Organic Matters of Limestones

Organic Matters of Limestones

In mining-regions where the ores occur in limestone, it is observed that in most instances the largest and most productive mines are in belts or zones of crystalline limestones which are either exceptionally pure lime-carbonates or, more frequently, dolomites with only a small amount of insoluble matter. Such formations, peculiarly favorable for ore, are rocks easily crushed by movements of disturbance, readily permeated by circulating-waters, and, from their chemical composition, rapidly attacked by solutions carrying carbonic acid. Where situated at the surface, they are cavern-forming limestones. Further, it is noted that both the magnesian limestones and the pure lime-carbonates usually contain some form of organic matter which, though small in amount, appears to have strongly influenced the deposition of the minerals in the strata.

Otherwise stated, ore-deposits in limestone, irrespective of the nature of the minerals constituting the ores, conform to the general law of selective deposition, namely, that “ Some geological formations appear to be everywhere barren of ore; others occasionally carry small deposits, workable where the conditions are exceptionally favorable; but in each mining region certain strata are ore-bearing in a degree exceeding all other formations combined.”

Among the causes that have induced the concentration and deposition of the ores in special formations, prominence may rightly be given to the deoxidizing action of the bitumen, bituminous coal, lignite, or other form of carbonaceous matter disseminated in the rock.

In Southeastern Missouri, the lead-ore now mined is mostly from deposits of disseminated galena in the dark-colored magnesian limestone, rich in bituminous matter, of the Cambrian formation at Bonne Terre and the Flat River mines. Practically all the zinc-ore, and the greater proportion of the lead, produced in the Joplin region, in the southwest part of the State, is yielded by the Cherokee limestone, the upper division of the Sub Carboniferous. The Cherokee is a soft, crystalline, pure lime-carbonate, carrying bitumen. Its average composition, from a number of analyses, is as follows:


In the Upper Mississippi lead-region the productive formation has been the Galena limestone, the upper member of the Trenton. It is a soft, crystalline dolomite; bituminous matter is present in relatively small amount, yet apparently more than sufficient to effect the precipitation of the metals and preserve the ores from oxidation below water-level. The deposits of blende in the mines near Mineral Point and Shullsburg, Wisconsin, occur in the underlying “ Blue” limestone of the Trenton, and the ores are concentrated about the intersection of the mineral-bearing fissures with thin strata of brown shale, saturated with petroleum,—the “ Oil-rock” of the miners.

A summary review of the occurrence of zinc- and lead-ores in the Mississippi valley shows that the formation of the deposits has been due to solutions of normal temperature; and that the chief agent in the primary deposition, and in the secondary enrichment of the ores, has been the bituminous substances contained in the strata in which the deposits are found. The larger part of the minerals constituting the ores has been deposited either by crystallization or by crystalline growth in the lime-rock or dolomite of the walls, or has impregnated the beds of specially favored geological formations.

In the limestone-area at Tintic district, Utah, the productive mines occur in two distinct belts in the Carboniferous formation; one, extending through the central part of the district, of dark-colored, magnesian limestones; the other, traversing Godiva mountain, on the eastern border of the lime-area, formed by beds of gray limestone with only a trace of magnesia.

On the central belt are situated the Gemini, Bullion-Beck, Eureka Hill and Centennial Eureka mines; and farther south, also in magnesian limestones, are located the Grand Central and the Mammoth. A number of these mines have been worked continuously since the early development of the district in 1870-71. All have reached a depth of 1000 to 1750 ft.; the Mammoth is now 2100 ft. deep.

It is not necessary to discuss the occurrence of the ore, beyond its relation to the magnesian limestones in which the deposits are found. Locally, these limestones vary somewhat in character; they are hard, crystalline, bluish-gray to bluish-black dolomites, the color being due to organic matter. In certain places the beds are filled with nodules and thin bands of hard, black chert. The average composition, from 7 analyses, of the limestones in the vicinity of the Bullion-Beck mine is (the organic matter and loss being estimated by difference):


In a number of analyses the silica varied from 6.75 per cent. to 13 per cent., and the total insoluble matter from 16 per cent, to 18 per cent.

