Grunerite in the Penokee Range of Wisconsin

Dr. Tom Fitz

Northland College

Ashland, WI

December 2013

UW - Madison

1

Ironwood Fm

Proposed mine

Location of Proposed Mine

2

This satellite image shows the area between Mellen, Wisconsin and

Ironwood, Michigan, with the location of the iron formation indicated in red.

This is the area of the proposed mine, with the outline of the mine property

shown as a dotted line. The area is about 4.5 miles long and bounded

approximately on the west by Ballou Creek and on the east by Tyler Forks

River. Most of the maps and cross sections in this presentation show this

same area. This map is from GTAC Bulk Sampling Plan of 11/25/13.

3

Cannon et al., 1996, USGS

Bedrock Geologic Map

4

This is a bedrock geologic map with the Ironwood Iron Formation shown

in red. All of the bedrock units have been tipped to north at an angle of

65 degrees, so the edges of the bedrock formations are stripes across

the area. The stripe of iron formation is about 1000 feet wide at the

surface. The iron formation was deposited about 1.9 billion years ago.

C. Hester and T. Fitz

Geologic Cross Section

Ironwood Fm

5

This geologic cross section shows a slice across the Penokee Range,

looking west. It shows how the formations dip 65 degrees to the north.

The Ironwood Formation forms the crest of the Penokee ridge.

T. Fitz

Quartz rich

Magnetite rich

6

This is typical Ironwood Iron Formation. It consists of dark-colored

iron-rich layers that are mostly magnetite, alternating with light-

colored layers dominated by the mineral quartz.

T. Fitz

Grunerite rich

Magnetite rich

Not all of the light-colored layers are quartz-rich though, in some areas

they are dominated by grunerite, which is an iron-rich amphibole mineral.

7

T. Fitz

Grunerite-rich layer

8

This is a closer view of a grunerite-rich layer.

T. Fitz

Grunerite-rich layer close up

9

This is a closer view of a grunerite-rich layer. The grunerite is

light grey to light brown slender crystals in a fan shape. The

ruler shows millimeters.

C. Hester and T. Fitz; October 2013

10

Grunerite-bearing iron formation was found in the summer and fall of

2013 in several locations in the Ashland County portion of the GTAC

project area.

C. Hester and T. Fitz; October 2013

11

This map shows the location of known grunerite-bearing rocks

in relation to the possible mine outline as of October, 2013.

D. White and T. Fitz; November 2013Asbestiform grunerite (green dots)

Locations with asbestiform grunerite known as of 12/5/2013

12

Additional bedrock outcrops with grunerite were found in November, 2013. No

grunerite-bearing rocks have been found just east of the county line, but amphibole

needles have been described just east of the proposed mine area.

Ashla

nd C

ounty

Iron C

ounty

J. Skulan and T. Fitz13

This is a grain of grunerite as seen through a microscope with light transmitted

from behind the sample. The entire grain is about 1 mm across. It is from Bulk

Sample Site 4 in Ashland County.

T. Fitz

Grains of Penokee grunerite

under transmitted light microscope.

The field of view is about 2mm.

14

M. Bjornerud

15

This is another microscopic view of a grunerite-rich rock from Bulk Sample Site 4

in Ashland County. The brightly colored mineral is grunerite, the black mineral is

magnetite. The grunerite constitutes approximately 75% of the rock, which is

common for the grunerite-rich layers of the iron formation. The photograph was

taken with polarized light shining through a thin slice of rock. Field of view is 4mm

M. Bjornerud

1 mm = 1000 μm (approximate scale)

magnetite

16

Here is a closer view of the same rock shown in the previous slide.

UW - Madison

SEM photomicrographs(Scanning electron microscope)

17

Here are more microscopic views -- these ones were taken with a

scanning electron microscope, which can zoom in very close. These

photographs were taken at the University of Wisconsin in Madison. The

next slide shows the image in the middle right side.

UW - Madison

18

This image shows much the same thing that we see at all scales, from

hand-sample scale to microscopic scale – the grunerite is long slender

crystals, most of which form a fan shape.

UW - Madison

X-Ray Diffraction Pattern from Penokee Grunerite

19

This is an x-ray diffraction pattern taken at the University of

Wisconsin Madison that confirms the mineral is grunerite.

Klein and Hurlbut, Manual of Mineralogy

Common Amphiboles

20

This diagram shows the composition of some of the common amphibole minerals.

Grunerite is in the lower right corner, the most iron-rich amphibole, which is what we

would expect to find in an iron-rich rock. All of these minerals can be hazardous

“asbestiform” varieties, but they are not always asbestiform.

Klein and Hurlbut, Manual of Mineralogy

Amphibole Cleavage

21

All amphibole minerals split along planes of weakness (at 124- and 56-degree

angles) when broken. With asbestiform varieties, the long, slender crystals will

break (or “cleave”) along their length, so if the mineral is crushed, it can make lots

of long, slender fragments.

Penokee grunerite under SEM

(scanning electron microscope)

22

One of the important questions is whether or not the crystals and fragments in the

Penokees are dangerous asbestiform varieties. Asbestiform generally means that

the pieces are longer than 5 microns (0.005 mm), and are long and slender –

specifically they have a certain ratio of length to width (called “aspect ratio”).

3:1

5:1

10:1

20:1

5 microns

23

There are various classifications of asbestiform -- different classifications

include anything that is at least three times longer than wide, others include

fragments 20 times longer than wide. As shown in this slide, the grunerite in this

part of Ashland County is commonly more than 20 times longer than wide,

which means it is asbestiform. Asbestiform grunerite is often called “amosite”.

