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GEOLOGICAL SURVEY CIRCULAR 842

The Potential for -Bearing Kimberlite in Northern Michigan and Wisconsin

Prepared in cooperation with the Geological Survey Division, Michigan Department of Natural Resources

The Potential for Diamond-Bearing Kimberlite in Northern Michigan and Wisconsin

By W. F. Cannon and M. G. Mudrey, Jr.

GEOLOGICAL SURVEY CIRCULAR 842

Prepared in cooperation with the Geological Survey Division, Michigan Department of Natural Resources

Description of a kimberlite pipe and possible cryptovolcanic structures and a discussion of the possibility that kimberlites in Wisconsin and Michigan might be the source for drift in Wisconsin

Washington 1981 United States Department of the Interior JAMES G. WATT, Secretary

Geological Survey Doyle G. Frederick, Acting Director

Library of Congress catalog-card No. 81-600130

Free on application to Distribution Branch, Text Products Section, U.S. Geological Survey, 604 South Pickett Street, Alexandria, VA 22304 CONTENTS

Page Page Abstract ------1 Possible cryptovolcanic structures ______7 Introduction ______1 Glover Bluff structure ------7 Diamond discoveries in Wisconsin ------­ 1 Structures near Pelkie, Michigan ______9 Eagle, Waukesha County------1 Limestone Mountain------9 Plum Creek, Pierce County ______3 Sherman Hill ------10 Saukville, Ozaukee County ______4 Areas of disturbed Jacobsville ______10 Kohlsville, Washington County (Theresa diamond) __ 4 Brule River outlier ------10 Oregon, Dane County ------­ 4 Possible relationship between cryptovolcanic struc- Burlington, Racine County ------­ 4 tures and kimberlite ------~------10 Collins, Manitowoc County ------4 Do Wisconsin drift diamonds have a local source ______11 Why no finds since 1913? ------­ 4 Exploration for kimberlite ______14 The Lake Ellen kimberlite ------5 References cited ------14

ILLUSTRATIONS

Page FIGURE 1. Index and location map of Wisconsin and northern Michigan ------2 2. Photograph of the Saukville and Burlington diamonds------3 3. Magnetic map of the Lake Ellen kimberlite area------6 4. Magnetic and gravity maps of the Glover Bluff disturbed area------8 5. Geologic map of the Pelkie, Michigan, area------9 6. Idealized cross section of a kimberlite pipe------11 7. Map of area and magnetic and gravity profiles across the Lake Ellen kimberlite------13

III

The Potential for Diamond-Bearing Kimberlite in Northern Michigan and Wisconsin

By W. F. Cannon and M.G. Mudrey, Jr. 1

ABSTRACT INTRODUCTION Between 1876 and 1913, diamonds were found in at least At least seven diamond discoveries were made in seven localities in southern and central Wisconsin. All were found in Pleistocene glacial deposits or Holocene river gravel. Wisconsin between 1876 and 1913. All diamonds The bedrock kimberlite source for the diamonds is unknown but were found in Pleistocene glacial deposits or Holo­ has been presumed to be in northern Canada, the only area cene river gravel. The diamonds must have been north of Wisconsin previously known to contain kimberlites. eroded from one or more kimberlite bodies because Recently, a kimberlite pipe, here named the Lake Ellen kimber­ kimberlite is the only primary bedrock source of lite, has been found in County, Michigan. That find sug­ gests the possibility that drift diamonds in Wisconsin have come diamonds. It has generally been presumed that from a more local source--kimberlites in northern Michigan and drift diamonds in Wisconsin were carried to their Wisconsin. discovery sites by glaciers that eroded them from The Lake Ellen kimberlite is very poorly exposed, but a the nearest known kimberlites in northern On­ strong positive magnetic anomaly indicates that it is roughly cir­ tario, 800 km (kilometers) (500 miles) or more cular in plan and about 200m in diameter. Although the kimber­ lite is entirely surrounded by Precambrian rocks, it contains away. abundant inclusions of fossiliferous , probably from the In this paper, we describe a newly discovered Ordovician Black River Group that overlay the area when the kimberlite in northern Michigan and suggest kimberlite was intruded. The post-Ordovician age of the several other areas where kimberlites may exist in kimberlite us to suspect that other possible cryptovolcanic the subsurface. We discuss the possibility that drift structures in Paleozoic rocks in the region were formed over kimberlite pipes that are not yet exposed by erosion. Such struc­ diamonds in Wisconsin were eroded from kimber­ tures include Limestone Mountain and Sherman Hill, in lites in Wisconsin or Michigan. Houghton and Baraga Counties, Michigan; Glover Bluff, in Mar­ quette County, Wisconsin; and possibly an area along the Brule DIAMOND DISCOVERIES IN WISCONSIN River south of Iron River, Michigan. No diamonds are known in the Lake Ellen kimberlite, but it Verified discoveries of diamonds have been made has not been adequately sampled. The cryptovolcanic structures at seven localities in Wisconsin (figs. 1, 2). Other could not be the source of the drift diamonds in Wisconsin be­ diamonds have been reported, but they were prob­ cause even if the structures are caused by kimberlites, those kimberlites have not yet been exposed by erosion. ably not from Wisconsin and are only briefly Elsewhere in the world, kimberlite is seldom found as a single discussed in the following sections. isolated body; clusters of bodies are more common, and the presence of one kimberlite implies that others may exist nearby. EAGLE, WAUKESHA COUNTY The discovery of additional kimberlites may be very difficult because of the extensive cover of glacial drift and the typical The first documented diamond discovery in small size of kimberlite bodies. If all are magnetic, they might be Wisconsin was made during the digging of a well in found by detailed aeromagnetic surveys. However, the mag­ 1876 by Charles Wood, who was a tenant on the netism of the Lake Ellen kimberlite appears to be caused by sec­ farm of Tom Devereaux near Eagle in ondary formed during serpentinization of olivine, so an unserpentinized kimberlite may not be strongly magnetic. southwestern Waukesha County (Hobbs, 1899; We suggest that one or more diamond-bearing kimberlites Vierthaler, 1958; Olson, 1953; Alden, 1918). This may exist in northern Michigan or Wisconsin, but the discovery first important Wisconsin diamond discovery is of such bodies is unlikely unless a very thorough search is under­ recorded in the files of the Milwaukee County Cir­ taken. cuit Court and the Wisconsin Supreme Court. The 1 Wisconsin Geological and Natural History Survey. digging passed through 10-15 m (meters) (33-44

