Exploration and Geology Techniques

Overview of Exploration

Richard L। Brown

In this chapter a number of authors describe the kinds of geological thought and exploration techniques applicable to the original identification and subsequent mensuration, in terms of tonnage and grade, of mineral deposits judged by the geologist to be suitable for surface mining. Many of these techniques are also applicable to grass roots or systematic reconnaissance style exploration. It is appropriate that there be discussion of reconnaissance techniques in this handbook since that activity in established mining districts often continues long after initial production.
Exploration geology seems to have separated, as a discipline, from mining geology, not because the one group has a greater or less need than the other to know and understand all these techniques as much as that each group has different objectives. The mining geologist seeks new veins or other new extensions to ore bodies and expects to find these on a regular basis, whereas the exploration geologist must find new districts and knows that he will be lucky if he makes one or two such discoveries during a career. The mining geologist assists in the day-to-day problems of production, and reacts to the discipline which accrues to achieving daily and monthly goals. The discipline accruing to the discovery of a new mineral deposit in a new district is of a different sort. At any rate, the separate disciplines of exploration geology and of mining geology merge at that point in the history of a mineral deposit, after discovery while it is being explored and determinations are being made of the tonnage and grade—that is to say, the period in the history of an ore deposit when data necessary for a feasibility study are being prepared. Because so many of the techniques discussed in the following pages apply both to the general reconnaissance-type exploration and to the business of drilling off an ore body, the authors have been rather general in their treatment of the various forms of exploration geology which apply to the commodity they have discussed. In this introductory section, exploration techniques common to most commodities are addressed with the purpose of providing an overview of the duties of the exploration geologist as he takes a prospect from discovery to the final economic estimates.
A review of the technology directed at the search and discovery of economic mineral deposits illustrates once more that there is “nothing new under the sun.” There has been a continuum from medieval times to the present day of man’s knowledge of mineral deposits and of ways of finding them. A quick trip across Europe shows that the Phoenicians, Greeks, and Romans were all adept at reading gossans, at panning heavy minerals from stream sediments, and at a variety of other exploration techniques still used by today’s geologists. It is also obvious that these ancients had more than a rudimentary grasp of many principles of economic geology.
The search for ore begins with the development of ideas as to where the search should be conducted. Application of the most modern geophysics, geochemistry, remote sensing, and other techniques cannot be made until the geologist has decided where the search should begin. The first things a geologist must decide are what the ore body he hopes to find looks like, what minerals are contained therein, and how it was formed. He must, in short, develop an empirical model before he leaves his office and goes to the field.
Most of the ideas, a few examples of which are described below, geologists use now and will use during the foreseeable future had been published by 1974. In March 1965, The Canadian Institute of Mining and Metallurgy held a symposium on volcanogenic deposits. The papers given at that symposium, published by the CIM later that year, form the bible used by most geologists as they plan their exploration for volcanogenic deposits. John Guilbert and David Lowell published their paper on “Mineral Zoning in Porphyry Copper Deposits” in Economic Geology during 1970. The so-called Red Sea book (Hot Brines and Recent Heavy Metal Deposits from the Red Sea, edited by Degans and Ross) was a 1969 publication. The term plate tectonics was firmly in place by 1970, and the Journal of Geophysical Research published its compendium of papers related to that subject in 1973. Kambalda in Western Australia was discovered in 1968, and the recognition that some nickel-copper deposits were derived from ultramafic volcanic rocks was made in print by a number of authors in the very early 1970s. Our knowledge of the so-called Mississippi Valley deposits lags far behind some of the other ore types mentioned above, but the sum of our knowledge of these deposits is pretty much contained in the August 1971 issue of Economic Geology.
