USGS Professional Paper 144 p 182-190
CALUMET & HECLA CONGLOMERATE LODE
LOCATION AND EXTENT
The Calumet & Hecla conglomerate (pl. 38) has been opened at numerous places from Portage Lake nearly to the end of Keweenaw Point. In most places it is a scoriaceous amygdaloid with a few inches to a few feet of overlying felsitic sand or grit. Only in the Calumet area has it been shown to be a well developed felsite conglomerate.
The lode has been developed in the Osceola, Calumet & Hecla, Tamarack, Tamarack Junior, and Centennial mules, now all owned by the Calumet & Hecla Consolidated Copper Co., for a distance of about 18;000 feet along the strike and to a maximum depth of 9,300 feet down the dip. (See pl. 75, D.)
The Calumet & Hecla conglomerate was discovered in 1864, and production from it began in 1865. The following table shows the production from the lode to the end of 1925:
|Mine||Period||Rock treated (tons)||Copper produced (pounds)||Dividends|
|Total||Per ton||Total||Per pound (cent)|
|Calumet & Hecla:
Tamarack Junior Mining Co
|Osceola Consolidated Mining Co||1897-1902||619,478||12,404,000||20.02|
Total production omitting figures for
which no corresponding tonnage is
Including copper reclaimed from tailings
|58, 213, 265||2, 983, 954, 295||51.26|
a - Estimated
The conglomeratic portion of the Calumet & Hecla conglomerate is made up of several kinds of rock varying somewhat in proportion from place to place. The common varieties are felsite, feldspar porphyry, quartz feldspar porphyry, amygdaloid, and trap, with other types of rock such as quartzite in small amount. The siliceous rocks make up probably more than 95 per cent of the lode. Amygdaloid boulders, which are most plentiful near the base of the lode, appear to have been derived from the amygdaloid immediately beneath the conglomerate and to have moved but a very short distance. A few boulders of more highly oxidized amygdaloid and of trap are present that may have come from other sources. No indication has been found that the siliceous pebbles were derived from earlier conglomerates. This and the absence of basic pebbles except of the amygdaloid immediately below makes it seem that the conglomerate must have been derived from sources outside the series of flows and conglomerates that make up the immediately underlying part of the series. Nearly all the material of the conglomerate is red to brown in color, so that the general color effect of the unmineralized lode is dark red to brownish red.
There is considerable, variation in the quantity of the different types of rock in different parts of the lode. For example, felsite and feldspar porphyries are relatively abundant south of the bar between No. 6 and No. 10 Hecla, whereas north of that bar quartz porphyry is relatively abundant. In the lower pant of the mine, to the south, the quartz porphyry also seems less abundant. In the upper levels in No. 5 Calumet a light-colored variety of felsite porphyry is especially abundant. This rock resembles in color the felsite intrusive south of Ahmeek, although it is not thought to have furnished material for the Calumet & Hecla conglomerate. A few quartzite boulders were noted in the region of the slope shaft and in North Tamarack. Such a local variation in the composition suggests rather near-by sources for the material.
IRON OXIDE OF THE FELSITES AND QUARTZ PORPHYRIES OF KEWEENAW POINT
The following notes relate to the source of the rather abundant ferric oxide in the lode. The chief purposes of the examinations here summarized were to determine whether the bulk of the iron oxides in the Calumet & Hecla conglomerate were there when the mineralizing solutions entered, or were introduced at an early stage of the mineralization, and to determine what oxides they are.
It is believed that the type of iron oxide and its abundance in the several felsites in place along Keweenaw Point throw considerable light on these questions. Polished surfaces of these felsites show that they all contain hematite to a greater or less extent. In several specimens where the reticulate structure characteristic of the magnetite-ilmenite intergrowths suggested that magnetite might be present, its absence was definitely established by such tests as treating the polished surface with hot concentrated hydrochloric acid and powdering the rock and testing with a magnet. It was shown that beyond a reasonable doubt the chief iron oxide of the felsites from the end of Keweenaw Point to the Porcupine Mountains is hematite.
