---

June, 1937

---

Concentrations of the ores of useful metals are rare features of the earth's crust, representing only a tiny fraction of its total bulk. Generally they have originated in the remote past, as a result of the action and combination of processes of sedimentation, igneous intrusion, fracturing, precipitation from hot solutions, erosion and the circulation of meteoric waters. Small wonder then that their history is anything but obvious, and that their discovery has been, and to some extent still is, more the result of good fortune than of accurate prediction. To man in earliest times the discovery of metalliferous deposits cannot have been anything but fortuitous. As experience in mining accumulated, glimmerings of some of the geological factors which controlled the shape and extent of the deposits emerged as a series of empirical rules for finding and following ore, generally differing from district to district. Geology, as applied to ore deposits, has systematised the former empirical knowledge, and by more intimate observation, has established many relations formerly unknown. However, the understanding of the genesis of such deposits still presents many unsolved problems, and unfailing prediction is therefore more than can be expected ; nevertheless, it is now universally admitted that the investigation of the subject is of more than academic interest. The geologist has, in fact, a real contribution to make to the practical success of metalliferous mining, by indicating the most favourable lines of development.

The purpose of this article and its sequel is to serve as an introduction to the principal lines of attack in the geological investigation of metalliferous deposits. The subject is so large that this outline cannot claim to be comprehensive.

At the outset a broad twofold division in the methods of investigation may be recognised. There is on the one hand the study of rocks and mineral deposits in the field and mine, which leads to a knowledge of physical relations, that is, of geological environment and structure. On the other hand, there is the investigation of the chemical constitution of the deposits, and the rocks associated with them, by mineralogical methods. Environment and structure are the subjects of this article ; a following article will be concerned with mineralogy.

Regional geology

Ore deposits cannot be understood apart from their geological environment. A knowledge of the regional geology is thus essential. The mapping of large tracts of country necessary for this purpose is seldom carried on by geologists employed by mining companies, though there are some notable instances such as the survey of Northern Rhodesia carried out for the Anglo-American Metals Corporation under Dr. Bancroft's supervision, and that of the Sudbury region of Ontario for the International Nickel Company, under the direction of Professor L. C. Graton and Dr. A. Yates. Generally, however, this work is beyond the scope of private enterprise, and in view of its national importance, is financed from the public funds.

The work of state geological surveys consists basically in the production of geological maps showing the distribution and structure of rock formations at the surface, and in the preparation of reports, memoirs or bulletins describing the characteristics of the rocks so that they can readily be recognised. The scale generally employed is small enough to permit a considerable area to be represented on a single sheet. The Geological Survey of Great Britain uses the scale of six inches to one mile in the field, and maps on this scale are published for areas of economic importance. The results of the survey for the whole country are published on the scale of one inch to one mile, and this scale, or one close to it, is widely used by the United States Geological Survey, and by the Surveys of the Dominions and Colonies.

The first matter of interest revealed by the regional survey is the nature of the rocks. One great class of ore deposits, the "syngenetic" deposits, have the same origin as the rocks with which they are associated; that is to say, they may be true sedimentary or igneous rocks. The continuity of such deposits at the surface is shown by the regional survey. The Jurassic ironstones of Cleveland, Lincolnshire and Northampton for example are of sedimentary origin, and are continuous over considerable areas. In the case of "epigenetic" deposits, those introduced into pre-existing rocks, the distribution of stratified rocks is again important because certain beds prove to be favourable "host" rocks for mineralisation, others definitely unfavourable.

The distribution of igneous rocks, especially in regions where they are younger than the sedimentary rocks, and invade them, is of even greater interest. During the past thirty years, evidence has been forthcoming from all parts of the world to show that there is a close connection between igneous activity and the deposition of ore of precious and semi-precious metals. Due credit here should be given to the United States Geological Survey, whose work firmly established this relation. The significant facts here are that metalliferous deposits occur in areas of igneous activity and in places, such as the metamorphic contact zones of igneous intrusions where they can only have originated from solutions emanating from the cooling igneous rocks. In addition to the "host" rocks, regional surveying thus indicates the distribution of "source" rocks. Fig. 1 shows a typical mining district of the south eastern Cordilleran region of the U.S.A, the regional geology of which was the subject of a recent study by the writer. The sedimentary rocks, which rest on a basement of Pre-Cambrian granite and schist, include Cambrian quartzite, Ordovician and Silurian dolomites, Devonian shales and Carboniferous limestones and shales. These were invaded by an igneous batholith in Tertiary times, in which there were three successive phases of major intrusion: (i) dark monzonite, (ii) quartz mouzonite, (iii) quartz-bearing monzonite The ore-forming fluids were associated only with the third phase, and the ore deposits are therefore concentrated in veins in this rock, and in sedimentary rocks near its contacts. (See Fig. 1.)

