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Item Bulletin No. 13. Surface Formations and Agricultural Conditions of Northeastern Minnesota(Minnesota Geological Survey, 1917) Leverett, Frank; Sardeson, Frederick W.; Purssell, U.G.This bulletin is a preliminary paper which treats the soils of only the northeast quarter of Minnesota. It will be followed by a report on the entire state, the field work for which already has been completed. The work has been done in accordance with the agreement for cooperation between the United States Geological Survey and the Minnesota Geological Survey, entered into, March, 1912. By this agreement the services of Mr. Frank Leverett were secured for surveying the surface formations and soils. Mr. Leverett has been engaged since 1886, or thirty years, in studying the surface geology of the Great Lakes region and because of his large experience in the greater area he was particularly well prepared to undertake the studies in Minnesota. He has spent, moreover, considerable time in the state studying its physiography in connection with the preparation of a monograph for the United States Geological Survey. Since the reorganization of the State Survey, the salary of Mr. Leverett has been met by the United States Geological Survey, while the greater part of his expenses have been paid by the State Survey. The State Survey has provided also for this work the services and expenses of Professor F. W. Sardeson, who has assisted in this work for the past five seasons. For a short period, also, the State has supplied the services of Dr. Arthur H. Elftman. We wish to acknowledge the generous assistance of the Division of Soils of the Department of Agriculture of the University of Minnesota and of the United States Bureau of Soils. The valuable contributions to the knowledge of the surface formations of Minnesota by the Minnesota Geological and Natural History Survey, under the direction of Professor N. H. Winchell, particularly those of Mr. Warren Upham of that Survey, have aided greatly in the preparation of this report. The section on climatic conditions in Minnesota has been generously contributed without any cost to the Survey by Mr. U. G. Purssell, Director of the Minnesota Section of the United States Weather Bureau. In the preparation of the maps and other data showing dates of killing frosts, lengths of growing season, rainfall, etc., Professor C. J. Posey has rendered efficient service.Item Bulletin No. 21. The Geology and Magnetite Deposits of Northern St. Louis County, Minnesota(Minnesota Geological Survey, 1926) Grout, Frank F.This bulletin treats the geology and mineral deposits of a large area in northern St. Louis County, extending northward to the Canadian boundary. It lies north of the Mesabi iron range and includes most of the Vermilion iron range. It lies south of the Rainy Lake area of Canada. Because it adjoins the world's greatest iron-producing area it has been the subject of much interest to explorers and prospectors seeking new deposits of iron ore and parts of the area have been surveyed by the United States Geological Survey and the earlier Minnesota state surveys. Much of the area, however, had not been surveyed in detail. The report includes a general map of the area (Plate I) several detailed maps of small areas of economic interest and 88 township plats. These township plats are essentially outcrop maps and cover the entire area. approximately 2800 square miles.Item Bulletin No. 24. The Geology of the Rove Formation and Associated Intrusives in Northeastern Minnesota(Minnesota Geological Survey, 1933) Grout, Frank F.; Schwartz, George M.In the large area of Rove formation along the boundary of north- eastern Minnesota and Ontario there have been two important silver mines, both on the Ontario side of the boundary - Silver Islet and Silver Mountain. The existence of a number of smaller mines and prospects makes it certain that mineralization of this kind occurred over a wide area, and several other mineral occurrences have long been known; but no other important deposits have been developed. Since much of the area is concealed under glacial deposits and since only a reconnaissance geological survey had been made in the part of the area that lies in Minnesota, it seemed advisable to survey the area in more detail. An effort was made to map and study all large outcrops and most of the smaller ones, to record where the veins are and what is their nature, to map the formations more closely than before, and on these records to base an estimate of the probable mineral prospects of the district. The Gunflint iron-bearing formation, which has been treated before, is here referred to only incidentally. The Minnesota area of Rove slate is a narrow strip south of the