Geology and ore deposits of the Chicago Creek area, Clear Creek County, Colorado

Harrison, J. E., Wells, J. D. · 1956 · Report

The Chicago Creek area, Clear Creek County, Colo., forms part of the Front Range mineral belt, which is a northeast-trending belt of coextensive porphyry

This 1956 document, Geology and ore deposits of the Chicago Creek area, Clear Creek County, Colorado, is preserved in the Mountain Man Mining Library for research and reference. Original source: pubs.usgs.gov.

TlN7r w. 1.1 Sd. G E 0 L 0 G Y A N D 0 R E D E P 0 S I T S 0 F T H E C H I C A G 0 C R E E K A R E A , CLEAR CREEK COUNTY, COLORADO By J. E. Harrison and J o D. Wells Trace Elements Investigations Report UNITED STATES DEPARTMENT OF THE INTERIOR GIDLOGICAL SURVEY +

OFFICIAL USE ONLY Geology and Mineralogy This document consists of 6 pages. Series A UNITED STATES DEPARTMENT OF THE INTERIOR GEOLOGICAL SURVEY GEOLOGY AND ORE DEPOSITS OF THE CHICAGO CREEK AREA CLEAR CREEK COUNTY, COLORADO* By J. Eo Harrison and J Do Wells March 1956 Trace Elements Investigations Report 432 This preliminar,y report is distributed without editorial and technical review for conformity with official standards and nomenclature o It is not for public inspection or quotation. *This report concerns work done on behalf of the Division of Raw Materials of the Uo So Atomic Energy Commissiono OFFICIAL USE ONLY

OFFICIAL USE ONLY USGS TEI=432 GEOLOGY AND MINERALOGY Distribution (Series A) Atomic Energy Commission, Washington o Division of Raw Materials, Albuquerque o Division of Raw Materials, Austino o ., .Division of ·Raw Materials, Butte o o o o o o Division of Raw Materials~ Caspero o o a o , 0

Division of Raw Materials, Denvero o ., o o o o o o o ., Division of Raw Materials, Ishpeming .. " .. o o o Division of Raw Materials, Phoenix " o o o o o o Division of Raw Materials, Rapid City" Division of Raw Materials~ Sto George ., Division of Raw Materials, Salt Lake City. " o o o

'Division of Raw Materials, Washington" o o o Exploration Division, Grand.Junction Operations Office Grand Junction Operations Office o o .. o o o o o o o Technical Information ~ension, Oak Ridge o o o Uo So Geological Surveyg Fuels Branch, Washington o o o

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Geochemistr.y and Petrology Branch, Washingtono Geopqysics Branch, Washington" o o o o o o

o Mineral Deposits Branch, Washington. Po Co Bateman, Menlo Park .. o o o o o ., o o A., L .. Brokaw, Grand, Junction No M .. Denson.!) Denver o o

., o o .. V o Lo Freeman~ College o o o o o o o

Ro Lo Griggs~ Albuquerqueo o o o o o o " o o o o

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o Ao Ho Kosehmann, Denvero o o Jo Do Love, Laramie o o o o o L o R o Page, Washington ., o o o o o o o o o o Qo Do Singewald, Beltsville., o o o o o o o o o o

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OFFICIAL USE ONLY GEOIDGY AND ORE DEPOSITS OF THE CHICAGO CREEK AREA, CLEAR CREEK COUNTY, COLORADO B.y Jo Eo Harrison and Jo Do Wells ABSTRACT The ~hicago Creek area, Clear Creek County, Coloo, forms part of the Front Range mineral belt, which is a northeast=trending belt of coextensive porpQyry intrusive rocks and bydrother.mal veins of Tertiar,y ageo More than $4o5 million worth of gold, silver, copper, lead, zinc, and uranium was produced from theminesin the area between 1859 and 1954 .. - This investigation was made by the Geological Survey on behalf of the Division of Raw Materials of the Uo So Atomic Energy Commissjon. The bedrock in the area is Precambrian and consists of igneous . rocks, some of which have been metamorphosed, and metasedimentar.y rocks. The metasedimentar.y rocks include biotite~quartz-plagioclase gneiss that is locally garnetiferous, sillimanitic biotite-quartz gneiss, amphibolite, and lime=silicate gneisso Rocks that may be metasedimentary or meta-igneous are quartz monzonite gneiss and granite gneiss and pegmatite o The granite gneiss and pegmatite locally form a migmatite with the biotitic metasedimentary rocks. These older rocks have been intruded by granodiorite~ quartz diorite, and associatedhornblendite, biotite-muscovite granite, and granite pegmatite. During Tertiary time the Precambrian rocks were invaded by dikes and plugs of quartz monzonite porphyry, alaskite porphyry, granite porpnyry, monzonite porphyry, bostonite and garnetiferous bostonite porphyry, quartz bostonite porp~, OFFICIAL USE ONLY

