The valuable results that have followed the application in recent years of microscopic methods of research to problems of ore genesis have been significant, but possibly the recognition of their practical importance is not as general as might be, and while, as a scientific method of investigating ores and rocks, the advantage derived through the use of the microscope is unquestioned, its utility as an aid in solving mining problems arising during actual every-day mine examination and operation may not be fully appreciated.
Perhaps the most logical method of showing some of these practical applications of the microscope will be to consider first the prospect stage of a mining venture, then the uses to which the instrument may be put during the operation of a working and developed mine, and, finally, to discuss some of the problems of a metallurgical nature that may arise during milling and smelting the ore.
The valuation of mining property as a preliminary to acquiring ownership or determining the advisability of investing time and money in an enterprise is one of the most important and delicate problems that come before the mining engineer. The many and variable factors that have to be taken into account, as, for example, the geology of the district, availability of supplies, probable mining costs, transportation facilities, and the quality of ore, require a careful balancing of costs against recoverable values.
Primarily, however, a study of the local geology is of first importance, since, in a prospect with little development work, the question of character and probable quantity of the ore and its value can only be approximated with any degree of accuracy by a thorough investigation of geologic conditions, and a general knowledge of ore deposits. Unless these are favorable and pay ore reasonably certain, other factors need not be considered. The character of the outcrop, evidences of mineralizing agencies in the adjacent rocks, and geological structure, are the data of chief significance.
Now, an early question which will present itself is: What sort of a deposit have we to deal with ? Is it a fissure vein, for instance; is it a replacement, or is it possibly a contact-metamorphic deposit?
At first thought it might seem as though the broad general structural characteristics would serve to differentiate these classes, and usually an idea may indeed be gained in this way which will be proved by subsequent investigation to be correct. Not always, however. Let us take, for example, the first two types, fissure veins and replacement deposits. Normally these have few points in common. The former has relatively well-defined walls, and comparative regularity of thickness and character; the latter appears in ramifying masses and irregular mineralized areas throughout the containing rock.
Fissure veins are formed when mineral-bearing waters or gase; traverse a pre-existing opening in the rock where some dynamic disturbance has caused fracturing and faulting. The channels for underground circulation so formed are gradually filled by mineral matter deposited from the solutions.
Replacement deposits, on the other hand, are formed by chemical interchange, or metasomatism. No open space for deposition exists except such minor cracks or fractures as are necessary to permit the solutions to penetrate the rock. The process is really one of substitution. Certain of the rock constituents are dissolved and, at the same time, metallic sulphides or foreign minerals are deposited. The reaction is simultaneous, molecule by molecule, solution of one and precipitation of the other, until finally the original grain is wholly replaced by a new constituent.
Now consider a certain type of replacement ore body often met with where rocks have been subjected to intense dynamic stress which has produced fractures along more than a single plane. The effects of the disturbance are distributed through a series of narrow parallel cracks, closely spaced in the zone of maximum movement and gradually separated by wider and wider intervals toward the outer limits. Under favorable conditions a complete replacement of the narrower tabular sheets of rocks toward the center of the zone might develop a solid mass of ore in which the original fissuring would preserve a structure closely resembling true crustification, while, toward the outer limits, a plate or wall of barren rock would separate adjacent areas of mineralization. In cases of this kind it might easily happen that errors of interpretation would result in misleading conclusions regarding genesis, and the presence of valuable ore might remain unsuspected.
Crustification, or banding, is one of the typical features of mineral deposition in pre-existing openings, but the type of deposit just described differs little in its general aspect from a true fissure vein.
And now, having indicated the problem, let us inquire how the microscope aids in its solution:
By study and investigation of many various types of replacement ore bodies geologists have detected certain common peculiarities, and have come to recognize certain criteria as being characteristic of metasomatic conditions. It is through observations of this sort, some structural and some microscopic, that questions of origin and genesis may be determined.
On examining a normal igneous rock in thin section some of the component minerals will be found in well-developed crystal form; In other words, they are idiomorphic toward others which show only an irregular outline.
In order that a pyrogenetic mineral may develop crystallographic faces on all sides it must have formed freely while suspended in a liquid of approximately its own specific gravity and without interference from neighboring crystals. Therefore the well-bounded minerals are believed to be early crystallizations from the magma. Iron oxides, apatite, and titanite, for example, are often idiomorphic.
But when we examine a rock, either igneous, metamorphic, or sedimentary, that has been mineralized, and find that the new mineral, introduced long after the rock was formed, has complete crystal outline, it is believed to indicate metasomotic replacement, since this theory of molecular interchange seems the only reasonable explanation. Had the new mineral started growth in the interstices between grains and forced them apart, the fact would be demonstrated by strain shadows developed in the adjacent minerals; or had the crystal formed in an open cavity, it would have been imperfect at its point of attachment to the walls. This evidence then, complete perfect crystal development of the new mineral in a rock, furnishes one important criterion of replacement. Its value, however, varies with the kind of rock in which the complete crystals occur, and other qualifying factors must be determined before reasonably safe conclusions can be reached.
