Butler USGS Professioanl Paper 144 pp 194-203
HISTORY AND PRODUCTION
The earliest notable production from the Kearsarge lode was made in the Kearsarge mine, now the North Kearsarge, in 1887, though development had begun some years before. Production from this mine wasfollowed by that of neighboring mines - namely, the Wolverine in 1895, the South Kearsarge in 1900, and the Centennial and Calumet & Hecla in 1904. At the north end of the lode the Mohawk began production in 1903, followed by the Ahmeek in 1904, the Allouez in 1905, the Gratiot in 1910, and the Seneca in 1921. Prospecting has been extended to the north by the Ojibway, Cliff, Miskowabic, Manitou-Frontenac, and Keweenaw Copper (Mandan) companies and to the south by the Laurium, La Salle, and Franklin Jr.
Rock treated (tons)
|Refined copper (pounds)||Dividends|
|Total||Per ton||Total||Per pound of copper (cents)|
|Calumet & Hecla, Kearsarge branch||2,331,524||67,351,569||28.89|
|Production omitting figures for which no corresponding tonnages are given||--------||60,551,104||1,158,075,429||19.14||--------||--------|
EXTENT AND CRITERIA FOR RECOGNITION
The Kearsarge flow (pl. 40) has been recognized from Atlantic to Mandan, in Keweenaw County, a distance of about 35 miles. The rock is a well-developed ophite. The identification of this flow is commonly based on the porphyritic character of its upper portion and on its position immediately above the Wolverine sandstone. With these aids it can be recognized more easily and certainly than most of the other flows.
The plagioclase phenocrysts are usually most abundant in the trap just below the amygdaloid, though they are present in the amygdaloid. They vary considerably from place to place, both in size and in abundance. Usually the tabular crystals do not exceed half an inch in length and make up but a small part of the rock. Locally, however, they reach an inch in length and over short stretches may form a considerable percentage of the rock. In the "west flow" they are less abundant than in the main flow. The concentration of phenocrysts at the base of the amygdaloid appears to have resulted from the rising of the crystals through the molten lava till they reached the solid or viscous portion near the top,where they were stopped. The lesser numbers in the thin "west flow" may be due to the fact that they collected from a smaller volume of lava.
What is classed as the Kearsarge flow does not everywhere consist of a single flow; in many place there are two or more flows at the Kearsarge horizon that contain feldspar phenocrysts and are grouped as Kearsarge. In the principal productive area, from the Centennial mine to the Gratiot mine, the amygdaloid of the lowest flow of the series has been mined, and only locally have the amygdaloids of the "west lodes" been shown to contain copper in paying quantity. In this area the higher flows are relatively thin, phenocrysts are few, and the amygdaloidal tops are of the cellular type and usually but little oxidized.
At No. 2 shaft, Ahmeek, the basal Kearsarge flow is 200 feet thick at the surface and has essentially the same thickness in the crosscuts on the Mass fissure in the lower levels. On the twelfth level, Centennial, the Kearsarge trap is 179 feet thick, with 6 feet of amygdaloid, a total of 185 feet. At the surface the total thickness of the flow as scaled from a cross section is 170 feet. The crosscut in the Wolverine mine showsthe Kearsarge bed about 200 feet thick. Between No. 1 and No. 2 shafts, Mohawk, on the twenty-first level, the flow is 185 feet thick. It thus seems that in the main productive area this basal flow has a thickness of 170 to 200 feet. Locally overlying the basal flow are thin flows of the Kearsarge type. How much of the lode is covered by such flows is not known, but there are certainly places where none are present, and in other places as many as half a dozen flows are present between the basal Kearsarge and the next heavy trap. These flows are usually thin, but a flow 85 feet thick, with phenocrysts, lies on the main Kearsarge flow at No. 4 shaft, North Kearsarge, and a flow of similar thickness has been noted in a crosscut on the Mass fissure in the Ahmeek mine. It appears, therefore, that in the productive area the Kearsarge flows may reach a total thickness of 300 feet.
The following observations were made north of the more developed area
Gratiot: No. 2 shaft, amygdaloid 29 feet, trap 169 feet, total 198 feet; No. 1 shaft, amygdaloid 9 feet, trap 193 feet, total 202 feet.
