Автор(ы):Karin Requia, Lluis Fontbote
Издание:PGS Publishing, Linden Park, 2002 г., 12 стр.
Язык(и)Английский
The Salobo iron oxide copper-gold deposit, Carajas, northern Brazil

The Salobo iron oxide copper-gold deposit is located in the Carajas Mineral Province, northern Brazil. The copper-gold ore is hosted by the Archean Salobo-Pojuca Group, which is formed by a sequence of amphibolites, banded iron formations, metagraywackes and quartzites. These rocks were deposited in a trondhjemitic basement, where a continental rift basin, that has been further described as a pull apart basin, was developed. Principal ore assemblages are magnetite-bornite-chalcocite and magnetite-bornite-chalcopyrite, with magnetite dominant and variable amounts of copper sulphides. The iron oxide copper-gold ore shows elevated concentrations of Ag, U, Co, Mo, F and LREE. Differences in geochemistry and textures between magnetite of iron-rich rocks and magnetite of banded iron formation suggest a hydrothermal origin for the mineralization. Fluid inclusion data for quartz veins and apatite indicate the involvement of highly saline fluids in the deposit formation. Adominantly magmatic source of the sulphur is indicated by isotope ratios determined for chalcopyrite and bornite (834S between 0.2%o and 1.6%o). Petrographic evidence supported by preliminary geochronological data indicates that the mineralization post-dates the metamorphism. Hydrothermal alteration effects on host amphibolites have been also investigated. The studied amphibolites occur as lenses or layers close to the contact with the gneissic basement or included in metagraywackes of the Salobo-Pojuca Group. Trace element chemistry of these rocks indicates that they are subalkaline basalts with tholeiitic affinity. Based on the K.,0 content, three alteration groups have been defined and informally named "less altered", "medium altered" and "very altered" types. They characterize rocks affected by different degrees of alkali metasomatism, resulting in major compositional changes. "Less altered" rocks (<0.5 wt% K.,0) show minor chemical modifications compared to the inferred average compositions of unaltered precursors. "Medium altered" rocks (0.5-3.5 wt% K20) show alkali metasomatism expressed by incipient sodic alteration (up to 4.5 wt% Na20) and superposed potassic alteration. "Very altered" rocks are characterized by extensive potassic alteration, with K-feldspar and biotite formation and high K20 (>3.5 wt%) values. The spatial association of "very altered" rocks with the main ore zone suggests a relationship between alkali metasomatism and mineralization. Similarities in the the hydrothermal alteration pattern combined with the ore mineralogy and chemistry indicate that the Salobo deposit belongs to the class of iron oxide (Cu-U-Au-REE) deposits.

ТематикаПолезные ископаемые
МеткиCarajas, Copper, Gold, Salobo
Автор(ы):Eduardo A.P.Vieira, Leonardo H.Souza
Издание:PGS Publishing, Linden Park, 2002 г., 12 стр.
Язык(и)Английский
Salobo 3 Alpha deposit: geology and mineralisation

The Salobo 3 Alpha Deposit is found in the southeast of the Amazon Craton, north of the Serra dos Carajas, in the State of Para, Brazil. The deposit is contained in supracrustal rocks of Igarape Salobo Group of Archean age, represented by iron-rich schists, metagreywackes, amphibolites and quartzites. This sequence overlies the basement gneisses of the Xingu Complex composed of partially migmatized gneisses. The original stratigraphic relationships are masked by intense ductile-brittle shear zones responsible for the generation of allochthonous rocks. The deposit extends over an area of approximately 4000 metres along strike (NW), is 100 to 600 metres wide and has been recognised to depths of 750 metres below the surface. The estimated mineral resources are of the order of 789 Mt with 0,96% Cu and 0,52 g/t Au. Copper mineralization occurs as chalcocite and bomite, with subordinate quantities of chalcopyrite, together with variable proportions of molybdenite, cobaltite, covellite, gold and silver, lodged in schists with variable proportions of magnetite, amphibole, olivine, garnet, biotite, quartz and piagioclase. Brittle-ductile shear zone deformation has resulted in lenticular shaped ore shoots that characteristically show close associations between copper mineralization and magnetite contents. The host rocks were progressively metamorphosed to pyroxene hornfels facies, at equilibrium temperatures of 750°C, resulting from sinistral transcurrent transpressive shearing accompanied by oblique thrusting. A first hydrothermal event developed at temperatures between 650 to 550°C causing partial substitution of chalcopyrite by bornite and chalcocite, accompanied by intense K-metasomatism. This was followed by sinistral transcurrent transtensive shear zone formation, causing green schist facies metasomatism, characterized by intense chloritization and partial substitution of bornite by chalcocite. Several hypotheses have been proposed for the genesis of the deposit. Based on similarities in the ore mineralogy and the hydrothermal alteration pattern, this deposit could be ascribed to the large class of iron oxide copper-gold deposits.

