MONTANA BUREAU OF MINES AND GEOLOGY Open File MBMG 627, Plate 1 of 1 A Department of Montana Tech of The University of Montana Geology of the Butte mining district, MT, 2013 GEOLOGY OF THE BUTTE MINING DISTRICT, MONTANA 376 377 378 379 380 381 382 383 384 385000mE 386 387 388 47 T i r B T l a t 4 T l l T l w 6 CORRELATION OF MAP AND CROSS SECTION UNITS LATE CRETACEOUS PLUTONIC ROCKS 112°35' W 112°32' 30" ° 5 T l w T l t 112°27'30" W 5

' Boulder Batholith T l l T l w 36 T l t 112°30' T l l b T i r T l l b N N Qal Qls

' T l l b Recent T l w 5 T l w 31 ° 5 36 6600 QUATERNARY Tmqp Modoc quartz porphyry plug, irregular intrusions and breccia - Light gray to grayish-green, 6 Q a l Recent to 6200 T l a t 5104 QTac 4 Q T a c 41 h h 7000 porphyritic rock contains phenocrysts of plagioclase, rounded embayed quartz, sparse ch lch Miocene l u 7000 u 5800 6200 large (6 to 25 mm) K-feldspar, and biotite, in a very fine-grained groundmass of quartz G G T l w ia 5104 b K b g T l t Tir and K-feldspar (64 Ma; Dilles et al., 2004). The Modoc quartz porphyry forms an irregular m p lu 43 K b g Q a l shaped plug that reaches a maximum width of ~300 m on the 2800 foot level of the ee o h C 35 Y S 6000 50 54 T l t a 7200 Trpu Leonard mine (Meyer et al., 1968). Contacts are sinuous with moderate fragmentation of T l w 34 n 57 Hail T l w ke the granite (Meyer et al., 1968). Breccia fragments contain older quartz-molybdenite 41 31 e Q a l T l w e Trpl Trpb Trpb Trpb veinlets characteristic of the deep porphyry Cu-Mo zone. 43 T l w 1 2 3 5600 T i r T l t Kqp Quartz porphyry dikes - Light colored, porphyritic rock (3 to 20 m wide) contains ca. 50 vol. % T l l 54 24 D 58 T l w 5800 25 o aplitic groundmass and 50 vol. % phenocrysts of euhedral plagioclase, rounded quartz 60 o Tll 80 20 19 d Eocene l e eyes, orthoclase and biotite phenocrysts enclosed in a fine-grained groundmass of 5103 Tllb 6800 C quartz and orthoclase (66 Ma; Lund et al., 2002). T l t re 6800 TERTIARY T 39 40 e Ka Aplite, granoaplite, and pegmatite dikes and irregular intrusions - Light colored to pink T l t L T l t T l w k Tlw U dikes consisting of orthoclase, quartz, oligoclase-andesine and trace amounts of biotite, 5103 A F 25 contain local occurrences of pyrite, chalcopyrite, magnetite, albitic plagioclase and very T l l Tlat 47 K 17 k Q T a c rare pyrrhotite in quartz in the core zones (Meyer et al., 1968; Smedes et al., 1962). 32 T l l b E T l t e 62 E re Black tourmaline and molybdenite also occur in miarolitic cavities and are most abundant R 76 C Tlt C T l w T i r T l w in the deepest plutonic exposures along East Ridge. Granoaplite dikes (described as a 51 T l w w Q T a c 55 6600 o granite-aplite hybrid rock; Roberts, 1975) contain phenocrysts of plagioclase, biotite, T l l b T l l N 32 48 k B Trdi Steep T l l b 31 U e K a 48 33 e r T l l T i r 80 R 39 r hornblende and rounded quartz eyes entrained in fine-grained aplitic quartz and alkali K a e Main Stage Mineralization Paleocene 33 Mountain C v 46 41 50 l T l t T l t i feldspar groundmass (76 Ma; Lund et al., 2002). Most dikes are gently-dipping and 87 30 L 6600 Tmqp Q a l L n S 5800 u 7000 make up parallel sheeted sets. Aplite dikes are cm- to m-thick and typically have sharp 20 77 76 U K a 5102 Pre-Main Stage Mineralization Q a l B 19 R contacts with the granite. Aplite dikes locally exceed 20 m thickness and commonly zone T l w T l