Agate deposits in Cretaceous magmatism in the Paraná Basin, southern Brazil

Abstract

The Salto do Jacuí Mining District, located in Rio Grande do Sul state (southern Brazil) is highlighted by the most important agate deposits in the world. This occurrence is associated with the Serra Geral Formation, an important Cretaceous magmatism in the Paraná Basin. Agate is a special variety of chalcedony characterized by a banded structure and found in geodes. It is important to register the presence of the named Umbu agate which has a great value by the uncommon dark blue color and high microporosity that makes it suitable for dyeing process. In this study we try to characterize the agate mineralization using chemical, petrographic and isotopic analytical techniques. Based on oxygen isotopes, the temperatures of agate formation range from 23 to 650 C, and these values can be related to parameters such as the total content of silica, the presence of impurities (like Fe₂ O3 ), the water content in the structure and the crystallinity. So, the analyzed agate samples are characterized by marked differences in major and trace elements, the crystallization temperature and degree of crystallinity indicating variations in the crystallization process despite the common occurrence in the SJMD.