USGS
Professional Paper 144 Butler & Burbank
FISSURE DEPOSITS
HISTORY
Apart from the incidental extraction of copper from fissures in mines that exploit the amygdaloid and conglomerate lodes, there are some mining operations devoted primarily to fissure deposits. The first copper produced in the district was taken from the fissure deposits of Keweenaw and Ontonagon Counties. (See pls. 50-52.) Production began in 1845, and fissure mines were operating till about 1890. Except for the production from the Mass fissure by the Ahmeek Mining Co., beginning in 1909, there has been little operation on fissure deposits since that time.
PRODUCTION
The total recorded production of copper from fissures to the end of 1925 amounted to 199,853,000 pounds; of this total 61,315,000 pounds came from the Minesota fissure, of Ontonagon County, leaving a total of 138,538,000 pounds from the fissures of Keweenaw County. (For the production of individual fissures see p. 72 and the reports on mining companies in the statistical section, pp. 76-98.)
Of all the fissures that have been worked independently of the lodes in the district, only three have yielded a profit - the Cliff, Central, and Minesota. The Cliff paid $2,518,620 in dividends; the Central $2,130,000; and the Minesota $1,820,000, plus National $320,000, or $2,140,000. The Copper Falls paid $100,000 in dividends, but that was less than the assessments collected. The Phoenix paid $20,000. The Mass fissure in the Ahmeek mine has undoubtedly been profitable and probably would have been profitable if worked alone.
DISTRIBUTION
Plate 50 shows the relation of the Greenstone flow and the Allouez conglomerate to the principal fissures worked in Keweenaw County from Ahmeek to Delaware.
The fissures in the north end of the district are tension cracks on the folds. Thus there is a series of fissures near the crest of the Allouez anticline. From the Gratiot River gap nearly to the North American gap the Greenstone flow, which is perfectly exposed, is massive and almost without fissuring. Near the North American gap the great Keweenaw anticline begins, and fissuring of varying degrees of intensity is present from that locality to the end of Keweenaw Point.
DISTRIBUTION OF COPPER IN THE FISSURES
The only fissures that the writers have been able to study in any detail are those that cross the Kearsarge lode in the Ahmeek and Mohawk mines. Some of these contain native copper, others arsenides. On the Mass fissure only has drifting been carried far from the Kearsarge lode, though on a few others drifts have been extended as much as 200 or 300 feet from the lode.
There is strong evidence that the Kearsarge lode has been a large factor in causing the precipitation of the copper in the Mass fissure. This evidence consists first in the distribution of the copper in the fissure. The copper is most abundant at the intersection o£ the amygdaloid and the fissure and decreases with increased distance from the amygdaloid. On the footwall side it extends but a short distance, but on the hanging-wall side it extends much farther. Between the Kearsarge amygdaloid and the Kearsarge conglomerate there are no thick amygdaloid lodes, and no relation between copper and any amygdaloid but the Kearsarge has been noted. The second evidence of relationship is the alteration of the lode adjacent to the fissure. In general the lode near the fissure is chloritized and sericitized and contains less copper.
So far as can be determined from the meager amount of development, the same conditions hold true for the other fissures that cross the Kearsarge lode, both copper and arsenide fissures. The parts of the lode adjacent to the arsenide fissures contain some arsenides, and there can be little doubt that the copper arsenic solutions were traveling along these fissures and precipitated their burden at the crossing of the Kearsarge lode. The gradations from fissures high in arsenic to those low in arsenic, like the Fulton and other fissures, strongly suggests a common age and origin for all the fissure deposits.
The Mass fissure is crossed by a bedding fault about 200 feet stratigraphically above the Kearsarge amygdaloid. Most of the copper is found below this fault, but no sharp change in the amount of copper in the fissure after passing the fault has been recognized.
The most productive fissure deposits in the north end of the district have been found beneath the Greenstone flow, though some copper has been produced from fissures above the Greenstone flow. There has been no opportunity for direct study of these deposits, and the descriptions in the literature are far from complete. From such information as is available, however; there seem to be at least two possible causes for the special richness of the fissures beneath the Greenstone flow. The first is the presence of a fault or "slide," immediately beneath the Greenstone flow, at the horizon of the Allouez conglomerate, whether the conglomerate is present or not. This movement has produced a rather thick gouge, which may have acted as a dam to rising solutions and caused them to concentrate just beneath the Greenstone flow. Locally there is a displacement of the fissures along this fault, which causes the fissures to end against the gouge and would favor concentration of solutions.
The second possible cause is the same as already discussed for the fissures crossing the Kearsarge lode - namely, that the amygdaloid lodes intersected by the fissures were an influential factor in causing the precipitation of the copper.
From drill-core records at the Delaware, Cliff, and Central mines and descriptions of the Cliff mine, as well as from material on the mine dumps, it appears that there are favorable amygdaloids in the belt under the Greenstone flow. In some reports mention is made of the favorable influence of amygdaloids on the vein, which seems to have been pretty clearly recognized by the early operators. Furthermore, some of the veins above the Greenstone flow are correlated, though not certainly, with those below, as the Petherick and the Northwestern, the Cliff and the North Cliff; and doubtless others have continuations above the Greenstone flow. Above the Allouez conglomerate horizon the veins seem to be poor or barren until they reach the amygdaloids at the horizon of the Ashbed lode, which are the next higher thick amygdaloids. The fissure deposits associated with the Ashbed are not as large as those below the Greenstone flow, but it seems likely that the solutions passed through the Greenstone flow and overlying beds but did not deposit much copper till they reached the favorable environment of the amygdaloids at the horizon of the Ashbed lode. If this is true, it would seem that neither the Greenstone flow nor the "slide" beneath it was everywhere a very effective barrier to the passage of ore solutions along these fissures. It may be noted, however, that apparently in neither the Cliff nor the Central mine has a largely productive deposit been found on the extension above the Greenstone flow. The extension of the Central fissure above the Greenstone flow has not been identified, but the supposed extension of the Cliff has been prospected without finding valuable deposits, though the fissure contains some copper where it crosses the Ashbed. The Cliff fissure is offset by a fault at the horizon of the Allouez conglomerate, and the same may be true of the Central. Such offsets would doubtless tend to limit the mineralization to the sections below the faults. From the detailed descriptions of the different fissure deposits beneath the Greenstone flow it appears that the fissures are not everywhere and probably not generally mineralized up to this flow or to the "slide" but have the maximum mineralization a short distance below the slide. The diamond-drill sections at the North American, Cliff, and Central mines show that the amygdaloids immediately beneath the Greenstone flow are not very thick or favorable as compared with those below the Houghton conglomerate. The fissures, however, probably averaged richer above the Houghton conglomerate than below it, so that while there is a general relation between thick amygdaloids and copper in the fissures, this relation is not close.