t pper Bull Kqp 87 R U Q a l 88 K b g inward to pegmatitic cores (Meyer et al., 1968). Most granoaplite dikes occur as massive 85 E 29 27 T l t Q a l P 5102 26 B’ 7400 gently dipping planar sheets that exceed 20 m in thickness. Contacts are gradational 37 85 75 P Ka 75 U Late 67 42 T l t K m e K a 6400 with the granite. Sheeted aplite dikes are abundant in the western and eastern parts of 54 6200 CRETACEOUS 17 59 58 Cretaceous the Butte district and intrude parallel to subhorizontal joint sets observed throughout the T i r T i r T l w 57 Kme Q a l 36 T l t Butte district. Q a l 20 50 51 Kbg Kme Mafic enclaves - Dark-green to black ellipsoidal nodules (15 to 50 cm dia.) containing crystals of T l l T l l b 33 34 6200 K a 56 T l l b 34 ? K a 86 plagioclase, hornblende, biotite, quartz, magnetite and minor apatite. South and adjacent 35 44 T i r Q a l 33 to Bull Run Creek, abundant nodules occur in a linear collection (~15 by 400 meters). At 62 60 15 DESCRIPTION OF MAP UNITS 53 6400 6000 24 34 7789' this location nodules are surrounded by fine-grained granite (Kbg). Elsewhere, isolated T l l b 28 31 T l a t 62 5101 T i r 22 nodules occur throughout the district. Enclaves cut by aplite dikes. Q a l 53 QUATERNARY and TERTIARY DEPOSITS 17 74 Kbg Butte Granite - Gray, medium- to coarse-grained equigranular rock consists of plagioclase B 22 25 6200 ull (35-40 vol. %), orthoclase (20-25 vol. %), quartz (15-25 vol. %), and about 20 vol. % 5101 R 66 36 un 38 k Qal Recent Alluvium deposits - Clay, silt, sand, and gravel fluvial detritus from eroded local C T i r r biotite, hornblende, magnetite, titanite, ilmenite, and apatite (76.5 Ma; Meyer et al., reek Q a l 6600 a T l w 79 P granitic and volcanic rocks. Includes colluvium, lacustrine and terrace deposits. 18 85 1968; Martin et al., 1999; Martin and Dilles, 2000, Lund et al., 2002). The granite 5 20 lk k Landslide deposits - Poorly sorted material derived from bedrock and surficial deposits Q a l E e Qls has distinctive euhedral poikilitic K-feldspar megacrysts up to 3 cm in length (Smedes et 27 T l l b 33 31 re T l l b 5800 28 C primarily occur as slump blocks and incipient rotational slumps. n al., 1962; Meyer et al., 1968). Occasional mafic enclaves and aplite dikes are present 38 o T i r ? iso Older Alluvium and colluvium deposits - Poorly sorted, unconsolidated to moderately 51 81 Trpb B QTac throughout the granite (Houston, 2001). 22 69 3 T l w Q a l T l l b Q l s 87 78 85 indurated, well-rounded, arkosic silt, sand, pebble and cobble detritus locally T l l T l l 40 49 35 36 Q T a c derived from granitic and volcanic rocks. Basin fill sediments near the town of 53 T i r 77 85 43 41 47 69 58 15 T l w T i r 12 47 87 88 Rocker are approximately 180 to 300 m thick (Feiveson, 1997; Hammar-Klose, Contact - showing dip; dashed where approximately located; and 58 76 60 79 71 5100 35 38 10 6200 1997). Recent to Miocene sedimentary rocks contain locally air-fall rhyolite tuffs. 66 53 76 45 6200 K a K b g 6400 58 dotted where concealed 50 Rare fossil camel, rhinoceros, horse, and oreodont teeth and bones are present in 78 ? D 65 36 Q a l 7200 deposits south of Silver Bow Creek (Smedes et al., 1973). Locally, includes talus 5100 60 T i r 6800 Fault - showing dip; dashed where approximately located; 65 4 36 ? T l w 6 deposits comprised of unsorted cobble to boulder angular rock fragments and debris 6600

