Geological studies conducted in northern Chile reveal that the enrichment of metal ores through the weathering of rocks, is a process mediated by bacteria
the weathering of rocks is a natural phenomenon which leads to the enrichment of various metal ores. The process was found to be mediated by bacteria, as research conducted in northern Chile revealed. According to the findings of the study, bacteria could be mediators in copper sulphide enrichment during the process of weathering.
Many of the world's major copper deposits were formed by supergene (high-grade ore) enrichment during weathering. Supergene chalcocite (Cu2s) enrichment during weathering is a very important process from the economic angle as it could lead to a several-fold increase in the copper content of sulphide deposits. The progressive replacement of hypogene (low-grade ore) sulphides, especially chalcopyrite (CuFes2) and pyrite by copper-rich sulphides of the chalcocite (Cu2s) group -- under reducing conditions -- could lead to enrichment at and beneath the groundwater table. Descending acidic solutions supplied the necessary copper by oxidative dissolution of cupriferous sulphides, above the groundwater table.
The above process was earlier believed to be abiotic in nature. But now samples of enriched sulphides from copper deposits like those in northern Chile, have suggested that the process of supergene enrichment like sulfide oxidation, is promoted by acidophilic bacteria. This came to be known from the results obtained by Scanning Electron Micrograph (sem) studies.
Observations made on representative samples of supergene enriched copper ores from rock-type deposits mined in northern Chile revealed the presence of massive, steely chalcocite group materials, a hallmark of mature enrichment zones. Only small amounts of unreplaced chalcopyrite and pyrite remnants were present in the chalcocite group minerals. The samples contained less than 50 per cent copper, in terms of weight. They represented the latest stage of the mid-tertiary enrichment process. Their cumulative nature had destroyed physical evidence of the earlier stages. To check for the presence of other sulphides, the researchers etched freshly broken pieces of the samples with 10 per cent hydrochloric acid for 30 minutes. The treatment readily dissolved chalcocite group minerals, leaving behind other sulphides.
sem examinations revealed an abundance of bacterioform bodies ranging in shapes from spherical to bean-like and in size from 0.03 to 0.2 micrometre (a micrometre is a millionth of a metre). The bacteria are fossilised dwarf forms of bacteria or nannobacteria. They occur in clusters as do others forms of bacteria. They flock towards food and multiply in geometric progression. Some faces of a single chalcocite crystal were found to consist of layers of mineral packed solidly with nannobacteria, while others had no nannobacteria. Other samples showed portions of massive chalcocite that appeared to be composed entirely of nannobacteria (whose density was around 1, 000 per four micrometre square), whereas some neighbouring samples had none. This suggests that these are different minerals of the chalcocite group.
Data from studies indicate that these bacteria are clustered closely along the replacement fronts between chalcopyrite and chalcocite and were scarce in the areas of chalcocite deposits which were more than two millimetre from the location of the hypogene sulphides. Nannobacteria appeared to have bored into the chalcopyrite and pyrite.
Bacteria, particularly those belonging to the genus Bacillus, concentrate a variety of metals on their anionic cell walls or occasionally within themselves. This process which is called biosorption, has many commercial applications like the treatment of acid drained from abandoned copper mines. Copper ions which can be supplied in various forms -- including sulphate solutions -- bind themselves to the cell walls in large quantities. Thus the most likely function of the bacteria in the enrichment process is the collection of copper ions. These bacteria are also capable of catalysing the electron transfer involved in chalcocite enrichment. Another important factor in the enrichment process is the microbial fixation of copper at active replacement fronts between hypogene sulphides and chalcocite. Finally, the most crucial factor of the process is the climatic or the hydrological conditions. Both these factors favoured the proliferation of bacteria in the studies conducted. In fact researchers attributed the cessation of the enrichment process, during the mid-Miocene period in northern Chile and southern Peru, to the arid conditions that prevailed during that period which may have been hostile to the proliferation of bacteria involved in the enrichment process.
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