Rare earth element signature modifications induced by differential acid alteration of rocks in the Iberian Pyrite Belt

The efficient use of REE as tracers of weathering processes requires further data acquisition to establish robust connections between mechanisms, mineral assemblages and REE concentration patterns. Here, results are pro-vided corresponding to rocks from four locations in the Iberian Pyrite Belt that were altered with different in-tensity by sulfuric fluids. The acidic alteration was mild (pH 5-7) in Calan similar to as and El Villar, while strong (pH below 3 to 5) in Quebrantahuesos and Tharsis. The main REE reservoir before alteration were phyllosilicates (and plagioclase/feldspar in El Villar only). Mild alteration generated mainly kaolinite while illite and some plagioclase/feldspar were preserved. Mildly altered rocks approximately preserved the relative REE concentra-tions of their precursors, interpreted as indicating a low fluid/rock ratio environment where binding sites in minerals outnumbered those in fluids. Strong acid alteration produced alunite, jarosite, beudantite and goethite, with partial illite preservation. Where illite and/or goethite were major minerals, this alteration generated REE patterns enriched in light REE over heavy REE or did not modify the relative REE concentrations of the original rock. However, samples where REE concentrations were controlled by alunite or jarosite had LREE > > HREE. In alunite the maximum concentration corresponded to Nd and Sm. In jarosite concentrations decreased from La to Lu, except that a positive Eu anomaly was preserved from the precursor. These REE concentration patterns are interpreted as due to crystallographic factors, controlled by the best fit of REE with charge 3+ in the 12-fold coordination site of K in jarosite and alunite. This site is larger in jarosite and fits La best, whereas the smaller site in alunite best fits Nd and Sm. Goethite REE signature is variable, perhaps due to two formation routes, (1) silicates -goethite and (2) silicates -jarosite -goethite. Our interpretations of REE concentration in alunite, jarosite and goethite are novel and provide new routes to further develop REE as tracers of acid alter-ation processes.

Automated summary

To effectively use rare earth elements as tracers of weathering processes, more data acquisition is needed to establish robust connections between mechanisms, mineral assemblages, and REE concentration patterns. This study provides results from rocks in the Iberian Pyrite Belt that were altered with different intensities by sulfuric fluids. The alteration process produced variable REE patterns, with mild alteration preserving the relative REE concentrations of the precursor rocks, and strong acid alteration generating REE patterns enriched in light REE. The interpretation of REE concentration in alunite, jarosite, and goethite provides new routes for the development of REE as tracers of acid alteration processes.