international boundary from Pigeon Point to a few miles west of Gunflint Lake in Cook County. Since the base of the formation trends northeast into Ontario and the beds dip south, it is believed that eastern outcrops in Minnesota are at a higher horizon than those near Gunflint Lake.Item Bulletin No. 28. The Geology of the Anorthosites of the Minnesota Coast of Lake Superior(Minnesota Geological Survey, 1939) Grout, Frank F.; Schwartz, George M.The northeast corner of Minnesota, lying north of Lake Superior and comprising Cook and Lake counties and part of St. Louis County, is underlain by Keweenawan rocks except for a narrow strip along the Canadian border. Early studies of these rocks were made principally by Winchell and by Irving, as shown by the list of publications given below. Because of the size and inaccessibility of much of the region, these early studies were confined to a narrow strip along the shore of Lake Superior. Later Grout and other members of the Minnesota Geological Survey made detailed studies of the Duluth gabbro. Between the areas covered by these workers lies a region in which only reconnaissance work had been done previous to the mapping presented in this report. It is planned eventually to map in detail this entire area, but as it embraces approximately 4,000 square miles of forested or brushy country, much of it not easily accessible for detailed work, it will take many field seasons to complete the task. The particular region mapped in this survey was selected because of unusual plagioclase feldspar masses of very high purity, for which it is hoped uses may be found, but the results are also significant as an example of the geology of a great Keweenawan area. The Keweenawan of this region consists almost entirely of igneous rocks. and even the few sedimentary rocks known are closely connected with extrusive igneous activity. It is about centrally located in the Keweenawan area of the north shore of Lake Superior and lies above the thickest part of the Duluth gabbro, but probably is not connected with it by continuous intrusive masses below the drift.Item Bulletin No. 32, The Geology and Underground Waters of Northeastern Minnesota(Minnesota Geological Survey, 1947) Thiel, George A.In 1924 the Minnesota Geological Survey began the task of compiling up-to-date information on the underground waters of Minnesota. Dr. Ira S. Allison carried on much of the work for the northwestern portion of the state and prepared Bulletin 22, The Geology and Water Resources of Northwestern Minnesota, which was published in 1932. Shortly thereafter, Professor Thiel began the revision of the data for the southern half of Minnesota which had originally been discussed in the United States Geological Survey Water-Supply Paper 256 by C. W. Hall, O. E. Meinzer, and M. L. Fuller. Dr. Thiel's revision appeared as Minnesota Geological Survey Bulletin 31 in 1944. The present bulletin completes the series on the underground waters of Minnesota and makes available in published form data on the underground water resources of every county in the state. These bulletins cannot be considered final because water will continue to be developed as long as people inhabit the state. The Minnesota Geological Survey will therefore welcome cooperation in keeping up its file of data on underground waters and will be pleased to answer inquiries not covered by the published material. The state owes a debt to Professor Thiel for the very large amount of painstaking work which has gone into Bulletins 31 and 32. The three bulletins arc available from the University of Minnesota Press at a moderate price. Attention should also be called to the fact that all legal matters regarding underground, as well as surface waters, are by act of the legislature placed under the jurisdiction of the Department of Conservation.Item Bulletin No. 39. The Geology of Cook County Minnesota(Minnesota Geological Survey, 1959) Grout, Frank F.; Sharp, Robert P.; Schwartz, George M.Cook County covers a triangular-shaped area at the extreme northeastern tip of Minnesota between Lake Superior on the south and the province of Ontario, Canada on the north. Its area is approximately 1680 square miles, of which about 274 square miles is covered by several hundred lakes. Its position north of Lake Superior is responsible for a rather moist and cool climate favorable to the growth of timber rather than agriculture. As a result, most of the area is covered by second-growth forest and this, together with the numerous rock-bound lakes, makes it an important vacation area. The area is hilly with a minimum elevation above sea level of 602 feet at Lake Superior and 2232 feet in the Misquah Hills. Much of the northern part of the county is characterized by long narrow lakes separated