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The Arizona Geological Survey is not responsible for the accuracy of the records, information, or opinions that may be contained in the files. The Survey collects, catalogs, and archives data on mineral properties regardless of its views of the veracity or accuracy of those data. 40 IGNEOUS ROCKS AND RELATED MINERAL DEPOSITS OF THE BARKER QUADRANGLE, MONTANA MAFIC ROCKS 17

~i= '----.J'---'-----'--~ch'-=-----."'! a: EXPLANATION o z

Minette-kersantite l >- 0:: « f- 0:: Rhyolite of Granite Mountain W f- ~..... Tb · ~ Barker Porphyry

UNCONFORMITY z « Il.. s... .0.. Il.. ~ .~~ { Lower partCIEEJ of Madi son Group til j .~ UNCONFORMITY til UJ ~ til ~ A C Three Forks Formation z « z 0 J efferson Dolomite > W 0

Maywood Formation

UNCONFORMITY

Pilgrim Limestone 4 --- z « 0:: B m Park Shale «~ u

Older Cambrian units I I- '«::::J 0- 0 0 0 0 0 0 (1) 8 0 0 0 0 0 0 ... 0 <0 N 00 0 r-- r-- <0 <0 ... "' "' "' Contact, approximately located

U o Fault, approximately located U, upth,-own side; D, downth"own side

14 --'-- B D Strike and dip of beds FIGURE 7.-Intermediate and mafic igneous rocks from the clase grain jacketed by alkalic feldspar enclosed in a micro­ Barker quadrangle. Scale of both hand specimens in centi­ o 1'2 MILE granular groundmass. C, D. Shonkinite. C, Hand specimen. I I meters. Photographs by R. B. Taylor, Sandra Brennan, and Dark-gray to black mafic rock marked by medium- to CONTOU R INTERVAL 200 FEET Louise Hedricks. A, B. Barker Porphyry. A, Hand speci­ coarse-grained texture. Large dark-gray to black partly men. Light-gray quartz latite porphyry characterized by rounded minerals are olivine. D, Photomi~rograph showing FIGURE 23.-The west flank of the Granite Mountain bysmalith. Base from U.S. Geological Survey, Mixes Baldy quadrangle, angular white laths of feldspar and black needles of horn­ ovoid olivine grain broken by curving fractures and en­ 1 :24,000, 1961. blende and flakes of biotite in a fine-grained groundmass. circled by biotite flakes. B, Photomicrograph showing sanidine, quartz, and a plagio- 18 IGNEOUS ROCKS AND RELATED MINERAL DEPOSITS OF THE BARKER QUADRANGLE, MONTANA INDURATED ALLUVIUM 39

SPECIMEN NUMBER santite, composed of biotite, orthoclase, and plagio­ clase; and (5) vogesite, similar to the minette-ker­ 00 N ~ santite, but including much pyroxene or hornblende (table 3). Petrographic details and modal and chem­ ical analyses of these rocks are given in table 4. Contact between laccolith (left) o 5 The mafic rocks do not seem to be distributed at Barker Mountai and Madison strata (right) & ~ random. Most of the shonkinite, plagioclase shonkin­ '" ~ 4 "' + ite, and syenite crops out in the northwestern part Cl) ~ ~O 3 ro iJ' of the Barker quadrangle (closer to the shonkinite­ ~~o - f- 2 rich Highwood Mountains); most of the minette-

EXPLANATION 5 ,------, o Barker <) l1 nilrangie 4 • Ne ih art quadrangle

3 6 0 o 0 [). Stanford· Hobson area :g"" 2 o o FIGURE 22.-View looking westward from Mixes Baldy at chiefly the Pilgrim Limestone. Heavily wooded area in o L-______~ the north and east flanks of Barker laccolith. The north­ left foreground is part of the west edge of Hughesville east flank of the laccolith is in intrusive contact-possibly stock. Rockslides in right foreground are part of the de­ 5 ,------, as a result of a preexisting fault-with Madison strata, 3 nuded southwest end of the Clendennin-Peterson laccolith. 8 9 ' 4 0 whereas all other flanks pass below Cambrian strata, Irene Peak is the upturned edge of a resistant sill. 4 6 " --BI O~2 (0 16 15 I) fti:::19 ~ 3 I P~OcJg\1 2 U ) 4 18 2 of about 5,750 feet. Here the indurated alluvium ex­ as 6 inches in size and rather well rounded, are tends down to and passes beneath the valley floor; incorporated in other masses of indurated alluvium. Na,O K,O the altitude of its base is unknown. If, however, the The deposits were clearly laid down by streams. gradient of modern Gold Run Creek, 100 feet to the

Iron mile, is accepted as the gradient of its ancestor, then The indurated alluvium includes poorly sorted an­ the altitude of the base of the indurated alluvium in gular to subrounded cobbles of Wolf Porphyry, of this locality can be calculated to be about 5,600 feet. the porphyry of Olendennin Mountain, and of the The 150-foot difference between exposed top and quartz monzonite of Hughesville. In these cobbles, calculated base is probably an average thickness for many of the feldspars have been intensely altered to the indurated alluvium. It seems doubtful that the white clay. Grains and seams of pyrite and chalco­ alluvium exceeds 200 feet in thickness. pyrite are in fragments of both the quartz monzon­ ite of Hughesville and the Wolf Porphyry. Commonly, the indurated alluvium stands as a steep or nearly vertical wall, and is veined by man­ The matrix invariably is brown and consists of a 60 62 64 66 68 70 ganese-stained fractures. Loc3ilIy, steeply tilted SiO,. IN PERCEl'JT motley of fine to coarse particles more or less blocks of the alluvium, 50-100 feet across, are en­ cemented by silica and limonite. Much of it consists EXPLANATION closed in the main mass and invariably dip toward of minerals derived from the disintegration of the the embankment. These features suggest that they Wolf Porphyry, of the quartz monzonite of Hughes­ o Hug hesville stock are slide blocks. vj,]]e, and of various metamorphic rocks. As a result, o L a ccoliths In most exposures the indurated alluvium is a round and angular quartz grains (from the Wolf(?) [). Sills MgO Na,O +K, Q light-brown to grayish-brown, firm, well-cemented Porphyry), as well as altered alkalic and plagioclase jumble of angular to well-rounded rock fragments feldspars (from the quartz monzonite.), are which range in size from a quarter of an inch to 1 common. Small amounts of garnet (from the meta­ FIGURE S.- Silica variation diagrams for intermedi­ FIGURE 9.-Comparison of some chemical characteristics of ate rocks from both large and small intrusions the intermediate rocks from the Barker quadrangle with foot; most, however, are about half an inch long. A morphic rocks) are present. Small specks of pyrite scattered through the Barker quadrangle. The comparable rocks from the Neihart quadrangle and from few beds are composed of moderately well sorted are also s·cattered irregularly through the matrix. grouping indicates that the intrusions came the Stanford-Hobson area. See table 2 for sample loca­ sand and pebbles; crossbedding is common; and here Mafi·c minerals are rare, presumably because most from the same magma. Specimen numbers are tions, and Vine (1956, p. 454-455) for location of samples and there cobbles of the' indurated alluvium, as much have been weathered and removed. identified in table 2. from the Stanford-Hobson area. 38 IGNEOUS ROCKS AND RELATED MINERAL DEPOSITS OF THE BARKER QUADRANGLE, MONTANA MAFIC ROCKS 19

TABLE 3,-Lamprophyre t erminology PLAGIOCLASE SHONKINITE (Sample 2 of table 4)

Dominant f eldspar Several minor plagioclase shonkinite intrusions around Barker Mountain are indistinguishable in Orthoclase Plagioclase hand specimen from the shonkinites; their greater plagioclase content is apparent only in thin section.

