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
I. INTRODUCTION
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:
CI=contact, intrusive
DF=dike, felsic.
FG=fault, inferred from outcrop gap.
FN=fault, normal.
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.
NC=vein, calcite-bearing.
NM=vein, miscellaneous assemblage; typically
combinations of quartz, hematite, calcite, and iron carbonate.
NQ=vein, quartz-bearing.
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.
VI. ACKNOWLEDGEMENTS
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
LTV—Bruce
Gerlock
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