Geological Society of Ameri

Special Paper 262

1991

Nonnan Russell*Rosario Dominicana, S.A., Apartado Postal 944. Santo Domingo, Dominican RepublicStephen E. Kesler

Department of Geological Sciences, University of Michigan. Ann Arbor. Michigan 48109

ABSTRACT

The epithermal precious metal deposits at Pueblo Viejo in the Dominican Republiccontain approximately 18 million ounces of gold and 95 million ounces of silver. Open-pit mining that began in 1975 has removed most of the oxide ore and exposed unweath-ered rocks at depths as much as 80 m below the original surface. Geologic mapping ofthese relatively fresh rocks has shown that mineralization is associated with a maar-diatreme complex that was emplaced in volcanic rocks of the Lower Cretaceous LosRanchos Formation.

Mineralization occurs mainly in sedimentary rocks that filled the maar crater.Diamond drilling to depths of 350 m did not reach the base of the sedimentary rocks,and the diatreme is interpreted to be present at greater depth. The initial maar-filldeposits consist of conglomerate, grit, and sandstone. The conglomerate contains clastsof plutonic rock and abundant quartz eyes, in addition to clasts of country rock. It isbelieved to be derived from an apron of pyroclastic rocks that were erupted from thediatreme and deposited on the rim of the maar. After an initial period of rapid sedimenta-tion from the steep walls of the crater, quieter conditions prevailed in the maar, andthinly bedded carbonaceous sandstones and mudstones were deposited. These rockscontain abundant plant fossils as leaf and bark imprints, and silicified logs occur inshoreline deposits.

Hydrothermal alteration and sulfide mineralization began during formation of thediatreme and continued through the filling of the maar crater. Alteration extends wellbeyond the maar-diatreme complex, but precious metal mineralization is within it orimmediately adjacent to it. The mineralization forms an incomplete ring around theperimeter of the intramaar sedimentary rocks, and the central maar area is barren. Theconfiguration of the ore zones indicates that hydrothermal fluids ascended to the maaralong the walls of the diatreme. In the maar-fill rocks, the form of mineralization wasinfluenced by lithology. Where a permeable conglomerate horizon is present, mineraliza-tion took place by pyrite replacement of the conglomerate matrix. In areas where theascending fluids encountered impermeable sandstone and mudstone layers, mineraliza-

.Present address: American Pacific Honduras, Inc., Apartado Postal 342,San Pedro Sula, Honduras.

Russell, N., and Kesler, S. E., 1991, Geology of the maar-diatreme complex hosting precious metal mineralization at Pueblo Viejo, Dominican Republic, inMann, P., Draper, G., and Lewis, J. F., eds., Geologic and tectonic development of the North America-Caribbean plate boundary in Hispaniola: Boulder, Colorado,Geological Society of America Special Paper 262.

203

204 IV: Russell and S. E. Kesler

tion is in the form of sharp-walled veins and veinlets. The veins are layered and exhibitmultiple episodes of mineralization due to repeated development of fluid overpressuresbeneath the impermeable beds.

The mineralized rocks were slightly eroded prior to their burial beneath UpperCretaceous marine limestone. Recent erosion has also been slight, and the southern partof the maar sedimentary sequence remains covered by the limestone.

INTRODUCTION REGIONAL GEOLOGY

The maar at Pueblo Viejo formed in the upper part of theLos Ranchos Formation (Bowin, 1966). The formation is a bi-modal volcanic pile consisting of spilite and keratophyre/quartzkeratophyre flows, tuffs, and debris flows (Kesler and others, thisvolume). The lower part of the formation consists largely ofsubmarine flows and tuffs, with some shallow quartz keratophyreintrusions. In the Pueblo Viejo area, the upper part of the forma-tion consists of the Meladito and Zambrana Members of mixeddebris flows and volcaniclastic rocks and the Platanal Member ofspilite flows. Lenses of black limestone near the top of the Mela-dito Member contain marine animal and terrestrial plant fossils,indicating that the limestone was deposited near-shore and incor-porated terrestrial detritus. The maar crater developed mainly inthe Platanal Member, on the southeast flank of the Platanal spiliteflows. The Pueblo Viejo Member includes the sedimentary rocksthat filled the maar. Plant fossils in these rocks, as well as theplant fossils in the Meladito Member, are considered to be ofNeocomian age (C. J. Smiley, 1982). The plant collection iscatalogued and stored at the University of Idaho.

The Los Ranchos Formation is unconformably overlain bythe Hatillo Limestone Formation (Bowin, 1966), which coversthe southern part of the Pueblo Viejo Member (Fig. 1). On theevidence of foraminifera collected from the limestone, Bowinconcluded that the formation was probably of Aptian-Albian age.Fossils that we collected from the basal beds of the limestone atPueblo Viejo include gastropods, pelecypods, ammonites, andechinoids. P. Kier (1982, personal communication) of the Smith-sonian Institution, Washington, D.C., examined the echinoidfauna and concluded that the assemblage was most probablyCenomanian, or possibly Albian. Genera represented in the echi-noid collection include Holectypus, Conulus, Holaster, Salenia,Cardiaster, Pygopyrina, and Hemiaster.