The amount of silica and insoluble matter is remarkable, for there is every evidence in the mines that these dolomitic limestones are rapidly decomposed and eroded by carbonated waters. The explanation of this marked solubility of the dolomite, notwithstanding the large proportion of impurities, is probably to be sought in the structure of the rock. In the upper levels of the Bullion-Beck and the Eureka Hill mines, where the subterraneous erosion by surface-waters has been greatest, large masses of residual dolomite sand, which have resulted from the disintegration of the fissured and shattered beds, occur, filling cavern-spaces or chambers in the limestone. Beneath the heavy wash of boulders filling the gulch, the limestone beds, for a depth of near 100 ft., without change in the stratification, are decomposed and altered in situ into soft, sandy dolomite, stained with iron and manganese oxides. Analyses showed that this decomposed rock had substantially the same composition as the loose deposits of sand; being dolomite, with the residual silica, clay, oxidized iron and manganese contained in the original formation. Surface-waters, carrying carbonic acid, appear first to attack the calcareous cement between the crystalline grains of dolomite, at the same time oxidizing the carbonates of manganese and iron present, and in this way rapidly disintegrate the rock.

Many caverns, mostly of small size and usually more recent in formation than the ore-bodies, occur in the limestone. It is noted that they are generally located along the course of the vertical faulting-fissures, which have been the channels followed by the solutions depositing the ore, the shattering and brecciation of the beds, due to the faulting movement, increasing the action of surface-waters in the erosion of the rock.

In a few instances the ore actually fills pre-existing caverns. The largest of these caverns in the Bullion-Beck mine, formed before the minerals were introduced in the primary deposition, was filled with argentiferous galena, in great part deposited by crystallization. The rock-floor of the cavern was covered by a horizontal stratum of chert nodules, overlain by sand-beds, 10 to 15 ft. in thickness, with disseminated pyrite and galena, the massive crystalline lead-ore resting on the sedimentary beds. The flat-beds covering the clay floor were unquestionably formed from the residual sand and chert contained in the rock, dissolved away by the circulating-waters in making the cavern. Such occurrences of ore deposited by crystallization are rare in these mines; practically all the quartz, and nearly all the lead- and copper-ores, are formed by replacement.

The limestones are much purer in the Grand Central and Mammoth mines. An average of 6 analyses gave (ferrous carbonate, alumina and organic matter not determined):


This is a bluish-gray, brown, or bluish-black, crystalline dolomite, free from chert. The weathered outcrop of certain beds shows the rock to be made up of the broken and water-worn joints of minute crinoid stems, one- to three-hundredths of an inch in diameter, with fragments of shells and an occasional small coral. The stratification is generally preserved; in some places the rock is cross-sheeted by movements of disturbance, and locally the beds have been brecciated and recemented by their own attrition material into a massive rock, with but traces of bedded structure.

This limestone, even where not mineralized, is easily distinguished by the appearance and fracture of the rock,—being dark-colored and crystalline, with spots and small vugs of calcite and stains of iron and manganese oxides on the joints. It is often sonorous, giving a clear metallic ring when struck with a pick. It crumbles under a blow into small, ragged, rough fragments, having the fracture of loaf sugar.

The beds are thin, brittle, easily shattered and crushed by faulting-movements. On account of the absence of clay in the rock, the breccias and attrition-material produced are permeable to circulating-waters; even small fissures and fractures in the lime keep open. Caves of considerable size are not infrequently encountered in the formation, but appear to have been formed subsequent to the ore. It is noteworthy that deposits of loose dolomite-sands are seldom formed in this pure dolomite; neither is the rock usually altered in place to iron-stained rotten dolomite. The action of surface-waters is to dissolve the rock completely, and not to disintegrate the rock by attacking the calcareous cement between the grains.

The ore-bodies afford evidence that this limestone, especially where fissured and fractured, is easily replaced by silica and by the other gangue-minerals, ankerite, siderite, barite and calcite. Not only has the limestone been metasomaticaly replaced by primary sulphide-ores, carrying lead, copper, iron, arsenic, silver and gold, but the complex products of their oxidation and redeposition have, also, more or less perfectly replaced the rock. It is observed that the darker and coarsely crystalline lime-beds are highly favorable for ore,—beds made up of comminuted fossil-remains, crinoid stems, broken corals and shells.

Eliminating the highly impure sediments, which seldom carry workable deposits, the structure of the rock appears to be far more important than the chemical composition in determining whether certain strata are ore-bearing or barren. Analyses of the magnesian limestones over a broad belt show that they vary but little in composition. The beds which have been found to be unfavorable for ore are usually dense, finely crystalline and impermeable; the rock generally breaking under a blow of a hammer with a “ dead ” sound, and a smooth, splintery fracture, like the fracture of quartzite. Organic matter is not at all prominent in this limestone, although it gives the color to the rock.

