Plumbogummite group minerals in Lower Devonian placoderm sandstones from Podłazie Hill, Holy Cross Mountains, Poland

Abstract

Samples of Lower Devonian vertebrate-bearing placoderm sandstones collected in a quarry at Podłazie Hill in the Holy Cross Mountains, central Poland, were found to contain numerous white and brownish aggregates of an unknown composition. Powder X-Ray Diffraction study has shown them to comprise plumbogummite group minerals (PGM). Gorceixite is most common, usually forming compact/porous aggregates. They are either found in voids within the quartz-rich zircon-, muscovite- and biotite-bearing matrix, or as a complete replacement after bone remnants. Goyazite aggregates are similar but rarer. Strontian crandallite is found as tiny zoned crystals closely associated with compact gorceixite (in the sandstone matrix) or as cores of fine-grained gorceixite aggregates (within the bones). All of the PGMs are enriched in Ce and La with two analyses marginally within the compositional field of florencite-(Ce). Ca enrichment, elevated F content and abundance of goyazite within the bone replacement suggest the primary bone apatite group as the source of these elements. Tuffites and claystones associated with the sandstones are probably the source of Ba and Sr, while Pb is possibly derived from local Devonian mineralisation.