USGS Professional Paper 144 Butler & Burbank pp 106-111



     Three types of amygdaloidal tops of flows have been recognized-banded cellular; brecciated, including some that combine both brecciated and nonbrecciated amygdaloid; and scoriaceous amygdaloid. These types differ from one another in those physical and chemical characters, especially in degree of permeability and hematite content, that determine the extent to which they are likely to be impregnated with copper. The banded cellular amygdaloid is common in the flows of the district, but it is not important as a copper producer except in the coalescing type. Scoriaceous amygdaloids have been still less productive. The brecciated amygdaloid is the dominating type as regards copper production, and it will be described first; the other types will then be contrasted with it.



     The structural and textural characteristics of the lava tops that were brecciated during solidification are described on page 29. Their chief features that concern ore deposition are variation from place to place in degree of fragmentation -- they range from layers of nearly massive material 3 or 4 feet thick to thoroughly brecciated layers 6, 10, 20, and even 50 or 60 feet thick; chilled or finer-grained borders for many of the fragments, especially near the top of the flow; and a high degree of vesiculation of most of the fragments. As a rule, the thick parts of these tops are well brecciated and therefore highly permeable; the thin parts are relatively dense and impermeable; the change from one to the other type may be gradual or abrupt. Commonly the thick breccia tops bulge up into the overlying trap and down into the underlying flow; the thin lode breccia, on the contrary, is pinched between a sag of the overlying trap and an upward bulge of the underlying trap. The presence of bars or slabs of trappy rock in the midst of the breccia gives in places the effect of a double lode.

     The brecciated amygdaloid type is well represented by the Osceola, Isle, Royale, and Baltic lodes. The Kearsarge lode, which represents a transition to the banded cellular type (see p. 30), has been subdivided into brecciated amygdaloid, intermediate somewhat broken cellular amygdaloid, and unbroken cellular foot amygdaloid.


     The red color shown by the tops of all the flows is especially conspicuous in those of the brecciated type. It is believed (see p. 34) that the countless tiny plates of hematite to which this red color is due were formed before the copper mineralization, having been produced while each flow was solidifying, whereas the copper was introduced after all the flows and sediments of the series had accumulated and had been tilted. In the amygdaloids, as in the conglomerates, the red color is present throughout the known extent of the lodes, continuing far beyond those relatively small parts of the lodes where copper has been deposited in important amount.

     The percentage of ferric iron as hematite in these brecciated amygdaloids exceeds the total percentage of iron in deeper parts of the corresponding flows -- in some places by as much as 40 per cent.


     Mineralogy. - The mineral composition of the top rock before it was acted on by the ore solutions was simple. Feldspar, hematite, and glass or its devitrified equivalent were the chief constituents. After the action by the ore solutions, however, the mineralogy was varied and complex. By far the greater number of the long list of mineral species found in this district were produced in these permeable lava tops by heated solutions. Most of these minerals are either present in small amount or are not intimately and significantly associated with the copper. Some characteristically fill vesicles, others occur chiefly in cracks and fissures, still others have replaced the rock material, and most varieties occur in all these ways.

     The minerals most intimately associated with the copper are quartz, calcite, epidote, pumpellyite (a greenish fibrous or prismatic mineral which resembles zoisite and which was called zoisite in making the records of drill cores), chlorite, red feldspar, and prehnite. Datolite accompanies the copper in parts of the Pewabic, Osceola, Isle Royale, and Evergreen lodes. Sericite is present plentifully in the. Isle Royale lode but is not. intimately associated with the copper. It is locally abundant in the Baltic lode with similar relations. Ankerite is abundant in parts of the Baltic lode. The hydrous magnesium-aluminum silicate, saponite, is common in some of the rich ground especially in the Kearsarge lode.

