Geology and Mineral Deposits of the Miners Gulch Area, Granite County, Montana Gold Placer Deposits and a Molybdenum Anomaly in Miners Gulch Area, Granite County, Montana Colluvial Gold Placers Near Miners Gulch, Granite County, Montana Geology and Mineral Deposits of the Miners Gulch Area, Granite County, Montana A. Gold Placer Deposits and a Molybdenum Anomaly in the Miners Gulch Area, Granite County, Montana By JEFFREY S. LOEN, MICHAEL J. BLASKOWSKI, and JAMES E. ELLIOTI B. Colluvial Gold Placers Near Miners Gulch, Granite County, Montana By JEFFREY S. LOEN This volume is published as chapters A and B. These chapters are not available separately. U.S. GEOLOGICAL SURVEY BULLETIN 1791 DEPARTMENT OF THE INTERIOR DONALD PAUL HODEL, Secretary U.S. GEOLOGICAL SURVEY Dallas L. Peck, Director Any use of trade names in this report is for descriptive purposes only and does not imply endorsement by the U.S. Geological Survey. UNITED STATES GOVERNMENT PRINTING OFFICE: 1989 For sale by the Books and Open-File Reports Section U.S. Geological Survey Federal Center, Box 25425 Denver, CO 80225 Library of Congress Cataloging-in-Publication Data Geology and mineral deposits of the Miners Gulch Area, Granite County, Montana. (U.S. Geological Survey bulletin; 1791) Bibliography: p. Includes index. Contents: Gold placer deposits and a molybdenum anomaly in the Miners Gulch Area, Granite County, Montana I by jeffrey S. Loen, Michael j. Blaskowski, and james E. Elliott - Colluvial gold placers near Miners Gulch, Granite County, Montana I by Jeffrey S. Loen. Supt. of Docs. no.: I 19.3:1791 1. Geology-Montana-Granite County. 2. Mines and mineral resources-Montana-Granite County. I. Blaskowski, Michael j. II. Elliott, james E. Ill. Title. IV. Title: Miners Gulch Area, Granite County, Montana. V. Series. QE75.B9 no. 1791 557.3 s 87-600290 [QE134.G7 [557.86' 88] Chapter A Gold Placer Deposits and a Molybdenum Anomaly in the Miners Gulch Area, Granite County, Montana By JEFFREY 5. LOEN, MICHAEL J. BLASKOWSKI, and JAMES E. ELLIOTT U.S. GEOLOGICAL SURVEY BULLETIN 1791 GEOLOGY AND MINERAL DEPOSITS OF THE MINERS GULCH AREA, GRANITE COUNTY, MONTANA CONTENTS Abstract At Introduction At Location and accessibility A2 Topography A2 Climate and vegetation A2 Previous investigations A2 Present investigation A2 Acknowledgments A2 Geologic setting A2 Structure of the Upper Willow Creek valley A3 Geomorphology AS Landscape development AS Piedmont alluvial plains AS Flood plain A6 Terrace remnants A6 Hills A6 Regional drainage development A6 Local drainage development A 7 Lithology A9 Middle Proterozoic sedimentary rocks A9 Snowslip Formation A9 Mount Shields Formation A9 Bonner Quartzite A9 Cretaceous-Tertiary igneous rocks A9 Miners Gulch stock and related dikes A9 Granodiorite of Miners Gulch A9 Cretaceous or Tertiary porphyritic granodiorite AlO Tertiary dikes and flows AlO Rhyolite AlO Miscellaneous dikes AlO Basalt All Cenozoic sedimentary deposits All Tertiary deposits All Older colluvium All Pliocene(?) or lower Quaternary deposits All Upper terrace deposits All Older fanglomerate A13 Quaternary deposits A13 Lower terrace deposits A13 Younger fanglomerate A13 Alluvium A13 Younger colluvium AlS Geologic history AlS Economic geology A17 History of mining A17 Production A18 Gold placer deposits A18 Types of placer deposits Al8 Alluvial placers A18 IV Contents Economic geology-Continued Colluvial placers A18 Terrace deposits A19 Descriptions of mined stream drainages A19 Alder Gulch A20 Niles Gulch A20 Homestake Gulch A20 Unnamed hillside placer A20 Deadwood Gulch A20 Miners Gulch A21 Cowan Gulch A21 Baboon Gulch A21 Coles Gulch A21 Sawpit Gulch A21 Scotchman Gulch A21 Vein deposits and altered areas A22 Stages of veining A22 Alteration A22 Geochemistry A22 Sampling and laboratory procedure A22 Correlation analysis A24 Discussion of geochemical maps A29 Analysis of gold samples A33 Character of gold A33 Electron microprobe study A34 Discussion of characteristics of gold grains A35 Molybdenum anomaly A41 Geophysical studies A42 Discussion of molybdenum anomaly A43 Comparison of molybdenum anomaly to known porphyry molybdenum deposits A43 Origin of gold placer deposits A47 References cited ASO PLATE [Plate in pocket] 1. Geologic map and sample localities of part of the Alder Gulch quadrangle, Granite County, Montana FIGURES 1. Map showing location of study area and local geologic and geographic features A3 2. Generalized geologic map of part of the John Long and Sapphire Moun­ tains, Montana A4 3. Benches east of Upper Willow Creek A6 4. North-dipping remnant of Pliocene(?) or early Pleistocene upper terrace deposits, between mouths of Homestake and Niles Gulches A 7 5. Longitudinal profiles of streams in study area A8 6. Schematic representation of