--> --> The mineralogy of an altered volcanic ash fall in a Siberian coal

AAPG Southwest Section Annual Convention

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The mineralogy of an altered volcanic ash fall in a Siberian coal

Abstract

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 pyrite, galena, sphalerite, ilmenite and REE minerals. 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, Mg- siderite has only been seen in a few Australian coals and the amount of Mg is consistently higher in concentration 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 kaolinite and Ca-rich magnesian siderite appear to have developed early in the diagenetic process, likely as a result of the dissolution of calcic feldspars, micas, and mafic minerals in the acidic peat environment, releasing aluminum, calcium, iron, and magnesium which reacted with carbon dioxide from the decomposing plant material. Sulfide minerals appear to have developed later in the diagenetic process as cleat fillings and intergrowths. This investigative approach can also be used to evaluate the contribution of volcanic ash to potentially valuable trace elements in coal deposits.