769

GEOLOGICAL SURVEY CIRCULAR 769

A Theoretical Basis for 

Exploration for Native Copper in 

Northern Wisconsin
A Theoretical Basis for 

Exploration for Native Copper in 

Northern Wisconsin

By Walter S. White

GEOLOGICAL SURVEY CIRCULAR 769

Certain geologic characteristics of the 

Michigan copper district can provide 

exploration guides for northern 

Wisconsin

1978
United States Department of the Interior

CECIL D. ANDRUS, Secretary

Geological Survey

V. E. McKelvey, Director

Library of Congress catalog - card No. 78-600001

Free on application to Branch of Distribution, U.S. Geological Survey 

1200 South Eads Street, Arlington, Ma. 22202
CONTENTS

Page 

Abstract .__ _________________ _ 1

Introduction .________ ___ 1

Geologic setting _____________________ 2

Geologic features common to younger lavas of

Michigan and Wisconsin ._________ _ _ 4

Shows of native copper or chalcocite _____ 4

Degree of metamorphism .____________ 4

Host-rock lithology__________________ 6

Paleohydrology of the copper deposits _______ 6

Degree of metamorphism _____________ 7

Permeability anisotropy ______________ 7

Regional distribution of most-porous rock ___ 7

Regional structure of the Keweenawan lavas in

Wisconsin__________________________ 10

General statement ._________________ 10

Kinds of information useful for stratigraphic

subdivision and correlation .__________ 11 

Data available for Wisconsin .__________ 12

Page

Regional structure of the Keweenawan lavas in 

Wisconsin Continued

Hypothetical model of a basin of lava accumulation ______ 12 

Areas favorable for copper in hypothetical lava 

basins _________________ - 13 

Hypothetical basin A .______________ 15 

Metamorphic grade _________-_-__- 15 

Permeability anisotropy ______________ 15

Regional distribution of most-porous rock 15

Summary for hypothetical basin A______ 15

Hypothetical basin B__i______________ 16

Portage Lake Volcanics .______________ 16

Conclusion __________________________ 16

Metamorphic grade_________________ 16

Stratigraphy ______________________ 17

Structure ________________________ 18

References __________________________ 18

ILLUSTRATIONS

Page

FIGURE 1. Generalized geologic map of Keweenawan rocks, northwestern Wisconsin ________________________________________ 3

2. Diagram showing stratigraphic distribution of critical metamorphic

minerals, Portage Lake Volcanics, Eagle Harbor section _______ 5

3. Diagram showing hypothetical plan of a Keweenawan flood-basalt flow 8

4. Generalized model of a Keweenawan lava basin _1 __________________ 10

5. Generalized geologic map of northwestern Wisconsin showing hypothetical basins of lava accumulation .____________________ 14

HI
A Theoretical Basis for Exploration 

for Native Copper in Northern Wisconsin

By Walter S. White

ABSTRACT

Exploration for native copper in the Keweenawan 

lavas of northern Wisconsin has been concentrated in 

areas of relatively shallow overburden that have 

sparse to numerous outcrops. Lack of success of this 

exploration suggests that if large deposits, comparable 

to those of northern Michigan, are present, they are 

more likely to be found in one or more of the large 

tracts that have few, if any, exposures, away from the 

"copper ranges."

A hydrologic model that may explain the mineralization of the classic native-copper district of Michigan 

could be helpful in suggesting that certain covered 

tracts are more favorable than others, and in narrowing the targets for physical exploration within these 

tracts. This model involves updip migration of a 

hydrothermal fluid of metamorphic origin, formed 

when ground water contained in the interstices of lava 

flows and conglomerate beds was carried to geat depth 

along the axis of the Lake Superior syncline. The 

largest reservoirs of such buried ground water would 

be expected in the peripheral rather than central parts 

of the basins in which the lavas originally accumulated 

because the ratio of porous fragmental flow tops and 

conglomerate beds to massive basalt should increase 

towards the margins of a basin. Thus, the areas most 

promising for copper deposits should be those updip 

from places where the trough of the later formed Lake 

Superior syncline crosses the marginal parts of a lava 

basin.

In Michigan, such migrating solutions deposited 

native copper in greatest abundance close to the 

boundary between epidote and pumpellyite metamorphic zones of Jolly (1974). To be most favorable, 

therefore, a potential target area should contain rocks 

of this metamorphic grade.

In order to locate areas favorable for native-copper 

deposits according to the hydrologic model described 

above, we must determine the outlines of basins of 

lava accumulation. This requires reasonably detailed 

stratigraphic information over a much larger area 

than that of individual tracts without outcrops. The 

requisite data can be obtained by conventional geologic mapping in areas that have exposures, and stratigraphic boundaries can then be interpolated or extrapolated across covered tracts with the help of ground 

and airborne magnetometer surveys, and possibly 

paleomagnetic correlations. Data from areas having exposures can also be used, in Wisconsin, to delimit 

fairly closely the covered tracts that might contain 

rocks of favorable metamorphic grade.

INTRODUCTION

Exploration for native copper in the 

Keweenawan lavas of northwestern Wisconsin 

has, for 130 years, been sporadically encouraged by small surface or near-surface showings. A special incentive for this exploration 

was the success, in Michigan, of mines in 

similar rocks. Exploration in Wisconsin has not 

been similarly rewarded. The most promising 

concentrations found there are tiny, even by 

comparison with deposits that were unsuccessfully exploited in Michigan. Butler and Burbank (1929, p. 67-98) listed nearly 100 Michigan mining companies that paid no dividends 

despite recorded production ranging from a 

few tons to as much as 12,000 tons (11,000 

metric tons) of metallic copper. In contrast, 

none of the prospects in Wisconsin has any 

recorded production, though small amounts of 

copper were presumably recovered where mill 

tests were made.

This unpromising record is consistent with, 

but does not confirm, the proposition that no 

significant native-copper deposits are present 

in the Keweenawan lavas of northwestern Wisconsin. In Michigan, all the major deposits and 

almost all the minor deposits were found at 

grass roots; many, perhaps most, from evidence 

of pitting by prehistoric Indians. Virtually all 

are in areas where outcrops are relatively 

common, and bedrock is probably covered by 

less than 6 m (20 feet) of surficial material 

over a significant fraction of any given square 

mile. In such terrain, the chance of finding 

copper stains in outcrop or soil is obviously far 

greater than in areas that have deep overburden and few outcrops; perhaps just as im-
portant, the relatively shallow overburden has 

made it economical to further investigate such 

stains by trenching. Most of the deposits were 

found by this procedure.

In Wisconsin, essentially all the copper 

prospects have the same kind of setting as the 

deposits in Michigan they are in areas of relatively shallow overburden characterized by rare 

to numerous bedrock outcrops. A comparison of 

the districts that contain the main prospects 

(see Grant, 1901, figs. 7-9, 13) with areas of 

similar size and exposure in the Michigan copper district suggests the following: if there had 

been significant copper deposits in the prospected areas, one or more of the deposits found 

by prospectors should have been comparable in 

size and grade to the many in Michigan that 

have produced at least a few tons of metallic 

copper. Therefore, if there are large deposits 

in Wisconsin, they probably are not in the areas 

that have numerous outcrops and copper prospects the "copper ranges" of Grant (1901). If 

major deposits do exist, they are, conversely, 

most likely to be in areas that have few, if any, 

exposures, away from the "copper ranges."

Within the area immediately underlain by 

Keweenawan lavas in northwestern Wisconsin 

are large tracts in which no outcrops were 

noted in the course of detailed traverses by the 

Wisconsin Geological and Natural History Survey (see distribution of symbols for outcrops 

on Dutton and Bradley, 1970, sheet 1). Large 

parts of many townships and a few whole 

townships appear to have no exposures (Dutton, 1972), and such well records as exist for 

these areas reveal deep overburden. The largest 

of these outcrop-free tracts are probably the 

best places in which to concentrate exploration 

efforts. Because the actual search for copper 

deposits must ultimately involve expensive 

drilling in greater or less degree, criteria that 

can help narrow target areas beforehand are, 

as always, highly desirable. The purpose of this 

paper is to present some criteria suggested by 

characteristics of the Michigan copper district 

and its regional setting.

