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Chalcocite Enrichment2 ECONOMIC GEOLOGY 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 secondarychalco- cite 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 chalco- cite, 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 ortho- clase. 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 solu- tion. 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 sul- phideminerals ensues, a seriesof reactionsbeing initiated of which seriesthe culminatingreactions involve the depositionof chalcocite. At first the waters contain free sulphuric acid fur- nishedby 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 reac- tions 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; some- what 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 sec- ondary 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 sul- phates, 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.ecan 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
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