There is evidence that this hematite is a primary mineral of the felsites. It appears homogeneous under the highest magnificationavailable both before and after treatment with hot concentrated hydrochloric acid, showing no islands of unreplaced magnetite such as are common where hematite has replaced magnetite. The rock contains intergrowths that resemble those of magnetite and ilmenite, but such forms are also assumed by hematite and ilmenite. Many examples of hematite with lathlike form were noted, suggesting cross sections of tabular hematite crystals; these are probably identical with the elongated opaque reddish forms in the felsites and acidic porphyries described by Irving under the name "ferrite." Hematite as a primary mineral occurs at many localities in siliceous and feldspathic rocks, where ferrous iron is absent or present in small amounts. That the ferrous iron in some of the Keweenaw Point felsites is low as compared with the ferric iron is shown by the following analyses.
|Felsite pebble from Allouez conglomerate, unaltered||4.88||0.58|
|Mount Houghton quartz porphyry||2.27||0.15|
|Daly's average quartz porphyry||1.57||1.02|
IRON OXIDES IN THE CALUMET & HECLA CONGLOMERATE
The chief modes of occurrence of iron oxides in the Calumet & Hecla conglomerate may be classified as follows: (1) Disseminated in rather fine particles in the felsite and quartz porphyry fragments and boulders; (2) in rounded grains in the sandy matrix of the conglomerate and particularly in black bands in the sandstone; (3) as hematite in soft, considerably altered boulders, many of which are iron-rich and form the familiar copper skulls when replaced; (4) as specular hematite in vugs and along planes of weakness.
The available information on these modes of occurrence may be summarized as follows:
l. All the polished sections show that the comparatively unaltered fragments of felsite and quartz porphyry - that is, those that are not softened or bleached - contain hematite in varying amounts, as scattered fine grains. In this respect they are the same, as closely as could be determined, as the felsite in place. The same tests to establish the absence of magnetite were made on such material with negative results.
2. Rounded grains of iron oxide occur plentifully throughout the finer material of the conglomerate. As very little or no magnetic material was found in repeatedly testing the powdered rock with a magnet, it is concluded that these grains consist of limonite, hematite, and ilmenite. This conclusion accords with Palache's determination on the black table concentrates of the conglomerate. It also agrees with the statement of one of the company's chemists who analyzed the dark material of the flotation slimes of the conglomerate, that it is a "dark-red powder and but slightly magnetic." His analysis is as follows:
Many of the larger of these elastic grains of oxide consist of limonite through which run reticulate bars of ilmenite. Two possibilities as to the mineral replaced by the limonite present themselves. In so far as it is detritus from traps, it was probably magnetite. If it is felsite detritus, it may have been originally hematite, as hematite-ilmenite intergrowths are common.
It is important in a consideration of the chemical aspect of the origin of the native copper to determine when and how this oxidation of the ferrous iron in the small quantity of magnetite in the conglomerate took place. The outstanding chemical alteration of the rock surrounding copper, both in conglomerates and amygdaloids, is the reduction and removal of ferric iron. Therefore, if oxidation of magnetite took place during the mineralization, it is a reversal of the commonly observed reaction. To test this point the iron oxide in some of the sediments, such as the Great conglomerate at Eagle Harbor, far away from copper deposits, was studied, and the magnetite was found to show the same oxidation to a varying degree, as the magnetite in the Calumet & Hecla conglomerate. Hence the change is independent of copper mineralization and probably took place in part soon after the crystallization of the magnetite in its parent lava flow (see p. 42) and in part during the time it was exposed to weathering.
3. Many pebbles and boulders, soft and much altered, are red with ferric oxide. Some of them contain feldspar and quartz phenocrysts. This hematite seems different, on the one hand, from the finely disseminated primary hematite of the hard felsite and, on the other hand, from the shiny black specular hematite which is to be associated with the recrystallizing effects of the mineralizing solutions acting on the rock. The explanation of the occurrence of these soft oxidized boulders is not clear. That there were phases of the porphyries with more abundant iron-bearing silicates than the prevailing types is probable. Irving describes many Keweenawan quartz porphyries as containing altered augite and says that there are gradations from the highly siliceous phases to the diabase porphyries. Hence, a possible source of these iron-rich boulders is not hard to imagine. The chief difficulty comes in explaining their presence in such a soft oxidized state in the conglomerate. If they were oxidized before deposition, how could they stand up under the rather severe mechanical abrasion to which they would be subjected? If they came to rest as hard unaltered pebbles but were later softened and oxidized, it would mean more intense weathering than is indicated for most of the conglomerate material, though such pebbles would be presumed to be relatively susceptible to oxidation.
