ECONOMIC GEOLOGY AND THE

BULLETIN OF THE SOCIETY OF ECONOMIC GEOLOGISTS

VOL. 81 JUNE-JULY, 1986 No. 4

Sedimentary and Diagenetic Controls on Red-Bed Ore Genesis: The Middle Tertiary San Bartolo Copper Deposit, Antofagasta Province, Chile

S. FLINT*

Department of Earth Sciences, University of Leeds, Leeds LS2 9JT, United Kingdom

Abstract

The middle Tertiary Pacencia Group of Antofagasta Province, northern Chile, consists of over 2 km of continental sediments. These include alluvial fan, playa, and lacustrine facies, representing deposition in an arid, closed basin. Copper mineralization is hosted by sheet and ribbon sandstone bodies, which were deposited in a high-energy playa-marginal sandfiat en- vironment. Ore minerals occur dominantly as a matrix to these lithic arkoses, replacing earlier carbonate and sulfate cements.

The main ore minerals are native copper, cuprite, and atacamite, although there is an along- strike passage into copper sulfide cements. In a structurally complex zone, copper-iron and copper sulfides were deposited in crosscutting fractures, commonly replacing evaporite veins. Primary chalcopyrite and bornitc exhibit progressive oxidation to anilite, digenite, and djurleite. Chalcocite is not found. Much of this material has oxidized further to covellite of normal and

blaubleibender varieties. Galena and native silver are present in uneconomic amounts. Ata- camite has a complex paragenesis, representing, at least locally, the earliest copper- bearing phase.

Provenance studies of the host sediments and lead isotope data for the sulfides indicate that both the sandstones and the metals were derived from a calc-alkaline volcano-plutonic complex to the east of the basin, an area now obscured by the recent volcanic Andes. Sulfur isotope results for copper sulfides show very light tSa4S values (-33%0), indicative of biogenic sulfate reduction under conditions of unrestricted sulfate supply.

Introduction

STRATIFORM, strata-bound deposits of copper and uranium are well documented in all continents. The

larger examples occur in siltstones and shales which are almost invariably associated with continental red beds. Some famous orebodies are, however, hosted by the red beds themselves (e.g., Nacimiento and Corocoro). These red-bed deposits are commonly of high grade, but in many cases their small size makes large-scale mining operations uneconomic. Copper sulfides are the dominant ores, commonly associated with organic matter and characterized by isotopically light, biologically reduced sulfur. Associated phases are native silver, gold, uraninite, native copper, and secondary oxides and carbonates.

Several recent studies have been concerned with

the identification of common geologic settings for many documented sediment-hosted copper deposits

* Present address: Koninklijke/Shell Exploratie en Produktie Laboratorium, Volmerlaan 6, Rijswijk, The Netherlands.

(Gustafson and Williams, 1981). Formation of many of the larger examples is thought to have been influ- enced by early rifting events within intracratonic ba- sins (Brown, 1981, 1984; Sawkins, 1984) wherein an appeal can be made to the commonly associated ba- saltic volcanism in such environments to provide a metal source.

Although this theory might be true for certain oc- currences, there are important, albeit smaller depos- its, found in avolcanic, late orogenic molasse basins. Andean examples include the historically famous Corocoro copper-silver district of the Bolivian alti- plano basin (Entwistle and Gouin, 1955; Ljunggren and Meyer, 1964), the conglomerate-hosted Coloso deposit in northern Chile (Flint et al., 1986) and the mines at San Bartolo which form the subject of this paper. The purpose of this paper is to describe for the first time the occurrence of what is essentially a high-grade oxide orebody and its relationship to sed- imentation and diagenesis of alluvial fan and playa sediments in an arid zone molasse basin.

0361-0128/86/536/761-1852.50 761

762 s. FLINT

The San Bartolo copper deposit lies at an altitude of about 3,000 m in the foothills of the High Andes of Antofagasta Province, Chile (Fig. 1). Abandoned mine workings are situated in the gorge of the Rio Grande (Fig. 2), some 20 km to the north of the oasis town of San Pedro de Atacama. The area forms a to-

pographic depression between the Cordillera Do- meyko to the west and the Andean cordillera to the east. Much of this basin is occupied by the recent Salar de Atacama, a large salt pan which has recently become an important lithium producer.

Geologic Setting

The San Bartolo copper deposit is hosted by con- tinental clastics of the middle Tertiary Pacencia Group (Fig. 2). The stratigraphy of the area around San Pedro de Atacama was first described by Bruggen (1934) and subsequently modified by Harrington (1961), Dingman (1967), and Hollingsworth and Rutland (1968). Recent work by Flint (1985a) has extended

the knowledge of Tertiary stratigraphy in this area, defining a series of lithostratigraphic members within the Pacencia Group which represents the whole basin- fill sequence (Fig. 3).

The Pacencia Group lies unconformably over con- tinental clastics of the Cretaceous Purilactis Forma-

tion. After deposition of over 2 km of alluvial fan and playa sediments, a compressive Andean tectonic event caused widespread folding which includes rare re- cumbant structures. Following this deformation, a se- ries of ignimbrite flows of the Pliocene San Bartolo Formation were deposited with angular unconformity on the clastic succession (Hollingsworth and Rutland, 1968). Postignimbrite thrusting has caused locally repeated sequences, some involving the Pacencia sediments.

Recent tectonic models (Coira et al., 1982) favor a period of molasse basin formation during the Oli- gocene. Radiometric dating of the overlying ignim- brites has yielded Upper Miocene-Pliocene ages

FIC. 1. The San Pedro region, showing main physiographic features of the central Andes. Contour heights given in meters.

ORE GENESIS CONTROLS, SAN BARTOLO DEPOSIT 763

• San Bartolo FM ':':-::-• Pacencia group undifferentiated

•-•--• Purilactis FM A B

3500 • 3250 ' -• • .•

• -_•-•.•::,.:...•,•.,,._• •, • :./;•111 •.'-•:.:.:?'...'....'..'....'.'?•

L ITHOLOGY SEDI,•ENT,•RY STRUCTURES

• CONCLOMFRATE-CLAS1 SUP[r>ORTEO • CONVOIUI{ I lMINllION

•C. 2. Generalized geologic map of the •ea •ound San Bar- rolo.

(Dingman, 1965; Guest, 1969) and thus assigns the Pacencia Group to the Oligocene-Miocene (Flint, 1985a). Geophysical studies over recent years (James, 1971; Ocola and Meyer, 1973; Stauder, 1975) indi-

cate a crustal thickness of some 70 km in this sector

of the Andes, and volcanological studies document an eastward shift in the focus of subduction-related mag- matism, from the present-day Pacific coast (Jurassic) to the recent stratovolcanoes that mark the border

between Chile and Argentina (Farrar et al., 1970; Coira et al., 1982; Pilger, 1984).