" 7400 U dotted where concealed; Names shown on principal faults

? ° 2 2 0 T r p u 55 K a flow deposits adjacent to the Rocker, Continental, Klepper, and East Ridge faults, ' 3 82

21 3 ' Q a l 64 43 35 K a 85 0 2 85 81 and Recent valley fill and alluvial terrace deposits (Pleistocene and Pliocene age; D ° 2

43 Trpb " 53 3 Vein or mineralized fault - showing dip; dotted where concealed; 6 T i r

4 Elliott and McDonald, 2009). 38 U Names shown on principal veins 45 26 24 K a K a RAMPART 24 Q T a c MOUNTAIN 42 Yankee Doodle Tailings Pond TERTIARY VOLCANIC ROCKS 25 Strike and dip of beds - shown in all bedded units including 19 N 7000 ida V 33 29 Flor sedimentary rocks and sedimentary interbeds within lava flows T r p u 5099 56 72 T r d i Lowland Creek Volcanics Formation - Modified from Smedes, (1962) S i l v e r 5800 63 L u c k 6200 50 K a K a V N 77 T ANN 15 Trpb MARGE Strike and dip of compaction foliation structure 3 N N 74 Goldflint V K V Big Butte Vent Complex ROC 5099 Trpb 81 CK SH. 3 6000 BLA ? Tir Rhyolite dikes - Light gray to red-brown, porphyritic rhyolite dikes and shallow irregular 10 Q T a c Elmorlu SH. Black Rock Strike and dip of rheomorphic foliation T r p u K a Alice Pit T i r SH. intrusive bodies contain plagioclase, quartz, and biotite phenocrysts surrounded in an ? W VN B INBO E aphanitic groundmass. Dikes south of Big Butte contain lineated flow foliation and T i r VN RA R Smith VN L Strike and dip of joint old STATE I K b g G N chilled margins parallel to intrusive contacts. North of Bull Run Creek a large rhyolite K a Moulton V 75 SH. AURA P N ? RIA VN N IL BUDD VN intrudes dacite lavas and may be the base of an extrusive dome. T r p l T i r C . E OT MAJOR Placer HI MIL S Bidirectional paleoflow indicator G P EF Y HA o ulch l S JO Badger SH. VN FT Q T a c Trpu Upper rhyolite pyroclastics - Light gray to reddish, moderately to strongly welded rhyolite in h r ace STAR VN H SE V F ce e Y PH N Six O'clock Pla r LT R VN Butte & London Q a l AU M SH. pyroclastic rocks (ignimbrite) contain 25 to 35 volume percent crystals of shattered 50 A F E Granite Mountain R 57 V SH. Anaconda/ARCO Deep Drill Hole Locality OR N JE 81 DDH-8 38 K a K a C 85 G SH. ES 85 crystals of quartz, oligoclase-andesine, biotite, and minor sanidine in a vitroclastic 43 RAY S 5098 ro 88 40 M RO E IE O T r p u IDN CK Corra SH. D V 74 78 IG VN IT N matrix. Abundant fiammé have an aspect ratio of approximately 6H:1V and are T r p l 72 75 Lexington HT H T r d i 71 V M Tuolumne 80 Productive Mine Shaft 80 T N A 54 38 SH. FAUL 88 Y circular in plan view, suggesting simple compaction. The upper rhyolite pyroclastic 65 65 ATA V SH. 80 72 LA PL N 5098 T i r 80 T r p b rocks are petrographically similar to the lower rhyolite pyroclastic rocks (Trpl), but are 57 57 73 56 75 1 K a 50 Gray Rock SH. Open-pit outline: Berkeley Pit and Continental Pit 87 79 H i s t o