4. Specular hematite is not uncommon in open spaces of various sorts in the conglomerate. It is regarded as a result of the rearrangement, of ferric oxide, shown to have been present in abundance before the ore-bearing solutions entered.
CHANGES IN THE IRON OXIDES DURING MINERALIZATION
The effect of the ore-bearing solutions upon the iron oxides is notable. Chemical analyses of bleached and unaltered felsite, study of polished sections of pebbles with bleached portions, examination of hand specimens and the interiors of copper "skulls," and observation of the bleaching on a large scale underground all point to the same conclusion-that the introduction of copper has been accompanied by the removal of ferric iron. Polished sections show that the bleached portions of pebbles, whose unaltered parts have swarms of tiny hematite crystals, are entirely lacking in ferric oxide. Hand specimens show that black iron oxide bands in the unbleached sand largely disappear on passing into a mineralized portion. Similarly, the soft iron-rich boulders, which analyses show may run over 12 per cent of ferric oxide, are bleached and contain practically no ferric oxide where the copper has been introduced.
The effect upon the ilmenite has not been observed in polished section, but the petrographic examination of thin sections indicates that much of it changes to titanite. In an epidotized sand nothing is left of many iron oxide grains but the skeletons of reticulate ilmenite surrounded by a transparent mineral, suggesting that the hematite or limonite went to build the epidote, leaving the ilmenite.
Therefore, while it seems clear that the ore solutions had the power of rearranging the iron oxide, the evidence indicates that the bulk of the iron oxide came in at the time of sedimentation as ferric oxide, much of it as primary hematite from and in the felsite and quartz porphyry. It was removed by the ore-bearing solutions, and this removal bleached the rock and was closely associated with the precipitation of copper.
The conglomeratic portion of the Calumet & Hecla conglomerate, so far as developed, is distinctly lenticular. The longitudinal axis of greatest thickness extends almost due north from about the collar of No. 1 conglomerate shaft, Osceola mine. Both east and west of this medial line the lode thins but not uniformly. There are, several minor axes to the east coming to the surface at. No. 5 Hecla, No. 1 Hecla, and No. 2 Calumet. (See pl. 38.)
The lens thins not only to the east and west but to the south or toward the outcrop. As far south as No. 8 Hecla, the lode has nearly feathered out to the east, and at most but a few hundred feet appears to have been eroded. South from No. 10 much more of it has been eroded. Along the main axis the conglomerate thickens toward the north, or down the dip, the thicker portion being found around No. 5 Tamarack and in North Tamarack. In passing from south to north the thicker part of the conglomerate lens widens notably, so that a cross section through No. 5 Tamarack would include a width of conglomerate having a thickness of 10 feet or more, at least four or five times as great as a similar section passing through the collar of No. 10 Hecla.
It is apparent that in passing downward from the surface successive sections across the entire lode would contain an increasing amount of rock. This is equally true whether the section is taken parallel to the present surface or at right angles with the major axis of the lode. A rough estimate of the relative amount of lode exceeding 5 feet in thickness at the surface and at the twenty-fifth, fiftieth, and seventy-fifth levels gives the approximate ratio 3:4:7:10.
Sections at right angles to the axis of the lode at equal intervals passing about through No. 10 Hecla and No. 1 Hecla and near Red Jacket shaft give a ratio of about 4:8:11.
The texture of the lode, like the thickness, varies from place to place, and in a broad way there is a rather close correspondence between thickness of lode and texture. Where the lode is thick coarse material is relatively abundant, and where it is thin fine material is relatively abundant. Thus, where the lode is less than 5 feet thick it is composed largely of coarse to fine sand; where it is more than 10 feet thick it is prevailingly a pebble to boulder conglomerate. The portion of intermediate thickness, over considerable areas at least, as is seen in the upper levels of the Calumet and of Hecla shafts Nos. 1 to 5, is intermediate also in texture, the prevailing rock being a coarse grit to fine pebble conglomerate. In the coarser parts of the conglomerate there is a varying but usually rather large percentage of coarse to fine grit that forms the matrix for the pebbles. The coarse and fine phases of the conglomerate, including sandstone, do not differ greatly in composition except that the finer portions contain considerable ferric oxide and ilmenite in grains. In the sandstone these form definite bands that are practically everywhere noticeable, but the grains are also present in the fine matrix of the coarse types of the conglomerate. The finer part, of the conglomerate are therefore richer in ferric oxide and in ilmenite than the coarser parts.
