Surface saline deposits and their substrates in a polluted arid valley (Murcia, Spain)

Abstract

This study reports the mineralogical and geochemical variations of surface saline deposits and their substrates along an arid valley with an ephemeral river system (Spanish: rambla) affected by pollution from the now closed Cabezo de San Cristobal mine. This mine, which is close to Mazarron, Spain, once produced Pb, Zn, Ag and Fe ores. Saline efflorescences and crusts were found on piles of tailings and on the mudflats surrounding acidic ponds associated with the mine. These were composed of jarosite (10–60%), haematite (up to 20%) and gypsum (5–20%), and the kieserite (20–40%) and halotrichite (5–40%) mineral groups. In the rambla deposits downstream of the mine, the mineralogy of the saline efflorescences was similar to that recorded upstream: gypsum up to 20%, thenardite 5–20%, halite 20–40% and bloedite 20–40%. However, the substrate was very different, being mainly composed of jarosite (40–60%), haematite (5–20%) and goethite (5–20%); these minerals were never found upstream of the mining area, where carbonates predominated. Downstream of the mine, the rambla deposits contained large quantities of Fe (158,251 ppm), Pb (2,720 ppm), Zn (3,064 ppm), Sb (240 ppm) and As (1,092 ppm), along with Co (29 ppm), Ce (323 ppm), V (112 ppm), Th (23 ppm) and Nb (19 ppm) (all figures are maxima). Upstream, these contents were insignificant. Neither were large quantities found in the actual stream water (only Cu, V and Zn were detected, and then at concentrations of <0.05 ppm). Leaching and washing out from the tailings and other mine wastes substantially altered the substrates, but not the composition of the saline efflorescences along the arid valley.