Evolução quimica e isotopica dos fluidos associados a mineralização de Fe-Cu-Au do deposito Alvo 118, provincia mineral de Carajas - PA / Chemical and isotopic evolution of the fluids from the Alvo 118 Fe-Cu-Au do deposit, Carajas mineral province (PA)

AUTOR(ES)
DATA DE PUBLICAÇÃO

2009

RESUMO

The Alvo 118 iron oxide-copper-gold (IOCG) deposit (170 Mt @ 1,0 wt% Cu, 0,3 g/t Au), together with the world-class Sossego and Cristalino deposits, lie in the southern sector of the Carajas Mineral Province (CMP) along a steeply-dipping, WNW-ESE-striking, 60 km-long shear zone, close to the contact of the Itacaiunas Supergroup metavolcano-sedimentary sequence (~2,76 Ga) and the basement (Xingu Complex; ~3,0 Ga). The Cu-Au mineralization at the Alvo 118 is hosted by mafic and felsic metavolcanic rocks and crosscutting granitoid and gabbro bodies that have been subjected to the following hydrothermal alteration sequence towards the ore zones: (1) sodic alteration represented by albite and scapolite, indicating the circulation of hot (>500°C) and highly saline fluids; (2) potassium alteration dominated by biotite or Kfeldspar, respectively, in mafic and felsic volcanic and intrusive rocks, accompanied by the formation of magnetite and silicification; (3) chlorite alteration spatially associated with carbonate-quartz ore breccia and vein stockworks that commonly display open-space filling textures; (4) local post-ore quartz-sericite alteration. The ore assemblage is dominated by chalcopyrite (80%), accompanied by bornite (10%), magnetite (10%), hematite (10%), and traces of Au-Ag tellurides, galena and cassiterite. Dissimilar from other IOCG deposits of the CMP where LREEs are commonly enriched in apatite, allanite and monazite, the Alvo 118 ore displays enrichments of HREEs. In this deposit the HREEs are mainly concentrated in apatite and in an Al-poor, Be-B-HREE silicate of the gadolinite group which has never been reported in IOCG systems worldwide. Fluid inclusion studies in quartz and calcite point to a fluid regime in which hot brine solutions, represented by <10 vol.% of salt-bearing aqueous inclusions, were progressively cooled and diluted by lower temperature, low-salinity (<10 wt% NaCl eq.) aqueous fluids defined by two-phase aqueous inclusions, by far the dominant type. d18OH2O values in equilibrium with calcite (- 1.9¿ to 10.7¿ at 300°C) suggest strong interaction between ore fluids and the host rocks, as well as prolonged mixing with meteoric fluids. The d34S composition of chalcopyrite (5.1¿ to 6.3¿) differs from a mantle/magmatic source adding the possibility of heavier sulfur sources (e.g., evaporite sulfate?) for the ore breccia and vein sulfides. The restricted high temperature sodic alteration, the pervasive overprinting of the potassic alteration minerals (biotite and K-feldspar) by chlorite proximal to the ore zones, quartz-calcite-chlorite ore breccias/veins with open-space filling textures in brittle structures, and the d18O data collectively indicate that the Alvo 118 IOCG system developed at structurally high levels and experienced the influx of evolved meteoric water

ASSUNTO(S)

serra dos (pa) isotopes isotopos deposits of oxide of fe-cu-au minas e recursos minerais - carajas serra dos (pa) geochemistry depositos de oxido de fe-cu-au - carajas geoquimica mines and mineral resources

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