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Introduction: Cerro Rico “I am the rich Potosi, the treasure of the world, the king of

mountains, and the envy of kings” (City of Potosi’s coat ofarms, bestowed by Charles V of Spain; Mining Journal, 1997).

Cerro Rico, located next to the city of Potosi in southernBolivia, is the world’s largest silver deposit. Its wealth helpedmake colonial Spain a great power in the 16th and 17th cen-turies. Cerro Rico has been mined for over 450 years and is anoted Bolivian landmark with its visage appearing on the na-tional seal, currency, and postage.

Initial silver grades at Cerro Rico (Spanish for “rich hill”)were extremely rich, averaging 7,000 oz/t (25 wt %) for thefirst 27 years of production (Wendt, 1890). Significant pro-duction of silver continued well into the late 19th century buthad virtually ceased by the 1930s (Wilson and Petrov, 1999).Total production of silver from Cerro Rico exceeded 1 billionoz and may have been as much as 2 billion oz (Evans, 1940).Substantial production of tin from Cerro Rico started in 1912,when a railroad was connected to Potosi (Evans, 1940), andcontinued until the tin crash in 1985. At present, severalthousand campesino miners remain at Cerro Rico, workingzinc-silver veins with primitive methods.

Veins have been mined from the summit to level –16, a ver-tical distance of 1,150 m. In the upper part of the mountain,swarms of parallel, centimeter-wide veinlets form sheetedzones of silicification and mineralization, comprising a re-maining resource estimated at 143 million tons (Mt) of 174 g/tAg and 0.1 to 0.25 percent Sn (Bernstein, 1989). This was putto tender as a proposed open-pit operation by Comibol, thenational mining company of Bolivia, in 1993, but concernsabout destroying the “sacred profile” of the Cerro Rico, aswell as the displacement of the small miners led to the tenderwithdrawal (Mining Journal, 1996).

There is another silver resource associated with Cerro Rico,composed of the eroded remains from the upper silicifiedzone of the cerro summit, which have been collected into

deposits of coarse gravels localized on the flanks of the moun-tain. These deposits, known locally as pallacos, do not formpart of the sacred profile of Cerro Rico; open-pit mining isnot considered a problem. These deposits have been workedon a small scale during the rainy season by mineros whowashed the gravel to recover cassiterite, using crude riffles,screens, and jigs. Silver was not recovered from the pallacos,owing to insufficient technology.

District GeologyThe district can be generalized as a shallow-level, single-

phase, funnel-shaped, dacite porphyry stock intruding a>400-m-thick section of Miocene air-fall tuffs, volcanic brec-cias, and waterlain sediments called the Cerro Rico Series(Turneaure, 1960a; Rivas and Carrasco, 1968). Cunninghamet al. (1996) interpreted the basal portion of the Cerro RicoSeries (Pailaviri Formation) as a phreatomagmatic explosionbreccia; this is overlain by the Caracoles Formation, whichthey interpreted as a tuff ring with associated ephemeral lakedeposits (Cunningham et al., 1996). The tuff ring portion ofthe Caracoles Formation occurs on the eastern and north-eastern sides of Cerro Rico; the ephemeral lake deposits(sandstone and ash member of the Caracoles Formation;Evans, 1940; Fig. 1) on the western and northern sides. Thedacite porphyry stock, dated at 13.8 Ma by Cunningham et al.(1996), appears to have been intruded in the crater wall sep-arating the two members of the Caracoles Formation (Fig. 2)and the available evidence (absence of foreign breccia clasts,flow bands, quench textures, or autobreccia at the margins)suggests that it did not breach the paleosurface.

