Summary:
The Salobo deposit is an example of an iron oxide-copper-gold (IOCG) deposit and is hosted in the Carajás Mining District within Carajás Province, a sigmoidal-shaped, west–northwest–east–southeast-trending late Archean basin.
The mineralization consists of mineralogical assemblages of magnetite–chalcopyrite–bornite and magnetite–bornite– chalcocite in a number of styles as disseminations, stringers, stockworks, massive accumulations, fracture fillings, or veins.
The deposit extends over an area of approximately 4 km along strike (west–northwest), is 100–600 m wide, and has been recognized to depths of 750 m below the surface.
Structure
The tectonic evolution of the Salobo area includes sinistral, transpressive, ductile deformation that developed under upper-amphibolite-facies conditions, followed by sinistral, transtensive, ductile– brittle-to-brittle shear deformation.
The Salobo deposit is situated within the Cinzento strike-slip system that reactivated older structures and formed a subparallel ductile–brittle shear zone in the northern part of the deposit and a brittle shear zone in the south. The brittle–ductile shear zone deformation resulted in lenticular-shaped mineralized shoots that show close associations between copper mineralization and magnetite content.
Metamorphism
Metamorphism consists of an initial high-temperature, low pressure phase followed by a retrograde greenschist phase. The initial phase included intense K-metasomatism and caused partial replacement of chalcopyrite by bornite and chalcocite. The retrograde phase is characterized by intense chloritization and partial replacement of bornite by chalcocite.
Alteration
The Salobo hydrothermal system has a core of massive magnetite that is surrounded by less intensely-altered rocks. Within the massive magnetite body there are small veins and irregular masses of secondary biotite. Garnet is completely replaced by magnetite, forming pseudomorphs. Away from the massive magnetite, the magnetite content gradually diminishes, giving way to biotite– garnet schist and/or garnet–grunerite schist. Alkali-metasomatism of the amphibolite facies rocks is expressed by weak sodium alteration with intense, superimposed potassium alteration (=4.6 wt% of K2O).
K-feldspar, biotite and oligoclase are the main alteration minerals. Potassium alteration in amphibolite was marked by replacement of calcium-amphibole and by formation of biotite and magnetite. The chemistry of the meta-greywackes at the deposit indicates that they also underwent significant iron and potassium alteration. Alteration assemblages are characterized by garnet, biotite and grunerite, with subordinate tourmaline and minor magnetite. The better-mineralized zones, located in the central part of the deposit, correspond to the most altered areas.
The Salobo deposit extends over an area of approximately 4 km along strike (west–northwest), is 100–600 m wide, and has been recognized to depths of 750 m below the surface.
Mineralization
Mineral assemblages occur in a number of styles: disseminations, stringers, stockworks, massive accumulations, fracture fillings, or veins associated with local concentrations of magnetite and/or garnet filling the cleavages of amphiboles and platy minerals, and remobilized in shear zones. Textural relationships indicate that mineralization was developed initially as an oxide stage, with a second, subsequent, sulphide stage.
There is a positive relationship between copper minerals and magnetite. Copper content is typically >0.8% in XMT and BIF, but in gneisses and schists it is
Sulphide mineralization typically consists of magnetite–chalcopyrite–bornite and magnetite–bornite– chalcocite. Accessory minerals include hematite, molybdenite, ilmenite, uraninite, graphite, digenite, covellite, and sulphosalts.
Chalcopyrite, bornite, and chalcocite occur interstitially to silicate minerals. These sulphide minerals are commonly found filling cleavage planes of biotite and the amphibole grunerite. Hematite is rare, but in places it can reach as much as 4% by volume. It exhibits tabular textures (specularite), with bornite infill, and partial replacement by magnetite.
Native gold occurs as grains in cobaltite, safflorite ((Co,Fe)As2), magnetite and copper sulphides, or interstitial to magnetite and chalcopyrite grains.
The gangue minerals are garnet, grunerite, and tourmaline, reflecting the intense ironmetasomatism. Minor amounts of fayalite and hastingsite are pseudomorphed by grunerite and magnetite. Ilmenite, uraninite, allanite, fluorite and apatite occur as accessory minerals.
Kinked biotite crystals are associated with potassic alteration, and spatially related to the copper– gold mineralization. Uraninite and zircon inclusions may be locally abundant in biotite.
Quartz is associated with biotite in better mineralized samples, and forms concordant veins within the host rocks.