Geodes and thundereggs are formed by the percolation of mineral-rich warm water through porous deposits. Such deposits include pyroclastic flows formed by rhyolitic eruptions. Some of these minerals are picked up from the pyroclastic deposits. Jefferson County in Oregon is one of the well-known sites for the collection of geodes and thundereggs. I do not know of any specific research about the development of geodes and thundereggs . . . but there is a great deal of research about the transport of minerals via hot water through porous rock (because that is how many mineral ore deposits form). A starting place is Chapter 12 of a newly published book: Volcanoes, Crucibles of Change by Fisher and Heiken. Also, almost any book about gemstones will lead you into a discussion about the formation of mineral ores.
Sincerely,
John Dvorak
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