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Corundum Deposits of Montana

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Corundum Deposits of Montana Corundum Deposits of Montana By STEPHEN E. CLABAUGH ^ GEOLOGICAL SURVEY BULLETIN 983 * .^ A description of one of the three commercial deposits found in the United States \ <r x fr K ORTON HALL LIBRARY UNITED STATES GOVERNMENT PRINTING OFFICE, ^A^HINGTON : 1952 ">'-«' UNITED STATES DEPARTMENT OF THE INTERIOR Oscar L. Chapman, Secretary GEOLOGICAL SURVEY W. E. Wrather, Director For sale by the Superintendent of Documents, U. S. Government Printing Office Washington 25, D. C. CONTENTS -t Page Abstract _ . ______________________________________________________ 1 Introduction _--_-----______-_-____-----_-_____--__--__--__-_____-_ 1 Location and types of deposits. _________________________________ 1 Economic importance _________-_____-_________________.:________ 2 Recent investigations. _________________________________________ 4 Purpose and methods of study. _________________________________ 5 ^ Acknowledgments _--_---__..-_------___________________________ 5 Corundum-bearing dikes and associated placer deposits___.______. ______ 6 ^ Yogo sapphire deposit. _________________________________________ 6 Introduction ______________________________________________ 6 ^ Geology----__------------------_-_--_-__._. _-----_--____- 7 Geologic setting. ______________________________________ 7 Sapphire-bearing dike_---_-----___-_----_----_------__- 11 . Sapphires. ________________________________________________ 17 Distribution __________________________________________ 17 *- ' Abundance- ------_-__------_----_---__-------_------- 17 Size and coating__-_._.. ________________________________ 18 * ' Crystallography__--__---_-_-___ _______________________ 18 Fracture and specific gravity_._____ _____________________ 21 Color. _.__---__-.:___--_---____._.___________________ 21 Value... --.._._..--..._-...--...-.-.-_-..-_..._- .. 22 Economic development- ____________________________________ 23 History. _____________________________________________ 23 Production methods ___________________________________ 27 « Production and reserveS--_-_________-.-p-_ __-______--__ 31 Missouri River sapphire deposits .-___________-____-________-_-._ 34 > Location __________________________________________________ 34 Geology. _________________________________________________ 35 Sapphires. _ ._______._.._.__-.._.___.__.__.._._.__.___-.___ 39 History and production ____________________________________ 40 * Roek Creek sapphire deposit. _ __________________________________ 45 Location __-._--_-_-_.____-__-__-____-_____-_--._______-__ 45 Geology. _________________________________________________ 45 y Sapphires. _ _______________________________________________ 48 History and production ____.__. __ _________.___.._..___-._-- 50 Dry Cotton wood Creek sapphire deposit---... ____________________ 52 Location __.._.__......_.__-__.________________.__________ 52 x. Geology. ____________^____________________________________ 52 Sapphires. _ _______________________________________________ 53 -**" History and production ____________________________________ 53 Other sapphire occurrences. _-_--_--__--____--____-_-____-__-___ 54 Quartz Gulch _____________________________________________ 54 Pole Creek_______:_______________-__________-_____-____-_. 54 Browns Gulch____. _______-_--_______________-_-___-_____-- 55 y, Chouteau County, _________________ _____________-________ 55 Origin of the corundum in dikes. _ _______________________________ 56 ni Gr 270$ IV ILLUSTRATIONS Page Corundum deposits in metamorphic rocks _ _._-__.._____.__.__.._____._ 58 Introduction,________________________________________ __________ 58 History and production_________________________________________ 59 Geology- ------.----_-------_______________.______..___________ 60 Regional geology._________________________________________ 60 Metamorphic rocks________________________________________ 61 General description____________________________________ 61 Major types._________________________________________ 61 Structure..-----_---.---------_--_-_-_________________ 64 Igneous rocks^____------____-_____________-__,____________ 65 Pegmatites.- - _________________________________________ 65 Younger igneous rocks. _-_---_-_-____-___-___:.-_____-_- 66 Metalliferous deposits-._____:________________________ ___-__ 67 Description of the corundum deposits.__-___--___'_--__________-__ 67 Elk Creek deposit. -____________-__.____.._...._________.__.. 67 Bozeman deposit.__________________________________________ 73 Bear Trap deposit..-'-_-_-___-_____-_____._______._______... 74 Other occurrences_________________________________________ 75 Economic importance of the deposits..___________________________ 76 Origin of the corundum deposits.____-_'_.___--_______.