DEPARTMENT OF THE INTERIOR UNITED STATES GEOLOGICAL SURVEY GEORGE OTIS SMITH, DIRECTOR BULLETIN 527 ORE DEPOSITS OF THE HELENA MINING REGION MONTANA BY ADOLPH KNOPF WASHINGTON GOVERNMENT PRINTING OFFICE 1913 CONTENTS. Page. Outline of report.... '. ..................................................... 9 Geography.............................................'................... 12 Location of the region................................................. 12 Physical features...................................................... 13 Climate.............................................................. 14 Historical sketch of mining................................................. 15 Production................................................................ 7 Field work and acknowledgments............................................ 17 Literature.................................................................. 18 General geology............................................... ............ 19 Character and distribution of the rocks.................................. 19 Proterozoic rocks...................................................... 20 Algonkian system.................................................. 20 Belt series.................................................... 20 Paleozoic rocks....................................................... 2.1 Cambrian and Devonian systems.................................... 21 Carboniferous system. .............................................. 21 Madison limestone............................................ 21 Quadrant quartzite............................................ 22 Mesozoic rocks........................................................ 22 Jurassic and Cretaceous sedimentary rocks.......................... 22 Igneous rocks..................................................... ' 23 Andesites and latites.......................................... 23 Occurrence................................................ 23 General character......................................... 24 Petrography.............................................. 25 Contact-metamorphic alteration of the andesites and latites.. 26 Age of the andesites ....................................... 28 Quartz monzonite.............................................. 29 General character......................................... 29 Petrography.............................................. 29 Contact metamorphism..................................... 31 Age....................................................... 33 Aplite........................................................ 34 Occurrence and character..................;............... 34 Origin of the aplites................................'........ 35 Cenozoic rocks........................................................ 35 Tertiary sedimentary rocks........................................ 35 Igneous rocks..................................................... 36 Dacites....................................................... 36 General character and distribution.......'.................. 36 Petrography................................................ 37 Age....................................................... 39 Rhyolites...................................................... 39 General character and distribution......................... 39 Petrography....'.......................................... 40 Age........................................................ 41 Quaternary system.................................................. 41 3 " 4 CONTENTS. Page The ore bodies.............................. '.............................. 42 General character...................................................... 42 Older ore deposits (late Cretaceous?)................................... 43 Classification...................................................... 43 Magmatic deposits................................................. 44 Contact-metamorphic deposits...................................... 44 Tourmalinic lodes.................................................. 45 Varieties of ores..................'.............................. 45 Tourmalinic silver-lead deposits................................ 46 Description................................................ 46 Chemical processes.. '. ..................................... 48 Tourmalinic silver-copper deposits............................. 50 Tourmalinic gold depoits ..................................... 51 Origin of the older ore deposits...................................... 51 Systematic importance of tourmalinic silver-lead deposits:.......... 53 Younger ore deposits (post-Miocene)..................................... 54 General character................................................. 54 Metasomatic processes.............................................. 55 Age and origin of the younger ore deposits........................... 58 Comparison of the metasomatic processes of the two periods of mineraliza­ tion.. .............................................................. 60 Descriptions of special districts.......'............,......................... 61 Method of treatment................................................... 61 Marysville district..................................................... 61 Location and history............................................... 61 Geology.......................................................... 62 .Ore deposits...................................................... 64 General features............................................... 64 Origin........................................................ 65 Descriptions of mines............................................. 68 Drumlummon mine........................................... 68 Cruse mine.......:........................................... 72 Belmont mine................................................. 73 Bald Butte mine............................................... 73 Penobscot mine................................................ 74 Empire mine.................................................. 75 Piegan-Gloster mine........................................... 75 Strawberry mine. .1........................................... 76 Elliston district....................................................... 76 Location........................................................... 76 Geology.......................................................... 76 Descriptions of mines............................................. 77 Julia mine..................................................... 77 Evening Star mine............................................ 78 Twin. City Mining & Milling Co.'s mine.......................... 78 Ontario mine.................................................. ' 79 Monarch mine................................................. 79 Blue Bell mine................................................. 79 Rimini district................... .-7.................................. 80 Location.......................................................... 80 Geology.......................................................... 80 Ore deposits...................................................... 81 CONTENTS. 5 Descriptions of special districts Continued. Rimini district Continued." , page. Descriptions of mines.............................................. 82 Valley Forge mine.-...................... ^...................... 82 Lee Mountain mine................. ^.......................... 83 John McGrew prospect........................................ 84 Peerless Jennie mine........................................... 84 Porphyry Dike................................................ 84 Helena district.........................................^............. 85 Historical sketch.................................................... 85 Geology.......................................................... 86 Sedimentary rocks............................................. 86 General features. ...................... r. ^. ................ 86 Algonkian system......................................... 87 Distribution and subdivision........................... 87 Spokane shale........................................ 88 Empire shale.............:... .\ ....................... 88 Helena limestone..................................... 88 Marsh shale........................................... 89 Cambrian system......................................... 89 Subdivisions and character............................ 89 Flathead quartzite..................................... 89 Wolsey shale................... r ..................... 90 Meagher limestone......................:.............. 90 Park shale...............:........................,.... 90 Pilgrim limestone................................ 1..... 91 Dry Creek shale...................................... 91 Yogo limestone....................................... 91 Devonian system........................................... 91 Jefferson limestone:................................... 92 Threeforks shale................................:..... 92 . Carboniferous system....................................... '92 Distribution and character.............................. 92 Madison limestone...................................... 92 Quadrant quartzite.................................... 93 Cretaceous system..........7.............................. 93 Tertiary system........................................... 93 Lake beds............................................. 93 Hot spring deposits................................... 94 Auriferous gravels.....................................