The Conterminous United States Mineral Assessment Program; background information to accompany folio of geologic, geoche

Dallas L. Peck, Director

For sale by Map Distribution

Federal Center, Box 25286 Denver, CO 80225

1. Index map showing locations of the Butte and adjacent 1°X2° quadrangles, Montana and Idaho ................................
2. Index map of the Butte 1°X2° quadrangle ......................................................................................................................
3. Principal maps of the Butte 1°X2° quadrangle folio.......................................................................................................
4. References to the geology and mineral deposits of the Butte 1°X2° quadrangle ...........................................................

Abstract ................................................................................................................................ . Introduction .......................................................................................................................... .

Purpose and Scope of Studies .................................................................................... .

Location and Geography ............................................................................................ .

Mining History ........................................................................................................... .

Previous Studies ......................................................................................................... .

Present Investigations ....... ..... .... .................. ......... ......... ....... .. ......... .......... ........ ......... Descriptions of Maps and Results of the Butte Cusmap Project..........................................

Geologic Maps (OF-86-292, MF-1925) ...................................................................

Geochemical Maps and Data (OF-82-0617-A to I, USGS-GD-85-006)................

Gravity and Magnetic Anomaly Map (1-2050-1).......................................................

Limonite and Hydrothermal Alteration Map (1-2050-A) ...... ............ ........................

Linear Features Associated with Metal-Bearing and Prospects Map (1-2050-B) .....

Mines and Prospects Map (1-2050-C) ........ ....... ........ ............. ...... ......... .................... 10 Mineral-Resource Assessment Maps (1-2050-D, E, F, G, and H)............................. 10 References Cited and Selected Bibliography for the Butte 1°X2° Quadrangle.................... 13

By JAMES E. ELLIOTI, CHARLES M. TRAUTWEIN, CHESTER A. WALLACE, GREGORY K. LEE, LAWRENCE C. ROWAN, and WILLIAM F. HANNA

The Butte 1°X2° quadrangle in west-central Montana was investigated as part of the U.S. Geological Survey's Conterminous United States Mineral Assessment Program (CUSMAP). These investigations included geologic mapping, geochemical surveys, gravity and aeromagnetic surveys, examinations of mineral deposits, and specialized geochronologic and remote-sensing studies. The data collected during these studies were compiled, combined with available published and unpublished data, analyzed, and used in a mineral-resource assessment of the quadrangle. The results, including data, interpretations, and mineral-resource assessments for nine types of mineral deposits, are published separately as a folio of maps. These maps are accompanied by figures, tables, and explanatory text. This circular provides background information on the Butte quadrangle, summarizes the studies and published maps, and lists a selected bibliography of references pertinent to the geology, geochemistry, geophysics, and mineral resources of the quadrangle.

The Butte quadrangle, which includes the worldfamous Butte mining district, has a long history of mineral production. Many mining districts within the quadrangle have produced large quantities of many commodities; the most important in dollar value of production were copper, gold, silver, lead, zinc, manganese, molybdenum, and phosphate. At present, mines at several locations produce copper, molybdenum, gold, silver, lead, zinc, and phosphate. Exploration, mainly for gold, has indicated the presence of other mineral deposits that may be exploited in the future. The results of the investigations by the U.S. Geological Survey

MINERAL RESOURCES MAPS OF THE BUTTE 1°X2° QUADRANGLE, MONTANA

ABSTRACT

indicate that many areas of the quadrangle are highly favorable for the occurrence of additional undiscovered resources of gold, silver, copper, molybdenum, tungsten, and other metals in several deposit types.

PURPOSE AND SCOPE OF STUDIES

This circular, together with a folio of maps (published separately, table 1), is part of a series of U.S. Geological Survey reports that contain information on the mineral resources and mineral-resource potential of the conterminous United States. The studies described in this circular were done for the Butte 1°X2° quadrangle as part of the Conterminous United States Mineral Assessment Program (CUSMAP). The main objective of CUSMAP is to provide information to support a sound, long-range, national minerals policy, and for Federal, State, and local land-use planning. In addition, this program will increase the geologic, geochemical, mineral deposit, and geophysical knowledge of the conterminous United States. The program also provides a regional geologic and mineral-resource framework for site-specific studies, such as wilderness investigations, and guidance for mineral exploration.

