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Introduction

THE ACCHA DEPOSIT is located in southern Peru, 70 km southof the city of Cuzco in the 30-km-long Accha-Yanque belt,where other Zn and Pb prospects are currently under explo-ration (Fig. 1). Accha is a nonsulfide Zn (Pb) deposit (withestimated resources of 5.1 Mt @ 8.2% Zn and 0.9% Pb) cur-rently owned by Zincore Metals. The nonsulfide Zn concen-trations of Accha are amenable to processing by leaching, sol-vent extraction, and electrowinning, using, at least partly, theprocess developed by Anglo-American at the Skorpion zincSXEW plant in Namibia. The increase in the Zn prices in2006 and 2007 resulted in the revival of several zinc oxideprojects. Those with metallurgy similar to Skorpion, where

the processing challenges have been successfully overcome(Cole and Sole, 2002; Gnoinski, 2007), have the best chanceof success. These deposits have a relatively simple mineral-ogy, good extraction recovery, and higher grades. Examples

are Angouran, Iran (Boni et al., 2007a), Shaimerden, Kaza-khstan, Jabali, Yemen, and most recently Torlon Hill,Guatemala.

Nonsulfide zinc is a very general term, which comprises awhole series of minerals (Large, 2001; Hitzman et al., 2003;Boni, 2005a). The only minerals of current economic impor-tance are the carbonates smithsonite and hydrozincite, andthe silicates hemimorphite, willemite, as well as Zn smectite.Zinc can also be hosted in other types of clay minerals differ-ent from smectites, as in chloritelike clays (Rule and Radke,1988; Blot et al., 1995; E. Belogub, pers. commun.). High

The Nonsulfide Zinc Deposit at Accha (Southern Peru):Geological and Mineralogical Characterization

MARIA BONI,

Universit di Napoli Federico II, Dipartimento di Scienze della Terra, Via Mezzocannone, 8 80134 Napoli, Italy, and

Geologisch-Palontologisches Institut, Universitt Heidelberg, Germany

GIUSEPPINA BALASSONE,

Universit di Napoli Federico II, Dipartimento di Scienze della Terra, Via Mezzocannone, 8 80134 Napoli, Italy

VERNON ARSENEAU,

Zincore Metals Ltd., 1650-701 West Georgia St., Vancouver, British Columbia V7Y 1C6, Canada

AND PAUL SCHMIDT

PRS Associates, 1213 Argreen Rd. Mississauga, Ontario L5G 3J2, Canada

Abstract

The Accha-Yanque zinc belt is located in the southern Altiplano of Peru, a major zinc-rich metallogenicprovince hosting a number of economic mineral deposits (porphyry copper and skarn ores). Several nonsulfide-type occurrences, showings, and mineral deposits are situated in a belt, peripheral to the northern, northeast-ern, and northwestern edge of the Oligocene-(Miocene?) Yauri-Apurmac batholith. Mineralization is hostedin breccias of both sedimentary and tectonic origin in the limestones of the Middle to Upper Cretaceous Fer-robamba Formation. Primary ores belong to the carbonate replacement deposit type and are at least in partstructurally controlled. Currently, the Zn mineralization is almost fully oxidized: the Accha deposit can be as-signed to both direct replacement and wall-rock replacement types. The mineralized zone (indicated resources5.1 Mt @ 8.2% Zn and 0.9% Pb) occupies the hinge of an anticlinal dome that has been exposed by erosion.The southern limb of the structure dips about 55 to the south-southwest, whereas its northern limb is trun-cated by faults. The nonsulfide concentrations, consisting of a mineralized zone 5 to 20 m thick, are continu-ous along strike to the west for at least 700 m.

The mineralogy of the Accha deposit shares many characteristics with that of the typical carbonate-hostedcalamine-type nonsulfide Zn ores. The nonsulfide mineral association consists mainly of smithsonite and hemi-morphite replacing both primary ore minerals and carbonate host rocks. Hydrozincite has been detected only

in samples near the surface. Smithsonite occurs in zoned concretions with goethite, Mn (hydr)oxides and Znclays, as well as replacive cement in the limestone intervals. One of the most peculiar nonsulfide Zn mineralsat Accha is a sauconite-like, zincian smectite, variably concentrated throughout the deposit. Locally sauconiteoccurs as replacement of detrital feldspars and/or detrital fragments occurring in marly sediments or in infillsof karst cavities. It also replaces both hemimorphite and smithsonite deposited during earlier stages.

The age of the supergene products in the whole belt is poorly constrained, although there is geomorphologicevidence that the formation of supergene minerals postdates by more than 10 m.y. the last large-scale sec-ondary enrichment event that terminated with central Andean climatic desiccation at ~15 Ma. The age of theAccha deposit may be consistent with a Pliocene K-Ar date of 3.3 0.2 Ma obtained for supergene alunite fromthe top part of the leached cap in the nearby Cotabambas Cu deposit.

Corresponding author: e-mail, [email protected]

2009 Society of Economic Geologists, Inc.Economic Geology, v. 104, pp. 267289


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amounts of Zn have been detected also in Mn (hydr)oxides, asin the Jabal Dhaylan prospect, Saudi Arabia (Hayes et al.,2000). However, these concentrations are relatively uncom-mon. The high-temperature mineralogical association offranklinite, zincite, and gahnite, occurring in the Franklin-Sterling Hill-type deposits in North America, is also not verycommon (Johnson, 2001). Willemite-rich ores (Beltana, Aus-tralia; Vazante, Brazil; Berg Aukas, Namibia, Kabwe, and StarZinc, Zambia), associated with Proterozoic carbonates, werealso probably deposited in higher temperature conditions ashypogene nonsulfides (Sweeney et al., 1991; Brugger et al.,

2003).Despite widespread distribution of surficial zinc oxides,economic zinc nonsulfide deposits are much less commonthan sulfide zinc deposits. Host-rock composition signifi-cantly influences the mineralogy (and therefore metallurgy)of nonsulfide zinc deposits. Those in limestone and dolomitetend to be dominated by smithsonite and hydrozincite, due tothe interaction of low-pH Zn-rich ground-water fluids withhost carbonates, whereas deposits in siliciclastic rocks (whereAl and Si are available) tend to contain hemimorphite- andsauconite-bearing assemblages. However, even with similar

host rocks, the mineralogy can vary substantially: it can be rel-atively simple (smithsonite, hemimorphite, hydrozincite), asin the oxidation products derived from low-temperature sul-fide deposits (Mississippi Valley- or Irish carbonate-hostedtypes) or far more complex when derived from the weather-ing of high-temperature, pyrite-bearing ores of carbonate re-placement deposits of skarn origin, owing to the wide rangeof metals associated in the primary mineralization (Hitzmanet al., 2003). Complex nonsulfide ores can contain Fe- andMn-rich zinc minerals such as Fe smithsonite, Zn dolomiteand/or minrecordite, manganosiderite, hetaerolite, as well as

Cu carbonates and arsenic, phosphorous, and vanadium min-erals (Borg et al., 2003; Boni et al., 2007b).The entire range of nonsulfide Zn minerals, with the ex-

ception of Zn spinels are leachable in sulfuric acid. However,because the differences in dissolution rates of the zinc miner-als present in the deposit may have strong implications for theproduction strategies and metallurgical requirements, it ishighly advisable to conduct detailed mineralogical and petro-graphical studies early in the exploration process.

The economic value of zinc nonsulfide ores is thus depen-dent not only on the geologic setting of each deposit but also

268 BONI ET AL.

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PERU

ACCHA

YANQUE

Quartz feldspar porphyry

Tertiary)

Yauri-Apurmac batholith

Oligo-Miocene)

Puno Group

E ocene-Oligocene)

Ferrobamba Fm

Cretaceous)

Mara Fm

upper Jurassic)

Zinc small) prospect

Zinc deposit

10 km

TITIMINAS

SW

MINASCCASA

OSCOLLO

YURAC

AZULCANCHA

PUYANI

STUDY

AREA

7200 W 7150 W

1400 S

1410 S

Accha

TITIMINAS

TITIMINAS

WEST

CAMP

ZONE

N

FIG. 1. Schematic geology of the Accha-Yanque mineralized belt in southern Peru.