Seneca No. 1 shaft: Diamond-drill hole No. 6, more than 127 feet in five flows; basal trap 63 feet. Diamond-drill hole No. 3, 74 feet from top cut, five amygdaloids and base of series not reached. Diamond-drill hole No. 1, basal trap more than 84 feet, sandstone not reached. Diamond-drill hole No. 5, 950 feet north of No. 1 shaft, 113 feet of Kearsarge flows with four amygdaloids, basal trap 24 feet. The old Seneca No. 1 shaft is sunk near the Wolverine sandstone, and crosscuts were extended from it into the hanging wall. First level, crosscut 150 feet; second level, crosscut 50 feet, drift on an amygdaloid; third level, crosscut about 350 feet, apparently cutting the entire series of Kearsarge flows, drift about 700 feet, apparently on top amygdaloid; fifth level, crosscut about 230 feet, drift on an amygdaloid, possibly the top amygdaloid.
Ojibway: Kearsarge apparently consists of two or more flows. Basal flow at No. 1 shaft reported 35 to 85 feet, at No. 2 shaft 260 to 360 feet.
Cliff: Diamond-drill hole No. 1 at shaft, Wolverine sandstone 33 feet, trap 93 feet, amygdaloid 6 feet, trap 28 feet, amygdaloid 7 feet (west lode?). Diamond-drill hole No. 11, 1,900 feet along strike from south boundary, Wolverine sandstone 2 feet, basal trap 107 feet, amygdaloid 22 feet, second trap 7 feet, second amygdaloid 10 feet, followed by a series of small flows. Diamond-drill hole No. 12, Wolverine sandstone 3 feet, basal trap 46 feet, amygdaloid 5 feet, second trap 12 feet, amygdaloid (Kearsarge) 14 feet, third trap 7 feet, amygdaloid (Kearsarge) 8 feet. Diamond-drill hole No. 3, 5,200 feet north of south boundary, Wolverine sandstone 19 feet, basal trap 174 feet, amygdaloid (Kearsarge) 24 feet, second trap 81 feet, amygdaloid 11 feet. Diamond-drill hole No. 4, 5,600 feet north of south boundary, Wolverine sandstone 5 feet, basal trap 113 feet, amygdaloid 18 feet, second trap 77 feet, amygdaloid 13 feet.
Central mine: Kearsarge as scaled from a cross section, 235 feet in three flows. Diamond-drill hole No. 2, Wolverine sandstone thin, basal trap 70 feet, amygdaloid 5 feet, second trap 57 feet, amygdaloid (Kearsarge) 33 feet.
Manitou-Frontenac: Diamond-drill hole No. 7-4-5, Wolverine sandstone 6-feet, basal trap 28 feet, amygdaloid 4 feet. Diamond-drill hole No. 3-5-5, Wolverine sandstone thin, basal trap 19 feet, amygdaloid 5 feet, second trap 63 feet, amygdaloid 3 feet, third trap 39 feet, amygdaloid 5 feet, fourth trap 27 feet, amygdaloid 14 feet, total Kearsarge flow 169 feet.
Mandan: Diamond-drill hole No. 9. Wolverine sandstone thin, basal trap 55 feet, amygdaloid 6 feet; appears to be one flow.
From the records given above it is apparent that north of the Gratiot mine the basal Kearsarge flow becomes decidedly variable in thickness, ranging from 24 feet in diamond-drill hole No. 5, Seneca, to 260 feet or more in No. 2 Shaft, Ojibway, 19 feet in the Frontenac, and 27 and 45 feet at Mandan. In contrast with. the main productive area, the basal flow is in most places thin, and the whole series does not maintain the thickness of the basal flow in the area from the Gratiot to the Centennial mine.
The following observations were made south of the productive area:
Laurium: Diamond-drill hole No. 3, 1,700 feet south of north boundary, Wolverine sandstone 10 inches, basal trap 110 feet, amygdaloid 10 feet, second trap 4 feet, amygdaloid 2 feet. Diamond-drill hole No. 2, 800 feet north of shaft, Wolverine sandstone 3 feet, basal trap 141 feet, amygdaloid 8½ feet. Diamond-drill hole No. 4, 900 feet from extreme south boundary, Wolverine sandstone 10 feet, basal trap 144 feet, amygdaloid 11 feet, second trap 4 feet, amygdaloid 7 feet.
Calumet & Hecla: Eighty-first level crosscut, Red Jacket shaft, two small flows over the main Kearsarge lode; basal flow 181 feet.
