The impact of the presence of acidophilic iron-oxidizing bacteria in south waste
rock dump has been investigated over a period of two years at la Mine Doyon.
During this period of time, samples were collected from various locations within
the dump. The waste rocks are made for a significant part of sericite schists and
their friability is such that pyrite grains are extensively exposed to chemical
weathering and microbiological oxidation.
The sampling of the south dump was done during the drilling of bore hole #1 by
removing drill cuttings over the whole height of the dump; water samples were
collected from the different bore holes (#1, #2, #3, #4 and #6) by pumping, and
microorganisms were “trapped” with especially designed devices in underground
water. The results showed that:
1- Thiobacillus ferrooxidans type bacteria are widely distributed within the dump,
in underground and seepage water, everywhere iron sulfides are exposed;
Thiabacillus thiooxidans and T. thioparus are also present wherever oxygen supply
is not limiting. Leptospirillum ferrooxidans, another iron-oxidizing bacterium, has
been isolated from a “mild” non-freezing area in the dump. Iron oxidizing activity
is particularly important in sections of the dump where bacterial growth
temperature is optimal, where pH lies between 2,0 and 3,5 and where porosity
permits aeration. Energy source (Fe2+, S2-), water (running and intersitial) and
carbon source (CO2) are abundant through the waste rocks. Fe2+ is continuously
supplied to bacterial metabolic reactions during chemical oxidation of pyrite by
Fe3+.
2- The fairly high temperature inside the dump does not prevent bacterial
colonization and activity, it provides a selective pressure for the thermophilic
microorganisms to develop and carry over the oxidizing activity of the mesophilic
bacteria.
3- We confirmed, for the first time in the field, that T. ferrooxidans, along with
some other unidentified strains, is able to oxidize sulfur and sulfides to sulfuric
acid in an anoxic environment under 30 m of rocks. This anaerobic biological
respiration uses ferric ions as electron acceptors.
