USGS Professional Paper 144 p 178-181
PEWABIC AMYGDALOID LODES
LOCATION AND PRODUCTION
The Pewabic amygdaloid lodes (pl. 36) have been mined mainly in the properties that now form the Quincy mine. The present Quincy mine (see pl. 74, A) includes the Old Quincy, Pewabic, Franklin, Mesnard, Pontiac, and others. Outside the Quincy mine the lodes have been mined in the Hancock mine, adjoining the Quincy on the south and in the Franklin Junior mine to the north. To the south the rocks at this horizon were extensively explored by the Naumkeag Copper Co. and to the north they were opened at the Rhode Island mine and have been cut by the vertical shafts at Calumet but only slightly explored.
The following table shows the production of the Pewabic amygdaloid lodes from the beginning of operations to 1925:
Production and dividends from Pewabic amygdaloid lode, 1855-1925
|Mine||Period||Rock treated (tons)||Copper produced (pounds)||Dividends|
|Total||Per ton||Total||Per pound (cent)|
|Albany & Boston||1862-1864||14,981|
|Franklin and Franklin Junior||1883-1919||a3,888,462||a80,186,631||a20.62|
b Much of the production from the Hancock mine for 1911 to 1918, amounting to 12,199,000 pounds of copper, was derived from the Pewabic lodes.
CHARACTER OF FLOWS
The Pewabic amygdaloid lodes at the Quincy mine consist of a group of relatively thin flows. These overlap so that it is possible to follow amygdaloid continuously and still pass from the amygdaloid of one flow to that of another. It is difficult to correlate the flows certainly from one point to another unless the top of the flow has been actually followed, as one flow may give place to another within a few hundreds or, in the thinner flows, a few tens of feet.
In composition the Pewabic flows fall toward the andesitic end of the basaltic Keweenawan series. Texturally they are glomeroporphyrites and feldspathic melaphyres, with ophitic texture in some of the thicker beds, such as the one below the Pewabic Far West lodes in the lower 50 levels at No. 2 shaft. All or nearly all the beds are porphyritic, containing well-developed feldspar phenocrysts.
CHARACTER OF AMYGDALOIDS
The amygdaloids of all the flows of the Pewabic lodes are characteristically of the coalescing type, though different lodes vary in the degree to which this character is developed, and there may also be a decided difference in its development in different parts of a single flow. Where it is well developed them are from 2 or 3 to as many as 8 or 10 cavernous bands or layers in a lode from 3 to 5 feet thick. It is not uncommon to see these bands continuous along a cross section of the lode for 10 to 15 feet or even more, and in the plane of the lode such layers must form connected openings for tens and probably for hundreds of feet. There is every gradation from the well-banded or layered lode to that in which the vesicles show only a moderate tendency to collect in layers and do not form continuous openings.
Fragmental tops are not characteristic of the Pewabic lodes as a whole. A little fragmental rock occurs at the top of the amygdaloids in many places, and over considerable areas the lode is typically fragmental, though more uniform in thickness than most of the fragmental lodes. A large area of this character is present in the south end of the mine, extending from the seventy-sixth to the eighty-first level and covering an area of 400,000 to 500,000 square feet. A smaller area, so far as opened, is present in the bottom (eighty-first to eighty-fifth level) near No. 2 shaft. So far as observed in the lower levels, the East branches of the series show more fragmental amygdaloid than the West branches. The upper and intermediate levels were seen in only a few places, but it is stated in old descriptions and by those familiar with the upper levels that the "main" lode was a thick, soft, chocolate-colored lode. Some of it was seen on the twenty-second level between No. 2 and No. 6 shafts, where for the most part it is a typical fragmental lode, showing all the characteristics of that type. The lode thins and thickens from place to place with bulges into the hanging wall and the footwall. Where the lode is thin it tends to become cellular and coalescing.
On the twenty-seventh level between No. 2 and No. 7 shafts the character of the lode alternates between rather tight cellular, tending to coalescing, broken in places into large blocks, and fairly well developed fragmental. The fragmental portions have been stoped and doubtless were considerably mineralized. From observations in and descriptions of the lode in the upper levels it seems probable that large areas of the "main" lode above, say, the thirty-fifth level were fragmental. Some of the other lodes where seen in the upper levels are as distinctly coalescing as any on the lower levels.
The oxidation of the coalescing lodes is less than that of the fragmental lodes, though the tops are distinctly reddened. The fragmental portions of the lodes are moderately well oxidized, though not so highly as many fragmental lodes elsewhere.
The mineralization associated with the deposition of copper in these lodes was simple. Silicification of the lode is pronounced, and quartz is abundant, commonly as well-formed crystals in the open cavities.
