Geologic Map of the Lemhi Pass Quadrangle, Lemhi County, Idaho
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IDAHO GEOLOGICAL SURVEY IDAHOGEOLOGY.ORG IGS DIGITAL WEB MAP 183 MONTANA BUREAU OF MINES AND GEOLOGY MBMG.MTECH.EDU MBMG OPEN FILE 701 Tuff, undivided (Eocene)—Mixed unit of tuffs interbedded with, and directly Jahnke Lake member, Apple Creek Formation (Mesoproterozoic)—Feldspathic this Cambrian intrusive event introduced the magnetite and REEs. CORRELATION OF MAP UNITS Tt Yajl below, the mafic lava flow unit (Tlm). Equivalent to rhyolitic tuff beds (Tct) quartzite, minor siltite, and argillite. Poorly exposed and only in the Whole-rock geochemical analysis of the syenite indicates enrichment in Y of Staatz (1972), who noted characteristic recessive weathering and a wide northeast corner of the map. Description based mostly on that of Yqpi on (54-89 ppm), Nb (192-218 ppm), Nd (90-119 ppm), La (131-180 ppm), and GEOLOGIC MAP OF THE LEMHI PASS QUADRANGLE, LEMHI COUNTY, IDAHO, AND Artificial Alluvial Deposits Mass-Movement Glacial Deposits and range of crystal, lithic, and vitric fragment (pumice) proportions. Tuffs lower the Kitty Creek map (Lewis and others, 2009) to the north. Fine- to Ce (234-325 ppm) relative to other intrusive rocks in the region (data from Deposits Deposits Periglacial Deposits in the sequence commonly contain conspicuous biotite phenocrysts. Most medium-grained, moderately well-sorted feldspathic quartzite. Typically Gillerman, 2008). Mineralization ages are complex, but some are clearly or all are probably rhyolitic. A quartzite-bearing tuff interbedded with mafic flat-laminated m to dm beds with little siltite or argillite. Plagioclase content m Paleozoic (Gillerman, 2008; Gillerman and others, 2008, 2010, and 2013) Qaf Qas Qgty Qalc Holocene lava flows north of Trail Creek yielded a U-Pb zircon age of 47.0 ± 0.2 Ma (12-28 percent) is greater than or sub-equal to potassium feldspar (5-16 as summarized below. Dating of Nd-enriched monazite and thorite from BEAVERHEAD COUNTY, MONTANA Qtg (LP-06; Table 3). Thickness as much as 60 m (200 ft). percent) in nine quartzite samples. Four others contained 19 to 35 percent the Lucky Horseshoe mine by electron microprobe analysis returned Qgt plagioclase but lacked potassium feldspar. One sample from immediately Carboniferous ages (300-350 Ma) for mineralization, much younger than Upper conglomerate (Eocene)—Poorly sorted, poorly indurated conglomerate Tcg3 east of the northeast part of the map contained about 15 percent the syenite-lamprophyre suite and not matching known intrusive events. Qls Qms north of Agency Creek and southeast of Lemhi Pass. Contains well-rounded plagioclase and no potassium feldspar, and one from the map itself Qtg QUATERNARY Zircons from the Lucky Horseshoe ultramafic to mafic sill yielded ages from 1 2 1 2 pebbles, cobbles, and boulders as large as 1.5 m in diameter. Most are contained about 20 percent plagioclase and 5 percent potassium feldspar. Neoproterozoic to Late Carboniferous. Neoproterozoic and Carboniferous Russell F. Burmester , Jesse Mosolf , Loudon R. Stanford , Qafo Qgto Qgoo Pleistocene quartzite, but includes distinctive cobbles and boulders of coarse-grained Top and bottom of unit not exposed, but thickness at least 900 m (3000 ft). cores appear to have younger Carboniferous overgrowths, but four zircon Reed S. Lewis1, Kurt L. Othberg1, and Jeffrey D. Lonn2 Mesoproterozoic biotite granite exposed 40 km (25 mi) to the northwest Correlated with an upper part of the Jahnke Lake member, Apple Creek grains yielded concordant results with dates between 317.9 and 315.1 Ma. ? ? (Burmester and others, 2016a). Rare garnet amphibolite cobbles also noted Formation, on the Goldstone Pass map (Lonn and others, 2009; Burmester ? north of Agency Creek. Boulders also form lag deposits west of the actual and others, 2016a) to the northwest. 