New Mexico Geological Society Annual Spring Meeting — Abstracts

Petrographic study of selected samples from the BB seam show that iron sulfides occur mainly as pyrite with minor marcasite. Marcasite occurs only as overgrowth on massive pyrite. Pyrite displays the following modes of occurrences: (1) pyrite framboids occurring in clusters or isolated with diameters ranging from about 1 µm to 45-50 µm, (2) euhedral to subhedral crystals isolated or in clusters with sizes ranging from about 1 µm to 5-10 µm, and (3) massive pyrite filling cellular structures and void spaces in inertinite, coating framboids and cementing pore spaces between clusters, and as cleat-filling. Following Renton's (1982) scheme of classification of mineralization stages in coal and based on some petrographic evidence and theoretical considerations, pyrite occurrences in the BB seam are classified as follows: 1) early syngenetic stage-framboids and euhedral crystals were formed during this stage of mineral emplacement. Such occurrences have been described as forming in recent peat deposits (Altschuler et al.,1987); 2) late syngenetic stage-more of the framboids and euhedral crystals may have continued to form during stage. Massive pyrite filling cellular structures and voids is believed to have formed during this stage as any degradable material that was present in the cellular structures would bave been removed at this stage due to bacterial degradation or other geochemical factors. During this stage, massive pyrite coating framboids and cementing inter- and intra-framboid spaces may also precipitate; 3) epigenetic stage-massive cleat-filling pyrite may have precipitated during this stage. Marcasite is closely associated with massive cleat-filling pyrite and may have formed later than or contemporaneously with the cleat-filling pyrite. Pyrite in the form of framboids, eubedral crystals and massive pyrite coating framboids, filling cellular structures and inter- and intra-framboid pore space is abundant in drill hole 2 in high sulfur zones (2 to 5.67% total sulfur) suggesting that most of the sulfur in these zones was fixed as pyritic sulfur during early-late syngenetic stage. The higher abundance of framboidal and euhedral pyrite, and high sulfur content may reflect elevated pH conditions that favor a higher degree of bacterial reduction. The high sulfur content also suggests that there was abundant supply of sulfate ions during the accumulation of the peat at this stage of accumulation was influenced by brackish (even possibly marine?) water source. Medium to high sulfur contents (0.6 to 1.4% total sulfur) adjacent to roof, floor, and partings, may indicate alkaline conditions that could promote accelerated microbial degradation of plant material and bacterial reduction of available sulfate. Framboids, euhedral crystals, and massive cleat-filling pyrite characterize these zones suggesting syngenetic and epigenetic iron sulfide emplacement. The presence of marcasite in relatively low sulfur drill holes (drill holes 1 and 3) closely associated with cleat-filling pyrite may indicate relatively acidic or intermediate alkaline conditions limited to fractures and microenvironments adjacent to clastic rock-coal boundaries such as near partings, roof, and floor. Bounding lithologies and partings have the lowest sulfur content. Their fresh water depositional setting and high iron content suggest that sulfate availability and Eh conditions may have been the main factors that controlled the presence of sulfur in these rocks.