r i c a l ULT locally overlying Trpl along a lithic-rich basal vitrophyre, up to 5 m thick. In the north 78 LL FA 72 64 88 l a n d f i l l 6000 LT BE K b g T i r 50 Trpb 80 T r p l 82 K FAU 78 3 55 T r p l ROC west part of the district, the occurrence of several vitrophyres, up to 5 m thick, 66 RAY 64 Tmqp Waste rock Tailings pond Historic landfill G N 86 ATE V M suggests multiple cooling units. NDIC T. CON V DIAMOND FAULT 83 SY N H A VN K b g K a 75 IG US Trpb 70 61 50 56 H High Ore COL 1 Pyroclastic breccia - Includes three subunits consisting of angular blocks (<1 m to 30 m in 79 67 Mountain O 58 T r p b 1 R East Colusa 72 88 53 Con SH. 77 E SH. Main Range 84 diameter) of granite, aplite, quartz latite ignimbrite, dacite lava, and welded rhyolite 58 43 72 64 V Homestake K q p 79 65 33 T r p b BA K q p N West Colusa SH. Trpb References 3 N D BERKELEY PIT SH. K q p 2 tuff surrounded in a poorly welded matrix pyroclastic rhyolite. The matrix contains Trpb 62 K E SH. 3 5800 18 ER R SH. Leonard 1. Dilles, J., 1998, 2002, 2012, 1:24,000, this study. Q T a c T i r 54 V B Kelley N Outline 1995 71 58 85 N L IE broken crystal fragments of quartz, plagioclase, sanidine, and biotite. Pyroclastic K a Big U R SH. 82 84 E SH. V Trpb 2. Dilles, J., Stein, H., and Martin, M., and Rusk B., 2004, Re-Os and U-Pb ages for the Butte Steward V N 86 5097 3 N Tramway SH. CONTINENTAL PIT breccia (<200 m wide) locally occurs at the contact of the lower rhyolite pyroclastic VN 73 66 Butte 76 Aspen SH. T Copper District, Montana: IAVCEI Meeting, Púcon, Chile. Q a l 73 T r p l 85 Trpb1 West Gagnon Original Anaconda SH. GA Outline 1995 L K q p (Trpl) and granite (Kbg). Three subunits are subdivided based on dominant clast Great 58 R VN ME 43 ONE VN SH. K q p 45 BE U 3. Dresser, H., 2000, Stereo Picture Guidebook to Big Butte Volcanics: Tobacco Root Geological Republic SH. 66 BO O Anselmo SH. P TT 55 M ARROT V A A lithology. Trpb , dominated by granite and aplite clasts. Trpb , dominated by quartz 80 78 76 EL RIGINAL VN NANAC VN DDH-3 1 2 5097 T i r S SH. O OND F 75 AN T i r K q p A VN Rarus SH. A’ Society, Annual Meeting, Butte, MT, August 3-6, 73 p. B latite ignimbrite blocks that locally contain distinctive eutaxitic foliation. Trpb , S VN EL K q p EAST FAULT 78 70 N Tropic 3 4. Duaime, T., Kennelly, P., and Thale, P., 2004, Butte, Montana: Richest Hill on Earth, 100 years ? T i r Trpb1 BRINU LON 86 V A 47 85 87 GAM N A V DDH-7 . 8 K q p . V T N O SH. R dominated by dacite lava and breccia clasts that contain planar flow aligned SO VN UL N of underground mining, Montana Bureau of Mines and Geology: Miscellaneous N E 75 ON V IA FA 45 Berkeley N Sarsfield ce G R B LE O O n A O M P epende Trpb T i r G SI U DD R . 6 84 plagioclase phenocrysts surrounded in an aphanitic groundmass. The pyroclastic Contribution 19, 1 sheet, 1:9,000. Ind 2 58 L OL MI K q p I SH. 30 VN SH.

