Geology of the Ore Deposits of Kennecott, Alaska

ECONOMIC GEOLOGY

WITH WHICH IS INCORPORATED

THE AMERICAN GEOLOGIST

VOL. XV JANUARY-FEBRUARY, 92o NO. I

GEOLOGY OF THE ORE DEPOSITS OF KENNECOTT, ALASKA.

ALAN M. BATEMAN AND D. H. MCLAUGItLIN. 1

CONTENTS.

PAGE. INTRODUCTION ...... 2 TOPOGRAPHY...... 4 ROCK FORMATIONS...... 5 STRUCTURE ...... I PHYSIOGRAPHICDEVELOPMENT ...... I•:, ORE DEPOSITS...... General ...... I7' Character of ore ...... I8; Types of deposits ...... I8. Mineralization in greenstone ...... 19, Mineralization in limestone ...... 20., Talus ore deposit ...... : ...... 23, Glacier ore deposit ...... 23; Structural relations ...... 24J. Relation of mineralization to rocks ...... 24- Behavior of fissures ...... 26 Relation of mineralization to fractures ...... 27 Relation of mineralization to folds ...... 29 Relation of mineralization to faults ...... 3o Shapes'and sizes of ore bodies ...... Relation to surface ...... 3t x This paper embodies the result's of field and laboratory studies of the Kennecott deposits carried on at various times during the last five years. The authors were together at Kennecott in the field seasonof I915, when the work was initiated in co6peration wit'h the Secondary Enrichment Investi- gation. Field work was continued by the first named author during the four following seas.ohsin his capacity of consulting geologist to the Kennecofi• Copper Corporation, while most of the det'ailed microscopic work was done. in the laboratoryby the other, in connectionwith researchfor the Secondary. Enrichment Investigation. .ALAN M. BATEMAN AND D. H. McL.,4UGHLIN.

Distributionof ore bodies...... 33 Alteration of host rocks ...... 34 Mineralogy and mineralography ...... 35 OXm•TIONo7 OREDEPosiTs ...... 44 General ...... 44 Minerals due to oxidation ...... 45 Distributionand depthof oxidation ...... 48 Relation of oxidationto structures...... 49 Relation to water level and climate ...... 50 Age of oxidation ...... 53 THEORETICALCONSIDERATIONS ...... 54 Origin of fissures...... 54 Suggestedhypothesis ...... 54 Origin of mineralization ...... 57 Source of metals in limestone ...... 57 Origin of copper in greenstone ...... 59 Agents of transportation ...... 6I • Met'hods of deposition ...... 65 Primary or secondaryorigin of the chalcocite ...... 66 Experimental evidence ...... 67 Mineralographic evidence ...... 67 Field evidence ...... 70 Discussion of evidence ...... 73 Conclusions ...... 77 SUMMARIZEDCONCLUSION OF ORIGIN OF ORE DEPOSITS...... 77

INTRODUCTION. General.--The copper depositsof the famous Bonanza and Jumbo Mines at Kennecott,Alaska, are unique for the character of their ores and 'their purity and size. Their occurrencepre~ sents many peculiar and interestingfeatures, and the origin of the great massesof .chalcocitehas long been a puzzle. Their ,decipheringinvo,lves the origin of f,racturesunusual in form; of a peculiar kind of primary mineralizationunparalleled in other deposits;of a sourceof metals not customarilyconsidered, as well as agentsof transportationbut seldomreferred to. No con- clusionscan be reachedwithout carefully weighing the primary or secondaryorigin of the chalcocite,and much of interest is gained regarding the distribution of oxidation and groun4water. It is with theseproblems that the paper deals,and' the facts and conclusionspresented, in the following pages are the result of a studyextending over a periodof five seasons,during which time the developmentof the .ore 'bodieshas been carefully followed and their detailsaccurately mapped. Location.--The Bonanzaand Jumbo Mines, the most impor- GEOLOGYOFORE DEPOSITS OFKENNECOTT, ALASKA. 3 tantof the mining properties ofthe Kennecott Copper Corp., are situatedatKenneco.tt, Alaska. (See Fig. •.) Thistown lies about200 miles northeast ofthe port of Cordova, onPrince Wil- liamSound, with which itis connected bythe, Copper River and NorthwesternRailro.ad, aroad which win.d,s around glaciers and is famousfor the difficulties of its co.nstruction. ThePort of

148ø lZ!G*

zs o.... _ zs,•_. so ;? ,oo•,,•, FIG.I. Indexmap to show location ofCordova, Copper River and Kennecott. Cordovaisn, aviga'ble theyear around and the railroad maintains continuousservice.with Kennecott, suffering onlyslight interrup- tion,inevitable with its 1.ocation. Themines are one mile .apart and each one is at a distanceof threemiles from the town of Kennecott 2and 4,000 feet above it, at anelevation of about 6,000 feet. Theyare connected with themill in the town by a•rial tramways. 2The U.S. Geol. Surv. terminology givesthe spelling asKennicott butthe postoffice and company name are spelt as. Kennecott. Asthe latter is com- monusage it is followedin thispaper. ALAN M. BATEMAN AND D. H. McLAUGHLIN.

•'tcknowledcdments.--Wetake pleasurein acknowledgingthe courtesiesand help renderedby the staff at Kennecottin •9•5, and the aid receivedfrom the geol.ogic maps prepared by W. E. Dunkle and H. D. Smith. The first named writer further records with appreciationthe benefit receivedin the seasonssince •9•5 from discussionswœth E. T. Stannard, Wm. Douglas, H. D. Smith, D.. C. Hoyt, and D. D. Irwin, and the helpfulnesso4 their information concerningthe detailed developmentof the ore bodies. Acknowledgmentis .due to Mr. StephenBirch, presi- dentof the.company, for placingall facilitiesat our disposaland for his permissi.onto publishthis material. The painstakingareal work by the membersof the United StatesGeological Survey facilitated 'our observations and' deduc- tions, and we ackr•owledgemore than the mere sourcesof the information recorded to the individual members in the foot- notes. To ProfessorL. C. Grato.nwe especiallydesire to record our grateful thanks for his many discussionsand sympathetic criticisms.

TOPOGRAPHY. The regionin the vicinityof the minesis oneof pronounced relief with high, steep-sidedmountains, rugged in detail, rising from KennecottValley (Plate I., ,z/). The valley itself .isoccu- piedby the Kennecottglacier, which has a width .of aboutthree miles at the town and extendsnorthwestward about twenty-four miles to its gathering groundson Mt. Blackburn, •6,•4o feet, and Mr. Regal, about •4,ooo (Plate I., B). The mountainsides on the Kennecottedge of the glacierrise steeplyto the sharpdivide of Kermecottspur at an elevationof about 7,ooo feet. The Spur juts southward fr.om the main WrangelMountains and is limitedon the westby KennecottVal- ß ley and on the eastby McCarthy,Creek, and risesfrom an ele- vation of 2,000 feet at the glacierand 2,650 feet at McCarthy Creek,to n'early7,0o0 feet (Fig. 3). The top of the divide where occupiedby the Chitisto,nelimestone is serratedinto per- pendicularpinnacles and hoodooforms of the shatteredbrittle PLATE I. ECONOMIC GEOLOGY. VOL. XV.

. . .____--

a. Kennecott spur and town of Kennecott, showing topography and location of mines (+). b. Kennecott glacier. Mr. Blackburn in background and greenstone-limestone contact at right. c. "Hoodoo" topography developed on fractured limestone adjacent to Bonanza Mine. PLATE II. ECONOMIC GEOLOGY. VOL. XV.

a. Contact betweengreenstone (dark) and limestone(light). Jumbo Mine at right. b. Greenstone-limestonecontact. l•ormal contact at left, down-faulted against greenstone. c. Quartz-diorite porphyry with inclusions of black shale of Kennecott formation, GEOLOGY OF ORE DEPOSITS OF KENNECOTT, ALASKA. 5 rock, resemblingthe DolomiteAlps (Plate I., C). Great talus slopes,at the angleof repose,reach down hundreds of feet below thepinnacles. The iower slopes underlain by thegreenstone, are less rough, with occasionalbenches and cliffs. The areas of porphyryweather to smoothsteep slopes covered by talus. The mineslie approximatelyI,OOO feet •belowthe highestpoint of the spur. The slopes.are scored by deep,steep-sided, V-shaped tributary gulches.c'on. taining dashingstreamlets which empty into the main valley at the edge of the glacier. The headsof these gulchescontain small hanging glaciers,.rock glac.iers,a or talus slopes. The Jumbo Mine is situatedin a basin at the edgeof one of theseglaciers (Plate II.• .4) and the Bonanzaore bodyout- crops along the sharp crest of a narrow transversespur lying between two such basins. On the McCarthy Creek sideof the divide, part way down the slope,lies the Mother Lode Mine.

ROCK FORMATIONS. The formations in the vicinity of Kennecottare ta'bularly shownas f,ollows. 4 (See alsoFig. 2.)

Quaternary Rockglaciers. Brokenrock an• ice. •Alluvium.MorMnes. GlacialFloodplaintill--partly.sorted,. gravels,sands and sil.ts. Jurassicor Quartz-diorite porphyry. Stocks,dikes and later. sills. Upper Jurassic. Kennecottformation. Shales,sand. stones, and conglomerate. [McCarthyshale. Shale width few thin-bedded J limestones. UpperTriassic/chitiston elimestone.Massivelimestone, mostly t_ magnesian,ore-containing. Triassic.' Nikolai greens'tone. Altered basalticlava flows. a Rock glaciers (see Moffit and Capps,Bull. 448, U.S. Geol. Surv.) are mixtures of rock and ice, or rather frozen talus slopes. They exhibit the characteristics of glaciers in their movements. 4 The formation names and geologic ages are taken from: U.S. Geol. Surv. Bull. 662, p. I64, I9x8, by Fred H. Moffit', and Bull. 448, •9•, by F. H. Moil:it and S. R. Capps. ZLZN M. BZTEM•4N ZND D. H. McLZUGHLIN. f....:.•:.:...' Alluvium

Rock glaciers

Moraines

Shales,sandstones, conglomerates

McCarthy shale

Chitistone limestone

Nikolai greenstone

Quartzdiorite poryhyry

1 o I z 3 MILES

Fro. 2. Sketch map of geology,vicinity of Kennecott, U.S. G. S. Bull. 622. GEOLOGY OF ORE DEPOSITS OF KENNECOTT, ALASKA. 7

Nikoli Greenstone.--TheNikoli greenston,e appears to. be a regular and massivelybedded, sedimentary series with certainharder bedsexpressed in abrupt cliffs and softer onesin Benches. It is, however, a successiono.f altered. basaltic lava flows, each flow varying slightly in compositionand ranging from about 25 to xoo feet in thickness. The total thicknessexposed in the vicinity of the mines is at least 3,500 feet, and the base cannotbe seen. Elsewhereit is estimated:to have a thickness.of 4,000 to 5,000 feet. 5 The formationis widespread,extending as a narrowbelt several tensof mileson either sid,eof Kennecott. Its uppercontact with the conformably overlying Chitistone limestoneis the most dis- tinctivegeol.ogi.c feature of the districtand canbe tracedby th.e eye over wide areas (Plate II., A, B). It is, 'therefore, older than the upper Triassic Chitistone limest.one,and Moffit and Cappsø considerit of probablyearly Triassicage. With closeinspection some beds are seento consist.of dense, hardgreenish gray lavas, and others o.f softer reddisl/or greenish coloredlavas. T'hetexture varies from denseto mediumgrained porphyritic. Both varietiesmay be amygdaloidal,•but the red- dishvariety is notablyso. Somebeds are not amygd,aloidal,and othersmay containthe amygdulesat oneplace but not at any other along the strike. The amygdulesare, however, fairly equally distri,butedvertically, within one.bed, and, not concen- trated at the top of the individualflows as in the Lake Superior amygdaloids,though there is. a suggestion, in places, of a grad.a- tion from denseto amygdaloidaltoward the top of a flow. The fillingsconsist of chlorites,serpentine, chalcedony and opal, calcite,laumonti•te, thompsonite, quartz and epidote. Specksof native copper,chalcocite, bornite, and chalcopyritehave also been observedin the amygdules.The bedsare traversedby numerous joint planesand occasional veinlets of quartzand epidote. Microscop.icallytherocks are seen to be,basaltic in composition • Moffit,F. H., andCapps, S. R., U.S. Geol.Surv. Bull. 448,x9xx, p. 62. • Op. cit., p. 63. ALAN M. BATEMAN AND D. H. McLAUGHLIN. with pronoun,ced ophiti.c texture which persistseven where the rock is amygdal.oid.al.They are invariablyaltered and now contain considerable'serpentine, chlorite, calcite, and epidote,and extremelysmall amountsof zeolites. Someof the reddishva- rieties seem.to owe their color to the presenceof mir•utescales of iddingsite,as a result.of alterati.onfrom olivine. ChitistoneLimestone.--The Chitistoneformation is of especial interestbecause the valuablecopper deposits .of the district are localized in it. It is a consp.icuous,heavy-bedded formation, widely distributedthrough.out the Kennecottregion. It is inter- sectedby severalsystems of fracturingwhich have divided it into a seriesof polygonalt)1ocks. Thesepermitted frost erosionto proceedwith unusualrapidity so that .o.nthe hillsabrupt cliffs and prominenthoodoo pinnacles f.orm a ruggedoutline (Plate I., C, II., B). The formationrests on the Nikolai greenstoneand is conformably.overlain by the McCarthy shale. In its typelocality the thickness. of theChitistone is 3,000 feet,• but near Kennecottis considerablyless. The lower part of the formationconsists o.f a 4-7 foot bed of shale,the lower part of which is red and the upper part green. Where it is exposedin the mines it is slickenslidedand crumpled and often locally thinned or thickened,giving evidenceof movementalong it. Althoughthin, thi.sshale bed is an importantfeature of the district, becauseit weathersto ,a p.rominentbench and often forms the only modeof travel alonga line of difficultcliffs. A,bove the shale is •2 feet of thin-bedded,smooth, hard, gray, argilla- ceouslimestone, then 23 feet of thimbedded,rough, pebbly lime- stonecontaining flattened, .cylindrical, fossil-like grains, and 3¸ feet or more of dull, dark-gray limestonein thicker beds. The remainderof the formation consistsof mass,ivebeds of sparkling light gray dolomiticlimestone with occasionalbeds of darker rock. The dolomiticbeds are notablyshattered and containnu- merousseams of calcite. The upperpart of t'heChitistone lime- 'stonebecomes thinner beddedand shaly,and gradesalmost insen- siblyinto the overlyingMcCarthy shales. ? Op. cit., p. 23. GEOLOGY OF ORE DEPOSITS OF KENNECOTT, ALASKA. 9

The gray limestoneis f,airlypure, yielding 3.9 percent. MgCOa, whereasthe sparklinggray dolomiterock averagesabout 30 per cent. MgCOa. The .dolomi.tizationwas not connectedwith the immediate ore deposition,for it is a widespreadfeature, neither is it a stratigraphicphase, for the cont,actbetween the dolomite and gray limestoneis an irregular surface,in someplaces cutting acrossbedding planes, f.ollowing them in others,or terminating abruptlyagain, st small faults along'or normal to the bedding planes. McCarthySha.le.--This formation, .of upper Triassic age, with its basalshale-limestone transition zone, has a proba,blethickness of at least3,00o feet. Its top is not exposed? It occursin the vicinityof the mines,but bearsno relationto the ore deposits, nor is it known to containvaluable minerals. KennecottFormation.--This formation,consisting of conglomerates,sandst.one, and gray and blackshales, with a thick- nessof probablymore .than 7,500 feet, rests unconform.ably upon all of the three earlier formations. Moffit 9 considersit. to be of UpperJurassic age. It doesnot occurin theimmediate vicinity of themines and 'bears no relationto theore .deposits. With the exceptionof the Quaterr•arygravels this formationcompletes the sedimentaryseries near the mines. Porphyries.---Lightcolored quartz-d,iorite porphyries intrude thegreenstone .and all o,fthe sedimentary rocks, in the form of stocks,_dikes, and sills. They occurmost abundantly about one mile from the BonanzaMine, where they form a largestock whichconstitutes Porphyry Mountain. This intrusioncontains numerouslarge inclusionsof blackshale, presumably of the Kennecottformation (Plate II., C). ,Theedges of the inclu- sionsare sharp.and the adjacent porphyry shows no evidenceof assimilation.The shalemasses probably represent blocks of the coverefigulfed in theporphyry ,by the process o•f magmatic stoping. Porphyryis strikinglyabsent in theimmediate vicinity of the 80p. cit., z9. 9 Moffit,F. H., 12'.S. Geol.Surv. Bulk 542,p. 8z, I9•3. IO .4LAN M. BATEMAN .4ND D. H. McLAUGHLIN. ore deposits,but one .dike was observed,cutting the Chitistone limestoneabout •,ooo feet from the Jumbo Mine, and .another two-foot dike cuts the limestoneand, the c.oppervein in the Erie pr.ospect. The porphyryconsists of phenocrystsof quartz and andesinc with small biotites .andhornblende, embedded in a microgranitic (or rarely glassy) groundmass. Alteration productsare practically alwayspresent. The universeallyfine-grained character of the intrusivesuggests intrusion under light .cover. No contact metamorphismhas beenfound aroundthe m.arginsof the intrusive in the Kennecott section.and no metalliferous depositsare known to occur in it. The relation the porphyry may bear to the copperdeposits at Kennecottwill ,bediscussed later.

