Geologic setting and characterization of coals and the modes of occurrence of selected elements from the Franklin coal zone, Puget Group, John Henry No. 1 mine, King County, Washington, USA

Abstract

Upper middle and upper Eocene coal-bearing strata in the Green River coal district of Washington state are in the Franklin coal zone of the undivided Puget Group. Coal-bearing strata accumulated in intertidal and deltaic environments along a tidally influenced delta plain. Numerous shale and tonstein partings within the coals indicate that the coal formed in low-lying peat mires. To evaluate variations in element distribution within these coals, vertical bench channel samples from the Franklin No. 7–8–9 ( n = 24) and the No. 10 ( n = 11) coal beds were collected. Coal beds in the John Henry No. 1 mine of the Green River coal district have an apparent rank of high volatile B bituminous. No. 7–8–9 and No. 10 have a mean sulfur content of 0.67 wt.% and 0.9 wt.% respectively. When compared statistically to other western U.S. Tertiary coals, the Franklin coal zone in the John Henry No. 1 mine Franklin coal zone is significantly higher in contents of ash, Si, Al, Mg, K, Ti, Ag, As, Cu, F, Ga, Hg, Li, Nb, Ni, P, Sc, Sr, V, Y, Yb, and Zr, and has significantly lower contents of Ca, Na, B, Nd, and Se. Mean contents of several elements in the Franklin No. 7–8–9 and No. 10 coal beds are uncommonly high compared to western U.S. Tertiary coals. The No. 7–8–9 bed is higher in As (6.6 X, 44 ppm), Cu (3.8 X, 42 ppm), F (3 X, 190 ppm), Hg (44 X, 4.4 ppm), Mn (3.4 X, 170 ppm), Nb (4.4 X, 10 ppm), Ni (3.5 X, 16 ppm), V (4 X, 68 ppm), and Zr (4.9 X, 88 ppm). The No. 10 bed is higher in As (11 X, 81 ppm), Cu (3.2 X, 35 ppm), Hg (75 X, 7.5 ppm), Ni (3.3 X, 15 ppm), P (5.4 X, 1300 ppm), Sc (4.1 X, 8 ppm), Sr (2.5 X, 680 ppm), V (3.7 X, 63 ppm), and Zr (2.7 X, 48 ppm). X-ray diffraction analysis of the low-temperature ash from these coals reveals the predominance of quartz and clays (kaolinite, minor illite, smectite group, and mixed layer) and minor plagiociase feldspar (albite, disordered, and ordered Ca-bearing), carbonate (ankerite, calcite, kutnohorite, and siderite), pyrite, and clinoptilolite. Minor crandallite group minerals, zircon, and β-form quartz phenocrysts were also found, indicating that volcanic ash was deposited within the peat-forming mires. The presence of augite, hornblende, and smectite group clays in the tonsteins suggests a volcanic source that was intermediate in composition. The higher contents of Ba, F, P, Sr, and Zr in the Franklin coals result from the alteration of volcanic ash in the mires. Apatite contains major amounts of F and P, whereas feldspar contains Ba and Sr. Zircon is the primary Zr-bearing mineral. Crandallite group minerals are major Ba- and P- bearing minerals in the coals and can contain Ca and Sr. Higher As and Hg contents in the coals result from post-Eocene hydrothermal mineralization. Mines in the nearby Green River Gorge contain cinnabar, metacinnabar, realgar, and orpiment. Realgar and orpiment were found in fractured sandstone above the No. 10 coal bed. Kleinite and cinnabar, both mercury-containing minerals, were identified in coal samples and native mercury was found in one parting sample. Thermal and XANES studies on coal samples indicate that there are at least two modes of occurrence for Hg in the Franklin coals: (1) organic complexes containing Hg, and (2) Hg-bearing minerals within the coal and associated partings. Pyrite may also contain trace amounts of As, Hg, and Ni.