Chalcocite Enrichment2

WITH WHICH IS INCORPORATED

THE AMERICAN GEOLOGIST

VOL.VIII OCTOBER,i9i 3 No. 7

CHALCOCITE ENRICHMENT2

ARTI•UR C. SPENCER.

PRELIMINARY STATEMENT. SinceI9OO , when Emmons, 2 Weed, a and Van H{se4 presented papersdealing with the downward enrichment of sulphideores, 5 thesubject has received, perhaps, more attention from working. studentsof oredeposits than any other single phase in thechem: {stryof oregenesis. Especially with respect to theenrichment of .cuprlferousmaterials by secondarydeposit{on 'the problem has beenworked out to theextent that .the chemical reactions involved maybe indicated in a generalway ,at least; besides which we have a fairlygood understanding of the geological conditions which favoror opposethe operation of recognizedprocesses ona scale • Froma forthcomingreport on the geology and ore deposits oœthe Ely district,Nevada. Published by permission of the Director oœ the U.S. GeoL Survey. • Emmons,S. F., "The SecondaryEnrichment of OreDeposits,". Trans.: _Am.Inst. Min. Eng., February meeting, 19oo , Vol., 30, pp. I77-217, i90I. a Weed,W. H., "Enr/chmentof Goldand Silver Veins," Trans. ,4m..Inst. Min. Eng.,February meeting, I9OO, Vol. 30, pp. 424-448, I9oI; "Enrichment of MetallicVeins by LaterMetallic Sulphides," Bull. 'Geol. Soc. ,4m., Vol. II, pp. I79-206 , I900 . ,•, * VanHise, C. R., "SomePrinciples Controlling the Deposition of Ores,"

Tpans.Am. Inst. Min. Eng., Vol. 30, pp. 27-127, 19Ol. ß ß ' 5For thebi•bliography of this subject the readeris. referred to ihe'follOw-" ingpapers: To!man, C. F., "SecondarySulphide Enrichment ofß Ores,'; /l/Zia}. $ci.Press, Vol. Io6, pp. i8o-i8i, I913; Emmons, W.H., "The Enrich•m.• .•.• $filphideOres,'" U. S. Geol.Survey, Bull. 529, I913. 62I 622 ARTHUR C. SPENCER. large enoughto pro.du.cethe segregationof copperminerals in bodies of commercial value. The climatic condi.tions under which the 1,argestor richestbodies of secondaryores have accum- ulated have not beenadequately discussed, so. that a full analysis o.f this portion of the subjectremains to be made. The processesof chalcociteenrichment and the conditionsthat controlthe depositionof chalcocitefrom oxidizedcopper-bearing solutionsare here consideredwith specialreference. to the dis- seminatedores of the Ely district, Nevada, which, as mined, carry from 2o to 6o poundsof copperper ton. The principal ore bodieshave been formed 'by the enrichmentof portions of great massesof uniformly pyritized and sericitizedmonzonite porphyry which carry not more than half of one per cent. of copperas primary chalcopyrite. In form and occurrencethe ore bodiesare blanketsthat lie beneathan overburdenor cappingof fully weatheredand .oxidizedmaterial and grade downward into primarily mineralized rock which has not been enriched. The depth of the cappingranges from a few feet to 250 feet or more and the thicknessof the materialcarrying the secondarychalcocite varies from a few feet up to a maximumof about4oo feet. The orescontain from 5 to perhaps•o per cent.of metallicsul- phides, of which usually somewhatmore than half is pyrite. Both chalcopyriteand pyrite havebeen partly replacedby chalcocite, but the coatingsof this secondarymineral are commonly much deeper on the chalcopyritethan on the pyrite, much of which has not been coated at all. The principal nonmetallic minerals--quartz, orthoclase,sericite, and brown mica--are the same as those of the pyritized rock from w'hichthe ores have been derived. All theseminerals persist in the weatheredcap- ping, thoughusually the brown mica hasbeen bleached and a little kaolin formed, mainly through the decompositionof the orthoclase. The chemistryof downwardchalcocite enrichment may be treated by following in imaginationthe various incidentsof the journey made by rainwater which, falling on the surface,soaks into the ground and penetratesan existing ore body. Rainwater CH•ILCOCITE ENRICHMENT. 623 carriesin solutionthe variousgases of the air, includingoxygen and carbondioxide. In arid and semi-aridregions it contains alsonoteworthy amounts of common salt, which may be regarded asof wind-blownorigin. As thesewaters pass into the soil and into the porousweathered capping that lies abovethe ore mass theycome into contact with orthoclase and mica and with oxidic compoundsof 'ironand copper(including limoni'te and its con- geners),basic sulphates carrying iron or copper,and basic car- bonatesof copper.Metallic copper and red oxi'de cuprite are also fairly commonin the overburden.The silicateminerals in the cappingtend to makethe wateralkaline, • but as theyare only slightlyattacked this tendency is likewiseslight. Of themetallic mineralsin the capping,those containing sulphate decompose, beingsomewhat soluble, the finalresult being to produceand to leave in place insolublelimonite and coppercarbonates and to furnish small amountsof ii'on and potashsulphates to the solution. Thus far the dissolvedoxygen does not enter largelyinto reactionbecause most of the mineralsof the cappingare already fully oxidized.The onlyexceptions to be notedare cupriteand nativecopper. Also,the carbonicdioxide has been by no means exhausted. Everywherethe cappingcolored characteristically red by ferric iron compoundsgives place by a shorttransition to grayor bluish ore. Above this horizonthe watersare able to accomplishlittle in addition to what had been done in advanceby similar waters. Justbeneath the cappingthe solutionencounters material rich in sulphideminerals, that are subjectto ready oxidation. Here then chemicalaction between the' oxygenated waters and the sulphideminerals ensues, a seriesof reactionsbeing initiated of which seriesthe culminatingreactions involve the depositionof chalcocite. At first the waters contain free sulphuric acid furnishedby the decompositionof pyrite, but graduallythe acidbe- comesneutralized by basesfurnished by the gangueminerals, and at sufficientdepth the solutionsbecome alkaline. If considered x Cameron,F. K., and Bell, J. M., Bull. No. 30, Bureauof Soils,LI. S. Dept. of Agriculture,p. I2 et seq.,I9O5; Clark, F. W., "The Data of Geochemistry," Bull. 49I, LI. S. Geol. Survey, pp. 454-459, I9II. 624 ARTHUR C. SPENCER. with respectto the minerals decomposedth.e reactions that occur beneaththe cappingpresent a successionof oxidations,whereas if consideredwith respectto the activesolution, the changesare of courseas consistentlyin the directionof reduction. The reactions may be consideredin three groups,assignable in a general way to higher, intermediate,and lower positionsin the body of sulphide-containingmaterial. In the upperpart of a sulphideore body, where •rtmosphericoxygen is the oxidizing agent; somewhat lower down, where free oxygenhas been exhaustedferric sulphatebecomes active; and after the oxygenmade available by this carrier has been utilized cupric sulphatefurnishes oxygen. The action of cupric sulphateon pyrite and chalcopyriteresults in the depositionof chalcociteand the consequentenrichment of material carrying the primary sulphides. The formation of secondary chalc.ociteprobably involves a series of transitions or stages,as' pyrite•halcopyrite--bornite--covellite•chalcocite. The following discussionis incompletein that the chemistry of the copperminerals that are characteristicof the capping is not considered. Though oxidationin the portion of an ore body that lies just beneaththe cappingresults in the compoundingof cupriferous solutionsthe fact must not b.e neglectedthat here also are formed the relatively stablebasic carbonatesand sulphates, and the .evenmore stable minerals cuprite and metallic copper.