The Pueblo Viejo gold-silver deposits have been mined byRosario Dominicana, S.A., for their oxidized ( oxide) ore since1975. Through 1989, 46 million tons of oxide ore have beenmined, essentially depleting these reserves. A sulfide reserve ofapproximately 100 million tons has been proved to underlie theoxide ore. The mineralization occurs in and beneath a sequenceof lacustrine sedimentary rocks in the upper part of the LowerCretaceous Los Ranchos Formation (Kesler and others, this vol-ume). Earlier reports on Pueblo Viejo (Kesler and others, 1981;Russell and others, 1981) dealt mainly with descriptions of themineralization, and the lithologic descriptions were based onwidely spaced diamond drill holes and surface exposures ofdeeply weathered, hydrothermally altered rocks. Almost all of theoxide ore has now been mined, and unweathered rocks are ex-posed throughout the ore zones (Fig. 1). Diamond drilling of thesulfide reserves has also provided further information on the sub-surface geology. These new insights into the geology of the districthave shown that the ore-hosting sedimentary rocks were depos-ited in a circular, steep-sided depression on the Los Ranchospaleosurface. This depression is considered to be a maar craterthat formed on top of a diatreme. Lorenz (1973) defined a dia-treme as "a pipe-like volcanic conduit filled with pyroclastic de-bris (tuff or lapilli~tutT) and blocks of wall-rock" (p. 184) and amaar as "a large volcanic crater cut into the country rock andpossessing a low rim composed of pyroclastic debris. It is under-lain by a correspondingly large diatreme" (p. 201).

Epithermal precious metal deposits are commonly asso-ciated with maar-diatreme systems, including Wau in Papua NewGuinea (Sillitoe and others, 1984), Cripple Creek, Colorado(Thompson and others, 1985), and Montana Tunnels, Montana(Sillitoe and others, 1985). In most of these systems the upperparts have been lost to erosion and only remnants of an intramaarsedimentary sequence are preserved. At Pueblo Viejo the sedi-mentary rocks that fill the maar have been only slightly erodedand the diatreme is interpreted to be present at depth and to haveacted as a conduit for hydrothermal fluids. Several hydrothermalcenters have been identified at Pueblo Viejo, including Moore-Mejita, Monte Negro, Cumba, and possibly Bench Five (Fig. 1).Each of these deposits is enclosed by an envelope of advancedargillic alteration containing quartz, pyrophyllite, kaolinite, andalunite, which are characteristic minerals in acid-sulfate epither-mal systems (Hayba and others, 1985).

GEOLOGY OF THE MAAR DEPOSITS

Geometry

The rocks of the Pueblo Viejo Member fill a funnel-shapedbasin with an inner, steeply walled portion of about l-km diame-ter and unknown vertical extent. The upper part of the PuebloViejo Member consists of well-bedded carbonaceous sandstonesand mudstones as thick as 200 m, which extend outward from th~

205Maar-diatreme complex. Pueblo Viejo, Dominican Republic

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Maar sedimentationcenter of the basin over a more gently inclined paleosurface; theyare now exposed over a distance of 2 km in an east-west direction(Fig. 2). In a north-south direction, the Pueblo Viejo Member isexposed over a distance of 3 km, but its southern limit is con-cealed beneath the Hatillo Formation (Fig. 1). These dimensionsindicate that the initial diameter of the maar crater at surface wasslightly greater than 1 km and that the diameter of the sedimen-tary basin was gradually enlarged by erosion of material from thecrater rim into the basin.

The deep inner part of the maar is filled with coarse-textured, poorly bedded, or unbedded rocks. These rocks areknown only from drill-core intersections (Fig. 2), and the avail-able information is insufficient to make stratigraphic correlationsbetween drill holes. Parts of the unit have a breccia-like appear-ance and consist of about 75 percent angular or semi-roundedclasts more than I cm in diameter, in a matrix of smaller clasts

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206 N. Russell and S. E. Kesler

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spilite boulders as much as I m in diameter, in a matrix of smallspilite clasts and grains. The conglomerate is pervasively alteredto pyrophyllite and kaolinite, but contains clasts of silicifiedspilite. The clasts are poorly sorted, but layering is evident in minefaces (Fig. 3e).

On the west side of Monte Negro, in the Arroyo Hondoarea, mining has exposed unweathered conglomerate of well-rounded, oval-shaped boulders of spilite, with their long axesoriented parallel to bedding. These boulders constitute about 50percent of the rock and are set in a matrix of spilite pebbles andcomminuted spilite. As at the north end of Monte Negro, theconglomerate also contains boulders and pebbles of silicifiedspilite. The matrix of the conglomerate is carbonaceous, andpieces of carbonized wood are present as irregular patches with acharcoal texture, usually lodged in the spaces between closelypacked boulders.