At several localities a slide has been reported at the horizon of No. 17 conglomerate and may have been a factor in the formation of fissure deposits in the Ashbed amygdaloid flows.
It appears that there is a very notable difference in the effect of the fissures on the amygdaloids near the crossing in the Kearsarge and on the amygdaloids to the north, under the Greenstone flow. The Kearsarge lode is relatively poor in copper near the fissure intersections, whereas both the Ashbed and the amygdaloids below the Greenstone flow are said to be best near the intersections of fissures. This difference is not necessarily inconsistent with the operation of the same causes. Apparently both the Kearsarge lode and the amygdaloids to the north were mildly mineralized near the crossings by the solutions that traveled along the fissures. To judge from what can be observed in the mines on the Kearsarge lode, these early solutions also altered the lodes where they mineralized them, destroying much of their favorable chemical character, so that when the later mineralizing solutions came up along the Kearsarge lode they encountered unfavorable ground near the fissures and hence did not deposit much additional copper in those parts of the lode.
In the Ontonagon district the fissures are nearly parallel with the strike of the beds. In the main Minesota or North branch, mineralization occurs in the fissure at and above the supposed intersection with the Minesota conglomerate, suggesting that in this deposit the character of the bed crossed by the fissure is the controlling factor. The position of the downward extension of the fissure is not certain. If the dip and strike of its known portion are maintained it cuts across the beds; but there is a possibility that it curves into the conglomerate bed and becomes a bed fissure.
From the foregoing statements it is apparent that a clear understanding of the cause of the localization of the copper in the fissures is of great importance in prospecting for this type of deposit. In the deposits at the Kearsarge lode crossings, for which the most definite information is available, the character of the lode seems to be the controlling factor. The deposits of which less is known, such as those under the Greenstone flow, afford reason to believe that the character of the lode is an influential factor, though the presence of the "slides" and the offsetting of the fissures, forming barriers, have aided in concentrating the flow of the solutions.
STATE OF DIVISION OF COPPER
The Mass fissure of the Ahmeek mine (pl. 51) differs from most of the other known fissures in the district in that it contains practically no stamp rock, whereas from the others the recovery from stamp rock was large. The approximate percentages which the several kinds of copper made of the total production from the different fissures are as follows:
Cliff, 70 per cent mass, 15 per cent barrel, 15 per cent stamp
Phoenix, 50 per
cent mass and barrel, 50 per cent stamp.
Central, 50 per cent mass, 50 per cent
barrel and stamp.
Northwestern (1854), about 30 per cent stamp.
Minesota (1866), 30 per cent mass, 43 per cent stamp.
Copper Falls, a large
amount of stamp copper.
Robbins, mainly stamp rock.
At nearly all the mines stamp mills were regarded as essential parts of the plants. Rock was stamped from the Stoutenberg, Clark, Eagle River, Madison, Amygdaloid Mining Co., St. Clair, and other fissures, in addition to those mentioned above. In the early days rock was calcined in stone kilns before being stamped.
EXPLORATION OF FISSURES
INCENTIVE TO EXPLORATION
The total production of copper from fissures has been small as compared with that from lodes. Each of the principal lodes of the district has produced nearly as much as all the fissures together, and most of them much more. The total amount of the dividends from fissure deposits has also been modest as compared with those resulting from lode mining. The dividend per pound of copper, however - about 3.5 cents - is about the same as that of the lode mines. It is worth while, therefore, to consider whether any considerable veins remain unopened or but partly developed.
Examination of the records and of the field indicates that by no means all the wide fissures that are exposed in the Greenstone bluff have been extensively explored; that almost nothing is known of the extensive areas comprised in the "gaps" in the Greenstone flow; that the experience gained in the Central exploration indicates that valuable deposits may be found in fissures that make little show where they cross the Greenstone bluff; and that a second favorable belt for fissure exploration may possibly be present east of the one under the Greenstone flow.
A distinct advantage in fissure exploration is that the promising zone beneath the Greenstone flow is very definitely outlined, and other areas of possible promise might be outlined with equal definiteness. The cost of exploration would be much less than in the early days, when much money was expended in promotion, in many separate organizations, and in roads, mills, and other features that could now be partly eliminated. Altogether there seems good reason to expect that a comprehensive campaign of fissure exploration would result in the development of mines that would give a moderate return for the effort. It is hardly to be expected that any great mines would be developed on fissures.
GENERAL FEATURES OF EXPLORATION
In any general plan of exploration of the fissures of Keweenaw County, there are certain features that should be considered. Among the more important are (1) significance of the gaps in the Greenstone flow; (2) significance to be attached to the character of the fissures as they are exposed in the Greenstone flow; (3) influence of thick, fragmental, well-oxidized amygdaloids on the precipitation of copper in the fissures; (4) influence of faults or barriers on the localization of copper.
1. There is little doubt that the gaps in the Greenstone flow have resulted from weakness of the rock due to faults or fissures. Definite indication of this is seen in the displacement of the rocks and in the shearing of the rock bordering some of these gaps, as the Eagle River gap and the Central gap. Two of the gaps have been prospected, the Madison and the Amygdaloid. In the Madison a fissure was exposed, but little was done on it, owing to the difficulty of drainage. In the Amygdaloid the Drexel fissure was developed by the Amygdaloid Mining Co. and yielded considerable copper. Diamond drilling across the North American, Arnold, and Central gaps has shown fissures present in each. Therefore, so far as developments have been carried in the north end of the district, there is reason to suppose that the gaps may mark the location of fissures.
