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The Geologyand Ore Deposits f the BullyHill Mining District,California

Of '. 2N 1V i. i S I T\

BY

ALBERT C. BOYLE, JR.,LAKAMIE. WYOMING

SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FORTHE DEGREE OF DOCTOROF PHILOSOPHY,N THE FACULTY

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58 BULLY HILL MINING DISTRICT, CALIFORNIA "* - *tI. INTRODUCTION

THEgeologicalield workof the Bully Hill district,uponwhich h^^ *paper s based,wasbegun uly 1, 1908, nd covered periodof thmonths. The time was found too short for a complete report and thdistrict wasagainvisited n the early part of July, 1912, rom which timdetailedstudy of the ocality continueduntil September f the same earThis paper s the result of observations ade n the mines n the immediatty of Winthrop, Shastacounty, Cal, during these wo periodsof

tudThe preparation of this report has been greatly facilitated bycourtesies rendered the writer by D. M. Riordan, President of the Bully

Hill Copper Mining & Smelting Co., and by John B. Keating and HerbertR. Hanley. managing officers actively in charge at the mines. Thelaboration and further study of the notes and collections were carriedin the laboratories of the Departments of Geology and Mining at Colum-bia University, and the writer takes pleasurealso at this point in express-ing his acknowledgments to Profs. James F. Kemp, A. W. Grabau,Charles P. Berkey, and William Campbell, for advice and assistance.In order that one may appreciate at first reading the significance ofsome of the details which follow, and for the sake of clearness, a briefoutline of the geologicalrelationships is here given, together with certain

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60 BULLY HILL .MINING DISTRICT, CALIFORNIArue. By secondary hangeswhich involved alt tion. the ores b

lied so that in som pper content of 8 pduced.Of considerab ! t in this district are th tionships betweenthe shearzonesand the variousdikes; h P nt. and genesis fthe ore bodies; he occurrence f lai m >f ulphat me, whichsuggestsormation by a process ot h gnizedof g mpt Ofg ,t economic ignificance re the associat f PPth the alaskite-porphyry,and the interpretation of some acts estabhed in prospecting the extensive massesof gypsum.The area covered by the map, Fig. 1, ex ds 2 miles east and westd 3 miles north and south. It braced in the Redding F f thU. S. Geological Survey, which was prepared by J. S. Diller, to whf i. tions the writer is deeply indebted. Th t d Pment of the mines has naturally revealed, h ever, somepoi ts of interestwhich t accessible when Mr. D bservations w *e made, andas a result some modification of his views will be supported.

II. LOCATION OF THE DISTRICTThe ully Hill district is one of several mineral belts situated in a

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BULLY HILL MINING DISTRICT, CALIFORNIA 61years and which is now under consideration lies in the immediate vicinityof the town. There are three local mining centers, designated as Bullhill to the north, Copper City to the south, and the Rising Star betweenthese, but somewhat nearer to Bully Hill.

History of the Mining DevelopmentThe existence of copper ores in this part of the mineral belt was knownin the first decade of the '50's. It was encountered in small quantitiesin tunnels which were opened primarily for the gold and silver veinswhich yielded the placer values. In 1853 placer gold was discovered nthe vicinity of Bully Hill1 and in 1862 gold was found in the altered surface

rock near the present site of Copper City.2 The copper deposits wereneglected for the time being, and little of importance happened before1895.

Oxidized ores from the surface soon failed to yield acceptable returns,and tunneling was employed with the hope of discovering sourceswhichpresumably supplied surface values. Tunnel No. 1, on the east slope ofBully Hill about 500 ft. from the top, was driven a short distance. Sincethe surface somewhat to the west and north had yielded profitable returnsin gold and silver, this tunnel was driven with the view of striking thegold-bearing rock in depth. As the working face advanced, the gossan

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62 BULLY HILL MINING DISTRICT, CALIFOBNIAIt is reported that in three years the company extracted nearly

$650,000rom the ores. Therich valuessoonbegan o decreaser changcharacter to such an extent that this mill was abandoned after a comiratively short period of operation. The equipmentbecamehe propty of Messrs. otterandHall,whohadpurchasedome f the adjoinirjlaims, but, like the Extra Mining Co., they were orced to discontmperationsbecause f the complexnature of the ores.The entire milling industry was established n the basis of returrom a 250-ton shipment of local ore sent to Swansean 1863. Itported that the ore assayed per cent, in copper,and showeda valof $40 in gold and $20 in silver to the ton. The copper content wasconsidered of no value.

In 1879 James Salee passed through this section of the country andincidentally visited Iron Mountain. He was a miner of wide experience,and in samples of the gossan which he had collected he found gold andsilver values. He became the sole owner of the property at Bully Hill.Years passedand occasionally new finds would be reported, which boomedthe Copper City district and gave a new impetus to speculation. Littledone,'however, until the latter part of the '90?s,when the propertytransferred to the Bully Hill Mining & Smelting Co. Such is,.in partsome of the early history of the camp

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BULLY HILL MINING DISTRICT, CALIFORNIA 63gressively toward the southern limits of the map. The topography isintimately related to the general geological structure of the district andresults from differential erosion of the various rock formations.The lower portions of the larger gulches have been partly filled withdetritus from the higher slopes, and in many parts of the district thesehave been worked as placers.

