PETROGRAPHY AND GEOCHEMISTRY OF THE SAN-MIGUEL LIGNITE, JACKSON GROUP(EOCENE), SOUTH TEXAS

The San Miguel lignite deposit (late Eocene, lower Jackson Group) of s outh Texas consists of four or more thin (generally < 1 m thick) ligni te benches that are separated by claystone and mudstone partings. The partings are composed of altered volcanic air-fall ash that has been r eworked by tidal or channel processes associated with a back-barrier d epositional environment. The purpose of this study is to examine the r elationship between the ash yield and the petrographic and geochemical characteristics of the San Miguel lignite as mined. Particular attent ion is given to 12 of the environmentally sensitive trace elements (As , Be, Cd, Cr, Co, Hg, Mn, Ni, Pb, Sb, Se, and U) that have been identi fied as possible hazardous air pollutants (HAPs) by the United States Clean Air Act Amendments of 1990. A total of 29 rock and lignite sampl es were collected and characterized by geochemical and petrographic me thods. The major conclusions of the study are as follows: (1) The dist ribution of Mn is inversely related to the ash yield of the lignite sa mples. This indicates an organic affinity, or an association with fine ly disseminated minerals in the lignite that contain this element. (2) On a whole-coal basis, the concentration of the HAPs' element Pb is p ositively related to ash yield in lignite samples. This indicates an i norganic affinity for Pb. (3) Average whole-coal concentrations of As, Be, Sb, and U in the San Miguel samples are greater than published av erages for these elements in other U.S. lignites. (4) The upper and lo wer lignite benches of the San Miguel deposit are both ash- and algal- rich, indicating that these intervals were probably deposited in wette r conditions than those in which the middle intervals formed. (5) The dominance of the eugelinite maceral subgroup over the huminite subgrou p indicates that the San Miguel lignites were subjected to peat-formin g conditions (either biogenic or chemical) that enabled degradation of wood cellular material into matrix gels, or that the plants that form ed these lignite benches were less woody and more prone to formation o f matrix gels. (6) An inertinite-rich layer (top of the B bed) might h ave formed from widespread oxidation of the San Miguel peat as a resul t of a volcanic ash fall which was subsequently reworked.