Geology of the Butte Mining District, Montana
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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" ° 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 ° 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 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 ° 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.