Butler USGS PP124 pp xi-xii
GEOLOGY
General features. - In Keweenawan time a series of basaltic flows accumulated to a depth of thousands of feet. Interbedded with the flows are felsite conglomerates. In the lower part of the series the conglomerates are rather abundant and thick; in the middle part they are relatively few and thin but persist for long distances; in the upper part they make up the bulk of the rock.
Intruded into and probably also interbedded with the basalts and conglomerates are siliceous, felsitic, and porphyritic rocks of similar composition to the material that makes up the conglomerates.
The location of the fissures from which the igneous material, both extrusive and intrusive, presumably came is unknown, but there are some reasons for supposing that they were situated toward the center of the Lake Superior structural basin, under the present lake, and that the flow outward from them was in a direction opposite to the present dip of the beds. Probably the felsite bodies from which the conglomerates were derived lay in the same direction as the fissures, and the material was carried down the same slopes as the lavas. Both the flows and the sediments are believed to have accumulated on a land surface, though probably some, especially of the later sediments, were laid down in bodies of water.
The flows range in thickness from a few feet to 1,300 feet and in extent along the strike from a few hundred feet to at least 40 miles. Nearly all of them are basaltic, but there are differences in composition, expressed in textural differences, which make it possible to divide the flows into several general groups.The intrusive rocks are mainly siliceous felsites or porphyries but there are some, like those of Mount Bohemia and others toward the Michigan-Wisconsin boundary, that approach the Duluth gabbro in composition and texture, and these and the felsite also were probably derived from the same source as that rock.
Lava tops or amygdaloids. - The dense rock that forms the greater part of most of the thick flows is commonly called "trap"; it is everywhere overlain by more open-textured material., which is known as "amygdaloid." The cellular tops were formed by the action of gas which was contained in the lava and was liberated during the progress of flow and consolidation. Part of this gas escaped from the surface, and part was caught as bubbles in the viscous top of the lava and thus formed a cellular mass which was later converted into amygdaloid by the filling of the vesicles. Several types of top are recognized. The most common is the cellular top, which was produced by the simple freezing in of individual bubbles of gas. Coalescing amygdaloid has resulted when many bubbles collected into an irregular layer of gas separated by material in which cavities were relatively few. This gave long connected passages in the tops. Fragmental top is due to the breaking up of the cellular top, during solidification and flow, into angular fragments which became so jumbled as to form an irregular hummocky surface. Where the fragments are piled above the general surface they also sink into the flow, so that lava top of this kind varies abruptly and irregularly in thickness. Scoriaceous top results from a working over of any of the other types by the process of erosion and sedimentation. It very commonly underlies felsite conglomerate but occurs also where there is little or no felsite sediment.From comparison with other regions it is thought that the different types have resulted from different conditions of the lava as it reached the surface. Lava that issued at a high temperature and had a low gas content produced cellular top. Lava that issued at a low temperature and had a high gas content produced fragmental top. Coalescing top was formed on lava intermediate between the other two.
Oxidation of lavas.-The tops of nearly all the flows are distinctly red, and the fragmental tops are decidedly red. Chemical analyses show that there is in general a steady decrease in the proportion of ferric iron and an increase in ferrous iron from the top of a flow nearly to the bottom. In the fragmental flows there is also more total iron in the top - as much as 40 per cent more than in the compact portion of the flow.
It is thought that the oxidation and concentration of the iron were accomplished in large part by the gases given off by the lava during solidification. The evidence indicates that at the temperature at which the lavas emerged the inclosed gases were either neutral or reducing in their action on ferric oxide, but as they cooled in their ascent through the flow they became strongly oxidizing toward ferrous oxide.
Copper in trap. - An examination of the freshest traps confirms previous observations that they contain copper in small amount. Both native copper and chalcopyrite are present. It seems probable that at least a part of the copper is a primary constituent of the traps.Structure.-The Michigan copper region is on the southern rim of the Lake Superior syncline or basin, which was probably formed during Keweenawan time. The early Keweenawan rocks of Keweenaw Point dip steeply and the later ones progressively less steeply northwestward, toward the center of the begin. Transverse to the general strike of the Lake Superior syncline are anticlines and synclines that pitch down the dip of the larger fold; among the folds are the Keweenaw anticline, the Ontonagon syncline, and the Bessemer anticline. On these broader anticlines and synclines are several subordinate folds of similar trend, such as the Allouez anticline, the Isle Royale syncline, and the Baltic and Mass anticlines.
The greatest fault of the region is the Keweenaw fault, which
bounds the copper-bearing series on the south from the end of
Keweenaw Point to Lake Gogebic. This is a reverse fault of
northwesterly dip, along which the basaltic series has been thrust
over the "Eastern" (Cambrian) sandstone. The dip of the fault is
nearly parallel to the flows, and it also follows the major anticlinal
and synclinal structure of the rocks. Many branch faults and fissures
are associated with the Keweenaw fault. Relatively small transverse
faults and fissures are also present around. the crests of the anticlines.
The movement on the Keweenaw fault probably did not begin till late
Keweenawan time, and much of it occurred after the "Eastern" sandstone was
deposited. The transverse folding and faulting
probably followed the Keweenaw faulting and preceded the period of
mineralization.
ORE DEPOSITS
The copper deposits are of two main classes - lode deposits and fissure deposits.
