USGS Professional Paper 144 p 190-194
HISTORY AND PRODUCTION
Important production from the Osceola lode in the Osceola mine began in 1879 and continued without interruption for any whole year till 1920. Exploration on the Osceola lode by the Calumet & Hecla Co. began in 1895 and steady production in 1904. The following table shows the production from the lode from the beginning of operations till the end of 1925.
|Mine||Period||Rock treated (tons)||Copper produced (pounds)||Dividends|
|Total||Per ton||Total||Per pound (cent)|
|Calumet & Hecla, Osceola branch||1904-1925||14, 598,914||213, 817, 584||14.65|
|Centennial Copper Mining||1891||28, 531||531,983||18.64|
|Do||1897-1900||153,661||1, 917, 901||12.48|
|Tamarack Mining||a9, 237, 000|
|24,295,050||416,398, 662||14, 789, 825||3.55|
|Production omitting figures for which no corresponding tonnages are given||24,295,050||407,054,861||16.75|
The Osceola flow (pl. 39), which is a well-marked ophite, has been traced with a reasonable degree of certainty from the Arcadian to the Cliff mine. Outside this stretch there is doubt of the accuracy of suggested correlations.
At the Cliff mine the flow is 35 feet thick; at the Ahmeek, 130 feet; at the Red Jacket shaft, seventy-eighth level, 210 feet; at the LaSalle, 180 feet; at the Franklin Jr., 98 feet; and at the Arcadian, 129 feet. It is the thickest part of the flow, near Calumet, that thus far has been productive. In this respect the Osceola is like the Kearsarge flow.
CHARACTER OF THE OSCEOLA AMYGDALOID
The Osceola flow throughout the area that has been extensively developed, and probably over a much larger area, has a distinctly rough or fragmental top. Its surface, which is much more irregular than that of the Kearsarge flow, consists of a series of hummocks and depressions of very irregular shape and variable size. Perhaps the tops of the hummocks and the bottoms of the depressions would average 12 to 15 feet above and below an average plane, or 25 to 30 feet from top of hummock to bottom of depression. In places the variation is considerably greater, and of course in other places it is less.
The lode is distinctly of the fragmental type. There is less of the non-fragmental, cellular lode rock than in the Kearsarge flow, but where present it commonly underlies the fragmental rock, as in the Kearsarge. The coarse cellular rock that is very common in the lower part of the Kearsarge lode is also less abundant in the Osceola. In many places the lode passes directly from fragmental lode to fine foot trap. There are areas where the lode is mainly of a rather thin cellular type.
There are considerable areas of the lode that contain much sandy material and are of the "scoriaceous" type.
Inclusions of partly resorbed amygdaloid are present below the lode, but they are scattered and not a conspicuous feature of the footwall as they are of the Isle Royale lode.
The lode ranges in thickness from as little as 1 foot to as much as 60 feet; perhaps in a few places it is even thicker. In general, the lode is thicker in the hummocks, consisting mainly of fragmental rock, that rise above the average level of the top of the flow, and is thinner in the depressions below the general level, as the fragmental material commonly extends deeper under the hummocks than it does under the depressions. This is what would be expected if the fragmental material floated in irregular masses on the lava-the higher the loose solidified material was piled above the general level the deeper it would sink into the liquid or plastic part of the flow, and it would remain there when the whole came to rest and solidified. It is the lower portion of these downward bulges that constitutes the so-called "foot lode."
In places the lode contains a bar of "vein trap" with fragmental rock above and below. This, however, has rarely been seen to be continuous over very large areas. Ordinarily the fragmental material underlying the thin trap layer is connected near by with that on the top of the flow, and in most places the lode is simply a continuous thick mass of fragmental rock.
The lode also shows variations that are on a larger scale than those mentioned above. Bands of relatively thick and thin breccia pitch in a southwesterly direction, in general, parallel to the southern boundary of the Osceola, ore shoot. Such a thin band is rather consistently present along the south. boundary of the Osceola shoot, and similar bands are present in both the Osceola and the Calumet & Hecla portions of the lode. Within these thin and thick belts the minor variations of hummocks and depressions are present.
The hanging wall of the Osceola lode is a heavy trap with. a rather indistinct contact between it's thin basal amygdaloid and the upper surface of the Osceola lode. In many places the hanging wall is jointed and sheet copper present on the joints. In both of these features - namely, the lack of a clear-cut upper boundary and the presence of copper in the hanging wall - the Osceola differs from the Kearsarge lode.
Except for the features of the lode itself, the structure in the Osceola mine is simple. There is no noteworthy folding or faulting, and only relatively few small fissures cross the lode..