Copper mineralization at San Bartolo occurs in the Artolla Member of the Pacencia Group. This member overlies unmineralized salt pan facies sediments of the Palicaye Member which crop out as the San Bar- tolo dome (Fig. 4). The junction of the Palicaye Mem- ber with the overlying Artolla sediments in the Rio Grande valley is tectonically modified, although its nature is unclear because the contact zone has been eroded to form much of the course of the Rio Grande

and Palicaye stream. The structure has been inter- preted as an anticline, faulted along its axial plane, which has brought sediments of the older Palicaye Member into juxtaposition with the Artolla Member (Hollingsworth and Rutland, 1968). The Artolla Member is conformably and gradationally overlain by

SAN BARTOLO FM

RIO SALADO MEMBER

BARROS ARANA MEMBER

E

!!• RIO SALADO MEMBER

CEMENTARIO MEMBER

ARTOLLA MEMBER

Cu

X X PALICAYE X X MEMBER

• PURiLACTIS FM

ß .

FIG. 3. Stratigraphy of the San Pedro basin (after Flint, 1985a).

764 s. FLINT

,

ß 2910

1

\

.!

/o

œ

?

? 7

Bß CEMENTARIO

/SAN BARTO LO.•

?

ß 3020

&3118

/

HEAD OF

PALICAYE MINES

[ / ß Major Workings ß Subsidiary Mines

• • ,• Rimof Gorge / • Anticline / ARTOLLA ß •r

MINE ] / Thrust Fault / , • 0 Km i

N

•+-•+• San Bartolo Fm ø•.• Rio Salado Member J• Cementario Member '::• Artolla Member • Palicaye Member

FIG. 4. Plan of the San Bartolo mining district showing the principal mines and structural features. Inset shows the local relationships between internal members of the Pacencia Group.

unmineralized, shallow lacustrine sediments of the Cementario Member. This unit is known to contain

thin ash beds (Flint, 1985a).

Sedimentology of the Artolla Member The Artolla sediments consist of over 400 m of red

mudstones and interbedded red and gray sandstone bodies (Figs. 5 and 6A). The growth of primary nod- ular anhydrite in the mudstones indicates highly arid conditions. Many secondary gypsum veinlets cut the succession at high angles to bedding. Sand bodies tend to be laterally extensive with erosive bases and bed- forms indicative of high-energy sedimentation. These internal structures include the common upward pas- sage from upper phase plane bedding, through trough crossbedding to climbing ripple crosslamination. The mudstones are interpreted as subaerially exposed playa mud-fiat material, deposited where shallow, high-energy sheets of unconfined flood water en- countered storm-expanded, ephemeral lake waters. Sandstones, exhibiting both sheet and ribbon geom- etries, represent a more proximal playa-marginal sandfiat. Periodic storms or seasonal climatic varia-

tions are envisaged to have produced high-energy, ephemeral flood waters which, after emerging from

distal alluvial fan channels, spread out over an un- confined subaerial sandfiat. This process results in the deposition of clastic material under conditions of waning flow (Flint, 1985a). Numerous desiccation cracks attest to dominantly subaerial conditions. De- tailed measurements of directional sedimentary structures indicate that the mineralized Artolla sands

were derived from provenance areas to the east and southeast (Fig. 5). Higher in the succession, transport directions swing to dominantly from the south, pos- sibly related to a change in drainage pattern (Flint, 1985a). Provenance areas to the east of the basin are now obscured by the recent volcanic Andes.

Mineralogy of the Host Sediments The sandstones of the Artolla Member are medium

to fine grained, consisting of poorly sorted, angular grains of low sphericity. Dominant minerals are quartz and sodic plagioclase feldspar with subordinate or- thoclase and rare microclines. Biotite and muscovite

micas are common, the former commonly showing chloritization. Detrital ferromagnesian minerals in- clude hornblende and rare augite. Heavy mineral lay- ers are common, noticeably picking out crossbedding foresets and dewatering structures. These opaque

ORE GENESIS CONTROLS, SAN BARTOLO DEPOSIT 765

CEMENTARIO MEMBER

SHALLOW EPHEMERAL LAKE

400

300

Cu

ARTOLL,• DRY PLAYA MUDFLAT MEMBER

200

]oo

Cu

Cu MARGINAL SANDFLAT

Cu

PALICAYE MEMBER SALINE MUDFLAT

FIC. 5. Representative sedimentary log through the Artolla Member at San Bartolo showing principal ore-bearing sandstones.

grains are well rounded and commonly include mag- netite, titanomagnetite, and illmenite which com- monly exhibit complex exsolution textures. Other ac- cessory minerals are rutile and apatite.

A characteristic feature of these sands is the abun- dance of igneous rock fragments, which tend to be subrounded and heavily weathered. They include volcanic and plutonic material of andesitic-dioritic to granodioritic compositions, and locally account for 20 percent of the mode. The Artolla sands have been interpreted as lithic arkoses, immature erosion prod- ucts of intermediate to acidic crystalline complexes (Flint, 1985a).

Premineralization Diagenesis of the Host Sediments

Microscopy, SEM, and microprobe studies have established a diagenetic scheme covering the period from deposition until after the mineralizing events. The earliest recorded diagenetic changes involve the reddening of the strata by production of thin hematite rims on detrital grains. Studies of this phenomenon in recent desert alluvium (Walker et al., 1978) have

suggested that hematite is derived from the break- down of unstable, iron-rich minerals such as pyrox- enes and amphiboles. Such minerals are only rarely found as intact, detrital phases in the Artolla sands.

Under SEM, plagioclase feldspars show progressive dissolution with subsequent replacement by clay minerals, ultimately resulting in complete clay pseu- domorphs. Following early dissolution events, a suite of authigenic minerals was precipitated as syntaxial overgrowths and discrete phases (Fig. 6B and C). Well-developed silica overgrowths in optical conti- nuity with the host detrital grain commonly preserve earlier hematite coatings.

Much of the sodium released during dissolution of plagioclase feldspars and rock fragments was precip- itated as small, euhedral, authigenic grains of analcime which have a distinctive cubo-octahedral morphology (Fig. 6B, D, and E). Analcime commonly nucleates on detrital quartz and feldspar grains and tends to line pore spaces. Formation of analcime from volcanic rock fragments is favored by relatively alkaline inter- stitial solutions (Aoyaki and Asakawa, 1984). These authors have also documented the transformation of

analcime to albite at temperatures above 138øC. Au- thigenic albite is present in the Artolla sandstones but occurs as overgrowths which predate analcime (Fig. 7), indicating that temperatures never reached this level in the host rocks to the ores. Thus the presence of abundant analcime in the host rocks allows an ap- proximate upper temperature constraint to be placed on the diagenesis and indirectly on the later miner- alizing events.

The earliest pore-filling cement in these sandstones is calcite (Fig. 6B). It occurs in a coarsely crystalline form and possibly represents the reprecipitation of calcium from dissolved feldspars. Associated gypsum cements indicate continued highly arid conditions.