On the Allouez anticline the strongest shatter zones, where they cross the Kearsarge lode, do not contain productive fissures. There are fissures in these zones, and what they contain in the belt under the Greenstone flow has not been determined.
The drilling of three of the gaps, the North American, Arnold, and Central, indicates that the gaps: are due to fissuring and faulting, although in none is a wide shatter zone present. In the "road gap" at the Central mine there is a mineralized fissure; in the main gap, a fault with only a narrow fracture zone about 8 feet wide.
2. The Greenstone bluff and some of the ridges north of it are practically the only places where the fissures are naturally exposed, and it is important to know whether the character of the fissures as shown in the bluff is any indication of what is to be expected, of them in the favorable zone below the Greenstone flow.
Naturally the fissures that appear largest in the Greenstone flow are the ones that have been chosen for prospecting. This prospecting has led to the development of one paying fissure mine, the Cliff; a second that would doubtless have paid under modern efficient management, the Phoenix; and several others that might have nearly or quite paid the cost of their development, as the St. Clair, the fissure mines at Delaware, and the Drexel. On the other hand, many of the other fissures prospected because of their prominence in the Greenstone flow have yielded little or no copper.
The most productive fissure, the Central, was not recognized where it crossed the Greenstone flow but was discovered in an outcrop about 600 feet south of this flow and in ancient pits which presumably were sunk on the outcrop of the vein. It therefore appears that too much reliance should not be placed on the appearance of a fissure in the Greenstone flow, but a more certain method and perhaps as cheap would be to drift on a favorable amygdaloid along chosen stretches of fissured ground with the idea of examining all the fissures of the area where they crossed the favorable zone.
3. There is reason to believe that the thick fragmental, well-oxidized amygdaloids had a favorable effect on the precipitation of copper in the fissures. Such amygdaloids are present in the belt under the Greenstone flow and may or may not have been the controlling factor in making this a favorable belt.. That it is a favorable belt has been well demonstrated. The practical question is therefore not so much whether the amygdaloids under the Greenstone flow have exerted a favorable influence as whether similar amygdaloids elsewhere that have not been explored have exerted such an influence. There are thick amygdaloids below this belt at about the horizon of the Osceola lode, in which fissures have been prospected but little. The Central fissure was mineralized at this general horizon, but other fissures have not been examined where they cross this belt. (See accompanying sections.)
4. A fault at the base of the Greenstone flow offsets some of the veins. Any interruption of the veins along such an inclined plane would tend to produce a barrier and concentration of solution beneath it. Such a barrier condition would favor the formation of deposits, although the evidence from the fissures that cross favorable parts of the Kearsarge, the Ashbed, and the conglomerates in the south end of the district suggests that barriers may not be essential.
METHOD OF EXPLORATION
Selected areas along a thick amygdaloid could be prospected either by sinking a shaft and drifting, or in places by driving an adit that would permit exploration. near the surface. The latter method would eliminate both pumping and hoisting and would serve in the examination of long stretches of territory, but probably sufficient depth to explore under the deeper gaps could not be obtained in this way, and it would give only very shallow exploration of the promising fissures, which would then have to be examined further by shaft.
SUGGESTED EXPLORATIONS
In the following paragraphs are suggested explorations which might determine the points outlined above and the results of which would of course influence any further consideration of the general problem.
At present all that is known about the influence of thick amygdaloids aside from that below the Greenstone flow is that the Osceola horizon where the Central fissure crossed it at depth was well mineralized and that the Vaughnsville fissure showed best in what was thought to be the Osceola amygdaloid.
It is of course possible to project any of the known mineralized fissures to the crossing of belts lower in the series and prospect at that point, but it would seem advantageous to use the information gained in prospecting the known favorable horizon in deciding how to prospect one that is little known.
FISSURE MINES AND PROSPECTS OF KEWEENAW AND ONTONAGON COUNTIES
Star. - Two miles south of Copper Harbor, in the E. 1/2 sec. 9, T. 58 N., R. 28 W., south of the Greenstone flow. Two shafts were sunk on the principal vein, 300 and 90 feet deep. These were connected by drifts and by an adit level for drainage. Little stoping has been done (1864). An amygdaloid belt 13 feet wide, carrying copper, was opened 150 feet east of the transverse vein. Another vein being prospected in 1864 carried rich barrel rock near the surface. D. S. Childs, the agent, reports opening two veins, one 700 feet to the west of the Star vein and one 600 feet to the east. The one to the west was opened 100 feet from the Greenstone flow, where it was 5 feet wide and well charged with copper. It was opened also 1,000 feet from the Greenstone flow, where it carried less copper. The recorded production of the Star Mining Co. is 17,938 pounds.
Clark. - South of Copper Harbor and north and west of the Star property. The veins bear N. 10° W. and have been mined on both sides of the Greenstone flow. Two veins were opened by adit and three shafts. Small masses of copper were frequently found, enough to give encouragement but not enough to pay costs. The recorded production is 187,915 pounds.
Iron City. - Southeast of Mosquito Lake, in sec. 14, T. 58, N., R. 29 W. Two shafts were sunk 30 feet apart, No. 1 to a depth of 300 feet, and connected by levels. The vein was wide but failed to yield copper.
Medora. - One mile southwest of Mosquito Lake, in the E. 1/2 sec. 17, T. 58 N., R. 29 W., immediately south of the Greenstone flow. The company was organized in 1851. Considerable work was done, but a relatively small amount of copper was found. Amygdaloid "floors" were said to be present as in the other mines.
Native Copper. -- North of Delaware, in sec. 10, T. 58 N., R. 30 W. Worked on a vein crossing the Ashbed horizon but found nothing encouraging.
