The Batalha (Piauí) quartz varieties and their relationship to the SiO2 mineralizations (chalcedony and opals) in the Parnaíba Basin, Brazil

Abstract

The Parnaíba Basin is a geological unit that hosts several occurrences and deposits of SiO2 minerals, especially quartz, chalcedony and opals, with highlight to the municipality of Pedro II, Piauí state. In the north of Piauí, in the municipality of Batalha, new occurrences of quartz were found, in the hyaline, morion, smoky, milky and citrine varieties, alongside the amethysts already known in the municipality. In order to better understand the occurrences of quartz in the region, some habitants approached us to study these occurrences with the objective of possible use. The work involved a field stage with mapping and sample collection, followed by description, imaging and cutting the quartz crystals. Field data show that the quartz occurrences are closely related to a hydrothermal alteration zone developed in the contact between sandstones and diabase. The quartz crystals occur as druse aggregates filling a fracture in the nearby sandstone and/or as isolated crystals in a hematite-rich breccia. The geological and mineralogical association of the crystals and rocks studied is relatively simple and similar to the occurrences and deposits of Pedro II and Buriti dos Montes, both located on the eastern edge of the Parnaíba Basin.