U.S. DEPARTMENT OF THE INTERIOR U.S. GEOLOGICAL SURVEY Mineral resource assessment of the Absaroka-Beartooth study area, Custer and Gallatin National Forests, Montana Edited by JANE M. HAMMARSTROM1, MICHAEL L. ZIENTEK2, and JAMES E. ELLIOTT3 Open-File Report 93-207 1993 This report is preliminary and has not been reviewed for conformity with U.S. Geological Survey editorial standards or with the North American Stratigraphic Code. Any use of trade, product, or firm names is for descriptive purposes only and does not imply endorsement by the U.S. Government. 'Reston, VA 3Lakewood, CO 2Spokane, WA NONTECHNICAL EXECUTIVE SUMMARY Mineral Resource Assessment of the Absaroka-Beartooth Study Area, Custer and Gallatin National Forests, Montana U.S. Geological Survey Open-File Report 93-207 The Absaroka-Beartooth study area (ABSA) is a tract of approximately 1.4 million acres (5,700 km2) of National Forest land in southwestern Montana adjacent to the northern part of Yellowstone National Park. It includes southeastern parts of the Gallatin National Forest and western parts of the Custer National Forest. Most of the mineral resources of both Forests are within the ABSA. The ABSA contains essentially all of the identified resources of platinum-group elements (platinum, palladium, rhodium, ruthenium, osmium, and iridium) and 75 percent of the identified chromium resources in the U.S., as well as significant gold, silver, copper, lead, and zinc resources. Mines have operated in the area since the 1860's and have produced gold, silver, arsenic, tungsten, copper, lead, zinc, chromium, and platinum-group elements. Mines in production in 1992 are the Stillwater mine (platinum, palladium, gold, rhodium, copper, and nickel), the Mineral Hill mine (gold and silver), and the Livingston Marble and Granite Works (travertine). Recent (since 1970) exploration led to the discovery and development of the Stillwater mine and the discovery of gold-copper-silver deposits with geologic reserves of more than 2 million oz of gold in the New World district (see map on p. ii). Exploration for gold, platinum-group elements, and chromium deposits is ongoing in several parts of the ABSA. Two exploration wells for oil and gas were recently drilled near the eastern edge of the study area. Continued exploration is likely to result in new discoveries for some commodities. The mineral resource assessment of the ABSA is based on consideration of the types of mineral deposits that could be present. Classes of mineral deposits with shared geologic, commodity grade, and ore tonnage characteristics are associated with specific geologic environments. Nine geologic environments are recognized within the ABSA that are permissive for the occurrence of 20 different types of mineral deposits. Favorable areas for undiscovered deposits are delineated within some of these geologically permissive environments. Estimates of undiscovered resources are made for gold, silver, copper, molybdenum, iron, and phosphorous for four deposit types by combining estimated numbers of undiscovered deposits with statistical descriptions of grades and tonnages for each deposit type (table 1). Estimates of undiscovered resources by these methods for the other 16 deposit types are limited by availability of appropriate mineral deposit models. Estimates of resources of copper, nickel, chromium, and platinum-group elements for three other deposit types in a geologic formation known as the Stillwater Complex (shown on map on p. ii) are constrained by exploration data and geology (table 1). Estimates of resources of chromium in deposits near Red Lodge, Montana, are based on detailed geologic investigations conducted previously by the U.S. Geological Survey (table 1). Resource estimates represent in-place amounts of metal. The most economically important types of deposits are those that contain precious metals. Economic analysis must be applied to these estimates to determine any net value that may be associated with these undiscovered metals as minable commodities. Other types of mineral deposits in the ABSA are described qualitatively. Base- and precious-metals are likely to be present in vein deposits. Parts of the ABSA have low to moderate potential for placer gold deposits, uranium, oil and gas, coal and coal-bed methane. Abstroka -Bear tooth Study Area r 45°45' Permissive tract for platinum ( palladium+rhodium, chromium, or copper+nickel in mineral deposits in the Stillwater Complex. Permissive tracts for gold+copper silver or iron+phosphorous in mineral deposits in Archean rocks. J45< 111' Travertine mine Y»llowston» National Park Red Lodge- WY 109° chromite district r -I 45°45' Permissive tracts forcopper+gold+ silver in skam deposits. Permissive tracts forcopper+gold+ silver+molybdenum in porphyry copper deposits. L J 45' 111° 109° New Worid district Permissive tracts for selected deposit types Table l.