V. XPV. RI•V.•TAL DAT• O• C•LCOCITV. DV. POSITIO•.

WinchelP and T.olman, and also Read2 obtained coatings of chalcociteby treating pyrite with slightly acid cupric sulphate solution in the presenceof SO2. In experimental work con- ductedat a temperatureof about200 ø C. Stokesa inducedthe formationof cuprousand cupricsulphides by treatingpyrite with • Winchell, H. V., Bull. Geol. Soc. Am., Vol. x4, pp. 269-276, t9o3. •' Read, T. T., "Secondary Enrichment of Copper-iron Sulphides," Trans. Am. Inst. Min. Eng., Vol. 37, PP. 297-303, I9o6. a Stokes, H. N., "On Pyrite and Marcasite," Bull. U.S. Geol. Survey No. I86, p. 44, •9oo; "Experiments on the Solution, Transportation, and Deposi- tion of Copper, Silver, and Gold," EcoN. GEot..,Vol. •, pp. 644-650, •9o6. CH,4LCOCITE ENRICHMENT. 625 cupricsulphate solution slightly acidified with sulphuricacid. The workOf Siokeswas followed by that of Readalready mer/- tioned,and by noteworthyobservations by Sullivan. Pulverized pyrite• andchalcopyrite shaken in a dilutesoluti.on of coppersul- phatecaused the solutionto loseits color. By contactwith 2o gramsof pyriteduring three days 4o cm.of cupricsulphate sola- tion lost.o4 gramsof copperout of .o97grams originally present. The experimentsof Sullivanare importantbecause they show that pyriteand chalcopyrit.e can cause the precipitationof copper fi'om sulphatesolution at ordinarytemperatures without the interventionof a strong reducingagent suchas was usedby Winchell,and it is sufficientlyobvious that the insolublecompound formed must '• a sulphide. But the action is ordinarily so re• luctantthat investigatorsof the subjecthave usuallyfailed to get visiblecoaiings on pyrite or chalcopyriteas the resultof treat- ment in the coldwith .simplesolutions of cupricsulphate. How- ever, by subjectingfragments of chalcopyriteto the actionof a weak soltalonof cupricsulphate during .threemonths, Welsh 2 and Stewart obtainedboth a tarnish,having the purpletinge of bornite, and "some thin black films." The writer has found that bornite reactsreadily with cuptic sulphateand that indigocoatings may be formedon that mineral in a few hoursby simplyimmersing it in a solutionof the copper salt. Under prolongedtreatment the indigo.colors first developed changesto a steelyblue which gradually fades until it givesplace to the gray color which is so characteristicof chalcocite. At- temptsto obtain similar resultswith chalcopyritewere not suc- cessfulwhen cupric sulphatewas used alone,but in the presence of ferrous sulphatethis mineral soon becomesmrni'shed and passesthrough a series0f colorchanges which repeated observation hasshown to occurin a definiteand predictableorder. First x Sullivan, E. C., "Discussion Relating to the Formation of Secondary Copper Sulphidesin Criticism of Read's Paper," Trans. Am. Inst. Min. Eng., Vol. 37, P. 894, •9o7. • Welsh, T. W. B., and Stewart, C. A., "Note on the Effect of Calcite Gangue on the SecondaryEnrichment of Copper Veins," Ecoa. G•.oL.,Vol. 7,. PP. 785-787, I912. 626 ARTHUR C. SPENCER. the natural yellow of the mineral darkens'slightly, then the'sur- facebecomes brownish with a bronzytone, then pink, light purple, darker purple, indigo, and various shadesof steelyblue with graduallylessening depth. The final result is a gray coatingof metallic luster which tums blue under the Stokes • test for chalcocite. Betweenthe blueand gray stagesthe surfaceof the mineral becomesyellowish or bronzy,the appearancebeing as thoughthe film first developedhad been dissolved. In various trials the wholetransition has required from five to ten days,and in certain trials the final chalcocitestage was not attainedat all. The same successionof colors,except the apparentreversion to chalcopyrite, may be obtainedby another method which will be described further on. By meansof a mixed copperand iron solutionspots of blueresembling covellite may be developedon pyri.tewithin a few hours, but so far as observedthis mineral does not become uniformly coated,as doeschalcopyrite. When the naturalacidity of a solutioncontaining cupric sulphate was increasedby adding a little sulphuricacid, chalcopyriteremained untarnished at the end of four months, and Winchell statesthat pyrite fragments immersedin acidifiedcupric sulphatesolution showed no visible alteration at the end of two years. While both bornite and chalcopyritehave been artificially coatedwith gray chalcocitefilms, the gray coatingshave beenpre- cededin all successfulexperiments by indigo-coloredfilms which are regardedas covellite. An unsuccessfulattempt was madeto form chalcocitedirectly by treating granulatedbornite with a solution containing ferrous sulphatein molecular concentration about four times that of cupricsulphate. In this experimentthe ferroussulphate solution was freshly reduced. The mineral and the liquidsemployed were freed from air by ebullitionunder reduced pressureat 6oø C. The apparatuswas sealed under ex- h.austand was cooledbefore the reagentswere broughtinto contact with the mineral. After the lapse of about four days the bornitehad assumeda deepindigo color which is takento indicate • A fragment of chalcocite boiled for a moment with Io per cent. ferric sulphatesolution turns blue. EcoN. GEOL.,Vol. I, p. 23, I9o3. CH,4LCOCITE ENRICHMENT. 627

the developmentof eovellite. After threeweeks had passedthe generaltone of the granularpowder was gray, but someof the grains still had blue surfaces. At the end of three months the material was removed from the apparatusfor examination. Underthe microscopemost of the grainshad the appearanceof chalcocite but certain surfaces showed the color of covellite. A sample of the original mineral was found to contain a small amountof chalcopyriteand specksof corellite,the latter being presentin aboutthe sameproportion as in the treatedmaterial. It thereforeappears that the borniteand chalcopyrite were both coatedwith chalcocitewhereas the original corellite remained unaltered. Like results were attained in a solution that carried equivalentconcentrations of copperand iron. In bothexperi- mentsthe concentrationof the coppersalt was about.02 of the formulaweight of cupricsulphate. Natural covellite was treated with solutionsof correspondingcomposition but no changeof color had resulted at the end of four months. The foregoingobservations have led to thesuggestion that the changeof pyriteor chalcopyriteto chalcocite may be considered asan alterationinvolving a seriesof steps,or perhapseven a con- tinuousprogression through indefinite compounds or mixturesof iron-coppersulphides. x Discussion of the suggested change pyrite to chalcopyriteis not offeredfor lackof adequatebasis furnished by experimentor observation.On theother hand, the change chalcopyritethrough bornite and covellite to chalcocitemay be broughtabout artificially in variousways, some of whichhave beenalready noted, and so far as the writer'sexperiments have goneit seemsto beimpossible to changechalcopyrite to chalcocite withouttraversing the bornite 'and covellite stages, or to convert borniteinto chalcocite except through covellite as an intervening ßproduct. Still it seemsprobable that in naturethe covellite stage maynot alwaysenter into the series,though the chemicalenviron- mentwhich would favor the more direct change to chalcocitecan not be stated'definitely at present.Experimental results with borniteand cupricsulphate indicate that the changecovellite to chalcocitemay occurand thereis a strongsuggestion that the • See table on page 638. 628 ARTHUR C. SPENCER. transformationproceeds in sucha way that graduatedmixtures of CuS and Cu2S are formed. This suggestionis supported by Graton and Murdock who have noted that natural "chalcocite".is not always gray in colorbut showsvarious shades of steely blue suchas might result from minute intergrowth of chalcocite and covellite. Specimensof copperore describedby Graton and Murdock• containthe seriespyrite, chalcopyrite,bornitc, covellite, and chalcocite, the mutual spacerelations of the several minerals being suchthat eachmineral of higher coppercontent appears to have been derived from the alteration of the mineral next below it in the series. Graton and Murdock state that all these minerals z have been .observedby them as descri:bedin different parts of a single polishedspecimen, under examinationby means of the reflectingmicroscope, but that all the copper-bearingphases were not noted about any single grain of pyrite. Mr. Bastin, of the GeologicalSurvey, has shown the writer a specimenof copper ore from Gilpin County,Colorado, which represents partly altered chalcopyrite. Certain surfacesshow chalcociteapparently lying directly on the chalcopyrite. On the other surfaces, and evi- dentlyof later origin, are complexfilms with covelliteoutside and a substanceresembling bornitc beneath. In different placesmay be notedbronzy effects,grading in tone from the normal color of bornitcto a purplehue intermediate .between bornitc and' corellite, as fhoughbornitc had beenplated over with a transluscentfilm of the indigo mineral. Similar color effects are to be obtained, accordingto Read,a by treating chalcopyvitewith coppersulphate solutionin the presenceof SO•.

"The enriched sulphidewas dark green in color; during the month it had becomesuccessively bronzy, purple, and dark steelyblue."