.2l The plagioclase feldspar is albite (An5_ 7 ), and it :;:; Minette Kersantite " 0 dominates the mineral assemblage. It forms elongate ""s'" iii ..,.. laths that are enveloped by alkalic feldspar. " '" Many of the olivine grains have been altered to a 'S" ~~ 0 "'.c ~ >< ", Vogesite Spessartite fibrous mesh of alteration products, which (because g S they probably include both antigorite and chryso­ Po."''' k 0 tile) are here grouped as serpentine.

kersantite and vogesite is in the southern half. I am SYENITE uncertain whether this pattern has more than local (Samples 3- 5 of table 4) significance, inasmuch as shonkinite is also exposed A few thin syenite sills are interlayered in the to the south at Yogo Peak (Weed, 1900, p. 397) sedimentary units of the Big Snowy Group exposed along the valley walls of both Little and Big Otter Creeks. And several more are exposed in the steeply SHONKINITE tilted beds that form part of the nose of the Clen­ ~w " \,. I 4 (Sample 1 of table 4) M I)< E ~BALDY- AND ~ R S DN PEAK dennin-Peterson anticline (sec. 20, T. 16 N., R. 9 E.) , LA CCO.LITH The largest shonkinite intrusion, a blunt plug (Witkind,1971). o 1;2 1 I I I about 1,600 feet wide at its base and some 400 feet In hand specimen, the syenite is light gray to CONTOUR INTERVAL 400 FEET high, bows up and alters Paleozoic (Big Snowy gray, holocrystalline, and medium to ,coarse grained EXPLANATION Group) and Mesozoic (Ellis Group and Morrison and is distinguished by abundant biotite flakes and ...:'" Formation) sedimentary rocks exposed along Little long thin needles of pyroxene scattered through a UNCONFORMI T Y Otter Creek in the SEl,4 sec. 18, T. 17 N., R. 8 E. light-gray ground mass of feldspar. 'm" w I Tv/ I 0;E { ...: FI.Sl (Witkind, 1971). In this general area shonkinite In thin section the rock is seen to be a crystalline, ""' Porphyry of "0 Vogesite ~ Galena Creek .. ~ sills, some a'S much as 160 feet thick, are ,common­ ;;:::" equigranular mixture of orthoclase, oligoclase o oj Mississippian units " ~ especially in the Heath Shale of the Big Snowy >< ~ (An2s ), biotite, augite, and sparse hornblende. The N (J) 00 ' G UNCONFOR MITY o o o Group. o o orthoclase predominates over the oligoclase and com­ Syenite q ~ q '" The shonkinite is dark gray and even grained, monly jackets it. Some of the plagioclase grains are Co ntact, approximately located ranging from medium to coarse ,grained with the altered to analcime. Apatite, sphene, magnetite, and ~ { r::I~ o ilmenite are common accessories. ;.a . >- Devonian units u coarser types much more common (fig. 7C, D) . ~ Qua rtz monzomte 0:: Fault, approximately located

TABLE 4.-The petrography and chemical composition (in percent) of mafic rocks from the Barker quadrangle, Little Belt Mountains, and of similar rocks from the Stanford-Hobson area

[Rapid-rock analyses. Analysts: P. L. D. Elmore, Samuel Botts, Gillison Chloe, Lowell Artis, H. Smith, John Glenn, and James K elsey]

Area ______Barker quadrangle !;.anford-Hobson area 1

Rock type ______Shonkinite Plagioclase Syenite Minette-kersantite Vogesite Vogesite Shonkinite. shonkinite. Peterson Mountain

ColoL ______Dark gray Dark gray Light gray to gray Gray Gray to dark gray Gray to Dark gray. / dark gray.

T exture ______Medium to M edium to Medium to coarse grained Fine to medium Very fine grained Very fine Medium to coarse coarse grained. grained. coarse grained. grained. grained. ---- Phenocrysts _____ Biotite, Biotite, Biotite and pyroxene None Biotite and pyroxene Biotite and Biotite, pyroxene, pyroxene, pyroxene. pyrox(>ne, olivine. olivine. olivine. -'--- Xenocrysts _____ None None None Quartz, feldspar, Quartz, sanidine, plagioclase, Quartz, None. mafics(?). feldspar (oligoclase (An 11) feldspar. ...>. to labradorite (An,,». 0... So Groundmass __ ___ Mixture of Mixture of Equi~anular mixture of Holocrystalline, Holocrystalline mixture of Mixture of Mixture of E biotite, biotite, ort oclase, oligoclase (An,,), anhedral grains pyroxene (augite, salite), pyroxene, biotite, "t olivine, olivine, biotite, augite, hornblende. of orthoclase, biotite, orthoclase, and biotite, olivine, Po. pyroxene pyroxene and zeolites. o.rthoclase plagioclase plagioclase microlitEs. and feld- pyroxene, cemented cemen ted iackets plagioclase. microlites, Some apatite, sphene, spar. cemented by ortho- by plagi- Sodalite(?). biotite, and opaque minerals. by ortbo- clase. oclase plus hornblende. clase. or tho- Some pyroxene. clase. -.---- Mode (percent)_ Orthoclase, Albite o.rthoclase, 51.0; oligoclase Alkalic and plagio- Alkalic and plagioclase feld- Not avail- Not avail- 39.2; (An3- 7)- (An,,), 32.4; biotite, 8.6; clase feldspar, spar, 66.7; clinopyroxene, able. able. olivine, 38.1; augite, 5.7; opaque min- 59.3; biotite, 26.0; biotite, 6.4; opaque 24.6; biotite, erals , 2.2; others (analcite, 22.7; clinopyrox- minerals, 7.0; others pyroxene, 22.2; or- heulandlte, thomsonite), ene, 7.7; opaque (including xenocrysts), 9.9. is formed by limestone beds of the Madison Group. The crest of the Barker laccolith shows along the skyline 15.7; thoclase, 0.1. minerals, 3.4; biotite, 17.6; others (including through the low pass-underlain by faulted Paleozoic rocks- between Mixes Baldy and Clendennin Mountain. 14.4; salite, xenocrysts), 6.9. albite, 10.7; 2.2; serpen- others, tinp.,9.6; co.untry reck has been severely defo.rmed and al­ The indurated alluviumco.nsists o.f a heteroge­ 4.0. others, 1.8. tered. There the reeks are intensely sheared and neous mixture o.f sedimentary, igneo.us, and meta­ , tho.ro.ughly impregnated with chlo.rite. No. faults mo.rphic rocks which cro.p out in scattered depo.sits Oxides were detected in the San Miguel area, and this cata­ alo.ng the sides o.f Galena, Daisy, and Geld Run dasis is attributed to. lo.calized defo.rmatio.n during Creeks (fig. 6). These depo.sits, part o.f a fill that was 2 S 4 5 6 7 8 9 10 11 12 emplacement o.f the pluto.n. once co.ntinuo.us acro.ss the valleys o.f these streams, Laboratory No ______163677 163679 D - lt2806 W-169661 W-169662 D-112805 W- 169663 163694 W-167705 W - 169696 D-1l2803 W-169051 either were net depo.sited or are net preserved do.wn­ Field No ______WL-31 WL-42 WL-1S WL-32 WL- 35 WL-ll WL-I08 WL- 120 WL-286 WL-417 WL-3 WL-495 The geo.lo.gic evidence, thus, suggests that the plu­ ---- stream fro.m the mo.uth o.f Geld Run Creek. Sio., ______48 . 3 56 . 3 51. 1 55 . 0 54.5 50 . 0 52. 2 49.2 53.7 56.9 50.7 50.1 ton is an o.val bo.dy elo.ngate to. the no.rtheast. So.me Aho.. ______10. 4 14.1 16 . 9 18.3 18 . 8 13.2 13.3 12.9 15.2 17.2 14 . 4 13 . 0 F "'o., ______2 . 0 2.3 3.7 2.9 2 . 6 4.2 4. 1 3.9 2.3 6.5 3.2 4.0 recent geo.physical wo.rk involving both gro.und grav­ In the upper reaches o.f Daisy and Galena Creeks, F eo. ______7.7 3.3 3.8 2 . 8 3.4 2 . 9 3 . 3 3.1 S.8 . 02 4.8 3.6 Mgo. ______16.0 7.6 3.1 2.2 2.0 6.8 5.6 6.9 3.4 1.0 5.7 9.8 ity and aeromagnetic surveys co.nfirms this interpre­ the indurated alluvium is preserved as thin discon­ Cao. ______6.8 5.4 5 . 6 3.3 3.9 8.9 6.7 9.4 5.4 5.0 7.2 7. 1 Na'o. ______tatio.n (fig. 31) (Witkind and ethers, 1970). tinuo.us layers 2-3 feet thick. So.uthward along Gal­ K,o. ______1.4 5.0 4 . 6 3.6 4 . 4 2.4 3.6 2 .1 3.4 3.9 2.5 2.8 H,o.-______3.6 3.2 6.6 8.2 6.0 4.9 3.2 4.6 3.3 3.7 4 . 1 4.7 ena Creek the depo.sit is thicker and finer grained. .31 .38 .30 0 .18 1.3 .93 1. 6 .94 1.3 2.6 1.2 Where expo.sed, the pluto.n is mineralized and co.n­ H ,o.+ ______1.7 1.0 2.4 2.1 2 .6 2 .2 2.2 1.4 2.3 1. 4 2.0 1.7 Tio., ______There, the base o.f the alluvium extends below the .75 .45 .96 .51 .62 1.4 . 73 1.3 1.1 .86 .78 .69 tains in different lo.calities, mo.lybdenum, galena, and p ,o., ______. 65 .57 .77 . 45 . 40 1.3 .52 1.6 . 56 .60 .65 .65 stream fleer . Mno. ______.16 .13 .12 .12 .14 .08 .12 .10 . 42 .13 .10 . 14 sphalerite. In the past, significant amounts o.f silver, Co., ______.08 .09 . 37 .36 .06 . 43 3.4 1.3 4.2 1.5 1.6 .35 r------lead, and zinc have been mined fro.m the pluto.n, and Sum ______99.85 99.82 100.32 99.84 99.60 100. 01 99.90 99 . 40 100.02 100.01 100.33 99.83 Inasmuch as the base o.f the indurated alluvium is it seems likely that co.mparable amounts are still no.where cleal'ly expo.sed, its to.tal thickness is uncer­ co.ncealed elsewhere in the pluto.n (see sectio.n en tain. N o.ne is preserved do.wnstream fro.m the mo.uth "Buried Mineralized Pluto.n"). of Go.ld Run Creek, and its absence must be due to. ero.sio.n by mo.dern Galena Creek. At the mo.uth o.f INDURATED ALLUVIUM Gold Run Creek, the bo.tto.m of the indurated al­ luvium-as indicated by truncated bedro.ck expo.­ Of the surficial depo.sits in the quadrangle, o.ne sures-must have been at o.r near an altitiude o.f unit-the indurated alluvium-is here considered in abo.ut 5,500 feet. Upstream and alo.ng the walls o.f detail because it o.ffers evidence en the relative ages Geld Run Creek, the to.p o.f the o.ld alluvium, trun­ of several igneo.us reck types. cated by pediment depo.sits, is at a maximum altitude 36 IGNEOUS ROCKS AND RELATED MINERAL DEPOSITS OF THE BARKER QUADRANGLE, MONTANA MAFIC ROCKS 21

