An Australian study in Geology of biofilms on gold finds that the bacteria-driven "dissolution, precipitation, and aggregation lead to the formation of bacterioform Au and contribute to the growth of Au grains under supergene conditions..." [right, credit USGS]
It sounds like the bacterial bio-films dissolve the gold into nanoparticles and redeposit them as grains that can be purer than the original source. News reports describe it as a way of growing nuggets.
Ref: Nanoparticle factories: Biofilms hold the key to gold dispersion and nugget formation, doi: 10.1130/G31052.1 v. 38 no. 9 p. 843-846