CELSIAN, (Ba,K)-FELDSPAR AND CYMRITE FROM SEDEX BARITE DEPOSITS OF ZAMORA, SPAIN

(Ba,K)-feldspars occur with cymrite as thin layers of massive fine-grained celsian interbedded with apatite, sulfides and barite, or diseminated in the thin-bedded barite-bearing siliceous and carbonate beds ranging in thickness from 0.15 to 0.40 m in three barite deposits in Zamora, Spain. The (Ba,K)-feldspars, which include celsian, hyalophane and K-feldspar, vary from 1.3 t o 96.4 mole % BaAl2Si2O8, but exhibit two discontinuities, at 8–24 and 40–92 mol.%. The associated cymrite shows 1.3–3 mol.% KAlSi3O8 in solid solution. Relict celsian has a similar K content, 1.2–2.8 mol.% KAlSi 3O8. The Ba-rich feldspars may have been deposited i) as an authigenic phase, or ii) by replacement of authigenic barite during a later diagenetic or metamorphic event. Cymrite and celsian were formed under low-grade dynamothermal metamorphism that affected Paleozoic sedex barite deposits of Zamora. Fluid-inclusion studies indicate 350 °–370°C and 1.5 kbars as conditions of equilibration. Those data, and the presence of vitrinite (350°C), indicate the upper limit of P–T reached. These conditions are close to the reaction curve for celsian + H 2O = cymrite. The stability field of cymrite confirms that this mineral can be stable in low-P – low-T metasedimentary environments. That reaction has geobarometric and tectonothermal importance.