The search for volcanogenic ores, many of which are mined from surface, has widened possibly further than any other type of exploration, and it may be well to describe the model which governs much of that exploration. In brief, the geologists who participated in the Canadian symposium in 1965 had noted that descriptions of synvolcanic and syngenetic ores described by German and Japanese workers corresponded closely with the results of our own mapping and observations of the Precambrian deposits in Northern Ontario and Quebec. As a result of this mapping, they were able to demonstrate that many of the Canadian deposits had been formed on ocean floors, apparently from brines derived from highly siliceous rhyolite domes. In addition, iron-rich silicate deposits often spread far from the volcanic dome and were deposited on the sea floor over wide regions. Thus the three elements of the volcanogenic model were: (1) the volcanic dome containing imbricate stringer zones; (2) the polymetallic sulfide deposits, formed on the ocean floor; and (3) widespread cherty pyrite bed. The complete system was usually covered by more recent volcanic rocks, often andesite or basalt. Thus the subsequent direction of massive exploration money at the andesite-rhyolite contacts in many shield areas throughout the world.
It would be difficult to pinpoint the decade during which geologists routinely began to map the distribution of clay alteration products around porphyry copper sulfide systems. A quick glance through bibliographies shows a number of papers on the subject were published in the 1930s. A number of company geologists were mapping such patterns routinely during the early 1950s. A number of papers authored by such people as S.C. Creasy, Richard and Courtright, and Paul Kerr demonstrated widespread interest in the subject during that decade. During the 1960s, Guilbert and Lowell, collectively and individually, published the results of their observations of alteration patterns in the southwest United States and in some other areas as well, and their 1970 paper cited earlier is now regarded by most North American geologists as the standard text on the subject. There is presently very little porphyry copper exploration near established mines or in new districts conducted which does not respond to the Guilbert and Lowell model.
Recently, it has become clear that predictable distribution of minerals containing fluorine, barium, and other elements, occurs around the previously known stacked intrusive complexes which host the molybdenum-porphyry systems. The discovery of molybdenum at Mt. Emmons in Colorado and at Pine Grove, Utah, can be attributed to recognition of this mineralogical distribution. Other important exploration programs, generated by recognition of similar features in other areas in the western United States, are in progress.
However, no such conclusive models have been developed for the Mississippi Valley deposit. While there is an excellent body of literature which describes most of the deposits in the six or seven type localities scattered around the United States and Canada, there is no single body of observations or of theory which is accepted by the majority of the workers in the field and which can be described as a common denominator underlying exploration activity. There is a need for additional work directed at these Mississippi Valley-type deposits.
It would appear that geologists engaged in the exploration of these deposits are paying increased attention to the study of paleosurfaces and paleoecological environments which are dominated by carbonate-rich rocks. Of course, the internal characteristics such as collapse breccias, limestone-dolomite interfaces, and recrystallized dolomitic rocks are recognized and mapped, and trigger intense exploration when they are seen. Possibly there is some consensus that the margins of carbonate platforms are good places to look for these deposits, and in southeastern Missouri, criteria implicit in both the old and new leadbelts are carefully adhered to.
Also, there is no commonly accepted rationale governing exploration for replacement-type polymetallic sulfide occurrences hosted by carbonate rocks. Possibly researchers and explorationists had not been interested in this group of hydrothermal ore deposits because they are relatively rare, and the metal content, dominated as it usually is by lead and zinc, is apt to be relatively low and unremunerative. The Mexican deposits such as Plomsas, Santa Eulalia, Naica, Charcas, Providencia-Concepción del Oro, Taxco, San Martin, La Encantada, Fresnillo, and Velardena are not well known, and the results of significant research, if any has in fact been conducted, are proprietary and locked up in mining company reports. Probably the most utilized lead in the search for these deposits is directed in the vicinity of veins and veinlet systems which can be classified either as feeder-type mineralization of the replacement bodies or as leakage from them.