1 0 100 MILES

0 100 KILOMETERS

EXPLANATION o POSSIBLE CRYPTOVOLCANIC + DRIFT DIAMOND LOCALITIES STRUCTURES 1. EAGLE, WAUKESHA COUNTY 1. GLOVER BLUFF 2. PLUM CREEK, PIERCE COUNTY 2. STRUCTURES NEAR PELKIE 3. SAUKVILLE, OZAUKEE COUNTY (INCLUDES LIMESTONE MTN 4. KOHLSVILLE, WASHINGTON COUNTY AND SHERMAN HILL) 5. OREGON, DANE COUNTY 3. BRULE RIVER OUTLIER 6. BURLINGTON, RACINE COUNTY 7. COLLINS, MANITOWOC COUNTY \t DIRECTION OF ICE MOVEMENT NEAR DIAMOND LOCALITIES ___ EXTENT OF ICE LOBES a. GREEN BAY LOBE b. LAKE MICHIGAN LOBE

FIGURE 1.-Map showing the location of diamond discovery sites, marginal moraines of major ice lobes and direction of ice movement, possible cryptovolcanic structures, and the Lake Ellen kimberlite.

2 ~---12mm---~ -

FIGURE 2. - The Saukville diamond (left) and Burlington diamond (right). ft) of and then through loose gravel of approx­ A boom resulted. G. F. Kunz, a noted gem­ imately 5 m (16ft), when a 2-m (7-ft) layer of a hard nologist, visited the area and noted that the newly yellow material was penetrated. A hard stone of found stones differed in size, color, and number of unknown identity was recovered. Clarissa Wood crystal faces from the original find. He identified took this peculiar pebble to Col. Samuel B. Boyn­ the new stones to be of African origin, and the bot­ ton, a jeweler in Milwaukee. Boynton identified the tom fell out of the great Eagle diamond rush. stone as "" and purchased it for $1.00. Once J. P. Morgan, the late 19th century financier, the true identity of the diamond was disclosed, purchased the Eagle diamond from Tiffany's and Clarissa offered to buy the diamond back, first for ultimately donated the gem to the American $1.10 and later for $1.50. Boynton refused, and Museum of Natural History in New York City. The Mrs. Wood sued. The Supreme Court ruled that stone was on public display at the Museum until the the stone was Boynton's because he had believed it evening of October 29, 1964, when the Museum was topaz when he bought it. He later sold the was broken into and the Eagle diamond and other stone to Tiffany & Company of New York for $850. gems stolen. On February 8, 1965, Jack Roland He had offered the stone to the State of Wisconsin Murphy, a Florida beach boy, also known as for $1,000, but his offer was declined. The crystal, "Murph the Surf," and two colleagues, admitted originally known as the Waukesha diamond and the theft, but the stone was not recovered. later as the Eagle diamond, is a warm yellow color and weighs 16.25 carats. Boynton formed a diamond-mining company and PLUM CREEK, PIERCE COUNTY later, in 1883, claimed to have discovered more In the period 1880-1887, G. H. Nichols and two diamonds, along with other precious and colleagues prospected for along Plum Creek semiprecious stones, according to Silvers (1978) in Pierce County. While sluicing for gold, one of writing in a recent issue of The Milwaukee Journal. the workmen detected a bright stone, which

3 proved to be a diamond. Over several years, this OREGON, DANE COUNTY group found about 10 small diamonds there (Kunz, In 1893, a 4.0 diamond was discovered on 1892, p. 337). the Charles Devine farm near Oregon on the In 1906, another discovery of diamonds was Johnstown moraine in Dane County (Alden, 1918). reportedly made downstream from the earlier find. It was later sold to Tiffany's for $50. Its present A company was immediately organized and stock owner is not known. advertised extensively in Chicago. Samuel W eid­ man of the Wisconsin Geological and Natural S URLINGTON, RACINE COUNTY History Survey contended that this was a bogus find and had been staged (letter to G. F. Kunz on In 1903, G. Pufahl recovered a diamond December 15, 1907). weighing 2.11 carats near Burlington, in Racine County (Alden, 1918). The last known owner is the Bunde and Upmeyer Co., in Milwaukee (Vier­ SAUKVILLE, OZAUKEE COUNTY thaler, 1958).