? VN 23 GLO CE C G . P T N O T son VN A VN 70 EX T IN Pennsylvania N LT 40 39

11 Car N U E breccia may represent the collapse of previously erupted surface rocks into the vent.

L O L A L Q a l Kit SAX U L FA W 5. Dudás, F., Ispolatov, V., Harlan, S., and Snee, L., 2010, Ar/ Ar Geochronology and

Trpb FA N SH. K q p IN L

U S DDH-6 U K a 85 2 Y O IL DL

58 46 T i r O . VE A N K Trpl Lower rhyolite pyroclastics - Light gray to reddish-brown, moderately to strongly welded A Nettie R S V A Geochemical Reconnaissance of the Eocene Lowland Creek Volcanic Field, west-central T R 6 B S 0 K q p

40 L 41 B B O V 5 F

W F 67 O Colorado A Belmont N . DDH-2 85 SH. U R L NO O rhyolite pyroclastic rocks (ignimbrite) contain 25 to 35 volume percent crystals of A 45 . N Montana: The Journal of Geology, v. 118, No. 3, p. 295-304. 44 O. BOY L SH. T SH. T 1 V F N GIR 35 IC L N 57 SH. HA T r p l U BOEHME VN L 55 E ORP V AN 66 shattered oligoclase-andesine, quartz, biotite and sanidine in a vitroclastic matrix. E BIRD VN T N A DRE 6. Elliott, C. and McDonald, C., 2009, Geologic map and geohazard assessment of Silver Bow BLU E V N W Pittsmont No. 4 Trpl F S A T i r L 71 N VN 5096 K U V S U T U A Orphan A Emma SH. The ignimbrite is intercalated locally with thin (<10 m thick) breccia lenses. Abundant M R N 84 E County, Montana: Open-File Report 585, Montana Bureau of Mines and Geology, 81 p., A F 82 45 O. 1 M A J. GALLATIN VN 4 VN N N K a E R .I. V C A W E Girl SH. SH. 47 F DDH-5 N 57 43 A V E (up to 20 vol. %.) cognate clasts of rhyolite and lesser amounts of partially flattened L U 4 I E L 2 2 Pittsmont 3 plates. Blue Bird T S K VN 1 . 6 N M NO. 1 . O I

U O T SH. . N No. 3 SH. fiammé (2H:1V to 1.5V aspect ratio) are orientated parallel to the near vertical fabric. A Otisco SH. DDH-1,1A N T 7. Feiveson, D., 1997, Seismic reflection investigation to support the existence of a normal fault in 5096 W T i r K . W AN 65 H L N R I E DDH-11 This steeply dipping foliation becomes highly contorted, locally. The rhyolite ELLA VN FAU DDH-10 Rocker, Montana: Unpublished Senior thesis, Northfield, Minnesota, Department of M I LT O 15 D . 8251' 40 39 W C pyroclastic rocks at Big Butte have a Ar/ Ar age of 52.9 ± 0.14 Ma (Dudás et al., K b g G . T i r T r p l 81 Geology, Carleton College, 4 p. r F CITY OF 5600 E ce contact a r bedrock DDH-4 2010). 8. Hammar-Klose, E., 1997, A high-resolution seismic reflection at Rocker Timber Treatment ? l e Travona SH. BUTTE & K a P ce a T i r l Extrusive P alluvium DDH-8 and Framing Plant site, Rocker, Montana: The College of Wooster, Keck Research N I

A V R of E

F N O F 84

38 RE S N O limit Tll Dacite lava - Brown to purplish red dacite lava (<120 m) consisting of abundant flow-aligned Symposium in Geology, 10th, Wooster, Ohio, April 1997, p. 302–305.

A Trpl C P G A Z T A T AR LA HIR

W D V Approx. U y V N U D E N Ophir I plagioclase phenocrysts (20 vol. %), biotite-rich glomerocrysts, orthopyroxene,

e ST 9. Houston, R., 1998, 1999, mapped at 1:12,000 and 1:24,000, compiled at 1:24,000; this study.

L d V L CITY OF ke h N R

e SH. T 115 40 T sk ch r i l h hornblende, quartz and trace amounts of apatite and opaques are surrounded in an ROCKER e 10. Houston, R., 2001, Geology and structural history of the Butte district, Montana: Unpublished

ch

v h u T i r l 87

o u

W G co aphanitic groundmass (70 vol. %). There is no age determination for these dacites 4 G

Silver Bow Creek sc M.Sc. thesis, Corvallis, Oregon, Oregon State University, 45 p., 10 plates. Q a l T i r i ? 6 a

N r

l e D T i r

u ce ° at Butte, but Dudás et al. (2010) suggest an age of 52.4 Ma based on a dacite 5400 r ° a B’’ 11. Lewis, R., 1998, Geologic map of the Butte 1° x 2° quadrangle, Southwestern Montana: o

l e

T i r 4

so N 6 P ce 5095000mN

6 ss

a i

l sample southwest of Butte. 4 T i r 8 Montana Bureau of Mines and Geology, Open File report 363, scale 1:100,000, 1 sheet,

M P 1 Tllb Volcanic breccia - Light-gray, brown to purplish red breccia consists of a lower matrix- Q a l Q T a c 17 p. Q T a c k supported part and an upper clast-supported part. The lower discontinuous part (up