III. STRATIGRAPHYIt is not the purpose of this paper to enter into an elaborate detaileddiscussion of the stratigraphy of the region, for it has received great care

and special attention from J. S. Diller,3 J. P. Smith,4 and H. W. Fairbanks.5In order, however, that the geological relations of the eruptive rocks andthe ore bodies may be fully comprehended, a brief outline of the separateformations is here introduced. (See Fig. 2.) The oldest rocks in the Bully Hill district which outcrop on the sur-face consist of a series of volcanics, everywhere tilted at a high angledecidedly to the southeast, although locally beds may vary so as to benearly flat.This series consists of alternate flows and tuffs which together meas-ure roughly 1,500 ft., and has been named by Diller6 Dekkas andesite.Above this and apparently conformable are the crumpled massive to thin-

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64 BULLY HILL MINING DISTRICT, CALIFORNIAThe freshest pecimens re dark gray, with numerous mallglisteningphenocrysts f labradorite,showingbest on fracturedsurfaces. The rock

exhibits a greenish inge, due to the presence f chlorite, epidote,andsimilar secondaryminerals derived from pyroxenes probably augite).On alteration the rock usuallybecomesighter colored, nd n many placeson first sight esembleshyolite. Occasionally parselyscattered esicleswhich have been filled with calcite occur, and the presenceof these gives

Black and grayjishale, with thinTriassic bedded sandstonesfa n d interbeddedtuff. I000

Aridesite fl o w sand tuffs with someTriassic. rhyolite tuff. Moreor less altered.

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BULLY HILL MINING DISTRICT, CALIFORNIA 65impossibleo trace he serieswhere he outcrops re obscured.Themassiveharacter f some f the lows s evidencedrom he presencefnumerousarge roundedboulders,which are o be seen n the gulchesasplacer gravels.Triassic Sediments.-ThePit Shales-The youngest ormation in thedistrict with which this paper s immediatelyconcerneds an extff shales, nterbedded occasionally with various volcanic tuffs. Ingeneral heseshales re ine-grained, ark gray to black, characteristicallythin-bedded, and frequently provided with remains of m m

ganismThis terrane s by far the most extensively eveloped f any ormatin the district and covers the entire area of the eastern part of the maThese shales underlie a limestone having an abundant and well-preservedauna which is known to be Middle or Upper Triassic.7 Because of thisrelationship the age of the shales s placed in Middle Triassic.The average strike is north and south when large areas are considered,but on the map it will be noted that the strike varies. The dip is decid-edly to the southeast, in common with the previous series, and in amountit is not far from 28 . Variable dips are encountered n the southeastpart of the maWhere the shales exist the topography is well rounded, and usually lowds result. This is not because he series s characteristically soft, f

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66 BULLY HILL MINING DISTRICT, CALIFORNIAcomprisingandesitesand andesitic tuffs. These rocks cover the entirewest half of the area under consideration. Irregular masses and longstrips extendsouthand east rom the main area. The entire areahasbeensheared o hat the rocksare usually of a pronounced nd thinly foliatedtype. Into the sheared ountry rock have entereddikes, which tend tcomplicate the structure. Natural outcrops are abundant and formsharp idges,persistent or long distances. The hilly tract containing hemines embraces his igneouscomplex and has abundant evidencesofmineralization. The ledges of altered andesite and dikes rise manabove the general profile, and obviously present what physiographers erma " young" topography. The rocks over wide areas are greatly decom-posed, and, except where exposedas already described, have weathered toa mantle of soft, reddish brown, chalky debris; and again they have be-come so thoroughly silicified that their original character is entirelybliterated. The extent and g tionships are given on thnap, Fig. 1.

StructureThe rock formations, which have a general southeasterly dip, arecomplicated by folds, faults, shear planes, igneous intrusions, and meta-

morphism to such an extent that in the immediate vicinity of the minesthese secondary features are of more importance, in the solution of the

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BULLY HILL MINING DISTRICT, CALIFORNIA 67

FIG. 3.-OPEN FOLDING IN THE PIT SHALES.

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68 BULLY HILL MINING DISTRICT, CALIFORNIAof the mines s that which follows rom compression. Within this zonethe strata have shortenedand the greater length of the various beds,previously isted and described, asbeen aken up by folding. The bend-ing became omplex and later reacheda condition n which the unitsyielded easier o the appliedstresses y breaking han by further crum-pling. As a result, n the westernpart of the Pit shales,he adjustment scharacterized y numerous reaks n the strata. Since he strata of thePit formation consist of shales, sandstones,and limestones, here isreason to suspect widely differing structures.The composition and thickness of the various beds determined in alarge way the behavior of the seriesunder the applied stresses. The unitsvaried greatly in thickness, and as a rule the thicker portions yielded bybreaking rather than by bending. On the other hand, where the stratawere thin and the composition favorable, complex folding and contortionresulted. These finally becamemore pronounced and were ater replacedby thrust and dislocation.Everywhere the Pit shales are folded, and so prominent is this featurethat the formation can be detected for great distances. These stratawere not originally deposited n this position, but have been subsequentlydeformed by lateral thrusts which crumpled the units and caused thefolds. As might be expected, folds occur of every degreeof complexity.Some are broad and open, others are narrow and compressed; in some

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BULLY HILL MINING DISTRICT, CALIFORNIA 69beenpredominantn this particular ocality. As such hey consisted

y of finedustparticles, ut there s evidencelso hat argeamountsf ash, as well as some glassy fragments, constituted part of thmulatThe massivebrittle rocks wereoriginally the flowsand thesehave hed the angular fragments of the breccias, while the tuffs, because oftheir porous exture, permitted adjustment o take place between hadividual particles without apparent crush. In extreme zones of sheand brecciation, however, all traces of former structures have bee]bscured,f not completelyobliterated. Thecomposition nd orm of thvolcanic ocksdetermined o some xtent heir behaviorunder he appliedstresses. Since there is such an abundance of folded structures in the

sedimentary formations on top of the igneous rocks, and a short distanceto the west in formations stratigraphically lower, it follows that thesedeformations exist in some orm, however badly preserved, n the igneousrocks as well, because he entire serieswas subjected to the same stressesand behaved as a structural unit. Just how far these deformations extendin depth is a matter purely conjectural, but it is believed that they haveaffected rocks very early in the geological column.As noted above, the general structure of the district is comparatively

simple, but there are minor features which tend to complicate it. As aresult of the degree of folding, beds which were in the beginning far below

