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. (± 20 m.y.). These results are supported by a fission track age on epidote of 1,044 ±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 87Sr/86Sr ratios between the low Rb/Sr phases suggest mixing of isotopically distinct sources of Sr during the generation and equilibration of the hydrothermal solutions.
Bornhorst, T. J., Paces, J. B., Grant, N. K., Obradovich, J. D., & Huber, N. K. (1988). Age of native copper mineralization, Keweenaw Peninsula, Michigan. Economic Geology, 83(3), 619–625. https://doi.org/10.2113/gsecongeo.83.3.619