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on the specific characteristics of the mineralogical associationand the nature of the gangue minerals (Boni, 2005a; Woollett,2005; de Wet and Singleton, 2008). As mineral processing fora number of deposits was not available only a decade ago, afew deposits (as Jabali in Yemen) are now about to enter intofull production. An exception was the Padaeng supergenenonsulfide zinc deposit at Mae Sod (Thailand), which has

been in operation since 1983, using the old Vieille Montagneextraction process (C. Allen, per. commun.).A thorough mineralogical and petrographic examination,

aimed at identifying which zinc minerals are present in orderto understand and facilitate the mineral processing, has beencarried out on the Accha nonsulfide mineralization, the re-sults of which are reported here.

General Geological Setting and Mineralization

The Mesozoic and Cenozoic stratigraphy of the regionsouth of Cuzco in Peru (Fig. 2) is chiefly made up of Jurassicand Cretaceous sedimentary successions. These were de-posited in two basins (western and eastern Peruvian basins)separated by the Cuzco-Puno basement high (Jaillard and

Soler, 1996). The northeastern edge of the Western basin in-cludes the Lagunillas and Yura Groups (Marocco, 1978) madeup of Lower Jurassic limestone and Middle to Upper Jurassicquartzarenite and shale of the Soraya and Mara Formations,whose total thickness is approximately 800 m. The top of the

succession consists of the massive to laminated micritic lime-stone of the Middle to Upper Cretaceous Ferrobamba For-mation (Marocco, 1978), containing intercalated black shalesand nodular chert. This formation hosts the Accha mineral-ization. The Ferrobamba Formation or local early Cenozoicsuccessions are unconformably overlain by the subaerially de-posited sedimentary San Jernimo Group and the dominantly

volcanic Anta Formation. These units of Eocene to earlyOligocene age are more than 1.5 km thick. The San JernimoGroup consists of two main formations (Kayra and Soncco)made up of red terrigenous and conglomeratic sediments in-terbedded with tuffaceous horizons near the top. The SanJernimo is equivalent to the Puno Group of the Peruvian al-tiplano southeast of the study region, where it is overlain bythe volcanic horizons of the Miocene Tacaza Group (Jaillardand Santander, 1992).

The main part of the region was affected by several LateCretaceous to Pliocene tectonic events (Marocco, 1975).Most significant are the Eocene to early Oligocene (Incaic)and Oligocene to Miocene (Quechua) events. The Mesozoicto Cenozoic strata were moderately to intensely deformed in

large, northwest-trending folds with dominantly northerlyvergence (Perell et al., 2003). The most intense folding inthe region typically involves the carbonate and shaly succes-sions of the Ferrobamba Formation and equivalent units thatwrap around the less deformed quartz arenites of the Yura

NONSULFIDE ZINC DEPOSIT, ACCHA, PERU 269

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25 km

7230 W 7200 W

1400 S

1430 S

VELILLE

LIVITACA

SANTO TOMS

ACCHA

COTABAMBAS

CUZCO

PARURO

URCOS

Late Paleozoic to early Triassic mainly

volcanic rocks (Mitu Gp.)

Miocene to Pliocene volcanics

Porphyry Cu cluster/deposit

Reverse fault Fault Syncline Anticline

Oligocene to Miocene continental sediments(Paruro, Punachanca Fms. and equivalentunits)

Oligocene to Miocene subaerial volcanic

rocks (Tazaca and Sillipaca Gps. andequivalent units)

Eocene to early Oligocene Apurmac-YauriBatholith

Eocene to early Oligocene volcanic andsedimentary rocks (Anta Fm.)

Eocene to early Oligocene red bedsequences (San Jernimo and Puno Gps.)

Mesozoic to early Cenozoic marinesedimentary sequence (Yura and LagunillasGps.; Ferrobamba Fm.)

N

Fe-Zn(Cu, Au) Skarn

cch

Non sulfide Zn deposit Town/Village

FIG. 2. General geologic map of the mineralized district around the Yauri-Apurmac batholith (modified from Perell etal., 2003).


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Group. Subsequently, the red beds of the San JernimoGroup were deposited in structurally controlled, northeast-trending synorogenic basins (Perell et al., 2003).

The Mesozoic sedimentary rocks, as well as the lower partof the Tertiary ones, have been intruded by the Yauribatholith (Fig. 2), whose age extends from Eocene toOligocene (Noble et al., 1984; Perell et al., 2003). This com-

plex and polyphase magmatic body, whose emplacement wassynchronous with the Incaic orogeny, is also known locally asthe Abancay (Marocco, 1978) or Apurmac batholith (Pecho,1981; Mendvil and Dvila, 1994). The batholith is composedof a multitude of intrusions (consisting of gabbrodiorites,monzodiorites, and granodiorites) that crop out discontinu-ously for over 300 km between the towns of Andahuaylas inthe northwest and Yauri in the southeast. Subvolcanic rocks ofdominantly granodioritic and/or dacitic composition repre-sent the last magmatic stage (Perell et al., 2003). Until thelate 1980s, the region around the Yauri-Apurimac batholithhad received only limited geologic interest and was mainlyknown for its Cu-Mo-Aubearing, magnetite-rich skarn de-posits located at the contact with intrusive monzonite and

granodiorite rocks. Examples are Millo, Iris, and Millohuayco(Santa Cruz et al., 1979; Einaudi et al., 1981).Later, important porphyry copper and gold deposits were

discovered in the area, occurring in association with multipleintrusive and effusive phases of the Yauri batholith, best ex-emplified by the Tintaya, Las Bambas, and Katanga ores (Sil-litoe, 1990). Zn-Pb and Ag carbonate replacement depositswere reported from the Ferrobamba Formation and equiva-lent units up to distances of several kilometers from the mainporphyry intrusions (Carman et al., 2000; Perell et al., 2003).

Perell et al. (2003) considered the calc-alkaline magmas ofthe Yauri-Apurmac batholith and associated porphyry-styleand skarn mineralization as generated during subduction flat-tening, which triggered a pronounced crustal shortening and

tectonism, followed by strong uplift. Recent K-Ar data con-firm that the porphyry copper mineralization, the skarn, andpossibly also most of the carbonate replacement deposits inthe region (including the primary sulfides in the Accha belt),have ages ranging between 41.9 1.1 and 28.7 0.8 Ma(Noble et al., 1984; Mathur et al., 2001; Perell et al., 2003).

Though supergene alteration affected most of these de-posits, secondary metal enrichments are of variable impor-tance. Partial to complete oxidation of sulfides in skarn andporphyry deposits in the region around the Yauri batholithmostly reaches depths of 30 to 50 m, locally even 150 m(Perell et al., 2003). However, most of the porphyry systemsin the area lack significant zones of supergene enrichment.This is due to relatively low pyrite contents and presence of

high neutralization capacities (Perell et al., 2003) in the hostrocks (potassic alteration zones as well as carbonates of theubiquitous Ferrobamba Formation). The best conditions forsupergene copper enrichment (not always available in the re-gion) include (1) country rocks other than carbonates, such asthe quartz arenites of the Yura Group, (2) appreciableamounts of pyrite in the alteration zones, and (3) a pluvialregime characteristic of the elevated Cordilleran topography(Sillitoe, 2005). Consequently, most supergene cap rocks ofthe orebodies in the Andahuaylas-Yauri district are immature,typically goethitic in composition, with only a few containing

appreciable amounts of copper in the form of carbonate, sili-cate, and associated Cu oxide minerals. Although the pre-dominance of carbonate strata (with their strong buffering ca-pacity) around the Yauri batholith is not favorable tosupergene Cu enrichment zones, it is ideal for the formationof nonsulfide Zn-Pb ores from carbonate-hosted sulfide de-posits. These are commonly associated with variably deep

karst dissolution.The age of the supergene products in the Andahuaylas-Yauriregion is poorly constrained, even though there is good evi-dence for a Pliocene (or even younger) timing for the mainweathering processes. A strong argument for Late Tertiary ox-idation events is the relationship of oxidation with strong up-lift pulses, which took place in this region of the Andes wellafter the middle Miocene (Schildgen et al., 2007; Garzione etal., 2008). In response to these uplifts and increasing runoffdue to a wetter climate recorded after 7 Ma (Thouret et al.,2007), distinct erosion pulses occurred in the area at an aver-age rate of 0.2 mm yr1. These erosion pulses resulted in deepvalley incisions, which reached a total depth of 2.4 km. An ad-ditional incision phase affected the already-established

