Alunite and REE Rich APS Minerals Associated to the Hydrothermal Clay Deposits in the Barker Area, Tandilia, Argentina

Abstract

Chemical bulk analyses of intermediate alunite-natroalunite minerals indicate anomalous high contents of REE, Sr, Ba, Ca and P. Alunite occurs as veinlets or nodules cross-cutting hydrothermal clay deposits, containing pyrophyllite, kaolinite and sericite in the west Barker area (Dristas & Frisicale 1983 ⁽⁵⁾ , 1992 ⁽⁶⁾ ). Petrographic studies reveal a pseudocubic habit for the alunite. No significant zoning has been detected in this mineral by EPMA backscattered images. Additionally, EPMA and EDX studies 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-like inclusions correspond to APS (aluminium phosphate sulphate) minerals, such as florencite, formed at early stages before alunite crystallization from a solution with a high phosphorous/sulphur ratio. The crystal structure of APS minerals are suitable for REE, Sr and Ba substitution since metal cations are situated in large cavities formed by 6-membered rings of M (O, OH) ₆ octahedra. A later increase of solution sulfurization would favor the formation of alunite that constrained the REE, P, Sr, Fe, Ca and Ba fixation.