On the west side of Monte Negro, the conglomerate is tran-sitional to well-bedded, carbonaceous sandstone, but on the eastside of Monte Negro, toward the central area of the maar, thesandstone overlies thinly bedded sandstones and mudstones. Thesandstone appears to have been deposited as an alluvial fan thatspread eastward from the margin of the basin over finer grainedlacustrine deposits (Fig. 2b). The coarsest sandstone beds, at thecenter of the fan, are as large as 1 m thick and frequently have abasal layer of spilite cobbles and boulders. Otherwise, the bedsare of uniform grain size and very similar in texture to hydro-thermally altered spilite from which they are derived. Thesandstone beds contain only minor amounts of carbonaceousmaterial, but are separated by thin (I to 2 cm) layers of blackmudstone containing abundant plant fossils (Fig. 3f).

Cumba-Mejita area. The thick sequence of conglomerateeast of Arroyo Mejita and south of Cumba (Fig. 1) is poorlyexposed, except for the ore zone at Cumba. Here the Platanalspilite is overlain by angular blocks of spilite in a matrix of thesame composition. No layering or sorting of clasts is evident, andthese rocks may be talus deposits on the edge of the maar. Fartherfrom the contact there is a more evident layering and the con-glomerate includes thin mudstone and sandstone layers. BetweenCumba and Mejita the conglomerate overlies volcaniclastic rocksof the Meladito Member, and the Platanal Member is missing,either through having thinned out or by having been eroded intothe maar.

The Mejita area appears to be represented by shoreline de-posits of the maar, and the stratigraphic relationships arecomplex. The conglomerate is carbonaceous and consists of lay-ers of spilite boulders, separated by beds of carbonaceous sand-stone. Glassy quartz eyes are common in the conglomerate andsandstone beds. Westward, the conglomerate grades into well-bedded sandstone and mudstone beds. These beds contain silici-fied logs as large as 1 m in diameter (Fig. 3c). Long, thin pieces ofsilicified wood may be tree branches but have been observedcutting across bedding planes and are possibly in situ root sys-tems. The lapilli tuff (described below) at Mejita rests on Platanalspilite and grades westward into carbonaceous sandstone. Quartz

and rock flour. Most of the larger clasts can be recognized asPlatanal spilite, and some of these have been pyritized and silici-fied prior to incorporation in the breccia (Fig. 3a). A few largeclasts are of a coarse-grained plutonic rock, now composed ofquartz, oxidized biotite, and altered feldspars in equal propor-tions. In the rock flour matrix are small angular clasts of chert andspilite, as well as glassy quartz eyes as large as 0.5 cm in diameter.Other unbedded parts of this unit are composed of mainlyrounded clasts of spilite, silicified spilite, and the same plutonicrock described above. Quartz eyes are also common in theserocks.

Bedded rocks are also present throughout the unit as hori-zons of sandstone and grit, sometimes showing graded bedding inbeds as much as 1 m thick. These bedded units appear to bemostly derived from spilite. Thin mudstone beds are present

infrequently.The rocks described above are considered to be mainly con-

glomerates, grits, and sandstones that were derived by rapid ero-sion from the rim of the maar crater in the waning stage ofexplosive volcanic activity. Following the maar-diatreme modelof Lorenz (1973), the maar crater would have been surroundedby an apron of pyroclastic debris that was ejected from the crater,and this material would have been the first to be eroded. Pyro-clastic debris would also have been deposited directly on the floorof the maar crater; but with only drill-core evidence available, it isnot possible to distinguish between pyroclastic deposits and epi-clastic deposits. Base-surge deposits are typical components ofmaar-diatreme formation (Sillitoe and Bonham, 1984), but havenot been identified at Pueblo Viejo. It is possible, however, thatsome of the bedded rocks were deposited as pyroclastic surgesand have been misidentified as grits and sandstones.

Overlying these coarse-textured rocks of possibly mixed py-roclastic and epiclastic origin is a horizon as large as 50 m thick,of well-bedded sandstone. The sandstone does not crop out butcan be correlated in drill holes across the central part of the basinfrom Monte Negro to Moore (Fig. 2). It is thickest in the westernpart of the basin and thins to zero beneath the Moore orebody.This sandstone horizon marks a change in the style of sedimenta-tion in the basin, separating the coarse deposits beneath fromoverlying carbonaceous sandstones and mudstones. From Figure2 it can be deduced that this change in style of sedimentation tookplace when erosion of the crater rim lowered the paleosurface togentle, inward-dipping slopes. Facies relations in the upper, car-bonaceous part of the intramaar sedimentary sequence are closelyrelated to local features of the crater rim and are described sepa-rately below for various areas.