Conglomerate. - Under the names Northwest Copper Co., Pennsylvania Mining Co., Delaware Mining Co., and Conglomerate Mining Co. the veins and the Allouez conglomerate at Delaware were worked at the explorations (1866) different times. The Northwest Copper Co. began operations in 1847 and developed three fissures. To the end of 1859 it had expended $611,000, and its copper sales had amounted to $328,000. In 1861 the Pennsylvania Mining Co. was organized to take over the property. This company opened three additional veins and undertook extensive surface improvements, expending $126,000, but produced no copper. In 1863 part of the territory was sold to the Delaware Mining Co., under the same management. The two companies are said to have spent nearly $2,000,000 in the next few years but produced little copper. In 1866 the property was taken over by the bondholders. The two properties were in 1876 united as the Delaware Copper Mining Co. This company operated with little production till 1881, when it was reorganized as the Conglomerate Mining Co. The new company is said to have expended $1,300,000 in making surface improvements and opening the Conglomerate mine. The mine at present belongs to the Calumet & Hecla Consolidated Copper Co. Altogether more than $4,000,000 was spent, and to 1886 a total of 7,188,000 pounds of copper was produced. At no period in its history was the mine operated at a profit. Three fissures were developed to a considerable extent - the Stoutenberg, Delaware, and Hogan. The Stoutenberg was opened to the eighth level, the Delaware to the ninth level, and the Hogan to the fourth level. The geologic relations in all the fissures seem to have been essentially the same. The productive part of each was close under the Allouez conglomerate, though the fissures do not appear to have been mined into the Allouez conglomerate. The reports state that several amygdaloid "floors" were present in the mine and that the amygdaloids had a marked influence on the vein. The "floors" were also mined and apparently furnished a considerable part of the production. The maps show that the stoping on the amygdaloids decreased away from the fissures, indicating that the richness of the ground decreased in the same direction.
Connecticut.-West of the Delaware. Owned by Amygdaloid Mining Co. Opened by three shafts No. 1, near Greenstone flow, 60 fathoms; No. 2, 20 fathoms; No. 3, 6 to 7 fathoms - and an adit. Adit level driven 551 feet; 10 fathoms below adit, drifted 181 feet; 20 fathoms below adit, 514 feet; 30 fathoms below adit, 147 feet. Total recorded yield 116,800 pounds of copper.
Amygdaloid. - Just west of the Delaware. Opened in 1860 and operated through 1878. No statement or map showing the amount of development has been found. Recorded production 1,541,180 pounds.
Eagle Harbor. - Explored a vein crossing the Greenstone flow, midway between the Madison and Amygdaloid gaps. It was reported to carry good copper above the Ashbed horizon in sec. 8. It crossed the Greenstone flow in low, poorly exposed ground, and failed to locate the fissure. A vein through Madison gap was opened in the low ground. It carried some copper but was not further explored (1865) on account of the difficulty of working. A vein west of the gap was opened by a shaft and adit to a depth of 72 feet. The vein was found 3 1/2 feet wide but was said to be in "hard rock" and unprofitable. The Essex fissure, which passes through a gap, was explored in sec. 16 by an adit. At 500 feet from the Greenstone flow a shaft 100 feet deep was sunk and a drift started north (1865?). The vein, where followed by the adit, was reported to be rich in stamp copper, but apparently further development did not reveal a valuable deposit. Another vein farther east, reported as 3 feet wide and yielding good stamp copper, was opened in 1866. No production is recorded from these explorations.
Madison. - The Madison Mining Co. operated on three fissures - the Perkins, 2,000 feet west of Madison gap, and the East and West veins, 4,000 feet west of Madison gap. The East or main lode has three shafts and an adit level. No. 3 shaft is opened to the 20 fathom level, No. 2 to the 30-fathom, and No. 1 only to the adit level. Shafts were connected on the adit, 10-fathom and 20-fathom levels, and some stoping was done. The West lode was opened by shafts and winzes to the 20-fathom level with some stoping. It is reported that about $250,000 was expended, The recorded production was 72,000 pounds. The Madison gap was not explored except for a pit that located a fissure.
Dana. - The Dana fissure, about 4,000 feet east of the Central mine, was opened by an adit 780 feet long and by three shafts, but it proved unproductive.
Copper Falls. - The Copper Falls Mining Co. was one of the earliest in the district. It conducted operations on several fissures and on the Ashbed lode. At different times tracts have been set aside from the original grant and recombinations made. At present the ground belongs to the Arnold Mining Co. Altogether the Copper Falls Co. collected $1,000,000 from assessments and paid $100,000 in dividends. It has the distinction of being the only mine in the north end of the district above the Greenstone flow that has paid dividends, but it evidently was not a profitable undertaking. The most prosperous period was in the late sixties, when a small but rich area in the Owl Creek vein was being mined. This vein was developed by an adit that starts near the base of the Great conglomerate and extends through the trap series, probably into the Greenstone flow. There is some uncertainty from the old records as to the relation of the ore to the different types of rock. In general it may be said that the fissure was productive only in the vicinity of the Ashbed lode. Apparently nothing encouraging was found from the Greenstone flow to a short distance below the Ashbed and from a short distance above the Ashbed to the Great conglomerate. According to Marvine's Eagle River section, there were no thick amygdaloids between the Greenstone flow and the Ashbed amygdaloid. Above the Ashbed are some amygdaloids but apparently none as thick as that flow. It seems logical to conclude that the Ashbed amygdaloid was a factor in the enrichment of the vein at this point. The Ashbed amygdaloid itself is mineralized and was mined for 1,000 feet or more on both sides of the fissure. No statement has been found to indicate whether it was notably richer or poorer near the fissure, though the old stope map suggests better ground near the fissure than at a distance. Datolite is abundant in the Owl Creek fissure and in the Ashbed amygdaloid. As the Ashbed was mined for part of the time the production from the fissures is not known accurately, but it is estimated as follows: Owl Creek fissure, 7,283,000 pounds; Copper Falls fissure, 731,000 pounds; Old Copper Falls fissure, 86,000 pounds; Hill fissure, 501,000 pounds; Childs fissure, 32,000 pounds. It is estimated that the Ashbed amygdaloid yielded 17,706,000 pounds of copper.
Petherick. - The Petherick fissure is about 2,000 feet west of the Owl Creek fissure and has been developed in the same general stratigraphic horizon. It has been opened, so far as known, continuously by adit and shaft for about 2,300 feet along the strike and to a maximum depth from the outcrop at No. 6 shaft to the adit level of about 225 feet. (See pl. 50.)
