Geology of the Wood and East Calhoun mines, Central City District, Gilpin County, Colorado

Drake, Avery Ala · 1955 · Report

The Wood-East Calhoun mine area is underlain by complexly folded Precambrian gneiss and pegmatite. The major fold in the area is an anticline that trends

This 1955 document, Geology of the Wood and East Calhoun mines, Central City District, Gilpin County, Colorado, is preserved in the Mountain Man Mining Library for research and reference. Original source: pubs.usgs.gov.

Geology of the Wood and East Calhoun mines, Central City district, Gil pin County, Colorado By A. A. Drake, Jr. Trace Elements Investigations Report 175 UNITED STATES DEPARTMENT OF THE INTERIOR GEOLOGICAL SURVEY

OFFICrAL USE ONLY Geolpgy and Mineralogy This document consists of ; 60 pag~sJ , including page .42 a, plus 3 figures 0 Series A UNITE:r> · STATES DEPARTMENT IDP tBE INTERIOR GEOLOGICAL SURVEY GEOLOGY OF THE WOOD AND EAST CALHOUN MINES, CENTRAL CITY DISTRICT, GlLPlN r, COLORADO* - A very A. Drake. Jr. March Trace' Ele~ents InveStigations Report 175 This , preliminary report is distributed without editorial and technical review for conformity with official standards and nomenclature. It is not for public inspection or quotation. *This repon'Concerns work done on behalf of the Division of Raw Materials of the Uo s. Atomic Energy Commission. When separated from Part U, handle Part I as UNCLASSIFIED. OFFICIAL USE ONLY

USGS - TEI .. 175 Distribution {Series A) Argonne National Laboratory GEOLOGY AND MINERALOGY Atomic Energy Commission. Wash!ngton Division of Raw Materials, Albuquerque Division of Raw Materials, Butte. Division of Raw Materials, Casper Division of Raw Materials, Denver Division of Raw Materials, Hot Springs ., Division of Raw Materials, Ishpeming Division of Raw Materials, Phoenix Division of Raw Mat~ria1s, St. George Division of Raw Materials, Salt Lak~ City Division of Raw Materials, Washington Exploration Division. Grand Junct~on Operations Office Grand Junction Operations Office Technical Information Servic~. Oak R~dge Tennessee Valley Authority, Wilson Dam Uo S" Geological Survey: Fuels Branch, Washington. Geochemistry and Petrology Branch, W~shington Geophysics Branch, Washington , Mineral Deposits Branch, Washington E. H. Bailey. Menlo Park Ao L. Brokaw. Grand Junction N. M. Denson, Denver R. L. Griggs. Albuquerque M. R. Klepper, Spokane A. H. Koschmann, Denver 1. Du Love. Laramie · L ... R. Page. Washington Q. D. Singewald. Beltsville A. Weissenborn, Spokane J'EPCO, Denver TEPCO, RPS, Washingtonr( including, ma~t~r) Nb. of ¢o2ies "1 1

Absuact o Introduction., Location o Previous investig~tions .. Field wmk Acknowledgments General geology Precambrian rocks o Biotite-·quartz-plagioclase gneiss Amphibolite Migmatite d Granite gneiss o Granite pegmatite Tertiary rocks ., Quartz boHonite ' Syenitic bostonite Structure Folds Lineation Joints Faults Calhoun fault Quartz Mill fault Wood fault o Minor faults Structure of Tertiary dike rocks Economic geology History and development East Calhoun mine Wood mine., Production ., East Calhoun mine _Wood mine. General character of th~ veins Calhoun vein Quartz Mill vein Wood vein 6 . Willowdale vein Mineralogy Quartz ., Pyrite Chalcopyrite Sphalerite o T etrahedrite '"tennant.ite Galena Gold and silver Pitchblende Bornite ., Sooty chalcocite Covellite Paragenesis GONTENTS Page f 0 .

Page Results of sampling o Trace elements e Wall rock alteration Zoning Ore shoots Origin Age io

future of the mines Literature cited Unpublished reports ILLUSTRATIONS Figure 1o Jndex maps showing location of W.ood-East Calhoun mine area, Central City district, Gilpin County, Colorado.. 2o Geologic map of the Wood-East Calhoun area, Central City ~istrict, Gilpi1;1 County, Colorado o 30 Contour diagram of 110 lineation~ in rocks of the East Calhoun and Wood mines o 40 Contour diagram of 386 joints plotted on uppe~ hemisphere of Schmidt net. 50 Contour diagram of 70 slickep.sides on the Wood vein plotted on lo~er hemisphere of Schmidt net o 60 Geologic maps of the East Calhoun mine, Central City district, Gilpin County, Colorado o o In envelope 7o Longitudinal projection of the East Calhoun mine, Central City clisttict, Gilpin County. Colorado 80 Vertical longitudinal projection of the Wood mine, Central City district Gilpin County. Colorado 9., Geologic map of the Wood crosscut and drift~ -- sixth level, East Calhoun mine, Central City district, Gilpin County. Colorado .. In envelope 100 Geologic sections across the Wood explora~ion drift, East CalhotJ.n mine, Central City district, Gilpin County, Colorado 110 Geologic section of the Wood 520-rai~e. 12. Graph showing the relation of gold to silver in samples from the Wood vein 130 Paragenesis of the vein minerals o 140 Vertical longitudinal section of the Wood vein, showing assay values In envelope

Figure 15. Distribution histograms of some elements in 132 samples from the Wood vein 16. Graph showing the relation of molybdenum and z~rconium to uranium in samples from the Wood vein 17. Graph showing the relation between zinc and cadmium in :samples from the Wood vein 18. Graph showing the relation between copper, arsenic, b~, and antimony in samples from the Wood vein TABLES Table 1. Production from the Jefferson-Calhoun vein 1902-15 2. Substantiated pitchblende production from the Wood vein , 30 Gold and silver eontent of pyrite, East Calhoun and Wood veins, Central Gity district, Gilpin County, Colorado 4. Gold and silver content of chalcopyrite, Wood and Calhoun veins, Centr~l City district, Gilpin County, Colorado Analyses of sphalerite, Wood and Calhoun veins, Central City district, Gilpin County, Colorado 6,_ S.ome chemical analyse~, fire assays, and semiquantitative spectrographic analyses of ore from the Wpod vein 7. Threshold values of elements included in the semiquantitative method Page 4 '7

GEOLOGY OF THE WOOD AND EAST CALiiOUN MINES, CENTRAL CITY DIS_TRICT,; GIL'PIN c :<DUN;TY, COLORADO By Av~ry A. Drake, Jr. ABSTRACT The Wood-East Calhoun mine area is underlain by complexly folded Precambrian gneiss and pegmatite. , The major fold in the area is an anticline that trends about N. 60° E. The Precambrian rocks ~re intruded by bostonite porphyry dikes of Te'rtiary age. All the rocks are cut by east .. to northeast-trending faults that have been filled by precious metal-sulfide veins which have been worked chiefly for gold. The Wood vein occurs in an east-trending fault; the Calhoun vein occurs in a northeast-trending fault. Much the uranium production of the Central City district has come ffom the Wood vein on Qu~rtz Hill. The veins consist chiefly of quartz; pyrite is the chief metallic mineral and chc;1lcopyrite is next in abundance. Sphalerite, g(\lena, tetrahedrite-tennantite, and pitchblende are locally present. Deposition began with alteration-sl!age quartz and pyrite followed in order by pitchblende, light-yellow pyrite, massive quartz. yellow pyrite, sphalerite, comb quartz, chalcopyrite, tetrahedrite-tennantite, galena, chalcopyrite, pyrite, and gray to light-brown fine-grained quartz. The veins of the Central CiW district are zoned, with quartz-pyrite veins near the center rand galenasphalerite veins on the periphery. The known pitchblende bodies are in the transition between these. but para genetically, the pitchblende is ·earlieT than all other metallic minellals. A trace element study of the ore indicates an association of zirconium and molybdenum with uranium, of bismuth, antimony, and arsenic w.ith copper. and of cadmium with zinc. The pitchblende and other ore minerals are concentrated in ore shoots. The shoots are in open spaces controlled by compt:(tency of the wall rocks, the presenc~ of a prevailing direction of weakness in the rocks and changes in strike and dip of the vein. The pitchblende is thought to be a local constituent of the quartz-pyrite ores and to owe its origin to residual solutions from the quartz bostonite magma.

INTR.OPUCT ION The Wood vein wa~ an intermittent SOlJrce of pitchblende from 1872 to about 1916. The East Calhoun was one of the largest gold mines in the C~ntral C~ty distri~t. Both properties have been idle since World War X. Because of the strategic importa~ce of t.traniu~ an4 beca~e little was known about the spatial and paragenetic ben.avior of pitchblende in the veins of the Cent~al City district, the Geological Survey examined both properties in 1950 and carded on a program of detaUed geolog~c study and sampling. The veins in the Central City district 4re ~oned with quartz-pyrite veins near the central part of the district, and galena-sphalerite veins on the periphery. The Wood veilJ is in the transit<;>n between these zones.. The pitchblende deposits are spatially and probably genetic~lly related to quartz bostonite porphyry dikeso Within the vein, pitchblepde bodies o~cur in ar w~sterly raking pre shoot that i,s controlled by the competency of the wall rocks, the presence of a pr~vailing direction of weakness in the rocks. and changes in strike and dip of the vein. The frequency of occurred1ce of ind~viqllal bodies decreases with depth, 300 feet being the economic limit. No appreciable q43ntity of pitcllblende was developed by the exploration, and it is improbable that ecopornic qup.ntiUes of pitchblende wi~l be found in tbe future. LOCJ\ TION The Wood and East Calhoup mines are on the south slope of Quartz Hill, near the head of Leavenworth Gulch, in unsurveyed ~ec. 14, T. 3 s R. 7q E. , about l. 5 miles southwest of <;:;:entral City. Gilpin County. Colo (:fig. 1 ). The mines are accessible from Colorado Highway 279 by 4bout a quarter of a mile of unimproved road. PREVIOQS INVESTIGATIONS Pearce ('18~5. p. 156-158), Moore and l<;ltnil (1913, p. 46), Alsdorr'(l916, p. 270), and Bastin and Hill (1917. p. 245), have described the pitchblende and ot}!Jer ores from the Wood mine, Guillotte (1944) examined the Wooq and East Calhoun dumps in 1943, rnd MOQfe and But~er (1952) mapped the accessible workings of both mines ip 1950 as part of the general re~onn~iss4nc;e investigation for radioactivity in the Colorado Front Range. Armstrong (in preparation) studied the surfa~e geology of Ol..lartz Hill.

