Geologic Map of the Maysville Quadrangle, Chaffee County, Colorado

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clear (possibly for alkali feldspar). Kouther (1969) mentioned the presence of beryl and trace scheelite in pegmatites in Proterozoic rocks south of the Maysville quadrangle. Smaller pegmatites are abundant throughout the Proterozoic terrane and over large areas it makes up the predominant rock float. Many of these pegmatites have been prospected and pegmatite material is ubiquitous on most of the waste dumps of the significant mines in the quadrangle. A northeast-trending pegmatite with small prospect pits, about 4,000 ft southwest of Maysville, contains minor epidote group minerals including both zoisite and thulite (mangenian zoisite) and possibly rhodochrosite (MnCO 3 ). Some pegmatite material on mine and prospect dumps contains disseminated chalcopyrite and secondary copper oxides and carbonates. ROUTT PLUTONIC SUITE- MISCELLANEOUS INTRUSIONS (Early to Middle Proterozoic) – This group of miscellaneous early Proterozoic intrusions includes diorite (Xd), gneissic granite and pegmatite (Xgp), and augite microdiorite (Xmd) with uncertain age relations to Denny Creek equivalent intrusions and/or to Berthoud-type intrusions. Xd Diorite (Early Proterozoic) – One roughly 750-ft diameter intrusion of diorite is present cutting Early Proterozoic gneiss about 9,000 feet southwest of Maysville. The diorite is dark gray, medium-grained (0.5 to 2.0 mm), equigranular and composed of black hornblende (about 55 percent) and white plagioclase (about 45 percent). At most, 1 to 2 percent of very fine-grained interstitial quartz is present and the rock lacks alkali feldspar, biotite, and magnetite. Thus, the rock is classified as a diorite on the basis of estimated modes (IUGS classification). The rock has a diffuse texture lacking wellformed plagioclase and hornblende crystals, possibly due to a weak metamorphic recrystallization. The diorite intrusion occurs in amphibolite gneiss (Xag) and calc-silicate gneiss (Xcs) but no contacts are exposed. Minor concentrated pegmatite (YXp) float suggests that the diorite is cut by Early to Middle Proterozoic Berthoud-type pegmatite dikes (YXp). A concentrated float zone of Tertiary () andesite dike (Ta) is present along the 116
eastern contact of the diorite intrusion but cross-cutting relations could not be established. Neither chilling of the diorite or development of foliations near the float contacts is obvious. One outcrop near the margin of the intrusion shows minor mineralogical banding of plagioclase and hornblende that may be weak localized gneissic structure. This diorite from the Maysville quadrangle is similar to the interior parts of Early Proterozoic quartz-diorite intrusions in the Buena Vista West (McCalpin and Shannon, 2005) and Buena Vista East quadrangles (Keller and others, 2004). The quartz diorite intrusions in these two quadrangles are mineralogically and texturally variable and have finer-grained textures and local development of foliation and gneissic structures near contacts. Xgp Gneissic granite and pegmatite (Early Proterozoic) – This unit consists of abundant sill intrusions of fine-grained granite with pegmatite that are spatially associated with a sequence of hornblende intermediate gneiss (Xhig). The two units are intimately interlayered (shown on plate 1 as a Xgp/Xhig fractional unit) and form a large 15,000-ft long and 2,000- to 3,000-ft wide crescent-shaped zone crossing the North Fork in the northwest part of the Maysville quadrangle. The Xgp/Xhig unit is about 1.5 percent of the surface area of the quadrangle. The granite and pegmatite concordant sills (about 60 to 75 percent) are interlayered with septum and screens of hornblende intermediate gneiss (about 25 to 40 percent). The hornblende intermediate gneiss (Xig) is described separately. The granite is white and ranges from very fine-grained to fine-grained aplite. It is composed of quartz, alkali feldspar, and plagioclase with minor (less than one percent) hornblende and/or biotite and trace fine disseminated magnetite. Slight variations in the distribution of hornblende and biotite locally produce a faint gneissic layering, and preferred alignment of mafic minerals creates a weak foliation. Pegmatites are white to slightly pinkish-white, simple, unzoned bodies consisting of quartz, alkali feldspar, plagioclase, biotite, and/or muscovite and local magnetite and red garnet. The pegmatites lack exotic minerals and evidence of prospects. The granite and pegmatite sills form linear, resistant-rib outcrops and concentrated block float while the septum of hornblende intermediate gneiss is very 117
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OPEN-FILE REPORT 06-10 Geologic Map
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TABLE OF CONTENTS Introduction…
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mylonitic, augen textures………
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INTRODUCTION LOCATION AND ACCESS Th
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SAWATCH RANGE RIFT-SHOULDER UPLIFT
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of the South Arkansas River (subseq
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EXPLANATION Rio Grande Rift Neogene
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in descriptions of the geologic uni
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During the Early and Middle Paleozo
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Mountains, and White River uplifts
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field are the largest remnants of t
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Figure 5. Detailed geologic-structu
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with the Colorado mineral belt (Boo
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Shannon, 2005). Widmann and others
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the Upper Arkansas graben (fig. 5)
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15-minute quadrangle, which is adja
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etween the northern and southern se
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on U.S. Forest Service color aerial
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We use fractional map units on the
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Qpt Pinedale till, undivided (late