5095000mN e 12. Lund, K. Aleinikoff, J., Kunk, M., Unruh, D., Zeihen, G., Hodges, W., Du Bray, E., and re C to 50 m thick) contains subangular (less than 1 m diameter) ash-flow tuff and dacite O‘Neill, J., 2002, SHRIMP U-Pb and 40Ar/39Ar age contrasts for relating plutonism and w Q T a c 20 o lava fragments (60 vol. %) supported by a light-brown to very pale orange devitrified mineralization in the Boulder Batholith region, Montana: Economic Geology, v. 97, Q a l T i r r B K b g

Silve ash matrix. This lower sequence may represent a volcanic debris flow or lahar. The p. 241-267. T

T i r T upper part (up to 120 m thick) of the breccia unit is dominantly framework-supported 13. Martin, M. and Dilles, J., 2000, Timing and duration of the Butte porphyry system: Tobacco

S S

A A k

k angular porphyritic dacite lava boulders (90 vol. %) up to 2 m in diameter that are Root Geological Society, Annual Meeting, Butte, Montana, August 3-6, 2000, p. 1.

E E e e

e

e enclosed in a lavender-gray dacite lava matrix. The upper breccia sequence is an r

r 5094 14. Martin, M., Dilles, J., and Proffett, J.M, 1999, U-Pb geochronologic constraints for the Butte C

C 15 autobrecciated lava along the base of the overlying dacite lava (Tll). 90 porphyry system [abs.]: Geological Society of America Abstracts with Programs, v. 31, 5600 T i r Tlw Quartz latite ash-flow tuff - Light gray, reddish-brown and olive, poorly to strongly welded no. 7, p. A380. a Q T a c 5094 d quartz latite ignimbrite contains shattered plagioclase, biotite, and hornblende 15. Meyer, C., Shea, E., Goddard, C., Jr., and Staff, 1968, Ore deposits at Butte, Montana, in a

n Q a l crystals in a vitroclastic matrix. Eutaxitic textures are defined by conspicuous fiammé. T a Ridge, J., ed., Ore deposits of the United States, 1933-67, v. 2: New York, American

L

C U Normal grading of lithic fragments occurs as repeated distinct zones upward in the A Institute of Mining, Metallurgical, and Petroleum Engineers, p. 1373-1416. F K b g section and correlates to 13 thin cooling units marked by distinct welding breaks. R 16. Proffett, J.M., Jr., 1973, Structure of the Butte district, Montana: in Miller, R., ed., Guidebook E

K 15 Locally, the ignimbrite is up to 300 m thick. In general, hornblende crystal abundance C for the Butte field meeting of the Society of Economic Geologists, The Anaconda