La Salle: Diamond-drill hole No. 12, opposite No. 2 shaft, Wolverine sandstone 2 feet, basal trap 131 feet, Kearsarge amygdaloid 9 feet, trap 75 feet. No. 2 La Salle shaft is on first amygdaloid. Diamond-drill hole No. 13, 1,700 feet south of No. 2 shaft, from top down, hanging-wall trap 60 feet, top Kearsarge amygdaloid 4 feet, trap 15 feet, amygdaloid 4 feet, trap 20 feet, amygdaloid 14 feet, trap 20+ feet, Wolverine sandstone not noted. Diamond-drill hole No. 14, 250 feet down dip from No. 13, hanging-wall trap 65 feet, Kearsarge amygdaloid 4 feet, Kearsarge trap 33+ feet. Diamond-drill hole No. 1, 5,200 feet south of No. 2 shaft, at No. 5 La Salle, Wolverine sandstone 7 feet, basal trap 28 feet, amygdaloid 4 feet, second trap 28 feet, amygdaloid 6 feet, third trap 10 feet, amygdaloid 12 feet, fourth trap 29 feet, amygdaloid 1½ feet, fifth trap 2½ feet, amygdaloid 6 inches, sixth Kearsarge trap 4½ feet, amygdaloid 7 feet. Diamond-drill hole No. 4, at No. 6 La Salle, Wolverine sandstone, first flow, lowest Kearsarge trap 42 feet, amygdaloid 3 feet; second flow, trap 8 feet, amygdaloid 7 feet; third flow, trap 14 feet, amygdaloid 4 feet; fourth Kearsarge trap 14 feet, amygdaloid 7 feet; hanging-wall trap 14 feet. Diamond-drill holes Nos. 6, 9, 10, 1,500 feet south of No. 6 La Salle; diamond-drill hole No. 9, Wolverine sandstone 1 foot, basal trap 96 feet, amygdaloid 2 feet, second trap 37 feet, amygdaloid 4 feet, hanging-wall trap 62 feet.Franklin Jr. mine, thirty-second level crosscut: Wolverine sandstone 10 feet, basal trap (broken) 10 feet, amygdaloid 16 feet; second flow, trap 22 feet, amygdaloid 30 feet.
Franklin Jr. new shaft: First Kearsarge amygdaloid 7 feet, trap 28 feet; second Kearsarge amygdaloid 6 feet, trap 33 feet; third Kearsarge amygdaloid 9 feet, trap 2 feet; Wolverine sandstone(?) 4 inches.
Arcadian No. 1: Total flow 109 feet; No. 2, total flow 101 feet.
Naumkeag: Diamond-drill hole J, Wolverine sandstone 1 foot, trap 57 feet, amygdaloid 17 feet.
Isle Royale mine: Wolverine sandstone 1 foot, trap 49 feet, amygdaloid 5 feet.
Atlantic: Wolverine sandstone 6 feet, trap 40 feet, amygdaloid 17 feet.
From these observations it is apparent that south of the Centenial mine there is a thinning of the Kearsarge flows, and that south of Nos. 1 and 2 La Salle the basal flow is thin and the series generally consists of a succession of thin flows.
CHARACTER THE KERSARGE AMYGDALOIDAL
Next to the Pewabic, the Kearsarge lode is the most regular of the large productive lodes of the district, yet it varies notably in character from place to place. In the main productive area the amygdaloid can be roughly separated into material of three types - namely , "fragmental lode," banded amygdaloid, and "foot lode."
The "fragmental lode" consists of irregular angular to subangular fragments of amygdaloid. The amygdaloid fragments are predominantly fine and contain very numerous but small amygdules, indicating relatively rapid solidification; but mixed with the fine fragments are some of coarser texture.
In the Kearsarge lode as a whole individual fragments rather rarely exceed a foot in greatest dimension, and most of the fragments range from 6 inches to a fraction of an inch. In the top portion the fragments average smaller than toward the base. In position "fragmental lode," if present, is always at the top, immediately beneath the hanging-wall trap. It gives place downward either to cellular banded amygdaloid or to "foot lode;" if the former, the transition is likely to be gradual; if the latter, it is more likely to be abrupt.
The thickness of the "fragmental lode" probably averages 5 to 6 feet but ranges from 20 feet down to the vanishing point. Very commonly, where the "fragmental lode" is thickest it bulges into the hanging wall, indicating that it formed slight elevations on the surface that was buried by the overlying flow, and it also extends deeper into the underlying material at such places.
The banded amygdaloid, as contrasted with the "fragmental lode," is an unbroken rock body over considerable areas. The amygdules are commonly more abundant at certain horizons, giving the rock a banded appearance in cross section. In texture it varies with distance from the surface of the lode. Where it forms the top of the lode the upper portion is fine textured and it increases in coarseness with increase in distance from the surface. Where it is covered by "fragmental lode" the finer-textured part is not present. Evidently the "fragmental lode" offered the same protection from rapid cooling as the chilled upper portion of the banded amygdaloid.
