geologia esperanza, maricunga

7/30/2019 Geologia Esperanza, Maricunga

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EPITHERMAL SILVER-GOLD MINERALlZATION AT THE ESPERANZA AREA,MARICUNGA BEL T, HIGH ANDES OF NORTHERN CHILE

TOMAS VILA G. Minera Anglo American Chile Uda . Avda. Pedro de Valdrvia 295,Clasificador 31, Correo 9, Santiago, Chile

ABSTRAeTThe volcanic and sediment-hosted precious-metal prospects in the Esperanza alteration area are located between

4,000 and 4,200 m elevation in the northwestern partof the MaricungaAu-Ag metallogenic belt, High Andes of northern Chile.This region consists essentialy of Miocene compound stratovolcanoes of calc-alkaline nature unconformably overiying anigneous and sedimentary basement of Paleozoic and Mesozoic ages.

At Esperanza, rocks of two main geologic units have been affected by hydrothermal activ ity probably associated withthe emplacement of a dacite flow-dome complex: Upper Triassic clastic sedimentary rocks of the La Ternera Formation andupper Oligocene-Iower Miocene dacite volcaniclastics and pyroclastics of the Esperanza unit. Reverse and normal faultingaffect these units.

Structurally and lithologically controlled Ag mineralization is confined to silicified tectonic and hydrothermal breccias(e.g. Chimberos prospect) and to permeable ignimbrite horizons (e.g. Arqueros prospect) hosted by Triassic and MiddleTertiary units respectively. Micron-sized Au mineralization is superimposed as a late metallization event, but is restricted tovein-like, NNE - to NW - oriented silicified structures. A vertically zoned hypogene ore zone which underwent deep in situsupergene oxidation is proposed for the Ag mineralized breccia at Chimberos prospect, where no sulfide precursors to theoxidized Ag ore were identified. The Chimberos breccia contains a proven reserve of 4.3 million metric tons averaging 320g/t Ag and 0.14 g/t Au at a 150 g/t Ag cut-off.The anomalous quantities of Hg, Sb, As, Bi, Baand Pb thataccompany Ag/(Au) mineralization, as wellas the associatedadvanced argillic and argillic alteration assemblages provide evidence for an acid-sulphate, epithermal type mineralizedsystem at Esperanza.Key words: Epithermal mineralization, Si/ver, Gold, Hydrothermal alteration, Triassic sediments, Miocene volcanism, Esperanza area,

Maricunga belt, Atacama Region, Chile.

RESUMENLos prospectos de metales preciosos albergados en rocas volcnicas y sedimentarias de la zona de alteracin

hidrotermal de Esperanza se ubican entre los 4.000 y 4.200 m de altitud en la parte noroccidental de la franja metalognicade Maricunga, Alta Cordi llera del norte de Chile. Esta regin est constituida, esencialmente, por estrato-volcanes miocenosde composicin calcoalcalina que cubren, discordantemente, un basamento rgneo y sedimentario de edades paleozoicasy mesozoicas.

En Esperanza, rocas de dos unidades principales estn afectadas por alteracin hidrotermal asociada probablementecon el emplazamiento de un complejo de domos dacfticos: rocas sedimentarias elsticas de la Formacin La Terneraasignadas al Trisico Superiory rocas volcanoclsticas ypiroclsticas dacfticas de la Unidad Esperanza, asignadas al lapsoOligoceno superior-Mioceno inferior. Fallamiento inverso y normal afecta a ambas unidades.

La mineralizacin de Ag est controlada estructural y litolgicamente y se emplaza en brechas tect6nicas ehidrotermales intensamente silicificadas (Prospecto Chimberos) o en horizontes de ignimbrita permeables (ProspectoArqueros), encajados en las unidades del Trisico y Terciario medio, respectivamente. La mineralizacin de Au, de granomuy fino, se sobreimpone como un evento tardro, restringido aestructuras vetiformes silicificadas de orientacin NNE y NW.

En la brecha mineralizada de Chimberos, se considera una zonacin vertical de la mineralizacin hipgena, que sufriuna oxidacin in situ profunda. En Chimberos no se han identificado sulfuros precursores de la mineralizacin oxidada deAg.

Revista Geo/g/ca de Chile. Vol. 18, No. 1, p. 3754, 11 Figs., 5 tables. 1991.


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38 EPITHERMAL SILVER-GOLD MINERALlZATION AT THE ESPERANZA AREA, MARICUNGA BELT

La Brecha de Chimberos contiene reservas probadas de 4,3 millones de toneladas mtricas con una ley media de 320g/t Ag Y0,14 g/t Au para una ley de corte de 150 g/t Ag.Las cantidades anmalas de Hg, Sb, As, Bi, Ba y Pb, que acompaan la mineralizacin de Ag-(Au), as como suasociacin con alteracin arglica y arglica avanzada evidencian, en Esperanza, un sistema mineralizado epitermal del tipocido-sulfato.Palabras claves: Mineralizacin epitermal, Plata, Oro, Alteracin hidrotermal, Sedimentos trisicos, Volcanismo mioceno, Esperanza, Franja

de Maricunga, Regin de Atacama, Chile.

INTRODUCTION

The volcanic- and sedimentary-hosted preciousmetal prospects at the Esperanza hydrothermallyaltered area are located in the High Andes of theAtacama Region, at elevations between 4,000 and4,200 m. It constitutes one of several Au-Agmineralized areas in the northwestern part of therecently defined Maricunga metallogenic belt (Vilaand Sillitoe, in press; Fig. 1).

The geomorphology of the area is characterizedby a western, north-south-trending chain of Miocenestratovolcanoes, separated from the subparalleleasternmost Quaternary locus of volcanism by theAltiplano or Puna, a 30-50 km wide subhorizontallandscape. Large salt lakes or salares, where modernevaporite deposits are presently being precipitated(Salar de Maricunga, Laguna del Negro Francisco),occur in topographically depressed areas.

The hydrothermal alteration area at Esperanza,characterized by an intense reddish-brown coloranomaly, was identified in 1980-1981 through a

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oo

grass-roots exploration program designed to locatepotential El Indio-type gold mineralized zones.Exploration started with an aerial reconnaissancealong the Andes of northern Chile by BT Exploraciones Uda. on behalf of Minera Anglo Cominco Uda.(ANCOM), a joint venture between Minera AngloAmerican Chile Uda. (MAAC) and Minera ComincoResources Uda. Based on anomalous Ag valuesfound in silicified outcrops at Esperanzaduring groundreconnaissance, a soil geochemical survey on an 80x40 m grid wasconducted duringthe 1981-1982fieldseason, covering most of the altered area (6x km).As a result of this survey, six main Ag soil anomalies(> 1.0 ppm Ag) were defined, namely Chimberos,Huantajaya, Potos, Arqueros, Bonanza, and SantaRosa prospects. Construction of access roads andbulldozertrenching on the soil anomalies was followedbydetai led geologic mapping and rock-chipsampling,as well as by drilling and tunneling in the Huantajaya,Chimberos and Arqueros prospects during the 1982

/

///

FIG. 1. Location map of !he Esperanza prospect. Boxed area is enlarged in figure 2.


