McKinstry and Noble-1932

8/12/2019 McKinstry and Noble-1932

1/22

ECONOMIC GEOLOGYVot.. XXVII SEPTEMBER, x932 No. 6

THE VEINS OF CASAPALCA, PERU.H. E. McKINSTRY AND J. A. NOBLE.

INTRODUCTION

IN connection ith the present nterest n zonal arrangement fmineralization, the CasapalcaDistrict affords unusual oppor-tunities for studyingan instructiveexample. Here a singleveinsystemhas been openedup through a remarkablevertical andhorizontal range. The structure of the country-rockand veinshas beenworked out and the sequence f mineralizationstudied.Developmentwork was followed by one or the other of thewriters duringa periodof ten years,so that there hasbeenampleopportunity o check heoriesagainstactual results..4cknowledgments.--Thewriters are greatly indebted o Pro-fessors . C. Graton and D. H. McLaughlin for their enthusiasticinterest,and for suggestingmany of the ideas herepresented; oMessrs.J. D. Campbelland 'C. S. T. Farish, Superintendent ndAssistant Superintendent t Casapalca, or facilities during thecourse of the .work; and to the officers of the Cerro de PascoCopperCorporation or permissiono publish his paper.Location.---Casapalcas situatedhigh in the central PeruvianAndes, just below the ContinentalDivide. The camp site andrailway station ie in the bottomof the canyon-like alley of theRimac River, a streamwhich at this point flows southward,butlower in its courseswings o the westand, passing hroughLima,enters he Pacificat the port of Callao.East of the to,wn of Casapalca,Carlos Francisco,one of thepeaks of the Western Cordillera, rises to an altitude of I7,OOO5oi


2/22

,502 H. E. MCKINSTRY AND J'. A. NOBLE.feet, and it is in this mountain that the Carlos Francisco Minehas beenopened. The name of one other mine needs o be bornein mind--the Aguas Calientes, so-calledbecause lows of hotwater have been encounteredn the workings. It is a mile and aquarter southof Carlos Franciscoon a continuation f the samegeneralvein system. From a point lower down in the valley atunnelthree miles ong cuts the vein system,which can be fol-lowedupward hroughconnected orkings or a .verticaldistanceof 3,500 feet to the outcropust below he summitof the peak.

GENERAL GEOLOGY.xA thick seriesof redbedss overlainby deep lows of extrusiveporphyry, he wholecompressedy Incaic folding alongnorth-7'-- c % .

Fro. x. Map of CentralPart of Casapalca istrict. C.M., CarmenMem-ber; T.V., TablachacaVoltanits; V.A., Veintiuno Andesite.x For a descriptionof the geolo of CtrM Peru and reference o earlier works,see: Meughlin, D. H.: BI. ol. S. Amen, vol 35, PP. 59x432, x924;Informaclones y Mmori de la Soe. de Ing. del Peru, vol. XXVII, Feb.,Steam,n, G.: Geologic won Peru, Carl Winters, Universitts Buehhandlung,

Heidelberg, x925, 448 Pp.z The post-Cretaceousrogeniemovementwhich accomplishedhe first phasethe development f the Andes is reined Andean folding by Steinmn (op.The secondphase (early Tertian) he terms "Ineaie folding."


3/22

THE VEINS OF CASAPALCA,PERU. 503west-strikingxes,ntrudedy magma f intermediateomp.osi-tion, utbyoreveins,nd inally xposedyerosion. Fig. x.)

The geologic olumn s shown n Table I.TABLE I.

GIOLOGIC COLUIN.SedOnentarlyand Extrusive Rocks.

tv. ET--Hot SpringsDeposits,Glacial Deposits.Qua?EtA--Glacial Deposits.?.?XAaV ( ?)--

(Unconforrnity.)Rio Blanco Forrnatwr.aRio Blanco Voltanits.Tapachuarmi Tuff.BellaVistaBedsthin imestonendshalenterbeddedithvoltanits).Yauliyacu Tuff.Carlos FranciscoPorphyry.Tablachaca Voleanies.

( Unconformity.)Rimac Formation.Carmen emberconglomerate,andstone,hale, nd imestone).Amygdaloidal lows (local).Casapalcaedbedsshale, andstone,ndsomeimestone).

( Unconformit.ctx?,covs--Machay Limestone.

Intrusive Rocks. -?x,--Tarucaorphyry. ictoria orphyryndVeintiunondesitcsills rob-ably related o Taruca Porphyry).

The history f sedimentationndvulcanismaybe sum-marized briefly as follows:A long periodof probablyerrestrial edimentationwas ol-lowed ya prolongederiod f volcanicxtrusion. receding Steinmann'sRimac ormation"ncludeshe redbedsndoverlyingoltanitso Western eru. McLaughlinassuggestedeparatinghe voltanits nder hename"Rio BlancoFormation" ecausef an unconformity. teinmannonsiderstheRimac ormation,ncludinghevoltahies,pper retaceousr EarlyTertiary.(Review f G. Steinmann'sGeologieonPeru"by D. H. McLaughlin,coa.'Gr. I..,vol.24, p. 667, 929.) The present riters re responsibleor the namesof the local subdivisions f the two formations.