Cerro Rico sits on a basement of weakly metamorphosedOrdovician shales. Other rocks in the area (Fig. 1) includeCretaceous red beds, the Tertiary Agua Dulce Formation(andesitic to dacitic volcanics), and the Tertiary MondragonFormation (coarse conglomerate and red calcareous sand-stones). These rocks predate mineralization but are distal inthe district and unaffected by alteration and/or mineraliza-tion. Postdating the Cerro Rico stock and mineralization is

SCIENTIFIC COMMUNICATIONS

THE PALLACOS OF CERRO RICO DE POTOSI, BOLIVIA: A NEW DEPOSIT TYPE

PAUL J. BARTOS†

ASARCO Inc., Latin American Exploration Division, 274 Union Blvd., Suite 450, Lakewood, Colorado 80228

AbstractPallacos are a new kind of ore deposit and are described here in detail for the first time. They are found on

the flanks of the Cerro Rico de Potosi, Bolivia, the world’s largest silver deposit. Fundamentally, pallacos aregravels whose clasts contain sufficient disseminated silver so as to be bulk minable. The silver-bearing clastsoriginated from the summit of Cerro Rico and are composed of dacite porphyry or air-fall tuffs or arkosic sandsof the Caracoles Formation that have been altered to vuggy silica or jasperoid. Analysis of facies, bedforms, de-positional textures, and clast sizes and percentages suggests that there are two types of pallacos, each with a dif-ferent depositional mechanism. One kind, the Huacajchi type, formed from multiple water-transported debrisflows. The other kind, the Potosina type, is interpreted as catastrophic landslides. Metal zoning, clast alterationstyle, and silver grade vary between the two pallaco types.

Economic GeologyVol. 95, 2000, pp. 645–654

† Email, pauljbartos@stanfordalumni.org

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FIG

. 1.

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the Huacajchi Ignimbrite and rhyolite dome complex (Steele,1996), exposed in the southern part of the district (Fig. 1).

Cerro Rico is located several kilometers west of the 12 ×32-km-diameter Kari Kari caldera (Francis et al., 1981,Schneider, 1987). Age dating indicates that eruption of theKari Kari ignimbrites occurred 7 m.y. earlier than intrusion ofthe Cerro Rico stock and associated mineralization (Schnei-der, 1987; Cunningham et al., 1996). Mapping by the authorsuggests that the Cerro Rico stock was intruded near, but notdirectly on, the ring fault associated with this caldera. A directlink to Kari Kari volcanism or structure appears denied.

There are extensive glacial deposits on the eastern side ofCerro Rico but none on the western side. None of the palla-cos contain evidence of glacial activity, such as striations, chat-termarks, polishing, or faceting of clasts.

Lode MineralizationThere are 35 veins and branches in the upper part of the

Cerro Rico. At depth, these merge into five principal vein sys-tems (Evans, 1940, Wilson and Petrov, 1999). All the veinsoccur in normal faults with little displacement. The veins are

steeply dipping, thin (widths typically between 10–60 cm;Ahlfeld and Schneider-Scherbina, 1964) and do not appear tobe affected by rock type (Evans, 1940).

There is a great variety of minerals at Cerro Rico (Wilsonand Petrov, 1999), and the vein ores are spatially and tempo-rally zoned (Turneaure, 1960b). Although the paragenesis iscomplicated in detail, a single ore-forming event is indicated(Cunningham et al., 1996). The earliest stage of the hy-drothermal event was the formation of the broad scale alter-ation (Sillitoe et al., 1975). Lode mineralization, consisting ofquartz, pyrite, cassiterite, and arsenopyrite, with wolframiteand bismuthinite in the deeper levels of the deposit, thencommenced. Quartz-sulfide veins of stannite, marmatiticsphalerite, chalcopyrite, and tetrahedrite followed. Vein-fill-ing assemblages containing pyrite, sphalerite, tetrahedrite,galena, andorite, and matildite then occurred, followed lastlyby ruby silvers, jamesonite, and boulangerite (Lindgren andCreveling, 1928; Evans, 1940; Turneaure, 1960b; Steele,1996). Alunite ± white clay veins, some of which are consid-ered hypogene by Cunningham et al. (1996) and Steele(1996), cut the earlier sulfide veins (Lindgren and Creveling,1928).

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Geology andAlteration

Looking South

FIG. 2. Cross section of Cerro Rico, showing geology and alteration zonation. Modified from Comibol plans and Sillitoeet al. (1998).