TABLE 4.-The petrography and chemical composition (in pe1'cent) of mafic rocks from the Barker quadrangle, Little Belt Mountains, and of simila1' rocks from the Stanford-Hobson area-Continued

1 2 3 4 5 6 7 8 9 10 11 12 ------._-- --- .. Laboratory No ______163677 163679 D- 112806 W - 169661 W 169662 D- 1l2805 ield No ______W- 169663 163694 \V-16770o "V-169696 D- 1l2803 W - 169051 F WL-31 WL-42 WL-15 WL-32 WL-35 WL- ll WL-108 WL- 120 WL-286 WL-417 WL- 3 WL-495

C.I .P.'V. norms

Q-or _-_-______------_____- - -- 0 0 0 0 0 0 1.28 0 .22 5.95 13 . 66 1. 60 0 ab ______21.27 18.91 38.88 48 . 53 35.60 28 . 95 19.58 27.18 20.47 21. 89 24 . 15 27.82 an. ______11.84 42.30 14 .73 21. 57 30 . 60 20.31 31. 54 17.76 30.20 33. 04 21. 08 18.66 C ______11.46 6.59 5.95 9.57 13.88 10.77 11.06 12.19 17.29 11 . 42 15.91 9.04 ne ______0 0 0 0 0 0 0 0 0 2.60 0 0 en ______0 0 13.04 4 . 84 3.67 0 0 0 0 0 0 2.75 5.18 5 . 00 4 .15 .45 . 61 8.00 6.49 5.56 1. 01 0 1. 53 6.63 fs ______1. 50 . 17 di wO . ______.97 1.38 . 41 0 .75 .17 . 67 0 .55 . 66 en ______7.31 6 . 64 6.01 .67 1. 06 9.26 8.17 6.58 1. 76 0 2.25 8.26 hy fs ______3.35 1.47 0 0 0 1. 29 7.95 11. 62 7.88 2.50 12 . 62 0 fo ______.97 .28 0 0 0 0 . 92 . 35 5.29 0 4. 50 0 01 22.03 8 .80 2 .49 3.53 3.08 5.36 0 4 . 82 0 0 0 fa ______12 .48 mt. ______7.03 1.87 .91 1. 51 2.27 0 0 .76 0 0 0 1. 37 ap ______2.90 3.33 5.35 4.21 3.79 5 . 55 6.15 5 . 66 3.50 0 4.62 5.81 1.54 1. 35 1. 82 1. 07 .95 3.08 1.28 3.79 1. 39 1. 4!! 1. 53 1. 54 i1. ______1.82 hm ______1.42 .86 .97 1. 18 2 .66 1. 53 2.47 .22 .04 1.48 1. 31 0 0 0 0 0 .37 0 0 0 6.51 0 0

1 See Vine, (1956, p . 454- 455). 7 . Sill(?) along nort h west flank, Granite Mountain. S 7!l sec. 31, T. 16 N., R. 1. Plug along Littlc Otter Creek, SE)4 sec. 18, T. 17 N., R. 8 E. 10 E. 2. Sill at Irene P eak, W y, sec. 36, T . 16 N., R. 8 E. 8. Dike, Frenchies Coulee, NWU sec. 18, T. 15 N ., R. 9 E. 3 . Sill along northwest flank, Clendennin-Otter anticline, sec. 20 T. 16 N., R. 9. St. Louis mine, S7\; sec. 7, T. 15 N., R. 9 E. (unsurveyed). 9 E. 10. Dike (altered), Gold Run Creek, C. sec. 18, T. 15 N., R. 9 E. 4. Sill at Hansen's Coulee, sees. 28 and 33, T, 17 N., R. 8 E. 11. Sill, Skull Butte, NE;1 sec. I, T. 15 N ., R . 11 E. 5. Sill along west vallcy wall Big Ot ter Creek, sec. 18, T. 16 N ., R . 9 E . 12. Dike, Skull Butte, Sy, sec. 35, T. 16 N., R. 11 E. 6. Sill along northeast flank, Clendennin- Otter ant icline, C. sec. 24, T. 16 N., R. 9 E.

FIGURE 20.- View from the crest of Granite Mountain looking westward across Lone Tree Park toward the south­ Scattered sills and small plutons of minette-ker­ N., R. 9 E. (unsurveyed) (Witkind, 1971). A voge­ east flank of the elongate anticline underlain by the C'lendennin-Peterson laccolith. The configuration of the anti­ santite intrude Cambrian shales and limestones ex­ site dike, exposed in Frenchies Coulee, cuts the Tay­ cline is believed to reflect the laccolith, part of which is exposed at Clendennin Mountain. The nose of the anticline posed along Dry Fork Belt Creek between Barker lor Mountain laccolith (NW% sec. 12, T. 15 N., R. 9 and Servoss Mountains (center T. 15 N., R. 8 E.) E.) ; anvther intrudes an old indurated alluvium (p. and congealed intl'lusions were punched upward by thickness of the pluton is unknown, but the presence (fig. 6). They range in thickness from a few feet to 41) exposed in the valley of Gold Run Creek younger subjacent masses of magma. Some .of the of orthoclase in various of the quartz porphyries as much as 100 feet; most can be traced for several (center sec. 18, T. 15 N., R. 9 E.). Still a third dike dikes that follow the fault may be apophyses of this that form it implies that it is floorless. miles. One sill contains much hornblende; and it is was intersected by the workings of the St. Louis younger magma. What is very likely the same pluton is exposed this sill, 4 miles west of Barker, that Weed (1900, p. mine (fig. 25; S% sec. 7, T. 15 N., R. 9 E., unsur­ beneath a 'cover of metamorphk rocks in the nearby 352) descr:ibed as a vogesite sheet. The Pilgrim veyed), as shown by vogesite fragments found in the BURIED PLUTON open pit of the Silver Dyke mine. Here, Precambrian Limestone seems to be the favored host rock, al­ mine tailings. though a few sills are in the Park Shale. A very large compound pluton has intruded and xenoliths in the porphyry, dikes of one or more por­ In hand specimen the vogesite is a .gray to dark­ raised the Precambrian crystalHne rocks that form phyries in the Precambrian cover, and brecciated One minette-kersantite sin that contains consider­ gray, very fine grained rock speckled with pheno­ the core of the Little Belt Mountains. In this quad­ porphyry indicate that many intrusive pulses able pyroxene is interleaved in Big Snowy strata cryst.s of brownish-black biotite. Locally near con­ rangle the pluton is completely concealed, and only formed the pluton. along the northeast flank of Peterson Mountain tacts, it 00ntains xeno.liths and xenocrysts of country here and there do small dikes and plugs of felsic Extending northeastward from this point into the (center sec. 24, T. 16 N., R. 9 E.); dikes of similar rock. In thin section those vogesites that are more or rock give some idea of its lithology. Its northern southwest corner of the Barker quadrangle is a zone rock are along the fault that encircles the Granite less free of inclusions consist of phenocrysts of clino­ flank is reflected by steeply tilted Cambrian strata of felsic dikes and plugs which intl'ude the Pre­ Mountain bysmalith (fig. 23). pyroxene (augite and salite) and biotite in a holo­ whkh wrap around the Precambrian complex. cambrian rocks (fig. 31, and samples 13-15 of table crystalline groundmass of the same minerals, and Wherever exposed, these beds dip at high angles 1). These minor intl'lusions , singularly like various abundant orthoclase and plagioclase microlites. VOGESITE away from the comp,lex; dips average 70 °, but lo­ of the rhyolite porphyries that ·constitute the Snow I Accessory minerals are apatite, sphene, and magne­ cally the beds are vertical or nearly so (Witkind, Creek Porphyry, form a zone about 1 mile wide and (Samples 8-10 of table 4) tite. 1971). To the south, however, in the Neihart quad­ 3112 miles long that trends N. 50° E. through the . \.. few sills and dikes of vogesite crop out in the All the vogesites examined 'contain xenocrysts rangle, a cupola of the pluton is exposed along Car­ SE1,4 T. 15 N., R. 8 E. (unsurveyed) (Witkind, south half of the quadrangle. One sill in the E % sec. chiefly of quartz, sanidine, and plagioclase feldspar. penter and Snow Creeks (fig. 31). In those areas it 1971). I interpret them as apophyses from the crest 24, T. 15 N., R. 8 E. follows the unconformity be­ An assortment of different plagioclase feldspars ex­ is a compound mass that consists of at least f.our of the central part of the pluton. tween the Flathead Sandstone and the Precambrian tending from oligoclase (An ) to labradorite bodies of quartz rhyolite porphyry, all strikingly l7 StiU farther to the northeast in the San Miguel crystalline rocks; and another, following the same (An6o) is included. Each xenocryst is rounded, cor­ similar (Johnson, 1964), which have been mapped as area (N% sec. 31, T. 15 N., R. 9 E., .unsurveyed) of unconformity, is exposed in the E1;2 sec. 31, T. 15 roded, and encircled by a distinctive rea:ction rim the Snow Creek Porphyry (Keefer, 1969). The the Barker quadrangle, the hornblende-biotite gneiss

