The Genesis of Vanadium-Rich Black Shales in the Richardson Trough, Canada

We explore the genesis of potentially economic vanadium mineralization in Middle Devonian shales on the Dempster property of the Richardson trough, Yukon, Canada. The local stratigraphy consists of the Vittrekwa Formation (1,253 ppm V; enrichment factor (EF) VEF 31.0), a transition zone (718 ppm V; VEF 14.7), and the Canol Formation (2,120 ppm V; VEF 78). Vanadium in these units is hosted mainly by muscovite, smectite, illite, and mannardite (BaTi6V2O16 · H2O), and V concentrations vary with the paragenesis (i.e., detrital to late diagenetic). Detrital muscovite is characterized by 0.2 wt % V, early diagenetic smectite (Vittrekwa Formation) contains 0.5 wt % V, and late diagenetic illite contains from 1.3 to 3.7 wt % V. Vanadium in the Canol Formation is hosted primarily by late diagenetic illite, whereas in the Vittrekwa Formation and the transition zone, it occurs in smectite and mannardite. Depositional processes and diagenetic reactions influenced the deportment of the vanadium. The illitization of smectite coincided with the release of organically bound vanadium. This was followed by a coupled redox reaction between V(IV) and sulfur, which led to the substitution of V(III) in illite and the precipitation of pyrite. Acid production via feldspar precipitation and the thermal maturation of organic matter were accompanied by calcite dissolution and the formation of a quartz-hyalophane ((K,Ba)[Al(Si,Al)Si2O8]) barium carbonate assemblage. The location and architecture of the Richardson trough created an environment conducive to vanadium scavenging, with a connection to the open ocean sufficient to prevent trace metal depletion (e.g., a basin reservoir effect) but with restriction sufficient to maintain deeper-water euxinia. Additional mechanisms, such as enhanced primary productivity, organic matter preservation, and a stratified water column, led to the partitioning of organically bound vanadium into illite and oxide minerals during late diagenesis. This study provides new insights into the complex geologic and geochemical processes responsible for vanadium enrichment in shale deposits and provides a genetic model for its mineralogical deportment.