USGS Professional Paper 144 Butler & Burbank pp 217-221
HISTORY AND PRODUCTION
Important development of the Baltic lode (pl. 49) began at the Baltic mine about 1898. The Trimountain mine and the Champion mine began production in 1902. Originally the three mines were operated under separate organizations, but at present the Copper Range Co. owns the Baltic and Trimountain and one-half of the Champion; the other half of the Champion is owned by the St. Mary's Mineral Land Co. The Copper Range Co. also owns the Atlantic mine. The following table shows the production from the mines and dividends from the beginning of operation to the end of 1925. A more detailed statement of production is given in the statistical section.
Production and dividends from the Baltic lode,a 1898-1925
|Mine||Period||Rock treated (tons)||Copper produced (pounds)||Dividends|
|Total||Per ton||Total||Per pound of copper (cents)|
|Baltic||1898-1916||8,672,553||200, 584, 164||23. 13||$7,950,000||3.96|
|Do||1917-1925||1,820,612||62, 939, 547||34.57||b 2, 084, 772||3.23|
|Do||1902-1923||6,703,079||139, 697, 289||20.84||3, 250,000||2.33|
|Champion||1902-1925||14, 156, 309||438, 403, 286||30.97||29, 070, 260. 96||6.75|
|31, 396, 177||842, 967, 589||26. 91||42,355 032. 96||5.02|
a Does not include production from Superior lode. b Copper Range Co. Estimated.
The relatively high copper content of the rock treated from the Baltic lode as compared with that from the other amygdaloid lodes of the district is due mainly to the present method of mining, which permits discarding underground a relatively large percentage of poor rock. The table of production by years (pp. 79, 82) shows the change in copper content of rock treated resulting from the development of this method of mining. The method has been described and the results discussed by Denton and Schacht,12 of the Copper Range Co., who show that for the period 1915-1921 as compared with 1908-1914 the yield per ton of rock treated increased from 24.8 to 37.2 pounds, or 50 per cent. The rock recovered per square foot of lode decreased from 1.54 to 1.40 tons.
The lode has been developed in the three mines for about 20,000 feet along the strike and in depth to the twenty-seventh level in the Champion mine, the thirty-sixth level in the Trimountain mine, and the thirty-ninth level in the Baltic mine.
EXTENT AND CORRELATION
The Baltic lode has been opened through the three mines of the Copper Range Co. - the Baltic, Champion, and Trimountain. North of these the Atlantic Mining Co.'s Section 16 explorations were presumably at this horizon, and farther north the Superior and Houghton Copper mines are at the general horizon of the Baltic lode and probably on that lode. Still farther north an amygdaloid at this horizon has been prospected by the Isle Royale Copper Co., and north of Portage Lake by the Arcadian Consolidated Copper Co. The identification and correlation of the Baltic lode becomes increasingly uncertain with increasing distance from the principal mines on the lode.
South of the Copper Range Co.'s mines an amygdaloid at the Baltic horizon has been prospected at the Globe and Challenge mines. The No. 3 conglomerate is not developed at these localities, and the correlation is somewhat doubtful, but the horizon has certainly been reached at the Globe, not so certainly at the Challenge.
The Baltic flow at the Copper Range Co.'s mines is an ophite 150 to 200 feet thick. It varies considerably in thickness from place to place.
Resting on the Baltic flow in the developed area are a number of thin disconnected flows. Some of these seem to thin out both up and down the dip and along the strike; others are as yet only partly outlined. Examples of those that seem to thin out are found in the upper levels of the Trimountain mine, and examples of both kinds in the south end of the Champion mine.
These small flows apparently occupy and tend to fill depressions in the surface of the main flow. This is indicated by the fact that the distance from the No. 3 conglomerate, beneath the Baltic flow, to the top of the highest of these flows and to the top of the Baltic flow where they are absent, is far more uniform than the distance from the conglomerate to the top of the Baltic flow measured both where the overlying flows are present and where they are absent. The relations suggest that these small flow's may be gushes from the main flow that were forced out after a considerable crust had formed on it but while most of the interior was still fluid. The gushes tended in part to fill hollows in the original surface of the flow, and in part they possibly caused the crust on which they flowed to settle into the fluid portion beneath, thus lowering the top of the main flow at these places.
Above the main Baltic flow and the "gush" flows resting on it is a flow that has a thickness of 40 to 60 feet where it has been cut in the developed area. The Baltic West lode is at the top of this flow.
