FACTORS AFFECTING THE GEOCHEMISTRY OF A THICK, SUBBITUMINOUS COAL BEDIN THE POWDER RIVER BASIN - VOLCANIC, DETRITAL, AND PEAT-FORMING PROCESSES

The inorganic geochemistry and mineralogy of three cores from the Ande rson-Dietz 1 coal bed, a 15.2-m-thick subbituminous coal bed in the To ngue River Member (Paleocene) of the Fort Union Formation, were examin ed (1) to determine if the cores could be correlated by geochemical co mposition alone over a total distance of 2 km and (2) to identify the major factors that influenced the geochemistry of the coal bed. Chemic al data (46 elements on a coal-ash basis) for 81 coal samples and 4 ca rbonaceous rock samples, with most samples representing a 0.6-m-thick (2-ft) interval of core, were grouped into compositional clusters by m eans of cluster analysis. Seven major clusters were produced; two of t hese clusters can be used to correlate the coal bed throughout the stu dy area. Data from scanning electron and optical microscope analyses i ndicate that several factors influenced the geochemistry of the Anders on-Dietz 1 coal bed. The majority of mineral grains in the coal bed ar e interpreted to be detrital (water borne); evidence includes the pres ence of rounded to subrounded quartz grains having two-phase, aqueous fluid inclusions characteristic of hydrothermal or low-to-moderate gra de metamorphic quartz. These quartz grains are found throughout the co al bed but are most abundant in samples from the midpart of the bed, w hich was influenced by detrital input associated with the deposition o f the clastic rocks that form the split between the Anderson and Dietz 1 coal beds 900 m to the east of the study area. In addition to the d etrital minerals mentioned above, volcanic ash that was fluvially tran sported to the sites of peat deposition or possibly deposited as air-f all volcanic ash also affected the geochemistry of the coal bed. For e xample, crandallite(?), a mineral reported to form as an alteration pr oduct of volcanic ash, is found in seven samples from the coal bed. Th e presence of quartz grains containing silicate-melt inclusions in eig ht samples from the coal bed provides further support for a volcanic a sh component. Other factors that probably affected the geochemistry of the coal bed include (1) detrital input associated with the depositio n of the roof rocks of the coal bed, (2) peat-forming processes and pl ant material, and (3) epigenetic ground-water flow.