AlterationThe upper 250 m of the Cerro Rico is composed of vuggy

silica alteration (Fig. 2), a hypogene alteration style associatedwith acid sulfate deposits such as Goldfields, Nevada (Ran-some, 1909; Ashley, 1974), and Summitville, Colorado(Steven and Ratte, 1960; Stoffregen, 1987). This is a porousrock in which feldspar phenocrysts have been leached awayand the groundmass matrix replaced by silica, such that therock commonly contains over 95 percent silica (Jaskolski,1933). The feldspar-shaped vugs are commonly empty butmay be lined with quartz, hematite, jarosite, or white clay.Quartz phenocrysts and the overall porphyritic texture of thedacite is preserved. Vuggy silica at Cerro Rico formed as amassive layer overlying the quartz-illite and sericitic alter-ation (Fig. 2), as opposed to the vertical pipes or ledges typi-cally seen at other acid sulfate deposits (Hayba et al., 1985).The vuggy silica at Cerro Rico typically contains between 200and 350 g/t Ag. Much of it has been directly eroded into thepallaco deposits, providing the silver (and tin) for these de-posits.

At the uppermost 10 to 20 m of the cerro, the dacite por-phyry has been pervasively replaced by jasperoid. This silici-fication is massive, with no residual vugs, and is composed ofvirtually 100 percent fine-grained, gray chalcedony. Chal-cedony veinlets can be seen cutting and replacing the resid-ual vuggy silica. The jasperoid appears directly comparable tothe compact quartz rock of Steven and Ratte (1960), which islocally present at Summitville.

Below the zone of vuggy silica is a 150-m-thick zone ofquartz-dickite-illite-kaolinite-pyrophyllite alteration (Fig. 2;Sillitoe et al., 1975, 1998; Steele, 1996). Svanbergite, a stron-tium and phosphate-bearing analog of alunite, is also found inthis zone (Steele, 1996). The quartz-dickite zone gradesdownward into quartz-sericite-pyrite alteration, which in turngrades into quartz-tourmaline alteration in the deepest partof the deposit (Fig. 2; Sillitoe et al., 1998).

Pallaco GeologyThere are six major pallaco deposits scattered radially about

Cerro Rico: Huacajchi, Diablo, Santa Rita, San Antonio, Pa-leosucu, and Potosina, as well as several lesser pallaco de-posits: Huacajchi Oeste, San Antonio Norte, and San AntonioNoroeste (Fig. 1). Total tonnage of pallaco material is esti-mated to be in excess of 80 Mt. On the order of 23 to 30 Mtof pallaco is believed potentially mineable for silver.ASARCO, Inc. has estimated reserves at 21.6 Mt, containing119 g/t Ag; screening this to –8 mesh and discarding the fineswould yield 12.1 Mt, averaging 174 g/t Ag, or over 67.7 Moz.Exploration and evaluation are continuing by Coeur d’AleneMines Corporation, to whom the project has recently beenvended. The majority of the present reserves are contained inthe Huacajchi deposit, with additional resources in Santa Ritaand Diablo.

There are two types of pallacos, each with a different ori-gin: the Huacajchi type and the Potosina type. In general,Huacajchi-type pallacos occur proximal to Cerro Rico andcontain clasts dominantly of dacite porphyry altered to vuggysilica. Huacajchi-type pallacos have abundant bedforms, sug-gesting aqueous transport. Potosina-type pallacos occur distal

to Cerro Rico and contain extremely large blocks of daciteporphyry and sands or tuffs of the Caracoles Formation thathave been pervasively silicified to jasperoid. Bedforms areconspicuously lacking in Potosina-type pallacos. Both pallacotypes contain silver; however, grades in the Huacajchi typeappear overall to be higher.

Huacajchi-Type Pallacos

Descriptive sedimentology

Huacajchi-type pallacos were first described by Browne(1997) and can be formally defined as silver-rich, poorly con-solidated, polymictic conglomerates. There are 30 to 80 per-cent coarse-grained (>5 cm diam), poorly sorted clasts in a siltto clay matrix. Clast diameter can be in excess of 1 m; 10 to40 cm is far more typical. The clasts are generally subangularto subround. Three lithologies typically make up the clasts:vuggy silica-altered dacite porphyry, air-fall tuff of the Cara-coles Formation, and Ordovician metasedimentary shales andquartzite. Jasperoid is not typically seen in Huacajchi-typepallacos.