Later in diagenesis the alkali feldspars show partial to complete dissolution, with subsequent potassic overgrowths on detrital and earlier authigenic phases. This paragenetic sequence of dissolution, replace- ment, and authigenesis has been observed in many arid-zone red-bed lithofacies (Walker et al., 1978; Turner, 1980).

No further diagenetic changes are found following alkali feldspar overgrowths in unmineralized sedi- ments; this is at variance with the main ore-bearing sands and spatially related, slightly mineralized beds, in which the most noticeable feature in both field and

microscopic studies is the partial to complete removal of hematite, giving the rocks a characteristic bleached, pale gray appearance. This color change associated with mineralization has been noted in many other red- bed-type copper deposits such as Corocoro (Ljung- gren and Meyer, 1964), Nacimiento (Woodward et al., 1974), and Caleta Coloso (Flint et al., 1986). A

'I

Cal

An . •

Cup

E

, At

II II

FIG. 6. A. Abandoned workings at San Bartolo in steeply dipping playa sandstones and mudstones of the Artolla Member. B. Photomicrograph of sandstone from the Artoila mine. This bed has not been subjected to the secondary porosity generation; early calcite cement (c) is preserved. Note the small, pore-lining analcimes (An). The large detrital grain in the center is a plagioclase feldspar showing advanced dissolution. The voids so produced are filled by atacamite (dark). Plane polarized light. Scale bar = 0.5 mm. (2. Photomicrograph ofmineralized sandstone, Artoila mine. Note syntaxial overgrowth on central quartz grain. The opaque cuprite matrix encloses earlier atacamite .(gray material at tip of arrow) which remains as corroded islets and early pore linings. Plane polarized light. Scale bar = 0.5 mm. D. Cuprite cement ((2up) enclosing earlier authigenic analcime (Ab) and calcite matrix ((2al). Reflected, plane polarized light. Scale bar = 0.5 mm. E. SEM backscattered electron image of fibrous pore-filling atacamite (At), postdating authigenic analcime (Ab) which has nucleated on detrital quartz (Qz). Scale bar = 0.5 mm. F. Detail of cuprite-cemented sand. Note the brecciation of silicates caused by displacive growth of the cuprite. Reflected, plane polarized light. Scale bar = 0.5 mm.

766

ORE GEHESIS COHTROLS, SAH BARTOLO DEPOSIT 767

#IliERAL/•V•liT EARLY OXIDATIVE R•DUCT1Oll LATE OXIDATIVE R•NOBILIZATIO• D1AG•liE$I$ D IAGE•I/• I$

Hematite

Clay lilaerals •lclte

Gy•um Analcime

Alblte

K Felds•r •l•tite

•artz

]obile r•uc•nt

(? orEanic acid) }•R•ITY GE]ERAT I0•

Chalcopyrite •rnite

]at lye Cop•r Chalc•ite Group Cuprite •vellite

Tenorire

[alachite

TI#E •

FIG. 7. Apparent paragenetic relationships of diagenetic events in the Artolla sandstones.

similar bleaching is also developed in sandstone ura- nium occurrences such as the Jackpile deposit in New Mexico (Adams et al., 1978; Turner-Peterson, 1985).

A pervasive dissolution of the early calcite cement appears to be temporally associated with this color change and commonly results in large, isolated calcite crystals in a later, mineralogically different matrix (Fig. 6D). This dissolution of early pore-filling min- erals caused a reduction of cementation in only slightly mineralized sands. The main ore-bearing sandstones are fully lithified owing to the precipitation of later copper-bearing cements.

These events may indicate a change in ground-wa- ter chemistry, prior to the introduction of copper- bearing fluids into the sediments. The early calcite cements appear to be replaced by poikilotopic celes- tite which is found only in mineralized samples. This relatively uncommon cement could be related to the release of strontium from the calcite lattice during dolomitization, but dolomite is not present in the Ar- tolla sandstones. A second possibility is the derivation of Sr during the dissolution of alkali feldspar; the greater stability of this mineral over plagioclase feld- spars in the diagenetic environment and thus its rel- atively late dissolution would be in accord with the late paragenetic position of celestite as a matrix min- eral. The presence of celestite as an immediately pre- copper cement possibly suggests that the change in ground-water chemistry associated with bleaching facilitated the dissolution of alkali feldspar.

Mineralization

The copper ores at San Bartolo are hosted by up to 15 sandstone bodies which range in thickness from 0.5 to 2.5 m. These sands have been mined along a strike length of about 4 km from the Artolla mine to the mines at the head of Quebrada Palicaye (Fig. 4).

The full complement of mineralized beds is exposed only at this latter location. They are found over a stratigraphic interval of some 75 m (Fig. 8A). Al- though the lateral continuity of individual orebodies is difficult to assess owing to recent land slips, the available evidence indicates a strata-bound but not always stratiform configuration. Mineralization is seen in some instances to lie at an angle to bedding while being restricted to individual beds.

Mining operations, carried out between the mid- nineteenth century and 1925 (Travisany, 1978), in- volved a combination of room and pillar and under- hand stoping methods. This resulted in the extraction of almost complete mineralized beds, accessed di- rectly from outcrops, although several shafts appear to have been sunk. The operation was discontinued owing to water problems rather than failing reserves.

Mineralized sands are recognized by a pervasive bleaching and poor cementation at outcrop. This color change contrasts strongly with the red, well-ce- mented, unmineralized strata. Traces of green mal- achite and atacamite are commonly found at outcrop. Underground investigation has revealed two different styles of mineralization. In the first style, ore minerals occur as a matrix to detrital silicates and commonly highlight primary sedimentary structures such as cross stratification. The second style of mineralization is observed only in the mines at the head of Quebrada Palicaye where the host rocks, a stack of channel sand bodies, dip vertically in a structurally complex zone and show local brecciation (Fig. 8A). Fractures are filled by evaporite and ore minerals in a veinlike man- ner. This style of mineralization can be traced over a stratigraphic thickness of about 25 m in the exposed lower half of the Artolla Member.

The main mineralization is considered to have no structural control. As discussed above, the main ore-

768 s. FL/NT

B

,. Dg

D

Dg

GR

FIG. 8. A. Vertically bedded, sulfide-bearing sandstones of the Artolla Member in the mining area at the head of Quebrada Palicaye. B. Reflected light photomicrograph of early pyrite (Py) partially replaced by digenite (Dg). The presence of paragenetically later covellite is consistant with progressive oxidation. Plane polarized light. Scale bar = 0.5 mm. C. Backscattered electron photomicrograph showing

ORE GENESIS CONTROLS, SAN BARTOLO DEPOSIT 769

bearing sandstones are located north of the San Bar- tolo dome, an anticlinal structure which exposes strata of the older, unmineralized Palicaye Member. How- ever, one mineralized bed is found in exposures of the Artolla Member to the south of the dome in the

Funica San Bartolo area (Fig. 2), which suggests that the copper ores formed before the Pacencia succes- sion was folded. The deformation occurred before deposition of the San Bartolo ignimbrites; assuming an Oligocene-lower Miocene age for the Pacencia Group, this relationship constrains the timing of min- eralization to pre-upper Miocene times.