A similar close relation between igneous intrusion and ore deposition is to be found in almost every important mining district of the Cordilleran region. Typical examples include the copper districts of Bisbee, Arizona; Santa Rita, New Mexico ; Bingham, Utah ; the lead-silver districts of Tintic and Park City, Utah; and the varied metalliferous deposits between Leadville and Jamestown, Colorado. An equally close relationship between igneous intrusion and ore deposition is found in the Hercynian belt of Europe, typified by the tin and copper deposits of Cornwall and the varied deposits of the Freiberg district.

It should, nevertheless, be noted that by no means all igneous intrusions have generated ore deposits. Rich deposits are associated with granites in Cornwall, while similar rocks, though of a different age in Scotland, are almost completely devoid of metalliferous ores. The Tertiary igneous activity of Western America produced notable ore deposits ; activity of the same age in West Scotland seems to have produced none.

Regional mapping of stratigraphic and igneous units reveals not only their distribution at the surface, but also yields information about the nature of the contacts between them, and their deformation. The picture of the regional structure so obtained is of great interest in the study of ore deposits. The importance of faulting and fracturing in localising deposits is evident in almost every mining district. Folding of the rocks may also play a part. Where there is a persistent stratigraphic horizon over a considerable area, the structure can often be investigated by means of contours on that horizon. Fig. 2, showing contours on the base of the Great Limestone in the Northern Pennines, brings out the fact that the broad structure of that region is a dome, truncated by the Pennine fault-system. It is suggested that there is a direct connection between the fracturing and mineralisation of this region and the formation of this dome, which would account for the downward pinching of the veins. The lead mining districts of West Yorkshire and Derbyshire are also situated on broad domes.

Mine geology

Regional geology provides the necessary background for the study of ore deposits, but it seldom shows what influences have controlled the formation of a particular deposit. The existence of favourable source rocks, of favourable host rocks, and of broad regional structures can usually do no more than point the way to wide regions which are likely to contain ore deposits. Understanding of the origin of particular ore deposits can come only from highly detailed studies of the deposits themselves. This is the work of the mining geologist. It is sometimes undertaken by government geologists, but it is more satisfactory when done by geologists in the employ of the mining companies because they are able to examine the deposits at frequent intervals while mining is in progress.

It is generally agreed that the most successful work done by mining geologists is that directed towards solving the detailed structure of particular ore deposits, for such investigations lead in many cases to specific predictions about the continuation of the deposits, and it is these, and not abstract genetic theories, which are needed by the mine operators. The work of a mining geologist is necessarily more restricted than that of a geologist engaged in general studies, for his primary object is to find and follow ore ; and he must avoid digressions, however interesting, which do not serve this end. In compensation he has the satisfaction of having his findings and predictions continuously tested as mining proceeds. His evidence, three dimensional rather than two as in the case of the field geologist, is also generally more detailed, and must be obtained with engineering exactness on a scale much larger than that used in field studies. Underground mapping of mine geology is usually done on the scale of one inch to forty feet, but in some cases the one inch to twenty feet scale is used. An effort is made to record even the smallest details of faults, folds, fissures, joints, slickensides, wallrocks and mineralised ground seen in the workings. The Brunton compass-clinometer, accurate to 1 deg., is used for the measurement of dips. Mineralised ground is distinguished from barren ground. In the system of mapping developed at the Anaconda Company's mines at Butte, Montana, and widely used in America, mineralised fissures are indicated in red, barren structures in blue. The evidence from mine workings and boreholes is combined on sections and projections, giving a picture of the structure of the ore deposit as a whole.

Some of the principal structures encountered are described below.

Faults and fissures

Ore deposits are related to fractures in the rocks of the earth's crust in the following four ways (1) the fractures may serve as channels for ore-bearing fluids; (2) they may serve as places of deposition; (3) they may block the path of fluids moving in other fissures which they cross; (4) they may. displace ore deposits already in existence. Fracturing is, then, the most important structural relation controlling the localisation of epigenetic ore deposits.

