Copper-rich “Halo” off Lake Superior’s Keweenaw Peninsula and how Mass Mill tailings dispersed onto tribal lands

Abstract Over a century ago, shoreline copper mills sluiced more than 64 million metric tonnes of tailings into Lake Superior, creating a “halo” around the Keweenaw Peninsula with a buried copper peak. Here we examine how tailings from one of the smaller mills (Mass Mill, 1902–1919) spread as a dual pulse across southern Keweenaw Bay and onto tribal L’Anse Indian Reservation lands. The fine (“slime clay”) fraction dispersed early and widely, whereas the coarse fraction (stamp sands) moved more slowly southward as a black sand beach deposit, leaving scattered residual patches. Beach stamp sands followed the path of sand eroding from Jacobsville Sandstone bluffs, mixing with natural sands and eventually adding onto Sand Point, at the mouth of L’Anse Bay. Dated sediment cores and a multi-elemental analysis of the buried Cu-rich peak in L’Anse Bay confirm a tailings origin. Copper concentrations are declining in the bay, yet copper fluxes remain elevated. The spatial and temporal studies underscore that enhanced sediment and copper fluxes around the Keweenaw Peninsula largely reflect historic mining releases. Mercury is correlated with copper, yet mercury concentrations and fluxes remain relatively low in Keweenaw Bay compared to nearby Superfund sites (Torch and Portage Lakes), perhaps reflecting the absence of smelters on Keweenaw Bay. Tribal efforts to remediate contamination are progressing, but are hindered by recent high water levels plus severe storms. The long-term repercussions of Mass Mill discharges caution against mine companies discharging even small amounts of tailings into coastal environments.