There are coarser and finer phases of the conglomerate present in practically every section. Thus, it would be hard to find a section that did not contain some bands of sandstone, and in the finer beds there are lenses of coarser material. There is no very clearly recognized regularity as to the stratigraphic position in the lode of the fine and coarse material. It is common to find sandstone on the footwall, on the hanging wall, or in intermediate positions, and, on the other hand, the coarser phases may occupy all these positions.
The general impression gained from looking at the lode as exposed in drifts is that the bedding is essentially parallel to the walls. Where mineralization has followed the. bedding, however, so that the mineralized rock is prominent for relatively long distances, there is a distinct appearance of cross-bedding of large pattern. Where best seen the divergence of bedding planes is to the north, parallel to the axis of the conglomerate. Cross-bedding of small pattern is seen here and there but is not common.
The hanging wall of the conglomerate over a large part of the developed area is a dense trap with a rather pronounced basal amygdaloid, from 2 to 6 inches in thickness, and with pipe amygdules extending upward from the contact of the conglomerate. The basal amygdaloid is commonly slightly oxidized for 1 inch to 6 inches from the contact. The actual contact is slightly irregular, the lava being molded around boulders in the conglomerate.
In the northeastern part of the mine there are frequently 1, 2, or possibly 3 small flows above the main conglomerate lode. In places felsitic sand or conglomerate overlies the small flows. On the twenty-ninth level, south of No. 5 Calumet, the drift follows a 3 to 4 foot bed of felsitic sand resting on a 3 to 4 foot lava flow that in turn rests on the main conglomerate lode. In No. 4 Calumet the hanging-wall flow was recognized as low as the forty-ninth level. The flows have been recognized to the south as far as .No. 2 shaft. A small hanging-wall flow (about 15 feet thick) is also present on the sixty-sixth level of the slope shaft where the crosscut extends to No. 3 Tamarack.
It would appear that small flows are present, locally at least, over a considerable part of the area north of No. 2 Calumet and above the sixty-sixth level. The small flows commonly show some oxidation and in places are rather well oxidized.
The footwall of the conglomerate is everywhere a scoriaceous amygdaloid, but it differs notably in different places. Over considerable areas there may be from 6 inches to 4 feet of soft red basic sandstone or shale immediately beneath the felsitic conglomerate.
Nearer its base this sandstone or shale begins to contain pebbles or boulders of amygdaloid, which gradually become more abundant. and within 3 or 4 feet it passes into scoriaceous amygdaloid filled with sandstone and shale; this in turn gives place to amygdaloid with decreasing amounts of clastic material; and the amygdaloid passes into the footwall trap. The felsite conglomerate may rest on rock of any one of these lower types, suggesting that where the upper members are lacking they have been removed by erosion. The data available do not indicate any very definite distribution of the different types where they form the immediate footwall. The sandstone and shale are, however, rather abundant where the lode thins out to the northeast and also to the south where the lode has been observed-namely, on the forty-ninth and fiftieth levels, south of No. 12 Hecla; likewise where the lode is thin on the seventh level east and west of No. 6 Hecla. These occurrences suggest that in a broad way erosion has been relatively slight where the conglomerate lode is thinnest.
The footwall along the main axis of the conglomerate however, does not seem to show vigorous erosion, as the scoriaceous amygdaloid is present. There seems, indeed, to be little evidence of vigorous erosion at the base of the conglomerate, although the scoriaceous amygdaloid is itself evidence of erosion and deposition at this horizon. The line of separation between the felsitic sediments and the underlying amygdaloid is usually sharp. Pebbles and boulders of the underlying amygdaloid are present in the lower part of the conglomerate, but rarely is felsitic material mingled abundantly with the basic sands of the footwall. The presence of the boulders of amygdaloid in the base of the conglomerate suggests that there has been some erosion and that the finer portions have been carried away or so disseminated through the conglomerate that they are not recognized. Beneath the thicker parts of the conglomerate the amygdaloid is usually dark and shows little oxidation. Where the conglomerate is thin, the amygdaloid is commonly distinctly reddened by oxidation.
