Rich deposits of blende, formed in great part by the secondary enrichment of smaller, or less mineralized, primary ore- bodies, are found near the surface in the Joplin, Mo., district, in the vicinity of Carthage, Lehigh, Central City and Reding’s Mill. At these localities the ore occurs in two ways: in the beds of soft, decomposed carbonaceous shales in the Coal Measures, occupying shallow basins in the Subcarboniferous limestone of the region; or, more commonly, in horizontal channels eroded in the underlying limestone and filled with soft, dark mud, intermixed with bituminous matter, derived from these same shales, crushed by faulting movements and washed into the openings by surface-waters. Without regard to the character of the enclosing formation, these forms of ore-deposit are known to the miners as “ mud-runs.” These occurrences are seldom far from the surface; the deepest “ run ” of this character observed, near Reding’s Mill, was at a depth of 90 ft. At Lehigh, similar deposits were found in the bottom-lands along the stream, within a few feet of the surface, where the permanent water-level came near to the top of the ground.
The ore occurs in minute crystals, thickly disseminated through the soft, shaly gangue, or enveloped in semi-fluid black mud. In some of the deposits the crystals of blende are agglomerated, forming irregular masses and sheets of pure ore. The crystals are of uniform size, usually from 1/16 to 1/8 of an in. in diameter, transparent, resin-yellow in color, with a shade of red, resembling in appearance the small garnets found in some placer gold-bearing gravels. The small size of the crystals is probably due to the concentration of the solutions from which they were deposited, and to their rapid formation. In some instances blende forms crystals in these shaly beds from ½ in. to more than an inch in diameter, presumably from a slower and more prolonged growth. In all the deposits of this type, the reducing agent has been bituminous matter acting upon surface-waters, which carried in solution the sulphates of the metals leached from ore-bodies at higher levels, that were undergoing oxidation. The following equation shows the character of the change:
ZnSO4 + 2CH + H2O + O = ZnS + 2H2CO3.
It has been observed that these shallow mud-runs usually occupy synclinal basins or troughs, and that surface-erosion has often removed a considerable depth of the ore-bearing formations from the immediate vicinity; the deposits being located where they have received the drainage from surrounding mineral-areas.
At the Britton mine, Central City, the ore occurs at a depth 40 to 50 ft. from the surface, in a broad, flat, compound-run, 75 to 150 ft. wide and 6 to 12 ft. high, developed on its course for a length of 550 ft., northerly and southerly. The ore is granular, crystallized blende in a stratum of soft, black mud and broken chert. The particles of blende are less than 1/16 in. in diameter, deep garnet-red in color, and, although thickly distributed in the mud, are not agglomerated. A number of similar deposits are developed in the vicinity.
Ore-deposits of this character are easily mined with pick and shovel; powder is only used to break up an occasional boulder in the ore-body. The ore requires very little crushing and readily concentrates, yielding from 12 to 20 per cent of clean blende. The product is very pure, assaying from 62 to 64 per cent, of metallic zinc.
Reding’s mine, about 4 miles southeast of Joplin, is situated in a shallow basin in a low range of hills, bordering the north side of the valley of Shoal creek. At the time of the examination, this mud-run was opened 75 ft. in length, north and south, with a width of 36 ft., and a height of 12 to 18 ft. The ore occurs in dark-brown mud, in finely disseminated crystals, and in crystallized masses and sheets of galena and blende. Beautiful specimens of galena are found in very perfect cubical crystals, 1 to 2 in. on a side. The upper part of the run is mostly crystallized galena, while the lower part is blende, with a smaller proportion of galena. Boulders of flint and black clay-shales are found in the ore, together with much coarsely crystalline rotten dolomite.
This ore-deposit is remarkable for its extreme richness. To the date of my examination, the yield of the mine, estimated upon all material extracted, had been 15 to 20 per cent, of clean ore. When first formed, it was evidently a small run of blende and galena in dolomite; subsequently, surface-waters eroded a cavern-like channel following the course of the run. Material washed from the surface formations of broken and crushed bituminous shale filled this channel and enveloped the ore-body. Finally, the ore primarily deposited was greatly reinforced by secondary deposition, both galena and blende being crystallized in the fluid mud by the reducing action of the hydrocarbon.
A small basin of coal-shales, near Belleville, Jasper county, Mo., carried beautifully preserved fossil-plants. The outer surface of the mass of shales, for a depth of about a foot, contained scattered crystals of blende, from ¼ to ¾ in. in diameter, mingled with a few crystals of galena and pyrite. The central portion did not carry any mineral, the mass having been mineralized from the outside, toward the interior, as far only as the mineral-bearing waters could penetrate the dense plastic clay. The crystals, in their growth, have distorted otherwise perfect fossil-plants, crowding parts of the fern-fronds to one side; giving evidence that the deposition of the minerals was of later date than the preservation of the plant-remains. These plants were determined by David White to belong to a horizon near the middle, or the upper part, of the Lower Coal- Measures.
On the Mine La Motte grant, in Southeast Missouri, deposits of disseminated galena in black shale of Cambrian age outcrop at the surface, and were worked for lead during the early period of mining in that region. The galena occurs in crystalline nodules, from ¼ to ¾ in. in diameter, thickly distributed in hands through the shale. This shale-bed appears to be a local formation. The ore-deposits closely resemble in mode of occurrence those of disseminated lead in the Cambrian limestone in the belt extending from Mine La Motte, through the Flat River district, to Bonne Terre.