4!)u -lRrl 0 - 73 - '1 22 IGNEOUS ROCKS AND RELATED MINERAL DEPOSITS OF THE BARKER QUADRANGLE, MONTANA BYSMALITH 35

most completely encircled by a high-angle fault (fig. floor of the laccolith and did modify the alkalic feld­ 2). The laccolith is about 2112 miles in diameter and spar. If so, erosion has not as yet cut deeply enough rises some 2,000 feet above the floor of Dry Fork to expose this basal part of the laccolith. I favor this Belt Creek. Its thickness is unknown; on cross sec­ second alternative. tion A-A' of figure 6 I have shown it as underlain by A gravity map of the Barker-Neihart area sug­ a younger pluton. gests that the north edge of an elongate northeast­ Virtually the entire sedimentary cover has been trending buried pluton does, in fact, underlie the stripped from the laccolith exposing the coarsely Mixes Baldy-Anderson Peak laccolith (fig. 31). phenocrystic Wolf Porphyry (samples 6 and 7 of table 1). Only a very thin layer of steeply dipping BYSMALITH Cambrian and Devonian beds is preserved around The term "bysmalith," coined by Iddings (1898), the north and northeast flanks. The laccolith, thus, is has been used to describe wholly different igneous even more denuded than the Barker laccolith and is bodies. In 'common usage (Am. Geol. lnst., 1960, p. far better exposed than the Clendennin-Peterson lac­ 41; Billings, 1942, p. 273), it describes a more or less colith. vertical cylindrical body of igneous rock that has ,., Locally, the evidence for the circumferential fault pushed the overlying strata up along one or more Quartz xenocryst is tenuous. On the north and northeast the evidence circumferential faults. Hunt (1956, fig. 31B), how­ is incontrovertible-the Cambrian and Devonian ever, has applied the term to the kind of intrusions beds wrapped around the laccolith have been raised here called asymmetric laccoliths, but I uSe the term and are in fault contact with the Mississippian Mad­ in its ,customary sense. ison Group. On the south and southeast, however, the Wolf Porphyry is juxtaposed with Madison GRANITE MOUNTAIN BYSMALITH strata. In that area the limestone beds adjacent to Granite Mountain, along the east edge of the the porphyry are unbleached and unbaked, and the quadrangle, consists of a plug of igneous rock encir­ texture of the porphyry in contact with the Madison cled by a high-angle fault-the Granite Mountain A B is as coarse as its texture distant from the contact, fault. I have called this plug the Granite Mountain sug.gesting a fault relation. On the SOuthwest the bysmalith; only its west flank is within the quadran­ relations between the laccolith and adjacent strata gle (fig. 2, and Witkind, 1971). are concealed beneath a pediment deposit, and on the The plug is composed of the extremely fine west the laccolith is intruded by the Hughesville grained rhyolite of Granite Mountain (sample 9 of stock (fig. 6). Possibly the laccolith crystallized at table 1) which is in fault contact with steeply up­ depth and then was punched upward by a younger turned Cambrian and Devonian beds (fig. 23). pluton; if so, it formed much like the trapdoor lac­ For most of its extent the circumferential Granite 'coliths in the Little Rocky Mountains (Knechtel, Mountain fault is concealed, but it is well exposed in 1944). According to this interpretation the south­ a roadcut along the south flank of the mountain eastern edge was raised the most, and the hinge was (N% sec. 7, T. 15 N., R. 10 E.). There, the fault along the west edge (fig. 6, cross section A-A'). .gouge is about 15 feet thick and consists of angular The general size, shape, and extent of such a plu­ fragments of rhyolite and Park Shale firmly held in ton is uncertain. Presumably if a younger pluton did a finely comminuted matrix chiefly of the rhyolite raise this laccolith, the resultant heat should have and of intermixed smaller amounts of shale. The converted the sanidine of the laccolith to orthoclase. fault dips valleyward at angles between 60 ° and 80 °, One possible explanation for the lack of such an but locally it is vertical. Plagioclase feldspar ·' inversion is that this younger pluton may have been In several places dikes of felsic and mafic rocks relatively thin- possibly it was a sill some 600-1,000 follow the fault. As an example, a dike of the por­ "-', ~ . " feet thick. (This thickness is implied by the strati­ phyry of Galena Creek follows the south flank of the graphic displacement along the Mixes Baldy­ bysmalith (east of quadrangle boundary), and Anderson Peak fault; fig. 6, cross section A-A'.) nearby is a dike of minette-kersantite (fig. 23). The ,causative magma could have risen in a nearly How the bysmalith developed is uncertain. One vertical fracture and then spread ,laterally beneath possibility involves the gradual dilation of a laccol­ the laccolithic floor. A second alternative is that this ith until its roof, partly congealed, is punched up­ younger pluton may be part of a much larger subja­ ward by rising magma. Another interpretation is cent body of magma and that it, in fact, did heat the that large segments of other previously emplaced c D 34 IGNEOUS ROCKS AND RELATED MINERAL DEPOSITS OF THE BARKER QUADRANGLE, MONTANA MAFIC ROCKS 23

9000' cO lith is over,lain by Cambrian strata. On the north­ (probably of clinopyroxene) which varies in thick­ 8000'-L_------east it is in sharp linear contact with the Madison ness from mineral to mineral (fig. 10). In general, it 7000' Mm Group, and its edge may conform to a preexisting is thickest around the plagioclase feldspars and thin­ fault. If so, the northeast flank is steep, whereas all nest around the sanidines. 6000 '...L_.-.- .-+-~+ -+-+-'-Tb-a~-­ + ++ + ++-- + ++ other flanks thin laterally and pinch out beneath the The source of the xenocrysts is uncertain; most 5000' + + ... Du + TAYLOR MOUNTAI N LACCOLITH + .. -l- + + +++-+-++ 4 +"-++ + + ++ +4- sedimentary 'cover. apparently are relics of country rocks, inasmuch as 4 000' £u The laccolith is semiconcordant, transgressing to xenoliths of both metamorphic and sedimentary rock younger beds. Thus, the northwest flank of the lac­ have been found in the vogesites. So, some xeno­ A colith underlies the Flathead Sandstone; farther crysts are obviously derived from the pulling apart south the laccolith underlies the Wolsey; and still of the xenoliths; others, however, may represent 12,000' farther south the laccolith has cut across the transferred phenocrysts-relics of blebs of salic 11 ,00 0' Meagher Limestone and Park Shale and underlies magma picked up as fingers of femic magma 10,000' the Pilgrim Limestone. These relations imply that breached and moved up through a salic one (Wit­ kind, 1970, p. C87). 9000' the floor of the laccolith, particularly its north half (Witkind, 1971, cross sections A-A' and D-D') , Mm The vogesite dike that cuts the indurated alluvium 8000' rests on Precambrian crystalIines, whereas its south has been thoroughly weathered to a light tan, and Na,O K, O 7000' half rests on one or more Middle Cambrian units. the included xenocrysts of quartz and feldspar give A 6000 ' The Barker laocolith, formed by the Barker Por­ it a superficial appearance strikingly like that of the Iron 5000' ...... r + + phyry (sample 4 of table 2), is classed as an asym­ porphyry of Galena Creek. Its texture and composi­ (Total iron expres sed metric laccolith chiefly because I' think it was tion (sample 10 of table 4), however, show that it is as FeO) B formed by magma that moved up a preexisting frac­ a lamprophyre.

