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ECONOMIC GEOLOGYAND THE

BULLETIN OF THE SOCIETY OFECONOMIC GEOLOGISTS

VOL. 50 MAY, 1955 No. 3

GEOLOGY OF THE ATACOCHA MINE,DEPARTMENT OF PASCO, PERU

ROBERT F. JOHNSONCONTENTS

PAGEAbstract .............................................................. 249Introduction .......................................................... 250Descriptive geology .................................................... 252Introductory statement .............................................. 252Triassic and Jurassic imestone ...................................... 253Cretaceous rocks ................................................... 253

Intrusive rocks ..................................................... 255Contactmetamorphism ............................................. 255Alluvium .......................................................... 255Structure ............................................................. 255General statement .................................................. 255Folds ............................................................. 256Faults ............................................................. 257Geologic history ...................................................... 258History and productionof the Atacocha mine ............................ 259General features of the Atacocha mine ................................... 259Ore deposits .......................................................... 262Replacementbodies ................................................. 262Veins ............................................................. 264Mineralogy and classification .......................................... 266Hydrothermal alteration ............................................... 268Depositionalhistory ................................................... 269Conclusion ........................................................... 270References ............................................................ 2?0

ABSTRACTThe Atacocha lead-zinc mine, 15 km northeast of Cerro de Pasco inCentral'Peril, was the object of a detailed geologic nvestigation n 1952by geologistsof the Instituto Nacional de Investigaci6n y Fomento Mi-neros and the U.S. Geological Survey. The mine is about 4,000 m abovesea level, in an area of steep opographywith about 900 m of relief.Rocks exposedat Atacocha consistof limestone,chert breccia, quartzsandstone, nd basalt that range from Late Triassic to Early Cretaceous

Publication authorized by the Director, U.S. Geological Survey.249

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250 ROBERT F. JOHNSONin age. These rockshave been ntrudedby dacite of probableTertiaryage.The most prominent structural features are the east limb of a largeantidine east of the mine, and a south-plunging syndine to the west.The two folds are separatedby a major fault zone. The rocks west ofthe fault have been dragged along it and are bent into a small cross fold,which seems o be the primary control for the localizationof the ore. Themain structural feature in the mine is a reverse fault on which limestonehas been thrust over quartz sandstone.The Atacochamine containsabout 30 km of undergroundworkings on16 levels,spacedover a vertical distanceof 700 m. The workingsex-plore portionsof a reverse ault and follow veins in limestone nd sand-stone. Productionwas 600 tons of ore per day in late 1953.Ore occurs in veins filling tension fractures in limestone and sandstonenear the axis of the cross fold and in irregular replacementbodies in anarrow limestonehorizon above and close o the reverse fault. Replace-ment bodies are also found west of the syncline in limestone dose to thecontact between imestone and overlying chert breccia.The typical sulfidemineral associations galena, sphalerite,and pyrite,with minor chalcopyrite and jamesonite. Gangue minerals other thanpyrite are calcite, rhodochrosite,clay minerals, and a minor amount offluorite. Late veins of realgat and orpiment cut the ore minerals. Verylittle oxidationor supergene nrichmenthas taken place.

INTRODUCTION

Purposeand Scope.--During the courseof a nationwidesurvey of Peru-vian lead and zinc resources, ndertaken ointly by the Instituto Nacional deInvestigaci6n FomentoMinerosand U.S. Geological urvey,a few districtswere examined in detail. The Atacocha area was one of those selected,both because f its present mportantproductionand becauset is little knowngeologically. This report describessome of the geologic eatures of theAtacochamine, the largest in the district.Locationand Access.--The Atacochamine is in the Departmentof Pascoin CentralPeril, 15 km northeast f the greatcopper-lead-zinceposit t Cerrode Pasco Fig. 1). Atacochas near the centerof a miningdistrictextendingabout 10 km from Machicn on the north to Milpo on the south. It is by farthe most mportant mine in the district with a productionof 600 tons of lead-zinc ore per day in late 1953. At that time Milpo producedabout 75 tonsof ore per day and Machictnabout 5 tons.Allweather roads ead from Atacochaand Chicrin (the mill site) to therailroad at Cerro de Pasco. A 3-kilometer aerial tram and a 20-kilometer roadconnectAtacochaand Chicrin. An underground onnection as also beenmade by meansof raises rom the Chicrin adit, or 3,600 level, which passesunder he principalmine workings. The nearestairport with scheduled erv-ice s at I-Iumuco, 0 km by road north of ChicrimTopo#raphy ndRelief.--The campat Atacochas 4,000m abovesea evel;mine workingsextend rom 4,300 m down o 3,600 m, the altitudeof the millat Chicrin.

The area s on the eastern dgeof the high centralplateauof Peril and is

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GEOLOGY OF THE ATACOCHA MINE 251characterizedy veryruggedopography;heslope ver hemineworkingsbetween,000and4,300m, for example,s 30-35 degrees.Quebrada hicrin,which rainshe area, s a tributary f the HuallagaRiver,a north-flowingtreamhat s partof theAmazoniversystem.TheHuallagaRiver s an actively owncuttingtream nd relief s considerable,400m in theareamappedn Figure andnearly 00m betweenheriverandthe higherpeaks.ClimatendVe#etation.--Aetanda dryseasonharacterizeheclimate,with hedryseasonastingromMay to October.During hedry season,showersooccur,ut n generalhedays reclear. Nights recoolhrough-out heyearwith rosts ommonbove,500m. Near heendof thedry

FIG. 1. Indexmapof Peru,showinghe ocation f the Atacocha ine.seasonhe carcityfwater ecomesproblemtAtacocha,utChicrin,eingnear he river,hasamplewaterall year.Mostof theAtacocharea s abovehe tree ine. Scatteredreesare foundin shelteredlaces p to an altitude f 4,000m, butabovehisaltitudeheonly roundovers grassesndherbs.Eucalyptusasbeenntroducedndis found s ar up heHuallaga iverasChicrin, utstands uitableor minetimberare not foundabove3,000m.FieldWork ndAcknowledgments.--Fieldork egann May1952 ndcontinuedntermittentlyntilDecemberf hat ear. A plane-tableapwasmade f the mmediateine rea, singhecompanyriangulationet orhorizontalndverticalontrol. heundergroundorkingseremappedgeologicallyna scalef 1:500 singompanyaps.Magneticorthsusedfor directionnall maps f this eport.