Monte Negro area. The greatest lithologic variety andmost abrupt facies changes in the intramaar sedimentary rockstake place in the Monte Negro area, on the northwest cratermargin. The Platanal spilite thickens to the north and northwestof Pueblo Viejo; this part of the maar lake was probably closest tothe areas of greatest topographic relief. The coarsest conglomerateoccurs at the north end of the Monte Negro orebody, overlyingPlatanal spilite. The conglomerate consists of poorly rounded

2U~ N. Russell and S: E. Kesler

IVlaar-alatreme complex. Pueblo Viejo. Dominican Republic 209

---

Figure 3. A, Photograph of drill core of early maar-fill epiclastic rocks.Clasts are partly altered Platanal spilite. Bright area at lower right issilicified and pyritized clast. B, Photograph of drill core of lapilli tuff atMoore, showing lenticular mafic clasts, rounded clasts of spilite, andangular chert clasts in a matrix of ash. Bright spots are quartz eyes thatare present in the matrix and in the lenticular mafic clasts. C, Photographof silicified log from the sandstone beds at Mejita. Swirl marks of a knotin the wood are visible immediately above the scale bar. D, Photographof a plant imprint on a bedding plane, identified by C. J. Smiley (per-sonal communication) as a previously undescribed species of Brachy-phyllum E, Photograph of spilite boulder conglomerate at the north endof Monte Negro, close to the contact with Platanal spilite. Large, poorlyrounded spilite boulders are supported by a matrix of spilite pebbles andcomminuted spilite. F, Photograph of carbonaceous sandstone from thecentral part of Monte Negro. Thick sandstone beds derived from spilitecontain pebbles and cobbles of silicified spilite. G, Photograph of tightlyfolded, barren, pyrophyllitized mudstone at the south end of MonteNegro. This type of folding is never present in silicified rocks, indicatingthat folding took place after alteration and mineralization. H, Layeredpyrite vein cutting silicified beds at the north end of the Moore ore body.

eyes derived from the tuff are found as much as 100 m from the

contact. Close to the contact, lenses as much as I m thick and

composed almost entirely of quartz eyes were observed, indicat-

ing a sorting process such as may have been provided by wave

action.

Bench Five area. West of the tailings pond, thinly bedded

carbonaceous mudstone overlies lapilli tuff. The contact is sharp

and contrasts with the northern margin of the maar, where thick

conglomeratic horizons are present. The Bench Five ore zone is ina lens of carbonaceous conglomerate that contains pebbles de-

rived from the lapilli tuff and from the carbonaceous mudstones.

Quartz eyes derived from the lapilli tuff are common in the

conglomerate matrix. The mineralized conglomerate lens wedges

out to the north, but continues south beneath the Hatillo Lime-

stone. Because the ore grades at Pueblo Viejo are suitable only for

open-pit mining, the continuation of this zone beneath the Hatillo

Formation has not been explored.

Central Maar area. Well-bedded carbonaceous sandstone

and mudstone fill the central part of the maar to a preserved

thickness of 200 m. The rocks are everywhere hydrothermally

altered, but their original textures are well preserved. Typically,

they consist of graded sandstone layers a few centimeters thick,

alternating with thinner, black mudstone layers containing

abundant leaf imprints (Fig. 3d). The cyclical deposition of

medium- and fine-grained beds may reflect a seasonal change in

sedimentation rates. Pyrite is present as disseminations in the

sandstone beds and as thin pyrite laminae in the mudstone beds.

Bedded pyrite is most abundant in the most intensely veined

zones, where pyrite layers as much as I m thick have been

observed. This correlation indicates that hydrothermal solutions

ascended to the surface at specific sites throughout the period of

sedimentation in the maar. The average pyrite content of the

Moore ore body is 15 percent, of which about 5 percent can be

attributed to vein pyrite and 10 percent to disseminated and

layered pyrite.As the maar filled with lacustrine sediments, the lake in-

creased its surface area, and eventually the lake beds were depos-

ited directly on the country rocks. At Monte Negro the

conglomerate thins to zero over a buried ridge of Platanal spilite

and thinly bedded carbonaceous sandstone and mudstone rest

directly on spilite (Fig. 1). These beds are unusually rich in bark-

like plant fragments, suggesting proximity to a wooded shoreline.

Lapilli tuff. Lapilli tuff crops out in the Moore ore bodyand has been traced southward for 2 km to the tailings pond. Asmaller body of lapilli tuff at Mejita is part of the same unit, now

separated from the main body by the Arroyo Mejita Valley. Thetuff consists of about 50 percent clasts that are more than 1 cm indiameter in a matrix of ash (Fig. 3b). The most common clastsare lenticular, with ragged ends. They are fine grained and ofmafic composition, but contain glassy quartz eyes as much as 0.5cm in diameter. The clasts are not sorted but are oriented parallelto a poorly developed foliation. Rounded clasts, identical in ap-

210 N. Russell and S. E. Kesler

pearance to the Platanal spilite, are present; some of these weresilicified prior to incorporation in the tuff. On occasion, the len-ticular, mafic clasts have been observed to bend around underly-ing silicified clasts. The lapilli tuff also contains clasts, as much as30 cm in diameter, of the same coarse-grained plutonic rock(?)found in the conglomerates filling the central part of the maar.The matrix of the lapilli tuff contains abundant quartz eyes,which are also a distinctive component of the early maar-fillrocks. An essential difference between the lapilli tuff and parts ofthe epiclastic maar fill is the absence of the lenticular mafic clastsin the epiclastic rocks.

