FIGURE 16. - Influence of varying permeability of rock on flow of solutions and formation of ore shoots
USGS Professional Paper 144
pp 112-120
HAS THERE BEEN ENRICHMENT OF THE DEPOSITS?
In either prospecting for or developing ore bodies it is important to know the
general behavior of the type that is under consideration. It is well established
that the richness of some types of deposits is greatest near the surface and
decreases downward. Some types have a barren zone at the surface followed by a
rich zone, which in turn gives place to ore of lower grade. Many copper deposits
are of the latter type, and it has been clearly shown that the barren zone near
the surface and the underlying rich zone have resulted from a leaching of metal
from the former and its deposition in the latter-a process known as enrichment.
In the following paragraphs the data relative to the behavior of the Lake
Superior copper deposits with increase in distance from the outcrop are set
forth, and the conclusion is reached that in these deposits enrichment has been
unimportant.
NO EVIDENCE THAT THE LODES ARE POOR AT THE SURFACE AND RICHER BELOW
In either prospecting or developing sulphide deposits, it is of fundamental
importance to recognize that an outcrop that is nearly barren of copper may give
place at depth to a zone of rich ore, which may grade off into material of lower
grade.
In the Lake Superior district there seems to be a general though far from
definite feeling that the best ore is some hundreds of feet below the outcrop -
that is, that there is a gradual increase of copper for some distance
below the outcrop to a maximum depth, below which there is a decrease. Lane
If it is true that there is commonly a leached zone near the surface, prospecting of a lode should be carried on at considerable depth, despite the heavy expense entailed by so doing; if it is not true, a given sum of money may be better expended in examining a greater stretch of lode near the outcrop. The location of property boundaries of course must affect any scheme that calls for extended exploration of a lode along the strike, but even so the presence or absence of surface leaching and downward enrichment will materially affect the planning of explorations.
Kearsarge lode. - At the South Kearsarge mine good ore was present practically at the outcrop, and although the ground varied in grade in different parts of the mine, there is no record of a consistent change from the surface downward. In the Wolverine and North Kearsarge mines and at least a part of the Ahmeek mine the upper levels were relatively poor and the intermediate and lower levels decidedly richer. In the south end of the Mohawk mine there was very good ground in the upper levels, notably south of No. 5 shaft, and apparently the rock in the upper levels of this part of the mine was as good as deeper in the mine or as in the North Ahmeek mine, which is opened below it. In the north end of the Mohawk mine surface pits encountered good rock, and so far as known the upper levels of that part of the mine averaged as good as the deeper levels, though this end averages poorer than the south end. It appears, then, that a part of the productive portion of the Kearsarge lode was as rich at or very near the outcrop as at greater depth, but that the remainder of the lode was relatively poor near the surface. A study of all available facts regarding the character of rock in relation to tenor of ore has shown, however, that the areas of low-grade ore near the surface are coincident with areas of relatively thin or cellular, impermeable lode rock. Areas of rock of similar character deeper in the mines are similarly low in copper, a fact which, together with the lack of evidence that these portions ever contained more copper, has led to the belief that the lower copper content is due to the character of the rock, and that depth below the present outcrop of the lode is not causally related to it.
Osceola lode. - The outcrop of the Osceola lode was richest at the north end of the Osceola mine, where, to judge from reports, the rock was as good as any found at depth on the lode. Throughout the Calumet & Hecla workings on the Osceola lode the top levels average as rich as at any greater depth. South of No. 3 shaft, in the Osceola mine, the top levels were poor. The ground was also poor in No. 6 shaft to the twenty-fifth level and farther south to greater depth. It is thus apparent that for the Osceola lode, as well as for the Kearsarge, the upper portion was good over part of the area and poor over the remainder. There is a bar of thin lode rock. along the south boundary of the shoot, and this, rather than any leaching of the lode, is thought to be the cause for the poor ground to the south. This relation is discussed under "Conditions determining position of ore shoots" (p. 115).
Calumet & Hecla conglomerate. - The Calumet & Hecla conglomerate lode was rich at the surface between No. 3 Calumet and No. 5 Hecla, and again from a point 400 feet south of No. 10 Hecla to and beyond No. 1 Osceola. In the Osceola mine and in No. 12 shaft of the Hecla the shafts went from good ore into poor ground with depth. Nos. 6, 7, and 8 Hecla went through poor ground into rich ore. No. 2 Hecla to No. 3 Calumet went from rich ore into lower-grade ground with increase in depth. Nos. 5 and 6 Calumet went through poor ground to the thirtieth level before they entered good ore. The outcrop of the Calumet & Hecla conglomerate lode was, in short, rich in some places and poor in others. A study of the character of the lode rock has shown that the differences in copper content are correlated with differences in the original. character of the rock; where the ore was poor, the felsite conglomerate was very thin or was represented by a thin bed of sandstone; where conglomerate was present at the outcrop in considerable thickness it was well mineralized. There is no indication that the poor areas were ever mineralized and subsequently leached or that the rich areas have been subsequently enriched. It is therefore logical to conclude that the variation in richness is dependent on the character of the rock rather than on position relative to the outcrop.
