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Chapada

Goias, Brazil

Main commodities: Cu Au
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The Chapada Cu, Au deposit and mine, is located in the state of Goiás in Brazil, approximately 320 km north of Goiánia and 270 km NW of Brasilia (#Location: 14° 14' 25"S, 49° 22' 50"W).

The Chapada deposit is located within the Mara Rosa volcano-sedimentary sequence which structurally overlies the western margin of the 600 x 150 km, NE-SW elongated, Goiás Massif, one of a series of microcontinents of Archaean (3.0 to 2.7 Ga) and Palaeoproterozoic (2.175 Ga) rocks exposed within the core of the Neoproterozoic Tocantins-Araguaia belt that separates the Amazon and São Francisco cratons in central Brazil. The Goiás Massif appears to abut the passive western margin of the São Francisco craton to the SE, where both are overlain by the thick Neoproterozoic sedimentary shelf sequence of the Brasilia Belt. To the NW, it is separated from the Amazon craton by the strongly metamorphosed Mara Rosa volcano-sedimentary sequence, which have been thrust to the SE over the Massif (Kuyumjian,1995; Palermo et al., 2000; Pimentel et al., 1997).

The Mara Rosa sequence consists of several NNE-trending belts of NW-dipping, 862±8 Ma meta-volcanic (meta-basalts to meta-rhyolites) and meta-sedimentary rocks (mica-schists, quartzites and cherts), separated by 856+13/-7 Ma tonalitic to granodioritic orthogneisses, intruded by 630±3 Ma tonalites, granodiorites, gabbros, pyroxenites and hornblendites. Both metasedimentary and intrusive rocks have been metamorphosed to amphibolite facies, and exhibit a conspicuous regional Brasiliano (late Neoproterozoic) event foliation. Younger, post-metamorphic pegmatites, quartz veins and dolerite dykes crosscut the sequence (Kuyumjian,1995; Palermo et al., 2000; Pimentel et al., 1997).

The Mara Rosa sequence has been divided into Eastern, Central and Western stratigraphic belts by Arantes et al. (1991). Throughout the Chapada region, the Eastern Belt comprises meta-greywackes, sillimanite-staurolite-kyanite bearing schists, quartz-feldspathic biotite schists and gneisses (metamorphosed acid volcaniclastic rocks), garnet-biotite schists, meta-cherts, banded iron formations, calc-silicate rocks and exhalites, with intercalations of calc-alkaline quartz amphibolites and quartz-garnet amphibolites. The Central Belt is dominantly composed by tholeiitic diopside amphibolites, with strained pillows, epidote amphibolites and garnet amphibolites, with minor intercalations of banded iron formation and metachert, while the Western Belt is predominantly staurolite-kyanite-garnet bearing schists, feldspar-biotite gneisses and calc-silicate rocks (Kuyumjian,1995; Palermo et al., 2000; Pimentel et al., 1997).

The ore zone is situated within the Eastern Belt of the Mara Rosa sequence. It is elongated ENE-WSW, parallel to regional strike, and comprises a folded, but overall flat lying, 2 km long, by ~500 m wide and 80 to 150 m thick body of low-grade, disseminated pyrite-chalcopyrite-magnetite mineralisation with associated metamorphosed hydrothermal alteration (Kuyumjian,1995).

The ore occurs as disseminated, foliated chalcopyrite which is associated with pyrite (2 to 5% sulphides), and is frequently intergrown with pyrite, magnetite and hematite. It is mainly found within biotite schist, and to a lesser degree, muscovite schist and amphibolite, and rarely quartzite. Pyrite is the dominant sulphide mineral, and although chalcopyrite is the only important ore mineral, bornite, chalcocite, sphalerite, galena, pyrrhotite and molybdenite are also present. Gold is very fine and occurs in chalcopyrite and rarely in between sulphide grains (Kuyumjian,1995; Richardson et al., 1986).

The deposit is concentrically zoned with a magnetite 'rich' core (generally >0.5% magnetite), a chalcopyrite-pyrite annulus (py:cp <2), and a pyritic outer zone (py:cp >2).

The sulphides frequently mimic the same metamorphic fabric as the silicate minerals following microfolds and crenulations of the schistosity with mica, as well as being boudinaged. Sulphides also occur also as inclusions in metamorphic minerals. All of these characteristics suggest the mineralisation was predated metamorphism (Kuyumjian,1995).