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252 ROBERT F. JOHNSONThe aid of two geologists f the Instituto Geo16gico el Peril is gratefullyacknowledged.Alberto Manrique P. assistedhe writer from May to Octo-ber, and Ing Eliodoro Bellido B. worked from October to 'December.

Wholehearted ooperationwas received rom all personnel f the CompafilaMinera Atacocha. Ing Edgardo Portaro, generalmanager,made the neces-sarypreliminaryarrangementsor the study. IngFelipe BautistaC., generalsuperintendent, rovided iving accommodationst Chicrin, and Ing CarlosValdiviesoS., mine superintendent, rovidedaccommodationsnd officespaceat Atacocha. Both gave freely of their time to discussproblemsconnectedwith the work, giving the writer the benefitof their long experience t Ata-cocha. Ing Ortiz T. providedassistants nd guides,as well as any informa-tion neededpertaining o company urveys. The other engineers t Atacocha,in particular Jorge Quintana S., who were in daily contactwith the miningoperations, ssisted he writer by calling his attention o various points ofgeologicnterest ound in the courseof mining. Officialsof HochschildCia., Ltda., kindly allowedaccesso an unpublishedeport on AtacochabyOtto Welter.The work wasgreatly acilitatedby the cooperation f Dr. JorgeA. Broggi,Directorof the Instituto Geo16gicoel Peril, who madeavailable he resourcesof the Instituto. Mr. Frank Simons,Chief of the Geological urvey party inPeril, madevaluablesuggestions hen visiting the party in the field, and alsoreviewed he manuscript or this report.Previous Work.--Earlier geologicwork in the Atacochaarea consisted fa surface econnaissancey J. B. Stone,of the Cerro de PascoCorp., in 1928

and studiesof the mine by Otto Welter in 1929 and by David Torres Vargasin 1948. The area was mentioned riefly n a report by Diaz (1).The report by Welter was made before the mine was developed o anygreat extent. His interpretationof the surfacegeologydiffered from that ofthe writer in that he consideredhe quartz sandstoneo be older than thelimestone,he reverseof the present nterpretation. As a consquencef hisstratigraphicnterpretation e consideredhe synclinal old in the sandstone,shown n Figure 3, to be an invertedanticline. In addition,a fault was neces-sary between the sandstoneand the limestone to the west and north. Thewriter'soppositenterpretationwill be discussedn subsequentections f thisreport.Geologicmapsof D. Torres Vargas were madeavailable y the company.The mapsare similar o the writer's,except or a differencen the correlationof the alteredrocks. Torres Vargas mappedmost of the alteredrocksasquartzite,whereas he writer believeshat they are largelyaltered imestone.

DESCRIPTIVE GEOLOGYThe generalgeologyof the Cordillera Central in the Department of Pascoand Juninhasbeendescribed y McLaughlin 3), Steinmann 5) and Harri-son (2). In brief, the historyof the region, rom the middlePaleozoic o theUpper Cretaceous,was one of intermittent marine sedimentation,with the

exception f somevulcanism nd the deposition f nonmarine ed beds n thePermian,and again n the Cretaceous. Orogenybegan n Late Cretaceous

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GEOLOGY OF THE ATACOCHA MINE 253timeandcontinuedntermittentlyhroughhe Tertiary. The sedimentaryrockswere folded and faulted,and intruded by igneous ocksof intermediatecomposition. Depositionof metallic sulfides ook place after the intrusions.The region was eroded to a low undulatingplain, which was subsequentlyelevatedo its present osition y a seriesof threeuplifts. The region s beingerodedso rapidly at present hat excepton the central plateau here is littleopportunity or supergene nrichmentof the mineral deposits.Triassic o Cretaceous edimentaryocksand Tertiary intrusives re theonly rocks exposednear the Atacochamine. The mine is locatednear theaxis of a small old caused y drag alonga major fault. Small reverse aultsprobablyassociated ith movements n the large fault are found n the mineand are important ore controls.TriassicndJurassicimestone.--Theldestocks xposedt Atacochaare limestone nd interbedded haleof Late Triassicage. They crop out overa wide area north, east,and southof the mine (Fig. 2). Gray to black ime-stone s the predominant ock, and black shalea minor constituent. Chertnodulesand lensesare common n the limestone. Individual beds range inthicknessrom 10 cm to 1 m. The entireseries s at least1,300and probablymore than 2,000 m thick, as 2,100 m of conformable imestone beds are ex-posed n the Chicrinadit, although here may be some epetitionby faulting.The base of the series is not seen at Atacocha but is found a few kilometerswest of the mine where the limestonesest with slight angularunconformityon the red bedsof the Mitu formationof Permianage.Limestone nd shaleof Early Jurassic ge rest conformably n the Triassicrocks near Chicfin. Fossils are rare and the contact could not be locatedprecisely. No Jurassicossilswere found near Atacocha, ut the presenceof Jurassicstrata cannotbe ruled out.In the minearea he lowerpart of the section onsistsf thick-beddedraylimestone. Thesebedsare overlainby 100 m of thin-bedded lack imestoneand shale, hat is overlainby 40 m of thick-beddedimestone. Conformablyoverlying his sectionare 40 to 80 m of interbedded recciated ed chert andclastic imestonehat is in fault contactwith quartz sandstone f the Goyl-larisquisgaormation. Ore hasbeen oundonly n and above he thin-beddedlimestone and shale; the lower thick-bedded imestone seemsto be barren.Cretaceousocks.--Resting ith slightdisconformityn the limestonesare bedsof the Goyllarisquisgaormation f Early Cretaceousge. The for-mationwas first namedby McLaughlin 3) from exposurest Goyllaris-quisga,northwestof Cerro de Pasco. In the mine area the formationconsistsof bedded hertandchertbreccia t the base, verlain y dark shale ndfine-grainedsandstoneontaining harredwood fragments; hese n turn areoverlainby white cross-beddeduartz sandstonehat makesup the bulk ofthe formation.The Goyllarisquisgaormation ropsout n the troughof a south-plungingsyncline t Atacocha nd s foundalsoabove he limestonelong he HuallagaRiver at Chicrin.Beddeddark-graychertcan be seen ying on the limestonen the Curi-ajashaarea. This chertand associatedhertbreciaontraststronglywith