by prominent ridges. The geology is controlled, in a broad way, by its position on the north limb of the Lake Superior syncline. With the exception of glacial deposits the rocks are all of Precambrian age, with the youngest in a general way occurring along the coast of Lake Superior and the oldest in the Gunflint district and near Saganaga Lake. The older rocks consist of the Ely greenstone, Saganaga granite and Knife Lake group of slates, graywackes, metamorphosed tuffs and various minor types. These form an area of exceedingly complex geology, limited to four townships at the northwest corner of the county. The next group in age, commonly called the Animikie rocks, consists of a thin quartzite followed by the Gunflint iron formation and this, in turn, by the Rove formation. These are correlated with the Biwabik iron formation and Virginia formation of the Mesabi district. The Gunflint formation is mainly limited to two of the four northwest townships noted above, but the Rove formation forms a narrow belt along the international boundary from Gunflint Lake to Pigeon Point, a distance of seventy miles. The beds of the Rove formation dip southward at low angles and have been intruded by numerous diabase sills. Erosion has left the sills standing as asymmetrical ridges between valleys occupied by long narrow lakes. Over two thirds of the county is underlain by rocks of Keweenawan age, consisting of a thin sandstone and conglomerate at the base overlain by an exceedingly thick series of lava flows. These, in turn, are intruded by the eastern part of the huge Duluth gabbro complex and by an extensive series of diabase sills, dikes, and irregular intrusions. The lava flows consist mainly of somewhat variable basalt plus a much smaller percentage of rhyolite. The oldest flows crop out near Grand Portage Bay and trend inland so that successive flows occur along shore to the west as far as Tofte, where the sequence is reversed. A total of 92 flows were mapped between Grand Portage and Tofte with an estimated thickness of over seventeen thousand feet. In the northwestern part of the gabbro exposed in Cook County there is a group of three granite and granodiorite masses of somewhat uncertain origin, but apparently of later age than the gabbro. During Pleistocene time glaciers probably invaded Cook County several times, but the drift now exposed to view represents deposits from the Rainy Lobe, which probably covered the entire county, and the Superior Lobe which covered only a narrow strip along Lake Superior. Drift is of sufficient thickness in some parts of the county to rather effectively mask the underlying rocks and leave unanswered questions about their detailed relations. Glacial lakes covered parts of the county during the waning stages of the glaciers and left abandoned beaches at several levels, as well as glacial-lake clay soils. In spite of the complex geology, Cook County has not furnished productive mineral deposits. Deposits of iron formation, titaniferous magnetite, copper sulfides, and lesser nickel and cobalt sulfides have been investigated from time to time. Forest resources and the resort business are the main sources of revenue, but fishing in Lake Superior and limited agriculture have added to the income. Recently the establishment of Taconite Harbor and a steam power plant by the Erie Mining Company at the end of their railroad from the Mesabi district has been an important addition to the economy.Item Bulletin No. 44. Geology of the Duluth Gabbro Complex near Duluth, Minnesota(Minnesota Geological Survey, 1964) Taylor, Richard B.Multiple intrusions at Duluth, Minnesota, form a rock series that is here called the Duluth Gabbro Complex. Each of the principal rock types transgresses one or more of the older units. The oldest rock, a coarsegrained anorthositic gabbro that was intruded into the Keweenawan flows, makes up the upper part of the complex. It was intruded by basaltic magma of a second period of magmatic activity which formed rocks that commonly are banded, and hence called the layered series. The gabbroic rocks of the layered series as well as the older anorthositic gabbro are cut by intrusive bodies of ferrogranodiorite and granophyre and by late dikes of basalt and aplite. The lower two-thirds of the complex, the layered series. is composed chiefly of troctolite, olivine gabbro, feldspathic gabbro, and syenogabbro. About 15,000 feet of layered rocks is exposed, locally with rhythmic banding, fluxion structure, and gravity stratification demonstrating bottom accumulation by crystallization under conditions of active magma circulation. A series of samples collected from bottom to top of the layered series shows only limited development of cryptic layering. This lack of cryptic layering may