MICROBIAL PRECIPITATION OF A STRONTIUM CALCITE PHASE AT A GROUNDWATERDISCHARGE ZONE NEAR ROCK-CREEK, BRITISH-COLUMBIA, CANADA

Mineralogical, chemical, and microscopic analyses demonstrate an intim ate relationship between epilithic cyanobacteria and the precipitation of a strontium calcite phase at a groundwater discharge zone located near Rock Creek, British Columbia, Canada. The groundwater flows out o f a serpentinite bedrock outcrop that provides a hard surface for the accretion of a coherent calcareous crust. The mean pH of water samples collected similar to 2.0 m above the base of the outcrop was 8.5, whe reas a value of 8.8 was recorded for samples taken near the outcrop ba se. This increase in pH cart be attributed to the growth of cyanobacte ria that carry out a HCO3-/OH- exchange process during photosynthesis. Calcium was present in the water samples at levels of 32-36 ppm, wher eas strontium occurred at lower concentrations (5.8-6.6 ppm). In each case, the lowest calcium and strontium values occurred in samples take n near the base of the outcrop, as expected for carbonate mineral prec ipitation. The crust itself is a porous thrombolitic matrix with inter growths of calcite and cyanobacterial colonies comprised of filamentou s Calothrix, and coccoid Synechococcus and Gleocapsa. Many of the cyan obacteria were completely embedded within fine-grained crystal aggrega tes. Examination of thin-sectioned samples by analytical transmission electron microscopy showed that tile cyanobacteria served as nucleatio n sites for calcite containing up to 1.0, wt% strontium. Bulk whole ro ck analyses on the crust using x-ray diffraction and x-ray fluorescenc e confirmed the electron microscopic observations. Based on these resu lts, the accretion of the strontium calcite crust is interpreted to be the result of microbial carbonate precipitation in groundwater with a n Sr2+/Ca2+ ratio that promotes solid solution of SrCO3 ill calcite.