The Butte quadrangle, which contains the worldfamous Butte mining district, was selected for study as part of CUSMAP because of its location in an important mineralproducing region in west-central Montana. During this study, new data on geology, geochemistry, geophysics, geochronology, mineral resources, and remote sensing were collected and synthesized, and a folio of maps and numerous reports were produced. This report is one of the products of the CUSMAP project and summarizes and provides background information for the folio of geologic, geochemical, remote-sensing, and mineral-resource maps.

The Butte 1°X2° quadrangle, which covers an area of approximately 6,550 mi is located in west-central Montana and is bounded by latitudes 46° and 47° and longitudes 112° and 114° (figs. 1 and 2). The quadrangle includes several major and minor mountain ranges separated by intermontane valleys. Major ranges include the Garnet, Flint Creek, and Anaconda Ranges and the John Long, Boulder, and Sapphire Mountains. The valleys are generally at elevations of 3,500 to 5,500 ft and mountain peaks at elevations of 7,000 to 10,500 ft. The highest point in the quadrangle is Mount Evans, at 10,641 ft, in the Anaconda Range, in the south-central part of the quadrangle.

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Figure 1. Index map showing locations of the Butte and adjacent 1°x2° quadrangles, Montana and Idaho.

The Continental Divide trends nearly south through the eastern part of the quadrangle. The divide forms the southern part of the boundary between Powell and Lewis and Clark Counties and the boundary between Jefferson County and the counties of Powell, Deer Lodge, and Silver Bow. The Continental Divide also follows the crest of the Anaconda Range in the south-central part of the quadrangle. East of the divide, the major drainages are the Boulder River and Little Prickly Pear Creek, which are tributaries to the Missouri River. West of the divide, the major drainage is the Clark Fork of the Columbia River. Major tributaries to the Clark Fork include Rock and Flint Creeks and the Blackfoot and Little Blackfoot Rivers.

The principal population centers in the quadrangle are Butte, in the southeastern part; Helena (State capital of Montana), on the eastern edge; and Missoula, near the northwestem comer of the quadrangle. Several other smaller towns, which include county seats of several counties, occur in the quadrangle. Most of the quadrangle is in Granite, Powell, Lewis and Clark, and Jefferson Counties, and smaller parts are in Missoula, Ravalli, Deer Lodge, and Silver Bow Counties.

Approximately one-half of the quadrangle is covered by National Forests, which include parts of the Deerlodge, Lolo, Bitterroot, Beaverhead, and Helena National Forests.

MINING HISTORY

The quadrangle has been a major producer of ores and mineral commodities since the 1860's. The largest and most famous mining district in the quadrangle is the Butte or Summit Valley district, one of the most productive mining districts in the world. The Butte district produced more than $6 billion (value of metals at the time of production). Other mines and districts in the quadrangle have a combined total production of about $400 million. Early prospectors discovered many placer-gold deposits in the quadrangle, such as Last Chance Gulch at Helena, and placers were rapidly developed and mined during the 1860's and early 1870's. About the same time, or within a few years after the discovery of placer-gold deposits, many of the principal lode deposits in the Helena, Marysville, Wickes, Gamet, Cable, and Philipsburg districts were also found. These lode deposits included rich gold, silver, silver-lead, silver-gold, silver-gold-lead-zinc, and gold-copper deposits.

During the early period of mining, before 1883, the development of lode deposits was hampered by the lack of milling and smelting facilities and the remoteness of supply centers. Only ores of highest grades could be mined because of the high costs of shipping to mills and smelters. The completion of a transcontinental railroad through the quadrangle in 1883 stimulated the mining industry and lead to the peak period (about 1883 to 1900) of production of silver and gold in most mining districts. Since 1900, except for the Butte district, mining activity has generally declined and most districts have not reached the high pre-1900 production levels.