The banded amygdaloid may lie immediately below the hanging-wall trap or it may grade upward into "fragmental lode." Downward it grades into "foot lode." In most places it is less than 6 feet thick, but it may exceed that thickness locally, as in some parts of the upper levels of the Wolverine, North Kearsarge, Mohawk, and other mines.
The rock in the zone between the banded amygdaloid and the footwall trap is called "foot lode." It differs from the banded amygdaloid in having a coarser texture, larger and fewer amygdules and less tendency for the amygdules to form in bands. It grades upward into cellular amygdaloid or underlies "fragmental lode.'" in a very few places it immediately underlies the hanging-wall trap. Downward it grades into the footwall trap. Locally it contains a few fragments of cellular amygdaloid. It rarely exceeds 6 feet in thickness.
From the foregoing statements it is apparent that, the lode may consist of "fragmental lode," banded amygdaloid, and "foot lode"; of "fragmental lode" and "foot lode"; of banded amygdaloid and "foot lode"; or rarely of "foot lode" alone. The thickest parts of the lode always comprise a fragmental layer and in many places all three types. The thin parts probably consist most commonly of banded amygdaloid and "foot lode," though a lava-cemented fragmental thin lode is a common type.
The average thickness of ground stoped varies somewhat in the different mines of the Kearsarge lode. In several of the mines the lode has been so largely removed from the part developed that it is possible to estimate roughly the average thickness stoped from the quantity of rock produced and the area mined. Such estimates are shown on page 199.
The amount of ground left in stoping differs considerably in different mimes; it probably ranges from 10 to 35 per cent. The South Kearsarge has one of the highest recoveries, and its greater stoping width as calculated is probably due in part to that fact. For the Mohawk the actual stoping width is said to be about 12 feet, whereas the calculated width is 9.5 feet, indicating that about 20 per cent of the lode is left.
It appears from the calculations that the average thickness of lode stoped decreases from the South Kearsarge northward. That this difference can mot be attributed to sloping wider than is warranted in the South Kearsarge and part of the Wolverine is indicated by the average high yield in copper per ton of rock from these mines. It must of course be recognized that there is a decided variation in the thickness of lode in different parts of individual mines. In the Mohawk, for example, the lode averages distinctly thicker at the south than at the north end.
The largest area of thin and cellular amygdaloid in the productive part of the Kearsarge lode extends along the outcrop from about the north boundary of the, South Kearsarge to the Mohawk mine. For much of the distance it extends but a few hundred feet below the outcrop, but in the North Kearsarge and Wolverine mines it reaches as deep as the twentieth level. (See pl. 40.)
Another large body of cellular rock is present in the north end of the Mohawk mine. At the surface it extends from about No. 4 shaft to the north boundary, and in the bottom levels from about No. 2 shaft to the boundary. Interspersed with fragmental rock the lode rock of this type apparently extends beyond No. 1 Gratiot shaft, though the lode in that area has been less developed. It is also predominant north of the new Seneca shaft in the deeper levels. Fragmental lode rock occurs along the Mohawk-Gratiot boundary in the upper levels of No. 2 Gratiot, and smaller areas in the upper and intermediate levels of No. 1 Gratiot and in the lower levels of the new Seneca shaft. A third body of cellular lode is found in the lower levels north and south of Allouez No. 1 shaft, and a smaller body is present north of No. 2 Allouez. Another stretch of thin lode is present in the North Ahmeek, extending from No. 4 shaft northward along the Mohawk boundary beyond No. 4 Mohawk. The bodies of thin lode included in the general area of thick lode thus far outlined have their greatest extent along the strike of the lode and relatively small extent on the dip. If this pattern proves to be typical, it may be possible to get some idea of the distance through a thin bar from its extent along the strike.
At Ojibway, according to Hubbard,6 there is an east lode and a west lode that are copper bearing. Both seem to be irregular in thickness and to be cut out in places. Hubbard suggests that the lode has been eroded and that the old stream channels are recognizable. According to his description the lode is present in the channels and absent from the bounding ridges. To account for this Hubbard7 suggests that after the erosion of valleys they were partly filled with rock material, which was later covered by the flow. Apparently there are two rather persistent copper-bearing lodes separated by trap. The report for 1909 states that at No. 1 shaft, "the east lode is from 24 feet in width on the 500-foot, level to 17 feet on the 650-foot level and contains copper on both levels. The west lode was reached by crosscutting through 17 feet of trap rock. The west lode is about 17 feet wide, horizontally on both levels and contains copper."
