Siliceous breccias with accompanying massive to banded metachert have been found hosted in metasandstones of the Fortuna Formation, the lower unit of the siliciclastic, mostly psammitic, Aguapeí Group. This group developed mainly in a late Mesoproterozoic aborted continental rift or aulacogen of the Sunsás-Aguapeí Province (1.2-0.95 Ga), whose evolution led to the stabilization of the southwestern fringe of the Amazonian Craton, promoting its final consolidation. Regionally, the breccias and related metachert form four main linear, NW-SE strike-parallel trends that lie symmetrically about the hinge zone of synclinal folds. These trends are known to occur for a length of 25 km and a width of up to 4 km, so that the main breccia occurrences are distributed over an area of ca. 100 km2 herein termed the Serra da Borda Breccia Field after the hills where they have been found in SW Mato Grosso state, western Brazil. The breccias display a multitude of textures and structures, e.g., massive, roughly tabular, bank-like outcrop morphology, dense to disperse packing and jig-saw to mosaic to chaotic clast distribution. They usually contain metachert fragments set in a fine-grained siliceous-clastic material, but fragments of crystalline quartz aggregates and of metasandstone are present as well. Stratified breccias with a clearly clastic sandy matrix also occur. Associated with the breccias are massive to banded metachert as well as sinter-like vuggy siliceous rocks with vugs following banding. This picture suggests strongly that the breccias constitute strata-bound syn-sedimentary bodies. On grounds of their outcrop and textural features, the breccias and associated metachert are provisionally interpreted as hydrothermal phreatic products of syn-sedimentary eruptive hot-spring activity completed by debris flow deposition, with the involvement of rift-related growth faults.
