goodell and petersen-1974

8/12/2019 Goodell and Petersen-1974

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348 PHILIP C. GOODELL .4ND ULRICH PETERSEN80 72

HUANCAYOINSET

PERU

PACIFIC OCEANI o

' '"uACAYO] ULCANIHUANCAVELICA

SAN GENARO

JULCANI LIRCAYHUACHOCOLII,.AMPAMALl200KmL 30KmLJ Hlgkwoy ':::: .... ::': Roilroodl

FIG. 1. Location map. Julcani district, Peru.

and depth of mineralizationn the copperdistrict ofthe upper peninsulaof Michigan, and his use ofthis ratio provided an understandingof the rela-tions between various depositsof the district. Anunderstanding f the zoningand inferred movementof mineralizing solutionswas suggestedby Brown(1935) through the use of Zn/Pb, Fe/Pb, andFe/Zn metal ratios in the Austinvilledistrict,Vir-ginia. The directionof flow of mineralizingsolu-tions was the objectof a study by Gross (1956) onthe Blyklippenmine, Greenland, singmetal ratios,decrepitation temperatures, asymmetric growthbanding of crystals, and structural parameters.Zoning in the Central City district, Colorado,wasestablishedy Sims and Barton (1962) usinggen-eralized metal ratios concurrentlywith changes nmineralogy,sphaleritecompositions,nd fluid-in-clusionilling emperatures.Koch and Link (1%7)appliedmathematical nd computer echniquesostudymetaland metalratio distributionn the veinsof the Fresnillo mine, Zacatecas,Mexico. Changesin the finenessof gold [Au/(Au + Ag)] in goldminingdistrictshave beensubstantiatedn studiestoo numerous o mention, and have generally beenrelated to zoning of the deposits. Finally, addi-tional studiesof Peruvianore depositsncorporatingmetal ratios have been made by the authors, andtheseresultshave in general substantiatedhe con-clusions resented ere.

Present WorkThe presentstudy s part of an extendedprojectdirected owardsa more completeunderstanding fthe genesisof polymetallicCordilleran ore deposits,

and, in particular, he Julcanimining district, Peru.This studyhasbeen n progress ince1965, and dur-ing this time numerous visits to the district havehave been made by both authors. The generalgeology nd structuralsettingare beingreportedonseparately Petersenand Arenas, n prep.). Detailsof the mineralogyand paragenesis f the ores willbe presented n a future publication. A geother-mometerusing the compositions f tennantite-tetra-hedritecoexistingwith enargite-famatiniteas beenapplied o the Julcani ores (Feiss, 1970). Investi-gationsof fluid inclusions, hasecompositions,ndisotopic ompositionsre currently n progress.Thispaper discussesnly those esultsobtained rom thedetailedstudy of metal ratios.Geologic Setting

Because he general geology, structure, mineral-ogy, and paragenesiswill be discussed n detailelsewhere, nly a brief summary s called or at thispoint. The Julcani district is located n the prov-ince of Angaraes, Department of Huanacavelica, nsouth-central Peru, about 300 kilometers southeastof Lima (Fig. 1). The district is locatedastridethe intersection of a faulted north-south anticline o


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.4 STUDY OF MET.4L R`4TIOS 349i ,,

TvTcK

PAL

QuaternaryTertiary volcanicsTertiary continentalCretaceousTriassic JurassicPermianPaleozoic

I 5 Km$. I

Tv/

PAL

/ Tv

I .'JU CANi". NINING DISTRICT'. Tv

PAL,. LIRCAYII

FIG. 2. Regionalgeologyof a portion of the provinceof Angaraes, ncludingthe Julcani district, Peru, from Petersen (1969).Paleozoicand Mesozoicsedimentswith a promi- Mineralization s in fractures within Tertiarynent transverseault (Fig. 2). flows, tuffs, and stocks,generally of dacitic and


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350 PHILIP C. GOODELL AND ULRICH PETERSEN

$Oo

3o

3&o

' 2Fro. 3. Vertical longitudinalprojection of Ag/Pb, vein 2, Herminia mine, Julcani district, Peru.Contoursseparateareas of differing ratio values.