STRUCTURE. The major structural features of the greenstone,Chitistone limestoneand McCarthy shale are almost identical, except that the last has been crumpledmore than the more resistantolder formations. The Chitistone limestone and the greenstoneare the formations w'.hose structure need be considered in detail. The contactbetween the two (Plate II., .4), accentuatedby their strong color contrast, forms the datum plane from which the larger structuralfeatures may be worked out. Folds.--The mines are on the northeast flank of a broad anti- clinal fold whoseaxis pitchesgently northwesterly. Part of the southwest limb occurs on the southwest side of Kennecott Val- ley; •thecer•tral part is occupied'by the valley, and the northeast lim,b, now dissectedinto rugged mountains,contains the mines (Fig. 3)- T'husthe greenstoneand Chitistonelimestone in the vicinity of the mines have a northwesterlys,trike and a dip of from 23 to 3ø0 N.E. (Fig. 2). Severalsmall open folds lie on the northern flank of the main anticlinewith their axes at righ.t anglesto the main.one and pi,tchingparallel to the dip of the beds formingthe major.anticline. This causesa flutingof fiat troughs and fiat ridgesup and d,ownthe bedscomprising the flank of the anticline, like slickenslideson the surface of a flatly dipping GEOLOGY OF ORE DEPOSITS OF KENNECOTT, ALASKA. II

fault. The major anticlinecan be clearlyseen from any height by following the greenstone-limestonecontact along the bare ßupper slopes,but the minor crossfoldsare not casually evident becauseof the steep and broken ,topography. They become known only 'by careful study. A structurecontour map of the

B ß •o4a' Fro. 3. Vertical cross-sectionat (,4) Bonanza Mine and (B) Jumbo Mine. No vertical exaggeration.

top of the greenstonewas made from all of the available data and it displayedseveral such unsuspected folds. Faults and Fractures.--The limestoneand underlyinggreen- stonesare more fractured and broken than the other formations; the former much more so than the latter, due probably to its more brittle nature. The fracturin,gmay be divided into three classes--faults,sheeted zones, and prominentsingle fissures without faulting. There is a-cer.tain• amount of interminglingof all three; faults or fissuresmay be accompaniedby sheetedzones, and faults may 'beaccompanied Iby sympatheticfissures showing no movement. T:hey fall into four fairly well d'efinedsystems, a northeastsystem, in part mineralized,an E.-W. systemslightly mineralized,and unmineralizednorthwest and N.-S. systems. The faults are mostlynorthwest and northeastand the sheeting northwest, northeast, an,d N.-S., while the mineralized fissures are northeastwith steepsoutheast dips. Nearly all the faults pass from the limestonedown into the greenstone,but a striking feature of the district is that the ils- 12 ALAN M. BATEMAN AND D. H. McLAUGHLIN. suresand sheetingmostly ter/ninate at, or slightlyabove, .the greenstonecontac,t. This feature will be discussedin more detail under the headingof Ore Deposits. ff"heconcen•tration of frac- turesin the limestonehas. rendered it more susceptibleof replacement by copperminerals. Faults form but a small proportionof the total fractures,and thosewith large displacementsare rare. One of them, the Inde- pendenceFault, south of tlxe Bonanza Mine (Plate II., B) has a northeaststrike, and .•he northwestside has been droppedat least 3oo feet. Another, the Flurry Fault, south of .the Jumbo Mine, has its southeastside droppedat least 60o feet and prob-

Fro. 4. Faulted contact near Erie Mine. ably muchmo.re. Numerousfaults with displacementsof from x to 25 feet may be seenalong the greenstone-limestonecontact at the Erie Mine, where the contactis steppedup and d.ownin closesuccession by normal,and rev[rse faults (Fig. 4). Numerousfaults of small.displacement are encounteredunder- ground,more beingfound in the Bonanzathan in the Jumbo Mine. They are characteristicallydiscon.,tinuous, horizontally and vertically,and varia'blein strike and dip. In the Bonanza Mine, faultsare equallydistributed in •he upperand lower levels, but thosethat show apprecia'bledisplacement are more numerous below. The displacementsof ,the veins and of the greenstone- limestonecontact has been worked out in .about5 ø per .cent.of the cases. The faults are mostly of small displacement,of dif- GEOLOGY OF ORE DEPOSITS OF KENNECOTT, ALASKA. 13 ferent dips, and with variablevertical and• hbrizontal.displacement. Most of the faults are pre-mineral, but many show post- mineral movemerit,and since partially altered sulphideswere depositedin the first, and draggedin the latter, the distinctionis often difficult. Conclusionsregarding the amount of displacement of ore bodiesare usually difficultto obtain and have to be judiciouslydrawn, for the post-mineralfaults may actuallybe pre-mineralf. aul'ts wi•h post-mineralmovement along them, and the displacementof beddingplanes or of the contactmay represent the total of ,bothpre- and post-mineralmovements. The region is characterizedby so-called"flat faults," which havean importantbearing upon the ore deposits. (See Fig. 7-) They are pre-mineral movements along the bedding planes, having•the same strike and.dip as the beds,but occasionallycut- ting acrossthe bedsa,t low anglesdown the dip, and,rarely along the strike. They appear to be the sliding of certain beds over others,and are .analogousto the movementpr.oduced when sheets of paper are moved over each other. Their displacemen.tis almostimpossible to determinebecause of the lack of veins, fissuresor other definit.efractures cr.ossing ,them. Such a strike fault lies betweenthe greenstoneand the limestone,and the thin shaleband separatingthem, ev.erywhere shows evidenceOf considera'blemovement by its cru'mpled,gouge-like, slickensided.character. An important one in the JumboMine, accompaniedby breccia, gouge and slickensides,lies about 4o feet abovethe greenstoneand forms the bottomof the ore (Figs. 6 and 8). A weakerone in the BonanzaMine lies about75 feet s,tratigra,phically above the greenstoneand in many placesforms the ,bottomof the ore. It also forms, in places,the boundary betweenthe gray and the dolomi'tizedlimestone. Several-other suchfla't faults usuallymarked by an inch or lessof black gouge. are scatteredthrough the mines,and in many placeshave caused termination of the ore. In our opinionthe fl•t faultsmay havebeen caused by a sliding of individualbeds or groupsof bedsover eachother, due to the x4 .4L.4N M. B.4TEM.4N .4ND D. H. McL.4UGHLIN. arching of the .major anticline. In such.an arching the upper- mostbeds would be subject.edto a stresswhich woul.d be relieved by breaking, or b.y sliding upward over the lower beds, in the sameway that the outersheets in a pile of papersarched upward have to slideupward over the lower onesto allow of the arching. If such be the case the flat faults would, be reverse.

PHYSIOGRAPHIC DEVELOPMENT. The physiographicdevelopment of the region bears upon the relations of the land surf.acesand previous water levels to the origin and superficialalteration of the ores. The record is any- thing but completeand the presentationof it here is, therefore, rather sketchy. ]>re-JurassicDevelopment.--As the base of the greenstonein this sectionis not known, there is no pre-greenstonerecord. The top of the greenstoneis as regular a surface as any sedimentary layer. The even..distributionof the amygdulesfrom top to bottom o,f the individualbeds is suggestedby Moffit anti Capps•ø as one reasonfor Believingthem to have been o.f submarine.origin. The relative coarsenessof grain of the flows and the entire ab- senceo,f ropy structure such as occurs in basic subaarialflows also suggestssu'txnarine origin. Further, the a,bsolu.telyun- weatheredsurface of the greenstoneas shownin •thedeeper un- dergroundexposures indicates th.at it was not subjectto wea.th- ering by the air. It would thus appearthat the uppersurface of the greenstonewas not a land one. The uninterrupted sedimentation.o.f the Chitis.toneand Mc- Carthy formationsindicates no land area until the completionof the Mc,Carthyshale at the cl,oseof the Triassic. Then uplif.tand tilting of the sea bottom initiated erosionalprocesses which cut downinlto the McCarthyan.d Chitistone formations and, inplaces, into.the greenstone. The resultingland surfaceappears to have beenone of ma,turity with appreciabler.elief, .consisting of rela- tivelyflat interstrea'mareas and fairly deep,,though gently sloped valley.s. The bouldersof fthebasal conglomerate,with a thick- xoOp. cit., p. 6o. GEOLOGY OF ORE DEPOSITS OF KENNECOTT, ALASKA. 15 nessranging up to 200 f.eet,TM in thesucceeding Kennecott f.orma- tion suggests.that the land from whichthey were derived had considerable relief. Jurassicto TertiaryVolcanics.--The land area next recorded isth• developedon the Kennecott formation, .and antedating the Tertiaryvolcanics which form the higherparts of the Wrangell Mountains to the north of the Kennecott area. This sur.face as seen from Bonanza Peak truncates the older beds and appears to be one of sl'ightrelief. It p.r0ba'blyextended over .the vicinity of the mines,although higher than the presentsurface. It is not knownwhether the Tertiary volcanicswhich lie on this surface evercovered the regionin the vicinityof the mines,but they do lie on the Kennecottspur about 9 milesnorth of BonanzaMine. The groundwaterlevel during t,his period of the development of a surfaceof sligh•rrelief hadprobably a gentlyundulating sur- facecorresponding with the •t.opogr.aphy,a comparatively shallow depth,and with a probableslow down:ward migration. Its relation to oxidation will b.e discussed later. Tertiary Volcanicsto Glacial Period.--Deformationof •the Tertiary volcanicsindica,tes disturbance subsequent to their extrusion, which, with the erosion that followed, appearsto have outlined the main features of the present topography. A ruggedtopography seems to have been•formeet before the start of the glacial periodand directedthe main lines of gla.cia- tion. The master sitreamsflowed in deep valleys and the main tributarieswere established. Erosion, apparently,was rapid and great relief was pro.d•uced.In fact, the main topographicfea- turesmust havebeen very similar t.othose of to-day, exce• for the accemuationand changesproduced by glacialand post-glacial erosion. Moffit and Capps•2 believethat the uplift of the present mountainousareas acted slowly over a long period of time and waslat. er thanthe Tertiary coalformati.ons of Alaska. During the carvingo.f .the master valleys, and the main tributaries,such as KennecottValley and McCarthy Creek,Kennecott • Op. cit., p. 38. a2Moffie, F. H., and Capps,S. R., Bull. U.S. Geol. Surv. 448, I9•, p. 74. 16 atLatN M. BatTEMatN atND D. H. McLAUGHLIN.

Spur,lying ,between these closely spaced parall,el tributaries, was formed,and musthave undergoneactive erosion. T'he cutting of these.tWO d'eep, closely spaced, tributari.es must have lowered rapi,dlythe groundwaterlevel in the narrow KennecottRidge lying beeweenthem. (See Fig. 3.) Glaciationand the Present Surface.--The land carved by streamerosion was profoundlymodified 'by glaciation,which in this countrystill persistsin lessenedform. The mastervalleys and major trit•utarieswere filled}by great glaci.ers, whichbroad- ened, straightened,and deepenedthem; spurs were truncated•, lower ridgesov.erridden, hanging valleys formed, and islandsof resistantrock left projectingfrom the floors.of the valleys. All thosefeatures characteristic of glaciationwere impressedupon the qountry. Moffit andCapps •a estimatethe amountof glacialdeepening to be betweenx,ooo and •,5oo feet and the top of the Kennecott Glacierto have•stood about 3,ooo feet higherthan i.t is m-day. This means(and examinationof the groundsubstantiates it) that the .oredeposits, lying appro•cim•tely4,ooo f.eet abovethe Ken- necottGlacier, were urmffectedby widespreadice sheetsor large valleygl.aciers. The upperparts of KennecottSpur were, however, vigr)rouslyattacked• by local moun,tainglaciatio.n, as is at- testedby the numerouscirques, many of which intersectedand producesharp arretes and steeppeaks. BonanzaPeak still sup- portsfive shortglaciers, one of .whichis evennow sappingthe outcropof the Bonanza•o.re bodies and enriching its morainewith copperminerals. Elsewherealong the spur, rock glaciersare frequent,and smallglaciers are active. The buildings.of the JumboMine are buil,tupon the edgeof one suchsmall glacier and are constantlymoving, while a power line which crossesit show an annual movement in the center. In additionto the localglacial erosion, frost actionplayed an importantpart in loweringthe generallevel of the spur and sculpturi.ngthe detailsof the presentrugged topography already described. How muchof the upper parts of the ore bodiesor their overlyingcover w:as..thus carried away cannot be told. •o Op. cit., p. 44. GEOLOGY OF ORE DEPOSITS OF KENNECOTT, ALASKA. 17

Post-glacialstream erosion in the vicinity of Kennecottis almostnegligible, because the post-glacialperiod is yet to come. Here and there shallownotches have beencu• by the streams.into frozen till .or softtrocks. Frost action, however, is active and is nowan importantager•t o'f erosion. Due to it manyof the slopes are mantled by great ,talussli•d'es resting at the angle of repose and extend,ingin a directionhur•dred*s or eventhousands of feet, downward from the base ,of the steep upper cliffs. The .talus slopesare continuallycreeping downward, due chiefly.to the action •of snowslidesin winters,or of meltingsnow in spring,and are as continuallybeir•g renewed. No evidenceof more than one period of glaciationhas been observedin .the Kennecottdistr/ct. If an earlier glaciationdid take place, its record has been obliterated,by the intenseerosion of the recentglaciers. It is probablethat here as in other parts of Alaska thesewere periodsof glacial advanceand retreatTM and the presentperiod may be oneof temporaryglacial retreat. The effect of .theglacial period on the ore bodieswas to bring •bouta rapiderosion of theirupper parts and t.o arrest all oxidation or chemicalchanges within the ore bodiesby freezingthe waters that would bring about suchchar•ges.

ORE DEPOSITS.

General. The ore bodiesat the Bonanzaand JumboMines are of the sametype, the same characterof ore, and exhibit similar behavior. They bothoccur in the samef. ormations and althougha mile apart, were formedby the sameprocesses. In detail and sizethey differ consi•derably.Other depositsof .thesame characterof ore and very similarin type,in .theimmediate district, arethe Erie and Mother Lode Mines, both of whichha4•e, as yet, madeonly small ore shipmen,ts. The Jumboand Bonanzahave beenthe great shippingmines. x4Such a conditionis consideredby Cappsto have occurredin the White Riverdistrict. Capps,S. R., Jour.Geol., vol. 23, p. 748,•9H, and.U.S. Geol. Surv. Bull. 63o, p. 63, •9•6. 18 ALAN M. BATEMAN AND D. H. McLAUGHLIN.

Character of Ore. The ore is worked for its coppercontent, but an appreciable amount of silver adds ,to its v,alue. No other valuable metals are extracted from the ore or occur in more than microscopic amounts. It is largely sulphidewith considerablecarbonate of copper scattered•promiscuously through it. Occasionally the latter, consistingchiefly of malachiteand minor azurite, constitute the chief l•art of .theore in particularplaces.. Of the copper producedin the last few years,about 2 5 per cent.has 'been derived from carbonates. The carbonateshave resulted emirely from the oxidation of the sulphide. The sulphideis almostwh•olly chalcocite • 5 anc•the greatm-asses and purity of this mineral are'one of the striking featuresof the deposits. The gangueconsists entirely of limestoneor dolomitic limestonecountry rock. In •laces, in and adjacentto the ore, the limestoneis recrystallizedin, to.white and mottledcalcite. Rhombs of pure white calcite are frequent. Shelter analysesof ore shipmen.ts,(•) first quarter of •9•5 and (2) last quarter of •9•8, show that it contains:

Copper S•O• Fe CaO MgO S Per Cent. SilverOzs. Per Cent. Per Cent. Per Cent. Per Cent. Per Cent.

69.11 Z5.96 O.60•6 O.8O'/ 3.42 -- 63.4317 13.73 ø'85•6 I o.61 6.16 0.29 I3.8

It will thus be seenthat the ore bodieswere formedby the depositionof ,coppersulphides, with practicallyno other minerals or gangue matter. The discussionof this feature will be taken up under "Theoretical Conditions."

Types of Deposits. ß General.--Inthe Kennecottan, d surroundingdistricts, copper depositsoccur in both limestoneand greenstone. Those in the x• Other sulphides,in almost negligible quantity, occur. For the amount and description of these see "Mineralogy." 16This silica is probably obtained from impurities of the lower limestone country rock. 17II.68 per cent.of this copperis in the form of coppercarbonate. GEOLOGY OF ORE DEPOSITS OF KENNECOTT, ALASKA. 19 greenstoneare mo.s,tnumerous but as yet have resulted in.no productivemines. The limestoneis the best rock of the only productive•bodies and most' of themare in thedolomitic phase. The on,lycommercial limestone o.res so far known occura•t Ken- necot-t,but the greenstonemineralization is widespread,being found over hundreds.of squaremiles. Although this paper is concerneelwith the ore depositsin limestoneit must treat briefly of the mineralization,in the greenstonebecause of its possible bearingon., the Kennecottdeposits. Copperin Greenstone.--Copperdeposits are foundin thegreen- stonein the immediatevicinity of Kennecottan, d also.scattered over the su.rrounding districts. All o.f ,themshow strikingly similar.characteristics.•he greenstoneitself, remotefrom ore bodies,invariably yields small amoun, ts of copper,by assay. J.D. Irving learnedin •9o7 from examinationsof many greenstone localitiesth.at hardly a greenstonespecimen c,ou•d, be foundwhich did not showappreciable copper, arid' numerous assays of green- stonefrom unmineralizedareas sh,owed . • • 'to .60 per cent.copper? The form in ,whichthis copperoccurs. was not deter- mined. The depositsof copperin the greenstoneoccur as (•) veins. (2) disseminatedreplacements an•d impregnation, (3) amygdulefillings. The veinsare themost importan,t .and the onlytype that give, promiseof becomingproductive. Considerablework has beer•. dorieon them in ,thed, istricts surrounding Kennecott. T.hey- are mostlyshort and shal}ow, terminating abruptly at somecross fractureor beddingplane in the greenstone.Their width is usuallymeasured in inchesrather than feet. The mineralsare extremelyirregularly distributed in, .theveins. Bunchesof ore maybe f.oun.d of suchsize that theireconomic possibilities seem realized,only to disappearinto a narrowstringer of no commer- cialvalue. Mostof theveins in greenstonecontain chiefly bor- nitean, d chalcopyri,tewith minor amounts of chalcocite.Quartz.,, calciteand lesser amounts of epidoteare the chief gangue minerals. Native copperfrequently occurs in the veins,and flat •8Statement contained in a privatereport by J. D. Irving. 20 ALAN M. BATEMAN AND D. H. McLAUGHLIN. sla•bsof it are often found in joint phnesunconnected with veins. Some veins exhibit combstructure with native copperlying between the projectingquartz crystals. Most of .theveins appear to have beenformed chiefly by a filling .of pre•xistin,g cavities, with minor replacementof the wallis. The disseminatedreplacement deposits are usuallynarrow .and elongatedzones of mineralizationf.ormec• By a partial replacement in and alongsheeted zones and • are very spotty. They consist of bornite,chalcocite, chalcopyrite, and native copper. The greenstoneis usuallyconsiderably altered, and chlori.te,epicdote, albite,calcite, and zeolitesoccur. A disseminateddeposit is re- portedby Irving•9 on the upperKotsina River in which native copper,averaging 0.65 per .cent.occurs in flakesand particlesup to ¬inch in diameter,associated with specksof quartz and minutegrains o.f epidote. ,Theaugire of the greenstonehas been altered'to hornblendeindicating deep-seated alteration by heated waters,bm .therock hasnot beenaltered by surfaceagencies. The amygdulefillings are of scientificrather than commercial interest. They occurin widely scatteredlocalities of greenstone, but are not numerousin any one locality. They containnative copper,born[te, chlorites, epidote, serpentine, quartz, Calcite, and zeolites.• 'The rock adjoining ,the amygdulesin placescontain, s •resh olivineand idd.ingsite. Native copperhas been reportedfrom a numberof localities -throughout{he Belt of ,greenstonerocks and is known .to occur as placercopper in all of the gulchesand streamswhich ct, t greenstones. Nuggetsweighing up to a poundare common,and some .havebeen found weighingseveral hund•red pounds. Mineralixation in Limestone.raThe valuable ore depositsat Kennecottoccur as fissureveins, irregular •massive replacements, .and stockworks in limes{one. Between the first two classes there are all gractations,the endphases being distinctly veins or massive rephcementsand the ir•termed,iate phasesbeing replacement veins. The third groupconsists of s,tockworks,irregular in outline, formed 'by the filling and a,dj,acer[treplacement of small •0 Private Report, •9o7. GEOLOGY OF ORE DEPOSITS OF KENNECOTT, ALASKA. 2I ramifyingfractures ar•c• joints. The veinlets:are rarelymore thana few inchesin length,less .than an inchin width,and a few inchesapart. 'Theseconstitute the lower grade milling ores. In all of .thegroups of depositsthe amountof ore formedby fillingof cavitiesis relativelysmall; by far thegreater part has beenformed by replacementof the countryrock. The fissures haveacted as ch,annelsfor .directingthe ci.rculationof the miner- alizir•gsolutions, thereby giving'the solutions access to the rock to a.ttackand replaceit. The veinsexhibit little evidenceof banding,crustification, or other structurescharacteris. tic of fissureveirrs, because their fillingsconsist chiefly of only the one mineral. In a few placesa prominent,banding of chalcocitean.d covellit.e occurs, and. occa- sior•alspecimens show microscopic ban.ding with otherminerals (seepage 36). Mostof theore in the veinshas been formed by replacementof the limestonewalls of individualfractures. The ore may 'bein the form .of onecorrtinuous band of solid,sulphide or of severalbands with braided structure. At one point there maybe threeor fourband.s of sulphideseparated. by n,arrowbands of limestone;a little further along thesemay coalesceinto two bands or even one, by replacementof the. interveningcountry rock. The individualbands vary greatly in width within short horizontaldistances, in placesp'inching to mere seams,and in othersswelling out to a wid,thof many.feet. The veinsmay be accompaniedby disseminated'sulphides in the walls. The replacementbodies are of three classes:(t) irregular massivereplacemen, t, (2) replacementveins, and (3) disseminated replacement,with all gradationsbetween. them. The massive replacementshave been localized. by fracturesor bedding,to be describedlater, and,their shapesare partly controlledby these features. In this classare many examplesof replacementso completethat not a vestigeof the original rock is to be seenin the ore. In the replacemen'tveins the ore •bodieshave beenlocal- ized by zonesof fracture in which the fractured rock has been wholly or partially replaced:'b37 ore, and•the unfracturedwalls alsoreplaced to varying widths,thus giving the ore Bodythe vein- 22 •IL•IN M. B•ITEM•IN •IND D. H. McL•IUGHLIN.

