Base modified from U.S. Geological Survey 1996 Digital RasterGraphic of the 1994 Mille Lacs Lake 1:100,000 quadrangle.

Universal Transverse Mercator Projection, grid zone 151983 North American Datum

SUPPORTED IN PART BY THE MINNESOTA MINERALS DIVERSIFICATION PROGRAM AS ADMINISTERED BYTHE MINERALS COORDINATING COMMITTEE FOR THE MINNESOTA LEGISLATURE

GIS compilation and cartography byTimothy E. Wahl and Joyce Meints

Graphic design by R.S. Lively

MISCELLANEOUS MAP SERIESMAP M-100, PLATE 1

Mille Lacs Lake 30 x 60 Minute Quadrangle

BEDROCK GEOLOGY OF THE MILLE LACS LAKE 30 X 60 MINUTE QUADRANGLE, EAST-CENTRAL MINNESOTA

MINNESOTA GEOLOGICAL SURVEYD.L. Southwick, Director

1 0 1 2 3 4 5 MILES

8 KILOMETERS

SCALE 1:100 000

1 0 1 2 3 4 5 6 7

CONTOUR INTERVAL 10 METERS

Pmks Pmkc

Pp

Pl

Piw Piwx

PgduPbbgPgm Ppd

Pti

Pdu

Pgs

Pd

PvdgPvas

Pgws

Pbs

Pcs

Pvgd

Pps

PdamPdmv

Phm

Pdi

Phmm

Amc

CORRELATION OF MAP UNITS

PALEOPROTEROZOIC

LATE ARCHEAN

MESOPROTEROZOIC

Keweenawan Supergroup

Pum

PgviPi Pvm

Psa

PasPi

INTERNAL ZONE(Includes the McGrath gneiss dome, the Hillman migmatite,

metagraywacke, and Penokean plutons)

MEDIAL ZONE(Includes the South range of the

Cuyuna district, the Moose Lake-Glen Township panels, and the Mi l le Lacs

monzoni te)

EXTERNAL ZONE(Includes the southern par t of the North range of the

Cuyuna district)

Prl Prv

UnconformityPmd

Unconformity Unconformity

Pq

Pim

P P

P

P

P P

P P

P

P

P

P P

P P

PP

P

P

P

P P

P

PP

P

P

P

P

P

P

P

PP

PP

P

P

Penokean Orogen

Fold-and-Thrust Belt

BEDROCK GEOLOGYBy

Terrence J. Boerboom and D.L. SouthwickMinnesota Geological Survey

Mark J. SeversonNatural Resources Research Institute

1999

46°00'

46°15'

46°30' 46°30'

46°15'

46°00'93°00'93°15'93°30'93°45'94°00'

47°00'

46°45'

47°00'

46°45'

94°00' 93°45' 93°30' 93°15' 93°00'

INDEX TO 1:100,000-SCALE QUADRANGLES, EAST-CENTRAL MINNESOTA

AITKINM-99

MILLE LACSM-100

shown only where intersected by drilling and are probably more abundantthan portrayed on the map.

Pgvi Metasedimentary and metavolcanic rocks—Graphitic schist and slate; alsolean iron-formation. Includes an unknown proportion of mafic tointermediate igneous rocks of unit Pvm.

Psa Metasedimentary rocks—Slate, argillite, and metasiltstone; unit includesa substantial but unknown quantity of mafic hypabyssal intrusions (unitPdu) and fragmental mafic volcanic rocks. May correlate in part withthe Mahnomen Formation. Metamorphosed under lower greenschist faciesconditions.

Pi Iron-formation—Oxide, silicate. and carbonate iron-formation intercalatedwith metavolcanic and graphitic rocks of units Pvm, Pgvi, and Psa. Shownonly where documented by exploratory drilling and extrapolated on thebasis of magnetic anomalies. Also includes a thin unoxidized, magnetic,slaty iron-formation beneath unit Pas.

