Download bacteriogenic deposits

BACTERIOGENIC DEPOSITS. (prepared by Uzochukwu Chidinma)
These are mineral deposits that are accumulated as a result of bacterial processes. The metabolic activities of
chemosynthetic bacteria lead to several chemical and biochemical reactions which in turn enhance the
concentration of certain valuable minerals in economic quantities. Typical elements that can be concentrated
by this process include Fe, Mn, S, As,P and these occur as sulphides, oxides and carbonates, forming minerals
like goethite, pyrite, siderite, native sulphure, arsenic sulphide, hydroxyl apatite( formed by bacterial action on
bones of vertebrates). Thus formation of bacteriogenic substances can be said to be a biologically induced
mineralization process.
MODE OF FORMATION.
These minerals can be classified into 2 broad groups based on their mode of formation and environment of
deposition-viz- those formed from oxidation process and those formed by reduction process.
BACTERIOGENIC OXIDATION MINERALS.
These are minerals formed when bacteria oxidizes organic matter in the soil. This occurs in transition zones
where deoxygenated water rich in organic matter from anaerobic environment flows into an aerobic
environment e.g. during the mixing of meteoric water, seawater and underground water as a result of tide
dynamics. Oxidation involves the increase in oxidation number of the element of interest by addition of O2,
removal of H 2 or by removal of electrons from it. Classic example of this type of mineral is Bacteriogenic Iron
Oxides (BIOS) formed by iron oxidizing bacteria (FeOB).
FeOB derive energy and nutrition they need to live by oxidizing ferrous iron, forming ferric oxide. They live in
environment where O2 supply is at least 0.3ppm in order to carry out this oxidation. E.g. Thiobacillus
ferrooxidans, Leptospirrilium ferrooxidans, Gallionella ferruginea, (produces bifurcating stalks of ferric
hydroxide), Leptothrix ochracea (forms sheath which are impregnated by ferric oxide and this accumulates as
the organism dies), Sphaerotilus natans. After this oxidation, poorly-ordered iron oxides precipitate inside and
outside the bacteria, especially in or on the cell wall and sheaths of bacteria. These bacteria accelerate the
iron oxidation rate and the accretion of iron minerals on bacterial sheaths or stalks, allowing the preservation
of the morphological characteristics of the bacteria. As time goes by, these amorphous BIOS can further
transform to more crystalline goethite or heamatite and therefore be preserved permanently, representing
the imprints of bacterial activity during the formation of iron ores. BIOs are usually associated with
hydrothermal vents around submarine volcanoes and mid ocean ridges.
Under initial reducing conditions, microbes with specialized enzymes reduce insoluble ferric oxide to soluble
ferrous hydroxide and use the oxygen released in the reaction to oxidize the remaining organic matter. The
acidic nature of the soil also causes ferrous iron to be leached from underlying rocks. During this time, the Ph
of the environment is low (acidic) and this prevents spontaneous formation of ferric oxides.
Fe2O3 + H2O = 2Fe (OH)2 + O2
When this deoxygenated water meets a source of oxygen, iron bacteria converts the soluble ferrous iron back
into ferric iron and the above reaction is reversed. The Ph at this time is neutral to slightly alkaline. Some
studies have indicated that iron-oxidizing bacteria accounts for at least 50% and up to 90% of Fe2+ oxidation in
neutral Ph waters, particularly under diffusion limiting conditions at the aerobic-anaerobic interface. High
organic content in swamp waters (due to decaying vegetation ) is the root cause of Fe bacteria. Similar
reaction occurs in manganese but less common because Fe (5.4%) is more abundant than Mn (0.1%) in the
earth crust.
SULPHUR OXIDIZING BACTERIA.
In the same way, S compounds can be formed by bacterial oxidation. SOBs are gram negative bacteria in the
species of Thiobacillus, Thiomicrospira, Thiosphaera. In this process, H2S released from decaying organic
matter is oxidized first to sulphur (biogenic native sulfur in sedimentary terrain is formed by this method),
then to sulphate. Thiobacillus Thioparus and T. Thioxidans are the two common sulphur oxidizing bacteria. The
process is as follows.
Organic sulfur compounds—H2S---S8---SO42-.