like appearance.In ,the disseminatedreplacements, grains and veinletsof ore are scatteredthrough massive or fractured limestone. They customarilyform a marginal zone to other ore bodiesor occupyt.he area betweentwo adjacentveins or massive bodies. In places,veins of massiveore may pass along their strike into disseminated ores. In general,the replacement was singularlycomplete where once started,and yet with a few exceptionsthe solutionsv•hich caused the replacemen'tappear to have been passive. They completely replaced•the rock with which they came readily in contact,but lacked'.the vigor to penetrate far 'into ma,ssivelimestone away from the fractures,an,d• were terminated •bruptly by insignificant slips and bedding planes. I•t is not uncommonto see a large bodyof solidchalcocite abruptly ended by a pre-mineralslip containing only .onequarter inch of gouge. The completeness.of the replacementand resultingpuri,ty of the ore may.bestbe ur•ders.tood'by the classesof ore mined. The "first class"or "high grade" consistsof practicallypure copper minerals Obtained from the massive repl,a.cements,and massive parts of the replacementveins. Large stopeshave been opened in whichpractically nc•thing else but pure chalcocitecould be seen, so that t•he stope resembleda 'black coal mine and the ore as drawn from the chuteshas run as high as 7¸ per cent.to 76 per cent. Cu? Examirmtion of car loa,d lots failed to disclose a speckof limestone. The "second class" ore consistsof bunc,hes or bandso'f copperminerals, mostly chalcocite,either a,ttached.'to

limestoneor dilutedwith ß it ir• the processof mining. This class comesfrom the margins of the massivereplacements or from the less.massive parts of the veins, and is shippeddirectly to the smelter. It cor•tains.from 20 to 5ø per cent. Cu. The milling ore con,si•ts of disseminated•grains of copperminerals or larger pieceswhich cannotbe mined without much admixed limestone. The in,dividual grainsor piecesof copperminerals are themselves practically free ,from in.cluded gangue matter, but are contained in, or attachedto, fragmentsof 'countryrock. This low-gra,de 2oPure chalcocit'econtains 79.8 per cent. of Cu. GEOLOGY OF ORE DE.POSITS OF KENNECOTT, ALASKA. 2 3 classcomes from ,themargins of other ore bodies,from the dis- seminatedreplacemen•ts and stockworks,or from thoseparts of the veinswhere the mineralizationis spottedor where the bands of copperminerals are too small to constituteany considerable proportionof the Brokenore. Talus Depositsor Slide Ore.--A un,iquedeposi.t of copperore occursalongside .the Bonanza Mine, mined by nature and gath- ered on the hillside as a talus slope. The Bonanza vein out- croppedas a great mass,of rich ore, mostlysolid ch/dcocite, along the top of a knife edge ridge projecting.out from Kennecott Spur. (See F,ig. 5.) The rapid and continuederosion of .the

Fro. 5. Diagrammatic cross-sectionat Bonanza Mine to show glacier ore body and falus slope ore. ridgeand the vein forming i,ts bacl•bone resulted in d•bris,com- posedof ore, falling downthe sideof this steepridge. Its ac.cu- mulationon lhe S.E. side formedthe talusslope ore knownas the "Slide ore body." It lies at the angleof reposeand forms a mass,irregular ,in outline,extending a few hun,dred feet up and down t-he hillside. The thickness .is several feet and, .the width severaltens of feet. More than 90,0o0tons of high-grade milling ore 'werethus formed, most of which has now been.mined by surfacescrapers. The rockfragmen, ts vary fromsmall grains up to the sizeof one'shead, and scatteredthrough it are more finelycomminuted pieces of copperminerals, chiefly chalcocite free from limestone,with a minoramount of coppercarbonate. GlacierCopper Deposit.--An ice glacierseems an impossible "countryrock" for an ore body,yet suchis the caseon the side 2 4 ALAN M. BATEMAN AND D. H. McLAUGHLIN. of the sameridge oppositefrom erheSlide ore deposit. Here the ridge falls away almost vertically, forming the side of a steep- wailed cirque,which containsa small glacier of no appreciable movement(Fig. 5). The c•dbrisresulting from the rapid frost disintegrationof the ridge and vein outcrop,accumulated on the glacierfor a distanceof a few hundredfeet from ietsedge, forming part of the glacialmoraine. The d'•ris accumulationand the buildingup .of the glacier went on hand in hand during the Glacial period, for the ore now occursdeep within the glacier. The glacier with a steeplysloping front has been exploredby means of three tunnel levels from which several cross cuts have beenrun, enabling,the ore body to be partially outlinedand sam- pled. So far, 220,000 tons of .copperore have been developed. This ore consists.of ice, limestone,and chalcocite,with a minor amountof carbonate;the countryrock is ice with considerable admixed rock d•'bris. The ore ddb•ris is of the same character as that describedfor the talus ore, except thaetlarger pieces are morecommon. Perhaps3 ø ,to60 per cent.of the materialwithin the ore'body is accumulatedddbris, the remainder'being ice. ,The ore body is quite compact,requiring blasting,and the workings required only moderate timbering. The passagefrom ore to wasteis a gradualone.

Structural Relations. Relation of Mineralization to Rocks.--In both the Bonanza ar/d Jumbomines the associationof ore and rocksis similar. No ore extends down into the greenstoneand none occurs in the lower few feet of the Chitistone limestone. Most of it occurs in the dolomiticphase of the Chi.tistonelimestone, but large bodies are minedin the dark gray limestonebeneath .the dolomitic phase. (See Fig. •2.) In the BonanzaMine the development'forsome years indi- catedthat the ore was co.nfinec[to the .dolomite,giving rise to the impressionthat the dolomite was the only favorable host rock, and that all developmentand exploration shouldbe confinedto this rock. The ore d,id not passinto the so-called"unfavorable" GEOLOGY OF ORE DEPOSITS OF KENNECOTT, ALASKA. 2 5 gray limestonebeneath the dolomite,but terminatedabruptly at the contact Between them. In the Jumbo Mine the samecondition existed to a certain extent, bu.tfurther developmentdemonstrated .that the ore passed uninterruptedlyfrom dolomitic to gray limestone (the la•er being as favorable to ore as the former), and:that the strati- . graphicdownward termination of .theore was due to causesto be discussed later. .This conclusionapplied, to the BonanzaMine later led to the developmentof extensiveore bodiesin the gray limestone. In the BonanzaMine (.comparewith Fig. 6) the ore bodiesare

4> ß 300

•, ,.,rß •00(• LEVEL

FTG.6. Longitudinal vertical projection from the 200 to 700 levels of Jumbo Mine to show inclined ore zone and its relation to the Flat fault and rocks. Shafts and•main levels, only, are shown. Lateral workings omitted. distributedthroughout an inclinedzone whose longer axis pitches d,owrrward parallel to the dip o.f the .bedding,and whoseshorter axis, normal to the bedding,extends up to a maximum distance of 340 feet •bove the greenstoneor 250 to 270 feet above.the limestone-d[olomitecontact. Within the main ore zone the veins bottom at the gray limestoneand.' Flat fault, {but the irregular replacementbodies occur in the gray limestonebeneath •rhe dolomite. The upperen(• of this inclinedzone intersects the surface for a shor,t distance;the lower has not yet been exposedby mining. 26 ALAN M. BATEMAN AND D. H. McLAUGHLIN.

In the JumboMine the conditionsare the same(Fig. 6) except that the irmlinedore zone extendsto a maximum distanceof 5o0 fee•above the greenstone,and the mainveins pass downward throughthe limestone-dolomitecon,tact and terminateabruptly againstthe Flat fault parallelto the bedding. The behaviorof ,themain vein whereit passesdown'ward from the Chitistonelimestone into greenston,e can be seenat ,theBo- nanza outcrop. The fissurepasses with diminishedstrength throughthe lower limestonemembers, but dies out in the green- stonea few feet from,.the contact. The ore stopsabout ten feet abovethe greenstone,but sparsemineralizatioix extends down into it. Even this ceases ,within a foot or so and it is so casual that it mightbe interpretedas being unconnected wi,th the mineralization of the vein a•bove. It is evident that the greenstone doesnot lend itself to the same fracturing .that gave rise to the prominentfissure in the limestone,and is decidedlyuncongenial for mi, neralization. The 'behaviorof the main vein ir• the g•eenstonecould not be observedin the Jumbo Mine, for it is not exposed. The ore doesnot extend to the greenstoneand it is doubtful if the fissure does. At the Erie prospect,the fissuredies out in the green'- stone within a short distance of the co,nta.ct;chal.cocite extends down,to ,the greenstone, but not into it and givesplace to a little chalcopyriteand quartz. Behavior o[ Fissures.--The fissures,which have been filled or replacedto form the veins, exhibit unusualbehavior. One cus- tomarily thinks of fissuresas extendingfro.m the surfacedown- ward. (except in the caseof bli,nc}veins), but the Kennecottfis- suresextend from the bot.tom upward and ,they have also been developedthat way. In horizontaldirection they resembleany fissure. They .are highly inclined•and cut across the strata normal to its strike. But in vertical direction,they start usually from oneof the inclinedbedding planes parallel to the greenstone contactand at variable,distances above it, and graduallydiminish and die out, or passupward from a clearly definedfissure to a slightlysheared zone of relativ'elyi.nsignificant fracturing. (See GEOLOGY OF ORE DEPOSITS OF KENNECOTT, ALASKA. 27

Figs. 5, 7, 8, •2.) Their strongest.develop.ment is below, and the weakestabove. The sympatheticshearing or fracturing which accompaniesthem also dies out upward,s. .Their Ul•vard extensionresembles the roots of the usual type of fissures. The fissuresmay thus•be pictured as t•bular bodieswhose Bottoms are terminatedabruptly and pitch downwardat 23 degreesparallel with the bedfling;whose tops die out in a frayed outline which a.lsopitches downward approximatelyparallel to the bedding; the wholeextending downward along its pitch an undetermined distance. ,(See Fig. 6.) Obviouslythe veins formed by the filling of thesefissures present the samebehavior. The mineral filledfissures exhibit little or no faulting. Relation of Mineralization to Fractures.--Three factors have beenimpor .tan.t in localizingthe ore; namely,the Chitistonelime- stone,steeply inclined N.E. fissures,and flat 'faults.2• The limestone,as already described,was the chief localizer of the ore, and also of the fissures an,d "flat" faults. The fissures governed,the position, shape, and size of the ore bodies. They guidedthe mineralizingsolutions through the liemstone, thusIocalizi, ng the veinsanti allowingopportunity for massive replacement,and the "flat" faults determinedthe bottom of indi- vidualore bodies and in placescaused local swellings of theveins (Fig. 8). Intersectingfrac.ture.s in placesdiverted solutions from the main channels,giving rise to smallerveins., stockworks, .or irregular massivereplacements. The fissures.next ,to the limestone,were the chief loci of ore andthus are a guldein exploration.for more ore. The massive irregularreplacement bodies in the BonanzaMine are separated from the mainvein, 'but clearly have been localized by other smallerfissures, al.though in some cases the fissures are no longer visible within the ore itself. The flat faultsstrike almost at rightangles with the veins, and theiri,ntersection •vith the veins pitches downward at theangle of .thedip o.f the flat fault. (See Fig. 7.) .Thisintersection ac.tsBoth as the d•ownwardtermination of the veinsand'a favor- 2x See p. I3. 28 ALAN M. BATEMAN AND D. H. McLAUGHLIN.

able locus for ore. The selvageor gouge from a fraction of an inch to a foot or more in thickness, contained in the flat faults, apparentlyactec• as damsto the solutions,for mineraliza.tionex- tendsbeneath'only a few of them. Also the damsof gougeby actingas barriers,to the progressof solutionsalong the fissure, impoundedthe waterson their uppersides, allowing greater replacementof the walls of the fissures.and causedenlargements of the veins, This is notablein the Jum'boMine •vhere the main

F•a. 7. Stereogram to show flat fault and,ore bodies. Note enlargement of ore body at int'ersection of steep fissures with flat fault, and diminution of ore upward and outward from it. flat fault with its thick gouge termina,tedthe veins in the upper levels,t)ut in the lowest levels apparentlyhad no influence,because the veins do not come in contact with it. The favorable locuscreatec[ .by the intersectiortof the two gave rise to the largest individualbody of solid'chalcocite ever mined, from which thou- sand•sof tons of ore were shipped,containing, as mined, over 7ø per cent. of copper (Fig. 8). In the BonanzaMi,ne a flat fault and,not ,thegray lime beneath the dolomiteterminated the ore. I,t •vasa coinciden,ce'that part way down the incline the flat fault and the contactbetween lime- GEOLOGY OF ORE DEPOSITS OF KENNECOTT, ALASKA. 2 9

stone and doloauiteconcurred, giving rise to the erroneousim- pressionthat .thedolomite was favorableand the gray lime u,nfa- vorable for ore. The Bonanza main flat fault, however, is so insignificantas to be almostovertooked. It is an unusuallyeven break, oocursalong a smoothbedding plane, and containsonly

:--:=-- Workin•

• C ha.lcocite .•:.:.... Disseminated ore 0 •0

incline • ....

ß FIG. 8. Plan of part of 500 level, Jumbo Mine showing relation of the "big chalcocitebody" to the Flat fault, and the shapesand sizes of ore bodies dis- closed on the level.

about one i,nchof compactgouge. Nevertheless,it appearsto havebeen effective in terminatingthe downwardex. tension of the main Bonanza vein from the fif.th level to the surface. From the fifth level downward•other smallflat faults appearto have done the samething. Nearly all of the irregular replacements,the so-called"flat ore," away from the main Bonanzavein, termi- natedownward against small fl•t faults,locally named "bedding planes,"because they occur along bedding planes, and the evidence of ,movementis not pronounced. (See Fig. I2.) Relationto Folds.--At the BonanzaMine, dips and strikes taken along the greenstonecontact indicate a sligh.tsynclinal foldingor e•e a monoclinalfold. The fold pitchesparallel .to thegreenstone contact and t.he trough coincides with theore zon. e. Und'ergrou,ndexposures of the greenstonedemonstra.te the same thing. It maybe consi•deredsimply as a downwardwarping of the inclinedtop surfaceof the greenstoneand overlyinglime- stone(Fig. I2). The ore appearsto be connectedwith this fold- 30 •IL,4N M. BATEM,4N ,4ND D. H. McLAUGHLIN. ing and it will be further discussect,under the "Origin of Frac- tures." Relation of Mineralization to Faults.--The 1.argerfaults already describedapparent'ly have no relation .to the ore but there are in the mines numerousfaults mostly o,f small displacement which have an important relation to it. ,Wherethe pre-mineralfaults or fault zonesin. tersect the N.E. ß mineralizedfisiures there are local enlargementsof ,the veins; the more brecciatedthe fault zone the greater is the ore enlargement, and where the intersectionis nearly at right angles,the enlargementis greater than where acu.te. The enlargementsare due to the opportunity'afforded the solutionsto be diverted from the main fissuresinto the crossfaults, bringing about their filling a,ndreplacement. Some pre-mineral faults, like .the flat faults, have acted as barriers to the further extension of mineralization. Post-mineral faults, or those faults which at least have post- mineral mo-kementalong ,them, have displacedore bodies. Shapesand Sizes of Ore Bodies.--In a generalway, the veins may be ,consideredas extremely .thin wedgesgradually tapering upward toward their apex, and their basesresting on an i,nclined' flat fault or beddingplane. ,The length of the inclinedwedges along their base is many times greater than their height from baseto apex. The heightof mostof the veinshas 'beendeter- mined, but lengthwisealong their base they are still being followed by inclined shafts (Fig. 6). In detail the veins are ex- tremelyirregular and departfrom the wed'ge-like.shape. The height varies from place to place along the incline; the wid,th alongthe base varies still more,and alsomay changegreatly from the basetoward .theapex, perhapsbeing wider midway up from the baseehan at the baseitself, sothat a cross-sectionat any one poin.tmay not show a simplewedge-like shape. The averageheight of the main Bonanzavein from the baseto the apex, measured normal to the incline, is about 2•o feet in the upper levels and • 50 on the lower levels. It has been fol- lowedfor a distanceof about•,9oo feet,measured along its base, an,dthe •vidth varies from 2 to 5ø feet. GEOLOGY OF ORE DEPOSITS OF KENNECOTT, ALASKA. 31

The .mainJumbo vein, exclusiveof its enlargementat the flat fault, averagesabout 360 feet in height, from 2 to 60 feet in width, and ,hasbeen followed down its base for x,5oo feet. The great variation of width in both veins is due to locally greater replacementof the walls at particularplaces, i.ntersections with fault zones,or junctions with other veins. .The lesserveins in both minesare, ifi general,similar to the ones described above, but not all bottom on the same flat fault, and their dimensionsare not as great as in the main veins. The lesserveins appearto die out along their pitch, as do the main. veins toward their apices,and where one dies out other parallel ones,have f. requently been found to star.t. Other veins occur in the Bona.nza Mine formedby the replacementalong verticalor highly inclinedfissures which are •bruptly terminatedabove and below,by bedding planes. They a'resimply :cracks across thick limestonebeds (Fig. x2). The big Jumbo ore 'body formed at the intersectionof the main vein and the flat fault extends down the incline for a distanceof 530 feet and has a maximumheight of xoo feet, measured normalto its base. Its baseis abruptlyt. runcated against theflat fault andthe shapemay 'be seen from part of the planof the 50o level shownin Fig. 8. The numerousirregular replacementbodies unconnected with the main vein in the Bonanza Mine are all limited on their bot- tomsby beddingplanes or smallflat faults. The topsof ma.ny of themare also.limited,the sameway so that theyextend as a seriesof moreor lessconnec,ted bodies up and downthe dip of the strata and enclosedwithin certain •bedsof limestone. Most of themhave a greaterwidth .than height, but in a fewthe dimensionsare reversed.Up to the presentthey have been developed fromthe surface down .to the 6oo level. Theirshapes are dia- grammaticallyrepreser•ted in Fig. 9. Otherreplacemen.t bodies occurin 'bothmines, but notablyin the BonanzaMine, as isolated blebsor bunchesof ch.alcociteup t(3 x 5 feetin diameter,apparently unconnectedwith fissuresor other ore bodies. Relationto Surface.--Themoderately pitching ore zone of the 32 ALAN M. BATEMAN AND D. H. McLAUGHLiN.