Grand GILPIN · osooo· · 1 to3°oo' oNederlond COUNTY Denver

ldoh Springs , 1

. Junction ~ff

I&#x27; I ,,

' 1..370 1oKLAHOMA ~00 Miles . INDEX MAP OF COLORADO SHOWING LOCATION OF GILPIN COUNTY To Nederland ,rr · : / t ) J 0 Tolland City EXPLANATION Wood- East Calhoun mine

To Idaho Spring$' '-

6 Miles To Denver INQEX MAP OF GILPIN COUNTY SHOWING LOCATION OF WOOD-EAST CALHOUN MINE AREA I; FIGURE I.-INDEX MAPS SHOWING LOCATION OF WOOD-EAST CALHOUN MINE AREAt CENTRAL CITY DISTRICT, GILPIN COUNTY, COLORADO

Phair (1952) noted a spatial relationsijip between uranium deposit$ and the quartz bostonite porphyry rocks of the Tertiary intrusive series of the Front Rangeo As a result of his worl<, he inferred a genetic relationship between the intrusion of these rocks and the uranium deposi~. Leonard (1952) found that two maltqypes of ore deposits give the Central City district a zonal arumg.ement in plan view: quartz-pyrite veins near the center. and galena-sphalerite veins on the periphery.. He noted that the pitchblende deposits of 01:1artz Hill lie in the transition zone between the two types of deposits .. FIELD WORK The present investigation began in July 1952 and continued through c:>ctober 1953. Geologic of accessible East Calhoun workings and the ,exploration headings were made and the working faces were examined and sampled after each round until the last week of January 1953.. Thereafter, the workings were visited at least once a we~k and back samples were taken at appropriate intervals. Office and laboratory work was cOJ;pleted at the Geological Survey offices in Denver. A total of 605 samples were cut. The samples were assayed for uranite.IIl, gold, silver, copper, lead, and zinc and a semi-quantitative spectrogrp:phic analysis was run on a split of each sample. The Defense Minerals Exploration Administration supervised the exploration, cut samples and coredrilled at three location~. The :U. s. Atomic Energy Commission mapped the exploration workings and sampled the ur~nium-bearing parts of the Wood vein. ACKNOWLEDGMENTS It is a1 pleasure to acknowledge the work of A. E. Dearth, who assisted the writer in the geologic mapping and sampling.. Every courtesy and consideration was shown the writer by the personnel of the Denver Realty Company. present owners~ All assays, chemical analyses, and spectrographic analyses were made by the Surveys Denver Laboratory. A. l. Martin, Supervising Engineer, Metals Economics Branch. U. S .. Bureau of Mineso furnished the production data. These investigate.Dns were made on behalf of the Division of Raw Materials of the u .. s. Atomie Energy Commissiono

(;ENpRAL GEOl,.OGY The Wood,.East Calhoun mine area is nearly astraddlE;: a northeast-trending anticline, which is the major structural feature of this part of the Central City di~trict (fig. 2). The area is underlain by interlayered Precambrian gneiss and pegmatite. The gneiss c~msists of layer$ of grani~e gneis~. (described by ~astin and Hill, 1917 o p. 30-32), with lesser amounts of biotite-qu~rtz-plagioela$e gneiss, migmatite, and amphibolite. Conformable layers and smaU irregular,.shaped pods of pegJDatite occur within the gneiss. These units, in general. dipgently away from the crest of the antic~ine and loc~lly are deformed into tight drag folds or broad genPe warps. The Precambrian rocks are intrude<;l by several ~uartz bostonite porphyry dikes of Tertiary age. The dikes occupy Q.Orthwest-trending fract~es (flg. 2). Steeply dipping faults that trend from e~st to N. 40o E. cut all tbe rocks (fig. 2). The faults are occupied by veins that average about one foot in width. The veins contain quartz-pyrite-gold-silver in the east and qu~r~z-pyrite-sphalerite ... chalcopyr~te-gray copper-galena-goJd-silver in the west. PrecamQrian ~ocks The Precambrian rocks of the Wood-East Calhoun mine area consist of granite gneiss, biotite-quartzplagioclase gneiss, migmatite, amphibolite, and granite pegmatite. All these rocks e~cept some of the granite pegmatite have been deformed and recry~tallized and have a metamorphic te:x:ture. The biotite-quartz-plagioclase gneiss and amphibolite are thought to be of met~sedimentary origin. Bastin and Hill (1917) and Lovering and Goddard (1950) included these rocks in the Precambrian Idaho Springs formation. The granite gneiss, as de~c;ribed by Bastin and Hill, is a metamorphic rock of uncertain origin. This rock has tbe average composition of quartz mon~QJ.l.ite; in this report, however, the writer follows the usage of Bastin and Hill and refers to it as granite gneiss. Th~ granite pegmatite has a simple composition and is younger than the other CJ;'OC'ks.

N Note: Sections .shown on figure 10. EXPLANATION Q:uartz bostonite porphyry rxJ:il - AmphiboHte Granite gneiss r's Contact~ showing dip z

.a: .m :E : 4 ·v "' cr Q.. Dashed wher~ opproxiniofely .located AnHcline. showing trace of O'l( iol plane Ooshed where opproxirnolely located 1 f 7f Vein, showing dip Ver-ticol -ve-in· IPJ Shaft · ·<l ·cr

UJ . GeoloQy after F.C.Armstrong FIGURE 2. -GEOLOGIC MAP OF THE WOOD-EAST CALHOUN AREA? CENTRAL CITY DISTRICT, GI·LPI·N COUNTY,COLORADO . 50 · o · 200 Feet

Biotite-quartz-p~agioclase gn,eiss Biotite-quartz-plagiocla$e gneiss does not crop out in the Wood-East <:;;calhoun mine area. but it is present in the workings of both mines The unit ()ccurs princip~llY in a layer between the fifth and sixth levels of the East Calhoun mine. Moore and Butler (1952) show schist, presumably equivalent to biotitequartz-plagioclase gneiss, as b~ing the principal rock typ~ in the upper workings of the Wood lll!ne. Many layers of the rock too small to be mapped occur within the granite gneiss. The biotite-quartz-plagioclase gneiss is light gray to dark gray. depending on the biotite content., The rock ranges from fine-to medium-grained; the medium-grained v,;~.riety predominates. The principal minerals in order of abundance are plagiocla~e (qear oligoclase), quartz, biotite, and microcline. Accessory minerals include magnetite. zircon, and apatite. In the East Calhoun mine, Wood drift east. a s~mewhat different phase of the unit is found with,in the granite gneiss in layers ranging from an inch to about one foot in thickness. It a fin~-grained salt-and-pepper rock contains as much as 5 percent pink garnet. Moderate to strong foliation is given to the rock by mineral orientation and compositional banding. Nearly all the rock contains conformable layers of pegmatite, a few inches to several feet thick. Amphibolite Amphibolite occurs as layers and lenses 1 to 24 inches thick within the granite gneiss. It is a darkto medium-gray, medium-to coarse-grained equigranular rock composed of nearly equal parts of darkgreen hornblende and oligoclase-andesine. Accessory minera~ include biotite, microcline. and some quartz. Many lenses of the rock have been altered to a "punky" biotite schist. Sub-parallel layering produces a fair to good foliation. Amphibolite lenses, now largely biotite schist, occur on the crests and troughs of northwest-trending~ warps in the more alaskitic granite gneiss. Migmatite Intimately interlayered rocks, termed migtnatite in tltis report, were mapped west of the shaft on the fourth level of the East Calhoun mine. The unit is composed of biotite-quartz-plagioclase gneiss in layers 1 to 2 inches thick separated by granitic layers averaging about 1 inch thic,k. The unit has an appearance somewhat similar to the so-called "injection gneisses."

Granite gneiss The most abundant rock in the Wood-East Calhoun mine area is~granite gneiss. It is dominant in the East ~calhoun mine above the first level and below the fifth level,. Several one-to ten-foot layers are also present in the metasedimentary body between the · first and fifth levels. Contacts of the gneiss are conform ... able to the layering of the metasedimentary rocks. The granite gneiss is a dark-and light-gray layered medium-grained rock composed largely of feldspar, quartz, and biotite. Individual layers range from about an inch to several feet in thickness and result from different proportions of biotite. The perfection of rock foliation varies and is due to primary layering and parallel orientation of biotite flakes; foliation in the more alaskitic phases of the rock is poor. Bastin and Hill (1917) classified the rock as granite gneiss, but the composition of the specimens and thin sections examined by the writer more :;nearly appro~imate that of quartz monzonite. The average composition is about 44 percent plagioclase (An25). 30 percent quartz, 20 percent microcline, and 5 percent biotite. Other min,erals, not always present, include magnetite (up to 1 percent in some ,specimens), muscovite. hornblende. apatite, sphene, rutile, and epidote. The texture is granoblastic., Discrete layers and lenses of metasedimentary rock, commonly only a few feet wide. are scattered through the granite at places. Younger granite pegmatite intrudes the gneiss. Granite pegmatite The granite pegmatite is a medium· to coarse-grained lighJ-gray alaskttic rock composed primar.Uy of microcline and quartz. Biotite and magnetite are accessory minerals; magn~tite is more widespread and aburi.dant. Granite pegmatite bodies are generally conformable to the foliation of the other Precambrian rocks .. Bodies of pegmatite within the granite gneiss are discrete layers along the foliation and small irregularshaped pods in the crests and troughs of small folds. Pegmatites in the biotite-quartz-plagioclase gneiss occur as distinct bodies in part large enough to map (fig. 6) and as discontinuous thin conformable layers.

Tertiary rocks The bostonhe dike rocks in the area mapped belong to tlle Tertiary intrusive sequence of u:he Front Range and are among the most radioactive igneous series in the world. Phair (1952) has divided the bost-onites into three sub-types~-quartz bostonite porphy~y. non-porphyritic quartz bostonite, and syenitic bostonite-- depending on the presence or absence of quartz in excess of 5 percent by volume and by the · presence ot absence of megascopic phenocrysts of pink potash feldspar. The quartz bostonite sub-type of this area contains from 0., 010 to 0., 025 percent equivalent uranium and is 10 to 20 times more radioactive than the .intruded Precambrian rocks. Quartz bostonite Four quartz bostonite dikes were mapped on the surface and underground in the Wood-East Calhoun mine area., The dikes trend north.weaterlj;l dip moderately to steeply to the northeast, and range in thickness from one to perhaps 10 feet. Quartz bostonite is a lilac-colored fine-grained porphyritic rock that has a characteristic trachitoid texture resulting from the subparallel arrangement of feldspar phenocrysts in a groundmass of quartz and feldspar, The margins of dikes are finer-grained than the interiors; at places glass is present at contacts. Syenitic bostonite In the Wood drift east, about 22 feet west of the face (fig. 9), a relict 2-to 3-inch bostonite dike was found in what is now a northwest-striking vein. The dike is badly broken and completely obliterated on the north wall of the drift. Thin-section study shows that this dike has very little quartz, is high in mafic minerals. and that the phenocrysts are plagioclase. Based on Phair's (1952) classification this rock is a syenitic bostonite. Structure The principal Precambrian structure in the Wood-East Calhoun mine area is a broad, east-northeast trending anticline with gentle to moderate dipping limbs. As~ociated minor fold structures .. -warps and drag folds~~essentially parallel the anticliqalaxis, or transect it at an angle of sixty degrees., Mineral lineations, in general. parallel the fold axes.,

Steeply-dipping Tertlary faults trend east, east-.npnheast~ and northwest. The northwest faults served as loci for dike emplacement and the east and east-nc;>rtbeast faults are metallic~d (fig. The northwest faults are cut and displaced by both the east and east-nort~ast t~ending faults; the east-trending faults are displaced by the east .. northeast faults. Some of the joints were for~ed by the same forces ~hat produced the faults There is no apparent genetic relaticm between the Precambrian structures and the Tertiary faults. Folds The major structural feature is the Q1,.1artz Hill anticline. ~he trace of its axial plane in this area trends about N. 60° E. (fig. 21, but dist~ict-wide it average~ about N. 30° E. (Sims, Drake, and Moench, 1953), Associated with this major structure are n~efpus small drag folds wt).ich pll.lnge gently to the northnortheast or east-northeast. Superpos~d on the major anti~line and its as~ociated structures are a series of warps that trend N. 10°-50° Wo. averaging about N. 30° W., and ~ gently to th~ northwest or SOJ.Itheast (not shown on figures). LineatioH Lineations measured in the East CalhoQll and Wopd mines include m~neral alinement, small drag fold axes. warps, rodding, and corrugations. A loweT hemisphere Schmipt net~ plot of 110 lineations (fig. 3) shows four ma~ima 100 N. 30° E. 1 5°s. 600 W. 1 50 N. 60° E., and 15° :s. 25° W. These lineations, in general, parallel the major and minor fol<,ling in the ar~a ~tudied. Joints To avoid overcrowding, joints have not been pl!Dtted on figures 6 and 9, Instead, the poles to joint planes were plotted and contoured on the upper herpisphere of a Schmidt net (fig. 4). Three strong maxima representing joint sets are present: N. soP E. 1 ~early vertical; N. 40° W., nearly vertical; and N 45° W. I 700 NE. The steep N. SUO:E~ set closely approximate~ the attitude of the Wood vein and possibly was formed by the same forces that produced the east-west fractures. Th~ two O.Qrthwest-trending ma;xima probably represent statistical peak readi1;1gs on the same joint set.

