Elemental mercury in copper, silver and gold ores: an unexpected contribution to Lake Superior sediments with global implications

Mercury and copper inventories are low in central Lake Superior and increase markedly towards the Keweenaw Peninsula. Total copper flux to Lake Superior sediments averages 5.0 ± 2.5 μg cm-2 year-1(mean ± 95% confidence limits), whereas mercury flux averages 7.5 ± 4.2 ng cm-2 year-1. In the Keweenaw Peninsula region, copper, mercury and silver inventories are elevated and highly correlated. High copper, silver and mercury inventories can be traced back to shoreline stamp sand piles, the parent ores and to smelters. Mercury occurs in elemental form, probably as a natural amalgam, in native metal (copper, silver, gold) deposits and was liberated as volatile Hg0during on-site copper smelting. Stamp mills discharged at least 364 Mt of ‘stamp sand’ tailings, whereas smelters refined 5 Mt of native copper, liberating together at least 42 t of mercury. The Keweenaw situation is not unique, as mineral-bound mercury is commonplace in US and Canadian Greenstone Belts and is of worldwide occurrence in massive base metal ores.