andesitic omposition.n general, he hostrock ofthe veinsreportedon here doesnot change ignifi-cantly. The ore contains ulfides f copper,silver,and lead, with significantamountsof arsenic,anti-mony, bismuth, and zinc. Galena, tennantite-tetra-hedrite,enargite, nd other sulfosaltsre the princi-pal ore minerals,with somesphalerite, halcopyrite,bismuthinite, nd arsenopyrite lsopresent. Pyriteis an abundantand ever presentassociate, ndgangue minerals are principally quartz, barite, andsiderite. In Herminia, the stage of polymetallicsulfosaltmineralizations preceeded y a stageofpyrite-wolframite-enargite-barite.n both Herminiaand Mimosa,a "lastgasp"stage onsists phaleriteand siderite; becausezinc ratios are not consideredfor theseareas,no problemarisesdue to the over-lapping of these parageneticzones. Wall-rock al-teration products comprise sericite and kaolinite.Several depositsare found within the district, andthe reader is referred to Figure 13 for the locationsof the various mines mentioned.

Metal Ratio CalculationsChemicaldata gatheredby mining companiesorore inventorypurposes enerallyconsists : 1) avery large number of channelor grab samples hatare analyzedor several lementsnd (2) weighted

averagesof the previous data for those blocks ofgroundbearingsufficientmineral value to be desig-natedas ore. The latter compri_seto 50 individualsamplesand are tabulated in periodic ore inven-tories. In Julcani,a typical block of ore would com-prise 20 x 15 meters in vertical projection. Bothdata types have been used for metal ratio studies,i.e., type 1 by Koch and Link (1967) and type 2 inthe present work.The use of type 2 data may be challengedon twogrounds: (1) that it only includes he richer ma-terial abovea certain cutoff grade and (2) that itaveragesout the detailed variations hat might beseenby using individual channelsamples. In orderto assess he first, a comparisonwas made severaltimes between the metal ratios in the original chan-nel samplessurroundinga block of ore with themetal ratios for the block from the ore inventory.This revealed no evident ratio differences betweenore and poorer vein material. As far as averagingout the detailed variations of individual channelsampless concerned,t is found that the ore blocksare sufficiently mall so that they do not obscure hesignificantvariations n ore characteralong a veinand throughout he district. Type 2 data ignoresonly very local variationswhich are not of interestor significance at the scale under consideration.


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STUDY OF METAL RATIOS 351Furthermore, in practice type 2 data is readilyavailable and can be plotted convenientlywithoutrecourse to computers.Silver, lead, and coppervalues are reported nthe Annual Ore Reserve nventory for most of the

DOCENITA$00

0

580Pb/Cu2- A

3o__.o2 oo/ 6 o-' ' -'-N55WAg/Pb // 100m B

30 IllIll

420- ' 000 Ag/Cu580 I0 CFro. 4. Vertical longitudinal projectioa of Docenitavein, Herminia mine, Julcani district, Peru, showing con-tours of metal ratio values; Pb/Cu (A), Ag/Pb (B),Ag/Cu (C): Docenita strikes N55W; dips 60W. Arrowsindicate inferred direction of solution movement.

VEIN 2;5O0

b/Cu ) I00420 'OX,o A

420 I IOOll1 B210

3,00 Ag/Cu ) I00

/

,420 /

\ \ Ag/Cu ( I00I

//

cFro. 5. Vertical longitudinal projection of vein 2,Herminia mine, Julcani district, Peru, showing contoursof metal ratio values; Pb/Cu (A), Ag/Pb (B), Ag/Cu (C).Vein 2 strikes N25W; dips 50SW. Arrows indicate in-ferred direction of solution movement.

veins in the Julcani district. These chemicalanal-yses were made over a period of 15 years, usingstandard wet chemicalmethods. From the averagesof metal contents rior to dilution to mining width,Ag/Pb, Ag/Cu, and Pb/Cu are calculated,usingsilver in ouncesper dry short ton and copperandlead in weight percent. Gold, where present, isalso calculatedn ounces er dry short ton..At thevery low and high ends of a ratio range valuesmaybecomeerratic as the limit of detectability n theanalysisof one of the elementsmaking up the ratiois reached. The ratio values are plotted on over-lays to vertical longitudinalvein projections n thepositionof their respective locks. In large singleveins the ratios commonly give evidence of con-


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Fig. 6. ContoursoPb/Cu, 360 level, Herminia mine, .lulcani district, Peru. Arrowsindicate inferred direction of solution movement.