Near the Milwaukee River, about 4 km (2.5 mi) COLLINS, MANITOWOC COUNTY north of Saukville, a diamond was discovered by Conrad Schaefer in 1881. It was not identified as a The Milwaukee Sentinel carried a short article diamond until1896 (Alden, 1904). This diamond is on January 19, 1913, about Peter Zagloba of Col­ 6.57 carats and about 12 mm (0.5 in.) in diameter. lins, in Manitowoc County. Zagloba had passed The last known owner is Bunde and Upmeyer Co., away several days earlier. While examining his in Milwaukee (Vierthaler, 1958). property, local farmers found several uncut diamonds in an old coffee pot. Zagloba was a her­ mit and spent his time digging into hillsides. This KOHLSVILLE, WASHINGTON COUNTY suggested that the diamonds might have come (THERESA DIAMOND) from gra.vel deposits in the Collins area. The Theresa diamond created the greatest in­ terest in the possibility of diamonds in Wisconsin. WHY NO FINDS SINCE 1913? It was discovered in 1888 by Louis Endlick of The seven localities in Wisconsin yielded a total Kohlsville on or near the Green Lake moraine. The of 16 diamonds. All diamonds were found in stone weighed 21.5 carats; it is the largest diamond Pleistocene. glacial deposits or Holocene river on record ever recovered in the State and the fifth gravel. Although none of the discoveries was made largest in the United States (Alden, 1918; by geologists and no further documented Sinkankas, 1959). discoveries have been made since 1913, the This diamond was of further interest because of Wisconsin diamond discoveries, in conjunction its uncommon color. One side of the crystal was with discoveries in other Midwest States, argue colorless, whereas the other side was cream that the diamonds did in fact come from the yellow. These two parts were separated by a flaw Midwest and do in fact provide evidence for or distinct plane. The crystal was almost kimberlite in the Midwest. spherical. A possibility exists that the finds may have been Shortly after the Theresa diamond was bogus. The diamonds could have been found discovered, the Endlick family moved away from elsewhere and subsequently brought to Wisconsin. Kohlsville, taking the diamond with them. Later Examples of such fraudulent discoveries include inquiries regarding the whereabouts of the dia­ the second Eagle discovery and the second Plum mond were fruitless until an article on the subject Creek discovery just discussed. In both cases, was published in a newspaper and read by a son of almost immediately after discovery of the Mr. Endlick (Olson, 1953). He explained that the diamonds, shares of stock were offered for sale. family had moved to Kewaskum, Wis. In 1918, the Kunz and Weidman were able to demonstrate that Theresa diamond had been cut into 10 stones at a these two finds were not in situ but rather were cost of $400. Total weight of the 10 stones was 9.27 salted. carats, divided as follows: 1.48 carats, 1.09 carats, Similarly, the lack of strong control at the 0.97 carats, 0.96 carats, 0.95 carats, 0.85 carats, seven discovery localities in Wisconsin leaves open 0.84 carats, 0.83 carats, and two stones weighing the possibility that factors other than geologic 0.65 carats each. processes were agents in their discovery. Gunn

4 (1968) summarized the known diamond discoveries of the answer may lie in the story of the Eagle dia­ in the Midwest. In order of number of diamonds mond, which sat unreported on a shelf for some 20 found, Indiana lists 34; Illinois, 25; Wisconsin, 16; years. Stones may well have been found in the past Michigan, 3; and Ohio, 3. This tends to suggest 80 years, but their significance is poorly that diamonds are geologically distributed understood by the finder, and the stone rests on a throughout the Midwest. The paucity of discover­ shelf or in a box awaiting rediscovery. ies in Ohio and Michigan suggests sources west of Lake Michigan. THE LAKE ELLEN KIMBERLITE In Wisconsin at least, none of the diamond discoveries was adequately documented at the time A kimberlite pipe, here named the Lake Ellen of discovery. The geologic details of the occur­ kimberlite, has recently been discovered about 15 rences are not known and can only be inferred km (10 mi) northeast of Crystal Falls, Mich., and from the geology in the area of the discoveries. about 1.5 km (1 mi) west of Lake Ellen, from which Most were found at shallow depths and presumably it derives its name. The kimberlite was apparently in the youngest glacial material. Similar relations first recognized by William H. Spence and Klaus J. are known for the discovery of drift nug­ Schulz about 10 years ago during a mining com­ gets in southeastern Wisconsin (Clayton, oral com­ pany exploration project, but its existence and mun.,· 1980). The drift copper has a source in the location have not been widely known, and it has upper peninsula of Michigan and is spatially close not been described previously. to the Lake Ellen locality discussed later. The only two known exposures of the kimberlite No effort was made at the time of the original are in the SW1/4 sec. 27, T. 44 N., R. 31 W. (fig. 3). diamond discoveries to exploit them, suggesting They consist of low exposures of rubbly reddish­ accidental finds offering little possible financial brown, highly weathered kimberlite along the return. The Kohlsville diamond may be a typical roadbed of an abandoned logging road. The two ex­ example, in that the stone was cut and mounted for posures are about 120 m (390 ft) apart and provide members of a family, rather than sold. a limit for the minimum dimension of the body. The Why then have so few diamonds been found in exposed kimberlite is strongly magnetic. A the last 80 years? Brummer (1978) reported that magnetic survey of the area (Gair and Wier, 1956) most of the diamonds found in the upper midwest defined an elliptically shaped positive magnetic were located by prospectors panning or sluicing anomaly about 180m (590ft) long in an east-west river gravels for gold, especially in Indiana. The direction and 120 m (390 ft) wide (fig. 2). Gair and Wisconsin diamonds were found by a prospector Wier did not know of the existence of the kimber­ sluicing at Plum Creek and by enterprising lite (the logging road had not yet been constructed) farmers at the other localities. and believed the magnetic readings were spurious, Since the last Wisconsin discovery in 1913, ex­ caused by magnetic boulders in glacial drift. cavations in Pleistocene materials have continued. However, the close spatial correspondence of the In recent years, nearly 30 million tons of anomaly to exposures of magnetic kimberlite Pleistocene and gravel have been produced us to conclude that the anomaly is caused by the annually for industrial use, and numerous excava­ kimberlite and roughly defines the size and shape tions for buildings, utilities, and roadways have of the body. been constructed. Although machinery is common­ The kimberlite is intruded into volcanic rocks of ly used, a significant percentage of hand labor is the Hemlock Formation, a part of the also used, and thus the chance of additional X Marquette Range Supergroup. The weathered diamonds should be quite high compared with the exposures of the kimberlite are heavily iron­ era of the diamond discoveries. stained rubble consisting of fragments about 1 em Why then, have no additional discoveries been (0.4 in.) in diameter. The material is soft enough to made? be dug with a shovel to a depth of about 0.5 m (1.5 The diamonds were found during the era of ft) below the surface, at which depth it becomes "gold" rushes. Although modern society harder, although still largely rubbly and disag­ remembers the rushes, few rushes have attracted gregated, and in places somewhat gray, in contrast the excitement of those in the 19th century. Since to the browner colors nearer the surface. The rub­ that time, rock hounds and collectors have ble fragments are a mixture of fine- to medium­ prowled pits, quarries, and streams for colorful grained kimberlite and inclusions of a variety of stones, but no diamonds have been reported. Part other rocks. The kimberlite is composed of olivine

5 R 31 W

r.a\~'3 v~.,. o"~' ...... ,'/f It II \\,, II I/ .~:~qOUTCROPS OF KIMBERLITE

SECTION T 44 N CORNER

0 2000 FEET

0 500 METERS

FIGURE 3.-Map showing the location of the Lake Ellen kimberlite and its magnetic expression. The magnetic data are from Gair and Wier (1956). Magnetic contours in the immediate area of the Lake Ellen kimberlite were drawn by W. F. Cannon in 1979 from the original data. Contours in gammas.

