246 ‘Engineering and Mining Journal-Press Vol. 117, No. 6
FEBRUARY 9, 1924
Upper Mississippi Lead and Zinc Ores—PART I
By J. E. Spurr
A RECENT FLYING VISIT to the Wisconsin lead and zinc district, under the expert guidance of W. N. Smith, of the Vinegar Hill Zinc Co., has interested me in reviewing again the literature of this old and much-discussed mining region. Mining began here as early as 1780, and has been going on ever since. Although the mining operations are not so extensive as formerly, new ore bodies are continually being found by churn drilling from the surface, and there are indications that still newer discoveries will be made for a long time to come.
The region is a classic one, which has been studied and described by very many geologists. The origin of the ores has been the chief problem discussed. The natural curiosity of mankind prompts us to seek the cause of unusual phenomena—and a workable ore deposit is in the large sense, a striking and unusual natural wonder—and leads us to tax our intellects to solve their problems. Marked concentrations of the metals always bring up the query, to the uneducated and the trained alike—how were they thus concentrated? The correct answer to this query has, of course, always a high practical, as well as scientific significance
Since these deposits were discovered and mined in the early days of economic geology, it is natural that there should have been a great diversity of opinion as to their cause, and many theories have been propounded. And also the deposits are undeniably puzzling, and lacking in striking collateral phenomena which might afford outstanding indications or views to the special processes which have gathered the metals, normally disseminated so thinly in the earth’s crust, into relatively concentrated and workable bodies. There are no igneous rocks, which are intrusive into the flat-lying, almost horizontal sedimentary beds, in which the ores occur.
While the sedimentary beds have been slightly disturbed, the disturbance or deformation has been relatively very little indeed. Thus, of the three allied phenomena which I have mentioned as usually characterizing metallogenic epochs1—intrusion, deformation, ore deposition—evidence of the first is missing, and the second is rudimentary. The conclusion is suggested, then, that the ore deposition was not connected with intrusion—that is to say, was not of magmatic origin; and of course this conclusion has been that of most workers in the field, for, after all, in working out geological problems, we are bound to reason from the facts on hand.
Igneous processes being thus by general (even if not universal) consent barred out, the general conclusion has been that the ores have been concentrated by ordinary ground waters, a conclusion led up to by the observation of the effects that the ground water has had in this district (as, indeed, in others), in dissolving and precipitating ores. Yet no general further unanimity of opinion has prevailed, so obscure have been the views to work from: and, furthermore, no explanation has been presented which seems to me sufficiently founded on facts, and sufficiently removed from highly theoretical assumptions, to be convincing: indeed, for each theory proposed, the objections seem stronger than the affirmative arguments.
The ores of this district are largely grouped by most writers with the essentially similar lead and zinc ores of Missouri, and other near-by states, as the “Mississippi Valley lead and zinc deposits: yet even in Missouri, there are occasional igneous intrusions which have served as one of the bases for some argument on my part, that the deposits were of magmatic origin.2 Thus it would seem indeed at first sight that there is little basis in this Wisconsin region for the magmatic theory, and that it should be excluded, as it has been hitherto. The trouble is that the non-magmatic theories actually adopted are apparently [also] little substantiated and as vague.
Among the distinguished geologists who have attacked the problem are Percival (1855), Whitney (1858), Chamberlain (1873 and later), Blake (1893), Jenney (1893), Winslow (1894), Leonard (1896), Bain (1900, 1906), Van Hise (1901), Grant (1903-1906), and Cox (1914). I will not attempt a complete summary of all these views; they are to be found in any of the painstaking later treatises on the subject, such as that of Cox. In reviewing the matter I will enumerate briefly the conditions under which the ores occur, and then mention some of the most widely accepted interpretations.