The general absence of extensive bleaching and subsequent ore genesis in the Funica-San Bartolo area could indicate that the fluids responsible for these events were concentrated in the San Bartolo-Palicaye area. This concentration is possibly due to the local- ization of ribbon sandstones with some bed amalgam- ation, which possibly allowed the passage of greater volumes of groun• water.

Ore mineralogy

Ore minerals were deposited in pore spaces fol- lowing the diagenetic events outlined above; their distribution was controlled by the generation of sec- ondary porosity.

Matrix mineralization: This style of mineralization, accounting for about 80 percent of observed occur- rences is perhaps best developed in the area of the Artolla mine (Fig. 6A). The important ore minerals in order of abundance are atacamite (Cu2[OH]3C1), tenoritc, cuprite, and native copper. Galena is wide- spread in small quantities. Native silver is associated with the native copper and copper oxides. Secondary malachite is present but not common. Copper oxides were probably the most economically important ore: black, oxide-cemented sands assay up to 28 wt percent Cu and contain up to 30 ppm silver (G. Walls, pers. commun., 1983). Detailed optical and microprobe studies show that two oxide minerals are present in this deposit. In subsurface samples, the oxide is red, contains 85 to 89 wt percent copper, and shows strong red internal reflections in polished specimens. These properties are consistent with cuprite. The black ce- menting mineral in sandstones contains less copper (78-80 wt %) and does not exhibit the red internal reflections. This mineral is considered to be tenoritc

(CuO), developed as a response to prolonged oxida-

tion. Texturally, the copper oxides occur as euhedral, interlocking crystals surrounding detrital and early diagenetic phases (Fig. 6D). They are also seen to envelop what appears to be paragenetically earlier native copper and atacamite (Fig. 6C).

Native copper is found only in unweathered un- derground samples. It occurs as large, sometimes poi- kilotopic crystals and is commonly rimmed by cuprite. This evidence for an oxidative overprint on the pri- mary ore minerals leads to speculation that the larger volumes of oxide-cemented sandstone originally had a matrix of native copper.

In sand bodies which carry uneconomic ore con- centrations, ore minerals tend to occur as distinct stringers and roughly spherical "concretions." The stringers are up to 1.5 cm wide, are commonly dis- cordant to bedding, and cause no disruption of the silicate fabric. Microscopic examination reveals abrupt lateral terminations to the stringers of matrix cuprite. Local brecciation of silicate grains is seen on a micro- scopic scale, suggesting that the ore minerals grew displacively in part (Fig. 6F). In rare cases organic remains are associated with stringers and tend to trace the outline of the structures. It is possible that these objects are the remains of invertebrate burrows. In one burrow, grains of early pyrite replaced the organic material. The pyrite itself is partially replaced by copper sulfides (Fig. 8B).

The roughly circular (0.5-2.5 cm) concretions of ore in bleached sands consist commonly of a central core of native copper, surrounded by small interlock- ing crystals of cuprite, again with abrupt outer ter- minations against unmineralized, invariably celestite- rich host rock. These nodulelike concentrations have

been documented in other red-bed copper deposits (Ljunggren and Meyer, 1964) and are known to cor- relate with the presence of local plant remains. To date no such debris has been identified at San Bartolo, making interpretation of these isolated patches of ore minerals problematical. Possibly original organic ma- terial could have been removed by diagenetic oxi- dation. In fact work on modern sediments of the Great

Salt Lake, Utah, shows that only 5 percent of the total organic carbon originally deposited in these playa sediments is preserved (Eugster, 1985).

The green oxychloride atacamite is widespread throughout the deposit; on textural grounds it appears to have at least two generations. This mineral is the

chalcopyrite (Cp)-bornite (Bn) botryoidal intergrowths rimmed by possibly supergene anilite-digenite (Dg). Palicaye mines. Scale bar = 0.5 mm. D. Galena (Gn) enclosing earlier djurleite (Dj). Much of the bounding djurleite is replaced by atacamite (At). Palicaye mines. Backscattered electron image. Scale bar -- 0.5 min. E. Digenitelike sulfide phase replaced along orthorhombic cleavages by atacamite. Palicaye mines. Backscattered electron image. Scale bar -- 0.5 min. F. Rare occurrence of insoluble organic residue (at tip of arrow) marking the former presence of the postulated organic mobile reductant which is thought to have promoted sulfide precipitation. Note the close association of copper sulfides (black). Palicaye mines. Transmitted, plane polarized light. Scale bar -- 0.5 mm.

770 $. FLINT

earliest copper phase found in bleached sandstones and is commonly the only ore mineral in slightly min- eralized rocks. It occurs as early rims around detrital grains, as intergrowths with authigenic silica, and fills early dissolution voids in calcic plagioclase feldspars (Fig. 6B). Early atacamite coexists with remaining he- matite and encloses partially dissolved relict crystals of the early calcite cement. In more heavily miner- alized sands, this early atacamite is commonly over- grown by later native copper and cuprite (Fig. 6C). Electron probe analysis shows this mineral to contain approximately 52 wt percent copper. Atacamite has never been synthesized. Its behavior is thought by Rickard (1971) to be Similar to its polymorph para- tacamite, which he produced under acid pH condi- tions by adding sodium hydroxide to cupric chloride solutions. Figure 9 shows that, given a chloride ion concentration of i M, atacamite could precipitate out of a chloride brine in moderate to highly oxidizing conditions and neutral pH conditions. This diagram also indicates that with a fall in pH values to acidic ground-water conditions, copper sulfides should pre- cipitate from this same chloride solution with little or no change in Eh values. A fuller discussion of atacam- ite stability relations is given by Flint (1985b and c).

Galena, although widespread throughout the de- posit, never attains economic quantities. Detailed mi- croscopic and SEM studies have shown it to have both early and late paragenetic positions.

A significant change in the mineralogy of matrix ores occurs some 3 km along strike northeast of the

.8

Cu 2+

ß 6 C u2(OH)3Ct

Eh.Z, cuo votts

Cu2S 0

-'2

pH

(CI- = 1.0 •. Mo•iGe• •te• Bose, 1976, •n• •ete•s, 1977.

Artolla mine where copper sulfides are common. Chalcocite-group minerals are dominant along with secondary covellite, but in rare cases chalcopyrite forms an early cement. The sulfides have chemical compositions corresponding to anilite, digenite, and djurleite; stoichiometric chalcocite is not present. These minerals share a paragenetic position identical to the volumetrically more important cuprite and na- tive copper. They have the form of small interlocking crystals, invariably showing a strong orthorhombic cleavage (Fig. 8B and C). Covellite commonly re- places the chalcocite-group minerals and occurs as overgrowths on these early copper sulfides.