4/22

504 H. E. MCKINSTRY AND J. .4. NOBLE.the volcanicactivity, erosionwas rejuvenatedand producedaseriesof pebbleand boulderconglomeratesCarmen Member).The vulcanismopenedwith eruptionsof ash and thick lava flowsthat were mingled or interbeddedwith coarsesediments Tabla-chaca Volcanics). Finally andesitic flows becamepredominantbut were interrupted at one stage by limestone sedimentationaccompanied y eruptionsof volcanicash.Since the principal ore depositsoccur in the redbedsand thelower portionof the volcanics,we will describe nly the RimacFormation, the Tablachaca Volcanics, and the Carlos FranciscoPorphyry.

Sedimentaryand Extrusive Rocks.CasapalcaRedbeds.--The lowest formation exposed n theimmediatedistrict consists f red sandstone nd shale; 3,0o0 feetare known to be exposed. Toward the top, coarse imestonebedsbecomemore and more abundantand thereappears horizonvarying in thicknessrom x5 or less o 90o feet (Carmen Mem-ber) in which lensesand beds of limestoneand conglomeratepredominate. A few small ensesof amygdaloidnear the bottomof the Carmen Member mark the beginningof vulcanism.Tablachaca Volcanics.--Transitional in character between theconglomerate f the .CarmenMember (on which it lies uncon-formably) and the overlying thick series of volcanics s a se-quenceof flows and pyroclasticswhich includesa great varietyof material; porphyry, andesite uff, breccia, agglomerate,con-glomerate,sandstone, uartzite, and limestone, he volcanicrockspredominating. Someof the agglomerate edsare most striking,as the "pebbles" range tip to a foot or more in diameter. Thethicknessof this formation, which is about 200 feet in the north,increases apidly southward o a measured2,30o feet.CarlosFra.ncisco orphyry.rePassing pward in the extrusiveseries, the flows of red porphyry common in the TablachacaVolcanicsbecomeessabundant, onglomeratesisappear, nd thepredominant ypes are massiveporphyry and porphyry breccia,which consists f angular fragmentsof porphyry,usuallygreen-ish, in a matrix of reddish porphyry. The massiveporphyry


5/22

THE VEINS OF CASAPALCA, PERU. 505beds regray o greenish,ocally earing ornblendehenocrysts.Microscopic xamination howsautomorphic henocrysts, mm.to 4 mm. long, of feldspar anging n compositionrom albitethrough ligoclaseo andesine.The feldspars highlysericitizedeven where remote from veins. The ferro-magnesians re usu-ally alteredbeyond ecognitionnd are now representednly byblotches f chlorite,calcite,and residualmagnetite. The ground-mass s mainly very fine-grained ut contains ome feldsp/trsupto o.x mm. in length n trachytic exture,with opaquenterstitialmaterial (limonite?).Some of the porphyry may be intrusive. The thicknessofthe formation' where measured s about x,3oo feet.

Intrusive Rocks._All the intrusive rocks of the district are similar chemically,but differ in texture and amount of alteration. All are inter-

mediate n composition nd characterizedby high soda content.As would be expected, he coarser-grained nd more acid typesoccur n stocks nd laccoliths, nd the finer and more basic ypesare in dikes and sills.Taruca Porphyry.--Particularly in the northern and easternpart of this.district there are stocks,sills, and dikes of grayislgreen medium-grained orphyry, consisting f blocky white togreenish feldspars and elongated hornblende aths in a graygroundmass.Quartz is rare.Thin sectionsshow a trachytic groundmass 60 per cent. ofthe volume) which consistsof somewhat sericitized feldsparmicrolites (oligoclase-albite) with small grains and shreds ofhornblendesparingly distributed. There is some magnetite insmall rounded to euhedral grains.Of the phenocrysts, eldspar is about twice as abundant ashornblende. Quartz is minor in amount, usually under 5 percent. The feldspar s oligoclase-albitewithcoarse lbite winningand a slight tendency o zonal structure. The phenocrysts resomewhat elongated and are partially sericitized. The horn-blende s greenishyellow, showing airly strongpleochroismr


6/22

506 H. E. MCKINSTRY AND J. A, NOBLE.olive green tones. The crystalsare decidedlyelongatedandsome are euhedral. Small apatite crystalsare included n thehornblende.

The rock, therefore, consists ssentially f albite-oligoclaseand hornblende, .e. acid plagioclase nd amphibolewith minorquartz. This might be calledan albite-dioriteor a soda-syenite,depending n the rock classificationsed.Veintiuno 4ndesite.--A number of sills of dark fine-grainedintrusive are found in the redbeds. Stocks of similar material(mappedas Victoria Porphyry) are intruded nto the Casapalcaformation n the northwestern ortion of the area.