No accurate record is available of the stoping on this fissure. There was some stoping on the Ashbed amygdaloid adjacent to the fissure workings, of which, too, there is no record. It is therefore not known how much of the production of the Petherick Mining Co. came from the fissure and how much from the Ashbed. As indicated by the material on the dump, datolite was abundant in the fissure and in the Ashbed adjacent to the fissure. In this respect the Petherick resembles the neighboring Owl Creek fissure.
Old Copper Falls. - The Copper Falls Co. began work in 1845 on the Old Copper Falls fissure. The outcrop was exposed in the falls of the stream. The work was continued till 1850, and a depth of 267 feet was attained. Some good ground was developed, but the production was small and the operations were unprofitable.
Northwestern. - The Northwestern Mining Co. began operations in 1845 on a fissure below the Greenstone flow and worked rather continuously till 1857; some work was also done later by this company on the original fissure and on the southward extension of the Central fissure. Assessments to the amount of $228,000 were collected, and copper to the value of about $75,000 was produced. The workings comprised four shafts, 109, 201, 215; and 225 feet deep, an adit 1,226 feet long, and levels Nos. 10, 20, and 30, respectively 944, 1,057, and 124 feet long. The recorded production is 313,000 pounds.
Central.-The description of the Central vein is compiled from the mine maps and from Hubbard's account. The Central mine was opened in 1854 and closed in 1898. During this period it yielded 51,875,527 pounds of copper, and the company paid about $2,130,000 in dividends. The mine is opened on a fissure vein striking nearly at right angles to the beds and dipping very steeply to the east in the upper levels and slightly less steeply in the lower levels. There is no pronounced gap or outcrop of vein in the face of the Greenstone bluff to indicate where the fissure crosses. It may be offset by a strike fault at or near the base of the Greenstone flow. The fissure was discovered by outcrops and in ancient pits about 600 feet south of the Greenstone bluff. The productive part near the surface was close under the Greenstone flow, though the mineable portion nowhere seems to have extended to or into the Allouez. conglomerate. With increased depth the mineralized portion gets farther and farther from the Greenstone flow. The ore shoot has its greatest extent along the strike of the fissure at about the 100-fathom level. Below that it narrows to about the 200-fathom level, where it again expands; it continues to the Kearsarge conglomerate at about the twenty-ninth level, where the fissure is displaced by a fault. The principal structural features are the main fissure and the strike fault at the Kearsarge conglomerate. The fault cuts out the Kearsarge conglomerate and offsets the vein below the fault 284 feet to the west. The fact that the thick conglomerate is completely cut out by the strike fault suggests a large movement of which the horizontal offset of the fissure is a small component. Below the fault what is regarded as the continuation of the fissure was found to contain but little copper, though the fissure appears to be wide. No direct statement is made in Hubbard's description regarding the age of the fault relative to the period of mineralization. As the breccia along the fault is mineralized, however, it is probable that mineralization followed the faulting.
No detailed description of the character of the amygdaloids intersected by the fissure has been found. Pumpelly briefly described those in the upper part of the mine. He mentioned three soft brown amygdaloids between the Calumet & Hecla conglomerate and the Houghton conglomerate. These are at about the horizon where the lode begins to widen out upward. The character of the amygdaloids below the Calumet & Hecla conglomerate is not known. Central. diamond-drill hole No. 9, west of the mine, and the one at Arnold Gap (No. 11.) show thick fragmental amygdaloids above and below the Houghton conglomerate.
The cause for the concentration at this point is susceptible of several explanations. If the faulting preceded the mineralization, it is hardly reasonable to suppose that the mineralizing solutions rose along the fissure below the fault and then jumped across to the portion above the fault. If mineralization preceded faulting and the movement was of great magnitude, a barren part of the fissure might be brought into contact with a mineralized part. The mineralized portion of the fissure is immediately above the point where the conglomerate is cut out. This, together with the fact that the crushed portion of the Kearsarge conglomerate is mineralized, has led to the suggestion that the solutions rose along the conglomerate and escaped into the fissure where the conglomerate was cut out. The reason for the variation in mineralization along the fissure is not clear, and little information is available concerning the character either of the fissure or of the inclosing beds to give a basis for an interpretation. The map shows, however, a distinct tendency for the expansions in the shoot to extend along the bedding rather than across it. This suggests an influence of the beds in precipitating copper. The soft brown beds noted by Pumpelly at about the horizon where the upper expansion occurs may have influenced the precipitation. Whether similar beds are connected with the lower expansion is not known. The mineralization did not extend to the Greenstone flow, and so far as can be judged from the map there is little evidence of the deflecting or damming influence of this flow.
There appears to have been some mineralization of several of the amygdaloids near the fissure, and the Calumet & Hecla conglomerate was well mineralized adjacent to the fissure at about the eighteenth level. Nowhere was commercial rock found to extend far from the fissure. In the Calumet & Hecla conglomerate ore extended about 40 feet on each side of the fissure. The Houghton conglomerate was reported as well mineralized, but no stoping was done on it.
Winthrop. - The Winthrop Mining Co. worked several years on a fissure in the SW. 1/4 sec. 23, T. 58 N., R. 31 W., west of the Central mine. No production is reported.
Eagle River. - The Babbitt vein, 1,000 feet east of the St. Clair, was opened in 1880 by the Eagle River Mining Co., which put down two shafts, 275 feet apart, one close to the Greenstone flow. The recorded production is 49,678 pounds.
St. Clair (acquired by the Phoenix Consolidated Copper Co.). - The St. Clair vein is east of the Eagle River gap, find the mine is immediately under the Greenstone flow. A fault or "'slide" branches from the Greenstone flow contact about 150 feet below the outcrop. It dips more steeply than the contact and thus diverges from it with increasing depth. At the 10th level the stratigraphic distance between the Greenstone flow and the fault is about 350 feet. Practically all the stoping has been done between the Greenstone flow and the fault. The stopes seem to extend up to the flow and in places cross the fault but have extended only a short distance below it. The mine was opened to the twelfth level. The vein is said to have been rich, carrying 2 to 3 per cent of copper, but it was narrow, and the mineralized area was relatively small. Under the conditions of operation it evidently did not pay.