Quechua II paleosurface between 5.1 and 2.3 Ma (Garzione etal., 2008). Geomorphologic evidence shows that the formationof several chalcocite blankets on top of the Cu deposits locatedaround the Andahuaylas-Yauri batholith are of Pliocene ageand, as in other regions of the central Andes, postdate by >10m.y. the last global scale secondary enrichment event that ter-minated with the central Andean climatic desiccation at ~15Ma (Sillitoe and McKee, 1996; Brimhall and Mote, 1997). Alsothe supergene Zn-Pb enrichment zones located at high eleva-tions (~4,000 m), as at Accha and Yanque, may be associatedeither with one of the Pliocene uplifts or with the multipletopographic rejuvenations, which have been reportedthroughout the region (e.g., Cabrera et al., 1991).

Absolute ages of the supergene minerals are sparse in this

part of Peru. The only existing age is a K-Ar date of 3.3 0.2Ma (late Pliocene, Perell et al., 2003) for supergene alunitefrom the top part of the leached cap in the Cotabambas Cudeposit.

Accha geologic setting

The Accha nonsulfide Zn (Pb) deposit is located at an ele-vation of 4,000 to 4,400 m above sea level, in the high Andessouth of the city of Cuzco. It occurs at the north end of the30-km-long Accha-Yanque belt, along the northwestern bor-der of the Yauri-Apurimac batholith (Figs. 1, 2). The area lieson a broad anticline oriented west-northwest, parallel to thepredominant regional Andean trend (Carman et al., 2000).The main nonsulfide concentrations occur in the Titiminas lo-

cality. According to the classification of Hitzman et al. (2003),these nonsulfides can be assigned to both direct replacementand wall-rock replacement.

Direct replacement deposits are equivalent to Zn-rich gos-sans, where smithsonite and hydrozincite replace sphaleriteand cerussite replaces galena; whereas wall-rock replacementdeposits, where the main ore mineral is microcrystallinesmithsonite, derive from the buffering reactions between thecarbonate host rocks and acidic ground water containing zinc.

A number of poorly known zinc and lead prospects, whichlocally contain copper and silver values, occur at several

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localities along the Accha-Yanque belt (Fig. 1). The Yanqueprospect, currently under active exploration and where Pbminerals (cerussite>>galena) prevail over Zn nonsulfides, oc-curs at the southwestern extremity of the belt. The inferredmineral resource estimate for the Yanque prospect is about10.3 million metric tons (Mt) grading 5.3 percent Zn and 5.4percent Pb (McMahon, 2008).

The geologic setting in the Accha area was first describedby Carman et al. (2000) and Hudson et al. (2000) and thensummarized by Winter (2006). Recent mapping conducted byMarsden (2006) offers new insights into the local geology(Fig. 3). The schematic stratigraphic succession and lithologicunits in the Accha area are shown in Figure 4. Formations ofmajor geologic importance in the mining district are thefollowing:

1. Soraya Formation (Yura Group, Middle Jurassic).2. Mara Formation (Yura Group, Upper Jurassic).3. Ferrobamba Formation (Middle-Upper Cretaceous).The Ferrobamba Formation consists mainly of carbonates

(limestone>>dolomite) with thin shaly intervals and was sub-

divided by Winter (2006) from base to top in (a) thin-beddedto laminated limestone with interbeds of massive limestoneand limestone breccia; (b) thin-bedded, commonly dark, lam-inated, locally shaly limestone with massive interbeds (b1 andb2). This unit is locally brecciated and the main host to zincmineralization; (c) massive- to thick-bedded micritic lime-stone (c1 and c2); (d) laminated cherty limestone with massivemicrite interbeds; (e) thin- to medium-bedded dark lime-stone with massive interbeds containing chert nodules; (f)medium-bedded micrite limestone, characterized by patchyyellow dolomite and/or ankerite alteration. The topmost partof this unit is generally lacking, except where unconformablyoverlain by the San Jeronimo Group sediments.

4. San Jernimo-Puno Group detrital succession (Eocene-

Oligocene).The sedimentary rocks belonging to this Group generallycrop out at Accha as a coarse conglomerate with polymicticclasts, consisting of intrusive rocks, limestone, and quartzite.The matrix of the conglomerate is a red mud-supported sand-stone. The San Jernimo Group has been named PunoGroup in internal company reports and maps.

5. Tertiary magmatic rocks associated with the onset of theYauri batholith.

The Tertiary Yauri-Apurmac batholith consists mainly ofintrusive bodies, with a minor contribution of porphyry stocksand tuffitic products. In the Accha area the composition isdioritic to granodioritic. The most common magmatic rocksof the area are: (a) hornblende-feldspar porphyry stocks; and

(b) feldspar porphyry and quartz-feldspar porphyry.

A local development of subrecent glacial tills, locally cover-ing the nonsulfide concentrations, has been recorded withinthe project area.

The main compressional deformation affecting the Fer-robamba limestone commonly resulted in northward-vergingfolds and thrust faults. The deformation of the thin-beddedlimestone (main host of the nonsulfides) is quite complex,with well-developed S- or Z-shaped folds (Winter, 2006).Whereas the Ferrobamba Formation carbonate rocks at

Accha are commonly tightly folded, the underlying SorayaFormation quartzites (Yura Group) are less deformed.

Main Characteristics of the Accha Deposit

The primary sulfide mineralization in the Accha-Titiminaszone, hosted in the Ferrobamba limestone, is genetically re-lated to Tertiary igneous activity (Bradford, 2002) and con-

trolled by the local tectonic setting (Carman et al., 2000).Owing to the location close to the northern outcrops of theYauri-Apurmac batholith and the elevated content of gra-nophile elements like arsenic, molybdenum, strontium, andthallium (present also in the nonsulfide minerals, Bradford,2002), the Accha primary base metal sulfide bodies can beconsidered a product of an intrusive-related mineralizationevent affecting carbonate rocks, basically similar to other dis-tal skarn deposits occurring in the region (Einaudi et al.,1981). The ore minerals consisted originally of sphalerite-pyrite > galena, locally associated with halos of silica and fer-roan dolomite. This dolomite, interpreted as hydrothermal inorigin, may have contained locally a fair amount of man-ganese, given the high content of Mn (hydr)oxides concen-

trated in the secondary deposits. However, Mn may also besourced by weathered fluid escape structures, which consistof Fe- and Mn-rich carbonate veinlets commonly occurring inthe distal areas of the skarn deposits (Meinert et al., 2005).

The thicker zones of mineralization are concentrated in the(a) and (b) carbonate units of the Ferrobamba Formation (Fig.4) and are hosted within strata-bound, brecciated, and lami-nated limestone. The main host to mineralization consists ofcarbonate-clay matrix-supported breccias and locally by verythin, quartz-rich conglomerate layers. The total thickness ofthe brecciated interval, visible both in outcrop and in drillcore, varies from 50 to 100 m, whereas individual brecciazones are continuous over 5 to 20 m downhole. The brecciasare polymict, consisting of angular limestone clasts and rare

quartzite. Most breccias are poorly sorted with little or no ap-parent grading. Unmineralized breccia intervals have beenrecognized in the same stratigraphic position up to severalkilometers away from the Titiminas main mineralized zone.

