DEPOSITS OF PRECIOUS OPAL: GENESIS AND SEARCH CRITERIA

Abstract

Origin, growth, and steady existence of monodisperse silica spheres (MSS) composing precious opal are possible solely in concentrated alkaline solutions of silica. In Australian deserts, such solutions result from dissolution of alkaline dust in periodic shower precipitation. The dust forms on decomposition of alkaline minerals and accumulates on their surface under conditions of hot and dry climate with drastic daily temperature variations. Alkaline solutions percolate down through high-porosity and fracture zones and thus give rise to MSS, which then grow, form suspensions, and undergo gravitational separation by size. At depths of 20-30 m, sedimentational concentration of the suspensions takes place in fractures and cavities, followed by their supramolecular crystallization to opal structures. Subsequent silicification and transformation of these structures into solid stone are due to the action of colloidal solutions of silica that form from MSS suspensions. Zones of depthward percolation of alkaline solutions are always conjugate with evaporation zones of ascending flows of solutions responsible for surficial silicification and deposition of Solonchak salts. Search for precious-opal deposits must be based first of all on alkali mapping of weathered surficial rocks in deserts and then on geomorphological and mineralogical reconstruction of the local hydrogeologic environment within alkali spots during the period of opal formation.