The Butte district is known primarily as a copper district but, before the discovery of rich copper deposits, the district was first, a placer-gold district and later, one of the most important silver-producing districts of the U.S. The first major discovery of copper at Butte occurred in 1882. By the early 1900's, Butte was the most important mining center in the U.S. and has continued, with short interruptions, to be major producer to the present. Copper was the principal commodity produced, but the district has also produced large amounts of gold, silver, zinc, lead, manganese, molybdenum, cadmium, bismuth, sulfuric acid, selenium, and tellurium.

Many areas in the Butte quadrangle, in addition to the Butte district, that have been large producers of metals and of other commodities. The most productive districts (and their principal products) are the Wickes (gold, silver, copper, lead, and zinc), Philipsburg (silver, manganese, zinc, lead, and copper), Marysville (gold, silver, and lead), Garrison (phosphate), Rimini (silver and lead), and Black Pine (silver). Other commodities produced in large quantities from the Butte quadrangle, in addition to those above, are tungsten, fluorite, barite, sapphires, limestone, and silica.

Mining and exploration is active at several locations within the quadrangle. Major deposits of copper and molybdenum are mined and milled at Butte; and, at the Montana Tunnels mine, about 17 mi south of Helena, a large ore body containing gold, silver, zinc, and lead is mined and milled. Phosphate ore is mined near Warm Springs, in the Gamet Range. Continuing exploration, especially for gold, has resulted in the discovery of gold deposits in several areas of the quadrangle.

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GRAVITY AND MAGNETIC ANOMALY MAP (I-2050-I)

mean sea level. Gravity and magnetic data were supplemented by measuring the densities and magnetization of 823 rock samples from the quadrangle representing a range of rock types that varied in density and magnetization.

High and low gravity anomalies were interpreted from a compiled Bouguer gravity anomaly map. In general, largeamplitude gravity lows correlate with thick, low-density sediment in intermontane basins and with relatively thick occurrences of felsic to intermediate intrusive rocks. Smallamplitude gravity highs correlate with relatively thick occurrences of Proterozoic and (or) Phanerozoic sedimentary rocks, which have higher densities than intrusive rocks or basin-fill deposits. Basin-fill deposits probably reach maximum thicknesses of about 3.5 km and intrusive rocks of the Boulder batholith are present to a depth of at least 5 to about 10 km. Other plutons extend from about 0.5 to 3 km in depth.

The compiled aeromagnetic map shows two contrasting regions in the quadrangle; (1) the southeastern part of the quadrangle contains a cluster of highs and lows that correspond with the region occupied by the Boulder batholith, the Elkhorn Mountains Volcanics, and the Lowland Creek Volcanics, and (2) the remainder of the quadrangle is characterized by many isolated highs that correlate with locations of plutonic igneous rocks of Cretaceous and Tertiary age. Magnetic lows of diverse amplitudes and wavelengths coincide with locations of plutonic or volcanic rocks that are inferred to be hydrothermally altered.

Using filter techniques, the gravity and magnetic data were combined to form an interpretative map that shows the distribution of surface and subsurface magnetic plutonic igneous rocks. This derivative map aided in the identification of areas of mineral resource potential in the mineral-resource assessment of several mineral deposit types in the quadrangle and is included in several of the mineral resource assessment maps (Elliott, Wallace, and others, 1992a, 1992b, in press).

LIMONITE AND HYDROTHERMAL ALTERATION MAP (1-2050-A)

The distribution of limonitic rocks in the quadrangle, some of which are hydrothermally altered, was mapped using a Landsat Multispectral Scanner (MSS) image (no. 2553-27331, recorded on July 28, 1976). Limonite, a combination of ferric iron oxide minerals, can be identified on MSS digital images because of its diagnostic spectral reflectance in the wavelength region of 0.4-1.1 micrometers (Rowan and others, 1974; Segal, 1983).