This statementis opento the interpretationthat the surface x Graton, L. C., and Murdock, Joseph, "The Sulphide Ores of Copper," Trans. ,'lm. Inst. Min. Eng., Vol. 4, PP. 754-755, I9t3. • Compare relations of bornite as described by R. H. Sales, EcoN. Vol. 5, P. 682, I9IO. a Read, T. T., "Secondary Enrichment of Copper-iron Sulphides," Trans. Am. Inst. Min. Eng., Vol. 37, P. 3o0, I9O7. CH.•ILCOCITE ENRICHMENT. 629

of thefilm deposited on the chalcopyrite passed through the stages bornite and corellite and that some chalcocite was formed. The colorsobserved by Readand presentin the Gilpin County specimenreferred to abovemay be readily obtainedby another simpleprocedure, and the observercan hardly fail to conclude that the colors obtained indicate the formation of the minerals bornite, covellite,and chalcocite. If any memberof the series chalcopyrite,bornite, covellite is touched,by a pieceof iron while it is immersedin a cupric.sulphate solution the mineral changes color almost instantly, and in a short time 'becomescoated with the mineral next aboveit in the series. The brilliant indigo of corellite changesto the dull gray so charracteristicof chalcocite, borni'teassumes a blue color unmistakablylike that of covellite, and chalcopyritetakes on a bronzyhue resemblingthat of bornire. Furthermore,within a very shorttime the bronzeplating on chal- copyritegives place to or is hiddenby a filmof covellite;then within an hour or so the surfaceturns to a chalcocitegray, and fin.allymetallic copper is deposited..When p.yrkeis usedin place of a cupriferoussulphide the effect of the iron is sufficientto throw downmetallic copper rather quickly,but by scrapingaway the metal and again placingthe mineral in contactwith iron in the solutionit is possibleto obtain depositsof coppersulphides. In this way spotshaving somewhat the samecolor as chalcopyrite and othersbronzy like borniremay be developedon pyritealong with unmistakablefilms of covelliteand of chalcoeite. If copper is used insteadof iron .the resultsare essentiallythe samewith chal'copyri.te,bornite and corellite. For instance,covelli.te may be coatedwith chalcociteby contactwith copperin a solutionof cupricsulphate. It .is obviousthat the speedof reactionmay be variedby employingdifferent metals •s inductors,or by employ- ing mineralsto causeelectrolytic ,action. Very pleasingresults have beenobtained by placingin a solutionof cupricsulphate a polisheds. pecimen of intergrownchalcopyrite and pyrrhotite. liere a pinkishbronze color resembling that of borniteappeared Withina few daysbut graduallych. anged to purple,deep purple andfinally to indigo-blue.On themost reactive grains the corel- 630 •IRTHUR C. SPENCER. lite colorwas fully developedin abouteight weeks, but the surface in generalbecame blue only after •2 weeks,and eventhen certainareas were still bronzy. At the end of four monthsno gray colorhad developedto indicatethe formationof chalcocite, but the colorwas a paler blue than that of natural covellite. It may be suggestedthat the resultsdescribed, which were ob- tainedunder ordinary temperatures, may actuallyepitomize the courseof reactionbetween the primary sulphidesand copper salts held in oxidizedsolutions penetrating from the surface. Even if the meansemployed to producethe resultsin a short time are not comparablewith tho'seinvolved in naturalprocesses, perhapsthe conditionsunder which the experimentswere made may be consideredto be lessunnatural than thoseprevailing in mineralsyntheses effected under high temperatures. Sincein naturecoatings of covelliteand of chalcociteare found on grainsof pyrite without observablefilms of other membersof the seriesbetween the primary and the secondarymineral, the contentionmight be made that intermediateproduct. s have not beeninvolved in the change,and that the suggestedstep process is thereforedisproved. This conclusiondoes not necessarilyfol- low, becauseintermediate phases may existin filmstoo thin to be observed,or if formedmay havedisappeared through conversion into somemineral standinghigher in the series. However, this may be, it would seemthat the true courseof the chemicalaction betweenpyrite and chalcopyriteon the one sideand cupricsulphate on the other can be determinedby sufficientlydetailed in- vestigationsin extensionof the experimentsmade 'by Sullivan which are referred to on page 625. In fu.turework •:heeffects of ferrous and ferric sulphatesand of sulphuricacid in known concentrations should be determined.

SOURCE OF OXYGEN.

Beforetaking up the threefold series of reactionsleading to chalcocitedeposition, it is of interestto inquire whetherthe oxygen dissolved in rainwater could alone have effected the oxidation of CH,zlLCOCITE ENRICHMENT. 63I the massof material which has contributedthe secondarycopper now containedin any given ore body. This query may be an- sweredin the negative. The ore body at CopperFlat, assumed to average•.5 per cent. copper,carries as much addedor secondary copperas couldbe furnishedby the completeleaching of 4o0 feet of primary material containing one half of one per cent. copper:but with about •oo feet of existingcap rock still con'tain- ing at least one half per cent. copperin oxidic minerals, the total depth of material which 'hasbeen oxidized t.oproduce the present massof chalcociteore can not havebeen less than 5oo feet. It is assumedthat all of this 500 feet of material passedthrough the chalcocitestage, so that a goodlypart of the copperhas been several timesdissolved and redeposited. This beingtrue, the amount of oxygen required would be the same as the amount necessary to oxidize 50o feet of ore such as now exists. By considering the amount of oxygenthat water can absorbby contactwith the air under a.tmospheric pressure at 7,000 feet elevation,and at the present mean annual temperature of the regi.on,it is found that, even allowingthat precipitationin the past has been25 per cent. greater than at present,and that so much as 60 per cent. of the rainfall could have penetratedto the ore body, the oxygen required to oxidize 50o feet of ore like that now existing would require the contributionsof rainfall during a }ongerperiod than physicistsand geologistsare willing to allow for the entire his- tory of the earth.x Although all the assumptionsmade tend to a minimum, the time required,as calculatedin this way, is still so inordinatelygreat as to demanda differenthypothesis in regard to the manner in which oxygenhas beendelivered I:o the place of sulphidedecomposition. It is thoughttherefore that a large part of the oxygen must have been derived from air that circu- lated through the oxidized capping. The pore space in this materialamounts to more than IO per cent.of the total bulk of the rock and there can be no doubtthat when the cellularopenings x Becker, G. F., "The Age of the Earth," Smithsonian Misc. Coll., Vol. 56, No. 6, pp. x-28, x9•o. This is the most recent review of various estimates by different methods. Dr. Becker regards6o million yearsas the figure most nearly in accord with the data now in hand. 632 ARTHUR C. SPENCER.

are not water-filledthey must be occupiedby air. It would 'seem then,that the greaterpart of the oxidationmust take placeduring such times as the sulphidesare merely moist rather than when they are flooded,because then the water could receiveoxygen from the air in contactwi. th it at the same rate at which oxygen is being taken out of solution by the reactionsof oxidation,. Thus the water would remain saturated under the conditions of partial pressurepertaining to the proportionsof various gases contained in the subterranean air. Within a short distance beneath the completelyoxidized and porouscapping the rock be- comesmuch lesspervious, and here the solutionswould be present only as capillary films essentiallyfilling the interspacesand therefore leaving no room for .air to penetrate. The train of reasoningmay be carried one step farther. Even if some air doespenetrate •t.he sulphide ore body the circulationwould be comparativelysluggish, and sinceall of the supplymust passby the sulphidesin processof oxidation it is apparentthat the oxygen might be entirely depleted,so that 'thegases reaching the ore 'mass would be merely the inert constituentsof the atmosphere plus carbondioxide.

OXIDATION B'Y FREE OXYGEN.

Beneaththe cappingthe sulphidesfirst encoun•teredby waters that carry dissolvedoxygen are pyrite and chalcopyrite. Just at the top of ,anyporphyry ore body grains of theseminerals persist after the removal of chalcocitecoatings which they carried at a time when the bottom of the cappingwas slightly higher than at present. Here too is to be foundsome pyrite whichnever carried more than the ,slightestcoating of chalcocite. The sulphidesnamed are of coursereadily decomposedby the oxygen-bearingsolu,tion. The' partial oxidation of pyrite with ferrous sulphateas a product has been indicatedby Gott•chalk and Buehler• by meansof the two equations: FeS2 q- 40 -- FeSO4 q- S, FeS2.q- 60 -- Feso4 q- SO•. xGottschalk, V. H., andBuehler, H. A•.,"Oxidation of Sulphides," Ecoa. GEOL.,Vol. 7, P. 16, 1912. CHALCOCITEENRICHMENT. 633 Thecomplete oxidation of pyrite may be considered astaking place in somesuch manner as is outlinedby the foregoingequa- tions and ,the following series,• in which equation(3) is derived from (•) and (2).

