FINAL REPORT TO THE MINERALS COORDINATING COMMITTEE
ON BEDROCK AND QUATERNARY GEOLOGIC MAPPING
OF THE MESABI IRON RANGE
October 7, 2005
Minnesota Geological Survey—University of Minnesota Project #523-6141
Funded September 2, 2003 through September 30, 2005
Submitted to: Dennis Martin, Minnesota Department of Natural Resources, Division of Lands and Minerals
From: Mark Jirsa, Carrie Jennings, Val Chandler, and Harvey Thorleifson, Minnesota Geological Survey
Bedrock and Quaternary geologic maps created for this project by the Minnesota Geological Survey form the backdrop for myriad digital databases that are intended to address issues of hydrogeology and land-use. Those databases document and interpret such observations as the measured orientation of bedrock structures, and the location of seeps and flows into the mines, buried glacial-sediment-filled channels, and hundreds of mine photographs. The following is a brief summary of work conducted and results obtained—it is an unedited text supplement to the accompanying geologic maps and ArcView projects on paper and CD-ROM. The project results are contained on two CD-ROMs—one with data from the study of bedrock geology is included with this report; and a second containing data about the Quaternary geology, which will follow shortly.
The project was funded primarily by the Minnesota Legislature on recommendation of the Minerals Coordinating Committee. It is the culmination of nearly 6 years of effort by the Minnesota Geological Survey to provide framework geology to iron range companies, agencies, and individuals to facilitate land-use planning. Map interpretations rely heavily on 2 prior biennia of funding by the Legislative Commission on Minnesota Resources. The maps and ancillary data associated with this project are considerably more detailed than was originally envisioned because of a significant contribution to the project by Minnesota Geological Survey State Special funding. Much of this funding was directed toward the Quaternary portions; specifically, mapping by a graduate student, Wade Reynolds, of mine pit lakes in much of the western part of the Mesabi range. Wade’s work was also funded in part by a grant from the U.S. Geological Survey under the 2004 EDMAP program.
II. CONTRACT REQUIREMENTS
This section describes the project objectives as expressed in the contract, and the methods employed and deliverables produced to address them. The contract states that the contractor shall:
1. Compile existing geologic data
2. Conduct fieldwork
3. Create bedrock geologic map
4. Create geologic map of Quaternary (surficial) materials
5. Create geologic sections
6. Produce maps and sections for publication
7. Provide final report containing compilation tables, maps graphics interpretations on 2 CDs with data for entire study area, all digital layers, and all attributes.
Data for mapping (contract items 1-4) were compiled from available sources, as demonstrated in the references cited on the geologic maps. These sources include published maps and reports, and those unpublished data available to us. It should be stressed that the geologic maps are not strict compilations, in that the interpretations depicted may differ significantly from the source maps. Compilation of existing works was augmented in most areas by field work conducted by the authors during 2003 and 2004. This work included canoeing more than 200 miles of mine shoreline and visiting nearly every abandoned and active mine. Considerably more field work was conducted than originally planned. In addition, various other types of data were incorporated that typically were not available to earlier workers. For example, the Quaternary map relied heavily on digital land surface topography and various soil surveys. The Quaternary map largely excludes depiction of the man-made surface materials (waste rock, tailings, stockpiles, etc), and instead depicts the inferred continuity of natural geologic units before modification by mining. The Mine Lands Database maintained by Department of Natural Resources, Division of Lands and Minerals should be consulted for location and content of the materials of mining. Interpretation of the bedrock geology was made in the context of recently completed bedrock topographic maps, mining company maps and data, contour maps of the base of iron-formation, and several derivatives of aeromagnetic data. The latter includes high-pass filtered images of aeromagnetic data, and data regridded specifically for this project from the original flight lines. Despite the great mass of data used in the creation of these products, we consider the geologic maps to be largely at the reconnaissance level of detail. Areas nearest to waypoints and stations, and continuous exposure as documented in the photographs are accurate and useable at a scale of 1:24,000. Areas now buried by mine tailings or at a distance from waypoints are less accurately mapped and should not be relied upon at that scale. If detail is needed in any particular area, you should consult the various databases on file at MGS and contact the authors for impressions and interpretations that are not reflected in the mapping.