The search for nickel-copper deposits during the 1950s and 1960s in general contemplated a Sudbury-type model in which it was supposed that a sulfide magma body had been injected from depth to a near surface position by any of a number of proposed mechanisms. The Sudbury, Ontario and the Thompson Lake, Manitoba districts both lie in the join between contiguous provinces of the Canadian Shield, and it has been widely assumed or hoped that additional deposits can be found along these sutures and supposed zones of weakness. Substantial exploration time and dollars have been expended in the search for deposits in these zones, and still continues, although on a much more limited scale. However, as noted earlier, subsequent to the discovery of nickel in the Kambalda district in Western Australia, it was recognized that some nickel-copper deposits are associated with ultramafic volcanic flows. The Travis and Wodell paper, published in the proceedings of the 12th Commonwealth Mining Congress, and A. N. Naldrett’s paper entitled “Nickel Sulfides, Classification and Genesis,” published by the CIM in 1973, each described this association. Nickel exploration has been at a low ebb during the last decade, due to unremunerative prices received by producers for that metal, but such nickel exploration work as does proceed is directed at both magmatic and the volcanically derived sulfides.
During the past 15 years, fair consensus at least regarding the morphology, if not the genesis, of uranium deposits hosted by sandstones and by quartz pebble conglomerates, has been achieved. Exploration, designed to test sulfide-rich as compared to oxide-rich portions of appropriate sandstone and quartz pebble conglomerate units, has developed. However, no such consensus has been reached in respect to the vein-type deposits of northern Saskatchewan or of northern Australia. These deposits appear to be characterized in both locations by high-grade pitchblende veins hosted in crystalline or in metamorphic rocks, covered by Proterazoic sediments in which are bedded carnotite or carnotite-type mineralization. While there is very little agreement amongst geologists as to how these deposits are formed, most organizations involved in the search for these deposits appear first (mainly by means of airborne electromagnetic and radiometric surveys) to search for signs of the bedded material in the overlying sandstones, and then to attempt to search for the veins.
Geologists who concern themselves with exploration, evaluation, and production of the various industrial minerals, as well as those involved in coal, oil shale, tar sands, and other similar materials, are obviously as interested in the genesis and geological environments of these deposits as are the hard mineral geologists. However, in most cases the deposits are huge and relatively easy to find and therefore the difficulty and cost of original discovery have not been as great as they have been in the case of the commodities discussed previously. The major challenge in the case of the industrial minerals, and in the case of the hydrocarbons, has been identifying major volumes of material which conform to various engineering and chemical standards. Therefore these geologists think not so much in terms of origin and empirical models, as they progress in their exploration work, as they do of quality control and engineering parameters.
The importance of these models, of course, is that they provide terms of reference and criteria to the geologist as he decides, for example, whether or not a given district or prospect warrants more expenditure. If the geologist maps a considerable number of features which conform to his model for a given deposit type, he may well decide to recommend drilling or some other form of physical exploration. If, on the other hand, his data shows that few of the elements of his model are present, he may decide that additional expenditure is not necessary or wise. Similarly, the geologist might use his model to tell him in which direction, either laterally or vertically, additional drilling should be planned. If he knows that the features he is mapping are usually found vertically above another feature of economic importance, he may decide to recommend deeper drill holes. The models, above all, give the geologists terms of reference and continuity of information which extend beyond the bore of the drill hole he is considering, and beyond perhaps, the geometry of the ore shoot being investigated.
The discovery by Newmont Mining Company of the Carlin gold deposit a decade or more ago generated a substantial amount of precious metals exploration in the basin and range province of the western United States. The primary exploration technique involved in this search has been the collection of samples, both geochemical and rock samples, which are assayed for gold, and a variety of other elements, such as mercury, which are thought by the geologists involved to be useful “indicator or pathfinder elements.” This search often requires a collection of samples from wide areas, without much geological discrimination. Recently, emphasis has been placed on hot spring environments, the so-called jasperiod environment; the interest in this type of environment being generated by recent discoveries at Alligator Ridge, Nevada, and in the Caldera environment, similar to the one at McDermitt, Nevada, where mercury mineralization has been mined for some time.
