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Amapari |
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Amapa, Brazil |
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Au
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Super Porphyry Cu and Au
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IOCG Deposits - 70 papers
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All papers now Open Access.
Available as Full Text for direct download or on request. |
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The Amapari gold deposit is located in Amapa State, northern Brazil, 15 km from the town of Serra do Navio and 135 km north-west of the state capital, Macapa. (#Location: 0° 51'N, 51° 52'W.)
The mine is located in the Vila Nova Greenstone Belt, part of the Guiana Shield that covers the northern part of Brazil, parts of the Guianas and extends northwest into eastern Venezuela. The terrain is comparable in many respects to the Birimian Greenstone terrains of West Africa, which hosts gold deposits such as Sadiola and Morila in Mali, and Obuasi and Tarkwa in the Ashanti belt of Ghana.
Archaean basement gneiss are found to the west of the Amapari deposit, which lies within the ortho-amphibolite and meta-sedimentary rocks of the Paleoproterozoic (1.75 to 1.9 Ga) Vila Nova Group, composed essentially of gneisses, granites, amphibolites, banded iron formations (BIF), massive iron formations, schists and quartzites. The Vila Nova Group comprises, from the top:
i). Clasto-pelitic Sedimentary sequence, comprising:
Muscovite quartzites - Muscovite-quartz schist, muscovite quartzite, locally containing fuchsite and/or sillimanite;
Pelitic quartz-mica schists - Quartz-biotite-muscovite schist with garnets, interspaced with lenses of calc- silicates, iron formation and muscovite.
ii). Clasto-chemical Sedimentary sequence, composed of:
Quartz-grunerite cummingtonite schist with garnets, chlorite and biotite, quartz-amphibole schists and amphibole schists with lenses of silicate facies iron formation and calc-silicates.
iii). Chemical Sedimentary sequence (William Fm.), comprising:
Ferruginous Domain - Silicate facies iron formation with garnet-hornblende-grunerite-diopside and magnetite-grunerite-hornblende schists; Oxide facies iron formation - (Grunerite)-quartz-magnetite/hematite, sometimes with garnets and diopside; Oxide-silicate facies iron formation - (Hornblende)-diopside-grunerite-quartz with magnetite, (diopside)-hornblende-garnet, quartz-magnetite-grunerite schist.
Calc-magnesian Domain composed of an upper Schists with diopside porphyroblasts lithofacies of actinolite-tremolite-diopsides, hornblende-diopsides, amphibole-diopsides, with epidote, biotite and garnets; and a lower succession of Marble and carbonate schists - including calcic marble, serpentine marble with tremolite, forsterite, fayalite, hastingsite, chlorite and magnetite; actinolite-tremolite-carbonate schists.
iv). Volcanic Unit - which includes Metabasics and ortho-amphibolites composed of plagioclase amphibolites, biotite-amphibole schists, plagioclase-cummingtonite-hornblende schists.
These units are intruded by granitic pegmatites, dolerite dykes and gabbro bodies.
The metasediments are similar to those of the nearby Serra do Navio Formation which hosts major mined manganese deposits in the Serra do Navio district.
The metasediments that host the gold mineralisation at Amapari lie along the limb of a syncline, which on the south end of the syncline strike north-south and dip at 65 to 85°W. To the north the dip is vertical to steeply northeast, with NNW strike.
Gold mineralisation is associated with iron- and carbonate-rich units of the chemical sedimentary sequence of the William Formation, predominantly the banded iron formation (BIF) facies, although carbonate and calc-silicate rocks also host economic mineralisation and sub-economic mineralisation may be hosted by any of the other lithologies within the sequence, with the exception of the late intrusions.
Laterite and lateritic colluvium are common within the district, tending to be thicker over topographic highs, with thick laterites developed in the proximity of BIF. Colluvium, which hosts secondary mineralisation is characterised by deeply weathered rock with no remnant original structure, commonly containing angular fragments.
Mineralisation occurs as: i). primary sulphide rich ore below the base of oxidation, ii). in situ weathered ore in the saprolite layer and iii). a blanket within the overlying colluvium, spreading out downslope from the hills.
The mineralisation at Amapari occurs as a series of deposits distributed over a 7 km strike length of a north-south trending shear zone, corresponding to a north-south line of topographic ridges. From south to north, these deposits comprise the Tapereba A, B, C, D and Urucum orebodies, each reflected by more intense developments of hydrothermal alteration. The Tapereba and Urucum clusters are separated by around 4 km.
The primary mineralisation is controlled by the shear zone which has a texture and mineralogy indicating intense high-temperature hydrothermal alteration, particularly silicification and sulphidation with associated auriferous pyrrhotite and pyrite. The alteration is most intense in the proximity of reactive meta-sediments which include BIF, followed by amphibolite, carbonate schist and to a lesser extent in calc-silicate rocks. Superimposed foliation, brecciation and silicification indicate remobilisation of the auriferous mineralisation. Shoots of sulphide ore follow the shear plane foliation, commonly crosscutting the BIF and other host meta-sediments, which are poor in sulphide and gold outside the shears and faulted zones.
Pyrrhotite and pyrite are the most abundant sulphides, with trace (<1%) chalcopyrite, arsenopyrite, sphalerite and galena. At Urucum the mineralisation is accompanied by intense silicification with pyrrhotite the dominant sulphide. At Tapereba, the gold is associated with masses containing 5 to 10% pyrite, with only trace pyrrhotite. The individual sulphide masses are several metres thick, and are elongated NNW to north-south, extending to depths of more than 400 m below the surface, down a 10 to 15° plunge with an azimuth of 350°. The dip of the shear zone ranges from almost vertical at Urucum, to 30° in the southern Tapereba orebodies. The gold is free and not tied to the crystal lattice of the sulphides, occuring primarily with pyrrhotite within the Urucum zone and with pyrite in the Tapereba bodies.
Intense tropical weathering extends to depths of 100 to 130 m, resulting in the development of saprolite from the in situ weathering and oxidation of the primary sulphides and host rocks. It is primarily composed of clay, silica, and the oxides and hydroxides of iron. Ore zones within saprolite follow the strike, dip and plunge of the primary sulphide mineralisation, with semi-decomposed primary sulphide remnants becoming more frequent with depth.
The colluvial deposits occur along, and down the slopes of the north-south trending ridges that straddle William Creek. The creek is at
an elevation of around 115 m, while the ridges reach 300 m. The crests and slopes of these ridges are covered by alluvium and colluvium. The grade of mineralisation within the colluvium tends to reflect the grade of underlying primary sulphide and saprolite ore shoots, with zones of low grade or barren colluvium generally reflecting low grade or barren underlying saprolite. Zones of mineralisation within the colluvium tend to be broader than in the underlying saprolite due to mechanical and chemical transport and the development of some secondary mineralisation.
Deep weathering and intense fixing of iron in the upper portions of the soil often creates a laterite horizon. The top of the colluvium profile is generally a rarely more than 1 m thick layer composed of silty, clayey and sandy materials, poor in limonite fragments. This immediately overlies a variable layer up to 10 m thick containing lateritic fragments rich in iron oxide dispersed in a ferruginous clay-
sand matrix which becomes rich in manganese at the base.
Resource figures at September, 2007 (Peak Gold, 2007) were:
Measured + indicated resource - 14.24 Mt @ 2.29 g/t Au for 32.5 t Au,
Inferred resource - 13.04 Mt @ 3.22 g/t Au for 42 t Au.
The most recent source geological information used to prepare this decription was dated: 2007.
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.
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