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Age of native copper mineralization, Keweenaw Peninsula, Michigan

November 21, 1988

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.

Publication Year 1988
Title Age of native copper mineralization, Keweenaw Peninsula, Michigan
DOI 10.2113/gsecongeo.83.3.619
Authors Theodore J. Bornhorst, James B. Paces, Norman K. Grant, J. Obradovich, N. King Huber
Publication Type Article
Publication Subtype Journal Article
Series Title Economic Geology
Index ID 70206764
Record Source USGS Publications Warehouse