-_____-_____ 77 A genetic classification of corundum deposits..- _.-__-_______-- 77 Examples of corundum deposits in other regions.-___._.____.__ 78 Origin of the deposits in southwestern Montana.__-.-_---_-_... 82 Hypothesis of igneous origin. ___.._______-__-__-_-_____._ 82 Hypothesis of hydrothermal origin.________. ___._____ .___ 88 Hypothesis of origin by metamorphism and metasomatism of alumina-rich sedimentary rocks. ____________________ 89 References cited._______-__-_-___-_-____--_______-__--------------_ 96 Index.... .._._-..__._.._._____.....___._._.._.._-_- _._......-_..-- 99 ILLUSTRATIONS Page PLATE 1. Map of the Yogo sapphire deposit, Judith Basin County, Mont._-----.-.-------.-_--------------------_--.-- In pocket 2. Geologic map of the corundum deposits of Gallatin and Madison Counties, Montana..--------.---.-----_..__. In pocket 3. Outcrop map and longitudinal projection of the Elk Creek corundum deposit._________________________________ In pocket 4. Mine workings and sections of the Elk Creek corundum deposit....-_----_--___----..--_--------------_--._- In pocket 5. Outcrop map of the Bozeman corundum deposit-___._._._ In pocket 6. Outcrop map of the Bear Trap corundum deposit-__-___^_ In pocket FIGURE 1. Index map showing location of Montana corundum deposits--.. 3 2. Property map of the New Mine Sapphire Syndicate---------- 8 3. View eastward along the old open-cuts of the British sapphire mine _________________________________________________ 9 4. View westward into the mouth of Kelly Gulch.__.___-_-_-___ 10 5. East wall of Yogo Gulch._.__._.._________________________ 11 6. Yogo sapphire crystals. _-_--_-___-___-__^..-_-_-_-_-_--._ 20 ILLUSTRATIONS V Pnge FIGURE 7. Longitudinal section showing approximate extent of under­ ground workings in the Yogo sapphire deposit.-___-__-__-_ 29 8. Sapphire crystals from Missouri River and Rock Creek deposits. 41 9. Map showing location of the Rock Creek sapphire deposit and placer claims of the American Gem Mining Syndicate. _____ 4.6 10. Coarsely crystalline corundum-bearing gneiss from the Elk Creek corundum deposit..------------------------------ 69 11. Corundum-bearing gneiss showing an abundance of coarsely crystalline aggregates of corundum and feldspar. __________ 70 12. Photomicrograph showing platy, skeletal crystals of corundum. 87 13. Photomicrograph of corundum crystals, transecting subradial sillimanite. ___---_-._----..-____---____--_--_-_-_-._... .. 87 CORUNDUM DEPOSITS OF MONTANA By STEPHEN E. CLABAUGH ABSTRACT Between $3,000,000 and $5,000,000 worth of corundum has been produced in Montana. The corundum deposits include occurrences of sapphires in dikes, placer deposits of gem and industrial sapphires, and deposits of abrasive corun­ dum in metamorphic rocks. Corundum-bearing dikes are associated with placer sapphires at French Bar on the Missouri River and at the Yogo deposit, and similar igneous rocks are assumed to have supplied the sapphires found in other placer deposits of the region. The sapphires in the Yogo dike are interpreted as a product of direct crystallization from the magma, possibly as a result of reaction between the magma and kyanite-bearing inclusions. Three deposits of abrasive corundum in the vicinity of Bozeman, Mont., have been explored and partly developed by prospectors and mining companies, and by the United States Bureau of Mines. The largest of the deposits has yielded only a few hundred tons of corundum-bearing rock, and the known reserves of the three deposits amount to no more than a few thousand tons of rock contain­ ing 10 percent or less corundum. The corundum-bearing rock occurs as lenticu­ lar to tabular bodies in metamorphosed pre-Cambrian sedimentary rocks. Silli- manite, feldspars, muscovite, biotite, and rutile are the chief minerals asso­ ciated with the corundum. A thin border zone of vermiculite-rich gneiss com­ monly separates the corundum-bearing rocks from the enclosing hornblende gneiss. The texture of the rock is varied, ranging from fine-grained and schis­ tose to very coarsely crystalline, resembling pegmatite. The origin of the abrasive corundum is not so easily deciphered as that of the gem and industrial sapphires which occur in unmistakable igneous dikes. The following three modes of origin are considered for the abrasive corundum deposits: 1) as sills of corundum-bearing syenite or pegmatite, 2) as hydrothermal veins, and 3) as metamorphosed alumina-rich sedimentary rocks. Metamorphic origin involving some metasomatism is favored as the hypothesis that best accounts for the form, distribution, and structural relations of the deposits, their variation in compo­ sition and texture, and the mineral relations. INTRODUCTION LOCATION AND TYPES OF DEPOSITS Nearly all commercial production of corundum in the United States has come from three regions.
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