Bay State. - The Bay State Co. operated on the Phoenix or Bay State fissure, south of the Phoenix mine, and was later absorbed by the Phoenix Co.
Phoenix. -- The Phoenix Copper Co., now absorbed by the Keweenaw Copper Co., was organized in 1844 and was one of the earliest companies in the district. It operated on numerous veins and on the Ashbed amygdaloid. The total assessments by the old Phoenix and the Phoenix Consolidated are given by Stevens as $2,385,500, and the dividends as $20,000. The only really profitable operations seem to have been conducted for a few years in the early seventies on the Phoenix fissure.
Phoenix fissure. - The only description found of the rocks under the Greenstone flow in the Phoenix mine is that by Marvine. This description does not give the impression of any particularly thick amygdaloids in the mine. There is a belt of thin flows, but the amygdaloids are apparently small. The Allouez conglomerate is represented by a "slide." Extensive stoping has nowhere been carried more than 1,400 feet from the Greenstone flow along the level, or about 700 feet across the beds. Nothing has been seen in the reports to indicate where the richest ground was found. The vein evidently varied in grade from place to place, but there is no statement of the conditions accompanying this variation. Neither is it stated whether or not there was a notable change in grade from the surface downward. The deepest workings on the inclined shaft are at a vertical depth of about 1,000 feet. Stevens states that the yield from 1872 to 1885 was 473 pounds per fathom, or about 1.5 per cent. No mention is made of operations on amygdaloids.
Robbins or West. - The Robbins or West vein was developed by the Phoenix Co. a short distance west of the Phoenix or Bay State vein. It has a more easterly strike than the Phoenix and cuts the beds at an acute angle. The mineralized belt, as indicated by the stope map, is 100 to 250 feet wide and 50 to 75 feet below the Allouez slide. The maps indicate but one amygdaloid belt, 300 to 350 feet below the scopes, but other amygdaloids are undoubtedly present. The mine has been opened to a vertical depth. of about 600 feet.
Some good ground was found, but as a whole it did not pay. The vein is said to have contained no mass copper, producing stamp rock only.
Old Phoenix. - A vein was encountered under the bed of Eagle River near the Ashbed crossing and followed for several hundred feet. One account of this vein describes the copper and silver as occurring with waterworn pebbles, suggesting a placer deposit. Others indicate that it is a vein that has been followed by the river. A few thousand pounds of copper and some silver was taken from this vein. It apparently was never developed at depth except where the Ashbed drift crossed its downward extension. There it is a thick calcite vein with some copper but not in commercial quantities.
Vaughnsville. - The Cliff Copper Co. opened the Vaughnsville fissure, nearly a mile south of the Greenstone flow. The strongest mineralization was found at the intersection of an amygdaloid supposed to be the Osceola.
Cliff. - The Cliff mine, on the Cliff fissure, was the first large copper producer of the Lake Superior region. Production began in 1845, when it yielded about 20,000 pounds of copper. The mine was operated, with the exception of an interval from 1870 to 1872, until 1883. It was idle from 1883 until 1906, when it was reopened and some amygdaloids in the upper levels were explored adjacent to the fissure. A total of 38,207,000 pounds of copper was produced, and the company paid $2,518,620 in dividends.
The productive portion of the Cliff fissure lies under the Greenstone flow. At the mine the Allouez conglomerate is represented by the "slide," a few inches of clay gouge. Immediately under the slide is a series of small flows numbered in the Cliff mine upward from 1 to 13. These have an average thickness of about 50 feet, and so far as can be judged from the old maps, from one-third to one-half of the rock in this belt is amygdaloid. The five beds next to the Calumet & Hecla conglomerate average about 90 feet in thickness. The series of thin beds beneath the Greenstone flow form a relatively weak zone, as is indicated by the pronounced depression in the bedrock surface just south of the Greenstone bluff. The thickest amygdaloid, as indicated on the maps, is No. 13, the one immediately beneath the "slide." No. 9 is also indicated on the maps as a thick amygdaloid. No detailed description of the amygdaloids has been found, and it is therefore not known whether they are fragmental or how well they are oxidized. Examination of the dump indicates that there is at least one fragmental, well-oxidized amygdaloid. The diamond drill section west of the fissure shows some thick fragmental amygdaloids below the Houghton conglomerate. The developments have been confined to beds above the Calumet & Hecla conglomerate.
The Cliff fissure cuts nearly at right angles to the beds and dips steeply to the east, with local reversal of dip. It is well exposed as a wide veined and shattered. zone in the face of the Greenstone bluff and has been followed from a point south of the Cliff mine workings to the lake shore. In places in the mine workings it branches. There are two "slides" noted in the mine , workings - one at the horizon of the Allouez conglomerate and one that reaches the surface at about No. 2 shaft, South Cliff. Both are essentially parallel to the beds, and both have displaced the fissure but slightly. The second slide has two branches for the first few hundred feet. Just north of No. 3 shaft, South Cliff, the lode is crossed by a strong zone of fissuring, which is represented on the maps as vertical. Little development work has been done south of this zone.
The mineralization was in the main confined to the fissure, though there was some mineralization of the amygdaloids. Here, as in the more productive part of the mine, the amygdaloids are closely spaced, and the old maps show no close relation between masses of copper and amygdaloids. Old reports speak of rich copper in the vein associated with the ninth and thirteenth floors; and on one of the old maps special mention is made of rich ground in the fissure where it intersects the thirteenth floor (just below the slide) from the 30-fathom to the 60-fathom level. In the lower levels, however, mineralization does not seem to have been strong in the thirteenth floor.
Mention is also made of rich ground where the fissure splits and includes a horse of the country rock. To the north the mineralization was limited by the slide. The vein passes through the Greenstone flow but has not been found to contain commercial copper there. The North Cliff workings are at the intersection of the fissure with the Ashbed amygdaloid. Here the vein is said to be a zone of fissuring 10 feet wide, but it contained relatively little copper.
