Microbially-induced K enrichments in hydrothermally altered andesitic tuffs

•Anomalous K enrichment in Neogene andesitic tuffs are biologically mediated.•Three generations of K-feldspars are distinguished.•Bioforms in K-feldspar host embedded organic matter.•Biosignatures were documented by high resolution analysis.•Representative K-rich bioforms are filamentous, vermiform and coccoid-like.•Microbial mediation of K-rich minerals can account for the observed features.