Little information is available as to the character o f the lode at the Cliff shaft. From an inspection of the dump the impression was gained that the lode is mainly of the cellular type, though some breccia is present. The diamond-drill hole at the shaft shows two flows, the bottom one 134 feet thick, with a 6-foot amygdaloid, and the top one 30 feet thick, with a 7-foot amygdaloid. In the main the lode seemed rather poorly oxidized. Feldspar phenocrysts are present but rather sparingly. Most of the work was done on the lower amygdaloid, from which most of the material on the dump was probably derived.
At the Rhode Island mine 10 feet of breccia lode, almost a "conglomerate," was noted by Lane. The thickness of the basal bed is 19 feet; the west flow is 86 feet thick, with 11 feet of red amygdaloid.
At the Franklin Jr. mine the Kearsarge amygdaloid was located,8 and the cores show a little copper.9 The thirty-second level crosscut showed enough copper on the footwall side of the lode to warrant further exploration, according to the report of the company. The explorations in the new Kearsarge shaft show three Kearsarge flows with cellular tops.
In the Delaware drill section the Kearsarge amygdaloid has been cut in several diamond-drill holes. In all it is a thin (3 to 4 feet), nonfragmental, poorly oxidized amygdaloid. Some of the holes cut "west" lodes also with thin, nonfragmental, poorly oxidized amygdaloids.
South of the main productive area the lode, so far as known, seems to be thinner and less fragmental than in the productive area. This change is apparent in the south end of the Centennial mine. Little is known of the character of the lode in the Calumet & Hecla, but farther south, in the La Salle mine, it is only locally fragmental and in general only moderately oxidized. The diamond-drill records show that the Kearsarge flow in La Salle ground consisted of three to five thin flows with amygdaloids. In the south shafts the development seems to have been on the upper amygdaloid of this series. The north shafts are evidently on the first amygdaloid above the Wolverine sandstone. In the Laurium ground the diamond-drill record indicates but one thick flow.
In the productive area of the Kearsarge lode the copper has practically all come from the first amygdaloid above the Wolverine sandstone.
From these notes it appears that nowhere outside of the main productive area have large areas of thick "fragmental lode" been discovered.
The major structural feature of the productive part of the lode is the Allouez anticline. The axis of this arch passes near No. 1 Ahmeek shaft; the beds flatten out at the south near the Centennial shafts and at the north between Nos. 3 and 4 Mohawk Within these limits the average divergence from a plane in the position of the Kearsarge bed outside this fold is about 5° for both limbs, or about 9 feet in 100 feet. The steepest dips are at the crest of the anticline, and if the dips continue down the lode at the same angle as at the surface, the anticline must gradually flatten, and at 10,000 feet down the dip it must nearly disappear. The anticline is much less pronounced in the higher beds of the series than it is toward the base.
FAULTS AND FISSURES
By far the most prominent series of fissures strike a little west of north and as a rule dip steeply eastward. Among these are the shatter zone, the Mohawkite, Mass, and Fulton fissures, and half a dozen fissures in the Mohawk mine. Fissures of this series are less abundant to the south but are present throughout the developed area.There has been some movement on many of the fissures. The shatter zone passing through Ahmeek and Allouez has the greatest displacement. It is a belt 200 to 400 feet wide that; contains many fissures. It has been traced through the Ahmeek and Allouez workings, and its persistence to the north and south is indicated by the low Allouez Gap, which doubtless has resulted from erosion of the weakened rocks of this zone. The displacement is about 100 feet, the lode to the north of the zone being that distance to the west of the portion to the south. Both the north and south boundaries of the zone are marked by strong red clay gouges; that to the north is the thicker, measuring from 6 inches to several feet. Associated with the north gouge is a strong prehnite-epidote-copper vein that has been greatly shattered, seemingly by the movement that produced the fissure.
The shatter zone that passes along the boundary between the North Ahmeek and Mohawk mines offsets the lode in the upper workings of the Mohawk, but this offset decreases with depth till in the North Ahmeek it is little more than the width of the lode. This shatter zone is narrower than the Ahmeek-Allouez zone but resembles it closely in other respects. It is considerably wider in the Mohawk than in the North Ahmeek. The fault south of No. 1 Gratiot offsets the lode about 50 feet to the west on the north side.