The discussion here of elements of the 

geology of northwestern Wisconsin is based 

entirely on published or open-file material and 

not on any original field observations by me in 

Wisconsin. It would be presumptious of me to

pretend that, as far as Wisconsin is concerned, 

the ideas presented here are any more than 

speculations, which is my reason for setting 

them out in a Circular rather than in a more 

formal publication. Field observations may 

prove or disprove some elements of these speculations, and thus the speculations suggest geologic and geophysical investigations that would 

be useful.

GEOLOGIC SETTING

The rocks likely to contain native-copper deposits are, as in Michigan, a thick series of 

mafic lava flows and a few interlayered felsitic 

conglomerate or sandstone beds. In Michigan, 

these copper-bearing rocks have been called the 

Portage Lake Volcanics (formerly "Portage 

Lake Lava Series"). In westernmost Michigan, 

the Portage Lake Volcanics overlie an older, 

more metamorphosed series of lavas, the Powder Mill Group (Hubbard, 1975), which is not 

known to contain significant concentrations of 

copper. As shown in figure 1, the Portage Lake 

Volcanics seem to pinch out near Mellen, Wis., 

the pinchout marking the western end of the 

basin in which they accumulated (White, 1966a, 

p. ElSandfig. 7; 1972).

In northwestern Wisconsin, west of Mellen, 

there also appears to be at least a twofold division of the mafic lava flows (fig. 1). A belt of 

conglomerate extending southwest from Davis 

Hill (sees. 2 and 11, T. 44 N., R. 6 W.; lat. 

46°19' N., long. 91°5' W.; near east end of 

unit labeled conglomerate of Davis Hill on 

fig. 1) was regarded by Aldrich (1929) as 

equivalent to the Copper Harbor Conglomerate 

(fig. 1), which overlies the Portage Lake Volcanics in Michigan and east of Mellen in Wisconsin. Aldrich thought that the volcanics to 

the south of the conglomerate belt of Davis 

Hill were the same as those to the north, and 

that the presence of two belts was due to repetition on a fault (Lake Owen fault). Hubbard's 

(1975, p. 529) recognition of rocks of his Powder Mill Group in the southern belt indicates 

that the conglomerate belt of Davis Hill separates an older group of lavas from a younger, 

and may mark an unconformity between the 

two. The "Lake Owen fault" of Aldrich may 

exist, but it is not necessary to explain the 

geologic relationships.
00

EXPLANATION

PALEOZOIC ROCKS 

Portage Lake Volcanics

Undifferentiated volcanics

Nonesuch Shale and younger

Conglomerate of Davis Hill

Copper Harbor Conglomerate 

(Michigan and Wisconsin) 

Sdor Church Formation of 

Craddock (1972) (Minnesota)

Powder Mill Group

Intrusive rocks

FIGURE 1. Generalized geologic map of Keweenawan rocks, northwestern Wisconsin. Modified from Bu

White, 1966a; and Craddock, 1972.
The Portage Lake Volcanics accumulated in 

a basin, part of the midcontinent rift of 

Keweenawan age, and figure 1 shows by a 

hachured dashed line the general outline of the 

western margin of this basin as reconstructed 

by White (1972). Termination of this basin at 

the pinchout of the Portage Lake Volcanics 

near Mellen implies that lavas of the younger 

volcanic formation (or formations) farther 

west in Wisconsin were poured out in a separate basin, and might or might not be contemporaneous with the Portage Lake Volcanics. 

For purposes of exploration for copper, contemporaneity with the Portage Lake Volcanics 

is probably less important than certain other 

characteristics that the younger volcanics of 

Wisconsin share with the Portage Lake. The 

following paragraphs discuss these points of 

similarity.

GEOLOGIC FEATURES COMMON

TO YOUNGER LAVAS OF 

MICHIGAN AND WISCONSIN

SHOWS OF NATIVE COPPER OR CHALCOCITE

One of the simplest but also, perhaps, most 

encouraging of the favorable characteristics of 

the younger volcanic formation(s) of Wisconsin is the common presence of megascopic 

specks of native copper or copper sulfide. In 

areas underlain by the Portage Lake Volcanics 

in Michigan, few large roadcuts lack copper 

stain on one or more joint faces. Similarly, 

specks of native copper are a very common 

feature of the amygdaloidal part of individual 

lava flows as seen in drill core. That this is not 

a universal characteristic of Keweenawan flood 

basalts is shown by the fact that few, if any, 

of the numerous roadcuts in mafic lava along 

the coastal highway between Duluth and Grand 

Portage, Minn., show any copper stains (personal observation), although chemically and 

mineralogically the mafic lavas of Minnesota 

overlap those of Michigan in composition. 

Green (1972, p. 331-332) noted the scarcity of 

copper-mineral occurrences in northeastern 

Minnesota. The great difference in abundance 

of visible traces of copper is probably due to 

differences in amounts of metamorphism and 

movement of heated water through the rock 

(White, 1957; Green, 1972). But whatever the

cause, the common presence of at least traces 

of megascopic copper minerals on joint faces 

and in amygdaloidal rocks would appear to be 

evidence that an environment favorable for 

migration of copper an essential condition for 

the formation of economic deposits was 

present in Michigan and not, to any notable extent, in northeastern Minnesota. The many copper prospects and recorded occurrences of copper minerals in northwestern Wisconsin 

(Grant, 1901; Button, 1972), on the other hand, 

suggest that an environment favorable for 

migration of copper did exist in this area.

DEGREE OF METAMORPHISM

The rocks of the Portage Lake Volcanics in 

Michigan have been regionally metamorphosed 

to grades that range from zeolite facies to lower 

greenschist facies. In general, metamorphic 

grade increases with stratigraphic distance 

below the top of the formation as a whole. In 

individual lava flows, minerals of metamorphic 

origin are abundant in the upper amygdaloidal 

portions, whereas the original igneous minerals 

 intermediate to calcic plagioclase, pyroxene, 

and even olivine are best preserved in the 

massive basalt of the middle and lower parts of 

flows; some of this massive basalt is remarkably fresh. This contrast in metamorphism can 

be attributed to the greater availability of water 

for metamorphic reactions in the originally 

porous vesicular and fragmental flow tops, and 

to lack of water in the relatively anhydrous 

massive interiors of flows.

The most conspicuous minerals of metamorphic origin are those that fill amygdules and 

other original open spaces in flow tops. Using 

these secondary minerals, Stoiber and Davidson 

(1959), following Broderick (1929), were able 

to map crude metamorphic zones in the area of 

principal copper deposits. They found (their fig. 

7) that all but two of the major copper deposits 

lie close to the top of a zone in which epidote 

and quartz are common amygdule fillings (fig. 

2). Considerable overlap exists between this 

zone and a higher zone in which prehnite is an 

abundant amygdule filling that is, rocks characterized by prehnite are found in the upper 

part of, and also above, the zone that has epidote-quartz fillings. All but one of the major 

deposits lie within the zone of overlap, and only
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FIGURE 2. Stratigraphic distribution of critical metamorphic minerals, Portage Lake Volcanics, Eagle 

Harbor section. Base of pumpellyite zone taken at 

top of Scales Creek Flow (Jolly, 1974, fig. 5), which 

is approximately 2.96 km (9,700 feet) stratigraphically below top of Portage Lake Volcanics.

one lies below the lower limit of prehnite, in 

rocks characterized by the amygdule-mineral 

assemblage, quartz-epidote without prehnite.

