Geology of Zinc Deposits MVD

Geology of Zinc Deposits MVD

I consider the lead and zinc deposits of the Mississippi Valley to be the result of descending waters. There are many features that we have not determined and yet when you see the general field relations, it seems to me that the cause and effect can be well explained by descending circulation. In the Joplin-Miami district, for instance, the ore deposits are connected with openings or with old cavity fillings which have a direct connection downward from the surface to practically 300 ft. In the Miami camp, in the western part of the field, some of the deepest orebodies are directly connected with water-channels; the same condition has been obvious and always present with the shallower ores. I speak of one deposit that I have examined more thoroughly than any other in that district, the Admiralty property, which contains large openings or caves at the deepest levels.

It seems to me that waters which would pass upward through 1800 ft. of dolomite would be saturated with lime and magnesia so far as the carbon dioxide of those waters would dissolve them, and therefore, on reaching upper levels, I do not see how they should suddenly form large cavities by additional solution. That is one of the features that I consider favorable to the descending theory.

The other feature is the black flint, that is connected with the ore. If the black color is due to organic material, as it may or may not be, I do not see the source of that organic material in anything but carboniferous shales that are directly above it. I recognize that, to make an interesting discussion of the matter of descending solutions, we ought to have a diagram showing the different formations and the different members of the Boone formation in which the ores occur, and that without it you cannot make a good presentation of the geology of the orebodies. While Dr. Nason believes that the deposits of lead and zinc are the result of waters that have come from an igneous origin, in this district we do not know of any igneous rocks above a depth of 1800 ft., and those constitute the old pre-Cambrian igneous surface that resulted from an erosion interval which lasted at least up to middle-Cambrian time; in other words, the rocks were very old and cold.

There is the argument that emanations from those rocks due to faulting might have sent up waters containing the minerals. In that case they have had to pass through 1800 ft. of dolomites and sandstones, and I do not believe that we could find water of that character coming from an igneous source, and rising through that extent of formations, without finding a certain amount of metamorphism either in the drill holes or in the area where we find these rocks outcropping in the Ozark hills. We do not find, in any of the older rocks, any metamorphism that would indicate waters from an igneous source; hence, I do not believe that the ascending deep water came from an igneous origin.

Zinc deposits of the Valley and of the Appalachian States are true fissures. Therefore, being true fissures, there is no reason why these deposits should not reach the productive depths of the deepest worked fissures in the United States, namely, at Butte.

Now it has always seemed to me that there is such wide difference in the character of fissure veins that they can hardly be compared; that is, although you may have fissures of several classes which extend to great depths, the chance for economic orebodies with depth depends very largely on the class of the fissure. The two vital factors in the formation of orebodies are:

First.—A condition that admits of free circulation of magmatic or other underground waters.

Second.—A condition that permits the free deposition of the salts of the heavier metals from such waters.

Other factors which have a bearing on deposition are:
Pressure and temperature of solutions.
Wall rocks favorable for replacement.
Wall rocks precipitating certain salts.
Brecciated or open condition of rocks through which solutions pass.
Weakness along certain strata or bedding planes.
Mechanical damming of solutions.
Contact of magmatic waters with surface waters.

Therefore, in looking for the continuation of economic orebodies whether of zinc or other metals, you do not necessarily look for the continuation of a fissure or for the continuation of a mass of eruptive rock; you look for the continuation of a certain set of conditions which has made the orebody possible. Let us take some specific illustrations.

The rock formation in the San Juan district, in southwestern Colorado, consists of a number of flows of andesite breccia. These flows are 3000 to 5000 ft. (914 to 1520 m.) total thickness and are capped at points with rhyolite. The rhyolite flows are for the most part eroded. Analyses have shown these breccia flows to be comparatively rich in basic metals. In cooling, these flows developed contraction fissures or simple straight fractures all the way from the surface down to the sedimentaries on which they rest. These fissures were filled with vein matter of which quartz, pyrite, rhodonite, sphalerite, galena, and calcite are the most abundant in the order named.