N Wr 4 E D

0-2.5 percent 2.5-3.5 percent s EXPLANATION

3.5-4.5 percent 4.5-5.5 percent 5.5-6.5 percent FIGURE 3-COUNTOUR DIAGRAM OF 110 LINEATIONS IN ROCKS OF THE EAST CALHOUN AND WOOD MINES

D o-1 cent

1-2 percent N s EXPLANATION

2-3 percent 3-3.5 percent FIGURE 4.-CONTOUR DIAGRAM OF 386 JOINTS Plotted On Upper Hemisphere Of Schmidt Net 3.5-4 percent

Faults The three principal faults in the Wood-East Calhoun mine area are the east-trending Wood fault, and the east ... northeast-trending Calhoun and Quartz Mill faults. , These faults are metallized and are the major ve!ns·ohhe areao Minor east-northeast northeast-trending faults als9 are metallized. Several barrel northwest-trending faults were mapped underground. The faults in order of from oldest to youngest are northwest..,trending, east.,.trending, and east-northeast ~rendins, Calhoun fault,-- The Calhoun fault strikes about N. 65° E. and dips an average of 700 SE. It is the most persist~t fault on the south slope of Quartz Hill and has been traced on the surface for 4, 000 feet (Sims,. Drake. and Moench. 1953). Minor ore brecciation and the dev~lopment of gouge indicate that s~me post mineral movement took place along the break. Neither the true direction nor amount of mo,vemdnt on the fault is known; but on the fourth level of the East Calhoun mine a bostQnite dike has an apparent horizontal separation of 10 feet, the south wall having moved west with respect to the north wall. On the first level · of the East Calhoun mine the Wood ·vein is displaced by the Calhoun vein, the south wall having moved horizontally about 12 inches to the west. On the same level. the south segment of a bostonite dike haS' been shifted a few inches to the west, From these data. it appears that relative movement on the Calhoun fault was strike-slip with a normal type Qf displacement. Quartz Mill fault, --The Quartz Mill fault crops out south of the Calhoun fault and intersects it just above the sixth level of the East Calhoun mine (fig. 6). The fault strikes about N. 65° E. and dips 70°-75° N, It is similar in character to the CalQ,ouQ fault. In the Bez~nt mine. which develops the Quartz Mill vein. the: Quartz Mill fault displaces the south segment of a bostonite dike about 7 feet horizontally to the west, Therefore. it appears that the movement along the fault is similar to that along the Calhoun fault, The Quartz Mill-Calhoun fault crossing cannot be seen now, but Sanderson (1909) observed no displacement at the intersection. The attitude of the two veins i$ such that their intersection is essentially horizontal with local flat plunges to the east or west.

Wood fault ~-The Wood fault has been tr~ced on the surface for about 1, 000 feet (Sims, Drake~ an'd Moench, It trends easterly and dips steeply either side of vertical. The presence of abundant gouge. slip planes. and numerous slickensides indicates that, in contrast to the Calhoun fault, there was much recijfrent movement along the break. The attitudes of 70 slickensides were plotted and contoured on the lower hemisphere of a Schmidt net (fig. 5). This diagram indicates nearly horizontal maxima in an east-west direction indicating that the movement along the fault was essentially horizontal. Several steep slickensides, too few to show a statistical maximum, plunge vertically down the dip of the fault Where measured, these dip-slip slickensides are l~ter than the strike-slip slickensides. The direction and amount of displacement on the Wood fault are not known definitely, but it appears probable that the movement was largely horizontal, the south wall having moved eastward relative to the north wall. Detailed geologic mapping of the surfac~ (Armstrong. in preparation) shows that the south segments of two bostonite dikes have been shifted 30-40 feet to the east along the Wmod fault. This apparent horizontal displacement, however. is probably exaggerated because c;:>f the poor ~xposures on Quartz Hill On the first level of the East Calhoun mine. the south segpJent of a bostonite dike is displac~d perhaps 3 inches to the east. .(\s the Wood fault is distinguished by lateral movement, is nearly vertical, and varies little in strike and much in dip. it is probably a wrench fault (Anderson, 1951). Minor faults --The Wood exploration drift east (fig. 10) does not follow the Wood vein but instead is on a northeasterly split that dips steeply either side of vertical. This fault alines rather well with the Willowdale patented claim, and accordingly ~qe writer herein names this split the Willowdale vein. Moore and Butler (1952) show a split off the Wood vein near the shaft on the 135-, 197-, and 275-levels of the Wood mine that bears So 75°-80° W o and dips nearly vertical. On the 275 level west, about 200 feet from the Wood shaft. they show another southwesterly-trending split from the Wood fault. The writers mapping shows numerous minor splits from the Wood fault that bear either northeasterly or southwesterlyo Many ·~connecting loops between parallel to sub-parallel veins hav~ a general northe~sterly strike Although these rninm faults have the appearance of splays from the Wood fault, the p~esence of gouge and abundant slickensides as characteristic of shearing complicates the picture of fault movement. This is mechanically resolved

D

12-15 percent 15-18 percent 18-21 percent FIGURE 5 .-CONTOUR DIAGRAM OF 70 SLICKENSIDES ON THE WOOD VEIN Plotted On Lower Hemisphere of Schmidt Net

in the following interpretation of the order of faulting: 1) fonnati()n of the Wood fault and associated tension fractures the movement being south wall east with respect to the north wall; 2) formation of the Calhoun fault, the -movement being south wall west with respect to tpe north wall; 3) stress was relieved by movement along the Wood fault and shearing along the previously formed tension fractures; such movement :would tend to realize any displaced units along the Wood fault and may account for the apparent lack of displacement. U the above type qf fractuJing took place, one would expect strong fracturing in the vicinity of Wood-tension fracture junctions. Such loci are actually strongly fractured.. The Mayflower fault, mapped on the surface in the Wood-East Calhoun area, strikes about N. 50~. and dips nearly vertical., Its relation to the Wopd fault is obscure, preparation) believes that this fault extends both north and south of the Wqod fault, but the writer mapped no structure underground that co'relates with the Mayflower. Structure of Terpary dike rocks The Tertiary dike rocks in the Wood-East Calhpun mine ~rea fill steeply dipping fractures in the - Precambrian rocks. The dikes have essentially tqe s,ame attitude as the northwest-trending joint set and barren northwest-trending faults. This suggests, at least for the SJDall area studied, that the dikes were emplaced in northwest - attending - fractw-es~ :-; , Because the dikes are cut and displaced by the veins, it is inferred that they fill a fracture system older than the veins. EC'ONOMIC GEOLOGY The gold. silver, urani4m, copper, lead, and ~inc depos~ts of the Wood-~ast Calhoun mine area are found in veins that are thought to have formed at moderate temperatures. These deposits are early Tertiary in age and have been related genetically tQ tbe intrusion of porphyr.itic dike rocks (Lovering and Goddard, 1950, Po 170-191; Phair~ 1952). Gold,s sil~r.and t,lianlu.m accounted for of the dollar value of the ore produced.

Two main types of ore deposits give the Central City district a zonal arrangement in plan view (Leon~rd~ A core. about two miles in diameter, of quartz-pyrite veins is surrol,lllded by a wide outer zone of lead~zinc--:silver veiQ.s. The area of overlap of these two zones includes the Wood-East Calhoun mine area where transition veins contain pitchblende in addition to ores of gold, silver. copper. lead. and zinc~ Phair {1952) noted a spatial between uranium cleposits and the quartz bostonite porphyry rocks of the Tertiary intrusive series of the Front Range. As a result of his work, he inferred a genetic relationship betweei) the intrusion of these rocks and the uranium deposits .. The uranium deposits of the Wood-East Calhoun area carry pitchblende and occur at places along the metalliferous veins as pods and lenses. These pods and lenses occur in the same westerly raking ore shoots as do the other metals. Pitchblende was deposi~ed earlier than sphalerite, galena. chalcopyrite, gray copper, and most of the pyrite and contains high trace amounts of zirconium and molybdenum. H is to r;r a n d , d e v e 1 o e me t East Calhoun mine The East Calhoun mine (fig. 6) probably was opened sometime in the 1860's. 'l'he mine was operated intermittently until World War I. It was last re<;>pened in l,949, and a Defense Minerals Exploration Administration loan was granted in 1951. The East Calhoun shaft is inclined abo1,1t 70° south from the surf~ce to just above the sixth level, where the Calhoun vein intersects the Quartz Mill vein. The original shaft was continued on the Calhoun vein for perhaps another 30 feet (Sanderson, 1909), but as the vein was valueless the miners started sinking on the Qu&rt~ Mill vein which dips to the north. This vein was developed to a depth of about 980 feet. Kn 19M \the mine was inaccessible below the sixth level. The positions of drifts and stopes are shown of figure 7 .. Wood mine The Wood is one of the oldest patented locations on Quart~ Hill-number 232., It has been worked intermittently from the late 1860"s to date.. Pitchblende was f~rst n<;>ted on the dump of th~ Old Wood shaft in 1871 (Pearce. 1895D p .. 157-158) ..

ezoo 1500' illed , ,1952 I'

, Second level Third level /

Fourth level NE---+ Collar of Eoat Calhoun atooft EXPLANATION Approximate outline of workings Inaccessible workings FIGURE 7 . - LONGITUDINAL PROJECTION OF THE EAST CALHOUN MINE, CENTRAL CITY DISTRICT, GILPIN COUNTY, COLORADO

I...Lqo 2..J90 FEET /)Ilium is IINtlll SH noo' 1100' IIOGI'

The Wood vein is developed by the Wood and Old Wood shafts on the 2) and by the Ross shaft that is 320 feet east of the Wood veinr-Calhoun vein juncUon. M0ore and Butler (1952) mapped the 135-, 197-. and 2..75-levels of the wood mine ~fig. 8); other l~vels that are inaccessible supposedly are present at vertical depths ·of 400, 500, and 600 feet. Neither the depth nor the amount of working from the Old Wood shaft is known The Ross shaft is about 120 feet deep and is inclined about 85° south (Armstrorfg. in The vein has been stoped from the surface to a point about 70 feet below the collar of the shaft (Armstrong, in .preparation ) AU these workings were inaccessible in 19540 The Wood vein is. accessible from the first level of the East Calhoun mine by a crosscut north and a short drift (figo 6), and by the exploration workings on the s~xth leve~ of the East Calhoun mine (fig. 9). p,ro d uc tion East Calhoun mine The total production of the East Calhoun mine p.ot known. Table 1 the production from the Jefferson-Calhoun vein from 1902-15. This includes production from the East Calhoun, West Calhoun. Kemp Calhoun. and Jefferson mines. Residents of the districtgener~lly assign a value of about $1, 500,000 to the .ore produced from the Calhoun vein. Little ore was produced from the Calhoun vein in the East Calhoun mine as most of the stopes are on the Quartz M~ll vein. So far as known, no pitchblende has been produced from the East Calhouq vein. Wood mine There are no records of the total production from th,e Wood mine. Table 2 shows the subst~ntiated pitchbleQ.de production. The writer could find recorc( of only one ~shipment of gold and silver, but residents of the clistrict assign a value of about $600,000 to the ore mined from the Wood vein. Bastin and Hill (1917, Po 245) state that the ore extracted in driving the 200-level to the east averaged $10 per ton (gold value in 1917, $20.67 per o~ce). and that exceptionally, the gold content was very high, one assay showing 35 ounces.