be explained as the result of periodic renewal of magma in the crystallizing chamber or by multiple small intrusions. Cross-cutting relations between the different types of gabbro in the layered series show that the mass originated by multiple intrusion. The lack of chilling effects indicates that the successive intrusions were not greatly separated in time. Near the top of the series a transition exists from gabbro to syenogabbro, and a similar transition may exist from syenogabbro to ferrogranodiorite. The rock series is similar in many respects to that of the Skaergaard Intrusion, but there are important differences that can be explained by dissimilar tectonic history. The poor development of cryptic layering and the absence of ferro gabbro at Duluth, in contrast to their remarkable development in the Skaergaard Intrusion, can be explained by differing tectonic stability. The Skaergaard magma apparently crystallized in a chamber under stable conditions, whereas the Duluth magma seems to have crystallized in an environment of tectonic instability manifested by multiple intrusions of magma. The various rocks of the Duluth Gabbro Complex can be explained by crystallization-differentiation of basaltic magma, although the origin of some, such as the anorthositic gabbro and intrusive peridotite, is puzzling. The rocks of the layered series probably were derived from a basaltic magma approaching the composition of analyzed late basalt dikes. Compared with the analyses of "marginal olivine gabbro" from the Skaergaard Intrusion, the Duluth parent magma seems to have been notably richer in K2O, TiO2, MnO, and P2O5. The Duluth Gabbro Complex is an immense sill-like mass that extends for 150 miles northeast from Duluth, and relations in other parts may differ from those at Duluth.Item Geologic Map Series 1. Bedrock Geology of Duluth and Vicinity St. Louis County, Minnesota(Minnesota Geological Survey, 1963) Taylor, Richard B.Duluth, Minnesota, is on the northwest limb of the Lake Superior syncline, a northeast-trending structure of Precambrian age. The north- west limb of the syncline dips 10-20 degrees S.E. toward Lake Superior and is dominated by the Duluth Gabbro Complex, a huge sill-like mass with crescentic outcrop that extends almost 150 miles from Duluth to near Hovland. At Duluth the gabbro complex lies on the Thomson Formation, and apparently was intruded along the surface of unconformity below the Keweenawan rocks. The gabbro complex was formed by multiple intrusion, and consists of an older anorthositic gabbro and a younger layered gabbro and related intrusions. Keweenawan flows above the gabbro mass are cut by diabase sills. The basalt flows at one locality currently are being quarried as a source of crushed rock for concrete aggregate.Item Geology of Grand Portage and Pigeon Point(Minnesota Department of Natural Resources, 1950-02) Schwartz, G.M.Brief summary of the geology of the Grand Portage State Park-Pigeon Point region, northeastern Minnesota.Item Guidebook 10. Field Trip Guidebook for the Western Vermilion District, Northeastern Minnesota(Minnesota Geological Survey, 1979) Southwick, D.L.; Ojakangas, R.W.The Vermilion Granitic Complex consists of granitic and migmatitic rocks of Archean age (2,700 m.y.) that are the westward extension into Minnesota of the Quetico gneiss belt of Ontario. The complex is chiefly light grayish-pink biotite granite (following the rock classification of Streckeisen, 1973) that grades into migmatite with increasing content of schistose inclusions. The major grayish-pink granite and its genetically related grani te rich migmati te were named the Vermilion Grani te by Grout (1923), who was the first to study the rocks in detail (Grout, 1923; 192~b,1926). Because migmatites are so abundant within the area Grout mapped as granite, and because other rock types such as quartz diorite and trondhjemite are important locally, the term Vermilion Granite has been replaced formally by the more inclusive term Vermilion Granitic Complex. Where the grayish-pink biotite granite that is the dominant component of Grout's Vermilion Granite is homogeneous, it has been renamed the Lac La Croix Granite, and it is understood to be a subunit within the Vermilion Granitic Complex.Item Guidebook 11. Field Trip Guidebook for the Keweenawan (Upper Precambrian) North Shore Volcanic Group, Minnesota(Minnesota Geological Survey, 1979) Green, John C.The Upper Precambrian plateau lavas of the Lake Superior region were produced in response to tensional rifting of North America roughly 12001120 x 106 years ago (White, 1972a). Geological and geophysical evidence shows that they consist of a group of about eight separate volcanic accumulations which partly overlap in time and space as the locus and activity of