2018 unit exposures. Equivalent to granite clast conglomerate (Tcg2) of CENOZOIC Blankenau (1999), but mapped higher in the stratigraphic section in the Ysyc Siltite of Yearian Creek (Mesoproterozoic)—Thinly bedded siltite and argillite, REFERENCES B Pliocene A' area north of Agency Creek. Blankenau (1999), Janecke and others (2000), and white-weathering, gray and green quartzite. Characterized by undulat- T B 26 F 45 ª and Chetel and others (2011) postulated that an Eocene paleoriver flowed ing, graded, uncracked couplets and rare couples of medium-green siltite 15 20 35 Y 55 30 WEST STRAND Anderson, A.L., 1958, Uranium, thorium, columbium, and rare earth deposits 45 through the Salmon basin and transported the granite clasts across the and light-green to gray argillite. Mud cracked bedding surfaces less common G B B Sedimentary 25 B BLOODY DICK 22 T 25 B B in the Salmon region, Lemhi County, Idaho: Idaho Bureau of Mines and 37 36 Qas ª Rocks than angular light-colored mud chips. Mud chips commonly in bases of beds 12 B Beaverhead Range at Lemhi Pass. As thick as 150 m (500 ft), but absent in 25 B 15 CREEK FAULT B T œ Geology Pamphlet 115, 81 p. A B T ¬ the southeast part of the map. along with well-rounded, fine to spherical medium quartz grains in dark 35 27 B 8 Yg? F Y B Qms Anderson, A.L., 1961a, Geology and mineral resources of the Lemhi quad- Yg? 25 A B Yajl? Oligocene green channels a few cm wide and as thick as 1 cm. Similar grains comprise 25 59 ª Intrusive Volcanic T T A ¶ Yg? Tcg Tsb Tqs Older quartz-sanidine welded tuff (Eocene)—Tan-orange to light-pink welded rangle, Lemhi County, Idaho: Idaho Bureau of Mines and Geology 45 ˜ 55 30 Yg? 4 T T B Rocks Rocks 1 quartzite beds a few cm to a few dm thick. Green to white, very fine- to B ˜ B µ tuff present north of Agency Creek and southeast of Lemhi Pass. Contains Pamphlet 124, 111 p. T Y fine-grained feldspathic quartzite in beds 1 to 3 dm thick is less common than B 20 B Qms B 10 25 ª TERTIARY sparse, euhedral to subhedral crystals of sanidine and quartz. Termed vitric Anderson, A.L., 1961b, Thorium mineralization in the Lemhi Pass area, Lemhi B 25 A 13 Tcr in the Agency Creek quadrangle to the west (Burmester and others, 2018). Six 20 T 20 ¬ B County, Idaho: Economic Geology, v. 56, no. 1, p. 177-197. B T Y T 50 tuff of Lemhi Pass by Staatz (1972, 1979), who estimated a thickness of F ª quartzite samples in the Agency Creek quadrangle and one east of Bear 20 45 20 21 Tlm 115-200 m (380-650 ft). Our estimate of maximum thickness is 60-90 m Blankenau, J.J., 1999, Cenozoic structural and stratigraphic evolution of the Q 25 T Creek had plagioclase exclusively or in excess of potassium feldspar, and C 20 B 45 Tt 48 T southeastern Salmon basin, east-central Idaho: Utah State University M.S. B B ˜ A 40 ª (200-300 ft). Equivalent to Tqs1 of Blankenau (1999). M’Gonigle and 15 15 T Tqs1? most had greater than 90 percent quartz. Lithologic correlation with the B B ˜A ˜ Td Tdu B 0 50 Eocene T 37 T 36 T 45 B Tcg3 40 39 thesis, 143 p., 3 plates. T A ˜ Dalrymple (1996) reported an Ar/ Ar weighted mean age of 47.95 ± 0.12 Lawson Creek Formation favored because of spherical medium quartz grains, Lucky 456 T 43 ª 5 B A T ˜ Burmester, R.F., Lonn, J.D., Lewis, R.S., and McFaddan, M.D., 2013, Toward a Horseshoe 6 46 T B Ma for this unit near Lemhi Pass. 25 ˜ 40 Tqs1 mud cracks, and chips. However, nowhere else have we seen such a thick B 40 25 µ 60 20 25 ˜ Qas ª Tqs Mine T T 18 40 Qls 1? grand unified theory for stratigraphy of the Lemhi subbasin of the Belt 6 Q T B ª section of siltite and argillite couplets in the Lawson Creek. An alternative is A Q 5 20 32 Tqt? B 56 B T ¹ ² 35 B Tcg Tcg Middle conglomerate (Eocene)—Poorly sorted, matrix-supported conglomer- Supergroup: Northwest Geology, v. 42, p. 1-19. QtgQ T T A 32 2 2 34 2 T TB B the coarse siltite member of the Apple Creek Formation, which is a thick unit 19 20 Qtg? Tqt? ate. Poorly consolidated and not exposed in outcrop north of Agency Creek A Burmester, R.F., Lewis, R.S., Othberg, K.L., Stanford, L.R., Lonn, J.D., and T B T B 34 B Tdu ª Tqt in the Salmon River Mountains but lacks large spherical grains. Thickness ¹ 6 38 T 29 where clasts consist of quartzite pebbles, cobbles, and boulders as large as McFaddan, M.D., 2016a, Geologic map of the western part of the Salmon Qls T B B' uncertain for lack of mapped lower and upper contacts, and deformation. ª T T Y 1 m in diameter. Silicified and finer grained in thin exposure immediately 30 x 60 minute quadrangle, Idaho and Montana: Idaho Geological Survey T B ¬ Qafo T 25 28 16 Tcg1 Upper contact appears gradational into quartzite of Grizzly Hill in the Goat Tlm ¹ T T 20 Yg? south of Lemhi Pass where subangular to subrounded clasts are quartzite Y Þ Geologic Map 52, scale 1:75,000. B 25 ¬ Tqt? 16 ˜ Mountain quadrangle to the south (unpublished mapping). There the contact ¹ Y 25 Qas B 20 B T ª ¬ 25 T ˜ Tcg3 T Þ and siltite; small feldspar clasts, originally volcanic-derived phenocrysts(?), Burmester, R.F., Lonn, J.D., Lewis, R.S., and McFaddan, M.D., 2016b, Stratigra- B ˜ T˜ B Tqt? is placed above the highest interval that contains recognizable siltite and ¹ Tlm T T T T 22B sy T T T T 30 43 Þ Þ mi CAMBRIAN are present in the matrix at this locality.