralisation is developed within shear zones cutting the Neoarchaean Crixás Group, and is associated with metasediments and
metavolcanics from the Ribeirão das Antas and Rio Vermelho formations respectively.
At Crixás, two main broadly concordant auriferous zones had been defined in 2001 (Jost and Fortes, 2001), 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.
As described in 2020 (Anglogold Ashanti, 2020), the geometry of the mineralised zones is typically complex with pinch and swell, and folded and boudinage shapes that dip at between 10 and 25°, mainly to the SW, with the greatest continuity along NW plunging structures with an azimuth of 290°. The deposit has been split into four main domains referred to as 'structures', namely Structure II, III, IV and Palmeiras representing deformed planar (near stratabound) structures that are stacked in the order listed, from bottom to top. Mineralisation within these structures occurs as a series of elongated lenses, with 'ore grades' generally concentrated where the same high deformation locations (i.e., folds or disruptions) affects one or more of these structures. Within Structure III, mineralisation occurs in quartz veins hosted in carbonaceous schists, with grades of up to 8 g/t Au as at Mina III (inferior or lower zone) and Ingá. Structure III also has associated massive and disseminated sulphides (mainly pyrrhotite and arsenopyrite) that are found in a sequence of hydrothermally-altered schists, commonly referred to as superior (upper) zones. Other mineralisation includes arsenopyrite associated with quartz as veinlets in carbonaceous metapelite. In Structure IV, the mineralisation comprises quartz veinlets and disseminated sulphide (pyrrhotite) hosted in graphite schists, as at Pequizã, and in sericite and chlorite schists where massive and disseminated sulphides are concentrated in folded zones. These ore shoots plunge to the NW with dips varying between 6 and 35°. The Palmeiras structure is associated with hydrothermal alteration of metabasalts, with sericite, chlorite, carbonate and massive sulphides (pyrrhotite).
Pre-mining drilling to 1986 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 an operating cost per ounce of $US 184.
The reserve at the end of 1999 was 0.56 Mt @ 9.5 g/t Au in the Mina III, and 0.50 Mt @ 4.7 g/t Au in Mina Nova.
In 2006, published Ore Reserves and Mineral Resources(Kinross Gold Corp.) were:
Proved + Probable reserves - 2.652 Mt @ 5.08 g/t Au for 13.5 tonnes of contained gold.
Measured + Indicated Resources - 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.
As at 31 December, 2020, remaining SAMREC compliant Ore reserves and Mineral Resources comprised (Anglogold Ashanti Reserves and Resources Report, 2020):
Measured + Indicated + Inferred Mineral Resources (inclusive of Ore Reserves) in 9 mine areas totalled:
38.35 Mt @ 3 g/t Au for 114.88 t of contained gold.
Proved + Probable Ore Reserves in 7 mine areas totalled:
6.99 Mt @ 2.79 g/t Au for 19.49 t of contained gold.
The most recent source geological information used to prepare this decription was dated: 2020.
Record last updated: 2/1/2022
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.
Serra Grande
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Selected References:
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AngloGold Ashanti Ltd. 2020 - Serra Grande: in Mineral Resource and Ore Reserve Report as at 31 December, 2020, (includes geology, mineralisation and reserves-resources), pp. 129-139.
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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.
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Bogossian, J., Kemp, A.I.S. and Hagemann, S.G., 2021 - Linking Gold Systems to the Crust-Mantle Evolution of Archean Crust in Central Brazil: in Minerals (MDPI) v.11, 35p. doi.org/10.3390/min11090944.
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Campos, L.M., Toledo, C.L.B., Silva, A.M., Ducart, D.F, dos Santos, B.A., Campos, M.P. and Borges, C.C.A., 2022 - The hydrothermal footprint of the Crixas deposit: New vectors for orogenic gold exploration in central Brazil: in Ore Geology Reviews v.146, 23p. doi.org/10.1016/j.oregeorev.2022.104925.
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Ferreira, B.F., Marques, J.C., Frantza, J.C., Souza, R.G., Campos, M.P., Figueiredo, F.L.A.R. and Padilha, N.L., 2021 - Stratigraphy and U-Pb detrital zircon provenance, Crixas Greenstone Belt, Goias-Brasil: Gold-bearing vs barren siliciclastic rocks: in J. of South American Earth Sciences v.105, 20p. doi.org/10.1016/j.jsames.2020.102994.
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Jost, H. and Fortes, P.T.F.O., 2001 - Gold deposits and occurrences of the Crixas Goldfield, Central Brazil: in Mineralium Deposita v.36, pp. 358-376.
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Jost, H., Apollo, J.F.H., Weber, W., Salles, R.D.R, Marques, J.C., Massucatto, A.J., Costa, D.A. and Santos, B.A.D., 2019 - Stratigraphic update, paleotectonic, paleogeographic, and depositional environments of the Crixas Greenstone Belt, Central Brazil: in J. of South American Earth Sciences v.96, 20p. doi.org/10.1016/j.jsames.2019.102329.
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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
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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
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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
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Ulrich, S., Hageman, S., Marques, J.C., Figueiredo, F.L.A.R., Ramires, J.E.F., Frantz, J.C. and Petersen, K., 2021 - The Orogenic Crixas Gold Deposit, Goias, Brazil: A Review and New Constraints on the Structural Control of Ore Bodies: in Minerals (MDPI) v.11, 27p. doi.org/10.3390/min11101050.
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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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