In summary, it should be reiterated that as mineral exploration develops and grows more sophisticated, increasing care will be given to the development of the empirical model. The model is simply a generalized amalgam of features of known deposits, the enclosing host rocks, and all the various alteration patterns which are usually attendant to the mineralization. Once the model is produced and agreed upon by everyone involved, the next exploration step is to decide which geological province might provide all the various factors and features in the model. The third step is to identify, through literature searches and through inspection of old exploration records and other geological material, where within the district chosen the various features called for by the model might be found.
Prospectors as well as modern explorationists have always had models in mind. In former years the prospectors looked for signs of direct mineralization in outcrop, and proceeded then to test these outcrops by drilling or other means. The modern explorationist still hopes to find mineralization in outcrop, and on occasion will do so for many years hence. However, increasingly his work will consist of testing models, once he has found areas in the field which conform in most respects to the model.
Mining company managements, increasingly, will have to get used to the idea of drilling concepts or models rather than mineralized outcrops. One management which has already adopted this idea is that of Western Mining, the Australian company. D.W. Haynes, its copper consultant, has explained in his paper entitled “Mining Technology in Mineral Resource Exploration” (published in Proceedings, Third Invitation Symposium on Mineral Resources in Australia, held in Adelaide in October, 1979 by the Australian Academy of Technological Sciences) how Western Mining geologists put together source rock theory with known information regarding the sedimentary basin on the Stuart Shelf to find the Olympic Dam deposit. The Western Mining geologists, Haynes explains, were looking for areas in the sedimentary basin in the state of Southern Australia where sediments similar to those hosting the Zambian Copper Belt might be found in proximity to basaltic rocks. In addition, a preconceived tectonic model was apparently postulated, and a lineament analysis, aided by data derived from Landsat images, was produced. Once the complete model had been settled upon, it was aggressively explored, and the important Olympic Dam copper-uranium discovery was made. Haynes points out that the mineral deposit which was discovered was not precisely the same type as was anticipated and this perhaps provides some food for thought. The Olympic Dam discovery is obviously not the first successful application of modeling. It is, however, one of the better documented cases of successful exploration generally designed to test a model.
Geological reasoning has improved and must improve even more if an adequate rate of discovery of mineral deposits is to be maintained. There has been parallel improvement of various geophysical, geochemical, and remote sensing techniques. Geophysics has progressed from Thomas Edison’s dip needle, successfully used at Sudbury, Ontario, early in this century, to satellite-mounted magnetometers. Geochemistry has progressed from the practice of early Scandinavians, who during the Middle Ages chased mineralized boulders up streams of glacial debris to their source to determinations of 25 or more metals in soil samples. Remote sensing, in its strict sense, has been developed from the day prior to the Second World War when Canadian geologists made interpretations from oblique aerial photographs to today’s interpretations made from enhanced remote sensed images from satellites.
Interpretation of leached outcrop and of gossan, which was started by Augustus Locke and Rowland Blanchard in the 1920s and carried on, among others, by Kenyon Richard and Harold Courtright in the 1940s and 1950s appears to be a dying art, simply because surface mapping of large porphyry copper systems is not now an everyday activity. However, during the 1960s, enhanced evaluation of potential drill targets was made possible by the interpretations of clay alteration patterns combined with that of leached cappings. The gossans in Western Australia are far different from those of the western United States, but an awareness of the technique and a fair ability to apply it to gossans in that country occurred. Later the Australian expertise was transferred to South Africa, and a number of discoveries were made there as well.