     Analcite is rather widespread flat rarely plentiful and as a rule not closely connected with the copper. Apophyllite is found sparingly with copper in the Phoenix mine on the Ashbed lode. Laumontite is the most abundant of the zeolites. It occurs mainly in. fissures with little or no copper, and where it is locally, abundant in amygdules the lode is generally poor; the mineral is regarded by the miners as a bad sign and apparently justly so. All told, the zeolites play a distinctly subordinate and inconsequential part in the mineralogy of the deposits. Prehnite and datolite, hydrous silicates transitional in character toward the zeolites, are far more intimately connected with the copper.

     Sulphides and arsenides of copper are present in veins that cut the lodes at various angles. So also is native copper. The copper of some of the lodes is somewhat arsenical, but in a lode of low arsenic content the proportion of arsenic may rise perceptibly in the lode copper close to an arsenide vein. Native silver is present along with the copper in all the lodes, generally in amount too small to pay for separation, but in some of the lodes or parts of them, especially the Pewabic, it adds in appreciable degree to the value of the product.

     Rock alteration. -- Alteration of the lava, tops, the filling of the vesicles to forth amygdules, and the cementation of the breccias were effected at the same time and by the same solutions. The vesicles were still unfilled when mineralization began, for pebbles of amygdaloid included in the felsite conglomerates contain elastic sand grains in their peripheral vesicles, and this sand has been partly replaced by the same minerals that filled the interior vesicles. Each of the minerals in the amygdules is present between fragments and replaces rock material; conversely all the minerals that replaced the rock after the magmatic stage are found also as breccia cement and as amygdule fillings; and all the minerals of the fissures have also replaced rock, cemented breccia, and filled vesicles. Very commonly the material of the amygdule continues beyond the original. vesicle boundary, having partly or completely replaced portions of the rock. The cementing of the breccia has also been accomplished largely by replacement of march of the finer material between. the larger fragments though there has been some filling of open spaces.

     The most common and abundant minerals of the amygdules are calcite, quartz, epidote, pumpellyite, chlorite, and copper. Epidote and copper favor the upper parts of the lode, or those which were richest in ferric iron; chlorite and pumpellyite are abundant in the lower parts of the lode, near the horizon where, it grades into trap. The same minerals, with the addition of sericite in the Isle Royale lode; are the chief constituents of the breccia cement and of the replacement products. Thus it is evident that the three processes of vesicle filling, breccia cementation, and rock replacement were inseparable and interrelated and were associated with the deposition of copper.

     In all the amygdaloid lodes, as in the conglomerate lodes, the most conspicuous alteration of the country rock accompanying the deposition of copper was a pronounced bleaching of the red color. Along with this destruction of the red color, which was due to removal of hematite, the rock suffered an even more profound alteration. This occurred around the copper in all the amygdaloid lodes, and in general the same minerals have resulted in all, though there is a marked difference in the proportion of the several alteration minerals in the different lodes and even in different parts of the same lode. Two general types of bleaching are recognized and have been called, from lodes in which each is characteristically shown, the Kearsarge type and the Isle Royale type, or, in descriptive terms, the iron-removal type and the quartz-pumpellyite type. Only here and there was copper deposited in red rock without attendant alteration and bleaching of one or the other of these types, but the deposition of copper occurred at places in the dark part of the lode near the underlying trap without bleaching, though accompanied by rock alteration.

     The alteration typical of the red, brecciated upper portion of the Kearsarge and other highly oxidized lodes effected a destruction of hematite and the formation of epidote, calcite, pumpellyite, chlorite, and quartz. The rock around the copper was bleached to a light-gray color of greenish or pinkish cast and commonly somewhat softened; areas rich in copper are thus conspicuous. The copper has been formed largely by replacement of the rock, and apparently having once started to form at a given place, it continued growing till considerable lumps were accumulated. The copper has clearly replaced the altered rock in the bleached zone around it. Roughly, the diameter of a bleached zone is three to four times the diameter of the copper which it surrounds, but both the copper and the bleached zone are very irregular.