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254 ROBERT F. JOHNSON

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GEOLOGY OF THE ATACOCHA MINE 255the brecciateded chertmentioned reviously, ut both may be in the samestratigraphic osition.Basalt lowsare interbedded ith and over i.he upper quartz sandstonebeds. The flowsare metamorphosedy a dacite ntrusion ear the mineandare indistinguishablen the field from fine-grained ordersof the intrusive;exposures ere not goodenough o map the two rocksseparately. Unmeta-morphosedlowsnear Chicrinare found o be olivinebasaltwith diabasictexture. Violet-coloreditanaugitephenocrystsp to 4 mm in diameterarea striking eatureof some lows.IntrusiveRocks.--The sedimentaryocksand lava flows have been n-trudedby dacite or tonalite orphyry). The largestntrusives an irregularstock hat cropsout from the San Gerardoadit northwesterlyo Curiajasha.Dikes and sillsof similarcompositionre foundon all sidesof this stockoveran area of severalsquarekilometers.The rock s greenish ray to dark gray on freshsurfaces, nd weathers oa reddishbrown. Phenocrysts f plagioclase nd quartz up to 3 min. indiameter,with smallercrystalsof hornblende nd biotite, are enclosedn afine-grainedgroundmass. In thin section he composition f the plagioclaseproves to be on the andesine-labradorite oundary, Ano:. Phenocrystsmake up 50 percentof the rock with plagioclaseorming about half of thephenocrysts.The percentagesf quartz, hornblende, nd biotite are aboutequal. Biotite s commonly ltered o an aggregate f carbonate, hlorite, ndepidote. Quartz and plagioclasehenocrystsenerallyhaveroundedcomersor are embayed howing he effectsof resorption. The groundmasss crys-talline and is largely plagioclase nd quartz; orthoclases probablypresentbut couldnot be positivelydentified.ContactMetarnorphism.--Contactffectsof the intrusive ock vary bothin the intrusive bodies hemselves nd in the intruded rocks. Along somecontacts he dacite has a chilled border very similar in color and texture tothe metamorphosedasalt,but on other contacts o chilledborder s evident.Quartz sandstone hows ittle change;alongsomecontacts he sandstone asbeenconverted o quartzite. Contactalterationof the limestone,with the de-velopmentof wollastoniteand garnet, was found in two areas, one on the4,103 level near coordinate 3,600 N, and the other on the mine road about 2km from the mine. Limestoneseems o be unchanged long someporphyrycontacts, articularlynear the smallerdikes.Alluviurn.--No rocksyounger han the daciteare foundat Atacocha. Un-consolidated lluviumcovers he bottomsof valleys,and slopewash and talusmantle from hillsides. This unconsolidatedmaterial is not thick, but it effec-tively obscureshe bedrockover extensiveareas.

STRUCTUREThe mine area can be divided nto three distinctstructuralgroups. Eastof the mine he sedimentsorm the flank of a largeanticline nd havea regular

east dip. South and west of the mine the rocks are folded into a south-plungingsyncline hat has been ntrudedby the igneous ocks. North and

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256 ROBERT F. JOHNSONwest of the mine the beds of limestone seem to be thrown into minor folds thatare difficult to relate to either of the other structures,but the major structureseems o conform o the synclinalstructure ound farther south. Faulting isextensive in the western blocks, and strike faults are found in the mine wherethe 3,600-leveladit crosseshe easternblock, although heseare not apparenton the surface. The ore bodies and veins seem to be confined to the south-western block.

Folds.--The mostprominent old in the area is a faultedanticline hat canbe traced for several kilometers. The east flank of this anticline is well ex-posedbetweenAtacochaand Chicrin; the anticline s faulted, probablynear

FIG. 3. View southward shoving he Atacocha mine.stone s visible at upper right and cross fold on the left.sents he approximate trace of the No. 1 fault.

: . :':- .

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GEOLOGY OF THE ATACOCHA MINE 257A small cross old formed by drag along a large fault was mapped n thesedimentary ock directly over the mine workings. This fold seems o beone of the principal controls n the localizationof the Atacochaore bodies.Ore is foundnear the axis of the fold, in veinsand in replacement odies.Faults.--A large ault zoneextends long he eastsideof the mapped reaand for several ilometers orth and south. Its position s well marked opo-graphically longpart of its length,particularlyon the east side of QuebradaChicrin., ut in manyplacest cutsacrosshe present opography, nd its posi-tion is markedby changesn strikeof the limestone edsand by a breccia one.A 60-meter brecciazone was found where the 3,900-level adit crosses he fault

zone.Total displacement nd direction of movementare not known. The eastsidemay havemovedup and to the north relative o the west side. North ofAtacochahe bedswestof the fault are bent sharplyand dragged o the north

showing hat the movement ad a horizontalcomponent.The fault appears o be premineralbecausehe structures hat control oredeposition re due to movements long t. In the 3,900-leveladit the faultbreccia s recemented,xceptalong he westerncontactwhere a gougezonemay representa later period of movement.The No. 1 fault s the chief ault exposed y the mineworkings. Its traceon the surfaces not well exposed, ut it mustcropout at the baseof the brec-ciated ed chert zone; blackgougemarksthe outcropof the fault just southof La Laguia. The No. 1 fault dips 45 to 60 degrees ast below the 4,103level and s the contactbetween riassicor Jurassicimestonen the hangingwall, and sandstone f Cretaceous ge in the footwall; this relationshipndi-cates hat the fault is a reverse ault. The fault closely ollows he strike ofthe bedding,even o the turn,made n the cross old. On and above he 4,103level the No. 1 fault steepens nd joins a steepwest-dippingault (Fig. 4).This latter fault is on the contactof the red breccia nd sandstonend closelyfollows he dip of the bedsas well as the strike.No direct evidencewas found for the premineralor postmineral ge Ofthe No. 1 fault. Veins terminateagainst t, but no evidencewas found thatindicated ffsetting f the veins. The abundant ouge ormedalong he faultmay have inhibited he formationof potentialore-bearing tructures. Mr.Bautista reported that on one level mineralization seemed o be continuousacrosshe fault zonebetweenhe Veta Prima (Main Vein) andthe No. 1 orebody, which also suggests premineralage for the fault. Smaller faultsbranching rom the No. 1 fault in the sandstone o offsetveinsa few meters,so possibly here was both premineraland postmineralmovement.Numerous maller aultswere observedn the course f surfacemapping.The most commonstrike directionsare within a few degreesof north andBI 60-80 W. Many other aultsare undoubtedly resentbut escaped otice.A probable ault exists n the San Gerardoarea where the quartz sandstonesouthof the porphyry mass n the quebrada s offset o the west relative to thesandstone orth of the porphyry. The porphyry may have been intrudedalong a zoneof weakness aused y faulting.