The most pronounced structural feature in the Copper Range Co.'s mines is a broad anticline whose crest passes between the Baltic and Trimountain mines. This fold causes a very sharp change in strike, amounting to about 30°, between these two mines and in the south end of the Baltic workings. In this area the ground is much broken by faults and slips, and these probably displace the lode considerably, though present developments do not make this certain. In the south end of the Baltic mine the lode swings around in a zone of considerable fissuring and assumes the general strike that it has in the Trimountain mine, but if continued on this strike it would not meet the lode in the Trimountain mine. This suggests an offset between the Baltic and Trimountain mine. (See pl. 10.)
CROSS FAULTS AND SLIPS
In all the mines there are faults or slips with strong clay gouges striking nearly across the lode; most of these have steep dips, though a few dip as low as 30°. The smaller slips have caused but little displacement of the lode, and a drift carried on the general strike usually reenters the lode a short distance beyond such a slip, or the displacement may be so slight that the lode is not lost.
On some of the slips, however, there is a displacement of as much as 60 to 70 feet-for example, in the lower levels of the Baltic mine between No. 4 and No. 5 shafts and in the south end south of No. 2 shaft. The faults do not show a uniform direction of displacement. In many of the smaller faults the block north of the break is offset to the west, but a strong fault south of Baltic No. 2 shaft has moved the south block relatively westward.
Examples of the cross breaks with heavy clay gouges, together with vein minerals, are to be seen at the south limit of developments in the Champion mine and the north end of the Baltic mine. Some of the fissures in the Trimountain mine also have strong gouges, and, as already noted, there is apparently much fissuring with gouges in the area between the Baltic and Trimountain mines, where the lode makes a sharp change in strike, and the area of the Atlantic Co.'s Section 16 exploration north of Baltic is said to be much broken and fissured.
Throughout the mines there are numerous fissures that strike with the lode or nearly so and dip more steeply than the lode, or from 75° to 90° where the lode dips 70°. These fissures are seen in the openings in the lode and in the crosscuts on both sides of the lode. In the long hanging-wall crosscut on the seventh level of the Champion mine such fissures are numerous near the lode but become less numerous with increasing distance from it. They are numerous and strong in the crosscut back of No. 4 shaft in the Baltic mine, but less numerous and weaker above the Baltic lode. This increase in abundance of strike fissures toward the Keweenaw fault, which is a few hundred feet southwest of the Baltic lode, suggests that the fissures may have been formed at the same time and by the same forces that produced the fault.
CHARACTER OF BALTIC AMYGDALOID
Although the writers were not able to examine the Baltic lode over large areas in the older worked-out portions of the mines, it is fairly clear from the examinations made and from descriptions by those familiar with the older workings that the lode is of the fragmental type.
Like all the other fragmental amygdaloids, the Baltic lode shows notable variations in the quantity of fragmental material from place to place. In many places the lode is 50 feet or even more in thickness and is composed of fragmental amygdaloid throughout; in other places it is but a few feet thick and is composed of rather coarse trappy fragments. Locally the spaces between the large fragments are filled with tongues from the, hanging flow that extend for some distance below the top of the amygdaloid. In still other places the lode is cellular, with slight tendency to fragmental character. As in the other fragmental amygdaloids, unusually thick parts of the lode tend to bulge both to the hanging wall and the footwall.
This variation appears in both a small and a large pattern. In an area of prevailingly thick, notably fragmental amygdaloid, there are areas of thin and trappy or cellular amygdaloid, and in an area of prevailingly thin trappy amygdaloid there are areas of fairly typical fragmental amygdaloid. In the larger pattern the largest areas of thick fragmental amygdaloid occur in the Champion mine and in the upper levels of the Baltic mine. In the lower levels of the Baltic, in the Trimountain, and in the lower levels of the northern part of the Champion mine the lode contains on the average less fragmental material, though there are areas of markedly fragmental amygdaloid. In the smaller pattern there is a distinct tendency for the areas of abundant and less abundant fragmental material to form "belts" or "shoots" with a rather low southerly pitch. This tendency is particularly apparent in the Trimountain mine but is also evident in the Baltic and Champion mines. Whether or not this tendency is also present in the larger pattern is not clear in the present state of development. There is, however, a suggestion of it in the lower levels of the Baltic mine, where the lode contains a relatively small proportion of fragmental rock.
Where the small overlying flows or "gushes" are present, the lode beneath them seems to average thinner than where they are lacking. In places, at least, there seems to be especially thick fragmental amygdaloid around the margins of such flows, as if they had filled in depressions underlain by relatively thin amygdaloid surrounded by ridges of thick amygdaloid. The amygdaloid of these "gush" flows averages thinner and is more cellular than that of the main flow, which is probably to be expected from their origin.