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70 BULLY HILL MINING DISTRICT, CALIFORNIA^mine workings the older fractures have been rehealed,while the latercracks are still open and furnish circulation channels.At only two points has faulting taken place on a scale hat meritsattention, and even here the data are somewhatobscure,so that theactual amount of dislocation is questionable. Further developmentsmay reveal some nteresting elationships. The shearzoneswhich standup as silicified edges n the hill-top apparently end n the south slopeof

the hill. It is quite possible,however, hat theseshearzonesare local;but there are several dikes, which will receive attention under "Intru-sives," which also outcrop on the south slope of the hill, and these alsoend rather abruptly. After apparent shifting to the west about 100 ft.,they continue in their normal direction southward and finally disappearunder the shales. This is very suggestive of a fault when taken in con-nection with the dikes and the shear zones, but the shales only a shortdistance to the east show no evidence whatever of any dislocation.Another possible fault, which is better characterized by being a" crush zone," is seen on the north slope of the hill where it crossesTowncreek just northwest of the main tunnel entrance and back of the companyhouses. Here the rocks are badly brecciated and the zone is apparentlyvertical and at right angles to the foliation planes of the rock massesoneach side. This zone has been partly explored in depth and some oreobtained. Aside from these two instances (and they are rather obscure)

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BULLY HILL MINING DISTRICT, CALIFORNIA 71terizedby three ndependent oneswhich pproximatelyparallel h other but rated by int f several hundred feet. Thzones are nearly vertical and have a north and south direct Theygular ledgeswhich project above the g 1 profile of the hid are shown n Fig. 5. The material constituting the rock of thdges is mainly silicified andesite tu dfl In this hardeneddition it resists the weathering processesand gives rise to rugged, teepslopes. In spite of the rock being hard it splits easily in a plane po the foliation and is often characterized by an abundanceof long pracks trending north and south.

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72 BULLY HILL MINING DISTRICT, CALIFORNIAhave racticallyodark ,t M f theout hibit altt d int bleach " so that th w ,thered, ough, projectingmassesrecolored y ironoxide, h gmg y fromythrough reds to pinIn the hand pecimenhe actual im on of thegrainss seldome than 5 mm. Much of the rock s ine-grained nd may be egardedas te-porphyry Of in gnifi are th y toliationplanesand he crack g which ore-bearing tions hdeposited ores in notable amouTh time of t t t definit know since th

V

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BULLY HILL MINING DISTRICT, CALIFORNIA 73course with the direction of the main ore bodies. In the deeper work-ings of the mines it is known to have a thickness of approximately 300 ft.To the south of the Rising Star mine it pitches under the shales anddisappears.In the hand specimen it is greenish to grayish in color and is usuallyvery fine-grained, although at times it is coarseenough to show distinctphenocrysts. It frequently exhibits amygdaloidal structure. The cav-ities in all cases are filled with calcite.In common with the alaskite this dike sends off branches whichundergound give considerable trouble and annoyance in mining opera-tions. Some of the smaller branches are finely felsitic in texture.The dike has been changed by shearing and alteration, so that in themines the dike material exactly resembles sheared masses of ore. Withreference to the age of this intrusion, it is the youngest igneous outbreakfound in the district mapped, and although a definite age cannot beassigned to it, it is known to intersect the lower members of the Pit shalesand therefore must be as young at least as the series, which is consideredas Upper Triassic.

IV. PETROGRAPHY OF THE ROCK TYPESThe chief object of the microscopic study of the rock types has been

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74 BULLY HILL MINING DISTRICT, CALIFORNIAThey are: sulphatization,arbonatization,ulphidation, hloritization,serpentinization, and silicification.

Andesite FlowsThe andesitesange n texture from distinctly porphyritic to finelyfelsitic and glassy varieties. The characteristic andesite s a close-textured, dense, rayish to greenish ock containingvisible phenocrystsof feldspar n a greenish roundmass. The representatives f this classaccumulated as thick flows, in some of which crystallization was welladvanced before movement ceased.The felsitic and glassyvarieties are without doubt thosewhich werepoured out on the surface as thin flows and sheets. In someof these theescaping gaseshave left the mass porous, but this feature is not alway

prominent. The microscopic features show the porphyritic varieties tconsist of labradorite with some hornblende in a felsitic ground mass.The feldspars are large and contain numerous scattered inclusions, pre-sumably ilmenite. In the finer-grained varieties the feldspar and horn-blende wane and in the glassy facies these minerals entirely disappear.Only fragmentary evidences of biotite remain and in the highlyaltered types this is often indicated by the arrangement of grains cmagnetite.

The transition of the porphyritic types to the finer-grained varietieis gradual but distinct. The alteration of the andesites is a very salien

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BULLY HILL MINING DISTRICT, CALIFORNIA 75ilcanic detritus which is clearly andesitic n composition. The texturethereforeof wide rangeand ncludes ocks madeup of lapilli as well asthose consisting of sand and dust. Under the microscoDe the clastharacter of the rock is easily recognized and the mass s seen o be composed of intermingled ragments of feldsparsand glass. The andesittuffs are badly altered or decomposedand this feature renders hem veidifficult to study. Although the alteration is well advanced and thparticles are very fine, yet the microscope hows traces of porosityMany of the fragmentsstill showevidence f originalglassy exture. Thtuffs alter to kaolin and sericite,and where he ferro-magnesian ineralshave been prominent these have given rise to considerable amounts ofchlorite and iron oxide.

