THE SERRA LESTE SEDIMENT-HOSTED AU-(PD-PT) MINERALIZATION, CARAJÁS PROVINCE

FERNANDO HENRINQUE BUCCO TALLARICO, CLÁUDIO RODRIGUES COIMBRA, CARLOS HENRIQUE CRAVO COSTA

Resumo


The Serra Leste Au-(Pt-Pd) mineralization is hosted by a folded low-grade metasedimentary sequence known as Rio Fresco Formation. The orebodies are located at the hinge zone of a recumbent syncline at the contact between carbonaceous metasiltstone and dolomitic marble. Ore mineralogy includes quartz, kaolinite, goethite, Mn-oxides, muscovite, amorphous carbon together with gold, Pd-Pt-(Hg) minerals and minor Cu-Co-Ni-sulfides. The orebodies are oxidized, and hypogene mineralization is not known beyond the base of supergene alteration. The widespread occurrence of the actinolite-calcite pair and the local association with diopside, indicate a minimum of 550°C for peak metamorphic temperature of the dolomitic marble. This temperature is inconsistent with the low-grade metamorphic assemblage of metasiltstones, thence with regional metamorphism, and is better accommodated in a thermal-aureole hypothesis. Cooling of the aureole triggered the breakdown of actinolite to calcite + quartz, at temperatures of about 400-450°C. Later hydrothermal activity, between 230°C and 360°C, developed chlorite alteration and sulfide precipitation in both dolomitic marble and diorite intrusion(s) that crosscut the metasedimentary sequence. The carbonaceous metasiltstone possibly acted as a chemical barrier, triggering focused deposition of precious metals through a rapid decrease of the oxidation conditions of the hydrothermal fluid. This process was possibly coupled with carbonate dissolution of dolomitic marble that triggered open-space-filling processes, developed an external jasperoid envelope around the orebodies and caused a pH increase.

Palavras-chave


Carajás Province; Serra Leste; Serra Pelada; Gold; Palladium; Platinum.

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