The requirement for geologic sections (contract item 5) is addressed by a series of photographs and photographic sections. Nearly 1000 photographs were taken during the course of this work, in part to document the appearance of mines and geologic features—a slice in time. For the sake of disk space and brevity, only a select set are incorporated into this report. The photos and photo-sections are keyed by unique station number to locations shown on the accompanying Arcview project. The Photo Catalogs list all the photos taken for each discipline (mostly M-prefix for bedrock; Q-prefix for Quaternary), with a color to indicate images included with this report. The photographic sections—in some cases composited from several individual images or taken from panoramas—use line and type overlays to convey the geologic interpretation. Most of these images have been significantly reduced in resolution from their original formats to fit on the CD-ROMs. If images of greater resolution are required, contact the authors.
This written report, associated data tables on CD-ROM, and the published maps satisfy contract items 6 and 7. In addition, we have included a brief summary of significant observations, separated into the two disciplines—bedrock vs. Quaternary geology—and a list of practical uses of the information to date. A PowerPoint presentation explains the mapping methods and provides a summary interpretation of the results.
III. BEDROCK GEOLOGY PRODUCTS
A. Digital and printed products
1) Final Report—printed and on CD-ROM
2) Bedrock geologic map—printed (also as PDF and part of ArcView project on CD-ROM)
3) Mine photo catalog of bedrock features—300 images on CD-ROM, hotlinked to project
4) ArcView Project for bedrock—on CD-ROM
5) PowerPoint presentation “Mesabi.ppt”—on CD-ROM
The published bedrock geologic map is referenced as follows:
Jirsa, M.A., Chandler, V.W., and Lively, R.S., 2005, Bedrock geology of the Mesabi Iron Range, Minnesota: Minnesota Geological Survey Miscellaneous Map M-163, scale 1:100,000.
B. Details of ArcView Project for BEDROCK data; themes include:
1) Stations.shp=Station numbers that relate to bedrock-related observations, mine photos, and structural measurements. Associated table lists station number and a general mine name.
2) Bgoc.shp=Bedrock outcrops. Includes several outcrop databases from prior projects of the MGS, and new mine outcrops mapped for this project. Some overlap of outcrops occurs locally.
3) Bgpg.shp=Geologic units coded by age, formation, and rock type.
4) Bgln.shp=Faults and unit contact lines.
5) Struclns.shp=Diabasic dikes and fold axes.
6) Intslate.shp=Approximate position of intermediate slate unit of Biwabik Iron Formation.
7) Gemstr.shp=Bedrock structure showing attitude of bedding, joints, faults, slickensides and fold axes measured at specific stations. Associated table records azimuth and plunge of dip for all planar structures; plunge direction and degrees from horizontal for linear structures. SEE below for more details.
8) Seeps.shp=Location of seeps and flows into mines. This database is not the result of a thorough search for seeps, and instead represents seeps discovered in the process of reconnaissance field work. Associated table shows station number (where one was assigned) and source of seeps—either from fractures in rock (rk), layers in the overburden (ob) that may include glacial materials or waste-rock, or at the rock/overburden interface (rk/ob). The visually estimated height in feet above 2004 water levels is given. File also includes the location of surface flows that were observed.
9) Quatchannel.shp=Approximate locations of depressions on bedrock surface filled with glacial material. Typically shown as a line along portions of mine shorelines or pit outlines. Depressions are commonly, but not everywhere filled with varying amounts of sand and gravel.
10) Pitlakes.shp=Mine pit lakes modified locally from DNR Mine Lands Database using Farm Service Administration air photography. Most are abandoned and flooded natural ore mines.
11) Tacmines.shp=Approximate outline of taconite mining; modified locally from DNR Mine Lands Database using air photography.
12) Btlines.shp=Bedrock topographic contours at 20-foot intervals; modified from Jirsa and others, 2002; 2005—see map for references.
13) Msbmag_lp=(Folder: mag_grids) A georegistered image of total field magnetic anomaly data constructed from the original flight lines specifically for this project, and modified using low-pass filter. The image is part of the ArcView project; and also is included on this CD-ROM as an ESRI grid.
14) Msbmag_hplp=(Folder: mag_grids) Magnetic grid image created by processing original flight line data, then applying a high-pass filter over the low-pass-filtered data from Msbmag_lp to remove much of the effect of high frequency anomalies and accentuate subtle contrasts of magnetic signature. The image is part of the ArcView project; and also is included on this CD-ROM as an ESRI grid.
