The deep and thorough oxidation of that portion of Globe hill extending in a southeasterly direction from the Summit shaft to about 100 ft. beyond the Plymouth Rock shaft, has received almost as much attention from geologists as that unique deposit of gypsum exposed in the Deerhorn shaft, forming, as it were, the core of this great area of oxidized and disintegrated breccia.
The following description of the occurrence culled from extensive notes made during an examination of some Globe Hill properties in 1905 may prove of some interest in the study of thermal action and oxidation of rock-areas. It is a matter of regret that on account of the numerous caves in the deeper levels of the Deerhorn mine I was unable to obtain complete data, more particularly as to the extent of the gypsum deposit along its strike southeastward.
The oxidation of the breccia and other wall-rocks of the Cripple Creek district is seldom referable to the water-level at any given place, but is solely attributable to the facility offered for the circulation of meteoric waters in the joint-plane, cracks, incipient fissures, and veins in the rock-mass. This is clearly seen in the section of the Chicago & Cripple Creek tunnel.
Taken in connection with the thorough oxidation of the breccia at the Deerhorn, and its disintegration and reduction to a mass of soft clay, this section affords an interesting study in rock oxidation and decay.
The portal of the Chicago & Cripple Creek tunnel is situated in Poverty gulch, close to the Abe Lincoln mine, at an elevation of 9700 ft. The adit starts in a thin band of schist passing in a short distance into the massive unoxidized breccia, which I shall, for distinctness, designate the 'sulphide zone.'
In this zone the breccia contains pyrite in considerable quantity, well distributed throughout the rock. Where veins occur, pyrite is the principal mineral and it appeared to me that there was also a heavier impregnation of sulphides in the wall-rocks of the veins, but even where no veins appear, the joint-planes of the rock were covered with films of fine sulphides, mostly pyrite. This sulphide zone may be considered the unaltered breccia of the district, it is the prevailing rock in which the sulpho-telluride veins occur, and it is assumed to have passed through no particular change or alteration since the formation of the veins and the deposition of the ore.
About 1400 ft. from the portal the adit passes out of the sulphide zone and enters one of partial oxidation, simply the same breccia with open slips, or joints, small fissures, and shattered zones through which circulating waters oxidized the exposed rock-faces, often filling the larger open joints and small fissures with iron and manganese oxides. Where the rock is shattered into small fragments, the oxidation may be complete; usually, however, a piece of rock no larger than a man's fist when broken open shows a core of blue unaltered breccia with shining pyrite.
This I shall designate the 'zone of partial oxidation.' It, of course, passes gradually over to the zone of complete oxidation when the pyrite in the rock-mass is all oxidized and the whole rock stained with iron and maganese oxides, either or both. Near the surface this rock is soft, but stained throughout with oxides and never shows the greenish blue of the breccia or phonolite groundmass. The zone of partial oxidation in the average Cripple Creek mine is usually reached at depths varying from 20 ft. to 150 ft. from surface, depending on the permeability of the rock-mass.
The Chicago & Cripple Creek tunnel passed through 240 ft. of this zone of partial oxidation and again entered the sulphide zone, through which it passed for a distance of 400 ft.; here copper sulphate was detected in several places on the tunnel-wall resulting from the oxidation of sulphides since the adit was opened. An examination of the rock near one of the copper colorations revealed crystals of chalcopyrite, the first I had seen at Cripple Creek.
Emerging from the 400-ft. sulphide zone, the adit enters a second zone of partial oxidation, which was penetrated for a distance of 1500 ft. at an average depth below the surface of some 700 ft. (see section), but gradually becoming more oxidized; the next 100 ft. of the adit is in the thoroughly oxidized zone; next comes that peculiar product of Globe hill, a thoroughly oxidized and disintegrated rock which I shall designate the 'zone of disintegration.'
At the time of my examination the tunnel was caved at a point slightly beyond the Plymouth Rock shaft after penetrating the latter zone for a distance of 120 ft. Prom the best information obtainable at the time, it appeared that the Plymouth Rock shaft had been sunk about 800 ft. in this loose disintegrated material, terminating about 60 ft. below the floor of the Chicago tunnel, the then water-level of this locality, and in the same loose material.