The other prominent fissures commonly show a little gouge, and in places there is some brecciation of the adjacent rock.. Not uncommonly the fissure zone widens out to 2 to 3 feet.
Another set of fissures strikes approximately east, or nearly at right angles to the north-south fissures, and usually dip steeply south. None of these are prominent, and few can be traced for more than a few hundred feet. There are numerous fissures in the mines that do not seem to fall into any well-developed regional system, but they are commonly small and traceable for only short distances.
ALTERATION AND MINERALIZATION
Oxidation was the earliest alteration of the lode. The oxidation of the Kearsarge and other lodes is fully discussed in the general section of the report (p. 34). The fragmental portions of the Kearsarge lode are highly oxidized and unusually high in ferric oxide. The oxidation is believed to have taken place during the cooling of the lava, earlier than and quite independently of the deposition of copper.
ALTERATION ACCOMPANYING MINERALIZATION
The alteration that took place subsequent to the oxidation may not all have occurred at the same time, but to separate it clearly is not easy. Much of it seems to have been closely associated with the deposition of copper, but certain phases of the alteration seem to have been more widespread than the copper deposition and not necessarily closely tied to it. For example, red feldspar, though present nearly everywhere in the Kearsarge lode, is distinctly variable in amount. In La Salle North Kearsarge, North Ahmeek, and Mohawk ground it is rather abundant, but in South Ahmeek ground there is less. Epidote is also far more widely distributed in the lode than important copper deposits. It may be added that what is true of the Kearsarge lode is also true of other lodes. Some that have very little copper show a more or less pronounced epidotization. This suggests that some of the changes may not have been due primarily to the copper-bearing solutions but probably to the general temperature conditions that accompanied the mineralization.
The mineralization that followed the oxidation of the lode bore a distinct relation to the compositions of the lode after oxidation. Thus epidote, a ferric silicate, is abundant in the more highly oxidized portion of the lode but is much less abundant in the "foot lode" that was less highly oxidized. Chlorite and pumpellyite, ferrous-ferric silicates, are characteristic of the less oxidized "foot lode" and also of the basal amygdaloid of the hangingwall trap, though they are also present in the more highly oxidized portion of the lode.
The change from epidote to chlorite in passing from the Kearsarge lode to the basal amygdaloid of the overlying trap is very sharp. Areas of these rocks but a fraction of an inch apart show the characteristic mineralization of each. In the west lodes, which are much less oxidized than the main lode, pumpellyite is relatively abundant. Minerals that do not contain iron show no such control as indicated above. Thus, quartz and feldspar are distributed throughout the lode, and so is laumontite where present; in fact, so far as recognized, the control is confined to the iron minerals.The most conspicuous and striking change that is closely associated with copper is a pronounced bleaching of the rock around the copper in the upper highly oxidized portion of the lode. In this portion the copper has replaced the rock in masses of diverse sizes, the largest weighing several hundred pounds. Surrounding the copper is an area of light-gray rock in which the hematite of the red lode has largely disappeared. Quartz, pumpellyite, epidote, calcite, and some of the feldspar of the rock are the most abundant minerals. The relation of the copper to these minerals indicates that it is later and that it continued to replace these minerals after they were formed. In the lower part of the lode, especially in the "foot lode," where the rock is much less oxidized, bleaching is less pronounced, epidote decreases in amount, and chlorite becomes abundant. The copper has replaced the rock to a less extent and is more abundant as an amygdule filling.
In general, the Kearsarge lode is richest near the hanging wall; the richest ore is not always, perhaps not usually, at the very top, but the first 3 to 5 feet is the richest part of the lode and grades downward into leaner rock. In the Wolverine mine the "foot lode" was not extensively mined in the earlier years, but it is said to have been in the last few years a rather large factor in the production. In the earlier years the rock mined averaged 25 to 30 pounds to the ton; in the last few years it has averaged 15 to 16 pounds, indicating that the "foot lode" is much lower in copper than the upper part, though some low-grade material from the top part has been mined at the same time. A similar but much less marked change occurred in the South Kearsarge when the "foot lode" began to be more extensively mined.
The copper derived from a given area of lode varies to a considerable extent with the thickness of the lode, though there are notable exceptions. For the larger mines on the Kearsarge as to which data are available, the following table shows the average calculated thickness of lode stoped, the average quantity of rock mined per square foot of lode area, and the average yield of copper.