Hydrothermal lteration nd offsetting f the contactbetween he che.rt

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258 ROBERT F. JOHNSONand limestonen the Curiajasha rea ndicate aulting,and smallnorthwest-striking aultshavebeen ound,but no large ault was found hat couldbe theonemapped y Welter (8). He mapped large ault on the contact, xtend-ing from near Curiajasha ast o La Laguiaand on to the north,and indeedsucha fault wasessentialo his stratigraphicnterpretationhat the sandstoneis older han he imestone.There s evidencef local aultingon thecontact,but in otherplaceshe chertcanbe seen yingconformablyn the limestone.

GEOLOGIC HISTORY

Limestoneof Triassic age is the oldest rock exposed n the mine area,eitheron the surface r in the mine workings. The limestones largelyre-crystallized, ut its association ith limy shales ndicates hat it is biochemicalin origin rather than clastic 4, p. 293). The presence f black imy shalealsosuggestshat stagnant onditions xistedat timesduring the depositionof the limestone.

The area was probablyuplifted and eroded n the interval between hedepositionf the limestone nd the Goyllarisquisgaormation, ven hough hetwo formations re largely conformable.A conglomerateontainingime-stoneand chert cobbless presenton the contactat Chictin. The contact smarkedby chert and chert breccia n the mine area, indicatinga possiblemarine basin that did not extend as far east as Chicrin. The isolated lime-stoneoutcrops n the Curiajashaarea, near the contact,may representanunevensurfaceon which the chert was deposited.The sea recededduring the period n which Goyllarisquisgaedimentswere deposited. The marinechert was followedby carbonaceoushaleandfine-grainedsandstoneepresenting eltaicconditions, nd these n turn werefollowed y crossbeddeduartzsandstoneshat probably epresentlood-plaindeposits. Volcanicactivity begannear the end of the period of sandstonedeposition nd brought that period to a close. A new marine invasion ol-lowed as shownby marine limestone edsabove he basalt lows. This lime-stonehas beeneroded rom the mine area, but is presentat Chictin.After a period of unknown duration the sedimentswere folded and faultedand subsequently ere intrudedby dacite. The dacite ntrusions n CentralPeril are generallyconsideredo be Tertiary in age.Sometime fter the intrusion, he rockswerealteredby hydrothermal olu-tions,and the metallicsulfideswere deposited. Galena-rich einswere formedin the quartz sandstone,nd galena-sphalerite-pyriteeins and replacementbodies in the limestone.A largeregion n centralPeril was reducedo an area of low relief byPliocene ime, accordingo McLaughlin (3) and Harrison (2), and subse-quentlyelevated o its presentheight n three stages.Glacierswere present n the Atacocha rea during the Pleistocene ut theirextent is not known. Excavations n Quebrada Chictin above the mine ex-posedunsortedgravels, probablyof glacial origin. Closedbasinsand U-shaped alleys oundabove ,000 are alsoevidence f glacialactivity.At present rosions proceedingorapidly hat there s little opportunity

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GEOLOGY OF THE ATACOCHA MINE 259for soil development, r for the development f zonesof oxidationor super-gene enrichmentof ore bodies. Sulfide minerals are found at the surface,notably n the San Gerardo area.

HISTORY AND PRODUCTION OF THE ATACOCHA MINE

The presence f mineralization t Atacochawas known in colonial imes,but no work was done due to the low silver content of the ore.Modern explorationof the depositbegan about 1910. J. H. Flemingabandonedhe propertyafter doinga smallamountof work near the present4,240 level. Anothergroupalsoabandonedhe propertywithoutdoingmuchwork.About 1920 a claim on the propertywas filed by Sr. J. F. Gallo of Cerrode Pasco. He carried on intermittentoperationsuntil 1929 when conditions

TABLE 1METAL PRODUCEDFROM THE ATACOCHAMINE, 1940-1952 (METRIC TONS)

SilverYear Oremilled Lead Zinc (kilos)1940 49,019 3,337 9,3351941 47,470 4,054 2,734 10,2081942 50,868 5,615 6,215 12,6281943 51,146 5,769 5,267 14, 7181944 57,329 7,710 9,195 16,6361945 58,519 7,240 9,657 17,8941946 52,257 5,768 7,373 16,9671947 54,902 5,628 4,529 13,6401948 56,613 4,409 2,242 10,9801949 59,263 6,105 1,852 14,5831950 111,162 9,306 4,419 26,1001951 155,106 11,357 5,135 28,9311952 192,692 14,950 6,698 35,216

Data from "Anuario de la Industria Minera en el Peril" and from company records.caused y the world economic risis orceda shut down. In 1936 he resumedoperations n the 4,000 evel,and subsequentlyound he No. 1 ore body. Onthe basisof this discovery he CompafiiaMinera Atacochawas formed. Thecompany evelopedhe mine rapidly,and expandedt to its present mportantposition.Production igures or the early operations re not known. Table 1, show-ing the production ince1940, gives some dea of the steadygrowth of theoperation.

GENERAL FEATURES OF THE ATACOCHA MINE

The mine is developed y about30 km of workingson 16 levels,coveringa vertical distance f 700 m. Vertical spacingbetween evelsranges rom25 to 55 m, with onegapof 175 m betweeenhe Chicrin unnel,or 3,600 evel,and the principal mine workingsabove. The altitudesused for numberinglevelsare basedon an arbitrary datum that is about 25 m too low, but the

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260 ROBERT F. JOHNSONdistances etween evelsare correct. All the levelsare connected y internalshaftsor raises. The workings follow veins in limestoneand quartz sand-stone,and in the upper evelsalsoexplorea strong ault zone extendingsouthtoward the San Gerardo area.

