U.S. DEPARTMENT OF THE INTERIOR U.S. GEOLOGICAL SURVEY MINERAL RESOURCE ASSESSMENT MAP FOR VEIN AND REPLACEMENT DEPOSITS OF GOLD, SILVER, COPPER, LEAD, ZINC, MANGANESE, AND TUNGSTEN IN THE BUTTE l 0 X2° QUADRANGLE, MONTANA By J.E. Elliott, C.A. Wallace, G.K. Lee, J.C. Antweiler, D.J. Lidke, L.C. Rowan, W.F. Hanna, C.M. Trautwein, J.L. Dwyer, and S.H. Moll Pamphlet to accompany MISCELLANEOUS INVESTIGATIONS SERIES MAP 1-2050--D Any use of trade, product, or firm names in this publication is for descriptive purposes only and does not imply endorsement by the U.S. Government CONTENTS Introduction 1 Purpose and scope 1 Geographic setting 1 Geologic setting 1 Mineral deposi~ 3 Vein and Replacement Deposits 4 Principal mining districts 4 Butte district 4 Philipsburg district 4 Black Pine district 5 Marysvi11e district 5 Helena district 5 Wickes district 5 Boulder district 6 Methodology of Mineral Resource Assessment 6 Acquisition, Compilation, and Interpretation of Data 7 Mineral occurrence data 7 Geologic data 7 Sedimentary and volcanic rocks 7 Plutonic rocks 9 Geochemical data 10 Sampling methods 10 Analytical methods 10 Data processing and interpretation 10 Geophysical data 11 Remotely sensed data 13 Hydrothermally altered rocks 14 Linear features 14 Computer-Based Data Processing 14 Description of the Geographic Information System (GIS) 14 Procedures used for data entry and processing 15 Maps 15 Tables 16 Gridded data 16 Previously digitized data 16 Data analysis 16 Mineral Resource Assessment 16 Descriptive model for vein and replacement deposits of gold, silver, copper, lead, zinc, tungsten, and manganese 17 Geology 17 Geochemistry 18 Ages of mineralization 18 Geophysics/remote sensing 19 Genesis 19 Recognition criteria 19 GIS submodels 19 Host-rock submodel 20 III Fracture potential 20 Composition 20 Contact metamorphism 20 Distribution of mines and prospects 20 Size of mine production 21 Summary host~rock submodel 21 Plutons submodel 21 Geochemical submodel 22 Precious- and base-metal vein and replacement deposits 22 Vein and replacement manganese deposits 24 Vein and replacement tungsten deposits 24 Summary geochemical submodel 24 Alteration submodel 24 Geophysical submodel 25 Fault submodel 25 Linear feature submodel 26 Composite mineral resource assessment model for vein and replacement deposits of gold, silver, copper, lead, zinc, manganese, and tungsten 26 Summary and Suggestions for Exploration 27 References Cited 28 FIGURES 1. Index map of the Butte 1°X2° quadrangle 2 2. Principal structures and plutons of the Sapphire thrust plate 3 3. Sketch showing age relationships and scoring of plutons 22 TABLES 1. Sample types and analytical methods used in the mineral resource assessment of polymetallic vein and replacement deposits 12 2. Favorable geochemical assemblages in the geochemical submodel for vein and replacement precious- and base-metal deposits 23 3. Favorable geochemical assemblages in the geochemical submodel for vein and replacement manganese deposits 24 4. Favorable geochemical assemblages in the geochemical submodel for vein and replacement tungsten deposits 25 5. Percentage of map area and normalized densities of known mineral occurrences of vein and replacement deposits for areas of very high, high, moderate, and low potential and areas covered by Tertiary sedimentary rocks and deposits 26 IV INTRODUCTION west of the divide, to the Clark Fork, which has its headwaters in the vicinity of Butte. PURPOSE AND SCOPE GEOLOGIC SETTING The purpose of this report is to assess the potential for undiscovered vein and replacement deposits of gold, silver, The Butte quadrangle contains igneous, metamorphic, copper, lead, zinc, manganese, and tungsten in the Butte and sedimentary rocks and deposits that range in age from 1°X2° quadrangle. This quadrangle, in west-central Mon­ Proterozoic to Quaternary. Proterozoic, Paleozoic, and Mes­ tana, is one of the most mineralized and productive regions ozoic sedimentary rocks are widespread, and Cretaceous in the United States. Its mining districts, including the world and Tertiary plutonic rocks and associated metamorphic famous Butte or Summit Valley district, have produced a rocks occur in the cores of most mountain ranges. Volcanic variety of metallic and nonmetallic mineral commodities and volcaniclastic rocks of Cretaceous and Tertiary ages are valued at more than $6.4 billion. Because of its importance found mostly in mountain ranges in the eastern and northern as a mineral producing region, the Butte quadrangle was parts of the quadrangle. Intermontane basins are filled with selected for study by the U.S. Geological Survey under the Tertiary and Quaternary sedimentary rocks and deposits. Conterminous United States Mineral Assessment Program The sedimentary record begins with rocks that were (CUSMAP). Under this program, new data on geology, deposited during Middle Proterozoic time when the Belt geochemistry, geophysics, geochronology, mineral resour­ basin occupied the area of the Butte quadrangle; clastic and ces, and remote sensing were collected and synthesized. The carbonate