Black Shale Geochemistry: Why it is trouble for the Coal Industry

Black shale, or “metalliferous shale” forms under reducing conditions (anaerobic) and commonly associated closely in the roof and floor of many coal seams. Such reducing environments could present in fresh, brackish, marine or even hypersaline waters. Sulfide minerals are common in black shales. The reducing conditions form from lack of oxygen usually caused by bacteria. Reducing conditions promotes the preservation of large amounts of metals and Rare Earth Elements (REE). Black shales form when the organic matter present in the system consumes all of the oxygen. Some of the black shale has been identified as host rock for syngenetic metal deposits.

Definition: Black shales are defined as fine grained (silt to clay size) dark colored mud-rock containing >5 % organic carbon ( often preserved as Kerogen).

Coal mining commonly disturbs the overburden completely for surface mining purposes. Even for a deep coal mine, some disturbance of the overburden material is required during the construction of slope, shaft etc. The coal permit application in the USA requires proper geochemical characterization of the materials to be disturbed by the coal mining activities. Commonly acid base accounting (ABA), Sulfur forms and selenium are analyzed for geochemistry purposes. High acid forming zones and total selenium > 1 mg/Kg in the rock is identified and marked as potential problem zones. Such materials are specially handled during the mining.

Black shales are typically enriched in organic matter (up to 6% in some formations). Sulfur content in the black shale could be as high as 16%. (Reference http://www.ias.ac.in/jessci/feb06/vin-3)

In most cases, black shales are known culprits for selenium. Black shales could have metal enrichment factor as high as 50 times for Ag, 10 for Mo.  Black shales are commonly enriched in:

  • Arsenic (As)
  • Copper (Co)
  • Chromium (Cr)
  • Molybdenum (Mo)
  • Nickel (Ni)
  • Uranium (U)
  • Selenium (Se)

These metals however do not form minerals of their own and cannot be identified by naked eye or even under the microscope using regular light microscopy. They are usually microscopically distributed among the organic matter in the black shale. Sometimes they substitute for other metals and present in trace amount in other mineral phases.

Black sea is a modern day example when black shale is forming. Black sea is shut off from oceanic currents and reducing condition exists below 150 meter death.

In short, black shale is a problem for the coal industry as they can leach high concentration of various metals (Including RCRA metals) in the surface and groundwater if the condition becomes acid producing. It is often recommended to perform leach tests especially on the black shale samples to evaluate the rate and fate of various elements present in the black shale.

Geochemical condition must be properly evaluated before mining begins. Please contact Ankan Basu, P.G (Geochemist/Hydrogeologist) [email: admin@coalgeology.com] with 6 years of experience in the coal industry for any geochemical investigations.

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