West 9300' 92.00' 9 1CO' 8900' eeoc· 870(.' r Wood 0 1d "'ood shaft ( ! ! Eost Calhoun shoft E':lst Ca lhoun FIGURE 8.- VERTICAL LONGITUDINAL PROJECTION OF THE WOOD MINE, CENTRAL CITY DISTRICT, GILPIN COUNTY, COLORADO .J :6 0 F~et O:XPLANAT iON ·-

t.c:;rcc:TIC! c / / -Nork :r.gs p~o: ec ted ::'ltJ ,eoione sec! IO!" / / Eost A~- prc,mo'e ! "T' ts of ;)tc~o 1 er.de ·-:re shoot 5.~ 9UXJ' aec::J' 8700'

Table lo --Production from the Jeff~rson,.Calhoun vein, 1902-l5l/ Crvde ore Concentrates shipped shipped Gold Silver Lead Year (tons) (tons) (ounces) (ounces) (pounds) (pounds) 1,020 5, 829 1,051 3,533 3,429 1,51'0 7&5. 00 10,786 2,043 2,280 1,944 9,676 3,021 4,630 5,843 1,, 7:~4. 341 4,079 'I 3,828 19,373 5,476 1,125 3,333 49.q6 4e 1,406 1,577 7,014 2,027 342"98 1,012 6,338

3.£6 TOTAL 12,634 1,307 5,901.57 22,720 79,421 y A. J. Martin, Metals Econoptics Stanch, U. s. Bureau of Min~s. Oenver,, Colorado.

Table 2. --Substantiated pitchblende production from the Wood vei!l. Crude ore Coijcentra 1es shipped shipped Gold Silver U308 Year (tons) (tons) (omices) (ounces) (pounds) Remarks 1871!! Hand sorted 3,720 Do. 2,254 18841/ 4,200 1894J/ ? ? Pearce purchased a quantity of ore. 1897 y 27,000 Not fully substantiated. 7,920 1916 Jl 1o.eo ) ' 1, 200 1940 Jl 35al 53. 6fE!, 77al 1953 g) TOTAL 47,549 !) Armstrong (in preparation) gj Operator's estimate, prob~bly high. This ore was not s'Dipped, as quantity was not great enough to be economic. A. J. iMartin, Metals Economics: Branch, u. S. Bureau of Mines, Denver, Colo.

General charact~r of the veins The veins of the Wood-East Calhoun mine area are for the most part single well-developed fault fissures ranging from half an inch to 24 inches in width, with few to many; subparallel veinlets and slips which form rather complex lodes. Outward from these fissures the wall rocks are replaced for a few inches to a few fe~t. Exceptionally, the wall ro~k is silicifi~d and pyritized to widths as great as 6 feet; on the average, however, the altered zone is about 18 inches wide. The east .. trending veins, such as the Wood~ show a marked tendency to form both looping and "horsetail typ~" branches. These branches generally are more common in areas of metasedimentary rocks. tt'Horses" of rock between splits and parallel veinlets are strongly shatter~d. silicified, pyritized, and in some cas~s nearly completely replaced by ore minerals. The lode zones are in general rather uniform in strik~ but highly variable in dip. Ore shoots commonly occur on the steeper parts of the veins. All the veins show fracturing, open filling, and replacement to a greater or lesser degree. Breccia fragments of both altered wall rock and vein filling, granulation of minerals--especially pyrite- .. an.d numerous sets of crosscutting veinlets and slip planes indicate repeated movement. Deposition in open cavities is shown by the pronounced development of comb structure in the vein quartz and the abundant deposition of ore minerals as crystals and encrustations on the walls of openings. The veins are stronger in the granite gneiss, and in pl)~ces pinch to a barren slip in the biotitequartz-plagioclase gneiss and amphibolite. Calhoun vein The Calhoun vein strikes about N. 60° E. and for the most part is a single well-developed fault fissure that ranges in width from half an inch to 18 inches. It consists of pyritized silicified wall rock, fragments, gouge, pyrite, and quartz, with spotty chalcopyrite and sphalerite 'apd, exceptic;mally. a little sooty pitchblende. individual streaks of quart~ and ore minerals seldom exceed 3 inches in width. The silicified and pyritized halo averages about 18 inches, but may be as much as 5 feet wide. The vein is rather tight;oo .. much more so than the Wood--and shows littlE) evidence of much post.,.rnineral movement; however, 1-3 inches of gouge is present along the walls and along a few oblique post-mineral slips, and ore minerals-- particu~arly pyrite--are granulated. Deposition in ppen spaces is indicated by the presence of comb quartz, pyrite, chalcopyrite, and sphalerite crystals in a few vugs. Silicified and pyritized wall rock" relict islands

of pyrite surrounded by chalcopyrite and sphalerite, and the corrosion of quartz crystal faces by sulfides indicate that the vein-forming minerals formed. in part at least, by replacement. Xn general. the Calhoun vein is : narrow and sparsely filled where it cuts metasedimentary rocks. except where these rocks were strongly migmatized. The vein seldom splits or loops except on the fourth and sixth levels west. where the vein is in biotite-quartz-plagioclase gneiss. For the most part the vein is stronger on the .. steepso" The vein steepens in dip to the w~st and is nearly vertical at the West Calhoun shaft (!Moor~ and Butler. 1952); all t~e major stopes are to the west of the are,a mapped. Quartz M'frT vern T'he Quartz Mill vein strikes about N. 65° E. and dips 70°-75° N. and ranges in width from about '3 inches to 3 feet. Where present in the East Calhoun and ~ezant mines the vein has been stoped and little is known about its character. .It consists of silicified and pyr~tized wall rock fragments, quartz, and pyrite. Presumably. where stope d. the vein also contained chalcopyrite and possibly some gray copper~ for samples cut by Sanderson (1909) show moderate amounts of copper. In the area studied, most of the stoping was done on the steeper parts of the vein. Wood vein The Wood vein trends nearly east-west, and is highly variable in dip (fig. 10). It consists of one principal veinofilled fis~ure with many parallel to subparallel fractures and veinlets; therefore it more properly should be termed a lode (Lindgren, 1933, p. 157 .. 158). The principal vein-filled fissure ranges from 2 to about 24 inches in width.. Discontinuous streaks of sulfides and quartz seldom exceed 6 inches in width. The vein shows abundant evidence of repeated fracturing, open filling, and replacement.. Breccia fragn:tents of altered wall rock 0 granulation of pyrite and pitchblende, numerous crosscutting veinlets and slip planes. abundant slickensides, and strong gouge indicate repeated movement (fig. 11). Deposition in open cavities is shown by colloform structure in the pitchblende, the pronounced development qf comb structure in vein quartz. and the abundant deposition of quartz, pyrite, and sphalerite crystals as encrustations in v~gs., Replacement is indicated by silicifiedand pyritized wall rock and the textures of the ore mineralso The vein thickens near acute fissure. intersections because of the shattering of the intervening wedge of rock.

9300' 9200' 9100' 9000' 8900' 8800' s- -Ns-- Wood 197 level

"0

Wood 275 level

! I !1 I t i It

Position of 1 1 if present t I + I+

: t A'

Wood drift I Sixth level East Calhoun

mine I

Wood mine if present -N sB' / /

Section along line B·· B' j ; u · N ­

/First level Eo,;·

mine 9400' c' 930o' noo' ifJI

f ' f J t f t t

Section along line C·-C' f f f 9100' 9000 ' EXPLANATION Barren fracture 8900' Vein Dashed where oppraximotely located

T Logging along shaft Approximate outline of workings Inaccessi ble workings Note. Lines of sections shown on figures 2 ,6, and 9. FIGURE 10.- GEOLOGIC SECTIONS ACROSS THE WOOD EXPLORATION DRIFT, EAST CA-LHOUN MINE, CENTRAL CITY DISTRICT, GILPIN COUNTY, COLORADO , Datum is mean seo level 120 Feat j

8850' 8825' 8800' 8775' 41n. quartz. pyrite, sphalerite~. EXPLANATION Vein 131n. gouge, quartz, pydte,chalcopyrite Sulfide stringers Vein splits Barren fractures . and slip planes Fractures serving watercourses. into many subparallel fractures · Cholcopyrlte, pyrite, galena, sphalerite, quartz. lOin. pyrite, chalcopyrite, galena, sphalerite. · 12 in. gouge with 1/8 to 1/16 in. pyrite stringers. From footwall to hanging wall: ~in. pyrite, chalcopyrite. 8 1/2 ln. gouge with pyrite-chalcopyrite strlngera. ~ln. quartz with chalcopyrite-pyrite stringers. 20 in. strongly serlcltized rock with s.tringera and patches of pyr-ite, chalcopyrite. Ore much stronger In back of stope, da much a a 12 ln. of solid sulfides. From footwall to hanging wall: 31n. gouge with pyrite stringers. 6ln. chalcopyrite, pyrite, quartz in lenses and stringers. 13 in. silicified rock with quartz-pyrite stringer's. a in. gouge' with pyrite atrlngera. From footwall to hongino wall 1 in. gouge with fragments of chalco!)yrite. 10 jn. silicified rock with diuemlnated pyrite. 9ln. chalcopyrite, galena, pyrite, ~phCJ!erlte, tetrahedrite, quartz. 10 ln. gouge with pyrite stringers. !2 in. gouge with disseminated pyrite. From footwall to hanging wall 12 in. gouge with pyrite stringers. 3 in. chalcopyrite, galena, pyrite, sphalerite, tetrohedrite, quC~rlz. IOin.gouge with pyrite-chalcopyrite stringers and disseminated pyrite. 12 in. quartz, pyrite, chalcopyrite, galena, sphalerite. Very strong chalcopyrite. From footwall to hanging wall: ~J..U "d~ 3 in. brecciated pitchblende with pale yellow pyrite 11/2 iro. c.hclcopyrite with lote galena. 7 1/2. in. ~tl' ongly serlcitized rock with 1/8 to 1/64 in. pyr lt~ stringent. Post··m!nerol .siip on hanging wall. Note: Wall rock is pre-Cambrian granite gneiss. Sill,olt. 8755 ft FIGURE !I.-GEOLOGIC SECTION OF THE .WOOD 520-RAISE o 20 Feet

Oatum is mean sea level 8860' 8826' 8800' 8776' Geolooy by A.A.Oroke,

Similarly thickened parts of the vejn are also present between parallel fissures. The vein is stronger on strike changes to the northeast indicating that wider fi~sures were produced by movement so oriented as to move the south wall east. No obvious dip control was noted; however, the vein appears to be stronger on steep dips to the north. Willowdale vein The Willowdale vein, a hanging-wall split off the Woo_d (fig. 9), strikes about N., 45° E., and is highly variable in dip north and south. The vein is similar to the Wood in that it shows abundant evidence of recurrent movement. In general, however. it is a much weaker struqtme and carries sparse amounts of ore miner also Mineralogy The veins in the Wood-East Calhoun mine area contain in order of decreasing abundance: quartz, pyrite. chalcopyrite. sphalerite, tetrahedrite-tenn,antite, galena, pitchblende, bornite, sooty chalcocite, and covellitea The suite is generally typical of those deposits thought to have formed at mQderate temperawre and pressure. Q1,1artz The gangue mineral in all the veins is quartz. At least four types of quartz are recognized. . The first type is a light-gray, very fine-~rained, cherty material that replace the wall rocke In polished surface observed under the microscope, a similar"appearing fine:grained quartz is seen to ce.ment fragments of pitchblende. White massive medium-grained quartz replaces the wall rock and fills fractures. White to clear. terminated qua11~z crystals, as much as half an Iri~h long, grow outward from the massive quartz. line vugs. and form comb st~ucture in the veins. The last type quartz to form was a brown to gray. fine-. grained, sometimes banded. variety (the horn quartz of the miners) whi~h fills open s·paces in the veins and forms thin coatings on crystals in vugs.