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to 1986 lield seasons.Exploration at Chimberos prospect inciuded 4

levels of adits (ca. 2.5 km), 800 m of diamond drillingand 4,400 m of reverse circulation drilling. At SantaRosa and Potos prospects, only geologic mappingand rock sampling with limited reverse circulationdrilling have been carried out.

As a consequence 01 this effort, a 4.3-millionmetrictons oxidized mineral deposit averaging320g/t AgandO.14glt Au wasdefinedatChimberos,while

39

two systems of NNE and NW-oriented Au-(Ag)-bearingsiliceous replacement veins were identified at Arqueros and Huantajaya prospects, respectively.

Consideringthatthe brief outlineoftheChimberosdeposit presented by Colley et al. (1989) is basedonly on preliminary MAAC exploration data and theirown very limited lieldwork, detailed descriptions ofthe Chimberos and Arqueros deposits are necessaryand presented below.

REGIONAL GEOLOGICAL SETTING

The regionalgeological setting inwhich Esperanzais located has been described by Segerstrom (1968),Zentilli (1974) and more recenily by Mercado (1982).Aguilar (1984), Zamora (in prep.) and Davidson andMpodozis (in press). Although no metallic depositswere known in the region prior to 1980, its potentialfor precious metals was suggested previously byZentilli (1975) and Llaumet and Henrquez (1976).

o 10 20 Km..--=====11

EL

Esperanza lies in the southern part of the AndeanCentral Volcanic Zone (CVZ; Thorpe et al., 1982),within the Maricunga belt (Vila and Sillitoe, in press).a NNE oriented Au-Ag metallogenic province whichconsists essentially of large Miocene compoundstratovolcanoes of calc-alkaline nature, uncon70rmably overlying an igneous and sedimentary basement01 Paleozoic and Mesozoic ages. Discrete episodes

Nt Paleozoic (mainly)sedimentary basementM Paleozoic r a n ~ i cintrusivesD Mesozolc to early. " . Tertiary sequences

ESPERANZA L,""j Oligoceneearly MioceneVolcn

Volcn .Dios de MoricunQo

VolcnSanto Roso

t2JW7A

r '"~)('

-- -

sequenc8s

Middle to late Miocenevolcanic cen:ers

Pliocene volcanlcs

Volcano summlt

Reverse jau!

Geological contact,dashed when inferred

Mine

San pan

FIG . 2. Regional geological setting 01 the Esperanza area in \he northern ha1f 01 the IlAaricunga Belt (from Davidson andMpodozis, in press). Boxed area indicates the hydrothermal alteration extension.


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40 EPITHERMAL SILVERGOLD MINERALlZATION AT THE ESPERANZA AREA, MARICUNGA BELT

of regional high-angle reverse faulting of lateCretaceous and early Miocene ages, resulted incompressional horsts and grabens, mainly of NNEorientation. Upfaulted blocks of pre-Andean and

Jurassic to lower Tertiary rocks also occur as NNEelongate strips, mainly along the western border ofthe Maricunga belt (Fig. 2).

GEOLOGY OF THE ESPERANZA AREAThe hydrothermal alteration at Esperanza is

probably related to the emplacement of a largelyconcealed dacite porphyryflow-dome complex. Rocksof two main lithological units are affected: 1. Clasticsedimentary rocks of the Triassic La TerneraFormation (Brggen, 1950, Zamora, in prep.), whichcrop out in the eastern half of the altered area, and2. The overlying late Oligocene-early Miocene daciticvolcaniclastics and pyroclastics which cover most ofthe western part (Fig. 3). The latter rocks, which arehere named informally as the Esperanza Unit, arewidespread distributed and are, in general, intenselyhydrothermally altered. South of Esperanza,chronological and compositional equivalent rocksconstitute an important host-rock for Au-Agmineralization (e.g. Santa Cecilia, La Pepa, LaCoipa;Vila and Sillitoe, in press). In the Esperanza area,they are overlain by middle Miocene (14-15 Ma)andesitic lava flows of the Cerros Bravosstratovclcanic complex (Cisternas, 1977; Zamora, inprep.)

STRATIFIED ROCKS

The lowest exposed part of the La Ternerasedimentary section crops out on the northern slopeof Chimberos hill. It is relatively unaltered andcomprises a minimum of 100 m of green, matrixsupported monomictic conglomerate with no visiblestratification. The conglomerate is composed mainlyof subrounded fragments of silicified sandstone,shale and granite in a coarse-grained, chlorite-richsandy matrix constituting 20-30% of the total volume.Epidote is also common. This sequence gradesupward to a 120 m section of yellowish-brown sandyconglomerate containing well-rounded, stronglysilicified sandstone and milky quartz fragments. Thematrix, 60-70% in volume, consists of coarse quartzrich sandstone.

The upper part of the sedimentary sequencecrops out at Santa Rosa and on the southern slope ofChimberos hill, and is represented by 400 m of light

gray, medium to fine grained sandstones and siltstones with thin lenticular intercalations of pebbleconglomerate, blackshale and fossiliferous limestone.Well-sorted detrital quartz (0.15 mm) in a matrix offine authigenic clay is characteristic for most of theserocks, along with fragments of plagioclase,volcanigenic lithics, muscovite (altered to chlorite)and carbonaceous-ferruginous matter.

The Esperanza Unit overlies the basal sedimentary sequence with angular (and erosional?) unconformity, and comprises most of the hydrothermallyaltered rock at the area. It is composed mainly of lightbrown to yellowish crystal and lithic tuff, block andash flows and volcaniclastic breccias with intercalations of lapilli and weakly welded ignimbrite aswell as thin lenticular layers of lacustrine volcanicsandstones. These rocks have been deposited indepressions in a rugged paleotopography and locallymay be folded weakly; the minimum thickness isestimated at 500 m.

According to stratigraphic and structural relationships, the Esperanza Unit is considered equivalentto the Quebrada Carrizo sequence (Bruce, 1989),south of Laguna del Negro Francisco area, and to theVillalobos sequence (Gardeweg and Mpodozis 1) onthe northern slopes of the Volcn Copiap.

Overlying the Esperanza Unit with an angularunconformity, a thick post-alteration-mineralizationvolcanic cover crop out, represented by dark greendacites to rhyodacites and rhyolitic ignimbrites. Theseare assigned to the Cerros Bravos volcanic complexof middle to late Miocene age (15.0 0.6 Ma; Zentilli,1974).Finally, a lacustrine sequence of probablePliocene age is preserved in the area as remnants of30-50 m thick horizontal volcanosedimentary beds ofalternating volcaniclastic and coarse grained volcanicsandstones.