This ore mineralogy, characterized by the complete absence of sulfides in samples from the Artolla mine, marks an interesting departure from almost all doc- umented economic red-bed copper occurrences which are dominated by copper and copper-iron sul- fides (Gustafson and Williams, 1981). Despite the ex- amination of over 80 specimens of fresh ore, there is no clear evidence of the nature of association between

matrix sulfides and matrix native copper-cuprite. Sul- fide mineralization tends to pass into texturally similar native metal-oxide mineralization along strike. In rare specimens containing an association of all the above mineral types cuprite, occupying a paragenetically late position, occurs as overgrowths to chalcocite- group minerals and covellite. This tentatively suggests that much of the original matrix could have been sul- fide minerals.

Veinlet mineralization: The second style of min- eralization occurs only in the mines at the head of Quebrada Palicaye (Fig. 4). Veinlets disrupt the fabric of host sandstones on a microscopic scale and are dominated by sulfide ore minerals. Atacamite is less common than in the matrix mineralization and cuprite is absent. Microscopic and SEM studies indicate that the earliest phase to precipitate was coarsely crystal- line chalcopyrite. Toward the edges of some veinlets, bornite occurs in botryoidal intergrowths with the primary chalcopyrite (Fig. 8C). In many sediment- hosted copper deposits bornite is commonly slightly sulfur rich (e.g., McCauley, 1964; Ripley et al., 1980). The bornite analyses (Table 1) show that the bornites at San Bartolo are also sulfur rich.

Chalcopyrite and bornite are present only in small quantities in the veinlet style ofmineralization at Pal- icaye where they are replaced along fractures by minerals of the chalcocite group (Fig. 8C). These re- placement textures indicate progressive oxidation of copper-iron sulfides to copper sulfides, a process known to produce metastable blaubleibender covel- lites and eventually stable covellite. Peters (1977) and Potter (1977) support the above theoretical studies. Galena is present in the veinlets but in lower quan- tities than in the matrix ores (Fig. 8D).

ORE GENESIS CONTROLS, SAN BARTOLO DEPOSIT 771

TABLE 1. Electron Microprobe Analyses of Ore Minerals from San Bartolo

Mineral Wt % Cu Wt % Fe Wt % Ag Wt % Au Wt % S Total Formula

Cp 34.295 29.611 0.003 35.358 99.940 Cuo.97Feo.96S2 Cp 34.060 30.562 0.019 35.656 100.919 Cuo.96Feo.ssS2 Cp 34.197 30.487 0.007 35.046 100.725 Cuo.9sFeo.9•S2 Cp 34.277 30.358 0.026 35.224 100.064 Cuo.9seo.ssSa Bn 60.610 13.075 26.260 100.249 Cu4.66Fe•.•4S4 Bn 61.125 12.906 26.316 101.261 Cu4.6sFe•.• aS4 An 77.324 0.181 0.067 0.273 22.282 100.472 Cu•.75S An 77.093 0.079 1.002 22.140 100.792 Cu•.75S An 77.018 0.025 0.058 0.301 21.912 100.283 Cu•.7sS An/Dg 77.035 0.039 0.219 0.080 22.042 99.415 Cu•.76S An/Dg 77.900 0.221 0.329 21.924 100.374 Cu•.7sS An/Dg 77.753 0.129 0.278 22.042 100.202 Cu •.7sS An/Dg 77.469 0.357 0.117 0.007 21.995 99.945 Cu •.•7S An/Dg 78.029 0.201 0.019 0.145 22.325 100.719 Cu•.76S Dg 77.262 0.178 0.007 21.610 99.057 Cu•.soS Dg 77,124 0.487 0.165 0.006 21.212 99.360 Cu•.soS Dj 78,685 0.296 0.021 20.139 99.141 Cu•.s6S Dj 79.801 0.226 0.191 20.120 100.338 Cu•.o6S Bb Cv2 72.551 0.222 0.096 27.238 100.107 Cu•.aaS Bb Cv• 73.212 0.178 0.218 0.030 27.386 100.996 Cu•.a•S Bb Cv• 68.888 0.164 0.078 30.763 99.893 Cu•.•aS Bb Cv• 68.110 0.212 0.101 30.661 99.084 Cu•.•zS Cv 66.226 0.068 0.042 32.696 99.032 Cu•.ooS Cv 66.904 0.092 0.117 33.289 99.990 Cu•.ooS Cv 66.213 0.212 0.039 33.326 99.790 Cu•.ooS

Abbreviations: Cp -- chalcopyrite, Bn -- bornite, An = anilite, Dg -- digenite, An/Dg -- intermediate phase between, or mixture of anilite and digenite, Dj = djurleite, Bb Cv• -- type 1 blaubleibender covellite, Bb Cva = type 2 blaubleibender covellite, Cv -- stoichiometric covellite

Underground investigation revealed the presence of thin (<1 cm), laterally extensive sheets of native copper which fill tectonic fractures in the vertically bedded sandstones at Palicaye. This phenomenon is identical to the sheets of native metal reported from the limbs of tight folds in the Corocoro deposit. Ljunggren and Meyer (1964) interpreted these sheets as a remobilization of earlier matrix ores during de- formation. Probably the veinlet and sheet material confined to the structurally complex area at the Pal- icaye mines has a similar origin.

Sulfide mineral chemistry

Electron microprobe studies were carried out to characterize the copper sulfide mineralogy and to de- termine if there were compositional differences be- tween matrix and veinlet sulfides. Electron micro- probe analyses (Table 1) indicate that, whereas there are no differences in mineral chemistry between ma- trix and veinlet types, all sulfides are characterized by lower Cu/S ratios than are found in many deposits (Ixer and Vaughan, 1982). Several compositions are common (Table 1), corresponding to anilite and dig- enite with minor djurleite. Compositions intermediate between anilite (Cu•.75S) and digenite (Cu•.8oS) are possibly due to fine-grained intergrowths of these two

phases. These results may be compared with micro- probe data on copper sulfides from sediment-hosted copper deposits of varying ages (Goble and Smith, 1973; Ripley et al., 1980) in which the most common Cu/S ratios fall between Cu•.76S and Cu•.78S. Theo- retical studies of the copper-sulfur system (Potter, 1977) indicate that no stable phase exists with a Cu/S ratio of 1.76 to 1.78.

Two explanations have been offered for these mea- sured ratios, one being that the measured phases are in fact mixtures of anilite with digenite and/or djur- leite. Goble and Smith (1973) suggest that originally compositions between chalcocite and covellite exsolve to give compositions close to the preferred Cu/S ratios of 1.12, 1.32, and 1.76. These ratios agree with the electron probe analyses presented in this paper (Table 1). A second explanation for the obtained ratios is that these sulfides were originally of anilite compo- sition. As demonstrated by Sillitoe and Clark (1969), anilite may be converted to a digenite-type solid so- lution by grinding at room temperatures. Therefore specimen polishing was possibly responsible for par- tially converting anilite to slightly more copper-rich phases or for initiating limited exsolution, as suggested by Goble and Smith (1973). Anilite in the oxidized zone of Chilean porphyry copper deposits has been interpreted as a product of supergene enrichment

772 s. FLINT

acting on hypogene chalcopyrite and bornitc (Sillitoe and Clark, 1969).

