Polymetallic interactions of Zn-Pb-Cu in blue/green-colored speleothems from Malaval Cave (France)

Abstract

Speleothems rarely exhibit stunning colors such as red, yellow, green, or blue. The colorations are often linked to elevated heavy metal ion concentration in the drip water and thus to a metal source/pollution in the catchment area. Here the blue-green speleothems coloration in Malaval Cave (Lozère, France) is characterized by a wide panel of optical, mineralogical and geochemical techniques. These techniques were applied on several small blue or white stalactites and a larger greenish stalactite. The speleothems are mostly composed of aragonite and contain variable amounts of Zn, Cu and Pb, which cause the colorations. Zn and Cu are mostly present in substitution in the aragonite and Cu²⁺ is the main cause of the blue coloration. Zn is also found in small amorphous gel particles, containing minor amounts of Mg, Cu and Si. These phases are responsible for microscopical scale variations in the blue coloration. Pb is present as Pb²⁺ ions in substitution within the aragonite, creating a saturated blue-to-greenish coloration. This coloration may depend on the Pb/Zn ratio due to metallic interaction. Pb, Zn and Cu ratios indicate that Pb likely deposited from distinct fluids and at a different timing than Cu and Zn. All three metals likely originate from the leaching of PbZn ores in the Jurassic formations surrounding the cave.