11
Downloaded by CORNELL UNIV on June 20, 2017 | http://pubs.acs.org Publication Date: September 10, 1984 | doi: 10.1021/bk-1984-0264.ch011
Geochemical Variation of Inorganic Constituents in a North Dakota Lignite F. R. KARNER, S. A. BENSON, HAROLD H. SCHOBERT, and R. G. ROALDSON Energy Research Center, University of North Dakota, Grand Forks, ND 58202 A study of the variation in the distribution of major, minor, and trace elements in lignite-bearing lithologic sequences is critical for both interpretation of deposi tional environments and utilization of the coal. Samples of lignite of the Kinneman Creek Bed, underclay and over burden from the Center Mine were analyzed by neutron acti vation, x-ray fluorescence, and x-ray diffraction techni ques. Major patterns of element distribution in the l i g nite include (1) concentration in the margins of the seam (Al, Si, S, Sc, Fe, Co, Ni, Zn, As, Zr, Ag, Ba, Ce, Sm, Eu, Yb, Th, and U), (2) concentration in the lower part of the seam (Cl, Κ, V, Cr, Cu, Ge, Se, Ru, Sb, Cs, and La), and (3) even distribution throughout the seam (Na, Mg, Ca, Sr, and Mn). Other elements exhibit indefinite or irregular patterns (P, Rb, Y, Cd). Elemental variation patterns in the lignite may be related to admixture of detrital material during deposition, secondary precipitation of minerals, and ionic exchange processes involving adsorption of ions from circulating groundwater by both organic and inorganic components. This chapter summarizes information on the distribution of major, minor, and trace elements in a lithologie sequence of sedimentary materials in a major lignite-producing portion of the Fort Union region of the northern Great Plains. The Kinneman Creek Bed and associated sediments at the Center Mine in the Knife River Basin exhibit several types of spatial patterns of enrichment and depletion of chemical elements. Previous studies of lignite-related geochemi cal variation in this region are summarized in the literature (1,2). The principal emphasis of this chapter is concerned with the patterns of elemental variations in lignite. These patterns are related to factors of accumulation of both organic and inorganic components during deposition and aqueous precipitation, dissolution, and ionexchange processes after deposition. Elemental variations in over burden and underclay are dependent upon relative abundances of quartz- and feldspar-rich s i l t fractions versus clay mineral-rich 0097-6156/ 84/ 0264-0175S06.00/ 0 © 1984 American Chemical Society Schobert; The Chemistry of Low-Rank Coals ACS Symposium Series; American Chemical Society: Washington, DC, 1984.
176
THE CHEMISTRY OF LOW-RANK COALS
fractions of the sediment, clay mineral and carbonate mineral variation, devitrification of volcanic ash, and distribution of authigenic cements and organic matter. The data and results presented in this chapter are part of continuing work on the inorganic constituents of low-rank coals and the factors that affect their distribution.