Prototypes of much of the geophysical equipment in use today were in field prior to the Second World War, for example as previously referred to, Thomas Edison’s dip needle, with which he discovered or, at least almost discovered, continuations of the nickel-copper ore at Falconbridge in the Sudbury District. Hans Lundberg, using an equipotential method which verged on electromagnetics, made a great discovery at Buchans Newfoundland, in 1926. Technology developed for military purposes during the war was put to work immediately thereafter, principally in the development of electromagnetics and of airborne electromagnetics. Geiger counters, rarities before the war, become commonplace shortly thereafter. In the mid-fifties, relatively trouble-free ground electromagnetics systems were used routinely in the field and torsion spring magnetometers also were developed; these instruments dramatically increased the rate at which readings could be taken. Airborne electromagnetics and magnetics also became routine during the early 1950s. The combined use of these two techniques resulted in a very impressive string of discoveries, mainly in Canada, which continued at least until 1975. In recent years the rate of discoveries by AEM and AM has declined, for a variety of reasons. However, one can wonder if the application of a single technique will ever again result in a list of discoveries such as Thompson Lake, Manitoba; Heath Steele, New Brunswick; Mattagami and Joutel, Quebec; Timmins, Ontario; Sturgeon Lake, Ontario; and Crandon, Wisconsin. This list, while incomplete, represents an extraordinary record of discovery. Also, in the mid-fifties, Dr. Arthur Brandt completed development of the induced polarization method. The Geiger counter largely gave way in the 1960s to the scintillometer. Drill hole logging by regular radiometric methods became commonplace in the search for rail front uranium deposits in sandstones, particularly in the western United States.
The advent of various microelectronic devices has given geophysicists the capability of gathering enormous amounts of data. For example, modern airborne electromagnetic equipment provides as many as six audio frequency channels, two very low frequency electro EM channels, four gamma-ray spectrometer channels, and a magnetometer channel. All 13 channels are recorded on magnetic tape every half second.
The proton magnetometer can now achieve a sensitivity of about one gamma. High sensitivity magnetometers are used for identification of extremely subtle features in areas of very low magnetic relief. Airborne spectrometer surveys, utilizing 49,161 cm3 (3,000 cu in.) crystals are now routine. These crystals yield much higher count rates than has been the case; the differentiation between earth generated radiation and atmospheric radiation is measured separately and identified.
Of course all the additional information collected by the newer equipment has provided enormous challenges for the interpreter of the data. Sulfide sources are easily confused with nonsulfide sources as much these days as previously.
In spite of improvement of the equipment, it has not yet been possible to make significant improvements in the depth penetration of electromagnetics equipment. During the past five years, transient electromagnetic and audiomagnetic telluric systems, the latter utilizing either distant thunderstorms or nearby controlled sources, have come into use. This system reputedly can achieve depth penetration of several hundred meters. Presumably additional research as to the application of these systems to mineral exploration problems will continue.
Additional research and improvement of in-hole electromagnetic and induced polarization systems will continue. Most current interest appears to be directed at fixed source time domain units, fixed source continuous wave multifrequency units, and single frequency moving transmitter-receiver systems.
The broad range of portable gamma-ray detectors employed in airborne radiometric surveys is also adapted to in-hole surveys. It is anticipated that improvements in simple nondiscriminating scintillation counters for channel differential spectrometers will be made.
In summary, geophysicists have been able to make remarkable improvements in the portability and accuracy of their equipment. They have not been able to make substantial improvements in the depth penetration of the equipment because increased depth penetration means increased volumes of rock energized and therefore increased numbers of nonsulfide features which can cause responses and consequently signal noise with concurrent difficulties in interpretation. Because as many mineral deposits remain to be found at depths beyond the reach of present geophysical equipment as have been found in the near surface, we can confidently expect that additional research and development of equipment capable of seeing deeper into the earth will continue.