Structure.The rocksof the district were nvolved n the last great o.rogenicmovement Incaic) which affected he Andesand which expresseditself in folding along northwest axes (Fig. 2). Typical ofthis regional structureare overturned olds which pass alongtheir strike into thrust faults. Locally a broad anticlinearchesoverthe RimacValley and passes astwardnto a syncline nder

Carlos Franciscopeak. Between he anticlineand the synclinethe dipsare steeply astward nd in placesoverturned.

..'"" '"x..'"'.."'"."'""'..?'. ', '"\1" ..'v ..k, ". '".'. ..'

Fro. 2. Section across CasapalcaDistrict looking northeast. bvl,Bella Vista Limestone; tv, TablachacaVolcanics; cc, Carmen Member;cfp, CarlosFranciscoporphyry; p, Taruca porphyry.

ORE DEPOSITS.Vein Str,wture.

The mainvein system xtends orthand south or 'more' c-curately . 40 E.). Branchingrom the main (CarlosFran-cisco-Aguas alientes)system rom pointsbetweenhe two


7/22

THE VEINS OF CASAPALCA, PERU. 5o7mines, wo other veins make off to the eastward--the Bella Unionand the Carmen Fig. I). Toward the hanging-wall, r west-ward of the main zoneand moreor lessparallel o it, is anotherless mportant ein (Rayo). All the veinshavegeneral orth-west dips.'Although fewcross-faultsave eceived inormineraliza-tion, the major faults are unmineralized. The veins cut at anacute angle across he northwest-trendingegionalstructure.passingrom the redbedsnto the overlying xtrusive orphyry.Although hey are productiven both formations,here s somedifferencen theirphysical ature n the differing ypesof wall-rock.In fracturing, he porphyry as evidently ehaved s a brittlerockand the veins n it are intricateand brandhing, ut the redshale asyieldedo plastic eformation,nd hrough-goingrac-tureshave formedonly 'where he stress as beenparticularlystrong. In fact, the only importantvein that extends ar intothis formations alonga fault fissure f some o0 feetdisplace-ment. One outlyingvein (the Carmen), which within a shortdistancepasseshrough three different formations, llustratesparticularlywell he physical ffects f wall-rock; n the CarmenMember,whosehard conglomerateedsappear o have beenparticularly asilybroken, t is widest;on passingnto shale heveinspinch o narrowslips;on enteringhe porphyryheysplitinto intricately ranching tringers,ich but narrow.Depthalsoappearso influencehe ypeof fracturing; heveinsare more regular on the lower levelsand more complicated earthe surface.

In the porphyryof the Carlos FranciscoMine, the veins liein en echelon r overlappingelation o eachother (Figs. 3 and4), locally termed "shingle structure." Where the hanging-wall "shingle" diesout downward,t is usually onnectediththe footwallveinby a verticalvein,whichmay be wide and rich.In some cases he "shingles" are "reversed ; that is, thefootwallvein is the one that dies out, and in this case here isusuallyno strong connecting ein. This type of structure s


8/22

508 H. E. MCKINSTRY AND J. A. NOBLE.shownnot only in the broad relationsof the large veins, whichmay be severalhundred eet apart, but also n the minor branchesof the individual veins. It is seen n plan as well as in section;where the offset s to the left, there s usuallya northward-strik-ing connectingein betweenhe t,wo ortheasteins;where heoffset s to the right, there may be no importantconnection.

//

$00T/' I00EET

/

?

//

//

Fro. 3. Vertical cross-sectionf Main Vein System, ookingnorth; toshow overlappingof veins and vertical connectingbranches.Fro. 4. "Shingle Structure" reversed rom that shown n Fig. 3, con-nectingvein lacking. Southernpart of Carlos FranciscoMine.Near the junctionsof thesesplits he veinsare particularlywide. Vertical portionsof the veinsare commonly trong,butportions latter han 50 are mostly oo narrow to mine.There hasbeen ittle post-veinmovement.The only fault ofany consequences one n the Carlos FranciscoMine which offsetsthe vein a maximum of about oo feet.


9/22

THE VEINS OF CASAPALCA, PERU. 509Typesof Vein Filling.