Jolly (1974) and Jolly and Smith (1972) 

studied the metamorphic alterations of the 

basaltic rock itself, rather than the amygdules, 

and were able to refine considerably the concept of regional metamorphic zoning. In the 

region occupied by the major copper deposits, 

they distinguish an upper laumontite (chlorite) 

zone, an intermediate pumpellyite zone, and a 

lower epidote zone (fig. 2). The epidote zone is 

characterized by the presence, particularly in 

the amygdaloidal tops of some flows, of masses 

of epidotized rock ranging in diameter from a 

few centimeters to more than a meter (epidote 

metadomains). Similar masses in the pumpellyite zone have pumpellyite, rather than epidote, 

as their dominant calcium-silicate mineral 

(pumpellyite metadomains). In the epidote 

zone, original calcic plagioclase in the flow tops 

is altered to albite plus epidote, and in the 

pumpellyite zone, calcic plagioclase is altered 

to albite plus pumpellyite. A transition zone 

between pumpellyite and epidote zones is characterized by both minerals. The boundary 

appears to lie about 1,500 m (5,000 feet) stratigraphically below Stoiber's upper limit of 

abundant epidote in amygdules, and 300-600 m 

(1,000-2,000 feet) above Stoiber's lower limit 

of prehnite in amygdules. Only two of the 

major copper deposits lie below Jolly's critical 

boundary, and all the others lie within 300- 

1,500 m (1,000-4,000 feet) above.

The literature on the Keweenawan lavas of 

Wisconsin contains no report of a detailed 

regional investigation of metamorphic zoning. 

Grant (1901, p. 62), however, noted the abundance of epidote, both as amygdules and in the 

masses that Jolly and Smith called "metadomains"; epidote metadomains are found associated with copper occurrences in all three of 

Grant's "copper ranges" (the Douglas, St. 

Croix, and Minong copper ranges, fig. 1). 

Grant also reported that prehnite is a common 

associate of copper ,on the Douglas copper 

range, but, though observed, is less common on 

the other two. Prehnite is noted as a common 

associate of copper in a more recent report on 

the Weyerhauser prospect (Minong range) 

(H. R. Cornwall, J. J. Runner, A. A. Stromquist, 

and R. W. Swanson, unpub. data, 1945). These 

observations suggest that the rocks known to 

contain copper in Wisconsin lie, in general, in 

Jolly's epidote zone and are, therefore, of higher metamorphic grade than those that contain 

most of the major copper deposits in Michigan. 

But the presence of prehnite as a common amygdule filling suggests, additionally, that Wisconsin copper prospects lie in the upper part of 

the epidote zone, and perhaps not far below 

the lower boundary of Jolly's pumpellyite zone, 

in which most of the major deposits occur in 

Michigan.

The metamorphic grade of the rocks at 

existing copper prospects may, therefore, 

explain why the rocks contain traces rather 

than an abundance of copper, but the grade 

is not so high as to preclude the existence of 

neighboring areas of slightly lower metamorphic grade that might be more promising. If 

one believes, following Stoiber and Davidson 

(1959) and Jolly (1974), that there is a genetic 

connection between copper deposition and metamorphic grade, then a logical early step in exploration would involve an attempt to find
areas in which the rocks belong to Jolly's 

pumpellyite zone, thus locating his epidotepumpellyite boundary. In Michigan, metamorphic grade decreases as stratigraphic distance 

increases above the base of the Portage Lake 

Volcanics, so areas of lower metamorphic 

grades should be sought where the rocks are 

stratigraphically higher than, or along strike 

from, areas where copper is associated with 

epidote metadomains. Such an investigation 

may have been carried out already, at least 

in reconnaissance by one or more mining companies, but, if so, the results have not been 

published.

HOST-ROCK LITHOLOGY

The major copper deposits of Michigan are 

either in flow tops characterized by an abundance of rubbly material the "fragmental 

tops" of literature on the Michigan deposits 

or in beds of felsite conglomerate that lie between certain lava flows. Both types of host 

rock are present in the lavas of Wisconsin, although because of lack of drill holes essentially the only practical way to build up complete 

stratigraphic sections of these rocks we do 

not know whether or not fragmental tops and 

felsic conglomerate beds occur with the same 

frequency as in Michigan. The only data adequate for a comparison come from a 300 m 

(1,000 foot) drill hole (No. 18) at the Weyerhauser prospect (H.R. Cornwall, J.J. Runner, 

A.A. Stromquist, and R.R. Swanson, unpub. 

data, 1945) ; the core from this hole was examined by H.R. Aldrich, B.S. Butler, and T.M. 

Broderick in 1925. The hole cuts rocks at the 

very top of the lava series. In this core, fragmental tops are as numerous and thick as in 

many parts of the Portage Lake Volcanics. 

Of the 20 flow tops intersected, 12 have at 

least some fragmental material, compared to 

an average of 21 percent fragmental tops for 

the Portage Lake Volcanics as a whole. The 

median thickness of these 12 fragmental tops 

at the Weyerhauser prospect is 1.8 m (6 feet), 

compared to 2 m (6.5 feet) for flows less than 

30 m (100 feet) thick in the Portage Lake 

Volcanics. The basalt of the thicker flows is 

generally ophitic, as are those of the Portage 

Lake Volcanics. All that can be said, therefore, 

is that in this sample of the lavas of Wisconsin, inadequate though it is, the lithology is 

similar to that of the Portage Lake Volcanics.

Individual township reports in the files of the 

Wisconsin Geological and Natural History Survey (1913-30) contain many references to 

"ophites." No quantitative comparison is possible, but the abundance of ophitic basalt is 

probably not vastly different from the 34 percent of ophitic basalt in the Portage Lake 

Volcanics (estimates -of White cited in Halls, 

1969, table 2).

A felsite conglomerate bed is the host rock 

of the largest native-copper deposit in Michigan, and other beds host smaller deposits. 

Similar conglomerate beds occur with the lavas 

in the Minong "copper range" and in the Minnesota extension of the Douglas range (Grant, 

1901, p. 9; Morey and Mudrey, 1972, p. 426- 

427), but the thickness and abundance of such 

beds in the series as a whole is not known. In 

Michigan, conglomerate beds tend to be poorly 

exposed compared to the lava flows with which 

they are interbedded, and their number and 

degree of continuity is known only because of 

extensive drilling.

PALEOHYDROLOGY OF THE COPPER 

DEPOSITS

Downdip projection of the Portage Lake 

Volcanics on opposite sides of the Lake Superior syncline in Michigan (Keweenaw Peninsula 

and Isle Royale) makes it clear that along the 

trough of the syncline, the lavas would be at 

sufficient depth to be metamorphosed at least 

to the greenschist facies (White, 1957, 1966b, 

1968; Halls, 1969). During burial metamorphism, leakage of ground water buried with the 

lavas from vesicles and interfragmental spaces 

in the flow tops would permit collapse of these 

openings. The water would leak toward the 

surface along any available channelways. This 

water, if saline like the deep formation waters 

common throughout the world, would inevitably be much enriched in copper by the metamorphic reactions (Stoiber and Davidson, 1959; 

Jolly, 1974), thus fulfilling the basic requirements for the ascending ore-forming fluid that 

deposited the native-copper deposits.

The distribution of native-copper deposits in 

Michigan can be explained by a hydrologic 

model involving updip migration of a hydro-
thermal fluid generated by the above mechanism. The maximum flow of fluid would be expected in areas where, other things being equal, 

the path from the depths of the Lake Superior 

syneline to the surface was shortest. The best 

reconstruction one can make of the structure 

of the Lake Superior syneline suggests that 

rocks of the Portage Lake Volcanics now exposed at the surface are closest to the deepest 

part of the trough of the syneline in an area 

extending southwest from the main copper 

district (White, 1957, 1968). The area that 

includes all the major deposits is farther northeast than one might expect if proximity to the 

deepest part of the basin were the only factor 

involved. One or more of at least three factors may explain this apparent displacement of 

the area of major deposits.