     In the Isle Royale lode and in many of the lodes in the south end of the district the alteration has resulted in a characteristic grayish-green rock of hard, compact texture composed mainly of pumpellyite, epidote, calcite, and quartz. The pumpellyite is the most abundant mineral and gives the color to the rock. The copper occurs in the midst of the bleached rock into which it grew, as in the Kearsarge type. Bleached rock of this type is conspicuous in the Baltic lode and, with more quartz and less pumpellyite, in the Pewabic lode.

     Alteration of a third type occurred in the dark-colored basal portion of the Kearsarge lode and more, or less commonlv in the other lodes. In the rock thus altered chlorite is abundant and intimately associated with the copper, which occurs mainly in amygdules, producing "shot copper" but also  forms films or plates along joints. Red feldspar may be plentiful in places. There has been no conspicuous bleaching, mainly because the rock near the foot of the lode was never very red, and because chlorite, the predominant alteration product, is dark like the trap, and epidote is far less common than in the upper red parts of the lodes, where it seems to have been formed through a recombination of part of the ferric iron. The lower few inches of the overlying trap generally shows alteration of this chloritic type, and in places a very little copper is present.


     The only lodes of the cellular type that have been extensively mined are the Pewabic amygdaloid lodes. These lodes belong to the "coalescing" variety of the cellular type; they are composed of layers in which amygdules are relatively scarce, alternating with layers in which the gas bubbles were so abundant that they coalesced so as to form irregular flat cavities. These cavities are filled with vein minerals, and the lode has the appearance of a banded vein. In places these cavities extend continuously in a cross section of the lode for as much as 10 to 15 feet, though their average extent is much less. (See pl. 58.) In the plane of the lode some of these openings must have been continuous for tens and perhaps for hundreds of feet. The Pewabic lodes are characterized by this type of top, but they contain large areas of fragmental top and some of the more ordinary type of cellular top. This "coalescing" type of flow is commonly flat-surfaced. The drifts in the Quincy mine that follow the narrow lodes are essentially straight for hundreds of feet and contrast strikingly with drifts following the tops of fragmental lodes.

     It is evident that the coalescing top, with its long continuous openings, would be relatively permeable and permit the passage of large quantities of mineralizing solutions.

     The coalescing Pewabic lodes are of a brownish color where least mineralized but distinctly less red than most of the fragmental lodes, including the fragmental areas of the Pewabic lodes. They are probably less completely oxidized than these other lodes, though some of the difference in the color may be due to the coarser grains of the hematite.

     In the coalescing Pewabic lodes quartz and pumpellyite are the characteristic minerals, though calcite and epidote are not rare. The rock alteration is of the quartz-pumpellyite type, and the lode is consequently hard. Bleaching is not as pronounced. as in. most, of. the other lodes. An unusual amount of the copper occurs in masses, some weighing many tons, that characteristically lie parallel with the lode and are associated with strongly developed amygdular bands. Most of the copper, however, occurs in small grains scattered throughout the amygdaloid, much of it having replaced the rock. The fine copper is also commonly associated with strong amygdular bands.

     In the fragmental Pewabic lodes bleaching is more pronounced and is in part of the iron-removal type. In the upper levels of the Quincy mine, probably in the fragmental lode, nodular masses of porcelanic datolite were present. No datolite was noted in the deeper levels.


     The Ashbed lode, which has been most productive in the Atlantic mine but has also yielded copper in the Phoenix mine, near Eagle River, and in the Copper Falls mine, farther north, is the principal representative of the amygdaloid lodes of scoriaceous type. Other examples of this type are numerous but so far as known contain little copper.

     As none of the mines on the Ashbed lode are active there has been but meager opportunity to examine the lode. Apparently it varies much from place to place.

     In the Atlantic mine its character is apparently rather typically "scoriaceous," as described on page 176. It is composed of amygdular fragments cemented with brown to red sediment and is distinctly soft. The copper was, distributed rather uniformly through the rock, and its deposition was accompanied by rather feeble bleaching.

