DENDROCHEMICAL VARIATION OVER THE CROSS LAKE VMS MINERALIZATION – A TOOL FOR MINERAL EXPLORATION AND DECOUPLING ANTHROPOGENIC INPUT FROM BACKGROUND SIGNALS

Dendrochemistry is an emerging technique in mineral exploration for buried ore deposits. Elements that are transported from the mineralized zone to the soil horizon through a variety of mechanisms are incorporated into the tissues of the tree via root uptake. Sampling tree cores is superior to soil sampling in the search for anomalous metal concentrations because trees provide the forth dimension of time that can decouple anthropogenic input from background signals.

Black spruce were sampled from west and south facing cores at the Cross Lake VMS property near Timmins, ON along an existing cut-line transecting the mineralization buried beneath ca. 50 m of glacial till. Tree rings were counted and 5-year segments representing pre- and post-smelter (Falconbridge Cu-Ni smelter in operation in 1972) conditions were dissolved in concentrated nitric acid followed by the removal of dissolved organics using hydrogen peroxide, and analyzed using HR-ICP-MS. Results show that Cu, Zn, As and Mg are anomalous over the mineralization. These trends are characterized by consistently low concentrations flanking the mineralization and up to a 5-fold increase over the mineralization. Other metals, such as Pb, Mn, Fe, and Ni, do not show the same response to mineralization, and are likely more influenced by bedrock lithologies and overlying sediments. Metal concentration trends are similar between the south and west facings of the trees, although the south samples are typically better for delineating the mineralization. A tree selected to represent background values at the property shows increases in most of the metals in tree rings representing 1920-25 to 1960-65, and 1960-65 to 1980-85, with larger magnitudes of change in the younger rings. This indicates that the Falconbridge smelter, and possibly other industrial and mining activities in recent years, is having an impact on metal contents of the surrounding soil. The results demonstrate that black spruce tree cores are useful for detecting metal anomalies from buried VMS mineralization under glacial till. Anthropogenic activity does not obscure these anomalies in younger tree rings, particularly for Cu and Zn, although the pre-contamination wood typically gives a better signal.