U.S. Department of the Interior

U.S. Geological Survey

Open-File Report 2020–1069

Mineral Resource Database for Deposits Related to the 

Mesoproterozoic Midcontinent Rift System, 

United States and Canada

Mineral Resource Database for Deposits 

Related to the Mesoproterozoic 

Midcontinent Rift System, United States 

and Canada

By Laurel G. Woodruff, Klaus J. Schulz, Connie L. Dicken, and 

Suzanne W. Nicholson

Open-File Report 2020–1069

U.S. Department of the Interior

U.S. Geological Survey
U.S. Geological Survey, Reston, Virginia: 2020

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Suggested citation:

Woodruff, L.G., Schulz, K.J., Dicken, C.L., and Nicholson, S.W., 2020, Mineral resource database for deposits related 

to the Mesoproterozoic Midcontinent Rift System, United States and Canada: U.S. Geological Survey Open-File 

Report 2020–1069, 20 p., https://doi.org/10.3133/ofr20201069.

ISSN 2331-1258 (online)

U.S. Department of the Interior

DAVID BERNHARDT, Secretary

U.S. Geological Survey

James F. Reilly II, Director
iii

Contents

Abstract...........................................................................................................................................................1

Introduction.....................................................................................................................................................1

MRS Mineral Deposit Database..................................................................................................................3

Structure of the MRS Mineral Resource Database.................................................................................3

Deposit Class.........................................................................................................................................4

MRS Tectonic Development Stage....................................................................................................4

Deposit Type ..........................................................................................................................................4

MRS Tectonic Stages and Related Mineral Deposit Types ....................................................................4

Plateau Stage Characteristics............................................................................................................4

Plateau Stage Magmatic Mineral Deposits .....................................................................................5

Conduit-Type Nickel-Copper-PGE Sulfide ..............................................................................5

Layered Iron-Titanium Oxide......................................................................................................5

Alkalic-Hosted Uranium-Niobium±Rare Earth Elements ......................................................5

Miscellaneous Magmatic Sulfide .............................................................................................6

Rift Stage Characteristics ...................................................................................................................6

Rift Stage Magmatic Mineral Deposits.............................................................................................6

Contact-Type Copper-Nickel-PGE Sulfide ..............................................................................6

Titanium-Iron Oxide ....................................................................................................................7

Miscellaneous Magmatic Sulfide .............................................................................................7

Late-Rift Stage Characteristics ..........................................................................................................7

Late-Rift Stage Magmatic-Hydrothermal Mineral Deposits..........................................................7

Copper Sulfide-Bearing Quartz Veins.......................................................................................8

Copper±Molybdenum Porphyry ................................................................................................8

Copper-Molybdenum±Gold Breccia Pipe................................................................................8

Post-Rift Stage Characteristics..........................................................................................................8

Late-Rift Stage and Post-Rift Stage Hydrothermal Mineral Deposits..........................................8

Veins in the Thunder Bay Area, Northern Ontario .................................................................8

Mainland Group Silver-Bearing Veins.............................................................................8

Island Group Polymetallic Silver-Bearing Veins............................................................9

Lead-Zinc-Silver Veins.......................................................................................................9

Amethyst-Bearing Veins....................................................................................................9

Lead-Zinc-Barite Veins ......................................................................................................9

Unconformity-Related Uranium-Bearing Veins .............................................................9

Copper Sulfide-Bearing Veins ..........................................................................................9

Timing of Thunder Bay Vein Mineralization....................................................................9

Sediment-Hosted Stratiform Copper......................................................................................10

Compressional Stage Characteristics.............................................................................................10

Compressional Stage Hydrothermal Mineral Deposits................................................................10

Native Copper±Native Silver in Basalt and Interflow Sedimentary Rocks......................10

Second-Stage Native Silver±Native Copper.........................................................................11

Chalcocite in Basalt ..................................................................................................................11

Unconformity-Related Uranium Veins....................................................................................11

Summary........................................................................................................................................................11

References Cited..........................................................................................................................................12

Appendix 1.....................................................................................................................................................19
iv

Figures

1. Map showing the distribution of major bedrock units of the Mesoproterozoic

Midcontinent Rift System in the northern United States and northern 

Ontario, Canada ............................................................................................................................2

2. Summary of the tectonic and mineralization history of the Midcontinent 

Rift System...................................................................................................................................12

Tables

1. Database of mineral deposits related to the Mesoproterozoic Midcontinent 

Rift System (MRS) in the northern United States and northern 

Ontario, Canada .......Available for downloading from https://doi.org/10.3133/ofr20201069

1.1. Definitions for mineral deposit categories in the database (table 1) for

deposits located in Ontario, Canada .......................................................................................19

1.2. Definitions for mineral deposit categories in the database (table 1) for

deposits located in the United States .....................................................................................20
v

Conversion Factors

International System of Units to U.S. customary units

Multiply By To obtain

Length

centimeter (cm) 0.3937 inch (in.)

millimeter (mm) 0.03937 inch (in.)

meter (m) 3.281 foot (ft) 

kilometer (km) 0.6214 mile (mi)

Area

square meter (m2) 0.0002471 acre 

square kilometer (km2) 247.1 acre

square meter (m2) 10.76 square foot (ft2) 

square kilometer (km2) 0.3861 square mile (mi2)

Volume

cubic meter (m3) 35.31 cubic foot (ft3)

cubic kilometer (km3) 0.2399 cubic mile (mi3) 

Flow rate

millimeter per year (mm/yr) 0.03937 inch per year (in/yr) 

Datum

Horizontal coordinate information is referenced to the World Geodetic System of 1984 (WGS 84).
vi

Abbreviations

Ga giga-annum (billion years ago)

GLIMPCE Great Lakes International Multidisciplinary Program on Crustal Evolution

Ma mega-annum (million years ago)

MDI Mineral Deposit Inventory

MNDM Ontario Ministry of Energy, Northern Development and Mines (Canada)

MPVG Mamainse Point Volcanic Group

MRDS Mineral Resources Data System

MRS Midcontinent Rift System

NSVG North Shore Volcanic Group

OUI oxide-ultramafic intrusion

PGE platinum-group element

PLV Portage Lake Volcanics 

REE rare earth element

USGS U.S. Geological Survey

USMIN U.S. Geological Survey Mineral Deposit Database
Mineral Resource Database for Deposits Related to the 

Mesoproterozoic Midcontinent Rift System, United States 

and Canada

By Laurel G. Woodruff, Klaus J. Schulz, Connie L. Dicken, and Suzanne W. Nicholson

Abstract

The Midcontinent Rift System (MRS) of North America 

is one of the world’s largest continental rifts and has an age of 

1.1 Ga (giga-annum). The MRS hosts a diverse suite of magmatic and hydrothermal mineral deposits in the Lake Superior 

region where rift rocks are exposed at or near the surface. As 

part of the construction of a database summarizing information 

on mineral deposits in the MRS, data from regional mineral 

deposits were downloaded from the U.S. Geological Survey 

(USGS) Mineral Resources Data System (MRDS), the USGS 

Mineral Deposit Database (USMIN), and the Ontario Ministry 

of Energy, Northern Development and Mines Mineral Deposit 

Inventory (MDI). Deposits related to MRS rocks or mineralizing events were identified and compiled into a database to 

develop a space/time classification for MRS-related mineral 

deposits. Information from MRDS, USMIN, and MDI records 

and from the extensive literature describing MRS mineral 

deposits was used to classify each entry by deposit type, host 

rock age and type, and estimated mineralization age. Most 

deposits were readily classified because of unique mineralogy, 

location, or well-constrained host rock. These deposits were 

then put into a tectonic evolutionary framework for the MRS, 

which showed that many deposits formed within discrete 

spatial and temporal stages of rift evolution.

Introduction

The Mesoproterozoic Midcontinent Rift System (MRS) 

of North America is approximately 2,200 kilometers (km) 

long and is one of the world’s largest continental rifts. It has 

a curvilinear shape that stretches from Kansas northeast to 

the Lake Superior region where it turns southeast and extends 

through Michigan (fig. 1). Although the rocks of the MRS are 

largely overlain by younger units, except in the Lake Superior 

region, the full extent of the MRS is highlighted by large positive aeromagnetic and gravity anomalies that were created by 

its huge volume of mafic volcanic and intrusive rocks (King 

and Zietz, 1971; Van Schmus and Hinze, 1985).

