Geology and mineral deposits of the Keweenaw Peninsula and vicinity, Michigan
No abstract available.
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Geology topics
Research about Keweenaw Peninsula
Source-linked reports with geographic coverage including Keweenaw Peninsula.
Evaluation of effects caused by high copper concentrations in Torch Lake, Michigan, on reproduction of yellow perch
Elevated concentrations of copper are present in Torch Lake, Michigan, an Environmental Protection Agency Superfund site. The sauger ( Stizostedion canadense ) population in Torch Lake has been extirpated and walleye ( Stizostedion vitreum ) populations are maintained by stocking. We used yellow perch ( Perca flavescens ) to determine if chronic exposure to elevated copper concentrations has reduced the reproductive success of percids. Ripe yellow perch from Torch Lake and Gratiot Lake, a reference site, were captured between 6 May and 6 June 1990. Crosses were made within each lake and between lakes (both combinations); five pairs of each type were attempted. Portions of each egg mass were reared in Torch Lake water (34 ng/mL copper) and reference site water (Rice Lake) until 4 days posthatch (20-d to 28-d). Copper concentrations were found to be over two times as high as in reference samples both in Torch Lake water and in gonads of perch from Torch Lake. The percentages of eggs hatched in Torch Lake and reference lake water were not significantly different, although a trend of reduced hatch in Torch Lake water was apparent. Likewise, no significant differences were indicated in hatching success among crosses. However, duration of hatching was significantly longer for Torch Lake egg masses than for reference lake egg masses, indicating that copper may affect reproductive success through disruption of hatch coordination with food availability or reduction of larval fitness.
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Age of native copper mineralization, Keweenaw Peninsula, Michigan
Amygdaloidal flood basalts and conglomerates are the host for substantial deposits of native copper within the Portage Lake Volcanics in the Keweenaw Peninsula of Michigan. A wide variety of gangue minerals are associated with the regional hydrothermal alteration-mineralization event. Application of the Rb-Sr method to amygdule-filling microcline, calcite, epidote, and chlorite suggests an age of mineralization between 1,060 and 1,047 m.y. (+ or - [asymp] 20 m.y.). These results are supported by a fission track age on epidote of 1,044 + or - 169 m.y. The age of native copper mineralization determined in this study is consistent with geologic evidence which suggests that mineralization postdated the deposition of most or all of the overlying Freda Sandstone. Variable initial 87 Sr/ 86 Sr ratios between the low Rb/Sr phases suggest mixing of isotopically distinct sources of Sr during the generation and equilibration of the hydrothermal solutions.
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