Faults. -- But one pronounced fault zone crossing the conglomerate lode has been recognized. This zone is at the surface between Nos. 3 and 4 Hecla and crosses the seventy-ninth level about 400 feet north of No. 6 shaft. The fault has not been cut outside the lode, and the strike can not be closely determined, but it seems to be essentially at right angles to the strike of the lode. The dip is everywhere steep but not uniform. If the strike is at right angles to the lode the average dip is about 82° S. The horizontal offset on this, fault on the upper and lower levels is about 8 feet. The same is reported for the upper levels of Tamarack No. 1. The block south of the fault moved to the west. There is a, rather strong gouge on the fault in the hanging-wall trap, which has been mineralized with calcite, laumontite, and copper.
Over a zone of 100 feet on each side of this fault there are minor faults with a throw of not more than a few inches where observed. Another fault, which has been traced for several hundred feet, crosses the lode a at the crosscut from No. 5 Tamarack on the eighty-first level. This fault strikes at a low angle with the lode, about N.45°E., and dips northwest. Where observed, the dip is irregular, ranging from 60° to 90°. The hanging wall is up about 6 feet; therefore the fault is reverse.
Not uncommonly there is a slickensided zone on the footwall or hanging wall of the conglomerate. This indicates some movement, but nowhere is the amount known.
Fissures and joints. -- Fissures and joints are abundant throughout the conglomerate lode. Many of them can be traced into the footwall or hanging wall, but show no displacement. They are commonly nearly vertical and at right angles to the lode. They are more abundant in certain areas, but close inspection over a few feet of lode almost anywhere will disclose fissures or joints. Many of them contain copper and calcite, and the adjacent rock is bleached, indicating that they are earlier than the mineralization. On a few of these joints a little chalcocite occurs, closely associated with the calcite, and where the lode was opened at Centennial small calcite-chalcocite veins were rather abundant.
The conglomerate, where unmineralized, is dark reddish brown, the prevailing color being due to the presence of ferric oxide both in the pebbles and as small grains in the finer matrix and in the sand. Where copper is not present the lode has a rather uniform appearance and seems to have suffered little alteration since its burial by the hanging-wall flow. The epidotization of some of the lean ground, especially along the borders of the ore shoots, is an exception which is mentioned on page 188.
In parts of the conglomerate soft boulders are rather numerous. These usually contain phenocrysts of red feldspar and quartz and appear to have been similar to the quartz-feldspar porphyry that is abundant in the conglomerate, as pointed out by Pumpelly and others. At present they show rather diverse characteristics. In. many of them a soft reddish-brown to dark greenish-brown material incloses the phenocrysts. These appear to be rich in iron, much richer than any of the fresh quartz-feldspar porphyry pebbles. In other pebbles a soft green chloritic material incloses the phenocrysts.
Nearly all the pebbles are softened and altered to the. center and give no evidence of the original character of the rock except for the phenocrysts. A few have relatively hard centers. In some of these epidote is abundant, suggesting that the original rock was first highly epidotized and later altered to its present condition. In others there appears to be little or no epidote. Magnetite, barite, and secondary specularite have been noted in these hard centers.
As the soft pebbles appear to be as abundant in the rock that contains no copper as in that which is well mineralized with copper, there seems no reason to connect this alteration of the pebbles intimately with copper mineralization.
The soft pebbles are usually in close proximity to other pebbles of quartz porphyry that show no similar alteration. This must mean either that there was some strong selective action or that the soft pebbles were altered in whole or in part before they were incorporated in the conglomerate. That they were incorporated in their present soft condition seems hardly possible. It may be that they had undergone some alteration and were in a condition to be further altered readily when they were incorporated, and that they were softened and oxidized before the conglomerate was buried. No satisfactory explanation of these pebbles has yet been suggested.
The most striking and characteristic feature of the mineralization is the pronounced bleaching of the rock that accompanied the deposition of copper. At nearly every place where the lode is exposed the presence of copper is indicated by the pale brick-red or salmon color of the mineralized portion as contrasted with the dark brownish red of the unmineralized lode. In the finer material the small pieces of rock may be largely bleached to the center, so that the lode has a rather uniform color. In the coarser material the pebbles have been bleached for a short distance from the surface, but the inner part retains its original color.
The coarser copper is commonly in the cement where it has partly replaced the rock material. Where the rock is bleached, an examination of the polished surface generally discloses the presence of copper.
Bleaching not plainly connected with copper has been noted only in lenses of epidotized sandstone. In such lenses pebbles may show a bleaching similar to that associated with copper, and such lenses are commonly surrounded by a narrow zone, rarely more than 1 inch wide, of bleached rock.
