Paleolimnological study of copper mining around Lake Superior: Artificial varves from Portage Lake provide a high resolution record

Abstract

On the Keweenaw Peninsula region of Lake Superior, -0.5 Gt of copper mine tailings were dumped along rivers, lakes, and connecting waterways between 1850 and 1968 -an era of active copper and silver mining. In the low-energy environment of Portage Lake, a part of the Keweenaw Waterway, clay particles from stamp sand discharges created artificially varved lake sediments that preserved a remarkably detailed record. Measurements of these varves, in conjunction with radiodating techniques, allow precise detcrminations of deposition rates, sediment, and Cu fluxes.Concentration and flux profiles produce fundamentally different patterns. Copper concentrations have remained high in sediments despite complete cessation of active tailings discharge. However, densitometer scans of X-rayed cores and flux calculations at sites closest to sluicing locations reveal a good correspondence with the historic record of stamp sand production. Sedimentation rates and Cu fluxes have declined steadily since cessation of native copper mining, although elevated Cu levels continue to circulate in the ecosystem. Comparisons of Cu profiles from sediment cores across eastern Lake Superior suggest widespread signatures of mining and major contribution to sediments.

Sediment radioisotope dating across a stratigraphic discontinuity in a mining-impacted lake

“…Intensive copper mining began in the 1840s and peaked around the turn of the century (Murdoch, 1943;Benedict, 1952;Lankton, 1997). The Keweenaw, with over 140 mines and 40 stamp mills, was for a time the most productive copper mining region in the country (Kerfoot et al, 1994). Because it provided a convenient means of transportation and had a large capacity for waste disposal, Torch Lake was chosen as the site for six stamp mills, the first of which began operating in 1866.…”

Light Detection and Ranging (LiDAR) and Multispectral Scanner (MSS) Studies Examine Coastal Environments Influenced by Mining

“…Marie regions of Ontario, Canada. Sediment investigations have revealed a major metal halo around the Keweenaw Peninsula related to historic copper mining activities [9][10][11]. Here, we use LiDAR and MSS, high definition side-scan sonar and cameras on remote operating vehicles (ROVs), i.e., instruments with increasing spatial resolution, to illustrate in detail how the coastal ecosystem is being disturbed by migrating mining wastes.…”

“…The stamps crushed the amygdaloid and conglomerate ores, and mills sluiced copper-rich stamp tailings into rivers and waterways. Between 1865 and 1968, approximately 360 million metric tons (Mt) of stamp sands were discharged from native copper stamp mills into interior waterways and along Lake Superior shorelines, including 64 Mt directly into Lake Superior [8,9,15]. Figure 2.…”

“…The amount of tailings discharged (Mt, million metric tons) from larger, coastline "stamp" mills (1895-1947) are shown. The locations of mines (circles) and mills (hollow triangles) are plotted along the Portage Lake Volcanic Series and on the coast (modified from Kerfoot et al, [9]). …”

“…The success at inland sites required the local damning of rivers, as the milling process required water to discharge large volumes of stamp sands [9]. In the late 1890s, larger steam-driven mills opened on the shores of the largest inland lakes (Torch Lake, Portage Lake) and along the coast (Figure 2 [3,8].…”