Hydrothermally opalized serpentinites in tethyan ophiolite sequence in central Serbia

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