Table of Contents
In the exploration of a copper deposit by drilling, obvious advantages are to be gained from a distinction between primary and secondary ore. Perhaps the chief of these is the aid which such a distinction renders in determining where a given hole should stop. The copper assay, often a sufficient guide, cannot always be exclusively relied upon. Primary ores may extend downward indefinitely and may fluctuate in value independently of depth: if primary ores are consistently lean for a considerable vertical distance, there is little reason to expect that, still deeper, they will increase to the commercial grade; on the other hand, rich primary ores may persist uninterruptedly downward or may come in again below a lean interval. Secondary ores, however, have a comparatively limited vertical extent; in a large way enrichment decreases with depth and finally gives out. It follows then, that when a hole descends from profitable ore into ore of less than the commercial minimum, the practical significance of the situation depends upon whether this decrease results from a decrease in secondary enrichment or is a variation in primary content: if the copper of the ore passed through has been chiefly secondary, little is gained in the average case by extending the hole much below the point at which the commercial limit is reached; but, when copper is chiefly primary, due weight must be given to the possibility that, still deeper, the ore may again improve.
The distinction between primary and secondary ore may of course be effected through an examination of the drillings. It has been found, however, to be facilitated by curves made by plotting assays against depths. In this connection, the data of several hundred drill holes in the copper districts of Ajo, Bingham, Ely, Miami, Ray, and Santa Rita, courteously furnished by officials of the local companies, have been studied in conjunction with the corresponding drill pulps and ore specimens. The accompanying curves represent holes chosen to illustrate both normal and abnormal conditions, which are respectively simple and difficult of interpretation; they are intended particularly to indicate how a presentation of the facts by their means readily permits deductions concerning secondary enrichment in copper ores.
Normal Conditions in Porphyry & Schist
The ores, whether primary or secondary, that are best adapted to exploration by drilling are those in which the valuable minerals are uniformly distributed. The result of enrichment on primary material of this kind is an ore not only richer but of somewhat more variable grade, though still sufficiently uniform to be suitable for drilling. Since a condition of uniform distribution is that most commonly met in drilled deposits, it may for the purposes of this paper be regarded as normal. Though present in some other rocks, it is most likely to occur in porphyry or schist.
The significant features of curve-shape are chiefly concerned with the abruptness of the change in copper content. In particular, a jagged profile is to be contrasted with a smoother one, and, according as they possess one or the other of these characters, curves representing normal conditions fall into two principal groups:
That in which the tenor undergoes no abrupt change throughout the sulphide part of the hole (e.g., curves 1, 2, and 3).
That in which the lower part of the curve has the characteristics of group I, while the upper part is richer and is marked by peaks (e.g., curves 4 to 8).
Primary ore commonly yields a curve of mild profile. A curve of secondary ore is commonly peaked, and, as might be expected, the more emphatic the enrichment, the higher and rougher are the peaks, and the more characteristic is the form of the curve. A curve, then, of group I presumably means primary ore, while a curve of group II presumably means primary ore below secondary ore. The curves noted as illustrating these two groups are, in fact, exactly what their forms thus suggest.
In group I, curves 1 and 2 well exemplify, by the smoothness of their profiles, the comparatively uniform tenor of primary ores under conditions defined as normal. They also illustrate a not unusual tendency of primary ore to decrease gradually in value with depth. Curve 3 indicates a somewhat less constant grade of ore, yet sufficiently uniform to
place the curve in group I; in any case, there is nothing in its shape to suggest enrichment.
In group II, curves 7 and 8 bring out strikingly the effect of strong enrichment on low-grade primary stuff; in curves 4 and 5, though the primary ore was of better grade and the ratio of enrichment somewhat lower, the effect of enrichment is clearly shown by the distinct difference in character as well as in tenor between the upper and lower parts of the curves; in curve 6, good primary ore, not very strongly enriched, yields a profile less strikingly different in its two parts, though even here the situation is plain. In all these five curves, the conditions are to be regarded as normal.
It is fortunate that these two types of curves, which are generally easiest of interpretation, are the ones most commonly encountered.
An interpretation of ore character, however, by means of curve shapes alone is often beset with difficulties. Departures from normal conditions may be encountered in porphyry or schist but are met more frequently in other rocks. They are most likely to be found in limestone, which is fairly common in deposits explored by drilling, and which, on account of the erratic nature of its mineralization, often introduces disturbing factors. Various kinds of difficulties that are experienced find illustration in curves 9 to 13, which may be classed as still another group:
That in which decided irregularities occur once or more, without systematic relation either to each other or to the top of the hole.
Curves 9 and 10, representatives of a type which is common, are of primary ore in limestone. If they were in schist or porphyry their moderately peaked character extending to the bottom would suggest deep and rather mild enrichment.
Curve 9 serves to illustrate also how the copper assay alone might lead to an erroneous decision as to the depth at which the hole should be bottomed: on the basis of copper assay alone, the hole might have been stopped at a depth of about 250 ft. or at a depth of about 600 ft.; if the decline in tenor just above either of these horizons had been due to a playing out of enrichment, bottoming at one or the other of these depths would have been justified; but since the ore is in fact all primary, it was wise to sink the hole deeper and actually there is no indication that the hole is yet deep enough.
Curve 11 is of slightly enriched ore in limestone. The existence of two peaks widely separated is not typical of enrichment in schist or porphyry, and suggests primary ore of erratic or bunchy distribution and this in turn suggests limestone. The two peaks in the curve might, for example, coincide with two of the limestone beds that were especially favorable for primary ore deposition. As a matter of fact, were the effect of enrichment removed from this curve, its shape would be only slightly changed. This situation well illustrates the impossibility of detecting enrichment in limestone from the shape of the curve alone.