Pewabic amygdaloid lodes. - Some of the Pewabic amygdaloid lodes were richest near the surface, and some at greater depth. In general, it may be said that commercial ore was found in the upper levels in the central and southern parts of the Quincy property, whereas to the north, in No. 8 shaft, a long stretch of lode was passed through before commercial ore was encountered, and farther north the profitable ore is still deeper. It appears, then, that the conditions in the Pewabic lodes are similar to those in the other lodes discussed..
Atlantic lode. - The maps of the Atlantic mine indicate that in places stoping was carried close to the outcrop. The grade of the ore in these places is not known, but as the ore in the deeper parts of the lode was of low grade it is reasonable to suppose that the ore near the surface was not much poorer, else it would not have been mined.
Isle Royale lode. - Stoping was carried close to the outcrop in the Isle Royale lode, and although a larger percentage of the lode has probably been mined at the lower than at the higher levels, this fact is apparently more the result of change of mining methods than of difference in character of the lode.
Baltic lode. - At all three mines on the Baltic lode the ground appears to have been good at the outcrop and to have shown no zone of decided enrichment at lower levels.
Evergreen and succeeding lodes. - The mines on the Evergreen and succeeding lodes in the south end of the district have practically all stoped close to the surface with apparently as good ground as at any greater depth.
Nonesuch lode. - In the Nonesuch lode as developed at the White Pine mine there seems to be no evidence to connect grade of the rock with its position relative to the outcrop of the lode.
Veins. - Without following out each important vein in detail, it may be said that in a general way the veins show the same relation as the lodes. Most of the productive veins that have been developed have had good copper near the surface in places, though not necessarily or usually throughout their productive extent. For example, the Cliff fissure was apparently rich in copper at the outcrop east of the Greenstone flow, but No. 4 shaft, which was sunk through the Greenstone flow, did not encounter ore till it had passed through that bed. It is evident that the rich and poor parts of the vein are related to the character of the fissure and of the adjacent rocks rather than to position relative to the present surface. Similar relations might be cited for many of the veins through the district.
NO EVIDENCE OF LEACHING AND REMOVAL OF COPPER NEAR THE SURFACE
Statistics of production indicate that the lodes at the outcrop have been as rich as at greater depths. The question may be asked, however, if there is physical evidence of leaching of copper near the surface or precipitation at depth. The first point to determine in answering this question is, What constitutes evidence of leaching of copper? At first sight the alterations of the rock associated with the copper might be taken for such evidence. The most striking of these alterations is the bleaching of the rock around the copper. In places there has been considerable bleaching of the pumpellyitization type, with which little deposition of copper was associated, and it might be inferred that this bleaching is of the same origin as that seen in rich copper ore and has persisted after the copper was removed. This barren bleached rock, however, seems as abundant on the deeper as on the higher levels. In rock affected by bleaching of the iron-removal type, copper seems always to be present. In the conglomerate there has been some bleaching associated with epidotization and accompanied by little deposition of copper, but this again seems to be no more abundant near the surface than deep in the mine. The amount of bleached rock associated with copper seems, indeed, to be rather less in the upper than in the deeper parts of the lodes. Nowhere has bleached barren lode rock that suggested that it had once been rich in copper been seen near the surface.
Another evidence of leaching might be the passage from rich ore at depth to lean rock near the surface without change in the character of the lode-forming rock. In the mines studied no example of such a change has been found. On the other hand, in the conglomerate lodes there is a rather steady increase in grade of rock from the deeper levels to the surface. In the amygdaloid lodes change in grade in the mines studied has been much more closely associated with change in character of lode than with increase in distance from the outcrop.
It is to be expected that the ordinary surface waters will dissolve some metallic copper, but the metal has not been so dissolved in sufficient amount to leave evidence that has been recognized. The deep salt waters are also known to contain copper, which will be deposited on iron, as has occurred in the waters from deep drill holes in the Baltic mine. (See "Chemistry of ore deposition," p. 121.) In a few places, as at Copper Harbor and the Algomah mine, oxidized copper minerals have been formed, but these are near the surface and afford no evidence of movement of the copper.
NO EVIDENCE OF ENRICHMENT AT THE SALT-WATER HORIZON
In the mines studied no evidence has been recognized of enrichment at the horizon of the change from fresh to salt water or to calcium chloride water. Lane points out that leaching has probably occurred in some shatter zones and cites particularly the one in section 16, Atlantic. The main evidence of leaching, however, seems to be the lack of copper. If such negative evidence is accepted as valid, the Allouez shatter zone also must be regarded as leached near the surface; but this zone has been opened well below the top of the salt water without encountering evidence of enrichment. The shatter zones are pretty clearly poor in copper, but, all evidence considered, it seems more likely that copper was not deposited abundantly than that it was deposited and later removed.
