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612 SCIENTIFIC COMMUNICATIONSCONTACT MET.dSOM.dTIC COPPER MINER.dLIZ.dTION

THE C.dLZ.dD.d MINE, PERUW. C. STOLL

The Calzada oppermine s 40 km S15Eof La Oroya, n the Cochasregion f theDepartmentf Junin, entral eru,and20 km eastof thecrestof the Cordillera Occidental, at an altitude of 4,300 meters. It is reachedthrough locllapampan the Oroya-Huancayoighway, y way of the settle-ment of Pachicuyo. Chalcopyrite re occursn siliceous onglomerateeds,and was worked for a time by the Volcan Mines Co. of Lima. In 1956about700 tonsof ore with an average radeof 5% Cu had beenmined nan open cut from a conglomerate ed at least 6 m thick.At the mine,a maximum tratigraphichicknessf 70 m of light coloredconglomeratesnd quartzites, ll more or lessmineralized,s exposed,hestrata trikingN60W anddipping 0SW. The bases concealedy groundmoraine. Overlying t with slightangularunconformitys a series, t leastseveralhundredmeters hick, of evenlystratifiedblack,brown and dark redbedsof which he lower members re graywacke. This formation esemblessomewhathe continental ertiary Rimacformation, xposedn the canyonof the upperRimac river, near Casapalca.No intrusive ocksare exposedat the mine.

The conglomeratesnd nterbeddeduartzites re compact, ementedndmetasomatized.They are little fractured and no mineralized aults wereseen. The conglomerateonsists f rounded, ray quartzitepebbles to 2in diameterand white chert pebbles,all in a matrix that has been almosttotally eplaced y silicatesndsulfides. Microscopicallyhe quartzite ebblesconsistof equal-sized, oundedquartz grains (0.1 to 0.4 mm in diameter)anda few chertgrains,with quartzcement ndwith smallquantities f intro-ducedsericite,calciteand chlorite. The chert pebbles, omeof which showsedimentary edding,are preferentially eplacedby the epigeneticminerals.The chert appears s a micromosaic f quartz with some ine, probablysedi-mentary, material, and traversedby microveinlets f coarser,recrystallizedquartz. Some of the matrix is quartz-cementeduartz grains. Most of itis composedof idiomorphicquartz and coarse white calcite, with smallerquantities f garnet,epidote,biotite,sericite,penninite,specularite nd coarsepyrite and chalcopyrite. The pyrite s cubic; he chalcopyriteorms rregularblebsamong he other metasomaticmineralsand small veinlets n the chertpebbles. It was evidently he last mineral to crystallize.Where not completely eplaced, he quartzite ayers interbeddedwith the'conglomeratesre,viewedmicroscopically,omposedf a mosaicf rounded.quartzgrains-0.04 o 0.12 mm in diameter,cemented y quartz and replacedby spotsof garnetwith lesserquantitiesof biotite, epidote,sericiteand calcite.One suchbed has beenconvertedalmost otally to brown grossularitegarnetoccurringas tiny idiomorphic rystalsshowing winning and growth zoning,togetherwith tracesof secondary uartz,sericite, hloriteand.calcite.

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SCIENTIFIC COMMUNIC.,'t TIONS 613The oligomictconglomeratesnd interbedded rthoquartzitesre inter-pretedas second-cycleediments, omposed f detritusoriginating n someolderseriesof quartzites nd cherty imestones.They may be transgresslyebeachdeposits, nd may be found to be associated, t somehorizon, with

limestones,s is indicated y the large quantities f secondaryalcite hatreplace them.The overlying raywackes composed icroscopicallyf a fine darkpastein which are scattered ngular ragments f quartz, chert and plagioclase,patchesof fine-grained arbonate nd pebbles nd quartz grains derivedfrom the conglomerateseneath. The graywackehas been altered appar-ently at the same ime as the underlying ormation. It containsmicro-veinletsof epidoteand spotsof mixed diopside, arnetand epidote.The suite of introducedminerals,rich in calcium, ron, copper andsulfur, s typicallyof contact-metasomaticrigin. It is probably elated oa concealed ranitic intrusive, which has also affected,at greater depth,calcareous eds. The high degree of replacement f the dense siliceoussediments ould indicate hat the solutions ossessedreat permeative ndreactivequalities.The Norma deposit,near Lake Quiullupa and a few kilometersnorth-west of Calzada, s a similar occurrence,n the sameconglomerateormation,which s here aboutvertical. The pyrite-chalcopyritere appearso followa bed alongwhich faultinghas occurredn a directionN50W, parallel othe bedding. The Manon mine, 10 km southwestof the Norma, containedsimilar eplacementhalcopyritere, containing,n 1956,as muchas 6% Cu,0.5% Zn and 2 gramsper ton eachof gold and silver.Theseoccurrencesf replacementresof chalcopyrite ight be akin tothe contact-metasomaticredepositsf southern eru (CuzcoandApurimac),suchas Tintaya, Ferrobamba nd others,which occur n limestonentrudedby smallmasses f dioriteor quartzmonzonite. They are obviously istinctfrom the other types of copper occurrencen Peru. These are: 1) thepolymetallic resof the Cerro de Pasco ype, generallywith silver and min-eralsof arsenic nd antimony, ) the secondarilynrichedporphyrycopperores of southwestern eru (Toquepala,etc.), and 3) the red bed copperdepositsof the Puno formation of southernPeru.

BUENOSAIRES, ARGENTINA,January 18, 1961