The basic principles of geochemical prospecting have been known for thousands of years. They were in fact successfully applied by early prospectors who traced visual indications of ore dispersion patterns in rocks, soils, and stream sediments back to bedrock sources. However, it was not until the 1930s that chemical emission spectrographic analytical methods began to permit trace element measurements. The Russians and Scandinavians began to use geochemical exploration techniques, as we know them, prior to the Second World War. Subsequent to the war, an explosion of geochemical exploration activity occurred, permitted by the development of inexpensive rapid colorimetric analytical techniques by the United States Geological Survey. Research activity spread to the United Kingdom and thence to other countries in western Europe during the 1950s. Students of Hawkes, Webb, Bloom, and Warren took the technique into almost every part of the world and many discoveries were made. As surveys were completed during the 1950s and 1960s, the various ways by which elements can become dispersed throughout the secondary environment became fairly well appreciated. During the 1970s startling advances, again made possible by the advent of microelectronics, were made in the analytical end of geochemical exploration. Atomic absorption instruments and techniques were refined and matrix corrections introduced as routine procedure for certain elements. X-ray fluorescence methods were greatly improved. Plasma spectrometry permitted significantly low detection limits for a number of elements. This improvement in analytical quality and reduction has resulted in a marked decrease in the dollar cost of exploration. Computer data handling capability has kept pace with analytical advances. Computerized data plotting as well as univariate and multivariate statistical procedures are widely accessible.
Many have pointed out that the improvement in equipment has not been matched by comparable improvement in the understanding of fundamental geochemical processes. Routine surveys have been laid out and interpreted without regard to the solid body of information which has been collected. Hopefully, future practice will catch up with the theory already in place.
The geologists also have improved hardware at their disposal and increased availability of this equipment should result in an increased rate of discovery. The microprobe is probably the most important of this new equipment. The ability to detect variations in metal content in individual minerals collected from various parts of districts and of mineral deposits will greatly increase the geologist’s ability to predict projections and to site exploratory drilling and other exploration activity. Increased use of the fluid inclusion stage will result in increased knowledge of fluid inclusions and of hydrothermal fluid temperature, pressure, and composition which is not unknown in respect of many mineral deposits. Collection of this fundamental knowledge will result in better definition of mineral zoning patterns and will, as a result, guide exploration. Fluid inclusion research will become a routine part of modern mineral exploration.
The microprobe will make it possible to determine phase compositions of sulfide-silicate vein and wall rock assemblages. These studies also will assist in determinations of deposit zoning and increase reliable predictability of exploration parameters. Sulfur isotope studies will also become routine and will assist in the classification of ores and the placing of these ores in appropriate models.
Application of nearly all the techniques listed in the previous pages will result in the production of enormous amounts of data. At present, geologists use computers to store data and make various calculations as to tonnage, metal content, and economic viability of mineral deposits, but they have not found a way to utilize the deductive capabilities of computers to find deposits. Possibly they never will, as the human brain is still the best computer of all. However, it is obvious that increased research as to the application of computer techniques to exploration will continue.
In due course geologists will be able to make much better use of images produced by sensors placed in satellites than they now do. Much of the science and technology required to produce images which will identify various rock types now exists. These sensors have been flown from time to time in U-2-type aircraft, and experimental surveys such as famous ones over Saindoc, Pakistan, and Goldfields, Nevada, as well as the various case histories flown by a GEOSAT-NASA joint experiment, provide impressive data. It is clear that the potential impact on mineral exploration of remotely sensed data will be significant.
The ultimate exploration tool is still the diamond drill. We have seen substantial improvement in the reliability, portability and, most important, the percentage of core recovery achievable by drilling equipment in the past few years. The retrievable core barrel has been a great cost saver. However, costs of drilling in the past decade have increased drastically, and the mining business needs, and needs soon, additional improvement. Probably the next routine improvement will be the retrievable bit. If some way can be found to change drill bits without removing an entire string of drill rods, important savings will be achieved. It is certainly to be hoped that the drilling industry will continue its search for ways in which it can keep the costs of diamond drilling under better control than is now the case.
Both the exploration geologist and the mine geologist must at all times have a reasonably accurate perception of the economics of the project. It goes without saying that this perception of economics should apply at all stages of a project and will be applicable initially when a grassroots or reconnaissance program is devised. If a discovery is made in a new district, even during the very early stages of the assessment of the prospect, the geologist must make a back-of-the-envelope calculation designed to demonstrate that under prevailing and under forecast economic conditions, presuming that all assumptions as to tonnage and grade materialize, the mineral deposit he is modeling, will yield a suitable return on investment. During the early stages of such a project, there are few hard facts and many assumptions. As first order drilling, designed to determine the outlines of the deposit continues, these assumptions are replaced by facts, and estimates become more reliable.