SUMMARY AND CONCLUSIONS
PERSISTENCE WITH DEPTH
All ore deposits must sooner or later show a decrease in metal content with increase in depth; there is however, a great difference in the depth at which such decrease becomes important in different types of deposits, and it is desirable to have some basis for judging what may be expected for a given type, with due allowance for the fact that each deposit has its own peculiarities.
For example, in the well-known type of disseminated copper deposit which owes its better portions to downward enrichment, it is reasonably certain that the foregoing details regarding leaching and enrichment may be summarized as follows:
Virtually every lode has been as rich at some place near the surface as in any of its deeper portions. Where the lode is lean near the surface no evidence is afforded by the way in which the rock has been altered to indicate that this portion of the lode was ever rich in copper.
In every lode examined, where lean ore near the surface gives place to rich ore at depth, it has been found that the change coincides with a change in the original character of the lode rock that seems adequate to account for the change in copper content.
No evidence of enrichment at the zone of change from fresh to salt water or to calcium chloride water has been found in any of the mines.
The enriched zone will be but a few hundred feet thick. It is also pretty well established that in the Tertiary type of gold-silver deposits the bulk of the precious metal is likely to be found within 1,000 to 2,000 feet of the surface and that many deposits fail before a depth of 1,000 feet is reached. On the other hand, in certain gold-quartz deposits, as those of the Mother Lode of California, the metal content of the veins persists at least to 3,000 feet and probably to considerably greater depth. In planning the development of mines to work deposits so extensive as those of the Lake Superior copper district, some basis of judgment as to what is to be expected at depth is of prime importance.
In appraising the effect of depth, it is necessary to consider geologic units and not property units. Thus a study of the mineralization of the Calumet & Hecla conglomerate based on the results attained in depth in the property of the Osceola Mining Co. would lead to conclusions quite different from those reached by considering the ore shoot as a unit.
Developments have been carried to so great a depth in certain deposits that there is a very considerable body of fact on which to base judgment as to what is likely to be found in deposits that have not been so deeply developed, as well as to what may be expected at still greater depth. Development has been carried to a vertical depth of about a mile in the Calumet & Hecla conglomerate and the Pewabic amygdaloid and to more than half that depth in other amygdaloids. To this depth there has been recognized no significant change in the type of minerals in the lode. In the conglomerate lode there seems to be less bleaching or removal of the iron in the higher levels; locally at least this lack of bleaching is rather pronounced. The same seems to be true for the Osceola lode. That it holds for the other lodes is less certain. Lane8 states that sodium minerals, like analcite and natrolite, are confined to the upper levels. This statement is not supported by the present investigation of the amygdaloid lodes, for analcite has been found on the deeper levels of the Osceola mine. In the Calumet & Hecla conglomerate lode zeolites are characteristically absent.
The literature contains numerous statements that silver is more abundant on the upper levels. These statements have not been verified. So far as regards the recovery of silver from the electrolytic treatment of copper, there seems to be no basis for the belief that the ratio of silver to copper decreases with depth. Shoots relatively rich in silver, for the conglomerate lode, are present in the lower levels at the south end of the Calumet & Hecla conglomerate body. It seems pretty certain that vugs rich in silver, which display the metal conspicuously, were more abundant at the higher levels, but it is not certain that silver was actually more abundant at these levels than lower.
It has also been stated that sulphides are less abundant in depth, but this also has not been verified. In the Isle Royale mine it does not seem to be true, and sulphides appear to be as abundant in the lower levels of the Baltic lode as at higher levels.
Some of the lodes lying near the base of the Keweenawan series were relatively high in arsenic, and it has been suggested that this may be so because the solution that mineralized these lodes had flowed for a relatively short distance in them and therefore had been less completely oxidized than the solution that mineralized the higher lodes. If this is true, it would be expected that there would be, in general, an increase in the arsenic content of a given lode with increase in depth. Such an increase, however, has not been definitely observed to the present depth of mining.
Mineralogically, therefore, there is no conspicuous change recognized to the present depth. The available evidence indicates that the native copper of the Lake Superior region was deposited through a wide vertical range. As the content of some of the great lodes has failed to show any systematic decrease with depth after having been followed down for thousands of feet, copper ore may be expected to persist to great depth-even greater than is now known. It is not to be expected, however, that all deposits of the type will attain the same depth, and the suggestion that some have been commercially bottomed is at least warranted by the rather general failure of the fissure deposits with increased depth. It is also possible that some lodes that show strong alteration with little copper may' represent the roots of deposits that were richer in their higher parts, which have been eroded.
CONDITIONS DETERMINING POSITION OF ORE SHOOTS
In general there are two main factors that have influenced the formation of the ore shoots - (a) permeability, which influenced the flow of solutions, and (b) the character of the rocks, which may have had a chemical influence in precipitating minerals. In addition structural relations may have had an important effect. The great ore shoots are related to structural features and peculiarities of the individual lodes. Chief among these are, on the one hand, the occurrence of impermeable or barrier conditions in an otherwise open-textured lode, and, on the other hand, the local opening up of a prevailingly tight, impermeable lode. The former type is exemplified by the Osceola lode; the latter by the Calumet & Hecla conglomerate and the Kearsarge lode.