The bleaching associated with the deposition. of copper has resulted from a removal of ferric oxide. Commonly there is no evidence of the ferric oxide having been converted to another mineral that has remained. There are, however, two exceptions. Sandstone lenses have been almost completely changed to epidote, apparently from a combination of the ferric oxide with other constituents of the sandstone to form epidote, though some of the epidotized. sandstone is higher in iron than the corresponding unepidotized sandstone. Copper is commonly present in these lenses, but it is rarely abundant.
The second exception is possibly the alteration of the iron-rich boulders to form "skulls" of copper. In this replacement the portion of the boulders rich in ferric oxide close to the copper has been altered to a soft gray to green material that appears chloritic. There has been a large removal of iron, but some remains as ferric or ferrous silicate.
There are few minerals in the conglomerate that can be regarded as gangue minerals in the sense that they were deposited with the copper.
Calcite is abundant in the lode, but its distribution does not seem to be closely connected with the distribution of copper.
Epidote, like calcite, is rather abundant, though usually not in the rich ground. It is in places plentiful near the margins of ore shoots, and it occurs around many areas of calcite, suggesting that the lime was obtained from the calcite and the ferric oxide from the conglomerate. It is perhaps most conspicuous where it has replaced lenses of sandstone. Such lenses are present in many parts of the mine, though they are decidedly more numerous where the ground is poor. In the conglomerate lode there is no indication of close association of epidote with rich copper ground.
Red feldspar is present throughout the mine in variable amounts. Like calcite and epidote, however, it shows no close association with rich ground, though copper is commonly present with feldspar. It seems rather more abundant on the lower levels.
Barite occurs characteristically in the soft iron-rich pebbles, locally in their bleached. portions, associated with copper. Barite in such pebbles is common in the lower levels of the Hecla, but it was rarely seen in the north end of the mine.
Chlorite is rare in the conglomerate except in the soft iron-rich pebbles, where chloritic minerals of undetermined composition are common; also in the altered amygdaloid pebbles.
Zeolitic minerals are characteristically absent from the conglomerate lode. Fissures passing from the lava to the conglomerate lose their zeolites at the contact.
DISTRIBUTION OF THE COPPER
The characteristic occurrence of the copper is in lenticular shoots flattened in the plane of the lode. The mineralized beds may occur in any part of the lode, as near the hanging wall, near the foot wall, or in intermediate positions. In the upper part of the mine, where the lode is relatively thin, the mineralized portion may form a large percentage of the lode. In the lower levels, where the lode is thick, the mineralized portion may be only a small part of the total thickness.
There are no obvious differences in the original characters of the mineralized and the unmineralized parts of the lode, and it seems probable that slight differences in permeability may have been the chief determining factor in producing the deposits. Mineralization may be influenced, however, by variations in composition, the effect of which may conflict in places with that of permeability. For example, it has been pointed out that the finer material contains more ferric oxide and ilmenite as grains than the coarser material. In the alteration of the lodes the smaller particles are completely bleached, while the pebbles are but partly bleached. It would follow that the finer sediment contains more ferric oxide available for reaction, and if this is a factor in the precipitation of copper, it would seem that, other things being equal, the finer-grained parts of the lode would be more favorable. On the other hand, the sandy portions are presumably not favorable to free circulation of solutions. There is then probably some combination of sufficient permeability and abundant iron oxide that is most favorable to precipitation of copper.
INFLUENCE OF FAULTS AND FISSURES
The lode is crossed by one rather strong persistent fault and by a great number of joints and fissures that show little or no displacement. The large fault and many o£ the joints and fissures are mineralized. Some very handsome specimens of arborescent copper have been taken from the large fault. There is no good evidence, however, that any of these were channels that admitted the ore-forming solutions to the lode. A zone of poor ground lies immediately north of the large fault, through part of the mine at least, but the ground to the south of it is generally good. This difference suggests that the gouge may have acted as a dam against ore solutions moving from the south through the conglomerate and thus caused a lean area north of the fault. There is little recognized evidence that the ground is particularly rich where the joints and fissures are abundant or particularly lean where they are few. Many of the fractures, however, are bordered by a zone of bleaching and were evidently channels for mineralizing solutions within the lode.
The crosscuts disclose many strike fissures outside the conglomerate that contain calcite, quartz, chlorite, laumontite, red feldspar, prehnite, and copper. A few cross fissures similarly mineralized have been noted.