Rhyolite Tuff.-In the essential features the rhyolite tuff does notdiffer markedly from the andesitic variety except n the presenceof quartzphenocrysts imbedded in a large amount of dust-like material. Thisdust may have been in part glassy, but the present metamorphosed con-dition of the products makes any trustworthy statement of the originalcondition uncertain. The range in size of fragments is variable. Rarelydo the particles exceed5 mm. in diameter and three-quarters of the entiremass is less than 1 mm. in diameter. The presence of quartz grainsmakes the identification of the rock comparatively easy. This rockinvolves glassy fragments as well as mineral grains, all of which are sharp,

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76 BULLY HILL MINING DISTRICT, CALIFORNIAOf the tuffs there is one specimen hat deservesspecial comment sinceit showssome nteresting features not commonly seen n the rest but which

are believed to be general throughout the entire class of tuffs. Thisparticular feature is to be noted when the sulphate encroachesupon andentirely replaces the quartz grains, Fig. 7. In the same section pyrite isseen encroaching upon quartz grains and completely replacing them. Itappears from these slides that the conditions under which the solutionsexisted were such that silica was dissolved and a sulphide as well as^a

\

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v>BULLY HILL MINING DISTRICT, IFO 77

attack near! all erupti ocks, d ne all d leavebehind, in m y cases,m kelet fTw ;pecimensf the 24 are ypicalbreccias,he fragm f whichgular n character, reenishn colorand cementedogetherwithgypsum. These rocks are of such haracter thatthey well merit special escript The ragmerj th ughoriginallyply angul w >P y sheared nd at fi dght resemble

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78 BULLY HILL MINING DISTRICT, CALIFORNIAdetailsof their originalexturearepreserved ith a sharpnesscarcelybe looked f ks which have had such a history

It is generally agreed h dy t is powerf g incompletelyobliterating all suchstructures,but in th tance t doesnotppear o hold Glassyocksareknown o bemoreeasily ttackedthan rn thers and biect to either alterat mplete desttion, so hat p ,tion of the glassybrecciawith PP t lossof itginal earmarksandquick-chill characteristicsmust be regarded

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BULLY HILL MINING DISTRICT, CALIFORNIA 79f the fragments ives o the larger ock masses distinctlybandedPPIn thin sectionhegreenishrains reeither sotropic r elsehey wdevitrification hanges.No chemical nalysis f the greenishragmenthas-been ade, ut sincehey are closelyelated o flow ocks heyprovisionally classedas acid glasses. In one thin section he greenishpatches howoriginalporphyritic habit generally ep

gmal glassy ocksand they are certainlybeing eplaced y the gyp mStringersand shredsof such ial are being encroached pon fromsides,and they remain as c Like the breccia, hese greenisharheavily charged with sulphides, and the conclusion s made that thsame mineralizers which introduced the sulphates were influential inintroducing the sulphides as well. The sulphate was introduced as ai filling, and this feature establishes ts secondarycharacter. As in the9 of the breccia, these rocks were originally flows and are best regardedicid glasses.One specimen out of the entire series s classed as a rhyolite flow. Itis badly sheared, and under the microscopeshows corroded and embayedgrains of quartz. Secondary carbonate has replaced some of the ferro-magnesian minerals, and this feature gives to the rock a mottled appear-ance. There are a few patches of divitrified glass which still show pro-nounced flowage. There is no evidence of introduced sulphate or sul-

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80 BULLY HILL MINING DISTRICT, CALIFORNIAmass has been fractured but has been later recernented by quartz. Thealteration productsare chlorite and epidote n small amount, derived nall probability from original flakesof mica. The dike varies n thicknessand on thin edges t is relatively fine grained, while in the center of thelarger masses he texture is rather coarsely grained.Although in places the rock shows conchoidal fracture it contains^glass. Diller10 egardedhis rock asa surfacelow and recordedt asbeing

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BULLY HILL MINING DISTRICT, CALIFORNIA 81Andesite Dike

1 ppearancehe andesitedike s a dark gray to greenishine-grainedk metimesshowing a tend y t mygdaloidal structur Thoscope p that the rock co f ph yst thmbedded n a fine-grainedground mas f lath-shapedeldsp Thhief acces minerals are magnetit rar rut Th kppears more basic than is actually th It varies in comp

* *tfrom point to point andshows n its northernmost t >phe pme pyroxenes. Like the alaskite dike, it also sends off branch tth try rock, and these are finely felsit Both dike rocks have bheared and have been replaced in part by sulphid 5S, ulphates, and

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82 BULLY HILL MINING DISTRICT, CALIFORNIAand sulphides. The carbonates fill irregular cavities, and while the sul-phide is often found along the borders of the calcite areas it is not con-fined to these alone. The cracks and fissuresare most heavily mineralized,although in one specimen he sulphide appears to be uniformly scatteredthroughout the mass. In only one specimen of the andesitic dike rockwas sulphate found. One specimen shows ntroduced copper carbonate.It is in the andesite dike and its apophyses that the greatest amountsof chlorite and serpentine are found. In all cases hese products havebeen produced by hydrous alteration from former ferro-magnesian py-roxenes as well as amphiboles.More than half of the specimenscontain ores,which are confined to thetuffs. Where the gypsum is found it appearsalso to favor the fragmentalrocks, especially the tuffs. In some of the specimens all original struc-tures are obscured in the various processes of sulphatization, sulphidation,and silicification. In a few there are satisfactory evidences of originalminerals and structures that prove the tuffaceous character of the originalrock. In the rhyolites the phenocrysts have resisted alteration where allother structures have been destroyed. In such cases the ores are essen-tially replaced rhyolites and rhyolite tuffs. Some of the specimens showmarked crushing and these have been preserved as breccias. The sul-phides and the sulphates were deposited n somespecimenssimultaneously,and in others the sulphides alone are promiscuously distributed through