rocks of the Belt Supergroup have a total thick­ field and laboratory studies were also supported by funding ness of about 16,000 m in the quadrangle. Mafic dikes and from the Geologic Framework and Synthesis Program and sills were intruded into the Belt rocks, probably during Late the Wilderness Program. The methods used in resource Proterozoic time. During Paleozoic time, near-shore and assessment include a compilation of all data into data sets, shallow-water carbonate and carbonate-bearing clastic the development of a descriptive model for vein and rocks were deposited and have a total thickness of about replacement deposits in the quadrangle, and the analysis of 2,400 m; Paleozoic strata are mainly in the north, central, data using techniques provided by the Geographic Informa­ and northeastern parts of the quadrangle. Mesozoic sedi­ tion System (GIS). This map is one of a number of reports mentary rocks were deposited in a foreland basin in which and maps on the Butte 1°X2° quadrangle. Other publications about 6, 700 m of clastic and carbonate strata accumulated in resulting from this study include U.S. Geological Survey the central part of the quadrangle and about 2,400 m of Miscellaneous Investigations Series Maps 1-2050-A equivalent strata in the northeastern part of the quadrangle. (Rowan and Segal, in press) and I-2050-B (Purdy and In Late Cretaceous time, numerous stocks and several Rowan, in press); Miscellaneous Field Studies Map batholiths were emplaced at mesozonal and epizonal depths. MF-1925 (Wallace, 1987); and Open-File Reports 86-292 The Boulder, Idaho, Sapphire, and Philipsburg batholiths, (Wallace and others, 1986) and 86--0632 (Elliott and others, composed of monzogranite and granodiorite, and numerous 1986). Reports on mineral resource assessment for several smaller stocks of diorite, granodiorite, and monzogranite other types of deposits in the Butte quadrangle are in were formed during this time. Hydrothermal activity during preparation. and following the waning stages of magmatism resulted in a variety of mesothermal and epithermal mineral deposits. GEOGRAPIDC SETTING The Elkhorn Mountains Volcanics, of Late Cretaceous age, occur as roof pendants and along the margins of the Boulder The Butte quadrangle is bounded by latitudes 46° and batholith and probably represent early extrusive phases of 47° N. and longitudes 112° and 114° W. (fig. 1). The city of the magma which later formed the Boulder batholith. Butte is in the southeastern part of the quadrangle; Helena, Volcanism and erosion, as well as sedimentation in the state capital of Montana, is on the eastern edge; and intermontane valleys, took place during the Tertiary. Exten­ Missoula is near the northwestern comer of the quadrangle. sive volcanism during early and middle Tertiary time Most of the quadrangle is in Granite, Powell, Lewis and formed the Lowland Creek Volcanics in the southeastern Clark, and Jefferson Counties and smaller parts are in part of the quadrangle and volcanic fields in the Gamet Missoula, Ravalli, Deer Lodge, and Silver Bow Counties. Range and east of Lincoln. Volcanics of equivalent age also The quadrangle includes a number of major and minor occur northeast of Deer Lodge in the northwestern part of mountain ranges separated by intermontane valleys. The the Boulder Mountains. Lacustrine and fluvial deposits continental divide trends nearly south through the eastern accumulated in intermontane valleys during mid- to late part of the quadrangle to a point near Butte, then trends Tertiary time, concurrently, in part, with volcanism which generally west near the southern boundary of the quadran­ contributed volcanic debris to the sediment. During late gle. East of the divide, drainage is to the Missouri River, and Tertiary time, extensive pediments formed, and gravel, 1 114° 112° 47° LEWIS AND CLARK ~ l.QQQ I c;, L ee.f ------1 I I I 1___ 1 POWELL l __ l , \ ) \ \ (/) GRANITE / )> ( "'tJ I _, r-- ~r t... ) \"-. i ' 0 m \ :I: " - z I '-... ... / s:: \ I 0 ....... J"'"' I §j~ c 0 r-/ z z I~ -1 \ G) ~ ,.,.I z ~I (/) ) ~I I RAVALLI ; t-------- ___1 _j I \ ( '_.,- ... , ( \ l r-- \ ' JEFFERSON ) ' BOW J \ '...1' 0 10 20 30 40 MILES 0 10 20 30 40 50 60 KILOMETERS Figure 1. Index map of the Butte 1°X2° quadrangle. some of which contains valuable placer deposits of gold, Disturbed Belt is in the northeastern part where it abuts was deposited on the pediments. faults of the Lewis and Clark line. The Lewis and Clark line Quaternary time was dominated by extensive alpine consists of a broad zone of east-southeasterly to southeast­ glaciation in most of the ranges in the quadrangle; icecaps erly trending faults that extends across the west-central and occupied high areas in the Flint Creek and Anaconda northeastern parts of the quadrangle. Some steeply dipping Ranges and the Boulder Mountains. Four glacial events faults of the Lewis and Oark line may have originated have been identified in the Flint Creek Range and multiple during deposition of the Proterozoic Belt rocks; however, glacial events probably occurred in the other ranges as well.