FeS• q- 70 q- H•O = FeSO4q- H•SO4, ( • ) 2FeSO• q- O q- H.oSO•: Fe•(SO•) a q- H•O, (2) FeS• q- •50 q-H•O: F•(SO4)a q- H•SO4. (3) For ½halcopyrite,.equations essentially .analogous to (x) and (3) are' CuFeS•q- 80 = FeSO4q- CuSO4, (•a) 2CuFeS•q- x6Oq- H.oSO•.=Fe(SO4)s q- CuSO4q- H•.O. (3a) Part of the ferric sulphateformed at the upper surfaceof the bodywhere free oxygen'is present decomposes to form basiciron. sulphat.esand hydratediron oxide,another part may be supposed to passdownward in solution.andto attack pyri.te, chalcopyrite, and chalcocite. The formationof iron hydroxidemay be repre- sentedin variousways, for exampleas follows' 6FeS0•q- 3¸ q- 3H•.O: 2Fe2(SO4)sq- 2Fe(OH)s. The e.qua•tions which have beengiven indicatethat the decompositionof pyrite.yieldi sulphuric acid, and ferric sulphate.. Under natur.al.conditions, solutions carrying these substances comeinto contact with chalcocite only a shortdistance below the- pointwhere pyrite and chalcopyrite are firstencountered. • Here, if anyfr• oxygenremains, chalcocite is decomposed with the formation of cupricsulphate Cu.oSq- H•.SO4q- 50 = 2CUSO4q- H•O. (4)'

. • Compare Stokes, H. N., "On Pyrite and Marcasite," Bull. No. I86, U.S. Geol..:Siirvey, pp.. •5 and I9, x9oI; Lindgren,W., "Copper Depositsof Clifton- liloi-endDiSti-ie_.ta,"' Prof. PaperU. S..Geol.•SurveyNo. 43,p. I79, i9o5. This author gives an .extendedbibliography'i'elating to pyrite Oxidation. Allen• E,.W.,."Sulphid•s of Iron andtheir Genesis,"Min. Sci. Press,Vol. xoj,-pp. 4x3-414, •9II; Gottschalk, V. H., and BUehler, H. A., "-Oxidation of $ul-- phiores,"RcO•:. Gkor,., V01. 7, p. x6,I9X}• ;. Tolman, C. F,, "SecondarySulphide Enrichment of Ores," Min. $ci. Press, Vol. 42, p.•4o, •9•3. 634 ARTHUR C. SPENCER.

The reactionindicated by this equat!onmay be consideredas usingup the last of the free oxygenwhich can reacha body of enriched porphyry ore under ordinary conditions. At points immediatelybelow those where the free oxygenhas been entirely consumed,ferric sulphateand cupricsulphate are present. Both thesecompounds are capableof oxidizing the sulphides,but the former is more readily reducedthan the latter, and it may be supposedto undergo almost completereduction to ferrous sulphate before the latter can comeint'o play as an oxidizing agent.

OXIDATION BY FERRIC SULPHATE. Where chalcociteand the other sulphidesoccur together, and especiallywhere, as in the Ely ore bodies,the chalcociteis in actual.contact with pyrite or chalcopyritein the form of coatings, it is the first mineral to be attackedby ferric sulphateand it acts as a temporary protection against the decomposition'of these minerals. The latter point has been proved experimentallyby Gottschalkand Buehler• and fully confirmedby Wells.2 Accordingto Vogt a the action of ferric sulphateon chalcocite, which would come into play below the shell of nearly complete oxidation is as follows:

Cu2Sq- 2Fe• (SO4)a-- 4FeSO• q- 2CuSO• q- S. (5) But if the sulphurset free reactswith more ferric sulphate,SO• might be formed and with still more ferric sulphatesulphUric acid will result, and eventuallywe may arrive at the equmtion suggestedby Weed,4 which may be deducedfrom equations(5), (7), and (8): Cu•S q- 4H•O q- 5Fe2(SO,)a--- IoFeSO4 q-4H2SO4 q-2CuSO•. (5a) • Gottschalk,V. H., and Buehler, H. A., EcoN. GEOL.,Vol. 7, P. 31, 1912. •Wells, R. C., "Electrical Potentials between Conducting Minerals and Solutions," four. P?ash.•Icad. Sci., Vol. 2, pp. 514-516, 1912. s Vogt, J. H. L., "Problems in the Geology of Ore Deposits," Trans. Am. Inst. Min. Eng., Vol. 31, p. 166, 19o2. 4 Weed, W. H., "Enrichment of Gold and Silver Veins," Trans. ,Jim. Inst. Min. Eng., Vol. 30, p. 429, 19Ol. CHALCOCITE ENRICHblENT. 6 3 5

Equation(5a) may be regardedalso as summarizinga seriesof oxidation stepssuch as the following: Cu2Sq- Fe2(SO•)a: CuS q- 2F.eSO• q- CuSO4, CuS q- Fe2(SO•)a---S q- 2FeSO• q- CuSO•, S q- Fe2(SO4)a: 2SO2 q- 2FeSO•, 41-I20 q- 2SO2 q- 2Fe2(SO•) a--- 4I-I2SO• q- 4FeSO4. The samereagent, ferric sulphate,may be consideredas acting on pyrite in a mannerrepresented more or lessadequately by the followinggroup of equationswhere equation(9) is derivedby meansof equations(6) to (8). FeS2q-Fe2(SO•) a-- 3FeSO• q- 2S, (6) S q- Fe2(SO•)a: 2FeSO• q- 2SO2, (7) SO2q- Fe2(SO4)a q- 2I--I20: 2FeSO• q- 2I--I2SO•, (8) FeS2q- 7Fe2(SO•)a-Jr-8H20-• t5FeSO• -Jr-8H2SO•. (9) It is to be notedt.hat equation (9) is comparablewith equation (I). For chalcopyritean expressionsimilar ,to (9) is: CuFeS2q- 8Fe2(SO4)a q- 8H20 -•. t7FeSO, q-8H2SO, q- CuSO• q-. (9a) In the studyof placesin the ore pits wherethe decompositions indicateda, bove are in progressthe observermust remark the short spacethat imervenes•between capping fully oxidizedand the subjacentore wherein to casualobservation chalcocite shows no sign of having beenattacked. In so far as the decomposing power of the downward m6ving waters dependsupon the presenceof free oxygenthat' power appears in generalto be almost spent within a shell of material scarcelymore than three feet thick, thougha certainamount of impoverishmentmay be going on to a considerablygreater depth through the action of ferric sulphateupon chalcocite (see equations (5) and (5a)). It should be noted}of course,that the under surface of the oxidized capping is very irregular: as a natural result of fracturesin the ore mass. A crosssection of the surfacebetween the cappingand the ore resemblesthe exaggeratedprofile comparing mountain heightswhich is to be found in our older schoolgeographies. 636 ARTHUR C. SPENCER.

PARTIAL RESUMe.

The foregoing discussionshould make clear the following points. First, that waters from the surfacepenetrating a body of porphyryore will continueto decomposestrongly the metallic sulphidespresent so long as they contain or can acquire free oxygen,or so long as t•heycontain ferric sulphate. Second,that where chalcocite,pyrite, and chalcopyriteare all presentthe chalcocitewill .belargely and perhapsfully decomposedbefore the other mineralsare attacked. T'hird, that the decomposition of chalc.ocite, pyrite, and chalcopyriteeffects the reductionof ferric saltscontained in {he solution. Fourth, that the decompositionof pyrite,chalcopyrite, and chalcociteeach tends to produce sulphuricacid. Fifth, thatthe decomposition of chalcocite and of chalcopyritefurnish cupric sulphateto the solution. Briefly then, when oxygen-bearingwaters reach the upper part of the massof sulphi.de-bearingrockthe consumption of the dissolved oxygen.,beginsat once,and beforethe waterscan progress down- . ward for any considerabledistance all of this free oxygenis used up in decomposingthe sulphides. Also within a short distance, ferric sulphatei.s largely reduced I•o ferrous sulphate. So long as free oxygenis presentthe decompositionof c.halcocitewill pro- gressuntil no sulphuricacid remainsuncombined. It shouldbe addedthat cuprousand ferric irons are .alwayspresent in small concentration• in any solution containing cupric and ferrous sulphates.