Severalunmineralizedaults containing bundantgougeare found in themine. The relationbetween our numbered aults of this type is best shownby Figures4 and 5. Number 1 fault is the main fault alongwhich he lime-stonehas been hrust over the quartz sandstone;t has been raced from the3,805 level up to the 4,103 level. At the 4,103 level the fault steepens ndjoins a steeplydipping fault separating he brecciated ed chert and reddishquartz sandstone. The downwardextensionof this latter fault is not known;it does not cut the "D" vein on the 4,048 level. Faults numbered 3 and 4

LIMESTONE146 LEVEL

_103

4._048

4--000$_70

LIMESTONE' OS %*'x _775 I00 200ETER JFro. 4. Cross section through the Atacocha mine.

seem o be splits rom the footwallof No. 1 fault as shownon the section(Fig. 4).The mportancef theNo. 1 fault nd tsextensionbovehe4,103evelis its relation o the replacement re bodies. The largestore bodies oundto date have been n the limestone angingwall within a few metersof thefault. The abundantgougemay have actedas a barrier to the ascendingsolutions rom which the ore minerals were deposited,concentrating hembelow the fault. In areas where the fault surface was broken by crossstructures, uchas Veta Prima (Main Vein) and possibly he Tres Mosque-terosdike, the solutions ad easyaccesso the limestone nd the replacementof favorablebedsby ore mineralswas possible.The No. 2 fault is a steep ault belowNo. 4 fault that seemso follow ashaleband between he quartz sandstonend the underlying hert breccia.The intersection f the two faultsplunges outheast.Neither the shalenor

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GEOLOGY OF THE ATACOCHA MINE 261

tiJ0

O

GEOLOGY BY R F JOHNSONAND E BELLIDO t ,1952

SANDSTONE,] CHERTNDHERT BRECCIAF LIMESTONE.' FAULT SHOIh'ING IP.... -' INFERRED FAULTw- VEIN SHOWING DiP//// OREBODY TOP OF WINZE BOTTOM OF WINZE

LU

ATACOCHA MINE

GEOLOGY OF THE

3900 LEVEL

215 50SCALE IN METERS

lOGi

FIG. 5. Geology of the 3,900 level, Atacocha mine.

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262 ROBERT F. JOHNSONa fault was found above the No. 4 fault. On the 3,900 level the No. 2 faultcontains bundant ouge,but on lower levels t is markedonly by shearingin the ndurated hales. The Veta Primaterminates n the westagainstNo.2 fault or its enclosing hale. This vein may be faulted,but, as it narrowsand becomes yritic on entering he shale,a more probableexplanationsthat the vein fissurediesout in passingrom the brittle sandstonesnto theincompetenthale, Not enoughwork hasbeendonewestof the shale o seeif the fissuredevelopsgain n the brittle chertbreccia.Innumerable small fractures are found in the mine. Some are unmin-eralized,otherscontainonly pyrite, and otherscontain phalerite nd galenaas well as pyrite.

ORE DEPOSITS

Ore is found as irregular replacementsn limestone nd as veins n eitherlimestone,quartz sandstone, r chert breccia.Replacement odies.--Several eplacement re bodieshave been foundduring the development f the mine. The majority have been ound n thelimestone boveor east of the No. 1 fault or its upper extension; he No. 5ore body s partly in red chert brecciaeast of the fault (Fig. 6). Ore bodieshavealsobeen oundbelow he 3,900 evel n the limestone estof the synclinein quartz sandstone. The contactof the chert brecciaand limestone s faultedon the 3,900 level, but seems o be a normal depositional ontactbelow.Veins havebeen ollowedwestward hrough he quartzsandstonend graychertbreccia, nd on entering he limestone omeof themhave openedup intoirregular replacements. Not enoughwork has beendoneas yet to determinethe sizeof thesebodies. A largeore body n limestonewas oundon the 3,600level west of the chert breccia.

The replacement eposits re the major sourceof ore. They consistofincomplete eplacements f limestoneby pyrite, sphalerite,and galena,withvaryingamounts f gangue, argelyclay minerals. They vary widely n bothsize and grade. The No. 1 ore body, now largely worked out, had a pitchlengthof about250 m, a breadthof 40 to 70 m, and a thickness f from 4 to10 m. The No. 4 ore body has a stope ength of 35 m and a thickness fas muchas 25 m but may not have he verticalextent of the No. 1 ore body.The grade of the ore ranges rom 8 to 20 per cent combined ead and zinc inapproximatelyequal proportions.A spatial relationship xists betweensomeore bodiesand the strongerveins. Ore body No. 1 and the Veta Prima are found togetheron oppositesidesof the No. 1 fault. Fault No. 3 splits from the No. 1 fault near theVeta Prima (Fig. 7) and may also have somerelationshipwith the localiza-tion of the No. 1 ore body. Ore bodies and 4 are located t the change nstrike of the No. 1 fault, near the axis of the cross fold mentioned earlier,andon the opposite ideof the No. 1 fault fromthe "D" vein. OrebodyNo. 5is adjacent o the Tres Mosqueteros ike that may cross he No. 1 fault.These elationshipsuggesthat the ore bodiesormedwherestrong racturescrossed the No. 1 fault and intersected the favorable limestone horizons above.

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GEOLOGY OF THE .4TACOCHA MINE 263

OREO(DY

+' DACITEr' SANDSTONEj-- BRECCIATEDED CHERT- LIMESTONE*' FAULT SHOWINGDIP...-" INFERRED FAULT VEIN SHOWING DiP..... CONTACT%-'" ORE BODY---.'j INACCESSIBLE WORKING

SS tt

tt

u

NO 5,OREBODY

ATACOCHA MINEGEOLOGY OF THE

4103 LEVEL

GEOLOGY BY R F JOHNSONAND A MANRIQUEP, 1952 0 25 510 I70iSCALE IN METERSFIG. 6. Geologyof the 4,103 level, Atacochamine.