FLOW-DOME COMPLEXES

Intense hydrothermal and supergene alteration1 1990. Geologa regional entre los prospectos Lobo y Soledad, Franja de Maricunga, Cordillera de Copiap. Minera Anglo

American Chile, Ltda. (Unpublished report), 179 p. Santiago.


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~ Paleozo ie sandstones .--l'':-- Sine line axe[ .T I Trlassie sandstones and shales ~ Fresh daelte porphyritic domes ~ Reverse fautt , dashed when Infered~ Oligoeene-Mloeene voleani

elastics; hatehed area showsopaline silieifieation

~ Mlddle Mlocene (?) volcano~ sedimentary lacustrine roeks __ Normal fault, dashed when inferedt1.I I ilieWied veins; at Potosi showspervaslvely silienled rocks ~ Geologieal profll" (shown in figure 8)Ao o No o o o o o o to o o o o o

o o Chimberos Faul!: . ~ ..... . ~ . .

o /. " ~ .I - . . '?---\vK. . ~ ' " ,CHIMBEROS. .;--------


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42 EPITHERMAL SILVERGOLD MINERALlZATION AT THE ESPERANZA AREA, MARICUNGA BELT

form, The rock is porphyritic and shows a bimodalsize distribution of phenocrysts (40 vol%) dominatedby plagioclase and accompanied by conspicuousround quartz, hornblende, and biotite; alignment ofcrystals is present.

STRUCTURE

The structural setting is dominated by ENE andWNW-trending reverse faulting of probable earlyMiocene age that places Paleozoic metasedimentsover La Ternera Triassic sandstones (Chimberosfault) along the northern slope of Chimberos hill. LaTernera Formation sedimentary rocks exhibit regional N30oW/20oS monoclinal attitude, although intensefolding is present locally at Chimberos, Potos andSanta Rosa hills as a result of the reverse faulting(Fig.3).Younger, normal fault systems of NNE and NWorientation are also present in the area, controlling agroup of siliceous replacement veins.

HVDROTHERMAL ALTERATION

The hydrothermal alteration at Esperanza showsclear structural and lithological controls. Moderate tointense, albeit patchy silicification with subordinatealunite and barite is present as vein-like fracturefillings or subvertical breccia bodies along the reverseand normal fault zones. Wall-rock alteration ischaracterized by district-wide argillization, with halosof kaolinite and alunite close to the silicified areas.These halos show a greater lateral extent whendeveloped within the Esperanza Unit, which in turnis extensively oxidized, limonitized, and bleached,probably due to a high original and secondarypermeability. Within the unit, nonwelded ignimbriteand lapilli tuff layers are commonly affected by moreintense, quasi-stratabound-like silica-alunitealteration.

Remnants 01 pe rvasive silicification, characterizedby milky white to pink opaline silica selectivelyreplacing parts of the uppermost layers of the Esperanza Unit, are present west of the Huantajaya hillarea (Fig. 3). This replacement zone is considered tohave originated in close association with a paleowatertable (Sillitoe2) and is represented by an opalinehorizon that probably was formed over most of thealtered area prior to erosiono

SOIL GEOCHEMISTRV

As mentioned previously, the soil geochemicalsurvey over the Esperanza area identified six mainAg anomalies in the central and eastern parts of thealteration zone that are generally related to silicifiedoutcrops. Silver anomalies show a well-defined northwest trend, and range in size from 100 x 300 m inChimberos prospect to 300 x 600 m at Huantajayaprospect. Some anomalies exhibit appreciabledownslope physical transport (Fig. 4). An apparentbutweak ENE orientation of some Zn-Pbgeochemicalanomalies is also observed at Santa Rosa prospect.Only one small and weak Au anomaly was identifiedat Arqueros prospect, and it is apparently unrelatedto Ag. Moderateto strong Zn anomalies are coincidentwith Ag at Bonanza and Huantajaya prospects,whereas the main Pb anomaly is centered on Potosprospect, which also encloses a small Ag anomaly.

MINERALlZATION STYLES

Two district styles of disseminated Ag (with subordinate Au) mineralization are recognized at Esperanza: hydrothermal and tectonic breccia hosted(e.g., Chimberos prospect), and strataboundignimbrite-hosted (e.g., Arqueros prospect). Gold (withsubordinate Ag) mineralization is restricted to steep,NNE- and NW-striking vein-like silicified structurespresent mainly in Arqueros and Huantajaya areas.These structures, which at Arqueros are superimposed on the ignimbrite horizon, are considered to bethe conduits through which most of the hydrothermal,mineralizing fluids ascended (Fig. 5).

Anotherstyleof Au mineralization, poorly exposedat Carachitas valley in the southernmost part of thealteration area, occurs as weak disseminations in astockwork of banded, millimeter-size, dark andtranslucent quartz veinlets, cutting an intenselyargillized dacite porphyry. A coincident, althoughweak Au-Cu-Pb geochemical express on, which isdistinctive for disseminated porphyry-type Au-(Cu)mineralization throughout the Maricunga belt (Vilaand Sillitoe, in press), characterizes the stockworkarea.

AGE OF AL TERATION-MINERALlZA TION

K-Ardating (Sillitoe etal., in press) ofthe alterationmineralization processes, as well as of the post-


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f4f!JfjldJffffw Ag ,. LO ppm~ Pb ,. l OOppm;.-". Zn

.l TJl!r.{([Ua11 ~~ , . " . ~ h$ ~ ~ i '

43

Nt. )1.0..._ , ... ,. 100 ppm CHIMBEROS

J J . w ~ 11 Au ,. O.o5ppm

\.. ' ...........HUANTAJAYA'\"\

/

o BO 400 m---,,;;=--=::--FIG. 4. Soilgeochemical anomales in the Esperanza prospecto

mineralization dacite dome at Esperanza indicates asequence of intrusive, volcanic, and hydrothermalevents Ihat spanned from al least 24- 20 Ma , withdom e emplacement and related hyd rolhermal activityoccurring close to 23 Ma The 23.2 0.7 and 20.7 0,7 Ma ages obtained from hydrothermal alunite alArqueros and Potosi prospects, respectively,suggestthat mineralization and alteration occurred within a 3Ma periodo The fresh post-alteration dacite dome alHuantajaya prospect yielded a 20.2-1.0 Ma age(Sillitoe et al., in press). The early Miocene ageascribed lo alteration-mineralization al Esperanza isconsistent with the western metallogenic sub-beltdefined in the Maricunga bel! (Vila and Sllitoe, inpress).

I v vLen - permeabl.volconic$ V1 LP,rmlobl.pYfoclo,l ic sJ L

O P ' J ; O ' ~ vv vi I v v v

FIG. 5. Schematic characterization of the di fferentmineralization styles al Esperanza Ifrom Sillitoe' }.