OXmAT•O• 3¾ CrOPInC SULP•IAT•-.

The progressof the surface derived waters has been followed to the pointwhere they containno free oxygenand 0nly a minor amountof ferric sulphate,but wherethey carry ferroussulphate, cupric sulphate,and sulphuricacid. T.hus far on its downward journey it may be assumedthat the cupricsulphate has been pro- x Stokes,H. N., "Experiments on the Solution, Transportation,and Depo- sition of Copper, Silver, and Gold," Ecoa. Gv.or.., Vol. I, pp. 644-650, •9o6; Wells, R. C., "Discussion on Secondary Enrichment," Ecoa. G•o•,., Vol. 5, pp. 481-482, •9•o. CHALCO CITE ENRICHMENT. 637 teeted from reductionbecause at ordinary temperaturesand in acid solutionit is lessreadily reducedthan ferric sulphate2 Beyondt'he place where the ferric sulphateconcentration in the solutionhas beenreduced to a certain minimum,cuptic sulphate comesinto play as an 'oxidizingagent. The sulphideswith which the solutioncomes into contactare chalcocite,chalcopyrite, and pyrite. At ordinary temperaturesin the absenceof oxygen, dilute sulphuricacid doesnot decomposechalcocite, and Wellsz hasshown that chalcopyriteand pyriteare probablynot at-tacked under the same conditions. It is to be noted, however, that at high temperaturescupric sulphate reacts with chalcocitea to form corellite and cuproussulphate, and on coolingthe latter decomposes,a productof its decompositionbeing metalli, c copper. On the other hand it would appearthat cupric sulphatedoes attack chalcopyriteand pyrite at ordinary temperatures,so that iron passesinto solutionas ferrous sulphate,sulphuric acid is formed, and a coppersulphide is deposited. The last part of this statement 'is supportedby t.he fact that covelliteand chalcociteare commonproducts in naturewhere copper sulphate solutions from the upperzone of oxidationhave encounteredpyrite and chalcopyrite. Although very little quantitativelaboratory work has beendone in connectionwith this subject,the work of Stokes4 has furnishedt•o equat'ionswhich aim to representthe changes pyriteto covelikeand pyrite to chalcocitein molecularproportions. The Stokesequation for chalcociteformed by replacementof pyrite throughthe actionof cupricsulphate may be built up em- pirically by the methodwhich has beenused in the derivation of equations(3) and (9) (seepp. 633 and 635). Thus:

2FeS.•--[- 2CuSO• -- Cu2S--[- 2FeSO4 --[- 3S, * x Stokes, H. N., Ecoa. GEOL.,Vol. 1, p. 646, 1906. • Recordof experimentsgiven by E. H. Emmons,Bull. U.S. Geol. Survey No. 529. a Stokes,H. N., loc. cit., p. 648. 4 Stokes, H. N., "Experiments on the Action of Various Solutions on Pyrite and IvIarcasite,"EcoN. Gv.ot.., Vol. 2, pp. 15-23, 19o6. 5 Vogt, J. H. L., "Problems in the Geology of Ore Deposits," Trans. Am. Inst. Min. Eng., Vol. 31, p. 166, 1902. 638 ARTHUR C. SPENCER.

3S q- 2CuSO4--- Cu2Sq- 4SO2,• 6H20 q- 5SO2 q- 2CuSO4:Cu2S q- 6H2SOo 12I-I20 q- 5FeS2 q- 14CuSO•---8Cu•S q- 5FeSO• q- 12H2SO•. (10) 2

In a similar manner the follow,ingequation indicating the replacementof pyri.teby covellitemay be deduced' 4HgO q- 4FeS2q- 7CUSO4• 7CuS q- 4FeSO• q- 4HSO4. (I I ) s

As thus derived.each of the Stokesequations appears to sum- marize a successionof oxidation effects. It is probablethat the completereactions involve severalsteps, including the temporary formation ,of chalcopyriteand perhapsthe progressiveformation of ,severalmineral speciesin.termediate in compositionbetween chalcopyriteand chalcocite. The mineralsof what may be called the pyri,te-chalcoci,te series are given in the following table in which the order is that of increasingcopper content.

PYRITE-CIIALCOCITE SERIES.

ßConstitution According Mineral. Formula. to Hintze. Pyrite ...... FeS= Chalcopyrrhotite ...... CuFe4S6 (FeS,): (Fe, Cu"), Barracanite ...... CuFe=S• (FeSs) :Cu" Cubanite ...... CuFe=S, FeSs(FeCu') Chalcopyrite ...... CuFeS• Cu:SFe=Ss Barnhardtite ...... Cu•Fe=S• 2Cu=SFe..S• Bornire a (x) ...... CuaFeSa 3Cu•SFe•S, Bornire (2) ...... Cu•FeS• 5Cu_oSFe•S• Bornite (3) ...... Cu•FeS6 9Cu•SFe•Ss Covellite ...... CuS CuS Chalcocite ...... Cu•S Cu,S

No discussionof ,the minerals standingbetween pyrite and x As intermediate between this equation and the one foregoing the following reaction should be considered: 4H•O q- 6CuSO4q- S -- 3Cu•SO•q- 4H•SO4. See Stokes, op. cit., p. 44. •' Stokes, op. cit., p. 22. 8 Hintze, "Handbuch der Mineralogie," gives the three formulas here presented in his discussionof bornire. The second formula• • the one adopted by B. J. Harrington, Am. Jour. Sci., 4th ser., Vol. I6, p. I5I, I9o3. CHALCOCITE ENRICHMENT. 639

chalcopyvitein the tableis hereattempted, but it may be suggested that the other irombearingmembers of the seriesare formed suc- cessivelyas stagesin the productionof corellite or chalcocite throughthe actionof cupricsulphate upon pyrite or chalcopyrite. The basi•sfor this suggestionis given in a foregoingsection. Here the contemplatedchange pyrite to chalcopyrite,though not yet adequatelyverified by experimentalwork, may be indicated by an equationwhich is readily deducedby the methodemployed in arrivingat. equation (IO): 8H20 nu 8FeS2n u 7CuSO4: 7CuFeS• d- FeSO4d- 8H•SO•. Next for the changeschalcopyrite to chalcoci.te.and to corellite we may have summary equationsanalogous to (Io) and (I I) thus'

8H•O q- 5CuFeS• q- I I CuSO4 -- 8Cu2Sq- 5FeSO4q- 8H•SO4, ( 13) CuFeS2q- CuSO,: 2CuSq- FeSO4. (14 ) Equation(I3) and analogousexpressions, which may be de- ducedto indicatethe same final productswhere barnhardtiteor any memberof the bornite group is taken as the mineral under- going replacement,are all reducibleto a 'singleexpression in whichthe activepart of the mineralmolecule is consideredto be solelythe iron sulphidewhich it contains. Equation(i 3) may be written thus'

I6H•O nt- 5Cu2SFe•Sa n t- 22CUSO4 =5Culls q[-ilCu•S q[-IoFeSO4 n u I6H_oSO4. As the expression'suggests that 5Cu2Shas undergoneno changewe have by subtraction'

I6H•O -3-5Fe•Sa q- 22Cu504= I ICu•S -1-IoFeSO,-{-_i6H2S04. (i5) Finallyby algebraically combining equations ( I 0) and,.( i i ) .or equations(I3) and .(I4) so as to eliminateFeS• from the first

ß 64o ARTHUR C. SPENCER. or CuFeS2from the secondpair, an expressionis found for the conversionof corellite into chalcocite,or the reverse:

4H20 -Jr-5CuS q- 3CUSO4• 4CugSq- 4HgSO4. (•6) Presen,ted in this essentiallyempirical way the chemistryof the secondarycopper sulphides centers about the three reactionsrep- resentedby equations(12), (15), and (16), and thesereactions are thereforeregarded as particularlyworthy of investigationby quantitativework in the chemicallabova,tory.