PERMEABILITY OF ROCK
The lode deposits are formed only in the more permeable beds, such as the conglomerates, fragmental amygdaloids, and coalescing amygdaloids, and are consistently lacking in the more abundant but less permeable cellular amygdaloids. The continuous breaks in which the fissure deposits occur also permit the ready flow of solution. Permeable rock, then, is essential to the formation of such ore deposits as these copper lodes. Roughly, the rocks of the district may be grouped in the following order of permeability: Fissured rock, fragmental amygdaloid, felsite conglomerate, coalescing amygdaloid, sandstone, "scoriaceous amygdaloid," ordinary cellular amygdaloid, trappy fragmental amygdaloid more or less lava cemented, trap, shale, fault gouge.
BARRIERS OF RELATIVELY IMPERMEABLE ROCK
The more permeable rocks are a positive factor in the formation of ore bodies, but the less permeable rocks may be an important negative factor in directing the movement of ore solutions. This is well shown in some of the ore shoots of the district.
The Calumet & Hecla conglomerate shoot is a relatively small lens of felsite conglomerate which thins in each direction along the strike. The sandy and shaly beds underlying the felsite conglomerate continue along the strike in both directions. The conglomerate body is thinnest and shortest near the outcrop and thickens and lengthens with increased depth, thus giving the effect of an inverted funnel. Solutions rising along the lode are converged by the less permeable rocks at the margins of the felsite conglomerate and by the relatively impermeable hanging-wall and footwall rocks into a steadily contracting channel of permeable rock, so that more solution passes through each unit volume of conglomerate in the upper part than in the lower part. (See fig. 16.) The richness of the ore at any depth is about in inverse ratio to the thickness and extent of the lode at that depth, a fact which indicates that copper was deposited in proportion to the amount of solution passing through the rock.
The funneling effect illustrated in the Calumet & Hecla conglomerate perhaps gives the most favorable conditions for the convergence of solutions and the formation of ore shoots; the Calumet & Hecla shoot, at any rate, is the richest yet opened. A concentrating effect may, however, result if a barrier of relatively impermeable rock interrupts a permeable lode. Such damming may result either from a change in character of the lode itself, as from fragmental to cellular, or from the offsetting of the lode by a fault.
In all the amygdaloid lodes there are repeated examples of cellular impermeable lava in the fragmental permeable lava, and consistently the cellular lode is poor, apparently because it has not permitted the ready passage of solutions but has diverted them to the more open rock. The south boundary of the Osceola shoot is formed by a southward-pinching inclined bar of cellular amygdaloid in the prevailingly fragmental amygdaloid. (See pl. 39.) The Osceola shoot is richest close under this impermeable bar, and it gradually decreases in richness northward and away from the bar. Above the bar the amygdaloid is fragmental and favorable in character but contains little copper. This relation suggests that solutions rising along the lode were diverted by and concentrated under the bar and that the most copper was deposited where the flow was greatest-that is, close under the bar.
The examples cited above illustrate two general conditions favorable to concentration of solutions and the formation of ore bodies - (a), a lode that is generally impermeable but has areas of permeable rock (see fig. 16); (b), a lode that is prevailingly permeable but has bars or areas of impermeable material so placed as to cause a diversion and concentration of solutions rising along the lode.
FIGURE 16. - Influence of varying permeability of rock on flow of solutions and
formation of ore shoots
In the first class are the Calumet & Hecla conglomerate and the Kearsarge lode. The rocks at the Calumet & Hecla conglomerate horizon have been examined for many miles along the strike, but only at Calumet has a well-developed felsite conglomerate been found. The Kearsarge lode for the 4 to 5 miles in which it is commercially mineralized is in the main a well-developed fragmental lode, but for miles to the north and south, so far as known, it is prevailingly cellular. This class perhaps includes other lodes about which less is known.
In the second class are the Osceola lode and the Allouez conglomerate. The Osceola lode is prevailingly fragmental and permeable for miles and is somewhat mineralized at many places, but it is known to contain a commercial ore shoot only at Osceola, where it is interrupted by an inclined bar of cellular amygdaloid. The Allouez conglomerate is a thick, well-developed conglomerate over long stretches but locally is represented by a clay "seam" or "slide." This conglomerate has been found to be so well mineralized as to encourage extensive development at the Franklin Jr., Allouez, and Delaware mines. The Arcadian lode should probably be assigned to this class and perhaps the Winona, of whose character less is known. The Ashbed is a lode that is permeable over long stretches and mineralized at several places, as at the Atlantic, Phoenix, Arnold, and Copper Falls mines. The details of the character of rock at these places are not well known. Some of the southern lodes may also be in this class, but too little was seen of them to warrant a classification. The rock at the general horizon of the Baltic lode northeast and southwest of the developed area is fragmental lava, but the correlation is somewhat uncertain. The principal developed area of the Baltic lode in the Copper Range mines is bounded in both directions by strong zones of fissuring and shattering.