The vein-matter consistsmainly of quartz-carbonate anguewith pyrite, sphalerite,galena, and tetrahedrite,but the propor-tions and textures of these minerals are far from uniform anda numberof intergrading ypesof mineralization ccur. _Aveinmay changealong the strike from one type to another, or twoor three ypesmay occur n streaks ideby side n the samevein.. Pyrite, with or without quartz gangue. The pyrite isusuallymassivebut may contain rugs and where crystallizedgenerallyoccurs n pyritohedrons.The walls of this type of vein are for the most part not welldefined,as vein-matter eplaceswall-rock n grains and veinletsalongthe margins. The pyrite may be accompaniedy a whitequartz gangue, which often shows comb structure n elongatedvugs. A little chalcopyritemay be present.2. Coarse-grainedyrite-sphalerite-galenaith little gangueorwith clear quartz. Where vugs occur, they are likely to beelongated nd showcombstructureof quartz. This type of veinis likely to be "tight," however.3. Fine to medium-grained phalerite-tetrahedrite-galenaein,with or without pyrite. This type may be entirely withoutgangue, n which case t is commonlyconfinedbetweengouge-walls and is narrow. Quartz, however, s usually presentassmall crystals in vugs. Crystals of sphalerite are common,galena, esscommon, nd tetrahedrite, are. Somevugs may belined with bournonitecrystals.4. Sphalerite-tetrahedrite-galenapyrite) in carbonate angue.The carbonate s white cleavablecalcite, manganiferouscalciteand rhodochrosite. Quartz may be present n minor amounts.5. Fine-grainedgalena-sphaleritepyrite). This type of veinis "tight," i.e. free from vugs,and replaceswall-rock n stringers.6. A type entirelydistinct rom the others,and probablyofa later stage. It consists f botryoidalcalciteand gray chertyquartz showingtiny crystal vugs. Stibnite and realgar com-monly occur.Occurrencef Types.--Except or type 6, noneof these arie-


10/22

5xo H. E. MCKINSTRY AND 3'. A. NOBLE.ties of vein-mattercut one another. They may exist side byside in the same vein with no distinct line of demarcation betweenthem. No one type has a consistent reference or. the hangingwall or footwall. Someof the typesare characteristic f certainportionsof the district, as will later be pointedout under ZonalArrangement.Micro-texture and Mineral Sequence.--Broad y peaking, heorder of deposition f mineralswas as follows: Gangueminerals(quartz and carbonates), yrite, sphalerite, alena, etrahedrite.bournonite. There is, in places, late generation f quartz,andquitecommonly late tufting of .calcite pon he sulphides.

Calcite, manganiferous alcite,and rhodochrosite re mostlyearlierthan quartz, whichcommonly eplaceshem in stringsofeuhedral grains. Quartz cuts the wall-rock in stringers andveins that may exhibit comb-structurend containcrystal-linedvugs. Thin sections how clusters f moderately oarsequartzgrains replacing he wall-rock.Sulphidesn the vugs may be perchedon the quartz crystalsbut commonly eplace he wall-rock behindthe quartz. Pyritein cubical rystals evelops y replacement ithin grainsof quartzand carbonate. Sphalerite orrodeshe prismsof the quartzandreplaces rains nterstitially. It rounds he .cubic yrite crystals.The dark variety of sphaleritenvariablycontainshe familiarchalcopyritelebs, ut he resin-coloredariety s free from them.Some of the galena,at least, s later than the sphalerite, l-though he boundariesre usually ounded nd non-committalas to order. In many instances, alenareplaces phaleriten"sea-and-island exture," and rare veinlets cut the blende. Inone casesphalerite orrodes nd replaces he lower part of aquartzcrystal, nd galena,whichpartially eplaceshe sphalerite,is molded round he terminalpyramidof the quartz,suggestingthat galenawas depositedn a vug, attackinghe sphalerite utnot the quartz. Galenamay traverse yritecrystals. It hasalsobeenobservedurroundingyritegrainsand sending ut veinletsinto inclosing arbonate.Tetrahedrite learly eplaces phalerite. It fills irregular rac-


11/22

THE VEINS OF CASAPALCA, PERU. 511tures which separate slands of blende from the main mass ofthemineral;t tendso seek ut heboundariesetweenphaleriteand pyrite and to form irregular veinlets n such places. Itreplaces arbonate anguedirectlyand, in the AguasCalientesore, s altered o chalcopyritelong he margins.The replacementextures f sphaleritey galena ndby tetra-hedrite differ, the tetrahedrite ending to confine tself morecloselyo fracturesndboundarieshereashegalenaxhibitsmore strongly eplacing ction, forming roundedcontacts, nd"sea-and-island texture."

Theres a lategenerationf carbonateangue hich ommonlycarries etrahedrite. n polished ection, einlets f gangue reseen o .cut he oldersulphides,ut rhombohedralrainspresentraggedboundarieso the surrotmdingetrahedrite, uggestingthat some etrahedriteamealongwith and slightly ater thanthis carbonate.The tetrahedriteor the mostpart canbe etched y KCN andthis, coupledwith its high silver content, ndicates hat it ismainly he variety reibergite.Bournonites commonly ssociated ith the tetrahedrite, ndits'characteristicositions in a bandof irregularwidth yingbetween alena nd the gray copper. Its color esembleshat ofthe tetrahedrite ut it can be distinguishedeadilyunderthepolarizingmicroscopey its beautiful winningbands. Thecommon ositionof bournonite etweengalenaand tetrahedritetogetherwith its chemicalomposition,Pb Cu.2)a b2S6,sug-gests hat both mineralsmay have contributedmaterial to itsformation.Beautiful ug liningsshowwell the relationshipsf someofthe minerals,he sequencen general heckinghat inferred rommicroscopic riteria. Some of the open cavities n the CarlosFrancisco Mine are a foot or more .wide and measure ten feet ormoreengthwise.hey re ined ithquartz,alcite,phaleriteand bournonite, ll well crystallized. Although n somecasesthe crystals f galena ndbournonitere intergrown s though