DEGREE OF METAMORPHISM

The area of Portage Lake Volcanics closest 

to the deepest part of the basin is an area in 

which the rocks have been metamorphosed to 

the grade of Jolly's (1974) epidote zone, and 

are thus of higher grade than the empirical 

optimum for major copper deposits. Within 

this area of higher metamorphic grade, there 

are actually a number of native copper deposits 

(see the two areas shown as containing minor 

deposits in fig. 1 of White, 1968); but though 

some of these deposits produced thousands of 

tons of metallic copper, their grade was generally marginal or submarginal, and they accounted for little more than 1 percent of the 

district production of native copper. If degree 

of metamorphism is, indeed, a major controlling factor, there may once have been major 

deposits in this area, but they would have 

been in rocks of lower metamorphic grade 

(pumpellyite zone) updip from the present 

surface, in rocks that have now been eroded 

away. According to this interpretation, the 

minor deposits now found in the area may be 

the downdip roots or tails of long-vanished 

major deposits.

PERMEABILITY ANISOTROPY

The easiest path from a point in depth to 

the surface need not be directly updip if the 

most open channelways rake at some lower

angle. In some of the major are deposits there 

is clear evidence of alternating zones of more 

and less permeable rock. These zones tend to 

be essentially parallel to the margin of the 

basin of lava accumulation, and generally rake 

westward in the main copper district. Westward-raking channelways would tend to produce a northeastward displacement of the center of maximum mineralization (see White, 

1957, 1968).

REGIONAL DISTRIBUTION OF MOST-POROUS ROCK

The existence of a reservoir of buried water 

is an essential feature of the hypothesis that 

the copper was deposited by water buried with 

the lavas and squeezed from them in depth 

during burial metamorphism. The principal 

reservoir would be the interfragmental spaces 

of fragmental flow tops and of conglomerate 

beds. I have estimated (White, 1968, p. 323) 

that original voids made up roughly 2.5 percent of the total lava column, an estimate based 

on the ratio of secondary fillings to solid lava 

in the part of the Portage Lake Volcanics now 

accessible to view. This ratio may or may not 

be typical of parts of the formation now 

deeply buried. The ratio depends on the relative 

abundance of fragmental versus smooth-topped 

flows and on the number and thickness of conglomerate beds; both factors were very likely 

different in different parts of the area of lava 

accumulation. An attempt to predict the abundance of voids in the deep parts of the Lake 

Superior syneline requires reconstruction of 

the paleogeography of the area at the time the 

lavas accumulated.

The Keweenawan lavas of the Lake Superior 

region are part of a belt about 1,600 km (1,000 

miles) long and 80-160 km (50-100 miles) 

wide in which there is a major accumulation 

of mafic rocks. This belt most probably represents a continental rift of some sort. Increasing 

evidence also shows that the mafic rocks are 

the product of a number of separate episodes of 

magma tic activity rather than a single major 

event (see Green, 1972, p. 292-297 and fig. 

V-5) and that separate basins of accumulation 

can be identified (White, 1972). Within an 

individual basin, such as that in which the 

Portage Lake Volcanics accumulated, the abundance both of fragmental flow tops and of
conglomerate would probably be greater toward 

the margins than in the center. Many of the 

arguments on which this conclusion is based 

are necessarily speculative, because there are 

no good modern analogs for many of the 

phenomena involved.

The Keweenawan flood-basalt flows that have 

fragmental tops differ in important respects 

from the aa flows whose outpouring has been 

observed by man. (1) A typical aa flow has a 

layer of rubbly debris underlying, as well as 

one overlying, the massive lava in the center; 

the underlying layer in aa flows is formed by 

overriding of fragments that tumble down 

the leading edge of the flow as it advances with 

action like that of a caterpillar tread. In contrast, typically little or no fragmental material 

is found at the base of the Keweenawan flood 

basalts. (2) The fragments of aa flows are 

typically spinose, whereas those of the Keweenawan flood basalts appear to be simply 

broken-up vesicular rock of the sort that caps 

the smooth-topped flows.

Flood basalts differ from those observed to 

flow from modern shield volcanoes primarily in 

respect to both the absolute volume of lava and 

the rate of extrusion. The average volume of 

lava in individual flood-basalt flows is about 

2 orders of magnitude larger than the average 

volume in modern flows, and the rates of extrusion and spreading of lava are comparably 

greater (White, 1960; Shaw and Swanson, 

1970; Swanson and others, 1975). The persistence and near-parallelism of flow tops for 

great distances in the Michigan copper district 

make it clear that the lava flows must have 

come to rest as nearly horizontal sheets on what 

was topographically a great plain, as was first 

suggested by Sandberg (1938).

On the assumption that fragmental flow 

tops are largely formed, as their appearance 

suggests, by crushing of ordinary smooth (pahoehoe-type) crust, one would expect to find 

fragmental tops primarily toward the margins 

of the area covered by an individual flow (fig. 

3). Near the vent area, the crust would be 

carried along passively on the fluid lava beneath, 

and nothing would impede its forward motion. 

Insofar as the flow increased in width away 

from the vent, the crust would be stretched, 

and cracks would probably form parallel to the

VENT AREA

100 KILOMETERS 

I

I

50 MILES

FIGURE 3. Hypothetical plan of a Keweenawan floodbasalt flow. Smooth-topped lava in the zone of spreading crust is presumed to grade into fragmentaltopped lava in the zone of buckling and crushing. 

Conglomerate beds overlying certain flows are interpreted as alluvial fans deposited by streams flowing 

into the basin from the margins.

direction of flow. But toward the margins of 

the flow where lava was ponded against a reverse slope at the basin margin, or against a 

levee of its own debris, a very different situation would exist. Even after the vent had 

ceased its activity, lava seeking hydrostatic 

equilibrium would continue to flow toward the 

margin, the additions of fluid near the margin 

being accommodated by increase in the depth 

of the fluid-lava pool. The crust carried forward by these additions of fluid lava would, of 

necessity, be ground against the crust already 

capping the pool. For this reason, I suggest 

here that fragmental flow tops form primarily 

where forward motion of a flood-basalt sheet 

decreases or ceases as the fluid becomes ponded, and that this phenomenon is to be expected most commonly near the distal parts of 

individual basalt floods the parts farthest 

from the vent area.

This model for the genesis of fragmental 

flow tops can be used to predict regional variations in the abundance of fragmental tops only 

if one knows the location of the vents from 

which the lavas came. This location in the 

Portage Lake Volcanics is not known with any 

certainty. Dikes are almost unknown within

8
the areas where rocks of the formation are 

now exposed. Inasmuch as all these areas lie 

near the periphery of the basin of accumulation, the vents are presumably either outside 

the basin1 or deep under Lake Superior, closer 

to the center of the basin. Bent pipe amygdules 

(observations first made by Butler and Burbank, 1929, p. 26-27, and confirmed in more recent work by the U.S. Geological Survey) indicate a more or less central source. Assuming, 

then, that the flows are derived from vents in 

the central parts of the basin, one would expect fragmental flow tops to be most common 

toward the margins.

The distribution of interflow conglomerates 

is presumably similar. Foreset beds and imbricate structure indicate that the interflow conglomerate beds of the Portage Lake Volcanics 

were deposited as fans by streams flowing inward from the margins of the basin (fig. 3) 

(White, 1957, 1960). The sedimentary rocks 

of the overlying Copper Harbor Conglomerate 

(fig. 1) are, for practical purposes, identical 

to those of the Portage Lake Volcanics, suggesting that they were derived from the same 

source terrain; these Copper Harbor sediments 

were also deposited by streams flowing inward 

from the margins of the basin (White and 

Wright, 1960; Hamblin, 1961; Hamblin and 

Horner, 1961; Wolff and Huber, 1973 (data 

from Isle Royale); Hubbard, 1972).