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THE CHIMBEROS SILVER MINERAL OEPOSIT

The Chimberos Ag mineral depost is developedwithin intensely slcfied and brecciated Triassicsedimentary rocks along the southern slope ofChimberos hill. Silver is the only metal of economicimportance. Its surficial expression corresponds to a50 x 40 m, dark-brow:1, intensely silicfied, oxidized,leached, and brecciated outcrop, which stands outfrom the argillized sedimentary country rock. A groupof smaller, ENE-trending silicified outcrops occur atthe southwestern part of Chimberos hill, while NNEfault-related silicified zones are present north of themain outcrop. The ENE and NNE linear configurationof the silicified zones at sUrface, subparallel to thenorthward dipping trace of the Chimberos reversefault, and to the general NNE structural trend,respectively, suggasts strongly a structural, faultrelated control of silicification, with the main outcroprepresenting the intersection of at least two structures(Fig. 6) .

SOIL ANO ROCK GEOCHEMISTRV

At Chimberos, only Ag anomalies were detectedby soil geochemistry. The east-west elongation of themain Ag anomaly is due apparently to downslopetransport from the main silicified outcrop.Silver valuesrangefrom 1.0to 16.0ppm. Nosignificant Au, Pb, Zn,As or Cu soil anomalies are present.

A single soil geochemical sampling transversefrom the top of Chimberos hill (4,200 m) to the lowestadit level (4 ,017 m) shows locally high CI (500-3.900ppm) and moderate I (2-15 ppm) anomalies, a p p a r ~ently related to silicification (H. Colley, writtencommun., 1986).

Although only fragmentary surface rockgeochemical data are available from trenches immediately abovethe Chimberos deposit, some generalgeochemical relationships may be obtained.Multielement rack geochemical anomalies show a

FIG. 6. General view 01 Ihe Chimberos Hill looking norlh. The dark oulcrop al Ihe righl corresponds lo slrongly siliciliedhydrolhermal ly breccialed sandslones and shales. II represenls!he surficial exposure ollhe Chimberos Ag deposit.


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_ Ag > 100 ppm-- Pb > 100 " Sb > 50 0 "__ As > 50 0 "__ Bi > 10 "

". . . - ..,.,. HO > 5 "

?

N

t

45

FIG. 7. Rockgeochemical anomalies overthe Chimberos Ag deposit.

o 10 50m1= ' ...3:==""""==-=="",,,.,.'

well-defined NNE orientation, the highervalues beingconcentrated along the southern part of the mainsilicified outcrop (Fig. 7). In this area, where moreintense hydrothermal brecciation is observed, highlyanomalous Ag (100-500 ppm), As (500-4,000 ppm),Bi (5-50 ppm) and Sb (80-600 ppm) values along withweak Pb (1 00-200 ppm) and Au (0.05 ppm) anomaliesare superposed. This anomalous geochemicalassemblage is enclosed by an extensive 200 x 50 mHg anomaly (> 5 ppm) which includes most of thesilicified outcrops in the area. Another small, but veryhigh As anomaly (>0.2% As) coincident with Sb andPb, is found north of the main anomaly. It appearsthat the increased contents of Ag and other elementsextend outward into the country rock as far asmicrofracturing are developed. Beyond this area,metal concentrations drop to background values.Approximate enrichment factors in oxidized ore vs.fresh host rocks are: Ag (>300 X), As (>40 X), Bi(> 100 X), Sb (> 100 X), Hg (>100 X) and Pb (>50 X).

STRUCTURE ANO HYOROTHERMAL ALTERATION

Underground workings at Chimberos haverevealed the mineralized breccia and host rocks onfour levels over a vertical range between 4,150 and4,000 m. The sedimentary host rocks are intenselyfractured and folded into a major syncline with asouth-plunging, north-east-directed axis. Thesuperimposition of NNE-trending normal, strike-slip(?)faulting observed at surface, increased the fractureintensity, producing a structurally controlled breccia.The breccia is difficult to recognize macroscopicallybecause of its highly silicified monolithologiccharacter,the tight fit ofthe angular clasts, the lackof distinctivematrix, and the presence of many large fragments,which were only shattered in situ or underwent verylittle displacement. Well-developed multiple episodesof hydrothermal brecciation is present locally, asrelatively small (10 x 5 m) irregular bodies withangular to subrounded fragments in a fine quartz


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A(SE)

CHIMBEROS HILL

CHIMBEROS FAULT

o Qo o /,j

pz // TR

u ~ Hydrothermol breccioT llllillilI Sonds!one ond sholeRe ( ~ir ~ Sondy conglomero!e

f.o ~ Conglomero!eu VvV ' " "~ Sondslone

~ e v e r s e loull,} / Normal foull lone

L'v.1 4tt4

4.200 m

4.100

4.000

PZ ..J o 100 200 30 0 m: I E = = = = = ~ ' = = = = = = ~ I ~ - = 'FIG, 8, Schematic cross-section 01 the Chimberos Hill ,

matrix, Open spaces are common through most ofthese breccia bodies, Silicification largely obliteratesoriginal rocktextures and mineralogy and isdominatedby dark gray recrystallized chalcedonic quartz, locallystrained and foliated, Interstitial clay and minor aluniteand barite occur intergrown with quartz, Partings inthe rock are filled with jarosite, and are banded withlaminations represented by o r i z o n s o f d ~ f e r i n g quartzgrain-size:. Pyrite boxworks lined with hematite arealso present; open spaces occur locally and are filledby banded silica, gypsum, siderite and jarosite(Lindsay3).

Original bedding in the sedimentary host rocks isdifficult to recognize due to moderate to intense,limonite-poor argillization developed as a halo to thesilicified breccia. Nevertheless, the available beddingmeasurements suggest that the host rocks of theChimberos orebody suffered complexfolding as resultof north-dipping reverse fauting (Fig. 8). Weak pyritedisseminations and veining occur locally in thenorthern argillized halo on an intermediate level.Boundaries of silicification are sharp perpendiculartothe axis of the breccia but are gradational along itslength.

A late magmatic event at Chimberos isrepresented by narrow, NNE-to NE-oriented, stronglyaltered dacite dikes and sills which intrude the foldedsed imentary sequence. Original mineralogy and

fragmental textures are poorly preserved. Relictfragments are composed of small quartz phenocrystsand irregularly shaped volcanogenic lithics upto 1 cmin size. Both fragments and fine-grained matrix arecompletely replaced by quartz, alunite and variableamounts of clay, gypsum, and jarosite. The spatialand temporal relationships with the hydrothermalbreccias are nor clear, although the dikes may berelated to the pre-mineralization(?) dacite dome inthe nearby Huantajaya area.