The sedimentary succession is represented by the accompanying figure (Fig. 1), which I reproduce from Cox’s report. The Cambrian sandstone rests upon a pre-Cambrian basement of quartzite, ancient rhyolites, and other rocks: this pre-Cambrian terrain immediately underlies the section shown. The formations from, and including the Prairie du Chien (dolomite and sandstone), up to and including the Maquoketa shales, are Ordovician; the Niagara dolomite is Silurian. Above this section come only the alluvial and residual clays of the driftless (unglaciated) area of Wisconsin. Whatever sedimentary strata that may have been laid down in this region since the Silurian, have long since been stripped off by erosion from the whole upper Mississippi Valley.
While a little ore has been found in all the sedimentary formations, above the Cambrian, shown in the section, the sandstones are practically barren, except that rarely ore has been taken from the top of the St. Peter sandstone. The ore has been found almost entirely in the dolomites and limestones—to a minor degree in the Prairie du Chien, and in not inconsiderable amount from the upper part of the Platteville (“Trenton”) limestone.
But the Galena dolomite is by far the most important ore-bearing horizon in this district; and large quantities of ore have been found in it, at levels ranging from the top to the bottom. Practically no ore has been reported from the Maquoketa shale; and the Niagara dolomite has been considered as, for all practical purposes, barren. The main ore-bearing horizon, then, is not more than 300 ft. thick, lying below the Maquoketa shales, in the dolomites and limestones comprising the whole Galena dolomite and the upper part of the Platteville (or “Trenton”).
“The great majority of occurrences of ore in this region,” says Ham, “are confined to the Galena formation. It is found at all horizons, from top to bottom.” It may be remarked here that it is the Galena formation which occurs at the surface over the larger part of the mining region; over most of it, the Maquoketa shale has been stripped off; and the Niagara dolomite occurs chiefly as residual and isolated buttes; moreover, that the formations below the Galena, are little known, and explored in the mining region, since most mining operations have been confined to the Galena.
The lead and zinc district, in which ores have been mined, not only lies in Wisconsin, but runs over to a slight extent into northwestern Illinois, and northeastern Iowa. (Fig. 2). Yet it is, taken altogether, a distinct unit area, and is relatively definite and restricted. Winslow’ says: “The area of the whole region is about 2,600 square miles, its length in an east and west direction, being about 65 miles, and its breadth in a north and south direction, being about 55 miles.” Cox’ estimates that the area of greatest ore deposition covers about 2,025 square miles.
Concerning this definite geological restriction of the ores, Ham very acutely remarks:’
“By referring to the description already given, it may be noted that there are two facts of geographic distribution which must be considered. The first is the limitation of the mining area to a certain definite part of Wisconsin, Iowa, and Illinois, and the second is the distribution of the deposits in camps or districts within this region. The reason for the major distribution is by no means obvious, since the Galena formation, in which the deposits mainly occur, has a wide extent through the other parts of the states concerned.
For many miles to the southeast, equivalent beds extend in characteristic development, with apparently abundant opportunity for secondary concentration of such ores, as may be present, but without any such concentration having taken place. In eastern Wisconsin, the Galena formation again appears well developed and without ore, and in Missouri, even where the same horizon is developed within a lead-zinc region, it is barren. If attention be directed to the underlying Platteville, much the same state of affairs is found. In the case of the Prairie du Chien, it is true that the ore occurs in equivalent beds in the Ozark region, and that there are small scattered occurrences of lead and zinc minerals in much of the area north of the Wisconsin River.
The fact, however, remains that outside the mining region this, as well as the Galena, is barren. “To say that the distribution is conditioned by the absence of the drift only partially satisfies the conditions, since there are broad barren areas of all these
Formations quite as free from drift, as are the productive areas. This leads to a consideration of the second important fact of geographic distribution—the segregation of the deposits into definite districts and camps, with barren areas between. This segregation, which was early recognized and which has become more and more definite as prospecting has gone forward, is one of the most difficult facts for any theory of genesis to explain.”
The ores consist essentially of lead, and zinc, and iron sulphides, in the unoxidized ores, with the usual secondary forms of lead, zinc, and iron, in the zone of oxidation—oxides, carbonates, and sulphates. The chief gangue is calcite; barite occurs, but not abundantly; quartz is practically wanting; the ores carry very little silver. Usually they carry little or no copper, which, when present, occurs in the form of chalcopyrite. But copper did occur in the district, especially at one locality, at Mineral Point, where it was mined to the value of half a million dollars. This intensive segregation of copper is another puzzle in concentration, to be added to the two, so well summed up above by Ham.