In a typical sedimentary basin, individual 

stratigraphic units would be expected to thicken toward the center of the basin. The situation would be entirely different, however, in a 

basin filled primarily with lava flows. In particular, if the lava flows are fed by fissures located 

in the central parts of the basin, streams flowing inward from the margins would be ponded 

or diverted sideways, sooner or later, against 

the slopes of the volcanic shields, however low, 

that would surround the area of the fissure 

eruptions. In fact, the basinward slope that 

would make any inflow of streams from the 

margin possible would presumably be due 

mostly to periods in which outpouring of lava 

did not keep pace with continued subsidence

x The swarms of Keweenawan dikes in areas of pre-Keweenawan rocks outside the basin, once thought to be a possible 

source of Portage Lake flows, are now known from paleomagnetic evidence to be older, perhaps more or less contemporaneous 

with all or part of Hubbard's Powder Mill Group.

of the basin (White, 1957, 1960). During such 

a period of volcanic inactivity, subsidence could 

reverse the initial outward (toward the basin 

margin) slope of the most recent lava flow, at 

least near the basin margin, permitting 

streams to course out over that flow toward 

the center of the basin. Conglomerate beds occur at intervals of roughly 250-300 m (800- 

1,000 feet) throughout the column of lava 

flows, suggesting cyclic fluctuation in the intensity of volcanic activity. Under these circumstances, the longer the interruption of volcanism, the greater the amount of subsidence 

before the next volcanic episode. The longest 

interruptions would thus be represented by the 

thickest conglomerate beds, which, other 

things being equal, would also be those that 

extended, as alluvial fans, farthest into the 

basin.

Another factor would work to the same effect. A stream pouring out over the rough 

permeable surface of a flow would quickly 

lose water to its bed and, of necessity, drop its 

load of gravel. The water seeping into the 

underlying lava would carry the sand and silt 

fraction of its load down with it, and gradually 

chink in the interstices. As this chinking 

process was completed up to the top of the flow 

at any place, subsequent loss of water to the 

streambed would be minimal, and the gravel 

deposit could be extended farther from the 

margin. The results of this process are clearly 

evident today: virtually all the interfragmental 

spaces in flow tops just below conglomerate 

beds are completely filled with fine-grained 

detrital material. As a result of this process, 

alluvial fans could extend only slowly out into 

the basin, and again one would expect the 

longest intervals of nonvolcanism to be represented by the thickest and most extensive 

conglomerate beds.

The total of all conglomerate beds, therefore, 

would include some that extend only a short 

way inward from the basin margin and others 

that extend far inward. The total amount of 

conglomerate in the lava column, therefore, 

would be greatest somewhere near the basin 

margin and would diminish inward. Field observations support this model. The ratio of conglomerate to lava is only about 3 percent on the 

Keweenaw Peninsula proper, where the Portage
Lake Volcanics are thickest and farthest from 

the margin of the basin. The ratio reaches 8-10 

percent a few tens of kilometers southwest, 

where the formation is much thinner and represents a part of the basin closer to the original 

margin.

In another example, the fine-grained character and relative scarcity of interflow sediments 

in the North Shore Volcanic Group of Minnesota 

(Green, 1972, p. 301) are consistent with 

Green's (1972, p. 308) observation that primary 

features of the lava flows suggest no uniform 

or preferred flow direction. These are all features that one would expect if the model for the 

Portage Lake Volcanics is applicable and the 

Minnesota shoreline (where most of the observations have been made) lies close to the axis 

of the basin in which the North Shore Volcanic 

Group were accumulated.

In summary, what is known and can be inferred about the distribution of f ragmental flow 

tops and conglomerate beds in the Portage Lake 

Volcanics suggests that, in general, they should 

be most abundant toward the margins of a lava 

basin, and should decrease toward the central 

part (fig. 4). To the extent that the primary 

voids in f ragmental flow tops and conglomerate 

beds form the main reservoir from which water 

of metamorphic origin the inferred ore-forming fluid for the native-copper deposits was 

derived, the largest reservoirs would be expected in areas where marginal parts of the 

lava basin lie in the trough of the Lake Superior 

syncline. (The "Ashland syncline" (Button and 

Bradley, 1970, sheet 3) is a local name for the

FIGURE 4. Generalized model of a Keweenawan lava 

basin.

Wisconsin segment of the Lake Superior syncline.) This syncline is a tectonic feature involving sedimentary rocks much younger than 

the various volcanic formations of the Lake 

Superior region, and is thus demonistrably 

younger than the basins in which the lavas accumulated. Where the rocks in the trough of 

the Lake Superior syncline represent the central part of the basin in which the Portage Lake 

Volcanics accumulated, the amount of water 

driven from the trough of the syncline might 

be relatively small. But the axis of the syncline 

crosses the margin of the basin at both its ends; 

one of these crossings is near Ashland (see fig. 

1); the other is deep beneath Lake Superior and 

cannot be located with any certainty, but probably lies somewhere northeast of the eastern 

end of the Keweenaw Peninsula (see map in 

White, 1972). Near both these crossings, the 

rocks in the trough of the syncline would be 

expected to contain relatively abundant fragmental flow tops and conglomerate beds, and 

thus to form important reservoirs for ore-forming solutions.

In conclusion, the speculations in this section 

of the paper were prompted by the need to 

explain why the major native-copper deposits 

lie farther northeast than might be expected if 

proximity to the deepest part of the lava basin 

were the principal control. In addition to the 

two other factors already discussed more briefly 

 degree of metamorphism and permeability 

anistropy we arrive at another possible explanation; namely, that the rocks in the trough 

of the Lake Superior syncline downdip from the 

major native-copper deposits are farther from 

the source area of the lava flows, and thus contained more fragmental lava and conglomerate 

and a larger reservoir of buried water than 

existed farther west. I have devoted a disproportionate amount of space to this third possibility in part because it has not been described 

previously in print, and in part because it has 

potential application to exploration for native 

copper in Wisconsin as well as in Michigan.

REGIONAL STRUCTURE OF THE 

KEWEENAWAN LAVAS IN WISCONSIN

GENERAL STATEMEN F

Existing knowledge of the geologic structure 

of the Keweenawan lavas in Wisconsin is not

10
adequate to allow one to distinguish areas more 

and less favorable for exploration on the basis 

of geologic structure. The configuration of the 

Lake Superior (Ashland) syncline is known, 

and it is clear that, in the trough of this syncline, the copper-bearing lavas overlying the 

conglomerate of Davis Hill (fig. 1) reach depths 

of more than 7,500 m (25,000 feet).2 But the 

configuration of the basin or basins in which 

the copper-bearing lavas originally accumulated 

is not known, and, as has been shown in the 

preceding section of this paper, some knowledge 

of basin configuration is essential for reconstructing such paleohydrologic systems as may 

have produced and channeled ore-forming fluids. 

Although concrete evidence for the configuration of the lava basin or basins is lacking, it 

may, nonetheless, be worthwhile to attempt a 

crude reconstruction using information that is 

available. The results of such an attempt may 

well contain gross errors, but they show, first, 

the kinds of data that are needed to make a 

better reconstruction. Second, the resulting reconstruction, right or wrong, can be used to 

illustrate how one might use more reliable 

knowledge of the structure to derive a paleohydrologic model that could, in turn, help delineate areas more and less favorable for exploration. By using this way of describing a 

methodology, I run some risk that a reader may 

take too seriously the actual results, forgetting, first, the tenuousness of the theory and, 

second, the inadequacy of existing geologic 

data. The hope is, of course, that the discussion and illustration will suggest essential 

elements of a proper investigation to individuals or organizations that may at some time 

be in a position to undertake an investigation.

KINDS OF INFORMATION USEFUL FOR 

STRATIGRAPHIC SUBDIVISION AND CORRELATION

As preface to the attempt at reconstruction, 

I shall discuss briefly the kinds of information 

that do exist or can be obtained for Keweenawan lava basins in general.

The stratigraphy of the Portage Lake Vol2 Lavas are shown extending to depths of as much as 15 km 

(10 miles) tn section J-J' of figure 3 in White (1966a), bnt 

this section is based in part on interpretation of gravity anomalies, and the volume of rock shown as "lava flows" includes 

lavas below the conglomerate of Davis Htll; these lower lavas 

are not known to contain significant concentrations of copper, 

as noted above.

canics is known almost flow by flow for a distance of 150 km (90 miles) east of long. 

89°30' W., due primarily to the enormous 

amount of diamond drilling that has been done 

in the search for copper deposits. The cost of 

this tool makes its use unthinkable as a means 

of stratigraphic mapping, but limited use is 

highly desirable for (1) establishing at least 

one standard stratigraphic section of the lavas 

(preferably made early rather than late in a 

regional survey), and (2) solving critical 

problems of stratigraphic correlation that cannot be resolved with data collected at the surface (presumably late in a regional survey).

