Abstract
The Sao Geraldo do Araguaia Opal Deposit is situated in the northern part of the Araguaia Belt and consists of small pockets and anastomosed opal veinlets, which are enclosed in the inner part of a sub-meridian, up to 6 m thick, highly brecciated and roughly zoned vein. This vein is composed of jasper opal with massive quartz margins and occurs in biotite schists of the Xambioa Formation (Estrondo Group) of Upper Proterozoic age. White to pinkish, fire and boulder opal varieties were identified in this deposit. Although the age of the quartz of the border is certainly Precambrian, jasper opal and the diverse opal types are more recent. It is thought that they were mainly formed in Jurassic- Cretaceous times, when distension tectonic and reactivation of Precambrian discontinuities, related to the evolution of the Parnaiba Basin and the opening of the Atlantic Ocean, took place and induced low temperature hydrothermalism. However, the more noble opal varieties apparently are more recent, related to Cenozoic supergene processes.
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