Geology and mineral deposits of the Poncha Springs SE Quadrangle, Chaffee County, Colorado

Abstract

The Poncha Springs NE quadrangle, which contains a major fluorspar district, is in the Arkansas Valley between the Sa watch Range and the southward extension of the Park Range. Altitudes in the quadrangle lie between approximately 7,290 feet and 9,290 feet. The geologic units mapped consist of Precambrian metamorphic and igneous rocks, Tertia•ry volcanic and sedimentary rock;s, and unconsolidated Pleistocene and Holocene deposits. Precambrian hornblende gneiss and banded quartz-feldspar-'bioti.te gneiss are intruded by a large mass of gneissic quartz monzonite. Quartzite and silicated marble are interlayered with the gneisses, which, together with other structural and mineralogic evidence, suggests ,that the metamorphic rocks were originally sediments. The gneissic quartz monzonite occupies a 1 bout 25 square miles of the quadrangle and is cut 'by ta:bular bodies of granite,. aplite, pegmatite, •lamprophy.re, dacite .pol'!phyry, and d•iabase, and by quartz veins that locally contain orthoclase, magnetite, specular hematite, py•rite, chalcopyrite, chlorite, and trace~S of purple fluorite.. The Tertiary volcanic and sedimentary rocks immediately overlie the Precambrian rocks. Volcanic rocks in the southern part of the quadrangle locally are more than 600 feet thick. The volcanic •rocks consist chiefly of a lower unlit of ush, mudflow deposit, and a rhyodacite porphyry flow of Eocene (?) age and an upper unit of ash-flow tuff that •has !black vitrophyre near the base and a grayish-pink to reddish. .. brown devitrifted tuff above. 'The ash-flow tuff• is early Oligocene in age, based upon potassium-argon dating of the black vitrophyre and upon pollen and spores found in tuff :immediately underlying it. A unit consisting of pyroclastic 'rocks, perlite, and a rhyolite flow that contains garnet and topaz, at the north edge of the quadrangle, is here named the Nathrop Volcanics. Dating by the• potassium-argon method indicates that the perlite and rhyolite flow are late Oligocene in age. Tuffaceous siltstone forms seven small ma,sses resting directly on Precambrian rocks in the south-"Central pa'rt of the quadrangle. Thts unit, here called the Browns Oanyon Formation, contains leaves und pollen comparable to the Oreede flora and is Miocene in age. Unconsolidated beds of clay, •silt, sand, and gravel of the Dry Union Formation underlie most of the western rpart of the quadrangle. These sediments locally contain shards of yolcanic glass, bentonite, and rhyolitic tuff layers. Vertebrate fossils indicate that the formation is Miocene and Pliocene in age. Pleistocene deposits mantle much of the Dry Union Formation • in the western part of the quadrangle; the deposits formed as outwash •:Crom multiple stages of moun.ta.in glaciation in the Sawatch Range to the west. In this quadrangle and the one im-2 GEOLOGY AND MINERALS, PONCHA SPRINGS NE QUADRANGLE, CHAFFEE COUNTY, COLO. mostly microgranular to fine grained and layered, and commonly has botryoidat, mammillary, and reniform surfaces and nodular structures. Pyrolusite, manganit...

Mineral resources of the Browns Canyon Wilderness Study Area, Chaffee County, Colorado

“…1) (Van Alstine, 1969). The fault zone along which the fluorite occurs trends northwest and does not extend into the study area.…”

“…The geology and known mineral deposits of the Poncha Springs NE quadrangle, which includes the wilderness study area, were described by Van Alstine (1969). A geologic map at 1:24,000 scale is included in Van Alstine's publication.…”

“…Proterozoic metamorphic and igneous rocks, Tertiary volcanic and sedimentary rocks, and Quaternary alluvium are exposed in the vicinity of the study area (pi. 1) (Van Alstine, 1969;Scott and others, 1975). North-and northwest-trending normal faults are the predominant local geologic structures.…”

“…Regional basin-and-rangestyle block faulting along a general northern trend during the Miocene and Pliocene probably formed the Arkansas River valley structural depression. This graben is considered to be a northern extension of the Rio Grande rift and is now filled with thick sediments (Epis and others, 1980;Van Alstine, 1969). Proterozoic metamorphic and igneous rocks, Tertiary volcanic and sedimentary rocks, and Quaternary alluvium are exposed in the vicinity of the study area (pi.…”

“…USBM data, Denver, Colo.; Van Alstine, 1969). Most of the placer prospecting was in Tertiary and Holocene sedimentary rocks and sediments in drainages west of the Arkansas River.…”