Curve 12 is of primary ore in alternating porphyry and limestone. It has the appearance of being a typical curve of primary ore in limestone, but there is no way of determining from its shape whether or not enrichment is involved in any part of it.
Curve 13 is of enriched ore in schist overlying primary ore in diabase. Altogether, the curve looks like one in porphyry or schist with irregular enrichment above primary ore; the barren stretch near the middle of the hole might be variously interpreted as the result of leaching or as due to primary poverty, and the small length of low grade at the bottom might suggest a return to primary conditions. As a matter of fact, the low grade at the bottom is due to the presence of the diabase into which the enriching solutions were unable to penetrate effectively, and the barren interval is the result of oxidizing influences, which have entered obliquely or laterally, and have thus undercut sulphides lying above.
Erratic Conditions in Porphyry & Schist
Abnormalities which are occasionally present in deposits contained in porphyry or schist may bring about conditions as difficult of interpretation as those due to the presence of limestone or of other disturbing rocks, or they may yield curves which, on the basis of shape, would fall in group II or even in group I, but which in reality owe their shape to causes other than those normally determining the profiles characteristic of groups I and II respectively. Curves 14 to 16 afford examples of such erratic or abnormal conditions.
Curve 14 is of primary ore in porphyry. Viewed broadly, it suggests enriched ore over primary ore; yet its upper and richer part lacks the strongly serrated outline usually found to accompany enrichment. Indeed, each of the two parts, taken independently, looks primary, and this is actually the case, the drop in tenor coinciding with the entrance of a porphyry of a different variety which, in the district in question, carries less copper.
Curve 15 is of ore in porphyry, only slightly enriched, and representing two primary grades, the poorer underlying the richer. The primary copper of the poorer is in chalcopyrite; that of the richer is in bornite as well as chalcopyrite. On the basis of shape, the curve is an especially good example of group II, which would ordinarily signify enriched ore overlying primary ore; yet, in this particular case, the effect of enrichment on the shape of the curve is only to accent the relief of the peaks in its upper, richer part.
Curve 16, of ore in porphyry, might be thought to indicate deep enrichment giving way to primary ore near the bottom. As a matter of fact, enrichment is confined to the upper part, where the irregularities are most accentuated; below the point, at a depth of about 170 ft., where the tenor suddenly steps down and the curve becomes smoother, the ore is wholly primary, growing distinctly leaner near the bottom.
Curves 17 and 18 are in porphyry and are slightly enriched throughout. They are not to be regarded as abnormal; they represent a normal condition which yields a somewhat erratic curve. Nothing in their shape, however, distinguishes them from curves of primary ore in limestone or even perhaps of primary ore of somewhat variable grade in porphyry or schist.
The causes of the lack of primary uniformity encountered in some ores in porphyry and schist are various and often obscure. Low-grade pervasive primary mineralization is in certain cases localized by the contacts between intrusive rocks and the rocks which they invade; vagaries in the distribution of sulphides may therefore be concerned with a neighboring contact. The presence of a mineralized dike, or of a vein, or an unaccountable increase of disseminated primary sulphides, may, for example, produce irregularities that look like the peaks of an enrichment curve. The fact, moreover, that, in the districts involved, enrichment always takes place through the replacement of one sulphide by another, and that it acts selectively, in such a way that bornite yields to its effects more readily than chalcopyrite, and both of these more readily than pyrite (to name only the three primary sulphides most commonly involved), causes peaks due to primary richness to undergo a misleading exaggeration, as is illustrated in curve 15.
Requisites for Correct Interpretation
After all, there are a number of varieties of knowledge, not yielded by curves, which are essential to an intelligent diagnosis of conditions. The study of curve shapes must be made with an understanding of the geologic habits of the district. Not uncommonly there is a decrease in the grade of primary ore with depth which may involve an entire district or affect only a certain portion; it may be related to the position of a contact or dependent on some other influence, but whatever its cause, it must be taken into consideration if the curves are to be correctly understood. A knowledge of the kind of rock traversed is invariably required and there must always be enough study of drillings and of other specimens to yield an intimate acquaintance with the district habits.
Such studies are facilitated by the microscope. Thin sections are sometimes necessary for the identification of the rocks and determination of the alterations they have undergone. Polished surfaces of opaque minerals are often indispensable for the recognition of enrichment and and of its character. Powders, such as drill pulps, can be investigated by the microscope, or by the binocular, a magnetized needle aiding in the separation of magnetite from chalcocite which, in the powdered form, it closely resembles. It is often advantageous to embed powders in sealing wax, in order that polished surfaces of their constituent grains may be prepared for microscopic examinations.
which is especially important to recognize on account of its disturbing influences—is sometimes difficult of identification in pulp form because of intense alteration. The common test of effervescence with acid may, of course, be ineffectual, because of the frequent destruction of the carbonate by the alteration. A familiarity with the conditions or character of limestone alteration in the district will usually serve in separating this rock from porphyry. The limestone, however much altered, is likely to have a characteristic aspect, due, for example, to certain habits of sulphide aggregation or of softness.
Altogether, the usefulness of a study of curve shapes will vary inversely with the amount of additional knowledge needed for their interpretation. Should the deposit or district be erratic in its habits, curves may not be very intelligible. But should the district possess a uniformity or regularity in its general geology and its mineralization, the curves are likely to be very significant and to constitute an important short-cut to an understanding of the ore distribution.