Authigenic mineral precipitates in acidic pit lakes and incubation columns related to biogeochemical factors

Abstract

Acidic Pit Lakes (APLs) formed in abandoned metal mines represent excellent natural laboratories for the study of authigenic mineral formation resulting from diverse biogeochemical processes in the water column and bottom sediments. Geochemical conditions include extreme to moderate acid pH (from ~2 to 4.5) in waters, oxidative to highly reductive Eh (800 to -200 mV), permanent isolation of deep anoxic waters (up to 130 meters deep) and its bottom sediments, as well as very high dis- solved sulfate and metals in water (e.g. Fe, Al, Zn, Cu, As, Pb, Cd) and the presence of sulfate- and Fe-reducing/oxidizing microorganisms. After a decade of monitoring waters and sediments of APLs in abandoned metal sulfide mines of the Iberian Pyrite Belt (Huelva, Spain) and La Unión (Murcia), together with in-lab simulations using incubation columns constructed with indigenous microorganisms and original waters and bottom sediments, a wide variety of authigenic precipitates were found. Fe-oxy-hidroxysulfates (schwertmannite, K- and H3O +-jarosite) are the main phases in acid oxic waters (pH 2.0-3.5), while hydrobasaluminite prevails in less acidic anoxic settings (pH>4.0); Zn, Cu, As, and Fe sulfides (e.g. wurtzite, covellite, chalcocite..) appear related to anoxic sulfidic layers formed as a result of sulfate reducing bacteria activity. The morphology and crystallinity of those precipitates greatly varies, from amorphous to nano- or microcrystalline, especially in sulfides and Al-precipitates, some of which incorporate other absorbed or substituting cations (Cd, Cu, Se...). The relation between mineral neoformation, biogeochemical conditions and mineral aging is explored using mineralogical techniques (selective separation, XRD, SEM, TEM).