Bradford (2002) interpreted the breccia bodies as hydraulicin origin, caused by fluid overpressure focused by preexistingfaults. Winter (2006) considered most breccias as being genet-ically related to tectonic processes. It is our opinion that severalbreccia packages, as well as the obvious conglomerates at thebase of the Accha succession, are synsedimentary or early dia-genetic in origin, possibly related to the instability of the lowerFerrobamba depositional environment. However, their charac-teristics are difficult to unravel, due to the generally strong ox-idation associated with the breccia intervals (Fig. 5a).

The Zn-Pb mineralization at Accha occurs in four areas:Titiminas (Figs. 2, 3), Titiminas West, the Camp zone, andTitiminas Southwest (Fig. 1), all located on the western sideof the main Titiminas zone. According to Winter (2006) theTitiminas mineralized bodies are hosted by a thick carbonatewedge between two major thrust faults, known as the Mainand Middle thrust zones and pinch out where the two thrustzones merge to the west and east of the main bodies. Themineralization occupies the hinge of an anticline that hasbeen exposed by erosion. This anticline plunges abruptly tothe east at about 50 and about 35 to 40 to the west (Fig. 3).


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0.1

18.4

AC-01

A

C-02

AC-04

AC-03

AC

-02

AC

-04

AC-03

AC-01

MET-03

MET-04

MET-02

MET-01

MET

-03

MET-04

MET-01

Legend

Drillholes

CollasuyoMETdrillholes

CollasuyoACdrillholes

Roads

0.0-0.1

0.2-1.0

1.1-5.0

5.1-100.0

Acchaoldworkings

Alteration

Dolomite

Dolomitebreccia

Dolomite/hematite

Fe(hydr)oxides

FeOx(stockwork)

FeOx(magnetite)

FoldA

xes

F1S

yncline

F1A

nticline

F2A

nticline

F2S

yncline

Faults

Limonitebreccia

Mineralization(ZincGossan)

Puno-SanJernimoGroup

(Eocene-EarlyOligocene)

f.Uppermediumbeddedlimestone

e.Upperthinbeddedlimestone

d.Chertylimestone

c2.Massivelimestone

b1&b2.Thinbeddeddarklimestone

a.Thinbeddedlimestonebreccia

80

0

20

40

60

100m

FerrobambaFormation

(Middle-UpperCretaceous)

RockDataZn%

MET-02

8453500N

TITIMINAS

PunoGroup(outcrop)

CoreDHSampleZn%

0.0-2.0%Zn

2.1-5.0%Zn

5.1-10.0%Zn

10.1-20.0%Zn

20.1-100.0%Zn

8453750N

8454000N

186750E

186500E

186250E

186000E

D

C

Section

c1.Thickbeddedlimestone

FIG.3.GeologicmapoftheAccha-Titiminasa

reawiththelocationoftheACandMETdrillcores(modifiedfromZincoreLtd.).


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Strike-slip and transtensional northeast-trending faults trun-cate the mineralized bodies in several places. Currently, theorebodies are completely oxidized and only rare sulfide rem-nants are recognizable in the Titiminas trenches (Fig. 5a) orin drill cores. In the resource area Fe-rich nonsulfide con-

centrations containing zinc carbonate, silicate, and oxide min-erals are exposed at the surface over an area measuring about300 by 200 m (Winter, 2006; Fig. 3). Drilling has shown thatthe oxide mineralization consists of zones 5 to 20 m thick, dip-ping 50 to 60 to the south, traceable along strike to the westfor at least 700 m and at depth for at least 400 m.

The mineralization generally consists of laminated, highlyporous, brown to yellow-brown lithotypes (Fig. 5b), contain-ing Fe (hydr)oxide and banded nonsulfide Zn minerals.Galena, with associated anglesite, can be found only locally.Stockwork to isolated pods of iron oxides and zinc carbonates

and/or silicates are also present throughout the succession.Due to the intense oxidation of the outcrops, the primary tex-tures of the host rock are obscured. Distinctive red lithologicunits occur, which host fine-grained detrital sediments con-taining abundant quartz and feldspar, possible karst infillings.The boundaries between the red, mineralized intersections,and the Ferrobamba limestone clasts and/or inserts are very

sharp (Fig. 5c).The Titiminas Southwest zone consists of a massive baritevein with local galena and iron oxide, along with nonsulfidezinc mineralization. This zone is hosted along a fault devel-oped at the contact between the Puno Group red beds andthe lower laminated part of the Ferrobamba Formation. Inthe Corrales area, 3 km southwest of Accha, a small strata-bound body crops out, consisting of a network of nonsulfideveins with galena and barite remnants in the matrix of lime-stone breccias. Along the same horizon also occurs a largebarite body with zebra texture. The zebra texture consists ofseveral layers of microcrystalline barite, dark dolomite, andalmost completely oxidized sphalerite.

Several mineralogical studies have been carried out on the

Accha nonsulfides; results are found mainly in unpublishedcompany reports (Pontifex and Assoc., 1999; Pasminco, 2000;Bradford, 2002; Boni, 2005b, 2007) and in the only publicationby Carman et al. (2000). All the quoted reports recognize the Znhydrosilicate hemimorphite and the Zn carbonate smithsoniteas the main ore minerals occurring at Accha. Also the local oc-currence of Zn smectite has been mentioned, as well as the pos-sibility of zinc being partly hosted within the goethite lattice.

Minor components also occur, like a few remnants of un-weathered honey-colored sphalerite and pyrite, galena,hematite, illite, and kaolinite clays. The possible occurrenceof the Zn silicate willemite has also been mentioned. Smith-sonite should have locally replaced the host limestones, whilehemimorphite occurs preferentially in the Fe-rich gossanous

parts of the deposit (Pasminco, 2000). A common associationexists between Mn oxides and smithsonite (in regular inter-growths), as well as Zn smectite in the fines (Pontifex andAssoc., 1999; Pasminco, 2000). Fair amounts of hydrozincitewere detected for the first time by Boni (2005b) in the gos-sanous samples from the Titiminas trenches.

Quartz as well as K-feldspar is ubiquitous and fairly abun-dant at Accha, whereas plagioclase is only a minor con-stituent. These mineral species are detrital and generally as-sociated with the mineralized intervals. The detrital mineralsmay have been concentrated in karst cavities and joints of theFerrobamba limestone. Another possibility is that they mayhave occurred as thin intercalations in the lower part of theFerrobamba Formation, near the stratigraphic contact with

the red beds of the Mara Formation. Such a stratigraphic po-sition has been recently hypothesized for the quartz- andfeldspar-rich conglomerates and breccias hosting the YanquePb-Zn deposit (Collasuyo geologists, per. commun.).

Sample Preparation and Analytical Methods

Mineralogical, petrographical, and geochemical researchreported in this paper has been performed on 80 samplesfrom the Accha prospect (Boni, 2007). The bulk of the ana-lytical work has been carried out on the MET 1, MET 2,MET 3, and MET 4 cores, spaced across a 200-m strike length


0361-0128/98/000/000-00 $6.00 273

Group Formation Stratigraphy Thickness Description

Puno-

San

Jernimo

Group

Yura

Group

Mara

Fm

Soraya

Fm

5 - 20 m Recent soils & glacial tills

> 1.5 kmFlysch

Conglomerate & Red Beds

> 8 km Quartzite & Shale

Red Bed Shale &

Conglomerate50 - 100 m

Quaternary

100 m

140 m

120 m

50 -100 m

80 -100 m

100 m

Silty (Marly) Medium

Bedded Limestone

Nodular Cherty

Laminated Micritic

Limestone

Fine Grained MassiveMicritic Limestone

Thin Bedded Limestone

Brecciated, Laminated &Foliated Bituminous Limestone

Fine GrainedMicritic Limestone &

Limestone Breccia

Upper

Unit (e - f)

Laminated

ChertyLimestone

(d)

MassiveLimestone

(c)

Transitional

Succession(b2)

FootwallLimestone

(a)

Eocene -

Oligocene

Upper

Jurassic

Middle

Jurassic

Ferrob

ambaFormation

Middle-

UpperCretaceous

Laminated

Succession(b1)

FIG. 4. Schematic stratigraphic section (not in scale) of the Accha area(modified from Winter, 2006).


