POSSIBLE GENESIS OF AN UNUSUAL MINERAL IN A SIBERIAN COAL

Volcanic ash is regularly found in coal and is particularly common in the Minusinsk coal basin in southern Siberia, Russia. Ash deposits in coal are usually observed in the form of thinly bedded, kaolinite-rich layers called tonsteins. The coal we studied by scanning electron microscopy with an energy dispersive detector contained many of the minerals typically found in tonsteins: kaolinite groundmass, volcanogenic phenocrysts such as K-feldspar, quartz, apatite, and zircon, along with secondary minerals such as galena, sphalerite, and REE minerals. However, in addition to these commonly observed minerals, the groundmass contains a rare calcium-bearing magnesian siderite in roughly equal proportion to the kaolinite. Although siderite is a common constituent in coal, Ca-Mg siderite has only been reported in a few Australian coals and never at these relatively high proportions. The ratio of Ca, Mg, and Fe in the siderite groundmass varies only slightly throughout the sample, but the amount of Mg is consistently higher concentrations than observed in other siderites in coal. The relative levels of K-feldspar, apatite, and quartz are consistent with a parent magma of felsic to intermediate composition. The Ca-rich magnesian siderite appears to have developed late in the diagenetic process, likely as a result of the dissolution of calcic feldspars, micas, and mafic minerals in the acidic peat environment, releasing calcium, iron, and magnesium which reacted with carbon dioxide from the decomposing plants. This investigative approach can also be used to evaluate the contribution of volcanic ash to potentially valuable trace elements in coal deposits.