Concerning this, Cox says: “The irregular distribution of the lead and zinc ores over the district, and the occurrence of copper near, and at Mineral Point, Wis., where it has been mined to the value of half a million dollars, and near Gratiot, Wis., whereas even traces are almost lacking elsewhere in the district, show conclusively that there were differences in the character of the solutions percolating through the rocks at different places, or that the precipitating agents were not everywhere alike.” The ores occur in definite horizontal shoots or “runs” in the strata in which they lie. These shoots are determined by vertical or diagonal fracturing and fissuring; and the diagonal fissuring often, but not always, shows alternate steep and flat portions, making a series of irregular steps.
Along these fissures, which, in the oreshoots, are branching and reticulating, the ores have formed mainly by replacement, but partly by fissure filling. Replacement of rock by ore also extends out irregularly, especially along certain horizontal beds, within the general shoot, but not beyond it. The ores now being mined are largely at a depth of between 100 and 200 ft. from the surface, and are essentially a mixture of blende, galena, marcasite, and pyrite, with calcite gangue. Among the metals, blende predominates.
Near the surface the lead ore was galena, mingled with zinc carbonates. The sulphide ores now being mined, are first treated by jigging, which separates the lead from the zinc-iron sulphides; then the zinc is concentrated by partial roasting and magnetic separation. The lead (galena) ores near the surface were separated from the zinc carbonate by washing; during the early part of the period when these ores were mined, there was no market for the zinc, which was wasted. Lead carbonate never occurred in large quantities so as to form an important ore, a fact which is significant.
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The favorite horizon, in the Galena dolomite, for the ores, seems to have been at the base of the formation, just above, (and to some extent in,) a bituminous shale called locally the oil rock, which is a few feet thick. This is the basal bed of the Galena. The ore also, in many cases, occurs below the oil rock, in a dense limestone which forms the upper part of the Platteville (Trenton), and from its conchoidal fracture, has been called the “glass rock.”
On account of the concentration of ore deposition in these horizons, the detailed stratigraphy has been much studied, and a section showing this is accordingly given herewith. Reverting to the published opinions concerning these deposits, we find that the origin from igneous rocks in depth was advocated by Owen (1839), Percival (1855), and Jenney (1898). All these believed that the ores came from below—that they rose from the depths, below the sedimentary series.
Percival says: “I have thus been able to trace the mineral in a series of crevices and openings, from the summit of the Upper Magnesian (Galena dolomite), to the depth of sixty to seventy feet in the Lower Magnesian (Prairie du Chien dolomite), and have found all the beds of limestone good mineral-bearing rocks, each with one or more openings, besides vertical or pitching sheets or veins. The small depth to which mining has been extended, does not allow one to trace the mineral through the whole of the extent downward in any one instance, but wherever circumstances permit of examination, the order of succession in the openings is found to be regular, and in multiplied instances, the vertical crevices and veins have been found passing down from one opening to another. It is then probable that the series is generally continued through the whole downward extent indicated, subject only to such interruptions as are more or less common in all veins.
And Jenney wrote: “The result of this investigation of the deposits of lead and zinc in the Mississippi Valley, has made it possible to announce the general law that all workable deposits of ore occur in direct association with faulting fissures, traversed by strata, and with zones or beds of crushed and brecciated rock, produced by movements of disturbance. The undisturbed rocks are everywhere, barren of ore.”
Concerning these views, Cox, practically the latest writer on the subject, writes that they are untenable, because (1) no general fissuring extending from one formation to another occurs; (2) there are one or more clay beds in the strata, which would have barred the way to the assumed rising solutions; (3) there are no rocks intrusive into the strata; (4) the ore in the Prairie du Chien dolomite has never been commercial; and (5) the Prairie du Chien dolomite and the St. Peter and Potsdam sandstones are porous, so that ore solutions rising through them would have been so greatly diluted that they would be unable to produce ore deposition above.