rifting changed along what is now the Midcontinent Gravity High (Green, 1977). Nearly all of these lava accumulations ("plateaus") contain preserved sections which range in thickness from 2.5 to 7 km and are made of hundreds of individual flows. They have subsided centrally during and/or after eruption. Basalt of various compositions predominates; the most common type is nonporphyritic olivine tholeiite with unusually high Al content and ophitic texture. Transitional to weakly alkaline basalt is also common. Quartz tholeiite, basaltic andesite and rhyolite are moderately abundant in most of the plateaus, and icelandite (intermediate quartz latite) is found in some.Item Guidebook 17. Field Trip Guidebook for Selected Areas in Precambrian Geology of Northeastern Minnesota(Minnesota Geological Survey, 1987) Balaban, N.H., EditorCONTENTS STRUCTURAL GEOLOGY OF THE BOUNDARY BETWEEN ARCHEAN TERRANES OF LOW-GRADE AND HIGH-GRADE ROCKS, NORTHERN MINNESOTA, P.J. Hudleston, R.L. Bauer, D.L. Southwick, D.D. Schultz-Ela, and M.E. Bidwell GEOLOGY OF THE KEWEENAWAN (UPPER PRECAMBRIAN) BEAVER BAY COMPLEX IN THE VICINITY OF SILVER BAY, MINNESOTA, James D. Miller, Jr. ROADLOG AND STOP DESCRIPTIONS FOR THE BEAVER BAY COMPLEX, James D. Miller, Jr., Paul W. Weiblen, and John C. GreenItem Guidebook 2. Field Trip Guide Book for Lower Precambrian Volcanic-Sedimentary Rocks of the Vermilion district, Northeastern Minnesota(Minnesota Geological Survey, 1972) Ojakangas, R.W.; Morey, G.B.The present investigation, carried out since 1962 as part of continuing studies of the Lower Precambrian rocks of northern Minnesota, has consisted of broad regional geologic mapping in the western half of the district and adjacent areas (Sims and others, 1968; Sims and others, 1970) and detailed mapping and study of critical areas (Gabbro Lake quadrangle, Green and others, 1966; and Green, 1970; Isaac Lake quadrangle, Griffin and Morey, 1969; Embarrass quadrangle, Griffin, 1969; Tower, Shagawa Lake, and Ely quadrangles, unpublished maps). Systematic geologic mapping has not been carried out in the eastern part of the district as a part of the present re-study; however, the excellent geologic map of Gruner (1941) in the type area of the Knife Lake Group remains a useful one, and S.S. Goldich, G.N. Hanson, and associates have examined critical areas in the Saganaga Lake Northern Light Lake area as part of regional geochronologic studies. This field trip starts a few miles south of Ely and ends a few miles west of Tower, and is designed as a two-day trip. Typical outcrops of all formations in the district are included, but most stops will examine the Lake Vermilion Formation. The rocks that will be seen on this trip are tightly folded and steeply plunging; therefore, we will be looking at a cross-section (but a structurally complicated one) of a volcanic-sedimentary pile. All the rocks in the area are metamorphosed, but for simplicity the prefix "meta" will generally be omitted.Item Guidebook 20. Field Trip Guidebook for the Geology and Ore Deposits of the Midcontinent Rift in the Lake Superior Region(Minnesota Geological Survey, 1995) Miller, James D., Jr.This guidebook has been prepared for the field conference and symposium on "The Petrology and Metallogeny of Volcanic and Intrusive Rocks of the Midcontinent Rift System" that will be held in Duluth, Minnesota in August of 1995 as part of International Geological Correlation Program (IGCP) Project 336 ("Petrology and Metallogeny of Mafic and Ultramafic Magmatism"). The four-year-Iong IGCP project was begun in 1992 to investigate the petrology and metallogeny of various intracontinental mafic igneous provinces with an overall goal of establishing geological criteria for targeting mineral deposits in such environments. The principal goal of the 1995 meeting is to showcase to the international scientific and exploration communities the significant advances in our understanding of the structure, mineralization, and magmatic history of the Midcontinent rift. Toward this end, three field trips have been organized around the three-day symposium to be held August 25-27, 1995 in Duluth. The field trips are designed to provide a broad overview of the geologic framework of the Midcontinent rift, where it is best exposed in the western Lake Superior region. Field trip I (August 19 -24) looks at the physical volcanology of the flood basalts exposed on Isle Royale. Field trip II (August 22-24) highlights the stratigraphy, structure and mineralization of the volcanic and sedimentary rocks forming the southern limb of the Midcontinent rift in Wisconsin and Upper Michigan. The post-symposium field trip ill (August 28-September 1) focusses on the geology and ore deposits associated with the intrusive rocks of the Midcontinent rift