Dendrochronology in mineral exploration: developing tools to see through anthropogenic impacts

Abstract

The Sudbury Igneous Complex (SIC) and its associated mining camp is one of the largest Cu, Ni, PGE ore systems in the world. Due to a long history of mining (>100 years), the SIC has been extensively explored making the discovery of new surface deposits difficult. This study explored the use of dendrochemical analysis as an economic means to locate deposits concealed by overburden. Tree cores combined with soil samples were collected and analyzed from two sites to provide a comparison between a known deposit (Broken Hammer) and a known contamination location (downwind site). Samples were collected from both White Pine (Pinus Strobus) and Scots Pine (Pinus Sylvestris) species, however White Pine proved to have a stronger affinity for the uptake of ore-associated metals (i.e., Cu, Ni, Fe, Co, Hg, Cd, Pb, Sb, S). Analyses of tree-core intervals revealed clear anthropogenic effects in the samples from both sites; however, due to the age of the trees the magnitude of these effects compared to pre-mining conditions could not be determined. The concentrations of S, Cu, Ni, Fe, Al, Pb and As in the downwind samples increased over time. On average, S increased from 6.37 to 8.98 ppm, Cu from 0.503 to 0.908 ppm, Ni from 0.605 to 1.18 ppm, Fe from 124 to 273 ppm, Al from 4.38 to 6.60 ppm and As from 0.008 to 0.012 ppm. Conversely, the concentrations of Cu, Fe, Ni, Al, S and As in the samples from Broken Hammer decreased over time. Copper decreased from 0.84 to 0.55 ppm, Fe from 2.45 to 1.19 ppm, Ni from 1.02 to 1.18 ppm, Al from 6.81 to 4.96 ppm, S from 7.62 to 7.57 ppm and As from 0.014 to 0.002 ppm. The total Pb concentration decreased over time in all samples from both sites. There were no trends in the Pb isotopic ratios with the exception of the 207 Pb/ 206 Pb. A comparison of the samples from each location revealed unique trends in the distribution of the data, which could suggest two sources. Soil (Ah and B horizon) collected proximal to the sampled trees near Broken Hammer showed the metal concentration increasing with depth. Copper averaged 33.6 – 187.9 ppm, whereas Ni had an even steeper trend, increasing from an average of 22.4 ppm–234.5. Sulphur and Hg also displayed a similar trend, however the difference was not as substantial. The highs and lows in concentration levels in the tree cores correspond to various historical events. During WWII (1939 – 1945), the Korean War (1950 – 1953) and the Vietnam war (1964 – 1973), the demand for Ni increased resulting in increased smelting activities and a spike in contamination levels. Conversely, the recession in the late 1970s early 1980s and the INCO strike in the late 1990s resulted in a decrease in Ni production and a drop in contamination levels. The distribution of metal concentrations at Broken Hammer indicates a lack of surface contamination and therefore a non-contaminated B-horizon. The higher concentrations in the B-horizon are anomalous to other studies in the Sudbury area suggesting a possible subsurface source. This is encouraging and the potential remains to develop a method for mineral exploration applications utilizing tree core samples, even in heavily mining impacted areas like the Sudbury.