A hydrothermal clay mineral assemblage at the Late Proterozoic unconformity Buenos Aires Complex-La Tinta Formation: Barker area, Tandilia Ranges (Argentina).

MARTÍNEZ, J.C., DRISTAS, J.A., MASSONNE, H.J. Y THEYE, T.

CLAY MINERALS (PRINT)

MINERALOGICAL SOC

Año: 2010 vol. 45 p. 209 - 224

0009-8558

ABSTRACT: A general alteration pattern of two transitional clay mineral assemblages wasdetermined by petrography and X-ray diffractometry studies at the Tandilia Late Proterozoicunconformity zone, around the Barker locality: (1) K-white mica + chlorite + calcite < anatase-rutile< secondary quartz (farther from the unconformity) and (2) pyrophyllite + K-white mica + Ti-richhematite < aluminium phosphate-sulphate minerals < tourmaline < anatase-rutile (closer to theunconformity). The local occurrence of Na in K-white micas and K-Na in pyrophyllite is describedfor the first time. Possible interlayering with intermediate K-Na mica and paragonite is indicated bydetailed X-ray diffraction analyses. A negative Eu anomaly (Eu/Eu* = 0.24), strongly positive Euanomaly (Eu/Eu* =11.7) and positive Ce anomaly characterized rapid changes from an unalteredbasement to the most altered basement and sedimentary rocks at the unconformity. Aluminiumphosphate-sulphate minerals dominate patterns of LREE. A model of hydrothermal alterationcomparable to that of unconformity-related uranium deposits is presented.