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264 ROBERT F. JOHNSON-

The dacite doesnot seem o have been important in the formation of orebodies, xcept or the Tres Mosqueteros ike, which orms he northern imitof the No. 5 ore body. The contact-metamorphicrea on the 4,103 level inwhich garnetand wollastonite ere formed s not ore bearing.

4-000 Level

3970

3935

3900

3870

t

$840

'. I

IIIIIIIiii

ii

Junction of No.Iand No.$ faults

0 25 50 metersI , I

Junction of No.I faultand Veto Prima

Fro. 7. Longitudinalsectionof a portion of the No. 1 fault, showing he pro-jectionof the No. 1 ore body o the fault plane,and he relationof the ore body othe Veta Prima and the No. 3 fault.Veins.--Veinsare found hroughouthe mine in any rock type. Theyrange n size rom rregular tringerso largeveins,suchas the "D" vein,whichhasbeen ollowedor over 150 m along he strikeand for over 300 mvertically. The veinsnowbeingmined re usuallyess han 1 m in thickness,but locally may be 3 to 4 m thick.Veins of minable width are found in limestone and thin-bedded shale above

the 4,000 evel,and eastof the No. 1 fault. They are mostnumerous nd

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GEOLOGY OF THE ATACOCHA MINE 265largestnear the axis of the cross old shownby the bend n the No. 1 fault(Fig. 6). The veinsnarrowand pinchdownward nd havenot been oundbelow he 3,970 level. The reason or their pinching s not known; the curvein the No. 1 fault is not as pronounced n the 4,000 level as it is above,sopossibly he cross old dies out in depth. Very little work has been donealong he No. 1 fault below he 4,000 evel. The veins n limestoneange ngrade from 15 to 28 percent combined ead and zinc, with zinc generallyslightly n excess f lead.Veins in quartz sandstone nd chert brecciaare found on all levels; belowthe 4,000 level they are the principal sourceof ore. The Veta Prima andthe "C" and "D" veinsare the largestof this type. The Veta Prima has beenfound on every level from the 4,000 to the 3,775 and it still continues own-ward. The No. 1 and No. 2 faults form the east and west limits of the VetaPrima, which strikesnorthwesterly nd has a steepsouthwest ip. The veinis 80 to 100 m long and 1 to 4 m wide.The "C" vein strikesnearly east and dips 50 to 70 N. It is limited onthe eastby the No. 1 fault and on the west by the "D" vein or by the No. 4fault. The segment etween he No. 1 and No. 3 faults s missingbelow he3,935 level. The "C" vein wedgesout above he 4,000 level against he inter-sectionof the "D" vein and No. 1 fault, but lengthens apidly down dip reach-ing a length of about 100 m on the 3,935 level. Below the 3,900 level onlysegments f the fault are known, but it has been followed to the 3,840 level.The vein is from 0.5 to 2 m wide.

The "D" vein has been followed from above the 4,192 level down to the3,900 level. It has a nearlyverticaldip and the strikevaries rom N 40-70W. The No. 1 fault is the eastern imit on most levels. On the 3,900 level,however,he vein hasbeen oundonlywestof the No. 3 fault; a diamond-drillhole drilled east of the fault failed to locate the vein. The vein dies out inchert breccia o the west on some evels,but on others it has not been followedbeyonda large fault. On the 3,900 level the vein has not been ound west ofthe No. 4 fault. The "D" vein hasa maximum engthof about150 m and awidth of from 0.5 to 2 m.The gradeof the veins n sandstoner chertbreccia anges rom 10 to 17percent ead and from 2 to 3 percentzinc. The reason or the difference nthe lead-zinc ratios in ores from the siliceous rocks and those from the lime-stone is not known.

Veins n dacite re mostlysmall,but ore-bearingractures re widespread.Many of the dacitecontacts re mineralized. The Tres Mosqueterosike isfoundon all the levels bove he 4,000 evel n the hangingwall of the No. 1fault. It contains einsof rainablewidth, and the No. 5 ore body extendssouth rom it. The dikehasnot beenexplored o see f it crosses o. 1 fault;a dacitedike mappedon the surfacewest of the fault may be the sameas theTres Mosqueterosdike.A suggestion f a radial pattern can be seen n the plan of the veins onthe 4,103 evel (Fig. 6). The veins n and near the Tres Mosqueterosikestrike nearly east, but those farther north strike a little north of west. Thispatterncouldbe due to tension ractures ormedduring the formationof the

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266 ROBERT F. JOHNSONcross old. Fracture filling seems o have been the principal process n veinformation; replacements of minor importance.

MINERALOGY AND CLASSIFICATION

A brief descriptions given of the modeof occurrence nd paragenesis fthe ore and ganguemineralsnoted during the Atacochastudy. A detailedmineralogicstudy of the ore was not attemptedso the list is probablynotcomplete.Sulfidesand Suljeosalts

1rsenopyrite.--Smallmounts f arsenopyritere found ntergrownwithand replacingpyrite, and in turn being replacedby sphalerite nd galena.The arsenopyrites massive;crystal faceswere not noted.Chalcopyrite.--Coppers a minor constituentof Atacochaore and is notof economic nterest. Chalcopyrite, he most commoncopper mineral, isfound hroughouthe mine n scatteredmallgrainsandas blebsn sphalerite.Galena.--Galenas one of the principalore minerals. Small crystalsarefoundbut the usualoccurrences in granularmasses ith typical cubiccleav-age. The cubesn a specimenrom the 4,263 evelare oriented o as to givethe specimen banded ppearance.Cleavage ragments re small and cubesover5 mmonan edge re rare. Galenawasdepositedftermostof thepyriteand probably ontemporaneouslyith the sphalerite. Galena-bearingeinscanbe seencuttingpyrite veins. The "D" vein hasa galena-rich enterbor-dered by sphalerite nd pyrite so at least someof the galena seems o be