Pyrite Pyrite is the most abundant mineral in the veins and four types are recognized The first type is yellow finely-crystalline cubic pyrite that impregnates the wall rock.. Another type--pale -yellow pyrite--is associated paragenetically and spatially with pit<;hblende in ~he Wood vein. l'his pyrite occurs in, veinlets and aggregates and is brecciated or fractured. Wher~ unbroken, it is largely cubic in habit Nearly everywhere observed, this pyrite veins pitchbleJJde or pit<;hblende-quartz breccias; but, in a few polished surfac~s it appears also lto be intergrown with pitchblende. The third type of pyrite~ . a yellow generally crystalline (mostly cubic) variety, is the principal vein-filling mineral. A fourth type of pyrite fo~;ms cubic crystals and encrustations on quartz and other crystals in vugs and is in turn coated with gray to br$Wn fine-grained quartzo Gold and silver occur type III pyrite (tC;lble 3). The other types of pyrlte also may carry go~d and silver. but data concerning them are not available. Table 3 .. --Gold and silver content of pyrite, East Calhoun and Wood veins, Central City district, Gilpin County. Colorado. Locality East Calhoun mine, 6th level, Calhoun vein, 800 feet west of shafto J! East Calhoun mine. 6th, level, Wood exploration drift, Wood vein, 500 feet t west of crosscut. 2J Y Armstrong (in preparation) Au Ag (Ounces per ton) J! Sized. superpanned, and hand picked. Analysis by u. s. Geological Survey Denver Laboratory .. Chalcopyrite Chalcopyrite is the second most abundant ore mineral in the Wood and Callloun veins. Veinlets of chalcopyrite cut sphalerite, pyrite, quartz, and galena. Under the reflecting microscope. a repl~cement texture composed of relict islands of sphalerite and pyrite was noted in chalcopyriteo Relict islands of chalcopyrite were noted in galena.. Two analyses of chalcopyrite (table 4) show a high silver and a moderate gold content ..

Table 40 Gold and silver conteflt of cllalcopyrjte. Wood and <:;,calhoun veins, Central Ci~y Gilpin County. Coloradoo Locality Au Ag (ounces per ton) East Calhoun· mine, Sth, level. Calhoun vein. 800 feet west of shaft. y E~s t Calhoun mine, 6th level, Wood exploration drift, Wood vein, 500 feet west elf crosscut., gj y Armstrong (in preparation.)., Denver Laborat~ry. Analysis by U. So Geological Survey 17.tW $phale\"iJe · Sphalerite veins cut quartz and pyrite. and sphalerite 1rystals fill fractures and vugs, The sphalerite is largely the darko iron-rich variety. marmatite (table 5), bt!lt "resin jack.. also is presema "Reslin Jaclk" is found in vugs as small tetrahedrons that have been deposed on dark sphalerite, Under the reflecHr1g microscope, sphalerite veins cut quartz and pyrite, fill opel spaces, and corrode pyrite along m uwal!oomdariecl suggesting replacement, Chalc9pyrite veinlets traveue sptlerite, and relict islands of sphaledte are in chalcopyrite. Tiny blebs and needles of chalcopyrite are distributed through the dark sphaJelt'Hea Mo£1t of thiS chalcopyrite is thought to be the result of replaceme~t, but some may have been exsolved, Sphalerite domains small amounts of gold and silver (table 5). Tenahedr~te .. Ternnanqte Gray copper is moderately a~UJidant in both the Calhol and Wood It is steel gray, has a red St't'e;a;k and CZ\ be mistaken for hematite. In polished secti0n the mineral nas a greenish cast suggesting that it may be near the teo nan ;ite end of the solid sol uti on, and !Hill ( 1917, p. 99) found that most of the grr.&y copper of the Cemral City district was tennantite. In ~polished section tentrahed!li11:e:=termandte veJi.m; chmkopyril!:e and sphallerite. Good "sea and island .. texture of gray copper in both and chalcopyrfJ:e

Table 50 --Analyses of $phalerite, Wood and Calhoun veins, Central City district, Gilpln County. Coloradoo Locality East Calhoun mine, 6th level, Calhoun vein, 800 feet west of shaft 1/o East Calhoun mine, 6th level, Wood exploration drift, Wood vetn: ~00 feet west of crosscut J._/. !J Armstrong (in preparation). Au Ag (ounces per ton) o. 76 J! Analysis by U s. Geological Su.rv~y , D~hve r Laboratory~ Fe Cu Pb (percent) Galena is sparse in both the Wood and Calhoun veins. Generally it is strongly crystalline and fills open spaces in the veins. Under the reflecting micro~cope galena veins sphalerite, tetrahedrite-tennanu:ite. and chalcopyrite, and in turn is q1t by chalcopyrite veinlets. Relict islands of pyrite, sphalerite. and chalcopyrite occur in the galen~. Galena streaks were also noted along cleavage of pyrite. sphalerite. and chalcopyrite. Relict islands of galena also were seen in chalcopyrite 10 Gold and silver Gold and silver are definitely carried by pyrite, chalcopyrite and sphalerite, and probably also by tetrahedrite-tennantite and galena. It is not known, however, whether these metals are carried mechanically or in solid solution. free gold also is present. Miner~ at the East Calhoun mine have panned gold from ore from the 520 stopes (figs. 8 and 9). The gold. is a "flour" type and. according to the miners. is lost by normal milling practice. No golcl was in polished section. The gold-silver ratio in samples from the Wood exploration drift is 1:5 (fig.

'1.25 ,

Lo

, ; .50 U' G / e G (J BG0 ye e (i) @ G (i) G G Q Silver 0%/ ton e --· -s / e I + 1'.22 12,$8 FIGURE 12:- GRAPH SHOWING THE RELATION OF GOLD .T O SILVER IN SAMPLES FROM THE. WOOD VEIN.

w en

Pitchble11de Hard lustrous pitchblende occurs as small qiscontirmous streaks and lenses on the footwall of the Wood veins The maximum thickness of individual lenses observed by the writer is 4 inches; however~ Bastin ( 1917 o Po 245) reported bodies as much as 2 feet thick on the upper levels of the -wood mineo The pitchblendelenses are separated from other parts of the vein hy a sn:ongp heavily slickensided fracture (figs 9 and Sooty pitchblende was observed in post-mineral slips near the hard pitchblende. Sooty pitchblende also was observed on fracture surfaces at the shaft on the first kvel of the East Calhoun mine; and on the same level it coats the vein wall just east of the Wood-Calhoun Vt"!in imersectiopo Megascopically. the pitchblende is hard. black, and lusttm.u;., It appears to be massive, but the ll.'reflecting microscope shows a coHoform structure The pitchblende occurs in veinlets and as aggregates of spheroidal grains., All of the pitchblende has been bmcdatedo Most grains have rounded ma1ruins, in part spheroidal. Many grains have been rotated. All spheroidal grain~ have both radial (syneresis) and circumferential cracks., In some sections tiny, apparently mass~ve grains of pitchblende are noted. These probably represent breccia fragments of larger areas of nearly uniform appearance. Most cracked grah'l.s are pre~ dominantly light gray and are in part healed by a darker gray pitchblende. which suggests brecciation during deposition~ Almost all grains have a slight color banding parallel to the colloform layering, the darker material occurring on the outside. This phenQmenon px:obably results from difference in oxidation., A little pale-yellow pyrite is intergrown with the pitchblende, bnt in most sections studied pitchblende is veined by pyritea In places the brecciated pitchblende is healed and cemented by quartz. Some pitchblende appears to replace quartz, but for the most part does not. In the pitchblendebearing sections studied, no minerals other than pitchblende. quartz, and pyri~e were noted. An analysis of pitchblende from the WooQ·mine (Hillebrand~ 1891) showed 58.51 percent U02 and 25.25 percent UO 3., The specific gravity is 8. 068., Lead /uranium age determinations on two specimens from the Wood mine (Stieff and Stern, 1952, p. 707) gave absolute ages of 57.3 and 60 million years.

Bornite Minor a.mounts of bornite were observed in ~he Calhoun vein west of the East Calhoun shaft; however none was detected under the microscope. The bornite probably is closely related to the chalcopyrite., S bot y c h-a 1 co cite Sooty chalcocite was observed in both the Wood and Calhoun veins in extremely wet areas. Kit is probably supergene. Covellite Thin brilliant-blue covellite coatings were noted on galena, pyrite, and chalcopyrite in both the Calhoun and Wood veins.. This supergene coatil;lg apparently is a district-wide phenomenon. as it has been observed by the writer in other mines in tlle district. ParageneSis The succession of events bringing about the formation of the veins is shown in figure 13. This sequence was determined from a megascopic study of the veins and examination of 20 polished sections. Mineralization began with the deposition of gray, fine-grained quartz which silicified the wall rock., Essentially contemporaneously with :the ·silicifilation., Jinely.crystalline pyrite was forme d in the wall 'rock, possibly from iron ions liberated from the rock by the altering solutions. Colloform masses of pitchblende were then deposited-in open spaces. Movements along the vein fractured the pitchblende, apd these fractures were healed by fine;..grained quartz and pitchblende. Paleyellow pyrite was deposited with the younger pitchblende. Strong' movement in the vein brecciated the quartz-healed pitchblende and the paleyellow pyrite. Subsequently ·white mass~ve quartz, yellow pyrite, comb quartz. and sphale;ite with exsolved chalcopyrite were deposited in open spaces. Sphalerite replaced pyrite to a limited degree. Renewed movement along the vein fractured the minerals and opened fissures in whi.ch chalcopyrite. tetrahedritetennandte. and galena were deposited. Clia1copyrite replaced sphalerite. pyrite, and. to a limit~d degree, quartz.. Tetrahedrite -tennan~ite followed chalcopyrite <;losely, replacing it and the sphaledre:o Galena filled open spaces around oth,er mineral grains and replaced sphalerite and gray copper., Galena was then replaced by late · chal,copyrite.