MINERALlZATION CONTROLSThe Ag-(Au) mineralization extends for about

400 m along a general north-east strike, is 100 mwide, and up to 150 m deep. Mineralization lim its arecoincident with those of brecciation and silicification.The NE-trending long axis of the mineralized brecciais subparallel to the reverse faults and folds axisalthoug ha downwards progressive clockwise rotationfrom N500E to N700E is observed in the undergroundworkings. A secondary NNE-striking vertical structureis also observed in the eastern part ofthe m neralizedbreccia which controls the Au distribution. As a resultof this structural control of the mineralization, thenorthern half of the deposit shows a subcircularshape in cross section which changes progressivelyto a 40-500N dipping tabular form in its southern part

3 1990. Notes on the petrology 01 selected samples from Esperanza. Minera Anglo American Chile Ltda. (Unpublishedrepor t) , 9 p. Santiago.


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(Figs. 9, 10). At the lowermost adit, a NNE/600N premineral(?) fault represents the southern limit of theorebody.

SILVER OCCURRENCES ANO OISTRIBUTIONThe Chimberos Ag orebody is almost completely

oxidized, probably aided by the intense fracturingand brecciation. Deep diamond drilling to explore forprimary Ag-bearing sulfides revealed that oxidationbecomes progressively deeper towards the centralpart of the breccia body, attaining a depth of 240 m,although Ag grades are lower (30-50 glt) . Hypogenesulfides include remnant pyrite veining with traces offine-grained argentite. No hypogene Ag-bearingsulfides were identified in oxidized Ag ore atChimberos.The argentiferous ore in the Chimberos breccia isextensively silicified, oxidized,leached, and enrichedin jarosite, limonite, and a number of silver-bearingminerals. Quartz (94-98wt.%) accompanied by minorto trace amounts of barite, leucoxene, rutile, zircon,garnet, apatite, altered feldspar, alunite, and sericiteare the gangue constituents (Glathaa4; Tables 1, 2).Quartz is predominantly cryptocrystalline and ischaracterized by the presence of small vugs (0.1-0.2mm) lined with botryoidal limonite or barite, whichapparently acted as favorable locations for depositionof Ag-bearing minerals.

Eight Ag-bearing minerals and an unknown SbBi-Pb-rich Ag mineral were identified at Chimberos(Glathaar4; Table 3). The bulk of these minerals arepresent as discrete, finely disseminated grainsthroughout the quartz-rich mass or intricatelyintergrown with jarositic limonite.

Native Ag is present as very small 5 microns)particles which cluster in a chain-like or botryoidalfashion, commonly associated with limonite. NativeAg particles enclosed in cerargyrite, bromerite, orargentite, and associated locally with barite, are alsocommon. Cerargyrite is present as free grains (300microns) or s included within limonitic material.lodyriteand argentite (acanthite) particles are generallysmaller and are enclosed commonly in cerargyrite orlimonite. The unknown Ag compound detected byelectron microprobe (Glathaar4) appears to beassociated preferentially with limonite and, to a lesserextent, with jarosite as discrete particles . Only a few,very small 5 microns) pyrite, chalcopyrite, and gold

47

particles are encountered, all completely endosed inquartz.

Limonite (and clay) are present mainly fill ingfractures and as coatings; disseminations aresubordinate. Based on color, at least three maintypes of limonite are distinguished in the Chimberosore: red, black and yellow . Jarosite and hematitepredominate in red limonite, along with alunite, barite,and gypsum; goethite and hematite predominate inblack limonite along with alunite and subordinateamounts of an unknown (Mn-AI-Co) mineral, andjarosite predominates in yellow limonitetogetherwithillite -montmorillonite clays. Most of the Ag-richsamples have significant amounts of hematite and/orgoethite. Underground workings show acrude lateralzoning from brown-yellowish limonite along theboundaries of the breccia that gradually changesinward to a dark-red and to iridescent black limoniterich innerzones. An increase in red limonite intergrownwith clay is also noticeable in the intermediate levels.

METAL ZONINGOn each underground level, Ag isopleths (e.g. 30,

70 and 150 glt Ag) are closely spaced transverse tothe long axis of the orebody but more irregular anddiffuse n a longitudinal sense and vary sympatheticallywith the intensity of silicification (Fig. 9). Gold values(1-3 g/t) define steep narrow north-east-orientedzones, which transect the main breccia at obliqueangles, generally confined to the most highlybrecciated parts of the orebody. The greaterpermeability of these highly brecciated zones mayhave focused late-stage, gold-bearing fluids. Ag-Auratios in Chimberos ore are generally greater than4,000 although in Au-rich zones, the Ag-Au ratioranges from 5-100.

In a vertical sense, composites of the richer innerzone taken from three of the underground levelsindicate a progressive decrease in the Ag and CIcontent with depth, whereas the opposite OCCJrs withS04, Fe203, and K20 (Table 4). The low K20/Na20ratio and its progressive decrease upward from 0.86-0.32 suggests the presence of natroalunite in theupper levels of the mineralized breccia. Average Mncontents are low, generally on the order of 0.01 wt.%.XRF analyses ofthe same samples (Glathaar4; Table5), suggest an increase of Pb and As w ith depth,while Ba, Sn, Te and Bi are enriched close to surface.

4 1984. Mineralogical examination of three Chimberos samples for the department of gold and silver. Anglo American Corp.,Ltda ., Research Laboratories (Unpublishad raport) 7 p. Johannesburg, South Africa.


12/18

N-500

N-300

N-500

00

N-300

8 4114 LEVEL

8'?'"

o

(l

c.::::: ...............oo

4096 LEVEL o

8

o'"

"

o

oo..J,

oo...:.

....o

o'

o

4072 LEVEL

84017 LEVEL

. ...... 30 g/ I Ag cul-off,-,_ 150 g/ I Ag cul-off. . . . - - 20 0 g/I Ag cul-off

o ~

()

N-500 _ _ _ _ Horizonlal Drill-holeso Verlical Drill-holes

' ~oo

00

o

~ o oo

'"

og.:.

FIG. 9. Horizontal distribution 01 Ag ore at the lour adi t levels, Chimberos Ag deposlt.

0.5 9/t Au cut-off

.:,

8

o 40 80 .(l , 1::::;;;;;;;; I

o

en

m"::J:m:Ds:::r-en

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13/18

T. Vila G.

D

G4180 13

12

4120

4060

L 40 80m1

D'

G'

LEVEL 4017

49

FIG. 10. Profiles of Ag ore distribution,Chimberos Ag deposit.

= = = = = = = = = = = = = = ~ = = -4000

Analytical data for Ag ore from the lower adit show upto 1.3 wt.% Pb and 1.1 wt % As, with Pb/Ag ratios onthe arder of 10-40.

ORE RESERVESMeasured reserves at Chimberos are 4.3 million

metric tons averaging 320 g/t Ag and 0.14 g/t Au at a150g/t Ag cut-off . In the 75-150 gft Ag cut-off range,TABLE 1. ESTIMATED MINERAL COMPOSITION,

CHIMBEROS Ag ORE"I Average (wt.%) at each levelMineral 4,114 m 4,072 m 4,017 mQuartz 98 .5 98.0 94.0Limonite 0.5 0.5 2.0Jarosite Tr 0.5 2.0Barite 0.5 0.5 1.0I Other minerals 0.5 0.5 1.0

Following Glathaar'

an additional reserve of 2.3 million metric tonsaveraging 108 g/t Ag was calculated.