Pvm Metavolcanic rocks—Mafic to intermediate flows and volcaniclastic rocks,metamorphosed under greenschist-facies conditions. These rocks are shownonly where intersected by drilling and are probably more abundant thanportrayed on the map.

Pgs Graphitic schist and slate—Graphite-bearing schist, phyllite, and slateinterbedded on a fine scale. Rocks typically display a crenulation cleavagesuperimposed on a layer-parallel schistosity.

Pps Metagraywacke and pelitic schist—Dominated by crenulated staurolite-garnet-muscovite-biotite schist derived from graywacke. Structurallylineated calc-silicate metaconcretions are typical of this unit, which containsvariable proportions of metamorphosed mafic volcanic rocks and associatedhypabyssal mafic intrusions that appear to increase in abundance to thenorth. Poorly exposed in map area, but more abundant outcrop and drillingdata exist along strike east of map area. On a regional scale, this unitoccupies the same structural position as the Little Falls Formation (Morey,1978) and may correlate with that unit; may also be equivalent to theschist xenoliths contained in unit Piwx. Metamorphosed under amphibolite-facies conditions.

MILLE LACS GROUP AND RELATED ROCKS

Pd Metadiabase—Fine- to medium-grained, dark-green; variably preservedtrachytic igneous texture carried by aligned tabular plagioclase grains.Metamorphosed under greenschist-facies conditions; primary pyroxenereplaced by variable proportions of bluish-green fibrous amphibole, chlorite,and clinozoisite. Accessory sphene and opaque oxides, biotite, andanhedral-interstitial quartz. Weakly foliated and locally sheared. Formsa subconformable sill emplaced between units Pq and Pgws.

Pvdg Metabasalt and metadiabase—Includes metabasalt flows and closely alliedsills. Flows grade upward from massive basal portions to pillowed middleand upper portions and are topped by breccia and localized interflowsediment. Pillowed rocks are amygdaloidal; flow-top breccia locallycontains chert clasts. Metadiabase, apparently more abundant in the eastern

part of the unit, ranges from thoroughly recrystallized epidote amphiboliteto amphibolitic schist that shows relict diabasic texture. Metamorphosedunder greenschist-facies conditions.

Pvas Mixed mafic volcanic and sedimentary rocks—Includes components of unitsPvgd, Pas, and Pgws.

Pas Carbonaceous argillite—Dominantly carbonaceous and graphitic argillite,slate, and fine-grained schist. Thin quartzite beds and hypabyssal maficsills are locally present; iron-carbonate is abundant near top and bottomof unit. Argillite contains variable proportions of bedded pyrite andpyrrhotite, with minor chalcopyrite and sphalerite in places where theymay form massive sulfide layers as thick as 10 centimeters. Unoxidized,thin-bedded, dark-gray iron-formation (unit Pi) locally present beneaththis unit, with which it shares a gradational contact.

Pgws Graywacke and slate—Sandy-textured tuffaceous graywacke and slate thatunderlies units Pas and Pi. Moderate cleavage axial-planar to fold axes.

Pbs Metabasalt and allied rocks—Inferred from meager drilling control to consistprimarily of variably amygdaloidal to massive metabasalt. Metamorphosedunder greenschist-facies conditions.

Pq Dam Lake quartzite—Quartzite and sericitic quartz schist. Massive- to thick-bedded quartzite typically lacks metamorphic foliation; micaceous,medium- to thin-bedded quartzose rocks typically are schistose. The unitincludes an unknown proportion of pelitic schist, as well as green, quartz-rich schist that has a substantial component of mafic volcanic rockfragments. Rocks contain variable amounts of secondary magnetite, whichproduces a moderately strong positive aeromagnetic anomaly.

Pvgd Metabasalt, metadiabase, and metasedimentary rocks—Massive to pillowed,grayish-green, variably porphyritic metabasalt interbedded with fragmentalvolcanic rocks and mafic tuff; medium-grained, grayish-green, variablyschistose, mafic hypabyssal intrusions; and graphitic-sulfidic argillite,cherty- and carbonate-facies iron-formation, and metagraywacke.Metamorphosed under lower greenschist facies conditions.