6 (in part serpentinized), , , , other stratigraphically higher Paleozoic and , and fine-grained serpentine (?) units could also have provided the fragments. At . present, we do not have an adequate fossil collec­ Preliminary petrographic and analytical studies tion to determine the precise age of the fragments, have been done by B. Carter Hearn, Jr., and Elaine but their Paleozoic age seems certain. Paleozoic S. McGee of the U.S. Geological Survey (written strata have since been eroded away, except for commun., 1980), and the following is their descrip­ scattered outliers of basal Paleozoic sandstone tion of the kimberlite: (Gair and Wier, 1956). The nearest exposures of Electron microprobe analysis of grains picked from a panned the Black River Group are now about 60 km (36 mi) concentrate and from fragments of weathered kimberlite show to the east, but the Black River Group could have the presence of typical kimberlite indicator : chromian been as little as 200 m (660 ft) stratigraphically garnet, magnesian ilmenite, chromian , and forsteritic olivine. above the present surface when the kimberlite was of 1-5 mm size were sorted into 6 color groups (pur­ intruded. ple, pink, , red-orange, orange, light orange) from which 3 to 6 grains of each color group were analysed. Sixteen garnets classified as , red, and pink are pyrope-rich and POSSIBLE CRYPTOVOLCANIC STRUCTURES -rich. These 16 garnets form a tight cluster on a Ca­ Mg-Fe plot with a range of 67 to 75 percent Mg molecule and 12 Several small areas of disturbed Paleozoic rocks to 20 percent Ca molecule and have the following ranges of have been known in Wisconsin and northern weight percent Cr20 3: 4.64-9.29 for purple garnets, 3.18-6.17 Michigan for many years (fig. 1), but the cause of for red garnets, and 1.72-3.43 for pink garnets. The ranges of the disturbance is not known. Until the recognition MgO and Cr 0 content are similar to those of garnet in garnet 2 3 of the Ordovician or younger age for the Lake zenoliths and garnet xenocrysts, which occur in kimberlites in (Mitchell, 1979; Hearn and Boyd, Ellen kimberlite, no intrusive or major structural 1975; McCallum and others, 1975) and in Africa (Dawson and events of Ordovician or younger age were known Stephens, 1975; Reid and Hanor, 1970). Ten red-orange, orange in the region. We now suspect that these disturbed and light orange garnets are more iron-rich and more widely areas may be collapse features formed over buried scattered on a Ca-Mg-Fe plot, and contain less than 0.79 weight kimberlite bodies. No intrusive rocks have been percent Cr20 3• One red-orange and one orange garnet plot in the Mg-rich cluster of purple, red and pink garnets, but contain found at the surface in any of the disturbed areas. We know of no drill holes in the disturbed areas. only 1.80 and 0.82 weight percent Cr20 3• The compositions of most red-orange, orange and light orange garnets fall into a compositional field which includes garnets from and GLOVER BLUFF STRUCTURE granulites. Ilmenite grains are either single crystals or granular The Glover Bluff structure is an area of dis­ multicrystal aggregates up to 6 mm in size. Five single crystals turbed Cambrian and Ordovician strata in Mar­ and two aggregates form a tight cluster near 50 percent quette County, Wis. (fig. 1). The structure was molecule (MgTi0 ) on a geikielite-ilmenite-hematite 3 mapped and described by Ekern and Thwaites plot (MgTi03-FeTi03-Fe20 3). Weight percent of MgO ranges from 12.85 to 14.96 and is typical of ilmenites in North (1930). The authors stated that the disturbed American and African kimberlites (Mitchell, 1973, 1977; Hag­ strata are exposed on three adjacent hills in the gerty, 1975). SW1/4 sec. 3, T. 17 W., R. 8 E. (fig. 4). The area of Six analysed diopside grains 1 to 3 mm size are green to bright the disturbance appears to be roughly circular and green, and contain 0.11 to 0.95 weight percent Cr20 3 and 0.87 to 1.31 weight percent NazO. These ranges are typical of diop­ about 500 m (1,600 ft) in diameter. Because ex­ side xenocrysts, diopside in garnet posures are discontinuous, the complete structural (Stephens and Dawson, 1977), and diopside from lher­ pattern has not been determined, but Ekern and zolite xenoliths in kimberlite (McCallum and others, 1975). Thwaites showed the structure to be folded and Al 0 and Na 0 are too low for typical eclogitic . CaO 2 3 2 complicated by several faults. The exposed dis­ content of pyroxenes indicates relatively low temperatures of equilibration, 900 to 1,000°C according to the Davis and Boyd turbed strata range in age from the Cambrian (1966) solvus. Galesville Sandstone to the Ordovician Oneota Olivine grains of 1-3 mm size contain 91 to 92 percent Dolomite. The structure is complex but generally molecule, typical of kimberlitic olivines. synclinal. Beds dipping as steeply as 30° are com­ A common type of inclusion in the kimberlite is mon, and locally bedding is vertical. The strata buff, gray, and black dolomite. Fragments are as within the structure have been dropped at least 60 much as about 10 em (4 in.) in diameter and m (200 ft) relative to their altitude in the surround­ generally angular to subrounded. The dolomite is ing undisturbed area, where they dip less than 1 o fossiliferous and may be derived from the Ordovi­ to the southeast. Outside of the structure Pro­ cian Black River Group that apparently overlay the terozoic rocks are at a depth of about 200m (650ft) area when the kimberlite was intruded, although below the surface.

