The Geochemistry of Gem Opal in South Gondar and South Wollo, Amhara Region, Ethiopia

Abstract

In this study, the geochemistry of four representative gem-opal and its host rock samples collected from South Gondar and South Wollo mining sites were investigated. The aim objective of this study was to understand the processes that led to the geochemistry and origin of gemopals in both study sites. The samples were identified as to whether they are real opals by different physical property measuring devices (viz., a refractometer, heavy liquid method and "MOHS HARDNESS PENCILS"), and characterized by flame atomic absorption spectrophotometer (FAAS), Fourier transform infrared spectrophotometer (FTIR), and X-ray diffraction (XRD). The performed physical analysis showed that the hardness, specific gravity and refractive index values were in line with natural opal properties. Results showed that the most abundant impurities in mg/L were: Fe (23.27-48.83), Mg (2.578-5.501), Ca (1.906-4.413), Cr (1.256-1.384), Zn (0.383-0.578) and Cu (0.220-0.296). Here, Fe appears to be the principal factor that determines the brown to red to orange to yellow colors. Opals and its host rocks from both study sites have a relatively lower content of Cu compared to literature values. There was no significant difference (p < 0.05) in the distribution of the investigated transition elements between gem opal and its host rocks; and so did the same for their mean values between the two sampling sites. FTIR spectra exhibited different sharp peaks in all samples, suggesting their volcanic origin. The peak observed around 3400 cm-1 confirmed -OH stretching; and a sharp peak appeared at 620 cm-1 in virtually all samples, except for opal sample from South Wollo, is indicative of their opal-C structure. XRD analysis highlighted the C and CT types of opals with variable contents of cristobalite and tridymite. In conclusion, opal and its host rocks from both study sites have volcanic origin. Thus, this study can provide baseline information on the origin identification and geochemistry of opals; and it will be of significant importance in creating awareness for the local community and people in the gemstone industry