The deposit occurs in the core of an anticline, between a persistent syncline to the NW and a 'basin-shaped' complexly folded and thrusted synform to the SE, part of which is occupied by the 1.5 km diameter interpreted metamorphosed altered diorite (biotite-plagioclase gneiss). The alteration assemblages follow the strike extensions of the mineralised zone, over widths of ~1 km, and for >5 km beyond the ore zone to the WSW, following the same aniclinal crest. To the NNE and then SE, it persists around the rim of the basin-shaped synform to form an annulus with internal dimensions of ~3 x 2 km, joining up with the WSW extension of the alteration zone described above (to form an 'eye' of a WSW-pointing 'needle'). The deposit is ~2.5 km NW of the NW dipping, Rio dos Bois regional thrust which forms the structural margin with the Archaean to Palaeoproterozoic basement of the Goiás Massif to the SE (Yamana Gold, 2011).

Metamorphosed alteration takes the form of: i). a potassic halo accompanying the main ore zone, represented by an assemblage of biotite-plagioclase gneiss and biotite schist, ii). pyritic quartz-muscovite (after sericite) schist reflecting an overprinting phyllic zone, iii). a more distal propylitic assemblages and iv). quartz-muscovite-kyanite schist representing an advanced argillic lithocap. v). On the SE margin of the basin-shaped syncline (opposite the Chapada deposit), the alteration zone includes a large lens-like zone of quartzites over a strike length of ~4 km, interpreted to represent vuggy silica, interfingered with argillic and phyllic assemblages, with associated anomalous gold values. vi). Localised zones of intense hydrothermal alteration, occuring as epidosite (epidote+quartz+sphene), is found within amphibolites (after basaltic rocks) (Yamana Gold, 2011).

The deposit has been variously interpreted to be a metamorphosed porphyry copper or sediment (volcanic) hosted deposit (Richardson et al., 1986).

Kuyumjian, (1989; 1995) suggests the Chapada disseminated Cu-Au mineralisation was deposited within metamorphosed, hydrothermally altered, calc-alkaline extrusive basaltic and dacitic rocks, in intimate spatial association with intrusions of tonalitic and dioritic composition, also of calc-alkaline affinity. He proposes that the deposit originally resulted from the interaction between deep hydrothermal solutions and fluids released by the tonalite and diorite intrusions, followed by extensive metamorphic redistribution in shear zones related to the top-to-the-SE thrusting of the Neoproterozoic Mara Rosa sequence over the Archaean Pilar de Goias greenstone belt during the late Neoproterozoic Brasiliano event. Gold enrichment, which is also locally found within late quartz veins crosscutting the Mara Rosa sequence, implies metamorphically derived fluids percolated the supracrustal rocks at the end of that event.

The resource has previously been quoted (Richardson et al., 1986) at between 100 and 200 Mt @ 0.44% Cu, 0.35 g/t Au. Commercial production commenced in early 2007.

Reserve and resource figures at December 31, 2007 (Yamana Gold, 2008) include:
  Proven + probable reserves - 310.82 Mt @ 0.34% Cu, 0.24 g/t Au, for 73 t Au,
  Measured + Indicated resources (in addition to reserves) - 119.82 Mt @ 0.22% Cu, 0.13 g/t Au,
  Inferred resources - 304.86 Mt @ 0.19% Cu, 0.11 g/t Au.

Reserve and resource figures at December 31, 2010 (Yamana Gold, website) include:
  Proven + probable reserves - 429.17 Mt @ 0.23 g/t Au, for 73 t Au, including 368.72 Mt @ 0.26% Cu,
  Measured + Indicated resources - 434.94 Mt @ 0.18 g/t Au, for 78.5 t Au, incl. 344.04 Mt @ 0.19% Cu,
  Inferred resources - 105.32 Mt @ 0.12 g/t Au, for 78.5 t Au, including 96.15 Mt @ 0.19% Cu.

The most recent source geological information used to prepare this summary was dated: 2011.     Record last updated: 6/9/2012
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
 References to this deposit in the PGC Literature Collection:
Kuyumjian, R N,  1995 - Diversity of fluids in the origin of the Chapada Cu-Au deposit, Golás: in    Revista Brasileira de Geociencias   v.25 pp. 203-205
Richardson S V, Kesler S E, Essene E J  1986 - Origin and geochemistry of the Chapada Cu-Au deposit, Goias, Brazil: a metamorphosed wall-rock Porphyry Copper deposit: in    Econ. Geol.   v81 pp 1884-1898


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