PORTAGE LAKE VOLCANIC SERIES
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 (try saying that five times quickly). 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.
Stratigraphy & Age: Portage Lake Volcanic Series, Bergland Group, middle Keweenawan Supergroup, upper Mesoproterozoic, 1.093-1.097 billion years.
Age of copper mineralization: ~1.05-1.06 billion years.
Portage Lake Volcanic Series - roadcut exposing parts of two successive basalt lava flows (1.094-1.095 billion years). The rubbly lower half of the cut is a vesicular basalt flow. The massive upper half of the cut is a nonvesicular basalt flow. Rt. 41 roadcut ~0.45 miles west of Delaware Copper Mine, northern Keweenaw Peninsula, UP of Michigan, USA (see map).
Portage Lake Volcanic Series - amygdaloidal basalt, ~1.095-1.096 billion years. Old quarry on southern side of Academy Road, 0.3 miles west of Rt. 26, just north of South Range, SW of Houghton, Keweenaw Peninsula, UP of Michigan, USA (see map).
Portage Lake Volcanic Series - ophitic basalt of the Scales Creek Flow, ~1.095-1.096 billion years. Ophitic basalt, or ophite, refers to a mafic igneous rock having individual plagioclase crystals enclosed within pyroxene (this feature can be seen in thin section). Roadcut on southern side of Seventh Street, 0.2 miles west of Agate Street-Seventh Street intersection, city of Houghton, Keweenaw Peninsula, UP of Michigan, USA (see map).
Portage Lake Volcanic Series - glacially grooved basalt knob, Scales Creek Flow, ~1.095-1.096 billion years. Outcrop across the road from the Houghton Gremlins' water tower, southern side of city of Houghton, Keweenaw Peninsula, UP of Michigan, USA (see map).
Amygdaloidal basalt from the Portage Lake Volcanic Series (field of view ~6.2 cm across). Dark material is basalt. Light-colored spots & masses are various minerals that have filled up former vesicles in the lava.
Amygdaloidal basalt (field of view ~5.7 cm across), from the upper Portage Lake Volcanic Series, immediately adjacent to the Allouez Conglomerate & just below the Greenstone Flow. This lava sample is ~1.094 billion years. Waste rock pile of the Delaware Copper Mine, north of Rt. 41, northern Keweenaw Peninsula, UP of Michigan, USA (see map).
Epidotic vesicular basalt (4.8 cm across), ~1.095 billion years, from a little above the Kearsarge Flow in the middle Portage Lake Volcanic Series. Most of the vesicles here are only partially filled with epidote (Ca2(Al,Fe)Al2O(SiO4)(Si2O7)(OH)). The sample comes from the Gratiot Mine, ~1 mile NE of Mohawk, Keweenaw Peninsula, UP of Michigan, USA (see map).
Cupriferous amygdaloidal basalt (copper ore) (7.0 cm across) from the Kearsarge Flow (Kearsarge Lode), ~1.095 billion years. The rounded reddish-brown masses are native copper (Cu) that has filled up former vesicles in the lava. This rock comes from the South Kearsarge Mine, southern side of Wolverine, NE of Calumet, Keweenaw Peninsula, UP of Michigan, USA. Sample generously donated by the Seaman Mineral Museum (Michigan Technical University, Houghton, Michigan).
Cupriferous amygdaloidal basalt (closeup of copper amygdules in sample shown above; field of view = 2.3 cm across) from the Kearsarge Flow at South Kearsarge Mine, UP of Michigan.
Cupriferous amygdaloidal basalt (field of view 4.3 cm across) from the Knowlton Flow (Knowlton Lode/Knowlton Amygdaloid), ~1.096 billion years. Native copper (Cu) has filled up former vesicles & fractures in the lava. This rock comes from the Caledonia Mine, WSW of Mass City, UP of Michigan, USA.
Cupriferous fractured basalt (field of view 4.6 cm across) from the ~1.096 billion year old Knowlton Flow (Knowlton Lode/Knowlton Amygdaloid) at Caledonia Mine, UP of Michigan.