This filling is evidently the result of direct, magmatic segregation from the breccia flows and presents an excellent illustration of that action. Several of these veins were reopened later by faulting, and there was considerable metamorphic action; Mr. Nason mentions one instance, the Sunnyside-Gold Prince.

I have never seen any other district where underground conditions can be estimated so far ahead of development. To begin with, there is little vegetation at this high altitude and the outcrops of these strong fractures can be followed sometimes for several miles. Where the outcrop is qarrow, we can expect with certainty the same condition in the vein underground. When the outcrop is iron-stained and widens out, we may predict with assurance the occurrence of wide sulphide stopes extending many hundred feet under these points.

If a tunnel is driven at, say, 11,500 ft. (3505 m.) altitude on a vein which shows fair gold content at that altitude, we are reasonably certain that at 1000 ft. (304 m.) above the tunnel we may look for high-grade ore, as at that elevation the vein passes through a breccia flow where the wall rock has a marked effect on precipitation and where there will be decided lateral enrichment. Ore extension in such fissures can be predicted and even estimated far ahead of development, because we know there is no change in the fundamental conditions which tend to produce that orebody.

Mr. Nason puts the Eagle mine in the class of mines in which the rich oxidized ores were formed more as a process of subtraction than of addition. That is, the decomposition of the original orebody resulted in the leaching out of the soluble minerals. I am not quite sure from this whether he believes the original ores at Eagle were derived from the dolomitic limestones or not. However, this is not our conclusion.

At the Eagle mine, we have first to consider an uptilting of the entire sedimentary strata to an angle of 12° at the time when the granite cores of Holy Cross and Gore ranges were upthrust through the sedimentary series. Probably at the same time a sheet of porphyry was thrust into the limestones which split the sedimentaries on their weakest plane namely, a shale series at the top of the blue limestone. There may have been some fissuring at this time but we cannot trace the connection with the orebodies. At this period, there was tremendous activity of magmatic waters, which followed parallel lines of weakness into the mountain and replaced the blue limestone directly underneath the porphyry. These ore-bearing solutions must have originated with the porphyry and must have come from depth, as the size of the orebodies is such as to dispel any idea of lateral secretion.

The original shape of these orebodies must have been flatly lenticular, 100 to 200 ft. (30 to 60 m.) in breadth and 10 to 60 ft. in thickness. There is evidence that aplite dikes were later thrust into the granite underlying the sedimentaries and created a bending stress in the sedimentary series above. These dikes did not reach the limestone, but fissured the quartzite lying between the limestone and the granite, forming deposits of siliceous gold-bearing ores in the bedding planes of the quartzite. At the same time, they caused a brecciation of portions of the orebodies and helped form pockets or rolls in the limestone which aided the collection of mineral along the lines of the primary ore already formed. The fracturing of the original orebody can be plainly seen. I believe that at this time we can recognize the primary ore as it occurred. We have at some points in the mine, especially on the lower levels, a hard glassy mixture of pyrite and ankerite (locally called siderite) carrying sphalerite and galena. This ore is massive and has every appearance of unaltered primary ore in which the pyrite and ankerite are contemporaneous. It has the following composition:


The upper part of the mine contains several million tons of non-commercial oxidized iron ore in which there were large bodies of lead carbonate and from 200,000 to 500,000 tons of iron-manganese ore. This iron-manganese ore is locally called Black Iron and large portions of it are plainly pseudomorphic after siderite or ankerite. All of these oxidized ores carry only a trace of zinc. Now if I am right in assuming that the primary ore had nearly a uniform composition, where did the zinc go? The relatively small bodies of zinc carbonate which we have found do not account for its disappearance. Where could it have gone except to enrich the sulphide bodies? This is the reasoning on which I base my idea that a large part of the sulphide body is secondary and that both oxidized and sulphide ores are built up largely by addition and not by subtraction, as Mr. Nason suggests.

