Hydrothermal paleoactivity in the unconformity between the Buenos Aires Complex and La Tinta Formation in the Barker area, Tandilia.

Abstract

The alteration present at the rocks in the unconformity between the Precambrian Buenos Aires Complex and the Late Precambrian- Cambrian La Tinta Formation was studied in eight profiles widespread around Barker locality, Tandilia Ranges. The petrographic study, the x-ray diffractometry, the treatment of whole-rock analysis and the fluid inclusion study allowed either to support or to question previous hypotheses about the genesis of such altered rocks. Mineral parageneses suggest asymmetric alteration patterns confined to the contact, basement - sedimentary succession, with local variations. Pyrophyllite + sericite (2M1-1M) + especularite (titanohematite) ± aluminum phosphate-sulfate (APS) minerals ± tourmaline ± rutile occurred where elements leaching and temperatures were higher (~ 260 °C), at the unconformity. Sericite (1M) + chlorite + calcite ± anatase-rutile ± kaolinite (1T) ± secondary quartz developed where lower temperature was reached, usually farther from the unconformity. Local silicification, at the unconformity rocks, postdates or was simultaneous with the alterations of lower temperatures (< 190 °C) of formation. By means of isocon diagrams the component mobility in the different types of alteration are shown. All mineral parageneses found could be satisfactorily explained by a hydrothermal alteration model in which the unconformity acted as a hydrothermal channel way, of multiple activity focus, on a regional scale.