Formation of black opal in the pegmatites from Volyn (Ukraine) – an example for interaction of silica with organic matter

Abstract. Black opal, a variety of opal pigmented by organic matter, from Volyn, western Ukrainian Shield, was investigated by means of scanning electron microscopy (SEM), electron microprobe analyses (EMPAs), μ-X-ray fluorescence (μXRF) element mapping, X-ray diffraction (XRD), and Fourier transformed infrared (FTIR) spectroscopy, and was dated by U-Pb by laser ablation (LA-ICP-SF-MS). Opal occurs as several centimeter large blocks and as cement in a breccia within pegmatites, which intruded into granites of the ca. 1.7–1.8 Ga old Korosten pluton. Opal samples were collected underground in miarolitic cavities of the pegmatites, which also host kerite, fossilized organic material (called Volyn biota). Inclusions in opal are typical pegmatitic minerals such as alkali-feldspar, Li-mica, and quartz. Buddingtonite and ammonium-bearing muscovite, as well as shape relicts of the fossils in opal, indicate interactions of organic material with the pegmatite. The XRD data indicate opal-CT with a complex pattern of the ∼ 4 Å peak. Its microstructure, as seen in SEM and back-scattered electron maps, consists of nanograins 15–35 nm in size, arranged in sphere-like structures, in some samples ∼ 5 µm-large and monodisperse, in others polydisperse with diameters of between 1 and 10 µm, and as lepispheres. Organic matter plus water makes up ∼ 10 wt %–15 wt %. The organic matter is situated within the core, in concentric zones in the sphere-like structures and in the matrix around them. Infrared (IR) spectra confirm the presence of silanol SiOH and CHn. At the contact with pegmatitic material, a pyrite-enriched zone was observed and interpreted as resulting from the infiltration of a late Fe- and Mn-bearing fluid along grain boundaries. Results of U-Pb dating yield a minimum age for opal formation of ∼ 200 Ma, and provide evidence for fluid overprint during the Jurassic (170–155 Ma) and Quaternary. These ages are much older than reported so far in opal from deposits worldwide. Furthermore, the here-reported occurrence of opal in pegmatitic rocks is unique, as all other opal deposits are either sedimentary or volcanic hosted.