At the tailings pond, the lapilli tuff rests on spilite and isoverlain by carbonaceous mudstone. The northern part of the tuffwedges out in carbonaceous maar-fill sedimentary rocks atMoore. Where the tuff overlies sedimentary rocks, the contactappears to be a stratigraphic horizon. On its west side the tuff unitis laterally gradational with carbonaceous sandstone and mud-stone over a distance of only a few meters, and quartz eyes arefound in the sedimentary rocks for only a short distance from thecontact.

The method and timing of emplacement of the lapilli tuffhas not been determined, but the similarity of clast types in thetuff and in the conglomeratic maar fill strongly suggests that it isclosely related to the maar-diatreme system. One possibility isthat the tuff was deposited outside the maar during the period ofexplosive eruptions in the diatreme. This interpretation wouldrequire that the northern part of the unit was emplaced as a faultblock that slid into the maar during the period of deposition oflake beds. Gravity sliding of blocks of tuff apron, and their incor-poration in intramaar epiclastic rocks, has been documented bySillitoe and others (1984) at Wau, Papua New Guinea.

An alternate explanation is that the lapilli tuff is a pyroclas-tic flow from a source to the south of Pueblo Viejo, which wasdeposited partly on the periphery of the maar and partly within it.A closer eruptive center, within the confines of the maar, isprecluded by the absence of fragments of carbonaceous sedimen-tary rocks in the tuff.

POST -MAAR GEOLOGY

Late Los Ranchos sedimentation

At the south end of Monte Negro, coarse-grained spilitewacke overlies carbonaceous mudstone and represents an abruptfacies change that took place about the time the maar craterbecame filled with carbonaceous lacustrine deposits. The wacke,which dips gently to the southwest and by extrapolation to theeast and north, is interpreted to have covered the maar deposits.The southern limit of the Monte Negro ore body encompassesbasal beds of the spilite wacke (Fig. I), indicating that it is not anearlier deposit that was thrust over the carbonaceous mudstone;the most recent exposures of the contact have revealed the pres-ence of conglomerate lenses at the base of the wacke. The spilitewacke is the youngest unit of the Los Ranchos Formation ex-posed in the Pueblo Viejo area. The change from deposition of

carbonaceous mudstone in the maar lake to coarse-grained wackesuggests that a quiet depositional environment within the maarwas replaced by a more regional sedimentation pattern. Thischange may have occurred when the crater rim was eroded downto surrounding topographic levels. Prior to deposition of theHatillo Limestone, a period of erosion removed most of the spilitewacke and the upper part of the Pueblo Viejo Member. Erosionwas greatest in the area of soft, barren, pyrophyllitized mudstoneat the center of the maar.

Hati/lo Limestone Formation

The Hatillo Limestone was named by Bowin (1966) for thegray, massive limestone that overlies the Los Ranchos Formation.Recent mine workings have exposed the base of the Hatillo For-mation and confirmed that the limestone is unconformable, witha basal transgressive sequence that is not always complete. Acomposite section (Fig. 4a) consists of a basal conglomerate,generally less than 10m thick, with as much as 50 percent chertcobbles in a fine-grained matrix. Above the conglomerate is auniform, fine-grained sandstone that is cut 'by an anastamosingnetwork of fractures that are bordered by concretionary ironoxide and silica (Fig. 4b). Sedimentary layers as much as 50 cmthick have also been converted to iron oxide and silica. Theselayers, as well as the fracture borders, are considered to be resid-ual paleosurface features. Overlying the conglomerate and con-cretionary zones are calcareous mudstones and sandstonescontaining abundant marine fossil fragments. The calcareousmudstones are overlain by the typical massive reef limestone.

In addition to the observed basal sections beneath limestoneoutcrops, the conglomerate and fracture-networked sandstonehave been identified overlying the Pueblo Viejo Member in thearea between the Moore and Monte Negro ore bodies. The fossil-bearing mudstones weather to soft clay and are unlikely to persistwhere they are not protected from erosion by overlying lime-stone; however, a small outcrop on the north side of the tailingspond contains echinoid fragments and is undoubtedly an outlierof Hatillo Formation.

Tertiary diorite

Stocks, sills, and dikes of pyroxene diorite of Eocene age(Bowin, 1966) intrude the Los Ranchos and Hatillo Formationsat Pueblo Viejo. The diorite most commonly occurs as sills at thebase of the Hatillo Formation. Small skarn magnetite deposits atdiorite-limestone contacts were mined during the 1950s. Feederdikes cut the Pueblo Viejo Member in the mine area. The largestof these is as much as 30 m wide and cuts the Monte Negro orebody. The diorite clearly bears no relation to the mineralizationand, although it cuts silicified, mineralized spilite and sandstone,is devoid of precious metals and hydrothermal alteration effects.