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BULLY HILL MINING DISTRICT, CALIFORNIA 83neralizations somewhatrregular,t is roughlynortheastn directiod hasa nearlyvertical dip. The shearzone asbeen ichl mineralizedd ore shootsof lenticular shae have esulted. The thicknessof thbodies aries rom a thin edge n he marginso 14 t. or morenear hmiddle. The width of the shoots varies from 30 to 150 t. The centrapart of the shoots generallyhe hickest. The onger xes oincide iththe general oliation of the inclosing ocks. Where he walls have been

mineralizedheyhavealso eenmined, ut in no nstanceave heybeenso productiveas o be egardedas mportant sources f ore.The principal ore bodieshave resulted rom the complete eplacemeof the country rock. The mine is, however, n the early stagesof itdevelopment and no ver extensive shoot has been mined out. The orconsists f a mixture of pyrite, bronite, chalcopyrite, nd an abundance sphalerite. There are also minor amountsof galenascattered hroughthe masses. In later years the sphalerite has become so abundant as tomake the extraction of the copper difficult. The workings have alsod a badly sheared intrusive dike, containing copperpparently not original. They are subsequently describedThe next property is about 2 miles to the north of Copper City, andlthough it is the latest of all to be worked, it is nevertheless of great

mportance. The mine is called the Rising Star and is situated near thehead of Buck gulch, which has its source n the south slope of Bully hill

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84 BULLY HILL MINING DISTRICT, CALIFORNIAthe hill in a general southwest direction. The shaft near the end of thetunnel is more than 1,000 ft. deep and connects with a number of driftsand cross-cuts. "The geology is similar to that of the Rising Star mine. The generalstructure is simple, but the details of the structure and the deformationwhich produced it are not only complex but in some nstances are beyondexact determination. This is due to the complex folding of the igneousmembers.The ore bodies are strikingly lenticular in shape and conform perfectlyto the foliation of the sheared country rock. The lodes lie much nearer

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BULLY HILL MINING DISTRICT, CALIFORNIA 85mmary.- It is plain from the precedin statements hat the minesare ocatedn a line correspondinglosel with the shear one. The shootre strikingly enticular n shape,with then- ongeraxes arallel o theliationof the nclosingockmasses;heir longer xes lsopitchsteeplyto the north, and the depositsn places ttain a thickness f 30 t. or mAdditionaldescriptionsf the orebodieswill begivenunder MetallifD "

VI. METALLIFEROUS DEPOSITSThe metalliferous eposits f the Bully Hill district, so ar asknown,aresimple and uniform, and constitute three principal tracts intimatelyrelated to an igneous ntrusive (alaskite-porphyry) ocally known asBully Hill quartzite. These tracts are represented on the map androughly embrace an area of 1 by 3 miles:The Bully Hill and Rising Star mines form a group comprising thmost important ore bodies of the district, and although ore is known aCopper City, and the mines have been operated at irregular intervals fo

many years, the composition of the ores has until recently made the minesmall importance. Detailed study of the ore bodies showsdisconnectebut often overlapping massesvarying in size rom mere flattened nodul

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86 BULLY HILL MINING DISTRICT, CALIFORNIAAlthough the movements ave or the most part beenof the minor orderthe subordinateracturing has aken placemainly parallel to the foliatloiof the country rock. It is alsobelieved,upon good evidencen the fieldthat important shifting has resulted at approximately right angles to theshear zone. Because of the number and character of the frac-tures it is impossible to determine definitely the ages of the differentmovements.

This condition is therefore favorable to the existence of faulted partsof the main ore body, and applies especially to the north end of the Dela-mar lode in Bully Hill. The evidence-though obscured by the badlybeared country rock, and therefore of rather unsatisfactory charactshows that in all probability the faulted portion can be looked for farthto the east than the main lode in Bully Hill. The strongest evidenbearing on such interpretation is twofold: first, the isolated massalaskite-porphyry near the eastern end of Bully Hill dump, which isthought to be shifted from a position which it once had near the mtunnel, eastwardly to its present position; secondly, the brecciatedand otherwise ground-up material just above the wagon road leading tothe Rising Star mine and directly back of the company houses as one goessouth. This broken and fractured zone is axially in line with an escaiment produced by an abrupt ending of alaskite-porphyry just west of thmain portal. Such brecciated zones, within the mineralized area, hav

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BULLY HILL MINING DISTRICT, CALIFORNIA 87The arealdistribution s shownon the map,but because f the charac-teristic weatheringof the surface ocks,and the subsequent pread

the gossan,here s no sharpdistinction betweenhe countryrock and thrmineralized ock. In very general erms t might be stated hat the D*-**de can be traced on the surface for a half mile, and the Anchorde of the Rising Star mine for a quarter mile. The width of thmineralized zone varies, but in round figures t is not far from 300to 500 ftCharacter of the Ores

In a number of instancespyrite appears to be the chief mineral, withchalcopyrite as a very common and characteristic associate. More rarely,and chiefly in the zones where oxidation and enrichment have been inprogress, we find chalcocite, bornite, covellite, and native copper. Thetypically oxidized ores if followed down to sufficient depth invariablygive way to secondary enriched products, and, when these are not promi-nent, to the unaltered sulphides. Although some grains of metalliccopper are found in the superficially changed rock, the decrease n assayvalues, going downward in the rocks, is doubtless to be explained by thewell-known fact that along open, water-bearing, shear and fracture zones,under the action of surface waters, copper minerals suffer rapid alterationand transportation, and only in the uppermost parts of the sheared zone,

88 BULLY HILL MINING DISTRICT, CALIFORNIA

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dicates that originally the masseswere rhyolites or rhyolite bThe surface rocks by early mining methods yielded notable amounts ofnative copper, but at present such materials are not an importantsource of the metal. Just how the copper accumulated is not known,but in all probability the presenceof organic compounds played an im-portant part in its reduction.ractically all the gold obtained by hydraulic mining occurs in thenative state. Although the particles are uniformly small they are usuallyvisible on close inspection in all the detrital accumulations.In the oxidized ore the gold occurs in a siliceous, limonitic matrix,which, without doubt, contributed to the placers of local areas. Fromthe intimate relation of the gold with the gossan it is evident that a veiclose association exists between it and the original sulphur-bearing merals. In the pyrites, however, the gold is in such a state of fine subdvision and is present in such small quantity that it has not been recognizedunder the microscope. While most of the gangue runs high in iron oxide,the gold is frequently closely associated with cryptocrystalline quartz.In the mine workings, and particularly in the vicinity of the basic dike,there appears to be a genetic relation and close association with barite andsulphides n which the gold is apportioned evenly through the mass. Onthe surface this mass alters to limonite, quartz, barite, kaolinite, and in afew places to hematite.


