Alunites and REE rich APS minerals associated to hydrothermal clay deposits in the Barker area, Tandilia, Argentina

Abstract

Chemical bulk analyses of intermediate alunite-natroalunite minerals indicate anomalous high contentsof REE, Sr, Ba, Ca and P. Alunite occurs as veinlets or nodules cross-cutting hydrothermal claydeposits, containing pyrophyllite, kaolinite and sericite in the west Barker area (Dristas & Frisicale1983(5), 1992(6)). Petrographic studies reveal a pseudocubic habit for the alunite. No significant zoninghas been detected in this mineral by EPMA backscattered images. Additionally, EPMA and EDXstudies indicate that the tabular-like inclusions in cores of alunite contain significant amounts of P, Ba,Sr, REE, Ca and Fe. Alunite shows epitaxial growth on the inclusion mineral. These tabular-likeinclusions correspond to APS (aluminium phosphate sulphate) minerals, such as florencite, formed atearly stages before alunite crystallization from a solution with a high phosphorous/sulphur ratio. Thecrystal structure of APS minerals are suitable for REE, Sr and Ba substitution since metal cations aresituated in large cavities formed by 6-membered rings of M(O, OH)6 octahedra. A later increase ofsolution sulfurization would favor the formation of alunite that constrained the REE, P, Sr, Fe, Ca andBa fixation.