younger than sphalerite.The silvercontentof the ore variesdirectlywith the galenacontent. Nosilvermineralswere seen,so probably he galena s argentiferous.]amesonite.--A ead-antimonyulfide, entatively dentified s jamesonite,was oundon the 3,900 evel. The mineralshows goodcleavagen one di-rection. It is associated ith galena,sphalerite, nd pyrite.Marcasite.--MarcasiteromAtacochas listed n the Catb,ogoMinera16gicoNacional,but nonewas notedduring he mine mapping.Orpiment.--Bladed and micaceous ellow orpiment is found associatedwith realgar. Orpiment s more abundant han realgar,but mostof it seemsto be an alterationproduct.Pyrite.--Pyrite is the mostabundant ulfiden the area,andwith sphaleriteand galena orms he characteristic ineralassemblage.Pyrite, however, sfound over a much wider area than the other sulfides. Concentrations ofpyrite are commonly resent long ntrusivecontacts, nd barrenpyrite veinsoccur. Massiveand crystallinepyrite are both common. Pyrite in the orebodies s massiveor forms small striatedcubiccrystals,but in the hydrother-mally alteredzones he disseminatedyrite s generally oundas smallpyrito-hedrons. Crystalsover 2 cm in diameterare rare. Pyrite was probably heearliestsulfide o form, and it continued o be depositedhroughoutmost ofthe period of sulfidedeposition.Real#ar.--Realgar and orpimentare abundanton the 4,240 level and were

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GEOLOGY OF THE ATACOCHA MINE 267noted n seams n the gougeof No. 1 fault as far down as the 4,103 level.They are also ound n fractures n limestone n the 3,600 levelbetween1,200and 1,300 m from the portal. Large specimens f realgar and orpimentwereobtained rom a vein in limestone n the 4,240 level. The realgar occursasorangeplaty masses nclosedn orpiment,and as small red crystals hat re-semble innabar. The larger realgar-orpiment einsare not found associatedwith lead-zincminerals,but realgar fills fractures n a small body of ore.Sphalerite.--Light-brown o nearlyblacksphalerite ccurs n varyingpro-portionswith galena. It occurs s a massivemineralwith goodcleavage, sroundedgrains, and as crystals hat may show multiple twinning. Age rela-tions of the sphaleriteare not well known. Specimensrom the ore bodiesare difficult o secure s the ore crumbles eadily. In the veins,sphalerite ndgalenaare later than someof the pyrite.Tennantite.--Tennantitewas dentified n one polished urfaceof Atacochaore. It was found replacingpyrite, but its age relation to adjoininggalenaand sphalerite ouldnot be determined.Calcite.--Veins of calciteare abundant n limestone,and someare found inquartz sandstone, herecalcite s a common anfluemineral. The calcite swhite or creamand mediumgrained with rhombohedral leavage. It is com-monly ntergrownwith rhodochrositen the ore-bearing eins. Thesecarbo-natesare among he latestminerals o form; they fill the centers f veinsandenclose ngular ragments f sulfides. Veins of fibrouscalcite to 2 cm wideare commoneatures n the uppermine workings. They are definitelypostoreand may be supergenen origin.Cerussite.Cerussitewas found n the Curiajashaarea as colorless rainsin a yellow earthy groundmass.Chlorite.--One or more membersof the chlorite group of minerals arepresent n two areasof the mine. Chlorite s abundantn a sheared oneonthe contactof the limestoneand chert west of the quartz sandstones n the3,935 level and alsooccursalong what may be the samecontacton the 3,600level.Clay Minerals.--White clay mineralsare common onstituentsf the hy-drothermally ltered ocksclose o and n the ore bodies. Positive dentifica-tion wasnot possible, ut the microscopeevealed lay mineralswith high andlow birefringence, o both the kaolin and montmorillonite roups may berepresented.Fluorite.Violet and green fluorite occurs n carbonateveinlets n theuppermine evels,both n ore bodies nd in country ock. Abundant luoriteis reported n the 3,840 evelbelowore bodyNo. 1, but the areawas nacces-sibleat the time of the examination. Fluorite alsooccurs n tiny fractures nporphyryand quartz saffdstone.Cubesand irregular masseswith grainsasmuch as 2 cm in diameter are associated with calcite and rhodochrosite.Fluorite is apparently ne of the last minerals o form, as the carbonate ein-lets in which it is found cut the ore minerals.

Gypsum.--Gypsum ccursas thin scalycrystals n fracturesand on ex-posedsurfaces, nd in massive ranularaggregatesn the alteration one ofthe ore bodies and veins.

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268 ROBERT F. JOHNSONLimonite.--Limonite occurspseudomorphousfter pyrite, and as stalac-titic and botryoidal orms n old workings. It is a common xidationproduct.Malachite.--Malachitewas foundboth in old workingsand on the surface.The bestoccurrence f malachites along he road about2 km from the mine,where t is found n old prospect renches ssociated ith chalcanthite.ManganeseOxides.--Black manganese xidesare commonon the walls ofsome workings,and in vein outcrops n limestoneadjoining the mine area.Psilomelanes the most abundantmanganese xide in the vein outcropswithpyrolusitepresent n small amounts. The manganese-bearingutcropsarenot indicatorsof ore below; someof the larger veins.of this type have beenexploredwith discouragingesults.Melanterite.--Fibrous green melanteritewas found in old workings.Muscovite.--Sericite s present n shearedzones n the hydrothermallyaltered rocks,but it is not a commonalterationproduct at Atacocha.Phlogopite.--Abundant hlogopitewas found n a fault zone on the 3,600level associated ith biotite, hematite,chlorite,and pyrite.Quartx.--Small acicularcrystalsof quartz are found in rugs in the veinsand ore bodies,but veins of quartz are not known. Silicificationof the lime-stone s the common ype of alterationnear No. 1 fault, but the silicamineralcouldbe either quartz or chalcedony.Rhodochrosite.--Rhodochrosites a commonganguemineral. It is gen-.erallymassive, ut a scaly orm linesvein cavities. The fresh mineral s pink,but it becomes early white on exposure.o air.Serpentine.--Serpentine odulesoccur in limestonesouth of Gueshguaand westof La Laguia. Massiveserpentine nd the fibrousvariety chrysotile

were found in a fault zone on the 4,240 level.The mineralogy f the Atacocha res s indicativeof deposition ndermod-erateconditions f temperature nd pressure. Realgarand orpimentare typi-cal epithermalminerals,but they were deposited fter the deposition nd frac-turing of the other sulfides. The galena-sphalerite-pyritessociations foundin all classes f hydrothermaldeposits, ut the association ith jamesonite ndtennantite,and particularlywith the gangueminerals hodochrosite nd flu-orite, indicate moderate to low temperatureand pressure. The depositsshouldprobablybe classified s leptothermal.HYDROTHERMAL ALTERATION

Well-defined alteration envelopes round the ore bodiesare not found atAtacocha. Alteration of the carbonatewall rocks s nearly everywherepres-ent, but it variesgreatly n intensityand showsno consistentelation o knownfeatures. The sandstone nd chert seem o be little affectedby hydrothermalalteration.The limestones ave beenaltered or varying distancesrom the ore bodiesand veins, and alteration s nearly continuousn the hanging wall of No. 1fault. Silicification s the commonest ype of alteration. Argillization isprominent earsomeore bodies ut may be weakor absent ear others. Dis-seminated yrite is present n both typesof alteration.

