Keweenaw Minerals

Minerals and mineral collecting are part of life in the Keweenaw. E. W. Heinrich, in his book Mineralogy of Michigan, recently revised by George W. Robinson, has compiled lists, descriptions and localities of Keweenaw Minerals. There are three groups of minerals in the Keweenaw.

The first are primary minerals, which make up the lavas and redbeds. These are the common rock-forming minerals (species such as plagioclase feldspars, olivine and pyroxenes). The second group, 63 hydrothermal minerals which are associated with veins, lava flow tops and conglomerates, is the most important for collectors, mineral enthusiasts and those wishing to mine. They grow from hot water passing through fractures and other open spaces in the rocks. The hot waters also bring about changes in the original rocks, which results in additional minerals. Geologists call these changes Zeolite Facies and Prehnite-Pumpellyite facies metamorphism. In the case of the Keweenaw, native copper is the most important of a large number of minerals formed during this episode. Others include native silver, quartz, prehnite, pumpellyite, various zeolites, calcite and datolite. These hydrothermal minerals are conspicuous in the lava flow tops of the Keweenaw and Isle Royale (amygdaloidal minerals) and their colors, forms and hardness variations make identification possible in the field. A third group of minerals are produced by weathering of the rocks and hydrothermal veins on Earth’s surface. These include malachite, cuprite, tenorite, clay minerals and a host of other rare species.

As mining in the region has declined and poorrock piles have been used for road construction, mineral collection sites have been depleted of high quality specimens. The best place to see these minerals has been assembled during the mining heydays by the A. E. Seaman Mineral Museum in Houghton. The museum has the largest and most well-documented collection of Keweenaw minerals anywhere, interpreted with extensive exhibits. It reflects the work of dozens of visionary collectors, led by Seaman himself, Lucius L Hubbard and John Thorley Reeder.