When dealing with igneous rocks it must first be ascertained with certainty that the mineral is really secondary, introduced after consolidation, and crystalline form, alone, can be used as a criterion only with those minerals not known to form under igneous conditions.
With other more typical ore minerals the problem is somewhat involved, since, under certain conditions, though rarely, they may be original. But if it is found that the mineral is thickly disseminated in the neighborhood of a crack and is not prominent in the normal rock, the fact of its secondary nature is established, and its relations to other grains may then be utilized to determine replacement; or, if the mineral cuts or intersects a quartz phenocryst in acid lavas, the criterion is of value, since quartz is here an early crystallization.
Metamorphic rocks present some difficulty, because even the original nature of the rock itself is obscure in many cases, and, unless the secondary minerals were introduced after the change and transformation were effected, very few indications will be found by which their origin may be determined.
But, if the new crystals intersect typical metamorphic minerals, or cut across the schistosity, and are entirely free from strain and fracture, it is reasonably certain that deformation of the rock was produced prior to mineralization, and the usual criteria may then be, applied.
With the sedimentaries the problem is simpler. Most ore minerals are too soft and friable to resist the extreme mechanical attrition during the formation of the sands and rock, and it is sufficient to prove only that the mineral has not grown by forcing the layers or bands of rock apart, but has really formed at the expense of the rock material.
Another criterion of value lies in the preservation of original rock structures. A dolomitic limestone, for instance, may be replaced by silica and still preserve the minute, rhombic, dolomite crystals in their original form; yet polarized light will show that they are now merely a quartz aggregate.
Still another microscopic criterion that is a rather definite indication of replacement is the presence of perfect pyrite crystals or of quartz crystals, surrounded by a mass of the same or other ore minerals. The inclosed crystals may or may not be due to replacement, hut the surrounding mass proves the gradual substitution of ore for rock. It must be demonstrated, however, that the deposit is not a magmatic segregation.
Suppose now we take up another problem connected with mining in the prospect stage or in early development. To vary the treatment, this discussion will apply more particularly to the fissure-vein type of deposit. Let us assume that a shaft has been sunk on the vein and that conditions have been exposed somewhat below the actual outcrop. A question of vital importance to the success of the mine, and one often submitted to the petrologist for an opinion, is the probability of extension with depth. What sort of conditions or what mineralogical changes may be expected at lower levels ?
The factors to be determined and the questions which depend for their solution upon observations of mineralogic relationship and structural characteristics of the vein, are, first, the nature of the primary mineralization, and, second, the probable extent of enrichment through secondary processes.
Primary ore, while reasonably certain to extend to great depth, is not always of commercial grade, and pay ore is more likely to result from a reworking of the deposit by later solutions. Descending surface waters penetrate the vein and cause changes in the upper zone through oxidation and solution, with redeposition as secondary sulphides at lower levels.
The original formation of a vein is by no means the end of the operation; alteration of some kind is always going on. It is doubtful if any marked change could take place, however, without the presence of water, and, unless aqueous solutions have free access, this change is very slow. It is evident, then, that the extent of alteration depends largely upon the character of the vein material as regards permeability to solutions. Another factor of importance is the mineralogic nature of the gangue. For example, a gangue prominently calcic retards secondary enrichment, probably through neutralizing the active organic or mineral acids in solution.
Microscopic examination reveals all this, sometimes clearly, sometimes less definitely; but, even if an original mineral be wholly removed, traces often remain, or characteristic alteration products are produced by which the original may be identified. Permeability of vein and wall rock is determined by evidence of brecciation, fracture, or distortion of the constituents.
Figs. 1 to 5 illustrate the mineralogic changes that may be found in a vein from the surface downward. The sections were made from samples from the Otter vein, Patagonia district, Ariz.
Most minerals are known to be capable of formation under many different agencies, but there are a few which, by their very presence in an ore deposit, serve to indicate certain definite physical conditions, and, where these are present, conclusions as to genesis are simplified.
Tourmaline, for example, is not known to form from aqueous solutions in the ordinary way, but is a typical pneumatolytic mineral which has been formed from vapors and under high temperature. Garnet is another, but it is particularly indicative of contact metamorphism.
Ores of this type, formed by deposition from gases and vapors, usually above the critical temperature and pressure of water given off by a rock magma during solidification under deep-seated conditions, are rarely of economic importance, and, where their original character is still preserved, with primary sulphides at or near the surface, it is rather useless to look deeper for profitable ore.
