Abstract
The Patinhas prospect, in southeastern Tapajos province, consists of a gold-bearing quartz vein, averaging 70 cm in thickness, hosted by a subvertical, NE-SW-trending, brittle-ductile, strike-slip shear zone, cutting a granodioritic gneiss of the Cuiu-Cuiu complex (~2.0 Ga). The quartz is milky massive and sulfide-rich in its central portion, and foliated at the contact with the host rock. This contact is also outlined by a narrow hydrothermal alteration halo, containing sulfide dissemination, and by the ductile deformation superimposed to the host rock during shearing. The structural evidence suggests emplacement of the vein at a moderate depth, in an active structure, whose activity outlasted the vein formation. Gold occurs in microscopic particles that fill microfractures in pyrite crystals. Under the microscope, large quartz crystals are surrounded by a mosaic of recrystallized small grains. Weak to strong effects of ductile deformation are observable. Fluid inclusions (FI) occur as intraand transgranular trails, in the boundaries of recrystallized grains, or in the cores of less deformed and/or preserved relic quartz, where they are isolated or form small clusters of randomly-distributed inclusions. FI containing CO 2 , mixed CO 2 -H 2 O, and H 2 O have been recognized. Among the aqueous inclusions two subgroups have been identified: a group of late FI, and a group of inclusions that are spatially associated with the CO 2 -bearing FI. The CO 2 melting temperature of the CO 2 -bearing FI clustered close to the temperature of the triple point of the CO 2 (-56.6oC). Based on structural, petrographic and microthermometric evidence, the fluid inclusions are interpreted as formed by a combination of phase separation and re-equilibration of an aqueous-carbonic fluid during and after their entrapment. Physico-chemical parameters estimated from the microthermometric results indicate that mineralization occurred between 307o and 389oC, at variable pressure (average of 2.1 kb, corresponding to a depth of ca. 7 km). The aqueous-carbonic fluid has bulk densities between 0.73 e 0.97 g/cm 3 , mean salinity of 6.6 wt. % NaCl equiv., and XCO 2 of 6-16 moles %. These characteristics are compatible with those of structurally-controlled, mesothermal / mesozonal gold deposits.
Paper version not known (
Free)
Published Version
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call