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GEOLOGY OF THE ATACOCHA MINE 269The first alterationeffect s a bleaching f the limestone,which may beslight and may be the only changenoted outwardfrom the ore. With moreintense lteration he limestone ecomes early white and loses ts distinctive

structural eatures. Highly altered imestonesre difficult o distinguishromfine-grained uartz sandstone, ut chert enses hat resemble hose n unalteredlimestonemay be present. In one area on the 4,000 level,north of coordinate3,760N, andeastof No. 1 fault,alterations so ntensehat he original ocktype couldnot be determined; he fault is shownas the contactbecause f simi-lar relations elsewhere.Zones f hydrothermallteration re favorableor exploration t Atacocha,but additional factors need to be considered. Alteration near the veins insandstones negligible, nd someore in limestones found n only slightlyaltered ock. On the other hand he limestonesear No. 1 fault are nearlyeverywhere ltered,so alteration lone s not an infallibleguide o ore. De-tailed microscopicxamination f altered rocksmay showan associatione-tweencertainalterationminerals nd ore, but withoutequipment nd trainedpersonnelhis is of doubtfulpractical id in prospecting.

DEPOSITIONAL HISTORY

The eventseadingup to the deposition f metallicsulfides t Atacocha renot well understood,ut a possible equences offered. The major fault zoneeastof the mine,and the subsidiaryhrust faultssuchas No. 1 fault, may haveoriginated uring he periodof orogenyn which he strong olding ookplace,probablyprior to the porphyry ntrusions. The intrusionsmay have accom-paniedrenewedmovements n the faults during which the cross old thatlocalized the ore bodies was formed or accentuated. No. 1 fault is conform-able in strike with the sedimentsn the cross old, so it may have been oldedwith them and would thus predate he cross old. The fractures n which theveins formed seem o have developed s a consequencef the cross olding.They couldnot haveremained pen or a long periodso t seems robable hatthey were formedafter the porphyry ntrusions. Silica and pyrite were thendeposited rom hydrothermalsolutions rom an unknown source. The por-phyry contactswere favorablechannelways t this time, as most of them aremineralizedwith pyrite. Later solutions arried lead and zinc sulfides hatfilled accessibleracturesand replaced avorable imestonebeds. The replace-ment 6re bodiesmay have ormed rom localconcentrationf solutions uidedby the gougezonesof the thrust faults, or in areaswhere intense racturingpermitted he access f abundantsolutions o the favorable imestonehorizon.Arsenic sulfideswere deposited s a later phaseof hydrothermalactivity.Their occurrencen the upper mine workings,or at somedistance rom knownlead-zincore on the 3,600 level, suggests oning, but where arsenicand oremineralsoccur ogether, he arsenicminerals ill late fractures n the crystal-lized lead and zinc sulfides, nd are not found intergrownor replacing hem.Postmineralmovements ppearto be minor. The "D" vein has been off-set as muchas 10 m, and a larger displacement ay have taken placeon astrong ault that cutsoff the vein on the west. Many veins erminateagainst

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270 ROBERT F. JOHNSONthe gougezonesof the thrust faults, but as there is no evidenceof faultedoreandno knowncorrelation etween einson eithersideof the faults, he veinfissures ayhaveendedn the incompetentouge ones.

Supergenenrichments of minor mportancenasmuch s the veinsdonot have much surfaceexpression, nd the ore bodiesdid not reach he zoneof oxidation.An exceptions at San Gerardo, ndtherebecausef rapiderosion, sulfide ores are found on the surface.COiXl CLUSIO ixl

Structural relationsare of prime importancen the Atacochamine.Knowledge erivedrom he mapping, uch sthe location f the majorore-bearing reanear he axisof thecross oldand he location f the largest rebodies ear the hangingwall of the No. 1 fault, should e of assistancen thelaying out of future exploration.The areasurroundinghe Atacochaminehasbeenwell prospected.Sev-eral smallore bodies avebeenmined,and otherore bodieswill probably ediscovered.An aditwasdriven n theCuriajasha rea n late 1952 o exploremineralized aults that crop out there. The faults were not found to be ofeconomicnterest t depth,but furtherwork s plannedo explore he contactof the limestone and chert breccia in that area. A small amount of ore wasfound along a contactof the dacite and lava near coordinates 420 N and5,110E (Fig. 2). Further work is alsobeingdone n that area.

U.S. GEOLOGICALURVEY,WAStllNGTON,D.C.,.4g. 10, 1954REFERENCES

1. Diaz, L. F., 1909, La regi6n rainera de Atacocha: Bol. del Cuerpo de Ing. de Minas delPeril, No. 74.2. Harrison, J. V., 1943, The geology of the Central Andes in part of the province of Junin,Peru: .Bol. Soc. Geol. del Peril, Tomo 16, p. 55-97.3. McLaughlin, D. H., 1924, Geology and physiography of the Peruvian Cordillera, Depart-ments of Junin and Lima: Geol. Soc. America Bull., vol. 35, p. 591-632.4. Petrijohn, F. J., 1949, Sedimentary rocks: Harper & Brothers, New York.5. Steinmann, Gustav, 1929, Geologie yon Peru: Heidelberg. Spanish translation by Jorge A.Broggi, 1930.6. Stone, J. B., 1928, Private report.7. Torres Vargas, David, 1948, Private report.8. Welter, Otto, 1929, Private report.