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0361-0128/98/000/000-00 $6.00 275

8454000

4250

4200

4150

4100

4300

4050

4000

4250

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4100

4300

4050

4000

8453900

8453800

8453700

8453600

8454000

8453900

8453800

8453700

8453600

N

MET-01

a

MassiveLimestone(c)

ChertyLimestone

ThinBeddedLimestone(b1-b2)

Ferruginous

Limestone

Mineralizedinterval

Soil/Overburden

BrecciaLayer

8454000

4300

4250

4200

4150

4350

4100

4050

4000

4300

4250

4200

4150

4350

4100

4050

4000

8453900

8453800

8453700

8453600

8454000

8453900

8453800

8453700

8453600

MET-04

N

8454000

4350

4300

4250

4200

4150

4100

4050

4000

4350

4300

4250

4200

4150

4100

4050

4000

8453900

8453800

8453700

8453600

8454000

8453900

8453800

845

3700

8453600

MET-02

N

4350

4300

4250

4200

4100

4150

4350

4300

4250

4200

4100

4150

8453900

8453800

8453700

8453600

8453500

8454000

8453900

8453800

8453700

8453600

8453500

8454000

4400

4400

MET-03

N

c

(d)

SECTION18

6650E

b

0

100

d

SECTION

186600E

0

100

SECTION

186550E

0

100

SECTION186750E

0

100

FIG.6.a-d.G

eologiccrosssectionsofthemineralizedzone

,withthelocationoftheMET1toMET4drillcores(modifiedfromExploracionesCollasu

yo).


10/24

parts of the samples with different color or aspect. Semi-quantitative XRD analyses were performed on 26 samplesfrom MET 1, 29 samples from MET 2, seven samples fromMET 3, and 18 samples from MET 4.

Quantitative XRD analysis (wt %) was carried out on 30samples, chosen among those analyzed by the semiquantita-tive XRD method, representing the better characterized min-

eralized lithologic units. The quantitative phase analysis(QPA) was performed using the Rietveld method (Rietveld,1969; Bish and Howard, 1988; Hill, 1991; Bish and Post,1993). The results are given in Table 1. These data were com-plemented with the assay data of the same samples, measuredat the chemical laboratory of Mintek (Johannesburg). Table 2shows the summary results obtained from the assays carriedout at the ALS Chemex laboratory in Lima on the equivalentintercepts of the AC one to four drill holes. The latter resultswere used for resource calculation.

X-ray powder diffraction data were analyzed using the Gen-eral Structure Analysis System package (GSAS; Larson and

Von Dreele, 2000) and its graphical interface EXPGUI (Toby,2001). The XRD data for QPA were collected using theSeifert-GE instrument with 2- and 3-mm divergence slits,0.1-mm antiscatter slit, and 1-mm receiving slits. The datawere collected from 2 to 100 2 with 18 s counts per step(step scan 0.02 2). The XRD spectra were converted toASCII format by ConvX software and then interpreted by

Expgui software. Multiple refinements were performed dueto the large number of phases present in most samples.Polished thin sections (~30m thick) were observed under

a petrographic microscope (transmitted and reflected light).Many samples were examined by cathodoluminescence (CL)petrography, utilizing a CITL 8200 Mk3 cold cathodolumi-nescence instrument at the Institut fr Geowissenschaften,Universitt Heidelberg (Germany), operating at 23- to 25-kVvoltage and a 500- to 550-A beam current. The use of acathodoluminescence microscope is an easy way to distin-guish different nonsulfide minerals, even if occurring in com-plex intergrowths.

276 BONI ET AL.

0361-0128/98/000/000-00 $6.00 276

TABLE 1. Selected Samples from the Accha Metallurg

Quantitative analyses

Sm Hem Sa Goe Qz Kf Cc Ka Ill Ha Cha Ce Ja Co Pr

(wt %)

MET 1MET1-22-83.45 36.1 22.1 36.6 4.0 1.2MET1-22-84.00 2.0 75.4 2.6 18.8 1.2MET1-22-85.4 38.0 23.4 2.9 35.0 0.6 0.1MET1-23-86 35.0 9.6 8.1 40.9 1.0 5.5MET1-24-90.8 43.2 6.4 10.4 39.9MET1-24-91.5 83.9 12.0 1.9 2.2MET1-26-98.25 76.0 17.8 6.1MET1-26-98.35 2.8 47.8 3.3 1.2 7.0 38.0MET1-26-98.55 2.6 36.0 22.6 38.9

MET1-26-99.25 2.0 92.3 0.5 2.7 2.6

MET 2MET2-15-54.7 76.2 8.5 4.9 2.8 1.6 6.0MET2-15-54.9 80.5 3.0 15.5 1.1MET2-16-59.1 49.5 10.2 17.4 15.3 1.0 6.6MET2-21-75.2 90.3 1.2 4.2 1.6 2.7MET2-21-76.55 90.1 1.3 2.6 1.6 4.4MET2-22-78.55 4.7 30.5 6.8 58.0MET2-22-80.25 20.5 30.7 48.9MET2-22-80.55 2.5 17.5 50.8 7.8 18.7 2.8MET2-28-99.3 1.8 63.8 7.6 25.7 1.2

MET 3MET3-27-102.1 17.9 22.6 35.9 23.6MET3-28-103.2 39.3 23.2 5.4 25.2 6.8

MET 4MET4-29-104.05 27.1 13.9 21.2 13.0 20.1 3.0 1.8MET4-29-105.45 25.1 9.2 27.9 7.6 25.8 2.9 1.5MET4-30-109 65.4 5.3 12.1 5.8 5.2 2.3 3.9MET4-31-111.4 88.0 4.1 6.0 1.9MET4-31-112.3 65.7 18.2 16.1MET4-31-113.2 86.0 4.6 6.1 3.2MET4-35-126.85 72.0 20.3 3.7 4.0MET4-37-136 7.6 13.6 9.6 3.5 64.0 1.8MET4-38-139.00 27.1 1.6 12.8 53.2 1.9 3.5

Notes: Cc = calcite, Ce = cerussite, Cha = chalcophanite, Co = coronadite, Goe = goethite, Ha = halloysite, Hem = hemimorphite, Sm = smithsonite,Sa = sauconite, Qz = quartz, Kf = K-feldspar, Ka = kaolinite, Ill = illite, Ja = jarosite, Pr = pyrolusite


11/24

Secondary electron imagining by scanning electron mi-croscopy (SEM) was carried out with a Jeol JSM 5310. Ele-ment mapping and qualitative energy-dispersive (EDS) spec-tra were obtained with the INCA microanalysis system(Oxford). To get the mineralogical composition of selectedphases, we used a wavelength dispersion spectrometry (fullWDS) on a Cameca SX50 electron microprobe operating at

15 kV, 15 nA, and 10-

m spot size. Silicates, oxides, and pureelements were used as standards.

Results

Mineral composition of nonsulfide ores

The data derived from X-ray analysis (partly complementedby EDS), including a listing of all minerals that have been de-tected at Accha in order of decreasing abundance, are shown inTable 3a. Table 3b lists all nonsulfide Zn and Pb minerals oc-curring in the deposit, with their stoichiometric Zn and Pb con-tent. The Zn clay sauconite is among the most common miner-

als found in several samples. According to Ross (1946),sauconite is a Zn-Na trioctahedral smectite. In this paper we usethe term sauconite as referring to the typical Zn-rich smectitefound at Accha, even though we were not able to determine theactual Na wt percent in this clay mineral due to strong Na K-Zn L peak overlap in both EDS and WDS analyses.

