Geologic processes affecting the quality of the Upper Freeport coal bed, west-central Pennsylvania

Abstract

The origins of organic and inorganic constituents of a coal bed cannot be determined from its bulk composition. Minerals such as quartz, calcite, and pyrite, as well as macerals such as the vitrinite group, can originate from a variety of processes that result from different depositional conditions of peat formation. Subunits (coal-bed facies) of the Upper Freeport coal bed have been mapped over a 120-sq mi area in west-central Pennsylvania. These facies were characterized by a variety of chemical, petrographic, and mineralogical analyses. Facies mapping was based on field description of the bed at mine faces and description of X-ray radiographs of core. Where thickest, the coal bed averages 83 inches and contains eight to ten facies; in other areas, the bed averages about 48 inches and is generally composed of four coal-bed facies. Each coal-bed facies is characterized by distinctive associations and concentrations of macerals and minerals. Area! isopleth patterns of ash and sulfur data for each coal-bed facies indicate different physical and biochemical conditions during peat formation that affected the concentration and distribution of premacerals and minerals. The different conditions inferred were (1) low to absent sediment influx and acid pH conditions that preserved humic tissues and leached mineral matter, (2) low sediment influx and near-neutral pH conditions that were conducive to degradation of plant tissues and formation of calcite and pyrite, and (3) moderate sediment influx and nutrient supply which produced both nonbanded coal and banded coal containing high-ash attrital layers alternating with thick vitrain under near-neutral pH conditions. The macerals, minerals, and elements can be statistically separated into two major groups which are interpreted to reflect genetic relationships that resulted from the botanical and geochemical conditions during paleopeat formation. In Group I, most minerals and non-organic elements are considered to be derived dominantly from plants. In those parts of the bed most influenced by detrital influxes, a mixed source existed for elements and minerals. Where detrital influxes dominated, the coal is poor quality (high ash and thin) and commonly not mined. For example, quartz in the Upper Freeport coal is interpreted to be derived from both authigenic (derived from phytoliths) and detrital sources. Group II constituents were probably derived from biochemical processes. Sporinite was probably concentrated by degradational processes and micrinite may represent a product of the degradation. In particular, pyrite formed at different times and under different conditions as indicated by trace element contents, petrographic associations, and isotopic data. Calcite also formed under different conditions of bacterial activity as indicated by isotope data; some calcite probably formed from fermentation reactions and other calcite appears to have formed from bacterial sulfate reduction or abiotic reactions. A geochemical model proposed for the origin of coal-bed facies of the Upper Freeport coal bed is consistent with interpretations of modern peat formation resulting from the interaction of climate, plant types, rainfall, ground water geochemistry, nutrient supply, and sedimentation. This model provides a means to evaluate and predict more precisely the variability of a coal resource ! s quality.