The disintegrated breccia around the Plymouth Rock shaft at the Chicago tunnel horizon is quite loose and soft, not unlike material that had filled a cavity. It contained much iron and manganese oxides, some rounded lumps of kaolin, a little gypsum, and occasional pieces of barite; in places manganese oxide predominates and the material is not unlike a pile of loose ashes. The red oxides of iron so common in the upper workings of the Deerhorn are not plentiful at this depth, brown and black colors predominating.
We can now summarize:
(1) Sulphide zone.—The practically unaltered rock.
(2) Zone of partial oxidation.—The rock mostly oxidized on the faces, joints, cleavages, and fissures.
3) Zone of thorough oxidation.—The pyrite all oxidized and the rock softened and stained with oxides, none of the original color being left.
4) Zone of disintegration.—Rock thoroughly oxidized and disintegrated, reduced in fact to ferruginous clays and talc.
This last zone extends for a length of about 1000 ft. in a southeasterly direction from the Summit shaft to some distance beyond the Plymouth Rock shaft and Chicago & Cripple Creek tunnel. The distinguishing feature in the Summit workings is the great amount of kaolin, resulting from the decomposition of the feldspars, and its accumulation in the veins, fissures, and channels of disintegration.
The pay-veins of the Summit mine occur in the zone of thorough oxidation and when followed southeasterly they disappear in the zone of disintegration near the boundary of the Deerhorn claim.
On referring to the section through the Deerhorn shaft, it will be noticed that the zone of thorough oxidation extends to about midway between the first and second levels; in this rock, well defined pay-veins occur in which pink fluorite is the most striking feature. The open-cut contains two such veins with numerous interlacing branches of almost solid fluorine and granular quartz filling. They show an average regularity in dip and strike, making allowance for the usual vagaries of Cripple Creek veins, and they have been, on the whole, fairly productive. On entering the zone of disintegration, however, these veins are extremely difficult to follow, they contain little value and as a rule speedily disappear in that soft heavily ironstained material forming the zone of disintegration.
On the third level of the Deerhorn shaft, three tongues of massive crystalline gypsum occur, apparently of great purity though surrounded by dark reddish and brown clays, kaolin, and other products of rock decomposition. The gypsum is quite white and contains numerous bunches of crystalline pyrite, some of the cubes having quarter-inch facets, there are also some minute pyrite crystals disseminated in the mass of the gypsum.
In places, fluorite occurs in sufficient quantity to give the rock a pinkish tinge; galena and other sulphides occur in occasional minute crystals disseminated in the gypsum; the outer boundary of the gypsum is everywhere corroded and plainly shows the action of some solvent, possibly water, about 400 parts of which will dissolve one part of gypsum, and with sulphurous acid present in solution this rock would be still more soluble.
Thirty feet below the third level, the Deerhorn shaft passes into a solid mass of gypsum and continues therein to the bottom, or sixth level. The western limit of the gypsum deposit is clearly determined by the westerly cross-cut on the fourth level, as is also the limit of the zone of disintegration. The eastern limit of the deposit is shown on the fifth level, where the station is cut in the east side and a cross-cut extended easterly through the gypsum; there are no workings on the west side, but by constructing the western limits from the fourth and sixth level exposures, the gypsum would appear to be about 240 ft. thick at the horizon of the fifth level.
The east cross-cut on the fifth level intersects two pipes, water-courses of oval shape, the longer axis being at right angles to the plane of section. These pipes had been at least partly filled with the loose ferruginous clay that surrounds the gypsum deposit in the zone of disintegration. The timbers of the level having decayed and broken down so as to allow the contents of the pipes to cave to a considerable extent, I was enabled to pass into some of the pipes and examine them closely.
They appear to be openings along lines of fissuring and were water-conduits of some sort, though now dry as a bone and quite warm. While it is possible these pipes may have been formed by the solvent action of descending waters—and there is strong evidence of such action—yet they so strongly resembled in form of crustification and in general appearance, the pipes of thermal springs I had seen in New Zealand that I had no hesitation in believing them to be of similar origin, though in places enlarged and changed by re-solution of the gypsum since the close of thermal activity and the lowering of the water-level of the district permitted the descent of surface waters in the pipes of the original hot springs.