No very clear description of the distribution of the copper in the fissure south from the Allouez slide has been found. The general impression gained from the old reports is that the vein was richest close to the Greenstone flow and grew leaner toward the south, and that the distribution was irregular, the richest portions being associated with the intersections of amygdaloids.
In some of the old reports the statement is made that the mineralization was cut off on the south "slide," and the maps show that very little stoping was done south of that slide. The vein may have about reached the limit of profitable working before passing the slide, and therefore it is not clear that the slide itself was an important factor in limiting the mineralization. The fissure and mineralization very evidently extended south of the slide. Exploration at No. 5 shaft, which is on the southern projection of the Cliff vein, has failed to disclose a strongly mineralized fissure.
Two possible causes for the richness of the fissure beneath the Greenstone flow have been suggested: 1. The fissure has been offset at the slide about 20 feet. If this offset occurred before mineralization, the interruption of the fissure and the presence of the gouge of the slide would form a barrier that might result in a concentration of solutions rising along it. 2. The area of greatest productivity is in a belt in which amygdaloids are very abundant, and there evidently was a relation between richness of the vein and some of the amygdaloids. The available description of the amygdaloids is too meager to judge of their character, but if there are several well-oxidized amygdaloids they might be an influential or even a controlling factor in the mineralization.
North Cliff. - The North Cliff Mining Co. was organized in 1858 to work the extension of the Cliff fissure above the Greenstone flow on lands set apart by the Pittsburgh & Boston Mining Co. The Cliff fissure had previously been traced by pits every few hundred feet from the most northerly Cliff shaft to the base of the Great conglomerate, a distance of 9,000 feet. In the Greenstone flow the fissure was found to be not more than 14 inches wide and to contain only a little copper. In the Ashbed lodes the vein was found broken into numerous branches for a width of 10 feet or more, and the branches were well filled with barrel and stamp copper. Three vertical shafts were sunk on the fissure near the Ashbed lode, one to the 30 fathom level, and an adit was driven south for more than 1,700 feet, cutting the Ashbed. The production to March 21, 1864, was 5,657 pounds of refined copper taken from the fissures as masses. An inclined shaft was also sunk on the Ashbed in or near the Cliff fissure, and some drifting was done on that lode, but there is no record as to the results obtained.
North, American. - The North American Mining Co. worked for four years on a fissure under the Greenstone flow in the E. 1/2 sec. 2, T. 57 N., R. 32 W. The workings are on the north side of the North American gap, and the bearing of the fissure, N. 58° W., is such that it soon enters the gap. It was worked to a depth of 415 feet. The total yield was 445,000 pounds of refined copper. The fissure is split into three parts in the upper levels. There appears to be a difference of about 30° in the trend. of the fissure and that of the gap.
Albion-Manhattan. -- The Albion-Manhattan Co. in 1848 sunk a shaft 115 feet through the Greenstone flow, drove an adit to connect with it, and sunk 200 feet below the adit level. The vein was 2½ feet wide but barren. Later a shaft sunk to a depth of 70 feet near the Greenstone flow yielded 5 tons of copper.
Mohawk. - Several arsenide fissures have been prospected in the north end of the Mohawk mine. The main mineralized area is at the crossing of the Kearsarge lode and extends for a short distance into the hanging wall of the Kearsarge. A fissure opened south of No. 6 Mohawk shaft contained some copper near the intersection with the lode. The lode is also reported as rich near the fissure.
Fulton. - The Fulton fissure was opened in 1853 in the SW. 1/4 sec. 33, T. 57 N., R. 32 W. The vein was 18 inches wide. The same fissure has been opened in the North Ahmeek mine, where some masses of copper have been found, but as it is arsenical little has been mined. What seems to be the same fissure has been opened under the Greenstone flow. The vein material on the dump contains copper, but no record of the work has been found. The recorded production from this fissure by the Forsythe Mining Co. was 1,255 pounds, and some copper has also been produced by the Mohawk and Ahmeek mining companies.
Mass. - The Mass fissure of the Ahmeek mine where it crosses the Kearsarge lode strikes about N. 20° W. and dips steeply to the east. The fissure has been opened to the twenty-sixth level. Along the strike it has been developed to maximum distances of about 1,100 feet to the northwest from the Kearsarge lode and about 600 feet to the southeast. A slide fault intersects the fissure about 200 feet stratigraphically and 300 feet horizontally above the Kearsarge lode. The fissure, however, is not displaced. The fissure varies considerably in character from place to place. Commonly the fissure zone has a width of 2 to 4 feet, but it may pinch to a few inches or may spread to considerably more than the average width. Where the fissure zone is broad the rock within is sheared and brecciated.
Calcite is the most abundant vein mineral. Locally a foot or more of calcite occurs in the fissure. Quartz, epidote, prehnite, and laumontite are locally present. Copper occurs typically in large irregular more or less lenticular masses. In places there is a solid mass of copper a foot or even more in thickness in. the vein. Some masses containing more than 200 tons of copper have been mined. Where the massive copper occurs the other vein minerals are usually sparsely represented. Chloritization is the most characteristic alteration of the wall rock and the breccia within the fissure, though there is some sericitization.
Most of the copper has been mined within 400 feet horizontally above and 200 feet horizontally below the Kearsarge lode. Copper is known to occur 1,000 feet from the Kearsarge lode, but the masses beyond 400 feet are small and scattering. Little copper has been mined above the ninth level. The general distribution of copper is shown in Plate 51. The Mass fissure to the end of 1925 is estimated to have yielded 11,567,000 pounds of copper.
Arsenide. - The Arsenide fissures in the Ahmeek, Mohawk, and Seneca mines have the same general dip and strike as the Mass fissure. They have been developed but a short distance away from the Kearsarge lode. The principal vein minerals are quartz, calcite, ankerite, and the copper arsenides. Albite, prehnite, and laumontite are present in variable amounts. The typical rock alteration associated with the Arsenide fissures is chloritization and sericitization. In places the veins contain a foot or even more of nearly solid copper arsenides at their intersections with the Kearsarge lode, but there has been little production from these veins.