7 R7E R8E 89"30' .------,~-.-,,---,---~----~----~--~7>------~ «000' 25

13

24 19 20 ~~ .g.~~ I I I I I

• POINTS AT WHICH GRAVITY OBSERVATIONS WERE MADE BOUGUER GRAVITY MAP A R7E R8E 89"30' .------,------,~--~-,--~--~r---~~-,------~ «000'

• POINTS WHERE MAGNETIC READINGS WERE TAKEN VERTICAL INTENSITY MAGNETIC MAP 8

0 3 MILES

0 3 KILOMETERS

FIGURE 4.-A, Bouguer gravity map showing the location of the Glover Bluff disturbed area (gravity data from Koenen, 1956). Con­ tours in milligals. B, Ground magnetic map showing the location of the Glover Bluff disturbed area (magnetic data from Koenen, 1956~ Contours in gammas.

8 A gravity and magnetic survey of the area was LIMESTONE MOUNTAIN made by Koenen (1956). Part of his maps are Limestone Mountain, in sees. 13, 14, 23, 24, T. 51 reproduced in figure 4. The Glover Bluff structure N., R. 35 W., is an outlier of Paleozoic strata is near the east end of a prominent west-trending overlying the Proterozoic Jacobsville Sandstone. positive magnetic anomaly and also is within the The Paleozoic rocks at Limestone Mountain, most­ area of a positive Bouguer gravity anomaly. ly dolomite and dolomitic limestone, form a promi­ Koenen has calculated that both anomalies could nent bluff that rises about 100m (330ft) above the be caused by a plug at a depth of about 2 km surrounding gently rolling topography. The outlier (1.2 mi) below the surface. He proposed that the is about 1.5 km (1 mi) long in a north-south direc­ Glover Bluff structure could be caused by faulting tion and about 1 km (0.6 mi) wide. The Paleozoic and collapse during intrusion of the plug. strata generally dip gently to steeply centripetally inward; therefore, the gross structure is a doubly STRUCTURES NEAR PELKIE, MICHIGAN plunging syncline. Several anomalous disturbances of Proterozoic The earliest detailed description of the area was and Paleozoic strata are in Houghton and Baraga by Case and Robinson (1914), who mapped the Counties near the town of Pelkie, Mich. (fig. 5). mountain and studied the paleontology to deter­ These may be cryptovolcanic features. mine the age of the strata. They identified forma-

I

I - f- I SHERMAN Yj 11 12 I ~HILL 8 9 I ; I Pzu I 8 45~ I I 16 14 13 18 17 r- --, 0' I I Yj I 0 I _}'Pelkie ~ LIMESTONE -- I 70)_ 11 6DY 5~ =>>::..\ MOUNTAIN 0 0 ,..., 0 0 T 51 N f7 I oo~ooo~o· 0 0 .. 0 uo I ro::;~( 0>0.... 0 I ~ I <\~ 24 19 20 21 23, I :E ~~oh ~~~ ulosl~ 0

I I R 35 W R 34 W 0 1 MILE

0 05 1 KILOMETER EXPLANATION ~ Paleozoic rocks, undivided-Mostly Contact-Approximately located Ordovician dolomite Fault ~ Jacobsville Sandstone-Reddish sandstone, , c:;) Area containing outcrops 6!l_ Strike and dip of bedding