exposed along the north shore of Lake Superior in Minnesota and Ontario.Item Guidebook 3. Field Trip Guide Book for Precambrian North Shore Volcanic Group Northeastern Minnesota(Minnesota Geological Survey, 1972) Green, John C.Detailed mapping of the 85th Minnesota shore of Lake Superior began with A. E. Sandberg's study (1938) of the section between Duluth and Two Harbors. Grout and Schwartz (1939) and Gehman (1957) studied the intrusions and flows in eastern Lake County; Grogan (1940) mapped the lakeshore between Two Harbors and Split Rock River; Schwartz (1949) studied the Duluth area; and Grout and others (1959) mapped most of Cook County. James Kilburg (1972) has recently mapped the wedge of lavas just west of Duluth. Most of the data reported in this account derive from studies by the writer who, starting in 1965, has mapped the shoreline between Silver Bay and Grand Portage, with considerable reconnaissance inland and to the southwest (Green, 1966; 1968a; 1968b; 1970). The report does, however, also lean considerably on Grout and others (1959) and, for the Duluth-Two Harbors area, on Sandberg (1938). The field studies have been supported by the Minnesota Geological Survey, and most of the laboratory studies have been .supported by the National Science Foundation (Grant No's GP-5865 and GA-134ll). Sincere gratitude for this support is extended to both agencies. The writer's ideas have benefited from discussions with many other geologists concerned with Keweenawan rocks, especially including Bill Bonnichsen, D. M. Davidson, Jr., H. Hubbard, G. B. Morey, W. C. Phinney, P. W. Weiblen, and W. S. White. Trip will leave Duluth and head up-section in the southwestern limb of the basin.Item Guidebook 6. Field Trip Guide Book for Precambrian Geology of Northwestern Cook County, Minnesota(Minnesota Geological Survey, 1972) Weiblen, P.W.; Davidson, D.M. JrAn exceptionally complete record of Precambrian history is recorded in the rocks exposed in Cook County, Minnesota. In northwestern Cook County, in the vicinity of the Gunflint Trail the Lower Precambrian is represented by a metavolcanic succession, which was intruded by the somewhat younger Saganaga Tonalite. These rocks are unconformably overlain by the Middle Precambrian Animikie Group, consisting of the Gunflint Iron Formation and the Rove Formation. In northeastern Cook County, a gently dipping angular unconformity separates Middle Precambrian and Upper Precambrian strata. There, a thin basal sandstone, the Puckwunge Formation, is overlain by volcanic rocks of the North Shore Group. The Logan intrusions and the Duluth Complex intrude and truncate Middle and Upper Precambrian rocks and comprise the major part of the Upper Precambrian section in northwestern Cook County. Although the geology of Cook County was summarized by Grout and others (1959), geologic mapping since 1962 has considerably revised the earlier geologic interpretation. Because much of this work is unpublished as yet, a comprehensive summary is presented here. The discussion is meant to provide a framework for the specific aspects of the geology which the chosen stops illustrate.Mileages for this trip are listed by stop as distances in miles along Minnesota 12 (The Gunflint Trail) going both northwest from Grand Marais and southeast from Trails End Campground, a round-trip distance of about 120 miles. Figure 1 indicates the location of the Gunflint Trail as well as the general geology of the area. A larger scale geologic map of the field trip area together with all the field trip stops is shown in Figures 2 and 3, while the cross section on Figure 2 and the block diagrams of Figure 4 represent the gross structural relationships between the units encountered on the field trip.Item Information Circular 10. Copper and Nickel Resources in the Duluth Complex, Northeastern Minnesota(Minnesota Geological Survey, 1974) Bonnichsen, BillThe Ely-Hoyt Lakes region, in northeastern Minnesota, contains very large quantities of disseminated copper and nickel sulfides that are potentially minable. The principal sulfides are pyrrhotite, chalcopyrite, cubanite, and pentlandite, and the average copper-nickel ratio in the sulfide concentrations is approximately 3:1. The sulfides are associated with the northwestern margin of the Duluth Complex, a large mafic igneous pluton of Late Precambrian age, and occur principally within the basal zone of the intrusion. A conservative estimate of the metal resources in the Ely-Hoyt Lakes region, based on assay data available from 24 drill holes, and a cutoff grade, or lower mining limit, of 0.5 percent combined copper and nickel indicates 13.8 million tons of copper and 4.6 million tons of nickel having a gross value of $27 billion. Although the copper and nickel deposits in the