Carbon has long been recognized as one of the most powerful reducing agents in the deposition of ores. Investigations, made by myself, of the zinc- and lead-deposits in Southwest Missouri, in the region centering about Joplin, where the formation of the metallic sulphides has been due to the action of bitumen, carbonaceous shales and bituminous coal, have afforded abundant evidence that the solid oxygenated hydrocarbons, particularly when in fine powder and in suspension in the waters circulating through the ore-bodies, are the most energetic and powerful reducing agents known.

Bitumen, liberated by the decomposition of the ore-bearing limestone, is found in the Joplin mines in all degrees of fluidity and hardness, dependent on the amount of oxidation it has undergone. From semi-fluid maltha it grades into partly oxidized mineral-pitch, which, by further oxidation, changes to hard asphalt, finally breaking up, from continued absorption of oxygen, into a fine powder resembling in appearance powdered coal. In this condition oxidized bitumen, from its light gravity, is transported readily in suspension in the underground circulating-waters.

Bituminous coal and black carbonaceous clays and shales occur as surface formations, often in intimate association with the ore-deposits, and when broken up, crushed, and faulted by crustal movements, furnish organic matter in a state of fine division. This is borne by the surface-waters and redeposited, often in large masses, in the channels and spaces in the ore-bodies.

Apparently the only pure form of carbon occurring in the ore-deposits of the Joplin region is the charcoal, which is found in inconsiderable quantity in the bituminous coal; all other carbon is combined with hydrogen, oxygen and nitrogen.

All these hydrocarbons readily absorb oxygen from the air contained in the subaerial waters, and undergo at ordinary temperatures, even below ground-water level, a slow combustion, the ultimate product being carbonic acid and water. They deoxidize the circulating waters by consuming all the free oxygen ; and they reduce to sulphides any sulphates that may be in solution. Even ferrous and ferric sulphates, on coming in contact, in solution, with any of the solid hydrocarbons, are at once re-formed as pyrite or marcasite,—minerals that reduce all other metallic sulphates, first, by being oxidized to ferrous sulphate, and then, by further absorption of oxygen, to ferric sulphate and limonite.

Bain, writing of these mines, says, “ The widespread presence of bitumen has been already emphasized. The ground-water is one great reducing solution.” Nearly every observer in this field has recorded the association of bitumen with the zinc- and lead-ores.

In my paper on “The Lead- and Zinc-Deposits of the Mississippi Valley” I called attention to the reducing action of bitumen and bituminous shales in the formation of the ore-deposits of the Joplin District, and noted the influence of organic matter in the rocks upon the selective deposition of the ores in certain beds in the Cambrian and Silurian of Central and Southeastern Missouri, and also of the Upper Mississippi lead-region.

In limestones containing organic matter, especially if the rock be easily dissolved by waters carrying carbonic acid, the efficiency of even so minute a quantity as a fraction of one per cent, of bituminous substances, in inducing the precipitation and crystallization of the ores, is very great. Small as the proportion of the carbonaceous matter may be, it is liberated by the rapid subterraneous erosion of the lime-strata, in quantity more than is required to consume the free oxygen in the ore-forming solutions, and to reduce all sulphates to sulphides. In limestones highly soluble in carbonated waters, two-tenths of one per cent, of any strongly deoxidizing material, as bituminous coal or bitumen, disseminated in the rock, appears to be ample to effect the reduction of the metals, and to induce the deposition of the ores in the special geological formation.

Compared with carbon, hydrogen has far greater reducing power, measured by the amount of oxygen consumed, for hydrogen, in the production of water, combines with three times the weight of oxygen that unites with carbon in forming carbonic acid. In fact, hydrogen stands first in reducing power, accomplishing nearly nine times the work of pyrite, the most efficient metallic sulphide in the redeposition of ores. Sulphur, oxidizing to sulphuric acid (SO3), requires less than one-fifth the oxygen that combines with an equal weight of hydrogen in forming water.

For illustration, the relative reducing power of hydrogen, carbon and sulphur may be compared with the heat generated by their combustion, although their calorific values do not run parallel with their respective powers in the deoxidation of mineral solutions.

A calory being the quantity of heat necessary to raise 1 lb. avoirdupois of water 1° C., the heat generated by the combustion of 1 lb. of the following substances is:

On account of the calorific value of the contained hydrogen, the heating-power of the highest grade of bituminous coals is greater than that of pure carbon.

It is not improbable that the bituminous coals occurring in association with the ore-deposits of the Southwest, on account of their purity and high percentage of hydrogen, have a somewhat greater reducing power than pure carbon, and are exceeded in power and intensity of action only by bitumen. The lignitic or finely divided coaly matter, disseminated in the black clays and shales, may be regarded for the purpose of this discussion as a form of highly impure bituminous coal. Anthracite may be classed with the bituminous coals, as it contains from 1 to 3 per cent, of hydrogen and 1 to 3.5 per cent, of oxygen.