In areas of good exposure, it is possible by 

surface geologic mapping alone to attain a very 

satisfactory degree of stratigraphic subdivision and correlation, as the work of Lane 

(1898) and Huber (1973b) on Isle Royale, and 

Green (1972) along the Minnesota coast has 

shown. Results for many areas of the Portage 

Lake Volcanics on 'the south shore of Lake 

Superior, on the other hand, would have been 

very much less satisfactory without the aid of 

diamond-drill data. Even less satisfactory 

results are to be expected for much of the 

Keweenawan lava accumulations of Wisconsin, 

particularly if, in contrast to the experience in 

Michigan, detailed mapping shows few, if any, 

individual flows or conglomerate beds that, by 

reason of unusual thickness or lithology, can 

be confidently traced for distances of many 

tens of kilometers.

The usefulness of outcrop mapping, particularly for stratigraphic correlations and local 

structure, can be enormously enhanced by magnetic surveys (see Hotchkiss and others, 1915, 

p. 75-136; Broderick and Hohl, in Butler and 

Burbank, 1929, p. 161-168; Aldrich, 1929, p. 

70-73; Lamey, 1938). The references cited all 

describe the use of the dip needle for magnetic 

mapping. Far more accurate and precise results and far greater speed and convenience 

can be obtained today from field magnetometers now available, but the principles of interpretation are the same. The upper amygdaloidal part of a mafic lava flow is normally 

much less magnetic than the massive basalt of 

the interior of the flow. As a result, successive 

measurements along a traverse at right angles 

to the strike of dipping flows yield a jagged 

curve in which pairs of highs and lows tend to

11
represent individual flows. One typically has 

little difficulty in correlating flows between the 

two, provided the distance between traverses 

is not too large, even though no two adjacent 

traverses would be expected to yield curves 

that are completely congruent. Exceptionally 

large highs, lows, or steps that fairly consistently mark certain flows or stratigraphic 

horizons can, of course, greatly strengthen 

intertraverse correlations.

The results of surface and airborne magnetometer surveys can be usefully combined, 

because the peaks and lows on an aeromagnetic 

profile represent groups of flows and tend to 

be traceable for longer distances than individual features of ground profiles.

Brief mention should also be made here of 

another tool for correlation, potentially very 

powerful, that to date has been used only to 

correlate major subdivisions of the Keweenawan rocks of the Lake Superior region. This 

is paleomagnetism (DuBois, 1962; Books, 

1972). The detailed flow-by-flow study by 

Books of a section of the Portage Lake Volcanics on the Keweenaw Peninsula suggests 

that the paleomagnetic direction characteristic 

of individual flows and groups of flows may, 

in places, be sufficiently different from one 

another to permit boundaries to be drawn 

and correlations made within a lava series. 

Books (1972, fig. 21) presented, as an example, the paleomagnetic evidence for correlation of a specific horizon between the Keweenaw Peninsula and Isle Royale, on opposite 

sides of the Lake Superior syncline. This tool 

should ultimately provide some of the best 

evidence for subdivision of, and correlation 

from place to place within, the Keweenawan 

lavas of Wisconsin.

Finally existing gravity and aeromagnetic 

maps of the region (Button and Bradley, 

1970, sheet 2; Ervin and Hammer, 1974) provide bi oad constraints for the regional stratigraphy and structure.

DATA AVAILABLE FOR WISCONSIN

Most of what is known about regional 

geology of the Keweenawan lavas of Wisconsin is the product of two major undertakings 

of the Wisconsin Geological and Natural History Survey. The most useful today are (1) the

report on Grant's (1901) investigation of the 

copper occurrences, and (2) the products of 

an extensive series of geologic and magnetic 

surveys of individual townships carried out 

between 1912 and 1930 for the purpose of 

mineral-land classification. Reports on a number of townships close to the southern border 

of the area of Keweenawan rocks have been 

published (Hotchkiss and others, 1915; Aidrich, 1929), but most of the reports, together 

with the field notebooks and specimens, are in 

the files of the Wisconsin Survey. Button and 

Bradley (1970) reviewed this material in preparing their report on the Precambrian rocks 

of Wisconsin.

Although the primary goal of the 1912-30 

survey was to determine as expeditiously as 

possible the "evidence that exists as to the 

presence or absence of iron-bearing rocks, and 

as to the geologic structure of the region" 

(Hotchkiss and others, 1915, p. 5), some of 

its results can be applied to the problem of 

regional stratigraphic correlation within the 

lava series. The location of the top of the volcanic sequence is known rather precisely for 

much of the length of its trace, providing a 

useful datum for stratigraphic measurements. 

And the magnetic surveys produced lines of 

magnetic anomaly that, where numerous and 

parallel, can be taken to represent the trend 

of bedding. (Button and Bradley (1970) show 

these magnetic trend lines as well as outcrops 

on their sheet 1.) And, finally, if the specimen 

collection is complete enough, study might 

well provide some basis for subdividing and 

correlating, particularly if there should prove 

to be some fairly distinctive units or lithologic 

boundaries within the lava series. This last 

effort has not, to my knowledge, been undertaken with the goal of correlation in mind.

HYPOTHETICAL MODEL OF A BASIN OF 

LAVA ACCUMULATION

It is possible to generate, primarily from 

the magnetic data, an example of the kind of 

configuration of a lava basin that one would 

hope to get reliably from a detailed knowledge 

of ihe stratigraphy of the lava series. Inspection of the magnetic trend lines compiled by 

Button and Bradley (1970, sheet 1) reveals 

a number of places where the lines converge
(or diverge) to a marked degree. The boundary 

between two areas whose trend lines converge 

or abut one against the other can obviously 

have more than one reasonable geologic explanation, such as a fault or an unconformity. 

Given good exposures and adequate stratigraphic knowledge, one can ordinarily discriminate between the possibilities, but at 

present this cannot be done for the region 

under discussion. Simply for the purpose of 

generating a model, therefore, let us assume 

that certain boundaries between areas of differing magnetic trend represent an unconformity. The boundaries between areas designated as units A and B in figure 5 were drawn 

to make such a separation.

The areas of unit A on opposite sides of the 

Lake Superior (Ashland) syncline are both 

lenticular, and their outcrop patterns suggest 

a lens-shaped body of lavas overlying those of 

unit B. By using such information as is available on the dip of the lavas and the breadth 

of outcrop shown, one can derive approximate values for the thickness of the hypothetical lava sequence within unit A at different points. If, further, one assumes that this 

lens-shaped body of lavas is elliptical in plan, 

the size and orientation that yield the best fit 

with the thickness estimates are suggested by 

the elliptical thickness contours (isopachs) in 

figure 5. All the uncertainties make it unwise 

to attach much value to this specific result. 

The orientation of the long axis, in particular, 

lies strongly athwart the trend of the midcontinent gravity high; the long axes of basins 

suggested primarily by interpretation of gravity anomalies, in contrast, tend to be subparallel to the midcontinent gravity high (see 

map in White, 1972). Furthermore, magnetic 

lines in the areas of unit A (the upper group 

of lavas) are more commonly cut by the 

boundary than those of the area of unit B, a 

situation opposite from that which characterizes most unconformities. Much of the northern 

edge of the basin as drawn in figure 2 does, 

however, follow a prominent low on the aeromagnetic map and is also subparallel to a very 

large northeastward-sloping declivity on the 

Bouguer gravity map (Button and Bradley, 

1970, sheet 2), both of which features may be 

reasonably attributed to the margin of a basin

of lava accumulation. As emphasized above, 

this attempt to define the extent of a lava 

basin in three dimensions was not carried out 

with any serious hope that presently available 

information is adequate for reaching a correct 

interpretation. It was made simply (1) to illustrate, by example, how stratigraphic information can be used to determine the geometry 

of individual basins of lava accumulation,

(2) to show the kind of situation that better 

stratigraphic information might reveal, and

(3) finally, to generate a hypothetical model of 

basin geometry that can be used to illustrate 

the use of such knowledge. The remainder of 

this section is devoted to the last-named topic. 