Normal and blaubleibender covellites replacing chalcocite are the most abundant sulfides in many Bartolo samples. Microprobe results (Table 1) show that two compositions of blaubleibender covellite are present, with Cu/S ratios of 1.1 and 1.4. This data is in striking agreement with compositions obtained by Goble and Smith (1973) and is in accord with the electrochemical studies of Potter (1977). Blaublei- bender covellites represent intermediate stages in the formation of normal covellite from chalcocite. They form due to the difficulty in nucleating hexagonally close-packed normal covellite from cubic close- packed anilite or digenite (Potter, 1977).

The copper sulfides almost invariably exhibit a strong orthorhombic cleavage (Fig. 8E), which indi- cates crystallization temperatures below 103øC (McCauley, 1961; Roseboom, 1966). This is in accord with the previously mentioned stability field of anilite and that of djurleite, which converts to high digenite and high chalcocite at 93øC. The absence of chalco- cite suggests that this reaction has not taken place in the Bartolo sulfides. Low temperatures are also in- dicated by the abundance of atacamite, which is com- monly paragenetically earlier than sulfides. Rose (1976) showed that the atacamite stability field on Eh-pH diagrams virtually disappears above 75øC.

Table 1 shows that the secondary copper sulfides are enriched in both silver and gold over the primary copper-iron sulfides. This enrichment has been doc- umented in other similar deposits such as Alderley Edge, Cheshire, England (Ixer and Vaughan, 1982), and Caleto Coloso, Chile (Flint et al., 1986).

Postmineralization Diagenesis After mineralization and subsequent oxidation of

copper-iron sulfides to "chalcocite" and covellite in the vein ores and after oxidation of sulfides and native

copper to cuprite in the matrix mineralization, a less important pulse of copper-rich chloride solutions is recorded by the precipitation of late atacamite over- growths on diagenetically modified detrital minerals and ore phases (Fig. 6E). Copper sulfides were clearly out of equilibrium with the latter diagenetic environ- ment, commonly showing progressive replacement by truly secondary atacamite (Fig. 8E). The presence of this later atacamite suggests that later ground water was at one stage chloride rich and highly oxidizing. After the termination of the mineralizing events, nor- mal diagenesis resumed in response to the physico- chemical conditions of burial and ground-water cir- culation. Further SEM study has indicated the pres- ence of silica and albite overgrowths on ore minerals, along with increased quantities of authigenic alkali feldspar which commonly overgrew earlier diagenetic phases (Fig. 7).

Source of the Metals

Many authors have speculated on the source of metals in sediment-hosted copper deposits. Ideas in- clude derivation from the mantle via hydrothermal fluids, erosion of earlier basement deposits, and con- centration of ore minerals from trace amounts in

common rock types by sedimentary, diagenetic, and metamorphic processes. Several workers have pointed out the higher than average Cu values in tholeiitic basalts and tried to fit red-bed deposits into early rift settings. Clearly the San Bartolo deposit is not related to continental rifting and basaltic volcanism; as dis- cussed earlier, the San Pedro basin, although exten- sional, is situated on a sector of the world's thickest continental crust.

Derivation of metals through magmatic-related processes is considered unlikely for the San Bartolo mineralization. There is no record of igneous rocks in the Pacencia Group or in the Cretaceous of the area; the middle Tertiary of this sector of the Andes is considered to have been a period of tectonic qui- escence following volcanism and compression during Eocene times (Coira et al., 1982). The overlying dac- itic ignimbrites have low copper values (B. Hooper, pers. commun., 1983) and mineralization occurs con- siderably lower stratigraphically than the sediments in contact with these volcanics. Furthermore, min- eralization shows no relationship to any of the nu- merous normal and thrust faults that cut the succes-

sion. Apart from the localized breccia-filling miner- alization in Quebrada Palicaye, there is no significant vein style of ore deposition. The simple ore miner- alogy and absence of gangue minerals also argues against an igneous hydrothermal origin.

Lead isotope studies

In recent years the measurement of lead isotope ratios in sulfide orebodies has provided a potentially useful tool for determining the age of mineralization (applicable mainly to Precambrian deposits) and for fingerprinting possible sources of lead, and by impli- cation, other metals in the deposit (Doe and Stacey, 1974; Sangster, 1976). Sediment-hosted ores with highly radiogenic, anomalous leads are commonly considered to have formed by remobilization of metals from older upper crustal sources. Mississippi Valley- type lead-zinc deposits and certain red-bed copper occurrences are thought to have formed in this way (Doe and Delevaux, 1972; Sangster, 1976; Ruxton, 1981; Bjorlykke and Thorpe, 1982).

Lead isotope studies were undertaken to charac- terize possible sources for the metals at San Bartolo. Possible source areas to the east of the basin are now

obscured by later volcanics. Seven samples of copper sulfides (four from veinlet mineralization and three from matrix mineralization) were obtained by acom-

ORE GENESIS CONTROLS, SAN BARTOLO DEPOSIT 773

bination of microdrilling and crushing to - 150 mesh, followed by heavy liquid separation. After hand pick- ing, 100 mg samples were dissolved in nitric acid. After centrifuging, residues were dissolved in HBr and passed twice through anion exchange columns. The lead was collected by 6M HC1, loaded on single rhenium filaments with phosphoric acid, and fused in silica gel. Measurements were carried out on a V. G. Micromass 30 solid-sourced mass spectrometer op- erated at 2.4 A and 3.5 kG over a temperature range of 1,150 ø to 1,400øC.

The isotopic values obtained (Fig. 10) are similar to those of Mesozoic to Paleogene calc-alkaline vol- canics from southern Peru (Barriero and Clark, 1984) and the Copiapo area of northern Chile (McNutt et al., 1979). They are not sufficiently radiogenic to have been derived from an earlier sediment-hosted ore de-

posit (Ruxton, 1981). The isotopic signatures obtained suggest that the ores were derived from volcanogenic sources which is in accord with provenance studies showing that the Artolla host sediments were derived

39.0 • 38.5

208p b Sediments 204•-•

38.0 37'5

15.7

15 '6

207pb 204-p-•pb

15.5

15'4

! I

• •"* *"*'**•c Sediments

f I I I I

18.2 18'4 18'6 18'8 19'0

206pb 204-js'-ff

FIG. 10. Lead isotope compositions of copper sulfides from the San Bartolo deposit. Circles represent matrix sulfides whereas squares are the measured compositions of veinlet sulfides. The fields of lead isotope compositions for Nazca plate basalts (npb) and Mesozoic-Paleocene volcanics (sp) from Peru (Barriero and Clark, 1984) and andesites from the Copiapo area of northern Chile (ca, McNutt et al., 1979) are shown for comparison. Other fields include Colombian porphyry coppers (Cpc) and sediments from the subducting Nazca plate (Pacific sediments) after Sillitoe and Hart, 1984.

from a calc-alkaline source area to the east of the basin.