BonanzaMine hasbeen intersected. by the steeplyrising surface to form an ore croppingwhich is a cross-sectionfrom bottom to top, almostnormal to the baseo4 the zone. T'husthe cropping displaysthe ,behavior of theveins in verticalextent, affording a cross-sectionof the wedge,with their bottomsspreading out on a beddingplane and taperi.ng to theirapex. (SeeFig. 5-) The relation,of the presentsurfa,ce.to the ore bodiesis purely accidentaland their exposureis dueto the rapidsurface erosion

Plan Cross-section Lon git udinal-se•tion, ¾-¾

Plan Cross-section Longlt udinal-section,

I I

Plan Cross-section Longitudinal-section, FIG.9. Diagramto showshapes of threetypes of irregularreplacement bodies and their relation to the beddingplanes.

which revealedthem. It is a coincidencethat, at the Bonanza, erosionhad progressed to •hepoint where two parallelveins and themineralized. ground between them formed an orebody much greaterin extentthan its continuation.undergroun,d. Had ero- sionbeen less deep the greatBonanz• cro?ping might not have beer}exposed, had it been.d,eeper 'the .big ore body might have beenworn away. In theJumbo the reverse is true;the cropping GEOLOGY OF ORE DEPOSITS OF KENNECOTT, ALASKA. 33 althoughstrong gives little hint of the great ore bodythat lies beneath. The vag.ariesof erosionwere such that it exposedon the surfacethe weakestpart of the vein, but had it extended deepermuch of the greatore bodymig•ht h.a•ve been dissipated by the processesof mechanicaldisintegration. The surfacecrop- pings,therefore, give but little indicationof what might lie beneath, nor does the absenceof croppingpreclude the possibility of ore below, for "blind" veins have been discovered underground. Hence the advisabilityof careful study of surface fractures. Distribution of Ore Bodies. Each mine contains one main vein and several lesser ones, the numberbeing greater in the Jumbothan in the Bonanza. The lesserveins of the Jumboare discontinuousvertically and ,horizontally,so that on differentlevels the num'bervaries (Fig. •o).

-•".;•.' I, '" ..... • ...... '? øol' t• ! f I ß,i,1' • f t I

t._•' ' "--'L','.?-.. •'"" * ' ' ore

Fro. zo. Plan of part of 2oo level, Jumbo Mine, to show shapesand distribution of veins.

Most of them are splits from the main vein that may coalesce with it again or fade out. On the too level are 4 separateveins: on the 200 level, 5; on the 300, 3; on the 400, 2; on the 5o0, 2; on the 600, 4 or 5, while on tbhe800 and 900 levels as yet only one vein is :beingworked on ea,ch,and theseare no.tthe sameone. With the exception of 't,he 900 vein all are containedwithin a narrow zone,and in the upperlevels the mineralizedwalls of the 34 ALAN M. BATEMAN AND D. H. McLAUGHLIN. differentones coalesce to fo,rmlarge connectedore bodies. The excessiveenlargement o.f the main Jumbovein againstthe fiat fault extendsas one continuousore body from above the 300 level to nearly the 600 level. No detachedirregular replacement bodiesoccur in the Jumbo Mine. In the Bonanza Mine detached.irregular replacementbodies ß are numerous,being found on all the levelsand the numberof separatefissure veins is less than in theJumbo. On the surface and downto the 15ø level are two large veinsand one smallvein; on the 200 and 300 thereis only one, and on the other levelstwo or more occur. At the Erie and Mother Lode mines similar ores are extracted. Small amountsof mineralizationof no presentcommercial value occur in other parts of the Kennec.ottarea. In more distant areas,mineralization in the Chitistonelimestone similar in many respectsto that at Kennecott,has been .discovered.

Alteration of Host Rocks. One of the most remarkable features of the mineralization in the limestoneis the lack of gangueminerals other than the country rock,and the absenceof silicificationof the wall rock. Some small crystalsof quartz were observedin the oxidizedminerals of oneore body,and microscopic grains of quartzoccur in a few chipsof .chalcociteand in the calciteof one thin section. Some of the wall rock is in placescrystallized along 'the veins, forming coarsemasses of calciteand dolomite,probably as a resultof the mineralizingsolutions. But beyondthe limits of the ore similar recrystallizationoccurs, and zonesof calciteare commonin various parts of the Chitis.toneformation, as wouldbe expectedin a limestonecountry. The recrystallizationevidently preceded the ,developmentof the sulphidesin someplaces, for occurrences are frequently observedwhere the rock carbonateshave been attackedalong their cleavagesby veinletso.f chalcocite. Only a part of the limestoneis recrystallized,for occurrences are numerouswhere not the slightestchange can be notedby the nakedeye, or microscope,in li,mestoneor dolomitelying against GEOLOGYOF ORE DEPOSITS OF KENNECOTT,ALASKA. 35

solid chalcocite. Thin sections have been mad,e of veinlets of chalcocite,in.cluding .parts of the walls on either side and no changein grain o,rch,aracter could be observedeven where a par- ticle of limestone rested'in intimate contact with chalcocite. The dolomitization of the limestone is not related to the ore formation,for it is a widespreadfeature, whereas the ore is local. Neither was the rock laid. down as a dolomite,for the contact betweenit and the dark gray limestonecrosses the 'bedding. Dolomitizationis consideredto havepreceded the ore formation.

Mineralogy and Mineralography. Relative •tbundance.--Theore minerals,with the exception of the obviousproducts of oxidation,are listed as follows, in orderof theirquantitative importance: (•) chalcocite,(2) .covellite, (3) enargite,(4) bornire,(5) chalcopyrite,(6) luzonite, (7) tennantite,(8) pyrite, (9) sp.halerite,and (•o) galena. Of these,chalcocite is by far the mostimportant. In the field, we estimatedthat it formed92 to 97 per cent.of the sulphideore. Covelliteprobably forms between2 an,d5 per cent. of the ore minerals. The remainingsulphides undou•btedly constitute less than • per cent. of the ore. Enargite may be prominentin places,an, d a few stringershave been predominantly of this mineral, 'butit is absentfor the mostpart in the g.reat chalcocite bodies. Bornitegrains over a centimeterin diameterare rare, butspecks of mic/'os•opicdimensions are fairly common.throughout muchof the chalcocite.Chalcopyrite occurred megascopic- ally onlyin a few bunches,which were removed for handspeci- mens. Luzonite and tennantite are not uncommon under the microscope,but are of no quantitativeimportance. Pyrite is noticeablyabsent in mostof •he.deposits.; only a few grainswere foundwhere the pyritifer%uslimestone was replaced by ore.. Sphaleriteand galena are microscopicrarities. Chalcocite.--Twodistinct types of chalcociteoccur, steely chal- cocitear•c[ crystalline chalcocite. The former is a compact,ma•-' sivemineral, with no apparentgrain, a conchoidalfracture .and a metallicsteely luster. Thecrystalline chalcocite has a granular 36 ALAN M. BATEMAN AND D. H. McLAU.•HLIN. texture of about the coarsenessof a medium-grainedmarble. The crystallineappearance is largely'due to an imperfect octa- hedralparting which disclosesthe size and arrangementof the individualgrains. The two types,common,ly grade into each other withoutsharp boundaries, but in somecases bands of the coarse inaterial occur in the fine and vice versa. On the freshlypolished surface, both types are identical,but when tarnishedor etchedwith nitric acid or potassiumcyani. de, the differencein grain is revealed. In the crystallinechalcocite a patternof trianglesor rectangles•vithin the grainsis revealed by the tarnish which prod.uceslighter bandsor stripsbounding areasof a deeperblue. Reagentsdevelop similar lattice patterns in the coarsergrains and here and t,herein the smaller(Plate III., .4). Occasionally,however, in the smaller grains only one set of strong lines is formed similar to the orthorhombicetch- cleavageof chalcocite. In muchof the steelymaterial, the rea- gents and tarnish merely reveal the fine grain without definite patternswithin the grains themselves. When etchedwit,h nitric acid, the steely chalcocitegenerally yiel.dsirregular patterns re- sembling cracked porcelain (Plate III., B). In mu,ch of the crystallinec. halcocite, however, the originalgrain of the material seems lost. The polished surface when slightly tarnished is finely mottled, with here and there a 1.argerbluish area. The mo.ttledmaterial when etchedyields patternslike the steelychal- cocite,or merely structurelesssolution surfaces; the blue areas usually yield lattice patterns. A rarer structure is a peculiar concentricform. Where developedon rather a fine scale,it •appearsto be madeu.p of an aggregate of small pea-like grains; the field name, pebblychalcocite, is descriptive. Chalcocitewith well-developedmammillary surfaces has been o'bservedforming the walls of s.mallopen spaces, commonthroughout the deposit,which givesstrong weight to the thesisthat the pebblych.alcocite was produced,'by open-spa.ce filling or by the replacementof earlier sulphidesformed in thaisway. On the polishedsurface the structureof this material is empha- sizedby concentriccracks in individualgrains or by scalloped veinlets of malachite or azurite. PLATE III ECONOMICGEOLOGY. VOL. XV.

a. Bonanza. "Crystalline" chalcocite etched with potassium cyanide, revealing variations in grain and triangular isometric etch-patterns. X 43. b. Bonanza. "Steely" chalcocite etched with nitric acid, crackled porcelain structure.

c. Chalcocite from greenstone near Bonanza Mine. Structure revealed by incipient alteration to malachite along orthorhombic cleavage planes. X 43. d. Bonanza Mine. "Diabasic covellite" (dark) with chalcopyrite (whit'e) and bornite (light gray) between laths. (Variation of tint of covellite due to pleocroism.) X 240. PLATEIV. ECONOMICGEOLOGY. VOL.XV.

c. a. BonanzaMine. Covellite plates (dark) in radial and scalloped patterns partially replaced by chalcccite(white). X 37. b.Bonanza. Covellite (dark) in radialplat'es, partially replaced by chalcocite(white). X 70. c.Bonanza. Covellite (cv) partially replaced bychalcocite (cc). Veinsof malachitein zonesof chalcocite.Bornite residues (bn slightly lighter than chalcocite) in two bands in chalcocite. X 70. d. BonanzaMine. Enargite crystals in limestone. X 40. GEOLOGY OF ORE DEPOSITS OF KENNECOTT, ALASKA. 37

The chalcociteis the latestsulphide • to form (PlatesIV., V.). There is clear evidencethat each of the other minerals, with the possibleexception of themicroscopic rarities, has suffered partial replacementby chalcoc.ite in someparts of the depositat least. Bornite, as usual elsewhere,is most readily altered, with ch.alco- pyrite a poor second(Plate V.). Enargite and tennantiteare lesseasily replaced, but are not as resistant.as the luzo.nite,which often remainsin the chalcociteas fine veinletsor rings, partially corroded,after all neighboringmaterial has beenconsumed. Careful chemical stu'dies of chalcocite from the Bonanza and Jumbomines have been made in the GeophysicalLaboratory. in Washington. Two analysesand specificgravity determinations havebeen published, == and are as follows:

Sp.Gr. at 25ø. PerCent. PerCent. PerCent. PerCent. Total. Cu. S. Fe. SiO2.

5.6•o 77.99 =L48 0.=6 o. x3 99.86 5.606 77.56 =L55 0.55 o.•8 99.84

The analysesof chalcocitecollected during our work in •9•5 are in striking ac.cordwith the earlier results. Material from the JumboMine yieldedthe following values

Steely Glance. Crystalline Glance. Cu ...... 77.90 77.88 Ag ...... 07 .09 As ...... o6 .04 Fe ...... 17 .17 S ...... 21-33 21.48 Insoluble Gangue ...... IO .09 CaO ...... MgO ...... 02 CO ......

Covellite.--Covelliteof two agesoccurs; part is earlier than the chalcocite(Plates III., D, IV.) and is clearly unrelatedto superficialagencies, and .a somewh,atgreater part is conclusively shown by its intimate associationwith malachite and li,moniteto 2• With the exceptionof somecorellite associatedwith oxidationproducts.. 2• E. Posnjak, E. T. Allen, and H. E. Merwin, Ioc. cit., p. 5o8. •a E. T. Allen, private communication, December, x915. 3 8 ALAN M. BATEMAN AND D. H. McLAUGHLIN.

be due to .oxidizingprocesses, arid to be a first product of the alterationof the chalcocite(Plate V., A). This covelliteof later origin will 'beconsidered' on the pagesdealing with oxid.ation. The earlier covelliteoccurs here .andthere in broad crystalline bandsthrough the ore, but it is commonestas aggregateso.f short laths scatteredin the chalcocite. Locally, the blunt crystalsmay be abundantenough to resemblethe distri'butionof feldsparlaths in a diabase(Plate III., D), but ordir•arilythey are very small and form only a subordinatepart of the total sulphide. Fine threads of covelliteare almost always present,even in the purest ' chalcocite. Bands, about an inch wide, of coarselycrystalline covellite penetrate'the massivechalcocite in a vein-like manner in several places, but when studied under the microscope,the chalcocite appearsto .belater, as it forms fine veinletsalong the Iboundaries of the covellite crystals and even breaks acrossthem, (Plate iv., 4). In a fewspecimens, believed when collected to beentirely covellite, the micros.coperevealed a complex aggregate of blunt ½ovdlitelaths with chalcopyriteand, bornite in thesmall angular spacesbetween them (Plate III., D), .a relation very similar to the feldsparlaths and the ferro-magnesianminerals in a medium grained dia•ase. The covellitelaths are sharply boundedand break acrossthe bornite-chalcopyritecontacts without the slight- .est change,a relationshipwhich suggestsuniformity of condi- -tionsof depositiono.f the three mineralsand-hence approximate ,contemporaneity. Elsewhere in the deposit, both the •bornite .andthe chalcopyriteare replacedby corellite of the earlier generation; the bornite m.uch more readily than the chalcopyrite. Bornite.--Bornite occurs in small grai.ns and patches with rather smoothoutlines, rarely exceedingan inch in diameter,but most abundantlyas tiny specksof microscopicsize. It is least abund,antor lacking, in,,the crystalline type of chalcocite,and most abundanti.n the steelyglance. The largergrains. are broken in manyplaces by veinlets'of chalcocitewhich afford clear evidence of replacement(Plate V., B). Some of the cracks fol- GEOLOGY OF ORE DEPOSITS OF KENNECOTT, ALASKA. 39 lowedby the chalcociteveinlets contain malachite or limonite cores,but the oxidizedmaterial rarely extends beyond the limit of the,bornite grain and only in thecase of the largerveins does the crack continueon into the chalcocitefield (Plate V., B). Malachiteveinlets in regular lattice patternsin the chalcocite either do not extendinto the •borniteor follow curvingcracks in it withoutyiel.ding definite patterns (Plate V., .4, B). The bornite etcheswith its usualfine-grained lines. but their orientation rarelyagrees with the structuraldirections of the surrounding chalcocite. .Definitelattice patterns 'between ,bornite and • chalcocite,similar to thosedescribbed in Bis'beeores, in Butte ores and elsewheredo not occur at Kennecott,,but in someplaces the distributionof

ß faint bluishresidues and thepatterns of blueand white chalcocite stronglysuggest the last stagesof replacementsof the lattice type. This impressiongains support from the occurrenceof re- sidualspines or platesof bornitein chalcocite,in symmetrical orientationto itd structure,which is strikingly similar to the association common in the .bornite-chalcocite ores of the districts mentioned,where the lattice structureis important. .Careful studyof the contactsusually yields eviden.ce of ,bornitereplace- ment by chalcocite,and esta•blishesthe resi.dualnature of the plates or spines. The lattice structureis developedalso by spinesof chalcopyrite in the bornite (Plate ¾., D). The chalcopyriteoften showsthe samedependence on chalcociteveinlets in the borniteas has'been noticedin other deposits,and it is fairly certain here also that the chalcopy.ritein this form is a by-productof the reactionsby which ,bornite was altered to chalcocite. Certainnarrow stripsof bornite,often with sharpoffsets, seem lesssubject to alterationto chalcopyrite,and usuallyremain un- touched(Plate V., D). They greatly resem,bleveinlets of bornite in chalcopyrite,and in the extreme casesof chalcopyrite .formation it is difficult to escapefrom the interpretation that chalcopyritewas the earlier mineral. However, by tracingthe relationsfrom the incipientstages, where a faint bandof chalco- 40 ALAN M. BATEMAN AND D. H. McLAUGHLIN. pyrite spinesis forming alongthe margins.of thesestrips o.f re- sistantbornite to the final stage,where the main field of bornite has been completelyaltered and only the strips remain, the se- quenceof borniteto chalcopyritemay 'befirmly established. By these changesintimate mixtures of .bornite and chalcopyrite are produced'. From them the 'borniteis altered in many casesto corellite or less commonlyto chalcocite(Plate VI., C, D). Extremely complex.patterns result with the covellitein fine lines,specks, or veinletsin chalcopyrite,sointricately and finely spacedthat under low magnificationsthe surface'appearsalmost as if it were a singlemineral with a peculiarshade of yellow. Graphicstructures .between b.ornite and chalcociteare rare, and have been observedwell developedin only one specimen,but in several cases•bornite grains in chalcociteoccur in forms similar to the sh.apesof the .blebsof graphicareas. Bornite continuedto form under conditionswhich produced the earliest covellite, if the reasoningpresented on page 38 is to be accepted. This weak continuationor renewal of bornite formation is also shown By certain feeble threads of the mineral, which break later structures, and later minerals as luzonite or, rarely, the covellite. Nodular Bornite.--A puzzlingoccurrence of bornitein nodules about the size and shapeof pigeons'eggs was noted near the Erie Mine. The nodules are in the lower siliceous limestone, and apparentlyhave no connectionwith the deposits,either in the limestone or in the underlying greenstone. No structure could be observedin the bornite whi.clxwould suggesta replacementof a fossilor of any impurity in the rock. In onenodule studied microscopically thebornite is cutby veinlets, the centers of which are usually malachite and calcite with limonite on the sides. They are bor,deredby thin margins of corellite which in turn is succeededinwardly by narrow bands of chalcocite. Many grainsof the bornite containfine latticesof chalcopyritespines which in places are almost submicroscopic size, and it seemsprobalble that grains with unusual yellowish tints are filled with chalcopyriteof this character,too fine to be detectedby our highestmagnification. PLATE V ECONOMICGEOLOGY. VOL. XV.

a.Erie. Bornite (bn) and chalcocite (cc)partially altered tomalachite withintermediate notcorellite in bornite.inthe chalcocite.3(43. Veinlets develop triangular isometric patterns inchalcocite, but b.Erie. Chalcocite (cc)and bornite (bn) containing veinlets ofchalcocite. Bothbroken bymalachite veinlets (darker gray) which develop triangular isometric patterns inchalcocite, butfollow curved irregular courses in bornite. 3(7o. nitec. Jumbo(dark). Mine.X 70. Chalcopyrite (light)of the earliest generation, partially replaced bybor- d.Jumbo Mine. Chalcopyrite (Fight)in feathery lattice structures inbornite (dark). Note apparentresis.rance ofbornite inveinlike strips, with characteristic angularoffsets. X z4o. PLATEVl. ECONOMICGEOLOGY. VOL.XV.

a. Bonanza.Chalcopyrite (light) with bornite (dark), including chalcocite (slightly lighter), in scalloped structures. X 43. b. Bonanza.Bornite (dark) and chalcopyrite (light) in scallopedstructures. Broken by azurite veinlets. X 70. c.Jumbo Mine. Complex association of bornite (dark), and chalcopyrite (light) and corellite(dark, mottled) in concentricstructures. Corellite is largelya replacementof bornite. X 70. d. Intricatechalcopyrite and bornite (mottled, light gray), with luzonite (?) (white),and coatsetcorellite (mottled gray) in concentricstructures. Crystals now mixtures of bornire, chalcopyrite,andcorellite with rims of luzonite.Corellite replaces bornite and forms fringes of radialplates about margins of curvedmasses of earlierminerals. The luzonite is in part later than covellite. X 43. GEOLOGY OF ORE DEPOSITS OF KENNECOTT, ALASKA. 41

Thenodule presents a complete record of thealteration of bornite. .Theslight stains on the surroundinglimestone indicate thatlittle except water, oxygen and calcium carbonate 'have been addedto the noduleand that little has been.re•noved. Conse- quently,the chiefwork of the oxidizingprocess has been to re- work the mineral combinationswith little addition or subtraction of material. As the alteration advances into the nodule along cracksand seams,the copperof the bornireis transposedinto chalcocite.Part of the iron liberated(by this reactionis forced aheadof the main alteration,and concentratedin the protected interiorof the grains,producing the latticeof chalcopyrite;part findsits way intothe principal channels, where, with the acidity of the solutionsreduced by calciumcarbonate from the limestone, it is oxidizedand hydrolyzed to limonite,and therefore little iron escapesbeyond the limits ,of thenodule. The continuingattack of oxidationyields covellite from the chalcocite,and malachite as the last product. The noduleaffords a clearpicture of the detailsof the alteration of borniteby superficialagents. It is not a case.of enrichment in the casualsense, if the entire noduleis considered,for nocopper has been ad.ded, (but its processesclosely resemble those whichtake place in the enrichmentof largeore bodies. Enar!7ite.--Enargiteis not unc.ommonas coarsecrystalline masses,but its usualoccurrence is in small amountsthroughout the ore, especiallyin the steelych'alcocite. It is usuallyasso- ciatedwith chalcocite,which partially replacesit, yieldinga peculiar brecciatedstructure, .with angul'arfragments of the enargite set in a cementof blue chalcocite. The replacementof enargite by chalcocitedoes not seemt.o be far advanced. The enargitein mostcases is clearlya replacementof the wall-rock. Small crystals have 'been observed in the unaltered limestone with no traces of other ore minerals (Plate IV., D). Certain crystals show signsof mechanicalchanges, being bent and fractured, The fractures are cementedby calcite or by covellite.of the early type. The .relationsto the other sulphidesare not definitely shown, but this deformation which was not observedin the others suggeststhat the enargitewas early in the series. 4 2 ALAN M. BATEMAN AND D. H. McLAUGHLIN.