Pcs Phyllitic schist and carbonaceous garnet-biotite schist—Fine-grained,dominantly gray to red, variably ferruginous phyllitic schist interbeddedwith gray to black, carbonaceous, and locally sulfidic garnet-biotite schist(meta-argillite). Thin disrupted chert beds locally present in argillite.Small equant garnets are variably pseudomorphed by chlorite. May beequivalent to unit Pas.

Denham Formation.Pdam Arkose, gritstone, conglomerate, dolomitic marble, and staurolite-garnet-

muscovite schist—This unit is exposed just east of the map area, wherethe arkose and conglomerate contain detritus derived from the adjacentMcGrath Gneiss (unit Amc). The dolomitic marble is tan and generallymassive, but in places contains thin beds of arkosic rocks. Schistoserocks display layer-parallel schistosity and well-developed crenulationcleavage.

Pdmv Metabasalt—Fine-grained, dark grayish green, amygdaloidal, and pillowed.Locally associated with flow-top breccia and volcaniclastic rocks.

Moderately to strongly deformed; contains abundant metamorphichornblende. Exposures immediately east of the map area show that theserocks are present within unit Pdam. A pronounced aeromagnetic anomalythat defines this unit and unit Pdam follows the northern margin of theMcGrath Gneiss (unit Amc); exploratory drilling on this anomalyintersected dark magnetic schists that are interpreted as metamorphosedigneous rocks. Unit mapped only where intersected by drilling; lateralextent probably greater than shown. Metamorphosed under amphibolite-facies conditions.

Pdi Slate—Black, magnetic, carbonaceous slate that has 1–3-meter-thick layersof cherty iron-formation in upper part. Preserved part of sequence isapproximately 10 meters thick; it is interpreted to overlie unconformably,and be folded into, the McGrath Gneiss (unit Amc).

LATE ARCHEAN ROCKSAmc McGrath Gneiss—Medium- to coarse-grained, pinkish-gray, variably sheared

augen gneiss and flaser gneiss derived from coarsely porphyritic biotitegranite. Original granite textures preserved where subsequent deformationis relatively less intense. Microcline megacrysts as much as 4 centimetersin diameter range from euhedral relict phenocrysts where rock is relativelyunsheared to elongate ribbons where flaser texture developed. Older shearfoliation that varies in intensity from place to place is cut by younger, narrow,discrete shear bands. Locally preserved trachytoid fabric, defined by alignedeuhedral microcline phenocrysts, is cut by shear foliation.

REFERENCES CITEDBoerboom, T.J., Southwick, D.L., and Severson, M.J., 1999, Geologic map of the Aitkin

30 x 60 minute quadrangle, east-central Minnesota: Minnesota Geological SurveyMiscellaneous Map Series M-100, 2 pls., scale 1:100,000.

Morey, G.B., 1978, Lower and Middle Precambrian stratigraphic nomenclature for east-central Minnesota: Minnesota Geological Survey Report of Investigations 21, 52 p.

Morey, G.B., and Morey, D.D, 1986, Distribution of iron-formations in the main CuyunaRange, east-central Minnesota: Minnesota Geological Survey Miscellaneous map seriesM-60, scale 1:48,000.

Southwick, D.L., Morey, G.B., and McSwiggen, P.L., 1988, Geologic map (scale 1:250,000)of the Penokean orogen, central and eastern Minnesota, and accompanying text:Minnesota Geological Survey Report of Investigations 37, 25 p.