TABLE 2. PERCENTAGES OF HEAVV MINERALS,CHIMBEROS Ag ORE"

Average (wt.%) at each levelMineral 4,114 m 4,072 m 4,017 mGold Tr Tr TrNative silver 6.1 1.2 0.8Cerargyrite (AgCI) 22.5 5.2 1.4lodyrite (Agl) Tr 1.5 0.4Argentite (AgS) Tr 2.0 0.9Limonite 19.2 48.7 35.9Jarosite 5.9 10.3 29.2Barite and othergangue 41.1 19.7 26 .1Rutile/leucoxene 3.7 6.0 5.3Pyrite Tr 2.2 TrCovellite - - TrZircon 1.5 3.2 TrTotal 100.0 100.0 100 .0


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50 EPITHERMAL SILVER-GOLD MINERALIZATION AT THE ESPERANZA AREA, MARICUNGA BELT

TABLE 3, Ag-BEARING MINERALS IN CHIMBEROSAg ORE"

Mineral Composition PresenceNative silver Ag majareerargyrite Ag el majarEmbalite Ag (Br, el) minarladyrite Agl minarArge'1tite A9 2S minarBramerite Ag Br minarArgentajarasite Ag Fe3 SO.)2 (OH)6 minarUnknown Sb-Bi-Pb-Ag-rich traceDyscrasite (?) A93Sb trace

Fallowing Glathaar'

TABLE 4. CHEMICAL ANALYSIS (AAS) OF CHIMBEROSAg ORE"

Average (wl.%) at each levelMineral

4,017 m 4,072 m 4,114 mAg(ppm) 223 365 1385Au (ppm) 1.33 0.72 0.60eu (ppm) 113 94 226eaO(%) 0.03 0.02 0.03MgO(%) 0.02 0.02 0.02Na2 (%) 0.12 0 .07 0 .07KP(%) 0.10 0 .05 0.02Fep3(%) 7.53 1.87 1.64AIP3(%) 0.47 0.63 0.31SO.(%) 0.67 0 .14 0.12el.(%) 0.09 0.12 0.15SiO a (%) 86.20 94.36 87.29

TABLE 5. XRF ANALYSES OF CHIMBEROS Ag ORE*Composite average at each level (ppm)Mineral

4,017 m 4,072 m 4,114 mPb 3798 445 203As 1931 155 248Ba 290 660 4366Sn 80 134 319Te 21 36 46Ni 6 4 3Ca 28 37 29Zn 23 4 3Se 8 3 4Sb 472 236 418Bi 178 99 315

THE ARQUEROS GOLD-SILVER PROSPECT

The Arqueros Au-Ag mineralized area is locatedat elevations between 4,170 and 4,050 m within theEsperanza Unit, which is exposed in the Bonanzavalley (Fig. 3). The Esperanza Unit here consistspredominantly of subhorizontal, monolithologicvolcan:c breccias of dacitic composition with a tuffaceous matrix, which are intercalated with epiclasticsediments and a 30-40 m thick, intensively oxidizedand leached ignimbrite layer (Figs. 3, 11). The latteris best preserved for about 100 m along the southernslope of the Bonanza valley, where it is limited to thewest by NNE oriented normal faulting. The high original permeablity of these rocks provided by pum ice

clasts in the ignimbrite (8ill itoe 1) localizedhydrothermal alteration and destroyed originaltextures. As a result of extreme hypogene leaching,the ignimbrite underwent intense, pervasive silicification and associated alunitization to leave only ahighly porous skeletaltexture (vuggy, residual silica).Replacementquartz is mainlychalcedonic, intergrownwith alunite and disseminted jarosite and hematite.

The silicified NNE-oriented vein-like replacementstructures, 5-15 m wide, pinch and swell but aretraceable over a strike length of up to 500 m. Thecontact between silicified and non-silicifid rock isgenerally sharp. In places, it is marked by a narrow


15/18

T. Vi/aG. 51

FIG. 11 . General view 01 the Arqueros area, looking south. The dark-brown outcrops correspond to a highly siliciliedalunitized ignimbrite layer with Ag mineralization. Several sub-vertical silicilied Au-bearing veins are seen cuttingthrough the ignimbrite.

zone (0.1-0.5 m) of fine-grained, light-gray silica;elsewhere, small, elongate hydrothermally brecciatedzoneswith asimilar light-gray silica matrixare present.

The inner zones of the silicified structuresgenerally contain voids where feldspars and pumiceframents have been leached out and locally filled byan advanced argillic assemblage: kaolinite, alunite,limonite, and jarosite, in a groundmass ofcryptocrystalline silica intergrown with alunite, bariteand jarosite. Some structures show strongly fracturedcentral suture zones (0.5-1.0 m) carrying abundantjarosite and local scorodite.

On either side of the vuggy sil ica zone is a zoneof alunite. The alunite close to the vuggy silicacompletely replaces the tuff. Progressively out of thevuggy silica zone, alunite replaces mainly feldsparphenocrysts and farther out changes to kaolinite.

This hydrothermal alunite from the altered Arquerosignimbrite and from Huantajaya silica veins yieldedK-Aragesof23.20.7 and 20.70.7Ma, respectively(Sillitoe et al., in press).

MINERALlZATION

Attention was focused originally at Arqueros ontwo E-W striking Ag anomalies on both sides of theBonanza valley and separated by a small, weak Auanomaly.

As noted aboye, Ag mineralization is mainlystratabound, associated with a subhorizontalunwelded ignimbrite, whereas Au-(Ag) mineralizationis present in NNE-striking replacement, vein-typesilicified structures. Stratabound Ag mineralizationwas explored by diamond drilling whereas Au-bearing


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52 EPITHERMAL SILVER-GOLD MINERALlZATION ATTHE ESPERANZAAREA, MARICUNGA BELT

silicified structures were investigated by twounderground levels separated by a vertical distanceof 50 m.

Both stratabound and vein-like Ag-Au mineralization are almost completely oxidized and leached to70 m beneath the Bonanza valley . Three Ag-bearingminerals have been identified : argentojarosite,electrum and native Ag . Argentojarosite is mostcommon and is generally disseminated in jarosite .Electrum is present as tiny free grains and native Agoccurs as 5 micron grains with botroidal texturessurrounded by limonite, jarosite, or silicate gangueo

Sulfide remnants in the oxidized zone are presentas finely disseminated pyrite with subordinate amountsof chalcopyrite, completely surrounded by silica.

