In an approximately 100-square-mile area east of Ely in the
center of the Vermilion district, three major stratigraphic units are
mapped. The oldest unit, the Ely Greenstone, contains at least
12,000 feet of dominantly metabasaltic rocks, and the base is
not exposed. Thin chert-siderite and chert-magnetite iron-formations
are interbedded with the lavas. Overlying this unit essentially
conformably (though with a basal conglomerate composed of Ely
Greenstone clasts) are at least 5,000 feet of graywackes, argillites,
slates, and felsic to intermediate pyroclastic rocks and their clastic
debris, which constitute the Knife Lake Group. Apparently stratigraphically
above the Knife Lake Group, as here defined, is a thick
sequence of felsic and intermediate volcanic rocks, mostly pyroclastic,
that interfingers to the west with metabasalts; this sequence
has been named the Newton Lake Formation. This unit, which may
be continuous with rocks to the east included by Gruner (1941) in
the Knife Lake Group, consists of at least 8,000 feet of strata, including
a thick sequence of mafic clastic rocks and a 500-foot thick
lens of recrystallized calcareous chert. Along the North Kawishiwi
River another thick group of metaconglomerates and metagraywackes,
now gneisses and schists, is faulted against the lower part
of the Ely Greenstone. These rocks are tentatively assigned to the
Knife Lake Group.
The stratified rocks are intruded by a variety of porphyries, including
a regionally widespread porphyritic dacite-rhyodacite that
was extruded onto the surface in Late Ely Greenstone time. The
Ely Greenstone and the Knife Lake metasedimentary rocks along
the North Kawishiwi River were metamorphosed and intruded by
the granitic rocks of the Giants Range batholith, which is dominated
in this area by two facies, a non-porphyritic (Clear Lake)
and a porphyritic (Farm Lake) type. Both facies are composed predominantly
of hornblende-biotite adamellite, monzonite, and granodiorite,
and contain many local variations including mafic types.
Most of the contacts within the batholith are unchilled, and many
suggest physical mixing of viscous magmas. Fine-grained biotite
adamellite is the youngest mappable phase, and aplite and pegmatite
dikes are common. A variety of dioritic and gabbroic dikes cut
the batholith but they are metamorphosed by it.
At the northwestern margin of the quadrangle are several small
outlying plutons of the Vermilion batholith, which has metamorphosed
the adjacent rocks. Southeast of Fall Lake, granitic rocks
have been faulted upward into the Knife Lake metasedimentary
rocks, and southeast of Stub Lake small bodies of pink quartz
syenite to granodiorite intrude the Ely Greenstone. Keweenawan
diabase dikes cut all the major stratigraphic units and the Lower
The entire structural deformation of the Lower Precambrian
rocks in this area is attributable to the Algoman orogeny (2.6-2.5
b.y. ago). The strata are nearly vertical, and depositional structures
indicate that tops are generally to the north, away from the
North Kawishiwi fault. There is local internal isoclinal folding,
however, in all the formations. Faults are of particular significance,
and at least some followed intrusion of the batholith. Several eastward-
to northeastward-trending faults of regional importance cross
the area, and lesser north-northeastward- and northeastward-trending
faults with apparent displacements of as much as two miles cut
the Ely Greenstone into many blocks. Strong, steeply-plunging
lineations are widespread in the northern edge of the area and
along the North Kawishiwi River. Kink-folds with gently-plunging
or vertical axes, which represent minor displacements and a higher
level of deformation, are superimposed on the earlier structures, especially
in a zone centered in the Knife Lake belt. Their age is unknown
but probably is late Algoman.
Metamorphism of the stratified rocks is generally of very low
grade (greenschist facies), and a lack of equilibrium is widespread.
Near the Vermilion and Giants Range batholiths, epidote-amphibolite
and amphibolite facies are attained, and next to the Duluth
Complex is a narrow zone of pyroxene hornfels.
Although much effort has been spent in the past in prospecting
the iron-formations. no economically viable deposits have been
found in this area. Sulfides, as disseminations and small veins, are
scattered through the greenstones and rarely in other rocks, and
exploration for sulfide deposits currently is in progress.
Green, J.C., Lower Precambrian Rocks of the Gabbro Lake Quadrangle, Northeastern Minnesota. Special Publication Series 13. 96 p.
96 p., 4 pls.
Green, John C..
SP-13 Lower Precambrian Rocks of the Gabbro Lake Quadrangle, Northeastern Minnesota.
Minnesota Geological Survey.
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