GRANITE MOUNTAI N ture, possibly one related to the Irene Peak fault. I 13,000' FAULT,,! suspect that the position of the laccolith-radial to CHEMISTRY OF THE MAFIC ROCKS , 12,000' £u the parent stock-was determined by an old high-an­ Compared to the intermediate rocks, the mafic , ' 11,000' + gle fracture which trended about N. 30° W. from the rocks contain less silica, soda, and potash, and more ~,.. " t"< -.j west flank of the Hughesville stock (fig. 6). The iron, lime, and magnesia. A comparison of the rocks 10,000' -+ .j ~ I former trace of the fracture is now followed by the within the mafi,c group indicates that a close relation 10 9000' ++ I d§, .linear northeast flank of the laccolith. Il • -t .,-i I rw~" exists between the vogesites and the minette-kersan­ ~7 . 9 t -t II =>- ~ 1 8~ 6 ()3 ()5 8000' Mm .,. -+ + <. Z to 12 Probably the magma that fed the Barker laccolith tites-both are characterized by high iron, magne­ 0 ;:" + « " 2 ()4 '" , < 1il 7000' ++t++ ":.~\;, rose in the same master conduit (now oocupied by sia, and lime, and low soda and potash (fig. 11). 1- t- ~ T b a 1- ... Du ) .J t' ~ ., TAYLOR MOUNTAIN LACCOLITH+- the Hughesville stock) responsible for the Clenden­ (Sample 10.of table 4, a weathered vog-esite, is disre­ 6000' + t t + 0- ~ + + + + .. ... 0- nin-Peterson and Mixes Baldy-Anderson Peak lac­ garded in this comparison.) c coliths. During its upward progress it was probably Possibly most of these lamprophyres are a result diverted westward into the high-angle fracture that of the differentiation of a parental femic magma MgO Na, O + K, O intersected the conduit. The magma rose in the fault that has the composition of shonkinite. As a result B J: GRANITE MOUNTAIN U 3 until its upward progress was slowed or stopped, of crystal fractionation-mainly settling of the EXPLANATION ~ o ~ Q) E NE then it migrated laterally along the Cambrian-Pre­ heavier olivine and pyroxene-the parental magma SW E u 7800' ro c Mm Barker quadra ngle Stanford-Hobson area Z TAYLOR MOUNTAIN ~ cambrian conta,ct and slowly raised and arched the differentiated into two contrasting magmas: a o Shonkinite I:; Shonkinite 7000' ~ Mm LACCO~TH LL -- beds. With the gradual addition of magma, those lighter fraction represented by the syenite, the pla­ ...... 5200' beds closest to the former fracture, which served as gioclase shonkinite, and some of the pyroxene-poor Iii Plag ioc lase-shonkinite 5 4 00'--L--±...--±.---±----!'--.JL-.!...-~-.!-....!:...~4~--"-'=--'.'--'--.p_ Du a feeder conduit, were raised the most. and hornblende-poor minette-kersantite; and a heav­ () Syeni te D GRANITE MOUNTA I N BYSMALITH ier fraction represented by the pyroxene-rich voges­ () Minette-kersant it.e o 2000 4000 FEET LI---'_.l.I_.l....--..J1 ite. MIXES BALDY- ANDERSON PEAK LACCOLITH • Vogesite • Vogesite This differentiation pattern is reflected by the Southeast of the Hughesville stock is the Mixes chemical analyses; the syenite, plagioclase shonkin- Baldy-Anderson Peak laccolith, an igneous mass al- FIGURE 10.-Xenoliths and xenocrysts in lamprophyres. All FIGURE 19,-Four sections offering one possible explanation B, The rhyolite of Granite Mountain invades the area and photomicrographs by R. B. Taylor and Sandra Brennan. for the structural relations between the Taylor Mountain differentially tilts both the sedimentary rocks and the A, Corona of clinopyroxene ( ?) grains about a rounded laccolith and the Granite Mountain bysmalith. p£r, Pre­ enclosed laccolith, C, With continued uplift, the sedimen­ quartz xenocryst. B, Corona of clinopyroxene ( ?) grains FIGURE ll.-Some chemical characteristics of the mafic rocks. cambrian crystalline metamorphic rocks; £ u, Cambrian tary rocks and the laccolith finally break along part of the about a corroded quartz xenocryst. C, Reaction rim about A, CaO-Na,O-K,O ratios. B, FeO-MgO-Alk ratios (FeO units; Du, Devonian units; Mm, Madison Group; Tba, high-angle circumferential Granite Mountain fault. D, Ero­ plagioclase feldspar xenocryst. D, Xenoliths and xenocrysts = Total iron expressed as FeO; Alk = N a,O + K,O). See Barker Porphyry; Tgm, rhyolite of Granite Mountain. A, sion strips some of the sedimentary cover from the lac­ derived from Precambrian crystalline fragments enclosed table 4 for sample locations, and Vine (1956, p. 454-455) Taylor Mountain laccolith develops as a planoconvex body, colith and the bysmalith. in vogesite. for location' of samples from the Stanford-Hobson area. 24 IGNE·OUS ROCKS AND RELATED MINERAL DEPOSITS OF THE BARKER QUADRANGLE, MONTANA LACCOLITHS 33 ite, and the minette-kersantite-the lighter fraction Little Belt Mountains, either concealed under the of the differentiated magma-contain, in general, thick foliage or unrecognized. The darker margins, more alkalies and less iron, magnesia, and lime than marked by conspicuous large round quartz grains, have been interpreted as ,chilled borders of felsic Granite Mountain bysmalith the vogesite (fig. 11). Taylor Mountain dikes. Their mineralogy and chemistry, however, / COMPOSITE DIKES differ completely from the mineralogy and chemistry of their felsic cores (table 5). Moreover, the field At least two composite dikes-characterized by relations of these bodies indicate that they are com­ mafic margins that ,contain quartz xenocrysts and by posite dikes, the products of coexisting salic and felsic interiors that contain quartz phenocrysts­ femic magmas. have been recognized in the Barker quadrangle, and at least one more is known in the Neihart (Witkind, The two composite dikes recognized in this quad­ 1970). It is highly likely that many more are in the rangle intrude the quartz monzonite of Hughesville. One, called the Maytee dike, is well exposed on an old mine road near the Maytee tunnel of the Block P TABLE 5.-Chemical analyses, in percentage, of the mafic mine (SW% sec. 6, T. 15 N., R. 9 E., unsurveyed) margin, transitional zone, and felsic interior of the May tee (fig. 6). The other, the Annie E dike, has been cut composite dike, secs. 6 and '1, T. 15 N., R . .9 E. (unsur'­ veyed), Judith Basin County, Montana through by the Annie E tunnel of the Liberty mine (fig. 12) (center sec. 7, T. 15 N., R. 9 E., unsur­ [Rapid rock analyses. Analysts: P. L. D. Elmore. Gillison Chloe, J am es Kelsey, S. D. Botts, H . Smith, Lowell Artis, and Jonn Glenn] veyed; see fig, 25 for mine locations),

