HR: 1400h
AN: V23B-04    [Abstracts]
TI: Contrasting compositions of amphiboles in two Neoproterozoic Banded iron formations of Uruguay
AU: Oyhantcabal, P
EM: epidoto2000@yahoo.de
AF: Departamento de Geologia, Facultad de Ciencias, UdelaR, Igua 4225, Uruguay
AU: * Sanchez Bettucci, L
EM: leda@fcien.edu.uy
AF: Departamento de Geologia, Facultad de Ciencias, UdelaR, Igua 4225, Uruguay
AU: Siegesmund, S
EM: ssieges@gwdg.de
AF: Geoscience Center of the University of Gottingen, Goldschmidtstr. 3, Germany
AB: Two occurrences of iron formations from the Precambrian of Uruguay were mineralogically and petrographically investigated. One of these deposits, located to the north of Minas City is associated with a low metamorphic grade Ediacaran succession (Minas Formation - Arroyo del Soldado Group) where glacial influence has been suggested. Fine grained quartz, Fe-rich amphibole (grunerite), magnetite porphyroblasts and hematite laths characterize these BIFs. Although an earlier increase of ferrous iron in solution in deeper water through the glacial episode is a probable factor, the identification of volcaniclastic acid rocks in the sequence indicates that Fe-fluids of volcanic origin played also an important role. A Rapitan type for this deposit is indicated by geological features and age constrains. The other analyzed deposit is associated with a highly deformed low- to medium- grade metamorphic supracrustal succession (Zanja del Tigre Formation) occurring as tectonic slices in a pre- Neoproterozoic basement. The age of this unit is not well constrained but field evidences and indications of glacigenic deposits indicate Cryogenian or Tonian age. This iron formation consists of about 2-5 cm thick layers, composed of alternating bands rich in quartz, magnetite and hematite, carbonate (calcite) and Mg-amphibole (cummingtonite). The difference found in the mineral chemistry of the amphiboles between both deposits suggests that, in the latter deposit, metamorphic changes in the paragenesis were associated with higher oxygen fugacity. An alternative explanation is that those rocks are not BIF deposits.
DE: 3660 Metamorphic petrology
DE: 3665 Mineral occurrences and deposits
SC: Volcanology, Geochemistry, and Petrology [V]
MN: 2007 Joint Assembly