(aoo) -(G rt r- {/)((). 361 ~ ­ ~ Wet Mountains, Colorado, T ·~ 52-19,54 By R. Ao Christman, M. R. Brock, R. C. Pearson, and Q. D, .~ingewald "ft / . y ~ '. i· .· .f\ t ./~- t~>· ' ~} ; .': l . \.___,/! I -· .. • . .;4'· lt '•' - ,. ;J;/~' ,. ;.- t: ~;· ~~: . __ /~ 'j\(1_. Trace Elements ln'Vestigations Report 354 UNITED STATES DEPARTME ~f t ' OF THE INTERIOR GEOLOGICAL SURVEY 0 FFICIAL USE ONLY Geology and Mineralogy This document consists of 52 pages, plus 2 figures., Series A UNRTED STATES DEPARTMENT OF THE INTERIOR GEOLOGICAL SURVEY WET MOUNTAINS . COLORADO, THORIUM INVESTIGATIONS, 1952-1954* By R, A, Ct.ud.stma.n, M .. R.. Brock, R. C. Pearson, and Q~ Dw Singewald August 1954 .Trac,e Elemenb; Investigations Report 354 {J~. ~ ~o/ GS-/7?~/?1.&-J'/_,- ~~~ ~ )~ .3"4//.r..r: This prelimmary .report is distributed without editorial and technical review for conformity with official standards and nomenclatureo It is not fOJC public inspection or quotation.. *This report concerns work done on behalf of the Division of Raw Materials of the U. s. Atomic Energy Commission., When separated from Part U, handle Part I as UNCLASSIFffiD· OFFICIAL USE ONLY 3 CONTENTS Page Abstract. 5 Introduction • 6 Summary of the geology. 8 Regional se~ting • 8 General geology • 8 Alluvium.,-talus. anti soil • 10 Rocks 10 Individual rock types • 10 Metasedi,mentary gneisses. 11 '· Hornblende ... plagiodase gneiss and hornblenae..:pyroxen~~pl~gid"Clase _ gne~s. 12 Biotite-quartz-plagioclase gneiss ' 13 Pyroxene-scapolite gneiss • 15 Quartzite • 15 Sillimanitic gneisses • 15 Garnetiferous gneisses 16 Granitic gneisses of unknown origin • 16 Migmatite. 17 Quartz monzonite gneiss 17 Alaskitic granite gneiss • 18 Leuco-granodiorite gneiss • 19 Igneous arid other rocks. 21 Metamorphosed gal;>broic and ultramafic rocks, 21 Pegmatite. 22 Breccia. 22 Albite ;syenite 23 Altered rock • 26 Dike rocks. 26 Bedrock map units .• 28 Geologic structure • 29 Foliation and lineation 29 Folds ,. 30 Fractu~es. 33 Mineral dep0sits. 35 Description of veins. 36 1'hoduti1 deposits. · 37 Batite deposits, 45 Ve.rmiculite deposits 45 Thor~um deposits outside the mapped area " 45 Suggestions for prospecting • 48 Literature cited • 5.0 ILLUSTRATIONS Page Figur~ 1. Index map of Wet Mountain area, Coloradoc ., 7 2. Geolp:gic map of the McKinley M~lilntain area. Wet Mountains. Colorado {2 maps: east half and west haif) ,. ., • • • • .. • • • • • • .. In envelope 3. Index map of localities of radioactive material. McKinley Mountain area. Wet Mountains. Colorado (2 maps: east half and west half). .. • • • • " c In envelope 4. Diagrams showing trends of veins and dikes. McKinley Mountain area. Colorado • 34 5. Map of known thorium deposits in parts of Custer and Fremont Counties. Colorado. excluding the McKinley Mountain area • .• • • • • • • • • • 42 TABLES Table 1. Modes (volume percent) of hornblende-plagioclase gneiss and hornblende­ pyroxene-plagioclase gneiss • • ., ., .. • • • .. • • ., .. • • .. 14 2. Modes (volume percent) of alaskitic granite gneiss 20 3. Selected analyses of albite syenite sto~k and syJnite dike. • • c. i!> • ., 0 25 4. Analyses of samples from the McKinley Mountain area. Custer and Fremont C.ountieso Coloraao • .. ., .. • • • • • • .. • 38 5• . Previously reported analyses from the McKinley Mountain area. Custer and Fremont Counties. Colorado .. • • • • • .. • • • • • • .. ., • 39 6. Analyses of the thorUe-like mineral fll'ont Pi.ne Ttee clainio C11$tei County. Colora_do .. • w. • c c. • . _ • 'Q • _9 v. o • C! ~ "' s • • ~ • 44 '1. Analyses of samples from outside the McKinley Mountain area. Custer and Fremont Counties. Colorado • , • • • • • • • • • • • • • ., .. 47 5 WET MOUNTAlNS 0 COLORAD0 0 THORXUM INVESTIGATIONS 1952-1_9.54 ~yR., A, Christ'triitlli~ · M.. R. Brock. R., C., Pea.Jl'SOtll0 and Q., D. Singewald ABSTRACT A 22-square mile tract (McKinley Mountain area) of pre~Cambrian rocks and veins containing thorium was mapped at the scale of 1:6. 000., This tract lies on the west flank of the Wet Mountains. Custer and . Fremont Counties. northeast of Westcliffe. Coloo ~he bedrock is a complexly interlayered sequence of gneisses of metasedimentary origin. migmatite and granitic gneisses that have been transected by an albite syenite stock and numerous northwest-trending dikes. veins. and fractures., Homblende~plagioclase and bioti te~ qua rt z~p l agioclase gneisses are the principal metasedimentary rocks; pyr o xen e~scapolite. gamet. sillimanite. and quartzitte zones are locally present. The most prominent rock is a poorly foliated. micmcline. alaskitic granite gneiss which occurs as layers ranging from more than 500 feet in thickness down to migmatitic " lit-par~ lit" type of injections less than an inch thick. Of less wide distdbution. but of similar occurrence. are quartz monzonite and leuco-granodiorite gneisses. Although the foliation of the rocks generally is steep and trends northeast over most of the area. several nonheast~uending folds have bee!J\ mapped in the northem half of the area; , a vert,ically plunging fold occurs in the southwest. The albite syenite is