Minesota and Branch fissures and Calico lode. - The occurrence of the ore in the Calico lode of the Minesota mine is closely associated with that in the fissures, and they are therefore described together. The statements here given are based on old descriptions and company maps and records, as there was no opportunity for underground examination. The copper occurs mainly in two northward-dipping beds and in strike fissures that parallel the beds or dip more steeply than the beds and cut them. The lower bed, the Minesota conglomerate (South Branch) is about 125 feet below the Calico amygdaloid (North Branch). Striking nearly parallel with the beds is the Branch fissure (Middle Branch), which cuts both the Minesota conglomerate and the Calico amygdaloid. The Minesota fissure is represented as diverging from the Branch fissure where the latter cuts the Minesota conglomerate and as following the top of the conglomerate to the outcrop. The operations on the Minesota fissure (pl. 52) were among the earliest and have been the most profitable in Ontonagon County. No separate record of the copper from the Minesota and Branch fissures and Calico lode is available, but most of the output came from the Minesota and Branch fissures. Production began in 1848. To 1880 a total of 53,000,000 pounds of copper was produced from the Minesota, National, and Rockland mines, and dividends of $2,140,000 were paid.
The Michigan Copper Mining Co., began operations in 1899. From 1899 to 1912 the Calico lode and the Branch vein were rather extensively opened and mined. The Branch vein was apparently the most productive. A total of 17,180,000 pounds of copper was produced. The yield from the stamp rock probably did not exceed 12 pounds to the ton, but there was much mass copper, especially on the Branch vein.
Production and dividends from Minesota and Branch fissures and Calico lode
| Mine | Period |
Copper produced (pounds) |
Dividends | |
| Total |
Per pound (cent) |
|||
| Minesota | 1848-1885 | 34, 707,000 | $1,820, 000 | 5. 24 |
| National | 1853-1895 | 11, 613, 000 | 320, 000 | 2.67 |
| Rockland | 1854-1880 | 5,821,000 | ||
| Branch | 1902-1912 | a9, 174,000 | ||
| Calico lode | 1900-1913 | a8, 006, 000 | ||
| 69, 321, 000 | 2, 140, 000 | 3.00 | ||
a Estimated.
The Minesota conglomerate is a few hundred feet above the Evergreen and succeeding lodes, and the Calico amygdaloid is about 125 feet above the Minesota conglomerate. Next above the Calico lode is the North lode, on which some development work has been done. The beds dip 47°-50° N., steepening slightly with increased depth and toward the southwest. Striking approximately with the beds but dipping at a steeper angle is the Branch vein or fissure. The outcrop is mainly in the hanging wall of the Calico lode except between shafts A and B. At shaft B the Branch vein is represented as diverging from the Calico lode above the fifth level and entering the conglomerate between the thirteenth and fourteenth levels. The dip varies considerably from place to place. At the contact with the conglomerate the Branch fissure is represented as splitting off from the South or Minesota fissure, which is shown as following the hanging wall of the conglomerate to the outcrop. There was apparently little displacement of the beds on the Branch fissure, though in places at least there seems to have been much shattering of the rock. Other fissures are mentioned in the records but do not seem to have been of great importance.
The Minesota conglomerate where seen on the outcrop is a pebble to boulder conglomerate composed mainly of felsitic material and similar to the other felsitic conglomerates of the area. In some sections it is represented as 25 feet thick and underlain by nearly as much sandstone, though it doubtless varies in thickness and averages less than 25 feet.
The Calico lode is apparently a fragmental lode varying in thickness and in character from place to place, as is characteristic of the fragmental lodes in general.
The Minesota fissure follows the hanging wall of the Minesota conglomerate from the outcrop to its intersection with the Branch fissure. The copper occurred characteristically in masses, some of great size. One mass of 527 tons, the largest ever found, was taken out in 1856. The masses are said to have occurred both in the fissure and in the underlying conglomerates. Apparently much of the copper replaced conglomerate but was closely associated with the fissure. Where seen on. the dump the copper-bearing conglomerate shows the bleaching by removal of iron characteristic of conglomerate mineralization elsewhere. The Minesota fissure was productive from the surface to about the 140-fathom level. Beyond the junction of the Branch fissure it appears to have been poor, at this junction and those of minor fissures it was apparently rich.
The developments on the Calico amygdaloid by the Michigan Copper Mining Co, showed the best ground to be at and above the intersection with the Branch fissure, especially in shafts A and B. There was some fair ground reported below the intersection in shaft C, but not much was stoped. The Calico lode yielded little mass copper, the output being mainly, stamp rock.
The Branch fissure was mined by the Michigan Copper Mining Co. nearly to the intersection of the Minesota conglomerate between shafts B and C, but at the intersection it was said to be lean. It is not clear from the descriptions whether the fissure continued on its normal dip through the conglomerate or flattened into the dip of the conglomerate. The copper occurred mainly as mass copper, but there was considerable production from stamp rock.
Norwich. -- The Norwich mine, 8 to 10 miles southwest of the Victoria mine, was productive intermittently from about 1852 to about 1870. The records show an output of 993,360 pounds of copper. The operations were mainly on fissures crossing the beds. The fissures contained some large masses of copper, but considerable rock was also milled. The fissures, are described as quartz-epidote veins. The Norwich property was later explored by the Cass Copper Co., mainly by diamond drilling. Northeast of the Norwich mine there were some early operations on fissures, but apparently little copper was produced.
LARGE-SCALE MAPS AND GEOLOGIC SECTIONS
The surface maps of the district were prepared on a scale of 1 inch=500 feet. A small edition of these maps was published in 1925. The cross sections were prepared on a scale of 1 inch= 200 feet. Part of them were prepared on tracing cloth and can be reproduced by blue printing. It was not found feasible to publish the detailed descriptions of sections, but two copies of such records have been prepared.
The original plates of the large-scale surface maps and of the cross sections have been deposited with the Michigan School of Mines and Technology at Houghton, together with one copy of the description of geologic sections. A second copy of the description of geologic sections has been deposited in the Calumet & Hecla library at Calumet. A few copies of the large-scale maps are available for distribution by the United States Geological Survey.
ADDENDA TO BIBLIOGRAPHY
The following entries were obtained after this report was in page proof
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