The isopachs of figure 5 define the original 

shape and extent of a hypothetical basin of lava 

accumulation which can be designated basin A. 

The lavas of unit B below it were presumably 

accumulated in an earlier basin or basins that 

will be designated as basin B. The eastern 

boundary of basin B is suggested by the doubly 

hachured line; this line simply connects apparent pinchouts of the lavas of basin B. The 

copper prospects in the areas described by 

Grant (1901) as the St. Croix and Minong 

copper ranges are in rocks of basin A, and the 

copper prospects of the Douglas range and 

probably of the Chengwatana Volcanic Group 

of Morey and Mudrey (1972, p. 425-430) of 

adjoining parts of Minnesota belong to basin 

B, provided there are no additional stratigraphic complications.

AREAS FAVORABLE FOR COPPER IN 

HYPOTHETICAL LAVA BASINS

Experience in the Michigan native-copper 

district has, as described earlier, suggested 

three factors that may affect the regional distribution of copper deposits formed by downwarp (in the Lake Superior syncline) of these 

hypothetical lava basins. These factors are 

(1) the grade of metamorphism, (2) the 

orientation of primary irregularities in flow 

tops (White, 1957), and (3) the location of 

areas where, by reason of above-average proportions of conglomerate and fragmental flow 

tops, the lavas in the trough of the syncline 

would have been especially porous and therefore provided an important reservoir of buried 

water. Evaluation of these factors in Wiscon13
91° 

EXPLANATION

Hypothetical subdivisions of 

undifferentiated Volcanics 

of figure 1

UnttB

Other units and symbols 

same as in figure 1

10,

Isopach, given in 

of hypothetica

.LL-LL.LL-

Margin of hy

lava bas

Inferred bound

in which Por

Volcanics ac

FIGURE 5. Generalized geologic map of northwestern Wisconsin showing hypothetical basins of 
sin can be used to suggest areas most favorable 

for copper deposits, first in hypothetical basin 

A, and then in basin B.

HYPOTHETICAL BASIN A

METAMORPHIC GRADE

The metamorphic grade of the rocks at the 

very top of the lava series is unfavorably high 

in the areas described by Grant (1901) as the 

St. Croix and Minong copper ranges. The main 

copper prospects of both ranges are in the central rather than marginal parts of basin A as 

defined by the isopachs of figure 5. Their higher 

metamorphic grade therefore offers a parallel 

to the situation in Michigan; as noted above, the 

area in which rocks of the Portage Lake Volcanics at the present surface lie closest to the 

area of deepest downfolding is also an area in 

which the metamorphic grade is unfavorably 

high. The prospects could conceivably represent 

the roots of larger deposits that once existed 

farther updip in rocks long since eroded. Furthermore, because the metamorphic grade tends to 

rise with stratigraphic depth, rocks deeper in 

the section than those that contain the copper 

prospects that is, to the northwest of those 

in the St. Croix range and to the southeast of 

those in the Minong range are even less favorable in terms of metamorphism. The most promising areas would lie at high stratigraphic 

levels in basin A, along strike to the northeast 

and southwest of copper prospects found to be 

associated with epidote metadomains.

PERMEABILITY ANISOTROPY

The trend of zones of more and less permeable rock in flow tops of the Portage Lake 

Volcanics tends to be more or less parallel to 

the margin of the lava basin (White, 1957). 

This parallelism may be a consequence of the 

crumpling of flow crust along the distal margin 

of individual basalt floods, postulated in an 

earlier section of this paper. In the model for 

basin A suggested by figure 5, the trend of 

permeable zones would be nearly down the 

present dip of bedding in all but the axial region 

of the basin. The trend would, therefore, be 

favorable for formation of copper deposits on 

both limbs of the Lake Superior syncline in all 

but a narrow zone along this axis, and this

criterion is not very exclusive for the basin as 

drawn.

REGIONAL DISTRIBUTION OF MOST-POROUS ROCK

According to the model suggested for the 

Portage Lake Volcanics, the principal reservoirs 

of water carried to great depth by downwarping of the Lake Superior syncline would have 

been in areas where marginal parts of basin A 

cross the deeper parts of the Lake Superior syncline. Water driven updip from these reservoirs 

would have passed through the rocks in the 

northeastern and southwestern parts of the 

areas of unit A on both limbs of the syncline, 

making these rocks most favorable for the 

occurrence of copper according to this criterion.

SUMMARY FOR HYPOTHETICAL BASIN A

All the criteria discussed above suggest that 

the places most favorable for copper deposits 

are in the northeastern and northwestern parts 

of the areas of unit A in figure 5 on both limbs 

of the Lake Superior syncline. The fact that the 

metamorphic grade is unfavorably high near 

the top of the lava series in the central part of 

basin A suggests that it is also likely to be 

unfavorable at deeper stratigraphic levels over 

a larger area, and perhaps over the whole basin. 

The most promising areas, therefore, are those 

underlain by rocks near the top of the lava 

series and within 15 to 25 km (10 to 15 miles) 

of the margin of basin A as drawn in figure 5. 

There are four such areas, two on the north 

limb of the syncline and two on the south limb. 

The westernmost area, which includes the 

southwest end of the St. Croix copper range, has 

at least scattered exposures, and for this reason 

it is doubtless the most thoroughly prospected. 

A large area east of the Minong copper range is 

underlain by rocks stratigraphically high in 

the lava series, and apparently has few, if any, 

outcrops; this is potentially the most favorable 

of the four areas. The other two areas are of 

intermediate promise.

The area east of the Minong range and, to 

some extent, also the one that lies across the 

syncline from it, would be favorable under a 

wide variety of assumptions about the size, 

shape, and orientation of basin A. These areas 

are, therefore, the most promising if no further

15
background studies are feasible. Discovery of 

epidote metadomains in either area would be 

discouraging.

HYPOTHETICAL BASIN B

So few inferences about the geometry of 

basin B are possible on the basis of existing 

information that only some general remarks, 

primarily on metamorphism, seem warranted. 

Unlike those of basin A, the copper prospects of 

basin B (the Douglas copper range of Grant, 

1901) are in rocks in the lower rather than 

upper parts of the stratigraphic column for the 

basin. Although the rocks that contain these 

prospects are characterized by epidote metadomains and are thus presumed to be unfavorable 

for the occurrence of major deposits, they probably do not lie very far below rocks of the 

pumpellyite zone (see fig. 2).

Grant (1901, p. 62-63) contrasted the abundance of prehnite in copper prospects of the 

Douglas copper range with the relative scarcity 

of prehnite in the prospects of the St. Croix 

range. A belt containing stratigraphically higher rocks of basin B lying south of the area of 

copper prospects, therefore, may well contain 

rocks of lower metamorphic grade. This belt is 

3 or 4 km (2 or 3 miles) wide, and apparently 

contains virtually no outcrops. The eastern end 

of this belt, at least, lies beyond the margins of 

hypothetical basin A as drawn in figure 5, and 

thus need not contain rocks of the same or 

higher metamorphic grade than those of the 

stratigraphically higher St. Croix copper range. 

The eastern end of this belt without outcrops 

could, therefore, be much more promising than 

the stratigraphically lower Douglas copper 

range.

Taking the double hachured line as a correct 

representation of the eastern margin of basin 

B, this area south of the easternmost Douglas 

range is also relatively favorable from a paleohydrologic standpoint. Like the favorable areas 

of basin A, it is fairly near the margin of a 

basin at a place where this margin crosses the 

axis of the Lake Superior syncline.

PORTAGE LAKE VOLCANICS

belt that wedges out westward in the vicinity 

of Mellen. According to the tectonic criteria 

applied above to hypothetical basin A, this belt 

of Portage Lake Volcanics should be favorabte 

for the occurrence of native copper, in both 

Wisconsin and Michigan. In terms of metamorphic grade, however, the area is less promising. Hubbard (1975) reported that all feldspars 

contain, or are altered to, epidote. Pumpellyite, 

however, does occur, at least locally, in epidote 

metadomains in the uppermost part of the Portage Lake Volcanics, so the top of the formation may lie fairly close to Jolly's (1974) 

boundary between epidote and pumpellyite 

zones.

