Geologic Map of the Maysville Quadrangle, Chaffee County, Colorado

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quadrangle surface area. No ages have been determined for the Proterozoic rocks in or adjacent to the Maysville quadrangle. The classification of Proterozoic intrusions into Routt and Berthoud-types is based on mineralogical and textural characteristics and field relations in comparison to type descriptions of these intrusive suites by Tweto (1987). However, the classification of Routt-type intrusions is supported by limited age determinations in the Salida region (Bickford and others, 1989). The Proterozoic rocks are exposed in the Sawatch Range rift-shoulder uplift on the west side of the Upper Arkansas Valley and South Arkansas grabens. They are present in two structural blocks in the western half of the quadrangle: a 24,000 ft long by 1,000 ft wide, N35° E-trending zone along the west margin of the Arkansas River Valley graben in the north; and a 18,000-ft by 15,000-ft zone along the west margin of the South Arkansas graben in the south. The relationship between the two structural blocks is unclear but they may be separated by the west-northwest extension of the Salida- Maysville fault, west of the Maysville quadrangle (See Structural Geology section). Proterozoic intrusive rocks are divided into three groups: younger intrusions related to the Berthoud Plutonic Suite; a group of miscellaneous intrusions of uncertain age and relation to the Berthoud and Routt Plutonic Suites; and older intrusions related to the Routt Plutonic Suite. BERTHOUD PLUTONIC SUITE (~1.40 Ga) – The Berthoud Plutonic Suite is represented by abundant, volumetrically minor but widespread dikes and small intrusions of coarse-grained pegmatite (YXp) and composite granite and pegmatite bodies (YXgp) that cut the older Early Proterozoic metamorphic and intrusive rocks. Pegmatite and granite and pegmatite dikes and sills are extremely abundant in the Proterozoic terrane in the southwest quadrant of the Maysville quadrangle. In the Proterozoic metamorphic rocks, no systematic criterion is recognized that could be used to distinguish whether the pegmatite-granite intrusions are related to the Routt- or Berthoud-type suites. Evidence for Berthoud-type granite pegmatites is supported by areas of abundant granite and pegmatite dikes cutting the two intrusions of Routt-type Early Proterozoic granodiorite 114
near the southwest margin of the quadrangle. These post-Routt-type granite and pegmatite intrusions may be related to a large Berthoud-type (Silver Plume-like) granite intrusion covering the southern part of the Garfield 15’ quadrangle (Dings and Robinson, 1957). YXgp Biotite granite and pegmatite (Early to Middle Proterozoic) – Biotite granite is a white to pinkish-white, fine- to medium-grained, equigranular rock. It is leucocratic with less than 1 to 2 percent biotite and locally a trace of red garnet. It is classified as granite b on the basis of estimated mineral modes (IUGS classification). Biotite granite usually occurs together with coarse-grained pegmatites as composite intrusive bodies (YXgp). The proportion of granite and pegmatite is variable, but pegmatite usually predominates. The granite and pegmatite bodies are abundant in the southwest part of the Maysville quadrangle where they cut all Early Proterozoic lithologies. The granite and pegmatite intrusions range from 1 to over 300 ft thick and tens of feet to over 1,200 feet long in bodies that are concordant and discordant to the enclosing gneisses. They generally make bold outcrops and persistent coarse-block float zones. YXp Pegmatite (Early to Middle Proterozoic) – Berthoud-type pegmatites are generally simple, unzoned, quartz-feldspar pegmatites usually containing minor biotite and/or muscovite and locally containing magnetite, red garnet, and minor black tourmaline. They are white to pinkish-white, coarse- to very coarse-grained rocks that lack foliations or preferred metamorphic fabrics. Quartz-alkali feldspar graphic intergrowths are locally present. The pegmatites occur as dikes and sills that cut all Early Proterozoic lithologies. They range from 1 to over 300 ft thick and tens of feet to over 1,200-ft long in bodies that are concordant and discordant to the enclosing gneisses. Pegmatites generally make bold outcrops and persistent coarse block float zones. A zone of larger pegmatite intrusions is present on the ridge between Willow Creek and Green Creek, near the Bon Ton mine (plate 1). The pegmatites are greater than 200 ft thick and about 500 ft long and most were prospected with cat trenches or cuts. No exotic minerals were noted in these pegmatites and the purpose of the exploration is not 115
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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
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 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: There is one additional Paleozoic s
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
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).
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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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