A second occasion for an economic review of the project might reasonably occur when plans are made for the expensive closely spaced drilling required to determine final tonnage and grade figures. This estimate, of course, will utilize many assumptions but substantial data will have been provided by the drilling completed. A crude estimate of tonnage and grade will be available, metallurgical data (supplied by bench tests on material from drill cores) will be on hand, assumptions concerning mining costs will be made possible, if only by comparison with other similar operations. Sufficiently accurate assumptions concerning the amount and cost of infrastructure can be made, and the cost of any necessary access roads and other similar items can be reasonably estimated. Presumably this estimate will be made by a team of construction and mining engineers, assisted by the geologist who should be on hand to provide data and interpretation of data concerning the nature and characteristics of the ore deposit, including among other things, continuity of grade, mineralogical zoning, and varying characteristics of host rock type.
There are, of course, various levels of economic estimates, leading from the first preliminary calculations to the full-scale detailed estimates as to the cost of a given project. In Fig. 2.1.1, various levels of economic estimates are defined. Probably, for reasons which will be explained later, the exploration geologist should be substantially involved during the early planning of a project, at least in those phases which involve mine design and planning. In the later design and planning stages, which involve mainly detailed estimates of construction, the geologist will be much less involved, but should in any case be available to the design and planning team until the mine opens.
Figure 2.1.1.
If the results of the feasibility study described in the preceding paragraphs should indicate that a deposit will be financially attractive (should assumptions concerning both tonnage and grade be later confirmed), the next obvious step is to cost out the kind of detailed drilling program necessary to generate accurate ore reserve estimates. The most important consideration in this regard is drill hole spacing. This is a critical matter, as the cost of the drilling can be (even in, say, the case of only a medium-sized porphyry copper deposit) on the order of $10 million or even substantially more. However, as the preproduction expenditure in the case of such a deposit would run into hundreds of millions of dollars, it can easily be recognized that the cost of diamond drilling and sampling of drill results is a very poor place to try to save money. On the other hand, the geologist does have a responsibility to recommend a program which will adequately sample, but not overdrill, the prospect. We would all like to think that it should be possible through the application of some statistical formula or other to precisely establish optimum drilling spacings. Sadly, this is not the case. In many districts long practice may have established the proper drill hole interval, but many of the mineral deposits being discovered during the 1980s are in new districts, and some involve types of mineral deposits for which no precedents are available. Obviously the first matter to be considered in reaching such a decision is a variation of the metal content from hole to hole in the drilling already completed. If there is a small variation, then obviously the drill spacing can be much larger than in the case where there has been large variation of metal content.
Another matter to which the geologist must give his most careful consideration is sample size, or the length of core which will be included in each sample to be sent for assay. Assay costs in case of even a medium-sized deposit will be astonishingly large, and if, for example, the decision is reached to sample every 3 m (10 ft) run of core, rather than each 1.5 m (5 ft) run of core, these very large numbers can be halved. Here again, the optimum choice will result in the least expensive alternative which will completely achieve the required results.
Other decisions, such as staffing, choice of assay office facilities, core storage, provision for constant, high-caliber geological input, and interpretation of data on a current basis are all highly important matters to be costed into the final total budget. There is temptation to scrimp on all of the above-listed matters, but inadequate attention to any of them can in the long run be very costly.
Aside from making the proper arrangements as to drilling intervals, sampling intervals, standards of assaying and the like, there are many other matters which need to be considered. Large volumes of core will be coming in from the drill sites each day. Good facilities will be needed for logging, photographing, and storage of the core; splitting or sawing, and preparing the samples for assay is no small job. Literally hundreds of assays will be returned and of course these assays will need to be plotted and posted on the appropriate maps and sections on a current basis.