Ely-Hoyt Lakes region are marginal economically, they constitute one of the United States' principal metal resources.Item Information Circular 37. Scientific Core Drilling in Parts of Itasca, St. Louis, and Lake Counties, Northeastern Minnesota, 1989-1991(Minnesota Geological Survey, 1993) Meints, Joyce P.; Jirsa, Mark A.; Chandler, V.W.; Miller, James D., Jr.This information circular is the fifth since 1980 to report on the basic results of test drilling (Fig. 1) by the Minnesota Geological Survey (MGS). As with the previous projects, this test drilling was conducted primarily to provide a better understanding of the Precambrian geology in areas of Minnesota where bedrock is covered by variable thicknesses of glaciogenic sediment, and thus could not be thoroughly mapped by standard outcrop mapping and sampling procedures. The drilling summarized here was conducted during 1989-1991 in areas where the bedrock is locally well exposed, but where exposures are separated by covered terrane. Thus, the selection of drill targets was constrained largely by the interpretation of geophysical surveys (Chandler, 1983a, b). Our procedure is to formulate a regional geologic interpretation based on geophysical maps, new and existing outcrop data and the archived records of previous drilling, and then to constrain and test the interpretation by selective drilling. The results have been incorporated into new geologic maps of the region including Jirsa and others (1991) and Southwick (in prep.). An important secondary objective of this project is to describe the Quaternary deposits that overlie bedrock, though no systematic regional interpretation of those deposits has been undertaken to date. This report summarizes the results of studies in two areas of vastly different geology: 1) Archean and Early Proterozoic rocks of the Cook area in west-central St. Louis and eastern Itasca Counties, and 2) Middle Proterozoic rocks of the central Duluth Complex in parts of St. Louis and Lake Counties. The geology of each area is summarized in the "results" section of this report.Item Information Circular 44. Geochemical Evaluation of Platinum Group Element (PGE) Mineralization in the Sonju Lake Intrusion, Finland, Minnesota(Minnesota Geological Survey, 1999) Miller, James D., Jr.This study was undertaken to evaluate the potential for stratiform platinum group element (PGE) mineralization in the Sonju Lake intrusion (SLI). The SLI is a well differentiated, tholeiitic, mafic layered intrusion that is part of the multiple intrusive Beaver Bay Complex, and is exposed near Finland, Minnesota. Geochemical analyses of outcrop and drill-core samples that span the 1200-m-thick SLI were interpreted in the context of (1) data from PGE reefs in other tholeiitic layered intrusions (most notably the Skaergaard intrusion of East Greenland), and (2) models of PGE and sulfide mineralization in tholeiitic intrusions. The geochemical data show that a PGE-mineralized horizon is present approximately two-thirds of the way up from the base of the moderately south-dipping, sheet-like SLI. PGE mineralization was apparently related to initial sulfide saturation and subsequent exsolution of sulfide melt from the SLI magma. The data also indicate that as it settled, the sulfide melt efficiently scavenged PGEs from the SLI magma, and concentrated them into a relatively narrow interval of gabbroic cumulates. The geochemical signature of the SLI is remarkably similar to that of the PGE-bearing Platinova reef in the Skaergaard intrusion. The highest Pd, Pt and Au concentrations in SLI outcrop samples are far from economic grade (360, 66, and 85 ppb respectively); but a more precise evaluation of peak grade, thickness, and lateral continuity requires drilling and analysis of the entire interval. When the results of the study reported here are combined with evidence for similar mineralization in the Layered Series at Duluth, they indicate that tholeiitic layered intrusions of the Duluth and Beaver Bay complexes are favorable exploration targets for reef-type PGE mineralization.Item M-199, Bedrock Geology of the Mark Lake Quadrangle, Cook County, Minnesota(Minnesota Geological Survey, 2018) Boerboom, Terrence J; Green, John CPortrays the bedrock geology of the Mark Lake quadrangle which prior to this effort was largely unmapped. The map shows the distribution of the various rock types, locations of bedrock outcrops, and structural attributes of the bedrock. Mapped outcrops were used to constrain the geology for the most part, but mapping was augmented by the use of geophysical maps, and lidar imagery. Lidar was especially useful in locating bedrock outcrops during field work, and also for delineating the various bedrock units during the map compilation stage following fieldwork.