Geology

Frequently the careful engineer puts the word porphyry in quotation marks or precedes it with “so-called,” when he writes of the Porphyry Copper mines. This is done by way of serving notice that he is aware that the term is not a precise one, though no one who is informed on the subject would in any event misinterpret it. The name originated from the fact that the rock containing the copper at Utah Copper, Morenci, Nevada Consolidated, and Braden—the first four to be developed—was in each instance known as porphyry. The term was convenient; and when Ray, Miami, and Inspiration came along they fell into the Porphyry category even though the ore consisted essentially of a rock called schist instead of porphyry. The word is not inclusive enough; nor as a matter of fact is it sufficiently exclusive. Numerous copper deposits occur in porphyry rocks that are not Porphyry Coppers, paradoxical as it may seem.

Desiring to be more accurate and at the same time to use a word that is in some degree descriptive, various engineers have adopted the expression “disseminated copper deposits.” This alludes to the characteristic scattering of grains or specks of copper minerals more or less uniformly throughout the mass of the rock. Yet here again the implication is not entirely accurate, for the following reason: in some of the ores, the proportion of copper minerals “peppered” in the rock is small, compared with that which coats the walls of cracks and minute fissures; and that which is found as veinlets along planes of fracture. Many geologists have extended their interpretation of the term disseminated to include both types of mineral occurrence. One of the most striking features of the Porphyry ores is the intense shattering and fracturing that the rock underwent at some period or periods prior to the deposition of the copper minerals. Still another significant term frequently used in conjunction with “porphyry” or with “disseminated,” or with both terms, is “low-grade,” for the comparatively low average copper tenor of the ore is a characteristic of all the deposits.

To frame a concise definition that will describe all the Porphyry deposits, and at the same time exclude all others, is impossible; but the following characteristics are common to all of the twelve. While the individual items are not distinctive of the group, any deposit that meets all of these specifications is a Porphyry; and vice versa, any that does not meet them can not properly be designated as a Porphyry.

1. The deposit is of such magnitude and shape that it can be mined advantageously by large-scale methods, either by under-ground caving or in open pits.
2. The distribution of the copper minerals is so general and uniform that “bulk” methods of mining are more profitable than selective methods whereby individual veins or thin beds would be stoped separately.
3. An intrusion of porphyry or closely related igneous rock has played a vital part in the genesis of the ore, though the porphyry itself may not constitute the major part of the deposit. The evidence is convincing that remarkably large, deep-seated, slow-cooling masses of rock were the source of the heat and energy and, directly or indirectly, of the metals in the deposits of the present day.
4. The process known as “secondary enrichment” has operated to concentrate the copper. At New Cornelia the zone of secondary enrichment is almost negligible but it exists.
5. The extent of the orebody is usually determined by economic limits rather than by geological structure. This is because the copper content gradually diminishes as progress is made either downward or laterally from the core of an enriched mass. At some point—which necessarily varies,, with the cost of production at the particular mine, with the price of copper, and with other economic conditions—a “cut-off” must be made between “ore” and “waste.” This may be 0.5 per cent copper or it may be 1.5 per cent in different mines; and, considered literally, it would vary widely with respect to the same mine at different times.
6. The average copper content of the mass is comparatively low (with 3 per cent as the maximum) and grinding and mechanical concentration are necessary to produce a suitable smelter feed, if the ore is sulphide in character.

Genesis of a Typical Porphyry Deposit

Without any attempt to go into the details of geologic process —interesting as it is—the genesis of a typical Porphyry deposit will be outlined. An igneous rock is one that has crystallized from a molten or fluid mass called a magma. Originating from unknown depths, it has been forced by internal pressure up through the crust of the earth, usually cooling and solidifying before reaching the surface. The process by which a plastic magma works its way up into or through the solid crust is called intrusion or invasion. The rock invaded may be another igneous rock, such as granite; a sedimentary rock, such as limestone; or a metamorphic rock, such as schist. Naturally, if one goes back far enough, all rocks once were igneous; the way sedimentary and metamorphic rocks descended from igneous rocks will not be discussed here. Scientific guesses indicate that the geologic eras in which these orebodies were made date back from 70 to 700 million years. Whether these figures are accurate or not, many millions of years have elapsed, and erosion in many places has exposed areas that once were thousands of feet below the surface.

