PETROGRAPHY AND GEOCHEMISTRY OF SELECTED LIGNITE BEDS IN THE GIBBONS CREEK MINE (MANNING FORMATION, JACKSON GROUP, PALEOCENE) OF EAST-CENTRAL TEXAS

This study examined the petrographic and geochemical characteristics o f two lignite beds (3500 and 4500 beds, Manning Formation, Jackson Gro up, Eocene) that are mined at the Gibbons Creek mine in east-central T exas. The purpose of the study was to identify the relations among sam ple ash yield, coal petrography, and trace-element concentrations in l ignite and adjoining rock layers of the Gibbons Creek mine. Particular interest was given to the distribution of 12 environmentally sensitiv e trace elements (As, Be, Cd, Cr, Co, Hg, Mn, Ni, Pb, Sb, Se, and U) t hat have been identified as potentially hazardous air pollutants (HAPs ) in the United States Clean Air Act Amendments of 1990. Eleven lignit e, floor, and rock parting samples were collected from incremental cha nnel samples of the 3500 and 4500 beds that were exposed in a highwall of pit A3 at the Gibbons Creek mine. Short proximate and ultimate and forms of sulfur analyses were performed on all lignite samples, and l ignite and rock samples were analyzed for 60 major, minor and trace el ements. Representative splits of all lignite samples were ground and c ast into pellets, and polished for petrographic analyses in blue-light fluorescence and reflected white light to determine liptinite, inerti nite, and huminite maceral group percentages. The following observatio ns summarize our results and conclusions about the geochemistry, petro graphy, and sedimentology of the 3500 and 4500 beds of the Gibbons Cre ek lignite deposit: (1) Weighted average dry (db) ash yield for the tw o beds is 29.7%, average total sulfur content is 2.6%, and average cal orific value is 7832 Btu (18.22 MJ/kg). Ash yields are greatest in the lower bench (59.33% db) of the 3500 bed and in the upper bench of the 4500 bed (74.61% db). (2) For lignite samples (on a whole-coal basis) , the distributions of two of the HAPs (Pb and Sb) are positively rela ted to ash yield, probably indicating an inorganic affinity for these elements, By using cluster analysis we found that Be and Cd were poorl y associated with ash yield, indicating a possible organic affinity, a nd that Ni, Sc, Hg, U, and Pb cluster with most of the rare-earth elem ents. (3) The dominance of the crypto-eugelinite maceral subgroup over the crypto-humotelinite subgroup suggests that all Gibbons Creek lign ites were subjected to peat-forming conditions (either biogenic or che mical) conducive to the degradation of wood cellular material into mat rix gels, or that original plant material was not very woody and was p rone to formation of matrix gels. The latter idea is supported by poll en studies of Gibbons Creek lignite beds; results indicate that the pe at was derived in part from marsh plants law in wood tissue. (4) The o ccurrence of siliceous sponge spicules In the lower benches of the 350 0 bed suggests the original peat in this part of the bed was deposited in standing, fresh water. (5) The petrographic data indicate that the upper sample interval af the 3500 bed contains more inertinite (3%) t han the other samples studied. Increases in inertinite content in the upper part of the 3500 bed may have been associated with alteration of the peat by acids derived from the volcanic ash or could have been ca used by fire, oxidation and drying, or biologic alteration of the peat in the paleo-mire. (C) 1997 Elsevier Science B.V.