Mt Bohemia
 

Jacobsville Sandstone

Portage Lake Volcanics

Mt Bohemia is a stock consisting of diorite and granophyre. It is strongly altered by hydrothermal solutions and has been extensively drilled and explored for its copper potential, being compared to other mineral deposit types called porphyry coppers. The alteration which affects the body appears to be related to its own heat and hydrothermal system, a situation where the stock “stewed in its own juices” after solidifying.  This alteration produced an interesting association of secondary minerals including chalcocite, which is the mineral of most economic interest at Mt Bohemia and other nearby volcanic and intrusive bodies near the Keweenaw Fault.

Many of the most interesting rocks here occur on the steep S facing slope of Mt Bohemia, in the midst of the ski trails.

excerpt from: GEOLOGIC MAP OF THE KEWEENAW PENINSULA AND ADJACENT AREA, MICHIGAN

By William F.  Cannon and Suzanne W. Nicholson


The Mount Bohemia intrusion has a somewhat different geochemical character. It is a small zoned stock which intrudes the lower part of the Portage Lake Volcanics near Lac La Belle. A massive medium-grained syenodiorite forms the outer zone around a core of fine- to coarse-grained granophyre. In the same area many subconcordant sheets of mafic intrusive rocks also are known from exploration drilling but are too small to show at the scale of this map (Robertson, 1975; Michcan Copper Company Ltd., 1982).