SILVER IN MICROBIAL MATS OF THE MIDDLE CAMBRIAN RAVENS THROAT RIVER DEPOSIT: AN ENVIRONMENTAL AND PRESERVATION FACTOR?

Burgess Shale-type Lagerstätten are early Paleozoic deposits with extraordinary fossil preservation. While the fossils have been studied extensively, the mode of preservation of these fossils is still controversial. The Ravens Throat River Lagerstätte in the Rockslide Formation (Drumian) of the Mackenzie Mountains, northwestern Canada, is nonmetamorphosed, relatively specimen-rich, but of low faunal diversity. In contrast to deposits like the Burgess Shale, this Lagerstätte conserves mainly just the robust body parts, but it similarly accumulated under an oxic to dysoxic water column. The sediments contain locally large amounts of organic matter, especially that seen as circular carbonaceous patches up to 5 cm in diameter on the bedding plane, which likely represent microbial mats. Articulated specimens of trilobites and hyoliths are commonly present on them, suggesting that they may be in feeding position.

One unique feature of this Lagerstätte is the high concentration of Ag in the fossil-bearing beds (up to 0.45 ppm), and an even higher concentration in the carbonaceous patches. Elemental Ag in the microbial remains range in size from a few to several hundred mm. The underlying lime mudstones contain much less Ag (0.07 ppm), no carbonaceous patches, and trilobites are rare and disarticulated.

The role of microorganisms in the reduction and accumulation of noble metals is thought to be important in the creation of some mineral deposits. Both algae and bacteria have the potential to absorb Ag nanoparticles out of the water column or create organic compounds from dissolved Ag. The enrichment of Ag in the Lagerstätte is interpreted to be due to preferential accumulation in benthic microbial mats. The source of the Ag is uncertain. However, the region hosts late Neoproterozoic and Cambrian SEDEX deposits, and the Lagerstätte is located near faults that were active during sedimentation.