sistentchanges nd contourscan be drawn separat-ing areas of generallydifferent values. In areas ofcomplex racturingand large numberof veins,as inthe I-Ierminia mine, it is necessaryo transfer alsothe data from the vertical projections o horizontalplans, hus obtaining three-dimensionalepresenta-tion of the zoning pattern.An exampleof the procedure sed n this studyis given in Figure 3, which showsAg/Pb plottedon a ve?tical ongitudinalprojection or vein 2 in}-Ierminia. Each number correspondso a block ofore. Consistentchanges n Ag/Pb values are seento occur, anging rom greater han 500 in the north-west to less than 1.0 in the southeast. This repre-sents atio changes rom only one stageomineral-ization. The contouredareas are labeledaccordingto their limiting values. In spite o he many pos-sible sources f error, from sampling o analysis o

averaging metal content for the blocks of ore, astrong and consistent hange n the chemicalchar-acter of the mineralization s revealed by this pro-cedure. In the following work, only the contoursand their metal ratio values are shown.The results from individual mines in the Julcani

district are discussed next.Herminia

The metal ratio variations in the I-Ierminia mineare presented n greater detail than for the othermines o he district. Herminia is in the southeast-ern part of Julcaniand contains large numberofmineralized fractures in an area 600 meters north-south by 700 meters east-west nd known over avertical interval of 450 meters. The earliest stageof mineralization is limited to the northwest andconsistsof pyrite-wolframite-enargite-barite. he


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STUDY OF METztL Rz'tTIOS 353

Agb , 1$60 level .0


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FIG. 8. Contours of Ag/.Cu, 360 level, Herminia mine, Julcani district, Peru. Arrowsindicate inferred direction of solution movement.

soutt/east. These same atio changes re seenon thehorizontal plan (Fig. 6) as concentricbands char-acterizedby monotonicallyncreasing b/Cu valuesto the southeast. The earlier enargite-bearing tageends at approximatelyP.b/Cu- 1.0; so, for valuesgreater than this we are looking at only one para-genetic stage.Ag/P.b increases n Docenita from Ag/Pb < 10in the northwest at depth to Ag/Pb > 100 in thesoutheast Fig. 4B). In vein 2 this ratio decreasesfrom northwest to southeast rom Ag/Pb > 100 toAg/Pb < 1.0 (Fig. 5B). On the horizontalsection(Fig. 7) these ratios show a gentle increase romnorthwest to southeast, eaching maximum valuesgreater han 100 in a centralband n Herminia, withvalues subsequently ecreasing apidly to less than2.0 further to the southeast. Contours of equalvalue on either side of this maximum are not con-

nectedor closed ecausehey correspondo differentmineralogiesand to different Pb/Cu values.Ag/Cu increasesn Docenita from Ag/Cu < 2.0in the northwest at depth to Ag/Cu > 100. to thesoutheast Fig. 4C). In vein 2 only one contourcan be drawn, with Ag/Cu > 100 to the northwestand Ag/Cu < 100 to the southeast Fig. 5C). Onthe horizontal section (Fig. 8) this ratio shows agentle increase rom northwest o ,southeast,each-ing a maximumgreater than 500 in the centralpartof Herminia, ahd decreasingmore rapidly further tothe southeast. Again, the contoursof equal valueon either side of the maximum are not equivalent.The Ag/Pb and Ag/Cu distributions ndicate aband of maximum values in the central part ofHerminia, whereas Pb/Cu increasesmonotonicallythroughout his area. The northwestand southeastsides of the maximum are neither symmetricalnor