Rowan and Segal (1989) processed the MSS image data, which covered nearly all of the Butte 1°X2° quadrangle, to display the characteristic limonite spectral reflectance and to distinguish limonite from dry vegetation, which has similar spectral reflectance. A color-ratio composite, instead of single-channel composite images, was used to subdue albedo and topographic illumination effects. Areas identified as having anomalously high limonite were transferred from the color-ratio composite image to topographic maps, and these areas were evaluated in the field to distinguish between limonite that resulted from weathering of hydrothermally altered rocks and limonite that resulted from weathering of unaltered iron-bearing rocks. The discrimination of limonitic rocks from Landsat imagery is limited by vegetative cover. Much of the quadrangle is heavily forested and vegetative cover greater than approximately 30 to 40 percent obscures the spectral response of limonitic rocks. Shadows on images caused by rugged topography also hampers mapping of limonitic rocks.

The limonite and hydrothermal alteration map (1-2050--A; Rowan and Segal, 1989) shows areas of limonitic rocks that were evaluated in the field and assigned to three categories: (1) hydrothermally altered, (2) unaltered, and (3) undetermined origin. The presence of hydrothermally altered rocks is a recognition criterion for several types of mineral deposits. The limonite and hydrothermal alteration map was used in the mineral-resource assessment procedure because of the observed association of hydrothermal alteration and mineral deposits. Hydrothermally altered limonitic rocks include zones adjacent to metalliferous veins, replacement bodies, and breccia and areas of pervasive silicic or argillic alteration. Limonite-stained but unaltered rocks were formed by diagenetic processes or secondary weathering of iron-bearing minerals in unaltered rocks. Areas classified as "undetermined origin" include some areas of poor exposure where the origin of limonite could not be specified and some that were not field checked.

The results of this study show that areas of limonitic rocks are commonly small and usually related to faults, breccia zones, contact aureoles of plutons, and areas of hydrothermal veins. Most areas of limonitic rocks are adjacent to known mineralized areas, such as the Butte district, but several hydrothermally altered limonitic areas have few or no mines or prospects. These areas may warrant further evaluation. Areas of limonitic rocks classified as "unaltered" include limonitic Proterozoic clastic and carbonate rocks of the Belt Supergroup, Paleozoic carbonate rocks, Cretaceous and Tertiary volcanic and volcaniclastic rocks, Cretaceous and Tertiary plutonic rocks, and Quaternary glacial deposits.

LINEAR FEATURES ASSOCIATED WITH METAL-BEARING MINES AND PROSPECTS MAP (1-2050-B)

Linear features were mapped from MSS and SLAR images and compared to the distributions of metal-bearing mines and prospects. After analysis and interpretation of the data, a map was prepared at a scale of 1:250,000 that shows linear features or intersections of linear features and their

MINERAL-RESOURCE ASSESSMENT MAPS (1-2050-D, E, F, G, AND H)

of gold, silver, copper, lead, zinc, manganese, and tungsten (Elliott, Wallace and others, 1992a, 1-2050-D), skarn deposits of gold, silver, copper, tungsten, and iron (Elliott, Wallace, and others, 1992; 1-2050-E), porphyry and stockwork deposits of copper, molybdenum, and tungsten (Elliott and others, in press; 1-2050-F), stockwork and disseminated deposits of gold and silver (Elliott and others, in press; 1-2050-F), Tertiary and Quaternary gold placers (Elliott and others, in preparation; 1-2050-G), and miscellaneous deposit types including strata-bound copper-silver and phosphate and vein and replacement barite and fluorspar (Elliott and others, in preparation; 1-2050-H). Each map is accompanied by a pamphlet that explains the methods used in the mineral-resource assessment and descriptions of the mineral deposit models used in the assessment. Most of these models were developed specifically for this study and designed to utilize the types of data available for the Butte 1°X2° quadrangle.

The methods employed for mineral-resource assessment of the Butte quadrangle required the use of descriptive mineral deposit models and the application of these models using the computer-based spatial data processing technology of GIS. The general procedure is:

1. Compilation of geologic, geochemical, geophysical, and other data pertinent to the occurrence of mineral deposits.
2. Determination of the types of mineral deposits known to exist and the types that possibly exist in the quadrangle.
3. For each deposit type, apply available descriptive models or develop models and recognition criteria as required.
4. Evaluate the areal distribution and relative importance of recognition criteria.
5. Assess the mineral-resource potential based on the presence and relative importance of recognition criteria. For the Butte quadrangle, most of the data needed for

resource assessment were acquired through new studies consisting of geologic mapping, geochemical and geophysical surveys, remote sensing and geochronologic studies, and examination of mines and prospects. These data, combined with data from previous published and unpublished sources, were compiled on maps at a scale of 1:250,000 or in tables and entered into a computer-based GIS.

