Colonization and weathering of natural sulfide mineral assemblages by Thiobacillus ferrooxidans

Selected polished thin sections were used as model substrata to investigate the influence of geochemical and electrochemical factors on bacterial colonization and weathering of mixed sulfide minerals. Naturally occurring sulfide assemblages including combinations of pyrite, chalcopyrite, sphalerite, galena, and pyrrhotite were studied. The distribution of Thiobacillus ferrooxidans was mapped using viable negative and positive staining techniques in combination with scanning confocal laser microscopy. The minerals were characterized using scanning electron microscopy and microprobe analyses. Inter- and intra-granular electrode potentials were monitored using microelectrodes. Results of the experiments indicated that the T. ferrooxidans strain preferentially colonized sulfide minerals that were the most electrochemically active. For example, when pyrite, chalcopyrite, and pyrrhotite were in combination, bacteria preferentially colonized the pyrrhotite. In each case it was shown that the mineral colonized was also the anode in a galvanic cell and the site of preferential weathering. Microelectrode measurements confirmed the existence of intergranular potentials. In addition, trace and minor element composition appeared to influence the electrochemical property of the mineral, thus its colonization and weathering pattern. Information of this nature should facilitate prediction of the fate of sulfide minerals in natural and mining environments.