4 For further deseriptlon f mineralogy ee McKinstry,H. E.: Amer. Min., vol.12, No. -, I927 .


12/22

512 H. E. MCKINSTRY AND J. A. NOBLE.they had started out from different centersand interfered witheachother, there are also crystalsof both tetrahedriteand bour-nonite that rest upon the galena. In somevugs the early sul-phides, alenaand sphalerite, re pittedand etchedbut the tetra-hedriteand bournonite re fresh. Occasional yritohedrons fa late generation f pyrite rest on the sphalerite. Vugs from theAguas Calientesmine showing similar etching of galena andsphaleritecontain tetrahedritecrystalsentirely coatedby chal-copyrite. Bournonite n these specimenss unetchedbut bearsa dustingof minutepyrite crystals. Tiny quartz crystals est onthe coated tetrahedrite but not on the bournonite. In fact onebournonitecrystal clearly grew around one of these quartzcrystals.Thus it appears hat, followingthe main period of depositionof sphalerite, alena,and tetrahedritehere was a stageduringwhich he sphalerite nd galenawereetched nd chalcopyritendquartz were deposited n the tetrahedrite. Following this, bour-nonite and still later pyrite were deposited. This sequencen-dicateshat theetching ndchalcopyriteeposition erehypogene,a conclusionwhich is supported y the observation f corrodedsulphides n the i,9oo-foot level of the CarlosFranciscoMine,far below the natural water-table.

In portionsof the lower levels,chalcopyrites fairly commonin veinletscuttingpyrite and alongmarginsof sphalerite eins.apparently,but not certainly, replacing he blende. Elsewherechalcopyrites sparse. The blebs n sphalerite,crustson tetra-hedrite, and microscopicallyisible films outlining tetrahedritecrystalsare quantitatively nsignificant. On the upper levels(200 ft. and above) occasional halcopyrite einletscut sphal-erite, tetrahedrite, nd quartz and are accompaniedy a littlecovellite. Such textures are not known on the deeper levelsand are probablysupergene.A reconstructed equence f mineralization s as follows:Gangue Minerals: quartz, calcite, manganiferous alcite, andrhodochrosite;Pyrite; Sphalerite; Galena; Tetrahedrite (andchalcopyrite?); uartz; (Chalcopyrite);Bournonite; Pyrite);(Quartz); Calcite.


13/22

THE VEINS OF CAS'APALCA, PERU. 513The minerals included n parentheses ave been observed na few places t the indicatedposition n the successionut do not

constitutemportant stagesof deposition. The order, especiallyin the repetitionof pyrite and quartz, probablyvaried somewhatfrom place o placeand there wasconsiderableverlappingn thetime of depositionof the sulphides. The simpler and moregeneralizedsequence t the beginning of this section,however,wasquite onsistentlyolloxed.Except for a minor portion of the chalcopyrite n the upperlevelsand a trace of covellite,no secondary ulphides ave beenobserved. There are, of course, oxidized minerals at the im-mediateoutcropand along fractures n the upper levels, butsupergene rocessesave playedno part .worthyof discussion.

I/Fall-Rock AlterationWail-rocknear he veinsshows lteration onsistingssentiallyof silicification and sericitization. Vhere alteration is intense

it is practicallympossibleo distinguish etween edbeds,uffs,and porphyries ithoutmicroscopictudy. Even the conglomer-ate of the Carmenmembercannotalwaysbe recognized.In CarlosFranciscoPorphyry and Taruca Porphyry.--Thechemical nd mineralogicalimilarityof these wo rockscausesthem to alter similarly. The hornblendes the first mineralattacked nd evenremote rom veins well preserved ornblendecrystals re rare. Hornblende s replaced y chloriteand calcite.The feldspars re sericitized,hose n the groundmassuccumb-ing first. The groundmassecomesn aggregate f calcite ndsericite pottedwith iron stains. (uartz is introduced nd tendsto congregate round he old hornblende henocrysts.The extremeproductof alteration s finely granularwhiterock with flaky fractureand cut by pyrite stringers. The feld-sparphenocrystsannot e distinguishedacroscopically.p.i-dote s absent. This rock consists f highlysericitized heno-crysts in a groundmass f fine-grainedquartz, sericite, and


14/22

514 H. E. MCKINSTRY AND J. .4. NOBLE.feldspar-remnants. Small angular patchesof calcite representhornblende. Pyrite is present in irregular grains, sometimeselongated, uggestinghat the mineral had started ts growth asveinletsbut had enlarged ts areasby replacement. Sericite lakesfollow irregular stringers.