A GIS, consisting of computer hardware and software components, was used to develop procedures for mineral-resource assessment, interpret compiled and processed data, and prepare the map-based products of resource assessment. Nearly all of the original data were either in map or tabular form. Maps, which include a generalized geologic map, mining district map, geophysical anomaly map, limonite map, and linear features map, were digitized and entered into a vector graphics subsystem of the GIS. Geochemical data and mine and prospect data were entered as tabular data in the GIS.

Descriptive mineral deposit models were developed based on the types of mineral deposits that are present or could exist in the Butte quadrangle. Each mineral deposit model consists of a description and a list of recognition criteria. For deposit types that are well represented in the Butte quadrangle, the recognition criteria are based mainly on observed characteristics of deposits in the quadrangle and in adjacent regions of southwestern Montana. For some deposit types that are not present or not well represented in the quadrangle, descriptions and (or) models for deposits in other parts of the Northwestern and Western U.S. were used. The kinds and quantity of data available for the Butte quadrangle limited the number and types of recognition criteria for each mineral deposit model. The recognition criteria are also limited to data that apply to the entire, and not just selected parts, of the quadrangle.

For each mineral-deposit model, GIS submodels were developed that correspond to recognition criteria of the deposit model. Within each GIS submodel, a scoring or weighting range was generated that expresses the degree of favorability for factors such as host rock, associated igneous rock, geochemical anomalies, and other factors. The scores are based mostly on observed or measured association of mines and prospects with certain classes of host rock, igneous rocks, geochemical anomalies, and so on. Each submodel has several levels with scores in the range of 0 to 5. The final mineral-resource assessment maps are derived by combining the GIS submodels into summary GIS models and then assigning levels (low, moderate, high, very high) of mineral-resource potential to the final maps.

The mineral-resource assessment maps show areas of low, moderate, high, and, in some cases, very high potential for undiscovered resources of each mineral deposit type. Many areas of the quadrangle are favorable for the occurrence of undiscovered resources and have moderate, high, or very high potential for the occurrence of one or more deposit types. Many of these areas are in established mining districts that contain many mines and prospects; but some favorable areas contain few or no mines and prospects, and these areas are possible targets for future exploration and for more detailed geologic, geophysical, and geochemical studies.

Mineral resource assessment map for vein and replacement deposits of gold, silver, copper, lead, zinc, manganese and tungsten (1-2050-D )-The mineral-resource assessment map shows four levels of potential-low, moderate, high, and very high-for the occurrence of undiscovered resources in polymetallic vein and replacement deposits in the Butte quadrangle. The areas of moderate (24.1 percent of the quadrangle), high (5.1 percent), and very high (1.0 percent) potential are all favorable for the occurrence of vein and replacement deposits. Approximately 30 percent (sum of moderate, high, and very high areas) of the Butte quadrangle is favorable for the occurrence of these deposits, and about 84 percent of the mines and prospects of this deposit type are within these areas. As expected, most of the areas of very high potential coincide with known mining districts, which have many highly productive vein and replacement deposits. Of particular interest for mineral exploration are areas of high or very high potential which have few or no mines and prospects. Nine such areas are identified and described in the mineral-resource assessment map.

Mineral resource assessment map for skarn deposits of gold, silver, copper, tungsten, and iron (1-2050-E)-The mineral-resource assessment map shows four levels of potential for the occurrence of undiscovered resources in skarn deposits in the Butte quadrangle. The areas of moderate (11.4 percent of the quadrangle), high (2.3 percent), and very high (0.4 percent) potential are all favorable for the occurrence of skarn deposits. Approximately 14 percent (sum of moderate, high, and very high areas) of the Butte quadrangle is favorable for the occurrence of skarn deposits and approximately 67 percent of the mines and prospects of this deposit type are located in the favorable areas. All the areas of very high potential for skarn deposits are in mining districts and are at or near the contacts of stocks with sedimentary rocks. Most of the areas of high potential are also restricted to mining districts, although many of these areas do not have productive skarn deposits. Eight areas of high or very high potential and with skarn mines or prospects are identified and described in the mineral-resource assessment map. Seven other areas of high potential and significant size (approximately 1 mi or larger) are not associated with known skarn occurrences, but these areas are also very favorable for exploration for undiscovered deposits; these areas are also identified and described in the map.