Mafic Transitional Felsic SIMILARITIES OF CLINOPYROXENES margin zone interior Rock Porphyr y of Kel'santite Kersantite (?) More than half a century ago Pirsson (1905, p. name Galena Creek 38) noted that clinopyroxenes from igneous bodies Granite Mountain bysmalith. View is northeastward across Lone Tree Park from the east flank of Mixes Baldy. Lab No. W-170181 W-171084 W-171083 scattered thro.ugh central Montana are similar re­ Field No. WL-270-c WL-552-a WL-551-a gardless of rock type. A study of clinopyroxenes con­ about 2 miles in diameter and some 700 feet thick. centrated from the diverse rocks exposed in this east end of the fault Madison strata are juxtaposed; Oxides Its original size and shape are unknown. quadrangle has confirmed Pirsson's observation. Py­ near the midpoint Cambrian and Devonian beds are SiO, ______57.5 68.6 75.9 roxenes from felsic rocks such as granite porphyry in fault contact with the Madison; and near the It is composed of a rock similar to the Barker Al,0 3 ______16.0 15.7 13.0 are remarkably like tho'Se from mafic rocks such as southwest end the Madison rests against the Flat­ Porphyry (sample 2 of table 2) but has more clino­ Fe,0 3 ______2.4 1.4 .74 shonkinite in optical and chemical properties and in head Sandstone. pyroxene than other samples of the porphyry. Much FeO ______._ ___ 2.8 .92 .60 the same is true of the porphyry that forms the MgO ______3.1 unit cell dimensions. Both faults, thus, increase differentially in strati­ .88 .29 graphi.c throw toward the stock. This increase is thiockened sill northeast of Irene Peak (sample 3 of CaO ______3.9 1.0 .11 It was concluded that this clOSe similarity of py­ table 2). Na,O ______3.3 2.9 .53 roxenes from widely disparate rock types implies a attributed to the pattern of dilation followed by the K 0 ______4.0 2 4.6 6.3 genetic relationship of the parent magmas. la·ccolith. The magma was probably fed northeast­ The laccolith was probably fed from the parent H,O- ______.22 .69 .51 ward from the parent stock. As the laccolith dilated, stock through the thickened sill. H,O+ ______.78 2.1 1.7 Detailed results of this study have been reported it first raised the overlying sedimentary strata, then ASYMMETRIC LACCOLITHS TiO, ______.89 .77 .15 elsewhere (Witkind, 1969). P ,O. ______.42 .33 .02 arched them until they finally broke to form the two BARKER LACCOLITH MnO ______.16 . 03 .00 faults . Due west of the Hughesville stock is Barker CO. ______3.9 < .05 < .05 THE INTRUSIONS OTTER LACCOLITH Mountain, a circular dissected dome about 3 miles in diameter which rises about 3,000 feet above the floor Sum 99.37 99.97 99.90 The 'configuration of the mountains and valleys The otter laccolith, here tentatively classed as a of Dry Fork Belt Creek (fig. 22). The mountain is reflects the distribution and s hape of the various satellitic laccolith, occupies the center of T. 16 N., R. underlain by the Barker laccolith (Witkind, 1971, C.I.P.W. norms intrusions (Weed, 1900, p. 384-400). Each upwarp, 8 E. (fig. 2). I believe it is closely related to the cross sections A-A', D-D') , Extensive dissection has anticline or dome, is more or less shaped by the thickened sill northeast of Irene Peak some 2 miles 19.33 31.74 48.12 stripped the sedimentary cover from the laccolith's orQ ------______------______23.79 27.18 37.23 subjacent int~usion; the intervening synclines or to the southeast in the NE1,4 sec. 36, T. 16 N., R. 8 ab ______crest and upper flanks, and the remaining sedimen­ 28.10 24.54 4.49 basins are formed of sedimentary strata squeezed E. (Witkind, 1971). Probably both features were an ______tary beds are now preserved chiefly along the lower o 2.49 .10 between adjacent intrusions (fig. 13). C ______once joined to form an elongate thin igneous mass flanks (fig. 6). 6.28 5.04 5.27 The 11 larger mapped plutons have been emplaced which encircled the north flank of Barker Mountain. 4.85 2.19 .72 Inasmuch as the floor is nowhere exposed, only a hy f ~n ------or are exposed at various stratigraphic levels. The The laccolith is ,completely stripped of sedimen­ l s ------2.00 o .24 general estimate can be made as to the size of the mt ______3.50 . 83 1.07 single stock intrudes strata of the Madison Group . tary cover and now appears as a pile of igneous rock ap ______laccolith. It is 'crudely circular, 3-3112 miles in diam­ 1.00 .78 .05 Three, and probably four laccoliths, are emplaced resting on various units of the Big Snowy Group, il ______eter, and possibly as much as 4,000 feet thick. 1.70 1.46 .29 along the Cambrian-Precambrian unconformity and chiefly the Heath Shale. Its floor conforms to the hm ______o .83 o so are roofed by the Flathead Sandstone or younger under,lying strata and slopes gently northward. It is On the north, west, south, and southeast the lac- 32 IGNEOUS ROCKS AND RELATED MINERAL DEPOSITS OF THE BARKER QUADRANGLE, MONTANA THE INTRUSIONS 25

EXPLANATION ~ ~ Q) D Felsic interior '" Quartz monzonite of Hughesville (porphyry of G alena Creek) u 2 Vein 3 t o 4 inches thic k in face ·iii Vein Crest of Wolf Butte o a. Dashed where approximately located E o D () Mafi c m argin (k ersantite)

, - ~~----- Porta~ ------===-= - - /-='-~ nze \" \ v v v Yy \ i:\Vv Vv v ~\: \ N 24 \t ---I 1--- Vein thins abruptly t o about ] · f oot thic kness V e in poorly exposed in roof of drift; pos itio n and where it c uts across dike thic kness uncertain; locally 2 to 3 feet w id e o 20 40 FEET IL _--L.._....II_--'-_---.JI

FIGURE 12.-The Annie E dike intrudes the quartz monzonite of Hughesville, as revealed in the Annie E tunnel of the Liberty mine. Mapped June 26, 1967 hy 1. J. Witkind, assisted by Mrs. Gwenllian Vaughan-Rhys McBride.

FIGURE lS.-Panoramic view of the compound dome underlain by both the Taylor Mountain laccolith and the Cambrian rocks. Two laccoliths are higher in the On the Colorado Plateau, sulfide mineralization is section, possibly resting on Devonian strata and concentrated in and near the stocks ; the laccoliths and possibly as much as 3,500 feet thick near the before the dike passes into the main mass of the roofed by limestone beds of the Madison Group. One are barren. Consequently, the stocks are considered stock (Witkind, 1971, cross sections B-B' and C-C'). laccolith. Southwestward beyond this point the line­ laccolith is in fault contlllct with comparable beds of to be far more significant than the encirding laccol­ iths. In the Barker quadrangle the main sulfide ore The oldest sedimentary unit that is domed by the arity of this flank of the laccolith implies that it the Madison, and another laccolith rests chiefly on developed against a preexisting fault. I interpret the Heath Shale, the uppermost unit of the Big deposits are in and near the HughesviJ,Ie stock. laccolith is the Flathead Sandstone. The laccolith Hence this :stock also is believed to be of special cuts younger beds, possibly because of faults, so that these relations to mean that even as the sedimentary Snowy Group. The lone .bysmalith is in fault contact significance. beds as young as the Jefferson Dolomite rest on its strata were arched and broken by the dilating laccol­ with the Park Shale of Cambrian age. A buried plu­ roof. Commonly, however, the Wolsey Shale directly ith, magma from the laCicolith invaded and obliter­ ton intrudes and domes the Precambrian crystalline overlies the laccolith. In sharp contrast to most of ated part of the fault trace. rocks and likely extends northeastward beyond the STOCK exposed Precambrian rocks (fig. 31). the other llllccoliths, those beds that overlie the crest The stratigraphic throw along the fault increases H UGHESVILLE STOCK and upper flanks of the thick end of the laccolith toward the Hughesville stock. So, at the northeast Near the >c enter of the quadrangle three laccoliths The Hughesville stock is bounded on the north, have been intensely metamorphosed. end of the fault, Madison strata are juxtaposed; and form a radial pattern about the circular stock. This west, and south by variably altered limestone beds Two maj Or faults cut the laccolithic flanks. The near the midpoint of the fault trace, Madison beds stock-laccolith grouping, interpreted as denoting a of the Madison Group and on the east by various northwest flank is broken by the Clendennin fault, are in fault contact with the Jefferson Dolomite. genetic relationship (fig. 13), has been noted else­ intrusions (fig. 6). On the north boundary, the lime­ and the southeast flank by the Peterson fault (fig. where, especially on the Colorado Plateau. There, stone beds directly adjacent to the stock dip :steeply 21). The field relations imply that the faults may P E T E RSON FAULT for example, eruch of the mountain groups that to­ toward the stock; on the west and south, they dip have formed during dilation of the laccolith (Wit­ The Peterson fault (along the southeast flank of gether constitute the Henry Mountains consists of a steeply away from it. On the northeast the exact kind,1965). the anticline, like the Clendennin fault is normal and parent stock and its surrounding tongue-shaped sa­ relations between the stock and the Clendennin-Pe­ dips valleyward (southeastward) at a high angle tellitic laccoliths (Hunt, 1946, 1953). The laccoliths terson laccolith are concealed beneath the aUuvial fill C L E NDENNI N FAU L T (fig. 21). The beds on the northwest side of the fault radiate from the parent stock much like spokes from of Green Creek. Locally, there appears to be an in­ The Clendennin fault is normal It dips valley­ have been raised. The fault can be traced southwest the hub of a wheel. Hunt's illustration showing the trusive contruct between the two, but elsewhere the ward (northwestward) at a high angle-at least 60 ° for about 2112 miles where it ends against the south­ ground plan of the intrus ions around the Mount two are separated by a narrow tight :syncline of locally- and the beds southeast of the fault have east flank of the llllccolith. Here also, the straight­ Ellen stock in the Henry Mountains is reprinted as Madison beds. On the east the stock intrudes the been raised. The fault can be trlllced northeastward ness of the laccolithic flank implies that the south­ figure 14A of this report. A similar pattern between west flank of the Mixes Baldy-Anderson Peak lac­ for about 2 miles from near Lost Creek (fig. 21, and east flank, like the northwest flank, is fault con­ parent stock and satellitic laccoliths was found in colith. Fingers of the stock extend into the laccolith, Witkind, 1971). Originally it may have extended for trolled. If so, this fault too may have been as much the Abajo Mountains by Witkind (1964); and an­ and other dikes and plugs of a latite porphyry simi­ a dist.ance of 4 miles, for southwestward from Lost as 4 miles long. other was reported by C. E. Erdmann (oral lar to that which forms the east margin of the stock Creek a thin dike of the porphyry of Clendennin The strati.,graphic throw, as along the Clendennin commun., 1964) in the Sweetgrass Hills of north­ (p. 16) intrude the laccolith. Mountain follows the fault trace for about 1 mile fault, increases toward the stock. So, at the north- central Montana. The stock, in turn, is intruded by felsic, mafic, and 26 IGNEOUS ROCKS AND RELATED MINERAL DEPOSITS OF THE BARKER QUADRANGLE, MONTANA LACCOLITHS 31