Rift volcanism lasted more than 20 million years, from 

about 1,112 Ma (mega-annum) to about 1,090 Ma, with 

minor eruptions lasting until about 1,083 Ma (Davis and 

Sutcliffe,1985; Palmer and Davis, 1987; Davis and Paces, 

1990; Heaman and Machado, 1992; Paces and Miller, 1993; 

Davis and Green, 1997; Zartman and others, 1997; Vervoort 

and others, 2007; Swanson-Hysell and others, 2014; Fairchild 

and others, 2017; Swanson-Hysell and others, 2019). A conservative estimate of the volume of rift-related erupted basalt 

is 2 million cubic kilometers (km3), possibly with an equal 

volume of intruded rock (Cannon, 1992). The large volume 

of MRS mafic igneous rocks as well as their geochemical 

and isotopic characteristics support the existence of a mantle 

plume as a source of thermal energy that helped create MRS 

melts (Hutchinson and others, 1990; Nicholson and Shirey, 

1990; Nicholson and others, 1997). Seismic reflection profiles 

from the Great Lakes International Multidisciplinary Program 

on Crustal Evolution (GLIMPCE) across the Lake Superior 

basin show that, during rifting, the Archean- to Paleoproterozoic-aged crust was thinned to less than half of its pre-rift 

thickness of about 50 km. The older crust was replaced by 

more than 20 to 25 km of rift-related basaltic lava flows overlain by up to 5 to 7 km of clastic sedimentary rock (see, for 

example, Behrendt and others, 1988; Cannon and others, 

1989; Hutchinson and others, 1990).
2 Mineral Resource Database for Deposits Related to the Mesoproterozoic Midcontinent Rift System, U.S. and Canada

Figure 1. Map showing the distribution of major bedrock units of the Mesoproterozoic Midcontinent Rift System (MRS) in the 

northern United States and northern Ontario, Canada. The MRS cuts through Archean, Paleoproterozoic, and other Mesoproterozoic 

bedrock (the “Non-MRS Precambrian rocks” unit). Much of the area shown in the figure is overlain by younger Paleozoic bedrock 

(the “Paleozoic rocks” unit).

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38°0'0" 38°0'0"

0 100 200 300 KILOMETERS

0 100 200 300 MILES

MRS intrusive rocks

MRS volcanic rocks

MRS sedimentary rocks

Paleozoic rocks

Non-MRS Precambrian 

rocks

EXPLANATION

Michigan

Wisconsin

Ontario

Minnesota

Iowa

Nebraska

Kansas

Lake Superior

Missouri

CANADA

UNITED STATES
Structure of the MRS Mineral Resource Database 3

MRS Mineral Deposit Database

The MRS hosts a wide variety of magmatic and hydrothermal mineral deposits. The types of deposits reflect 

changing magmatic and tectonic events that marked the development and evolution of the MRS. An extensive literature 

describes MRS mineral deposits, both regionally and locally 

(see, for example, Cannon and McGervey, 1991; Nicholson 

and others, 1992; Miller and others, 2002; Smyk and Franklin, 

2007; Miller and Nicholson, 2013). To capture information 

on these numerous and variable deposits, a comprehensive 

database summarizing MRS mineral deposits (table 1; available as a separate file) was compiled from the U.S. Geological Survey (USGS) Mineral Resources Data System (MRDS; 

U.S. Geological Survey, 2011) and the Ontario Ministry 

of Energy, Northern Development and Mines (MNDM) 

Mineral Deposit Inventory (MDI; Ontario Ministry of Energy, 

Northern Development and Mines, 2019). MRDS is a searchable collection of archived reports that describe metallic and 

nonmetallic mineral resources of the United States and other 

countries. Data include deposit name, location, commodity, 

deposit description, and geological characteristics. However, 

MRDS is no longer supported by the USGS and is being 

replaced by the USGS Mineral Deposit Database (USMIN; 

U.S. Geological Survey, 2019), an updated national mineral 

deposit database. Deposit information in the Ontario MDI 

includes deposit name, occurrence type, and primary and 

secondary commodities. Deposits in MRDS and the MDI are 

included in this compilation; three additional deposits recognized through recent exploration that are in USMIN (Burger 

and others, 2018) are included here for completeness.

A few known MRS-related deposits described in recent 

literature that are not listed in MRDS, USMIN, or the MDI 

are not included in table 1. In addition, several MRDS entries 

for the commodity “copper” in Michigan and Wisconsin—

many of which refer to native copper mineralization in MRS 

basalts—are called “unnamed copper prospect” or “unnamed 

copper deposit” or “unnamed copper mine.” All such entries 

have sparse information and are ranked with a quality grade of 

“D” (out of a scale ranging from “A” to “E,” where “A” is a 

most reliable and informative record and “E” is a least reliable 

and informative record) (Schweitzer, 2019). Because of their 

low quality, these records are not included in table 1.

Structure of the MRS Mineral Resource 

Database

The compiled MRS mineral resource database (table 1) 

lists 495 individual mineral prospects, occurrences, producers, 

and past producers that are characterized by columns of 

descriptive information. All but three columns contain 

explanatory information derived from MRDS, USMIN, or the 

MDI. These data attributes include:

• a unique entry identifier assigned by either MRDS 

(column A), USMIN (column B), or the MDI 

(column C);

• deposit name (column D) and alternate name(s) 

(columns E and F);

• deposit category (column G) (see appendix 1 for an 

explanation of categories); 

• country (column H);

• State or Province (column I);

• county, township, or area (column J);

• location defined by latitude and longitude coordinates 

expressed in decimal degrees (datum: World Geodetic 

System 1984 [WGS84]) (columns K and L);

• major commodities in order of their relative economic 

importance (columns M and N);

• minor commodities in order of their relative economic 

importance (columns O through R);

• ore mineralogy (if known) in order of their relative 

economic importance (columns S through X);

• gangue mineralogy (if known) (columns Y 

through AC);

• tectonic setting (inferred from host formation; 

column AD);

• host rock lithology, formation name, and age (if 

known) in order of their relative economic importance 

(columns AE through AJ); and

• associated rock lithology, formation name, and age (if 

known) (columns AK, AL, and AM).

The final three columns in the database (table 1) contain 

our interpretation of each deposit’s place in MRS space/time 

groupings. The entries in these three columns were informed 

by data in MRDS, USMIN, and the MDI, as well as by the 

extensive MRS mineral deposit literature. These last three 

columns are the focus of this publication:

• deposit class, defined as magmatic, hydrothermal, or 

magmatic-hydrothermal (column AN);

• MRS tectonic development stage during which the 

deposit formed (column AO); and

• specific name for a descriptive deposit type 

(column AP).
4 Mineral Resource Database for Deposits Related to the Mesoproterozoic Midcontinent Rift System, U.S. and Canada

Deposit Class

Attributes in column AN of the database (table 1) 

describe the deposit class for the mineral deposits. Most 

deposits are easily classified as either “magmatic” or “hydrothermal” based on mineralogy, setting, and the nature of host 

rocks. Magmatic deposits can form directly in place from a 

melt or consist of minerals crystallized from and transported 

by melt. These deposits are genetically related to their host 

rocks, and thus the mineralization age is the same as or is very 

close to the age of the host rocks. In contrast, hydrothermal 

deposits form by precipitation from hot metal-bearing fluids. 

Because fluids and metals may have originated far from the 

deposit, the timing of mineralization is often poorly constrained when compared with the timing of magmatic deposits. 

Deposits classified as “magmatic-hydrothermal” represent a 

hybrid of the two processes. Intruded magma can release highpressure, metal-bearing magmatic fluids that hydrofracture the 

overlying country rock. Mineralization in a permeable, veined, 

and brecciated carapace can result from mixing of magmatic 

and local hydrothermal or meteoric waters, and (or) a decrease 

in temperature and (or) pressure. Although mineralization 

often is distal to an intrusion, the timing of mineralization in 

these hybrid systems is closely related to the timing of related 

intrusive events.

MRS Tectonic Development Stage

Attributes in column AO of the database (table 1) identify 

our assigned tectonic development stages of the MRS. Based 

on age, physical volcanology, magnetic polarity, and geochemistry of MRS rocks in the Lake Superior region, three main 

magmatic stages of rift development are delineated: the early 

Plateau Stage (~1,112 to ~1,105 Ma), followed by the Rift 

Stage (~1,102 to ~1,090 Ma), and the Late-Rift Stage (~1,090 

to ~1,083 Ma). Prior to high precision uranium-lead (U-Pb) 

zircon dating, the magnetic polarity of MRS rocks was a major 

tool used to distinguish what is now named the early Plateau 

Stage from the later Rift Stage. Extensive characterization of 

the magnetic properties of MRS rocks (Books, 1972; Halls 

and Pesonen, 1982) showed that early MRS Plateau Stage 

basalts and intrusions mainly have reversed magnetic polarity, whereas the more voluminous Rift Stage flood basalts and 

intrusions have normal magnetic polarity. The change from 

reversed to normal magnetic polarity may have fluctuated 

over a short interval between about 1,102 Ma and 1,100 Ma 

(Swanson-Hysell and others, 2014); however, MRS rocks 

dated at ~1,102 Ma and older have reversed magnetic polarity and rocks dated at ~1,100 Ma and younger have normal 

magnetic polarity. This difference makes magnetic polarity of 

undated rock units (if known) a very useful tool for tectonic 

stage classification.