TIME Primary . Minerals Quartz i White,massive comb Pyrite Pitchblende Sphalerite

· Crystalline :;1 Pole

'll

. 'I Exsolutlon ~ha lcopyrite Tetrahedrite-Tenna ntite Galena ' Covel lite Fine grained CrystOiiine Minerals 13.-PARAGENESIS OF THE VEIN MINERALS '

Coarse pyrite crystals were then deposited on earlier minerals in vugs. The ~equence ended with gray to brown fine-grained quartz which (~oared ore minerals in vugs. The final event in the his~ory of the V(;'!~n formation was tpe coating of ore minera~s by covellite and the deposition of sooty chalcocite. These $econdary minerals were precipitated from cuprifemus supergene solutions. RESULTS OF SAMPLING Figure 14 presents in graphic form the results of assays for uranium, gold. silver. copper, lead, and zinc in samples taken from the Wood vein. Table 6 presents assay data from 132 individual samples. In the past, it w~s thought that pitch;>lende occurred in small bodies more or less randomly distributed through the vein. A study of figure 14 shows that uqmium valu~s are concentrated in one limited area of the vein, near the 520-raise. Only rtegligible quantities are detected elsewhere. The zone of uranium occurrence li~s .' on the projection of the Wood ore shoot (fig. 8) as Ol!tlh1ed by Moore and Butler (1952). The ore-shoot relations will be discussed later. The highest gold and silver assays come from samples cut in the uranium-bearing zone. The gol9uranium relation is of considerable interest in the Centr~l City districto Many workers in the area have remarked on the inverse relation bet,ween gold and uranium content of the veins~RiCRan:l (1913, p. 853) states: "it is axiomatic in these mines that as the pitch~ende comes in the gol~ goes out. and., as a matter of fact. the pitchblende ores seldom contain more than $2. 00 to $4. 00 in gold. '' This reported inverse relation does n<)t hold true for the Wood vein, at least f01: that part sampled in this study. Perhaps, the "old timers" sampled pitchblende and ~got podr gold: assays , Anotller possible ·explanation Is that pitchblende~ which ·was deposited before the: main; gold·bearing miner~ls~ almost completely filled available, open spaces where1it was <;leposited leav1ng little> rQ9m:;for subseque'nt ~gold-bearing ,·mineral5J c As less-pitchblerrde was deposited at depth1 there was more space for later minerals. Both pyrite and chalcopyrite have been shown to be auriferous and argentiferous (tables 3 and 4). The pyrite. however, does not appear to ·r;:arry sufficient gold to constit~te oreo Numerous samples of almost soli~ pyrite vein material showed very low gold assayso If pyrite were auriferous enough to (nake ore. one would

Sample number W-2 W-31 W-36 ' rJ-39 W-42 W-44 W-50 W-53 W-58 W-59 W-61 W-62 ri-66 W-70 W-74 'W-76 W-82 W-84 W-89 W-98 W-102 W-112 W-117 W-122 W-125 W-135 W-139 W-142 W-146 W-147 W-154 W-156 W-160 W-172 W-178 W-179 W-180 W-182 W-187 W-192 W-196 W-ZQl Width (inches) U Table 6 .-Some chemical analyses, fire assays, and semi-quantitative. spectrographic analyses dt ore from the Wood vein. !/ eU Au Ag Cu Pb Zn o.oo3 0.12 1.20 0.35 2.03 42.04 o002 I <.10 0 , .35 12.57 .1) 24.35 Tr. .000· Tr. .002 1.oo 1.74 Tr. Tr. Tr. ,002 Tr. Tr. .42 53.78 1.00 53.79 Tr. 1,46 1,16 Tr. Tr. Tr. Tr. ,001 Tr. Tr. Tr. Tr. 0.002 Tr. .002 Tr. .003 Tr. .001 Tr. ,001 Tr. .002 Tr. o·.24 70 .002 Tr. .004 Tr. .001 Tr. .000 Tr. .003 Tr. .003 Tr. .004 Tr. .001 Tr. .001 Tr. .003 Tr. .ooo .004 Tr. Tr. ,20 ·48 Tr. Tr. .)6 Tr. .26 18.44

1.52 31.61 , .10 19.03 .74 29.95 .17 13.97 2.03 26.72 0.14 32.36 .oa 16.12 30.9l: 1.93 3.31 2.77 25.96 1.37 2.10 9.60 22.17 .25 14.90 .38 14.11 .17 17.75 As Q Ba Be Tr. 'o Tr. Tr. Tr. Tr. ELEMENTS 2 Bi Co Cr Ga '5 Mo 5 , Ni 5 , Sb Sc , Sr '6 5 , y , 5 p Zr Cd B 0 , p ' o' Sn ' Ce 1J All assays and spectrographic analyses made by P. R. Barnett, G. W. Boyes, Jr., P. J. Dunton, S. P. Furman, W. D. Goss, R. G. Havens, E. C. Mallory, Jr., J . McGurk, W. Mount joy, w. w. Niles, J. p, Schuch, J, 1. Siver1ey, D. 1. Skinner, arxi J. \'lah1berg of the u. s. Geological Survey Denver Laboratory. y Au and Ag, in oz./ton. U, eU, Cu, Pb, Zn, and Fe2o3, in percent. Other elements reported by "group-number" method. GPO 837347

Sample number W-209 W-215 W-222 W-225 lf.-229 W-231 W-232 W-235 W-236 W-243 W-248 W-297 W-312 W-320 w .. 324 W-336 W-345 W-347 W-357 'lll-369 W-370 W-373 W-374 W-377 W-385 W-390 W-391 W-395 W-400 W-406 W-424 W-425 W-427 W-439 W-444 W-456 W-460 W-463 W-464 W-465 W- 472 Table 6 .-Sorre chemical analyses, fire assays, and semi-quantitative spectrographic analyses of ore from the Wood vein. · -Continued Width (inches) U eU Au Ag Cu Pb .002 1.40 Tr. Tr. !l'r. lo08 001 1.20 Tr. Tr. ,002 Tr. Tr. ,08 Tr.

Tr. Tr. .o6 ,004 ,002 Tr, Tr. Tr. Tr. Tr. 24 ,001 .04 10.14 .04 35.5f:l 2.82 18.49 .03 32.12 .01 13.69 .07 10.65 .50 25.80 1.47 10.01 .51 18.68 2.32 10.52 1.91 11.42 .17 10.13 1.19 25.08 .TO .30 17.99 .47 12.76 .09 1.66 20,28 .07 12.66 .68 47.11 .51 40.13 .45 30.73 5+ 5+ 45t ,001 Tr. .so 46.90 06 .23 31.08 .17 30.11 Mn 3+ 24t Jt 3t 3+ 3+ As 3t 34t 3Ba 6+ 6t 3+ 5Be Tr. Tr. Tr. Tr. Tr. Tr. Tr. Tr. Tr. Tr. Tr. Tr. Tr. Tr, Tr. Tr. Tr ELEMENTS Bi Tr. 5+ 4Co 6t Cr Ga Mo Tr. Tr. Tr. 6t 6+ Tr. Tr. 6+ 6+ 6t Tr.

Tr. Tr Ni Sb 4t Sc Tr. Sr Tr. Tr. Tr. Tr. y 6t 6+ 6+ 6+ Zr 6t 55-' 5+ 5Cd B Sn Ce GPO 837347

Table6 .-Some chemical analyses, fire assays, am semi-quantitative spectrographic analyses ot ore !rom the Wood vein. -Continued Sample number W-475 W-476 W-485 W-486 W-515 W..;621 W-524 W-53.3 W-5.34 W-5.35 W-5.37 W-55.3 . W-561 'W-562 W-56.3 W-568 Width (inches) u q f 0 42tV .eU .ooo .ooo .os Tr. Tr. Tr. Tr. Ag 26

Cu Pb Zn Ti 5t 4+ M 4Mn 3f 5t 70 Tr. .12 · Tr. ,4 5+ 4+ 5f W-572 DDH-:2 core. 0 .82 10.86 4W-574 Grab. Tr. W-575 Tr. W-576 Tr. W-577 9.94 9 .3 W-578 .010 .021 W-579 Grab (rock) 002 Tr. Tr. 4+ 4+ W-580 021 .026· Tr. 4t 4W-581 W-582 W-58.3 W-584 W- 585 W-586 W-587 W-588 W-589 W-590 W-591 W-592 W-59.3 'W-594 'W-595 'W-596 W-598 W-599 w-600 Core. .010 .017 Tr. .006 .006 .3.76 ;3.60 1.10 5.02 .19 .3.32 Tr. 45t .008 .012 Tr. Tr. 3t Tr. Tr. Tr. .oo6 .ooe Tr. .002 .006 1.28 28.72 6.71 1.67 .55 Tr. Tr. ,.105 1.57 Tr. Tr. .002 .005 .004 .007 Tr. Tr. Tr. Tr. Tr. u ~22 ; 0 4+ 44t 34+ 34t 4t 4t. 4+

4t As 1P 33+ Ba 5t 6t 35+ 6+ Be Tr. Tr 3Tr. Tr. 5t Tr. 5t 5t Tr. 5f 5f Tr. 5t 6t 4+ 4+ 5f 4t 6t 5t 6Tr. ELEMENTS Bi 56t 5t 45t 5t 5t 6f 45+ Tr~ 4+

4t .34f 4t 5Tr . Co 6t 6+ 56t

6f. 6f. 6+ 6t 56t 6t 56t 6t 6t 6t 5Cr 56+ 5Ga Mo Tr. 5t 5Tr. Tr. 6+ 5Tr. 56+ 6t Tr. Tr. 5Tr. 6f 5Tr. 6Tr. 6Tr. 6+ 6Tr. 55+ 5t 66+ 54~ 6f. 6+ 6t 56+ 6t 6t Tr. 6+ Tr. 6t Tr. 4o ' 55.,- Ni 6+ 6+ 6+ 6t 55t 5t &10 56t 56+ '56t 6t 55t 56+ 5Sb Sc Tr. Sr 5+ o . 55t 5Tr. 6t Tr. . 6f Tr. 6t Tr.

Tr. Tr. 5+ 6+ Tr. Tr. Tr.

6t 5t Tr.- 5f 5t 46t 56+ 6+ 6f. 56t 56+ 6t 65f 5t 55f 5+ 56t y 6t 55+ 6f 6+ 6-f. Zr 5t r 55+ 46t 5t 5f 5t 45t 5t 5f 5-f.-

5Tr. 5t 55t 45f 5t 5t 4t Cd 5+ 5f B Sn Ce .o 4+- 4+ . GPO 837347

expec!l samples. largely of pyrite. to show ore-grade assays. It has been long known that gold and copper values nm together (Basrdn and Hill, 191 '7). This is generally true for the Wood vein; however. the analysis of nearly clean chalcopyrite from the Wood vein (table 4) is low in gold. The writer believes that free goldpcssiply was deposited nearly contemporaneous with chalcopyrite, and that the copper~gold relation may be more apparent than real. Silver is carried by both gray copper and chalcopyrite. Galena is presumably argentiferous, but is too sparse to be of much interest either for lead or for silver. Moderate quantities of zinc are found. but at the present time zinc is of little economic interest. TRACE ELEMENTS Distribution patterns of trace elements in the ore from the Wood exploration drift were made from semiquantitative spectrographic analyses to guide uranium prospecting and to aid in the interpretation of ore genesis~ Table 6 presents the data obtained from the analysis of 132 samples. Results of this study show an association of zirconium and molybdenum with uranium; bismuth. antimony, and arsenic with copper; and cadmium with zinc. Threshold values, shown in table 7, are those attained in the spectrographic laboratories of the Geologicr al Surveyo A "group-number" method of reporting results of spectrographic analyses (Riley and Shoemaker, 1952, p. 18) is used in this report and is shown by the following data: Group No. Percent 10 to 100 1 to 10 o. 1 to 1 0. 01 to 0.1 o. 001 to o. 01 o. 0001 to o. 001

Table 70 ~~Threshold values of elements included in the semiquantitative method Element Percent Group number Parts per million Ti o .. oo1 Mn As IBa .,0001 lBe iBi o001 Co .,0005 Cr Ga .,001 Mo Ni .,0005 Sb Sc Sr y .,001 Zr Cd .,005 B Sn .,001 Ce .,05