The position of the lenses of mineralized ground in the lode is distinctly suggestive of solutions moving upward through the conglomerate.
MINERALIZATION OF WALLS
The basal amygdaloid of the hanging-wall trap locally contains some copper. It is said that in the northeastern part of the mine the thin amygdaloidal beds were in places sufficiently mineralized to have been taken for ore. Certain joints in the hanging wall contain sheet copper.
Ordinarily there is no copper in the footwall immediately below the conglomerate. Where mineralized conglomerate rests on the basic sand or on the scoriaceous amygdaloid the mineralized rock cuts off sharply at the contact. In the lower part of the amygdaloid, however, there is in many places a little copper. This is associated with chlorite, and the occurrence is similar to that near the base of the Kearsarge amygdaloid.
Where the conglomerate is very thin or lacking and the amygdaloid is moderately oxidized, as on the forty-ninth level south of No. 12 Hecla, the amygdaioid may contain more copper. Here it is associated with bleached areas, and the. occurrence, in general, is similar to that of the amygdaloid lodes.
CHANGE IN GRADE OF ORE
From the surface to the lower levels there has been a very pronounced decrease in the grade of the rock Several possible causes for this decrease are suggested - that the precipitation of copper was controlled by distance below the surface that existed when the deposits were formed; that the decrease is due to a change in character of the conglomerate; or that deposition was influenced by the contraction of the conglomerate body upward. It is possible that all these causes and perhaps others have been operative.
There is little doubt that distance below the surface existing at the time of mineralization was a factor in determining where deposition of copper began. Doubtless there was a range of depth in each lode that furnished the conditions of heat and pressure most favorable to deposition, and probably the deposition was less both above and below that region.
Parts of the lode are physically and chemically more favorable than others. The finer rock, provided it permitted free passage of the solutions, would seem to be more favorable than coarse conglomerate. This may, in part, account for the very rich ground in the upper levels of the Calumet mine, where the conglomerate is fine, and it may likewise account for the low grade in some of the thick parts of the lode.
The conglomerate as developed is a tapering body, increasing both in thickness and in width, and consequently in volume, down the axis. It is roughly estimated that the relative amounts of the conglomerate at the surface and the twenty-fifth., fiftieth, and seventy-fifth levels are in the ratio 3:4:7:10. This upward contraction of the body and cc consequent decrease in the volume of conglomerate suggest that the change in grade may be due in, part to the converging of the ore-depositing solutions through a steadily decreasing volume of rock. It has been found that sections across the shoot at different levels, as the twenty-fifth, fiftieth, and seventy-fifth, contain approximately the same amount of copper, but that in the lower levels the ore, being distributed through more rock, is of lower grade.
The grade map (pl. 38) shows very clearly that there is a steady decrease in the grade of the rock from the surface downward and that in general the successive belts of different grade lie roughly parallel to the present surface.
By far the richest rock in the mine is in the "pocket" lying between the north boundary of the ore body and the bar of thin conglomerate between No. 5 and No. 10 Hecla shafts: From this bar north rich shoots come to the surface between the small bars of thin conglomerate. This relation suggests that there were minor outlets through these channels. Below the bar of thin conglomerate - that is, below the twenty-fifth level - the grade belts are nearly horizontal except in the South Hecla, where good ground extends up along the main axis of the conglomerate body.
In the lower levels of the mine a bar of poor ground with a general northerly trend lies just south of No. 5 Tamarack shaft. This broadens to the north. Likewise, a bar of poor ground seems to come in at the northern extension of the North Tamarack workings, though the facts necessary to outline this ground are not available.
MAP SHOWING "COPPER PER UNIT AREA OF LODE"
The data on which the maps showing grade of rock and thickness of lode are based have been combined on Plate 38 by multiplying pounds of copper to the ton of rock at each point by the thickness of lode at the same point and dividing the product by the number of cubic feet of rock to the ton, thus obtaining figures that represent the amount of copper per square foot of lode at the different points.
An examination of this map indicates that the areas of maximum mineralization do not coincide with the areas of richest rock. The area of maximum mineralization trends northward and down the lode, roughly parallel to the main axis of the conglomerate body but somewhat above it, and the mineralization decreases in intensity northward and down the axis. There are some exceptions to these general tendencies. The most pronounced are a tendency for the intensity to increase in the pocket toward the outcrop of the Hecla, and the presence of a rather large area of relatively low mineralization in the vicinity of the lower workings of the Tamarack.