Rock mined and copper produced per square foot of Kearsarge lode
|Mine||Thickness of lode (feet)||Rock (tons per square foot)||Copper (pounds)|
|Per square foot||Per ton of rock|
|Ahmeek Nos. 1 and 2||10.1||0.84||17.26||22.15|
|Ahmeek Nos. 3 and 4||9.3||0.77|
The relatively large amount of unprofitable ground in the Wolverine, Centennial, and North Kearsarge make the figures for these mines less accurate.
There is a rather regular decrease in the average stoping width from, the South Kearsarge mine northward to the. Mohawk, the average in the latter mine being fully 3 feet less than that in the former. Except in the South Ahmeek (Nos. 1 and 2; there is also a decrease in the copper per square foot of lode. The Wolverine has the highest-grade rock, and in part of the mine the lode is as thick as in the South Kearsarge, so that this part probably has the highest yield per square foot of lode. The grade of rock in the Ahmeek has averaged distinctly higher than that in the South Kearsarge, but the average thickness stoped is 2 feet less, so that the average per square foot of lode is slightly less.
It appears that although the lode is mineralized from the Centennial to the Mohawk, two areas richer than the average have been developed. One includes the South Kearsarge and Wolverine with adjacent parts of the Centennial and North Kearsarge; the other includes the South Ahmeek, South Mohawk, and parts of the Allouez. There are also certain poor areas that in general correspond to the areas of thin and cellular lode already outlined and associated with the strong fissures and fissure zones. (See pl. 40.)
PROBABLE CAUSES OF RICH AND POOR GROUND
Three principal causes seem to have been operative in determining the richness of the ground-character of rock, structural relations, and relation to strong fissures. Of these the character of rock and relation to strong fissures are the more conspicuous.
CHARACTER OF ROCK
Within the productive portion of the Kearsarge lode the thin parts of the lode and those consisting of dense, trappy amygdaloid are consistently poor. In the larger areas where the lode is poor it is thin or tight and relatively impermeable. The richest ore has been formed in thick masses of strongly developed fragmental top. The thick parts of the lode are not all rich, nor are parts of apparently equal thickness similarly rich, but rich parts of the lode are always thick or loose and fragmental in texture. Outside of the main productive area there are, so far as available data indicate, no large areas of thick fragmental top, most of the outside exploration having disclosed only thin or cellular amygdaloid.The parts of the lode that are above the average grade in the Ahmeek mine and the south end of the Mohawk are decidedly of the fragmental and well oxidized type. The same is apparently true of the Wolverine-South Kearsarge shoot. In the North it had the same cross section as the central portion of the productive part of the lode but at a lower stratigraphic horizon is a body of intrusive felsite. This is another factor which is not regarded as essential but which may be favorable.
EFFECT OF DEPTH ON THE COPPER CONTENT OF THE LODE
The study of the Kearsarge lode has shown no evidence of leaching of copper near the surface and reprecipitation at greater depth. Where the lode is poor at the. surface, as in parts of the Wolverine and North Kearsarge mines, the rock is impervious and unfavorable, and there is no indication that it was ever well mineralized. Where similar rock is encountered in the deeper workings, as in the lower part of the Allouez and in part of the North Ahmeek, the low copper content is encountered.Where favorable lode rock reaches the outcrop, as in the south end of the Mohawk mine, it is well mineralized, and the same is true of favorable rock at greater depth.
If the mineralizing solutions in the main were traveling up the lode, the lode in general must continue permeable down to the connection with the source of the solutions, otherwise they would not have found their way to the level now developed. That mining will disclose areas of unfavorable rock and variation in the copper content at greater depth is to be expected from the known conditions in the developed areas. The encountering of an area of unfavorable rock should not be regarded as discouraging for the lode as a whole, though it may be so for an individual property. Poor ground, due both to character of rock and to the influence of fissures, is likely to be found here and there at greater depth as it has been to the present depth.
It is not intended to imply that a general decrease in the copper content of the Kearsarge lode will not be found at depth, but there is no reason to think that it will be other than a gradual falling off for the lode as a whole. So far as known, the causes that resulted in a decrease in the grade of the rock on the Calumet & Hecla conglomerate lode - namely, an increase in the thickness and extent of the lode with increased depth - will probably not affect the Kearsarge lode. The conditions are more likely to approach those of the Quincy lode, where a depth nearly as great as on the Calumet & Hecla conglomerate has been attained with no notable decrease in copper content.