Under the reflectingmicroscope, the polishedsurfaces of certain sectionsof the enargite.crystals were found to be .distinctly pleochroic.When thelonger axis is parallelto theplane of polari- zation (the plane of the reflectingmirror), the mineral assumes a pinkishtint; at right anglesit becomeswhite or slight'lybluish when contrasted,with no.rmalenargite white. Chalcopyrite.--Besidesthe chalcopyritepreviously described, which forms lattice replacementsin the ¾ornite,the mineral also occursin a subordinateway in small grains usually of mutual boundariestoward the bornite (Plate III., D), but occasion.ally corrodedby the bornireor brokenby its veinletsin ways which convincinglyshow its earlier,origin (Plate V., C). Chalcopyrite of the two agesusually may 'bereadily separatedin typical cases. Luzonite.--A ,pinkishmineral, which has not •beenpositively identified,has been observedin veinlets or small grains in the chalcociteor the less commonminerals. It agreesin mineralographic propertieswith the luzonite in Dr. Murdoch's standard collection. It resemblesenargite in generalproperties, but pos- sessesa deeper p!nk color in reflectedlight and is never pleochroic. Qualitative tests on small fragmer•tsind.'icate the presenceof arsenicand the a,bsenceof antimonyand bismuth. It is rather a late product, for its veins break all .otherore minerals exceptchalcocite. It replacesbornite in the complexchal- copyrite-4bornitemixtures, resulting in a lattice of chalcopyritein a field of luzonite (Plate VI., D). Tennant•te.--Tennantite occursmicroscopically in certain complex mixturesof 'bornite,chalcopyrit.e and covellite. It is in part a replacementof chalcopyrite, for corroded cores of this mineral are frequently found in the tennantite grains. The grains usually possesscrystalline o.utlines. The tennantite is often partially replaced'by luzonitewhich developsin one or two instancesa lattice structure identical in appearancewith that which occurs between bornite and later minerals.' The ten- nantire is relatively resistant to replacementby covellite and chalcocite. Pyrite.mPyrite is widespreadas a sparsescattering 'of small GEOLOGY OF ORE DEPOSITS OF KENNECOTT, ALASKA. 43

cubes in the lower siliceous beds of the Chitistone limestone, where it apparentlyhas no connectionwith the mineralization whichproduced the copperdeposits. Under the flat fault in the JumboMine, a few smallg. rafi'rs were fou. nc• associated with partially.oxidized copper ores and much limonite, apparently residues f,rom the replacedpyritiferous limestone. Sphalerite.--A few grains of sphaleritehave been observed microscopicallyin severalspecimens, but they yield no information concerningthe placeof the mineralin the sequence,except that it is earlier than chalcocite. Galena.--Onlya few small specks of galena,doubtfully identified ,bymicrochemical tests, were observed. Concentricand BandedStructures (Plate VI.).--Information concerningthe mineralsequence has beengained chiefly from certainpeculiar kn, ots in themass of monotonouslyuniform chalcocite,in .whichthe rarer mineralsof the depositare concen- trated. In the majority of thesecases, the varioussulphides possessremarkable concentricor banded structures,which on accountof their geneticsigrfificance a.re worthy of separatedis- cussion.Chalcopyrite and bornitea,pparently form the groundmass. They weresucceeded b.y the depositionof tennantite,'both as a partialreplacement of the earliersulphides and as crystals apparentlyforming drusy coatir•gs on the wallsof smallrugs. Changesin the mineralization•processes again produced chalcopyrite, closelyfollowed 'by covellitein abund[ance,,with a little luzoniteand very smallamounts of bornite. The chalcopyrite of thesecond generation forms ri.ms about the tennantite crystals or cutsthem, in veins. It is mostabun,dant, however, as replacementsof 'bornitein complexaggregates of spinesas previously descri'bed.The covelliteis alwaysa'bundan, t. It is prominent in largeradial flakes (Plate VI., D) in the centersof the con- centricareas. From the relationsto the crystallinetennantite bandsit seemsprobable that it constitutesthe final filling of .open spaces.A littlebornite in feebleveinlets cutting the laterchal- copyriteand the tennantitewas apparen, tly formedat aboutthe sametime as the corellite.:The luzonite is in partcontempo- 44 •tL•tN M. B•tTEM•tN •IND D. H. McL•IUGHLIN.

raneo.usand in part later than the crystallineco.vellite. It forms veins in the tennantite and covellite, but also occurs as sharp crystals euhedral toward the covellitefields. In a few instances, fine rims of the luzonite.were observed following the serrateout- lines of the covelliteblades. ,Chalcocitereplaces all the .preced- ing minerals and structures. In some specimens,it is ,present only as veinsor bandsparallel to, or breakingacross, the complex aggregate; in others, more extreme replacementhas left only coarsesheaves of covellite (Plate IV., /-/j B) or rings or bands of resistant luzonite or of tennantite. Some traces of these minerals are usuallypresent in the pebblychalcocite, o.r in the mam- millary chalcocite,and it is very probable that these peculiar forms were inherited from these complex masses of earlier sulphides. OXm^T•ON OF ORF• r)EVOS•TS.

General.

Oxidation in cold northern countries is of scientific and economic importance becauseof the contrast it .offers to oxidation in more temperate climates. Usually ore deposits situated within the northern regionsof glaciationhave so insignificanta zone of oxidation that the subject is quickly dismissed. The Kennecott depositsare particularly fortunate because,situated as they are, in the midst of an intenselyglaciated region and containing a zone of deep and pronouncedoxidation, they afford unusual opportunity for this cornfast, as well as a study of the relation between oxidation and past climates. They indicate that the •presenceor absenceof ,oxidationand its attendant economic resultsin northernregions is not as much one of removal by glaciation or lack of time or temperature conditionssince the glacial period, as a study of the amount of glacial erosion and time and temperature conditionsprecedin 9 the glacial period. Such a study also involvesconsideration, o.f the situation, extent, and movementof the ground water in presentand past geologic times and in this respectthe Kennecott depositsalso offer inter- esting data. GEOLOGY OF ORE DEPOSITS OF KENNECOTT, ALASKA. 45

Minerals Due to O•4dation. ,The minerals, obviouslyformed by oxidizing processesare: ( • ) malachite, (2) limonite, (3) covellite, (4) antlerite, (5) azurite, (6) arsenatesof copper, (7) cha}canthite,(8) cuprite, and (9) possi•blybrochantite. They are listed in the order of their probableabundance. Malachite.--Malachite is abundant as a replacementof chalcocite,o.r as a cavity fillings in vugs or other open spaces. It penetratesthe chalcocitein veinlets, whose orientation is con- trolled'by the structureof the sulphide,,producing lattice patterns, or scalloped•bands in .material•with concentricstructure. Less commonlyit is also a replacementof limestone;coarse crystals of calcitewit.h malachite (fa,ding into a stain) developedalong their cleavageswere observed. •tzurite.--Azurite is unusuallyabundant in proportion,to malachite in the Ken,necottores as corn'paredwith most regions. Vugs in the chalcocitelined with azurite crystalsare common, and in nearly all placeswhere malachite is developed,there is someazurite formed with it. .No definiteage sequencebeeween the two carbonatescould be establishedhowever. In someplaces one is the older,in othersthe reverseis true. There is a slight tenden.cy for the ratio of azurite to malachite to increasein the heart of massive oreJbodies,and to decreasein, the zones of more intense oxidation. Limonite.--Li•moniteis smallin amount,but is widelydistributed throughoutthe deposits. Stainspenetrate the limestoneor are diffusedthrough gouge for nota•bledistances. Pure massive limoniteis unknowr•. It is nearlyalways mixed .with malachite or azurite,or with residuallimey material or gouge. No limonite gossan occurs. Covellite.--Covelliteis the first productof the oxidationof c'halcociteat Kennecott. Its relation to the chalcocite is con- clusivelyshown by the field associationsand confirmedby its distributionalong the malachiteveinlets observed under the microscope.Zones of chalcocitepartially altered to covelliteexist between unaltered chalcociteon one side and oxidized minerals 46 ALAN M. BATEMAN AND D. H. McLAUGHLIN.

ßon the other. Under the microscope,rosettes and sheavesof covellitecommonly accompany malachite veinlets in chalcocite .(Plate V., A). The spotsof covellite,sp.reacting out from the channel-ways,resemble the productsof decayadvancing into wood (Plate VI., D). It is usuallyclosely followed by the carbonates,and there is no doubt that they are successiveproducts of the samegeneral processes. Antleritc and Chalcan.thite.--Antlerite and chalcar•thite are hssociatedwith secondarycorellite in the JumboMine. A large, roughlypipe-shaped mass of oxidizedore oc.cursparallel to the dip of the beddingat the baseof one of the large Jumbochalco- cite body. The chalcocitepasses without sharpboundary into a purplish friable material, resemblingcovellite, but porous and with a low specificgravity. In the intermediatezone between them, chalcanthite occursin an intermittent band of one half to one inch in thickness. The following resultsof studieson this purplishmaterial have been kindly suppliedby Doctor E. T. Allen and Doctor H. E. Merwin, of the GeophysicalLaboratory, throughthe courtesyof L. C. Graton: The compositionof the ci•velliticmaterial is: Per Cent. CuS ...... 50.44 CuSo•.sH20 ...... 45.•o Quart'z ...... 4.56

100.20 The 45 per cent. coppersulphate was a surpriseas it was not detected under the microscope. Each grain of corellite is very minute and is probably surroundedby a zone of sulphate? The presenceof the quartz was not suspected. None was contained in the overlying.chalcocite, consequently it may have been introducedwith the oxidizing solutions. The crystalsare well formed and'each is doubly terminated. In the fid'd, covellitecan be detectedcutting the chalco.citein a networkof fineveinlets. ,Thematerial is diffi.'cultto polish,but ' even on imperfect surfaces;the relation to chalcocitewas clear. The covelliteis avithoutquestion a replacementof chalcocite. 24E. T.' Allen, private communicationto L. C. Graton, January, x9x6. ß

GEOLOGY OF ORE DEPOSITS OF KENNECOTT, ALASKA. 47

That the reactingsolutions. contained copper sulphate is indicated by the presenceof the chalcanthite. Chalcanthite,however, is uncommonunder normally humid conditions,and its formationhere mustbe attributedto the ,paucityof water at the beginningof the glacialperiod. The zone of covellitic material which is a foot or so in thick-

'1 I I Longitudinal section Transverse section Fro. II. Anterlite-covellite-chalcocitebody, Jumbo Mine.

ness,surrounds an underlyinglarger massof light greenishma- terial, which in the field we believed was malachite contaminated with sulphates. .The boundary between the corellite and this oxidizedproduct is fairly sharp,..but fine s'trir•gersof the sulphide extend in lacy massesseveral feet from the line of contact. Specimensof the purestmaterial whichwe .collected.were studied by DoctorE. T. Allen, who wroteconcerning the mineral:=5 The particularbasic sulphate of copperturns out to be 3CuO.SO32I-I20, and is in all probabilityidentical with antleritc,which is in turn identical with stelznerite. It is very difficultlysoluble. I am inclinedto connect its formationwith the presenceof limestone.... Of course,basic copper sulphateis formedby the actionof water on a dilutecopper sulphate solution which contains no free acid.

In laterwork in the GeophysicalLaboratory, it wasfound that thepr.oduct of the reactionof a •o per cent.cupric sulphate solu- ßtion on marble in thelump at roomtemperature yielded, a product muchlike the material from the JumboMine. From this evidenceit seemsprobable that the basicsulphate is theproduci of solutionscarrying copper sulphate reacting with •s Private communication to L. C. Graton. 4 8 ALAN M. BATEMAN AND D. H. McLAUGHLIN. theunderlying limestone. The fieldrelatio.ns are in accordwith this interpretation.The lacework of covelliteveinlets may be residuesof chalcociteveinlets in originalli.mestone. Very similar conditionshave been observed or• the edgesof unalteredchal- cocite ore-bodies. Cuprite.--Onelittle nest of cupriteoctahed. ra coatec•with malachite was found in the Bonanza Mine, but the mineral was not observedelsewhere except in small amountsunder the microscope. The commonoccurrence of cupriteasso,ciated with native copper,described in con•tectionwith the greenstonedeposits, offersan interestingcontrast, and indicatesthe controlof wall rock o.n the character of the oxidation. Native copperhas not beenobserved in the depositsin the limestone. •lrse'natesoJ: Copper. MArsenates of copperare fo.rmed in small quantitiesby theoxid,ation of theenargite-bearing ore. Mostof these are mixed with malachite, and have not been identified mineralogically.Mixtures of enarõiteanc} chalcocite have been observed'.to oxidize more readily than the pure enargite. .The chalcociteis apparently.the more resistant.

Distribution and Depth of Oxidation. There is no singlezone of oxidation,as distinctfrom a sul- phidezone, in the Kennecottmines. Oxidationis just as widely distributedas are the sulphides. Oxidation of sulphidesis no- wherecomplete, yet neverabsent, except in the interiorof massive chalcocite,bodies where oxidizing •'aters have had little opportunityto enter. .Thereis no relation•be.tween the amount arid distri'bution of oxidation to the surface, and the lowermost workingsshow just as muchoxidation as the upper,though lo- ßcally, in bothupper and lower levels, certain places may be much more completelyoxidized than the adjacentground. Even blind veins ,which do not reach within several hundred feet of the surface displaypronoun. ced oxidation. The amountof oxidation alsoseems equally dis.tributed between, the baseand. apex of any individual vein. The oxidation is thus distributed widely. The alteration, ho.wever,is feeble; it is rarely thorough,and in most GEOLOGYOF ORE DEPOSITSOF KENNECOTT,ALASKA. 49 casesonly where an isolatedchalcocite stringer happens to be attacked,does it approachcompletion. Nevertheless,the oxi- dizedcopper minerals in the aggregateform 25 per cent.of the ore and led to the buildingof a specialplant for their treatment by an ammon4aleaching process? The actualdepth of oxii:lationmay be consideredeither as the distancefrom the outcropd, own the inclinationof the strata,or thevertical distance from. the surface to anypoint. The former isprobably the route by .which, most of theoxidizing waters trav- elledand the latter is anomalousbecause the 400 Bonanzalevel hasgreater depth vertically below the surfacethan parts of the 600or the700 levels, since the' workings extend beyond the ridge whosecrest is overthe 400 level(Fig. 3). In theBonanza Mine, oxidation occurs at a maximumdepth of aboutx',ooo feet vertically below the present surface, on the600 level,and at 775 feeton the lowest or 700level. Measured• from th.eoutcrop down the incline, the maximum depth is aboutx,35o feet. In theJumbo Mine thelowest or 900 levelshows strongly oxidizedore at a depthof aboutx,5oo feet vertically below the surfaceand of a'boutx,6oo feet measured from the outcrop down the incline. The depthof oxidationbeneath the surfacethat existedat the timethe oxidation took place must have 'been considerably greater thanthat beneath the present surface for, as will beshown later, the oxidation,was pre-glaciaI, consequently much has been strippedfrom above the present surface by thevigorous erosion of the Glacial Period.

Relation of Oxidationto Structures. Fractures.--Areasof intenseoxidation are to be foundwhere thesul.phides are intersected by faults,fissures, and shear zones. Thesestructural features have acted as channels, which allowed the oxidizingwaters to gainready contact with the oreand to bringabout its oxidation. Where the fracturing ispost-mineral •6A treatmentdevised and pat'ented byE. T. Stannard,the general manager. 50 ALAN M. BATEMAN AND D. H. McLAUGHLIN. and the ore itself also shattered,the oxidation is more thorough and more spread out. This is especia.lly pronouncedin wide shear zonessuch as the Azure fault. Also the steeperthe dip of the fracturing,the more pronouncedis the oxidation. In general, the crossbreaks which allow unchangedsurface solutions to descendto great depthsthrough barren limestoneare more importantagencies in promotingoxidation...than the ,breakspar- allel with the ore. In these, descendingsolutions are quickly robbedof their powers of oxidation by the easily altered sulphidesin the upperparts of the veins. Cross fracturestan almostinvaria,bly 'be spotted by the oxid,ation which a.ccompanies them. The alteration throughout the massivesulphide ore is confinedto the surface of joints, small cracks, or to the .walls of rugs. Curiously enough, all the intersectingfractures contain a large amountof limoniticstain. In placesthe wholedrift along sucha fracture may be yellowishred ir• color,while the sulphide itself containsonly a fraction of a per cent. o.f iron. While it is fully appreciatectthat a small amount of iron may yield an extensivelimonitic stain, neverthelessthe amountpresent is striking. This is also found to be.the casein workings remote from ore and is evenpronounced over wide areasof surfaceexposures of the Chitistonelimestone apparently devoid of known ore. Beddin# PIanes.--Beddir•gplanes and flat faults .with 'their bandsof gougehave actedas dams,protecting ore beneaththem and directingthe flow of oxidizing ,•Tatersdown their upper surfaces, thereby causingpronounced oxidation of the lower por- tions of the ore bodies. This is especiallytrue of .themain flat fault in the Jumbo where the upper end of the inclinedbase of the big chalcociteore body is completelyoxidized to unusual oxidationproducts. These relationsmay øoeseen in Fig. • I.