Without isotopic measurements of the now inac- cessible crystalline source rocks it is difficult to rule out the possibility that the measured ratios represent the mixing of lead from two sources. One alternative could involve the mixing of a mantle-derived com- ponent (e.g., midoceanic ridge basalt, MORB) with radiogenic lead from the host arkoses. However, geo- logic evidence indicates that the metals were derived from the same source as the host rocks. The Pacencia

Group is of Oligocene-Miocene age and the timing of ore genesis is pre-upper Miocene. Thus, there was probably insufficient time for the sandstones to gen- erate much radiogenic lead through the decay of ura- nium or thorium. Therefore, while a mixing event cannot be dismissed on the available isotopic data, the similarity of lead in the ores to that of Andean andesites supports a common source.

In a recent isotopic study of porphyry copper de- posits in Colombia, Sillitoe and Hart (1984) discussed possible ultimate sources of lead in such Andean en- vironments. Many of these Colombian porphyry data (Fig. 10) lie in a field similar to the San Bartolo leads. The descending Nazca plate is discounted as a lead source, as is the overlying mantle wedge (assuming it does not possess the unusually radiogenic lead isotope signatures of certain oceanic islands such as St. He- lena). Sillitoe and Hart pointed out that the sediments overlying the Nazca plate can be isotopically divided into two categories. The basal metalliferous sediments overlying the basaltic crust contain lead that is largely derived from the oceanic crust itself thus having mid- oceanic ridge basalt values. These are overlain by pe- lagic sediments which contain lead derived through the weathering of continental crust (Fig. 10). It is this second lead which the above authors conclude rep- resents the major source of Pb in porphyry systems and, it appears, in the ores from San Bartolo. Varia- tions from this field may be attributed to selective contamination by upper crustal leads of variable iso- topic characteristics. An example of such contami- nation is given by Barriero and Clark (1984) where relatively nonradiogenic lead isotope ratios in andes- ites are explained in terms of contamination from metamorphic basement.

Transport and Deposition

Major problems in the understanding of low-tem- perature base metal deposits include source of metals, the mechanism of transporting copper, and related constituents to sites of deposition and subsequent fix- ation of the metal(s), commonly as sulfides, in what is generally a highly oxidizing environment. Several authors (e.g., Brown, 1971; Rose, 1976) have shown that the solubility of copper in terrestrial ground wa- ter at neutral pH levels is ostensibly less than 1 ppm,

774 s. FLINT

but its solubility is considerably increased in chloride- rich solutions. Dissolved copper values of 6 ppm or greater may be quite adequate for the eventual for- mation of an ore deposit, given a suitable concentra- tion mechanism.

Almost without exception, economic sediment- hosted copper deposits are hosted by successions that contain evaporites (Gustafson and Williams, 1981); this striking correlation has led several authors to suggest that there is a genetic link between evaporites, chloride brines, and copper mineralization (Davidson, 1965; Renfro, 1974; Rose, 1976). In this context the early paragenetic position of the oxychloride atacam- ite is interesting. It has recently been suggested that atacamite represents an early phase in certain red- bed deposits, precipitated from an evaporite-derived cupric chloride solution under conditions of high Eh and moderate to low pH, with temperatures probably less than 50øC (Flint, 1985b and c). As already dis- cussed, evaporites form a significant proportion of the sedimentary succession in the San Bartolo area and atacamite, although occupying a complex paragenetic position, does appear on textural grounds to be the earliest copper phase in many sandstones.

A possible mechanism of copper transport in this setting could be via ferric oxide grain coatings (Holmes et al., 1983), derived through diagenetic breakdown of copper-rich detrital minerals (e.g., au- gites and hornblendes), although such phases were probably uncommon in the granodioritic provenance areas of the Artolla sediments. Furthermore, because of the low metal concentrations in such minerals, a large volume of sediment is needed for scavenging by chloride solutions. Holmes et al. (1983) argued that the redistribution of metals during red-bed dia- genesis could provide an adequate source of copper to form ore deposits. In their example, a 100-m thick- ness of the Sherwood Sandstone Group of central England, having an areal extent of 3.4 X 109 m e would, assuming an average copper mobilization of 1 ppm, yield over 800,000 metric tons of copper metal.

An alternative model could involve the erosion of

large areas of low-grade bed-rock mineralization. The petrography of the host sands and the lead isotope data fit with the erosion of a calc-alkaline volcano-

plutonic complex. If this source area carried por- phyry-style low-grade mineralization, then such ma- terial would be reflected at an appropriate level in the unroofing sequence. At San Bartolo there is, how- ever, no evidence ofmineralized clasts or detrital sul- fides. Possibly the relatively distal sedimentological setting would make the continued survival of such material unlikely, although detrital bornite grains have been reported in graywackes of the Zambian copperbelt (Binda, 1975). This model would explain the apparent stratigraphic and sedimentological con- trols on mineralization with copper found only in

sandstones and only over a restricted stratigraphic in- terval, coincident with the unroofing of a favorable source area.

The paragenetic position of galena is interesting in this discussion. In the first model, where copper is released from detrital ferromagnesian minerals during arid weathering, lead, which is normally present in resistant alkali feldspars, is released later during deeper weathering (Bjorlykke and Thorpe, 1982). This greater resistance to weathering of alkali feld- spars compared to ferromagnesian minerals has al- ready been noted in the Artolla sediments. This pro- cess should result in lead being either lost from the system due to its greater solubility than copper in terrestrial environments (which may be why lead val- ues are commonly uneconomic in this class of deposit) or precipitated as paragenetically late galena. The galenas at San Bartolo appear to be both precopper (found in early evaporitic cements) and postcopper (as small interstitial grains to blue copper sulfides).

A third alternative model could involve the trans-

port of metals in solution the entire distance from the crystalline source area to the San Pedro basin. A mod- ern example of this process is found in the La Exotica deposit near Calama (Fig. 1) where chrysocolla-ce- mented gravels were mined for some years near the giant porphyry deposit of Chuquicamata (Munoz, 1975). The chrysocolla is thought to have been pre- cipitated from copper-bearing ground water derived through weathering of the adjacent porphyry (New- berg, 1967). A similar process could have supplied copper in the San Pedro basin. However, when the relative sizes of Chuquicamata and Exotica are con- sidered, it is questionable whether this process alone could transport copper in solution for the much greater distances to form the much higher grade ores at San Bartolo. It is the author's view that some form

of particulate transport, as discussed above, is nec- essary to introduce the metals into the basin. Fluid transport is certainly an important process during the later intrabasinal ground-water circulation.

Sulfur isotope studies

Sulfur isotope studies were carried out on copper sulfides and evaporite minerals in an attempt to elu- cidate the mechanism of sulfide fixation in the sand- stones of the Artolla Member and to test the theory that the evaporites provided a sulfur source for later diagenetic sulfide fixation. Samples were obtained by a combination of microdrilling and crushing to -150 mesh followed by heavy liquid separation, in a manner similar to that for lead isotope studies. Sulfur was ex- tracted from the samples for analysis as SO2. Sulfides were oxidized at 1,070øC with cuprous oxide (Rob- inson and Kusakabe, 1975) and sulfates were prepared for analysis as outlined by Coleman and More (1978).