Every reasonable effort has been made to ensure the accuracy of the factual data on which this map interpreta-tion is based; however, the Minnesota Geological Survey does not warrant or guarantee that there are no errors.Users may wish to verify critical information; sources include both the references listed here and information onfile at the offices of the Minnesota Geological Survey in St. Paul. In addition, effort has been made to ensure thatthe interpretation conforms to sound geologic and cartographic principles. No claim is made that the interpreta-tion shown is rigorously correct, however, and it should not be used to guide engineering-scale decisions withoutsite-specific verification.

pale-green clinopyroxene, and 15 percent subhedral titanomagnetite inpoikilitic groundmass of albite and K-feldspar. Contains accessorynepheline, biotite, hornblende, apatite, chlorite, sphene, pyrite, andchalocopyrite. Forms east–northeast-trending dike or set of forking dikestraceable by aeromagnetic data for about 90 kilometers. Exposed alongand in Snake River, where one 7-meter-thick dike is chilled against WarmanGranite; a second dike, 400 meters north, is at least 13 meters thick.

Piw Isle and Warman Granites—Biotite-bearing, medium- to coarse-grained,equigranular to weakly porphyritic, light-gray to pinkish-gray granite totonalite. Formerly mapped as two separate granite bodies—the Isle Graniteto the east of the Warman Granite, but on the basis of geophysical andoutcrop data, the Isle and Warman are here considered to constitute oneintrusion.

Piwx Inclusion-rich Warman Granite and pegmatite—Granite contains numerousinclusions of tonalite similar to the Hillman Migmatite (unit Phm) andgarnet-biotite schist similar to unit Pps in terms of protolith, metamorphicgrade, and deformation. Inclusions are large and abundant; they mayrepresent an in-place carapace to the Warman Granite. Abundant pinkgarnet-biotite-muscovite pegmatite and aplite are present within thiszone as irregular masses at least 10 meters wide, sills of 3 meters ormore thickness, and thin dikes. Rare occurrences of a fine-grained,bright-green mineral, tentatively identified as beryl, noted in pegmatite.

Pbbg Bradbury Creek Granodiorite—Biotite- and hornblende-bearing, medium-to coarse-grained, light-gray granodiorite to tonalite. Moderate foliation,interpreted as primary, is carried by preferentially oriented elongate quartzgrains, and biotite and hornblende grains.

Pgm Granite at Mille Lacs Lake—Chlorite- and biotite-bearing, medium- tocoarse-grained, grayish-maroon granite. Extensively recrystallized andcut by narrow ductile shear bands. Primary mafic minerals, such ashornblende, are altered to fine-grained aggregates of chlorite, epidote,sphene, and biotite. Primary apatite and opaque oxides are associatedwith pseudomorphic assemblages. Shear bands, recrystallized texture,and retrograde metamorphism indicate that the intrusion was emplacedprior to the end of Penokean deformation.

Ppd Porphyritic hornblende diorite—Medium-grained, greenish-gray, plagioclase-and hornblende-phyric diorite; moderate trachytoic foliation carried byaligned plagioclase and hornblende; retrograde metamorphic overprinthas recrystallized primary pyroxene or hornblende phenocrysts to clotsof fine-grained hornblende. Biotite varies in abundance; primary apatiteand opaque oxides and secondary sphene and epidote are accessoryminerals. Unit inferred from meager outcrop data in vicinity of SkunkCreek to intrude Hillman Migmatite.

Pgdu Granodiorite—Inferred from geophysical data to form the core of a north-trending structural dome; rimmed by gneissic rocks of Phm unit.

Phm Hillman Migmatite—Medium- to coarse-grained, irregularly layered, light-to dark-gray tonalitic gneiss; contains enclaves of biotite-garnet-cordieriteschist and hornblende schist and is cut by slightly porphyritic, weakly

INTRODUCTIONThis map and the adjoining geologic map of the Aitkin quadrangle (Boerboom

and others, 1999) were primarily constructed to provide digital coverage of the easternend of the Minnesota portion of the Penokean Orogen. They cover the northeasternpart of a previously published geologic map of the Penokean Orogen (Southwick andothers, 1988), in which the geologic framework was established in map and report form.Although the new maps are similar to and retain the overall stratigraphic framework ofthe 1988 map, they differ in lithologic and structural interpretations, as they are basedon more recent exploratory and scientific drilling and much-improved geophysical maps.The detailed descriptions and interpretations of the stratigraphic and structural elementsgiven in Southwick and others (1988) are still valid for this map, and the user shouldconsult that publication for a more thorough treatment of the subject.