Relation of Oxidationto Water Level and Climate. Present Climate.--During ten monthsof the year, and often longer, the ground at the mines is frozen and snow lies upon it. Frost may occuralmost any night in the summermonths and the GEOLOGY'OF ORE DEPOSITS OF KENNECOTT, ALASKA. 51 surfaceseepage water is alwaysicy col•d. In fa•ct,'water is difficult to obtainand the amountnecessary for operationat the BonanzaMine is largelyobtained from meltedsnow. These conditionsare most unfavora•blefor presentsurface oxidation. Broad facesof chalcocite,broken by mechanicaldisintegration, havenot developedmore than a thin tarnishfrom their exposure to the air and wet, and•pyrite cubes in the lowerlimestone show almostno alterationwhere exposed. The persistenceof fragmentsof sulpl•idesin the loosematerial of the slideore body, often as 'brightas that freshlybroken underground, expo.sed as favorablyas possi•ble to the influence of oxidation;attests forcibly to .theneglig?ble extent of superficialchemical •decompositio.n under presentclimatic con,ditions. Under9roundTemperature.--Undergroun, d, the conditionsare evenless favorable for oxidationthan on the surface. Tempera- ture readingstaken in the middle of summerat different places 'on different levelsshowed an averageof 30.4ø F. in the upper levels,with a range from 3ø0 .to 320 F. Only at the portals of the tunnels did the thermometer rise above 320 F., and in one confineddrift where a number of men were working. In the thetwo latterlowest figure levels beingof each uncommon.mine the range 3•.5 ø is F. from might 3 •ø be to taken 320 F., as; the averagefigure on .the70o Bonanzaand the 900 Jumbolevels.. The mines are dry an,d dusty and standingwater freezes. Ice is a commonmineral, occurringas splendidcrystals lining the walls of drifts near the surface,and as a fissurefilling. .Slicken- sided ice was also seen. There is no circulation of water at present,consequently no appreciableoxidation is possible. Water Level--Past a.nd Present.--From the fact that the Ken- necottspur owesits elevationto the rapid pre-glacialcutting of the McCarthy Creek Valley on the one side and the Kennecott GlacierValley on the other side (Fig. 3), an.d.that thesevalleys must have constitutedthe local hyd'rostaticbase level, it follows thatthe lowering of thegroundwater in theridge must have been' faster than erosion on the ridge itself. Un,der conditions of groundwaterprogressing'downward morerapidly than er0sion• 5 2 ALAN M. BATEMAN AND D. H. McLAUGHLIN. incompleteoxidation usually takes place and this feature may explainthe lack o.f completeoxidation in the Kennecottdeposits. Sucha conditionof deepcuttin,g on either sideo.f a narrow ridge would tend'to producea deep water level within the ridge. It would also causeconsidera'ble motion of the groundwaterdown- ward to the nearby deep valleys and the flow would be more rapid along channelsof easymovement, such as fractures,than in the inter-fracture areas. Such conditionsof deep and relatively rapidly.downward moving groundwater would alsogive rise to an irregular surface of the water table; lower where prominentfractures occur and higher in the inter-fractureareas. These featuresare exactlythe condlitionsrequired to explain the circumstances,already describbed,of intenseoxidation produced along fractures,the paucityof it in inter-fractureareas, and the great depth to which it has gone. Under such conditionsof topography, the water level can hardly be consideredto have been the customarysea of slow moving or stagnant.water, for the great head would propel it toward the adjacent deep valleys and the r•umerous fractures would facilitate and accelerate the downward movement toward a lower outlet. It might 'belikened to a .bath-tu2bfull of water, with water pouring in and also running out by drain pipes, so that there is a continuous,rapid, ,downward motion. •Under suchcircumstances, oxygenated waters couldtravel downward in conduitsfar below the surfaceof the groundwaterand produce deep oxidation. Beforethe oxygenatedWaters had completedtheir .work of oxidizing the sul.phidesthey were arrestedby the frigid climate of the Glacialperiod, an, d, frozen to form a "fossil groundwater" which has remainedto the present. The mine workingsdisclose that all fractures,vugs, cavities,and poresare partly or completely filled by ice. It was the last mineral to form and en- closes oxidation products. But the two lowest levels of the Jumbo Mine and the lowest one of the Bonanzaare completely free from ice or ,water and the second lowest level of the Bonanza is almostfree from.it. They are dry and dustywith openfrac- GEOLOGY OF ORE DEPOSITS OF KENNECOTT, ALASKA. 53 tures and vugs and show considerableoxidation. The mine workings have thus penetrateda zone of frozen water, which in pre-glacial times was liquid. •The bottom of the zone is wavy and exhibitsprominent peaks and sags. Where the rock is not greatly fractured the ice zone extendslower and forms sags,but in prominentfractures like the Azure fault the peaksrun ,wellup into the frozen zone. The interestingquestion arises as to whether this was originally the •bottomof the groundwater,such as has 'beenfound in many deep.mines. The abundantoxidation on the bottom levels, and the shapeof the cross-sectionsof the ridge rather precludes this, however. If the mine temperatureswere to be raisednow, these openings,would unquestionablybecome filled 'by water. The conditionmay be conceivedas being due to frost encroaching downward and freezing the waters, •)ut before the waters occu- pying openingsin the lower levelscould become frozen and fixed, they drained down•wardand left open cavitieswhich could not againbe filledby the overlyingfrozen waters.

•lt7e of Oxidation. The foregofrogparagraphs indicate that present oxidation on the surfaceis negligible,while nonecan take placeunderground. It antedates,therefore, the freezing of the waters underground. This freezing must have taken place shortly after the inaugura- tion of the low temperaturesof the glacial period, so that all of the oxidationmust have occurredin pre-glacialtime. This con- clusion is further supportedby the fragments of oxidized ore sealedwithin the ice of the Glacierore body. They must have fallen in during the building up of the glacier in the Glacial period and since they cannot have becomeoxidized since then, they musthave been oxid'ized •before that ,time. Their presence in the glacieralso indicates that the ore body outcroppedin the Glacial period and ,was not first exposedby the erosionof the Glacialperiod. All of the oxidationis, therefore,pre-glacial. Oxidationof the ore depositsprobably started during the developmentof the mature surfaceupon which the volcanicswere 54 ALAN M. BATEMAN AND D. H. McLAUGHLIN. extruded. (See p. I5. ) The water level was probablyshallow at that time and .migrateddownward very slowly,giving those conditionsunder which thoroughoxidation usually takesplace. The greater part of .the oxidation at presentexposed, however, most pro,b•blytook place during the time of the rapid erosion immediatelypreceding the Glacial period. Then,the topography and erosion,were suchas to give rise to the incompleteand deep oxidation so characteristic of the Kennecott deposits. The processof oxidation was vigorouslyoperating until it was arrestedby the Glacialperiod.

THEORETICAL CONSIDERATIONS. Oft#in. of Fissures. SuggestedHypoth.esis.--The origin o.f the fissuresof suchun- usual behavior has 'beena puzzle since•hey were first studied; various hypotheseswere tested and discardedin the successive seasons. The usual theories of origin of fissuresdo not apply, becausewedge-like fissures of little or no faulting, starting from the bottom and dying out upward, are decidedlydifferent from ordinary fissures. One hypothesisconceived in the springof •9I 7 and testedin the fieldthe followingseason appears to be borneout by the field relation's,namely: that the fissureswere formedby the stretching of the limestonebeds due to a sligh.t synclinal folding, or possibly a mono.clinalfold, superimposedupon the flankof the .majoranti- cline, with its axis parallel to the dip of the limestonestrata and normal to their strike. Careful mappingby structurecontours underground and on the surface at the Bonanza Mine demon- stratedthat sucha synclineor possiblya monoclineexists, and that the ore bodiesare located,in its trough. The lack of suffi- cientexposures at the Jumboprevented a si.milartest being made there. Sucha synclinalfold would causea stretchingof the lower bedsor the lower part of any group of beds,and a com- pressionof the upperbeds or the upperpart of any groupof beds. In brittle rockssuch as limestone,under light load, the tensionwould rbe relieveg •by fissuring at or nearthe place where GEOLOGY OF ORE DEPOSITS OF KENNECOTT, ALASKA. 55

the tensilestress was greatest. The fissuresthus formed would trend alongthe' line of greateststress, or alongthe troughof the synclineor monocline;they would start at the bottomof some strata and extend upward to a poin.t where the tensile stress diminishedsufficiently that it no longerneeded relief by rupture; the fissureswould be strongestand most numerousat the bottom and taperupwards to .thepoint wherethey died out; they would also probably start from the hot.toms of more. than one bed. Beds,or groupsof bedsacting as a unit, would, if of consider- •ble thickness,contain fissures of considerabledimensions; if of small thickness, fissures of small dimensions. Fissures formed in this manner would start (and thus be abruptly terminated downward)at anyprominent parting in the beds,such as strongly developedbedding planes or flat faults. More or lessparallel fissureswould Be formed,staggered with eachother., longitudi-

Dol.

Ls.

Gy.

Gro

Fro. x2. Diagrammatic vertical section to illustrate probable method of formation of wedge-shapedfissures (black) near 'axis of slight synclinal fold. Bedsdip away from reader. Bottomof fissuresfollow same'dip as beds;the topsapproximately so. Greatestdimensions of fisst/resis alongdip.

nally and vertically. The aboveconceptions are diagramma.tic- allyshown in Fig. •2, whichis alsoa diagrammaticcross-section ' of the ore zone. 56 ALAN M. BATEMAN AND D. H. McLAUGHLIN.

This hypothesiswould adequately account for the inverted, wedge-likeshape of the veins; their downward tenuination at the flat faults, and ,theshorter fissures extending across a few beds only. •The upward termination throughouttheir length, at an' approximatelyuniform distanceabove the base, is readily ex- plained'by the cessationo.f the rupturing due to lessenedtension a.t a definite height from the bottom. The distribution of the mineralizedfissures and the lack of appreciablefaulting along them is in keepingwith the theory, anti their confinementto the brittle limestoneis to be expected. In short, the suggestedhy- pothesisappears to explainreadily all the featuresof the fissures previouslydiscussed. A comparisonwith someof the structural features of the Mis- sissippiValley leadand zinc deposits is enlightening. Discussing the origin of the fractures in the Wisconsinarea, Chamberlain states,2• "where the beds .were (originally) bent downward, as in the case of the ore-bearingbasins, .the ho.ri'zontal force would causethem to bend more deeply,an(i thesewould likewise be most fractured along the bed of the depression. In each casethe fractureswould gapemainly on the outer curve of the strata, i.e., on the under side of the sag." Again Chamberlain states,2s "that they were formedas here indicatedby the neces- sary fracturing of the bent strata on the outer side, is, I think, placedessentially beyond question by a considerationof .thesig- nificantcircumstances, (I) that they lie in stratigraphicdepres- sions,(2) that both their upperand lower flats habituallysag, (3) that they lie almostwholly in the basalbeds of the stratum bent..." On the following page he concludes."that the strata shouldfurnish a suitablereceptacle for the ore at a given horizon and not aboveor below, is clearly indicativeof some general physical cause." The most notable difference in. the structural relations of the two is that in the Wisconsin area the synclinaldepressions are in the for,m of basins,whereas a.t Ken- necott it is a long trough. rrhis differencewould accountfor 27Chamberlain, T. C., "Geology of Wisconsin,"vol. IV., x879,p. 484. 28OP. cit., p. 486. GEOLOGY OF ORE DEPOSITS OF KENNECOTT, ALASKA. 57 the moreelongated fissure-like character of the Kennecottfrac- tures. Anotherprocess m.ay have operated, along •with the sligh.tfold- ing, to help producethe fissures,namely, in the slippingof the individualbeds of limestoneover eachother and particularlyof the lower .part of the Chitistoneformation over the greenstone, along the shale contact,bumps or other irregularitieson the planesof sliding would causeone part of the overlyingbeds to be differentiallyelevated or depressedwith respectto the other, therebyproducing stresses which would result in rupture. Dif- ferential movementalor•e, greater nearer t.hegreenstone and less removedabove it, similar to that which operatedin the San Fran- ciscoearthquake faul•t, could also produce such rupturing without the necessityof irregularities. The connectionbetween mineralized fissures,many of which do not outcrop,and synclinesor other irregularities,could, if positivelyestablished, ,be used as a criterion for indicating other localitiesfavorable for explorationfor more ore, by accurately locating from surface exposuresall d,ownwarps.

Origin o/Mineralixation. ' Underthis heading will be consideredthe immediatesource of the metalsoriginally deposited in the limestone,as well as their ultimatesource. The understandingof the depositswill not be completeunless the ager•ciesof transportationby which the metalswere broughtto the limestone,and their routesof travel, are also discussed,and finally, the meansby which the metals were transferredfrom their carriersto their restingplace as the minerals of the ore deposits. Sourceo]: Metals in the Limestone.--In a mineralizedregion where igneousintrusions occur, the natural tendencyis to attrib- ute •theore to the igneousrock, not from halbit,.but becauseit usually provesto be the case. Indeed, almost all of the large copperoutput of the United,States comes from depositswhich are clearly related to igneousintrusions. In the Kennecott district, however,there is no apparentrelation betweenthe ores and the 58 zlL.4N M. B.4TEM.4N .4ND D. H. McLAUGHLIN. porphyry. The porphyry shows no traces of mineralization, either in the heart of the Porphyry Mountain stock,or near its edges,or in its apophyses. The three-foot dike of porphyry at the Erie prospectwhich cutsthe ore, is absolutelyunmineralized, and the large dike in the limes,tonea•oove the Jumbo, situated in the rock most susceptibleto mineralization,shows no traces of mineralization. There is no evidenceindicating that the mineralization was derived from the porphyry. On the contrary, th'e absolutb lack of silicification of the wall rocks, the absence of silica and iron in the ores, and the mildnessand monotonyof the mineralization, make it difficult for us to believe that emanations, even from a remotemagma, produced ore depositsof the Kenne- cott type. The Erie dike which cuts the vein, indicatesminerali- zation antedatingthe intrusive,though it doesnot definitelyprove that the mineralizationwas earlier than the main porphyry stock, for thestock and its apophysemay have been s.eparated by quit.e an interval of time. On the other hand, the presencein the ore. of a mineral like enargitesuggests ,that the porphyrymagma might have been the source of the metals. This, consideredwith the occurrenceo.f some copper at Bisbee, Arizona, in unaltered limestone remote from the porphyry,but believedto be geneticallyconnected with it, indi.cates,that the porphyry at Kennecott cannot be entirely eliminated as the source of .the metals. The barren limestonemay be dismissedas a source,and the remotenessof the Tertiary volcanics,and the lack of any known mineralizationassociated with them, makesit extremelyimprob- able that the copperwas derived from their activity. The greenstoneis copper-bearingwherever exposedand the associationof copperwith it is'just as constantover the hundreds of squaremiles of its exposureas is the well-known association of iron with the Clin.tonformation. Hundreds of sampleshave demonstratedan appreciablecopper con,tact. The widespread disseminatedcopper in the greenst0necould hardly have t)een formedsimultaneously with the copperconcentrations in the limestone,for its wide d.istriq•u.tion precludes this. But the cop)er GEOLOGY OF ORE DEPOSITS OF KENNECOTT, ALASKA. 59 concentratedin the greenstonein the form of veinsand stringers may havebeen part of the sameconcentration that producedthe depositsin the limestone,for veinletsbreak acrossthe greenstone limestonecontact at the Erie. The widespreadcopper, however, is not concentratedin veins,but existsin an ind.istinguish•ble unknownform. Thus, the greenstoneseems the mostplausible sourceof sup•>ly.Moffit and Capps 2øalso believed .that the deposits'were derived from the widely disseminatedcopper in the greenstone. .The derivation of the copper from the greenstoned. oes not necessarilyimply ,that it came from the immediatelyadjacent greenstone;in all prob,a'bilityit was chiefly derived from more distantparts, both verticallyand laterally. Sourceof Copperin Greenstone.--A distinctionmust be made betweenthe copperwidely distributedin the greenstoneand tha.t concentratedinto veins or stringers; the former is a phenomena as widespreadas the greenstoneitself, and is essentiallya feature of the greens.toneformation, ,the latter is localizedand is a result of some localizing agencieswhich operatedwithin the greenstone. It is ,believedthat the veins and stringers in the greenstonemay have been formed at the same.ti. me and ,by the same processes.that operated to produce the concentrationof copperin the li.mestone. Certainly, at the Erie, veinletsof copper minerals pass from the greenstoneinto limestone. The occurrenceof the small quantitiesof xvidelydistributed copperin the greenstoneis similar to that describedin numerous locfi.litieswhere copperoccurs in basiclavas? ,The form• as- sumedand the metallic and gangueminerals are all characteristic of this world'-wide,type of mineralization. 29U.S. Geol. Surv. Bull. 448, •9•, P. 8•. a0Lake Superior region; White River District, Alaska; Coronation Gulf, CanadianArctic; Eastern Oregon; Triassic Traps of New Jersey and Con- necticut; near Lurray Val; South Mr., Pa.; the Bay of Fundy, Nova Scotia; the Faeroer,north of Scotland;Sterling in Scotland;Oberstein a.d. Nabe, Germany; Sao Paulo, Brazil; the Kristiania Region, Norway; New Guinea; the Transbaikalian provinceson the Dochida River; Monte Catini, near Li- vorno, Italy. (For general discussion and references to literature of the abovedistricts see Lindgren's"Mineral Deposits,"p. 392.) 60 ALAN M. BATEMAN AND D. H. McLAUGHLIN.