Two additional stages of rift evolution are important 

for mineralization related to the MRS. The Post-Rift Stage 

(~1,083 to ~1,060 Ma) represents a time interval dominated 

by clastic sedimentation between the older Late-Rift Stage 

and a younger Compressional Stage (~1,060 to ~1,040 Ma), 

which was a time of prolonged tectonic activity. All stages are 

described in more detail below.

Deposit Type

Attributes in column AP in the database (table 1) describe 

our deposit type names for each of the entries. The 495 MRSrelated mineral deposits in this database are classified into 

specific deposit types based on host rock, deposit characteristics, dominant commodities, and metal affinities. Individual 

deposit types are assigned to an MRS tectonic stage based on 

a combination of host rock age or magnetic polarity, mineralization age, and cross-cutting relationships. For some deposit 

types, detailed geologic descriptions in published literature 

were critical to distinguishing among them.

For example, the Eagle deposit in Michigan, now 

an operating nickel mine, is hosted by an MRS reversedmagnetic-polarity ultramafic intrusion that has a crystallization 

age of 1,107.2±5.7 Ma (Ding and others, 2010). It is described 

by Schulz and others (2014) as a conduit-type nickel-copperplatinum-group element (Ni-Cu-PGE) sulfide deposit. Thus, 

the Eagle deposit is classified by its deposit class (Magmatic; 

column AN), MRS tectonic development stage (Plateau Stage; 

column AO), and specific deposit type (Conduit-type Ni-CuPGE Sulfide; column AP).

MRS Tectonic Stages and Related 

Mineral Deposit Types

Plateau Stage Characteristics

The early Plateau Stage of MRS development extended 

from ~1,112 to ~1,105 Ma. MRS volcanic activity in western 

Lake Superior began with the eruption of high-magnesium 

pyroxene-phyric basalt onto a thin layer of mature quartz 

sandstone. Compositions of the earliest MRS basalt in western 

Lake Superior are consistent with low-percentage partial melts 

(~1 to 3 percent) produced near the base of the lithosphere at 

depths of ≥120 km (Schulz and Nicholson, 2016). Lateral and 

stratigraphically higher contrasting basalt compositions denote 

melting at shallower depths of ≤80 km (Schulz and Nicholson, 

2016). Such rapid transitions in melting depths are consistent 

with a subcrustal lithosphere in the Lake Superior region that 

was of variable thickness prior to the development of the MRS 

(Faure and others, 2011).

During this early stage, subaerial basalts erupted from 

fissures and low-angle shield volcanoes across a broad area 

to form a ≤10-km-thick volcanic plateau (Green, 1989). 

There is little evidence of actual rifting during the Plateau 

Stage (Green, 1983; Allen and others, 1995), although basalt 
MRS Tectonic Stages and Related Mineral Deposit Types 5

sequences appear to thicken toward the eventual rift axis in 

Lake Superior. Intrusions of sills and plutons accompanied 

volcanic eruptions. The original spatial extent of Plateau 

Stage lava flows is unknown because their terminations outward from the Lake Superior region are extensively eroded 

(Cannon, 1992). The distribution of magnetically reversed 

dikes, however, is evidence that Plateau Stage basalts extended 

south beyond the Lake Superior region into Minnesota and 

central Wisconsin (Green and others, 1987).

In Ontario, Canada, alkaline intrusions were emplaced 

into rocks of the Archean Superior Province, and diabase sills 

intruded conformably into flat-lying Paleoproterozoic and 

early Mesoproterozoic sedimentary sequences. In northeastern 

Minnesota, gabbroic layered intrusions and granophyric felsic 

bodies marked the beginning of intrusive magmatic activity 

that would result in the formation of the large Duluth Complex 

(Miller and others, 2002).

Plateau Stage Magmatic Mineral Deposits

Conduit-Type Nickel-Copper-PGE Sulfide 

Conduit-type Ni-Cu-PGE sulfide deposits are defined 

as magmatic sulfide mineralization restricted to small- to 

medium-sized (typically hundreds of meters to several kilometers thick and hundreds of meters to tens of kilometers long), 

mafic and (or) ultramafic, irregularly shaped sills, dikes, or 

chonoliths that served as pathways for the flow-through of 

picritic and (or) magnesium-rich basaltic magmas (Schulz 

and others, 2014). Conduit-type sulfide deposits often contain 

more metal and sulfur than could be derived just from the 

limited volume of magma that formed their host intrusions. 

This implies that the sulfide deposits were derived from a 

much greater volume of magma than the current volume of the 

intrusions that host them. Thus, the sulfide deposits represent 

the product of a large volume of magma that moved through 

open conduits and enriched an immiscible sulfide liquid 

with metals. Economic minerals in conduit-type deposits are 

primarily pentlandite, chalcopyrite, and pyrrhotite (Schulz and 

others, 2014).

Examples of this deposit type in the MRS include the 

Eagle deposit in northern Michigan (the only operating primary nickel mine in the United States since production began 

in 2014), the Tamarack deposit in east-central Minnesota, and 

the Thunder Bay North (also known as Current Lake) deposit 

in northern Ontario. The mineralized Eagle East intrusion 

is a possible extension of the mineralized Eagle intrusion, 

although drilling has not located any shared feeder connections. These metal-rich deposits typically vary from massive to 

semi-massive sulfide to disseminated sulfide with Ni>Cu and 

have only been found outside the central MRS rift where they 

intrude older basement rocks.

The Marathon Series of the Coldwell Complex is host 

to several Cu-Ni-PGE mineralized gabbroic and ultramafic 

intrusions, including the Two Duck Lake, Geordie Lake, and 

Area 41 intrusions (Good and Lightfoot, 2019). The Marathon 

Cu-PGE sulfide deposit in the Two Duck Lake intrusion has a 

different metal endowment than the other known conduit-type 

sulfide deposits, such as Eagle or Tamarack, as it has little or 

no Ni and extreme local enrichment of PGE (Good and others, 

2015). Mineralization is thought to have developed by multiple injections of crystal mush carrying sulfide droplets; thus, 

the Marathon Cu-PGE deposit is described as a conduit-type 

sulfide deposit (Good and others, 2015), although it differs in 

detail from other MRS sulfide deposits of this type.

Host rocks for MRS Ni-rich conduit-type sulfide deposits 

are the product of the high magnesium magmas that erupted 

only during the Plateau Stage of rift development (Nicholson 

and others, 1997; Ding and others, 2010; Taranovic and others, 

2015). Dates of intrusions that host conduit-type deposits are 

1,107.2±5.7 Ma for the Eagle deposit (Ding and others, 2010) 

and 1,105.6±1.2 Ma for the Tamarack deposit (Goldner, 2011). 

Recent dates for gabbro intrusions that host the Marathon 

Cu-PGE deposit are reported to be between 1,107.7±2.1 Ma 

and 1,106.1±2.2 Ma (Good and Lightfoot, 2019).

Layered Iron-Titanium Oxide

Layered Fe-Ti (iron-titanium) oxide mineralization 

occurs as banded segregations in the reversed-magneticpolarity Poplar Lake intrusion, a large layered intrusion of 

dominantly gabbroic cumulates along the northeastern contact 

of the Duluth Complex, Minnesota. The intrusion was dated 

at 1,106.9±0.6 Ma (Paces and Miller, 1993). Grout (1950) 

described drill core from the intrusion as a series of olivine 

gabbro layers grading to peridotite layers, with intervening 

segregations of magnetite-ilmenite oxides. Some oxide layers 

have up to 35 percent iron and 18 percent titanium dioxide 

(Grout, 1950).

Alkalic-Hosted Uranium-Niobium±Rare Earth 

Elements

In northern Ontario, along the trend of the Big BayAshburton fault—which Sage (1991) proposed was part of a 

much larger, generally north-south-trending Trans-Superior 

Tectonic Zone—is a series of diatremes, alkalic intrusions, 

and carbonatites. Among these are the alkalic Coldwell 

Complex, the Dead Horse Creek diatremes, and the Killala 

Lake Complex. In addition to sulfide mineralization, the 

Coldwell Complex has radioactive columbite-bearing syenite 

dikes along fractures that cut mafic intrusions (Scott, 1987). 