Mineral resource assessment map for porphyry and stockwork deposits of copper, molybdenum, and tungsten and stockwork and disseminated deposits of gold and silver (1-2050-F)-The mineral-resource assessment map includes separate maps for porphyry copper-molybdenum-tungsten and disseminated gold-silver deposits that each show three levels of mineral resource potential (low, moderate, and high). For porphyry copper-molybdenum-tungsten deposits, the areas of moderate potential cover 23.9 percent and the areas of high potential cover 5.1 percent of the quadrangle. Therefore, 29 percent of the area of the quadrangle is favorable for the undiscovered deposits of porphyry coppermolybdenum-tungsten. For disseminated gold-silver deposits, the areas of moderate potential cover 32.7 percent and the areas of high potential cover 2.3 percent of the quadrangle. Therefore, 35 percent of the area of the quadrangle is favorable for undiscovered deposits of disseminated gold-silver.

For porphyry copper-molybdenum-tungsten deposits, the favorable areas (moderate plus high potential) are located within and adjacent to mining districts and are related to granodiorite and monzogranite stocks and batholiths. The largest favorable areas are located in the eastern part of the quadrangle and are associated with the Late Cretaceous Boulder batholith. Other favorable areas are found in the northeastern, central, northwestern, and southwestern parts of the quadrangle.

For disseminated gold-silver deposits, the largest favorable areas are in the eastern and southeastern parts of the quadrangle, mainly in well-known districts and in areas that were extensively hydrothermally altered and mineralized. These areas have plutonic rocks of the Boulder batholith and Late Cretaceous and Eocene volcanic rocks. Smaller, widely scattered, areas of favorable potential are located in the northeastern, northwestern, central, and south-central parts of the quadrangle.

Mineral resource assessment map for Tertiary and Quaternary placer gold deposits (1-2050-G)-The mineral-resource assessment map includes separate maps for Tertiary and Quaternary placer gold deposits. The map for Tertiary placers shows three levels of potential: low, moderate, and high. The map for Quaternary placers shows four levels of potential: low, moderate, high, and very high.

Areas of favorable potential for Tertiary placer gold deposits are located along the margins of large basins with thick sections of Tertiary rocks. Factors that control the location of favorable areas include favorable lithologies in Tertiary rocks, proximity to source areas with gold-bearing lode deposits, and proximity to range-front normal faults. The largest favorable areas are located in the northwestern, north-central, central, and west-central parts of the quadrangle.

For Quaternary placer gold deposits, the mineral-resource map displays many portions of streams and valleys that are favorable (moderate, high, or very high potential). The most favorable areas are along the margins of intermontane basins and wide valleys in mountainous areas. Many placers have been mined in the quadrangle and future discoveries will probably be of buried placers that have little or no surface expression. Many of the placers in mountainous areas are narrow valley-type placers of small size and high grades of gold. Larger volume, lower grade placers could exist in wide valleys and on the margins of intermontane basins. The most promising areas in the quadrangle for Quaternary placer gold deposits are in northwestern, central, and northeastern parts of the quadrangle.

Mineral resource assessment map for miscellaneous deposit types ofphosphate, copper, silver, barium, and fluorine (1-2050-H)-The mineral-resource assessment map for miscellaneous deposit types displays areas of low and moderate potential for strata-bound copper-silver deposits in Belt rocks, areas of high potential for strata-bound phosphate deposits, areas of low, moderate, and high potential for fluorine deposits, and areas of low and moderate potential for barium (as barite) deposits.

Strata-bound copper and silver minerals are present at several localities in Middle Proterozoic Belt Supergroup rocks. Most of these occurrences are in either the Spokane

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