The lode varies greatly in thickness. In the Champion mine in numerous places it is 50 feet thick and in some places even more; the average stoping width is about 26 feet.13 In the Trimountain mine the average is about 16 feet. In the upper levels of the Baltic mine the lode averaged 20 to 23 feet, but in the bottom of the mine it is relatively thin and cellular.
The Baltic lode is well oxidized though distinctly less so than the Kearsarge lode. The markedly fragmental rock, where not bleached by mineralizing solutions, is brown to reddish and well oxidized throughout. Trappy fragmental rock and cellular rock are usually less oxidized, though the tongues from the overlying flow that fill the spaces between coarse, trappy fragmental rock are commonly distinctly oxidized and give the lode a reddish appearance.
The minerals of the Baltic lode are few. The abundant minerals associated with copper are quartz, pumpellyite, epidote, and carbonate - both calcite and an iron-bearing carbonate. Locally sericite is plentiful as in the south end of the Champion mine on the ninth and tenth levels. It is present but not abundant in many other places in all the mines. Laumontite is present in many fissures and locally in the lode. Other zeolites were not noted and certainly are relatively rare. Prehnite is present but is not abundant except in some fissures. Orthoclase, which is abundant in many other lodes and in the Superior mine, to the north, is almost entirely lacking in the Copper Range Co.'s mines. Datolite was not noted. Sulphides of copper are unusually abundant in all three mines. Their characteristic occurrence is in the fissures that dip 75° to 90° and strike nearly parallel with the lode, and they are likewise present in cross fissures. They also occur in the lode in the same manner as the copper, but this type of occurrence is rare. It was seen best in the Trimountain mine, on the twelfth level, in the stope south of No. 4 shaft from the end of the stope to No. 2 mill, where most of the copper in the lode is chalcocite associated with iron carbonate of the same type as occurs in the fissures. Chalcocite is by far the most abundant sulphide in the fissures, but bornite is not rare. Chalcopyrite is reported to occur in a few places.
The mineral most characteristically associated with the sulphides, both in lode and in fissures, is an iron-bearing carbonate. This mineral usually forms on the walls of the fissures, with the sulphides in the center. A similar relation exists where sulphide forms in the lode; the sulphide is surrounded by the iron carbonate. The iron carbonate is far more widespread than the sulphide. Over large areas it has permeated and replaced the amygdaloid, so that the rock, when exposed on the dumps, quickly turns brown from oxidation of the iron in the carbonate, as can be readily seen on the dumps of the Baltic mine. In many places copper is associated with the sulphides. Usually it occurs at the margins of the sulphide veins, but it may occur with quartz in the center of a vein. Where a vein branches and dies out in fragmental lode rock, chalcocite may give place to copper in the extreme ends. In the mass copper fissure in Trimountain No. 2 shaft, twenty-fifth level south, the mass copper was found near the intersection with the lode; where the fissure was followed into the footwall trap chalcocite appeared, though copper was still present. Only a relatively small amount of copper was present in both forms away from the lode.
Barite has been found in the Baltic lode but here as elsewhere is rare except in some of the fissures. The principal commercial occurrence of copper is in the lode, where it is irregularly distributed through the amygdaloid in bodies ranging from minute specks to masses several tons in weight. Most of the copper in this lode is in relatively small masses. Schacht 14 gives the percentage of the various sizes of copper produced as follows:
20 pounds or more -------------------------- 6
Less than 20 pounds but over 1/2 inch ------- 9
1/2 to 1/4 inch ------------------------------ 22
1/4 inch to 20 mesh ------------------------- 13
20 to 60 mesh ------------------------------ 25
Below 60 mesh ----------------------------- 25
Large masses of copper have been found in two fissures at and near their intersection with the lode. Near Trimountain No. 2 shaft a mass fissure was mined from above the twenty-fifth level to the twenty-seventh. It is said that most of the copper was found just below the Baltic lode. Mass copper has also been mined from a fissure in the West lode of the Baltic mine, above the twentieth level. The fissure strikes nearly parallel with the lode but dips more steeply. The copper was in the hanging wall of the West lode. The fissure is of the same system as those that carry chalcocite, but it carried no sulphide so far as was noted. Mass copper in considerable quantity is said to have been recovered from strike fissures in the Champion No. 2 shaft. The copper is said to have occurred mainly in the trap below the lode.
The characteristic rock alteration is of the quartz-pumpellyite type, with varying amounts of epidote. Wherever copper is present the lode is bleached, usually to the light-green color characteristic of abundant pumpellyite. This type of rock alteration is more fully discussed on page 107. Alteration of the same type occurs along the fissures both in the lode and in the adjacent trap rock.