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STUDY OF MET.4L R.4TIOS 355equivalent s they have differing mineralogies ndchemical character. The maximum area has ex-tremely rich silver mineralization, ndicating thedumpingof silver in a limited area by the mineral-izing solutions. Knowledgeof the spacialextensionof this region is of obvious nterest in mineral ex-ploration. Earlier mining efforts n Herminia weredirected chiefly toward this region, and when theband of rich ore appeared o narrow in depth, hemine was considered o be approachingdepletion,and operations easedor a period. If the presentstudieshad been imited to the Ag/Pb and Ag/Curatios, the movementof mineralizingsolutionsmighthave been nterpreted o have spread rom the areaof maximum silver ratios outward. Such an inter-pretation was made in earlier studies.The shapesof the ratio contours n vertical sec-tion (Figs. 4 and 5) are roughly those of a wavefront moving from northwest to southeastwith alarge lateral component. That is, the curves aregenerally convex toward the southeastwith theiraxes close o horizontal. In those portionsof themine where developmentwork has not reached henecessary epth,only the top portionsof thesecurvesare seen,and these shapesmight be interpretedaspart of a domal zoning pattern. In horizontal sec-tion the concave ratio contours (convex to thesoutheast)sweep out an arc from N120E to duesouth. This is suggested y the curved bands ofdifferent ratio values as seen on the horizontal sec-tions in Figures 6, 7, and 8; however, t can bestbe seen by he superimpositionf thesecontours ormany levels.The shapesof the ratio contoursappear to indi-cate the direction of movementof mineralizing solu-tions, the concavedirection pointing toward their'source'. This statement is based on theoretical con-siderations and on field observations. In the firstplace, it is noted that the contoursare more sepa-rated and further from the presumed source' inzones of greater permeability. For a given vein,such as Docenita or vein 2, the fracture generallynarrows both upward and downward. In planview and for Herminia as a whole, the contours ad-vance' further where there is a greater concentrationof fractures or where wide veins predominate. Inprinciple,one would expect hat the more permeableareasallow a greater solution low and consequentlyrequirea longer distance f travel to reach he sametemperatureor degreeof modification hrough wall-rock alteration han would be necessaryor a smallerflow volume along a narrower path.Additional evidence suggests hat mineralizingsolutions moved from northwest to southeast inHerminia. Monotonically ncreasingPb/Cu valuesreflect the widely recognized oning sequencerom

copper n the center to lead toward the margin.To the northwestof Docenita,surfacemappingsug-gests he presence f a small intrusiveprobably heremnant of an erodedvolcanicneck (Petersen andArenas,n prep.). A smaller reaof mineraliza'tion to the north of the intrusive, Lucrecia, to bediscussedbelow, indicates, from similar evidence,that solutions here moved with a northerly com-ponent. Projecting these wo presumed low direc-tions backwardpoints directly to the intrusive andits associated rea of fracturing as a likely conduitfor the hydrothermal solutions. We thus con-clude that the shapesof the ratio contoursdo ap-parently reflect the direction of movement of themineralizingsolutions,with the convexitypointingtoward the marginal zones.An observationmade repeatedlyupon detailedanalysisndicates more rapid changen ratio valueswhen solutionsmove into a minor channelway roma major one, as is indicated n the upper left por-tions of Figures 4B and 4C.Lucrecia

The Lucrecia mine lies 600 to 800 meters north-northwestof Herminia (Fig. 13). The major partof 'the mineralization is in the Carmencita veinstriking north-south,dipping 50E, and consists ftennantite-tetrahedrite, alena,and barite, with minoramountsof an earlier barite-enargitestage. Valuesof the metal ratios are summarized in Figures 14,15, and 16. The contourshave a domal shapewiththe axis of the dome tilted 30 north from the verti-cal and the convex directionsof the contourspointupward and to the north. The monotonicallyn-creasing Pb/Cu values, the shapes of the metalratio contours, the mineralogical changes, and thepresenceof a fractured zone and associatedn-trusive to the south, all suggest hat mineralizingsolutions rose and flowed north in the Carmencitavein.T entadora

The Tentadoraarea occupies centralposition nthe district. The dominant structure is the Rosariovein which is two kilometers long, strikes north-west-southeast,nd is nearly vertical. Only thecentral 400 meters has been well mineralized, andthe oresconsist f crustified yrite-sphalerite-galena;coppervaluesare very low. To the southwest fRosario are a numberof parallel veins with wolfra-mite, pyrite, and quartz. The Rosario vein hasbeenmined out for 25 years, so that the most com-plete data consists f lead, zinc, copper, ilver,andgoldvaluesmadeon 104 channel amples istributedthroughout he vein. The crustifiednature of themineralsequenceliminates ertain atiosas signifi-


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356 PHILIP C. GOODELLIND ULRICHPETERSEN

Fro. 9. Vertical longitudinal rojectionof Estela vein, Estela mine, Julcani district,Peru, showing ontours f severalmetal ratio changes.Estelavein strikesN70W; dips45SW.

