Environmental mineralogy and geochemistry of waste dumps at the Pb(Zn)-Ag Bottino mine, Apuane Alps, Italy

Abstract

The Bottino mine (Apuane Alps, Tuscany) had been exploited for silver and lead since at least Renaissance times. Detailed field work has led to the recognition of several mine dumps which differ in size, age, and types (rock waste dumps; jigging and handpicking wastes). In the dumps, the primary sulfides are sphalerite, galena, and pyrite ± variable amounts of chalcopyrite, pyrrhotite, arsenopyrite, and a wealth of other sulfides and sulfosalts. These mine- rals are associated with gangue quartz, white micas, chlorite, and carbonates, mostly of the siderite-magnesite and dolomite-ankerite series, whereas calcite is scarce. Supergene alteration led to the development of secondary mine- rals such as goethite, lepidocrocite, pyrolusite, cerussite, and clay minerals (kaolinite, montmorillonite, and vermi- culite). Two main types of supergene effects have been observed: (a) development in situ of pseudomorphic replacement of primary minerals, and (b) leaching and dissolution. Acid generation and metal release are mainly pro- duced by the replacement of pyrrhotite by iron hydroxides, the partial dissolution of siderite-magnesite carbonates, and the extensive dissolution/replacement of galena and sphalerite. Given the scarcity of calcite in the primary assem- blage, the most effective attenuators of acidity seem to have been dolomite-ankerite and siderite-magnesite. The mainly unpolluted character of the Bottino waters may be explained by a number of concurring factors, including the moderate volume of wastes, their overall coarse grain size, the low abundance of acid-producing phases (pyrite and pyrrhotite) relative to acid-consuming phases such as dolomite-ankerite, and the steep topography.