USE OF ION MICROPROBE FOR PROOF OF LIFE IN SEDIMENT-HOSTED ORE DEPOSITS

The ion microprobe, or secondary ion mass spectometry (SIMS) is capable of determining trace-elemnent concentrations and elemental isotopes in spots as small as ~10 microns. Recent research has shown that extremophile bacteria may have significant geochemical effects on metals and redox-sensitive elements in the crust, and certain ore deposits may owe their genesis to such bacteria. At the very least, bacteria may have contributed to the conditions necessary to make certain sediment-hosted ore deposits. Sulfur isotopes have traditionally been useful in fingerprinting the source of sulfur in sulfide minerals that apparently were derived from the reducion of sulfate to hydrogen sulfide in some ore deposits by sulfate-reducing bacteria (SRB). Our research using the ion microprobe points to potentially an even better fingerprint of in situ SRB metabolism during sulfide formation. We have conducted detailed micro-traverses across pyrite grains formed by SRB in a low temperature (groundwater) setting and hypothesize that our results could be used to interpret if in situ bacterial sulfate reduction occurred in some sediment-hosted ore deposits. We found that the cores of individual pyrite grains were isotopically lighest with repect to S-34, and got increasingly heavier toward the rims. This is the classic Rayleigh distillation trend which is the hallmark of biogenic sulfate reduction in a mostly closed system. We further hypothesize that the "closed sytem" is a function of rapid crystal precipitation caused by the introduction of biogenic hydrogen sulfide into metal-rich solution. Certain mineral textures may result, and we are currently investigating if sphalerite is a mineral that might preserve these bacterial "far-from equlilbrium" textures and characteristic Rayleigh distillation trends, and thus might be useful in documenting biogenic ZnS in ore deposits.