I Vein hoq......... :- .... __. X'- ,.,,'. ".......... z\_.:_'..X,-,,.x

., '% oq - I. /Pb/Cu MIMOSA I I00 m

+5

Fro. 10. Projectionntohorizontallanof Pb/Cu throughout imosamine,Julcani istrict,Peru.See text for description f veins. Arrows indicate nferreddirection f solutionmovement.


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STUDY OF METAL RATIOS 357

/ --- ChoqI +o '4:50\,..... ......... x\ +o

/

Porvenir Vein4-I0 .

Ag/PbMIMOSA I I00 m I +5+1

FIG. 11. Projection onto horizontal plan oAg/Pb throughout Mimosa mine, Julcani district, Peru.See text for description of veins. Arrows indicate infer. ed direction o olutionmovement.

cant,suchas Pb/Zn. Other ratios,suchas Ag/Pb,show regular changes n value, and their contourshave a convexupwardshape. Vertical ascendencyof the mineralizingsolutions s inferred in theRosario vein.Estela

The Estelamine s in the west-central art of thedistrictand is characterizedy a singlemajor veinwhich strikesN70W and dips 45SW. Mineral-izationconsists f a major stageof tennantite-tetra-hedrite,chalcopyrite,rsenopyrite,nd bismuthinite,with minor apatite. Early pyrite-wolframite ndlater galena-sphalerite tagesare found in limitedareas. The ores containappreciable old but virtu-ally no lead, providing for Ag/Cu, Ag/Au, andCu/Au ratios obtainable rom the ore reserves. Acompilation f the contoursof these ratios is givenin Figure 9. These contoursdefine a dome in thecenter of the vein exposedby mining, with a rela-tively copper- and gold-rich core and a silver-richhalo. It is inferred that the mineralizing solutionsrose upward and outward in the vein.

MimosaThe Mimosa mine is located in the northwestern

portion of the Julcani district. Mineralization iscurrently known over an area of 600 meters east-west by 600 meters north-southand a vertical ex-tent greater than 250 meters. The three majorveins form in plan a roughly triangular pattern.San Demetrio vein forms the eastern imit, strikingN40W and dipping75SW. Porvenirvein strikesE-W and dips 80S. Mimosavein strikesN60W,dips 60NE, and lies to the southwestof the othertwo major veins. If these fractures are extendeduntil they intersect with each other, an invertedtriangular pyramid would be formed, with its apexon the 640 level, some 150 meters below the lowestpresent mine workings. Several smaller veins liewithin this invertedpyramid, but mineralizationsnot thought to be confined o its limits. In general,the mineralizationbelongs o one major stage, con-sisting of tennantite-tetrahedrite, alena, and bis-muthinite.

Silver, lead, and copper analysesare availablefrom ore inventories throughout the mine, and


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358 PHILIP C. GOODELL 4ND ULRICH PETERSEN

.... , ...... +20, .

N

+1ooo k

Ag/Cu MIMOSA

-20 -2o PorvenirVein-t20 / +20+20

+ 20 t -20\\ x ' '\ +20

Choq ] '+20I I00 m I

Fro. 12. Projection onto horizontal plan of Ag/Cu throughoutMimosa mine, Julcani district, Peru.See text for descriptionof veins. Arrows indicate inferred directionof solutionmovement.

vertical ongitudinalrojections f Ag/P.b,Ag/Cu,and P.b/Cu for all the veins were plotted. Thisinformations summarizedn Figures10 through 2and Figures'14-16. Figures 10-12 are projectionsof ratio contours hroughoutMimosa onto a com-positehorizontalplan showing he veins on threedifferent levels, the 490, 430, and Choquisuela(Choq). Thesediagramsndicate onsistenthangesof metalratios rom the area of the pyramidal pexat depth upward and outward in the shapeof asimpledome. The changes re more gradual n theflatter Mimosa vein.Pb/Cu values increasemonotonicallyrom lessthan 1.0 in the centralportionat depth o greaterthan 1.0 in Porvenir, greater than 10 in San Deme-trio, andgreaterhan1,000 n Mimosa. The generalcopper to lead zoning sequenceogether with theshapesof the ratio contours substantiateshe ideathat mineralizingsolutionsmovedupward and out-ward from the pyramidal apex at depth. Themaximumvalue of the Pb/Cu zoning ndex foundin eachvein may be used as a measureof the rela-tive positionn the zoningsequenceeached o date