The Dead Horse Complex is a diverse group of highly altered 

heterolithic breccias that is being explored for uranium and 

rare earth elements (REEs) (Zurevinski and others, 2019). The 

Killala Lake intrusion has a syenite core surrounded by a rim 

of olivine gabbro; pegmatitic phases of the syenite contain 

pyrochlore (Sage, 1988).
6 Mineral Resource Database for Deposits Related to the Mesoproterozoic Midcontinent Rift System, U.S. and Canada

Miscellaneous Magmatic Sulfide

This group of magmatic sulfide-bearing mineral occurrences hosted by Plateau Stage intrusive rocks is not well 

characterized, although further exploration may better define 

its nature. Occurrences include sulfide mineralization within 

or near small mafic-ultramafic intrusions, such as Disraeli, 

Hele, and Seagull, all of which are in the Nipigon Embayment, 

Ontario (Hart and MacDonald, 2007). The Disraeli intrusion 

has been dated at 1,109±1.4 Ma, the Hele intrusion has been 

dated at 1,106.6±1.5 Ma, and the Seagull peridotite intrusion 

has been dated at 1,112.8±1.4 Ma (Heaman and others, 2007). 

Additional Plateau Stage magmatic sulfide deposits and occurrences include Fairbairne and Geordie Lake in the Coldwell 

Complex (~1,108 Ma) and Sandspit-Papaver in the Killala 

Lake Complex (undated).

Rift Stage Characteristics

A second magmatic stage, the Rift Stage, is constrained 

between ~1,102 and ~1,090 Ma. Following an interval of little 

apparent magmatic activity, rifting began with voluminous 

subaerial flood basalt eruptions within rapidly subsiding basins 

along the trend of modern Lake Superior and along the eastern 

and western arms of the MRS. Basalt compositions changed 

from magnesium-rich during the Plateau Stage to aluminumrich during this Rift Stage. Rift Stage basalt compositions 

throughout the Lake Superior region are generally similar and 

consistent with large quantities of melting at a relatively shallow depth (~45 km).

Eruptions were generally very frequent over the given 

timescale, with individual lava flows commonly occurring 

one after the other and keeping pace with subsidence such 

that successive flows erupted onto relatively flat surfaces 

(White, 1960). Along the Keweenaw Peninsula in northern 

Michigan, the 5-km-thick Rift Stage Portage Lake Volcanics 

(PLV) is composed of more than 200 individual basalt flows, 

many of which are remarkably uniform in terms of thickness 

and strike length (White, 1960). Swanson-Hysell and others 

(2019) used precise dating to estimate median eruption rates of 

1.2 millimeters per year (mm/yr) for the PLV and 2.1 mm/yr 

for normal magnetic polarity basalts of the North Shore 

Volcanic Group (NSVG) in northeastern Minnesota. Breaks in 

volcanism are indicated locally by the presence of sedimentary 

interflow conglomerate and sandstone units, several of which 

can be followed along strike for more than 100 km (Merk and 

Jirsa, 1982). These interflow sedimentary units are consistent 

time markers in the Rift Stage volcanic section in western 

Lake Superior, help to constrain flow stratigraphy along strike, 

and are an important host rock to hydrothermal native copper 

deposits in Michigan (Butler and Burbank, 1929).

Rift Stage mafic intrusions into the middle and upper 

crust formed the Duluth and Beaver Bay Complexes in 

Minnesota and the Mellen Complex in Wisconsin. Rift Stage 

Duluth Complex intrusions are tightly constrained between 

1,099 Ma and 1,098 Ma (Paces and Miller, 1993). Intrusions 

of the Duluth Complex are at the surface along its western 

margin, but the lower contact plunges southeastward beneath a 

carapace of the NSVG and the younger Beaver Bay Complex. 

The Beaver Bay Complex consists of mostly non-cumulate, 

hypabyssal mafic intrusions emplaced into the NSVG package (Miller and Chandler, 1997). Two intrusions from the 

Beaver Bay Complex have been dated at 1,096.1±0.8 Ma and 

1,095.8±1.2 Ma (Paces and Miller, 1993).

The timing of declining Rift Stage subaerial volcanism 

is established by the Greenstone flow in the upper part of the 

Portage Lake Volcanics. A recently published date for differentiated pegmatite in the flow is 1,091.59±0.27 Ma (SwansonHysell and others, 2019). Conglomerates indicating a shift in 

rift behavior from mainly magmatic to mainly sedimentary are 

only ~600 meters (m) stratigraphically above the Greenstone 

flow, which suggests that Rift Stage magmatism may have 

ended around 1,090 Ma.

Rift Stage Magmatic Mineral Deposits

Contact-Type Copper-Nickel-PGE Sulfide 

Contact-type Cu-Ni-PGE sulfide deposits (Zientek, 2012) 

are exemplified by mainly disseminated sulfide mineralization 

along the basal contacts between MRS Rift Stage intrusions 

and Paleoproterozoic and Archean country rock. Interstitial 

disseminated sulfides—including pyrrhotite, chalcopyrite, 

cubanite, and pentlandite—can be extremely erratic in spatial 

extent and ore grades along the basal units of the intrusions 

(Miller and others, 2002). Rift Stage contact-type sulfide 

deposits have Cu>Ni likely because Rift Stage magmas were 

lower in MgO (magnesium oxide) and richer in aluminum 

than magmas of the Plateau Stage that gave rise to the conduittype Ni-Cu±PGE sulfide deposits.

There are currently 11 identified contact-type sulfide 

deposits along the basal contact of the Duluth Complex 

where the South Kawishiwi, Bathtub, and Partridge River 

intrusions are in contact with Animikie Group metasedimentary rocks and Late Archean igneous, sedimentary, and 

metamorphic rocks (Miller and others, 2002). Major named 

deposits include Birch Lake, Dunka Pit, Maturi, Mesaba, 

NorthMet, Serpentine, and Spruce Road. Disseminated and 

local more massive sulfide mineralization of the Duluth 

Complex is estimated to contain about 4.4 billion metric tons 

of ore with average grades of 0.66 percent Cu and 0.2 percent 

Ni at a 0.5-percent Cu cut-off (Listerud and Meineke, 1977). 

Contact-type disseminated sulfide mineralization—consisting 

of chalcopyrite, cubanite, and pentlandite in thin patches and 

veinlets—also occurs near the base of the Mellen Complex 

in Wisconsin where it is in contact with the Paleoproterozoic 

Tyler Formation, a unit equivalent to the Virginia Formation.
MRS Tectonic Stages and Related Mineral Deposit Types 7

Titanium-Iron Oxide 

Titanium-iron oxide mineralization in normal magnetic 

polarity Duluth Complex intrusions is found in two spatially 

separated areas. The most important titanium-iron±vanadium 

(Ti-Fe±V) oxide deposits are late, discordant intrusions along 

the southern basal section of the Duluth Complex. These 

bodies are called oxide-ultramafic intrusions, or OUIs, and are 

pegmatitic, plug- and dike-shaped intrusive bodies that cut the 

Partridge River Intrusion and other Duluth Complex intrusions 

to the south (Severson, 1988). Deposits including Longnose, 

Longear, and Water Hen also carry minor Cu-Ni sulfide mineralization. Several OUIs are in faults or are close to fault zones, 

typically with nearby inclusions of Biwabik Iron-Formation 

(Hauck and others, 1997). These relationships suggested to 

Severson (1988) that there was a genetic connection between 

OUI Ti-Fe±V mineralization and inclusions of iron-formation. 

A separate group of titaniferous magnetite deposits along the 

northern basal contact of the Duluth Complex was interpreted 

to be the product of alteration and metamorphism of inclusions 

of the Paleoproterozoic Gunflint Iron-Formation (equivalent 

to the Biwabik Iron-Formation) by Duluth Complex magma 

(Broderick, 1917; Grout, 1950). One puzzling aspect of both 

types of Ti-Fe oxide mineralization, if it is related to assimilation of iron-formation, is the relatively high Ti content of the 

oxides, as iron-formation typically has very low Ti content 

(Broderick, 1917; Hauck and others, 1997).

Miscellaneous Magmatic Sulfide

Locations of massive to disseminated chalcopyrite and 

pyrrhotite mineralization, some known only from limited 

exploration drilling, are related to MRS gabbro and diabase 

dikes south of the Thunder Bay region, Ontario (Lightfoot and 

Lavigne, 1995; Smyk and Franklin, 2007). Most of the dikes, 

such as the Pigeon River swarm, have normal magnetic polarity; thus, these deposits are included in the MRS Rift Stage. 

