Origin of the tuff parting and associated enrichments of Zr, REY, redox-sensitive and other elements in the Early Miocene coal of the Siniy Utyes Basin, southwestern Primorye, Russia

Abstract

Miocene coal beds in the Siniy Utyes Basin, South Primirye, in eastern Russia, are interbedded with volcanic ash layers and are enriched in rare earth elements and Y (REY, or REE if Y is not included), Zr, redox-sensitive and other trace elements around these layers. However, high contents of fresh volcanic glass and alkalies as well as no evidence of leaching suggest that the source of these elevated concentrations of these elements are not interpreted to be derived from the volcanic ash fall layers, as is found in many other coals worldwide. In contrast, the broader geochemical signature of the coal indicates that these elements sourced from weathered granites. Interestingly, both the coal and volcanic ash contain pyrite and, less commonly, barite, REE phosphate (rhabdophane?), silver sulfate and sylvite, all of which indicate a post-depositional emplacement of these trace elements, possibly through an initial evaporative process in the peat before burial. Instead of leaching of the adjacent tuff, enrichment of REY, Zr, Mo, V, and other elements in the coal proximal to the volcanic ash layers may be a result of higher permeability and a geochemical barrier at the coal/volcanic ash interface, which concentrated element migration in these zones during diagenesis. U-Pb zircon dating (LA ICP-MS) of the volcanic ash layer indicates an age in the 18–25 Ma range. The younger (5 of 22 grains) cluster of dates, however, is in the 18–19 Ma (early Miocene) range, which may indicate the age of the actual eruption.