File:Cupriferous amygdaloidal basalt (Kearsarge Flow, Portage Lake Volcanic Series, upper Mesoproterozoic, 1.095 Ga; South Kearsarge Mine, Upper Peninsula of Michigan, USA) 1.jpg

English: Cupriferous amygdaloidal basalt (copper ore) from the Precambrian of northern Michigan, USA. (field of view: 7.0 cm across) (sample generously donated by the Seaman Mineral Museum, Michigan Technical University, Houghton, Michigan, USA)

The Portage Lake Volcanic Series is an extremely thick, Precambrian-aged, flood basalt deposit that fills up an ancient continental rift valley. This rift valley, analogous to the present-day East African Rift Valley, extends from Kansas to Minnesota to the Lake Superior area to southern Michigan. Unlike many flood basalts (e.g., Deccan Traps, Siberian Traps, Columbia River), the Portage Lake only filled up the rift valley. The unit is exposed throughout Michigan's Keweenaw Peninsula, in the vicinity of the towns of Houghton & Hancock.

The Portage Lake succession thickens northward through the Keweenaw, up to >5.5 km worth of section in places. The dominant rock type is basalt - vesicular basalts, for the most part. Most of the original vesicles (gas bubbles) have since been filled up with a wide variety of different minerals. A vesicular basalt that has had its vesicles filled up with minerals is called an amygdaloidal basalt. Keweenaw amygdaloidal basalts have long had significant economic importance because native copper (Cu) is one of the more common vesicle-filling and fracture-filling minerals. Keweenaw has (had) the highest concentration of native copper anywhere on Earth. Numerous Keweenaw-area copper mines have exploited these cupriferous amygdaloidal basalts. Almost all of the copper mines have since shut down.

Basalt is the not the only lithology in the Portage Lake succession - coarse-grained siliciclastics (conglomerates, sandstones) were occasionally deposited atop the basalts between lava flow events. These beds are fairly similar to the coarse-grained siliciclastics in the overyling Copper Harbor Conglomerate.

The rock shown above is an amygdaloidal basalt with native copper (= reddish-brown masses) filling the former vesicles in the lava.

Stratigraphy: Kearsarge Flow (Kearsarge Lode), Portage Lake Volcanic Series, Bergland Group, middle Keweenawan Supergroup, upper Mesoproterozoic, ~1.095 Ga

Age of copper mineralization: 1.05-1.06 Ga

Locality: South Kearsarge Mine, southern side of the town of Wolverine & northeast of the town of Calumet, Keweenaw Peninsula, Upper Peninsula of Michigan, USA