Agates from the Uruguay River: Distribution, Abundance, and Characterization.

Abstract

This study evaluates the potential of agates found within the Plio-Pleistocene deposits of Entre Ríos Province (Argentina) for use as ornamental stones. A total of 19 sediment samples were collected from quarry pits and natural outcrops along the Uruguay River and analyzed through grain size distribution and petrographic techniques. The studied gravels exhibit a near-Gaussian size distribution, with an average grain size of 25 mm, corresponding to coarse pebbles. The coarser fractions (>32 mm), including very coarse pebbles and cobbles - those most suitable for lapidary purposes - are most abundant between the towns of Santa Ana and Concordia. Agates (both whole and fragmented) constitute approximately 28% of the total gravel analyzed, although only 4.6% exhibit the quality and size necessary to justify commercial lapidary work. This corresponds to a grade of 60kg/m3, which increases significantly in stockpiles from deposits exploited for construction purposes, where gravel smaller than 50 mm has been removed. Petrographically, the most
common agates display concentric banding of chalcedony with one or two generations of crystalline quartz. Horizontally banded agates are less frequent, while spherulitic varieties are rare.