Geologic Map of the Maysville Quadrangle, Chaffee County, Colorado

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Trp Rhyolite porphyry dikes (early to late Oligocene) – At least four rhyolite porphyry dikes are present in the northwest quadrant of the Maysville quadrangle (plate 1). They cut Early Proterozoic gneisses (Xhig, Xag and Xgp) and granodiorite (Xgdf). The rhyolite porphyry dikes range from whitish to light pinkish gray and are generally massive. They have porphyro-aphanitic texture with variable, moderate to high phenocryst contents (30 to 45 percent). Quartz phenocrysts (12 to 15 percent) are generally euhedral (1 to 3 mm) and commonly have a slight smoky color. Alkali-feldspar phenocrysts (7 to 12 percent) are subhedral to euhedral (2 to 4 mm) and plagioclase phenocrysts (8 to 15 percent) are generally subhedral (1 to 3 mm)). Small biotite phenocrysts (1 to 3 percent) are typically intergrown with, or partly replaced by muscovite. The dikes are characterized by trace to 2 percent accessory orangeish garnet and trace to 1 percent fine disseminated magnetite. Orangeish garnet appears to be a primary accessory mineral disseminated in the rhyolite but also locally occurs as fine coatings on fracture surfaces suggesting some is late magmatic or deuteric. Two rhyolite porphyry dikes occur in a northeast-trending set subparallel to aphyric rhyolite (Tr), fine-grained granite (Tcf) and quartz latite porphyry hybrid (Tqlp) dikes. The main dike occurs in three segments, suggesting a continuous or nearly continuous dike for about 11,500 ft across the Maysville quadrangle and continuing for another 6,500 ft in the Mount Antero quadrangle. The central portion of this dike, on the ridges north and south of Squaw Creek (fig. 20), is composite consisting of both rhyolite porphyry (Trp) and fine-grained granite (Tcf). Outcrops of the dike show thicknesses ranging from 10 to 35 ft, and some concentrated, large block float zones are up to 150 ft wide. Exposed contacts indicate that the dike strikes from N2° to 37°E with dips from 35° to 68°NW. The overall orientation is about N25°E with a 50°NW dip. The second northeast-trending rhyolite porphyry dike occurs as an almost continuous float zone for about 6,000 ft along the range front associated with the Sawatch fault zone north of Squaw Creek. Similar rhyolite porphyry float on the Blank Mine adit dump, about 7,000 ft southwest, suggests that a rhyolite porphyry dike was also emplaced along the Sawatch fault zone in that area. North of Squaw Creek, two outcrops of the rhyolite porphyry are present but no contacts are exposed. A rib of silicified rhyolite porphyry breccia at the northern Maysville map boundary trends N45°E and is the same 76
as the overall N47°E trend of the Sawatch fault zone. A second set of northwest-trending rhyolite porphyry dikes consists of two 1,500 to 3,000 ft long dikes, about 6,000 ft southeast and 11,000 ft east of Mount Shavano. The dikes are similar to the northeast-trending rhyolite porphyry dikes but are thinner and generally have lower phenocryst content. One subcrop and the narrow float zones suggest the dikes are about 5 to 10 ft thick. The relative ages of the two rhyolite porphyry dike sets, the aphyric rhyolite dikes, and the quartz latite porphyry hybrid dikes could not be established by cross cutting relationships. The main northeast-trending rhyolite porphyry dike cuts the Mount Princeton pluton less than 1,000 ft north of the northern boundary of the Maysville quadrangle. Trp Figure 20. View of Shavano Lake in Squaw Creek glaciated valley overlooking the southern Upper Arkansas Valley graben. Note Tertiary rhyolite dike (Trp) in distance. 77
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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
Page 25 and 26: with the Colorado mineral belt (Boo
Page 27 and 28: Shannon, 2005). Widmann and others
Page 29 and 30: the Upper Arkansas graben (fig. 5)
Page 31 and 32: 15-minute quadrangle, which is adja
Page 33 and 34: etween the northern and southern se
Page 35 and 36: on U.S. Forest Service color aerial
Page 37 and 38: We use fractional map units on the
Page 39 and 40: Qpt Pinedale till, undivided (late
Page 41 and 42: weathered, but Mount Princeton quar
Page 43 and 44: stratified, matrix-supported, bould
Page 45 and 46: 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: Tr Rhyolite dikes (early to late Ol
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 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
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Many of the calc-silicate gneiss zo
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typically expressed by intermittent
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The field relations and characteris
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the north side of all three segment
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eplaced by muscovite. The western m
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sillimanite-quartz-muscovite that a
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sillimanite gneiss (Xmsg) and the m
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Figure 35. Outcrop of Proterozoic h
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samples) indicates they are compose
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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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