15) msb_fsa.img=Clipped version of Farm Service Administration air photography. Imagine image format (Imagine extension is standard with ArcView).
16) Basemap datasets=Compiled and modified from 1990 TIGER/line digital files maintained by the U.S. Bureau of the Census. Subdivided into east and west halfs of the study area. East is in folder eastbase, West in folder westbase.
17) BRphoto.shp=(in folder BRHOTOS) Point locations that generally correspond to the camera position from which the photos were taken. Photographs are linked to the APR so that users can select a location using the “Hotlink” icon and view the image. Image resolution is generally quite low—if higher resolution is needed, please contact the authors.
18) Legends—folder containing ArcView legend files for shapefiles (.avl)
19) Export—folder containing ArcInfo file interchange datasets (e00) for the bedrock linework, bedrock polygons, dikes and the maparea.
20) Iview—folder containing the jpeg viewer used for the hotlinks in the ArcView project.
21) m163bg.apr—the ArcView project file.
STRUCTURES associated with the theme Gemstr.shp
Structures were measured using sun compass locally. In most places, sun compass was used to verify lack of significant magnetic deflection, and magnetic compass was used. For most planar structures, the azimuth of dip and amount is given (note that this differs by 90 degrees from strike). Lineations (anticline and syncline axes, and slickensides) are recorded as azimuth and plunge.
Definition of structure codes:
FG=fault, inferred from outcrop gap.
FT=fault, thrust (includes reverse faults locally).
FZ=fault or shear zone, undifferentiated.
IM=igneous modal layering.
IU=igneous fabric, undifferentiated.
JJ=joint, prominent set; where more than one measurement is given, they were entered into database in approximate order of prominence—from most to least prominent.
JM=joint with mineralized coatings; typically hematite, less commonly hematite and quartz.
JW=joint with wall-rock alteration; typically oxidation; includes weathered zones along joints, faults, and narrow collapse zones where bounding faults are too close to be shown at scale.
LA=lineation of anticlinal axis.
LS=lineation of synclinal axis.
MO=monocline, axial plane.
NM=vein, miscellaneous assemblage; typically combinations of quartz, hematite, calcite, and iron carbonate.
SF=lineation of slickensides; arrow indicates inferred sense of upper plate movement.
C. Details of Mine Photographs and Photo Catalog
Mine photographs record the status of mine pits during the field season 2004; providing a “slice in time” of the nature of pit walls and water levels. Most of this set of photographs is intended to convey the appearance of particular bedrock geologic features, such as bedding, joints, veins, faults, folds, alteration, and mineralization. The associated Excel table contains the following fields:
· Photo Number—Typically corresponds to the field station number, which represents the location from which the photo was taken. In many places, the station location is on the outcrop; whereas the photo may have been taken some distance away from the outcrop to capture the target feature. Those photographs highlighted are included on the CD-ROM, others are available on request.
· Mine/Pit Name—Mine names are a combination of specific names taken from Skillings Mining Directories, and more informal pit complex names assigned by the authors to represent multiple mines now flooded into a single pit lake.
· Subject—Description of the geologic feature of interest. Contains numerous abbreviations; users are encouraged to contact the author if explanations are needed.
· Vector—Direction that the camera faced; ie., “N” indicates that camera was pointed north when photo was taken.
· Topic—One- or several-word categorical descriptor.
Photographs are linked to locations (BRphoto) on the ArcView project—they can be viewed by using the “Hotlink” icon to select the image location. Photographs may be copied for digital display, with credit given to Mark Jirsa, Minnesota Geological Survey; however, the author requests that you contact the MGS for permission and a higher resolution version of the image if they are needed for print or other publication formats.
D. Highlights of field observations about the bedrock geology
1) Animikie Group strata along much of the Mesabi range are surprisingly undeformed, with many miles of straight, gently dipping strata that is neither obviously faulted nor folded.
2) Many faults have little affect on stratigraphic continuity; most are simply joint trends with minor offset and irregular zones of oxidation, brecciation, solution, collapse, and weathering—all to varied degrees of development.
3) Some structures carry or control hematite mineralization; others are the focus of intense oxidation; still others are loci of deep chemical weathering. Distinguishing which structures control which of these mineralizations is needed in future work. We probably have the data needed to start this evaluation.
4) Low angle structures are rare. Some have dip-slip movement that indicates thrusting, others are normal.