Parenthetically, I think we may cancel reason No. 5, above, for certainly the ore solutions imagined by Cox (waters which leached from the rocks, metals which were present in them, only in traces) were excessively diluted also.
Whitney is the first geologist who has been mentioned with approval by later writers. Cox says: “Whitney showed his genius by going beyond the general views of his day and advancing a sedimentary origin for the ores, upon which practically all the later theories, including those in vogue today, are based.” Whitney believed that the ores were in solution in the sea water and were precipitated by organic matter; and that they were later concentrated by meteoric waters.
Chamberlain, accepting the main features of this explanation, saw in the organic matter of the oil rock (see above), the material which produced precipitation of metals in the seas. He computed that the amount of lead and zinc necessary to have been leached from the whole Galena formation, to form the known richest ore bodies, was very minute indeed. Concentration into ores was by meteoric waters, according to him. Winslow” believed in the same general origin and, indeed, so have subsequent writers, including Bain, Grant, and Cox.
Bain and Grant believe that the original source of the lead and zinc was in the Pre-Cambrian crystalline rocks of the Lake Superior region, which contributed the Paleozoic sediments in which the ores occur; that lead and zinc, as well as magnesia, were transferred in solution, from the land to the Ordovician seas, by the inflowing streams; that the metals were deposited in thinly disseminated form in the Galena dolomite through the reducing action of gases derived from the decomposition of the oil rock below, which gases bubbled up through the mud; that after the long subsequent elevation of this formation, to become dry land, the disseminated metals were leached out by waters derived from the surface, carried laterally to openings and there deposited.
But Cox rejects the theory which postulates the close genetic connection of oil rock and ore, as advocated by Chamberlain, Bain, Grant, and others. And Cox states truly, “It is impossible to prove that any lead and zinc compounds ever existed as original minerals in the Galena limestone of this region, and therefore, that this is an ample source of supply for the ore.” And as to the relation of oil rock and ores he says:”
“Whereas the distribution of the oil rock corresponds somewhat closely to the area of the Upper Mississippi Valley district, and the thickest oil rock is found in the general neighborhood of the best mining areas, as at Platteville and Hazel Green, Wis., the lateral distribution of the ores within the district does not correspond to the thickness of the oil rock.
Thus whole townships in the vicinity of Lancaster, Wis., are practically barren, but others near Highland, underlain by less thickness of oil rock, are heavily mineralized. The thickness of the oil rock in the vicinities of Fenimore and Dodgeville, Wis., is practically the same, yet but little ore has been found near Fenimore, whereas the Dodgeville area has been a large producer.” Cox concludes that “the relation between the oil-rock basins, and the ore deposits, is therefore probably structural, and not genetic.” He thereupon submits a newly built theory—that the original disseminated metals were deposited in the Maquoketa shale, which overlie the Galena dolomite, rather than in the Galena dolomite where they now mainly occur; and that they have been carried downward into the dolomite by meteoric waters. Cox states that the richly organic portion of the Maquoketa shale (which organic character is assumed to have precipitated the metals from the Maquoketa sea) is roughly limited to the lead and zinc mining district.”
The explanation for the fact that some areas in the Galena formation show ore, while others do not, is thought to be due to the concentration of solutions along surface valleys and underlying fractures, during erosion. The slightly waving contour of the oil-rock bed (and the other sedimentary formations), has produced generally imperfect shallow basins, along with other structures. Chamberlain27 concluded that these varieties were primarily depositional, and secondarily structural, and, further that the ore deposits were in the main confined to the synclines.
Bain and Grant agreed with these conclusions. Grant observed: “It is not intended to imply that all of the deposits of the Wisconsin zinc and lead region are confined to these synclinal basins, but it is very evident that a large number of the important deposits are so located, and there can be no question as to the causal relation of these structural forms to the ore deposits.” But Cox points out that certain mines are located on top of oil-rock structural ridges, and others on oil-rock structural slopes; and from my perusal of many underground mine maps in the office of the Vinegar Hill company, which maps usually show also the depth at many points of the oil rock, as well as from the oil-rock contour which had been made by Grant and others, I have concluded that while ore deposits may occur in basing, and perhaps preferentially thus, yet they occur almost equally easily where no such definite basins exist.