1 The lode deposits consist of conglomerate lodes, which are mineralized beds of felsite conglomerate interbedded with the lava flows, and amygdaloid lodes, which are the mineralized vesicular or brecciated tops of the lava flows. The fissure deposits are veins along fractures that are in part parallel and in part transverse to the beds; all of them are of narrow tabular form. The commercially important known deposits with one exception are confined to the portion of the Keweenawan series composed predominantly of lava flows. They are distributed through most of that portion of the series. The more productive lodes, from the base upward, are the Baltic amygdaloid, Isle Royale amygdaloid, Kearsarge amygdaloid, Osceola amygdaloid, Calumet & Hecla conglomerate, Allouez conglomerate, Pewabic amygdaloid, and Ashbed amygdaloid. The Nonesuch lode, named for the formation that contains it, is in the upper sedimentary portion of the series. The fissure deposits are in the same portion of the series as most of the lodes.Conglomerate lodes. - Only two of the conglomerate beds have been extensively explored, the Calumet & Hecla and the Allouez. The Calumet & Hecla conglomerate over most of its known extent is a sandy or "scoriaceous" bed with a little felsite sand at the top. At Calumet it opens into a well-developed felsite conglomerate lens that increases in thickness and extent with increased depth. The Allouez conglomerate is thicker and more persistent than the Calumet & Hecla but in places is represented only by a clay seam. The valuable mineralized portion of the Calumet & Hecla conglomerate is confined to the conglomerate lens, and the copper content decreases abruptly where the conglomerate changes to sand. The copper occurs as native metal and mainly replaces the finer matrix of the conglomerate. The common minerals associated with the copper are feldspar and epidote, mainly deposited earlier than the copper, and quartz and calcite, contemporaneous with the copper. Zeolites are conspicuously absent. The characteristic rock alteration effected by the ore-bearing solutions was the removal of hematite, with a resultant bleaching of the lode from brown to pinkish.
Amygdaloid lodes. - All but two of the largely productive amygdaloid lodes, as well as most of those that have given some encouragement, are of the fragmental type. One, the Pewabic, is mainly of the coalescing type, though in part it is fragmental, and the Ashbed is mainly a "scoriaceous" lode, though locally fragmental. No mines have been found in the cellular amygdaloids, which are by far the most abundant.
A greater variety of minerals is associated with the copper in the amygdaloid lodes than in the conglomerates. Chlorite, feldspar, epidote, and pumpellyite are the abundant minerals that mainly preceded the copper in deposition. Abundant quartz and calcite and some prehnite and datolite were locally deposited with the copper; the zeolites laumontite and analcite, together with saponite and other minerals in small amount, were deposited later than the copper. Sericite was deposited apparently both with and later than the copper.
The rock alteration effected during the formation of the amygdaloid lodes was of two types - a removal of hematite similar to that in the conglomerate, and a partial removal of the iron of hematite and a partial chemical reduction of ferric oxide to ferrous oxide and recombination to form ferrous and ferric compounds such as pumpellyite, chlorite, and epidote.
Ore shoots. - The minable copper occurs in shoots, of which many are large but all far less extensive than the lodes that contain them. The ore of the shoots was deposited in the more permeable parts of the lodes by solutions whose movement
was directed by barriers of relatively impermeable material. Two types of environment have most commonly favored the formation of shoots - a "bed" conglomerate or amygdaloid, that is prevailingly impermeable but contains permeable portions of great downward extension, like the Calumet & Hecla conglomerate shoot and the Kearsarge amygdaloid shoot, and a "bed," such as the Osceola amygdaloid, that is prevailingly permeable but contains impermeable streaks which cause a concentration of solutions beneath them.Mineralized fissures. - Must of the veins in the north end of the district are in cross fissures on the Keweenaw and Allouez anticlines. The mineralization of the fissures has occurred near the intersections with strong amygdaloids and under the "slide" at the base of the Greenstone flow.
The fissures at the south end of the district are mainly strike fissures dipping more steeply than the beds; they also are mineralized near the intersection with strong conglomerates or amygdaloids.
Changes with depth. - All the larger known ore shoots have had as rich ore at some place near their outcrop as at any greater depth. Any changes in grade to the present depth of development may be attributed more to changes in the character of the lode rock than to distance from the outcrop. Decrease in grade in the fissures and possibly in some of the lodes may be due mainly to depth.Genesis of the deposits. - Two differing explanations of the genesis of the deposits have been advanced. One assumes that they are due to descending solutions, that the copper was derived from the lavas or from overlying beds, and that reaction of the oxidized copper-bearing solutions with the ferrous iron of the lavas produced metallic copper and ferric compounds. The other assumes that they were formed by ascending potential sulphide-bearing solutions which derived their copper from an igneous source, and that the reaction of these solutions with the ferric iron of the rocks resulted in the oxidation of the solutions, the reduction of the ferric iron, and the precipitation of native copper.
The theory of deposition by descending waters appears untenable for several reasons. There is, in the first place, no adequate source of the copper, for although copper is present in the traps there is no evidence of its removal. It is difficult, also, to believe that gravity circulation could have been adequate to form the deposits, for the gravity circulation of solutions in the deep levels of the mines is almost nil, and many of the deposits are on the under sides of impermeable barriers. The deposits, moreover, were formed in beds rich in ferric iron and poor in ferrous iron. The ferric iron was partly removed and partly reduced to the ferrous state - a reaction which does not seem likely to occur in the presence of oxidized solutions.In the ascensional hypothesis it is assumed that the copper solutions originated in the underlying Duluth gabbro and that they either entered the lode-forming layers directly where the downward extensions of these layers were in contact with the magma or passed from the magma to the places of deposition by way of fissures. Solutions thus originating must have been highly heated, in the early stages gaseous, and under great pressure, and they could therefore easily make their way along fissures and permeable layers.
The principal facts that cause the authors to favor this hypothesis are that the solutions became concentrated and deposited ore on the undersides of barriers and that they were reducing in character - they carried sulphides, and they deposited native copper in beds rich in ferric iron, which they partly reduced or removed.
1 The terms "lode" and "fissure" are here applied according to local usage; for definitions see section on ore deposits, p. 101.