USGS Professional Paper 144 pp 175-177
The Ashbed amygdaloid (pl. 34) has been prospected from the Atlantic mine, south of Portage Lake, nearly to the end of Keweenaw Point. Whether the developments on this stretch are on the same bed is not known, but they are at the same general horizon, a little below No. 17 conglomerate. The most extensive developments have been made at the Copper Falls, Arnold, and Phoenix mines, in Keweenaw County, and the Atlantic mine, south of Portage Lake.
The following table shows the production from the Ashbed amygdaloid to the end of 1925:
Production and dividends from Ashbed lode to end of 1925
|Mine||Period||Rock treated (tons)||Copper produced (pounds)||Dividends|
|Total||Per ton||Total||Per pound (cent)|
CHARACTER OF ROCK
The Ashbed flows are toward the andesitic end of the basaltic series and are everywhere porphyritic, containing rather abundant small feldspar phenocrysts, which are usually collected in nests.
The amygdaloid is prevailingly of the scoriaceous type and is ordinarily cited as the typical example of scoriaceous amygdaloid. There is, however, a notable variation in the amount of the elastic material in the Ashbed at different places. At the Atlantic mine, so far as indicated by the dump material, it is distinctly of the scoriaceous type with pebbles and boulders of amygdaloid in a sandy matrix. At the Phoenix mine the upper part, or "gray" bed, is of the same character, but the lower part, or "red" bed, is not described as scoriaceous. At the Copper Falls mine the bed is locally sandy and approaches the scoriaceous type, but for the most part it is rather typically fragmental. At most of the places in Keweenaw County where it has been cut by the diamond drill the lode is scoriaceous. At the Copper Falls mine it is well oxidized, and at the Atlantic it is said to be chocolate-colored. The Ashbed seems, therefore, to be a fairly well oxidized amygdaloid.
The mineralization of the Ashbed does not seem to differ materially from that of other lodes. The more abundant minerals at the Copper Falls mine are calcite, quartz, epidote, and pumpellyite. Near the fissures datolite is apparently abundant, and in Keweenaw County it appears to be generally more abundant on the Ashbed than on the other lodes.
At the Phoenix mine the basal portion of the lode is chloritized. Not enough of the Atlantic lode has been seen to give a very clear idea of the mineralization. At the Copper Falls mine much of the bleaching associated with copper is of the iron-removal type.
At the Copper Falls mine the upper part of the lode seems to contain more copper than the deeper part, and there is some copper in the basal amygdaloid of the overlying flow. At the Phoenix mine the copper seems to occur throughout the "gray" lode, but the top of the "red" lode is richest. At several places in Keweenaw County where the lode has been cut by the diamond drill the copper seems to be in amygdules in the lower trappy part of the lode.
COPPER FALLS MINE
At the Copper Falls mine of the Arnold Mining Co. the Ashbed had been opened in 1891 to a depth of about 1,500 feet and had been opened and stoped for about 2,400 feet along the strike. The workings extend about 1,000 feet east and 1,400 feet west of the Owl Creek fissure, Work continued for a year or so after 1891, but the extent of the latest openings is unknown. The Ashbed was also opened adjacent to the Petherick fissure but to what extent is not known.
The Ashbed lode as seen in the upper levels of the Copper Falls mine is of the fragmental type of amygdaloid with some sandy material and is well oxidized. The width stoped varies greatly but probably averages 6 to 8 feet. The lode seems richest near the top.
A report by Wadsworth states that the lode averaged 10 to 12 feet in thickness and yielded about 1 per cent of copper. The yield in 1859 is given as 22.8 pounds to the ton. There is no record of the output from the Ashbed separate from the output from the Owl Creek fissure, which was operated at the same time, but the production from the Ashbed was certainly considerable.
The relation between the Owl Creek and other fissures and the mineralization of the Ashbed is not very clear, but the mineralization seems to have been strongest near this fissure.
At the Arnold mine the lode has been opened by No. 1 shaft to the eighth level. On the fifth level a drift has been carried 210 feet to a fissure, which was opened for a short distance. The drift has been extended also about 300 feet south, to a fissure which was opened for about 400 feet. The levels above the fifth were not carried far from the shaft.
The Phoenix mine of the Keweenaw Copper Co., on the Ashbed lode, was not operating when this report was, in preparation. The following notes are taken from the company records and statements by Mr. C. A. Wright, superintendent.