Huacajchi-type pallacos formed lobate aprons around theslopes of Cerro Rico (Huacajchi, Santa Rita, upper Diablo) orfilled topographic depressions such as drainages (San Anto-nio) or fault grabens (lower Diablo). The surface of the palla-cos is commonly hummocky and there may be a thin, organic-rich soil or colluvium covering. Where incised by erosion, abadlands topography typically results.

Four facies are present: (1) massive nonstratified conglom-erates, (2) thick-bedded stratified conglomerates, (3) thin-bedded pebble and sand beds, and (4) thin lenses of sand orclay (Fig. 3). Massive nonstratified conglomerate is mostcommon closest to the cerro summit. It has a high percentageof coarse clasts (up to 70–80 vol %), is commonly clast sup-ported, and very poorly sorted (Fig. 4a, b).

Stratified beds tend to consist of crude layers 1 to 5 m thick.The percentage of coarse clasts varies considerably, from 30to 70 vol percent but is typically less than in the massive non-stratified beds. Subhorizontal orientation of clasts is common.Scouring is present at the base of some beds, although sub-horizontal planar bed surfaces are also common. Channelingis locally present, with very coarse material cutting into un-derlying stratified beds. Meter-scale planar cross bedding andgraded beds are locally seen (Fig. 4c, d).

Thin-bedded pebble and sand layers (Fig. 4e) are volumet-rically insignificant in the pallacos. Individual beds are on theorder of 5 to 15 cm thick and commonly occur at the distalends of the pallaco deposits, farthest downslope from the sum-mit. Neither clast composition nor rounding vary significantlyfrom underlying stratified coarse conglomerates; only clast size.Interbedded finer grained sandy or pebbly layers commonlyoverlie beds with a high coarse clast fraction percentage.

Small lenses of arkosic sand or brown clay are locally inter-digitated within the pallaco deposits. These have limited lat-eral and vertical extent and low silver contents, constitutinginternal waste blocks.

The source of the silver for the Huacajchi-type pallaco de-posits is the vuggy silica alteration zone of Cerro Rico (Fig.4f). Grades of these pallaco deposits are directly related to thepercentage of vuggy silica clasts contained within them.

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Clasts of sericitically or quartz-dickite-altered rocks can alsohave significant silver values, particularly where they containveinlets. The matrix clay and silt is believed derived predom-inantly from the sericitic and quartz-dickite-altered rocks andcontains low-grade silver values, typically less than 30 g/t Ag.However, the vast majority of the silver resides in the coarseclasts. At Huacajchi, 59 wt percent of the pallaco material iscoarser than 2.4 mm (–8 mesh); this coarse fraction contains83 percent of the total silver. The percentage of silver in thecoarse fraction increases to 88 percent in Huacajchi materialwith an overall grade in excess of 60 g/t Ag. Size distributionsof silver at Santa Rita and Diablo follow a similar pattern. Inthese deposits, the proportions of coarse clasts and the per-centages of total silver contained in the coarse fraction aresomewhat lower, which is reflected by lower overall grade. Ascreening operation to remove the fines would eliminate asignificant portion of silver-poor material and significantly up-grade the material processed by a plant.

Pallaco gravel units containing clasts up to 40 cm in diame-ter have wedge-shaped geometries, with considerable verticaland lateral extent (Fig. 5). Pallaco gravel units containing verylarge clast diameters (greater than 40 cm), clay, and sand fa-cies typically form small intercalated lenses. There is no cor-relation between increased size of clasts (once gravel isreached) and silver grade. Rather, silver grade appears to bea function of proximity to the cerro summit and to the overallpercentage of vuggy silica clasts. Thus Huacajchi, particularlythe northern part closest to the summit, has the highestgrades. Higher grade portions of a particular pallaco also tendto coincide with the thickest intervals. Silver grades within the

mineralized portions of a given drill hole or shaft were rela-tively consistent, with no large nugget effect.