The silicified structures at surface carry Au valuesof 1-3 g/t, increasing to 10 gft Au at shallow depths(20-40 m beneath surface) as found in the upper(4,058 m) underground level. Downwards, Au gradesdiminish and tend to disappear near the top of the

sulfide zone in the lower (4,000 m) undergroundlevel. Silver contents are on the order of 15-20 gft,although isolated and sporadic high values, upto 90gft are obtained. The Ag/Au ratio in Arqueros veinsranges typically between 0.1 and 10. Taking intoaccount that underground Ag values are generally anorder of magnitude lower than in outcrops, it isthought that Ag (and silica?) was enriched at surfacein the leached capping.

Lead contents range generally from 0.02% to0.15% and follow closely the outlines of the silicifiedstructures and the Au distribution; however higher Pbcontents tend to occur near the outer margins of thesilicified structures. No significant Bi or Sb anomaliesare present, but relatively high Hg (5-10 ppm) isrecorded at surface.

Silver values in the ignimbrite range typicallyfrom 20-100 g/t Ag, although a uniformly mineralizedvolume of rock has not been defined.

DISCUSSION

The Esperanza precious-metal prospect isinterpreted to be related genetically to a largelyconcealed pre-mineralization dacite porphyry flowdome complex, which crops out locally in the northernpart of Huantajaya hill (Sillitoe2). Precious-metalbearing breccias and veins were probably generatedin a hydrothermal system related to the domes.Fracturing provided permeabili tyforthe hydrothermalfluids that caused pervasive alteration and metalintroduction.

Hydrothermal brecciation is considered to becaused by magmatically heated meteoric waters(phreatic breccias), probably as a result of fluidoverpressures generated by intermittent self-sealingof the system through pervasive silicification andmultiple events of explosive boiling. The structuralcontrol on the emplacement of the Chimberos orebodystrongly support this brecciation mechanism.

Deposition of sulfides was probably initiated duringhydrothermal alteration . At least 5 volume percent ofpyrite, along with subordinate amounts of Ag-bearingsulfides were introduced in the breccias and mantosas interpreted from the amount of transported limonitepresent at both Chimberos and Arqueros (Sillitoe2).The relatively high content of Pb in the deeper centralportions of the Chimberos orebody (up to 1.3%)

implies the original presence of galena, in which Agmay have been present in solid solution. The unknownAg-Sb-Bi-Pb compound detected in Chimberos oremay reflect substitution of Pb by Ag-Sb-Bi in thegalena lattice as at Real de Angeles, Zacatecas,Mxico (Pearson et al., 1988).

The fine-grained size of sulfide remnants, as wellas the presence of chalcedonic silica suggest rapidprecipitation from transporting fluids due to abruptdecreases in temperature and pressure. In contrastto disseminated Ag mineralization at Chimberos, Auis present in steep tabular bodies at Chimberos,Arqueros and Huantajaya, which lack Sb and Bi. Thetextural characteristics of vein fill and vein brecciasindicate that Au metallization followed fracturing,probably as a late-stage event. Complexities of thiskind, involving repeated fracturing events and passageof dissimilar fluids are probably commonplace duringthe evolution of epithermal vein systems (Sawkins,1988).

Progressive oxidation and Ag enrichment of theChimberos breccia during Middle Miocene(?) loweringof the water table was probably favored by therelatively high original pyrite content of the rock. Incontrast, only supergene Au mobilization is thoughtto have taken place at Esperanza.


17/18

T. Vi/a G.

In common with upper parts of many oxidized Agdeposits elsewhere (e.g. Wonder, Tonopah; Boyle,1968), at Chimberos cerargyrite is the predominantmineral giving way downward to argentojarosite. Thepredominance of cerargyrite over native Ag in theoxidized zone is ascribed to high chloride ion activityin the local groundwater along with the stability ofsilver chloride complexes at low temperatures(Sangameshwar and Barnes, 1983).

Silver (and Au) mineralization is restricted to alimited vertical interval, from about 4,000 to 4,200 m,which transects the stratigraphy of the area and maybe observed at other similar Au-Ag deposits in theregion (e.g. La Coipa; Rivera, 1988; Oviedo et al., inpress). Within this vertical interval, mineralogic andanalytical data point to a metal zoning involving, atChimberos, an upward progression from Pb-rich toAg-rich limonite mineralization and, atthe shallowestlevels to a barren opaline-chalcedonic silica horizonwhich probably represents the paleowater table. The20-40-fold decrease in Pb content towards the upperAg -enriched zone, suggests a hypogene zoning ratherthan supergene enrichment or transport becausewidespread supergene leaching and downwardmigration of Pb is unlikely (Sillitoe2).

Afterthe alteration and mineralization processeswaned, a dacite porphyry dome was emplaced in thePotos-Huantajaya area. The emplacement of thispost-mineralization dome is probably responsible forthe disruption of the Au-Ag bearing silicified veins atHuantajaya.

The geochemical suite As, Sb, Bi, Ba and Hgaccompanying Au-Ag mineralization at Esperanza,the low-temperature character of the replacementsilica (opal, chalcedony) and the sulfur-rich alterationmineralogy, suggest a high level epithermal system(Buchanan, 1981; Beger and Eimon, 1983), probablygenerated in a paleogeothermal system of highsulfidation (Bonham, 1986)or acid-sulfatetype (Healdet al., 1987).Although the Middle-Tertiary paleosurface is

53

absent and not characterized by strong preciousmetal enrichment, the erosion level at Esperanza isconsidered to be relatively shallow, probably notmore than 200 m beneath the paleosurface. Evidencefor this estimate includes the subhorizontal ratherthan inward-dipping flow banding in the northern partof the post-mineralization dome, which suggests thatthe upper flat-surface of the dome is beingapproached, and the relatively thin, subhorizontalchalcedonic replacement zones, which are believedto represent paleowater tables (Sillitoe2).

COMPARISON WITH OTHER AG-AU DEPOSITSIN THE AREA

In the overall Esperanza are a (Fig. 1), twoepithermal disseminated precious-metal deposits arebeing mined: the Eocene-Oligocene El Hueso Audeposit (Brook et al., 1987; Colley et al., 1989) andthe volcanic- and sediment-hosted Oligocene-Miocene La Coipa Ag-Au deposit (Rivera, 1988; Sil litoeet al., in press; Oviedo et al., in press).

According to published descriptions and personal observations, La Coipa deposit also correspondsto an acid-sulphate or high-sulfidation epithermalsystem, which shares remarkable similarities withEsperanza in terms of geological-structural control ofmineralization, hydrothermal alteration patterns, AgAu metallization timing, and ore mineralogy.

Besides the obvious difference in mineralizedrock volumes between these deposits (La Coipahosts an aggregated 61.2 million tons orebodyaveraging 1.37 g/t Au and 77 g/t Ag, Oviedo et al., inpress), a distinctive Au-rich (up to 70 g/t Au) multidirectional array of alunite veinlets cutting Triassicshale occurs locally at the La Coipa orebody. Considering that the Triassic basement beneath thevolcanic cover has not been systematically exploredat Esperanza, interesting prospecting possibilities fordisseminated Au mineralization at the Santa Rosaand Potos areas are envisaged.