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when observedby this method, are best identified in sliced sections ratherthan by reflected light.Pyrite.-Detailed studies show that in the freshest specimensofalaskite-porphyry there are no metallic sulphides which crystallizedfrom fusion as did the other componentsconstituting the rock mass.Pyrite is indeed ound disseminatedhrough he alaskite,but it isbelievedthat it was formed at a time after the alaskite had solidified. In thisurrence the m y forms aggregat dg Ind iduyst f free growth and ffi tly r recognition of th yst


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shaft. These studies show that pyrite was the first of the ore minerals tocrystallize. Whether or not the pyrite is auriferous s not easily deter-mined. That it is not highly so s certain, since tests of the most pyritousmatter often show little or no gold, and an abundance of pyrite is gen-erally a sign of poor rather than of rich ore.Where the pyrite is in masses nnumerable branching cracks traversethe specimen n every direction (Fig. 12). As far as could be determined,no other metallic mineral of importance could be seen. This fact does


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"characterized y the introductionof chalcopyrite Fig. 13). Thisprimary mineral closely ollowedpyrite, although n rare instances,asstatedby L. C. Graton,12small grainsof chalcopyrite. . . are nclosedin the pyrite individuals." Studies made by the writer point to theconclusion that chalcopyrite has been the last mineral to enter. Thismineral completely surrounds the pyrite grains and in some instancesenters the minutest fissuresof the partly crushedpyrite masses. Attimes the pyrite and the chalcopyriteare so inextricably mingled thattheir physical appearancesare almost identical, and it becomesdifficultif not impossible to distinguish, in the hand specimen,one mineral fromthe other. In such casessimultaneous deposition may be employed toexplain these complex relationships, although from the examination ofmany specimens he evidence indicates chalcopyrite as being later thanthe pyrite.Sphalerite.-Like pyrite and chalcopyrite, sphalerite is one of theprimary minerals and is everywhere crystalline. Sphalerite is later thanpyrite and is seen o surround particles of this mineral. Unlike the pyritewith which it is so intimately related, it does not form well-boundedcrystals even where conditions of free growth appear to have been favor-able. The blende is so common throughout some of the ores, especiallythose near Copper City, that a zinc-extraction plant is now being con-structed for its recovery. Near the surface it is easily oxidized, and,aside from a few occurrences, its products appear as sulphates on the


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the surface were not observed. It is quite probable that the gthe blend tiferous, and that Id explain the pf high silver values n the g well as n the regularmine-run ofIt is plain from a detailedstudy of th ,tionships f the minerals hatgalena h sulphides,esp y where blende s in larg mChal The cuprous sulphide is not a common m al in ofthe ores of the district. It occurs as dotted m timately tedwith bornite. Mr. Graton13states that chalcocite and bornite apparently

.


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SecondaryEnrichment The oxidizedzone n the Bully Hill district continues, n general, depths. The lower limit is known to be as low as the 1,000-ftlevel. The ore in this zone s considerably nriched,and the enrichmentwas materially favored by the openness of the mass because of the abdanceand relative sizeof the cracksand issures,hesebeingpossibleonlyin materialswhich are brittle, suchas quartz and pyrite.The profound modification resulted chiefly through the action ofdescending waters which in their downward course traversed zones inwhich small but important amountsof copper-bearing ulphidesexisted.The following equation illustrates the probable chemical react

CuS04 + 2 FeS = CuFeS2 + FeS04.As an illustration of the principles of ore deposition, or, rather, oreconcentration, this chemical reaction can hardly be overestimated, andsince the conditions assume he presenceof copper minerals near the sur-face, this phenomenon is usually related to the second concentration.The first is believed to have beenbrought about chiefly through the action

of ascending hot waters, while the second concentration is almost exclu-sively related to cool descendingwaters. It follows from this that if theconcentration processesbe regarded as cycles the first is far the more


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the ores,especially of the Bully Hill and Rising Star mines, s the hydratedsulphateof calcium,gypsum. L. C. Graton16eports anhydrite as well,but so far, the examination of many slides has not revealed its presenceextensively. The explanation advanced for the gypsum will also accountar the anhydrite, and this will be given in "The Gypsum Masses."Genesis of the Deposits" -The preceding pagesset forth the problems awaiting solution, and only

a brief recapitulation is deemednecessaryof some of the most significantfacts with which an adequate explanation of the ore genesis mustharmonize.The association of the ore deposits with the intrusives is interesting,and a study of the lean ores s one hat promises to give most light on theirorigin. Due consideration must also be given to the mechanical alter-ation and subsequent changes wrought in the dike materials throughwhich suitable solutions passed, and by which the concentration andenrichment of the ore bodies were made possible.