MET 1

The Zn-rich interval of this core (2050% Zn) occurs be-tween 80- and 100-m depth (Figs. 6a, 7a). A fairly high con-centration of smithsonite has been detected in MET 1 startingfrom 82.3 down to 99.25 m. Hemimorphite (occurring from85.899.50 m) is generally spatially associated with Fe(hydr)oxides, but not always with smithsonite, occurring withtwo different associations: hemimorphite-smithsonite andhemimorphite-sauconite. Sauconite is ubiquitous (from 79down to 98 m) and locally very abundant. Quartz and K-feldspar have also been detected. The occurrence of K-feldsparin most samples is commonly correlated with the presence of


0361-0128/98/000/000-00 $6.00 277

ical Drill Cores MET 1, MET 2, MET 3, and MET 4

Chemical assays

Mg Al Si Ca Mn Fe Zn Pb S K Na As Cl Cd Tl

(wt %) (ppm)

0.08 2.06 8.09 0.49 0.53 21.60 22.30 0.05 0.01 0.43 106 28 89 150 41

0.05 1.04 4.45 0.53 0.79 24.70 25.71 0.07 0.01 0.75 95 16 93 139 310.12 0.88 4.93 2.41 0.52 26.30 21.52 0.06 0.01 0.84 146 22 105 115 380.05 0.33 5.96 16.10 1.38 9.28 19.72 1.03 0.01 0.26 78 25 125 95 290.05 0.31 11.32 0.77 1.42 6.95 43.21 1.95 0.01 0.11 93 19 89 186 270.05 1.26 13.21 0.43 2.95 0.42 43.52 0.09 0.01 0.16 109 20 68 192 400.05 5.34 12.32 0.52 11.40 1.01 24.61 0.05 0.01 0.29 126 19 95 125 280.13 3.42 12.31 0.75 15.80 0.74 21.31 0.05 0.01 0.68 158 16 112 119 61

0.05 0.26 12.01 0.09 2.55 0.19 51.81 0.09 0.01 0.25 58 28 82 195 15

0.05 5.08 0.36 0.36 3.22 6.15 36.70 1.29 0.01 0.25 83 19 85 175 690.11 1.45 11.52 0.31 1.02 11.60 34.70 0.43 0.01 0.68 103 23 125 162 570.44 3.07 14.41 0.47 0.84 5.82 32.90 0.07 0.01 0.45 125 28 121 135 280.12 0.56 5.51 7.48 0.28 5.56 35.50 0.11 0.01 0.23 70 15 96 185 650.05 0.26 1.38 0.39 0.71 5.01 45.80 0.13 0.01 0.19 78 35 105 124 290.19 0.77 3.55 21.20 0.82 20.01 2.43 0.38 0.01 0.25 85 42 112 58 520.32 5.41 19.00 0.41 0.74 14.42 8.63 0.12 0.01 0.19 69 28 89 102 240.69 3.89 16.80 0.29 0.05 26.41 2.01 0.35 0.01 0.83 126 22 99 63 160.05 0.44 8.45 0.15 0.85 23.12 32.22 0.45 0.16 0.13 63 19 103 182 28

0.05 0.09 10.90 0.18 0.11 25.11 1.42 23.50 1.62 0.11 531 36 135 25 810.11 1.38 9.77 0.47 0.65 7.81 37.70 1.32 0.02 0.32 83 28 129 176 65

0.78 6.32 21.22 0.62 1.15 6.93 14.70 1.49 0.01 2.25 378 41 121 87 590.62 5.06 16.71 0.63 0.40 18.40 11.30 1.01 0.01 2.06 348 45 68 69 480.21 2.21 13.41 0.49 2.95 5.71 31.70 1.02 0.01 0.31 126 28 79 135 710.05 0.39 2.56 0.44 2.97 4.97 44.10 1.42 0.01 0.13 60 36 125 182 380.05 0.25 1.93 0.52 1.51 11.91 39.80 0.66 0.02 0.09 54 29 132 165 690.08 0.91 3.81 1.05 0.39 14.70 32.30 1.23 0.16 0.28 60 15 98 157 480.05 0.36 4.32 0.28 0.08 5.74 43.60 0.91 0.38 0.09 43 12 115 191 350.06 1.74 7.93 26.01 0.33 1.83 7.42 0.07 0.04 0.73 119 27 128 58 62


12/24

smectite and other clays, which may be derived from weath-ering of the same feldspars. Crystalline goethite is also abun-dant, as well as possible forms of amorphous Fe (hydr)oxides(typical evidence is the consistently high background de-tected on the X-ray traces). Abundant Mn (hydr)oxides

(chalcophanite?), containing scavenged lead and thalliumoccur in the smectite-rich samples.

MET 2

The Zn-rich zones of this core (about 3040% Zn) are be-tween 54- to 60- and 75- to 78-m depth (Figs. 6b, 7b). TheMET 2 core seems to be of lower Zn grade, though, compared

to MET 1. Irregular amounts of Pb minerals, mainly cerussite,but also beudantite and plumbojarosite, are also contained inthis core; the latter is particularly abundant in the samples be-tween 65 and 75 m. Smithsonite commonly occurs in thelower part of the mineralized core, from 75 to 100 m, whereashemimorphite could be detected from 55 to 102.8 m. Znsmectite can be locally very abundant. Crystalline goethite isubiquitous, as well as amorphous Fe (hydr)oxides. Illite andkaolinite exist in minor, though detectable concentrations.

MET 3

The higher Zn values of this core (max 37% Zn, containedin smithsonite, hemimorphite, and lesser Zn smectite) havebeen detected between 102- and 112-m depth (Figs. 6c, 7c).

Plumbojarosite and cerussite prevail in the top of the sampledsection. Quartz, K-feldspar and illite-sericite (the latter de-rived from the hydrothermal alteration of the K-feldspar)have been detected locally.

MET 4

The higher Zn values of this core (1044% Zn) are localizedbetween 102- to 119-m and between 126- to 136-m depth (Figs.6d, 7d). Smithsonite is very common, whereas hemimorphiteoccurs only sporadically. Goethite is ubiquitous, and sauconitehas been detected in several samples. K-feldspar is locallypresent and coexists with Zn smectite, quartz, and smallamounts of illite, kaolinite, and probably halloysite.

278 BONI ET AL.

0361-0128/98/000/000-00 $6.00 278

TABLE 2. Summary Results Obtained from the Pb-Zn Assayson the Intercepts from Drill Holes AC-01 to AC-04

Hole no. From (m) To (m) Interval (m) Zn (%) Pb (%)

AC-0160.93 65.10 4.17 1.63 0.13

79.00 90.20 11.20 4.51 0.1882.75 86.42 3.67 8.09 0.02

118.27 119.96 1.69 6.46 0.42

134.35 136.60 2.25 13.67 0.45

AC-0247.81 82.70 34.89 9.31 1.2752.20 61.03 8.83 17.88 0.6469.78 73.05 3.27 22.67 1.02

84.50 98.80 14.30 1.95 0.08

98.80 104.77 5.97 11.00 0.17

AC-03101.80 114.85 13.05 8.65 2.41103.00 106.70 3.70 23.37 2.78

AC-04 102.84 120.10 17.26 8.65 0.97109.90 118.40 8.50 13.64 1.07

125.35 130.66 5.31 5.87 0.18

188.45 193.40 4.95 5.64 1.30

TABLE 3. Most Common Ore and Gangue Minerals Occurring in the Accha Deposit (a) andIdeal Chemical Formula of Nonsulfide Zn and Pb Minerals (b)

(a)

Minerals

Drill core Sm Hm Goe Ce Sa Ja Ox-1 Ox-2 Qz Cc Kf Ka Ill Ha

MET 1 MET 2 MET 3 MET 4

Notes: Cc = calcite, Ce = cerussite, Goe = goethite, Ha = halloysite, Hem = hemimorphite, Ill = illite, Ja = jarosite, Ka = kaolinite, Kf = K-feldspar, Ox-1 = Fe-Zn-Mn hydr(oxides), Ox-2 = Mn-Pb hydr(oxides), Qz = quartz, Sa = Zn smectite, Sm = smithsonite; = very common, = common, =

rare, = very rare

(b)