The largest known deposit in the region is the Great Lakes 

Nickel Cu-Ni deposit in the Crystal Lake Gabbro. The Crystal 

Lake Gabbro is an approximately Y-shaped intrusion about 

5 km long with a shorter 2.75-km-long arm extending to the 

southeast (Smith and Sutcliffe, 1987). A date for the intrusion 

is 1,099±1.2 Ma (Heaman and others, 2007). Mineralization 

at Crystal Lake has characteristics similar to contact-type 

sulfide deposits of the Duluth Complex (for example, intrusion age, sulfide mineralogy, disseminated sulfides with Cu>Ni; 

see Cogulu, 1993). However, the intrusion’s small size and 

internal layering of the northern limb—which suggests a tilted 

cone open on the western end (Smith and Sutcliffe, 1987)—are 

more similar to host intrusions of conduit-type sulfide deposits.

Late-Rift Stage Characteristics

With the end of voluminous magmatic activity, subsidence within the central rift basins continued due to thermal 

relaxation as mantle plume heat and intrusive activity waned 

(Cannon and Hinze, 1992). The Late-Rift Stage, from ~1,090 

to ~1,083 Ma, was a time of diminishing magmatism and 

increasing sedimentation. The shift to a sedimentary regime 

began with the deposition of coarse conglomerates in alluvial 

fans into the central rift region sourced from highlands away 

from the rift basins. This early sedimentary sequence is called 

the Copper Harbor Formation, which is composed of coarse 

conglomerate and red sandstone and has a maximum exposed 

thickness of ~2 km (Daniels, 1982).

There was localized volcanism around Lake Superior 

during this stage. Near the end of the Keweenaw Peninsula, 

the Lakeshore Traps in the Copper Harbor Formation are 

made up of ~60 individual lava flows which are 1 to 30 m 

thick and range in composition from tholeiitic basalt to 

andesite. Davis and Paces (1990) obtained a 207Pb/206Pb 

date of 1,087.2±1.6 Ma for an andesite flow in the Lakeshore Traps; Fairchild and others (2017) obtained a more 

precise 206Pb/238U date of 1,085.57±0.25 Ma for the same 

andesite flow. Products of late felsic volcanism in eastern 

Lake Superior include felsic porphyrite and a rhyolite flow 

on Michipicoten Island; their emplacements have been 

dated at 1,086.5+1.3/-3.0 Ma (Palmer and Davis, 1987) and 

1,083.52±0.23 Ma (Fairchild and others, 2017), respectively. 

Rhyolite at a depth of 7,238 feet (2,206 meters) near the 

base of a borehole in Michigan’s eastern Upper Peninsula 

was dated at 1,083.0±3 Ma (Ojakangas and Dickas, 2002). 

The Schroeder-Lutsen tholeiitic basalt sequence, the youngest flows of the NSVG in Minnesota, are unconformable on 

older volcanics and are likely younger than 1,091.61±0.14 Ma 

(Fairchild and others, 2017), a date for the Silver Bay intrusion 

in the Beaver Bay Complex that is overlain by the SchroederLutsen sequence.

Late-Rift Stage Magmatic-Hydrothermal Mineral 

Deposits

The three deposit types listed in this section are within 

or near the MRS Mamainse Point Volcanic Group (MPVG), a 

6-km-thick sequence of basalt flows, rhyolites, and conglomerates along the eastern Lake Superior shoreline that unconformably overlies an Archean granite-greenstone terrane. Mineralization near Mamainse Point is associated with local MRS-related 

felsic intrusions and cross-cutting felsite dikes. The area is 

also cut through by a series of mineralized quartz veins and 

breccia pipes that are thought to be cogenetic, which would 

indicate different levels of exposure of a porphyry-related 

hydrothermal system (Richards and Spooner, 1989). The three 

deposit types listed below are from shallow to deep in the mineralization sequence of Richards and Spooner (1989).

The timing of this felsic volcanism and mineralization during the Late-Rift Stage is not well constrained by modern dating 

methods. Roscoe (1965) reported a potassium-argon (K-Ar) 

date of 1,055±55 Ma for muscovite from the mineralized 

Breton pipe breccia in the Tribag area. Two samples from drill 

core into the Jogran porphyry yielded a rubidium-strontium 

(Rb-Sr) date of 1,070±50 Ma (Van Schmus, 1971).
8 Mineral Resource Database for Deposits Related to the Mesoproterozoic Midcontinent Rift System, U.S. and Canada

Copper Sulfide-Bearing Quartz Veins

Copper sulfide-bearing quartz veins that make up the 

Coppercorp and Lutz deposits (two epigenetic vein sets) cut 

basalt flows of the MPVG. Mineralization is mainly chalcocite with minor bornite, chalcopyrite, and silver in quartz- and 

carbonate-filled cross-cutting veins. Between 1965 and 1972, 

the Coppercorp mine produced about 1.3 million tons of 

ore at 2.1 percent copper (Grunsky, 1991). Fluid inclusion 

data from the Coppercorp deposit suggest that hydrothermal 

mineralization in these deposits was related to the same 

intrusive activity that created the Jogran porphyry and coppermolybdenum±gold (Cu-Mo±Au) breccia pipes (Richards and 

Spooner, 1989).

Copper±Molybdenum Porphyry

Copper±molybdenum porphyry mineralization is related 

to the Jogran intrusion that contains quartz and feldspar 

phenocrysts. The Jogran porphyry has a surface outcrop of 

~200 m by 120 m; drilling into the porphyry reached a depth of 

~200 m, but the true dimensions of the intrusion are unknown 

(Armbrust, 1980). The Jogran porphyry, one of several in the 

area, is pervasively mineralized with disseminated chalcopyrite and minor molybdenite along its margins; narrow quartz 

veins that cut the porphyry also carry molybdenite flakes 

(Blecha, 1974). The average grade of the Jogran porphyry is 

0.19 percent Cu and 0.032 percent Mo, with locally high grades 

of silver (Ag) up to 2.08 ounces per ton (Grunsky, 1991).

Copper-Molybdenum±Gold Breccia Pipe

Copper-molybdenum±gold breccia pipes developed over 

MRS felsic intrusions that cut an Archean granite-greenstone 

terrane (Armbrust, 1969; Norman and Sawkins, 1985). Five 

separate breccia pipes are recognized along a 2-km trend, but 

only two (Breton breccia and West breccia) have supported 

mining. From 1967 until 1974, the Tribag Mining Company 

produced 16,900 metric tons of copper and 246,000 ounces 

of silver from the Breton breccia (Grunsky, 1991). Drilling in 

1974 identified the Palmer breccia, which is about 6 km southwest of the Tribag area. Breccias contain angular fragments of 

granite, diabase, felsite, and metavolcanic rock, all of which 

were locally derived (Blecha, 1974). Exploration through the 

Breton breccia pipe encountered a quartz-feldspar porphyry at 

about 600 m depth (Norman and Sawkins, 1985). The spatial 

relationship to felsic intrusions, sulfide mineralogy, and fluid 

inclusions, as well as stable isotope data, led Norman and 

Sawkins (1985) to conclude that the major component of 

mineralizing fluids at the Tribag district was most likely magmatic, related to local porphyry intrusions.

Post-Rift Stage Characteristics

Two additional stages of rift evolution are important 

for mineralization related to the MRS. The Post-Rift Stage 

represents the time interval between the Late-Rift Stage 

(which we estimate ended at ~1,083 Ma) and the Compressional Stage (~1,060 to ~1,040 Ma). The sedimentary 

sequence that began with the deposition of the Copper Harbor 

Formation during the Late-Rift Stage was followed by a 

marine or lacustrine incursion which resulted in the deposition 

of the ~240-m-thick Nonesuch Formation (Daniels, 1982). 

The Nonesuch Formation is overlain by a sequence of clastic 

sediments (the ~1,500- to ~3,600-m-thick Freda Formation; 

Daniels, 1982). Both the Nonesuch and Freda Formations 

were deposited into subsiding central rift basins. The sedimentary sequence of the Copper Harbor, Nonesuch, and Freda 

Formations is called the Oronto Group. A synsedimentary carbonate bed in the Nonesuch Formation has a Pb-Pb isochron 

date of 1,081±9 Ma (Ohr, 1993); a composite sample collected 

from outcrop yielded a less precise rhenium-osmium (Re-Os) 

depositional age of 1,078±24 Ma (Cumming and others, 

2013), which overlaps the age obtained by Ohr (1993). The 

top of the Freda Formation is not exposed and it is overlain by 

unconformable sandstone units (all possibly somewhat time 

equivalent) including the Bayfield Group in Wisconsin, the 

Jacobsville Sandstone in Michigan and Ontario, and the Fond 

du Lac and Hinkley Formations in Minnesota.

Late-Rift Stage and Post-Rift Stage 

Hydrothermal Mineral Deposits

Hydrothermal mineral deposits have been the most 

productive MRS deposits to date in terms of metrics like total 

value and volume of metal extracted; however, exploration, 

development, and production in the region are shifting to 

magmatic sulfide deposits. The broad array of MRS hydrothermal deposits are classified based on ore mineralogy, setting, 

and age, with some groups containing multiple, more specific 

deposit types. For many of these deposits, there are few definitive constraints on the timing of mineralizing events.