The sixth level (575 ft. from surface) is the bottom of development in the Deerhorn mine; no work had been done in an easterly direction, while the west cross-cut was (at the time of my visit) caved at about 60 ft. from the shaft and the level filled with reddish earth, whether from a pipe or the outer boundary of the gypsum I could not determine. Rickard, however, calls attention1 to an outlier of gypsum at this point, which he examined while the original development was in progress.
This is my authority for the western column of gypsum on the sixth level; the eastern wall of the pipe was, however, exposed for some height above the level and appeared in every way similar to those I had the opportunity to examine. A drift extended 50 ft. southerly from the station (along the strike of the deposit) through solid gypsum. At this point another pipe is revealed; it also was filled with reddish clay which had in part caved, closing the level, but permitting a complete examination of the pipe for 30 ft. above; it looked like a water-course. My note reads: "It is truly water action that has dissolved this space in the gypsum, which is simply a water-course filled with red ash and debris from above, brought down from the oxidized ground surrounding the gypsum."
The study of the other pipes led me to the conclusion that they were first thermal conduits, afterward enlarged in places by the solvent action of descending waters. All the workings described on the sixth level are in massive gypsum of varying purity, that at the bottom of the shaft is white and crystalline, shading toward pink in places, possibly from the presence of fluorite; no bedding structure could be observed; very fine pyrite crystals are disseminated all through the white gypsum, while the impure variety contained some dark sulphides, mostly pyrite, on the joint-plane and surrounding pieces of altered breccia.
West of the station the rock has the appearance of a fine breccia saturated with, and cemented by, gypsum differing much from the white variety at the shaft. This gypsum rock stands quite well even without timbers, it does not appear to soften or slab in the least. The workings were dry and the temperature I would judge at about 80° F.; on account of the caves there was no circulation of air, nor was there any accumulation of carbonic acid gas so prevalent in other mines in this vicinity.
At the time of the geological examination of the Cripple Creek district by Cross and Penrose, developments in the Summit mine had reached a depth of 200 ft. and in the Deerhorn 280 ft.2
In discussing the evidence of fumarole action Cross says: "Gypsum occurs in the Deerhorn mine on Globe hill, in ore veins, and is most probably the result of aqueous action." Penrose considers the gypsum "possibly, though not necessarily, the product of the action of sulphuric acid or sulphates derived from the oxidation of sulphides on carbonate of lime or other calcium compounds derived from the alteration of certain eruptive rocks."
Regarding the occurrence of the gypsum, this observer states: "In a vein on the 280-ft. level of the Deerhorn mine considerable quantities of a compact, fine grained crystallized gypsum, containing more or less fluorine, occurs; while on some of the upper levels thin layers and crystals of gypsum are found in the cracks in the breccia."
Rickard, in speaking of the Deerhorn gypsum deposit, says: "There can be no doubt as to the nature of these masses of gypsum. Thermal springs that have become extinguished are marked by just such accumulations of lime, although the carbonate is, under such circumstances, more common that the sulphate. The flow of hot water encountered in the deep workings of the Comstock carried a notable percentage of gypsum."
Lindgren proves3 the presence of sulphates in the circulating water during ore deposition, by the "almost universal occurrence of celestite or sulphate of strontium in the veins, as well as by the more sparing development of barite. . . .
Calcium sulphate is not found as a primary constituent of the veins, but the peculiar occurrence of large masses of gypsum associated with pyrite and fluorite at the Deerhorn mine, leads to the belief that waters rich in sulphates of the alkaline earths appeared as one of the latest phases of thermal activity. The closest analogy to this would perhaps he found in the hot waters of the Comstock lode, which are exceptionally rich in these constituents."
It appears that the lower levels of the Deerhorn were not accessible during the examination of Globe hill by Lindgren and Ransome, who quote from Rickard's description of the gypsum deposit and zone of disintegration. Regarding the latter, we find on page 197:
"The explanation of this unusually deep oxidation is probably that extensive shattering preceded a local and very intense thermal-spring action, which by dissolving much material rendered the breccia so porous as to fall an easy prey to oxidizing processes." The gypsum in "the Deerhorn mine is, however, probably not formed during oxidation, but is more likely a primary deposit by hot waters."
The authorities quoted are in agreement that the gypsum deposit is the result of thermal-spring action occurring toward the close of volcanic activity; it must be noted, however, that pyrite and fluorine were deposited with the gypsum and very probably throughout the space now represented by the zone of disintegration. Gypsum is of common occurrence in sulphate springs, hot and cold.