An attempt has been made to determine the relative amounts of copper in different horizontal cross sections of the lode. This has been done by multiplying the "foot-pound" figure (obtained by multiplying pounds per ton by thickness of lode) by the length of lode in the sections at the different levels. One estimate took the general average for the level as the "foot-pound" figure; another estimate calculated each line in several sections, which were combined to obtain the "foot-pound" figure. The greatest difference, on the seventy-fifth level, resulted from differences in estimate on the undeveloped area at the north end of that level and may be taken as representing the uncertainty as to that area.
The average of the two estimates gives the following ,relative amounts of copper on the different levels, in a unit section: Twenty-fifth, 133; fiftieth, 133; seventy-fifth, 115; average, 127. The seventy-fifth level thus shows 10 per cent below the average.
It is apparent, then, that so far as these data are trustworthy they indicate a decrease in mineralization with increase in depth, though the decrease shown is certainly not beyond the limits of error in the data used. What they seem to show with considerable certainty is that they tendency toward a decrease in total mineralization with increase in depth is not marked, but that the large decrease in grade is due mainly to the dissemination of approximately the same amount of copper through a larger volume of rock.
A calculation of the amount of copper in the lode from the data indicated on the maps, with the assumption that 75 per cent of the lode has been stoped, gives approximately the amount that has been actually recovered and thus tends to corroborate the approximate accuracy of the data used on the maps.
If it is assumed that the same amount of copper is disseminated through the lode at each level and further that the grade at the seventy-fifth level is 30 pounds to the ton, then at the fiftieth level it should be 43 pounds, at the twenty-fifth 75 pounds, and near the surface 100 pounds. The recorded grades show a rather surprising approach to these figures.
There seems no reason to expect a change in these general relations with increased depth. The lode appears to be increasing both in extent and width, and it would follow that there should be a decrease in grade of ore. There does, however, appear to be some tendency to break up more with depth into poor and rich shoots. The appearance of the lode underground suggests this rather more strongly than the data brought out on the map.
CAUSE OF CONVERGENCE OF SOLUTIONS AND INFLUENCE OF BARRIERS
As pointed out on page 101 and indicated on the map showing thickness of lode (pl. 38) the part developed is a projection from a much larger body of conglomerate. As no striking relation has been found between fissures and mineralization, it is believed that the solutions gained entrance at some point below the area developed and rose along the lode. If movement were unimpeded the solutions would probably rise directly along the lode toward the outcrop. If, however, they met a barrier they would be deflected and move upward below the barrier.
It is of interest, then, to note what such barriers may be. As is well known, the finer a sediment the less readily a solution moves through it. Toward the edges of the conglomerate lenses the material becomes finer, and where the lode is less than 5 feet in thickness it is mainly sand. There appears to have been little movement of the solutions through these sandy parts, and they may be regarded as under the circumstances effective barriers to the movement of the solutions. The favorable influence of the converging barriers on the grade of ore is discussed under "Ore shoots" (p. 115).
COPPER BOULDERS AND SKULLS
Many of the pebbles o£ porphyry rich in iron and others largely altered to a chloritic material that lie in the ore shoots have been very favorable to replacement by copper. Pumpelly long ago described the characteristics of the pebbles thus replaced. The inner part of the pebbles and presumably the part least affected by the copper solutions is a red porphyry. The material varies considerably but is commonly rather soft and friable, dark red-brown to greenish black, and evidently rich in ferric iron. Next to the copper of the "skulls," which are only thin envelopes of copper surrounding the pebbles, is a zone of gray-green soft chloritic rock, which grades into the red central part. There is much less iron in the chloritic part of the pebbles, and much of that present is ferrous iron.
In other pebbles the copper penetrates to the center and only a small proportion of the rock material remains. This consists of phenocrysts of feldspar and quartz and some green chloritic material similar to that described above. The feldspar is somewhat altered, but the nature of this alteration has not been determined.
Very little silver was seen in the conglomerate lode. That observed, principally in the southern stopes on the lower levels of No. 10 Hecla, is similar in mode of occurrence to the amygdaloid. It seems to be slightly later than the copper. It is said that silver was much more abundant in the upper levels of the Hecla shafts.
Collector's Corner Previous section Table of Contents Next section