The following outline of the five steps in the formation of a typical Porphyry Copper deposit is adapted in part from W. H. Emmons (Trans. A.I.M.E., vol. 75):

1. The intrusion of a magma and its solidification forming large masses of quartz diorite, monzonite, granite, diorite porphyry, monzonite porphyry, granite porphyry, or quartz porphyry. The term “porphyry” refers to the texture of the rock and not to its mineral composition. When, in the process of solidifying from the magma, some of the minerals formed large crystals that are conspicuous in a groundmass of much smaller or even invisible crystals, the rock is said to have porphyritic structure. The average miner who knows little geology calls almost any rock that he is asked to identify “a kind of porphyry”; and many times he is unknowingly correct. Such intrusive masses may extend to unknown depths; and their horizontal dimensions may be measured by many miles.
2. The intense fracturing or shattering of large portions of the intrusive, usually near the top. This fracturing may be the consequence of shrinkage when cooling; or of the play of various other stresses within the rock.
3. The primary deposition or precipitation of metal-bearing minerals, brought about by the upward circulation of hot aqueous or gaseous solutions through the cracks and fractures. These metal-carrying solutions usually emanate from igneous masses that may invade immediately following the intrusion of the original porphyry or that may, on the other hand, be of a much later date.
   The essential minerals deposited are the sulphides of iron and of iron and copper together: respectively, pyrite and chalcopyrite. These sulphides fill the minute and closely spaced cracks; and also, by replacement, they deposit as small shot and specks “disseminated” in the rock. The copper content of this so-called protore rarely exceeds 1 per cent, and 0.5 per cent is more usual.
   “Protore” is a comparatively new term coined to fit a case of this kind. The objection to “primary ” ore—the metal-bearing minerals are primary—is that “ore” implies a rock that is worth enough to be exploited at a profit. Protore need not and usually does not contain enough copper to warrant mining. On the other hand, it sometimes does. At New Cornelia, for example, the protore and primary ore are identical. And there is no telling when the protores at many of the other mines will enter the “ore” class as a consequence of economic readjustments.
4. The deposition of chalcocite by the process of secondary enrichment. Between steps 3 and 4 there, elapses a period long enough to allow for the erosion of the overlying rocks, whatever they may be, and the exposure of the top of the mass of protore. This may require, say, 100 million years or more; but this is not a particularly long period, as geologic time goes.
   Surface waters in conjunction with atmospheric oxygen bring about the oxidation of the copper-bearing sulphides in the near-surface zone. Solutions of copper sulphate are formed and percolate down through the mass of rock. The burden of copper in the solutions continues to increase so long as the supply of oxygen survives; but when it is exhausted there ensues a reversal of the operation, and in the presence of unaltered pyrite and chalcopyrite the copper solutions give up their burden in the form of chalcocite, the sulphide of copper that is richest in copper. The chalcocite replaces the other two sulphides volume for volume, and as it contains 80 per cent copper compared with 34.5 per cent in chalcopyrite and none in pyrite, a substantial enrichment results. Chalcocite consequently may be deposited as specks or grains, or it may form a coating around particles of pyrite or chalcopyrite.
   Normally, the zone of secondary enrichment extends from an elevation slightly above the permanent ground-water level downward to one several hundred feet below. The explanation is that between seasons of wet weather the comparatively porous rocks absorb a supply of air and the oxygen in the air tends to prevent the precipitation of chalcocite in great quantity above the water level.
   The chemical reactions involved in secondary enrichment are complex; indeed, the generally supposed sequence may be inaccurate, but the net result is a persistent downward migration of copper. Chalcocite itself is re-dissolved and re-precipitated again and again at successively lower horizons, thus forming an enriched layer or blanketlike mass lying between the leached or oxidized zone above and the unaltered protore zone below. It is extensive, undulating, and irregular as to thickness and copper content; though,for obvious reasons, the normal experience would be to find a gradual decrease in the proportion of chalcocite to chalcopyrite as depth is gained. Theoretically, replacement would be complete at the top and absent at the bottom. Though this ideal condition is never attained, the tendency is for the secondary zone to show progressively lower assays with depth.
5. Progressive concentration of copper in the zone of secondary enrichment. Erosion continues to wear away the surface rocks and as this process continues the zone of secondary sulphides continues to move downward. It tends to become thicker and richer.