Calcite is also nearly everywhere present and rather abundant. Pumpellyite is present as a product of rock alteration and also in open cavities, and epidote is common but less abundant than pumpellyite. Chlorite is abundant in amygdules in the base of the flows and throughout the trap part of some of the flows. Near fissures the chlorite may be replaced by the light-colored minerals, quartz or calcite, which gives the rock adjacent to the fissures a much more amyg dular appearance. Zeolites are sparsely represented. Laumontite is present in the Main Spar crossing and was noted in other small fissures. In the lode it is rarely seen on the lower levels but is rather abundant for a considerable distance from the Spar crossing on the upper levels. No other zeolites were noted. Prehnite is present but not common. Nodular masses of porcelanic datolite are said to have been of common occurrence in the upper levels of the mine, but no datolite was seen in the lower levels.
The bleaching and alteration of the rock is characteristically of the quartz-pumpellyite type common in the coalescing lodes. In the well-oxidized fragmental portions, especially where seen in the upper levels, the quartz-pumpellyite alteration is less conspicuous and the bleaching is due more to the removal of iron, as in the Kearsage and Osceola lodes.
Before the mineralization, when the cavities were empty, the well-developed coalescing amygdaloid was much more permeable than the cellular amygdaloid and would permit the passage of much more of the mineralizing solutions than the cellular type. It is evidently for this reason that the richer copper ground is in the well-developed coalescing lodes, and although these are not everywhere rich, the poorly developed coalescing and cellular lodes are pretty consistently poor.
Considered in more detail, the coarse copper and masses very commonly are associated with strongly developed bands in an amygdaloid, though the copper and associated minerals generally replaced the rock adjacent to the openings. The finer copper also is as a rule closely associated with well-developed bands, though it may extend some distance from them; and some copper is of course found in cellular amygdaloid and even in amygdules in the trappy portion of the lode.
Fragmental lode rock is of course distinctly permeable and readily replaceable and therefore favorable to mineralization. The masses of such rock exposed in the bottom of No. 2 shaft are pretty consistently rich. The fragmental amygdaloid in The "main" lode in the upper levels of the mine was evidently well mineralized, and in places it was rich. Smaller areas of fragmental lode and areas where a foot or so of the top of the lode is fragmental are commonly good to rich ground.
It is pretty clear, therefore, that a fragmental lode in this series can be as favorable and possibly more favorable than a well-developed coalescing lode.
There is some minor fissuring both parallel to and across the lodes. The fissures are commonly mineralized and evidently were favorable to the movement of solution.
MINERALIZATION IN DIFFERENT LODES
The Pewabic lodes are separated into three groups by the Quincy Mining Co. for the purpose of the underground mapping. The grouping is based on position, and the three groups are called the East lodes, the West lodes (including the "main" lode), and the Far West lodes. The East lodes are east of No. 2 shaft in the upper levels. The shaft was in the "main" lode, which apparently corresponds to the West lodes in the lower levels. The Far West lodes are stratigraphically above or west of the West lodes.
The East lodes are the most persistent and regular in the lower and middle levels. They consist of a lower or foot branch and an upper or west branch. The flow whose top forms the lower branch lode rests on the Old Pewabic flow. It is usually not more than 30 to 40 feet thick. It consists in places of one flow, and in places apparently of more than one. The upper flow is even thinner; in places only 10 feet of trap, or even less, separates the two branches, and it is possible that the upper flow is not everywhere present.
In the south end of the mine the upper branch of the lode has been most extensively opened. Stoping on this branch extended from the tenth level, between No. 7 and No. 2 shafts, to the bottom of the mine. To the north there is little work on this branch to the twenty-seventh level in No. 6 shaft, and somewhat lower in No. 8. Below this to about the sixty-fifth level in No. 8 and No. 6 shafts not more than 50 per cent of the lode has been taken. From about the sixty-fifth level to the bottom in the north end more of the upper branch has been taken, and in the south end it has been very largely stoped.
The lower branch was mined but little to about the fortieth level. From the fortieth to about the sixty-fifth level it was extensively mined north of No. 6 shaft, and considerable was taken in the south end of the mine. Below the sixty-fifth level this branch has been mined very little in the south end but some has been taken between No. 6 and No. 8 and considerable north of No. 8.
Above the sixty-third level both branches have in places been mined in the same area, though not as a rule. Below this level in the north end of the mine both have been mined. This difference apparently is due in part to a change in mining method rather than to a sudden change in the character of the lodes.
Down to about the thirtieth level the "main" branch was the one most extensively mined. This is above the East lodes and, apparently corresponds to the West lodes in the lower levels. In the lower levels it has been but slightly developed to the north of No. 8 shaft and to the south of No. 7 but has been rather extensively mined from No. 6 and No. 2 in the central part.
There is more than one lode in this West group. What is known as the hanging-wall branch has been most extensively mined. These branches in the lower levels do not seem to have the long stretches of continuous favorable rock that are present in the East lodes, but where favorable rock is present it is very well mineralized.
