SP-13 Lower Precambrian Rocks of the Gabbro Lake Quadrangle, Northeastern Minnesota

Abstract

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 Precambrian structures. 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.