stratigraphic relationships of Cenozoic sedimen­ tary deposits in Miners Gulch area A12 7. Stratigraphic column of upper terrace deposits south of Sawpit Gulch A14 8. Stratigraphic column of Quaternary alluvium in Sawpit Gulch A14 Contents V 9. Placer workings north of mouth of Alder Gulch A20 10. Map of vein attitudes in study area A23 11. Geochemical distribution of antimony and arsenic from 133 rock samples A36 12. Geochemical distribution of gold from 133 rock samples A37 13. Geochemical distribution of barium from 133 rock samples A38 14. Geochemical distribution of molybdenum from 133 rock samples A39 15. Geochemical distribution of tungsten from 133 rock samples A40 16. Gold grain with attached quartz crystal from alluvium in gulch south of Sawpit Gulch A41 17. Rounded gold nugget from alluvium in Homestake Gulch A41 18. Wire gold and angular nuggets from placer mine in Upper Sawpit Gulch A41 19. Wire gold from barite vein south of Sawpit Gulch A41 20. Histogram of gold fineness data, study area A42 21. Graphs showing variations in gold fineness from rims to cores of grains A43 22. Gravity map, John Long and Sapphire Mountains, southwest Montana A45 23. Aeromagnetic map of study area A46 24. East-west geologic cross section through study area in vicinity of Miners Gulch A47 25. Distribution of average fineness of gold grains with fineness >900 in stream­ sediment samples A49 TABLES 1. Lithologies and estimated frequency of 4-6 in. cobbles in upper terrace deposits A13 2. Lithologies in alluvium in study area based on visual examination of clast lithologies and their characteristics seen in mine tailings of placer-mined stream channels A16 3. Production data for Miners Gulch area and neighboring placer districts A19 4. Semiquantitative spectrographic analyses for selected elements and atomic absorption analyses for gold in grab samples of altered rock or vein A25 5. Atomic absorption analyses for selected elements in grab samples of altered rock or vein A29 6. Semiquantitative spectrographic analyses for selected elements and atomic absorption analyses for gold in panned concentrates of stream-sediment samples A30 7. Semiquantitative spectrographic analyses for selected elements in panned concentrates of stream-sediment samples, fraction magnetic at 0.6 amperes A31 8. Semiquantitative spectrographic analyses for selected elements in panned concentrates of stream-sediment samples, hand-magnet fraction A32 9. Semiquantitative spectrographic analyses for selected elements in panned concentrates of stream-sediment samples, fraction magnetic at 2.0 amperes A32 10. Semiquantitative spectrographic analyses for selected elements in panned concentrates of stream-sediment samples, nonmagnetic fraction A33 11. Semiquantitative spectrographic analyses for selected elements in panned concentrates of vein samples A33 12. Semiquantitative spectrographic analyses for selected elements and atomic absorption analyses for gold and fluorine in soil samples A34 13. Positive linear correlation coefficients for pairs of elements from geochemical analyses of rock samples A35 14. Linear correlation coefficients between gold and 24 selected elements A35 15. Fineness ranges for sample types analyzed by electron microprobe A44 VI Contents Gold Placer Deposits and a Molybdenum Anomaly in the Miners Gulch Area, Granite County, Montana By jeffrey S. Loen, Michael j. Blaskowski, and James E. Elliott Abstract of veining is the indicated source of low-purity gold (659-900 fine) observed in placers, whereas high-purity gold (>900 fine) Mineral occurrences in the Miners Gulch area include in placers appears to have been transported into the area from placer gold, barite veins, base and precious metal veins, and external sources. a molybdenum anomaly. Placer miners, working mainly be­ A molybdenum anomaly showing many similarities to tween 1867 and 1890, mined gold-bearing deposits along 17 known fluorine-deficient, stock-type, calcalkaline porphyry linear mi of stream channel. The amount of gold recovered is molybdenum deposits exists in the Cretaceous Miners Gulch estimated from the extent of workings mapped in this study stock. Concentric zones of anomalous metal values from at about 2,200 ounces, then worth approximately $46,000 at geochemical samples, and aeromagnetic and gravity lows are $20.67 per fine ounce. At least one lode mine apparently pro­ centered on a pervasively altered porphyritic granodiorite plug. duced a small amount of gold during the 1890's. Barite has a low resource potential, and the molybdenum anomaly has not yet been sufficiently investigated to confirm the presence INTRODUCTION of a porphyry molybdenum system. Results of geologic and geomorphological mapping, geo­ This report appraises the mineral resources of the