The distri;butionof the copperthrough all parts of the Copper River greenstones,supported by the commonassociation of copper with basic extrusiveselsewhere, points to the lavas them- selvesas the original sourceof ,themetal. Lindgren writes Basic igneous rocks such as gabbro, diabase, basalt, some andesites and basaltic flows designatedmelaphrye or amygdaloidsprobably contain copper, in somecases as much as o.x or 0.2 per cent., but commonly about 0.02 of the metal. According to Volhey Lewis and F. F. Grout, the copper is presentas a sili.cate,possibly in part as a sulphidesuch as t>ornite or chalcocite. Instancesfrom the literature might be multiplied to showthe prevailingopinion that the basicrocks contain copper. •Considerabledifference of opinionexists concerning the mech- anism by ,which the concentrationof copper in the lavas was affected. As the ,copperis intimatelybound up with zeolitesand the latter are relatively diagnosticminerals, their elu.cidationmay explain the oc.currencesof copper. Commonlythey are considered as being formedby descendingwaters near the surface,and are so regarded:by Whitney, Pumpelly and Wadsworth for the Lake Superiorregion. Van Hise32 regards them as productsof descendingsurface waters, or similar percolatingwaters in the zone o.f cementation,al,though he later attributesthem to ascend- ing waters. Weect3• believesthose in Virginia and Pennsylvania to have been formed by surface infiltrations. Lane•4 believes the Lake Superiorlavas to have been submarine,and attributes the concentrationof copperto the.action of heatedsea water on the beds. Fenner•5 regards them as the result of lava flows encounteringwater in shallow lakes or playas. Lewisaø assumes hot solutionsof cuproussulphate releasect from the magma to accountfor ,thecopper in New Jerseytraps, while Knopfa7 at- as Op. cit., p. 393.. a2Van Hise, C. R., U.S. Geol. Surv. Mon. 47, •9•4, p. 333. aaWeed, W. H., "Types of Copper Deposits in the Southern United Stat'es," T..4. I. M. E., vol. 30, •9oo, p. 449. 24 Lane, A. C., Proceedings Lake Superior Min. Inst., vol. •2, x9o6. 85Fenner, C. N., .4nnals N. Y..4cad. Sci., vol. 20, Pt. II., •9•o, p. 97. 86Lewis, J. V., Geol. Surv. N.J., •906, p. I3•. 87Knopf, Adolph, EcoN. GEox..,vol. 5, •9xo, p. 247. GEOLOGY OF ORE DEPOSITS OF KENNECOTT, ALASKA. 61 tri:butesthe disseminated.copper in the lavas at White River, Alaska,to hot solutionscirculatir•g through the lavassoon after their extrusion. As. proof of the latter point, he describesa copper-.bearinga.mygdaloid covered by a coarsepyroclastic bed, the breccia of which contains fragments of the cupriferous amygdaloid,proving that the mineralizationtook place .during the interval between successiveflows. Lindgrenas points out that zeolitesare unstablein the zone of weatheringand must have been formed at somedepth, and states: Following Lane (p. 4o6), I believe that the water of seas or lakes, minglingwith the exhalationsfrom the magma,decomposed the copper silicatein the pyroxenes,and that the resultingchlorides of iron and copperwere decomposedby silicatesor carbonatesof calcium, with the formation of native copper,ferric oxide and calcium chloride. The developmentof albite, epidote,chlorite, iddingsiteand datolite in the Nikolai greenstoneindicates they are not the product of .coldmeteoric waters, '.butrather of heated waters, probably volcanic after effects, and such waters must have been as widelydistributed' as the greenstone.They cannot,therefore, havebeen a resultof a few localporphyry intrusions. The su,b- marineextrusions may alsohave played an importantpart in the formationof the copperin the mannerindicated by Lane and Lindgren,for, aspreviously pointed out, the evidence is strongly suggestivethat theseflows were extendedbeneath water. ,dt7en.tsof Transportation.--Aqueoussolutions are the only agencythat couldhave searched the. copper out of thegreenstone and transportedit to its presentabode in the limestone. That the solutionswere not cold,is suggestedby theenargite in theore, a mineralwhich in Bu,tteoccurs most abundantly in thezone where thehighest temperatures are supposedto haveprevailed. Lind- gren,su however, says enargite "rather favors the deposits f.ormed relativelynear the surface,"and Clarke 4ø implies that it is a low temperature mineral. But no instance is known to the writers s80p. cit., pp. 397 and 399. aoOp. cit., p. 597. 4oClarke, F. W., "Data of Geochemistry,"U.S. Geol.Surv. Bull. 616. •9•6, 66•- ß 62 ALAN M. BATEMAN AND D. H. McLAUGHLIN.

where it is considered to have been formed from cold solutions. The strongestproof of the temperatureof the solutionsis to be deduced from the chalcocite. Investigations of sulphidesof copperat the GeophysicalLaboratory 4x haveshown that chalcocite is dimorphous;an isometricform is producedabove 91 o C. which passesinto an orthorhombicform below that temperature. At temperaturesbelow 9 Iø C. only the orthorhombiechalcocite is produced, and it is, therefore, the stable form under conditions pertaining at the earth's surface. They also found that chal- cockemay containcovellite in solid solutionand when the amount of dissolvedcovellite exceeds8 per cent., there is no inversion. Thus chalcocitecontaining over 8 per cent. covellite, formed move 9 Iø C., remainsin the isometricform at normal temperatures. Among the coppersulphides investigated by them was someBonanza chalcocite, in regard to which they state: It is further significant that analyzed samples of the Bonanza ore show about 9 per cent. of dissolvedcupric sulphide (covellite), a quantity sufficientto prevent isometric crystalswhich may have formed above , 9 t per cent. from going over into the orthorhombic form as cooling progressed. They also state: If solid solutions containing even t5 per cent. of covellite could be crystallizedat temperaturesjust below 9t per cent. they would probably etch like ordinary (orthorhombic) chalcocite. Etchedsurfaces of the Bonanzachalcocite show isometric patterns (Plates Ill., V.) which resemblesthose yielded by etching the artificial isometric chalcociteproduced at the Geophysical Laboratory, thus suggestingthat the Bonanza chalcocitewas formed above a temperatureof 9 tø C. The isometricpatterns were first describedby Gratonand Murdoch,42 who believedthem to be inherited from bornire (isometric). Tolman•a also described some .Bonanza chalcocite and• .concluded tha.t its isometric etch pattern was inherited from replacementof bornire. These 4xE. Posnjak, E. T. Allen, and H. E. Merwin, "The Sulphides of Copper," Ecoa. GEOL.,vol. •O, p. 49•, •9•5. 42 Trans. Amer. Inst. Min. En•7rs., vol. 45, p. z9z4. • Trans. Amer. Inst. Min. Engrs., vol. 54, P. 402, z9z6. •EOLOGYOF ORE DEPOSITS OFKENNECOTT, ALASKA. 63 conclusionsthus throw dou'btsupon the original isometricform of the ch,alcociteand consequently upon its temperatureof forma- ß tion havingbeen over.9• ø C. A specimencollected by the first- named.writer in the summerof I918 showedan isometric•)at- tern in the hand specimen,and one piece measuredby the goniometerproved' to be octahedral. .Thesemeasurements were later continnedBy Dr. Merwin, of the GeophysicalLaboratory. Whetherthe ocmhedralangles are to be interpretedas cleavage or parting couldnot be conclusivelydetermined. In examining material from a I9I 5 collection,Dr. Merwin found a numberof twinnedcrystals in :thechalcocite, which he believesfrom .meas- urementsof the anglesto be octahedraltwins of chalco.cite. It is thefirst recognition of isometricchalcocite crystals in nature.44 The evidenceof isometricchalcocite presents a strongcase for the thesisthat the solutionswhich formed the Kennecott ores had a temperatureof at least9I ø C. (See alsopages 73-77.) The characterof thealte[a,tion of the greenstonealso indicates that it was formedby thermalsolutions. This alterationmay have,been brough',t about by theagencies. that produced the origi- nalwidespread dissemination of copper,or by the solutionswhich gatheredup the copperfrom the greenst;o.neand concentratedit in largebodies in thelimestone anc• as small veins in the greenstone. The latter seemsmore probable, and if true, the solutions that carriedthe copperinto the limestonewere hot. ,Theabove evidence, though not conclusive,is stronglyindic- ative'thatthe solutionswere heated, and. since magmatic waters givenoff from the porphyryintrusives do not seemprobable (seepage 58), someother source must be soughtfor thewarm solutions. The thesisis presentedthat the mineralizingsolutions were probablymeteoric w•ters with their activity increased By addition of heatand solutes. ,The-waters in travellingthrough .the rocks would have acquiredcar,bonates, sulphates, chlorides, carbon dioxide,and probably some hyr•rogen sulphide and would, therefore,be competent solvents for thecopper in thegreenstone. In- 44E. T. Allen, private communication. 64 ALAN M. BATEMAN AND D. H. McLAUGHLIN. creasedtemperature .would accelerate their dissolvingpower. is possiblethat the heat retainedin the greenstonelavas may have raised their temperature,although this is not probable,*because suchheat would, likely havebeen lost during the longinterval of the depositionof the limestone. However, the temperature gradientin this regionmust have been high at a'boutthe time the ores were formed. Precedingthe ore-depositionwas great volcanic activity, which gave rise to the Nikolai greenstone;at z•boutthe sametime as the mineralization,o.r slightly later, the porphyry intrusionstook place, anc•in the early Tertiary were extensive volcanic eruptions, .which are still evident in Mt. Wrangel, a short distanceaway. A magmaticreservoir must have extendedbeneath this region which, from time to time, emittedmagma, part of which was pouredout as a moltenfluid on the surface,and part was arrestedbefore reaching the surface, giving its heat to the surround'ingrocks. Heat emanationsmust have precededthe upward movement of the porphyrymagma and the intrusions,which were not deep- sea,ted ones,may have,been suffikiently close to the time of ore formation to supply sufficientheat to raise the temperatureof meteoricwaters, even if. they did not comein direct contactwith the magma. Suchsolutions would not containthe large amounts of silica, iron, and smaller quantitiesof other metals, so typical of waterswhich emanatedirectly from magmas. The heatedwaters searched.the copperout of the greenstone and in their circulation'took advantageof establishedchannel ways wherecopper would Ibe depositect if conditionswere favor- •ble. Favorable conditions did exist in the small fractures in the greenstoneand in the larger onesin the limestone,giving rise to veinletsin greenstoneand large bodiesin limestone. Under suchan hypo.thesisit might well be arguedthat deposits shouldoccur in many placesthroughout the greenstoneand lime- stoneareas,. Perhapsthey do, but noneof great commercialim- portanceother than those mentione4have yet been .discovered. Severaldeposits have been found in the Chitistonelimestone, and the countryis dottedwith prospectsin the greenstone,some of GEOLOGYOF OREDEPOSITS OF KENNECOTT,ALASKA. 65

whichhave excited high hopes, and a few mayyet be smallpro- ducers. The greenstone,under most favoralbleconditions o•f solutionsand supplyof copper,is uncongenialfor ore, and the formationof deposksthat do occurin it tookplace in spiteof this difficulty. It is a noteworthyfact that the bestof .thede- positsin greenstoneare usuallylocated near intrusivessimilar to thoseat Kennecott,which were capable of heatingthe waters. The wide.distribution of smalldeposits in the greenstonemay well'be due to thefact that the solutions were present, tile copper was available,but the temperatureconditions were not proper for concentrationinto largerbodies. Wherelarger bodies do occurin thegreenstone, temperature conditions were suitable, but the rock was too uncongenialto permit oœthe formation of extensivebodies, such as mighthave occurred had the country rockbeen limes.tone instead of greenstone. Methodof Deposition.--Mostof the deposition of thecopper mineralsin thelimestone took place 'by a replacementof therock in andadjacent to .thechannelways; some was open filling. The conditionswhich brought about the deposition, and, the reactions involved,are unknown. It is plannedthat thesefeatures be consideredmore fully in a laterpaper by thefirst named writer. Tolmanand Clark 45 point out that most ore solutions are prob- ablycombinations of colloi.dal suspensions and electrolytic solutions,and that colloidal dispersions of copper sulphides may be inducedby hydrogensulphide or otherdispersing substances (especiallyin alkaline golutio.ns), and, flocculated •bycalcium carbonate. Precipitationby calciumcarbonate offers an attractive explanationfor the localizationof the oresin the Chitistonelime- stone. ,Thehypothesis finds support in theconcentric and mam- millarystructures, o.ccasionally observed in theore, which resemble colloidalproducts, and some of it mayhave been formed in thismanner. Clarkeand Menaul •ø point out that colloidalsolu- tionsof sulphidesmay be so highly dispersed that they can pene- tratethe most minute fissures and pores, and their experiments 45Ecoa. G•-oL.,vol. IX., pp. 559-572,1914. 46Clarke, J. D., andMenaul, P. L., "TheR61e of theColloidal Migration in Ore Deposits,"Eco•r. G•.o•.., vol. 1I, p. 37, I916. 66 ALAN M. BATEMAN AND D. H. McLAUGHLIN. show that copper in a colloidal solution can replace calcite. Whether the great ore bodiesat Kennecottcoulct be formed by this meansis unknown,but if they could,many of the unusual features o.f those depositswould be understood•. It would explain (t) the concentrationof the copperin .thelimestone; (2) the lack of silicificationor otheralteration of the wall rock; (3) the remarkablepurity of the ore; (4) the concentricstructures whichresemble diffusion structures; (5) the com.binationof open spaceand replacement;(6) the inability, displayedby the COl>- per, to migrate far from the channelways;and (7) the lack of virility of the solutionswhich preventedthem from penetrating through small gouge slips. 'The presenceof small amountsof enargiteand other.minerals now or originally presentwould not be a stumbling•block to the acceptanceof the colloidalhypothesis, for the experimentsof Clark and Menual demonstratethat they also replacecalcite. Nor doesits acceptanceexclude precipita- tio.nby chemicalreaction, from electrolyticsolutions.

Primary or'Secondary Origin of tl,e Cha.Icocite. General.--At the outset it must be admitted that the evidence concerningthe hypogene.(primary) or supergene(secondary) originof the chalcociteis conflicting.Strong arguments can •be advancedfor andagainst both origins, and our knowledge of the generalquestion is not sufficientlyadvanced to overthrowcom- pletelyeither hypothesis;certain features can be interpreted with equalforce to applyto eit'herorigin. Field evidence,without corroborativemicroscopic study of polishedspecimens, has led to the assignmentof an hypogeneorigin by Moffit and Capps,4' andlaboratory microscropic study of polishedsurfaces withoutcorro%orative field evidence led Tolman48 to implya supergeneorigin, though no direct statementto' this effect was made. Gratonand .Murdoch, •9 who were the firstto study nanzachal•coc.ite by means of •polishedsui-faces, thought that most 47 Op. cit., p. 82. 48Tolman, C. F., Trans.Am. Inst. Engrs.,vol. 54,pp. 402-435,I917. 40 Personal discussions. GEOLOGY OF ORE DEPOSITS OF KENNECOTT, ALASKA. 67 of i.twas hypogene, ,•utgreater refinement of methods later rev.ealed certain structures which led them to believe it to be mostlysupergene. It is thus obviousthat the correctinterpre- tation canbe reachedonly by correlatingand properlyweighing the different lines of investigation,such as field evidence,petro- graphicstudy, microscopic examination of polishedsurfaces, and experimentalevidence. In noneof theseis our knowledgesuffi- cientlycomplete to allow of .conclusivedeductions. In the final summarythe weightgiven the variousarguments will throw the scalesone way or theother, but dueto the.differentvalues given to the evidenceby .severalinvestigators, a close agreementis hardly to be expected. In the following discussionof the different lines of evidencethe writers beg toleranceof repetition in order to render clear the pointsat issue. œ;rperimentalEvidence.--TheGeophysical Laboratory investigations show that becauseKennecott chalcocite contains over 8 per cent.dissolved corellite =ø it shoulddisplay isometric etch patterns if formed above 9 •ø C. Much of it does d/splay octahedral partings (Plates III., IV., ,/t, B), although"etched sur- facesof someof the analyzedsamples contain grains in irregular veins which etch like chalcocite (orthorhombic) and are thus shownto be the low temperatureform. They may have formed above 9 •ø and.contain too little cupric sulphideto prevent their inversion, or they may have formed below the temperatureof inversion."• Either explanation.is possible.' The twinned octahedral crystals (see page 63) are the strongestindication of isometric chalcocite. If it *be established that the Kennecott chalcociteis isometric,it follows that it was formed above9 xø C. and therefore hypogenein origin. Mineralo!Iraphic Evidence.--The experimental .work demon- strates the conditionsunder which chalcocitemay form, but microscopicexamination of polishedsurfaces must be used to determineif the ore exhibitsthe featuresshown by the experimental work to be diagnostic. 5oSee pp. 62. 5xPosnjak, Allen, and Merwin, op. cit., p. 527. 68 ALAN M. BATEMAN AND D. H. McLAUGHLIN.

The microscopereveals 1TlUC•I chalcoci•e that strongly points to a replacemento.f bornite (Plate V., B), and some so absolutely free from residualsof former mineralsas to suggestthat it 'was originally formed in the limestoneas chalcocite. The latter can, of course,be interp.retec•as the first stagecarried to completion,and there is much to suggestthat this may be the case. Etching demonstratesthat most of the chalcocitehas isometricpatterns. (Plates III., A, V., A, B). The isometric patternsmay be interpretedas (•) partings inheritedby replacement of an isometricmineral suchas bornite, and (2) cleavage indigenousto isom'•tricchalcocite. That secondarychalcocite doesretain the cleavageof the mineral it replaceswas first showp by Graton and Murdoch,52 who describedcleavage lines passing uninterruptedlyfrom bornite to the replacingchalcocite. It is now a well-known occurrence. Upon the correct interpretationof the isometricetch patterns lies the decisiveclue to the primary or secondaryorigin of the Kennecottchalcocite for, if the first, the isometriccleavage is not diagnosticand merely proves that the .chalcocitereplaced bornite, and if l;hesecond, it was formed above9 •ø and is un- questionablyprimary or hypogene. Tolman5a presents.strong proof to establishhis positivestate- ments that all the chalcocitereplaces bornite and inherits its cleavages. He summarizesthe mineral relationsof the Bonanza depositas follows: First group: Bornite, klaprothite (klaprotholite) and galena; temperatureof formationprobably high. Seco.ndgroup: Blue and white chalcociteof the first generation, secondaryafter bornite; temperatureand sourceof solutions unknown. ' Third group: Minerals formedprogressively under conditionsof increasing oxidation--second generation of white chalcocite--> covellite and cSalcopyrite--> tenorite (and cuprite)--> malachite--•azurite; temperature"at about oø C., as much 5.0p. cit., p. 58. 5a Op. cit., p. 408. GEOLOGY OF ORE DEPOSITS OF KENNECOTT, ALASKA. 69

