Geology, Vol. 1 [of 3]

by Thomas C. Chamberlin; Rollin D. Salisbury

Solution also continues to take place varyingly as the water descends below this zone of dominant solution, and extends probably to the full depth of water circulation, but in the deeper circuit, precipitation also takes place and the action becomes complex. With the waters taking up and throwing down material at the same time, it is difficult to estimate the balance of results.

When waters that have been mineralized near the surface descend, they often take on a precipitating phase at no great depth below the upper level of the ground-water; thus sulphides that were oxidized and dissolved near the surface are reprecipitated, often at horizons not greatly below the permanent water-level. Waters that dissolve metallic substances in the upper levels often become charged with sulphuretted hydrogen and other precipitants within a few scores or a few hundreds of feet of the surface, as deep wells abundantly prove. The freshness of surface which metallic sulphides often exhibit at these levels is fair ground for inferring recency of deposition and absence of solvent action. Actual demonstrations of depositions in progress are not wanting.

Short-course action.—The concentration which thus takes place by solution in the upper zone, followed closely by reprecipitation within a few score or a few hundred feet, may well be termed the short-course mode of ore concentration. It finds its most important illustration in what is commonly known as the “secondary enrichment” of ore-deposits. The ores in the outcropping edge of the vein or lode are dissolved by the surface-waters, carried a short distance down the ore tract and redeposited, causing enrichment at that point. This is only a special case of what takes place generally at this horizon. It is effective in this case because it has a previous partial concentration to work upon. Secondary enrichments of this kind often contain most or all the workable values of the ore tract. If instead of a previous concentration in a vein, lode, or similar ore tract, there had been partial concentration in the country rock by sedimentation, as in the case of iron-ore beds and perhaps lead-, zinc-, and copper-impregnated sediments, the short-course method may give working values not before possessed. In some of the more obscure cases of previous partial concentration in the crystalline and other rocks, it is probably this short-course action that brings the concentration up to working value. It is probably effective also in concentrating the metallic contents of certain igneous rocks that were rich in metallic material when extruded. How far this is true has been, and still remains, a mooted question.

Long-course action.—After the surface-waters have once passed through a cycle of dissolving and precipitating action, as they are apt to do within the first few hundred feet of their courses below the water-level, they are liable to pass through a succession of dissolving and depositing stages, each reaction resulting in a state that makes a new reaction possible. This is especially true if the waters pursue deep courses. Strictly speaking, the precipitations usually concern only a part of the substances dissolved. New substances are often taken up in the very act of throwing down those already held, and the way thus prepared for further changes. If the water pursues a deep and devious course, it may receive additions by solution and suffer losses by precipitation at many points in its course, both descending and ascending. The changes are very complex, and in the case of a deep or long circuit where various rocks, pressures, and temperatures are encountered, the history becomes one long succession of complexities, the full nature of which is not yet revealed.

In the deeper circuits, each individual current usually takes on a descending, a lateral, and ascending phase, the three being necessary to complete a circuit. The chemical conditions of the waters in the three phases are probably not sharply distinguished from one another, and hence there seems to be no defined horizon of concentration comparable to that near the water-level already described. The chief distinctions in the deeper regions relate to pressure, temperature, length or depth of penetration, and duration of contact. It seems safe to assume, as a general truth, that, other things being equal, the solutions become more complex and more nearly reach general saturation the farther and the deeper the waters penetrate.

It has long been a mooted question whether ore-deposits are due chiefly to descending, to lateral, or to ascending currents. The question in its usual form is too undiscriminating for advantageous discussion, but if the ore-deposits due to surface or short-course concentrations and reconcentrations be set aside, as in some sense a separate class, the relative functions of the descending, the lateral, and the ascending portions of the deeper circulations become a measurably definite question. Two great working factors enter into the comparison: (1) much greater circulation in the upper zone, where lateral movement most prevails; (2) much greater heat and pressure in the lower zone, where the circulation must be chiefly vertical.

Heat and pressure in general favor solution, and hence so far as this factor goes, descending water is likely to be increasing its mineral content, rather than diminishing it by deposition. But this is only general; particular elements of the solution may be deposited. In ascending, as the same water must later, it is predisposed to deposition from loss of solvent power through reduction of pressure and temperature. The theoretical balance is here clearly in favor of preponderant deposition by the ascending portion of the current. So far as precipitation is dependent on the mingling of differently mineralized waters, descending and ascending currents seem to be situated much alike, in general, for both are subject to accessions and mutual unions.

The amount of water that circulates in the deeper horizons is much less than that nearer the surface. Allowing a few hundred, or at most one or two thousand feet for the special short-circuit zone next below the water-level (it is known to reach 1000 to 1500 feet in some cases), the water circulating through the next 1000 or 2000 feet is probably several times greater than all that circulates at greater depths, and this greater circulation above doubtless offsets, in greater or less measure, the intensified action of the deeper circulation. Much of the upper and more rapid circulation is lateral, being actuated by the sloping surface of the ground-water, which in turn is determined by topography, precipitation, and other surface conditions. Theoretical considerations, therefore, favor the view that lateral flow is an important factor in the concentration of ore material. But as descending and lateral currents almost inevitably meet and mingle with ascending currents, it is difficult to distinguish, in the ore-deposits, the special functions of each phase of action. It is even more difficult to determine whether the different phases are not alike essential to the mutual reactions on which the deposition depends. It may be as necessary to have a precipitant as to have a metallic constituent in solution to be precipitated, and what is more, this precipitating agency may be a substance of no economic value in itself and of no obvious relations to the substances that form the ores. If the deposition is due solely to a physical state, as relief of pressure or lowering of temperature, these considerations do not hold.

Summary.—The general results are probably these: In the deeper circuits, more ore material is brought upward and deposited than is carried downward and deposited, so that metallic values are shifted toward accessible horizons. In the lateral currents, more metallic values are shifted toward the trunk-lines of circulation—the great crevices and other waterways—than are carried from these into the rock and distributed, and lateral segregation results. At the same time the atmospheric waters acting at or near the surface concentrate ore values downwards. The sum total of these processes is to promote the development of the higher ore values in accessible horizons, and along the main lines of circulation.

The influence of contacts.—As ore-deposits depend on a dissolving state followed by a depositing state of the waters, and perhaps on a complex succession of these alterations, it is obvious that conditions which favor changes of state and the commingling of different kinds of water are apt to be favorable to ore production. At any rate it is observed that many important ore-deposits occur at the contact between formations of different character. The contact of igneous rock with limestone is a rather notable instance. It is not to be inferred that such contacts are generally accompanied by workable ore-deposits, but merely that a notable proportion of workable ore-deposits occur at such junctions. It is rational to suppose that where the chemical nature of the two formations is in contrast, the waters that percolate through the one are likely to be mineralized very differently from those that course through the other, and hence that on mingling at the contact, reactions are specially liable to take place, and that when a valuable metallic substance is present it is liable to be involved and by chance to suffer precipitation. Reactions are the more probable because the contact is likely to be a plane of crustal movement, and hence more or less open and accompanied by fractures, zones of crushed rock and other conditions that facilitate circulation and offer suitable places for ore formation.