Occurrence and origin of opaline silica in the Mesoarchean Bangur chromite deposit, Singhbhum Craton, India

Occurrence of opal is being reported here from the Mesoarchean Bangur chromite mines area in the Boula–Nuasahi ultramafic complex (BNUC) of Odisha, India. The opal shows colour bands in mm to cm scales. From the X-ray diffraction pattern, it is identified as a variety of opal-CT consisting dominantly of α -tridymite and α -cristobalite with very minor quartz. This is the first report of opal from BNUC area. High-resolution field emission scanning electron microscopy (FE-SEM) reveals that this opaline silica is nano-crystalline and consists of silica spherules (10–20 nm) with occasional ill-defined cubic and tetragonal crystallites. Selected area electron diffraction (SAED) pattern obtained through transmission electron microscope (TEM) reveals that it is polycrystalline in nature. Multi-point analysis by electron micro-probe (EMP) indicates its composition to be ~95 wt.% SiO 2 . From its mode of occurrence in the field and the type of mineral inclusions in the opal, its genesis can be coined with the second phase of magmatic event (the Bangur gabbro intrusion) and the related hydrothermal alterations. We interpret that the silica has been derived from the host mafic and ultramafic rocks at a high-temperature regime (1000–500°C) during the Bangur gabbro intrusion. During this magmatic event when the host rocks were hydrothermally altered, SiO 2 was released and precipitated as opal-CT. Graphical abstract Research highlights This is the first report of occurrence of opaline silica in form of opal-CT from Boula–Nuasahi ultramafic complex, Odisha, India. This opal-CT is semicrystalline, and primarily consists of nano-crystallites (10-20 nm) of α -tridymite and α -cristobalite. The silica material could have been released from the mafic–ultramafic host rocks of the area in relatively high-temperature regime (1000–500°C) due to hydrothermal activity caused by the second phase of magmatic intrusion of ‘Bangur gabbro’. The opal-CT is interpreted to have formed due to rapid cooling from a siliceous sol/gel.