MAYFLOWER-OLD COLONY MINE
The Mayflower-Old Colony Co. was formed by a consolidation of the two properties indicated in the name. The property covers the portion of the copper-bearing series extending from a horizon a short distance below the Wolverine sandstone to the Keweenawfault. It lies east of the Calumet & Hecla (Centennial and South Kearsarge) and Mohawk (Wolverine) ground.
The rocks conform to the general strike and dip of the beds in this part of the range down to about the horizon of the "St. Louis" conglomerate, where the normal structure is interrupted by the Mayflowerfault. This fault is apparently a branch of the. Keweenaw fault, which it seems to join's short distance south of the Old Colony tunnel. The Keweenaw anal Mayflower faults diverge from a point south of the Old Colony tunnel (see pl. 8) to a maximum known separation of about half a mile in the north end of the Mayflower-Old Colony property. At the Old Colony tunnel the Mayflower fault is about 700 feet east of and several hundred feet stratigraphically below the "St. Louis" conglomerate. To the north the fault approaches the "St. Louis" conglomerate, and in the north end of the property near the surface it cuts out that conglomerate and some of the overlying "Big" trap. The dip of the fault is slightly steeper than that of the beds, and at depth the "St. Louis" conglomerate is present in the north end of the property, as indicated by diamond drilling. The continuation of this fault north of the area drilled can only be inferred. The "St. Louis" conglomerate crops out about half a mile north of the northern drill holes, and it is pretty certain that the fault lies east of the conglomerate and very probably connects with the area of felsite in that region (see pl. 8), with which, indeed, it may be associated in origin.
In the block between the Keweenaw and Mayflower faults the rocks are broken and displaced by minor faults of diverse attitude, but in general the beds are horizontal or have a gentle eastward dip - the reverse of that normal for the formation west of the faults. The block contains many beds of melaphyre and glomeroporphyrite, with some rather thick ophitic beds and two persistent beds of conglomerate. It is not positively known where these rocks belong in the general series, but there can be little doubt that they are higher in the series than the rocks adjacent on the opposite side of the Mayflower fault. Similar traps are present in the series above the "Big" trap, but in the nearest sections observed there are no conglomerates above the "Big" trap that correspond with those below the fault. There are, however, sedimentary beds in the series both to the south and north that might have developed into conglomerates. Rocks similar to the series of melaphyres and glomeroporphyrites immediately above the "Big" trap do not again appear till. the Greenstone flow has been passed, and it seems more probable that the rocks in the fault block correspond to the series above the "Big "trap than to the series above the Greenstone flow.
The earlier development of the Old Colony or southern portion of the property consisted of several shafts and a tunnel. The Old Colony tunnel starts near the Keweenaw fault and was driven across the series a distance of about 2,500 feet, or to the fourth flow above the "Big" trap. The No. 1 and No. 2 shafts were on amygdaloids about midway between the Kearsarge amygdaloid and the "Big" trap. No. 2 was on a bed a little higher in the series than No. 1.No very clear record is available of the results of these earlier operations, though so far as known no copper was produced. On the dumps of No. 1 and No. 2 shafts there is some fragmental lode rock with a little copper. The pump shaft is on an amygdaloid a few hundred feet lower than No. 1 shaft. The North shaft, in the northern part of the Old Colony property, was in the area between the Keweenaw and Mayflower faults. The Haddy shaft opened two amygdaloids a little higher in the series than the No. 2 Old Colony shaft. The several operations apparently failed to open a lode that gave much encouragement.
About 1910 an extensive drilling campaign was undertaken by both the Mayflower and Old Colony companies to prospect the lower portion of the copper bearing series on their properties. After this tract had been more intensively drilled than any other area in the Copper Range, the two companies were merged, and the present No. 1 shaft was sunk to a depth of 1,760 feet to prospect the Mayflower lode. The shaft starts in the "Big" trap, but the prospecting has been in the rocks below the Mayflower fault, on the 1,450 and 1,700 foot levels. The Mayflower amygdaloid occurs above two well-defined conglomerates, which have been a great help in working out the structure of this much-faulted block of ground. At the end of 1924 a crosscut was being run N. 70° W from the seventeenth level, No. 1 shaft, to prospect above the "Big" trap at the horizon of the "St. Louis" amygdaloid.
6 Ojibway Mining Co., Repts., 1909-1912,
7 Lake Superior: Mining Inst Proc. vol. 17, p. 234, 1912.
8 Franklin Mining Co. Rept. for 1905, p.12.
9 Idem for 1916, p8.
10 Marvine, A. R. Michigan Geol. Survey, vo1. 1, pt. 2, p. 61, 1873.
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