FAULT BARRIER
A fault offsetting a lode and so situated that the intersection with the lode makes an angle with the dip of the lode would constitute a barrier similar to an inclined bar within the lode itself. No example of an ore shoot under such a fault barrier has been clearly established; but the Hancock fault forms such an intersection with the lodes it crosses, and practically all the ore from the lodes of the Hancock mine and the Pewabic amygdaloid lodes in the vicinity of the fault has come from the north or lower side of the fault.
The productive portion of the Baltic lode is bounded by faulted zones with heavy gouges, but their attitude, so far as known, does not seem particularly favorable to the converging of solutions.
The fissures in the north end of the district have been most productive for a short distance under the Greenstone flow, which has been regarded by Smyth, Lane, and others as a barrier to rising solutions, but there has been slipping or faulting between the Greenstone flow and the Allouez conglomerate, which has produced fault gouge or "slide," and this gouge is probably the true barrier so far as one exists. There can be no doubt that the fissures are relatively rich under the Greenstone flow or "slide," but the cause may be a combination of barrier and chemical effect, as is shown on page 118. In some of the fissures the copper is said to spread out under the "slide."
At the White Pine mine the Nonesuch shale is thought to have acted as a barrier. Solutions rising along fissures have spread out when they reached the shale, forming an ore shoot in the sandstone under the shale. Lane, who favors the hypothesis of downward moving solutions, regards the White Pine fault as a barrier, with the ore shoot above it.
CONDITIONS DETERMINING POSITION OF ORE SHOOTS
FOLDS
The broad pitching anticlines, like the Allouez, Baltic, Winona, and Mass anticlines, might have a converging influence on rising solutions and thus produce ore shoots. If solutions were rising along a lode they would have a tendency to concentrate toward the crests of such anticlines. Several important ore shoots, including the Kearsarge, Baltic, Winona, and Mass, are situated on such anticlines, though their richest portions are not consistently near the crests, and in detail the distribution of rich and poor ground is far more dependent on character of rock than on structural position. The Isle Royale shoot, indeed, has its richest portion near the trough of a syncline, and here too the character of the lode rock seems to have a more effective control than position on the syncline. The effect of the anticlines is therefore somewhat uncertain.
STRIKE FISSURES
Both Hubbard and Lane have suggested a relationship between mineralization and fissures parallel to the lodes, and they have cited the Baltic lode particularly as an example. Most of the numerous strike fissures in the Baltic lode are slightly steeper than the dip of the lode and cross it at a low angle, so that it seems unlikely that they were produced by simple slipping between beds of trap, as suggested by Hubbard. They may, rather, have been formed by the forces that produced the Keweenaw fault. Regardless of how the fissures were formed, however, the kinds and relation of the minerals in the fissures and in the lode indicate that the mineralization in the lode and that in the fissures had a common origin. A somewhat similar relation between strike veins and lode mineralization exists in the Isle Royale mine.
The Branch fissure, in the Minesota mine, crossed the Calico lode at a low angle. The Calico lode was mined profitably above the intersection but was found poor below. In the Pewabic, Osceola, Calumet & Hecla conglomerate, and Kearsarge lodes, however, strike fissures are not conspicuous, and there is no evidence to suggest that the mineralization of these lodes is dependent on strike fissures. It seems, on the whole, that strike fissures may well be a cause for ore shoots in lodes, but they do not appear to be present in all the lodes nor to be a necessary condition of mineralization.
CROSS FISSURES
Fissures crossing the strike of the lodes have also been regarded for a long time as a possible cause of ore shoots, and such fissures and their relation to intrusive felsite bodies have been emphasized particularly by T. S. Woods.9 Steeply dipping fissures striking generally across the lodes are numerous on all the anticlines, and practically all. the lodes in the district are crossed by such fissures. They are abundant in the Kearsarge lode north of the North Kearsarge mine, in the Isle Royale mine, in the mines on the Baltic lode, and in the Evergreen and succeeding lodes on the Mass anticline; they are present but not abundant in the Quincy mine, the Osceola lode, and the Calumet & Hecla conglomerate.
Cross fissures are especially abundant in the north end of the district on the Keweenaw anticline. The available descriptions of the fissure deposits in this end of the district indicate that in many places the lodes were mineralized near the fissures and that some were mined for a short distance from the intersections but were soon abandoned because of marked falling off in richness. This relation suggests that the lode rock was mineralized by solutions leaking from the fissures, and so does the fact that in the Kearsarge lode the ore near the arsenic fissures is distinctly arsenical. Here, however, the lode is consistently poorer near the fissures than away from them, and the general relations do not indicate that these fissures served as feeders for the main mineralization. One prominent fissure crosses the Calumet & Hecla conglomerate, but the mineralization does not seem to have been related to it; in general, the lode is perhaps poorer near the fissure. There is also a prominent fissure crossing the Pewabic lodes, and the lodes were distinctly poorer near it in the upper levels and apparently also, though to a less marked degree, in the lower levels. A prominent fissure is also present in the Atlantic mine, but its relation to mineralization is not known. On the whole there seems to be no close relation between the mineralization and the cross fissure.