by mining. For example,extensiveexploration nPorvenir vein could be recommended from thisstudybecausehe maximumvalue of Pb/Cu for thisvein was just greater than 1.0, in contrast to amaximumvalue of over 1,000 n Mimosa, suggestingthat the completesequence f mineralization n thePorvenir fracturehad not yet beendiscovered.Sub-sequentdevelopment onfirmed his suggestion. histype of reasoningcan be applied in other districtsonce the typical behaviorof a ratio is known.Summary of the Inferred Plow Directions ofMineralizing SolutionsSeveral centers of mineralizationare present inthe Julcani district, and the inferred directionsofmovement f mineralizing olutionsor eachof themare summarizedn Figure 13. In the southeasternpart of the district solutionsapparently rose fromthe generalarea of fracturing associated ith an in-trusive and spreadoutward, at times with a largelateral component,providing mineralization o veinsin Herminia and Lucrecia. In Tentadora and Estelasolutionsapparently rose vertically within major


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A STUDY OF METAL RATIOS 359

N9000IERMINIA

I km

Fx6. 13 Summary of inferred solutionmovement n the Julcani district, Peru. Arrows indicate solution movementwith lateral component;diamond shapes ndicate orly vertical ascendancy.

fractures esulting n simpledomalpatternsof metalratios; these are indicatedby diamond shapes nFigure 13. Domal zoning with solutionsmovingupward and outward is indicated for the fracturesin Mimosa. The time relations between the variouscenters s of interestbut, becausehere s no overlapin space,evidence egardingcross-cuttingelationsis not available.General ConsiderationsRegarding MetalRatio Changes

This study eveals monotonicncrease f Pb/Cuvaluesalong the apparentpath of mineralizingsolu-tions throughout he Julcani district, except inTentadora. This change n Pb/Cu is in agreementwith generalized oning observationsor polymetal-lic depositswith normal zoning sequences. Thischaracteristic ehaviorof Pb/Cu makes it a good

indicator f the positionwithin the zoningsequenceof a particular ore block or vein stretch, and it isproposedhat this ratio be usedas a zoning ndex.Under specialconditions his parametermay bedirectly related to the differentiationof a hydro-thermalsolution long ts path. The rate of changeof this parameter along several inferred solutionpaths is indicated n Figure 14 where log P.b/Cuis plotted against distancealong the path. Zeropoints on the distancescaleare determined y thefirst appearance f blocksof ore along the solutionpath as exposedby presentdevelopment, nd henceare quite arbitary. In Herminia, d(log Pb/Cu)/d(dist) = 0.25/100 meters one order of magnitudechange n 400 meters) for the first 300 meters,andaverage 0.5/100 meters for the remainder of thezoning sequence. Values of 0.5/100 meters for thefirst 150 meters, and 1.0/100 meters for the re-mainder are seen for Mimosa. Carmencita has a


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360 PHILIP C. GOODELLlIND ULRICHPETERSEN

log Pb/Cu 0 /200 3000 600 0000 '/ Herminia-'I'- distancelong solutionaths,2.o_A- metersFig.14. LogPb/Cuversusistancelong olutionath,or hree ifferentdepositswithin the Jul. ani district, Peru.

changeof approximately .4/100 meters or itsshort path.It thus appearshat subsidiaryolution athsexhibitgreater atesof change f ratiovalues ersusdistancehan majorpaths. If thesedepositsavesimilarmineralogiesndparageneticequences,hensuch ariationsn ratesof change f the zoning n-dexmayreflect ifferencesn the relativentensityof depositionalactors.The changesn Ag/Pb and Ag/Cu in Herminia,Carmencita,nd Mimosaare shown n Figures15and 16. Theseratiosdefinea broadbandof values,but if a particularpath within a mine is considered,a more narrow band results. Ag/Pb and Ag/Cu

pass hrough a maximum in Herminia, reflecting hearea of high silver valuesnoted earlier. Both ratiosdecrease n Carmencita. In Mimosa, Ag/Pb de-creasesmonotonicallywhereasAg/Cu has a maxi-mum superimposed pon its otherwise constantlyincreasingvalue. The vertical or lateral displace-ment of thesecurves elative to eachother may pro-vide an indication of differences n bulk chemistryof the solutionsor of depositional egimes. Thegeologic similarity between Herminia and Car-mencita suggests he interpretation that the Car-mencitacurves n Figures 15 and 16 correspondothe latter half of the Herminia curves, with just,such a lateral displacement. It then follows that