In the tabula.1tions of the elements in various samples on table 6 0 plus and minus signs after group lillUmbers show the relative position within the group: plus .. -toward the upper limit of th~ range indicated a~nd minus~-toward the lower limit A zero (0) has been used to signify that the element was looked for but nolt found The most marked apparent variation in trace element content occurs 0 as one would expect. between samples of strong vein and samples of altered and sparsely mineralized rock. The vein samples contain a neady uniform suite of trace elements. and~ except for arsenic. antimony. bismuth, cadmium. zirconium, and molybdenum. show no appreciable variation in quantity (fig. 15). The two samples containing the highest uranium also show high zirconiull1--("group number" 3) (table 6 and fig. 16). The average zirconium content of all samples is within group 5 or 4, (fig., 15). A chemical analysis of pitchblende from the Wood mine showed 5. 47 percent zirconia (Phair. 1952. p. Hillebrand (1891) found that a pitchblende from the Central City district contained 7. 59 percent zirconia. tlo Ha Campbell (oral communication. 1954) found that samples from Gold Hill. Coloa. containing appreciable uranium also were high in zirconium. T. G. Lovering (oral communication, 1954) has found similar uraniumzirconium relations in the spectrographic study of pitchblendes from several localities. The direct relation of uranium and zirconium is of some interest. Phair (1952, p. 45) believed that most of the pitchblende now found in veins of the Central City district had its source in late stage uraniumrich differentiates of quartz bostonite magma These solutions mingled with regional solutions and rose along the planes of weakness provided by 'the porphyry dikes and l'acted with and leached part of the .uranium presento ,If. ~Phair believe~ ,(1952,- Po 22), the . uranium in the quartz bostonites is tied: up in zircon. it is not un~ reasonable to-assume that zirconium would also be picked up by the .solutions a.nd deposited with the pHchblendeo A tenuous relations~ip between uranium and molybdenum is present in samples from the Wood veins Samples of the uranium-bearing part of the vein near the 520-raise contain more molybdenum than is normal (figs 14. 15. and 16). A molybdenum-uranium relation has been noted in other areas--at Climax (Ro Uo King. oral communication. 1954). in the Henry Mountains (Riley and Shoemakero 1952, pl" 47). att the Happy Jack mine. Utah (To Go Lovering. oral communication, 1954), and in British Columbia (Drake, previous Sparse molybdenite has been found in the Central City district (Bastin and Hill, 1917. Po 105). and where present. was the first mineral to crystallize_ The writer can do little more than point out the molybdenum-uranium association and note that they both were deposited early in the paragenetic sequence

.Q E :J z Bi Sb Zr.. Amount present in group numbers ·0 Cd Co FIGURE 15.-DISTRIBUTION HISTOGRAMS OF SOME ELEMENTS ·1 N 132 SAMPLES FROM THE WOOD VEIN ::n

Cd · 6 04 W

Sam pi n um bera FIGURE i·r.- GRA~H S HOWING THE RELATION BETWEEN ZINC AND CADMI UM IN SAMPLES FROM T HE WOOD VEIN. Mo

A~/

:1 Zr c t a.e

E :1 a c

I!

c a e u :1 :1 0 ·- e 't; Sample numbua FIGURE 16.- GRAPH SHOWING THE RELATION OF MOLYBDENUM AND ZIRCONIUM TO URAN IUM IN SAMPLES FROM THE WOOD VEIN. 1~2

Cadmium i'S locally present in the Wood ores (table,, 6) an may be quantitatively correlated with zinc (figo 1 7) 0 Sphalerite contains a large proportion of all ccad~ iUJTI, up. to 4·0 5 perceJit (Ra:nkama and Sa,hama, 1950.,JPo 708), and is thought to occur in sphalerite in the Kn 1the Wood mes, arsenic. antimony, and bismuth occu in varying quantities (fig" Kn general. they 'elate rather well with copper (Ug~ 18) 0 Inasmuch s tetrahedrite-tennantite is present in the vein. itt seems reasonable tto relate the amount of arsenic and anti ony in samples directly to the amoun1t of gray copper presento Bismuth is grouped with arsenic and antimo y. and, in general, behaves in a simli.lar manner (Rankama and Sahama. 1950. p. 738). Tetrahedrite may c ntain bismuth, commonly less than 2 percent. whereas bismuthian tennantite may contain as much as 13 p rcent bisrnuth (Palache, Berman. and Frondel. 1944o Pc 379). It is thought, therefore. that the bismuth pr sent in the Wood ore is in tetrahedrite·' tennamite. Nickel and cobalt minerals frequently are associated wi h uranium deposits (Everhart and Wright, 1953). Minerals containing these elements have not been identified however, in the Central City district., Samples of the Wood ore contain (reported by group number) 6 to 4 n'~kel, averaging 5; and 6 to 5 cobalt, avex:~ging 6 (figo The four high nickel samples were taken from ltered rock that contained hornblende., H i$ not certain that all the reported nickel and cobalt was origin lly in the ~samples as they were pulverized in steel grinding machines and the amounts of these constitu nts present are within limits of contamination set by Myers and Barnett (1953. p. WALL ROCK A TERATION Particular effort was made to detect a difference in the all rock alteration between pitchblende;' bealfing and non~pitchblende bearing parts of the Wood veins Unfortt,mately. no difference wa~ Megascopically. the wall rocks are silicified, pyritized and altered to argillic mineralso The width of alteration varies directly with the vein strength, ranging f om about 6 inches to 5 feet, and averaging about 18 inches lin general, the alteration progresses outta d from a pyritized and silicified zone, through a silicified zone. to an argillic zone At most places, pyrit zation is stronger where the wall rock origin~ ally contained a large proportion of mafic minerals, suggest' g that at least part of the pyrite was formed from iwn ions Hberalted by the altering solutions.

"' . Sb 5 r r

r L.JLJ!

I f

c: g.

4 "' 81 c: ·;. c: 0 eo Q) CD o '- s:.

..; c: G; c:

y -

, Sample Numbers FIGURE 18.-GRAPH SHOWING THE RELATION BETWEEN COPPER,ARSENIC, BISMUTH,AND ANTIMONY IN SAMPLES FROM THE WOOD VEIN +:' w

Micmscopk swdy that the 'fe'ldspa:n; were a generally the firut to be affected~~they are se:riciHzed and in part altered to gre~nmo.ntmorilloniteo Mafic minerals are bleached and altered to chlorite (?)o MicmcHne is last to be affected and is in part sericiti~edo Some kaolinite is present and appears to be A detailed study of wall rock alteration in these and other veins in the Central City district cunem.ltly is being carried on by E., Wo Tooker of the Geological Survey, and the results of this WO!!'k will be p!resumed in a later publication ZONING The mineralogy of the Wood, Calhoun 0 Willowdale, and Quartz Mill veins changes from essentially quartz ~pyrite in the east to quart~ '"'PYrite .,.chalcopyrite -sphalerite -galena -tetrahedrite -tennantite in the west This change in mineralogy along strike is part of the larger . hypogene zoning pattern of the Central City district (Leonardo In simplified form, the zoning consists of a core of quartz -pyrite veins surrounded by a shell of galena-sphalerite veins. The veins mapped in this study lie in the care transitional between these major zones. It also is - expected that changes in mineralogy should occur vertically along the veins Evidence suggests that such a change does take place for the Wood-Calhoun group of veins. The Calhoun veins on the upper levels of the East Calhoun mine, is largely filled by quartz and pyrite. Chalcopyrite, gray copper galena. and sphalerite are moderately abundant in the Calhoun vein below the fourth level Similarly. the Wood vein above the 197-level consists principally of quartz and pyrite (Moore and Butler. 1952); below that level galena and sphalerite are presento This change in vein min~ralogy with depth suggests that the transition zone at this locality dips to the east. Leonard (1952) found that the known major pitchblende occurrences of the Central City district ate concentrated in the transition zone; Wallace and Campbell (oral communication1 1954) believe that pitchblende is genetically related to a zoning sequence. Emmon~ (1927. p. 35), in his classic zoning sequence placed uranium. above copper and below zinc, overlapping both; consequently. it is to expected that pitch~ blende will be closely related to copper-bearing minerals and sphalerite both in space and Umeo in the

Wood vein" the spatial relation generally holds true. In specimens studied by the writer. however the pitchblende is earlier in the paragenetic sequence than both chalcopyrite and sphalerite. Bastin and Hill ( 1917" p. 123~ 124) report that chalcopyrite is intergrown with pitchblende in specimens from the Wood veins suggesting that they were deposited essentially contemporaneously., These conflicting data suggest that the questionable relation between pitchblende and the zoning theory remainopen for further study. ORE SHOOTS Mapping and sampling in the East Calhoun and Wood mines show that minable ore is not uniformly dli.snibuted throughout the veins.. Most of the ore bodies occur in shoots that apparently are the result of deposition within open spaces along pre-mineral faults. Factors influencing the development of open spaces include: the competency of the wall rock. the presence of a northwest-trending joint set. changes in stri,ke and dip. and intersections and near-intersections of individual fractures .. Openings along the fractures were best developed where the faults intersected relatively brittle wall rocks. In order of favorability. at the Wood and East Calhoun mines. the most favorable wall rocks are granite gneiss. pegmatite. migmatite. bostonite, amphibolite, and biotite-quartz-plagioclase gneiss. Another feature favorable for the development of open spaces is the presence of a strong northwest~ ~:rending joint set (fig. 4) in the c.ountry rocks. The intersection of the veins with joints of this set plunges about 70° s. 80° W. This plunge approximates the rake of the shoots shown by the stope outlines on figures 7 and 80 Theoretically. the rocks shoulCL.be inteliselybroken,at the intersection of the two fractures. The favorable open space so created should occur along the plunge of the intersection. A relationship between the steepness of dip and strength of vein was noted~ particularly on the Calhoun amd Quantz Mill veins. En general. the steeper the dip. the stronger the veins The Wood vein is suonger and richer where it changes strike toward the northeast.

Small1 local ore bodies are found near the intersections and near-intersections of branching and sub~ parallel fissuresa These bodies are attributed' to open spaces produced by the shattering of the rock between The principal ore shoot in the East Calhoun mine-between the sixth and tenth levels==Ji.s on the Mill veina The stope outlines on the longitudinal projection (fig. 7) suggest that the ore body raked about '15° .. 80°. Sa 70° Wa Xt had a stope length of 100 .. 300 feet, and a rake length of 450 feeto Miners at the propeny generally ascribe this shoot to the Calhoun and Quartz Mill vein intersection h is difficult to conceive. however. how such a flat intersection could create a steeply raking open space Sanderson (1909). in a prllyau:e report on the Bezant mine. outlined an ore shoot in the Quartz Mill vein with the !lame general attitude as the shoot mined in the E~st Calhoun mineo As has previously been mentioned. this shoot lines up with the stoped ground below the East Calhoun sixth levelG The writer believes that the ore mined below the East Calhoun sixth level was a continuation of the Bezant ore shoot The Wood vein has been stoped on the upper levels over most of its explored length It is thought thatt much of this stoping was done in the oxi~ized zone, and that the ore here was mechanically enriched in golda M,oore and Butler (1952) outlined a pitchblende shoot on the upper levels of the Wood mineo Pitchblende bodies found in the exploration done, during the writer:~s study, line up rather well with the projected rake of ll:his shoot (figo As has been previously mentioned, gold. silver, and other values also were found to be greatest in this areao Therefore it is thought that the pitchblende shoot outlines a general me shoot lit must be remembered that pitchblende occurs as relatively small pods, lenses, and kidneys scattered through the _vein; therefore, the pitchblende shoot only outlines ground favorable for the occurrence of pitchblende ORIGIN The Woodo Calhoun. Quartz Mill. and Willowdale veins were filled by minerals deposited from hydro~ thermal solutiomo Repeated movement along the veins throughout the period of ore deposition reopened channels through which the vein -forming solutions could migrate a Deposition of minerals by fraclttue filling and by replacement of earlier minerals was generally restricted to the zone of fracturingo although the wan rock is silicified and pyritized over widths as much as 5 feeto Mineral deposition occurred in two general stages==quanz=pyrite and galena=sphaleriteo

Pitchblende. one of the first minerals emplaced, apparently was a local phase of the general quartzpyrite stage of mineralizationo Alsdorf ( 1916. p. 273) thought that the precious metal-sulfide veins were later than the pitchblende veins. and that (1916, p. 270) the pitchblende veins were cut across. followed. and obliterated by subsequent faulting and precious-metal vein filling. The pitchblende bodies observed i!DI. thu study cenainly show every indication of being deposited prior to the main precious metal-sulfide filling. but lie in the same vein. and even in the same ore shoot as the later minerals. Therefore. the writer believes that the pitchblende represents an early local variant of the quartz-pyrite stage of mineraU~ zationo It is difficult to reconcile :the onset.wed·t - field - and'parag~netiC : rel.\uibns with Leonard"s (1952). hypogene zoning theory. Of course, such factors as telQs'coping be applied to show that it is possible to have pitchblende deposited as the first metallic mineral and still be genetically related to a zoning sequence. No evidence of telescoping was noted in the mines. The writer prefers Phair's (1952) hypothesis that re~idualsolutionsfrom the differentiation of a quartz bostonite magma mingled locally with regional hydrothermal solutions and rose along the faults. Pitchblende was deposited when the temperature was lowered sufficiently bring about reducing conditions. The pitchblende was preceded in deposition by quartz and a little pyrite. Recurrent· movement reopened the veins to allow the deposition of the later precious metal-sulfide ores. It is· difficult to assign these deposits to one of the usual pres:sme-temperature classifications. Armstrong (in preparation) classified the ore deposits of Quartz Hill as xenothermal. largely on an infened temperature of formation. His temperature-of-formation refe+ences were based on the presence of exsolved chalcopyrite in sphalerite (chalcopyrite and sphalerite supposedly unmix at about 350°-4000 C, Edwards. 1947) and on the uo2~uo3 ratio of the pitchblende (pitchblende with a relatively high uo2 percentage presumably indicate a high temperature origin. Tomkeieff. 1946). The writer prefers to classify the deposits as leptothermal (Graton. 1913, p. 536-540). The deposits have many characteristics of the mesothermal zone. yet plentiful vugs and the development of comb structure in places indicate that the conditions of formation were somewhat shallower than most mesothermal types.