It is, then, fairly clear that there was some mineralization of lodes where crossed by well-mineralized fissures; and it is shown on page 110 that the lodes also affected mineralization in the fissure. It does not seem likely, however, that the main mineralization of the larger lode deposits extended outward from cross fissures.
In suggesting a relation to felsite intrusives, Woods has called attention to the felsite bodies, including that of Mount Bohemia, under the Keweenaw anticline; that under the Kearsarge mines, below the Allouez anticline; that east of Calumet; and that at the Indiana mine. He believes that the cross fissures reached the intrusive bodies and that the ore solutions passed outward from these bodies along the fissures.
The presence of felsite under some of the anticlines and the suggestion that intrusive bodies map underlie other anticlines; where not exposed, is discussed under "Structure" (p. 50). On page 124 it is shown to be possible that the ore solutions originated from the same source as the small intrusive bodies and that some of the sulphide veins may bear a close relation to certain exposed intrusive bodies such as the one at Mount Bohemia. With due consideration of all these suggestive relations, the evidence does not seem sufficient to prove that the main lodes were mineralized by solutions conducted into them, by way of cross fissures, from the small exposed intrusive bodies, or from unexposed bodies of the same type. The most serious objection seems to be the lack of mineralization in many favorable lodes crossed by the fissures. If the solutions passed along such fissures and from them into the permeable lodes, it would be expected that every lode that was physically and chemically favorable would be mineralized, at least to some extent, where crossed by a mineralized fissure. For instance, two fragmental well-oxidized amygdaloids have been opened in many places between the mineralized portions of the Calumet & Hecla conglomerate and the Osceola amygdaloid, but nowhere have they been found to contain much copper. This fact is difficult to reconcile with the hypothesis of mineralization by cross fissures, but it may be explained if the solutions found entrance deeper in the lode.
It seems clear that the lodes have influenced the mineralization in cross fissures and that there has been some mineralization of the lodes outward from cross fissures, but it also seems unlikely that the chief mineralization of the great lodes has been effected through the cross fissures. The helpfulness for prospecting in recognizing a relation of ore shoots to cross fissures, to anticlines, or to felsite bodies is apparent, but this fact makes it all the more necessary to beware of exaggerating the closeness of such relations.
INFLUENCE OF THICK FLOWS
Lane has pointed out that some of the ore shoots may lie below unusually thick flows. This relation has long been recognized for the fissure deposits beneath the Greenstone flow, but, as already suggested, its real cause may be the "slide" at the Allouez conglomerate. An examination of the geologic map does not seem to give strong support to this idea for other known deposits.
The Kearsarge lode is a long distance below the Greenstone flow, which, moreover, is not very thick over the southern part of the productive area of the Kearsarge lode, and the "Big" trap above No. 8 conglomerate is a heavy flow below this lode. There are no particularly thick beds above the Calumet & Hecla and Osceola lodes at Calumet. The beds above the Pewabic amygdaloid lodes at Quincy are rather thin. The flow above the Isle Royale lode is thick but not exceptionally so; the thickest and most massive flow in the section at the Isle Royale mine is the Mabb ophite, which lies below the Isle Royale lode but above the Baltic lode. Farther south, at the Copper Range mines on the Baltic lode, the thickness of the Mabb ophite has decreased to ordinary size and the thickest bed lies near No. 8 conglomerate, at a considerable distance above the Baltic. Altogether, therefore, there seems to be no clear indication of a relation between known ore deposits and thick overlying beds.
CHEMICAL COMPOSITION
The ferric iron of the rocks is believed to be a controlling factor in the precipitation of metallic copper. Ferric iron is abundant in all the amygdaloids and conglomerates that have a physical character favorable to ore deposition and was therefore present in all the beds through which large volumes of solution passed, and because of this wide distribution it may not have greatly influenced the localization of the ore shoots in the lodes.
Ferric iron may, however, have had a much more active part in forming the ore shoots in the fissure deposits. Developments show that in the arsenide fissures and in the Mass fissure, crossing the Kearsarge lode, most of the copper and arsenide was precipitated near the lode, chiefly above it. Descriptions of the fissure deposits in the north end of the district indicate that a similar relation between copper and a group of well-oxidized fragmental amygdaloids was recognized by those who explored the fissures. In the south end of the district the fissures worked by the Minesota and adjoining mines carried. their copper at and above their intersection with the Minesota conglomerate, a typical felsite conglomerate. A similar relation is shown in some of the fissures crossing the Baltic lode. Ore shoots in fissures, then, appear to occur most commonly at the intersection of the fissure with a thick, well-oxidized bed.