STRUCTURAL GEOLOGY

Development of the maar-diatreme system appears to havebeen one of the last phases of volcanic activity in the Los RanchosFormation, but there is no known paleostructure that can be

invoked to explain its location. Regional deformation after for-mation of the maar-diatreme included southwestward tilting of theLos Ranchos Formation to dips of 20° to 30°. The overlyingHatillo Formation also dips gently to the southwest and furtherregional tilting probably took place after deposition of the HatilloFormation.

In the thinly bedded sedimentary rocks of the maar, small-scale folding and faulting is common. The degree and style ofdeformation varies with the type of hydrothermal alteration pres-ent, indicating that deformation took place after mineralization.In silicified, mineralized zones the bedding is generally undulat-ing, and where more pronounced folding did take place-such asat the north end of Moore (Fig. I)-the folds are relatively largeand symmetrical. In contrast, the soft, pyrophyllitized mudstonesare nearly everywhere crumpled into small, tight folds with de-velopment of axial plane cleavage. Crumpling is most severearound the margins of the ore bodies, and the silicified ore zonesappear to have acted as buttresses against which the softer, barrenrocks were deformed (Fig. 3g).

No major faults are known in the country rocks surroundingthe maar, and, because of the lack of marker horizons, the conti-nuity of faults in the intramaar sedimentary rocks is difficult todetermine. The most important fault mapped cuts the Moore orebody and offsets the west side of the ore body to the north (Fig.1). The fault defines the contact between ore and barren rocks, asdoes a secondary, east-west-trending fault.

The local deformation at Pueblo Viejo, aside from the re-gional tilting, probably took place during the Eocene, which wasconsidered by Bowin (1966) to be the time of development of theHatillo Thrust Fault. This fault, which is one of the major tectonicfeatures in central Dominican Republic, thrusts metamorphicrocks northeastward over the Hatillo Formation. The surfacetrace of the fault passes 2 km to the south of Pueblo Viejo.

ALTERATION AND MINERALIZATION

Origin of alteration and mineralization

The presence of clasts with different types of hydrothermalalteration in rocks that were later cut by ore-bearing veins showsthat alteration and mineralization overlapped diatreme formationand filling of the maar crater. Rocks containing silicified andpyritic clasts in matrices that are not silicified are present in all ofthe maar-fill units, as well as the lapilli tuff. It is not clear if all ofthe hydrothermal alteration in the Pueblo Viejo area is related toformation of the diatreme, although the zones of pyrophyllite,kaolinite, and illite, which extend for at least 10 km to the west,appear to be centered on Pueblo Viejo (Bowin, 1966). The doc-umentation by Muntean (1989) that the Monte Negro depositunderwent at least two stages of hydrothermal alteration suggeststhat periodic hydrothermal events affected the area.

The close association of maar-diatreme formation, altera-tion, and mineralization strongly suggests that the explosive vol-canic activity in the diatreme was driven by magmatic processes.The coexistence of alunite and pyrite, the abundant pyrophyllite,and the isotopically heavy sulfur in the alunite (Muntean, 1989)clearly place these deposits in the acid-sulfate epithermal class(Hayba and others, 1985). Deposits of this type form from unu-sually acid hydrothermal fluids containing similar amounts of re-duced (HzS) and oxidized (SO4-Z) sulfur, and phase relations inthe Monte Negro deposit require fluids with a pH of 1.5 to 2 at atemperature of 2500 to 300°C (Kesler and others, 1988). Theonly geologic environment capable of generating fluids of thistype in quantities large enough to account for the extensive altera-tion and mineralization at Pueblo Viejo is one in which a mag-matic vapor plume mixes with an overlying groundwater system(Henley and McNabb, 1978; Brimhall and Ghiorso, 1983; Hayba

212 N. RussellandS. E. Kesler

At Moore, mineralization consists of a network of sulfideveins and vein lets that cut thinly bedded sandstones and mud-stones. The veins have sharp walls, and the sulfide minerals arelayered inward from the walls, indicating multiple pulses of min-eralization. On the east side of the maar, a thick conglomeratelayer is not present, and it is apparent that fluid overpressuresattained beneath the impermeable sedimentary rocks were thecause of repeated fracturing. Self-sealing and reopening ofthe fractures resulted in the bilaterally symmetrical layered veins

(Fig. 3b).

DISCUSSION

Geologic history

and others, 1985). The principal source of the acidity and sulfur ismagmatic S02 in the vapor plume, which reacts with water as itcools, to form H2S and S04-2. These reactions require that theoxidation state of the magma be sufficiently high to produce avapor containing significant proportions of S02 (Ohmoto andRye, 1979). Most island-arc magmas are relatively unoxidized,although residence at high levels in the crust, possibly beneath adiatreme, would probably cause a magma to oxidize as it mixedwith meteoric fluids.