The ore occurrence at the Hanover mine is interesting and might be mentioned as an instance of a large orebody of which the extension with depth cannot be definitely predicted. Here we have vertical dikes cutting sedimentaries as well as metamorphic granite. I believe that in such cases the ores are mainly to be found in the more favorable sedimentary rocks only. The fact that dikes or masses of metamorphic rock go to a great depth does mean that the ores will extend to the same depth but that they will replace only the more favorable strata along the contact.

The occurrence of ore in the Spring Mountain range in Nevada is most unusual. Here we have a district roughly 30 miles long with a great mass of porphyry near the center, near Good Springs. The rest of the district is all sedimentary and no other intrusives can be seen. It has always seemed very probable to me that the porphyry at Good Springs was the mother of all the zinc deposits in that country. All mines except those in the vicinity of Good Springs are plainly the result of infiltrating surface waters following along a shale bed and making workable ore deposits wherever there were favorable rolls or faults which loosened up the surrounding rocks, thus forming pockets where the solutions could deposit their contents. The Potosi mine is a splendid illustration. The intrusives mentioned may have originally spread over a wide area. Later removed by erosion, their leached metallic contents found their resting place as above noted.

The Yellow Pine mine at Good Springs offers a striking contrast. Here we find a mine on the circumference of the porphyry body at its contact with the upturned dolomitic limestone. This is a different story. Here is every reason to expect large replacement bodies along the contact and a deep-seated origin of the solutions-which will lead to deep extension of the orebodies. Furthermore, the only chance for deep ore-bodies in the district is and must be around the porphyry core. Such ore has been found in the Yellow Pine mine, and it is interesting to speculate whether any more deep orebodies will be found in other prospects around the rim.

At Butte, probably the best known examples of fissure veins occur. They are aplite dikes cutting a rock mass of granite, with accompanying fissuring and faulting. We find the same kind of dike cutting the same kind of rock to unlimited depth. We should expect the same kind of vein at depth as at near the surface, after necessary allowances have been made for secondary enrichment, difference in temperature and pressure, etc.

But can these veins formed directly by contraction and fissuring and magmatic segregation in impervious rocks be compared to veins formed by deep-seated external influences in limestone? There is this difference: fissures in sedimentaries have a much greater tendency to close after formation, due to the weight of the rock mass and the yielding nature of some of the strata, which flow out under pressure and fill up any opening. The deposits are naturally erratic and make away from the fissure following lines of weakness along bedding planes of the limestone or precipitating along favorable strata. This tendency with depth is bound to become more pronounced and the ore occurrence still more uneven. Free circulation of solutions in deep sedimentaries does not seem to be nearly so likely as in a free fissure in eruptives. Would this not also apply to orebodies?

A dike of eruptive cutting sedimentaries has somewhat the same characteristics. You may observe it at some point where its contact with the sedimentary rock is dry as a bone and without trace of metamorphic action. Several hundred feet deeper you may find solutions from the dike freely permeating a receptive bedding plane. I have seen well- marked instances of this at Leadville. On the other hand, fissures by dikes or other influences in plutonic rock masses are more uniform and the magmatic solutions stick closely to their original source.

The importance of fundamental conditions has always impressed me very forcibly. Several times I have had options on prospects and have spent much time and eloquence in trying to persuade some plutocrat to buy them. Unfortunately, there never has seemed to be enough ore in sight to make them attractive, but the conditions for deep-seated ore-bodies were right. They have become great mines. A prospect with good conditions for ore extension is often a better purchase than a mine with tonnage developed, but where the story has all been told.

As to western fissure veins in eruptive rock, I expect many more large zinc mines to be developed. They may be prophesied at Butte among the many unexplored vein systems. Similar districts such as Chloride, Arizona, should produce deep mines. Such mines will, of course, require a great deal of faith, patience, and money to find and develop, but when they are needed badly they will be found.