Downloaded by CORNELL UNIV on June 20, 2017 | http://pubs.acs.org Publication Date: September 10, 1984 | doi: 10.1021/bk-1984-0264.ch011
Geologic Relationships The Center mine, Oliver County, North Dakota, is in the Fort Union coal region, which contains one of the largest reserves of lignite in the world. The Fort Union region encompasses portions of the northcentral United States and south-central Canada, including both subbituminous coals of the northern Powder River Basin and lignites of the Williston Basin (2). Important lignite beds of the Fort Union Region occur within the Ludlow, Slope, Bullion Creek, and Sentinel Butte Formations of the Paleocene Fort Union Group. The Sentinel Butte Formation has been described as a "lignitebearing, nonmarine, Paleocene unit whose outcrops are somber gray and brown" (3). Rock types include sandstone, siltstone, claystone, lignite, and limestone. The lignite beds typically vary from less than one meter to 3-5 m in thickness (4). The Oliver-Mercer County district is located in eastern Mercer and northeastern Oliver Counties with the major lignite seam, the Hagel Bed, ranging in thickness from 3-4 m and the overlying seam reported on here, designated by Groenewold and others (4) as the Kinneman Creek Bed, up to 2.5 m thick. The lignite of the Kinneman Creek Bed is a black to brownishblack coal that slacks rapidly on exposure to the atmosphere and typically contains dark carbonaceous clay or gray clay partings. The underclay is a gray-green clay with lignite fragments. The overburden is gray fine-grained sediment primarily consisting of silty clays and clayey s i l t s with minor concretionary zones and sands. Logan (6) has described the overburden sequence as the Kinneman Creek interval and has interpreted i t to be of lacustrine origin. Generally, the early Cenozoic sediments in this region are believed to have been deposited in a coastal complex of stream channel, flood plain, swamp, lake, and delta environments. Inorganic Constituents Inorganic constituents in the Sentinel Butte sediments are present as 1) detrital mineral grains and volcanic glass fragments, 2) authigenic mineral grains and cement, 3) inorganic components of plants, and 4) ions adsorbed by clay, other minerals, and organic material (7-10). A summary of minerals observed in the Sentinel Butte Formation is given in Table I (9). Major original detrital constituents include montmorillonite, quartz, plagioclase, alkali feldspar, biotite, chlorite, volcanic glass, and rock fragments. Major minerals formed during deposition and diagenesis by conversion of original detrital constituents consist of montmorillonite, chlorite, and kaolinite. Pyrite, gypsum, hematite, siderite, and possibly calcite formed through post-depositional reducing and oxidizing reactions related to changing conditions of deposition, burial, and groundwater movement
Schobert; The Chemistry of Low-Rank Coals ACS Symposium Series; American Chemical Society: Washington, DC, 1984.
11.
K A R N E R ET AL.
177
Inorganic Constituents in North Dakota Lignite
and chemistry. Occurrences and modes of formation of these postdepositional constituents are currently under study. Table I.
Inorganic Constituents in the Sentinel Butte Formation (from Schobert and others, 9)
Downloaded by CORNELL UNIV on June 20, 2017 | http://pubs.acs.org Publication Date: September 10, 1984 | doi: 10.1021/bk-1984-0264.ch011
Constituents Alkali feldspar Augite Barite Biotite Calcite/Dolomite Siderite Chlorite Gypsum Hematite Hornblende Illite Kaolinite Magnetite Montmorillonite Muscovite Plagioclase Pyrite Quartz Volcanic Glass Rock fragments
Overburden
Lignite
Underclay
XX
X X X
X
X XX XX XX X XX XX X XXX X XXX XXX X XX
X X XXX XX
X
X XXX
X XXX
X
XXX
X XX XXX
X XXX
XXX = Abundant XX = Common X = Minor Sampling The samples used in this study are a subset of those collected in a sampling program (5) which was designed to obtain stratigraphicallycontrolled specimens for a study of the character, distribution, and origin of the inorganic constituents in lignite. The objectives of the field sampling were to obtain: 1.
Incremental samples of underclay, lignite, and overburden;
2.
Duplicate vertical sections and lateral samples; and
3.
Samples of specific beds, lenses, fault zones, concentrations, or other materials of unusual
mineral aspect.
Lignite, lignite overburden, and underclay were collected from a vertical section of a freshly exposed high wall at an actively mined pit in the Center mine. Figure 1 illustrates the lithologie sequence.
Schobert; The Chemistry of Low-Rank Coals ACS Symposium Series; American Chemical Society: Washington, DC, 1984.
178
T H E CHEMISTRY O F LOW-RANK COALS
SAMPLES
•
23
CONCRETION ZONE LAMINATED MUDSTONE TAN-GRAY MUDSTONE
6.0
~
TAN-GRAY
CLAYSTONE
Downloaded by CORNELL UNIV on June 20, 2017 | http://pubs.acs.org Publication Date: September 10, 1984 | doi: 10.1021/bk-1984-0264.ch011
CLAYEY BROWN LIGNITE GRAY CLAY BROWN LIGNITE 5.0
GRAY GREEN CLAY BROWN LIGNITE
Ε < LU CO
4.0 114
Ζ
GRAY MUDSTONE AND GRAY CLAYSTONE
ο LU >
Ο
CD