--Estimated resources for selected commodities in selected mineral deposit types in the Absaroka-Beartooth study area reported as in-place amounts of metal [Results are reported in metric tons; estimates are not additive] Note: Metal tonnages reflect the size and grade of different types of mineral deposits and do not reflect the relative importance of different metals mined in the past or likely to be sought in the foreseeable future; for example, iron is less likely to be of interest than gold or platinum-group elements. Commodity Estimates of undiscovered resources based on computer Estimates of geologic Estimates of geologic simulation for 4 types of mineral deposits reported in resources in the Stillwater resources in Red Lodge terms of percent probabilities and mean expected values. Complex based on chromite deposits from For example, there is a 90% chance that undiscovered evaluation of exploration previous USGS studies. deposits contain 20 or more metric tons of gold. data for 3 types of mineral deposits. 90% (more 50% (median) 10% (less Mean Undiscovered Extensions of Undiscovered Extensions of likely) likely) identified identified resources resources Gold 20 90 400 200 43 Silver 50 700 5,000 2,000 Copper 500,000 3,100,000 17,000,000 6,500,000 220,000 1,200,000 Molybdenum 0 35,000 300,000 130,000 Iron 1,600,000 62,000,000 1,300,000,000 650,000,000 Phosphorous 0 3,000 640,000 30,000 Palladium 2,400 Platinum 690 Rhodium 24 Chromium 6,500,000 4,000,000 12,000 9,000 Nkkel 230,000 1,300,000 IV NONTECHNICAL SUMMARY FOR LAND AND RESOURCE MANAGERS Mineral Resource Assessment of the Absaroka-Beartooth Study Area, Custer and Gallatin National Forests, Montana U.S. Geological Survey Open-File Report 93-207 DESCRIPTION OF THE STUDY AREA occurs are excluded from permissive tracts for chromium deposits. Some types of copper, gold, and The Absaroka-Beartooth study area (ABSA) is silver deposits do form in limestone if certain geologic a tract of approximately 1.4 million acres (5,700 km2) processes operated. Thus, limestone may be included of National Forest land in southwestern Montana in permissive tracts for some types of mineral adjacent to the northern part of Yellowstone National deposits. Nine tracts that illustrate geologically Park. The ABSA includes southeastern parts of the permissive areas for 20 different types of mineral Gallatin National Forest and western parts of the deposits are described in this report. Much of this Custer National Forest. This report describes a report describes the different types of data that were mineral resource assessment of the ABSA that was considered in delineating permissive tracts. This data conducted during 1990-1992 by the U.S. Geological includes compilations of previous studies and new data Survey as part of a study of the mineral resources of acquired by the authors for this study. A new the entire Gallatin and Custer National Forests. Most geologic map for the ABSA shows the spatial distribu­ of the mines and prospects in both Forests are located tion of different rock types. Geophysical data are within the ABSA. The remainders of both National used to interpret the geology below the earth's Forests that lie outside of the ABSA will be studied surface. Sediments collected from streams are during 1992-1994. chemically analyzed to look for associations of elements that may indicate the presence of various types of mineral deposits. Mining and exploration METHODOLOGY history and locations and descriptions of mines, prospects, and mineral occurrences are examined to This mineral assessment is based on several basic determine where the known resources are, what types assumptions. First, mineral deposits can be grouped of mineral deposits they represent, and how they into classes or types that share essential features that relate to the geology. result from fundamental geologic processes. Second, As a second step in the assessment procedure, a class of mineral deposits is typically associated with frequency distributions of grade and tonnage for the a particular geologic environment in which ore- classes of mineral deposits under consideration are forming processes may operate. Third, the size constructed or reviewed to determine if they are (deposit tonnage) and grade (concentrations of various appropriate for the study area. Four of the 20 classes commodities) of a class of mineral deposits reflect the of deposits in the ABSA have appropriate grade and geologic environment and the mineralizing process. tonnage information that is used for estimating Estimates of quantities of undiscovered resources amounts of undiscovered resources. are based on a three-part form of assessment. First, Third, an estimate is made of the number of all available geologic information is compiled and used undiscovered mineral deposits of each type expected to identify areas in which one or more classes of in a tract permissive for that deposit type. This mineral deposits may occur. These areas are called subjective estimate is made by a team of scientists permissive tracts and are drawn such that the possi­ who consider all of the data described for the first bility of a mineral deposit belonging to the class part of the assessment, along with knowledge of the occurring outside of the tract is negligible.