FIGURE 5.--Map showing the general geology of an area near Pelkie, Mich., including Limestone Mountain, Sherman Hill, and areas of disturbed Jacobsville Sandstone. Geologic data modified from Case and Robin­ son (1914) and Roberts (1940). tions ranging in age from Proterozoic to Late Or­ bank, possibly because of slumping, some strata dovician in outcrops and also found talus dip as much as 15° E." In July 1979, the outcrop fragments of rocks as young as Early could not be located, perhaps because of high water that could not be found in outcrops. The area was in the Brule River or slumping of the river bank. remapped by Roberts (1940), whose detailed work About 2 km (1.2 mi) northwest of the limestone ex­ confirmed the structures identified earlier by Case posure, basal sandstone (Cambrian?) of the and Robinson. Paleozoic section is exposed on hills about 50 m (160ft) higher than the limestone, suggesting that SHERMAN HILL the limestone has been dropped down on the order of 100m (330ft) relative to surrounding Paleozoic Sherman Hill is in sec. 7, T. 51 N., R. 34 W., outliers. about 2 km (1.2 mi) northeast of Limestone Moun­ We suggest that this could be a cryptovolcanic tain (fig. 5). The area was included in the studies of structure. Although the downdropping of the Limestone Mountain by Case and Robinson (1914) limestone could have been caused by faulting, and Roberts (1940). Like Limestone Mountain, it is faults having post-Ordovician throws of 100m (330 an outlier of deformed Paleozoic rocks surrounded ft) are not known in this region. The 15 o dip by Jacobsville Sandstone. Strata dip from about 5o reported by James and others (1968) might be a to as much as 30° inward along a semicircular true dip into a cryptovolcanic depression rather ridge, suggesting that a roughly conical depression than a slump feature. about 0.5 km (0.3 mi) in diameter exists in the area, although its topographic expression is not as com­ POSSIBLE RELATIONSHIP BETWEEN plete. The area was included in the studies of Case CRYPTOVOLCANIC STRUCTURES AND and Robinson (1914) and Roberts (1940). KIMBERLITE The four areas of deformed Paleozoic rocks AREAS OF DISTURBED JACOBSVILLE SANDSTONE described have several features in common: Several areas where the Jacobsville Sandstone 1. All disturbances occurred in Ordovician or has been disturbed and dips steeply were identified later time. At Limestone Mountain, the by Roberts (1940). Along the north, west, and disturbance was apparently of Devonian or south sides of sec. 17, T. 51 N., R. 34 W. (fig. 4), younger age. scattered small exposures of Jacobsville, con­ 2. All disturbed areas are relatively small, at sisting of reddish-brown sandstone and con­ most about 1.5 km (1 mi) in diameter and are glomerate, have dips ranging from 45° to 70°. In surrounded by apparently undisturbed rocks. the rest of the region, the Jacobsville is generally At least three of the areas are roughly cir­ nearly horizontal or locally has dips as steep as cular. The fourth, the Brule River outlier, is 20°, apparently as initial dips on large-scale not adequately exposed to determine the crossbedding. shape or size of the disturbed zone. Exposures of the Jacobsville are widely 3. The three well-exposed areas are synclinal or separated, and there is no obvious topographic ex­ basinal structures characterized in general pression of structure, so the size and shape of the by moderate to steep inward dips and local disturbed area is not known. complications caused by faulting and small­ scale folding. BRULE RIVER OUTLIER 4. Strata in all structures have dropped at least 50 m (160ft) below their normal altitudes in An outlier of limestone about 10 m (33 ft) in surrounding undisturbed areas. diameter is exposed in the bed and banks of the The area of disturbed Jacobsville Sandstone near Brule River on the Michigan-Wisconsin border, in Pelkie is also similar in that generally flat-lying sec. 27, T. 42 N., R. 35 W., about 8 km south of sedimentary rocks are locally highly deformed. Iron River, Mich. (fig. 1). The limestone was first Lack of outcrops, topographic expression of struc­ described by Allen (1910), who dated it paleon­ ture, and marker beds do not permit the deter­ tologically as Ordovician. It apparently lies on and mination of the size and shape of the disturbed is surrounded by Proterozoic X metavolcanic area or the amount of downdropping that might rocks. James and others (1968) described the have occurred. The close proximity to Limestone limestone as "* * * light gray to tan, flaggy and ir­ Mountain and Sherman Hill, however, suggest a regularly bedded. The general attitude of bedding common origin for all three structures in Devonian is probably horizontal, although, in the stream or later time. 10 We suggest that all five disturbed areas could be scribed. The Lake Ellen kimberlite would repre­ cryptovolcanic structures formed over kimberlite sent the deepest level of erosion, at which the pipes. Although this is certainly not proved with kimberlite containing downdropped xenoliths of available data, we offer the suggestion as a star­ Paleozoic rocks is exposed. Limestone Mountain, ting point in the search for kimberlites. Sherman Hill, and the Brule River outlier would Alternatively, the disturbed areas might be ex­ represent higher levels of erosion, which have ex­ plained as (1) solution collapse features or (2) posed subsided Paleozoic strata above the main grabens related to faults in the Proterozoic base­ kimberlite intrusion. Glover Bluff would be a still ment. Alternative 1 seems unlikely because, for higher level of erosion, at which Paleozoic strata most features, no soluble rock is known are downdropped and deformed but erosion was stratigraphically beneath the disturbed strata. not deep enough to expose Proterozoic rocks or an Alternative 2 is possible, but we know of no faults underlying intrusion. in the region having well-documented post­ Ordovician throws of 100m (330ft) or more. Also, DO WISCONSIN DRIFT DIAMONDS HAVE A the circular, rather than linear, nature of the LOCAL SOURCE? downthrown areas make this alternative seem unlikely. In the early 1900's, shortly after the discovery of We feel that a more likely explanation