°0 .... .' . A C A S S • Aro Ar9 •• 4rg Hackensack - 4rg c . Arg N c McGregor / / / / / Geology modified from Plole I, Schmidt, 1963 EXPLANATION PENOKEAN ALGOMAN Mafic intrusive roc ks I :·:. ::·· ::··: ~ Mafic intrusive rocks t/;·/:·J °. 00 0 •• ° <.;,:,; Freedhem tonal i te ~ ~;;;~~f,~~ Gran i ti c intrusive ro cks l, n , ", ",_ .. I: : : :: :1H illman tonalite l::::::::1Giants Range granite P\:!iDJWorman monzonite o 8 16 24 M iles < ~.••..• v< Mc Grath gne iss ~====== ---- Probable western boundary of Middle Precambrian Rocks ..... Magnetic anomaly trend --- Trommold Formotion Figure IV-17. Generalized geologic map of the Cuyuna district and adjoining areas showing the locations of the Emily, North C uyuna, and South C uyuna ranges (modified from Schmidt, 1963). 226 MIDDLE PREC AMBRIAN CUYUNA DISTRICT Ralph W. Marsden The Cuyuna district is about 100 miles west-southwest ward-trending, generally parallel belts of iron-formation ex­ of Duluth, in Aitkin, Cass, Crow Wing, and Morrison tending from near Randall northeast for about 60 miles. In Counties, and is defined here to include the Emily, North, addition to the three named ranges, several linear magnetic and South iron ranges, inasmuch as the rocks in the three anomalies occur within the Cuyuna district that may indi­ areas are lithologically, stratigraphically, and structurally cate other, as yet unexplored, areas of iron-formation (fig. similar (fig. IV-I). The proposed boundaries for the Cuyuna IV-17). district differ from those used by Schmidt (1963), but are The geologic relations of the Precambrian rocks are ob­ consistent with those of Harder and Johnston (1918). The scured by a nearly complete blanket of glacial drift. which. Emily range extends from the Mississippi River northward together with local Cretaceous strata, is from 20 to 450 feet to the north line of Crow Wing County and into southern thick. Except in the mine areas of the North range. the Cass County, and comprises an area of about 450 square geology of the Cuyuna district is pieced together from data miles. The North range includes the principal iron ore-pro­ obtained from a number of explorations for iron and man­ ducing area of the Cuyuna district in the vicinity of Crosby, ganiferous ores made during the past 70 years. Early ex­ Minnesota. The South range includes an area of northeast- ploration work consisted of magnetic surveys followed by Table IV-3. Stratigraphic sequences in the Cuyuna district and westernmost Mesabi range. WESTERNMOST CUYUNA DISTRICT MESABI RANGE Pleistocene Des Moines drift Des Moines drift unconformity ---------- -unconformity ---- -- Upper CretaceoLls Coleraine Formation Coleraine Formation ---------- unconformity----------- Keweenawan ? acidic volcanic rocks'? unconformity ---------- -unconformity ---- -- Middle Precambrian Animikie Group Rabbit Lake Formation Virginia Formation Upper 'Member argillite Emily Member ferruginous slate and iron-formation Lower Member argillite Trommald Formation Biwabik Iron-formation Mahnomen Formation Pokegama Quartzite possible unconformity ---------------- -_ pre-Animikie Trout Lake formation slate and quartzite? unconformity ---------- -unconformity --- -- Lower Precambrian granite and greenstone granite and greenstone CH. IV / GEOLOGY OF MINNESOTA 227 drilling to determine the cause of magneti c anomali es. More Knowledge of the pre-M iddle Precambrian rocks in th e recent work in the 1940's and 1950's utilized detail ed grav ­ Cuyuna di strict is limited. Lower Precambrian rocks (fi g. it y survey to supplement airborne and ground mag netic IV-I S) include granitic rocks ex po ed near the Pine Ri ver surveys and drilling. The geologic studies show compl ex fo lding and some local fa ulting. mark ed lateral vari ati on in the magneti c character and the fac ies of the iron-forma­ tio ns, and longi tudinal variati on in th e lithology of rock units. These factors alone, in the absence of glacial drift and Cretaceous sediments, would require careful geologic mapping to show the geologic relati onships. Much interpre­ tation of limited in fo rm atio n is required to give continuity to the geology, so the geologic maps represent an approx i­ mati on of the distribution of form ati ons and of the rock structures . Diffe rent interpretati ons of th e geology can be expected if additional geological, geophysical, and explora­ tion work is done. Thi report has utilized data from all published and un­ published o urces to which I have had access. In pl aces where data are in confl ict, I have exercised my