of the ore is frozenthroughout the year. It is the resultof the presentvadose circulation, and, the copperis migratory largely as malachite." Tolman's mineral sequenceis confirmedin a generalway by our own 'work. No klaprothite or tenorite was encountered, however,and the extremerarity of galenahardly entitles it to be mentionedwith bornite. Limonite is not mentionedthough fairly abundant, and .much corellite was observedto be earlier than the chalcociteand amongthe first mineralsto form. It is dou,btfulif azuritecan be considereda rep.lacementof malachite throughoutall of the ore. ,Coppermigrating as malachiteby meansof a frozenvadose circulation is rather surprising. Our microscopicexamination of hundred•sof specimenssuggests that muchof the chalcocitemay havebeen derived' from. bornite, but not necessarilyall of it. On the otherhand•, some specimens suggest that the isometric structureis original. A specimenfrom the Mother Lode shaft collectedin t9•8 showswell-developed octahedral twinning, and also twinning on larger faces which are themselvestwinned. After examiningthis specimen,Dr. Merwin states:54 The larger structureis octahedral,this gradesoff into the minute structureof the fresh chalcocite,even to the structurewhich etcheson a polishedsurface .... Thatthe structure here represented is pseudo- morphic,I cannotbelieve . . . chalcociteformed at temperaturesabove. about93 degreesby replacingbornire or by anyother process provided! it containedabout Io percent. or moreof covellitein solution,would be: expectedto havejust the structure which this Kennecott ore presents'. ß. . If actualunmixing (of thesolid solution) takes place on lowering the temperaturethen'new crystals of bothcovellite and chalcocite form andare thus contemporaneous. Or if purechalcocite is being altered to covellitethe surfaceof the chalcocitewill firstbecome a solidsolution of chalcociteand covellite before actual crystals of covelliteappear. Fromthis saturated chalcocite, crystals of corellitewill growduring furtheralteration and the two sets of crystalsthus resulting (that is thecovellite and the saturated solid solution) will be essentially contemporaneous.Of course,there may be covelliteformed later from sur- roundingaqueous solutions. If thealteration and recrystallization take •4Merwin, H. E., letterfrom Geophysical Laboratory, Washington, dated April I6, •9•9. 70 ALAN M. BATEMAN AND D. H. McLAUGHLIN. place below the inversion temperature of chalcocitewe would expect orthorhombic chalcocite to be found in contact with the corellite; above this temperatureisometric chalcocite would be expected. In the polished specimenwhich I have just examinedthe minute octahedraletch pattern can be seenin the chalcocitepractically touching the corellite in places, and no orthorhombicpattern was seen. Dr. Merwin's examinationof the specimenstrongly suggests an original octahedralstructure. It is also significantthat the octahedral partingsof the artificial chalcociteformed a,box•e9 Iø C. yield etch .patternson the polishedsurface similar to the etch structures of the Kennecott chalcocite. The specimenjust describedyields the most diagnosticevi- denceof all themicroscopic examinations of the Kennecott specimens. Most of the microscopicevidence previously presented shows that the isometric etch patterns of the chalcocitecannot themselvesbe consideredconclusively diagnostic of high temperature formation, therefore, of either the supergeneor hy.po- geneorigin of the c'halcocite,but this specimen•trongly suggests an original isometric structure of the chalcocite,which in turn meansits formationabove 9 Iø C. and by hypogenesolutions. Field Evidence.--It is obvious that secondaryenrichment is impossi•bleunder ,presentclimatic conditionsat Kennecott. It is also unlikely that any was associated,with the oxidation now prominentthroughout the deposit. The o.xidationis limited for the most part to fractures upon which the chalcociteshows no dependence. It also causesalteration of the previouslyformed chalcocite. 'The period during which the oxidation is consideredto have been fo.rmed and particularly the one of mature development preceedingit, offered conditionsthat would have been favorable for any secondaryenrichment to take place. Also, if the ore bodiesextended up to the then existing surface on their present flat dip, a large amountof ore would be subjectto oxidation and solution, for reprecipitationas secondarysulphides, because a lowering of IOO feet vertically would release2oo feet of ore. The field conditionswere thus favora{blefor secondaryenrichment on a large scaleto take place. The presenceof chalcocite GEOLOGY OF ORE DEPOSITS OF KENNECOTT, ALASKA. 71 of unquestionedsupergene origin in the bornitenodule (page 40) showsthat secondaryenrichment can ta,ke, place in limestone, althoughthe alterationin this caseis on a smallscale. Opposedto the supergeneorigin of the chalcociteare the following arguments,.based on field.evidence: I. There is no actualfield evidence whatever (apart from mi- croscopicor chemicalevidence) to pointto. downward secondary enric'hment,except that whichhas just b•en mentioned. 2. There is no distinctzonal arrangement,of oxidation,secondarysulphides, and primary ore, usually found in any deposit wheresecondary enrichment has takenplace. There is only one zone of sulphides,with partial oxidation scatteredall throughit. 3- Thereis an absenceof coatingsof chalcociteor sootychal- cociteon other minerals,such as occursin practicallyevery depositthat h,as been second.arily enriched. If the deposithad undergoneenric'hment to the completenessrequired to account for the overwhelmingproportion o.f chalcocite,it is difficultto understandhow areas of enargiteshould remain absolutely fresh, withouteven a .thincoating of chalcociteon the outside. One exception occurs,above the too level and near the surface of the Jumbo,where, in a shearzone a littlesurface water drips for part of theyear, sooty chalcocite coats some enargite. It resembles theusual secondary enrichment. This exception, where clearly definectsecondary enrichment does occur, suggests a different origin for the rest of the chalcocite. 4. If the chalcocitebe attributedto descendingwaters, it im- pliesthe movement of largequantities of coppersulphate waters .throughthe ore and ad,jacent limestone. Limestonehas been foundin otherplaces to inhibitthe'formation of secondary chalcocite.It hasbeen establisbed. from chemicaland geological datathat when copper sulphate solutions meet calcium carbonate, theylose their copper content by formingcarbonates and basic sulphatesof copper.'Thus copper sulphate solutions, which journeyed'considerable distance through limestone, would not haveany availa•ble copper to formsecondary copper sulphides. 7 2 ALAN M. BATEMAN AND D. H. McLAUGHLIN.

The absenceof malachite,azurJ•te, or basicsulphates in the limestone walls, unconnectedwith sulphides,and especiallyalong the little veinlets of chalcociteisolated in limestone, is a strong argument againstthe formation of .thechalcocite from descend- ing coppersulphate solutions. 5- If descendingsolutions prod*uced the chalcocite,the lime- stoneshould show evidenceof acid attack or of attackby ferric sulphate. Although the enrichmentof bornire producescom- paratively little acid, the great quantitiesinvolved, if all the chalcocite were formed in t'his way, would seeinto have been suffi- cient to have produceda pronouncedeffect on the limestonewalls. It should'be pointed out, however,that Zeiss,Allen and Merwin 55 esta•blishedan equationby whichbornire may alter to a mixture of chalcociteand covellitewith the productiono.f no sulphuric acid, as follows: CusFeS4 -1I- CuSO4: CuES--[- 2CuS --[-FeSO4. There is not sufficientcovellite in the ore, however,to explainthe formation of chalcociteby this equation,al, though if it were- invokedto accountfor •he propor.tion of corellitethat doesexist, it would meanthat.less acid would.'be formed than by the usual equations. The only alterationof the walls is a slight amountof sandy limestonewhich occursin but a few places. Its absenceis an argumentagainst the formationof chalcocitehaving taken place by descendingsolutions. 6. With the assumptionof secondaryenrichment, the replacement of suchlarge masseso.f bornireby chalcocite,through the agency of descendingsolutions, would involve fhe releaseof a great quantity of iron, as sulphates. In neutral solutions,such as thosein a carbonaterock, limonite would form readilyif mild oxidizing conditionswere encountered;if the iron remained in the ferrouscondition reaction with the wall rock might be expectedto yield siderite.'The limonitein the depositsis insig- nificantin amountas comparedto that whichwould be expected if the chalcocitewere a supergenereplacement o.f bornite. Fur- ther, the distributionof limohiresuggests that it is largelya • Op. cit., p. 48I. GEOLOGY OF ORE DEPOSITS OF KENNECOTT, ALASKA. 73 productof later oxidationand connectedwith the destructionof the presentore andnot with its formation. Sideritehas not been observed. These considerationsoffer objectionsto a supergene processinvolving transportation of muchiron throughlimestone in the form of solublesulphates. 7. No changein the characterof the mineralsor the proportion of chaleocitetakes place with increaseddepth; in secondarily enrichedores there is usually a gradual changefrom su.pergene to hy.pogeneminerals. 8. Several blind veins occur in the mines,the upper parts of someof which do not comewithin •,ooo feet of the surface,and the characterof ore and proportionof chalcocitein them is ex- actly the same as in the veins which outcrop. Also, there is no differencein upper and lower parts.,and no oxidized zone lies above them. Such would not ..beexpected if the chalcocitewere formed by descendingsolutions. 9. In the main veins' the chalcocite pinches out upwards, normal to the stratification, into barren limestone, and extends down the dip to unknown depth, so that from the 2oo levels downward(see Fig. 6) h,ooutcrops or zonesof oxidizedore lie vertically above them. Therefore, the chalcocitein them could not have 'been formed by vertically descendingwaters. It is difficultto understandhow any descendingsolutions could have producedthe chalcocitein the tips of the inverted wedge-like veins unless the waters be considered to have moved almost laterally,•which seems improbable. A summationof the fieldevidence shows that it stronglyfavors a hypogeneorigin for the chalco.cite,and that there is no direct evidencesupporting a supergeneorigin, thoughin past geologic timesconditions did existwhich were suitablefor secondaryen- richmentto have taken place. Discussionof Evidence.--The microscopicevidence shows that an importantpart of the chalcociteexhibits featureswhich may be consideredan indicationof replacementof bornire. If such be the case the chalcocite is later than the bornite. It does not necessarilyfollow, however, that the chal.cocitemust have 74 ALAN M. BATEMAN AND D. H. McLAUGHLIN. beenformed as a resultof descendingsolutions, even though the occurrence of almost identical relations between chalcocite and bornite in depositsof unquestionedsecondary enrichment, does suggesta supergeneorigin for the Kennecottdeposits. A replacementof bornite by chalcocite,only, is indicated,and this •may have taken place either by .descendingsolutions or by the final productof hypogenesolutions. The field evidencestrongly favors the latter. Such an origin for the chalcociteinvolves no overthrowingof establishedprocesses, except a customary tendencyto regardas supergeneall chalcocitewhich replaces •bornite. No conclusivereasons are known to the writers why .chalcocite cannot replace'bornite by means of hypogeneas well as supergenesolutions. The hypogenereplacement of pyrite by bornite, chalcopyrite,an, d other primary sulphides,also of chal- copyriteby bornite (Plate V., C), is a well-knownand accepted fact. The hy.pogenereplacement of bornite by chalcocitemay take place in the same manner. The processesinvolved in the replacementof one set of sul- phidesby another,in primary solutions,however, are not known. Probably it involvesin pant the applicationof Weigel's solubility t•bles, in which a sulphidein solution,of lesssolu'bility replacesone already formed, of higher solubility. Since iron and iron-coppersulphides are more solublethan coppersulphide, the'tendency would be for chalcociteto replacebornite or other iron•bearingsulphides. This appliesequally well to secondary or to primary solutions. That the replacementof bornite by chalcocitemay havetaken place %y hypogene solutions is further supportedby the findingof considera[lechalcocite which shows no evidenceof havingreplaced other minerals, but is strongly suggestiveof havingbeen originally depositedas such. In the specimenfrom the new shaft of the Mother Lode Mine there are relationssuggesting graphic structures between borni.te (p. ri- mary) and chalcocite,which have been interpreted in somecases as typicalbetween minerals of primary ore. It is reasonableto supposethat depositionfrom hypogenesolutions would not be uniform, especiallyif derivedas previouslysuggested, and that GEOLOGY OF ORE DEPOSITS OF KENNECOTT, ALASKA. 75 from certainparts of the solutionsmore iron-containingminerals might .bedeposited than from others. W'hereother parts of the solutionsmeet suchiron•bearing sulphides, they being of greater solubility,would be replaced.by chalcocite;but where suchsul- phidesare not encounteredchalcocite would be depositeddirectly. Thus, chalcociteof both replacementorigin and direct deposition might be formed in the samedeposit. The processwould not be unlike that which•has taken placein mostore depositswhere primary borniteor chalcopyritehas been deposited,in part by replacementof primary pyrite, and in part without replacingit. That the isometricstructures of the chalcocite(Plate III., A) may be inherited .by replacementof bornire, would appear to invalidatethe diagnosticcriteria that might be applied from the experimentalinvestigations of the GeophysicalLaboratory. But chalcocitewhich •a's apparentlyreplaced enargite also exhibits. i.s.ometric structures. In these cases the isometric structures could not have .beeninherited, 5ø and suggestchalcocite formed above 91 ø 'C. Also, in Plate V., A and B, malachite has developed along triangular isometric structuresin the chalcocite,but alongcurved, irregular courses in the •bornite. If the triangular isometricstructure of the chalcocitebe inherited from replacement of the adjacent bornite, should not the .bornire also show the alterationto malachitein triangularpatterns instead of curvedlines ? .Thefact thai it doesnot, suggests that the isometric triangular pattern of the chalcociteis due to its having been formed above91 ø C. If replacementof 5ornite.be considered as havinggiven the chalcociteisometric structure, it seemsstrange that other chalcocite, also a replacementof bornite, shouldexhibit, not isometric but orthorhombicstructure (Plate III., C). The latter is usually consideredto be indigenous.to chalcocite, and why mightnot the former also .be considerec}indigenous rather than inherited? The orthorhombicstructure cannot be inheritedfrom bornite,for borniteis known only in the isometricform. Although'it is •6 Unlessby somechance the chalcocitemay possiblyhave replacedbornite associatedwith the enargitein replacementrelations. 76 ALAN M. BATEMAN AND D. H. McLAUGHLIN. possiblethat bornire may have another crystallographicform, nonehas yet :beenfound experimentally, and naturalbornite has never yielded any ofher crystallographicstructure than the isometric. The explanationas given by Posnjak, Allen and Mer- win,5' that the orthorhombicchalcocite may have been formed above9 •ø and containedtoo little covelli•eto preventits inversion, seemsa simpler and more direct one.58 It is a rule that depositionfrom mineralizing solutionsis not uniform, and it wouldbe expectedthat in placeschalcocite would containover 8 per cent.covellite, and in other placesless, thus allowingboth the isometricand orthorhom,bicforms to persist. If the possibility that the isometric structure of the chalcocitewas inherited, be excluded,the conclusivecriteria establishedby the Geophysical Laboratorycan then be appliedto the Kennecottchalcocite to show that it was formed above9•ø C., which also meansthat it is hypogenechalcocite. The evidenceoffered by the. Mother Lode shaft specimenand the conclusionso.f Doctor Merwin regarding it show that, in this specimenat least, the isometric structure is in all proba•bilityoriginal. Although the evidence from this one specimencannot correctly be appliedto all the isometric structures,it is, nevertheless,a strongly suggestiveargu- ment that the other isometric structures were formed the same way, and that the geophysicalcriteria can be applied. The large amount of dissolvedcovellite in the Kennecottchal- cocite (or the high sulphurcontent) also suggestsa high temperatureof formation. Posnjak, Allen and Merwin59 .produced solutions.of covellitein chalcocite,down to 80 ø C., althoughthe solutionwas more completewith higher temperatures. No solution was obtainedat room temperatures,and suchsolid solutions 57Op. cit., p. 527. 5s In arguing for an indigenous rather than inherited isometric structure for the Kennecott chalcocite, the authors by no means intend to exclude all isometric structure from being inherited, as its occurrence has been conclusively proved by Graton and Murdoch, and numerous examples of it are familiar to the authors. Their contentions apply only to the Kennecott chalcocite. 59 Op. cit., 506. GEOLOGY OF ORE DEPOSITS OF KENNECOTT, ALASKA. 77 have not been found in chalcocitedefinitely 'kno.wn to be supergene. Aside from all discussionof evidence,the fact stands forth clearlythat the Kennecottdeposits show characteristics different from all depositsof unquestionedsecondary enrichment by descendingsolutions. Conclusions.--Thegeneral discussions of thedifferent lines of evidenceindicates rather clearly a hypogeneorigin for the chalcocite. The field evidencestrongly suggestsit and offers no opposingevidence, though it doesshow that certain conditions did exist wherebyenrichment might have taken place. The microscopicevidence offers much in favor of a hypogeneorigin and nothingdemonstrating a supergeneorigin. That much of the chalcocitereplaces bornite is shown%y the microscope,but this doesnot indicatea sup.ergeneany more than a hypogeneorigin. The experimentalwork of the GeophysicalLaboratory offers diag• hosticcriteria applicable tothe origin of chalco.cite,and the argu- mentsgiven above ind'ic•tte that it canin all probabilitybe applied to indicatea hypogeneorigin for the Kennecottchalcocite. The authors thus conclude, after a careful examination of the different lines of evidence,that practicallynone of the Kennecottchal- cocite has been formed by cold descendingsolutions, but that it is the result of heatedhypogene solutions ;0o part of the chalcocite having been formed by replacingearlier sulphides,part formed directly in solid solutionwith covellite,and part having beendeposited as chalcocite.

SUMMARIZED CONCLUSIONS OF ORIGIN OF ORE DEPOSITS. Su,bsequent to the conformabledepos{t{on of the Ch{t{stone limestone upon the N{ko]a{ greenstone,the formations were folded {nto a large ant{cl{ne,upon the flanks of wh{dh were super{rnposedre{nor syncl{nal troughs,' or poss{blymonoc]ines. The formationof the latter was accompan{edby ruptur{ngof the br{ttle l{mestone,giving rise to wedge-shapedfissures, by a o0A d{fferent {nterpretat{onof the or{g{n of the chalcoc{teis held by Pro- lessor L. C. Graton, who w{ll later d.iscussthe subject from another v{ewpo{nt. 78 ALAN M. BATEMAN AND D. H. McLAUGHLIN. slidingof the bedsover eachother, and to separationsalong bedding planes. The openingsthus formed were the loci for copper depositionwhich took place chieflyby replacementof limestone and adjacent to the openings,and to a minor extent by filling the open cavities. The solutionsare consideredlikely to have beenmeteoric waters which had their temperature raised within the earth, probably by contactwith rocksheated •y intrusionsof porphyrymagmas, or by contactwith the magmas'themselves. The possibilityof the solutionshaving been direct emanationsfrom the porphyry magma cannot, however, be eliminated. The warmed solutions then searchedthe copperand the small quantitiesof other metals out of the greenstone,the latter rock having obtainedits copper contentat, or shortly after, the time of extrusion. The warmed copper-bearingsolutions circulated through the greenstoneand depositeda small part of their load in the occasionalsmall fractures or other openingsthey encountered,thus giving rise to small,irregular, and widely scattereddeposits in greenstone. In a few placesthe solutionspenetrated into the Chitistonelimestone and encounteringfractures and other openingsin this congenial host rock, depositedtheir load•in suc•hconcentration as. to give rise to the Kennecottore deposits. Where the openingsin the limestonewere few and small, or where the coppercontent of the solutionswas low, minor depositswere fo.rmed, such as are widely scatteredin the limestonenear the greenstonecontact. The relatively great concentrationof co.pperin the limestoneas comparedwith that in the greenstoneis 'believedto be duelargely to the •physicaland chemical differencesof the two rocks; the limestonebeing more readilyreplaced and.containing larger and more continuousfractures, whereasthe greenstoneoffered no large or continuousopenings and did not react readily with the mineralizing solutions. It is believedthat all of the sulphideminerals were deposited at thistime .by the prim.arysolutions; the rarer sulphidesbeing depositedfirst, and corellite, chalcopyrite,and bornite at •t)out the sametime or slightly later. The chalcocitedeposition has in GEOLOGY OF ORE DEPOSITS OF KENNECOTT, ALASKA. 79 largepart takenplace by replacementof borniteand otherpre- viouslyformed. sulphides, but in manycases. it seemsto havebeen depositeddirectly without replacement of othersulphides. It is believedthat the evidence,though not absolutelyconclusive, is sufficientlystrong to indicatea primary or hypogeneorigin for the chalc.ocite,and that if all the argumentsfavoring its hypogene origin were to be placedin one pan of a balancescale, and all favoringa supergeneorigin be placedin the other,the former would overwhelminglyoutweigh the latter. At a later periodthe regionwas erodedto a stateof maturity, and conditionswere made favorable for oxidation and any en- richmentto take place,provided the ore depositscame within reach of oxidizing processes.Still later, under rejuvenated erosion,deep valleys.were cut and Kennecottspur was rapidly lowered. The ore depositswere partially convertedinto carbonatesof copper;small amountsof limonite and oxidationproducts of arsenic were formed. The ox•datmn' '' extended to the greatestknown depthsof the depositand is no more complete neare.r the surfacethan at greaterdepth. In no placenow existing is it complete,although it is greateralong channelways than in unfracturedareas o.r in massiveore. The oxidationgives the impressionof havingbeen produced by fairly rapidly moving waters,confined largely to channeiways,and part'of a deepcircu- lation whichsought outlet to the deepvalleys •below. With the exceptionof a negligibleamount of sootychalcocite found in the uppermostworking of the JumboMine, and.possibly the chalcocitein the nodules,no secondarysulphides appear to have been formed at this time. Oxidation was arrested with the advent of the Glacial Period by the freezingof the groundwatersand they have remained frozenuntil thisday. Oxidationsince then has been negligible; none having taken placeunderground, and that on the surface consistingonly of a thin tarnish. Mechanicalerosion of theore deposits has continued actively throughthe GlacialPeriod up to the present. During part of 80 ALAN M. BATEMAN AND D. H. McLAUGHLIN. the Glacial Period the accumulationof disintegratedore d.dbris fromthe Bonanza deposit, along with ice,gave rise' to theGlacier ore body. On .theother side of the slopethe breakingdown of the limestoneand the containedore depositformed a talus slope rich in copper,giving rise to the slide ore bodies. YALE UNIVERSITY', N•w HAVEN, CONN.