Mn oxidation also occurs and typical bacteria involved include cyanobacteria and Lepthotrix.
BACTERIOGENIC REDUCTION DEPOSITS.
These deposits are formed by the reducing action of anaerobic bacteria on organic matter in order to obtain
energy. The metal or element involved is reduced to a lower oxidation state through a series of redox
reactions. The most common of these bacteria are sulphate reducing bacteria. Sulphates are reduced to
sulphur and hydrogen sulphides. The hydrogen sulphide released is a strong reducing agent, so it attacks and
reacts with metallic ions in the environment to form metallic sulphides such as pyrite, galena, chalcocite,
sphalerite etc. Sulphate reduction process is a direct opposite reaction of sulphur oxidation. Environment of
sulphur reduction are acidic, waterlogged with fine grained sediments and high in organic content.
Sulfate reducing bacteria can also oxidize methane anaerobically to form carbonate.
CH4 + SO42- = HCO3- + HS- + H2O.
This methane is formed by another group of chemotropic bacteria called methanogens produced from
decaying organic matter in the sea bed. The carbonate ion formed then reacts with highly electropositive
metals to form carbonates. Examples of sulphate reducing bacteria are Desulfotomaculum, Desulfosporosinus.
Spirillium disulfuricans. Reduction deposits are more widespread because reduction bacteria are more
abundant in nature and they have been found to survive in extreme environments such as the bottom of
ocean trenches, where there is high pressure and no oxygen. This process is also believed to play a major role
in the formation of banded iron deposits (BIF).
The role of reducing bacteria in formation of hydrocarbons is also very important as hydrocarbons are formed
by anaerobic decomposition of organic matter e.g. methane is formed by methanogens.
FACTORS INFLUENCING THE FORMATION OF BACTERIOGENIC DEPOSITS.
The factors that affect the formation of this type of deposit are Ph, Eh, Grain size of sediments, concentration
of dissolved oxygen, time, physical and chemical nature of the elements and compounds involved
(compounds that are easily hydrolysed in water are more strongly adsorbed to the surface of bacteria cells).
DISTINCTION BETWEEN BACTERIOGENIC DEPOSITS AND THOSE FROM OTHER SOURCES.
The major way of recognizing these deposits and distinguishing them from deposits formed by other processes
is by microtextural and isotopic analysis. Microscopic analysis of the rock is carried out to determine its
microtexture. Biogenic deposits usually have a large concentration of bacteria filaments and cocci which
branch extensively through the rock and gives the rock a fibrous texture when seen under the microscope. The
filaments can occur as straight, twisted, branching or dendritic in nature. Some of these bacteria filaments are
fragmented while others occur wholly. They usually have directed growth pattern parallel to sediment laminae
and they are made up of cylinders of Fe oxide. These filament morphologies can therefore be used as
biomarkers for bacteriogenic Fe oxide precipitation in rocks.
Isotopic analysis of sulphur is also used to distinguish these minerals. It has been found that there is large
variation in the spread of S34 in bacteriogenic sulphides which is not so for sulphides and sulphates from other
sources e.g. hydrothermal sulphides. This is said to be due to large scale bacteriogenic fractionation of sulphur
and the difference in migration rates between the heavy and light isotope (Jensen and Nakai, 2010).
Bacteriogenic sulphides are more enriched in S32 than the heavy isotope. It is believed that sulphur isotopes in
sulphate are reduced unidirectionally to sulphide and that the lighter isotope reacts 2% faster than the heavier
one.
Case study from the work of Jensen and Nakai (2010).
Hydrothermal dep.
Cu deposits of butte Montana
Newfoundland deposits in montana
Marysvale, Utah
Tintinc Distric in Utah.
Sed. Minerals
Dorchester Mine in Utah … Cu red Beds
Jackpile Mine in New Mexico…..Uranium sandstone deposits.
ECONOMIC IMPORTANCE
Bacteriogenic deposits are source of various useful metals such as Fe, Mn, Pb, As, S. examples of bacteriogenic
deposits are extensive bog iron found in Northern and Northeastern Europe, Petroleum in several parts of the
world, BIOs accumulation in Hard Rock Lab, Sweden.