ALTERATION OF COVELLITE TO CI-IALCOCITE. The supposedchange covelli. te to chalcociteunder the actionof cupricsulphate (equation •6) hasnot beenestablished by experi- mentsin which essentiallypure natural corellite vcasused, but as already statedthe transitionensues where '.bornire which has been coatedwith corelliteis further treated'with cupricsulphate solution, and the changein color from indigo to gray takes place more quicklyin a solutionalready containing ferrous sulphate in additionto cupricsulphate. The experimentdescribed on page 625 was devisedwith the expectationthat chalcocitewould be directly formed without the intervening corellite stage. This expectation was based upon the known reversible reaction between ferrous and cupric sulphateswhich producescuprous and ferric ions in the solution:

2CuSO4 q- 2FeSO• • CugSO4q- F%(SO4)8.1 While ch,alcocitewas not directly formed,the observedhasten- ing of the final chalcocitestage strongly suggests .that the rela- tive concentrationsof cupric'and ferrotis salts in the downward movingsolution may be •oneof the principalfactors in determin- ing whethercorellite or chalcociteis to be the end stagein the sulphideenrichment of copperores. In this experimentthe x Stokes,H. N., "On Pyrite and Marcasite,"Bull. U.S. Geol.Survey No. I86, pp. 44-45, I9ox. This reversiblereaction was studiedby Stokes,who says:"It is doubtlessto the reductionof the cupticsulphate by sulphurand by ferrous sulphatethat the formation of chalcocitefrom pyrite is to be ascribed." CHALCOCITE ENRICHMENT. 64x color change from covel.lite•blue to chalcocite-grayoccurs by gradationthrough steely-blue. The possibleimport of this suc- cession of colors which is observed also when natural covellite is touchedby iron or copperwhile immersedin a cupric'sulphate solutionhas been discussed on page627 . In addition to the experimentswith bornitc, the attempt was made to changecovellite to chalcociteby treating it with a mixture of cupric and ferrous sulphate. In each of three experiments one gram of powderedcorellite was treated in sealedtubes with about 55 cm.a of solution from •vhich all air had been exhausted. In one experiment the strength of the solution with respectto copper was about .02 F., • and with respectto iron ßx7 F.; in anotherexperiment copper .02 F.; iron .oi 7 F.; while in the third experimentno cupricsulphate was used,ferrous sul- phatebeing present in .about.09 F. concentration.At the end of three monthsthe mineral showedno notablechange in color in any of the tubes. Chalcocitesimilarly treated with ferrous sulphatesolution also remainedapparently unchanged. If the reactioncontemplated by equation(I6) whenread from left to right could be established,the result would be of note- worthy importancein the theory of chalcocitedeposition.

H•O q- 5CuS q- 3CuSO4• 4Cu•S q- 4H•SO4.

Inspectionof this expressionindicates that in a solutionalready containingsulphuric acid in any considerableconcentration, corellite, rather than chalcocite,would be the sta'olemineral. In other words, a solution of rel.ativelyhigh acidity would tend to the forma,tion and persistenceof covellitewhile decreasingacidity would favor depositionof chalcocite.Thus, ,theoreticallycon- sidered, it follows also that where silicate or carbonate minerals are present.to take up free sulphuricacid, chalcocitewould be the more stableof the two sulphides,a conclusionwhich is in har- mony with the yery considerableaggregate loss of basesduring the alterationof pyrite porphyry through chalcociteore to the leachedand fully oxidizedcapping, and also with the observed alkaline reaction of the Brooks mine water. x Here the symbolF. is used to indicate formula weight. 642 •IRTHUR C. SPENCER.

VOLUME RELATIONS IN CHALCOCITE DEPOSITION. The repl,acement of pyrite by chalcocite,if consideredas taking place accordingto equation (IO), would involve a material in- creasein volume,w'hereas the correspondingreplacement of chalcopyrite by chalcociteaccording to equation (I3) would occur with a slight decreasein volume. Definite figurescan not, of course,be given to representthese theoretical changes in volume but if the limits ,arecalculated • the figuresfor pyrite to chalcocite show expansion ranging between 54 and 75 per cent., while those for chalcopyriteto chalcoci.teshow contractionranging from 6 to 15 per cent. From this it would appear that the replacementof chalcopyriteby chalcocitewould take place more read.ily than the replacementof pyrite by ch,alcoci'teif cupricsulphate is the only reagentin the solutionto be considered. That chalcopyriteis actually more susceptiblethan pyrite to replacement by chalcociteis strikingly illustrated by relations to be noted in specimensof poryphry ore from the Ely mines. De- tailed examinationof polishedsurfaces and of thin 'slicesshows that as a rule most of the secondarychalcocite has formed on chalcopyrite. In certain specimen.scontaining no visiblechalco- pyrite the pyrite has,been deeply changed to chalcocite,but where both .of .theprimary sulphidesare presentand where the chalcopyrite grains are deeply coated with chalcocite,the neighboring grainsof pyri.teare eithercovered 'by very thin filmso3 have not been coated at all. Similar relations of sensitiveness to chemical attack are exhibi,ted under the following experimentalconditions: If fragmentsof chalcop.yriteand of pyrite are touchedsimultane- ously by piecesof iron while immersedin a solution of cupric sulphate,a very noteworthyeffect is immediatelyapparent on the surf,aceof the chalcopyritebut the pyrite at first seemsto be inert. It is possibleby fhis procedureto form on chalcopyritea brilliant blue coating,presumably covellite, and to withdraw the iron pieces•vhile the pyrite is still .brightand fresh. From the volume relations which have been indicated and the x In these calculationsthe figures used for specific weight are: pyrite, 4.83-5.2; chalcopyrite, 4. I-4.2; chalcocite, 5.5-5.8. CHALCOCITE ENRICHMENT. 643 observedscanty replacement of pyrite •bychalcocite in the porphyry ores,the writer is inclinedto the belief that in generalthe replacementof pyrite'by chalcocite does not take placeunless the activesolution contains, in additionto cupricsulphate, some other substancecapable of attackingeither the pyriteor the chalcocite. Many observationsindicate that metasomaticreplacement of one mineralby anotherusually takes place in sucha way thatthe, new mineral occupiesthe samespace as the old mineral. This rela- tionhas been convincingly set forth by Lindgren,• andby Bastin.•' With referenceto secondarychalcocite, Ransome s makes the following statement:

"In theMiami-Inspiration ore bodies of the Globedistrict, many of the specksof chalcociteare solidly embeddedin the silicifiedschist and in veinletsof quartz,so that theyhave the appearanceof beingproducts of the earliestperiod of mineralization. Yet the evidencehere is conclusive thatthese specks of chalcocitehave resulted from the alteration of pyrite and chalcopyrite,and probably in mostcases exactly fill the spaceonce occupiedby the presentmineral. All stagesmay be seenfrom pyrite thinly coatedwith chalcociteto solid chalcocite."

If the relationof equalvolume holds as it is believedto hold in the replacementof pyrite by chalcociteit is obviousthat not all the changesinvolved are summarizedby the Stokesequation. In replacementby equalvolume the productcontains less chal- cocitethan would correspondmolecularly with the pyrite that has been destroyed,and it seemsthat .the removal of the excess material must be accounted for as a result of the attack of somereagent other than cupric sulphate. It is possiblethat chalcocitereplacement 'of pyriteis promot.edby the presenceof a minor concentrationof ferric sulphatein the active solution. It is difficultto obtain a visibledeposit of coppersulphide on pyrite or chalcopyriteby treatmentwith a solutionof cupricsulphate •Lindgren,W.; "The Natureof Replacement,"EcoN. GEor•., Vol. 7, PP. 5:•x-535, x9•:•. • Bastin,E. S., "Metasomatismin SulphideEnrichment," EcoN. G•-or•., Vol. 8, pp. 5•-63, •9•3. a Ransome,F. L., "Criteria of DownwardSulphide Enrichment," Ecoa. GEot..,Vol. 5, P. 2x8, x9xo. 644 ARTHUR C. SPENCER.

alone,yet 'bluecoatings may be readily formedon ei,thermineral if ferroussulphate i,s added. This fact may havea directbearing on the problemunder consideration. The reactionis accelerated by heating,but it takesplace within a few hoursat ordinarytem- perature. During the progressof the reaction insolubleiron compoundsare precipitatedand after sulphidedeposition has takeI•p'l'ace the solutionresponds to the test for ferric iron with potassiumsulphocyanide. As already not.ed(p. 64o) ferric sulphateis formedby the interactionof ferrousand cupricsulphates in solution,so that obviouslysecondary corellite and chalcocite are .bothformed in the presenceof at leastminute concentrations of ferric sulph'ate.

COMPOSITION OF MINE WATER.