J. E. Spurr has pointed out that in the case of the Ray mine the average thickness of the overlying leached zone is 217 ft. Below is a zone of secondary sulphides averaging 100 ft. thick, with an average copper content of 2.3 per cent; and below this lie primary sulphides averaging 0.29 per cent copper. He has calculated that a layer at least 500 ft. thick above the present surface must have existed to supply the additional copper found in the secondary zone. Obviously, this rock has been removed by erosion. This calculation assumes that there was no loss of copper whatever in the process of solution, migration, and enrichment.

The usual condition of the leached zone is that the copper is almost entirely removed; but that some residual iron, in the form of the oxides (hematite and limonite) remains and stains the surface with a characteristic brownish color.

This leached decomposed outcrop, typically iron stained, and containing quartz and kaolin, is called gossan. It was such gossan that attracted J. Parke Channing to the Miami deposit. He had seen specks of chalcocite in a short adit at the far western end of what became the Inspiration property. That gave evidence of copper and, most important, of course, of secondary enrichment. Two miles to the east he saw large surface areas characterized by intense silicification, not too much iron oxide, and scarcely a trace of copper. His deduction that the outcrop, lean and hungry to the eyes of the average prospector, but impressive to the careful student of geology, would be underlain by a valuable deposit of chalcocite ore proved to be accurate.

S. F. Emmons and W. H. Weed, as early as 1910, presented the first comprehensive papers dealing with the secondary enrichment of copper ores. Clearly, it is the most important geologic phenomenon in the Porphyry story.

Fig. 23 is a typical cross-section of the Copper Flat-Liberty orebody that is being steam-shoveled by the Nevada Con-

solidated company at Ely, Nevada. The outline of the pit is omitted. An interesting point is the comparative regularity of the upper boundary of the orebody, effected, to a considerable extent, by the normal level of the ground water. By contrast, the lower boundary is extremely irregular. This is due, doubtless, to the physical and structural features of the rock which tended to permit the free circulation of the copper-bearing solution in some areas while resisting it in others.

The porphyry outcrops in the Ely district cover a belt about seven miles long and one mile wide, the Copper Flat-Liberty

deposit lying at the east end of one of the larger porphyry areas. While the evidence is by no means conclusive, W. H. Emmons is of the opinion that all of the outcropping porphyry masses are upper projections from a single deep-seated batholith—the original porphyry intrusive that broke through the Paleozoic limestone which constitutes the crust of the earth in the region. The most intense shattering and fracturing took place, according to his hypothesis, in and near the domes or cupolas that project upward from the top of the batholith; and consequently it is here that the orebodies are found.

In essential respects, the genesis of most of the Porphyry Copper deposits is as just outlined. Table 29, summarizing main features of each, bears out this statement; but at the same time it calls for a brief explanation on some points.

In the first place, it should be noted that the orebody may be almost exclusively in the intrusive porphyry or in the invaded rocks; or it may be partly in each, on both sides of the contact between the two. The controlling factor is largely the degree of shattering, cracking, and fracturing, which makes the particular rock hospitable to the mineral-bearing solutions. At Miami, for example, the schist was altered, silicified, and cracked by the intrusion of granite and granite porphyry. The schist was metallized and became ore; but the granite, speaking generally, did not become ore. At Andes, on the other hand, both the intrusive porphyry and the invaded quartzites, and at Chino,

not only porphyry but limestone and quartzite became loci for ore deposition. The position of the pits in the sections shown in Fig. 24 illustrates the point clearly.