There are a few veinlets of .calcite and adularia with somequartz. Quartz alsodevelopsarge irregular grains. Small rodsof a prismaticaterialstraightx[inction,egativelongation.low birefringence),possibly oisite,are scattered ere and there.In many places,particularly n the southend of Carlos Fran-cisco,a pink phaseof alteredrock appears. This is fine-grainedand porcelain-like,breakingwith a subconchoidalracture. Mi-croscopicallyt consists f a groundmass ainly of sericite,withscattered uartzgrains. Epidoteoccurs n irregularpatches ndpyrite in shapeless rains, commonly carrying inclusionsofquartz.With increasingdistance rom the vein, epidotebegins o ap-pear and feldsparphenocrystsegin o be visiblemacroscopically.This less altered type shows, in thin section, feldspars onlyslightly ericitizedndgroundmassairly fresh. Ferromagne-sians are completelyaltered to chlorite and epidote. Magnetiteis absentutpyrite rains rrangedfi stringersraversehe ield.In general, he sequence n going away from the vein is:

x. White, granular,pyritized, to 3 feet.2. Pink, granular, with someepidote,o to 3o feet.3. Green.,epidotizedwith somepyrite, o to 3 feet.4. Purplish, little-altered porphyry.In the south end of the Carlos Francisco mine the altered zone isin places s muchas x50 feet wide. In the north end, or fartherfrom the centerof the district, he greenand purple ypesmayoccur .within a few centimeters of the vein.

To summarize: intense alteration .consists of sericitization withpyritizationand silicification. Less intensealteration s propy-litic (calcite, hlorite, pidote, nd some ericite ndpyrite). In


15/22

THE VEINS OF CASAPALCA, PERU. 515the "green" stage, propylitic alteration continueswith somesericitizationnd a little pyritizationbut with very little silicifica-tion.In Redbeds.--The most intenselyaltered phaseof this forma-tion is associated ith the silver-leaner ortionsof the veins be-tween the Aguas Calientes ore bodies and the 'Carlos Franciscoshaft. The original red .color s bleached nd the rock is silicifiedover a width of oo feet from the vein. Pyrite is developed othin stringers nd in isolated ubes. The rock becomesn aggre-gate of quartz and calcite veins with interstitial epidote andzoisite. The original ounded uartzgrainsof the sediment reoftenbuilt out by additiono ydrothermaluartz. Pyrite mayreplace uartz n cubes r mold itself around he grains.Adjoining the richestparts of the vein (the ore bodies nAguas Calientes) the alteration is much less intense. In someplacesherock s bleachedor as much s thirtyfeetfrom hevein; in others, the vein walls and even inclusions withih thevein are not strongly ilicified. There are placeswithin five feetof the vein where the shale is still red and unbleached. Onestrikingvariety of the milder sort of alterationproduces greenmottling due to epidote.In CarmenMember.--In their most intensephaseof alterationthe conglomerateedsare so stronglysilicified hat the pebblescannotbe distinguished. The limestone edsalter to fine-grainedmarble.

Zonal ArrangementThe Principal Deposits.--The centeraboutwhich the differenttypes of mineralizationand wall-rock alterationare arranged na rudely symmetricalway is at depthbetween he Aguas Cali-entes and Carlos FranciscoMines; above this the alteration isbroadest and most intense. North.ward and southward from this

centerthere is a gradual but definite change n mineralizationaccompaniedy increasingeeblenessf alteration:35


16/22


17/22

THE VEINS OF C,'tS,'tP,'tLC,'t, PERU. 517not appearabove the supposed enter of deposition, nd thoughthere s an increaseupward in the silver contentof the tetrahedritewith decreasen pyrite along with a tendency o finer texture ofthe ore, the change s less marked than in a corresponding is-tance horizontally.Horizontal rs. Lateral Zonint7.--Here obviously he tempera-ture gradientexpressedn the zoningwas due essentiallyo theheat of the mineralizingsolutions2 Were the zoningdue to thenormal increase of temperature with depth, one would expectthe zones o be essentially orizontal. The facts, however,pointto an arrangementof zonesaround the largestand most abundantchannelways or the ascent of ore solutionsand indicate that,insofar as the zoning s dueto temperature radient, he heat wassuppliedby the solutions hemselves. If it were possibleo addto the sectionshown n Fig. 5 the portionsof the vein-system

PITE RMF_DIRT / Sphllerlre, 6oilrio,Terre- (JOrt. end Gelcite hearIre, Bour.nlre, /Pyrlte, Terre- / Heih Silver' /hearire, Sribnite, / ' ,"GF_ "IT 1 LRealget, potty/

// Sphnlnr*te.olenl, nen BerryeM41aleIrentlrrnedQte Zone .d tll/nltl bigln/ / Les6ilver *(wtherealcite,to ppear//ji PreeenrLowestevelFIG. 5. Longitudinal Section (looking east) along Main Vein Systemshowing onal relations. The southernportion s along Aguas Calientesvein, which is nearer the observer han the plane of M-vein.

which erosionhas removed,a flattening of the zones near theoriginal surface would be expected. Since there is no hint ofthis in the now-visible ortion of the vein-system, contemplationof Fig. 5 suggests hat the area labelled "intermediate zone"extendedat least I,OOO eet, and probablymore, above he highestexposures nd that this was coveredby an unknown thicknessof the "outer zone."

