Geologic Map of the Maysville Quadrangle, Chaffee County, Colorado

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Mount Princeton pluton. Age determinations on the Mount Princeton pluton and a potentially correlative volcanic unit, the Wall Mountain Tuff, were compiled by Shannon (1988). The average of six K-Ar ages, five fission-track ages, and one Pb/Th age on the Mount Princeton pluton is 36.6 Ma. The Pb/Th zircon age determination of 36.6 +/- 0.4 Ma (Ed DeWitt, 1987, written communication; Fridrich and others, 1998) may be the most reliable indication of the age of the Mount Princeton pluton because other age determinations may be partially reset by younger magmatism. The average of six K-Ar ages and three fission-track ages for the Wall Mountain Tuff is 36.6 Ma, the same as the indicated age of the Mount Princeton pluton. McIntosh and Chapin (2004) presented new high precision 40 Ar/ 39 Ar age determinations for the Wall Mountain Tuff and the Mount Princeton pluton. The mean of five ages for the Wall Mountain Tuff is 36.69 +/- 0.09 Ma. New age determinations for the Mount Princeton pluton mostly record thermally reset ages associated with the Mount Aetna cauldron magmatic event (Chapin, 2003, personal communication). Tmpp Mount Pomeroy subunit (late Eocene) – The Mount Pomeroy subunit is present in a 400 to 1,000 ft long, 300 to 400 ft wide zone in the northwest corner of the Maysville quadrangle. It is separated by the outer ring dike of Mount Aetna quartz monzonite porphyry (Tma) from a small area of Mount Princeton finer-grained quartz monzonite subunit (Tmpf) in the very northwest corner of the quadrangle. The Mount Pomeroy subunit is a medium-dark-gray to slightly purplish-gray, medium to coarse-grained rock (fig. 24). It is distinctly darker in color than all the other Mount Princeton pluton border and interior subunits. The rock is characterized by a slight porphyritic to seriate texture with abundant 3 to 10 mm tabular plagioclase grains, some 3 to 7 mm alkali feldspar grains and 2 to 4 mm hornblende-biotite grains. The plagioclase grains have a distinct medium-gray to purplish-gray color. The matrix is finer grained, about 0.5 to 2.0 mm and practically merges with the smaller sizes of the larger crystal population. It is distinctly more quartz- and alkali feldspar-rich (felsic) than the larger crystal population. Modal analyses of the various Mount Princeton pluton subunits were compiled by 98
Shannon (1988). The average of seven analyses of the Mount Pomeroy subunit shows 20.9 percent quartz, 32.4 percent alkali feldspar, 34.9 percent plagioclase, 10.8 percent hornblende-biotite, and 0.8 percent magnetite. It has slightly less quartz and more alkali feldspar in comparison to other Mount Princeton subunits. The Mount Pomeroy subunit is classified as granite b (IUGS classification) on the basis of modal mineralogy. One thin section of Mount Pomeroy subunit from the Maysville quadrangle shows predominantly biotite with lesser hornblende as the mafic minerals and abundant accessory magnetite and sphene, and minor to trace allanite, apatite, and zircon. Plagioclase is andesine with about An 37 content (Michel-Levy method, 20 grains). Minor clinopyroxene, mantled by hornblende was noted in one sample of the Mount Pomeroy subunit (Shannon, 1988). Figure 24. Billets of Mount Pomeroy subunit (Tmpp, left) in comparison to Tertiary quartz monzodiorite (Tqm) medium grained (middle) and fine grained (right) dike. Stained for alkali feldspar. 99
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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
Page 47 and 48: stream-channel deposits of all pere
Page 49 and 50: Figure 10. View west up South Arkan
Page 51 and 52: Figure 11. View west up South Arkan
Page 53 and 54: valley near the eastern map boundar
Page 55 and 56: lithologies are variable and depend
Page 57 and 58: Arkansas River in parts of Redman C
Page 59 and 60: angular to subangular. Deposits gen
Page 61 and 62: elations, age constraints from foss
Page 63 and 64: 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: the margins of the pluton, includin
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 and 116: near the southwest margin of the qu
Page 117 and 118: eastern contact of the diorite intr
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
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of the Proterozoic metamorphic rock
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that the present structural configu
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elated to rotation of domain blocks
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sometimes terminate at a cross stru
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limestone that is exposed in a wind
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quadrangle are intimately associate
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of the quadrangle. The north-northw
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quartz monzonite ring dikes (Tma).
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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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