Veins in the Thunder Bay Area, Northern Ontario

In the Thunder Bay area in northern Ontario, MRS rocks 

include sills from the Plateau Stage as well as dikes from 

the Rift Stage. The Logan sills intruded into flat-lying siltstone and shale of the Paleoproterozoic Rove Formation and 

Gunflint Iron-Formation of the Animikie Group and the Nipigon sills intruded into flat-lying sandstone of the early Mesoproterozoic Sibley Group. Northeast-southwest-trending Rift 

Stage Pigeon River dikes parallel the Lake Superior shoreline. 

A later generation of northwest-southeast-trending dikes cut 

the Pigeon River dike swarm (Cundari and others, 2011).

Mainland Group Silver-Bearing Veins

Silver-bearing veins near Thunder Bay are often called 

the Mainland group to distinguish them from polymetallic 

silver-bearing veins called the Island group. Mainland group 
MRS Tectonic Stages and Related Mineral Deposit Types 9

silver mineralization is within an area where MRS Plateau 

Stage Logan diabase sills conformably intruded into the lower 

section of the Paleoproterozoic Rove Formation. Mineralization in veins occupies fault sets that are roughly parallel to 

the Archean/Proterozoic boundary, cutting both diabase and 

Rove shale and siltstone (Franklin and others, 1986). Silver 

occurs as the native metal and as acanthite accompanied by 

galena, sphalerite, pyrite, and chalcopyrite, with a gangue of 

calcite, quartz (sometimes amethystine), barite, and fluorite 

(Sergiades, 1968).

Island Group Polymetallic Silver-Bearing Veins

Polymetallic silver-bearing veins in the Island group 

occur where Rove Formation greywacke and shale were 

intruded by a swarm of mainly east-northeast-striking Rift 

Stage diabase dikes. Veins in northwest-trending faults perpendicular to the strike of the dikes host silver deposits along the 

shore and on several islands of Lake Superior, including the 

large vein exploited by the Silver Islet mine located on a small 

island in Lake Superior off the tip of the Sibley Peninsula. 

Ores in the Island group are characterized by native metals 

and a unique group of five elements: silver-bismuth-cobaltnickel-arsenic (Ag-Bi-Co-Ni-As) (Tanton, 1931; Kissin, 

1992). Ore minerals for the polymetallic veins include native 

silver and argentite, as well as niccolite, galena, sphalerite, 

marcasite, cobaltite, skutterudite, domeykite, chalcopyrite, and 

tetrahedrite, with a gangue of dolomite and quartz (Tanton, 

1931). Silver-bearing veins in the Mainland and Island groups 

produced 300,000 pounds of native silver in the mid- to 

late-1800s, with the Silver Islet mine being accountable for 

more than 60 percent of this total production (Franklin and 

others, 1986).

Lead-Zinc-Silver Veins

Lead-zinc-silver veins, also called the Port Coldwell 

veins, are in an area of sheared Archean metasedimentary 

rocks and metavolcanic rocks cut by MRS dikes that are likely 

related to the Coldwell Complex (~1,108 Ma). Lead-zincsilver-bearing veins are commonly in north-south faults that 

cut all rock types. These deposits contain argentiferous galena 

and sphalerite, along with pyrite, chalcopyrite, covellite, and 

digenite (McCuaig and Kissin, 1997).

Amethyst-Bearing Veins

Gem-quality amethyst crystals are being mined from 

parallel and subparallel north- northeast-trending brecciated 

vein systems that cut Archean and Sibley Group rocks near 

the unconformity between the two units (Smyk and O’Brien, 

1991; Garland, 1994). Veins are typically zoned with multiple 

generations of chalcedony, quartz, and amethyst, along with 

secondary chlorite, hematite, and calcite, and more rarely 

chalcopyrite, galena, sphalerite, and pyrite (McArthur and 

others, 1993; Garland, 1994). Amethyst also occurs in leadzinc-barite veins.

Lead-Zinc-Barite Veins

Lead-zinc-barite veins mainly carry galena and sphalerite with minor pyrite, marcasite, chalcopyrite, and chalcocite 

in a gangue of quartz (locally amethystine), calcite, barite, 

and minor fluorite. Galena, sphalerite, and chalcopyrite are 

paragenetically early, and barite and amethystine quartz are 

paragenetically later (Tanton, 1931). Pitchblende (also known as 

uraninite) also occurs at the Enterprise and Dorion Amethyst 

deposits, which suggests a link to unconformity-related 

uranium-bearing deposits (Franklin and Mitchell, 1977; 

Scott, 1990).

Unconformity-Related Uranium-Bearing Veins

Unconformity-related uranium deposits occur in veins 

and vein breccias that occupy north-trending regional-scale 

faults and shear zones, as well as in lithologic pinchouts along 

the unconformity between Archean rocks and early Mesoproterozoic sandstone of the Sibley Group (Franklin, 1978; Smyk 

and Franklin, 2007). The Sibley Group is a ~950-m-thick 

sequence of relatively flat lying, siliciclastic and chemical 

sedimentary rocks (Rogala and others, 2007) that was 

deposited in a north-trending intercontinental basin between 

1,537+10/-2 Ma (Davis and Sutcliffe, 1985) and 1,339±33 Ma 

(Franklin and others, 1980). Regional fault zones or onlap 

of the basal Sibley against Archean highs may have acted as 

stratigraphic traps for migrating hydrothermal fluids (Franklin, 

1978). Ruzicka and LeCheminant (1984) obtained a date of 

1,090±20 Ma for pitchblende and hematite mineralization in 

the Black Sturgeon Lake area in the Nipigon Embayment.

Copper Sulfide-Bearing Veins

Four vertical veins that carry chalcocite-bearing calcite 

stringers with trace amounts of chalcopyrite and silver occur 

in flat-lying amygdaloidal basalt flows of the Plateau Stage 

Osler Volcanics. Veins similar to those in the Mainland 

Group in Ontario continue into northeastern Minnesota (Grout 

and Schwartz, 1933). In this part of Minnesota, the geology 

consists of flat-lying rocks of the Rove Formation intruded by 

Logan sills, which are cut off to the south by younger rocks 

of Plateau Stage Duluth Complex intrusions and overlying 

reversed-magnetic-polarity volcanic rocks. Veins described 

by Grout and Schwartz (1933) include calcite veins, calcitebarite veins, calcite-sulfide veins, and calcite-barite-sulfide 

veins; silver was not reported. Most of these veins were of 

insufficient size and economic importance to be included in 

the USGS MRDS database. An exception is a small copper 

sulfide vein with chalcopyrite, bornite, chalcocite, and pyrite 

in calcite gangue that cuts a Rift Stage diabase dike and Rove 

Formation argillite on Susie Island off the Lake Superior 

shoreline (Schwartz, 1928).

Timing of Thunder Bay Vein Mineralization

The diverse group of mineralized veins in the Thunder 

Bay area differs in setting, metal assemblages, and gangue 
10 Mineral Resource Database for Deposits Related to the Mesoproterozoic Midcontinent Rift System, U.S. and Canada

contents, but these veins presumably relate to a single, extended 

hydrothermal event with metal assemblages that indicate 

varying metal sources (Smyk and Franklin, 2007). The heat 

source driving the regional hydrothermal system on the north 

side of the MRS was not Plateau Stage sill intrusions or Rift 

Stage diabase dikes, as these would not have had enough heat 

to reach the high fluid inclusion temperatures reported by 

Kissin and Jennings (1987). There is evidence, however, of 

possibly contemporaneous regional intrusive events. The Moss 

Lake intrusion that cuts the reversed-magnetic-polarity Osler 

Volcanics has a baddeleyite date of 1,094.7±3.1 Ma (Heaman 

and others, 2007). A small group of zircons from gabbro 

near the margin of the St. Ignace Island Complex, which also 

intrudes the upper section of the Osler Volcanics, was dated at 

1,089±3.2 Ma (Smyk and others, 2006). These are within error 

of the 1,090±20 Ma date obtained from pitchblende mineralization (Ruzicka and LeCheminant, 1984), which suggests that 

the magmatic heat of intrusion related to local late Rift Stage 

or Late-Rift Stage magmatism could have driven the hydrothermal system that created the diverse vein mineralization in 

the Thunder Bay area.