nonfoliated and is about 59'& million years old (late pre-Cambrian) by the Larsen zircon method of age determination., Many of the dikes are related to the stock., More than 800 radioactive occurrences were found along the northwest~trending veins., Almost all the radioactivity is due to thorium which in its purest form occurs as a hydrated thorite~Iike mineral. The veins also contain carbonate minerals. ba rite~ quartz. red and yeHow iron oxides. and minor sulfides; no genetic relation of these minerals to the thorium has been established.. Although most of the deposits .are only weakly radioactive. richer concentiration:; are scattered as pockets and lenses along the veins. INTRODUCTION Du'ring the summers of 1952 and 1953 geologic mapping at a scale of 1:6~ 000 was done in an area. designated in this report as the McKinley Mountain area. about 10 miles northeast of Westcliffe on the western flank of the Wet Mountains in Custer .and Fremont ~ounties. Colo.. (fig., 1). The pre~Cambrian complex of igneous and metamorphic bedrdGk was mapped in great detail (fig. 2) in order to discover new vein-thorium deposits and to determine the structural setting of these deposits., More than 800 radioactive localities were found (fig. 3) within the 22 square miles that were mapped. The deposits occur irregularly atong northwest trending fraetttres without apparent relation..either to the country rock or to changes in strike of the fractures. Previous work in the McKinley Mountain area consislted of reconnai$sance of a few of the thorium deposits and a plane table map of a small tract at the Tuttle ranch., This worko as well as descriptions of some other thorium deposits and of a drilling program at Hapu1ta ranch. is set forth in "Thorium investigations. 1950-52. Wet Mountains. Coknado"' by Christman. Heyman. Dell wig and Gott (1953). The 1953 report outlined the known thorium province as at least 20 miles long and 10 miles wide • .It now is known to b~ at least 30 miles long and 12 miles wide~ To the southwest the Rosita and Silver Cliff mining dlstricts. containing silvero lead. goldg zinc. and copper veins associated with Tertiary volcanic rocks. have been described by S., F., Emmons ( 1896). and Cmss (1896). About two~thirds of the work by the U., s. Geological Survey was done on behalf of the Division of Raw Materials of the U. s. Atomic Energy Commission; the remainder was done by the State of Colo.rado Geological Survey Board and the Geological Survey on a cooperative basis. This report was .- prepared by Christman. Brock 9 and Pea.rson. under the general supervision of Singewald. DEPARTMENT OF THE INTERIOR TRACE ELEMENTS INVESTIGATIONS UNITED STATES GEOLOGICAL SURVEY REPORT 354 CITY _.., DENVER • COL 0 R AD 0 Canon City .Pue~lo /----J 0 ~ ~ FIGURE I.-INDEX MAP OF WET MOUNTAIN AREA, COLORADO. : ~ ~ 8 SUMMARY OF THE GEOLOGY Regional setting The pre-Cambrian rocks in the McKinley Mountain area are part of a north-trending belt of gneisses. schists, and granites which are exposed as a result of the Wet Mountain uplift during the Laramide revo­ lution. This belt is the southward~ echelon extention of the ancient Front Range highland (Lovering and Goddard. 1950). Most of the belt lies east of the mapped area and forms the higher portions of the Wet Mountains. Just east of the mapped area is the northwest -trending llse fault which apparently represents a structural break between the northeast-trending rocks in the McKinley Mountain area and the rocks on the crest of the Wet. Mountains. Most of the area is fairly rugged; relief ranges from 7. 900 to 9. aOO feet~ Water is scarce. Rather thin. somewhat scrubby stand~ of evergreen cover most of the ridges. but good stands cover some of the north slopes. West and southwest of the mapped area a discontinuous belt of Tertiary volcanic rocks lies between the pre-Cambrian rocks on the east and the alluvial fill in the Wet Mountain Valley on the west., F~rther to the west fire the high Sangre de Cristo Mountains composed of folded Paleozoic sedimentary rocks and several small Tertiary-intrusions (Burbank and Goddard. 1937)~ Associated with the Tertiary volcanic .roc_ks are vein depo.sits of gold. silver. lead. zinc 0 and copper near Rosita. Silver Cliff. Querida. and Westcliffe. Although one of the volcanic bodies near Hillside is slightly radioactive. Tertiary minerali­ zation appears unrelated to the thorium deposits in the pre-Cambrian rocksQ General geology The bedrock consists of metasedimentary. migmatitic. and granitic; gneisses which form complex interlayered sequences of interfingering and gradational rock types (fig. 2). Except along the crests and troughs of several folds, ·the layers of gneiss dip steeply and form narrow bands which trend northeast across the area. Distinctive lithologic "marker beds" in some places are important aids in determining the geologic structure. Units of homblende~plagioclase gneiss. biotite~qua.r~z:,P.lagjoclas :e , g_neiSsp and- undi~ided metasedimentary gneisses probably are the metamorphosed rep.resentatives of the oldest pre~ Cambrian rocks in the area., Units of migmatite.