Efflorescences associated with acid-saline drainage from a dump of copper-leached ores

This study reveals the mineralogical complexity of efflorescences in heap-leaching residues dump of the copper mining industry, focusing on their relationship with saline acid drainages, particularly during rainfall events that cause rapid dissolution of the efflorescences. Using Raman spectroscopy and SEM-EDS techniques, various secondary minerals of hydrated sulphate and chloride (Cu, Mg, Na, Fe, and Al) were identified, including aubertite, magnesium-aubertite, tamarugite, natrojarosite, pickeringite, gypsum, alunogen, sanderite, and kieserite. These minerals suggest an intrinsic connection with the chemical composition of the dump drainage, as reflected in the results of the mineral wash water, indicating acidic (pH 3.8) and saline drainage (total dissolved solids, TDS > 10,000 mg/L). A distinctive colour sequence was observed in the efflorescences, ranging from bluish and greenish crystals of aubertite-magnesium-aubertite to yellow botryoidal layers of natrojarosite and globular white layers of tamarugite and acicular pickeringite. The saline crusts on the slopes of the dumpsites showed extensive areas of white and brown gypsum, with crystals of aubertite and magnesium-aubertite, covered by simple white sulphate salts of Al and Mg. Furthermore, this study presents a generalized spectral pattern for the aubertite—magnesium-aubertite series, highlighting key vibration modes of sulphate, as well as the potential influence of interstitial chloride ions on the vibration modes of water molecules. This study provides valuable information on mineral variability and environmental conditions in the dump sites, offering a perspective for understanding the formation processes of saline acid drainages in copper mining.