In a study of any class of ore deposit an examination of the associated rocks is always necessary. Wherever mineralization has been extensive enough to form profitable deposits the rocks will be found considerably altered, not by ordinary surface change, weathering and oxidation, but through reactions occasioned by the mineral-bearing solutions themselves. Where rocks are fresh, mineral deposits will not, as a rule, be found worth developing.
Figs. 6 and 7 are micro-sections illustrating rock alteration.
In discussing the field of usefulness of the laboratory microscopes during active mining operations it will be evident that the problems are of a somewhat different nature from those we have been considering. Questions of genesis and origin are no longer of real economic importance. The ore is exposed to some depth, and has been explored along the various levels and has been blocked out. Its quantity can be estimated with some accuracy and its metallic content ascertained by systematic sampling and assay; and since the zone of secondary enrichment may have been passed through, the primary mineralization may be exposed for study.
The success with which microscopic methods may be used to aid in solving the new problems is, however, just as definite, but in this case the work is more one of identification than one of interpretation. The structural geology of the district and of the mine become important factors.
The most economical system of stoping, the proper location for shafts and tunnels, the distance between levels and other purely engineering problems depend upon geologic structure, and upon the nature of ore and rock formations.
These factors are best determined by a careful survey of accessible workings and the compilation of accurate mine maps showing all geologic features. A certain dike, fault, or ore body, for example, may be plotted and correlated through similarity of microscopic characteristics with another exposure on a different level. Its strike and dip can then be computed, and its probable location at a desired point determined.
With these data the miner is enabled to reach the particular formation with the least possible excavation; or avoid it, should this be necessary. But, if the identification be faulty and general appearance alone be depended upon, it may happen that two separate and distinct
features will be assumed to be identical. Location on this basis may prove to be in error by hundreds of feet. These problems become particularly important in regions where the structure is complex.
Figs. 8, 9, and 10 illustrate changes in rock texture produced through differences in the rate of cooling.
The identification of rocks and ores, then, is one of the fields of usefulness for the microscope in the operation of producing and developed mines, and it was the accuracy with which rocks could be identified and correlated by petrographic methods that led to one of the first practical applications of it in an actual mining problem.
The particular question involved related to the identity of certain formations in the Eureka district, Nevada, and to the definition of the terms “ vein ” and “ lode ” as used in the U. S. Mining Statutes.
A body of mineralized limestone in Ruby Hill, near Eureka, had been located and mining operations conducted for some time. Unfortunately, a dispute arose. One company, which had followed down a more or less connected vein within the limestone, claimed ownership of a certain mass of ore on the 5th level, by virtue of the law of extra-lateral rights; the other company contended that the limestone itself was a lode as defined by the statute, and therefore that they were entitled to that portion of the ore located on their side of a compromise line between the two claims.
During litigation it developed that a definite foot-wall of quartzite formed one boundary of the mineralized zone and that a shale and limestone formed a definite hanging-wall. The rock embraced between the two differed in its characteristics from all other limestone in the vicinity. It was extensively altered; crushed, brecciated, and recemented by calcareous matter and silver-lead ores. No traces of stratification remained.
These facts, demonstrated by microscopic methods, helped to establish that the mineralized Prospect Mountain limestone itself constituted a lode or vein of ore in place.
Besides the usual results to be derived from this sort of work, quite unexpected facts of profitable import are sometimes brought to light. A case that came to notice not long ago serves to illustrate the point:
Having had occasion to determine the nature of a mineral occurrence in southern Arizona, a microscopic investigation of the ore was undertaken. Samples of the neighboring rocks also were taken, to help in the solution. One of these proved to be a diorite, of normal appearance at first sight, but, on more careful examination, it was discovered that throughout the rock were some peculiar opaque white crystals, perfectly developed, and generally surrounded by magnetite. They were identified as native silver. An assay of the rock, after careful sampling, gave an average silver content of 7.5 oz. per ton. The silver value might easily have been overlooked except for its accidental discovery in thin sections. (See Figs. 11 and 12.)
The last phase of the subject which will be considered relates to metallurgical treatment of the ore after reaching the surface. In many cases the success of a mine depends on the solution of this very, problem, how can the ore be most economically treated to extract its values. Here again the microscope may profitably be utilized to assist in the solution.
The nature of the work differs from either of the two previous classes. The important things to be determined relate to the texture of the ore, the nature of the mineral aggregate, the size of grain; in short, to the answer to the question how, and not why, are the minerals related to one another.
For example, let us consider a recent milling problem at a Canadian gold mine. Upon microscopic examination of the ore it was found that most of the gold was closely associated with pyrite, as an inclusion or intergrowth, in a finely divided state. An appreciable proportion, however, occurred in the quartz gangue separate from other minerals. It was justifiable to anticipate that, with this proportion, amalgamation would accomplish a good recovery, but that, with the other and principal part of the gold, amalgamation would prove ineffective without extremely fine grinding. Hence a combination with the cyanide method suggested itself as a solution of the problem.