A second phenomenon that requires explanation is the formation of zones of oxidized ores. At Inspiration, the ore that goes to the leaching plant is described as mixed ore; that is to say, it contains chalcocite as well as copper silicate (chrysocolla) and sundry carbonates of copper. This mixed ore is the product of geologic events subsequent to secondary enrichment. The grains of chalcocite have been oxidized to produce the other minerals. This oxidation may be surficial, producing only a thin coat of greenish stain; it may have proceeded to such an extent that a small residual kernel of sulphide remains; or the

chalcocite particle may have been entirely replaced by chrysocolla or malachite. The question arises why the chalcocite was not dissolved and carried down as in the normal process of secondary enrichment. The reason is that the supply of pyrite and chalcopyrite necessary to produce copper sulphate had been exhausted. Probably also the climate had become dryer, so that there was comparatively little percolation of water through the rock.

One should not, by any means, get the impression that oxidized ores are totally absent in other Porphyry mines. Miami has them and so has Ray; but to date they have not been of sufficient moment in these mines to warrant separate treatment. Incidentally, Miami lists in its ore reserves 7,000,000 tons of “mixed ore,” averaging 1.83 per cent copper, whose treatment has for some years engaged the attention of the company’s staff. Recently a scheme of beneficiation has been developed.

New Cornelia

New Cornelia presents a geologic picture that is unique in the Porphyry group. Secondary enrichment occurred to only a negligible extent. Ira B. Joralemon {Trans. A.I.M.E., vol. 49, 1914) gives an unusually concise description of New Cornelia geology from which the following is quoted:

The genesis and geologic history of the Ajo ore seem unusually easy to trace. After the monzonite intrusion had uplifted the rhyolite, the slow cooling of the porphyry was accompanied by considerable contraction. This resulted in a thorough jointing and Assuring of the monzonite, especially near the rhyolite contact, and in a less-complete fracturing of the rhyolite itself. Near the center of the intrusion, some of the fissures continued to great depth. Probably soon after the solidification of the outer layer of porphyry, hot mineral-bearing solutions rose along these deep fractures. The solutions were heavily charged with iron, sulphur, silica, and later copper. The iron and sulphur were carried through the joints and fissures in the rock far from the source of the mineralization, and caused a general, though scanty, pyritization of the monzonite mass.

Later, as the proportion of silica and copper in the solutions increased, these solutions rose through the larger fractures in the monzonite until they encountered the impervious, less thoroughly fractured overlying beds of rhyolite. These dome-shaped beds acted as a dam, stopping the upward flow of the mineralizing solutions and causing them to spread out through the jointed monzonite on both sides of the large, deep-seated fractures. Here they remained imprisoned until they gave up their mineral content, depositing veins of quartz, chalcopyrite, and bornite along the fissures and joint planes, and partly replacing the rock itself with the same minerals. The mineralization was greatest near the large central fractures through which the solutions had risen, and in the more thoroughly jointed portions of the monzonite near rhyolite contacts. In this manner was formed the mushroom-shaped disseminated orebody, grading on the sides and bottom to rock less thoroughly mineralized, or mineralized with the iron instead of copper sulphides.

Probably soon after the end of the period of mineralization, before rhyolite capping of the ore had been eroded away, the whole country was covered by a great thickness of andesite lava and basalt. This covering protected the ore from surface alteration until the present geologic period.

Erosion in early Quaternary times must have been very rapid. Deep canyons were cut in the recent lavas. The covering was stripped off from the Ajo monzonite, and much of the orebody itself was rapidly washed away.

