LYONS, P. C., U.S. Geological Survey, Reston, VA, A. T. CROSS, Michigan State University, East Lansing, MI, Z. GAO, University College of Cape Breton, Sydney, Nova Scotia, Canada, K. GILLIS, Nova Scotia Department of Natural Resources, Stellarton, Nova Scotia, Canada, J. H. CALDER, Nova Scotia Department of Natural Resources, Halifax, Nova Scotia, Canada, E. L. ZODROW, University College of Cape Breton, Sydney, Nova Scotia, Canada, and R. D. CONGDON, U.S. Geological Survey, Reston, VA
ABSTRACT: Discovery of In-Situ Carbonate Petrifactions (Coal Balls) in the Foord Seam (Westphalian C, Upper Carboniferous), Stellarton, Nova Scotia, Canada: Implications for Origin of Sulfur in the Foord Seam
Carbonate petrifactions (coal balls) were discovered in situ in the 13-m-thick Foord Seam (Westphalian C) at the Westray open-pit mine at Stellarton, Nova Scotia, Canada. These are the first in-situ coal balls discovered in Nova Scotia. This bed, the thickest and oldest coal mined in the Carboniferous coal basins of the Maritime Provinces of Canada, is the uppermost seam of the Albio Member of the Stellarton Formation and is known for its low sulfur content (mean = 0.5% total sulfur), the lowest of all Maritime Canada coals.
The coal balls are up to 60 cm in length and are scattered abundantly from the bottom to the top of the seam, including the shale parting. The principal minerals contained in the coal balls (n = 6), as determined by semiquantitative x-ray diffraction (XRD) analysis, are siderite (70-100%), dolomite (0-20%), quartz (0-5%), and traces of a clay mineral (illite?). Calcite and pyrite were detected in trace amounts by SEM-EDAX and by single-crystal XRD analysis. As determined by whole-rock and coulometric methods, the coal balls are composed of Fe2O3 (43.6-60.0%), MgO (1.5-3.8%), CaO (1.8-7.2%), MnO (0.13-0.35%), SiO2 (0.9-11.7%), Al2O3 (0.3-6.2%), TiO2 (0.03-0.23%), K2O (<0.01-0.8%), Na2O (0.09-0.16%), P2O5 (0.09-0.27%), CO2 (29.5-36.4%), and organic carbon (1.1-3.4%). The following ge era were tentatively identified in the coal balls: Sigillaria, Lepidodendron, and an unidentified fern(?).
The almost complete absence of pyrite in the coal balls suggests a chemical link with the pyrite-poor Foord Seam. We hypothesize that sulfate-rich marine water or recycled marine sulfate from evaporites from the Lower Carboniferous Windsor Group were unavailable in the peat-forming mire, and, therefore, siderite was favored over pyrite. A nonmarine origin of the siderite also is suggested by the nearly pure end-member nature of the siderite (Fe0.94+/-0.03 Mg0.02+/-0.02 Ca0.04+/-0.01 Mn<0.01 Sr<0.01 Ba<0.01; n = 48), a composition consistent with siderite of freshwater origin. Because of the lack of sulfate or H2S to form pyrite, sulfur combined almost exclusively with the organic molecules of the lycopod-rich peat, and this lack of sulfate or H2S favored the low-sulfur conten of the Foord Seam.
AAPG Search and Discovery Article #90995©1993 AAPG Eastern Section Meeting, Williamsburg, Virginia, September 19-21, 1993.