SPHEROIDAL DOLOMITES IN A VISEAN KARST SYSTEM - BACTERIAL INDUCED ORIGIN

Spheroidal dolomite crystals occur in the karstified top of a Dinantia n dolomite sequence in eastern Belgium. The spheroidal dolomite crysta ls are best developed at the base of the karst system. The dolomite cr ystals are characterized by a spherulitic or dumb-bell inclusion patte rn, and are overgrown by dolomite cements with a rhombohedral outline. They are considered to be bacterially related precipitates based on, (1) textural similarities with documented bacteriogenic precipitates, (2) the presence of 'bacterial' microspheres and framboidal pyrite emb edded within the dolomite, and (3) their general geological setting. T he geochemical characteristics of the dolomites and associated mineral s support a bacterial origin. The ubiquity of framboidal pyrite, deple ted in S-34 (delta(34)S = - 22.4 to - 25.5 parts per thousand CDT), te stifies to a period of bacterial sulphate reduction. The isotopic comp osition of the spheroidal dolomites (delta(13)C = - 2.4 to - 3.2 parts per thousand PDB and delta(18)O = - 3.8 to - 3.4 parts per thousand P DB) suggest a contribution from oxidized organic carbon produced durin g bacterial sulphate reduction. Sulphate reduction may also result in a concomitant O-18 depletion if the system is nearly closed. It is how ever, evident from the sulphur isotopic composition of associated fram boidal pyrite that the system was fairly open. The O-18 depletion of t he spheroidal dolomite crystals (delta(18)O = - 3.8 to - 3.4 parts per thousand PDB) and their occurrence adjacent to, and within karst cavi ties suggests a mixing zone origin, with a significant proportion of f reshwater in it. The rhombohedral cement-overgrowths have calculated d elta(18)O values in the range of 0 to + 5.3 parts per thousand PDB, wh ich reflect precipitation from normal to slightly evaporated contempor aneous seawater.