The heatdue to mereproximityof intrusions ithout he aid of heat transferby solutions is probably small, since the thermal conductivity of rocks is low. SeeIngersoll, I. R., and Zobdel, O. J.: Theory of Heat Conduction with Engineeringand Geological Applications, x9 x3.


18/22

518 H. E. MCKINSTRY AND J. A. NOBLE.OutlyingDeposits.--Away from the centralportion of the dis-trict whichhas ust beendescribed, numberof outlyingmines

and prospects howmineralization f severalsomewhat ifferingand probablymilder types,accompaniedy less ntensewall-rockalteration.From one to three kilometers to the east there are three small

deposits:a. Intricately branching narrow, rich, vuggy veins of the cooler CarlosFrancisco type, with much bournonite and some stibnite. Rubysilver and argentite have been reported (San Antonio Mine).b. Clearable carbonateveins with pyritohedrons, nd tetrahedrite in clear-

cut little tetrahedronsperched n the rugs (Americana mill).c. Gash veins of sphalerite, chalcopyrite, and a little tetrahedrite, withbarite and clearablecarbonaten mildly alteredporphyry (Capri-chosaMine). Mineralization of the same type is found also onthe opposite ide (i.e. to the west) of Casapalca.To the north of Casapalcaoccursstill another type of de-posit. In line with an extensionof a vein of the main system(:Rayo), the Corina Mine is in a stockworkof interlacingnarrowstringersn little-altered orphyry. The gangue quartz-calcite)

s subordinate.The quartz s replacedy tiny crystals f arseno-pyrite, and the two are replaced y sphalerite. Within the sphal-erite and apparently eplacing t, as well as forming matted ag-gregates round t, are slenderneedles f boulangerite.To the south, veins in the Bella Vista limestonecarry bariteand manganese ioxide (probably an oxidation product afterrhodochrosite)with the samesulphides s thoseof the Casapalcaveins. The appearanceof barite suggests hat these small de-positsare related o the .cooler haseof mineralizationoccurringin the southernend of the Aguas Calientesmine.

Origin.Source.--The ore of the main Casapalcaveins cannot becertainly referred to any exposed gneous ntrusion. In factthe zonal arrangementpoints to a magmaticsourcedeep belowthe presentsurface.


19/22

THE VEINS OF CASAPALCA, PERU. 519There is a slight localizationof small veins and altered areasaround some of the smaller stocksand sills. Around the larger

stocks the association of veins is less definite. In fact thethrough-going eins cut through the stockswith no consistentch.angen mineralization. In the adjacentdistrictof Morococha,the ore is believed O have come from an intrusive later andslightlymoreacid han hat correspondingo the Tamca porphyrybut sufficiently imilar chemically o suggest hat the two aregenetically elated. This later intrusive is not exposed n theCasapalca istrictbut may occur n depth.Themineralizationt Casapalcas of loweremperaturehanthat of Morococha (where the early phasesare of the contact-metamorphicype) andwouldsuggesthat i't formedwell abovethe apex of an intrusive insteadof adjacent to it.

The main fractures that have been mineralized are later thanthe regional folding and not directly related to it. They mayhave been formed in readjustments ollowing the folding andintrusion. In the fractures, gan lue and sulphideshave beendeposited ontinuously nd progressivelyn a fairly definitese-quence. At a late stage portions of the veins were re-openedand a generationof carbonates nd quartz deposited. Hot wateris still encounteredn portionsof the mine lying above he sup-posedcenterof zoningand, before the flow was tappedby mineworkings, was issuing at the surface and depositingcalcite,suggesting hat open fractures still reach to the vicinity of awarm intrusive and that the feeble dying phasesof mineraliza-tion may still be.goingon.Age of Ore Deposition.--Theveins cut the volcanics nd arelater than Tertiary (Incaic) folding. Material of the magmabe-lieved o be responsibleor the ores s petrologicallyuitesimilarto that of the flows,and on regionalevidence,he emplacementfthe ntrusivess inferredo have ollowedolding ut to havebeenconnected ith the sameorogenicmovements.There is also definite regional evidence hat mineralization ofthis type was earlier than the formation of the Puna erosionsurface at the dose of the Pliocene.