Depositional environment

Huacajchi-type pallacos are interpreted as thick, areally ex-tensive debris flows (Browne, 1997). Individual Huacajchi-type deposits are actually comprised of multiple overlappingdebris flows; total thickness locally exceeds 30 m. There areabundant sedimentary bedforms indicative of rapid aqueoustransport and deposition. These bedforms distinguish Huaca-jchi-type pallacos from landslides (cf. Crandell et al., 1984;Francis et al., 1985). Huacajchi-type pallacos probably origi-nated as talus which was later mobilized by intense rainfall(Browne, 1997) or by the intermittent melting of glacial ice(cf. Eyles and Miall, 1984) in multiple events to depositionalsites farther downslope. The massive nonstratified facies, lo-cated farthest uphill, probably represents extremely rapiddumping of the sediment load such that bedforms did not de-velop. The stratified conglomerate facies, typically locatedfarther downslope, contain meter-scale cross bedding andgraded beds; these are features common to proximal debrisflows (cf. Spearing, 1974, Rust and Koster, 1984). Reducedflow velocity during the waning stages of deposition at thetoes of the pallaco deposits is suggested by interbedded finergrained sandy or pebbly layers overlying beds with a highcoarse clast fraction percentage.

Mineralogy

Silver occurs finely disseminated throughout the vuggy sil-ica clasts, dominantly as native silver and chlorargyrite, with

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FIG. 3. Schematic facies diagram for the Huacajchi- and Potosina-type pallaco deposits.

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FIG. 4. Huacajchi-type pallaco features. a. Massive, nonstratified pallaco. Man for scale (circled). Cliff is about 30 m high.Vuggy silica alteration on skyline to left. b. Closer view of massive nonstratified pallaco. Hammer for scale (circled). c. Pla-nar cross bedding in stratified pallaco. Hammer (circled) and llama for scale (left). Channel in center is 30 cm wide.d. Graded bedding in stratified pallaco. Pencil is 15 cm long. e. Thin-bedded pebble and sand layers. Pencil is 15 cm long.f. Close-up of higher grade pallaco, showing clasts of vuggy silica in a matrix of silt and clay. Knife is 9 cm long.

a b

c d

e f

lesser amounts of pyargyrite, argentojarosite, and argentianpsilomelane (R. Honea, writ. commun., 1997). The materialis very strongly oxidized, with less than 1 percent relict sul-fide. This relict sulfide is dominated by pyrite, but chalcopy-rite, pyrrhotite, marcasite, bornite, digenite, chalcocite, andcovellite have also been detected (R. Honea, writ. commun.,1997).

There is a considerable quantity of tin in the pallacos. Atthe Huacajchi deposit, an overall grade of 0.11 percent Sn,with a higher grade portion of 0.17 percent Sn, was estimated.Tin grades of the other Huacajchi-type pallaco deposits arebelieved to be similar. The tin occurs in two modes—as fine-grained hypogene intergrowths of cassiterite with quartz, oras very fine powdery aggregates of secondary cassiterite in-tergrown with stibiconite, which was likely derived from theoxidation of stannite or tin sulfosalts (R. Honea, writ. com-mun., 1997). Neither type of cassiterite is easily recoverable.Historically, the tin washers recovered cassiterite from thefine fraction of the pallaco. This cassiterite was probably de-rived from veins and veinlets as opposed to being dissemi-nated; however, there does not appear to be enough cassi-terite to warrant processing on a bulk scale.

Dominant pallaco gangue phases are quartz, barite, andhematite. Table tests were able to produce a concentrate con-taining about 60 percent barite (Servicio Nacional de Geolo-gia y Mineria [Sergeomin], writ. commun., 1998), which isnot economical at this time.

Potosina-Type Pallacos

Descriptive sedimentology

Potosina-type pallacos are typically located farther awayfrom Cerro Rico than Huacajchi-type pallacos, up to dis-tances of 4 km. Potosina-type pallacos contain extremely largeboulders and blocks chaotically set in a matrix of brick redclay, or locally, tuffaceous material (Fig. 6a). Boulders reachup to 20 m in diameter and are typically subangular to sub-round (Fig. 6b). These boulders are significantly larger thanclasts found in Huacajchi-type pallacos. Clasts with diametersgreater than 2 cm comprise 5 to15 percent of the total volumeof Potosina-type pallacos. Potosina-type pallacos can be dis-tinguished from the massive nonstratified conglomerate (fa-cies 1) of Huacajchi-type pallacos by the much larger clasts(commonly an order of magnitude larger), much lower

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FIG. 5. Facies diagram showing horizontal and vertical variation in the Huacajchi deposit.