RELATION OF MINERALIZATION TO CHARACTER OF LODE
In a broad way there is a close relation between character of amygdaloid and degree of mineralization. Rich ground is formed in thick fragmental amygdaloid. Thin trappy fragmental amygdaloid and cellular amygdaloid are commonly poor. The thick fragmental amygdaloid is not invariably rich, but thin or cellular lode is nowhere known to be rich over more than small areas.
The largest areas of thick fragmental amygdaloid opened are in the Champion mine and in the upper part of the Baltic mine. These are also the richest areas developed on the lode. In the Trimountain mine and in the lower part of the Baltic the lode is less fragmental and is not so well mineralized. In this respect the Baltic lode does not differ from other mineralized fragmental lodes.
RESULTS OF OPERATION
The variation of copper content with thickness of lode is shown by the data published by Denton15 and Schacht 16 on the results of operations to 1921 on the richest and poorest units of operation on the Baltic lode.
Results of operation of mines on Baltic lode
|Average thickness of lode------------- feet||26||16|
|Ore per square foot of lode ---------- tons||1.36||0.98|
|Copper per square foot of lode-----pounds||44||21|
|Productive lode area ------------- per cent||66||48|
It is readily seen that the thicker parts of the lode have been the more productive. The Baltic mine is intermediate in character and productivity between the other two.
BALTIC WEST LODE
The Baltic West lode is the first amygdaloid above the Baltic lode. The underlying trap is from 40 to 60 feet thick, fine grained, and brownish. The amygdaloid of the portion, opened, mainly in the Baltic mine, is of the fragmental type and is similar in character to the thinner parts of the fragmental amygdaloid on the main Baltic lode.
There have been no large areas developed on this lode comparing in thickness with the thick fragmental Baltic lode of the Champion and upper levels of the Baltic mine. There is some suggestion in this lode, as in the main lode, that the amygdaloid is in areas or belts of moderately fragmental and slightly fragmental or cellular lode. The present developments do not give much idea of the trend of such belts, but apparently if present they do not pitch very steeply.
The results of mineralization and rock alteration are entirely similar to those shown in the main lode, and there is the same general relation between mineralization and character of rock.
RELATION OF FISSURE MINERALIZATION AND LODE MINERALIZATION
Hubbard 17 long ago suggested that the strike fissures in the Baltic lode were the main channels through which the mineralizing solutions passed. He regarded the deposit as a true lode in the sense that it is a zone of closely spaced mineralized fissures rather than a mineralized amygdaloid.
The close similarity of the minerals of the fissures and of the amygdaloid-the presence of abundant iron carbonate and. quartz in both, and the same type of rock alteration in both -- certainly gives good reason for supposing that the solutions that produced the fissure mineralization and the amygdaloid mineralization were very similar. The main difference - namely, the relative abundance of sulphides in the fissures and their relative scarcity in the amygdaloid may be due to the oxidizing effect of the ferric iron of the amygdaloid, as has been discussed on page 129. This is particularly indicated where fissures finger out and are lost in fragmental amygdaloid. The mineral in some such fissures changes near the ends from chalcocite to native copper. It is also suggested by the presence in the amygdaloid over small areas of chalcocite instead of native copper, as has been noted especially in the Trimountain mine. These relations also suggest that the minerals in the veins and in the amygdaloids were formed during the same general period of mineralization, but there can be no doubt that some of the minerals in the fissures were deposited later than some of the minerals in the amygdaloid, and the minerals that now fill the fissures may have been deposited largely after the mineralization of the amygdaloid. Indeed, this must have been the case if the fissures were really the channels through which the mineralizing solutions entered the amygdaloid. The general subject of lode mineralization is discussed on page 124, where it is pointed out that in some of the lodes of the district there is no evidence to the present depth of development as to how the mineralizing solutions entered the lodes, but the mineralization, seems to have been effected by solutions rising along the lodes.
It is entirely possible that solutions may have entered the Baltic and other amygdaloids with which mineralized fissures are associated, in part through the fissures that are now exposed and in part through fissures that entered the lode below the present depth of the mines.
12 Denton, F. W., Development and extraction methods for Lake Superior copper deposits: Lake Superior Min. Inst. Bull., 1922, pp: 26-42. Schacht, w. H., Mining methods of the Copper Range Co.: Am. Inst. Min. and Met. Eng. Trans., vol. 72, pp. 346-370, 1925; Lake Superior Min. Inst. Bull., 1922, pp. 56-79.
13 Schacht, w. H., op. cit., p. 5
14 Op, cit., p. 5.
15 Denton, F. W., op. cit., p. 42.
16 Schacht, W. H., op. cit., p. 5.
17 Hubbard, L. L., Lake Superior Min. Inst. Proc., vol. 17, p. 229, 1912.
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