The principal ore-bearing formation in Godiva mountain is a stratum, 200 to 400 ft. in thickness, of massive, gray, very coarsely crystalline limestone. It is a pure lime-carbonate, carrying some undetermined form of bituminous material, which gives to the rock, when broken, a sulphurous, fetid odor. On dissolving the limestone in acid, the bitumen separates in clots. An analysis of this limestone gave (the organic matter and loss being estimated by difference):


The weathered outcrop of the gray limestone is, in many places, bluish-black from the concentration of the bitumen in the surface of the rock. The beds are formed of water-worn grains and broken fragments of small shells. A coral (zaphrentis), 1 to 2½ in. long, is its characteristic fossil. It exceeds in chemical solubility all other ore-bearing limestones in this section of the district. The perfect manner in which great masses of the rock are replaced by the ore is evidence of this.

The gray-limestone formation extends northerly and southerly through the whole length of Godiva mountain, bounded on either side by limestones more or less magnesian in character. Many of these magnesian limestones, interbedded in the series, are highly impure sediments, with 25 to 35 per cent, of silica, 5 to 10 per cent, of iron and alumina, 6 to 12 per cent, of magnesia, and 20 to 30 per cent, of lime. These formations have been found unfavorable for ore.

On this gray-lime belt are located the Uncle Sam, May-Day and Yankee Consolidated mines,—properties developed since 1897. The deepest workings have attained a depth of 800 ft. The Uncle Sam mine has been noted for its large output of high-grade lead-ore, carrying silver, in a gangue mainly composed of lime-carbonate.

Quartz-ores prevail in the other mines of the belt, with lead, silver, and usually a small amount of gold.

The ores have been introduced in the strata through belts of nearly vertical faulting-fissures. Along the course of these fissures the ore-bodies have formed in the limestone. In some places the ore-deposits take on the form of fissure-veins, the ore being confined within the walls of the fissure and deposited in a more or less tabular sheet, pitching like the ore-shoots in quartz-veins in the metamorphic rocks. More commonly the faulting movements forming the fissures have so fractured the beds that the mineral deposits are not limited by the fissure-walls and extend irregularly into the limestone. The largest ore-bodies have formed in spaces of multiple fissuring, where the belt of master-fissures cuts through lime-beds, broken and rifted in different directions by the complex intersection of sheeted belts, due to cross-fissures and to diagonal fissures.

The ores of primary formation are mostly deposited by replacement of the limestone. Quartz occurs in many varied forms,—from massive limestone, more or less completely altered to quartz, with little change in structure, to the white, crystalline mineral, grading insensibly into soft, crumbling, pulverulent quartz, in appearance resembling granulated sugar.

In certain places in these mines the limestone appears to have been sheeted and broken into large, thin and sharp fragments before it was replaced by the ore; the sharp edges of the pieces of limestone were not rounded in the conversion; there has not been any solution of the rock without the perfect pseudomorphic replacement of its structure by the minerals.

This has occurred not only in the replacement of the lime-stone by quartz, but also in its replacement by massive argentiferous galena, as fine-grained in its crystalline structure as steel. The galena reproduces the shape of the original limestone fragments, so that they are fossilized by lead-sulphide, as wood is petrified by the infiltration of silica, only less perfectly.

The largest body of galena of this character occurred in the Uncle Sam mine. The ore-body, 50 ft. long, 13 to 20 ft. wide, and 50 to 60 ft. high, was formed entirely of pure lead-sulphide, with no other minerals except calcite and the lime wall-rock. The ore averaged 75 per cent, of lead and 50 oz. of silver per ton. In this ore-body the massive limestone, prior to its mineralization, had been fractured vertically in large, sheeted fragments, some of which would measure 10 to 15 ft. long and 10 to 25 ft. high, but only 6 to 15 in. thick. Even the largest masses of rock were altered throughout to steel-galena. Numerous vertical open seams and fractures, from the thickness of a knife-blade to 2 in. in width, separated these irregular sheets of ore one from another. A vertical fracture, 12 to 18 in. wide, passed through the ore-body; it was more recent in formation than the primary ore and was filled with coarsely crystalline galena, with cleavage faces 2 to 3 inches across, deposited by crystallization.

In general, in the mines on Godiva mountain, fine-grained galena, replacing the limestone, is of primary origin, while coarsely crystalline galena is usually secondary; although some of the lead-ore deposited by crystallization (or crustification) appears to be primary.