O

R increases slightly from 1 to 5 vol. % upwards in the section. A vitrophyre up to 30 cm 5800 Company, Butte, Montana, p. G-1 to G-12, plus figures. 40 39 Q a l thick locally marks the basal contact. This ignimbrite has a Ar/ Ar age of 52.9 ± 17. Proffett, J.M., Burns, G., and Anaconda staff, 1982, Geologic map of the Butte district, 0.14 Ma (Dudás et al., 2010) Montana: Anaconda Archives, American Heritage Museum, Laramie, Wyoming, scale 5093 5800 Q T a c Tlat Rhyolitic air-fall tuff and base surge deposits - Light gray, unwelded laminated and 1:12,000. cross-bedded crystal-poor ash air-fall and base surge deposit (<50 m) with up to 15 18. Reed, M., 1979 a and b, written communication. vol. % lithic fragments. This upper unit is interpreted to be the base surge and airfall 19. Roberts, S., 1975, Early hydrothermal alteration and mineralization in the Butte district, 5093 15 90 deposits precursor to the overlying quartz latite ignimbrite. Montana: Ph.D. thesis, Cambridge, Mass., Harvard University, 173 p. T i r Tlt Volcaniclastic sedimentary deposits - Fluvial mudstone, siltstone, sandstone, and 20. Sales, R., 1914, Ore deposits at Butte, Montana: American Institute of Mining and pebble conglomerate sedimentary sequences containing reworked volcaniclastic Metallurgical Engineers Transactions, v. 46, p. 4-106. h sedimentary rocks. Laminated micaceous mudstones and siltstones grade upward 21. Smedes, H., 1962, Lowland Creek Volcanics, an Upper Oligocene formation near Butte, lch S u Q T a c a into coarse-grained and rippled arkosic and tuffaceous sandstones. Cross-bedding, Montana: Journal of Geology, v. 70, p. 255-266. G n k d ripples and petrified logs indicate a SSW-NNE bi-directional fluvial flow-direction. 22. Smedes, H., 1967, Preliminary geologic map of the northeast quarter of Butte South e C T i r e r r B This sedimentary unit (<50 m) was deposited on an irregular erosional surface, locally e Quadrangle, Montana: U.S. Geological Survey, Open File Map, 1:24,000. C 5600 lack e e kta 5092 marked by deeply weathered granite (grus) surrounding intensely weathered 112°35' W v k il 23. Smedes, H., 1968, Preliminary geologic map of part of the Butte North Quadrangle, Silver o Cr r ee 6400 boulders of granite and aplite. G B k 7000 Bow, Deer Lodge and Jefferson Counties, Montana: U.S. Geological Survey Open-File a 7400 Intrusive si Report, Scale 1:48,000. i 376 377 n Trdi Rhyodacite dike of Rampart Mountain - Gray, porphyritic rock contains phenocrysts of 24. Smedes, H., Klepper, M., Pinckney, D., Becraft, G., and Ruppel, E., 1962, Preliminary SCALE 1:24 000 C Q a l r plagioclase, quartz, and biotite and rare 2 cm orthoclase set in a fine-grained aplitic 6200 K b g e geologic map of the Elk Park Quadrangle, Jefferson and Silver Bow Counties, Montana: e Q T a c 6000 7800 groundmass (Meyer et al., 1968). From east to west, this dike transects the district k 1 0.5 0 1 km 7600 U.S. Geological Survey map MF-246, Scale 1:24,000. 5800 8000 Q a l for >7 km at depth. Normal displacement along the Continental fault has exposed 25. Smedes, H., Klepper, M., and Tilling, R., 1973, The Boulder Batholith, Montana: in Miller, R., 7200

8200 this dike on Rampart Mountain and further east in a I-15 roadcut. In the hanging wall Contour Interval 40 feet Q T a c ed., Guidebook for the Butte field meeting of the Society of Economic Geologists, The Datum is Mean Sea Level of the Continental fault, at approximately the 2,000 foot level, a breccia at the apex Anaconda Company, Butte, Montana, p. E-1 to E-16. 18.5° K a of the dike, known as the Mountain View breccia, continues upward ~100 m, but is 26. Smedes, H., Klepper, M., and Tilling, R., 1988, Preliminary map of plutonic units of the BUTTE DISTRICT LOCATION not exposed at surface (Meyer, et al., 1968). Available 40Ar/39Ar and 206Pb/238U 5091 Boulder Batholith, southwestern Montana: Montana Bureau of Mines and Geology, Timber 6000 monazite ages suggest emplacement at ~59 Ma (Dilles et al., 2004). Open-File Report 88-283, Scale 1:200,000 (& digital reprinted USGS Data Series 454, Butte 90 6800 5091 6600 2009). Q a l MONTANA 6000 5600 5800 27. Weed, W.H., 1912, Geology and ore deposits of the Butte district, Montana: U.S. Geological True North 1122°32' 30" 112°30' Survey Professional Paper 74, 262 p. Magnetic North 112°27'30" W Approximate Mean Acknowledgments Declination, 1962 379 380 381 382 383 384 385000mE 386 387 388 This research was supported by grants from the Tobacco Root Geological Society to Houston, and by NSF grants EAR-9614683 and EAR-0001230 to Dilles and Cyrus Field of Oregon Base map modified from four U.S. Geological Survey 7.5 minute topographic quadrangles; State University. We greatly thank the many geologists who contributed scientific research to Butte North (1989), Butte South (1989), Elk Park Pass (1985), and Homestake (1978). advance the knowledge of the geology of the Butte deposit over the last 140 years, and we acknowledge the research sponsorship until 1986 by the Anaconda Company and its successor A Big Butte Post-Ore Anaconda Dome Pittsmont Dome A’ the Atlantic Richfield Company. Steve Czehura of Montana Resources Inc. kindly provided many West Rhyolite Intrusion East B A to A’ B” informative discussions as well as unpublished data of the Butte district, including geologic maps B’ made by George Burns and John Proffett. Review of an earlier version by Professors Cyrus Field, N344°W Section South S344°E North Edward Taylor and Robert Yeats, when it was part of a M.Sc. thesis by Houston, is greatly appre-

