Distribución de tierras raras en soluciones y coloides de un sistema natural de aguas ácidas (Arroyo del Val, Zaragoza)

Abstract

Acidic waters of the «Arroyo del Val» stream are naturally generated by oxidation of sulfide minerals dispersed in the sedimentary materials of the drainage basin. Mixing of this acidic stream with neutral-alkaline tributaries yields spectacular iron and aluminium rich flocculants that precipitate at the confluences. Previous analysis of rare earth element (REE) concentrations of sorne 0.1 I-tm-filtered water samples and colloids are shown. The shale-normalized REE patterns of acidic waters show significant HREE enrichments in terms of Lu_n/La_n (≈ 2) and they are tipically enriched in the intermediate REEs (IREEs, Eu-Gd-Tb). So, in detail, these patterns are characterized by HREE enrichments between La and Gd and HREE depletions between Gd and Lu. The REE contents of two colloid samples are enriched by 103_104 relative to acidic waterso Different REE patterns were found in these colloids, sampled at differents distances from head of Arroyo del Val stream. Colloid sampled between the acidic waters analysed (CV-l) shows a very similar REE pattern to those of the waters. The REE composition normalized to shale of second colloid sample (CV-2), collected down stream, has a flatter profile although it is still slightly convexo Significant REE removal (about 50 %) caused by colloid coagulation is observed at the confluence and mixing point of the Arroyo del Val acidic waters with the first alkaline tributary. At this point the CV-l colloid sample shows HREE enrichment relative to acidic waters.