The hanging wall is marked by a clay slip that dips and strikes with the formation. This possibly represents No. 17 conglomerate. Below the clay slip is the amygdaloid of a trap averaging about 15 feet in thickness. Next is the "gray" lode, with a slip at the top. The "gray" lode is described as an amygdaloidal trap that grades downward into dense trap. In places there is sandstone at the top of the "gray" lode, ranging from a fraction of an inch to several feet in thickness; the lode averages 6 to 7 feet. The trap of the "gray" lode is at some places 15 feet thick, but at other places it appears to be, absent. Next is the "red" lode, described as amygdaloidal conglomerate with no mention of sandy material. The thickness of the "red" lode averages 15 to 20 feet but is variable. This suggests that the "red" lode here, as at the Copper Falls mine, may be more a fragmental than a scoriaceous amygdaloid. The trap of the "red" lode is about 60 feet thick. The next amygdaloid is described as narrow, and reddish gray; it is mineralized in spots but seems very " bunchy."
The Ashbed is crossed in the Phoenix workings by the Phoenix and Armstrong fissures, and near the surface a third fissure was opened. A fault west of the Phoenix fissure offsets the beds to the east of the fissure about 60 feet to the north.
The Ashbed has been opened at the Phoenix mine to a depth on the dip of 1,600 feet and along the strike for about 2,500 feet. Most of the development has been on the "gray" lode, and most of the production has apparently been from this lode. The copper in the "gray" lode is said to be fine.
From October 13, 1916, to December 31, 1917, 67,215 tons of rock was hoisted, of which 18,993 tons was discarded; 48,063 tons stamped yielded 449,416 pounds of copper, a recovery of 9.35 pounds to the ton, with a loss in tailings of 5.65 pounds to the ton. This can probably be taken as representing the grade of the "gray ".lode.
The "red" lode has also been opened in several places, at most of which it is reported to show an encouraging amount of copper near the top of the lode, in a zone as much as 4 feet in thickness. It is said that the copper is coarser than that in the "gray" lode. Some mineralization is reported to have occurred also near the base of the lode. Apparently not enough stoping has been done on the "red" lode to give a clear idea of the copper content. The lode close to the fissures is said to be poor, but there is some suggestion that it may be better in the general vicinity of the fissures than it is away from them.
The Ashbed was also opened at Garden City, east of the Phoenix workings. Old reports indicate that amygdaloids similar to those at the Phoenix were present and that the copper content was encouraging. The production, however, was small. The "red" lode appears to have been the one most worked at Garden City.
The Ashbed has been opened at the Hancock mine but is described with the other lodes of that mine on pages 177-178.
At the Atlantic mine the Ashbed lode has been opened for about 5,800 feet along the strike and dowel to the thirty-fifth level. (See pl. 34.) Here the lode was of the scoriaceous type, was soft, averaged 12 feet or more in thickness, and seems to have been rather uniform in character and in mineralization in the central part of the mine but became poorer both to the north and to the south. The copper is said to have been rather uniformly distributed through the lode, and the developed ground has been nearly all stoped.
The only structural feature of importance at this mine is a strong fissure or "crossing" with a steep northerly dip extending through the mine just north of No. 2 shaft. The filling of this fissure, as judged by descriptions and some material on the dump, was a coarsely crystalline calcite, in places at least with a pinkish tinge.
It is said that in the upper levels the lode near the fissure was poor, and a rather wide pillar was left. In the lower levels the pillar left was much narrower. In the early years of operation of the Atlantic Mining Co. the rock averaged 18 to 20 pounds of copper to the ton, but for much of the productive period it did not exceed 15 pounds and dropped almost to 11 pounds. For the last years of operation it was as follows: 1902, 11.09 pounds; 1903, 12.78; 1904, 13.63; 1905, 13.72; 1906, 14.69. When the mine was closed by caving in 1906 it was reported that the central portion in the deeper levels shows no indication of a decrease in the grade of ore. The profitable operation of the Atlantic mine on a lode that averaged so low was due partly to the rather uniform mineralization, which made mining cheap, and partly to the softness of the lode, which was favorable to cheap mining and milling.
5 Longyear C. S., Michigan Geol. Survey Pub. 24, ser. 20, 1916.
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