is that the diamonds in glacial deposits of Wisconsin, it was disturbed areas are collapse features formed over generally assumed that the bedrock source for the kimberlite pipes. Figure 6 shows an idealized cross diamonds was far from Wisconsin, probably in section of a kimberlite pipe in which massive northern Ontario. Six of the seven diamond occur­ kimberlite at depth grades upward into a rences in Wisconsin are in the marginal moraines with kimberlite matrix and, still higher, grades in­ of the Green Bay and Lake Michigan ice lobes (fig. to a zone of downfaulted and downfolded sedimen­ 1). Hobbs (1899) concluded on the basis of the tary rocks. Various levels of erosion of such struc­ shape and character of the diamonds, and known ture could account for the Lake Ellen kimberlite glacial transport directions, that the ultimate and all the other areas of disturbed strata de- source of the diamonds was probably in the James

FIGURE 6.-Idealized cross section of a kimberlite pipe and overlying disturbed strata. Dashed lines suggest the stages of erosion of various features mentioned in the text.

11 Bay lowland of northern Ontario, at least 800 km terranes in northern Michigan and Wisconsin. It (500 mi) distant. The existence of kimberlite in the seems, therefore, that the principal bedrock type James Bay lowland lends support to this idea. from the James Bay lowland makes very little con­ Hobbs' conclusions seem to have persisted over the tribution to drift in the study area. We feel that the years (see, for instance, Gunn, 1968), and the rare kimberlites in the lowland are very unlikely to possibility of a kimberlite source in Wisconsin or contribute diamonds to the study area. northern Michigan has been given little considera­ We feel that the discovery of the Lake Ellen tion. However, we feel now, that in view of the oc­ kimberlite places a new perspective on a possible currence of one kimberlite in Michigan and the source for drift diamonds in Wisconsin. suspicion that several other disturbed areas may be Throughout the world, kimberlites are found in related to kimberlites, diamond-bearing clusters rather than as a single body. Well­ kimberlites could exist in Wisconsin and Michigan. explored kimberlite fields, as in southern Africa The fact that diamonds have been found at seven and , contain tens or even hundreds of in­ or more localities in Wisconsin, all by accidental dividual bodies in regions that commonly have discoveries, suggests that diamonds may be a dimensions of a few tens to a few hundreds of widespread, although certainly very rare, consti­ kilometers. The discovery of one kimberlite is a tuent of drift in Wisconsin. To our knowledge, no strong, but not necessarily conclusive, indication concerted effort to find diamonds in drift has been that others exist nearby. On the basis of the ex­ made, and we suspect that such a search could istence of the Lake ElleD-kimberlite and five possi­ yield substantially more diamonds. That is, the ble kimberlite-related structures, we suggest that diamonds already found are almost surely only a one or more kimberlite fields exist in Wisconsin or very small sampling of a much larger number that northern Michigan. The fact that many of the dia­ may be scattered widely through the drift. mond finds are in the area of the Green Bay lobe Although glacial ice is known to transport further suggests that sources are concentrated in material for hundreds of kilometers and could have northeastern Wisconsin or northern Michigan. carried the drift diamonds in Wisconsin from Brummer (1978) stated that, on a world-wide Canada, several factors suggest that a more local basis, about 1 kimberlite in 10 contains diamonds source is likely. and about 1 in 100 contains diamonds in commer­ cially recoverable quantities. Glacial drift commonly is composed largely of If we are correct in our suggestion that the five material eroded from nearby bedrock that has been areas of disturbed strata described are underlain transported only a few kilometers or less and by kimberlite, then at least six kimberlites are in material from very distant sources forms a minor the region. However, those proposed to exist part of the drift. This is clearly demonstrated in beneath the disturbed areas are not exposed at the many areas of Wisconsin 'and Michigan. Although surface and could not be the source of drift it has recently been shown (Gwyn and Dreimanis, diamonds in Wisconsin. No diamonds have been 1979) that heavy mineral assemblages may reflect found in the Lake Ellen kimberlite, but it has not bedrock sources 50-150 km (30-100 mi) away, we been adequately sampled to our knowledge. Dia­ question whether a kimberlite source at least 800 mond is a very minor constituent in kimberlite, km (500 mi) from Wisconsin is adequate to explain even at economic grades, and many tons of rock the number of diamonds that probably exist in drift might have to be processed to determine if in Wisconsin. diamonds are present. As glaciers transport material from its source, Because most of Wisconsin and northern the material tends to become progressively diluted Michigan is covered by glacial drift, much less than as it becomes mixed with rocks from other areas. 1 percent of the bedrock is exposed at the surface. Diamonds, which would be a very small fraction of Kimberlite pipes, which typically are nonresistant the drift, even at their bedrock source, may to erosion and at best would be small bodies, pro­ become so diluted through 800 km (500 mi) or more bably no more than 500 m (1,600 ft) in diameter, of transport that we question whether a source as are unlikely to be exposed. remote as northern Ontario is adequate to account The discovery of one kimberlite, the tendency for for the large number of diamonds that probably ex­ kimberlites to occur in clusters, the very limited ist in the drift in Wisconsin. amount of exposed bedrock, and the presence of In the James Bay lowland, the predominant rock diamonds in the glacial drift suggest that more type is Paleozoic carbonate rock. Paleozoic rock kimberlite bodies, some of which are diamond bear­ fragments are very rare in the drift on Proterozoic ing, may exist in the area. 12 R 31 W