Flourite has also been detected in thermal springs of low temperature, as, for example, the mineral springs of Schwollen, in the Duchy of Oldenburg, to the extent of 0.0005 gram per litre, and at those of Baden to the extent of 0.0021 gm. per litre, while the deposit from the Sprudel water at Carlsbad gives practically 1% calcium fluoride.4
The source of the gypsum on Globe hill is probably from below, that is, from a greater depth than is revealed by any workings today; as a possible source of sulphate waters, Gautier has shown5 that granite heated in a sealed tube with water to a temperature of 250 to 300°C. yields sulphur compounds in solution, resembling the sulphur waters of hot springs. He ascribes the sulphur to reactions on sulpho-silicates such as haüynite and lazurite, and not to metallic sulphides.
This conjecture is interesting; from the granite could be obtained both the gypsum and the fluorite, as it is well known that fluorine compounds are specially characteristic of plutonic rocks, and, indeed, it has been recognized in Cripple Creek granite. The enrichment of the zone of disintegration by iron and manganese oxides remains to be considered.
The composition of the average rock of the Cripple Creek volcano is computed by Lindgren at about 3% mixed ferric and ferrous oxides. Practically all of the spongy rock-mass in the zone of disintegration is heavily saturated with iron oxide; some of it is almost a low-grade iron ore. Where, it may be asked, did this great quantity of iron come from?
It saturates a channel of porous rock about 1000 ft. long, perhaps a similar depth, and, say, 600 ft. wide, all of which contains much more iron and manganese oxides than the average oxidized mineral veins of the district. Two sources suggest themselves: one by descent from oxidation of the superincumbent breccia, since removed by denudation; secondly, from the oxidation of pyrite deposited in the altered breccia itself, say, at the beginning of solfataric action. I am rather in favor of the latter hypothesis.
It must not be forgotten that gypsum is much more soluble than celestite and may easily have been leached out of the veins by circulating solutions, hence, the possibility of the closing period of vein formation being coincident with solfataric action, at Globe Hill, for example, where the simultaneous deposition of pyrite, galena, fluorite, and gypsum affords at least some evidence in favor of such phenomena.
It may be claimed that the gypsum deposit is a local occurrence, and to a large extent it is, for I take it that the sulphate solutions that built up the great mass of gypsum rose along lines of fissures, dissolving and replacing the breccia in their neighborhood. I could not conceive a cavity representing the proportions of the gypsum deposit remaining open during the long period necessary for the deposition of the gypsum mass; furthermore, we have in the partial replacement of breccia in the gypsum at the sixth level what I take to be direct evidence in favor of a replacement process.
The periphery of the gypsum deposit, at the few places where I could examine it, showed strong evidence of partial solution; at such places it is surrounded by very loose iron oxide conforming to the configuration of the gypsum, while the latter is pitted with numerous cavities and corrugations, but is quite solid, showing no graduated passage from 'the solid gypsum to the decomposed breccia; hence I believe the deposit was much larger before the period of oxidation set in, and also extended much higher than the present section shows; the peculiar fin-shaped pieces exposed on the third level are in themselves evidence of solution.
The phenomena of volcanic action and rock oxidation on Globe hill suggest the following tentative cycle of events:
(a) Fissuring of the breccia and general shattering of the rock in the main channel, followed by fumarole action with kaolinization of the feldspars, closing with the deposition of minerals in the veins and pyrite in the partly decomposed breccia; this is perhaps more correctly classed as the early solfataric stage.
(b) Solfataric stage, deposition of gypsum, pyrite, etc., from sulphate waters along lines of fissuring, and, in part, in the spongy breccia of the shattered zone, closing with thermal waters rising in pipes to the then surface of the ground, said pipes being somewhat centrally placed in the mass of gypsum.
(c) Oxidation period. Descending meteoric waters dissolve the upper portions of the gypsum and the less solid portion around its periphery and to some extent enlarging the pipes of the original springs. At the same time, the pyrite in the shattered breccia channel was oxidized, completing the disintegration of the rock and forming the zone of disintegration as we now see it, much of the iron, of course, was removed in solution, but enough remains to show that at some time this great channel was, as I take it, very thoroughly impregnated with pyrite.