Minerals Formula Wt % Zn Minerals Formula Wt % Pb

Smithsonite ZnCO3 52.15 Cerussite PbCO3 77.54Hemimorphite Zn4Si2O7(OH)22H2O 54.29 Plumbojarosite PbFe6(SO4)4(OH)12 18.33Zn smectite (sauconite) Na0.3(Zn,Mg)3(Si,Al)4OH2nH2O 33.81 Coronadite Pb(Mn4+,Mn2+)8O16 24.41Hydrozincite Zn5(CO3)2(OH)6 59.55Chalcophanite (Zn,Fe2+,Mn2+)Mn4+3O73(H2O) 17.09


13/24


0361-0128/98/000/000-00 $6.00 279

120

110

100

90

80

70

60

50

40

30

20

10

160

150

140

130

HOLE ID: MET - 01

Lithology

symbol code

NORTHING: 8453672

ELEVATION: 4287Final Depth: 168

EASTING: 186758

Lithology

symbol code

Lithology

symbol code

Lithology

symbol code

HOLE ID: MET - 02

NORTHING: 8453734


EASTING: 186661

HOLE ID: MET - 03

NORTHING: 8453694


EASTING: 186556

HOLE ID: MET - 04

NORTHING: 8453704


EASTING: 186606

a b c d

Pbmax23%

Pb1.3

%

2A

-2B

-2C

Zn20to50%

2A

-2C

Zn30to40%

2

A

Znma

x37%

2A

Zn10to44%

2B

2C2

A

2B

2D

170

83.45- 22.3% Zn

85.40- 25.7% Zn

86.00- 21.5% Zn

90.80- 19.7% Zn91.50- 43.2% Zn

98.25- 43.5% Zn98.35- 24.6% Zn98.55- 21.3% Zn

54.70- 36.7% Zn54.90- 34.7% Zn

59.10- 32.9% Zn

75.20- 35.5% Zn

76.55- 45.8% Zn

78.55- 2.45% Zn80.25- 8.65% Zn80.55- 2.01% Zn

99.30- 32.2% Zn102.10- 1.42% Zn103.20- 37.7% Zn

105.45- 11.3% Zn

109.00- 31.7% Zn

104.05- 14.7% Zn

111.40- 44.1% Zn

112.30- 39.8% Zn113.20- 32.3% Zn

126.85- 43.6% Zn

136.85- 7.44% Zn

99.25- 51.8% Zn

0

50

40

30

20

10

0

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0

LLS

LLS

BX

BX

BX

LLS

LLS

ORE

ORE

ORE

ORE

ORE

ORE

ORE

ORE

ORE

OREORE

ORE

ORE

ORE

ORE

ORE

ORE

ORE

ORE

ORE

ORE

ORE

ORE

ORE

LLS

LLS

LLS

LLS

LLS

LLS

LLS

LLS

LLS

LLS

LLS

LLS

LLS

LLS

LLS

LLS

LLS

LLS

LLS

LLS

LLS

LLSLLS

LLS

LLS

LLS

LLS

LLS

LLS

LLSLLS

SO

SO

BX

BX

BXBX

BX

BX

BX

BX

BXBX

BXBX

BX

BX

BX

BX

BX

BX

BX

BX

BX

BX

GOUG

GOUG

GOUG

GOUG

MLS

MLS

MLS

MLSMLS

MLS

MLS

MLS MLS

MLS

MLS

MLS

MLS

BX

BX

BX

BX

BX

GOUG

GOUG

LLS

LLS

BX

LLS

LLS

LLS

NR

NR

BX

GOUG

GOUG

LLS

BX

MLS

BX

BX

BX

BX

BX

BXNR

SO-soil; GOUG-gouge; ORE-mineralized zone;

LLS-laminated limestone; BX-breccia;

MLS-massive limestone; NR-no recovery.

Colors as in Fig. 3 : Zn interval; : Pb interval; 2A: sublithotypes

LLS

LLSBX

FIG. 7. a-d. Graphic core logs of the MET 1 to MET 4 drill cores with Zn assay data (modified from ExploracionesCollasuyo).


14/24

In conclusion, as a result of both semiquantitative andquantitative analyses, we have been able to detect in theMET cores several different combinations of Zn minerals.The most common mineral association in MET 1 consists ofsmithsonite (3538%) and hemimorphite (923%). A similarcombination has also been found in MET 3 (smithsonite 39%- hemimorphite 23%) and MET 4 (smithsonite 6688% -

hemimorphite 420%). In MET 2, smithsonite prevails in themineralized sections.The association between hemimorphite and sauconite is also

commonly present in all cores: MET 1 (hemimorphite 4376%- sauconite 618%), MET 2 (hemimorphite 50% - sauconite10%), and MET 4 (hemimorphite 2565% - sauconite 514%).In a few samples from MET 4, sauconite is associated withother (barren) clays, as illite and halloysite, and with mineralsof the jarosite group. In the section comprised between 98.35and 98.55 m of MET 1, sauconite is particularly abundant(3648%) and complex Mn oxides of the chalcophanite grouphave been found here together with sauconite. Small amountsof chalcophanite (which can contain up to 17% Zn) have beendetected also throughout the MET 1 core.

Goethite is ubiquitous, except where Mn (hydr)oxides pre-vail, in MET 1 (1041%), MET 2 (337%), MET 3 (1825%),and MET 4 (428%) and might contain variable, though lowamounts of Zn. Quartz is another abundant mineral phase. Itcan range from


15/24

larger cavities and empty geodes. It contains smithsonite as

the prevailing Zn phase and is less common at Accha than2A.Sauconite and hemimorphite are very scarce or absent. Sev-eral smithsonite generations can be detected in this sub-group, from the early ones mixed with goethite and clays tothe zoned, clear crystals growing in the cavities. The CL col-ors of the different smithsonite generations vary greatly be-tween strong red tones to drab blue (Fig. 9e-f). A latereplacement of Zn carbonates by hemimorphite has beenobserved only in a few samples. Zn concentration in this sub-group can be higher than 45 percent Zn. (2C): This sublitho-type is represented by fine-grained, yellowish-brown, clay-rich

sediments with local zones of blackish Mn concretions and

veins (Table 4, Figs. 5e, 10a) and has been detected in theMET 1, MET 2, and MET 4 cores. It consists generally of Znsmectite (Fig. 10b-d), commonly mixed with Fe and Mn(hydr)oxides, as well as minor hemimorphite and smithsonite.Associated with the Mn (hydr)oxides, high amounts of lead(up to few %) and traces of thallium have been detected.Abundant quartz and K-feldspar fragments have been alsoobserved in this lithotype. Several Zn smectite types in theform of crystals and concretions (Fig. 11a-b) growing in cavi-ties occur in MET 1. An interesting mineral association, stillbelonging to the2C sublithotype and consisting primarily of


0361-0128/98/000/000-00 $6.00 281

Ca b

b

10 cm0 cm

3 cm c

c

E1 cm

f g

0.5 mmd Ee

0.5 mm0.5 mm

e

Hydr

Sau + Sm

hem 2

Sau + Sm

hem 2

Sau + Sm

Sm

Sm

Sau + Sm

Sm

Sm

Fd

QzQz

Qz

Qz

0.1 mm

0.2 mm

0.1 mm

0.1 mm

FIG. 8. a. Siliciclastic intercalation in the Ferrobamba Formation limestone with prevailing quartz clasts, MET2-14-54.7thin section N+. b. Same as (a) with quartz and K-feldspar clasts, MET4-37-136.3 thin section N+. c. and d. Lithotype 2A,fragments from the drill cores MET1-21-82.3 and MET3-28-103.2 consisting of smithsonite, goethite, and sauconite: in thecavities and microfractures white crystals of hemimorphite (hem 2). e. Banded concretion of hydrozincite (from Titiminastrench). f. Lithotype2A: zoned concretion of smithsonite (several generations) and sauconite, MET3-28-103.2A, thin sectionNII. g. Same as f, under CL


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Zn smectite with at least two hemimorphite generations,

could be locally observed in MET 1. The first generation (hm1) occurs as small concretions (stars) with a dusty appear-ance, growing in fine-grained siliciclastic sediments (Fig.10e). The second generation (hm 2) appears as clear elon-gated crystals growing in veins and in cavities together withMn (hydr)oxides (Fig. 10f). Generation hm1 is partly alteredto Zn clays. Zinc concentration in subgroup 2C is generallydirectly correlated with that of manganese. (2D): This sub-lithotype consists of dark-red laminated sediments, with detri-tal quartz and feldspars (Table 4). Numerous hemimorphitestars (hm 1) are seen to grow in a groundmass of sauconite

(with Mg, Fe, and Mn added) and Fe (hydr)oxides, but in

MET 4 they have also been locally detected in the carbonatehost rocks. The hemimorphite stars are up to 1 mm in di-ameter and have been completely transformed into sauconite.Spotty manganese concretions, commonly associated withmoderate Zn enrichments (chalcophanite), have been de-tected locally. 2D is not very common in the MET cores,where it represents a special variation of the2C sublithotype.