judgment in accepting or rej ecting interpretations and information; for example, some of the earl y reports appear to use the term " slaty iro n fo rmati on" fo r oxidi zed, red-brown slate having a low iron content. The Trommald Formati on is the main m arker bed used in interpreting the structure and strati ­ graphy of the Cuyun a district. I nasmuch as iron-formations commonl y are id ent ified in drill samples without in forma­ ti on on the associated rocks, an iron-bearing member that occurs in th e Rabbit Lake Form ati on locall y may be mI s­ takenl y identified as the T rommald Formati on. STRATIGRAPHY The generalized sequence of Middle Precambrian rocks in the C uyun a di strict is rather simple (table IV-3) . It con­ sists of a central iron-formation that is underl ain by clas ti c strata and a dolomite fo rm ati on and is overl ain by cl astic, locall y carbonaceous strata that include an intercalated fe r­ ruginous slate and iron- fo rmati on un it. Detai ls of th e litho­ logic, strati graphic, and sedimentati o nal relati onships with­ in the fo rmatio ns are poorl y known, fo r much of th e avail­ EXPLANAT IO N abl e info rmati on is from geophysical surveys and expl ora­ Penokeon iIIIIIIiIIIIIIl ChlOf,I,zed In l (USI'"e Rock c=J Rabbi' Loke Formation. undifferentiated A'g"",e, slote, ti o n drilling designed to di scover iron or mangani fe rous iron graphlt,c argillite and slote WIth rnterbedded luffs ores. As most drill holes were located to check magnetic or and possibly flo ws and ferruginOus orgllll le and slote and "on-forma t ion ;n t he Sou th and Nor th ranges. grav ity anomali es or to outline areas of potential ore, drill­ V-:.:·.:j Upper Rabbll Lake Formation ing was done la rgely along the iron- fo rmati on zo nes. Deter­ ~ Emily Member - RabbIt Lake Fo r mallon minati on of the strati graphy was a secondary considerati on. .~ ,. ; y'i Lower Rabbit Lake Formation The stratigraphic terminology defined by Schmidt ~ Trommo ld Forma tion ThIck · and thm-bedded Cherty (1963) for the C uyuna North range can be applied with 1...--- Iron -formO/lon ~ Mahnomen For mation ArgIllite, siote, sills/one and some modi ficati on throughout th e di strict. The rock se­ quar tz"e; local phyll"es and schis ts quence, from old est to youngest, includes ( I) Lower Pre­ , - - uncon formIty --, cambrian granites a nd greenstones, and (2) Middle Precam­ ~ Trout Lake for motion brian rocks consisting of a possible lower unit of clastic t---j Lo wer Closlics ? '-- uncon formity - - , strata, a dolomite here informally termed the 'Trout Lake Early Precambri an c=J Greenslone, gronlfe SC hIsts, etc formation," the Mahnomen Formation, the main iron-for­ Con/acts be'ween formations are not well defmed In large par's of 'he map area and represent 'he "kely forma/Ion boundofles. The Trommold Formallon IS shown m black With areas quened where mation (Trommald Formation), and the Rabbit Lake For­ dolo are par/lculorty scan/yo ~ mation. The Rabbit Lake Formation includes a lower clastic and volcanic member, a ferruginous slate and iron-forma­ 2 l 4 5 6 7 8 Miles ! tion member here termed the " Emily member," and an up­ per slate, graywacke, and argillite member. The stratigraphic Figure IV -IS. G eologic map of the Cuyuna distri ct (com­ seq uences in the Cuyun a d is trict and th e Mesabi range are piled from various sources including shown in T abl e IV-3. Schmidt, 1963). 228 MIDDLE PRECAMBRIAN Table IV-4. Chemical analyses, in weight percent, of selected samples of dolomite from the Trout Lake formation. Drilled Thickness Depth Locati on (in feet) (in feet) CaO MgO CO 2 Si0 2 Sec. 6, T. 137 N ., R. 26 W. 38 555-560 25.96 18.62 2.95 Sec. 31, T. 137 N ., R. 26 W. 120 350-355 10.53 9.40 21.64 50.56 445-450 13.55 18.08 36.12 14.86 Sec. 32, T. 138 N ., R. 26 W. 124 396 28.72 12.98 43.87 4.20 420 29.04 19.16 45.14 1.98 420 29.86 18.76 45.96 1.40 and in th e southeastern part of T. 138 N., R. 28 W ., mafic R. 28 W. is simil ar to the dolomite cut in the drill holes and intrusive rocks ex posed in th e central part of T. 138 ., R. is the only known probable exposure of the dolomite in the 29 W ., and greenstone ex posed in the north ern part of T. region. The glacial errati c was fi rst described by H arder 136 N., R. 29 W. In addition, it is reported th at a drill hole and Johnston (191 8, p. 63). Some of the dolomite contains in the SW I;4 NEI;4 sec. 14, T . 137 N., R. 27 W. intersected siliceous layers and patches th at have a granular tex ture greenstone, two drill hol es in the NW I;4 SEI;4 sec.