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mittee on Digital Cartography. This committee was chartered by the Office of Management and Budget to reduce the duplication of effort by Federal agencies in digitizing map data and developing geographic data files. The Department of the Interior chairs this committee through a representative from the Survey.

A Standards Working Group within the committee has been testing methods of exchanging digital data to help reduce or avoid unnecessary costs of data conversion. This working group has proposed the Federal Geographic Exchange Format as a standard format for the exchange of digital cartographic and geographic data among Federal agencies.

A National Committee for Digital Cartographic Data Standards, which includes representatives from Federal, State, and local governments and private companies, has been undertaking similar investigations at the national level. This committee, funded by the USGS, is under the auspices of a professional society, the American Congress on Surveying and Mapping. USGS representatives on this committee have served as a coordinating element between the national and Federal efforts.

During 1987, the digital standards activities of the Federal and national committees have been combined into a task force. The goal of the task force is to merge the draft proposed standard produced by the national committee and the data exchange format developed by the Federal committee. The merged document will be reviewed by both parent committees and then published for public review and comment. After review and testing, the USGS will submit the standard through the formal Federal Information Processing Standard approval process as defined by the National Bureau of Standards.

Progress in Ongoing
Program Activities

Major activities in support of the National Mapping Program are: • Primary mapping and revision, which include the production and revision of 7.5minute 1:24,000-scale topographic maps in the conterminous United States and Hawaii and 15-minute 1:63,360-scale topo

graphic maps in Alaska. A few maps have been prepared at 1:25,000 scale in selected areas. During fiscal year 1987, about 1,190 revised and 1,050 new primary quadrangle maps were completed and made available. Topographic maps are available for about 96 percent of Alaska and for 92 percent of the other 49 States (fig. 4). Thirty-one States have complete 7.5-minute series map coverage.

• Intermediate-scale, small-scale, and special mapping, which includes the preparation of maps and map products from the intermediate-scale (1:50,000 and 1:100,000) series to the small-scale (1:250,000) series and other smaller scale U.S. base maps. Complete topographic coverage of the United States is available at 1:250,000 scale and in State base format (except for Alaska) at 1:500,000 scale. The State of Alaska is covered by four smaller scale State base maps ranging in scale from 1:1,584,000 to 1:12,000,000. Planimetric base coverage of the conterminous United States in quadrangle format at 1:100,000 scale became available in 1986. Topographic coverage is available for at least 50 percent of the conterminous United States and Hawaii in one or more of the following intermediate-scale series: 1:50,000-scale quadrangle maps, 1:50,000- or 1:100,000scale county maps, and 1:100,000-scale quadrangle maps (fig. 5). More than 200 topographic-bathymetric maps have been published for coastal area planning. Land use and land cover maps are complete for the conterminous United States and Hawaii and are available in the 1:250,000scale or, in selected areas, in the 1:100,000scale quadrangle format.

• Digital cartography, which includes the production of base categories of cartographic data at standard scales, accuracies, and formats suitable for computer-based analyses. Categories include hypsography, hydrography, transportation, boundaries, Public Land Survey System, and digital elevation model data from 7.5-minute and 1:100,000-scale quadrangle maps; boundaries, census tracts, hydrologic units, Federal and State land ownership, land use and land cover, and digital elevation model data compiled at 1:250,000 scale; and boundaries, transportation, and hydrography data from 1:2,000,000-scale National Atlas sectional maps.

• Information and data services, which include the acquisition and dissemination of

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Geologic Investigations


Applications of Mineral Deposit Models to Resource Assessments

By Donald A. Singer and Dennis P. Cox

A major step in identifying and assessing areas favorable for mineral resource exploration is the assembly of a comprehensive group of mineral deposit models. The U.S. Geological Survey, through its own conterminous U.S. and Alaska mineral assessment programs, and in cooperation with the U.S. Bureau of Land Management and the U.S. Forest Service, is continually working to improve its capabilities in mineral resource assessment.

Resource assessments are performed to help plan economic development, consider alternative uses of land, plan exploration, and estimate the availability of minerals. In order to respond to each of these diverse problems and to use a variety of information and resource assessment methods, a three-step assessment process, including the use of deposit models, is described below and in figure 1.

