Reduction of Cu2+ in mine waters by hydrolysis of ferrous sheet silicates
Markl, Gregor; Bucher, Kurt
European Journal of Mineralogy Volume 9 Number 6 (1997), p. 1227 - 1236
published: Dec 2, 1997
manuscript accepted: Jun 11, 1997
manuscript received: Mar 11, 1997
ArtNo. ESP147050906014, Price: 29.00 €
Abstract Zoned sequences of secondary copper minerals (malachite, cuprite, native copper) forming at the present day are found in thin smectite-filled fractures and faults of basement gneisses in an abandoned fluorite mine (Baumhalde mine near Todtnau, Schwarzwald, Germany). The gneisses host fluorite-quartz-(barite) veins with chalcopyrite and galena as principal ores. The copper mineral sequences are formed by the reaction of descending Cu2+-bearing meteoric waters with smectite fillings in thin fractures. Initially, the waters are undersaturated with respect to copper minerals. The key process in the formation of the zoned copper mineral sequences is the hydrolysis of smectite in the fractures. This process increases pH and decreases po2 by release of Fe2+ ions into the water. In an early stage of reaction, the pH-increase resulting from smectite hydrolysis, together with a sufficiently high pco,, produces the malachite (Cu2(OH)2CO3) zone. Subsequently, copper is reduced and precipitated from the waters in two steps, first as cuprite (Cu2O) and then as native copper. Cuprite and native copper formation is also related to the partial hydrolysis of smectite, which decreases the Eh of the water down allowing cuprite and finally copper saturation. If the zoned sequence of reaction products that forms in the veins is exposed to air and fresh water in the mine, previously formed cuprite and native copper become unstable with respect to malachite, and react to form malachite within a few weeks. The displacement velocity of zoned copper mineral sequences in thin smectite veins is of the order of 1 m in 50 years. Thus, the observed zonation of supergene copper mineral deposition is highly transient, with an estimated life span of the order of tens of years.