Hydrothermally Opalized Serpentinites in Tethyan Ophiolite Sequence in Central Serbia

Intensely fractured serpentinite protrusions in Vardar tectonic zone of central Serbia are a part ofaccreted Jurassic Tethys ophiolite sequence. Primary ultramafic rocks presenting remnants of oceanic crust wereregionally metamorphosed into serpentinites after middle Jurassic. Tertiary magmatic activity of calc-alkalinecharacter occurred along deep faults marking the zone of closure of Tethys ocean trench. Hydrothermal activitydriven by magmatic heat, through fracture and fault zones, has caused opalization of ultramafic host and veindeposition. Results of various examinations of these opalized serpentinite are presented: field examinations,chemical, microscopic, X-ray diffraction (XRD) and X-ray fluorescence (XRF) analyses. Opalized serpentinite hasbeen identified as predominating tridymite-cristobalite type opal-CT. Practical significance of these opal-CToccurrences lies in possibility for its use as a gemstone and in prospection of genetically and spatially relatedhydrothermal deposits (cryptocrystalline magnesite veins and other siliceous gemstone types). Occurrences ofopalized serpentinite of this type have been discovered on many locations along Tethys suture zone