Sediment-Hosted Stratiform Copper

Sediment-hosted stratiform copper deposits are located 

on the south shore of Lake Superior, near the Porcupine Mountains of Michigan. Economic mineralization is fine-grained 

disseminated chalcocite restricted to the lower 1 to 4 m of 

shale and siltstone in the Nonesuch Formation (Mauk and 

others, 1992). The two principal stratiform copper deposits 

are Copperwood, located within the Western syncline 

(a N. 60° W.-plunging open fold southwest of the Porcupine 

Mountains), and White Pine, located to the northeast of the 

Porcupine Mountains.

Primary mineralization at both Copperwood and White 

Pine is attributed to chalcocite replacement of pyrite during 

diagenesis by compaction-driven, Cu-rich hydrothermal fluids 

moving upward through underlying Copper Harbor Formation 

red beds and into pyrite-bearing gray and black shale of the 

lower Nonesuch Formation (Ensign and others, 1968; Mauk 

and others, 1992; Swenson and others, 2004; Bornhorst and 

Williams, 2013). Copper in the mineralizing fluid was leached 

from volcanic clasts in the underlying immature Copper 

Harbor Formation; compaction from deposition of the overlying 4 km of Freda Formation created a single-pass mineralizing event of the lower Nonesuch (Swenson and others, 2004).

Mineralization at the White Pine deposit is interpreted 

to have occurred during late diagenesis of the Nonesuch 

Formation (Ensign and others, 1968; Mauk and others, 1992), 

perhaps several millions of years following sediment 

deposition. Based on reported ages of Nonesuch Formation 

deposition (1,081±9 Ma, Ohr, 1993; 1,078±24 Ma, Cumming 

and others, 2013), mineralization of these sediment-hosted 

stratiform copper deposits falls within the MRS Post-Rift 

Stage. The reported timing of Nonesuch diagenesis and a 

date of 1,090±20 Ma for pitchblende in a Thunder Bay area 

uranium unconformity vein overlap. If the diverse vein mineralization in the Thunder Bay region was driven by Late-Rift 

Stage magmatic heat, however, it is probable that the formation of the multiple mineralized veins in Ontario pre-dates 

the stratiform copper mineralizing event in Michigan. This 

assumption is supported by the relative timing of the change 

of the MRS from a primarily magmatic regime to a primarily 

sedimentary regime.

Compressional Stage Characteristics

Thrusting and inversion in parts of the MRS are attributed 

to the renewal of northwest-directed compression caused by 

a phase of the Grenville Orogeny to the east (Cannon, 1994). 

This is the Compressional Stage of the MRS that created the 

geometry of older basalts thrust over younger sediments along 

reverse faults, which is seen today in the Lake Superior region. 

Faulting occurred around 1,060±20 Ma (Cannon and others, 

1993), as indicated by thermal resetting of Rb-Sr biotite ages 

in the upper plate of a listric fault system in Michigan and 

Wisconsin that is part of the series of reversed normal faults 

that bound central rift basins.

Compressional Stage Hydrothermal Mineral 

Deposits

Native Copper±Native Silver in Basalt and 

Interflow Sedimentary Rocks

Native copper accompanied by minor native silver mineralization is the most famous of all MRS mineral deposits. 

Native copper showings and occurrences are ubiquitous 

in exposed normal-magnetic-polarity Rift Stage basalts, 

although economic deposits are found only in Michigan’s 

western Upper Peninsula. Comprehensive descriptions of 

native copper mineralization are given in Butler and Burbank 

(1929), Broderick and others (1946), White (1968), and 

Bornhorst (1997).

Native copper and native silver are the only ore minerals in this deposit type. Copper and silver occur predominantly as native metals rather than in sulfide minerals 

because Cu-bearing metamorphic fluids were derived from 

deeply-buried subaerial basalts that had degassed most 

volatiles, including sulfur, during eruption (Cornwall, 1956; 

White, 1968). Native copper ore bodies are mostly stratiform 

deposits, locally called “lodes,” and are found in amygdaloidal 

basalt flow tops—which are often fragmental or rubbly—or 

within interflow conglomerate beds. Some steeply dipping 

veins, locally called “fissure veins,” that cross-cut layered 

basalt flows also carry native copper (White, 1968).

Native copper mineralization is epigenetic, with both 

copper and mineralizing fluid thought to have originated from 

the thick buried basalt sections within the central rift axis in 

Lake Superior (Stoiber and Davidson, 1959; White, 1968; 

Jolly, 1974; Bornhorst, 1997). Precipitation of native copper 
Summary 11

may have been caused by mixing with cooler, dilute meteoric 

water (Livnat, 1983; Bornhorst, 1997) that was accompanied 

by reactions between mineralizing fluid and rock that resulted 

in the removal of iron in the immediate vicinity of copper, 

thus causing a bleaching effect (Butler and Burbank, 1929). 

Brecciated and recemented vein-filling minerals suggest 

that mineralization was contemporaneous with faulting and 

minor folding of the Rift Stage basalts during the Compressional Stage (White, 1968). Bornhorst and others (1988) dated 

amygdule-filling potassium felspar, calcite, epidote, and chlorite 

using Rb-Sr techniques to constrain the timing of native copper 

mineralization as between 1,060 and 1,047 Ma (± ~20 Ma). 

Thus, the timing of native copper mineralization is some 30 to 

40 million years after eruption of the host basalt section.

Despite the common occurrence of native copper 

showings around Lake Superior, 97 percent of the total native 

copper production came from only 10 lode deposits located 

within a small 45-km by 5-km section of Rift Stage PLV along 

the Keweenaw Peninsula of Michigan. One lode deposit, 

the Calumet and Hecla Conglomerate, accounted for nearly 

40 percent of the production from that section (White, 1968).

Second-Stage Native Silver±Native Copper

A “second-stage” mineralizing episode at the White Pine 

deposit—which locally increased the grade of the initial 

“main-stage” (Post-Rift Stage)sediment-hosted stratiform copper mineralization—is temporally related to the much larger 

native copper mineralization event of the Compressional Stage 

(Mauk and others, 1992). During regional faulting related to 

the Grenville Orogeny compression, copper-bearing fluids 

moved along multiple strike-slip and thrust faults that cut the 

White Pine mine. Ruiz and others (1984) analyzed calcites in 

late veins that cut the chalcocite mineralization at the White 

Pine mine and obtained a Rb-Sr date of 1,047±35 Ma for 

second-stage mineralization. The abundance of silver in sandstone at the contact between the Copper Harbor and Nonesuch 

Formations created a short silver boom from 1872 and 1876 

that saw the opening of a number of small silver mines and a 

stamp mill (Rominger, 1876; White and Wright, 1954).

Chalcocite in Basalt

Chalcocite is widespread in trace amounts throughout 

much of the Keweenaw Peninsula, especially in fissures 

cutting basalt (Butler and Burbank, 1929). Within the lower 

section of the PLV, however, near the northeast tip of the 

peninsula, there are seven chalcocite deposits and all but one 

are hosted by basalt. The deposits are restricted to an area 

2 km by 13 km that is stratigraphically below the Bohemia 

Conglomerate, a prominent interflow sedimentary layer just 

above the Keweenaw fault that can be traced along strike for 

more than 100 km.

There is no spatial overlap between economic native 

copper mineralization and the chalcocite mineralized area, 

although there are similarities between the two. Alteration 

minerals related to chalcocite include epidote, quartz, hematite, pumpellyite, chlorite, albite, potassium feldspar, and laumontite, which matches the minerals found with native copper 

(Robertson, 1975). Both native copper and chalcocite are most 

abundant in fragmental basalt flow tops. The major difference 

between the two, other than native metal versus sulfide, is 

the presence of the dikes and the high degree of fault-related 

fracturing that controlled the distribution of sulfide mineralization within the chalcocite zone. It is likely that the same 

hydrothermal event that resulted in the deposition of native 

copper higher in the PLV section also deposited chalcocite in 

this limited area.

Unconformity-Related Uranium Veins

Unconformity-related uranium veins occur along sheared 

contact zones between MRS diabase dikes and an Archean 

gneissic tonalite suite in the Theano Point-Montreal River area 

along the eastern shore of Lake Superior, Ontario. Pitchblende 

occurs with hematite in gangue calcite veins along the contact 

between diabase and Archean rocks, as well as in tension 

cracks in diabase; pyrite as well as lead and copper selenides 

are also found in some of the veins (Robertson and Gould, 

1983). The uranium source is the Archean gneiss, which is 

known to be locally radioactive (Grunsky, 1991). The timing 

of mineralization is unknown, although the steep northerly 

dip of the dikes and shearing along their margins suggest 

that mineralization could be related to hydrothermal activity 

concurrent with faulting during the Compressional Stage.

Summary

The MRS in the Lake Superior region is a diverse suite 

of rocks which have been categorized into different stages of 

tectonic activity and rift development, each having characteristic host rocks, metal endowments, and mineral deposit types 

(fig. 2). Development of the MRS began with magmatic activity of the Plateau Stage and the Rift Stage, which were then 

followed by minor magmatic activity of the Late-Rift Stage. 

