Significance and role of chemosynthetic bacteria in the formation of Carboniferous, deep-water, laminated carbonates.

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1. Thin section analyses and close inspection of hand specimens, which were collected during the field survey in 1991, have revealed that 1) carbonatemounds characterized by chemosynthetic communities distributed not only in New Foundland, but also in New Brunswick, and 2) methane-seepage from the sea floor prevailed in very wide sedimentary basin in the Atlantic Canada during Carboniferous age.2. Carbon and oxygen isotope compositions were determined on calcite from tube worm mounds. Skeletal calcite of bryozoa and brachiopod have 1.1 to -1.4 o/oo delta^<13>C (PDB) and -3.6 to -6.6 o/oo delta^<18>O (PDB), whereas sparry calcite cement filling interstices of shells and cavities have 2.3 to -1.9 o/oo delta^<13>C and -8.6 to -10.7 o/oo delta^<18>O. Light oxygen of sparry calcite may suggest higher temperature of precipitation.3. Sulfide minerals occur as various forms in tube worm mounds. Disseminated pyrite has delta^<34>S of 0.75 to 6.37 o/oo, whereas massive sulfide ore a few m in diameter has delta^<34>S of 28.3 o/oo. This suggests that massive sulfide ore was precipitated from the more evolved "sea-water" at higher temperatures compared with disseminated pyrite.4. Combined textural analyses, chemical analyses, and isotopic signatures lead us to conclude that tube worm mounds were formed in the following processes : 1) Microbial process controlled by enhanced seep and expulsion of cold, methane and sulfide containing solutions, 2) development of calcite cement due to increased temperatures and mixing of connate waters, and 3) formation of massive sulfide ores caused by migration of high-temperature brines derived from deeper levels.