The mine tailing (MT) materials are the main REE source, with peak concentrations of 300 mg/kg ¦²REE. PAAS-normalized patterns of MT are characterized by light REE (LREE) enrichment with respect to heavy REE (HREE). The waters flowing out of the MT heaps are near-neutral (6.2 to 7.0 pH), with Zn-sulfate dominant composition, and a mean concentration of 53 ¦Ìg/l ¦²REE. Drainages from MT and an adit flow in the Naracauli stream headwaters (mean 1.4 ¦Ìg/l ¦²REE). Concentrations of REE in the Naracauli stream decrease dramatically about 400 m downstream of source. The REE decrease matches Fe and Zn decrease in waters, probably because the REE are sorbed on freshly precipitated solid phases.
Sorption processes, and/or co-precipitation with secondary phases, appear to control the REE geochemistry in the studied waters under near-neutral conditions. Also, despite the non-acidic environment, it is worth to observe that small changes in pH seem to affect the mobility of REE at Naracauli.
The PAAS-normalized REE patterns in the waters generally reflect the PAAS-normalized REE patterns in the solid materials with which the water interacts, either MT or secondary phases such as Fe-hydroxide and bio-hydrozincite. As it is expected in waters under oxidizing conditions, patterns with negative Ce anomalies are developed due to the poor solubility of Ce+ 4 species, and its consequent scavenging in solid phases. However, there is an exception: Ce shows a poor affinity for the bio-hydrozincite mineral, probably reflecting the role of bacteria in the precipitation of the Naracauli hydrozincite.