B-B’ Section Continental fault 2.5 km 2.5 km ciated. We gratefully acknowledge two Economic Geology reviewers John Proffett and David Klepper fault Cooke, who reviewed the map as part of the formal review process of the manuscript (Houston Rocker fault Berkeley Pit and Dilles, 2013). Finally, the senior author is indebted to the residents of the study area who Tlcv Tailings freely gave their permission to traverse their properties. Middle fault Badger 2 km Supergene Mineralization Middle fault Leonard (proj.)

Orphan Girl DDH-10 Stewart DDH-1, 1A Nettie Anselmo Chalcocite dominant (proj.) DDH-2 DDH-7 DDH-6

Blue Bird DDH-7 Rarus fault Maps may be obtained from: 0.03 Mo Publications Office 1,500 ft. level Montana Bureau of Mines and Geology 0.03 Mo 0.03 Mo ZnS Interior 1300 West Park Street Butte, Montana 59701-8997

Top of Mo .5Mo0.05 1 km 1 km 2,800 ft. level Phone: (406) 496-4167 Fax: (406) 496-4451 0.03 Mo http://www.mbmg.mtech.edu 0.03 Mo 0.05 Mo 0.05 Mo East Ridge fault East fault Cu Ore Front

Mt Front Ore Cu Tmqp zone Index Map showing credit for geologic Montana Bureau of Mines and Geology Milwaukee fault Kqp Limits of Limits of Kqp mapping noted by reference number Open File MBMG 627 0.005 Mo Modern 0.05 Mo Modern Rarus fault Exposure 0.02 Mo Trdi 0 Tmqp 0.05 Mo 0 0 0 1, 9, Exposure East 0.01 Mo GEOLOGY OF THE BUTTE MINING DISTRICT, 0.02 Mo and 23 9, 17, Cupola MONTANA West (Inferred) and 23 Trdi Cupola 1 2 No. 15 fault (Inferred) Robert A. Houston and John H. Dilles Inferred Granite Pre-Main Stage Cupola 2013 1 km Modoc qtz ppy 1 km 1 km Inferred Granite Cupola (Inferred) Main Stage Cupola 9, 17, 16, 1, 27, 1 Lowland Creek Volcanics 24, 22, 15, 26, Oregon Department of Geology and Mineral Industries, Albany, Oregon Boulder Batholith 2 Explanation Rock Types Lowland Creek Volcanics Boulder Batholith Pre-Main Stage Mineralization Explanation Formation Pre-Main Stage Mineralization 4, 11, 20 and 18 Oregon State University, Corvallis, Oregon Fault, showing Quaternary & Formation Big Butte Vent aplite dikes Fault, showing Lower quartz latite pyroclastics quartz porphyry dikes aplite dikes Rhyolite pyroclastics & vent quartz porphyry dikes Sparse py/Gs veins Magnetite vein (Cu) zone Tlcv Sericite alteration zone Complied by Complex (Ka) slip direction & sediments (Tlw, Tlat, Tlt) (Kqp, Tmqp) (Ka) slip direction Tertiary sediments breccia (Tiv, Tvcb, Tvwb) (Kqp, Tmqp) R.A. Houston (Qal, QTac) Rhyolite dikes granite cupola Main Stage vein or Middle Paleocene granite cupola Butte Granite Main Stage vein or Trdi Middle Paleocene Butte Granite Sericite alteration zone Trdi Magnetite vein (Cu) zone This map supplements Houston and Dilles, 2013, Structural mineralized fault (Tir) rhyodacite dike & breccia carapace (Kbg) mineralized fault rhyodacite dike & breccia carapace (Kbg) 25 9 Geologic Evolution of the Butte District, Montana: Economic Geology, v. 108, no. 6, p. 1397-1424.