Structural and Chemical Characterization of Placer Gold Grains: Implications for Bacterial Contributions to Grain Formation

Abstract

Gold grains collected from the Rio Saldaña River, Colombia were hundreds of micrometers in size and discoid-ellipse in shape. Fourteen of 63 grains contained an iron oxyhydroxide coating that occurred as ca. 50 to 100 nm thick lamina while thicker coatings were comprised of colloids 200 nm to 4 μm in diameter. Bacterial-size casts were observed throughout the thicker iron oxyhydroxide coating and intuitively represent relic impressions of bacterial cells. The surface textures of gold grains were generally smooth with surficial depressions or crevices containing detrital material colonized by bacteria. Focus Ion Beam (FIB) milled cross-sections demonstrated that the detrital material contained nanophase gold particles. Biofilm attached to this detrital material contained ca. 2 to 3 nm colloidal gold attached to exopolymeric substances. Cross sections of grains revealed solid cores with vesicular voids near the grain edge including a bacterial-size cast interpreted to be a permineralized bacterial cell. Synchrotron-based elemental mapping indicated that grains contained heterogenously distributed Ag and Cu. While strong Ag and Cu signals (relative to Au) were detected in the core, a stronger Au signal occurred at the edge of grains demonstrating enriched rims of secondary gold. The preservation of bacterial casts and biofilms associated with secondary gold structures at the surface of grains suggest that bacteria may contribute to gold enrichment and growth in this placer environment. Bacteria, occurring on the surface of 13 of 25 gold grains, were enriched by “inoculating” individual grains into separate test tubes containing R2B growth medium. Enriched growth of bacteria on gold grain surfaces demonstrated preferential attachment onto detrital material within creviced regions. The dominant bacteria from these enrichments were transferred to solid R2A medium to obtain pure isolates. The isolates were identified as one of four bacterial species: Nitrobacter sp. 263, Shewanella sp. YM-8, Sediminibacterium sp. B2-10-2 and sp. I-32 based on 16S ribosomal DNA sequencing.