     In the Phoenix mine the upper or "gray" lode appears to be similar in character but the lower or "red" lode is possibly less scoriaceous.

     At Copper Falls much of the amygdaloid is apparently rather typically fragmental, though containing some sediment. It is well oxidized, and bleached rock of the iron-removal type is conspicuous in association with the copper.



     None of the important amygdaloid lodes are mineralized over more than a small part of their known extent. The Kearsarge lode has been traced for 40 miles or more but is known to be commercially mineralized for only about 5 miles, though it contains notable amounts of copper for double that distance. The Osceola lode has been developed for but little more than 3 miles along the outcrop, although its length is known to be several times 3 miles. Similar conditions exist in the other lodes. The copper, then, is concentrated in shoots within the lodes.

     With the possible exception of the Allouez conglomerate and the Ashbed amygdaloid, neither of which has been shown to be rich at any point, no lode is known to be commercially mineralized in two widely separated parts. This probably does not result from any fundamental cause but is the outcome of generalization from a small number of cases. It is frequently stated in the district that but one mineralized lode is to be found in any one section across the series. This statement, however, is by no means true, as is indicated by the overlapping of the Calumet & Hecla conglomerate and Osceola ore shoots. Moreover, in the closely spaced series of lodes, like the Pewabic amygdaloid lodes and the Evergreen and succeeding lodes, several ore shoots have been developed in the same cross section. Immediately above the main Baltic lode is an ore shoot in the Baltic West lode, and above the main Superior lode is an ore shoot in the Superior West lode. The distribution of ore bodies in each lode appears to be independent of the adjacent lodes and to be governed by local causes, which are discussed under "Causes of ore shoots" (p. 115).


     Within the mineralized portion of a lode or what may be regarded as a major ore shoot, the copper shows considerable inequality of distribution, both in the plane of the lode and across the lode.

     In drifting along the lode through a major ore shoot patches or zones of lean ground, in part too lean to be worked, are generally encountered. Some of these are areas in which the lode is thin; some occur along fractures or shatter zones. The areas of profitable ground between such lean areas may be regarded as shoots of a second order. An example is afforded by the rich South Kearsarge-Wolverine shoot, which lies within the main shoot that extends from Centennial to Gratiot on the Kearsarge lode. These better shoots differ greatly in size, but all are large as measured by the standards of the usual mining district. Their size, indeed, is generally so great that, although they have been developed to depths and lengths of several thousand feet, neither their shape nor the pattern of their distribution is satisfactorily known. There is strong indication, however, that many of the better shoots are of elongated shape and that some of them pitch rather steeply within the plane of the lode.

     Variations across the plane of the lode permit closer investigation. The amygdaloid lodes are generally richest near the hanging wall and. decrease in value toward the footwall. This is notably true of the Kearsarge lode. It is also true of the Osceola lode, though in the Osceola. and Baltic lodes there are well-mineralized areas near the footwall.

     The Pewabic amygdaloid lodes are narrow, and the variation in distribution of copper within them is not so noticeable. No definite information is available for the Ashbed lode. In the Isle Royale lode, in contrast to the others, the richest ground is pretty consistently along the footwall.

     Permeability seems to have been a controlling factor in determining the place of deposition of copper in the lode. In general the top of an amygdaloid was the more permeable, and the greatest volume of solution passed through that part of the lode. In highly fragmental lodes some of the deeper parts are loose and permeable and therefore were well mineralized.

     The reason for the reversal of the usual relation in the Isle Royale lode is not entirely clear. The cause that seems most likely is the presence of joints or shearing planes parallel to the lode near its base. These joints did not develop in the friable most thoroughly brecciated portion of the lode, and the presence of the mineralized zone in and near the joints along the footwall suggests that these fractures, which are continuous for long distances, made this the most permeable portion of the lode.