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1 cm

1 cm

1 cm

1 cm

FIG. 6. Potosina-type pallaco features. a. Typical Potosina-type pallaco with unsorted boulders and no sedimentary bed-forms, San Antonio Norte. b. Large subround boulders of finely silicified (jasperoid) dacite porphyry believed derived fromabove present-day exposures of vuggy silica at Cerro Rico (skyline peak at left). Pallaco deposit extends to top of hill in cen-ter of photo, Paleosucu. c. Pervasively silicified (jasperoid) air-fall tuff (Caracoles Fm.) with internal jigsaw brecciation,Potosina. d. Pervasively silicified, finely laminated siltstone (Caracoles Fm.), Paleosucu. e. Vuggy silica alteration veined andreplaced by jasperoid (center), Potosina. f. Brecciated, rounded clasts of silicified Caracoles Fm. and vuggy silica dacite por-phyry. Matrix consists of clay-quartz gouge and sulfides (predominantly arsenopyrite and pyrite), Paleosucu.

a b

c d

e f

abundance of clasts (typically 2–16 times fewer), differentclast types (below), and different matrix (red clay in Potosina-type; light gray sand, silt, and clay in Huacajchi type).

The surface expression of Potosina-type pallacos is com-monly a dense boulder field on positive topographic features,where they drape over and armor hills and shallow rises. Sed-imentary bedforms, clast size sorting, and graded bedding areabsent, although boulders of a given original lithology do tendto occur together (Fig. 6b).

Potosina-type pallacos include the type locality, Potosina, aswell as Paleosucu, San Antonio Noroeste, and San AntonioNorte. (The main San Antonio pallaco appears to be a Hua-cajchi type, although field relations are obscured by the ex-tensive working of this pallaco by tin washers and furthercomplicated by superimposed modern fluvial processes.) AllPotosina-type pallacos occur on the western side of the dis-trict, which may reflect paleotopography (deposition awayfrom the crater wall?).

Potosina-type pallacos contain boulders of pervasively sili-cified air-fall tuff and arkosic sand of the Caracoles Formationand dacite porphyry. These jasperoid boulders are typicallycomposed of fine-grained, gray silica that is chalcedonic in ap-pearance. The silicified Caracoles Formation boulders arebrecciated in a jigsaw style, both on a small (Fig. 6c) and largescale. All of the jasperoid appears to be preexisting rock thathas been replaced by fine silica (Fig. 6d); no sinter, geyserite,or other indications of surface hot springs were observed.

The jasperoid alteration of Potosina-type pallaco clastsstrongly contrasts with the vuggy silica alteration of daciteporphyry clasts seen in Huacajchi-type pallacos. A transitionfrom vuggy silica alteration to jasperoid replacement of daciteporphyry can locally be seen in some of the Potosina-typeboulders (Fig. 6e); this is identical to outcrops seen in theupper 10 to 20 m of the Cerro Rico. This transition stronglysuggests that the jasperoid boulders and blocks in the Poto-sina-type pallacos were derived from a higher level of CerroRico than the vuggy silica dominated clasts of the Huacajchi-type pallacos.

Commonly, the jasperoid boulders in Potosina-type palla-cos contain barite with associated high silver values; individ-ual boulders and blocks have been mined locally. As in Hua-cajchi-type pallacos, the matrix contains only very low gradesilver values. The overall grade of Potosina-type pallacos islower than the Huacajchi-type owing to a lower abundance ofsilver-rich coarse clasts.

