THE HYDROTHERMAL GENESIS OF OPAL IN THE STATE OF PIAUÍ, BRAZIL

Abstract

There are two sets of theories that propose different origins for Pedro II's opals. One group proposes a similar origin to Australian opals, resulting from climatic variations under environmental conditions in an arid climate. This work demonstrates the hydrothermal genesis for these opals from Piauí, whose geological context is marked by the association of sedimentary rocks, sandstones and intrusive igneous rocks, diabase. It all starts with the fragmentation of the supercontinent Pangea, formation of the Atlantic Ocean and a reactivation of the Transbrasiliano Lineament crossing the Parnaiba Sedimentary Basin, occurring a magmatism forming sills lodged below the sandstones. This magma heated the water contained in the sandstone pores, developing fluid convection cells, solubilizing the silica from the quartz grains of the sandstones and from the outer layer of igneous rock, releasing much silica to form the opals. As the temperature decreases, the silica-saturated solution becomes a supersaturated gel solution, precipitating opal in the fractures, above the clay layer in contact between the sedimentary rocks and the diabase, and between the altered facies and the unaltered diabase. The set of geological, mineralogical and geochemical evidence found associated with opals are indicative that they originated in this hydrothermal environment.