The foregoing statements regarding the conditions favorable to the formation of ore shoots imply, of course, that mineralizing solutions rose along the lodes and fissures. That even under the most favorable conditions solutions in sufficient volume to form ore bodies traversed all the favorable lodes and all the fissures is not probable.
How the solutions may have gained access to the lodes and fissures is discussed on page 125. It may be stated here that the answer to this question would be very helpful in the search for ore bodies, but at present the problem is obscure, little recognized evidence being available on which to base conclusions. It seems certain, however, that many favorable lodes, in fact, most favorable lodes over long stretches - were not traversed by ore-bearing solutions in large volume and therefore do not have ore shoots.
GENESIS OF THE DEPOSITS
SIMILAR ORIGIN OF ALL TYPES
The outstanding trait that is common to all the copper deposits of Michigan - to fissure deposits as well as to lode deposits in widely differing rocks - is the fact that the copper is mainly present as native metal. In copper deposits the world over the occurrence of native copper, except as an alteration product of other minerals, is unusual and has evidently resulted from conditions that are not widespread. Native copper is abundant, however, in the deposits of all types in this district - a fact which points to common conditions or at least a close similarity in conditions of deposition for all the deposits.
FEATURES COMMON TO ALL TYPES
A common result in the deposits of different types might come either from a similar solution regardless of the kind of rock in which it acted or from some feature common to all the rocks that produced the result. The idea of the copper being carried in solution as native metal and precipitated as such is regarded as unlikely. (See p. 129.) If the features common to all the types can be separated from the numerous features that are not common to all, the causes of copper precipitation and the nature of the mineralizing solutions are more likely to appear. Such a comparison may be first made between the conglomerate and amygdaloid lodes and extended to the others.
All the rocks that have been mineralized to form lodes were originally relatively permeable. The lode deposits are in the conglomerates, in fragmental and coalescing amygdaloids, and in sandstone, which, how ever carries ore only near fissures. They are consistently lacking in the relatively impermeable shales, traps, and cellular amygdaloids. The fissures were of course permeable regardless of the type of rock through which they passed.
In original mineral composition the conglomerates are much simpler than the amygdaloids, yet as the copper in the lodes of both types is native, all the conditions essential to the deposition of native copper must have been present in both.
The outstanding feature that seems to have been common to the two types of rock before the copper mineralization was the presence of abundant ferric oxide in the form of hematite. The sandstone lodes also are red and hematite bearing, and the fissures seem to be productive only near the points where they cross red conglomerate or amygdaloid.
The minerals of the ore-depositing period that are common to the conglomerate and the amygdaloid lodes are comparatively few. A red potash-soda feldspar was formed early in lodes of both types. This feldspar is not conspicuous in all the amygdaloids, but in some of those in, which it is lacking a potash mica, sericite, is present, though in the Isle Royale lode, where the sericite is particularly abundant, it seems to have been formed, in part at least, late in the mineralizing period. Epidote and pumpellyite are present in both conglomerates and amygdaloids. Chlorite is abundant in the amygdaloid lodes and much less plentiful in the conglomerates. Calcite and quartz are abundant in both. These comprise the minerals that are at all common in the two types of lodes. The whole series of zeolites and allied minerals, which range from rare to abundant in the amygdaloids and the fissure deposits, are practically absent from the conglomerates. There can be little doubt that they have resulted mainly from a recombination of elements in the amygdaloid lodes, which, in contrast with the conglomerates, were chemically unstable in contact with the ore-forming solutions. The boron of datolite, the fluorine of apophyllite, the potash of feldspar and sericite, and perhaps other elements besides the copper doubtless came from the solutions and not from the rock. Most of the minerals introduced into the amygdaloids and the fissures, whether filling vesicles or fractures or replacing the rocks, can not be regarded as essential to the formation of the native copper, but as something is known of their conditions of occurrence elsewhere and, from experimental work, something of their range of stability, these minerals indicate the physical conditions under which the native copper was formed.
The striking alteration that is common to the lodes in conglomerate, in sandstone, and in amygdaloid is the bleaching of the red rock in the immediate vicinity of copper and as a rule nowhere else. This bleaching has resulted in different minerals in different lodes, but the chemical change in all lodes is in the same direction - namely, toward a removal or recombination of the ferric iron. In the fissure deposits a similar tendency is shown by the alteration of part of the hematite and the formation of chlorite in the red lodes near the fissure intersections.
The essential features shown by all the types of deposits are therefore permeability and presence of ferric iron originally and removal or recombination of the iron close to the introduced copper.
SINGLE PERIOD OF MINERALIZATION
The mineralization has been assigned by various writers to different times according to their conception of the origin of the ores.
Pumpelly, believing that the copper was leached from the sandstones overlying the trap series, supposed that the ore was deposited later than the traps and the overlying sandstone. The general abandonment of the idea of derivation of the copper from the upper sandstones removes that basis for the dating of the ores.