OFFICIAL USE ONLY trachytic granite porphyry~ and biotite-quartz lati te porphyry. Solifluction debris of Wisconsin age forms sheets filling some of the high basins, covering some of the steep slopes, and filling parts of some of the valleys; talus and talus slides of Wisconsin age rest on or are mixed with solifluction debris in some of the high basins. Recent and/or Pleistocene alluvium is present along valley flats of the larger streams anq gulches. Two periods of Precambrian folding can be recognized in the area. The older folding crumpled the metasedimentary rocks into a seri~ s of upright and overturned north-northeast plunging anticlines and synclines. Quartz monzonite gneiss, granite gneiss and pegmatite, granodiorite, and quartz diorite and associated hornblend.ite . ~.t·e metamorphosed during this period. The second period of folding appears to have been the reflection at depth of faulting nearer the surface~ it resulted in crushing as well as some folding of the already folded rocks into terrace and monoclinal folds that plunge gently east-northeast. The biotite-muscovite granite, which is the youngest major Precambrian rock unit, is both concordant (phacolithic) and crosscutting along the older fold system and has been fractured by the younger fold system. Arching of the Front Range highland during Laramide time is believed responsible for the development of a regional joint pattern consisting of a north-northwest trending longitudinal joint, a related cross joint, and two related diagonal joints. Joints of this regional pattern can be distinguished from Precambrian and Tertiar,y joints. Northwest-trending faults known as the "breccia reef" system, formed possibly during or following the archinge During Tertiary time the OFFICIAL USE ONLY

OFFICIAL USE ONLY bedrock was intruded by porphyritic dike rocks that probably were emplaced under tensional stresses Later regional shear stresses caused east= to north-northeast-trending fractures in the bedrock and at places reopened the vwbreccia reef'' faults o These openings were the loci of deposition of hydrothermal veins The fractures formed under the regional shear stress are as much as 2! miles long and are relatively straight fault~fissures that follow the "grain" of the bedrock Many faults are subparallel to foliation, axial planes of folds, contacts between rock units or pre-existing joints The veins in the district are typical mesothermal fillings of fault-fissureso Some of the veins are lodes that have smooth bounding walls and abundant slickensides, the faults containing these veins are fair~ regular in strike and dip, and irregularities, where present, commonly provided -favorable structures for the deposition of the ore mineralso A series of five vein types and one sub-type can be recognized in the areao Most of these veins contain quartz~ carbonate minerals, pyrite, chalcopyrite, tetrahedrite=tennantite, galena, and sphalerite, and many contain minor amounts of gold, silver, or polybasiteo The proportion of the metallic minerals in any vein is used as ,the basis for classifying the veins into types The five vein types are: (1) pyritic; (2) pyritic with copper sulfides; (3) pyritic galena~ sphalerite with copper sulfides and copper and silver sulfo~salts; (4) galena-sphalerite with pyrite and/or marcasite and copper and silver sulfO-salts; and (5) galena=sphaleriteo The sub-type is similar to the OFFICIAL USE ONLY

OFFICIAL USE ONLY pyritic t ype but is t elluride=bearing Many veins are composite and contain t wo or more t ypes of oreo Wherever composite veins have been observed the older type of ore is always more pyritic than a younger type The wr i t ers beli eve that the veins resulted from repeated .fract uring, each fracture being filled at the time of opening by solut ions that changed with time from predominantly iron-depositing through copper-depositing, to lead-zinc depositingo Some of the veins, pegmatites, and Tertiar.y porpQyries are abnormally radioactive o All t.he known occurrences in the porphyries are of too low grade to be important commercially Most of the abnormally radioactive material in veins and pegmatites is either too low-grade or in too small quantities to be of commercial importance Some of the occurr ences in veins have been explored inadequately and could not be evaluated in 1955; one mine (the Martha E winze) within t he district shipped 2,920 pounds of 0.14 percent ore that was hand~ picked from high-grade spots. Structural control of some of the ore shoots in the area is welldefinedo Openings favorable ~or the deposition of ore were formed at vein intersections, along deflections in strike or dip of veins, and along deflections where a vein enters a rock of different competency About 60 mines, ranging in size from those containing a few hundred feet of workings to those containing several miles of workings 3 are scattered throughout the areao Most of the mines have not been worked for years, many since before 1900. During 1953, 1954, and the f i rst half of 1955 only one mine was in continual operation, and only three others had a small amount of exploration work done in theme OFFICIAL USE ONLY

Plates & figures from the original