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Fire agates from Deer Creek are highly appreciated gemstones due to the presence of optical phenomena and rainbow colours that cause fiery effects to be observed on their characteristic brown base. The specific morphology of poorly ordered chalcedony (crystallinity index = 0.1–1.5) with an admixture of mogánite (av. 6.6%), micro-quartz and opal-C forming a colloform texture seems to be responsible for the presence of fire effect in these agates. The multi-layered silica spheroidal forms ('bubble'-like structure), already noted in hand specimens, could be the centres of reflection and interference of white light. Numerous, microscopic inclusions of Fe and Ti compounds randomly scattered within some silica zones, together with microstructural features of agate, could determine the colour and size of the domains with the optical effect. Deer Creek fire agates form veins within their host volcanic rocks. The silica mineralisation filling the network of fissures in the host rocks was supplied cyclically with aqueous fluids of varying composition, enriched periodically in CO2, Fe, Ti, Mn, Zn and Ca. As a result, the red-brown colour of fire agates was created by scattered pigments of tiny iron oxides (magnetite, maghemite) and titanium oxides (rutile, anatase) within the silica matrix. The precipitation of strongly disordered silica with a characteristic colloform texture is diagnostic for boiling processes in this area. [ABSTRACT FROM AUTHOR]

Studies dealing with the response of subaqueous volcanic and hydrothermal activities to carbonate sedimentation in hemipelagic environments affected by tectonic processes are comparatively rare. Here, a microfacies record with combined chemical data from the neptunian dykes found at an intrabasinal ridge (Tatric Ridge; Carpathian domain of the Western Tethys), close to a source of alkaline volcanism with possible hydrothermal vents (Zliechov Basin), is presented. The characteristic features of the neptunian dykes, up to 20 cm thick, in the middle Albian echinoderm-foraminiferal limestones (Tatra Mountains, Inner Carpathians) are their red fillings. Microprobe and x-ray diffraction analyses show that this reddish material, partly mixed with sparitic clasts coming from the host limestone, consists mainly of hematite crystals which are associated with low crystalline silica and quartz. The microfacies data suggest that the reddish infillings of the dykes is partly related to dissolution processes inside the fissures that could have taken place during the transport of FeCl3 fluids together with silica gel. The fluids could have been derived from hydrothermal vents occurring along the extensional faults in the neighbouring Zliechov Basin. Rare Earth element (REE) signatures of the reddish infill (i.e. low values of total REE content, chondrite- and Post-Archean Australian Shale-normalized REE + Y patterns with negative Ce anomaly) and a high Y/Ho ratio suggest authigenic removal of REEs from the water column. This suggests that the fissures were open to the sea bottom and were in contact with sea water during their filling. [ABSTRACT FROM PUBLISHER]