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.
CUPRIFEROUS CONGLOMERATES
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.