During a Post-Rift Stage, clastic sediments were deposited 

into thermally subsiding basins. A final compressional event 

(the Compressional Stage) reversed motion along originally 

normal faults as clastic sedimentation continued. Each stage of 

the rift evolution is characterized by specific ore deposit types 

that are largely confined both by their physical location in rift 

rocks and by the type and timing of mineralization. Controlling factors for magmatic deposits include sulfur sources, 

magma compositions, and magmatic propagation styles. 

For hydrothermal deposits, fluid compositions, favorable 

structures controlling fluid flow, and the controls on mineral 

precipitation all contributed to metal endowment and richness. 

We present a database (table 1) of known MRS deposits taken 

from public deposit databases (categorized by deposit type 

and stages of rift development). This database has resulted in a 

space and time framework for MRS mineralization events.
12 Mineral Resource Database for Deposits Related to the Mesoproterozoic Midcontinent Rift System, U.S. and Canada

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1120 1110 1100 1090 1080 1070 1060 1050 1040

R N Magnetic

Polarity

Plateau

Stage

Rift

Stage

Magmatism Thermal subsidence Compression

Late-Rift

Stage

Clastic sedimentation

Post-Rift

Stage

Age (Ma)

MRS Evolution

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1. Conduit-type Ni-Cu-PGE sulfides

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5. Ti-Fe oxides (OUIs & inclusions)

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8. Cu±Mo porphyry

9. Cu-Mo±Au breccia pipes

10. Veins in the Thunder Bay area

11. Sediment-hosted strataform Cu

12. Native Cu±native Ag

13. Second-stage native Ag±

 native Cu

14. Chalcocite in basalt

15. Unconformity-related U veins

(2)

(6)

(4)

(5)

(7)

(8)

(9)

(10) (11) (12)

(13)

(3)

(1)

(14)

(15)

Figure 2. Summary of the tectonic and mineralization history of the Midcontinent Rift System (MRS). In the “Tectonic 

Stages” section, colored boxes and bars represent the timing of rock emplacement and deposition for the corresponding 

stages; arrows represent the estimated timing of the Post-Rift Stage and the Compressional Stage. Magnetic polarity is 

indicated by blue for reversed (R) and red for normal (N) magnetic polarity. For MRS mineral deposits, numbers refer to 

the name of the deposit type (given within the corresponding box) and dashed arrows represent the assumed duration of 

mineralization. Timing of mineralization is estimated from the age of host rocks and (or) timing of mineralization, where known. 

Abbreviations: Ag, silver; Au, gold; Cu, copper; Fe, iron; Ma, mega-annum; Mo, molybdenum; MRS, Midcontinent Rift System; 

N, normal magnetic polarity; Nb, niobium; Ni, nickel; OUIs, oxide-ultramafic intrusions; PGE, platinum-group element; R, 

reverse magnetic polarity; REEs, rare earth elements; Ti, titanium; U, uranium.
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Appendix 1 19

Appendix 1

Tables 1.1 and 1.2 give definitions for the mineral deposit categories (column G) in the database (table 1; available as a 

separate file at https://doi.org/10.3133/ofr20201069). Deposits located in Ontario, Canada, correspond to the definitions in table 

1.1 and deposits located in the United States correspond to the definitions in table 1.2.

Table 1.1. Definitions for mineral deposit categories (column G) in the database (table 1) for deposits located in Ontario, Canada.

[The mineral deposit categories and definitions are those listed in the Ontario Geological Survey (OGS) Mineral Deposit Inventory (MDI; Ontario Ministry of 

Energy, Northern Development and Mines, 2019). They were obtained from appendix 3 of Wilson and others (2008)]

Mineral Deposit Category Definition

Discretionary Occurrence An occurrence or deposit that does not meet any of the defined criteria but is entered into the 

MDI database based upon a subjective decision by a MNDM [Ontario Ministry of Energy, 

Northern Development and Mines] geologist.

Occurrence Mineralization is present in 2 dimensions as indicated by surface rock sampling (channel or grab) 

and (or) isolated diamond drill intersection(s). At least one sample must meet the minimum 

requirements for a mineral occurrence.

Prospect Mineralization is present in 3 dimensions, indicated by diamond drilled intersections ± surface 

rock sampling. Mineralization occurs for significant distances along strike and down dip with a 

minimum of 3 intersections that meet the minimum requirements for a prospect.

Developed Prospect with Reserves Mineralization is present and defined in 3 dimensions as outlined by a delineation diamond 

drill program. Mineralization occurs for significant distances along strike and down dip with 

multiple intersections that meet the minimum requirements for a Prospect. Reserves must 

meet the minimum requirements for a producing mine, a past-producing mine, or a developed 

prospect with reserves.

Developed Prospect without Reserves Mineralization is present in 3 dimensions as outlined by a delineation diamond drill program. 

Mineralization occurs for significant distances along strike and down dip with numerous 

intersections that meet the minimum requirements for a Prospect. Reserves figures have not 

been published or do not meet the minimum requirements for a producing mine, a pastproducing mine or a developed prospect with reserves.

Past Producing Mine Commodities were in the past extracted for sale. No defined in-situ reserves. Past production 

must have met the minimum requirements for a producing mine, a past-producing mine or a 

developed prospect with reserves.

Past Producing Mine with Reserves Commodities were in the past extracted for sale. Total past production plus in-situ reserves must 

meet the minimum requirements for a producing mine, a past-producing mine or a developed 

prospect with reserves.

Producing Mine Commodities are currently extracted for sale. Total of the commodity produced plus the 

published defined ore reserves must meet the minimum requirements for a producing mine, 

a past-producing mine or a developed prospect with reserves.
20 Mineral Resource Database for Deposits Related to the Mesoproterozoic Midcontinent Rift System, U.S. and Canada

Table 1.2. Definitions for mineral deposit categories (column G) in the database (table 1) for deposits located in the United States

[The mineral deposit categories and definitions are those listed in metadata for the U.S. Geological Survey Mineral Resources Data System (MRDS; 

Schweitzer, 2017)]

Mineral Deposit Category Definition

Occurrence Ore mineralization in outcrop, shallow pit or pits, or isolated drill hole. Grade, tonnage, 

and extent of mineralization essentially unknown. No production has taken place and 

there has been no or little activity since discovery with the possible exception of routine 

claim maintenance.

Prospect A deposit that has gone beyond the occurrence stage. This is subsequent work such as surface 

trenching, adits, or shafts, drill holes, extensive geophysics, geochemistry, and (or) geologic 

mapping has been carried out. Enough work has been done to at least estimate grade and 

tonnage. The deposits may or may not have undergone feasibility studies that would lead to a 

decision on going into production.

Producer A mine in production at the time the data was entered. An intermittent producer that produces on 

demand or seasonally with variable lengths of inactivity is considered a producer.

Past Producer A mine formerly operating that has closed, where the equipment or structures may have been 

removed or abandoned.

Unknown At the time of data entry, either the development status was unknown or the data source this 

record came from did not specify this value.

References Cited

Ontario Ministry of Energy, Northern Development and 

Mines, 2019, Mineral deposits (MDI [Mineral Deposit 

Inventory]): Ontario Ministry of Energy, Northern Development and Mines database, accessed January 22, 2020, 

at https://www.mndm.gov.on.ca/en/mines-and-minerals/

applications/ogsearth/mineral-deposits-mdi.

Schweitzer, P.N., 2017, Metadata [as questions and answers 

for the] Mineral Resources Data System [MRDS]: U.S. 

Geological Survey metadata for a database, accessed 

January 22, 2020, at https://mrdata.usgs.gov/metadata/

mrds.faq.html.

Wilson, A.C., Rowell, D.J., Seim, G.W., and Debicki, R.L, 

2008, Procedural guidelines for provincially significant 

mineral potential mineral resource assessments: Ontario 

Geological Survey, Open File Report 6141, 91 p. [Also 

available at http://www.geologyontario.mndm.gov.on.ca/

mndmfiles/pub/data/imaging/OFR6141/OFR6141.pdf.]
Manuscript approved on July 20, 2020

For additional information, contact:

Mineral Resources Program Coordinator

U.S. Geological Survey

913 National Center

Reston, VA 20192

Email: minerals@usgs.gov

https://www.usgs.gov/energy-andminerals/mineral-resources-program

Publishing support provided by the Reston 

Publishing Service Center
Woodruff and others—Mineral Resource Database for Deposits Related to the Mesoproterozoic Midcontinent Rift System, U.S. and Canada—OFR 2020–1069

ISSN 2331-1258 (online)

https://doi.org/10.3133/ofr20201069