Actual mill tests along this line proved the value of these observations. A preliminary study might easily have saved a considerable amount of money spent in unprofitable experiments.
Or take the case of disseminated iron in schists and gneisses, a class of ores that is coming into prominence. Their value and importance lie wholly in the ease of concentration through magnetic separation. But if, instead of the normal magnetite, part of the iron is present as martite, a mineral which is only slightly magnetic and yet which resembles magnetite closely in general appearance, it may readily be seen how the ore might prove a very unprofitable thing to handle.
Polished sections studied with vertical illumination would reveal the mineral character and would suggest the difficulty of concentration, notwithstanding the fact that a chemical analysis might show a favorable percentage of iron.
When deciding upon methods of concentration for any ore, microscopic examination will, in general, indicate which process will be most likely to give the best results. If two or more components are found to be very closely associated and minutely intergrown, chemical treatment, as a rule, will prove most effective, since fine grinding usually introduces difficulties; while an ore with a comparatively coarse-grained fabric will prove more susceptible to mechanical separation.
One recent development in the preparation of ores for the market, the sintering of fine concentrates, flue dust, and other by-products to render them amenable to furnace treatment, is worthy of notice. Many devices have been invented to accomplish this result, and, when comparing the qualities of the sintered product from the various processes to determine the most available treatment for a certain class of material, microscopic characteristics will be found of no little value in arriving at conclusions.
In this way may be determined, for instance, whether the material is sufficiently porous to permit the ready access of furnace gases, or whether the cohesiveness of the product is due to the formation of refractory silicates which would retard reducing action.
The study of mattes, slags, and speiss is another field, but one in which the problems are within the province of the metallurgist rather than the mining engineer; nevertheless it is an unusual problem that may not be at least simplified by microscopic investigation.
In conclusion, it may be of interest to note that petrographic methods were first applied less than 60 years ago, and, when we realize the great development of the science and its wide application to various lines of research since Dr. Sorby in 1858 published his first paper on the microscopic structure of crystals and of rocks, the possibilities of future development may be the better appreciated.
I am perhaps one of the few present who are exceedingly interested in the details of microscopic work, but I am sure we have all enjoyed very much the general features of this paper. I will not attempt to express my own feeling in regard to it beyond saying that it seems to me that this kind of microscopic investigation and study has been overlooked and slighted and that its possibilities are very great.
There are one or two details in Mr. Apgar’s paper which impressed me because they were novel to me : one of these was the development of secondary galena with dull luster. None of the galena which I have seen has seemed to be secondary and I have never seen any, whether secondary or primary, that did not have a bright luster.
The present-day mining engineer is expected to “ see into the ground farther than the point of the pick ” and he is justified in making free claim to his ability to do so. Mr. Apgar has recorded the development of the application to the work of the engineer of a very useful but tardily recognized instrument—the petrographic microscope, an instrument which is helping to shatter the proverbial “point of the pick” limitation—and he has shown its use and its possibilities very clearly. Some further field examples of the use of the microscope in engineering problems may add to the appreciation of Mr. Apgar’s scholarly paper. In exploration by diamond drills in the Sudbury, Ontario, nickel district it is often of great importance to determine when the drill has passed out of the norite and into the foot-wall formation. Structural calculations may give a clue to the expected point of contact, but the variations are considerable, and since to stop the drill before reaching the contact is futile, and to continue beyond it is needless, it becomes of theoretic and economic importance to determine positively the lithological nature of the core at critical stages in the work. In some places the foot-wall is an earlier norite, only distinguishable from the nickel norite (Sudburyite) by means of an examination of the rock slides with a petrographic microscope. Again, certain diabase dikes which cut the norite and the foot-wall formations cannot be easily or with certainty distinguished megascopically from the norite. The determination, only possible with the high-power microscope, that the often highly altered rock gangue associated with outlying pyrrhotite masses in the Sudbury district is a phase of the nickel norite, is of evident importance in view of the fact of the invariable association of the norite and the nickeliferous pyrrhotite. From these random examples from actual working notes, it is evident that the microscopic laboratory is often quite as essential to the engineer in charge of explorations as is the assay laboratory to the metallurgist or to the miner.
Another point of application of the microscope should be considered. Often it is of no great importance, from a commercial point of view, that the exact species of the associated rocks of a mineral deposit should be determined, but the engineer owes a recognition to the science on which his profession is founded which should restrain him from incorrect, careless, or indefinite rock nomenclature in his reports and records. A few “standby” rock names, frequently wrongly used in mining literature, often cover obvious ignorance or indicate culpable carelessness. It is not to be expected that the busy field engineer can offhand “ read ” all the multitudinous rock species, but he can obtain a slide of the important rock formations he describes at a very insignificant cost, and likewise can have a lithological determination made by a specialist for a nominal charge, if desired.