Distribution and form of mineralization

Figure 1 shows that the ore zones bear a close spatial rela-tion to the perimeter of the maar. The principal ore bodies atMoore-Mejita (Kesler and others, 1981) and Monte Negro (Rus-sell and others, 1986) have been shown to be funnel shaped (Fig.2), and the smaller ore body at Cumba also extends downward ina funnel shape. These three zones contain the highest grades ofmineralization and were defined as ore bodies prior to start-up ofthe mine in 1975, based on a cut-off grade of 2.4 ppm of Au.During the life of the mine, the cut-off has been lowered to itspresent level of 0.7 ppm of Au, and new zones have been addedto the reserves. These low-grade zones are in conglomerate westof Monte Negro and in lapilli tuff and spilite south of Moore (Fig.1). None of these zones has any appreciable vertical extent thatmight indicate a separate hydrothermal system; they are low-grade fringes of the principal ore bodies. A possible exception isthe Bench Five ore body (Fig. 1). The exposed part of the orezone is stratigraphically controlled and does not have a root zone,but this may be present farther south beneath the HatilloLimestone.

The central part of the intramaar sedimentary sequence isbarren, and economic mineralization forms a ring around theedge of the maar. Because of the silicification that is associatedwith mineralization, the ring shape is evident in the present to-pography. The ore zones lie on a circular ridge that runs from thesouth end of Monte Negro clockwise through Cumba, Moore-Mejita, and Bench Five, and is broken only by the Arroyo Mejitadrainage system (Fig. 1). Before mining, the tops of the hills hadelevations of 400 to 500 m above sea level, whereas the elevationof the Arroyo Margajita at the north end of the Hatillo Formationoutcrop is 200 m.

The most probable explanation for the circular configura-tion of the ore zones is that hydrothermal fluids ascended verti-cally along ring faults on the diatreme walls and, at higherelevations, were influenced by the lithologies of the intramaarsedimentary rocks. This influence is most evident in the MonteNegro zone, where mineralization was largely controlled by theconglomerate unit at the base of the sedimentary rocks. Sulfideveins are uncommon in the conglomerate and mineralizationtook place by sulfide replacement of the conglomerate matrix.The lack of vein-filled fractures at Monte Negro suggests that theconglomerate acted as a permeable conduit for hydrothermalfluids. The funnel shape of the ore body is distinctly asymmetrical

(Fig.2b).

An interpretation of the geologic history of Pueblo Viejo,following the maar-diatreme model of Lorenz (1973), is shownschematically in Figure 5. The diatreme developed in theMeladito/Zambrana and Platanal Members of the Los RanchosFormation, near the end of Lower Cretaceous time. Phreatic orphreatomagmatic eruptions in the diatreme resulted in the forma-tion of a maar crater with a surface diameter of 1 to 2 km and adepth of more than 300 m. Pyroclastic debris accumulated on thefloor of the maar, and a tuff apron of ejecta was deposited on thecrater rim (Fig. Sa).

As explosive activity in the diatreme waned, erosion fromthe crater rim transported most of the tuff ring into the maar,depositing it as conglomerates, grits, and sandstones on the floorof the crater. As the crater rim was worn down and the floor ofthe maar was raised, the peripheral paleoslopes were smoothedand sedimentation in the maar changed to quieter, lacustrineconditions. Vegetation was reestablished, and the maar was sur-rounded by wooded slopes, which contributed abundant carbon-aceous detritus to the lake beds. Sediment was derived mainlyfrom the north and west sides of the maar, where the Platanalspilite continued to form a topographic high. Thick accumula-tions of carbonaceous, spilite boulder conglomerate were depos-ited at the edge of the maar in these areas. During a late stage offilling of the maar with lake beds, lapilli tuff was deposited in themaar in the Moore area. It is unclear whether the tuff is part ofthe original tuff ring that later slid into the maar as a block, or if itwas deposited during renewed volcanic activity peripheral to themaar (Fig. 5b).

Hydrothermal alteration at Pueblo Viejo commenced priorto or during formation of the diatreme, and the earliest maar-fillsediments of the Pueblo Viejo Member contain clasts that weresilicified prior to their deposition. Precious metal mineralizationwas a late stage of the maar-diatreme system and is concentratedin the upper part of the intramaar sedimentary sequence. The orezones lie in a roughly circular zone on the periphery of the maar,indicating that the mineralizing solutions ascended along ringfaults at the margins of the diatreme. In the maar sedimentaryrocks, the form of mineralization was influenced by the lithologyof the host rocks. At Monte Negro, mineralization was concen-~

. -~~.~ . .~J~' ~v. ..~..,. . '~Y"V'"