Faults of large displacements are seldom favourable channels for ore-bearing solutions, probably because they are largely choked with gouge. Fissures with displacement measurable in feet or tens of feet provide the commonest type of mineralised veins. Such fissures are remarkably sensitive to the physical properties of the rocks they traverse. In closely folded ancient rocks, they tend to appear in broad shear zones which include much country rock, rather than as single fissures. The Hollinger and McIntyre gold mines in Ontario, for example, are situated on one such shear zone which reaches a width of 1,800 ft. In bedded rocks fissures tend to change in direction according to the hardness of the bed they are traversing. On the Mother Lode of California, there is a distinct change in direction both in strike and dip, when the veins pass from slate to greenstone. Similarly, the Pennine lead veins tend to stand nearly vertical in hard limestones and sandstones, where they are productive, whereas they have a low dip in shales and are barren. (Fig. 3)

In many districts, mapping of the veins reveals a definite pattern from which it may be possible to establish a unique direction in which they are productive. Thus in the Northern Pennine field, shown in Fig. 2, there are three dominant directions of fissures, north-east, north-west and nearly east-west. Although veins in all directions are mineralised, the good ore-shoots are confined to those running north-east. Veins crossing at 90 deg. in either dip or strike directions are very common ; examples are found in Cornwall, in the Freiberg district, and in the Grass Valley district of California. In some cases, both sets of fissures originated simultaneously as a" conjugate" system, and may have been mineralised at the same time. This is probably true in the Pennines. In Cornwall, on the other hand, east-west veins were formed before north-south veins.

What appear to be minor features of fissure veins may prove to be very important controls of ore-shoots. Small changes in direction have often left open spaces which have been filled with rich ore. Intersections with insignificant-looking joints may have far-reaching results on the productivity of the ore-shoot. Hence the importance of recording as much detail as possible as mining proceeds.

Space does not permit a lengthy digression on the methods used to solve the problems raised by post-mineralisation faults, for they are many and varied. It must suffice to say that in a great many cases, search will reveal geological evidence from which a definite estimate of the amount and direction of displacement can be made.

Folding

Ore-deposits may be introduced along channelways provided by folding; they may replace a previously folded bed; or they may themselves be folded. The last case seldom applies to epigenetic deposits, but there are many instances of folded syngenetic deposits. The iron ore field of Lake Superior presents examples. The structure here is solved by regional rather than by local mapping. Folded beds, which have been replaced by metallic minerals after deformation, present some interesting problems, because ore shoots depend not only on the presence of the favourable bed, but also upon its position in the fold. In the Homestake (South Dakota) gold deposit, which is a replacement of a closely folded and sheared bed, the ore-shoots occur along the crests of pitching anticlines. Folds acting as channels and as loci for ore deposition are exemplified by the "saddle reefs" of the Bendigo district (Victoria) and of Nova Scotia. These also occur in the crests of anticlines.

Ore deposits may also occur in the troughs of synclines. The copper ore body at Roan Antelope (Northern Rhodesia) is perhaps the outstanding recent example. The silver ores at Colquijirca (Peru) occur in a similar fashion.

Minor folds intersected by fissures may play their part in localising mineralisation. Fig. 3 shows a fissure vein which cuts an earlier minor fold. Ore has been found in the fold, replacing the limestone, as well as in the vein in this case:

Formational Contacts

Contact between different rock formations have frequently controlled the position of ore-shoots. The case of a permeable bed under an impermeable one is well known; the effect of the impermeable cover is to concentrate the flow of solutions near the top of the permeable bed, which is thus in time replaced Deposits of this type occur in the Organ district (Fig. 1) in the Silurian dolomite under the Devonian shale. Fissure veins may have rich ore bodies where they cross such contacts, or they may act as channels carrying the solutions which replaced the permeable beds at their contacts with impermeable beds. The rich deposits of the Millclose mine (Derbyshire), and the Boltsburn mine (Durham), are replacements of limestone below impermeable beds; they are associated with veins which acted as feeders.

The contact between dykes and the rocks they invade may be the loci of ore bodies Small gold shoots were found at the margins of epidiorite and felsite dykes in the Organ district (above) ; while at Pribram, and in Shropshire, ore shoots occur at the contacts between diabase dykes and sedimentary rocks.

SELECTED REFERENCES

• Lindgren, W., "Mineral Deposits," 4th Ed., New York, 1934.
• McLanghlin, D. H., Sales, R. H., and others, "Utilisation of Geology by Mining companies," in "Ore Deposits of the Western States," pp. 683-729, New York, 1933.
• Rastall, R. H., "The Geology of Metalliferous Deposits," London, 1923.
• Stoces. B., and White, C. H., "Structural Geology, with special reference to Economic Deposits," London, 1935.

Drawings and Photographs accompanying the article