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weathered, but Mount Princeton quar
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stratified, matrix-supported, bould
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as rockfalls, rock avalanches, rock
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stream-channel deposits of all pere
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Figure 10. View west up South Arkan
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Figure 11. View west up South Arkan
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valley near the eastern map boundar
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lithologies are variable and depend
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Arkansas River in parts of Redman C
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angular to subangular. Deposits gen
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elations, age constraints from foss
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elated to the inferred Squaw Creek
Page 65 and 66: Figure 14. Crude bedding in coarse
Page 67 and 68: microcline, and muscovite. The latt
Page 69 and 70: Volcanic Ash Figure 17. View of eas
Page 71 and 72: Figure 18. Close-up view of intense
Page 73 and 74: The western zone of brecciated Pale
Page 75 and 76: Tr Rhyolite dikes (early to late Ol
Page 77 and 78: as the overall N47°E trend of the
Page 79 and 80: One age determination on the rhyoli
Page 81 and 82: northwest-trending fault. Thus, fie
Page 83 and 84: on the North Fork leucogranite intr
Page 85 and 86: A small 400 by 400 foot body of Nor
Page 87 and 88: N12°E, 62°NW (about 6,000 ft sout
Page 89 and 90: The age of the quartz latite porphy
Page 91 and 92: Tma Mount Aetna quartz monzonite po
Page 93 and 94: of the Maysville quadrangle and the
Page 95 and 96: Mount Aetna ring dikes (Tma), one i
Page 97 and 98: the margins of the pluton, includin
Page 99 and 100: Shannon (1988). The average of seve
Page 101 and 102: indicating a granite b composition
Page 103 and 104: analysis by Crawford (1913), two an
Page 105 and 106: locally tanish to pinkish gray. It
Page 107 and 108: quadrangle (tables 1 and 3) were fr
Page 109 and 110: slices of Paleozoic sedimentary roc
Page 111 and 112: south-southwest of Maysville is a 9
Page 113 and 114: There is one additional Paleozoic s
Page 115: near the southwest margin of the qu
Page 119 and 120: endoskarn zones. The two widely sep
Page 121 and 122: Xhig) along the southeastern contac
Page 123 and 124: Xgd Granodiorite (Early Proterozoic
Page 125 and 126: Hybrid granodiorite is locally deve
Page 127 and 128: Xcs Calc-silicate gneiss (Early Pro
Page 129 and 130: Many of the calc-silicate gneiss zo
Page 131 and 132: typically expressed by intermittent
Page 133 and 134: The field relations and characteris
Page 135 and 136: the north side of all three segment
Page 137 and 138: eplaced by muscovite. The western m
Page 139 and 140: sillimanite-quartz-muscovite that a
Page 141 and 142: sillimanite gneiss (Xmsg) and the m
Page 143 and 144: Figure 35. Outcrop of Proterozoic h
Page 145 and 146: samples) indicates they are compose
Page 147 and 148: Xal Lineated amphibolite (Early Pro
Page 149 and 150: of the Proterozoic metamorphic rock
Page 151 and 152: that the present structural configu
Page 153 and 154: elated to rotation of domain blocks
Page 155 and 156: sometimes terminate at a cross stru
Page 157 and 158: limestone that is exposed in a wind
Page 159 and 160: quadrangle are intimately associate
Page 161 and 162: of the quadrangle. The north-northw
Page 163 and 164: quartz monzonite ring dikes (Tma).
Page 165 and 166: that truncates the Upper Arkansas g
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as at the mouth of Squaw Creek. How
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adjacent to the southwestern edge o
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graben and the Upper Arkansas Valle
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GEOLOGIC HAZARDS Potential geologic
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In addition, older landslides may h
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sparse vegetation. One such area is
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Table 7. Historical (pre-instrument
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4262272N. ECONOMIC GEOLOGY Currentl
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A number of the larger pegmatite bo
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River placers near Buena Vista (Van
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The small mines and prospects along
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elatively recent (post-1970) explor
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contact of two Quaternary gravels (
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quadrangle. The first area is a clu
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y Sharp (1976) to be a fault slice
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quadrant of the Maysville quadrangl
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the large Paleozoic rock outlier) i
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WATER RESOURCES Water resources on
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water near Maysville is being produ
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to Rudy C. Epis: Colorado School of
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University, M.Sc. thesis, 50 p. Din
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Hutchinson, R.M., and Hedge, C.E.,
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Litsey, L.R., 1958, Stratigraphy an
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County, Colorado, in Berg, R.R., an
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Stark, J.T., and Barnes, F.F., 1935
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Tweto, O., 1987, Rock units of the
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quadrangle, Colorado: U.S. Geologic
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94A 391996 4263422 Prospect YXp;Xbf
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336A 396189 4274961 Prospect Xgdf B
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854 391849 4268852 42 Adit; Geochem
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Geologic Map of the Maysville Quadrangle, Chaffee County, Colorado

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