Depositional environment

The absence of sorting, the extremely large size of theblocks, and the lack of bedforms in Potosina-type pallacossuggest a single, instantaneous depositional event that trans-ported debris rapidly up to 4 km from the source at the sum-mit of Cerro Rico. A catastrophic debris avalanche (cf. Cran-dell et al., 1984; Francis et al., 1985), or possibly lahar origin(based on the local appearance of tuffaceous matrix), is in-ferred for these deposits. Deposition of Potosina-type palla-cos is believed to have occurred shortly after the formation ofthe Cerro Rico hydrothermal system, while the volcanic edi-fice was still in place, prior to significant erosion. Potosina-type pallacos are inferred to be older than Huacajchi-type

pallacos because they are composed of clasts derived from ahigher level of the Cerro Rico alteration system.

Mineralogy

Pervasive fine-grained silicification locally preserves sul-fides in Potosina-type pallacos. These sulfides include pyrite,arsenopyrite, tetrahedrite, and locally, jamesonite. Bariteblades, up to 1 cm in length, are common. Breccia in brecciatexture is locally present with sulfides contained in the ce-menting matrix (Fig. 6f), the matrix in turn cut by late bariteveinlets. Barite-poor jasperoid is commonly low grade (lessthan 40 g/t Ag); barite-bearing jasperoid commonly grades140 g/t Ag or more.

The sulfide mineral and metal associations of Potosina-typepallacos are distinct from the Huacajchi-type (significantlymore barite, arsenic, and perhaps, antimony in Potosina-type,and significantly more tin, copper, and perhaps, silver in Hua-cajchi-type). This is believed to represent the original verticalzonation within the Cerro Rico hydrothermal system, withPotosina-type pallacos representing a higher source regionthan the Huacajchi-type pallacos.

Exploration ImplicationsThere are few analogs to the pallaco deposits. Perhaps the

best is the talus Au-Ag ore at La Coipa, Chile, which was de-rived from the silicified orebody cropping out at the top of themountain (D. Cadwell, pers. commun., 1999). The tonnagewas small and is believed to have been mined out. The de-posit was entirely loose talus, forming a small rock glacier. Itis suspected that if the La Coipa talus ore were subjected tointense rainstorms, as at Cerro Rico, then small Huacajchi-type pallaco deposits would have resulted.

Another variant is the glacial Quinua deposit in the Yana-cocha district, Peru. This is a significant orebody, with re-serves containing 7.3 Moz of gold (J. Rota, pers. commun.,1999). Quinua has well-defined glacial facies, including ter-minal and lateral moraines, as well as outwash fans and fill.Significant differences between the pallacos and Quinua arethe interpreted environment of formation—glacial versus flu-vial debris flow and avalanche, and the fact that at Quinua,the matrix contains gold (J. Rota, pers. commun., 1999),whereas the pallaco matrix is low grade and constitutes wasteor dilution.

As a specific type of bulk tonnage silver deposit, the palla-cos of Cerro Rico are unique. Other pallaco occurrences mostlikely exist, but to date, nothing of the magnitude of the CerroRico pallacos is known. This may be a reflection of the origi-nal Cerro Rico deposit, a deposit so large that even its ero-sional residue constitutes a significant resource. But as thereis only one Cerro Rico, so there well may be only one districtwith a large quantity of mineable pallaco material.

AcknowledgmentsRuben Terrazas and Miguel Montaño of Expromin brought

the pallacos to the attention of ASARCO and helped negoti-ate the arrangements with the cooperatives and Comibol. Theproject would not have been possible without them. Figuresand rock photos were prepared by Mike Asplund, draftsmanextraordinaire. Quentin Browne provided photos and sedi-mentological insights. George Steele generously provided a

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copy of his thesis. Reviews by F. Graybeal, E. Seedorff, M.Einaudi, J. Thompson, R. Allen, and S. Bull were appreci-ated. Permission to publish was given by ASARCO, Inc.

April 7, December 10, 1999

REFERENCESAhlfeld, F., and Schneider-Scherbina, A., 1964, Los yacimientos minerales y

de hidrocarburos de Bolivia: Bolivia Departmento Nacional de GeologiaBoletin 5, 388 p.

Ashley, R.P., 1974, Goldfield mining district: Nevada Bureau of Mines andGeology Report 19, p. 49–66.

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