Stable isotopic constraints on Kuroko-type paleohydrothermal systems in the Mesoproterozoic Serra do Itaberaba group, Sao Paulo State, Brazil

摘要

Mesoproterozoic oceanic paleohydrothermal systems developed in the volcanosedimentary Serra do Itaberaba Group, which comprises part of the Ribeira fold belt. Hydrothermal alteration associated with these systems was responsible for large premetamorphic chloritic alteration halos (CZ1 rocks), overprinted by restricted premetamorphic chloritic (CZ2 rocks), argillic, and advanced argillic alterations that correspond to intensely leached rocks within feeder zones. Well-defined trends of increasing δ~(18)O values with the progressive intensity of the alteration process are observed for igneous metabasites, metabasic hydroclastic rocks, and intermediate metamorphosed igneous and volcaniclastic rocks from CZ1. Systematic stable isotope variations evince that, in the Serra do Itaberaba metamorphosed hydrothermalized rocks, the preexisting isotope signatures of the hydrothermal systems were at least partially preserved. Highly evolved hot seawater is suggested for the genesis of the CZ1 rocks, whereas for the CZ2 rocks and marundites, the ~(18)O fluid enrichments are interpreted as due to the major contribution of evolved seawater-derived fluids with a subordinate magmatic water component. An early near-seafloor, low-temperature alteration in a mid-ocean ridge environment was responsible for heterogeneous ~(18)O whole-rock enrichments and followed by steady hydrothermal circulation with discharge of hot fluids, which previously underwent isotopic exchange with the ~(18)O enriched volcanic rocks in the deeper part of the system with high temperatures and low water: rock ratios in a backarc environment. The subordinale magmatic water component derived from andesitic and rhyodacitic intrusions. The extremely high δ~(18)O anomalies from the CZ1 rocks suggest an associated base metal massive sulfide ore body. The lower δ~(18)O values related to the CZ2 rocks represent alteration by a higher temperature fluid, which might indicate the proximity of possible ore zones. The identification of several premetamorphic hydrothermally altered zones, similar to those of Kuroko-type base metal mineralizations, expands the mineral potential of base metal deposits in the Serra do Itaberaba Group and the volcanosedimentary sequences from the Ribeira fold belt.