AGE Bastin and Hill ( 1917. pG 93) considered the ore deposits of the Central City district to be early Terriary agea Recent absolute age determin~tions on two pitc:;hblende specimen,l,s from the Wood mine by the Ph u238 ratio method gave ages of 570 3 and 60 million years respectively (SHeff and Stem. 1952. Po These data confirm the early Tertiary age postulated by Bastin and Hillo FUTURE OF THE MINES The exploration carried on during this investigation faile~ to develop economic quantities of pitchblende in the Wood veins Pitchblende apparen,tly decreases in abundance with depth and. as the upper parts of the Wood vein have been rather well explored, it is thought that there is little chance for substantial new manium production The ore shoot oJ,ltlined in this study contains moderate gold and silver values. but the vein material generally is not now economic. If conditions for gold mining become favorable in the future. small quantities of pitchblende probably will produced as a byproduct.

LITERATURE CITED Alsdorf$ P e R", 1916Q Occurrence, geology, and economic value of the pitchblende deposits of Gilpin County. Colorado: : Econ" <Geology, v .. 11, 266-275. Anderson, Eo M .. , 1951, The dynamics of faulting and dyke format~on with applications to Britain. Oliver and Boyd. Ltd ... Edinburgh. 206 p .. Bastin. Eo So, and Hill. J .. M .. , 1917, Economic geology of Gilpin County and adjacent parts of Clear Creek and Boulder Counties, Colorado: ; U. s .. Geol. Surv~y Prof. Paper 94, 379 Po Edwards, Ao B ... 1947. Textures of the ore min~rals. Melbourne, Austrablsian Inst .. Mine and tallurgy One.,). 185 Po Emmons. W .. He, 1927, Relation of metalliferou~ lode systems to igneous intrusions: Am .. Inst .. Min" Met. Eng. Trans v. LXXIV, p. 29-70. Everhart. Do L and Wright, . R. J" , 1,953, The geologic character of typical pitchblende veins: Econ" Geology, Vo 48, Po 77 ... 99/ Graton. L" C". 1913, The depth-zones in ore deposition: Econ. Geology, v. 28, p .. 513-555" Hillebrand, W. F" ; 1891, On the ~ccurrence of nitrogen in uraninite and on the composition of uraninite in general: U. s .. Geol. Survey Bull. 78, p. 65-66. Leonard, B. F., 1952, Relation of pitchblende deposits to hypogene zoning in the Front Range mineral belt. Colorado (abs.): Geol., Soc., America Bull., v. 63, p. 12~4-1275. Lindgren. Waldemar. 1933, Mineral deposits. 4th ed., McGraw-Hill, New York. p .. 157-158. Lovering. To So. and Goddard, E. N", 1950, Geology and ore deposits of the Front Range. Colorado: u. s. Geot Survey Prof. Papet 223, 319 p. Moore . F" Bo. and Butler, C., R ... 1952, Pitchblende deposits at the Wood and Calhoun mines. Central City mining district. Gilpin County, Colorado: u .. S., Geol .. Swvey 186, 8 p" Moore. Ro B", and Kithil, K. L.,, 1913, A preliminary report on uranium, radium, and vanadium: U" s. Buro Mines Bullo 70. p. 46., Myers. Ao T and Barnett. P. R. , 1953, Cc;mtamination of rock samples during grinding as determined spectrographically: Am. Jour. Sci., v. 251, no. 11, p .. 814-830. Palache. Charlt:~s . Berman. Harry. and Fronde!, Clifford, 1944, Dana's system qf mineralogy. 7th edo. Vo 1, John Wiley and Sons, New York, p. 374-384. Pearce. Richard, 1895, Some notes on the o<;;currence of uraninite in Colorado: Colorado Sci. Soco Proc .. V 0 5 0 Po 156-158 0 Phaill'. George. 1952, Radioactive Tertiary porphyries in the Central City district. Colorado. and their bearing upon piltchblende deposition: U., S., Geol., Survey TEl-247, u. s. AtomiC Energy Commo. Tech. Info

Rankama, Kalervo, and Sahama, Th. G., 1950, Geqchemistty. Univ. Chicago Press~ Chicago, p. 708-738. Rickard,, Forbes,. 1913, Pitchblende from Quartz Hill, Gilpin County, Colorado~ Mining and Scientific Calif.), v. 106, no. 23, p. 8p1-856. Stieff. L. R., and Stern, T. W., 195~, The identification a11d lead-uranium ratio ages of massive uraninite from theSliiharu.mpcongl:omerate, Utah: Science, v. 115, p. 706708. Tomkeieff, s. I., 1946, The geochemistry of uranium: Sci~nce progress (London), v. 34, p. 696-712. UNPUBLISHED REPORTS Armstrong. F. C. in preparation, Pitdbblende deposits in the Central CitY district, Gilpin County. Colorado: u.s. Geol. SurveyTraceElE~mentslnv. Rept. 177. Guillotte. G. B 1944, Uranium deposits of Gilpin County, Colorado: Vnpublished typescript Union Mines Development Corp, report, 18 p. Riley,' L. B., E. N., 1952, J;>istribu~ion of ore depOBits and spectrographic analyses of some , rocks and ores on the Colorado Plateau: u. s. Geol. Sprvey Trace Elements ·lnv. Rept. 278. Sanderson, H. s., 1909, R'eport on the prppert~es o( the Bezant mining Company: Unpublished typescript private mining report, i5 p. Sims, P. K.1 Drake, A, A. 1 and Moenc'tl, R. HH PreliQl~nary geologic a('ld vein maps of part of the Central City district, Gilpin and Cle~t.r Creek Counties, Colo~ado: tr. s. Geol. Survey Trace Elements Inv. Rept. 304 (3 illustrations opep.f1led,. Jan. lr954).

Reserves a Recommendations Literature cited a Unpublished report OFFICIAL USE O~LY USGSTEJ-175e Parr 11 CONTENTS 'RESERVES :. Page Moore and Bulter (1952} on the basis of 5 channel and 3 chip samples, estimated 40 tons of indicated ore below the 135-level and 40 tons of inferred ore above the 135-level of the Wood mineo For both indicated and inferred ore they estimated average width of 6 inches and an average grade of Oo 27 percent Armstrong (in preparation) believes, and the writer concurs, th~t previous operators would not have left much high~grade ore in faces and backs, and that an:y s'ampling don~ in such workings would not give a true picture of the unmined vein material. A method of arriving at the average grade of material excavated from a mine is to divide the total production by the total material removed, and to extrapolate this grade to the unmined portions of favorable areas in the mine. Using this method, Armstrong inferred that the Wood vein combined 85$000 tons of Oa 189 percent U308 over a mining width of 5 feet above the East Calhoun sixthlevel crosscuto The crucial factor of this;imethod of calculation was the assumption tbat pi1tchblende-rich areas were randomly distributed through the Wood vein. Unfortunately~ exploration disproved the random distribution theory. at least for the lower portion of the Wood vein. OFFlCIAL USE ONLY

OFFICIAL USE ONLY Approximately 600 feet of drifting west of the sixth level crosscut (the east drif1t is ignored in this treatment as it is not on the Wood vein) exposed an estimated 800 pounds of uranium Using the same method of calculation as Armstrong used, 25~ 200 cubic feet of material, or 2. 100 tons. was ·excavated in driving this drift" This rock an average grade of something less than Oo 0002 percent uranium UsJng the same method and taking an average vein width of 6 inches. the vein matter removed had an average grade of abou~: 0., 003 percent uraniuJO About 115 tons of vein material were removed by raising. subleveling. and stoping in the' favorable pitchblende area (vein width 6 inches. tonnage factor 12) to get 800 pounds of uranium Therefore. this material had an average grade of 0 0 0034 percenturaniumo Combining the tonnage removed to reach the favorable area. 170 tons~ and the tonnage removed from the favorable areai 115 tons. it may be i:Seen that 285 tons were moved.tQ gee 800 pounds (0. 4 tons) of uranium. This material had an average grade of o. 0014 percent uranium . Approximately 500 tons of vein material should be present in the favorable area between the sublevel off the 520=-raise and the 275-level of the Wood mine. Using the average grade of material removed from "'"""' I) 13 the favorable area, "0 003;) about 3, 000 pounds of uranium could be recovered. if the above material was mined Therefore. the writer assigns 3, 000 pounds of inferred reserves to the Wood veins Pitchblende bodies may. of course. occur more frequently at higher altitudes in the favorable area, but there is no assurance that they willo Gold and silver values also uccui in this area, but a test stope did noli: pay expenses. The writer feels that the economics are such as to disallow any further work on the Wood vein. RECOMMENDATIONS The writer believes that economiC co1;1siderations fail to !justify any further exploration on the Wood veins Kf. however. it i.s decided to carry on more work, it is suggested that it :be concentrated in the shoot ~trueture as outlined on figure 8. This structure can best be explored by a raise up its projected rake The drift on the reported fifth level of the Wood mine might be encountered in this raising It is not known if the Wood mine is completely drained; therefore, suitable precautions should be taken in raising. OFFICIAL USE ONLY

OFFICIAL USE ONLY Another place favo~able for exploration is on the first level of the East Calhoun mine (fig" 6). The ~ast face of the drift on the Wood vein should be a~vanced about 150 feet east~ This drifting might cut the pH:chblende shoot that was mined in the Ross shaft. The writer thinks ihat'any further exploration on Quartz Hill should be confined to the veins that have been little developed between the Wood and German veins. He can ~ee no reason why any veins in the area should not contain pitchblende; "(therefore. those that have not been mined offer the most promise., LITERATURE CITED Moore, Fo B. o and Butler. c. R 1952, Pitchblende depodts at the Wood and Calhoun mines, Central City mining district, Gilpin County0 Colorado: U. s. Geol. Survey Circ .. 186 0 8 Po Phair. George. 1952, Radioactive Tertiary PQrphyries in .!be .Centr.al City districl, Colorado, and their bearing · upon pifdhbleirde' ~ep6$ition: ' 'U. s.··:Geol.' Survey 'TEI~24.7, TU_. S. A-comic Energy Comm ... Tech. Inf. Service, Oak Ridge, Tenn. UNPUBLISHED REPORT Armstrong. f. c .. in preparation, Pitchblende deposits in the Central City district, Gilpin <;::ounty, Colo,radp: U., So Geol. Survey Trace Elements Inv. Rept. 177. OFFICIAL USE .:ON;l;,l / !

Plates & figures from the original