The hydrocarbons may be divided into the petroleums, or fluid non-oxygenated compounds of carbon and hydrogen; the bitumens and asphalts, solid oxidized hydrocarbons, soluble in chloroform or other solvents of the resins; and the pyro-bitumens, which also contain oxygen and nitrogen, including anthracite, bituminous coal, lignite, etc.

Oxygenation of Petroleum

 

Protective action of Carbon and of Hydrocarbons

Hydrogen and carbon have affinities for oxygen stronger than those of any other chemical elements, under conditions normally occurring in ore-bodies. By consuming the free oxygen in the circulating-waters, they act to preserve and shield from decomposition all metallic sulphides. Otherwise stated, all forms of carbon, and of the fluid and the solid hydrocarbons, when present in excess, owing to their superior affinity for oxygen, prevent the oxidation of the ores; although many of the minerals in the ore-bodies under other conditions, where carbon and its compounds are absent, or present only in a sub-ordinate degree, are powerful deoxidizing agents.

In the mines at Joplin, Mo., the metallic sulphide-ores, blende and galena, and the associated minerals, chalcopyrite, pyrite, and even marcasite, are protected from decomposition below ground-water level by the bitumen and the bituminous shale contained in the wall-rock and present in the ore-bodies. Near water-level, on the boundary between the zones of oxidation and reduction, these hydrocarbons are consumed in places; although, a few feet distant in the same ore-body, they may occur in great excess. In such places the metallic sulphides undergo oxidation, only to be re-formed a new on coming in contact with the hydrocarbon.

This protective action has long been understood with respect to the rocks which contain organic matter. In the black-band iron-ores, and in many bituminous shales, the iron occurs as the proto-carbonate (siderite), and is preserved from oxidation by the hydrocarbon. In the outcrop, such rocks are bleached by weathering, and the iron is oxidized to limonite or hematite. That organic matter preserves the strata from oxidation is a fact familiar to persons engaged in collecting fossils, particularly fossil-plants. The beds of black and gray shales, where the iron occurs as carbonate or sulphide, are carefully searched, while bright-colored strata, in which the iron is peroxidized, are given only a slight examination.

Contributory Action of Carbonic Acid Gas

In the formation of ore-deposits, carbonic acid gas may, under special conditions, displace and expel the air from the cavities, channels and interspaces in the rocks, and in this way, by mechanically excluding the air, materially aid the reduction and precipitation of the ores by the ordinary deoxidizing agents. Conditions also occur in the oxidation and re-formation of an ore-body, particularly at those points where the zones of oxidation and reduction merge one into the other, under which carbon dioxide would be an efficient auxiliary in the process of reduction.

The specific gravity of carbonic acid gas, compared with air as the standard, is 1.524. Its action in displacing air is not unlike that of water; a rise in the ground-water from any cause, as is often observed, drives out the air from all the openings in the ore-bodies and checks the oxidation of the minerals.

In the Parker mine, Wood River, Idaho, the country-rock is a lime shale, heavily charged with graphite. Above the permanent water-level all the seams and joints in the rock are filled with carbonic acid gas, produced by the oxidation of the carbon. Carbon dioxide is found in a zone reaching from ground-water level to within 100 ft. of the surface; in this zone it fills all the rock-openings as perfectly as water fills similar openings below.

Sulphuretted hydrogen gas, which has a specific gravity of 1.19, may act in much the same manner in excluding the air, although in itself it is a strong precipitating and reducing agent.

Natural gas, notwithstanding its low specific gravity (0.558), may he held under pressure in the strata in much the same way as it occurs sealed in the interspaces of the Trenton limestone of the Ohio and Indiana gas-field.

At the Silver-Islet mine, Lake Superior, pockets filled with hydrocarbon gas were struck in drifting on the vein on the 8th (440 ft. vertical depth) and 10th levels (610 ft. depth). The gas was held confined in the seams of the slate and in the openings in the vein under a water-pressure of from 440 to 600 ft., equal to a pressure of from 190 to 265 lbs. per square inch.

Observations at many gas-wells show that the pressure greatly exceeds that which would be due to a water-column equal in height to the depth of the gas-producing strata. There is no record of any measurement of the gas-pressure at Silver-Islet.

The Stability of Carbonic Acid and of Water

 

Occurrence of Carbon and the Carbon-Compounds

 

The Occurrence of Carbon combined with Hydrogen

Lignite

 

Bituminous Shales

 

Limestones Containing Organic Matter