The Mille Lacs Lake quadrangle includes the southeastern part of the Cuyuna Northand South ranges; both mining districts that are no longer in production. The geologicinterpretations of Morey and Morey (1986) were used for those parts of the map.

DESCRIPTION OF MAP UNITSMESOPROTEROZOIC ROCKS

KEWEENAWAN SUPERGROUP

Pmks Clastic sedimentary rocks, undifferentiated—Includes rocks assigned tothe Hinckley Sandstone and the Fond du Lac Formation. Quartzosesandstone of the Hinckley is medium to coarse grained and buff to tan.Arkosic sandstone of the Fond du Lac is fine to coarse grained and duskyred to dark reddish brown; it is interlayered with substantial but variablythick sections of siltstone and shale. Drill cores show that a weatheredresiduum is developed on older rocks beneath the Fond du Lac Formation.

Pmkc Quartz-pebble conglomerate—Reddish-orange, matrix- to clast-supported;dominated by rounded to subangular, white quartz clasts set in matrixof dusky-red, silty sand. Unit also contains angular clasts of red quartzsandstone, fine-grained gray and red flow-banded volcanic rocks, and fine-grained red argillite. Minor amounts of native copper are present in latebrittle shears.

Diabase dikes—Northeast- and northwest-trending dikes of fine- to medium-grained subophitic olivine diabase. Delineated by linear aeromagneticanomalies of both normal and reversed polarity.

PALEOPROT EROZOIC ROCKSPp Mafic to ultramafic intrusions—Small plugs and irregular masses mainly

composed of ultramafic to mafic rocks that include serpentinized peridotite,pyroxenite, hornblendite, hornblende- and phlogopite-phyric diorite, andgabbro. One drill hole located at the south edge of the map intersectedappinitic hornblende diorite; this plug produces the same characteristicpositive aeromagnetic anomaly as other more mafic bodies that are wellknown from drilling to the west and south.

Lamprophyre—Fine-grained, dark-green dike of reversed magnetic polarity;contains 30–50 percent small prismatic crystals and rare phenocrysts of

foliated to massive biotite-hornblende tonalite. The schist enclaves areslab-like to angular in shape and display an internal foliation that in placesis discordant to the regional northeast-striking foliation of the enclosingtonalite gneiss. The external foliation of the tonalite gneiss (here regardedas a fabric imposed on a syntectonic intrusion) and the discordant internalfoliation of the enclaves are cut by northwest-trending dikes of weaklydeformed granite pegmatite and poorly defined, irregularly shaped bodiesof weakly deformed tonalite. Some schist enclaves contain hornblende-rich meta-concretions similar to those observed in unit Pps east of themap area.

Phmm Magnetic phase of Hillman Migmatite—Areas of Hillman Migmatitedelineated by positive aeromagnetic anomaly.

NORTH RANGE GROUPRabbit Lake Formation.

Prl Slate, siltstone, and very fine grained graywacke—Gray to very dark gray;includes irregularly distributed layers and lenses of carbonaceous slate,as well as carbonate- and sulfide-facies iron-formation, chiefly in thelower part of the unit.

Prv Mafic volcanic and volcaniclastic rocks—Interlayered near the base ofthe Rabbit Lake Formation.

Pti Trommald Formation—Upper part is composed of thick-bedded, cherty iron-formation that includes chert beds as thick as 2 meters. Lower part isthin-bedded, unoxidized, oxide-silicate-carbonate-facies iron-formation;moderately deformed and weakly metamorphosed; oxidized zones of poroushematite contain variable proportions of chert in beds as thick as 30centimeters. Thick-bedded unit locally absent in areas where thin-beddediron-formation is thin.