ALTERATION AND MINERALIZATION OF THE LODE
Oxidation. - In its general character the oxidation of the Osceola lode does not differ from that of other fragmental lodes. As in the other lodes, it is earlier than and independent of the copper mineralization and varies with the character of the rock. The highly fragmental lava is consistently well oxidized; the cellular and trappy rock is much less oxidized.
There is not the same change in degree of oxidation from the hanging wall toward the footwall in the Osceola that there is in the Kearsarge lode. The Kearsarge lode is most highly oxidized near the trap, but the Osceola lode is highly oxidized throughout some of its fragmental "foot lode" is apparently oxidized as much as any other rock in the lode. Oxidation has extended rather deeply into the "foot trap," which for some distance from the lode is distinctly redder than the "hanging trap." In this respect, as in some others, this lode resembles the Isle Royale lode.
Alteration and mineralization later than oxidation. The results of the alteration and mineralization of the lode that were later than the oxidation do not differ in general from those of the Kearsarge lode. Prehnite and datolite are more abundant than in the Kearsarge. Epidote is distributed throughout the Osceola lode, whereas in the Kearsarge it is largely confined to the upper part. This is apparently a result of the deep oxidation of the Osceola lode.
Bleached rock very similar to that of the Kearsarge lode is associated with the copper and differs from that found in the Pewabic, Isle Royale, and Baltic lodes in containing a relatively small amount of pumpellyite.
The more abundant minerals are feldspar (rather irregularly distributed), calcite, quartz, epidote, pumpellyite, chlorite, and prehnite. Prehnite is most abundant in the sandy areas. The less abundant minerals, which are rather unevenly distributed, are datolite, most plentiful toward the south boundary of the ore body; laumontite, mainly in fissures; analcite, also largely in fissures; and a little saponite in the lode.
DISTRIBUTION OF COPPER IN THE LODE
Distribution through thickness of the lode. - In the Osceola lode, as in the Kearsarge, by far the richest and most uniformly mineralized portion lies against the hanging wall. The mineralized rock, however, extends irregularly downward, in places 50 feet or more from the hanging wall. No quantitative data are available as to the relative amount of copper at different depths in the lode, but the general impression is that the top 4 to 5 feet contains fully 75 to 80 per cent of the copper that is present in the upper 10 feet of lode and that the 1 foot just below the hanging wall is by far the richest part of the lode. The copper that lies deep in the lode is irregularly distributed. It is confined to the areas where the fragmental rock extends to considerable depths, but there is too little information as to the grade of rock from the foot workings to determine whether or not copper is always or usually present in commercial quantities in those areas.
Five samples of red lode rock that showed no copper on ordinary inspection were taken to determine if copper is present in rock in which it is not readily seen, and, if so, in what amount. Assays of these samples ranged from 2.8 to 4.8 pounds to the ton, with an average of 3.76 pounds.
As a general rule, "thick lode" and "deep foot lode" occur where the lode bulges up into the hanging wall and, conversely, are not to be expected where the hanging wall bulges down into the lode. If experience shows that copper in commercial quantity is present in a sufficient number of places where "deep foot lode" occurs to warrant prospecting for it, then this relation to the hanging wall would seem to offera very economical method for locating the areas of thick lode.
Distribution in the plane of the lode. - It has already been noted that there are bars of thin amygdaloid pitching southwest in the lode. These bars are usually poor in copper, though their thicker portions contain some commercial ground. Along the south boundary of the ore shoot, where it has been examined, the lode seems to be rather uniformly thin and tight. This thin streak is regarded as an inclined barrier that has prevented the direct upward movement of the ore solutions along the lode and caused a concentration beneath it. This belief is supported by the fact that south of this barrier the lode is lean, even though it is moderately or even in places decidedly thick and has the other physical characters that are regarded as favorable.
The best ground in the shoot has in general been found close to the barrier, and the grade has decreased with increasing distance from it; but the grade varies notably with the character of the lode, and too little definite information is available to give a very clear picture of the variations and their causes. The following facts; however, have a bearing on the matter.