The fissures produced a weakened zone which not only permitted thesolutions to circulate, but determined also the place where the intrusives(dikes) should pass. While it is believed that chemical alteration hasfacilitated important surface changes, the larger structures of the rockmasses n depth are believed to have been caused primarily by deep-


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mately 1,000 t. They are seenn Fig. 6, lookingnorthward,as a series ridges in the profile of the hill. The eastern zone incloses the alaskitdike and contains he Delamar ode of Bully Hill.The rough, rugged ridges are forms which have resisted erosion morffectively than the surroundingmaterials. With the exception o bted, all such ridges are referable to silicified sheared ock, chiefhdesite flows and tuffs. It appears that the materials are main]tuffaceous, f rather wide rangewith regard o fineness f grain; and itis believed hat such materials, because f their porous character,per-mitted siliceouswaters to circulate more freely than the more massiveandesite flows. The solutions came from below and are believed to havetransferred silica, which was derived from silicate minerals in the deeperparts of the crushed zone, to its present position. Weathering has alsoinfluenced the composition of such materials by dissolving out solubleconstituents, so that in someareas he mass s rather porous.The exception referred to above is found in the disconnected masses ofalaskite-porphyry. This material, because of its position and externalphysical appearance, closely resembles the masses in the shear zones.Close inspection, however, shows the latter to contain small phenocrystsof quartz, while the former contains only secondary quartz. In someinstances, especially where shearing has been excessive,and the rock isbadly stained by iron oxide, identification can be established only bymicroscopic study. In the field the alaskite is often mottled with darker


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ture of calcite nclosingminor amountsof chal opy ite and pyrite. Thefissured rhyolites were thus cemented by these minerals.(2). Long before the alaskite had completely solidified stressesagainbecame operative, this time breaking the alaskite-porphyry and alsodisturbing the andesites. Mineralization and impregnation of this rockby metallic sulphides resulted It is therefore logical to make the statment that this is the first stage n the genesisof the copper ores. Accom-panying this change, although not definitely connected with the deposi-tion of the ores, was more or less sericitization, silicification, etc., in the

rock masses.The brecciation forces which acted on the alaskite-porphyry finally

roduced essentially the same shear planes that were observed in theg andesites, so that both rock types behaved practically as astructural unit to all later phenomena. These were important stages iidecomposing the constituent minerals, and they were followed by chemical modifications which were later represented by completely alteredk masses nd the production of secondary silicates.(3). Shortly after the production of the foliated structures igneoustivity was again characterized by the intrusion of an andesite dikwhich also vfolio wed the foliation planes, and in places cut across thalaskite-porphyry by intruding into the shear planes developed in itmass. Of all the rocks in the Bully Hill district this andesite dike was the

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BULLY HILL MINING DISTRICT, CALIFORNIA 97that pressure nd temperatureare mportant factorsat depth,a decreasein either of thesemight furnish conditionsunder which depositionwouldtake place. Whatever the chemical nature of the solutions that trans-orted he metallic sulphides,t is certain hat they weredepositedn thesheared alaskite-porphyry mass.The magmatic solutions that followed the andesite dike contributedadditional metallic sulphides, hief of which were blende,bornite, andgalena. Important amounts of chalcopyrite were also introduced whichgreatly added to the richness of the ores. It should be stated that in a]itages of mineralization the shearedrocks were impregnated indiscrimThe element of control seems to have been the sheared zone.In several places the andesite dike is partly mineralized, but so far asknown the ores appear only as a thin crust or veneer on the dikerock.

Several diamond-drill holes were driven through the andesite dike toascertain conditions on the opposite side. Xo oreswere found, and so faras known only the hanging-wall side of the dike contains sulphides. Inthe opinion of the writer the opposite side of the dike rock should be pros-pected, although it is quite possible that the position of the dike partlyinfluenced the course of the solutions in such a way as to leave the rockson the other side practically barren. So far, marketable ores have beenfound only on the west side of the dike.


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pointed out by Clarke,17 re solvents or most of the commonminerals,including quartz. It is believed hat someof the solutions, hus carbon-ated, in passing hrough the disturbedzone attacked the acid as well asthe basic rocks and left in most instances only the resistant minerals.Carbonate was in this way introduced, as shown in Fig. 15. It wasaccompaniedn some nstances y notableamountsof chalcopyrite.Simultaneously with the introduction of carbonate, mineral hydrationand removal went on, and in some rocks, especially the dike, considerable


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"processes. The study shows that there was a certain definite order orquenceof deposition,although these elations are partly obscuredbythe deposition f some ompoundshichbeganwith the earliest tagd continued persistently hrough the entire processo the lastSuch minerals as pyrite and quartz are the most important representtives of this class.

The important phasesare those genetically related to carbonatizatkd sulphatization. From a study of thin sections, the former was aprocess by itself, while the latter included pyritization in addition tsulphatization.The presence of certain minerals and their various mutual relat

hips suggest a common source, and also that the source was rich in thelements which form the compounds now found in the rocks. Garbates, sulphates, and sulphides are seen n nearly every slide in vaiamounts.

The most important geological factor which assisted the circulatingsolutions was undoubtedly the sheared structure of the rock masses.The solvent power of the solutions, under high temperature and pressure,was very great. The ferro-magnesian minerals were the first to bcompletely altered, and these changed chiefly to chlorite. The feldsparsapparently were the next to yield, and alteration has gone on so completelythat in most cases very little remains to suggest their former p


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Gypsum-

The monoclinic hydrous calcium sulphate (CaS042H20) is first no-ticed in the mine workings near the 300-ft. level and from this pointit continues downward to the lowest levels of the mine. It is usually ofa light gray color, massive to banded, and rarely found in crystals. Itsbanded appearance s not due to any variation in its composition but isproduced by inclusions of other rocks, the fragments of which have beenarranged by shearing processes nto roughly parallel positions.At some points it is found in larger masses n a comparatively purestate, but the greater amount in the drifts and tunnels is streaked andlocally contains pyrite. The larger massesare soft, usually white andeverywhere crystalline. At times the pure fragments are translucent,and it is possible to secure masses several feet in dimensions. Where therock is in such large masses t is very difficult to mine becauseof its softcharacter and its resistance to the action of explosives. For this reason,and also because of the expense of drifting through it, the limits of suchmassesare not known. Drifts in such bodies are characterized by beingnoticeably dry.It is of interest to note that the gypsum as a whole is found limited tothe shear zone in depth, and thus far has not been reported from any pointon the surface. It is found only in the richest mines, intimately associ-


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having itreous uster. This mineral s the only sulphateoundon thesurfaceand s usually detected y its glisteningcrystal acesand tttively great weight. According to Diller18 the barium had its sourcethe feldsparsof the alaskite-porphyry. After a number ofTcarefulesthe writer was unable o detect any barium in the feldsparsof this dikrock. It is believed that the barium which furnished the barite had itorigin in the same ourcewhichsupplied he sulphides. Since his minersis so sparingly present it will not receive further discussion.