     Several ore bodies in the district have given indication of decreasing copper content with increase in the depth to which they have been mined. The main Superior lode and the Superior West lode were found too poor in depth to justify continued operations, and the Calumet & Hecla conglomerate is known to have decreased rather steadily downward in grade of rock. The Baltic mine is of low grade in the bottom levels, though the Champion mine, on the same lode, has rich ground in its lower levels; the North Winona shafts went into poorer ground in the lowest levels, though there is indication that the southern shafts encountered the ore shoot pitching downward.

     Are such changes in grade due to increase in depth, or are they due to local conditions peculiar to each case? This subject is more fully discussed under "Cause of ore shoots," but it may be said here that where the ore bodies are best known the decrease can be accounted for by conditions peculiar to the particular deposit.

     Thus the Calumet & Hecla conglomerate shoot increases in size down the dip and decreases in grade in about the same ratio. The amygdaloid in the lower levels of the Baltic is relatively thin, cellular, and unfavorable and has the same effect on copper content that similar rock does higher in the shoot, even where surrounded by rich ore. The bottom of the Superior West lode is also relatively thin and cellular. The condition of the bottom of the main Superior shoot is not known. It thus appears that in the amygdaloid deposits the decrease in tenor with depth is due to changes in rock texture that result in similar changes along the strike as well as in depth.

     The Quincy mine has been opened for 8,000 feet down the dip. In this mine one lode was found most productive in the upper levels, another in the deeper levels, also one in the north end of the mine, another in the south end. On the whole there is no evidence that there is a notable decrease in the grade of rock to the present depth. In the deep levels, as in the upper levels, favorable lode rock is usually well mineralized and unfavorable rock is poorly mineralized.

     In the Kearsarge lode, which has been opened continuously for 4 to 5 miles along the strike, some stretches in the top levels were poor, such as parts of the North Kearsarge and Wolverine ground, but other, stretches were rich, such as the South Kearsarge part of the Wolverine, the Ahmeek, and the South Mohawk. These differences are pretty clearly due to differences in character of the amygdaloid. Similar differences occur in the lower workings on the lode and are apparently due to the same cause, and not to change in depth.

     No change in the character of minerals or the type of rock alteration has been noted even in the deepest workings, such as those of the Quincy mine.

     There is little doubt that for a lode of uniform size and character a depth exists at which the degree of mineralization would change -- probably for the worse because of factors correlated with increase in depth; if, for example, the lode were fed by fissures, it might be expected that a change would occur where such feeding fissures were passed. The fissure deposits, with the single exception of that on the Ahmeek Mass fissure, have decreased in richness downward to such an extent that they have been abandoned, and in this fact there is certainly a suggestion that depth is influential. There is no clear evidence, however, that the changes of grade observed in the lode mines up to the present time are primarily due to increase in depth.



     Fissure deposits or veins have been of relatively slight importance, in the Michigan copper district as compared with the lode deposits, though a number of mines, notably the Cliff, the Central, and the Minesota, were operated profitably on fissures, and the Ahmeek Mass fissure is being worked by the Calumet & Hecla Co. in conjunction with its amygdaloid mine. Many other fissures have been developed and have yielded considerable copper. But the combined production from fissure mines has been less than 3 per cent of the total production of the district, and the dividends from these mines have been only about 2.5 per cent of the total dividends.

     The fissures in the north end of the district strike across the beds and dip steeply. They are apparently tension cracks that developed during the folding of the beds transverse to the Lake Superior syncline. The fissures in the south end of the district strike nearly parallel with the beds and dip somewhat more steeply. They may possibly be related in origin to the Keweenaw fault.

     The principal fissures, in the miner's sense, are not single breaks through the rock but rather zones of parallel or interbranching fractures. The width of the fissures or fissure zones ranges from that of a tight crack up to 10 feet or more. At many places there is some brecciation of the rock within the fissure walls, but little gouge is present.

     In respect to mineral content, the fissures may be separated into three types - those in which copper is predominantly native, to which all the commercially important fissures belong; those that carry copper arsenides; and those that contain copper sulphides. There is no sharp line between the types, but commonly the copper in a single fissure occurs chiefly in one of these conditions.