Chemical analysesof samplesof water from the Ruth and Br,ooks porphyry mines are givenin the subjoinedtable'

Composition of Waters from Porphyry Mines. (Parts per million.) I. II. Na ...... 35 33 K ...... i x 27.6 Ca ...... i26 lO5.7 Mg ...... trace 26.3 Fe" ...... I9 36.4 Fe'" ...... none 42.0 SO, ...... 256 556.0 CI ...... 8.5 55.4 CO• ...... 38• HCO8 ...... 117x sio ...... 28 Total ...... 638.5 976.4 Reaction ...... alkaline not stated

I. Water from Brooks inclined shaft of the Giroux Consoli- dated Company collectedin September,I9IO. The preceding summerhad beendry and the water stood about 23o feet below x Total CO8 distributed between CO• and HCO• in proportions to balance the reacting values of bases and acids. As reported, the analysis shows HCOa I56 parts per million. CHALCOCITE ENRICHMENT. 645 the surface. A test was made for copper with negative result. Analysis made in the laboratory of the GeologicalSurvey by Dr. Ch'asePalmer. Ferrous iron was determined immediately after taking the sampleto be 22 parts per million s'otha't .the iron was all in the ferrous state. For this determination the w,riter is indebtedto Mr. A. J. Sale. II. Water from Ruth mine. Compositioncalculated to parts per million of ions, from analysisby Dr. Harry East Miller, recordedby A. C. Lawson? The analysisof the Ruth mine water was originally statedin the following form: Analysis of Ruth Mine Water Grains per Gallon of 232rCu. In. SiO, q- insoluble matter ...... 5.50 NaC1 ...... 4.88 KC1 ...... ' ...... 55 K•SO, ...... 2.96 FeSO, ...... 5.77 Fe•(SO,)3 ...... 8.78 CaSO ...... 20.99 MgSO ...... 7.6t Organic matter and water of sulphates ...... 6.96 Total ...... 64.oo

In regardto this analysisProfessor Lawson a says:

"It is probablethat the proportionof ferric sulphateis higher than is actuallythe casein the mine, owing to the difficultyof preventingoxida- tion of the ferrous salt. The result is interesting as an indication of the materialswhich are beingleached from the porphyryby the descend- ing meteoricwaters. Another sampleof the mine water analyzedin the mine laboratory as expeditiouslyas possibleafter collection,by Mr. Herbert Ross,gave resultsfrom which it .wouldappear that the iron of the mine water is nearly all in the ferrous state."

The form in whichthe Ruth analysisis stateddoes not enable classificationof the water, but the Brooksanalysis indicates the followingch'aracteristics :: x Lawson,A. C., "The CopperDeposits of the RobinsonMining District, Nevada,"Pub. Bull. Univ. of Cal.,Dept. of Geology,Vol. 4, p. 332,z9o6. • Palmer,Chase, "The GeochemicalInterpretation of Water Analysis,'i Bull. U.S. Geol. Survey No. 475, P. z3, z9zz. 646 ARTHUR C. SPENCER.

Per cent. Primary salinity ...... eo.6 Secondary salinity ...... 42.8 Primary alkalinity ...... oo.o Secondary alkalinity ...... •'9.o Sub alkalinity ...... 7.6

It sh,ould be notedthat the mine'waters that havebeen analyzed are in a way the complementof the pyri'teporphyry for they contain in solution 'the bases which are known to have been leached out of the rock during the processesof enrichment and of ore decomposition.The high proportionof lime in both of the mine waters indica. tes that the mines have received contributions of water through percolationfrom the limestonemasses which ad-. join the ore ma,sses,:because the porphyryores are essentiallyfree from lime. The indicatedabsence of magnesiumin the Brooks mine water appearsto ,beanomalous for the analysesof samples indicatethat the ore porphyry of the district carriesmore of this element than it does of 1.ime. The alkaline reaction of the Brooks mine water bears out the suggestionthat has been made on another page that the waters from which chalcocitehas 'been depositedwere proba'blynot strongly acid.

CHEMICAL CHARACTER OF SOLUTIONS WHICH DEPOSIT

CHALCOCITE. Among the productsof the oxidation of pyrite is sulphuric acid so that the solutions•vhich carry copperfrom the placesof decompositionto those,of chalcocitedeposition must be distinctly acid when they are first formed, and it is well known that waters collectedfrom the upper levels of mines in pyritic ore bodies usually show an acid reaction. However, carbonateand many silicategangue minerals are decomposedby acidsand when such minerals are long in contactwith the descendingsolutions their basesare taken into combinationand the original acidity of the solution gradually disappears.Attention has been recently directedto the fact that the waters in certain coppermines are CH.4LCOCITE ENRICHMENT. 647 lessand lessacid at lower and lower levels/ and are alkaline in someof the deepestworkings, and therecan be little doubtthat thisstate of affairsis general. A sampleof watercollected from a pointnear the top of thewater table in oneof theEly porphyry mineswas found to bedistinctly alkaline (see p. 644). Although theseobservations have led ,to the suggestionthat a reductionof acidityin descendingmetalliferous solutions may be importantin connectionwith secondarysulphide deposition 2 it is not obvious, merelybecause solutions in a givencase have •become alkaline as •a resultof reactionwith gangue minerals, that the conclusion mustfollow that no part of the dissolvedcopper could have been deposited'before the alkalinestate 'had been reached. The oppo- site.contention that aciditymay be necessaryfor the progressof coppersulphide enrichment is suggestedby Welsh8 and Stewart who foundby experimentthat, while chalcopyritemay be coated with films resembling.bornire by treatmentwith cupriosulphate solution,the presenceof calcite preventsthe reaction. These writers state that, "The ordinarycopper sulphate is acidand the effectof the calcitemay be to neutralize this acidity." The presentwriter hasobserved that artificialcoatings may be readily formed on chalcopyrkein solutionspossessing only the slightacidity due to thepresence of cupricand ferroussulphates, but that when a little sulphuricacid is added the mineral remains untarnished. Also Winchell 4 states that, while he obtained chalcocitein the presenceof SOs, wi,thoutthis reducing agent pyrite was not visiblyattacked by acidifiedcupric sulphate at the end of two years. From the foregoingobservations it appearslikely that under natural .conditionschalcocite will not replaceprimary sulphides • Emmons, W. H., "The Enrichment of Sulphide Ores," Bull. U.S. GeoL Survey No. 529, pp. 6o and 89, z913. : Wells, R. C., discussionof a paper by F. L. Ransome,on "The Criteria of Downward Enrichment," Ecoa. GEo•..,Vol. 5, P. 482, z9zo; Emmons, W. H., loc. cit., p. 60. s Welsh, T. W. B., and Stewart, C. A., "Note on the Effect of Calcite Gangueon the SecondaryEnrichment of Copper Veins," Ecoa. GEo•..,Vol. 7, pp. 785-787, •9•2. 4 0p. cit., p. 274. 648 ARTHUR C. SPENCER. when the acidity o,f the cupriferoussolutions exceeds some rather small minimum, but that an alkaline condition is not n•cessary in order that •rhereaction may proceed.

CIIALCOCITE DEPOSITION IN TtIE PRESENCE OF CALCITE. A't severalplaces in the F•]y district chalcocitehas been noted {n the form of secondarycoatings on pyr{teand cha]copyr{ted{s- semin.atedthrough massesof .partly weatheredmetamorphosed limesgonelying beneatha surfic{alshell of fully oxidizedmaterial. Considerable bodies of enriched material {n altered limestone masseshave been developedin the Old Glory mine where the originalmatrix of theprimary sulph{des,though consisting largely of lime silicateminerals, also comprisesconsiderable calcite. In material of this s,ortthe depositionof chalcocitewould appear to be uncommonaccording to D.C. Bard• who states,as a result of his observations,that sulphideenrichment {s usually lacking where the ganguecomprises much calcite. The explanationoffered {s that coppersulphate reacts with calciteto form coppercarbonate, so that only an excess,of the coppersalt abovethat requiredfor replacementof calcitecan finally appearas covell{teor chalcocite. Welsh and Stewart2 treated cha]copyritein the presenceof calcite, and also cha]copyritemixed with quartz, with moving solutionsof cupric•ulphate in uprighttubes so arrangedthat the lower part of the column•vas .continuallyflooded. These writers state that,

"In the ca]cite chalcopyritemixture the calcite abovethe water level was distinctlytinged with green, while none of the chalcopyriteshowed any change. In the tube containing quartz and chalcopyriteno change above the water level was noted, but the chalcopyritefor four inches below this level showedclearly the purple tinge of born{te, and upon closer examination some thin black films were found."