Origin of theDeposits,with SpecialReferenceo GypsumBefore proceedingo a discussion f this subject a summaryof therelationshipsof the gypsum s given. An effort has beenmade o bring

together all the data bearing directly on the problem and also to condense,as far as possible, the descriptions of numerous slides examined. Thestatements represent facts gathered from the field as well as from detailedmicroscopic study. A few of the most important relationships are givenbelow:Field occurrences of gypsum:1. It is found only in the greatest shear zones.2. It is never found on the surface in the vicinity of the mine:

3. It begins on the 300-ft. level of the mine and extends to th


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mode of formation as set forth by the first and fifth theories is not appli-cable. The secondand fourth only demand careful consideration, and forconvenience of discussion will be taken up in the reverse order.Deposition through the Action of Pyrites upon Carbonate of Lime.This method is perfectly possible and has taken place without doubt in anextensive way in nature, but there are certain initial conditions which donot appear n this region. First of all there must be deposits of limestonesufficiently large to form the gypsum deposit as it now appears. Thatsulphides oxidize to sulphates is well known. If we start with pyrite thefinal oxidation products are indicated by the following chemical equation:

FeS2 + 7 0 + H20 = FeSO4 + H2SO4.It is seen that one molecule of pyrite will yield one each of FeSC>4 nd

H2S04. To be on the safe side, it is assumed that both FeS(>4 and H2S04operate to produce sulphate of lime, although it is very doubtful if theferrous sulphate acts in this manner. The molecular weight of gypsumis 172 while that of pyrite is 120. If these be converted into molecularvolumes it is found that for 74.8 volumes of gypsum we require 24 vol-umes of pyrite, or that for every 100 volumes of gypsum there must be 32volumes of pyrite, on the basis that H2S04 only goes o make up the sul-phate, Under these conditions, and also assuming that the gypsum interritory not yet explored is as thick as that already found, a uniform bed


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" "through which the emissions f the igneousmagma assed,t is ptly reasonablehat the rock would suffermore or lesssolution by thd liquid solutions. These may have been in part water containinCO2, S02, S03, or S04. Any of these alone would dissolve the lime cajbonate and transport large amounts to higher points. Whether the limwas carried as carbonate or sulphate or as a mixture of the two is a matteabout which we can only speculate. It is possible that certain solutionhiefly carbonatewhile othersweresulphate n generalcompositThat both solutions existed is shown by the presenceof carbonate as wellas sulphate in the rocks, but since sulphate is in largest amount it foll< wthat solutions of this character were dominant.

Whether the sulphate was deposited as anhydrite or as gypsum is notcertain, although Graton21 rom his studies concludes that anhydrite isprimary and that gypsum is a secondaryproduct derived from anhydrite.There is room for further study on this point because of the well-knownfact that gypsum may be transformed into anhydrite,22 or that the re-verse reaction, anhydrite into gypsum, may readily take place. Further-more, it is known that anhydrite thus formed may be reconverted intogypsum.23The writer is fully aware, however, that within the limits of experi-ment the solubility of sulphate increasesup to about 38 C. and then de-creases for additional increments of temperature. This is somewhat


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2. The copper ores of the Bully Hill district were originally depositedrom magmatic solutions in which the metals were transported assoluble sulphides and were deposited as such.3. From the character of the metallic sulphides, the associated minerals,and the structural relationships, it appears that replacement has beenthe dominant process in these deposits.

4. The accumulation and concentration of the ores as they are nowd involves secondary enrichment, changes in which precipit

tion by mingling solutions and reactions on wall rock are of greatestimportance.5. The mineralization processes have apparently taken place underconditions which were entirely independent of rock mass control, astuffs, flows, and dikes, differing widely in physical and chemical makeup, are indiscriminately replaced.6. The lime as calcite, and the sulphate either as anhydrite or gypsum,had an origin in deep-seated sources, and both these minerals weregenetically related to the sulphidation processwhich has given rise tothe ores.

IX. BIBLIOGRAPHYGeology of Shasta County, Cal.

DILLER, J, S.: Folio No. 138, U. S. Geological Survey (1906).


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Gypsumm, QuarterlyJournal of 172-173,339; vol. vi, p. xlix, (1849, 1850).HUNT,T. ST Gypsum8 (1874).ROSE H O Solubility of Gypsum f(1854).MARIGXAC C.: On Solub Gsum A

(1874).McC On the Solubilit of G m American ChemicalJ al, vol. xi, p. 31(1889).ROGERS,A. F.: Occur e f G m and An Munty, Nevada, E 5-189 (1912).SURR G N: Notes on Oc m Minin W

xxxiv, pp. 1283-1284 (1911).HESS, F. L.: A Reconnaissance of the Gypsum Deposits of California, Bulletin No.413, U. S. Geological Survey (1910).

MiscellaneousG m

(1911).IE JA N

HARKER, ALFR Natural History of IgneouIDDINGS, J. P.: Rocks (New York, 1909)."KEMP, JAMES F. : Secondary Enrichment in Ore-Deposits of Copper, Economic Geol-

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ALBERT CLARENCE BOYLE, Jr. (Salt Lake City, 1879)B.S., E.M., Utah School of Mines, 1906

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