Mahnomen Formation.Pum Upper member—Gray siltstone, argillite, and fine-grained quartzose

sandstone; member includes irregularly distributed beds and lenses ofiron-formation and allied anomalously iron-rich sedimentary rocks. Baseof unit is arbitrarily defined as the top of the lowermost mappable iron-formation layer of the Mahnomen iron-formation member (Pim).

Pim Iron-formation member—Thin-bedded, variably green, oxide-silicate-carbonate iron-formation; lithologically similar to map unit Pti, and tosmaller lenses of iron-formation within units Pum and Plm. Rockscontain strong metamorphic cleavage, and bedding is only preservedin fold hinge zones. Contains rocks formerly assigned to the TrommaldFormation and the Emily Member of the Rabbit Lake Formation (Morey,1978).

MISCELLANEOUS ROCK UNITS

Pdu Metadiabase—Medium-grained, grayish-green, variably porphyritic. Unitchiefly forms sills and irregular sheet-like intrusions; metamorphosed underlower greenschist facies conditions. Sills lack geophysical definition;

DESCRIPTION OF MAP SYMBOLSInferred contact—Mainly located from aeromagnetic data,

except where locally constrained by information fromoutcrops.

Inferred concealed contact beneath Mille Lacs Lake—Located from aeromagnetic data.

Inferred fault—Mainly interpreted from aeromagnetic databut locally based on drill-hole data.

Inferred thrust fault or structural discontinuity—Sawteethon upper plate. Interpreted to have mainly involved thrustdisplacement.

Inferred concealed thrust fault or structural discontinuitybeneath Mille Lacs Lake—Located from aeromagneticdata.

Inferred trajectory of foliation or layering in gneissic rock—Based on linear aeromagnetic anomalies.

Synclinal fold axis—Showing troughline and plunge.

Anticlinal fold axis—Showing crestline and plunge.

Pti PqPdu

Pbs

Pvm

Pdu

Pd

PqPd Pcs

Pvgd

PasPgs

Pps

Pgws

Pgm

Pdam

Pmkc Pdam

Amc

Amc

Pbs

Pq

Pcs

PdiPp

Phm

Pps Amc

Piw

PpPiwx

Pp

Pp

Pmks

Pp

Pp

PpPp

Phm

Piw

Pbbg

Ppd

Phmm

Pmks

Pmks

Pl

Pl

Pl

Pl

Piw

PdamPp

Pgdu

Pp

Amc

Amc

Pmd

Pp

Pmd

Pmd

Pmd

PmdPmd

Pmd

Pvgd

Pcs

Pcs

Pvgd

Pq

Pgws

Pgws

PasPi

Pas

Pi

Pas

Pas

Pvas

Pgws

Pq

Pgs

PgviPi

Pi

Pi

PiPi

Pdu

Pdu

Pdu

Pgs

Pi

Pdu

Pi

Pdu

Pi Pi

Pi

Pi

Pdu

Pi

Pdu PiPgvi

Pdu

Psa

Pum

Pim

Prv

Pp

Phm

Ppmm

Pp

Pps

Pdam

Pdam

Pvdg

Pvdg

Pvdg

Pvas

Pq

Pgvi

Pi

Pdu

Pdu

Pdu

Pi

Prv

Prv

Prl

PrlPum

Prv

PrvPrl

Pdmv

46°00'

46°15'

46°30' 46°30'

46°15'

46°00'93°00'93°15'93°30'93°45'94°00'

93°45' 93°30' 93°15' 93°00'94°00'

PP

P

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P P

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P

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P P

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P P

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R 25 WR 29 W R 28 W R 27 W R 26 W R 24 W R 23 W R 22 W R 21 W

R 29 W R 28 W R 27 W R 26 W R 25 W R 24 W R 23 W R 22 W R 21 W

T 46 N

T 45 N

T 44 N

T 43 N

T 42 N

T 41 N

T 46 N

T 45 N

T 44 N

T 43 N

T 42 N

T 41 N

Pmd

Pl

LOCATION OF MAP AREA©1999 by the Regents of the University of Minnesota

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