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  Definitions for mineral resources and reserves from U.S. Department of theInterior, U.S. Geological Survey,  Mineral Commodity Summaries 2001 (Washington, D.C.: GPO, 2001), Appendix C. Resource .—A concentration of naturally occurring solid, liquid, or gaseous material in or on the Earth’s crust in such form and amount that economic extraction of a commodityfrom the concentration is currently or potentially feasible. Original Resource .—The amount of a resource before production. Identified Resources .—Resources whose location, grade, quality, and quantity areknown or estimated from specific geologic evidence. Identified resources includeeconomic, marginally economic, and sub-economic components. To reflect varyingdegrees of geologic certainty, these economic divisions can be subdivided into measured,indicated, and inferred. Demonstrated.—A term for the sum of measured plus indicated. Measured .—Quantity is computed from dimensions revealed in outcrops,trenches, workings, or drill holes; grade and/or quality are computed fromthe results of detailed sampling. The sites for inspection, sampling, and measurements are spaced so closely and the geologic character is so welldefined that size, shape, depth, and mineral content of the resource arewell established. Indicated .—Quantity and grade and/or quality are computed frominformation similar to that used for measured resources, but the sites for inspection, sampling, measurement are farther apart or are otherwise lessadequately spaced. The degree of assurance, although lower than that for measured resources, is high enough to assume continuity between pointsof observation. Inferred .—Estimates are based on an assumed continuity beyond measured and/or indicated resources, for which there is geologic evidence.Inferred resources may or may not be supported by samples or measurements. Reserve Base .—That part of an identified resource that meets specified minimum physical and chemical criteria related to current mining and production practices, includingthose for grade, quality, thickness, and depth. The reserve base is the in-placedemonstrated (measured plus indicated) resource from which reserves are estimated. Itmay encompass those parts of the resources that have a reasonable potential for becomingeconomically available within planning horizons beyond those that assume proventechnology and current economics. The reserve base includes those resources that arecurrently economic (reserves), marginally economic (marginal reserves), and some of thosethat are currently subeconomic (subeconomic resources). The term “geologic reserve” has been applied by others generally to the reserve-base category, but it also may include the   Energy , Ch. 10, extension 4 Definition of resources and reserves 2inferred-reserve-base category; it is not a part of this classification system. Inferred Reserve Base.—The in-place part of an identified resource from which inferred reservesare estimated. Quantitative estimates are based largely on knowledge of the geologiccharacter of a deposit and for which there may be no samples or measurements. Theestimates are based on an assumed continuity beyond the reserve base, for which there isgeologic evidence. Reserves .—That part of the reserve base which could be economically extracted or  produced at the time of determination. The term reserves need not signify that extractionfacilities are in place and operative. Reserves include only recoverable materials; thus,terms such as “extractable reserves” and “recoverable reserves” are redundant and are nota part of this classification system. Marginal Reserves .—That part of the reserve base which, at the time of determination, borders on being economically producible. Its essentialcharacteristic is economic uncertainty. Included are resources that would  be producible, given postulated changes in economic or technologicalfactors. Economic.—This term implies that profitable extraction or  production under defined investment assumptions has been established,analytically demonstrated, or assumed with reasonable certainty. Subeconomic Resources .—The part of identified resources that does notmeet the economic criteria of reserves and marginal reserves. Undiscovered Resources .—Resources, the existence of which are only postulated, comprising deposits that are separate from identified resources. Undiscovered resources may be postulated in deposits of suchgrade and physical location as to render them economic, marginallyeconomic, or subeconomic. To reflect varying degrees of geologic certainty,undiscovered resources may be divided into two parts. Hypothetical Resources .—Undiscovered resources that are similar toknown mineral bodies and that may be reasonably expected to exist in thesame producing district or region under analogous geologic conditions. If exploration confirms their existence and reveals enough information abouttheir quality, grade, and quantity, they will be reclassified as identified resources. Speculative Resources.—Undiscovered resources that mayoccur either in known types of deposits in favorable geologic settingswhere mineral discoveries have not been made, or in types of deposits asyet unrecognized for their economic potential. If exploration confirms their existence and reveals enough information about their quantity, grade, and quality, they will be reclassified as identified resources. Restricted Resources/Reserves .—That part of any resource/reserve category that isrestricted from extraction by laws or regulations. For example, restricted reserves meet all   Energy , Ch. 10, extension 4 Definition of resources and reserves 3the requirements of reserves except that they are restricted from extraction by laws or regulations. Other Occurrences .—Materials that are too low grade or for other reasons are notconsidered potentially economic, in the same sense as the defined resource, may berecognized and their magnitude estimated, but they are not classified as resources. Aseparate category, labeled other occurrences, is included in figures 1 and 2. In figure 1, the boundary between subeconomic and other occurrences is limited by the concept of current or potential feasibility of economic production, which is required by the definitionof a resource. The boundary is obviously uncertain, but limits may be specified in termsof grade, quality, thickness, depth, percent extractable, or other economic-feasibilityvariables. Cumulative Production .—The amount of past cumulative production is not, bydefinition, a part of the resource. Nevertheless, a knowledge of what has been produced isimportant to an understanding of current resources, in terms of both the amount of past production and the amount of residual or remaining in-place resource. A separate spacefor cumulative production is shown in figure 1 [in srcinal; not shown here]. Residualmaterial left in the ground during current or future extraction should be recorded in theresource category appropriate to its economic-recovery potential.
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