ACKNOWLEDGEMENTS

Several geologists worked with the author duringfield work at Esperanza. E. Viteri (Exploration Manager), J. Betzhold, M. Snchez, D. Pacci, J. Rodrguez,A. Rojas, R. Albornoz, A. Heresman, and R.H. Sillitoe,who carried out diverse geological studies in the areaand from which the author benefited.

Special thanks are given to R.A. Lyall, AngloAmerican Consulting Geologist for South America,who made major contributions to the organizationand development of exploration in the Maricunga beltand encouraged the submitting of the manuscript.

Helpful comments and discussions that improved


18/18

54 EPllHERMAL SILVER-GOLD MINERALlZAllON Al lH E ESPERANZA AREA, MARICUNGA BELl

the manuscript were provided by N. Lindsay, C.Cunningham, W. Hildreth, V. Maksaev and R.H.Sillitoe. V. Garrido and M. Maeztu provided timely

and careful drafting and typing services.Publication of this paper is authorized by Minera

Anglo American Chile Ltda.

REFERENCESAguilar, A. 1984. Geologa de los Cuadrngulos Cerro

Vicuita y Salar de Maricunga, 111 Regin de Atacama,Chile. Unpublished Thesis. Universidad del Norte, Departamento de Geociencias, 150 p. Antolagasta.Berger, B.R.; Eimon, P.L. 1983. Conceptual models 01epithermal precious metal deposits. ln Cameron volumeon Unconventional Mineral Deposits (W.C. Shanks,W.C.; editor). American Institute of Mining, Metallurgyand Petroleum Engineering, p. 191-205. New YorkBonham, H.F., Jr. 1986. Models lor volcanic hostedepithermal precious metal deposits; a review. InInternational Volcanological Congress, Symposium No.5, Proceedings, University of Auckland CentreContinuing Education, p. 13-17. Hamilton, New Zealand.

Boyle, RW.1968. Thegeochemistryolsilveranditsdeposits.Geological SurveyofCanada, Bulletin, No. 160,264 pp.

Brook, M.; Pankhurst, RJ.; Shepherd, T.J.; Spiro, B. 1987.ANDCHRON Report. British Geological Survey, Openfile Report, 190 p. London

Bruce, M. 1988 Geologa de la Alta Cordillera de Copiapal suroeste de la Laguna del Negro Franscisco.Unpublished Thesis. Universidad de Chile, Departamento de Geologfa y Geofsica, 143 p. SantiagoBrggen, 1. 1950. Fundamentos de la Geologa de Chile.Instituto Geogrfico Militar, p. 374. Santiago.

Buchanan, L.I. 1981. Precious metal deposits associatedwilh volcanicenvironments n Ihe southwest. In Relations01 Tectonics to Ore Deposits in the SouthewesternCordillera (Dickinson W.R; Payne W.D.; editors).Arizona Geological Society Digest, Vol. 14, p. 237-262.

Cisternas, M.E. 1977. Geologfadel sector La Ola, al surdePedernales. Unpublished Thesis. Universidad de Chile, Departamento de Geologa, p. 152.Santiago

Colley, H.; Treloar, P.I; Daz, F. 1989. Gold-silvermineralization in the El Salvador region, northern Chile.Economic Geology, Monograph, No. 6, p. 208-217.Davidson, J.; Mpodozis, C. (In press). Regional geologicsetting 01 epithermal gold deposits, Chile. EconomicGeology.Heald, P.; Foley, N.K.; Hayba, D.O. 1987. Comparativeanatomy 01 volcanic-hosted epithermal deposits: Acidsullate and adularia-sericite types. Economic Geology,Vol. 82, p. 1-26.

Llaumett, C.; Henrquez, F. 1976. Fajade mineralizacin deoro-plata-cobre-arsnico en la Cordillera Andina de las11 1 y IV Regiones de Chile. In Congreso Geolgico

Chileno, No. 1, Vol. 2, p. E175-E191. Santiago.Mercado, M. 1982. Geologa de la Hoja Laguna del Negro

Francisco, Regin de Atacama. Instituto de Investigaciones Geolgicas, Carta Geolgica de Chile, No. 56,73 p.Oviedo, L.; Aguilar, A.; Grez, E.; Fuster, N.; Ribba, L.;Tschischow, N.;Zuccone,A. (Inpress). General Geology01 La Coipa precious metal deposit, Atacama, Chile.Economic Geology.Pearson, M.F.; Clark, K.F.; Gonzlez, O. 1988. Ore mineralogy and silver distribution at Real de Angeles, Zacatecas, Mexico. In Silver-exploration, mining andtreatment, Symposium.lnst itute of Mining MetallurgyofLondon, p. 73-100. Mxico.

Rivera, S. 1988. Exploracin del depsito de plata-oro LaCoipa, Regin de Atacama. In Congreso GeolgicoChileno, No. 5, Actas, Vol. 1, p. B135-B150. Santiago.

Sangameshwar, S.R.; Barnes, H.L. 1983. Supergeneprocesses in zinc-Iead silver sullide ores in carbonates.Economic Geology, Vol. 78, p. 1379-1397.

Sawkins, F.I. 1988. Anatomy 01 a world-class silver systemand implications lor exploration: Fresnillo district,Zacatecas, Mexico. In: Silver-exploration, mining andtreatment, Symposium. Institute of Mining Metallurgy,London, p. 33-40. Mexico.Segerstrom, K. 1968. Geologadelas Hojas CopiapyOjosdel Salado, Provincia de Atacama. Instituto de Investigaciones Geolgicas, Boletn, No. 24, 58 p.Sillitoe, RH.; McKee, E.H.; Vila, T. (In press). Reconnaissance K-Ar geochronology 01 the Maricunga goldsilver belt, northern Chile. Economic Geology.

Thorpe, RS.; Francis, P.w.; Hammill, M.; Baker, M.C.w.1982. The Andes. In Andesites: Orogenic Andesitesand Related Rocks (Thorpe, R.S.; editor) John Wileyand Sons, p. 198-206. New York.Vila, T.; Sillitoe, R.H. (In press). Gold-rich porphyry systemsinthe Maricungabelt, northem Chile. EconomicGeology.

Zentilli, M. 1974. Geological evolution and metallogenicrelationships in the Andes 01 northern Chile between26and 29South. Unpublished Ph. D. Thesis. Queen'sUniversity, p446. Kingston, Ontario, Canada.

Zentilli, M.1975. Zonacin regional y evolucin metalognicade Los Andes entre las latitudes 26 y 29 Sur. InCongreso Iberoamricano de Geologa Econmica,No. 2, Actas, Vol. 5, p. 531-544. Buenos Aires.

geologia esperanza, maricunga

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