In this experimen'tthe solution draining from the calc{te- chalcopyrite mixture still contained cupric sulphate, and the authorssuggest that secondarysulphide was not precipitated • Bard, D.C., "Absence of Secondary Copper-sulphideEnrichment in Cal- cite Gangue," EcoN. GEOL.,¾O]. $, pp. $9-6I, • Op. cit.

/ CHALCOCITE ENRICHMENT. 649

causethe solutionmay have *beenrendered alkaline through reaction with the calcite. Whether the explanationoffered is the true oneor not,'on its face the experimentbears out the sugges- tion presentedby Bard that the presenceof calciteis unfavorable for the depositionof secondarycopper sulphides. However, the presentwriter has found that calcite doesnot precipitatecopper from a solutioncontaining both cupric and ferroussulphates, and, furthermore,that by meansof sucha solutionpyrite and chalcopyrite may be readilycoated with secondarysulphide films in the presenceof large amountsof calcite. Becausespecimens of the chalcocite-bearingore from the Old Glory mine now contain no calcite, the possibilityremains that here the secondarysulphide was not actually depositedin the presenceof calcitebut only in material from which calcitehad previouslybeen 'removed as the result of solutionby sulphuric acid. Observations more detailed than those that have been made would be requiredto determinewhether the last of the original calcitedisappeared before or after chalcociteenrichment occurred.

WATER TABLE RELATIONS DURING CI-IALCOCITE DEPOSITION. The positionof standingwater in t'heporphyry mines of the Ely dis,tr{ctvaries from timeto time. No systematicrecord of rise and fall {s at hand,but in the auntutahof I9Io the water in some of lhe mineswas about 3 o feethigher than in the autumnof I9o9. In generalthe chalcocite porphyry, though lying mainly above the water table, continuesdownward below the highestposition of standingwater, and possiblybelow t'he levels to which the water top may sink at the lowest stage. It seemsprobable, however, that no porphyrycontaining so much as o. 5 per cent.of secondary copperoccurs at lower levelsthan thosewhere the top of ground water may have stoodduring the dry epochsof the Quaternary period. Beforethe first expansionof the Great Basinlakes and in the intervalbetween the two expansionsthe climatewas prob- ablysomewhat drier than it hasb•en during the present epoch, and the meanposition of standingwater may havebeen for con- siderableperiods from 5o to Ioo feet lower than it is now. In the Alpha mine, wherethe ore bodiesare not of the disseminated 650 ARTHUR C. SPENCER. type but are rather of the nature of compactsegregations, the water tableis now fully 200 feet higherthan it hasbeen formerly. Here oxidation has penetratedmore than x,200 feet below the surface,while the naturallevel of standingwater is at a depthof about x,020 feet. On the other hand, during thoseepochs when the Great Basinlakes were in flood stage,in the porphyryore massesthe water level certainly stood much above its present mean position. Deposits of iron-coppersinter that occur near someof the mines furnish good evidencethat the chalcociteore was onceat least submergedto a height so great that part of the productsof its oxidationescaped into the surfacedrainage. This event is correlatedon physiographicevidence with the first Qua- ternary humid epoch,when rather steadystreams were undoubtedly flowing in the district, and as the next later humid epoch seemsto have beenwetter than the first, the ore 'bodiesmay have been submergeda secondtime. If the later flood stage of the groundwaterwas higher than the earlier, the top of the ore body would have been below water level, a suppositionwhich would adequatelyaccount for the lack of sinter depositsin the swales which were erodedduring the secondhumid epoch. Presumably similar alternationsof low and high water were characteristic of late Pliocenetime when the principalsegregation of chalcocite is believed t.o have occurred. It wouldseem that a considerablebody of sulphideslying above the water ta.blewould offer the best possibleconditions for the retention of copper taken into solution by oxidizing surface waters. This suggestionis *basedin part upon the conception that the speedof reactionbetween copper solutions and primary sulphidesis slow under the most favorable conditions,and pre- sumablyextremely slow after grains o.f pyrite and chalcopyrite have receivedmoderately deep films .of chalcocite. A long journey involvingcontact with a very large numberof primary sul- phidegrains would 'bemore favorableto precipitationof a larger proportionof the dissolvedcopper than a short journey during which correspondinglyfew grainswould be encounteredby any unit volumeof solution. A low water table and a high position of the ore top would tend to producea thick ore body of rather CHALCOCITE ENRICHMENT. 65 t

uniform grade from near its top to somewherenear its bottom. So long as the s•lutions could be exhaustedabove standing water, the.enrichedma.terial would grade imperceptibly into the primary material. On the other hand, if copperwere not exhausted above standing water, precipitationshould continue within a relativelythin shell of material just below the water table,because the soluti,onsin this situationwould necessarily movelaterally rather than downward,and their flow or passage wouldbe at a diminishedrate, permitting much longer contact with reactingprimary sulphidesin a limited massof material. Duringsuch times as the groundwater may have stood well up in material previouslyenriched, .the tendencywould have beento producelayers of ore carryingmore than the averageamount of copper. Wha,t is possiblythe recordof a former high stageof groundwateris seenin the localoccurrence of particularlyrich layersof orebeneath from 2o to 5ø feet of medium-grademate- rial lyingjust underthe 'oxidizedcapping.

DATES OF CI-IALCOCITE ENRICHMENT. In forego{rigsections which deal with the chemistry of c•halco- citeenrichment, the idea has been developed that the porphyry ore bodiesof theEly districtare currently subject to wastagein their upperportions, and to accretionof copperin theirlower portions. A similardownward transfer of copperhas undoubtedly been goingon in thepast, and it is obvioust'hat the present ore bodies are the successorsof similar segregations which formerly ex- istedabove the placeswhere the enrichedmaterial is now found. The rocksalong the metamorphosedzone were undoubtedly chargedwith pyri.te and chalcopyritefor severalhundred, and perhapsfor severalt'housand, feet above the present surface, and the earliestformation of disseminated.chalcocite ores may be thoughtof as havingoccurred when, as a resultof erosion,the massesof pyritizedrock firsl• came under the action of oxidizing solutions.This conception would carry the operations of chalcociteenrichment a long' way back into the past. It is heldthat the volcanic rocks, of probablePliocene age, which•nowoccur from place to placewithin the district formerly 652 ARTHUR C. SPENCER.

extendedover much.wider areas, and probablycovered all of the ore belt. But the pyritized rocks had been •hdergoing erosion for a long time before the period of volcanicextravas. ation and there can be no reasonabledoubt that the processesof chalcocite enrichment'had beengoing on hand in hand with the gradual loweringof the land'surface. However, it is not safe to conclude that ore bodiescomparable with those of the presentexisted at any particulartime, and it is not possibleto state whetheror not thick masses of disseminated chalcocite ore existed when the rhyolite lavas flowed out upon the old land surface. The processesof weathering and chalcocitedeposition must havebeen in abeyanceduring a long perioddevoted to the erosion of the rhyolite, but when the rocksof the metamorphosedzone had beenpartially strippedthese processes again came into play and have since been operative except for such interruptions as resulted when the ore •bodiesmay have been overtoppedby groundwater. On physiographicgrounds it seemscertain that there has been practicallyno erosionof the groundabove the ore bodiessince the -'•beginningof the pre-Bouneville epoch (the first division of Quaternary time) during which great alluvial coneswere .built up throughout the region a'boutthe mouths 'of lateral canyons that open out into the broad north-southvalleys. All of the secondarycopper contained in the presentore bodieshad therefore been segregatedbelow the now existing surface before the close of the Pli'ocene,and the fact that thick massesof enriched material exist is ,dueto conditionsduring the latter part of Plio- cene time that were prevailingly favorable for the precipitation and accumulationof copperin the form of chalcocite. While essentiallyall of the secondaryor added copper now presentin the disseminatedore bodieshad beenbrought together before the beginningof Quaternarytime, it seemsalmost necessary to •believethat redistribution of the metal has:since been goingon with the resultthat the top• of the orebodies-have been gradually falling t'o lower and lower positions. No 'basishas' beenfound, however, for determiningwhat portion Of th• dipth of oxidizedcapping is attributableto Quaternarydecomposition.