The Role of Geologic Mapping in Mineral Exploration
George H Brimhall, John H. Dilles, John M. Proffett, 2005. "The Role of Geologic Mapping in Mineral Exploration", Wealth Creation in the Minerals Industry: Integrating Science, Business, and Education, Michael D. Doggett, John R. Parry
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Geologic mapping provides many types of information essential both in exploration for new mineral deposits and during subsequent mining. Geologic mapping of outcrops is used to describe the primary lithology and morphology of rock bodies as well as age relationships between rock units. This information allows delineation of ore-bearing host rocks and postore rocks that obscure or truncate ores. Mapping gathers structural information, including attitudes of veins and postore faults that can be used to predict the geology in the subsurface or laterally under postore rocks, and improves the utility of geophysical data for refinement of subsurface targets. Mapping of the mineralogy of hydrothermal alteration zones, ore minerals, igneous rocks hosting ores, and oxidized and leached rocks that commonly occur at the surface above sulfide-bearing ores can be used in conjunction with geochemical data to produce zonation patterns to target potential ore or to define prospective corridors of exotic mineralization. Similarly, regional geologic mapping in regions with both Paleozoic-Mesozoic overthrusts and Cenozoic normal faults such as the Paleozoic and Mesozoic thrust belt of the United States Cordillera and Basin and Range Province can define prospective windows into basement where mineralization such as Carlin-type gold deposits may occur. In general, geologic mapping underpins the construction of three-dimensional geologic models or hypotheses that guide exploration and discovery and, when geologic time is considered, produces the fourdimensional space-time models necessary for understanding of primary ore formation processes and postdepositional modification by secondary surficial and tectonic processes.
Geologic mapping has been used extensively for exploration for more than 100 years and we predict it will continue to be essential although the tools for recording, compiling, and synthesizing data are evolving rapidly and improve data integration in the office and most recently in the field. Both traditional and future methods rely on field identification skills of the geologist to record salient new geologic data. This review describes the traditional paper- and pencil-based mapping system developed and used extensively by the Anaconda Company from 1900 to 1985 and, because of its versatility, adapted by many other geologists in industry and academia. This and similar systems allow geologically complex and diverse data to be recorded and plotted on a base map, including lithology, rock alteration and mineralization features, relative age relationships, and structural features such as faults and veins. Traditional paper-recorded geologic mapping data are now commonly converted to digital format in the office. We document use of mapping at different stages of the mine-life cycle from general regional-scale geologic mapping to regional- to district-scale exploration targeting, to deposit assessment and ore-reserve definition, through mine planning and production. Examples of mapping described herein include the Ann Mason porphyry copper deposit, Yerington district, Nevada; the Bajo de la Alumbrera mine; Argentina; the El Abra-Fortuna-Chuquicamata districts of Chile; and the Pioneer Mountains of Montana.
Beyond the use of traditional paper-based mapping methods, recent technological advances include global positioning systems, pen tablet computers, palm computers, and laser ranging devices that all support direct (paperless) field-based digital geologic mapping. Improvements in computation speed, memory, data storage, battery life, durability, screen visibility, and portability have made digital mapping practical in general field mapping, mine sites, and advanced projects. Portable digital-electronic instrumentation allows the field geologist rapid access to digital data bases that include geologic maps and photographic and remotesensing imagery with automatic registration and scale independence. Another example described here, using digital mapping systems in the heavily forested portions of the Pioneer Mountains of Montana, shows how on-line GPS communicating directly to the pc tablet and digital orthophotographs made mapping sufficiently effective so as to discover a previously unknown granitic pluton with a concentric breccia zone.
These new digital mapping tools may thus improve the efficiency of mapping and support a scientist in the field with unprecedented opportunities to map where field work has been difficult before. Visualization of geophysical or geochemical data together with geology and synoptic aerial imagery at any scale while mapping provides an integrated data base that facilitates identification of crucial geologic relationships. Digital techniques improve the potential for making conceptual leaps by exploring the available integrated data sets as a field geologist maps, and may in the future lead to more comprehensive three-dimensional geologic models for mineral deposits by effectively using information technology. The authors conclude that both paper and digital systems are powerful and each has certain advantages. However, the central challenge remains the training and nurturing of highly skilled field geologists motivated to practice their profession, welcoming both the rigors of intensive field work and the excitement of scientific discovery. It is surmised here that digital mapping technology may help attract an increasingly computer-literate cadre of new practitioners of mapping into mineral resource exploration.
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Global political and economic developments shape both the demand for minerals and primary metals and their supply. Overall, demand has moved broadly in step with economic activity over the past 30 years. Notwithstanding the collapse of the Soviet Union and Eastern Bloc countries, demand grew more rapidly in the second half of the period than the first. The performance of individual products within this general trend largely reflects the specific nature of their main end uses. The geographic center of demand has shifted away from the mature industrial economies of North America, Western Europe, and Japan toward the newly industrializing countries of the Pacific Rim, China, and India. Mine production rose with demand, but not always in precise step. New capacity was required not just to meet demand, even where that was static, but also to offset the continuing effects of ore depletion. There were also changes in the location of production in response to geopolitical forces, the depletion of ore reserves, and the changing economics of extraction and processing. The number of mines contracted, especially during the 1990s, and the scale of mining operations was increased in order to achieve the requisite cost savings. Prices fluctuated in response to changing balances between supply and demand, trending downward from the early 1970s until the early 2000s. Most products witnessed at least one sharp price spike during the period, usually with continuing repercussions. Prices picked up from 2003, but generally not back to their earlier peak in real terms. Profitability varied according to the products concerned. In many years the average rates of return on capital employed have been insufficient to cover the risks involved.