Step 1. Areas are delineated according to the types of deposits their geology will permit;

Step 2. The number of deposits of each type within each delineated tract is estimated; and,

Step 3. The amount of metal and some characteristics of ore are estimated by means of grade-tonnage models. Areas or domains are delineated that

may contain particular deposit types as

inferred by analogy with deposits in similar

geologic settings elsewhere. In order to construct the boundaries, it is necessary to have a geologic map and it is desirable to have mineral-occurrence, geophysical, exploration, and geochemical information (fig. 2). This information must be integrated with information about the geologic environment of different types of mineral deposits to perform the delineation. The keystone to combining the diverse information is the mineral deposit model. Documented deposit models have been published in USGS Bulletin 1693; this publication allows many geologists to link deposit types to geologic environments.

The Bulletin contains 87 descriptive models and 60 grade-tonnage models compiled by 38 authors. The models are based on data from over 3,900 individual deposits located in 110 countries. Because every mineral deposit, like every fingerprint, is different from every other in some way, models have to progress beyond the purely descriptive in order to represent more than single deposits. Deposits sharing a relatively wide variety and large number of attributes come to be characterized as a "type," and a model representing that type can evolve. The organization of the models constitutes a classification of deposits. The arrangement used emphasizes easy access to the models by focusing on host-rock characteristics and tectonic setting—the features most apparent to the geologist preparing a map.

The descriptive models have two parts. The first, the "Geological Environment," describes the environments in which the deposits are found. The headings "Rock Types" and "Textures" cover the favorable host rocks of deposits as well as source rocks believed responsible for some deposits. "Age" refers to the age of the event responsible for the formation of the deposit. "Tectonic Setting" is concerned with major features or provinces. "Associated Deposits" are listed as deposit types whose presence might indicate suitable conditions for deposits of the type

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portrayed by the model. Thus, this part uses information from the geologic map, the geophysical maps, and the known deposits and occurrences.

The second part of the descriptive model, the "Deposit Description," provides the identifying characteristics of the deposits themselves, particularly emphasizing aspects by which the deposits might be recognized through their geochemical and geophysical anomalies. For 25 models, a cartoon-style map or cross section illustrates ore controls, zoning patterns, or other features of the model. Thus this part is primarily used to classify the known deposits and occurrences in and near the area being assessed. In appendixes to the Bulletin, the models are extensively

indexed by frequency of occurrence of minerals and associated geochemical anomalies. The 3,900 individual deposits referred to in the models are also indexed, and their model classification is shown.

Frequency distributions of tonnages and average grades of well-explored deposits of each type are employed as models for grades and tonnages of undiscovered deposits of the same type in geologically similar settings (fig. 2). These models are useful for correctly identifying known deposits in an area being assessed and for providing information about the potential value of undiscovered deposits in the area. The grade-tonnage models are presented in graphical format to make it easy to compare deposit types and to display the data.

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This publication (Bulletin 1693) represents the largest collection of models that can be used for resource assessments.

The current version of models has been used as a framework for organizing mineral deposit compilations and interpretations for 2-degree quadrangles in Alaska and the conterminous United States. Explorationists have benefited by the new recognition of significant differences in some gold deposit types associated with volcanic rocks. The models are also providing a basis for economic analysis for land and policy planning in cooperative projects with other Federal agencies.

Deposit models tie together diverse information on geology, mineral occurrences, geochemistry, and geophysics that is used in mineral resource assessments. The Survey's ability to make better resource assessments, therefore, depends directly on the quality of the models available. The present 3-step mineral resource assessment mode represents an important step in the improvement of these models.

A New Approach to Determining How Much Coal is Available for Mining

By Jane R. Eggleston and M. Devereux Carter

According to many energy forecasts, the United States currently has over 400 billion tons of coal available for mining. This estimate was determined by applying bed thickness, mining depth, and reliability factors to a geologically derived coalresource base. However, this methodology for calculating "available" coal, defined as coal which could be removed by mining, ignores many geologic, land-use, engineering, and economic factors that restrict the minability of coal. When these factors are

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