Following this period of rapid erosion, very stable conditions have lasted up to the present time. The water level has remained nearly stationary, at a rather shallow depth, long enough to allow thorough oxidation of the rock down to this level. Where the mineralization was chiefly with pyrite, much sulphuric acid was formed by the oxidation of the pyrite to limonite. This acid thoroughly softened and kaolinized the feldspars of the monzonite, and helped take into solution the copper from the traces of chalcopyrite which accompanied the pyrite. This copper was carried down by the descending surface waters and was redeposited at water level to form the thin layer of chalcocite ore. [This layer is so thin as to be of scientific rather than economic importance.]

In the siliceous chalcopyrite and bornite orebody, it seems that little sulphuric acid was formed during the oxidation. The descending surface water, probably always small in amount in that arid climate, contained an excess of lime and of carbonic acid. The result of this condition was that as soon as the copper sulphides were oxidized, the copper was precipitated almost in place in the form of malachite. The iron of the chalcopyrite and bornite remained in place in the form of limonite and hematite. The silicified monzonite was very slightly altered, because of the small quantities of sulphuric acid, and remained hard and resistant to weathering, forming steep hills. The little sulphuric acid which was formed probably reacted at once with the calcium carbonate in the oxidizing waters to form calcium sulphate. Thin seams of the resulting gypsum occur here and there in the carbon¬ate ore, and there is much gypsum in the gravel of the valleys.

The result of this cycle of changes was a large body of low-grade primary bornite and chalcopyrite ore, oxidized above a horizontal plane at the present water level to a malachite ore of exactly the same grade as the sulphides. The ore was deposited from rising deep-seated solutions. There is no overburden of lean or barren rock, since the oxidation was accompanied by practically no leaching or transportation of the copper from the primary ore. The ore rises above the surrounding barren ground, and continues without any considerable increase or decrease in values from the actual surface to the bottom of the orebody.

The same phenomena that account for the absence of important secondary enrichment also account for the absence of a leached cap or overburden. These are: (1) the aridity of the climate and the consequent meager supply of surface water; and (2) the presence of calcium carbonate in such water as was present to attack the ore. According to Joralemon’s hypothesis, dissolved copper was promptly re-precipitated as malachite, and the sulphur as calcium sulphate or gypsum. The decomposition of calcium carbonate in Nature’s laboratory supplied the missing elements to make both of these minerals.

Chuquicamata

Chuquicamata also presents what may be regarded as a mild geological vagary, in that the protore contained, instead of chalcopyrite, the mineral called enargite, a sulpharsenite of copper. The major intrusive of porphyritic granite may be only a later phase of the granite that constitutes the country rock, or it may be an intrusion of distinctly later date. The intensely fissured zone that constitutes the “largest known orebody in the world” occupies the greater part of an area having approximate maximum dimensions of 3000 by 9000 ft. The accompanying sketch (Fig. 26) is a generalized column of the orebody; but one must not get the impression that the deposit is a series of uniform horizontal layers of the thickness shown, covering an area 3000 by 9000 ft. Ore deposits, particularly in igneous rocks, tend to display all kinds of structural irregularity, and these in turn have influenced the deposition and migration of the copper minerals.

Chuquicamata experienced its era of secondary enrichment, doubtless during a geologic period when rainfall was abundant. Presumably the leached cap was then eroded away and oxidation under conditions of aridity ensued. The chalcocite was converted principally into brochantite, a basic sulphate of copper. The process was accompanied or followed by the deposition of sulphates and chlorides. These were dissolved to produce solutions which reacted to convert part of the brochantite into the other minerals that are shown in the chart: atacamite, a basic chloride of copper; krohnkite, a mixture of sulphates of soda and copper; chalcanthite, which has the same composition as the ordinary bluestone of commerce; and natrochalcite, a rare mineral of copper. From this zone has come the 87,000,000 tons of leaching ore that had been treated up to the end of 1931.

Next below, as might be expected, is an intermediate zone in which the chalcocite has been converted to brochantite only in part; and this is followed by a substantial sulphide zone in which chalcocite predominates. Some time in the future, no doubt, this ore will provide food for a stupendously large concentrator.

Underlying this ore is the primary zone wherein the valuable minerals are the two sulphides, enargite and cupriferous pyrite. Presumably, this will prove to have a somewhat lower copper content than that just above, but expectations are that immense tonnages will be available for concentration.