5) Though rare, thrust features include north-verging folds (in intermediate slate), thrust faults, and slickensides with south-over-north vergence (in Lower slaty member).
6) Much of the deformation in natural ores is hosted by paired faults bounding collapse grabens.
7) Many of the natural ore zones are focused along structures that vary from faults to collapse structures to monoclines.
8) In many places, monoclines traced along strike become faults and visa versa; and many monoclinal structures are locally “broken” by faulting locally.
9) Much of the deformation observed in mine pits is likely a near-surface phenomenon; most taconite company geologists report little or no mappable offsets in taconite mined beneath natural ore mines (Hibtac, Keetac, Minntac, United). However, this is based largely on drilling with spacing that may not give a clear picture of faults having only minor offset of a few feet to tens of feet.
10) Most magnetic lows identified by aeromagnetic maps can be linked to visible oxidized, leached, and weathered zones on the ground. This could not, however, be established for North Shore Taconite and parts of the abandoned LTV mines. Some of those lows may be the product of removal by mining of significant thicknesses of magnetic materials. High-pass filtered data provide excellent detail in areas of strong magnetic intensity. In most areas, these details can be related to specific structural or stratigraphic observations on the ground.
11) Veining by quartz and hematite is most common in natural ore mines. Veins occur in both bedding-parallel trends (for some 100 or more feet from vertical faults) and as vertical, locally brecciated veins in fault zones. In the natural ores, there is clear evidence that veins are mobilizing hematite (paragenesis indicates both synchronous with and after quartz). It is not clear whether these veins represent primary leaching, or introduction of hematite; or simply secondary remobilization. Authors intend to compare and contrast orientation of veins having different paragenetic sequences.
12) In many natural ore mines, hematite occurs along nearly all joints in both solid-blue and botryoidal forms.
13) Quartz-hematite veins are present in nearly all monclines, broken monoclines, collapse structures, and faults—in both natural ore and taconite mines. Both steep veins and bed-parallel veins occur. The Minntac taconite mine and the adjacent Mountain Iron natural ore mine provide a proximal contrast of veining between unaltered and altered rock. Veins in taconite (ie “unaltered”) contain varied percentages of quartz, magnetite (intergrown w/qtz), iron-carbonate (likely ankerite), pyrite, and chalcopyrite. The continuation of those veins in oxidized rock at Mountain Iron contain quartz, hematite (as primary massive material intimately associated with quartz), and secondary hematite as botryoids and coatings, and exsolution pits exist that have the shape of carbonate.
14) McKinley mine contains early asymmetric folds, rotation, and veining consistent with north-vergence (thrusting). Many slickensides in slaty units show similar slip kinematics. Thrusting is also evident locally at Hibbing Taconite, United Taconite, and several LTV mines.
15) LTV 6 mine contains a large nappe structure, and sheath folds in Lower slaty member. These structures appear early, yet they contain veining that implies some brittle behavior. Perhaps not coincidentally, both underlie the Aurora sill that is inferred to have been emplaced during the Mesoproterozoic, and may have played a role in the deformation.
16) Oxidation zones at Hibbing Taconite were mapped by company personnel using closely spaced drilling and Davis tube analyses. These zones occur along steep structures, and are mapped at several different mining levels. In most cases, field work indicates these structures are faults. In all cases, the width and continuity of oxidation decreases with depth—implying that oxidation is a comparatively near-surface phenomenon.
17) Cross-bedding is common in cherty units of the iron-formation, especially in the Upper cherty member. Most dip basinward to the south and west, but local bimodal-bipolar examples occur. Cross-bedding is best preserved in oxidized rocks where the foreset beds consist of white weathered granular chert, capped by thin beds containing abundant hematite.
18) Monoclines are important structures in the localization of oxidation and leaching, and many of the natural ore mines lie along them. The trends of many of these structures cross the strike of bedding at a small angle, as displayed on aeromagnetic data. Examples include monoclines in Forster, Hartley-Burt, Hawkins, and LaRue mines. The Alpena monocline exposed in the Sauntry and Enterprise mines (N of Rouchleau) is one of the most spectacular structures on the range. The fold limb dips to overturned, and the fold axial zone contains en echelon lenses of asymmetric intrafolial folds inferred to be the product of drag. Folding is spacially (and perhaps temporally) associated with quartz and hematite “flooding”, especially in a sandy zone at the base of the Upper cherty member.