     The gangue minerals of the fissures are the same as those associated with the copper in the amygadaloid lodes. In some, like the Mass fissure, the mineralogy is simple, the copper being accompanied chiefly by calcite, quartz, epidote, and chlorite, with only minor quantities of other minerals. In other fissures, like the Copper Falls (Owl Creek), prehnite, datolite, analcite, and some rarer zeolites are plentifully associated with the commoner minerals. Apophyllite is present in some, and laumontite is common in many. Certain fissures carry copper that is distinctly arsenical. Some of the arsenide fissure deposits are essentially quartz-arsenide veins, though most of them contain carbonate, which is in part iron bearing. Many of the sulphide veins contain dolomite and siderite or ankerite, together with a little specularite. The minerals of the fissure veins are similar to those of the lodes, though there is perhaps more extreme variation among the fissures than among the lode deposits.

     Rock alteration along the fissures is not extensive where they cut through the dense traps. Chlorite is the most abundant alteration product in these rocks, but pumpellyite, epidote, laumontite, and calcite are common, and sericite is formed plentifully along some of the sulphide and arsenide veins. Alteration of the amygdaloids where they are cut by the fissures is more difficult of recognition because of their complex mineralogy. The most noteworthy change is a darkening of the red amygdaloid lodes for distances in some places as much as 100 feet on each side of the fissures. This darkening is occasioned chiefly by the development of chlorite but in part from removal of hematite, especially that of finest grain.


     The distribution of copper in the fissure veins is even more irregular than that in the amygdaloid lodes. In some of the fissure veins most of the copper formed large masses weighing a ton or more, finely disseminated copper occurring in but relatively small quantity. In others small masses were commonest; and in a few "stamp copper" (copper disseminated through the vein material in small grains) predominated. Several masses estimated at nearly 500 tons each have been encountered in fissure veins.

     It is now impossible to study the underground relations in the more extensively developed old fissure mines, which have been closed for some years, but certain deductions may be drawn from the descriptions and reports concerning them and from examination of the fissures that cross the Kearsarge lode.

     In the fissures that cross the Kearsarge lode, both those that contain native copper and those that contain arsenides, the copper was deposited in greater part at and near, mainly above, the crossing of the lode. Few if any masses worth the cost of getting them have been found more than 400 feet from the lode.

     Descriptions of the old mines strongly suggest that the fissures in them likewise were more productive where they crossed a series of thick, well-oxidized amygdaloids below the Greenstone flow and at the Ashbed horizon, above the Greenstone flow. A similar relation is suggested in the Minesota fissure, at the south end of the district. This fissure and the associated Branch fissure have been found to be mineralized at and above but not below their intersections with the Minesota conglomerate. Likewise, in the fissures carrying chalcocite, which are rather abundant in the Baltic lode and less so in the Isle Royale lode, the chalcocite seems to be most abundant where the fissures are within the lode or near the points where they cut the lode.

     Thus the general rule for copper occurrence in fissures of all types seems to be that it is most abundant at and near the intersections of the fissures with thick, well-oxidized amygdaloids. This concentration of copper at such intersections suggests that hematite is needed for precipitation of copper in fissures as well as in conglomerates and amygdaloids. According to descriptions of old fissure mines, the amygdaloids and conglomerates were commonly mineralized for a short distance from the fissures. In a few places this lode mineralization formed commercial ore. What happened to the Kearsarge lode near the intersecting fissures is not so clear, because of the mineralization of the entire lode for several miles along the strike by solutions that ascended along the lode apparently independently of the fissures. The net result of the mineralization of fissures and lode is that the lode is darkened near the fissures because of chloritization and the removal of hematite and its copper content is somewhat lower than away from the fissures. The reasons for the abundant precipitation of copper at f he intersections of some of the fissures with the lodes and the impoverishment of lodes near intersections of other fissures are discussed in the section on genesis (p. 135).

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