I / 25oAg/Pb

/ \

-I.0 -0.5 0 0.5 1.0 1.5 2.0 -I.5 -I.0 -0.5 0 0.5 1.0 1.5 2.0log Pb/Cu log Fvo/Cu

FIG. 15. Ag/Pb versus og Pb/Cu, for three differentdepositswithin the Julcani district, Peru. FIG. 16. Ag/Cu versus og Pb/Cu, for three differentdepositswithin the Julcanidistrict, Peru.


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.4 STUDY OF MET.4L R.4TIOS 361the first half of the Herminia sequencemight befound at depth and to the south n Carmencita.As a final statement, t is emphasized hat the'techniquesnd ideasdevelopednd used n thisstudy could have a more general application,and theauthors would like to encourage heir use in otherore deposits. Which ratios are studieddepends neach particular case upon the metal content dataavailable, and thus may vary from one part of amineralizedarea to another. The Pb/Cu ratio is alimited zoning ndex in that it cannotbe applied toareas that are impoverished n one of these metals.In addition, careful paragenetic studies must bemade before decifering metal ratio results.

ConclusionsThis work leads to the following conclusions:1. Major element metal ratios can be used ef-fectively to describequantitativelya zoned ore de-posit and suggest he directionof movementof min-eralizing solutions.2. In certain depositshe monotonicallyncreasingbehavior of Pb/Cu values servesas a suitablezon-ing index. The rate of changeof the zoning indexvariesaiongdifferent athsand from depositodeposit.3. Other metal ratios plotted against the zoningindex may provide insights into similarities anddifferences mongmineralizingsolutions nd deposi-tional regimes.

AcknowledgmentsCompafiia de Minas Buenaventuraprovided theopportunity for the study of the Julcani district,

Peru, and agreed o complete ccesso all availableinformation on the area. To the numerous officials,geologists,engineers, and other individuals theauthors would like to express their gratitude andappreciation. Our thanks go particularly to Al-berto Benavides, orge Benavides, duardoRubio,Mario Arenas, and Franco Peracchio.P. C. G.

DEPARTMENT OF EARTH AND PLANETARY SCIENCESCITY COLLEGE OF THE CITY UNIVERSITY OF

NEW YORKNEW YORK, NEW YORK 10031U.P.

DEPARTMENT OF GEOLOGICAL SCIENCESHARVARD UNIVERSITY

CAMBRIDGE, SSACHUSEVrS2138.tune 25, October 26, 1973REFERENCES

Broderick, T. M., 1929, Zoning in the Michigan copperdepositsand its significance: EcoN. GEOL.,v. 24, p. 149-162.Brown, W. H., 1935, Quantitative study of ore zoning,Austinville Mine, Wythe Co., Virginia: EcoN. GEOL.,V. 30, p. 425--433.Feiss, G. P., 1970, Arsenic and antimony partitioning be-tween the tennantite-tetrahedrite and enargite-famatiniteseries: Ph.D. thesis, Harvar.d Univ., 187 p.Gross, W. H., 1956, The direction of flow of mineralizingsolutions, Blyklippen Mine, Greenland: EcoN. GEOL.,V.51, p. 415--426.Koch, G. S., and Link, R. F., 1967, Geometry of metaldistribution in five veins of the Fresnillo Mine, Zacatecas,Mexico: U.S. Bur. Mines Rept. Inv. 6919, p. 1--64.Link, R. F., and Koch, G. S., 1967, Linear discriminantanalysis of multivariant assay and other mineral data:U.S. Bur. Mines Rept. Inv. 6898, p. 1-25.Petersen, U., 1969, Regional control of Julcani: PrivateRept., Cia. de Minas Buenaventura, Lima, Peru, 2 p.Sims, P. K., and Barton, P. B., Jr., 1962, Hypogene zoningand ore genesis, Central City district, Colorado: Geol.Soc. America Spec. Vol. D, Buddington Vol., p. 373-395.

goodell and petersen-1974

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