CONCLUSION

Although the specific conclusions about favorable and unfavorable areas reached in the preceding exercise should not be taken very 

seriously because of the weakness of the underlying assumptions and, data, the principles of 

interpretation could be very profitably applied 

to a more substantial body of basic data. The 

nature of such a body of data is the subject 

of this concluding section.

As is implicit from all the foregoing, the 

basic information needed falls into three main 

categories: metamorphic grade, stratigraphy, 

and structure.

METAMORPHIC GRADE

Within areas containing outcrops, rocks of 

the epidote and pumpellyite zones are fairly 

easy to differentiate (Jolly and Smith, 1972). 

For a given outcrop this can be done by simple 

inspection if the altered bodies described by 

Jolly and Smith as "metadomains" are present. 

The bright pistachio green of epidote metadomains is very different from the drab grey 

green of pumpellyite metadomains.3 If no 

metadomains are present in outcrops that 

are critical for determining a metamorphic 

zone boundary, petrographic study of the 

altered rock between amygdules in flow tops 

can be used to discriminate between rocks of

* In older drill logs of the Michigan copper district In company flies, the rock of pumpellyite metadomains Is commonly

As Shown in figure 1, lavas belonging to the called "zolsitlzed." Metadomains tbat have both epidote and

Portage Lake Volcanics are exposed in a narrow pumpeiiyite pumpellyite characterize a transition zone between epidote and 

zones.

16
the different metamorphic zones (see Jolly and 

Smith, 1972, p. 288-291).

The fragmentary information in the literature suggests that most, if not all, the rocks in 

outcrop areas belong to the epidote zone. It 

might be profitable, first, to study specimens of 

amygdaloid and altered rock in the collection 

of the Wisconsin Geological and Natural History Survey and then to make field examinations in critical areas to see if any rocks of the 

pumpellyite zone are represented in outcrops. 

They are to be expected high rather than low, 

stratigraphically, within any given lava basin. 

And if experience in Michigan is a reliable 

guide, a metamorphic-zone boundary is not 

likely to cut across stratigraphy at a high 

angle on a regional scale; the highest gradient 

observed in Michigan is about 40 m/km (1,000 

feet/5 miles). For example, if the rocks at the 

very top of the lava series belong to the epidote zone at point X, rocks stratigraphically 

600 m (2,000 feet) or so below the horizon of 

X can be safely assumed to be in the epidote 

zone for at least 8 km (5 miles) along strike 

from X.

These principles are of some help in the vicinity of outcrop areas, but there is probably 

no simple way, short of diamond drilling, to 

determine metamorphic grade in the larger 

areas of northern Wisconsin that are devoid 

of outcrops. Needless to say, if drilling is done 

in such areas for any other reason, an important extra dividend may be critical evidence on 

metamorphic zoning.

STRATIGRAPHY

For the purpose of determining the geometry of a basin of lava accumulation, it is not 

necessary to have as much stratigraphic information as is available, primarily from drill 

records, for the Michigan copper district (see 

Butler and Burbank, 1929, pi. 15). A desirable 

level of stratigraphic subdivision and correlation is best illustrated perhaps by Huber's 

(1973b) geologic map of Isle Royale National 

Park. The important feature of this map is 

that four lava flows were traced from one end 

of the island to the other, and a number of 

other units for more than 15 km (10 miles), 

providing a solid network of stratigraphic 

datum planes. By 'combining measurements of

dip and breadth of outcrop, Huber (1973a, fig. 

23) was able to prepare a longitudinal stratigraphic section for the whole island that provides important information about the north 

flank of the basin in which lavas of the Portage 

Lake Volcanics accumulated. Individual flows 

are farthest apart in the middle of the island 

and tend to converge toward the ends of the 

island. This suggests that the ends of the 

island are closer to the margins of the lava 

basin than the middle or, in other words, that 

the exposures on the island lie along chords 

bounding a segment of arcuate basin margin, 

convex toward the north.

In northern Wisconsin, scarcity or absence of 

outcrops in many, perhaps most, areas makes 

it impossible to attain this degree of stratigraphic subdivision from outcrops alone. If, 

however, a few horizons can be reliably traced 

across the areas that do contain outcrops by 

some combination of lithologic and magnetic 

mapping (Grant's map (1901, pi. 13) of 

the Minong range suggests that this is feasible) , it is not unreasonable to expect that correlations can be made across large areas devoid of exposures. With luck, one might be 

able to make strictly lithologic correlations 

of individual flows or flow sequences in some 

areas. Correlation of a substantial fraction of 

the Portage Lake Volcanics between Isle 

Royale and Keweenaw Peninsula across a gap 

of 80 km (50 miles) (Huber, 1973a, p. C25- 

C27) shows that long-distance lithologic correlations are possible.

In the absence of reliable lithologic correlations, aeromagnetic lines, reinforced by surface magnetic work in areas of ambiguity, can 

provide a basis for extrapolating stratigraphic 

boundaries for long distances. For some areas, 

the earlier magnetic work with dip-needle 

showed large numbers of lines of magnetic 

anomaly that could be followed for kilometers 

(see Button and Bradley, 1970, sheet 1). 

Where such lines are parallel and fairly 

straight, they can be accepted today as tools 

for correlation. Over much of the area underlain by lavas, however, the mapped lines are 

short and of irregular strike, or even nonexistent. Far more satisfactory results probably 

can be obtained by using the more sensitive 

field magnetometers now available. For the

17
purposes envisaged here, to repeat all the 

1913-30 traverses using modern magnetometers would not be necessary or desirable. It 

would be far more efficient to follow a few distinctive anomalies by short traverses, at right 

angles to the strike, making these traverses at 

far closer intervals than the half-mile intervals between traverses of the earlier surveys. 

Traverses 150 m (500 feet) apart have proved 

very satisfactory for geologic mapping of 

lavas in Michigan, but there will always be 

some places where ambiguities, such as those 

due to faults, can be resolved only by closer 

spacing. The results of earlier dip-needle surveys could be very helpful in suggesting anomalies or groups of anomalies that have sufficiently distinctive characteristics to make them 

easy to follow along strike.

As noted earlier, paleomagnetic measurements might provide a particularly useful tool 

for reliable long-distance correlation across 

covered areas. A basic requirement is the existence of one or more horizons that can be 

more or less uniquely identified by the paleomagnetic directions in a single flow or in two 

or more contiguous flows. To find such a horizon would initially require measuring the 

paleomagnetism of a large number of flows in 

as nearly complete a sequence of flows as one 

or more of the better outcrop areas afford 

(see, for example, Books, 1972, figs. 19 and 

20). Once a useful horizon (or horizons) is 

found, fewer measurements should be needed 

to firmly establish correlation with the rocks 

in another area, assuming a rough correlation 

can be made by other means.

STRUCTURE

The structural measurement most necessary 

for the purpose envisaged here is the dip of 

bedding, inasmuch as the strike is manifest 

from the results of the various kinds of mapping described. If interflow sandstone or shale 

beds can be found, there is little problem. The 

dip of lava flows, on the other hand, may be 

difficult to obtain, particularly in exposures 

with small relief, because the boundaries between flows tend to be poorly exposed. Even 

where exposures are good, flow contacts may be 

sufficiently irregular to rule out a unique dip 

reading. A single measurement on a lone exposure is probably worth very little. A large 

number of readings made over a relatively 

limited area, on the other hand, should generally make possible an estimation of the dip 

within 5° or so. For many areas, an improvement on the estimates that already exist in the 

townships' reports of the Wisconsin Geological 

and Natural History Survey may not be possible. The matter of dip is mentioned here only 

because anyone doing field work for any of the 

purposes described above should be constantly 

aware of the need to record every available 

clue to dip.

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Butler, B. S., and Burbank, W. S., 1929, The copper 

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18
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19