Can we asume you've already exhausted the resources of the Church of Latter Day Saints (LDS)?

If not, one of the microfilms referenced in the following links may be of help.
http://www.familysearch.org/eng/search/IGI/individual_record...
http://www.familysearch.org/eng/search/igi/individual_record...

I assume you know the Guarani people are native to what is now Paraguay, and to surrounding regions of Argentina and Brazil, and are in no way connected to Chile, other than by long distance migration.

The surname (de la) Barrera is that of a Spanish Conquistador. There may be multiple generations of mestizo involved in the tree you are researching, and you'll have to establish how many generations prior to Rosario the migration from Paraguay to Chile occurred.

A sometimes useful resource for Chilean genealogy is genealog.cl

Good luck


Philip