RQ~~­ i1000 e,\;,-:::-\-~G------.:::::-.:::::- < (.!) (jl; 3500ft T 44 N o /f z ....., II- 3000ft \\ >-.. \\,, ...... 1\ ~ 500 OUTCROPS OF )!..__ 2500ft 0 ~ z -'/~ < ,.;,o'///////7Jk- 2000 ft u w~ SEC. 27 s 1..4- z CORNER (.!) 0 0 500 1000 1500 2000 2500 3000 3500 /f 1500ft SEC. 34 ~ SEC. 27 ~ C/.:) II B DISTANCE ALONG LOGGING ROAD, IN FEET II -...... \ ~ 0.8 z OUTCROPS OF ""'' < 0.4 """ , '----, / - - ''-....,., '-..... 0::: KIMBERLITE~ w :::::> (.!) 0 :::::> 0 500 1000 1500 2000 2500 3000 3500 0 2000 FEET 0 A ICC c DISTANCE ALONG LOGGING ROAD, IN FEET 0 500 METERS

FIGURE 7.-A, Map of area around Lake Ellen kimberlite. B, Total intensity magnetic profile across the Lake Ellen kimberlite. C, Bouguer gravity profile across the Lake Ellen kimberlite. EXPLORATION FOR KIMBERLITE stronger than would be expected from The general scarcity of bedrock exposures in kimberlite. An anomaly caused ·by a northern Michigan and Wisconsin and the relative­ kimberlite would be difficult to recognize in ly complete geologic mapping in northern such magnetically "noisy'' regions. Michigan during which most exposures have been Gravity surveys have had limited success in find­ examined suggest that few, if any, kimberlite ing kimberlite (Macnae, 1979) but will probably not bodies remain to be found in outcrop. If additional be very useful in Wisconsin and Michigan .. Figure 7C shows a gravity profile across the Lake Ellen ~imb~~lites. are discovered they will probably be Identified first by geophysical techniques that will kimberlite. Even in this rather precise survey in recognize a diagnostic signature of kimberlite which station elevations were surveyed to a preci­ beneath glacial deposits. sion of less than 0.1 ft, no anomaly was detected Macnae (1979) has reviewed the state-of-the-art over the kimberlite. The of the kimberlite is of geophysical exploration for kimberlite. Elec­ apparently very nearly the same as that of sur­ trical and magnetic surveys, and in some cases rounding rocks. gravity surveys, have been successful in finding Apparently, then, magnetic surveys offer the buried kimberlite. best hope for finding buried kimberlite, but new Electrical methods depend on the existence of surveys that have closely spaced flight lines will be conductive residual clay overlying the kimberlite. needed. Problems of recognition of kimberlite­ Such clay cappings are common on kimberlite in related anomalies from abundant anomalies from unglaciated regions but are unlikely to be pre­ other sources must be solved. Magnetic surveys could be done more efficiently if relatively small s~rved in Wisconsin or Michigan because glacia­ tion has probably eroded them. No residual clay is areas of high potential could be identified. known on the Lake Ellen kimberlite. Regional studies of heavy minerals in drift using Magnetic surveys have also been successful. kimberlite-indicator minerals, such as pyrope garnet, magnesian ilmenite and chrome diopside, ~ost fresh (unweathered) kimberlite is magnetic. , might be used to define zones likely to contain FI~re 7A shows the location of a ground magnetic profile over the Lake Ellen kimberlite. A promi­ buried kimberlite. Such studies have been· used nent positive anomaly of about 1,000 gamma 'with some success in Canada (Brummer, 1978). amplitude, shown in figure 7B, clearly marks the We conclude that the discovery of kimberlite bodies will be difficult but that detailed magnetic ~imberli.te. The magnetism is apparently caused by fine-gramed magnetite produced during serpen­ surveys of areas where kimberlite indicator tinization of olivine. Ilmenite, a somewhat abun­ minerals are in glacial drift holds some promise of dant , is apparently not magnetic. success. For each kimberlite found, however, there The amplitude of the magnetic anomaly may de­ may be only about one chance in 100 that it will pend, therefore, on the degree of serpentinization contain economically exploitable concentrations of of the olivine and could vary substantially from one diamonds. body to another. The use of magnetic surveys to find kimberlite in REFERENCES CITED Wisconsin and Michigan is hindered by two fac- tors: · Alden, W. C., 1904, The Delavan lobe of the Lake Michigan 1. The relatively small size of kimberlite bodies glacier of the Wisconsin stage of glaciation and associated requires close flight-line spacing of phenomena: U.S. Geological Survey Professional Paper 34, 106 p. aeromagnetic surveys to locate bodies. --1918, The Quaternary geology of southeastern Wisconsin Aeromagnetic surveys have been done for with a chapter on the older rock formations: U.S. Geological much of the region of interest at a line spac­ Professional Paper 106, 356 p. ing of one-half mile, but at that spacing, even Allen, R. C., 1910, The Iron River iron-bearing district of Michigan: Michigan Geological and Biological Survey a. relatively large body (500 m (1,600 ft) in Publication 3, 151 p. . dmmeter for instance) might not be in­ Brummer, J. J., 1978, Diamonds in Canada: Canadian Mining tersected by a flight line. Hence, to be effec­ and Metallurgical Bulletin, v. 71, no. 798, p. 64-79. tive, magnetic surveys that have a closer Case, E. C., and Robinson, W. I., 1914, The geology of flight-line spacing would be needed. Limestone Mountain and Sherman Hill in Houghton Coun­ ty, Michigan: Michigan Geological and Biological Survey 2. Many of the Proterozoic rocks in the region Publication 18, p. 165-181. have strong magnetic expression and com­ Davis, B. T. C., and Boyd, F. R., 1966, The join monly cause anomalies as strong as or MgzSiz06-CaMgSiz06 at 30 kilobars pressure and its applica- 14 tion to pyroxenes from kimberlites: Journal of Geophysical McCallum, M. E., Eggler, D. H., and Burns, L. K., 1975, Research, v. 71, no. 14, p. 3567-3576. Kimberlitic diatremes in northern Colorado and southern Dawson, J. B., and Stephens, W. E., 1975, Statistical classifica­ : Physics and Chemistry of the Earth, v. 9, p. tion of garnets from kimberlite and associated xenoliths: 149-161. Journal of Geology, v. 83, no. 5, p. 589-607. Macnae, J. C., 1979, Kimberlites and exploration geophysics: Ekern, G. L., and Thwaites, F. T., 1930, The Glover Bluff struc­ Geophysics, v. 44, no. 8, p. 1395-1416. ture, a disturbed area in the Paleozoics of Wisconsin: Mitchell, R. H., 1973, Magnesian ilmenite and its role in Wisconsin Academy of Science, Arts, and Letters Transac­ kimberlite petrogenesis: Journal of Geology, v. 81, no. 3, p. tions, v. 25, p. 89-97. 301- 311. Gair, J. E., and Wier, K~L~ 1956, Geology of the Kiernan --1977, Geochemistry of magnesian ilmenites from Quadrangle, Iron County, Michigan: U.S. Geological Survey kimberlites in and Lesotho: Lithos, v. 10, no. Bulletin 1044, 88 p. 1, p. 29-37. Gunn, C. B., 1968, Relevance of the Great Lakes discoveries to --1979, Mineralogy of the Tunraq kimberlite, Somerset Canadian diamond prospecting: Canadian Mining Journal, Island, N.W.T., Canada, in International Conference on v. 89, no. 7,p. 39-42. Kimberlites, 2d, Santa Fe, New Mexico, 1977, Proceedings, Gwyn, Q. H. J., and Dreimanis, A., 1979, Heavy mineral v. 1: Washington, D.C., American Geophysical Union, p. assemblages in tills and their use in distinguishing glacial 161-171. lobes in the Great Lakes region: Canadian Journal of Earth Olson, E. E., 1953, Historyof diamonds in Wisconsin: Gems Sciences, v. 16, no. 12, p. 2219-2235. and , v. 7, no. 9, p. 284-285. Haggerty, S. E., 1975, The chemistry and genesis of opaque minerals in kimberlites: Physics and Chemistry of the Reid, A. M., and Hanor, J. S., 1970, Pyrope in kimberlite: Earth, v. 9, p. 295-307. American Mineralogist, v. 55, nos. 7, 8, p. 1375-1379. Hearn, B. C., Jr., and Boyd, F. R., 1975, Garnet peridotite Roberts, E., 1940, Geology of the Alston district, Houghton and xenoliths in a Montana, U.S.A., kimberlite: Physics and Baraga Counties, Michigan: Pasadena, California Institute Chemistry of the Earth, v. 9, p. 247-255. of Technology, unpub. MS thesis, 48 p. Hobbs, W. H., 1899, The diamond fields of the Great Lakes: Silvers, Amy, 1978, When Eagle pinned hope on diamonds: The Journal of Geology, v. 7, no. 4, p. 375-388. Milwaukee Journal, November 9, 1978, Accent section, p. 1 James, H. L., Dutton, C. E., Pettijohn, F. J., and Wier, K. L., and 10. 1968, Geology and deposits of the Iron River-Crystal Sinkankas, John, 1959, of North America: Falls district, Iron County, Michigan: U.S. Geological Princeton, N.J., Van Nostrand, 675 p. Survey Professional Paper 570, 134 p. Koenen, K. H., 1956, Geophysical studies in south central Stephens, W. E., and Dawson, J. B., 1977, Statistical com­ Wisconsin: Madison, University of Wisconsin, unpub. MS parison between pyroxenes from kimberlites and their thesis, 44 p. associated xenoliths: Journal of Geology, v. 85, no. 4, p. Kunz, G. F., 1892, Gems and precious stones of North America: 433-449. New York, The Scientific Publishing Co. (reprinted by Vierthaler, A. A., 1958, Wisconsin diamonds: Wisconsin Dover Publications, New York, 1968), 336 p. Academy Review, v. 5, no. 2, p. 53-55.

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