110·45' TAYLOR MOUNTAIN LACCOLITH I believe that the dome is underlain by a planocon­ 47"15' 1_' _---.;;::: _ _ ~-=:::::------__,_~-----~------.--:1~10 ·30' vex laccolith whose general composition is suggested .---~ Taylor and Granite Mountains together form a -...... -2600 __ .compound dome which dominates the east edge of by a plug and sill which intrude the strata a:long the ---- the quadrangle. The dome, astride the junction of north flank. The igneous rock (sample 12 of table 2) four townships (Tps. 15 and 16 N., Rs. 9 and 10 E.), is like the Barker Porphyry. is underlain in its southern half by the Taylor The dome was tested unsuccessfully for oil in 1957 Mountain laccolith and in its northern half by the by the Oien Oil Co. The test, known as the J. W. T. 17 N. Granite Mountain bysmalith (fig. 18). Field rela­ Bodner 1, was collared on the crest of the dome at tions indicate that after the laccolith was emplaced, an altitude of 5,761 feet in the SWl,4 sec. 2, T. 16 N., it was bowed upward and likely broken in two by R. 8 E. It was abandoned as a dry hole in the J effer­ the developing bysmalith (fig. 19). The northern son Dolomite at a total depth of 1,887 feet. Inas­ half of the laccolith has since been eroded away, much as the test did not penetrate any igneous rock, leaving the southern half tilted to the southwest (fig. the laccolith must have formed at greater depth, pos­ 19 D and Witkind, 1971 cross section D-D'). siblyalong the unconformity between the Pre­ cambrian basement .complex and the overlying Mid­ The dome is about 5 miles in diameter and rises dle Cambrian strata (Witkind, 1971 cross section some 2,200 feet above the floor of Lone Tree Park; A-A'). only the western two-thirds is within the Barker quadrangle. Originally the laccolith may have been a somewhat linear body that trended about N. 55° E. I DRY WOLF LACCOLITH estimate it to have been some 4 miles long and about A small part of the north flank of the Dry Wolf 2% miles wide. It probably had a maximum thick­ laccolith is exposed in the southeast corner of the . 16 N. ness of about 1,800 feet. quadrangle; this exposure occupies most of sec. 6, T. The laccolith rests on the Jefferson Dolomite and 14 N., R. 9 E. (fig. 2). Practically all the sedimen­ directly underlies the Madison. In the N1J2 sec. 12, T. tary ,cover has been stripped from the laccolith; and 15 N., R. 9 E., (unsurveyed) a thin sliver of Madi­ the la'ccolithic floor, sloping gently northward, is ex­ son is preserved along the laccolithk ,crest (fig. 19 D posed along the east valley wall of Dry Wolf Creek. and Witkind, 1971 cross section D-D'). The floor truncates limestone beds (which here dip The laccolith is composed of the Barker Porphyry about 10° NE) of the Madison Group. A few lime­ (samples 5 and 6 of table 2) but contains orthoclase stone beds of the Madison Group are still preserved on the laccolith's flank; the implication is that the RANITE MOUNTAIN rather than sanidine as in the other laccoliths. This BYSMALITH laccolith is the only floored intrusion that contains laccolith was emplaced wholly in the Madison orthoclase; possibly it is connected at depth with a Group. RANITE MTN FAULT part of the stock. The la:ccolith is composed of the Barker Porphyry To determine whether any significant variation or (sample 9 of table 2). differentiation occurred as the la:ccolith dilated, one sample was collected from the floor of the laccolith SATELLITIC LACCOLITHS

(sample 5) and another from the roof (sample 6). CLENDENNIN-PETERSON LACCOLITH The rocks are alike mineralogically and chemically, implying that the magma was emplaced rapidly The north flank of the mountains is dominated by enough to preclude any differentiation. the nose of an elongate northeast-trending anticline whose general shape, length, and width are believed

LACCOLITH(?) UNDERLYING THE LIMESTONE BUTTE DOME to result from the emplacement of the Clendennin­ Limestone Butte dome, in the northwest corner of Peterson laccolith (fig. 20). The anticline, which trends northeast from the HughesviJ,le stock (fig. 2), the quadrangle (NE%, T. 16 N., R. 8 E.), is about 4 miles ·in diameter and almost perfectly circular. Its is about 6 miles long and 3 miles wide; it rises top rises smoothly and evenly some 1,000 feet above about 2,500 feet above the adjacent valleys. Parts of the adjacent stream floors. Softer strata have been the laccolith are exposed in various streams; all ex­ stripped, leaving a surface held up by the more du­ posures are believed to be parts of the laccolithic o 2 3 MILES roof. The laccolithic floor is not exposed. ~I __~ __~I __~ I rable limestone beds of the Madison Group. The flanks of the dome are outlined by moderately dip­ On the basis of exposures and the general configu­ FIGURE .I3.-Distribution and shape of the various intrusions as shown by structure contours which are drawn on or ping strata of the Big Snowy Group (Witkind, ration of the anticline, I suggest that the underlying proJected ~o the top of the Madison Group, Shaded areas are exposures of various igneous and metamorphic 1971). lacc~lith is about 5 miles long, about 2 miles wide, rocks. ProJected fault planes are hachured. Contour interval 200 feet; datum is mean sea level. 30 IGNEOUS ROCKS AND RELATED MINERAL DEPOSITS OF THE BARKER QUADRANGLE, MONTANA STOCK 27

Inasmuch as only the roof is exposed, the shape Most of the laccolith underlies the Flathead Sand­ a few composite dikes, all of which follow fradures ther individual joints nor joint zones are traceable and size of the laccolith can only be surmised. Likely stone, but locally it cuts across the sandstone to that break the stock. Locally, veins of Sulfide miner­ for more than a few hundred feet. it is planoconvex. I estimate it to be about 3 miles in younger Cambrian .strata. The youngest rocks de­ als follow the contact between the dikes and the The fracture pattern was not studied in detail, but diameter and a maximum of about 3,000 feet thick. formed by the laccolith belong to the Big Snowy stock (p. 45). None of the dikes and veins extend the attitudes of many conspicuous joints were meas­ The laccolith and its related sills are 'composed of Group, and remnants of these are preserved in a beyond the margins of the stock. ured. The poles of perpendiculars to these joints the Barker Porphyry (samples 7 and 8 of table 2). triangular trough north of Butcherknife Mountain. The stock is intensely jointed. Many of the joints have been plotted and then contoured on an equal­ are isolated curved fractures which are either verti­ area stereo.graphk net (fig. 15). This plot shows that calor steeply dipping. Others, however, are very all the joints are very steep with the great majority closely spaced and form zones 2-6 inches wide. All being vertical or nearly so. Two conjugate fracture are stained by limonite, and locally the adjacent rock systems are postulated. One system consists of a is sericitzed (p. 49). Because of thick foliage, nei- joint set that trends -about N. 60 ° E. and that is vertical; at almost right angles to thi.s set is another set that trends N. 40° W., and dips about 80 ° SW. The second fracture system, locally the more domi­ EXPLANATION Butler Wash nant of the two, consists of a set of almost vertical laccolith joints, which trend in a general northerly direction