It will be seen, as I remarked above, how absolutely wanting are clear indications as to the origin of the ores; and how extremely hypothetical the explanations have consequently been. The reasoning has been, briefly, that since there are no igneous rocks, the ores must have been derived from the sediments. Since metals occur in these sediments in minute quantity, they must have been concentrated into certain areas by meteoric waters, as there is no record of any other kind of waters having traversed the rock. That leaves it to be reasoned out only what horizon the ores were originally precipitated in, and leached and concentrated from; and how and why they were finally precipitated where they are.
Broad hypothetical generalizations of this type are subject to broad and general criticisms. With this license, there is, in my opinion, nothing but negative evidence available as to the origin of the ores as above stated. Ham had certain limestones analyzed for lead and zinc,” finding an average percentage of 0.00376 Pb, and 0.00043 Zn, from nine analyses. Winslow” had already had similar analyses made of Missouri sedimentary rocks, and underlying Archean rocks, and found lead, zinc, and copper in all of them. Out of thirteen analyses of Silurian and Lower Carboniferous limestone, Winslow got an average of 0.001009 Pb and 0.00239 Zn. He did not trouble to average the copper in the same analyses (possibly because the ores whose genesis he was pursuing were lead-zinc ores, and not copper ores); but I have averaged his copper analyses for the same thirteen samples, and get 0.001693, or more than the lead. The easy solubility of copper in meteoric waters is well known, and the facility with which concentration is effected, from a disseminated condition by surface water.
Why, then, do we not have more copper than lead in the sedimentary rocks of Missouri? Four analyses of Archean rocks by Winslow, gave an average of Pb 0.01587, Zn 0.00901, and Cu 0.01050, showing more metal in the Archean basement than in the sedimentary rocks whose leaching is supposed to have formed the immense quantities of lead and zinc in Missouri. Go figure.
To be concluded.
The Ore Magmas,” Vol. I, p. 387.
The Ore Magmas,” Vol. I, p. 402.
USGS Bull. No. 21, ‘Lead and Zine Deposits of Northwestern Illinois,’
1914, p. 63.
Platteville.—Mining in the Wisconsin zinc-lead districts is in a helpless condition and one from which it may be a long time recovering. Two mines operated by the Vinegar Hill Zinc Co., the Yewdall mine at Livingston, and the Unity mine at Day Siding, continued outputting low-grade zinc ore through the month of April, the ore going to the National Separators at Cuba City, for magnetic separation, and the manufacture of commercial sulphuric acid.
Early in the month the New Jersey Zinc Co. unexpectedly began pulling up the pumps at a number of mines, soon after, power, mining, and milling plants were being dismantled. The mines owned and controlled by the New Jersey Zinc Co. were producing regularly at the time of the shutdown last autumn, and it is difficult to understand the apparent abandonment.
At Mineral Point, where is an establishment, representing an investment of several million dollars, for the manufacture of zinc oxide and sulphuric acid, a force of men was kept at work until the middle of April. The sulphuric-acid works suspended operations entirely during April, and the department making zinc oxide was run with a divided force. One half of the men working each two weeks out of the month. It Is rumored that the plant at Mineral Point will be dismantled and removed, but no official confirmation Is obtainable.
At the Penna-Benton mine in the Benton district, a force of miners has been kept at work constantly, and considerable ore is being mined and milled, but no shipments are being made. The Black Jack mine, in the Galena district, has been abandoned, and the pumps removed.
Some improvement was shown in the offerings for zinc ore and lead ore the latter part of the month, but it is of little material interest as there was but one company in the field in a position to take advantage of improved offerings, this being the Vinegar Hill Zinc Co., which made delivery of about 400 tons of high-grade ore.
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