As a result of the necessity to perform all the tasks listed on a current basis, there is also need to provide for housing and eating facilities for as many as 30 or 40 people. Supervision must be provided not only for the geological end of the job but also for the logistical end. It is important to reiterate that the final authorization for expenditure should be constructed from costs carefully computed from realistic assumptions, and the recommended procedures should be completed as inexpensively as possible. However, reductions in costs that result in the production of poor samples and poor interpretation can lead, at the very least, to severe cost overruns of the finally completed exploration job. And worst of all, if, as occasionally has happened, it is discovered after the mine has been opened that the ore “was not there,” the entire preproduction cost has been wasted.
Obviously, the end result of the entire exercise is the production of an ore reserve. The geologist in charge of the project will ultimately have to certify the reserves since the success or failure of the entire mining project is, in the last analysis, dependent upon them. Obviously there are, in the case of any modern mining organization, a number of checks and the exploration geologist will not stand alone in this exercise. However, some one individual eventually is required to sign-off on the ore reserve. There are two kinds of numbers which will be generated and the meaning of these two following definitions should not be confused. The first is the mineral inventory. The mineral inventory simply is a listing of the pounds of metal confined within a given volumetric limit. This reserve may be stated without reference to the cost of the extraction. The second number, the ore reserve estimate, it best described by reflecting that ore is often defined as “naturally occurring materials which can be removed from the ground at a financial profit.” This estimate must be made in connection with determinations of mining plans, the geometry of the ore itself and of internal waste, and obviously in reference to forecast metal prices.
Of course, the ore reserve estimate is the prime objective of detailed drilling. Secondary objectives include provision of material for metallurgical testing, rock mechanics information, and of course, the best possible determination of the geology. The determination of tonnage is a fairly routine arithmetic calculation. Practice varies from organization to organization, but it is common to make a measurement of the specific gravity of each run of core. If the metric system is in use, the volume of material expressed in cubic meters is simply multiplied by the specific gravity and the tonnage thereby determined. If the English system is in use, a cubage factor must be determined, and the volume of rock, as expressed in cubic feet, divided by this factor.
Determination of grade is a much more complicated matter. Corporate policy may dictate the method to be used, regardless of the type of mineral deposit under study. In the vast majority of operating mines in the United States, a simple polygon system is employed. Under this method of ore reserve calculation, the volume of ore closest to a sample point is assigned the grade of that sample. Efforts are made to design polygons so that the division point between adjoining sample points is more or less equidistant between them. The polygon technique has served many mines well over many years, but it sometimes fails to adequately predict trends within ore bodies which may, in fact, greatly influence grade. If such a condition should be suspected, it might be well to consider some form of moving average method similar to that devised by Dr. J.G. Krieg, the famous South African geologist and statistician. Under this technique, it is possible to assign a value of metal content in an area where there is no sample point, by considering the values of all the existing adjoining sample points. Sometimes, because all adjacent sample points are considered in calculating the grade of a block, some blocks are assigned higher values than are indicated by a sample point within the block. In extreme circumstances, ore can be extrapolated through a block in which there is a blank drill hole. Some engineers have extreme difficulty accepting a mathematical or statistical estimate which will draw ore grade contours through a blank drill hole. Regardless of their discomfort, Krieg’s method or trend surface analyses may be the only way to accurately determine grade.
As previously noted, the geologist must remember in addition to the ore reserve data he must produce, he must also produce data which will aid in such matters as pit design and predictions of wall stability.
It is apparent from the foregoing that the exploration and mining geologist, to be really good at his job, must be expert in a wide number of fields। He must know geology and must be current in new geological understanding. He must be a good pragmatic prospector, must understand basic finance and economics, must be good at logistics in order to organize complete drilling and other kinds of exploration projects under his direction. In the following sections various authors describe in greater detail those factors which are important in the geological and exploration work concerned with deposits containing various important minerals and commodities.

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