In the lower levels there is a notable change in the grade of rock northward from the south boundary. The area south of Osceola No. 6 shaft is of decidedly better grade than that; of No. 5 shaft. In the accessible levels near the bottom of No. 5 the lode is of only medium thickness, a condition which was probably a factor in producing the lower grade, but regardless of the difference in character of lode, there seems to be a decrease toward the north in general, which is observed in the Calumet & Hecla Co.'s Osceola ground as well as in the Osceola mine. Taken as a whole, there is a decrease in the grade northward, till in No. 18 shaft there is a much lower proportion of pay rock than in the south side of the shoot, and in the Centennial it apparently was not sufficient to encourage development below the twelfth level. A shaft farther north in Wolverine ground is apparently out of the shoot and encountered no encouraging copper content to the fifth level. Tamarack No. 1 in its upper levels encountered some commercial rock, but this continued for only a few levels, when the rock became of too low grade to pay.
The available facts thus suggest that the commercial rock will be found to fail toward the north along a line roughly paralleling the southern boundary of the ore shoot. This means, of course, that the shafts from the north to south will reach unprofitable ground at progressively greater depths. No. 18 even at the outcrop is at about the margin of pay ground. No. 17 averaged poor below the tenth level. No. 16 seems to be getting leaner in the lower levels. If this trend continues, No. 15 will soon be getting into poorer ground, though the Tamarack found some good ground between Nos. 15 and 14 at considerably greater depth. Nos. 14 and 13 would seem to have a good distance to go before they encounter poor ground.
No. 5 Osceola is bottomed in rather poor ground, and the question naturally arises whether this is the bottom of the shoot. The general trend would suggest that it is not. Furthermore, there is a rather wide bar of thin lode in Calumet & Hecla ground that, if projected, would about intersect the bottom of No. 5 shaft, and it is possible that the poor grade in the bottom of No. 5 is in part due to this bar of poor, thin lode rock, and that the bottom of the shoot is at very considerably greater depth.
VARIATION IN COPPER CONTENT WITH DEPTH
In any discussion of the variation of the copper content of the Osceola lode with depth, it is necessary to consider the ore shoot as a whole and not a section as represented by any single shaft. Considering individual shafts, there is little doubt that all the Calumet & Hecla shafts and Nos. 1, 2, and 3 Osceola would eventually show a decrease in copper content with increase in depth. Nos. 5 and 6 Osceola, on the other hand, pass through lean or nearly barren ground for several hundred feet before entering the ore shoot, and there are indications that the ground along No. 5 shaft is growing leaner in the lower levels.
If, however, an inclined belt parallel to the trend of the ore shoot is examined --- for example, a belt 1,000, 2,000, or 3,000 feet north of and parallel to the south boundary of the ore shoot -- no clear evidence is found of a decrease in copper content with increase in depth down the shoot.
The grade of the ore recently mined in the lower levels of the Osceola mine is lower than that of the ore formerly mined in the upper levels, but this is, in part at least, a matter of changes in mining method and policy, as is discussed below, rather than a real change in the copper content of the lode.
There are no exact data available to check the grade of the ore in different parts of the mine, but the impression gained from inspection of the lode is that the copper content of the rock in the ore shoot in the lower levels south of No. 6 shaft compares favorably with that in the higher levels of the mine.
CHANGES IN METHOD OF MINING
There have been decided changes in the method of mining lodes of this type in the district and in the Osceola mine itself, as is very evident to one going through the workings of different periods and of different depths of mining. In the early days of mining on the Osceola lode the method seems to have been to make the highest possible recovery from the hanging side of the lode; the hanging wall was stripped clean, and the pillars were small. No great effort was made at that time to get a high recovery of the copper deep in the lode. A relatively narrow stope was carried against the hanging wall, and where copper was exposed near the footwall it was apparently followed; but where it was not exposed little if any effort was expended in search for it. Thus there was a high percentage of recovery from the hanging side of the lode and a low recovery from the foot side.
Moreover, in the early mining a much larger proportion of the thin and poor portions of the lode was left untouched than in the later operations. The ore under this early type of mining over a distance of 2,000 to 3,000 feet north of the south boundary of the shoot yielded 22 to 29 pounds of copper to the ton.
In the lower levels of the mine - south of No. 6 shaft, for example - the width stoped averages distinctly greater than in the upper levels, and more of the lode has been mined, despite the facts that there has not been the former high recovery of the rock just below the hanging wall, a few inches to a foot of lode being commonly left on the hanging wall, and that the pillars are more numerous and larger than in the upper workings, as is required by the greater depth.
The results of this change from the earlier method are not very certain, but the impression is that considerably more lode rock has been mined per unit area, with perhaps little if any greater recovery of copper per unit area. For example say the average thickness mined in 1920 was 12 feet, including foot work, with a recovery of 18 pounds to the ton, or about 18 pounds to each square foot of lode; in the early mining 8 feet of lode was mined with a recovery of 27 pounds to the ton, or 18 pounds to the square foot.
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