PEWABIC FAR WEST LODES
The Pewabic Far West lodes have been developed principally below the fiftieth level near No. 2 and No. 6 shafts. In the upper part of the developed area at No. 2 shaft they are separated from the West lodes by an ophitic trap, which is as much as 130 feet thick in this part of the mine, but apparently thins very much down the dip, up the dip, and toward No. 6 shaft. The Far West lodes are above this trap, in the coalescing amygdaloidal tops of several small flows. So far as opened, these tops appear to have shorter stretches of favorable amygdaloid than the East or West lodes, but where their character is favorable they are well mineralized.
In the preceding paragraphs the changes in the lodes in different parts of the mine have been noted in some detail to bring out the idea that in a series of lodes like the Pewabic too much emphasis should not be placed on local changes in character of amygdaloid and copper content of individual lodes in their effect on the favorability of the series as a whole. It is apparent, so far as the present extensive developments show, that some of the lodes were richer in the upper levels, some in the intermediate levels, and some in the lower levels. Some were most extensively mined in the north end of the developed area and some in the south end. The series as a whole appears to have been about equally productive in the upper and in the lower levels of the mine.
FISSURES OR "CROSSINGS"
A strong cross fissure extends through the nine, dipping north at a high angle (see pl. 36) and crossing No. 6 shaft at the fifty-fifth level. The strike of the fissure, as indicated in the fifty-first level footwall crosscut, is about 10° from right angles with the strike of the lode. This crosscut intersects the Allouez conglomerate and in fact all the lodes cut by it only a short distance from the fissure. Where it crossed the "main." lode in the upper levels little stoping was done adjacent to this fissure, and the lode was evidently poor. This condition continued to about the fiftieth level. Below that stoping was carried close to the fissure, which is said to have had little effect on the copper content of the lode for more than a few feet on each side, and in places good ore is present against the fissure. Where seen in the lower levels the fissure has displaced the lode very little, though the rock is broken for several feet. The fissure filling consists mainly of calcite, laumontite, and quartz. Chloritization and pumpellyitization of the wall rock are conspicuous in and near the fissure but extend only a short distance. Where the "Spar crossing" was seen on the upper levels, laumontite is abundant in the lodes for a considerable distance from the fissure. Pink calcite is the most abundant mineral in the fissure; quartz and epidote are present locally. Copper is present in the fissure wherever examined, and at one place a good-sized mass was seen. The copper content of the fissure has not been sufficient to encourage work on it.
Two other crossings are noted on the mine maps; one of these passes between No. 6 and No. 8 shafts and is about parallel to the main crossing; another crosses No. 8 shaft at the twelfth level and dips about with the north boundary of the ore shoot. Neither of these were seen. The first seems to have had no notable effect on the grade of the lode adjacent to it. The second is mainly outside the stoped area.
The only fault of note that affects the Pewabic lodes is the Hancock fault, which cuts the lodes in the south end of the Quincy mine and in the Hancock mine. This fault is represented in the cross section of No. 7 shaft as crossing the shaft it the fifteenth level and reaching the surface in the hanging-wall side of the shaft. These two points give the fault a dip of about 75°. At the fifteenth level it is marked on the map as having a dip of 76° 30' and a strike of S. 17° 48' W. for a short distance. The fault is more fully described in connection with the lodes of the Hancock mine (p. 177) and shown in the cross section of that Mine.
The Allouez ("Albany & Boston") conglomerate, as represented in the old adit, is offset approximately 600 feet on the level. The Pewabic lode on the sixteenth level is indicated as having similar offset.
FRANKLIN JR. MINE
The production from the Pewabic lode and that from the conglomerate lode in the Franklin mine were not separately recorded for part of the time. After the closing of the conglomerate lode in 1909 the Pewabic lode produced about 16,000,000 pounds of copper. During this period the yield was about 9 to 12 pounds of copper to the ton of rock.
The writers have had no opportunity to examine the lode in the mine.
The lode has been opened by four shafts, but most of the operations have been conducted from the No. 1 shaft, which extends to the thirty-seventh level. No. 3 shaft goes to the fifteenth level. The lode has been opened for about 3,800 feet along the strike, but most of the development has been within 1,00 feet north and 1,000 feet south of No. 1 shaft.
In the upper levels most of the ground stoped was north of No. 1 shaft; in the lower levels the productive ground extended south of the shaft, suggesting a southward pitch of the south boundary of the ore shoot. The ground was stoped over a longer stretch in the lower levels than in the upper, and between the upper and lower levels there was an area in which comparatively little of the lode was stoped.
RHODE ISLAND MINE
The Pewabic amygdaloid has been opened at the Rhode Island mine by two shafts - No. 1 to the fourth level, No. 2 to the tenth level. The openings extend about 2,700 feet along the strike for about equal distances north and south of No. 2 shaft. A few tons of mass and barrel copper has been shipped from the Rhode Island mine. There has been no opportunity for the writers to examine the lode in the mine.
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