Braden

Braden geology is much like that of the Porphyries in the United States so far as genesis is concerned; but structurally, the deposit is unique. The rock of the immediate vicinity of the mines is a huge intrusion of andesite porphyry. To paraphrase Waldemar Lindgren and E. S. Bastin (Economic Geology, March-April, 1922): The event of supreme importance in the geologic history was a great explosion that produced a volcano-like vent and shattered the bordering andesite porphyry in such manner as to provide ample spaces for the subsequent deposition of the ore minerals. The explosion probably was caused by the meeting of a mass of upward moving hot plastic rock with water-filled fractures in the diorite and the consequent generation of steam. The resulting vent was roughly circular and about 3000 ft. in diameter. Later, the debris of the explosion was washed back into the vent, where it solidified as Braden tuff.

Subsequently, hot mineralizing solutions rose around the periphery of the vent, depositing pyrite and chalcopyrite in such sections as had been subjected to the maximum shattering and fracturing. The plug of the vent, composed of tuff cemented with clay, was exposed to these solutions, but was comparatively impervious, so that virtually no ore is found in the tuff. The result was the impregnation of a half dozen crescent-shaped bodies of protore spaced at intervals around the periphery of the tuff and extending as much as 1500 ft. into the andesite porphyry. The largest became the Fortuna and Teniente ore-bodies of the Braden mines.

Other periods of metallization followed. The history is complex; but finally came a period of secondary enrichment. Incidentally, the chalcocite zone is wholly above the present ground-water level, which gives rise to the conclusion that, subsequent to the enrichment, the area about the volcano was subjected to an “uplift” that in effect put the orebodies above the water level. The protore assayed from 1 to 1.25 per cent copper; whereas, after secondary enrichment, grades ranged from 1.5 to 4 per cent. The richer ore, other things being equal, lay in the upper portion of the secondary zone, where the ratio of chalcocite to chalcopyrite is highest. To what extent the protore may become ore remains for the future to demonstrate.

That the genesis of the Porphyry ore deposits in its scientific aspects is of great interest to the geologist is unquestioned. Geologic deduction helped the engineers who recommended the early exploration with churn drills at several of the properties. But to what extent has geology contributed in a practical way to the intelligent exploitation of the Porphyry Coppers?

Twenty-five years ago the mine superintendent of the traditional “old school” regarded the geologist as an unmitigated nuisance in helping to run a mine. The superintendent argued that the ge’ologist could see no further into the ground than he himself. What interested the mine superintendent was not the chemical and dynamic events of 200,000,000 years ago; he wanted to know the quantity and richness of the ore that he could find in the twentieth century A. D.

However, this attitude, for various reasons, underwent a decided change. Mine managers, whose objectives were just as realistic as those of the superintendents, discovered that in the long run they generally found more ore and found it more cheaply if they followed the advice of a highly competent geologist, instead of the hunch of an individual who might be the best mine foreman in the world but who knew nothing about the art (for it is an art) of geologizing a mine.

Of course, there are inherent differences between the typical “vein” mine and the Porphyry Copper mine. The position and extent of the deposit as a whole and of rich and lean portions is determined by drilling and boring. The opportunity for the geologist to exercise his talents to practical advantage is somewhat restricted in connection with a Porphyry deposit.

Nevertheless, many of the Porphyry companies maintain a small geological department. It may consist of only one or two men, or it may be larger. The principal function frequently is to direct the prospect drilling; to maintain the drill logs and the geologic maps; and to advise the operating staff on questions of future development and the sequence of mining operations. In some instances the actual “drawing” of ore in underground caving operations is controlled by members of the geological staff; but usually this is done by the engineering department, where the work seems logically to fall. Probably close attention to operations on the part of a geologist is less necessary in a Porphyry Copper mine than in most other mines; and yet it seems probable that a geologist with an investigative turn of mind is, or would be, a good investment at most of the Porphyries.