IV. QUATERNARY GEOLOGY PRODUCTS (PENDING-October 21, 2005)
This portion of the Final Report is likely to be modified somewhat before submission—an updated version will accompany those products.
A. Digital and printed products
1) Surficial geologic map— printed (also as PDF and part of ArcView project on CD-ROM)
2) Geologic Sections—stratigraphic interpretations at several locations drawn on photographs of mine walls.
3) Mine photo catalog of Quaternary features—CD-ROM only
4) Table of sample analyses—CD-ROM
5) ArcView Project (APR) —CD-ROM
The Quaternary geologic map is referenced as follows:
Jennings, C.E., and Reynolds, W.K., 2005, Quaternary geologic map of the Mesabi Iron Range, Minnesota: Minnesota Geological Survey Miscellaneous Map M-164, scale 1:100,000.
B. Details of ArcView Project for Quaternary data
1) Waypoints.shp=Waypoint numbers that relate to bedrock-related observations, mine photos, and structural measurements. Associated table lists M-number and general mine name.
2) Q-number locations. Locations from which samples were taken. Textural analyses were conducted for many—the results are tabulated in an Excel file.
3) Qgpolys.shp=Geologic units coded by age, mode of deposition, and material type.
4) Qglns.shp=Various line symbols and unit contact lines.
C. Highlights of field observations about the Quaternary geology
V. USES OF THE DATA
During the course of this project, our observations have been used by a number of agency and company individuals—some of the practical uses of the data that we are aware of include the following:
1) Bedrock structural control for the development of Well Head Protection plans for municipalities by the Minnesota Department of Health.
2) Estimation of material thicknesses such as waste piles and stock piles for infrastructural considerations at the MSI-Butler Taconite property.
3) Bedrock structure and processed aeromagnetic imagery were used to qualitatively refine taconite resource estimations and distribution at the MSI project near Nashwauk.
4) United Taconite used bedrock structure and processed aeromagnetic imagery to refine location of mining to avoid areas of oxidation in new northeast part of Thunderbird mine.
5) Academic training exercise for Mining Engineering Department at University of Utah; mine planning near Biwabik. Structural data were used to constrain the hypothetical model.
6) Barr Engineering used historic maps and other data for environmental impact studies of LTV site.
7) Bedrock topography matches magnetic maps in outlining faults.
8) Both bedrock topography and magnetic maps allowed more accurate mapping of geologic contacts and faults.
9) Products, though reconnaissance level, are being used for current LCMR-funded work in the central iron range. Quaternary geology and bedrock topography acquired during earlier LCMR and MCC funding identified major channels in the bedrock surface that may play a role in maintaining mine water levels.
10) A comprehensive structural history of the Biwabik Iron Formation and associated rocks is emerging, using bedrock data acquired for this project in tandem with rock strain work being conducted by a professor at Macalester College, St. Paul, and similar studies of the adjacent Gunflint Iron Formation by a Canadian academic researcher.
Geologic mapping was supported by grants from the Minnesota Legislature on recommendation of the Minerals Coordinating Committee and the U.S. Geological Survey EDMAP program, and by the State Special Appropriation to the Minnesota Geological Survey. Map construction relied heavily on drilling information, mine maps, advice, and property access provided by a great number of individuals from mining companies and state agencies, including the following:
Cliffs Mining Services—Alan Strandlie and Ronald Graber
Department of Natural Resources-Division of Waters—John Adams
Department of Natural Resources-Division of Lands and Minerals—James Sellner and Joseph (Tim) Pastika
Eveleth Fee Office—Dan England
Great Northern Iron Ore Properties—Roger Johnson
Hibbing Taconite—Mike Orobona, Bill Everett
ISPAT Inland Steel Company—John Arola and Steven Mekkes
Keewatin Taconite—Jeff Price and Joseph Scipioni
Meriden Engineering—Dave Meineke and Beth Hooper
Midland Labs (Butler Taconite)—Elmer Rhode and Frank Kangas
Natural Resources Research Institute—Mark Severson, Julie Oreskovich
Northshore Mining Company—Douglas Halverson
United Taconite—Peter Jongewaard
U.S. Steel Corporation, Minntac—Jerry Dombeck, Frank Pezzutto, and Bruce Kniivila