Granite , (ranging from N. 15° E. to N. 15° W.), and a second Ri dges. '.. lac set, at right angles to the other set, which trends ;:; easterly. These two fracture systems seem to be Z y dominant in the stock; most of the other joints ap­ pear to be diversely oriented. A B LACCOLITHS

Copper Ridge Most of the large intrusions in the Barker quad­ laccolith rangle are here classed as laccoliths, although only three are demonstrably floored. It ~ould be argued, therefore, much as Goddard (1950) has done for comparable intrusions in the nearby Judith Moun-

N

o 2 Miles I I

A

c D

FIGURE 17.-Four ways in which an asymmetric laccolith. Here also the fault does not extend w E laccolith might develop. In the first three (A, below laccolithic fioor. From Cross (1894, fig. B, and C), the magma (arrow) moves generally 30). C, Magma rises in an inclined conduit toward the fracture; in the fourth (D), it which crosscuts host rocks. The greatest thick­ moves away from the fracture. A, Magma in­ ness of the laccolith develops opposite the truded along a near-horizontal bedding plane mouth of the inclined conduit owing to the moves laterally to form first a sheet and then intrusive force of the magma. Fault does not a tonguelike satellitic laccolith. As the distal extend below laccolithic fioor. From Howe edge of the laccolith dilates, the overlying (1901, p. 299). D, Magma rises in a preexisting B strata are raised, folded back, and finally fracture. Where the upward progress of the broken. Fault does not extend below laccolithic magma is halted-possibly because the fracture FIGURE 14.-Tonguelike satellitic laccoliths. A, "Ground fioor. From Hunt (1956, fig. 31B). B, Magma closes-the magma moves laterally along a plan of intrusions around the Mount Ellen stock in Henry rises in a nearly vertical conduit which cross­ bedding plane. Dilation of the laccolith causes Mountains, illustrating radial distribution of the lacco­ s cuts host rocks. When upward progress of the overlying beds to break; consequently these liths." From Hunt (1956, fi g. 30). B, "The laccoliths * * '" magma is halted, it moves laterally to form are offset more than those beneath the lac­ are tongue-shaped and are bulged linearly and radially FIGURE 15.-Joints in the Hughesville stock. Lower hemi­ first a sheet and subsequently an asymmetric colith. from the litocks." From Hunt (1956, fig. 31A) . sphere plot; 127 poles. Contoured in percent. LACCOLITHS 28 IGNEOUS ROCKS AND RELATED MINERAL DEPOSITS OF THE BARKER QUADRANGLE, MONTANA 29 tains, that those masses without visible floors are In the Barker quadrangle this stock-laccolith rela­ not laccoliths but rather are deep-seated plugs. But I tion is suggested by the three intrusions that seem to prefer to think of them as laccoliths, as did Weed radiate from the HughesviUe stock (figs. 2 and 13) : (1900), because they are similar in both structure the Barker laccolith, the Clendennin-Peterson laccol­ and petrography to the floored bodies, and because ith, and the Mixes Baldy-Anderson Peak laccolith. the overall character of this intrusive complex is so Of the three intrusions, however, only the Clenden­ much like that of demonstrable laccolithic complexes nin-Peterson laccolith appears, on the basis of its elsewhere, such as in the Henry Mountains (Hunt, deformed sedimentary cover, to have the requisite 1953) and the Abajo Mountains (Witkind, 1964) in tonguelike shape of a satellitic laccolith. Utah. Moreover, the alkalic feldspar in all but one of the ASYMMETRIC LACCOLITHS laccoliths is marked by low to moderate optic angles Asymmetric laccoliths resemble half a planocon­ A B (._30° to _50°) that imply sanidine, rather than by vex (mushroomlike) laccolith; one flank is blunt and high optic angles (75°) that imply orthoclase. The the other flanks taper to thin edges (fig. 16E, F). presence of orthoclase suggests slow cooling and These igneous bodies have been given various gradual {!rystallization, whereas the occurrence of names: Hunt (1946, 1953, and 1956, fig. 31B) re­ sanidine implies high temperatures quickly ferred to them as "bysmaliths"; Pratt and Jones quenched. Conditions favoring sanidine would pre­ (1961, p. C164) called them "trap-door laccoliths," vail during the formation of a laccolith. Being iso­ and Cross (1894, p. 237), Weed (1899c, p. 25), Pirs­ lated 80mewhat from the main heat source, laccol­ son (1898, p. 581) and Harker (1909, p. 66-67), D ~~--~~=---==---=____ ~~~~~ '~"~"'~" ~ iths should crystallize before their sanidine could among others, called them "asymmetric laccoliths." \ I I t;---=-~-- ::::-----.. . transform to orthoclase. Stocks, on the other hand, This latter name is used here. ------are connected at depth with a continuous source of How they formed is uncertain; there are several ~ I .' ~=--==------===--j heat, and the transformation is likely to occur as it \ possibilities. One explanation, advanced by Hunt .. did in the Hughesville stock. -- d (1956, fig. 31B), involves the di,lation of the distal \ \ \ The previous work done on laccolithic complexes end of a tonguelike laccolith (fig. 17 A). A second D has demonstrated that laccoliths assume at least explanation, offered by Cross (1894, p. 237) and by c three different shapes: (1) a planoconvex (mush­ others involves either a vertical or an inclined feeder roomlike) type (fig. 16A, B), (2) a tonguelike satel­ beneath the laccolithic floor (fig. 17B, C). I would litioc type atta1ched to a parent stock (C, D), and (3) add a third alternative: The laccolith developed ad­ an asymmetric (half a mushroom type (E, F). It j acent to and along a preexisting fracture which seems likely that all three types are in this quadran­ served as a channel for the ascending magma (fig. 17 gle. D). PLANOCONVEX LACCOLITHS Of the laccoliths in the Barker quadrangle, only Gilbert's (1880, p. 19) original work in the Henry the Barker laccolith is asymmetric. Weed (1900, p. Mountains gave rise to the classic concept of a lac­ 355, fig. 43) considered the north flank to be the colith-a lenticular planoconvex (mushroomlike) blunt end, whereas I think the east flank near the body presumably fed by a nearly vertical dike or stock is the blunt side (fig. 6, cross section A-A', pipe (fig. 16A, B) . and fig. 13). This type of la'0colith probably underlies Taylor F DESCRIPTIONS OF LACCOliTHS E and Butcherknife Mountains and possibly the Lime­ EXPLANATION stone Butte dome (fig. 13). PLANOCONVEX LACCOLITHS u D -.-+-- Strike and direction BUTCHERKNIFE MOUNTAIN LACCOLITH F ault Syncline TONGUE-SHAPED SATELLITiC LACCOLITHS of dip of beds U, 1tpthrown "ide ; D. Showing trou ghline and These include those linear laccoliths (Hunt, 1946, An intensely dissected dome known as Butcher­ downth?'own side direction of plunge 1953) that are attached to a parent stock. They are knife Mountain occupies the southeast corner of the FIGURE 16.-Three types of laccoliths. Dark arrows 70) . D, Diagrammatic map of satellitic laccolith tongue..:shaped masses, thick where they are attached quadrangle (fig. 2). The dome, astride Tps. 14 and and dashed lines indicate direction and general a ssuming erosion has reached line D-D' on section to the stock and thin at their distal ends (fig. 16C, 15 N., R. 9 E., is almost 41/2 miles across and rises sequence of magma emplacement. A, Cross section C. Line C-C' represents plane of cross section D, and fig. 14B). As the individuallaocoliths may be about 2,200 feet above the floor of Dry Wolf Creek. of the simple planoconvex (mushroomlike) lac­ shown in C. E, Cross section of an asymmetric laccolith developed adjacent to and along a pre­ It colith (from Gilbert, 1880, fig. 8). B, Diagram­ emplaced in different stratigraphic units, some lac­ is underlain by the ButoCherknife Mountain laccol­ existing fracture. F, Diagrammatic map of' asym­ ith, from which much of the sedimentary cover has matic map of the planoconvex laccolith assuming coliths can be either wholly or in part 'above or be­ erosion has reached line B-B' on section A. Line metric laccolith assuming erosion has reached line neath others. been stripped. A-A' represents plane of cross section shown in F - F' on section E. Line E-E' represents plane of A. C, Cross section of tonguelike satellitic laccolith cross section shown in E. attached to the parent stock (from Hunt, 1953, fig.