Native copper (Cu) (4.7 cm tall) from the Portage Lake Volcanic Series, 1.05-1.06 billion year Cu-mineralization age.
Glacial float copper (6.1 cm across), 1.05-1.06 billion year Cu-mineralization age. During the Pleistocene, glaciers scoured the copper-bearing rocks of the Keweenaw Peninsula. Copper was often picked up and then dropped once the glaciers melted away. Oxidized copper masses have been and still are frequently found in the Pleistocene glacial sediments. The reddish material is cuprite (Cu2O - copper oxide), an oxidation (weathering) product. The greenish material is malachite (Cu2(CO3)2(OH)2) the result of copper oxide alteration with water and carbon dioxide gas.
Glacial float copper (6.2 cm across) with reddish cuprite oxidation coating, from Pleistocene glacial drift in Michigan's Keweenaw Peninsula. 1.05-1.06 billion year Cu-mineralization age.
Glacial float copper with greenish malachite alteration coating - large mass recovered from the floor of Lake Superior, and on display at Quincy Mine, Michigan's Keweenaw Peninsula, USA. 1.05-1.06 billion years Cu-mineralization age.
Silver-copper halfbreeds (left: 2.1 cm across; right: 2.0 cm across). These are Ag-Cu stampings recovered from outside the Caledonia Mine. Native silver (Ag) is sometimes found closely associated with copper in rocks of the Portage Lake Volcanic Series. In northern Michigan's copper mining heyday, mine supervisors often advised miners to be on the lookout for “white copper”. The uneducated miners didn't know this referred to valuable silver. Miners handed samples to the mine foremen who often pocketed them for personal profit. These specimens are from a foreman's or supervisor's old hidden stash that was never recovered. Using a metal detector, the stash was discovered relatively recently.
Some of the coarse-grained siliciclastic interbeds of the Portage Lake Volcanic Series have also been impregnated with native copper. Below are photos of two such interbeds - the Allouez Conglomerate and the Calumet and Hecla Conglomerate.
The Allouez Conglomerate is sufficiently cupriferous that some copper mines were located on its outcrop belt. The Delaware Copper Mine, though long inactive, is still accessible underground as a tourist site. The mine targeted copper that occurs in the conglomerate as intergranular masses and as clast coatings ("skull copper").
Stratigraphy & Age: Allouez Conglomerate (formerly the No. 15 Conglomerate), just below the Greenstone Flow, upper Portage Lake Volcanic Series, Bergland Group, upper Mesoproterozoic, 1.094 billion years.
Delaware Copper Mine - this is the No. 1 Shaft adit. This mine is located just north of Rt. 41, near Delaware, northern Keweenaw Peninsula, UP of Michigan, USA (see map). The shaft follows the dip of the beds here. It descends ~100-120 feet at a 24º angle to the first level (~60 vertical feet).
Allouez Conglomerate, as exposed along the walls of the Delaware Copper Mine's 1st level, the only accessible level. All levels below this (levels 2-10) are flooded with water. Unlike many mine waters around the world, the waters here are not acidic or significantly polluted. Why? The minerals present in the rocks are not sulfides.
Allouez Conglomerate, as exposed along the walls of Delaware Copper Mine's 1st level. Most of the large pebbles in the conglomerate are composed of rhyolite/felsite lava. This fluvial lava-pebble conglomerate was deposited in an ancient continental rift valley.
Allouez Conglomerate-Amygdaloidal Flood Basalt contact. The reddish material at top is fluvial, lava-pebble conglomerate. The brownish material at bottom is vesicular flood basalt with many of the vesicles subsequently filled up with minerals.
Fault plane in the Allouez Conglomerate, as exposed in the roof of Delaware Copper Mine's 1st level. The direction of the striations (fault slickenlines) indicates the direction of fault movement. A fair number of slickenlined fault surfaces are present in the rocks at this mine.
Calumet & Hecla Conglomerate - cupriferous felsite conglomerate (cut & polished surface, ~15.5 cm across), Calumet & Hecla Conglomerate, Portage Lake Volcanic Series, 1.094-1.095 billion years. This came from the Centennial # 6 Mine in Houghton County, UP of Michigan.