Mineral chemistry of ore and hydrothermal alteration at the Sossego iron oxide–copper–gold deposit, Carajás Mineral Province, Brazil

Lena Virgínia Soares Monteiroa, , , Roberto Perez Xaviera, , Murray W. Hitzmanb, , Caetano Julianic, , Carlos Roberto de Souza Filhoa, and Emerson de R. Carvalhoa,

aInstituto de Geociências, Universidade Estadual de Campinas, R. João Pandiá Calógeras, 51, CEP 13083-970, Campinas, SP, Brazil

bDepartment of Geology and Geological Engineering, Colorado School of Mines, Golden, Colorado 80401, United States

cInstituto de Geociências, Universidade de São Paulo, Rua do Lago, 562, CEP 05508-080, São Paulo, SP, Brazil
Received 10 January 2007; revised 13 October 2007; accepted 31 January 2008. Available online 3 March 2008.
Abstract

The Sossego iron oxide–copper–gold deposit in the Carajás Mineral Province comprises two major orebodies, Sequeirinho and Sossego. Sodic alteration (albite–hematite) and sodic–calcic alteration zones represented by albite, ferro-edenite/hastingsite (up to 3.8 wt.% Cl), actinolite/magnesiohornblende, magnetite, titanite, epidote, and calcite are predominant at Sequeirinho. Magnetite bodies with envelopes of apatite-rich actinolitite were formed with the sodic–calcic event at high temperatures (~ 500 °C at 1.4 kbar). In the Sossego orebody, potassic alteration with orthoclase and Cl-rich biotite (up to 3.1 wt.%) and chloritization are the main alteration types.

Mineralized breccias in both orebodies have coarse-grained zoned actinolite/ferro-actinolite, Cl-apatite, and magnetite within the matrix. Sulfides occur in equilibrium with a paragenetically late calcite–quartz–chlorite–epidote–allanite assemblage. The AlIV contents of the chlorite indicate crystallization at temperatures below 300 °C. Chalcopyrite occurs associated with pyrite (up to 2.3 wt.% Co and 0.2 wt.% Ni), native gold (up to 14.9 wt.% Ag), siegenite, millerite, vaesite, Pd-melonite, and hessite.

Dilution and cooling of the hot metalliferous fluid (> 500 °C) by mixing with meteoric fluids may have been the main mechanisms responsible for the deposition of metals transported as metal chloride complexes in both orebodies of the Sossego deposit.

Keywords: Sossego deposit; Iron oxide–copper–gold deposit; Carajás Mineral Province; Brazil; Mineral chemistry; Geothermobarometry

Article Outline
1. Introduction
2. Geological setting of the Carajás Mineral Province
3. The Sossego iron oxide–copper–gold deposit
3.1. Sequeirinho orebody
3.2. Sossego orebody
4. Analytical methods
5. Mineral chemistry
5.1. Amphibole
5.1.1. Sodic–calcic alteration
5.1.2. Mineralized zones
5.2. Feldspar
5.3. Biotite
5.3.1. Halogen activities in the hydrothermal fluid
5.4. Chlorite
5.5. Magnetite
5.6. Sulfides
5.6.1. Chalcopyrite and pyrite
5.6.2. Siegenite, millerite and native gold
6. Geothermobarometry and stability fields of the hydrothermal associations
6.1. Sodic–calcic alteration
6.2. Chloritization
7. Discussion
7.1. The Sossego hydrothermal system evolution
7.2. Amplibole chemistry: comparison with Carajás IOCG deposits
7.3. Halogen contents in the hydrothermal fluid, metal transport and mechanisms of ore deposition
7.4. Magnetite–Cu–Au–Co–Ni–Pd–REE association
8. Conclusions
Acknowledgements
References

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