Van Hise and Leith10 regarded the main mineralization as confined to middle Keweenawan time and therefore as of essentially the same age as the mineralized rocks. They state, however, that mineralized boulders derived from underlying beds are present in some barren conglomerates and thus indicate an earlier period of mineralization, before the conglomerates were laid down. They do not mention where or by whom this observation was made. In the work on which the present report is based no pebbles or boulders that had been mineralized before being incorporated in a conglomerate were seen. Mineralized amygdaloid boulders in conglomerate were found, but there was good reason to believe that the mineralization occurred long after the conglomerate was deposited. The hypothesis of a pre-conglomerate period of mineralization needs verification before it is entitled to credit.
Lane believes that the ores were deposited while the basalts were still hot and therefore, presumably, fairly soon after their formation, but his conceptions of imbibition of surface waters and of the dependence of copper deposition on the present position of water zones appear to imply mineralization after tilting had been accomplished. He regards the arsenide and sulphide veins as possibly of later and independent origin.
The conclusion reached in this work is that the major mineralization was effected during a single period of somewhat complex activities that followed the completion of all the essential deformation that the rocks reveal. This deformation, it is thought, began during the outpouring of the Keweenawan lavas and the deposition of the associated sediments and was completed shortly after their accumulation was finished; the mineralization probably followed immediately afterward, perhaps overlapping the last structural adjustments. The evidence upon which this conclusion is based is summarized below.
The Keweenaw fault bends with the cross folds, such as the Keweenaw anticline, the Allouez anticline, and the Isle Royale syncline, and its changes in direction are too great to be reasonably explained in any other way than to assume that the fault itself was folded. It is therefore probably older than at least the last of this folding. The cross fissures from Allouez northward are tension cracks radial to the cross folds and produced at the same time. The countless minor fractures that cut the rocks are also most plausibly ascribed to the stresses that produced the faulting and cross folding, because these minor breaks are more conspicuous and numerous near the places of major deformation and are disposed conformably to them.
Copper mineralization did not occur in the Keweenaw fault itself, so far as known, but it affected the highly fractured zone adjacent to the fault and therefore was undoubtedly later than the fault and this attendant shattering. The mineralization in the cross fissures was of course later than the fissures. The mineralization of the lodes was similar in character to that of the fissures, and the presence of arsenides and sulphides in the lodes adjacent to fissures containing these minerals is a further indication of their close relation.
The same mineral association in all these types and the interrelation of the types indicate mineralization of all types from a single source and during a single period. The sulphide veins contain intergrown metallic copper or merge into native copper veins. The same is true of the arsenide veins, with the modification that the copper is arsenical, as are some of the copper fissures themselves, and the arsenide veins carry some chalcocite. The sulphide and arsenide fissures are thus apparently of the same source and of the same age as the deposits of metallic copper.
The domical uplift of the Porcupine Mountains is not closely connected with the other structures of Keweenaw Point and is probably due to intrusion. Faults of varying magnitude are associated with the uplift. The age relation of the dome to the other structural features of Keweenaw Point is not known, though the latest rocks of the region are involved in all of them. The precipitation of native copper and subordinate chalcocite along and near the faults of the Porcupine Mountain region may therefore have occurred in a different period from that of the main mineralization farther northeast, though there is no evidence that it did. Similarly the sulphide fissures that extend out from the intrusive mass of Mount Bohemia may belong to an independent period, but it seems more reasonable and in accord with the known facts to assume that all the mineralization was accomplished in a single period.
Such a period doubtless was of considerable length. Some of the minerals were formed earlier and some later in the period, and the same minerals were deposited at different times in different places. Some veins apparently were slightly earlier than the mineralization of the lodes they cut, and some were later. There was, however, no cessation of mineralization from the beginning to the end.
As all the rocks of Keweenawan age known on Keweenaw Point were affected by the structural deformation already described, it is evident that the mineralization was later than the Keweenawan rocks of the district. I t is believed, however, from evidence given on page 124, to have been accomplished within. the limits of Keweenawan time.
HYPOTHESES OF ORIGIN
It is clear that the copper is later than the rocks in which it is found and was therefore introduced into them. The important elements of genesis of such epigenetic deposits are the source of the copper, the method of transportation, and the cause of precipitation. Two contrasting views of the origin of the Michigan deposits are presented in the literature and may be designated as that of descending origin and that of ascending origin.
NOTES
7 Lane, A. C., Lake Superior Min. Inst. Proc., vol. 17, p. 134, 1912.
8 Michigan Geol. Survey Pub. 6, p. 871, 1911.
9 Woods, T. S., The porphyry intrusives of the Michigan copper district: Eng. and Min. Jour., vol. 107, pp. 299-302,1919
10 U. S. Geol. Survey Mon. 52, p. 581, 1911.
Collector's Corner Previous section Table of Contents Next section