The secondary formation of metallic sulphides is now taking place in the Missouri mines; blende and galena, oxidizing to sulphates in the ore-deposits near the surface, are carried in solution by the subaerial waters to the deeper horizons, and there regenerated by the deoxidizing action of bituminous matter. The subject is too extensive to admit of discussion here, and must be left for a future paper.
A single instance, however, may be cited of the reproduction of blende from mine-waters. An old tunnel, driven through bituminous shale on the Banker’s Tract, near Joplin, became filled with water draining from adjoining mines on which work had been suspended. The tunnel remained closed and submerged for ten or twelve years, until the mines were un-watered in 1898. When reopened, the surface of the shales, on the roof and sides of the tunnel, was found to be thickly encrusted with minute crystals of blende, one- or two-hundredths of an inch in diameter. In places, the blende was deposited on the pick-marks made when the tunnel was run.
The paper of T. A. Rickard, on “ The Enterprise Mine, Rico, Colorado,” shows the existence of a strongly-marked resemblance between the occurrence of the flat ore-deposits at Rico, carrying silver and gold, and the ore-formation of Southwest Missouri, where galena and blende occur in simple runs, and also in compound runs, formed in like manner, in the favorable beds, by mineral-depositing solutions introduced through vertical fissures from an unknown source in depth, and where bituminous matter, contained in the ore-bearing strata, has likewise been the precipitating agent in the deposition of the ore.
Experiments made in the laboratory, to determine the reducing action of the black shale associated with the ore-bodies, are described by Rickard, as follows:
“ A piece of the Rico shale was put into a weak solution of sulphate of silver (Ag2SO4) containing some free acid intended to neutralize the lime (CaCO3) in the shale. The precipitation of metallic silver became visible in three days. The parallel experiment with gold was more interesting. A piece of ore (assaying 1147 oz. of gold per ton) obtained from Cripple Creek, was taken, and its gold was extracted by a solution containing ferric sulphate (Fe2O3,3SO3), common salt (NaCl) and a little free acid (H2SO4). This Cripple Creek ore carried the black oxide of manganese (MnO2) in visible quantity, and thus the chlorine used to form the gold-solution was liberated in a manner simulating natural conditions. Of the gold, 99.91 per cent, was extracted and subsequently precipitated on the Rico shale by inserting the latter in the solution thus formed. The gilding of the black shale by the deposit of gold became visible within four hours.”
W. Nicholas has made a series of similar experiments in the precipitation of gold by black carbonaceous shales from the Victorian quartz-reefs.
The peculiar vein-formation known as the “ Indicator ” is an example of the localization of rich deposits of free gold, due to the reducing action of carbonaceous shales. Rickard defines the Indicator as “ a very thin thread of black slate, which is remarkable on account of its extraordinary persistence, and also because the quartz seams which cross it are notably enriched at the point of intersection.” It is also referred to by Dr. Don. The Indicator is the most important member of a series of thin seams of black shale, more or less impregnated with pyrite and arsenopyrite, traversing the slate and sandstone formation of the district.
Different views have been expressed as to whether the carbon or the pyrite in the Indicator seam was the reducing agent in the formation of the rich bunches of gold-ore. But pyrite was deposited in the black shale by the reducing action of the organic matter; so that, in any event, it was the presence of carbon compounds which, directly or indirectly, caused the local accumulation of the gold.
Owing to the far more powerful action of the hydrocarbons, reduction by pyrite contained in carbonaceous shales must always be subordinate, even where local conditions, such as the presence of free oxygen and oxidized metallic salts, admit of the pyrite being decomposed. Any ferrous sulphate produced, at once re-forms pyrite, giving up its oxygen to the carbon. In the depths of the strata, below water-level, wherever any form of carbon is in excess, it absolutely protects the pyrite by consuming all the free oxygen.
Many instances might be cited to illustrate the influence of the organic matter contained in bituminous shales upon the formation of ore-deposits.
A stratum of black shale in the Mono silver-mine, Dry Canon, Utah, formed the hanging-wall of the rich ore-shoot, from which large masses of horn-silver and high-grade sulphide-ores were mined, carrying from 500 to 3000 oz. of silver per ton. It is reported that a carload of ore from this mine yielded over $55,000.
Emmons says: “ Most famous, in view of the enormous values taken from them, are the rich silver-bodies of the Mollie Gibson and Smuggler mines of Aspen, Colo.; but, in their case, there is sufficient organic matter present to explain the reduction of the oxidized solutions to sulphides. They occur along a vertical fault, formed since the original mineralization of the district, and consist of great masses of polybasite and pink barite, which, in places, have been further reduced to native- silver. On one wall of the ore-body is the limestone, of which it is a replacement, and on the other a black bituminous shale.
Rickard, discussing this subject, writes: “The idea of the precipitation of the ore through the agency of carbonaceous matter has been advanced in connection with ore-deposits in other regions. I may quote, as instances, the black Silurian slates of Bendigo, Victoria; the Devonian slates of Gympie, Queensland; the Jurassic slates of the ‘Mother Lode ’ region in Calaveras and Amador counties, California; the black shale enclosing the gold-specimen ores of Farncomb Hill, Breckenridge, Summit county, Colorado; and the celebrated Indicator series of Ballarat, Victoria.”
In this connection, reference is made to the well-known occurrence of copper-ores at Mansfeld, Prussia, in beds of bituminous slate and bituminous limestone. Certain of the lower limestone beds are fetid. These copper-bearing formations extend over a large district. The ore contains so much bituminous matter that only a little brushwood is required in roasting the ores in piles.