ТематикаПолезные ископаемые
МеткиCarajas, Copper, Gold, Salobo 3 Alpha
Автор(ы):Claudinei Gouveia de Oliveira, Edison Tazava
Издание:PGS Publishing, Linden Park, 2002 г., 10 стр.
Язык(и)Английский
The Igarape Bahia Au-Cu(-REE-U) deposit, Carajas mineral province, northern Brazil

The Igarape Bahia Au-Cu-(REE-U) deposit is located in the Carajas Mineral Province - Northern Brazil - and is hosted by an Archaean low-grade metamorphosed volcanosedimentary sequence characterized by metavolcanic rocks of the footwall and metavolcanoclastic/metasedimentary rocks of the hangwall. An intense hydrothermal alteration occurred in this sequence, promoting intense chloritization, Fe-metasomatism, Cu-sulphidation (chalcopyrite and bornite), carbonatization, silicification, tourmalinization and biotitization.

ТематикаПолезные ископаемые
Автор(ы):Anselmo D.V.Soares, Celio F.Barreira, Marcos Giovanni S.Dos Santos, Petronila C.Ronze
Издание:PGS Publishing, Linden Park, 2002 г., 12 стр.
Язык(и)Английский
Alemao copper-gold (U-REE) deposits, Carajas, Brazil

The Alemao copper-gold deposit is located within the Carajas Mineral Province of Northern Brazil and was discovered in 1996 by DOCEGEO using geophysical and geological techniques. Alemao is hosted by the Igarape Bahia Group, which comprises two lithological and stratigraphic domains: a lower metavolcanic unit composed of metavolcanic rocks and acid to intermediate volcanoclastics; and an upper clastic-chemical metasedimentary unit with volcanoclastic rocks. The Alemao ore body is covered by a 250 metres thick unconfonnable siliciclastic unit referred as the Aguas Claras Formation. The ore body, which is 500 metres in length and 50 to 200 metres wide, strikes NE-SW and dips steeply to the NW, being emplaced along the contact between the two stratigraphic domains of the Igarape Bahia Group. In the ore zone, the hydrothermal paragenesis is marked by ferric minerals (magnetite-hematite), sulphides (chalcopyrite, pyrite), chlorite, carbonate (siderite, calcite, ankerite) and biotite, with minor quartz, tourmaline, fluorite, apatite, uraninite, gold and silver. Sericite and albite are rare. The mineralisation is represented by hydrothermal breccias and "hydrothermalites" classified into two types: (1) the BMS type, composed of massive bands of magnetite and chalcopyrite and by polymitic breccias with a matrix comprising magnetite, chalcopyrite, siderite, chlorite, biotite and amphiboles; (2) the BCLS type breccia which comprises brecciated hydrothermalised volcanic rocks with chalcopyrite, bornite, pyrite, chlorite, siderite, ankerite, tourmaline and molybdenite in the matrix, as well as dissemination in the rock. The geochemical association of Fe-Cu-Au-U-REE in iron rich, heterolithic, hydrothermal breccias at the Alemao Cu-Au Deposit, as well as its possible association with an extensional tectonic setting, suggests a correlation with Olympic Dam type mineralization. The total estimated ore resources based on a krigging method is 170 Mt @ 1.5% Cu and 0.8g/tAu.

ТематикаПолезные ископаемые
МеткиAlemao deposit, Carajas, Copper, Gold
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