WDS analyses of important mineral phases

The results of the WDS analyses of the most common eco-nomic and noneconomic minerals at Accha are listed in

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a Bb

c

e f

d

1 cm

Sm

Sm

Sm

GoeSau + Goe

Sau + Goe

SmSm

Sm

hem 2hem 2

Sm + Sau + Goe

Sm + Sau

Sm + Sau + Goe

Sm + Sau

Lms

0.1 mm

0.1 mm

0.1 mm0.1 mm

0.2 mm

FIG. 9. a. Lithology2B consisting of a network of smithsonite associated with goethite and/or hematite. Fragment fromthe drill core MET4-35-126.85. b. Concretion consisting of several generations of smithsonite; the first generation (dark) isintergrown with sauconite, the following generations are relatively clean; thin section from MET2-21-76.55A NII. c. Smith-sonite intergrown with sauconite (dark), bordered by pure smithsonite crystals followed by late diagenetic hemimorphite (hm

2; blue) filling the cavity; thin section from MET1-22-85.80, N+. d. hm 2 filling a vug in the host limestone; thin section fromMET3-28-103.2, N+. e. Concretion consisting of several generations of smithsonite intergrown with sauconite, MET2-21-76.55A NII. f. Same as e, well-zoned under CL.


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Tables 5 and 6. Due to the common intergrowths between

mineral phases, it was not always possible to measure thecomposition of single minerals. Therefore, some analyses pos-sibly record a mixture of minerals.

Smithsonite: Smithsonite is commonly finely intergrownwith Fe (hydr)oxides and clay; as a result WDS measurementsdo not always conform to the stoichiometric value of Zn car-bonate. Nevertheless, most analyses show a metal contentranging between 60 and 62 percent ZnO (stoichiometricvalue for smithsonite is 64.9% ZnO). FeO is also commonlypresent in the lattice of the Accha smithsonite, as is MgO andCaO (Table 5). CdO values are generally below 1 percent and

not strictly correlated with Zn. Minor to trace contents of Mn,

Pb, Sb, and As have locally been recognized.Hemimorphite: The abundance of this mineral at Accha sug-gests similarities with many other nonsulfide deposits (Table 6).In hemimorphite ZnO ranges between 65 and 70 percent (sto-ichiometric value for hemimorphite is 67.58% ZnO), whereasSiO2varies between 25 and 26 percent. Both hemimorphite-1(early diagenetic stars in sediment) and -2 (late diageneticcrystals in veins and cements) show similar compositions.However, Al2O3 amounts up to 2 percent, K2O up to 0.5 per-cent, FeO up to 1 percent and MgO up to 0.5 percent havebeen detected throughout the concretions of hemimorphite-1.


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E

F

Ce

hm 1

hm 1

3 cm

Mn-Ox

1 cmcm

sauconite

A

Da

Zn-clay

b

FE

Zn-clay

Zn-clay

c d

c

3 cmcm

Bhm 1

hm 2

Zn-clay

fFIG. 10. a. Lithology2C, consisting of Zn clays (mainly sauconite) and Mn (hydr)oxide concretions, fragment from the

drill core MET1-26-98.25. b. Alternating Mn (hydr)oxides (dark) and Zn clays, thin section MET1-26-98.35, NII. c. Ag-glomerate of Zn clays (mainly sauconite), thin section MET1-26-98.25, NII. d. Same as c. N+. e. hm 1 concretions (stars),gradually altered to Zn clays, in a fine-grained sediment, thin section MET4-30-109, NII. f. hm 1 remnants in a Zn claygroundmass. In the upper part a vein of clear hm 2, thin section MET1-26-98.25, NII


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We took this as evidence of partial alteration of hemimorphite-1 to Zn smectite, as observed in thin section. Traces of MnO,PbO, CdO, and Sb2O3 have been locally detected.

Sauconite (Zn smectite): Sauconite is one of the most com-mon authigenic minerals detected at Accha (Table 7). TheSiO2 content of this trioctahedral smectite ranges between 30

and 36 percent, generally higher than published values, andZnO between 34 and 44 percent. The Al2O3value of the mostreliable analyses is between 4 and 7 percent. FeO, CaO, and

MgO are below 2 percent, thus confirming the prevalence ofthe Zn ion in the octahedral site. Sauconite is also intergrownwith several other minerals, coexisting with it. These were

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TABLE 5. Wavelength Dispersion Spectrometry (WDS) Analyses of Several Smithsonite Types

Sample ZnO FeO MgO CaO MnO PbO CdO CO21 Total

MET 1 61.96 0.50 0.52 0.26 0.17 0.02 0.08 34.87 98.3862.95 1.56 0.13 0.22 35.39 100.2564.42 0.16 0.36 0.78 0.03 0.27 34.19 100.2162.71 0.23 0.34 0.21 0.15 34.79 98.43

MET 2 60.00 0.06 0.02 0.88 0.16 0.42 35.52 97.0661.47 0.34 0.02 0.19 0.13 35.62 97.7761.59 0.27 0.01 0.93 0.78 34.86 98.44

60.85 0.41 0.02 0.83 0.75 0.08 0.14 35.46 98.54

MET 3 59.94 0.63 0.23 0.59 0.52 0.70 34.22 96.8360.41 0.23 0.20 0.58 0.62 0.17 0.33 34.54 97.08

MET 4 60.87 0.25 0.40 0.12 33.65 95.2960.22 0.13 0.80 0.80 0.20 33.99 96.1461.34 0.13 0.49 0.46 0.33 34.12 96.8761.99 0.08 0.43 0.48 0.35 0.29 34.53 98.15

1 Calculated from stoichiometry

b

10 m 5 m

a

FIG. 11. a. and b. Scanning electron microscope (SEM) images of the Zn smectite (sauconite) at Accha (from MET1-26-98.35).

TABLE 6. Wavelength Dispersion Spectrometry (WDS) Analyses of Hemimorphite

Sample SiO2 Al2O3 ZnO FeO MgO CaO K2O MnO Total

MET 1 25.50 67.22 0.08 0.02 0.02 92.8424.72 70.02 0.32 0.08 0.02 1.36 96.5225.87 0.27 65.81 0.16 0.05 0.06 0.07 92.2926.63 0.63 65.42 0.32 0.07 0.12 0.08 0.04 93.31

MET 2 25.89 65.08 0.05 0.06 91.0825.67 0.24 64.75 0.37 0.05 91.08

MET 3 26.51 67.81 94.3225.65 64.07 0.06 0.25 0.33 0.07 90.43

MET 4 26.74 0.75 59.50 0.55 0.09 0.08 0.16 0.13 88.0026.41 62.52 0.14 89.0725.18 0.52 67.29 0.29 0.11 0.08 93.4726.44 0.69 60.91 0.17 0.21 0.17 0.04 0.04 88.67


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July 1, 2008; February 18, 2009

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