Deposition of Ores in Limestone

Deposition of Ores in Limestone

A study should be made of the structure of the ore-bearing limestones, with the special object of determining the causes that have made certain strata favorable for ore, while other beds in the same geological formation, having an almost identical chemical composition, and so situated that they are traversed by the same fissures, through which the mineral-depositing waters have been introduced, have remained barren. In many instances the productive and the barren strata are interbedded and so situated that the ore-bearing fissures cut through all the beds alike, without any change in this selective deposition of the ores.

Analyses of these ore-producing limestones are needed to determine the amount and character of the carbonaceous substances present, and also the minute traces of other elements, some of which may be found to have had an influence on the formation of the deposits. That such an influence may have been exerted seems probable, when we consider the enormous masses of the highly soluble limestones that have been dissolved or replaced in the creation of the ore-bodies. It has been shown, for example, that the small percentage of bitumen or other hydrocarbon contained in the rock, and set free by its dissolution, has strongly aided in the deposition of the ore.

Prof. Church, discussing the deposition of ores in limestone, says: “ The operation of solutions whose composition we do not know can be judged only by their effects. When metasomatic replacement takes place in limestone, it is generally assumed that lime-carbonate goes into solution, while its place is taken by the ore-substances,—that is to say, that the action is molecular substitution, and not atomic; but it is conceivable that the change should begin by an interchange of acidic elements—that SiO2 should drive out CO2. Subsequent changes might remove the lime-silicate by another process of substitution, since it is more soluble than silica; but the point is that CO2 would be liberated, and, though the original ore-solution were free from CO2, it would immediately become charged with that agent and exert the well-known dissolving power of carbonic acid solutions. In this way a solution which would have but feeble power in other rocks may in limestone set up a chain of reactions that would intensify its effects. Limestone contains the elements for self-destruction, since the breaking-up of one lime-carbonate molecule may cause the solution of another; and as this cannot be said of any other rock, we reach a possible explanation of the comparative frequency of ore-bodies in limestone. The dolomites would, of course, present similar reactions.” Prof. Church continues, respecting “ The selection of a favored stratum for ore-deposition. In some situations the solutions, before reaching the stratum of actual ore-deposition, must have passed several strata suitable for their action, if they had possessed from the beginning the power of solution which they showed ultimately.

Ore-solutions exhibit a selective power which is extraordinary in a water fully supplied with dissolving qualities, but quite explicable in a solution which lacks this power.”

Many contributory causes have in all probability cooperated in the deposition of the ore, such as decrease of pressure and reduction in the temperature of the solutions, the mingling of mineral-bearing waters of different chemical composition entering the limestone formation through distinct fissured belts, etc.; but the important factor appears to have been the great solubility of these limestones and dolomites in the waters which brought in the minerals, joined with the chemical activity of the contained hydrocarbons released in the dissolution of the rock.

In the solution of the limestone, the incidental liberation of large volumes of carbonic acid, ever dissolving more and more of the rock, set free a constantly renewed supply of carbonaceous matter, whose function was to remove all free oxygen and reduce the sulphates in the waters to sulphides. At the same time, the calcium- and magnesium-carbonates, when dissolved, neutralized the acids and destroyed the chemical equilibrium, so that the mineral-saturated waters could no longer hold the metals in solution, after the addition of the elements derived from the limestone. The combined action of the carbon, hydrogen, lime and magnesia contained in the rock was to deoxidize the solutions and bring them to the “ critical-point,” when deposition of the ores rapidly took place.

In conclusion: The ores of primary formation in the Tintic mines have been, in most of the occurrences, deposited from highly heated solutions by the metasomatic replacement of the limestone; only in relatively subordinate amount have the metallic sulphides been formed by crystalline growth in the rock, or by crystallization in the interspaces of the ore-bodies.

In the instance cited, in the Uncle Sam mine, in the large body of steel-galena, without quartz, replacing the fractured lime strata, the deposition seems to have been from solutions either free from silica, or more probably of so low a temperature that the chemical reaction in the substitution of quartz for the lime-carbonate could not take place.

In Tintic, the limestones, when unaltered, retain the included carbonaceous matter deposited with the sediments. In the ore-bodies, all forms of the hydrocarbons have been destroyed, either in the primary formation of the minerals or in the subsequent oxidation; the deepest mines in the district (1700 and 2100 ft., vertical depth) not having reached ground-water level.

Whatever may have been the role of the volatile hydrocarbons in the original creation of the deposits, no evidence has been found of their ever having been present. Tintic district has been a center of intense volcanic activity, and it seems almost inevitable that, with the presence of notable quantities of bituminous matter in the rocks, volatile hydrocarbons would have been formed.

Many examples might be given of limestones, especially of magnesian limestones, which carry more or less organic matter and constitute the favored geological formations in the selective deposition of the ore. The zinc- and lead-deposits of Missouri and the lead- and copper-ores of Tintic District, Utah, carrying silver and gold, were chosen, owing to the author’s more detailed acquaintance with the ore-deposits of those regions.