-- tuff apron

~~=:--maar

A. MAAR - OrA TREME

FORMATION

(LR.CRETACEOUS)

carbonaceous sandstone splliteand mudstone wacke

.. '...' ' "" '. ,._.~::::::::::::: ".;'

v \lopil'i - tuffcarbonaceous

~ '" t.mlnera Iza Ian

coarse

B. MAAR SED/MENTATION

AND M/NERAL/ZAT/ON

(LR. CRETACEOUS)

~

D. PRESENT

0 , 2KM, I ,

VERTICAL DIMENSIONSNOT TO SCALE

Figure 5. Schematic sections illustrating the geologic history of Pueblo Viejo, from development of themaar-diatreme in Lower Cretaceous time to the present. A, Formation of the maar-diatreme duringLower Cretaceous time. B, The maar is filled with an upward-fining sequence of sediments. Mineraliza-tion and alteration take place during sedimentation. At the close of Lower Cretaceous time, spilitewacke is deposited over the maar and the surrounding area; mineralization ceases. C, Erosion and slightregional tilting take place. The silicified ore wnes form topographic highs at the time of a marinetransgression in the early Upper Cretaceous. The Hatillo Limestone is deposited unconformably on theLos Ranchos Formation. D, At present the maar sedimentary deposits and ore bodies are exposed atsurface, due to the ongoing uplift of Hispaniola. The level of erosion only recently has reached the maarsedimentary deposits and ore wnes, which are still partly covered by the Hatillo Limestone.

214 N. Russell and S. E. Kesler

sulfate alteration is scarce in the Water Island Formation of theVirgin Islands, which is correlative with the Los Ranchos Forma-tion (Kesler and others, this volume), although pyrophyllite alter-ation has been described around some quartz keratophyre plugs(Donnelly, 1966), and gold values have been detected in streamand rock samples (Alminas and Tucker, 1987). The descriptionsof the geology do not indicate the presence of breccias or maar-type lacustrine deposits, however. More intense acid-sulfate alter-ation has been reported from the northern part of the WaterIsland-equivalent rocks in southeastern Puerto Rico, betweenCerro La Tiza and Colonia Junio (Hildebrand, 1961; Mattson,1973). Lithologic descriptions of these rocks refer to the occur-rence of breccias, but not to possible maar sedimentation. Acid-sulfate alteration has also been reported in Lower Cretaceousvolcanic rocks south of Camaguey in Cuba (Ve1nikov and others,1983), although the lithologic descriptions do not refer to brecciasor other features that might indicate association with a maar-diatreme complex.

trated in a conglomerate horizon deposited on Platanal spilite.Gold-bearing pyrite replaced the conglomerate matrix, and sul-fide veins are rare. At Moore, on the east side of the maar, whereconglomerate is rare or absent, the precious metals are in pyrite-sphalerite-enargite veins that cut thinly bedded sandstones andmudstones and show multiple episodes of vein filling. These veinswere probably emplaced when fluid overpressures periodicallydeveloped beneath the impermeable lake beds (Fig. 5b).

When the rim of the maar crater was lowered by erosion tothe levels of the surrounding area, a regional sedimentation pat-tern was established and spilite wackes were deposited on thecarbonaceous maar-fill rocks. Mineralization ceased at about thesame time and only the basal beds of the wacke carry economicmineralization. In Aptian-Albian time, erosion took place, possi-bly because of uplift and tilting of the Los Ranchos Formation.During this period of erosion, the upper part of the Pueblo ViejoMember was removed. Erosion was greatest in the soft, pyrophyl-litic zones, and the silicified ore zones persisted as topographichighs. During the Albian or Cenomanian, a marine transgressiontook place and the Los Ranchos paleosurface was covered bymarine limestone of the Hatillo Formation (Fig. 5c). Furthertilting of the Los Ranchos and Hatillo Formations took placeduring Eocene time, and stocks, sills, and dikes of diorite wereintruded in the Pueblo Viejo area. Recent erosion has uncoveredonly part of the maar-fill rocks, and their southern extent isconcealed beneath the Hatillo Formation (5d).

ACKNOWLEDGMENTS

Greater Antilles region

Maar-diatreme complexes have not been described fromother areas of Cretaceous volcanic rocks in the Greater Antilles,but descriptions of acid-sulfate alteration and precious metal min-eralization may indicate their presence. Well-documented acid-

This work summarizes the geologic observations made atPueblo Viejo over the course of development of the mine andincludes contributions from all the geologists who have worked atPueblo Viejo, including Cornelius Monster, Mel Essington, JoseRivera, Mike Seaward, Juan Jose Rodriguez, Karr McCurdy,and Jose Polanco. Many of the geologists who have visitedPueblo Viejo also contributed with observations of similarities to(and differences from) other epithermal deposits.

Salvador Brouwer kindly prepared drill core for illustration.The figures were drafted by Jose Dolores Encarnacion. Reviewsof the manuscript were provided by H. Schellekens, J. Lewis,R. Moore, and C. Henry.

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