Temperqture nd Depth of Minera.lixation.mThe bviously


20/22

520 H. E. MCKINSTRY .AND J. A. NOBLE.epithermal ealgar-stibnite-bearingineralization s clearly alate phaseand not a safe criterion of the conditionsunder whichthe main ore deposits were formed. Nevertheless there is agradation at the outer edgesof the district from the standardtypes of ore into vein-filling that carries botryoidalcalcitewithstibnite and appears o be transitional toward this late phase.The mineralization s a whole has many pointsof similarity tothat of the San Juan veins of Coloradoand the widespread res-enceof rhodochrosite nd tetrahedriteare suggestiveof a fairlylow-temperature rigin. On the other hand, lack of fine band-ing and crustification, redominance f sericiticrather than pro-pylitic alteration, and absenceof abundant adularia, as well asthe transitionnear the center nto coarsepyrite-blende eins inkthe oreswith those formed at intermediatedepths. In some re-spects, he mineralizationresembleshat of the silver-zinc zoneof Butte, Montana. It is concluded hat the depositsstraddlethe indefiniteboundarybetweenepithermaland mesothermalde-posits. They would be included n the class or which Gratonhas proposedhe name "leptothermal."6Structural evidence s pertinent insofar as one is justified inassuminghat mineralization mmediately ollowed folding. Onthis assumption rosion, n order to have exposed he presenthighestoutcropsmust have removednot only the major part ofthe CarlosFranciscovolcanics ,3oo feet thick) but a greatthickness,estimatedas at least o,ooo feet, of the younger flowsof the Rio Blanco formation, all of which were involved in Incaicfolding. Actually someof this thicknesswas doubtless rodedbefore ore deposition ook place. Thus, while it would not beinconsistentwith .what is known of the rock-structure o assignan original depth well in excessof Io,ooo feet to the highestknown portionsof the veins, the evidence or so large a figureis not compelling. Zonal arrangement, however, suggests hatthe minimum depth was a thousand, or, more likely, severalthousand feet.

One figure,at least, s definite: hereare oresof the tetrahedrite6 Graton, L. C.: "The Depth-Zones in Ore Deposits." Pres. Address, Soe. Eton.

Geol., Tulsa Meeting, i93x.


21/22

THE VEINS OF CASAPALCA, PERU. 52Itype (i.e. those of the lower workings) which were formed ata depthof not less han 3,5oo feet, for they now lie at that dis-tance below the outcrop. How much deeper than this theywere at the time of formationcan be inferred only in a generalway by adding o this the indefinite ange suggested bove.

SUMMARY.

The ore at Casapalcaoccurs n veins that follow fractures inseveralkinds of wall-rock,and the differing physicalnature ofthe rock s clearly eflectedn the vein structure. In plasticshalethrough-goingracturing ookplaceonly underexceptional tress;in the more brittle conglomerate any small fracturesdeveloped;and in the porphyry, stress produced an intricate system ofbranchingvein-fissures. In this branchingsystem, he broadestportionsof the veins are along definitedirectionsof dip andstrike and the broadest zones of mineralization are found at thejunctions of fractures where the rock would have been moststronglybrecciated. At deeperhorizonsbelow he zoneof easiestfracturing, the fissures re more regular and are less nfluencedby physicalnature of wall-rock.The rock alteration is most intense about a certain center,and herethe veinsare broadest. Corresponding ith the intenserock-alteration s the type of mineralizatio.nn the veins whichis believed o have formedunder he conditions f highest em-peratureand pressure. Passingupward and in both directionsoutward from this center there is a decreasen intensityofalteration and a correspondingchange in texture and mineralcontent of ores. Cutting the earlier mineralization there is atone placea vertical zone of mammillarycalcitewith realgar andstibnite.

On a regionalscaleboulangerite, tibnite,and barite appearin the outlying mines a kilometer or more from the center of thedistrict.

Almosteverygradatio.nmay be traced, rom the coarse yritic; The present Rimac Valley was cut during two topographic stages, both later

than the Puna Peneplane which, in turn, is younger than the mineralization.


22/22

522 H. E. MCKINSTRY AND .1'. A. NOBLE.ore through he mammillarycarbonate angue o the calcareoustufa now beingdeposited y the nearbyhot springs, nd hot wateris still encounteredn deepcentral parts of the workings.Correspondingo the zonal sequence f deposition,he oremineralsn any one ocality howa time-seriesrom pyriteandsphaleritehrough alena ndtetrahedriteo late calcite.The replacementexturesare in general of the "sea-and-island" type showing mooth ounded oundaries,nd the wallsof veinletsare irregular rather than parallel.Conditions f deposition re believedo haveranged rom mildmesothermalo epithermalat depthsof several housand eet.Mineralization ollowed he last of the great periodsof foldingand intrusion which formed the Andes (early Tertiary) butpreceded liocene rosion.

TIMMINS ONTARIO,AND

LEAD, S. D.

Date post:	03-Jun-2018
Category:	Documents
Upload:	haro-velasquez-sanchez
View:	220 times
Download:	0 times

McKinstry and Noble-1932

Documents