USGS.

UICC amosite asbestos standard

24

Let’s compare it to asbestos minerals from elsewhere. This is the

amosite standard used by the Union for International Cancer

Control. Notice that the aspect ratio is about the same as that

seen in the previous slide from the Penokees.

Amosite standard

25

Penokee grunerite (amosite)

This is the grunerite (amosite) from the Penokees shown along with the

amosite standard. The photographs are at approximately the same scale.

Penokee grunerite and anthophyllite asbestos at the same scale

Penokee grunerite

Anthophyllite asbestos from Georgia

26

Here is another comparison -- the Penokee sample is on the left, a

standard anthophyllite amphibole asbestos on the right, shown at

approximately the same scale. Notice that they have similar aspect ratios.

Cannon et al., 1996, USGS

So, why is there asbestiform grunerite (amosite) in the Penokees? Grunerite

commonly forms in rocks rich in silicon and iron when they are heated (or

“metamorphosed”) to the right temperature. These rocks were probably heated by

magma intrusions when the region had large volcanic eruptions 1.1 billion years

ago. Some of the intrusions have been mapped to the north of the area, and

others are likely present but not visible at Earth’s surface today.

Magma intrusions

27

Cannon et al., 2006

The volcanic rocks are shown on the right of this cross section. Magma

had to come up through the iron formation on its way to the surface,

which made the conditions correct for the crystallization of grunerite.

28

T. Fitz

This heating and recrystallization of the rock is why the asbestiform

mineral is an integral part of the layers in the iron formation and does not

occur as thin isolated veins the way does is in some iron-rich rocks

elsewhere, such as in the Peter Mitchell iron mine in Minnesota. That mine

has been carefully studied for possible asbestos-like minerals.

29

M. Bjornerud

So what does all this mean? It means there definitely is asbestiform grunerite

(amosite) within the area of the mine proposed by GTAC. Dust from this rock would be

a health hazard if it is generated in large quantities during excavation or mining. That

needs to be considered during the permitting and possible development of a mine.

30

References

Aldrich, H.R.,1929, The geology of the Gogebic Iron Range of Wisconsin,

Wisconsin Geological and Natural History Survey, Bulletin 71.

http://digicoll.library.wisc.edu/cgi-bin/EcoNatRes/EcoNatRes-

idx?id=EcoNatRes.WGB71Econ24

The amphibole reference is on page 211, near the top of the page.

Cannon et al., 1996, Bedrock geologic map of the Ashland and Northern part of the

Ironwood 30x60 quadrangles, Wisconsin and Michigan: U.SG.S. Miscellaneous

Investigation Series Map I-2566.

Cannon et al., 2008, The Gogebic Iron Range – A sample of the northern margin of

the Penokean fold and thrust belt, U.S.G.S Professional Paper 1730.

(With detailed geologic map. Available on-line and in paper.

http://pubs.usgs.gov/pp/pp1730/

GTAC Bulk Sampling Plan of November 25, 2013. Posted on the Wisconsin DNR

web page:http://dnr.wi.gov/topic/Mines/documents/gogebic/BulkSamplePlan20131125.pdf

Hurlbut and Klein, 1993, Manual of Mineralogy, 21st edition, John Wiley and Sons.

Additional information about selected slides

(by slide number)

1) Photomicrograph of grunerite from the Penokees. See notes on slide 17 for more

information (below).

3) This map is cropped from Gogebic Taconite’s (GTAC) Bulk Sampling Plan

submitted to the Wisconsin DNR on November 25, 2013:

http://dnr.wi.gov/topic/Mines/documents/gogebic/BulkSamplePlan20131125.pdf

5) This geologic cross section was based on the map in USGS Professional Paper

1730 by Cannon et al., 2008. There is no vertical exaggeration in the topography. The

cross section was drawn by Tom Fitz and drafted by Cyrus Hester of the Bad River

Natural Resources department.

6 and 7) The rocks in these photographs are located along the main access road in

the proposed mine area in Ashland County.

8) This rock is from Bulk Sample Site 4 in Ashland County.

9) This is a close-up of the same rock shown in slide 8.

10 and 11) These maps were drawn by Cyrus Hester.

12) The amphibole needles are described on page 211 in the publication by Aldrich,

1929 (see references above).

15 and 16) These photomicrographs were taken under cross-polarized light by

Marcia Bjornerud at Lawrence University. The rock is from Bulk Sample Site 4.

17 and 18) These SEM (scanning electron microscope) photomicrographs were

taken at the geology department at University of Wisconsin in Madison. The rock is

from Bulk Sample Site 4.

19) This is from the rock shown in slides 17 and 18.

24, 25, and 26) These SEM images of asbestos were taken at the U.S. Geological

Survey Microbeam Laboratory in Denver. http://usgsprobe.cr.usgs.gov/picts2.html

29) The Peter Mitchell mine in Minnesota has been studied because the fibrous

minerals in taconite waste rock at Silver Bay, Minnesota come from that mine. The

fibrous minerals there are ferroactinolite amphibole, ferrian sepiolite, and small

amounts of grunerite.

This slide show was presented by Tom Fitz on December 5, 2013 at the Iron

County Citizen’s Forum in Oma, Wisconsin. The slides have been slightly

revised and detailed captions have been added for posting on the web.

For more information, contact Tom Fitz at Northland College:

tfitz@northland.edu