Salobo Sequence, Carajas, Brazil: Geology, Geochemistry And Metamorphism

The Salobo Cu (Au, Ag, Mo) deposit is in the Carajas basin, located in the eastern Amazon craton, southern Para state, Brazil.;The Salobo sequence at the deposit area comprises metamorphosed basaltic rocks, iron formations, graywackes and arkosic arenites deposited at 2758+/-2 Ma on a trondhjemitic basement dated at 2851+/-4 Ma. The amphibolites are continental tholeiitic basalts, indicated by Fe, LIL and LREE enrichment, LREE fractionation and almost flat HREE chondrite-normalized patterns. Silicate-type iron formation grades to an aluminous type and finally to metagraywackes. Geochemical studies show a continuous compositional variation between both iron formation types, with gradual Al, K, Ti and Zr increase and Fe and Si decrease, defining a mixing line between chemical iron formation and volcaniclastic contaminants of the Grao Para Group.;The highly fractionated REE chondrite-normalized pattern of Type I iron formation (La/Lu{dollar}sb{lcub}rm cn{rcub}{dollar} = 440) and strong positive Eu anomaly (Eu/Eu* = 6.3) indicate a hydrothermal origin. Hydrothermal provenance for Cu is indicated by a positive correlation between Fe and Cu.;The Salobo sequence underwent progressive metamorphism close to pyroxene hornfels facies, followed by two episodes of hydrothermal alteration, at amphibolite and greenschist facies. The first metamorphic episode took place at low pressure (2-3 kbar) with a temperature of 750{dollar}spcirc{dollar}C (garnet-hornblende geothermometer); the associated fluids had P{dollar}sb{lcub}rm CO2{rcub}{dollar} {dollar}>{dollar} P{dollar}sb{lcub}rm H2O{rcub}{dollar} and log f{dollar}sb{lcub}rm O2{rcub}{dollar} from {dollar}-{dollar}18 to {dollar}-{dollar}22. Garnet-biotite and muscovite-biotite geothermometers indicate temperatures of 650 and 550{dollar}spcirc{dollar}C for the first hydrothermal alteration episode. The garnet-plagioclase geobarometer shows a pressure of 2.5 kbar for this event, which is also the pressure of the first metamorphism. Mass balance calculations indicate that related fluids were slightly oxidizing, acidic and highly saline promoting hydration accompanied by leaching of Ca and deposition of Si, K, Na and B. The temperature during the second hydrothermal episode was 370{dollar}spcirc{dollar}C (garnet-biotite geothermometer). Mass balance calculations show that the associated fluids were acidic, oxidizing, with moderate salinity, depositing Si and leaching Na, K and Mn. The lower temperature (550{dollar}spcirc{dollar}C) stage of the first hydrothermal event and the second hydrothermal episode occurred at the time interval of 2581-2551 Ma, which also represents the age of basement ascent.

Name thesis:degree_name

Ph D

Year dc:date.issued

1990

Author dc:creator

• Lindenmayer, Zara Gerhardt

• Geology

Handle dc:identifier.uri

https://hdl.handle.net/20.500.14721/21593

OAI identifier oai:identifier

oai:uwo.scholaris.ca:20.500.14721/21593