ORE GEHESIS COHTROLS, SAH BARTOLO DEPOSIT 775

Measurements were carried out on a V. G. Micromass

602C gas-sourced mass spectrometer fitted with a heated inlet system to avoid machine isotopic frac- tionations. All gases were run against internal stan- dards and the data were corrected for instrumental

and isotopic effects (Coleman, 1980). Results for copper sulfides (Table 2) show a spread,

but all are enriched in the light isotope. A wide range of values would be expected from biogenic reduction of sulfur, while the isotopically light signatures would be consistent with reduction under conditions of un-

limited sulfur supply. There is minimal variation in isotopic values between primary chalcopyrite (-33.788%0) and its chalcocite group oxidation prod- ucts (-33.527 to -33.786%0) in the veinlets, but ma- trix sulfides do appear to show generally heavier val- ues (-12.634 to -27.603%0). A possible reason for the heavier values in these early matrix sulfides could be the limited amount of sulfate available in preex- isting intergranular gypsum and celestite cements. With limited sulfate available, the system could shift toward closed system fractionation which is known to result in the production of progressively heavier values as more sulfate is used up (Rye and Ohmoto, 1974). The veinlets of ore minerals commonly replace evaporite minerals deposited in the tectonic breccias around the Palicaye mines. In these veins the rela- tively large supply of sulfate probably ensured the preservation of an open system, leading to light iso- tope enrichment.

Isotopic results for the sulfate minerals are consis- tent with this model of mineralization. The evaporites are considerably lighter isotopically than those pro- duced by evaporation of normal marine water. The data support the sedimentological interpretation of these deposits as continental playa deposits; sulfur

was derived from mixed sources which could include

evaporites of the older Purilactis Formation.

Mechanism of bleaching and reduction As stated at the beginning of this paper, the ma-

jority of the ores at San Bartolo are not obviously associated with preserved organic material. Nev- ertheless, the sulfur isotope results strongly suggest that the sulfides were precipitated through biogenic sulfate reduction. It is possible, therefore, that bac- teria can exist in sufficient abundance to reduce large quantities of sulfate without the necessary preserva- tion of large volumes of organic substrate. The feeding trails and rare algal laminae preserved as trace fossils in the Artolla sandstones indicate that environmental

conditions on the playa surface-subsurface were not sufficiently hostile to inhibit the development of in- vertebrate (and therefore probably bacterial) life.

Some of the sulfide veinlets at Palicaye are, as dis- cussed above, an infilling of invertebrate burrows, with localization of ore due to the former presence of syngenetic organic material. The elongate stringers and circular nodules of native metal-oxide ore were

probably formed as a response to a reducing microen- vironment surrounding small concentrations of similar syngenetic organic material, as documented on a larger scale in other deposits (Woodward et al., 1974; Ripley et al., 1980). It is difficult, however, to explain the deposition of the greater fraction of the ore by this mechanism. There is no evidence of extensive

syngenetic organic remains and yet extensive bleach- ing and secondary porosity generation have occurred. As outlined above, copper distribution is controlled by sedimentary facies and subsequent diagenesis. The mechanism of bleaching and reduction must therefore involve the movement of a fluid, whose passage was

TAI•LE 2. Sulfur Isotope Compositions of Copper Sulfides and Earlier Evaporite Minerals in the San Bartolo Deposit

Copper sulfides

Sample number Mineral Lithology Del a4S (per mil)

5904 Chalcopyrite Brecciated sandstone -33.788 44497 Digenite Brecciated sandstone -33.527 44620 Digenite-anilite Brecciated sandstone -33.786 45507 Djurleite Brecciated sandstone -29.680 44620 Bornitc Brecciated sandstone -36.483

5894 Digenite-anilite Undisturbed sandstone -27.603 5879 Djurleite Undisturbed sandstone -23.458

45517 Digenite Undisturbed sandstone - 17.444 5895 Covellite Undisturbed sandstone - 12.634

Preore evaporate minerals

44618 Gypsum Vein in sandstone 2.445 45480 Anhydrite Nodular, bedded 4.572 45459 Celestite Matrix to sandstone 6.589

45543 Gypsum Bedded, after anhydrite 3.413

Veinlets

Matrix

776 s. FLINT

controlled by these host-rock characteristics. The re- mains of a brown, semiopaque organic substance is found in rare specimens (Fig. 8F). This material occurs as a precopper matrix which appears to replace earlier calcite. Textural relationships including rare, cor- roded calcite crystals enclosed within the organic ma- terial indicate that this substance was introduced after

the generation of the calcite-destroying secondary porosity. The paragenetic position of this organic ma- terial, enclosing earlier, largely dissolved matrix ce- ments and following overgrowth formation is re- markably similar to that of similar substances docu- mented in sandstone uranium deposits of the southwestern United States. Bleaching has been used as a field aid to identify uraniferous sandstones in these deposits (Granger et al., 1961). Studies of the host sandstones in the Jackpile and other deposits of the Laguna district, New Mexico (Moench and Schlee, 1967), show that the ore minerals occur as a matrix to detrital and earlier authigenic minerals. The fact that the organic matrix at San Bartolo encloses relict crystals of earlier calcite cement and embays the sil- icates suggests that a similar preore secondary po- rosity generation took place in these host rocks. The corrosive nature of this process points to a fall in pH levels from the relatively alkaline interstitial condi- tions, characteristic of red-bed diagenesis, which would have favored the earlier calcite cementation.

The similarity of the San Bartolo deposit to these uranium occurrences may indicate a related genesis. The porosity generation and localization of ore in the latter case was due to the introduction of acidic, re- ducing ground water. The mobile (epigenetic) re- ductant is considered to be a water-soluble humate

derived from decaying plant matter. Much of this ma- terial remains as an insoluble residue in the uranifer-

ous sandstones (Moench and Schlee, 1967; Turner- Peterson, 1985). As previously stated there are only small amounts of a similar epigenetic organic matrix in the San Bartolo host sandstones. In light of the oth- erwise general similarities with the urahiferous sand- stones, possibly much organic material has been re- moved during later oxidative diagenesis, which is in- dicated by late atacamite and alkali feldspar overgrowths to sulfides.

Furthermore, the origin of dissolved humates has been attributed to a source in overlying boggy ma- terial in the New Mexico example. The modern Salar de Atacama is host to several large areas of vegetated standing water which, during burial, could act as a source for dissolved humates. Thus, although the San Pedro area is believed to have been arid since the Cretaceous, this need not rule out the existence of boggy areas.

Discussion and Conclusions

The San Bartolo copper deposit, although small by the standards of economic red-bed copper occur-

rences, is interesting for several reasons. Firstly, it is at least economically, an oxide orebody, with only minor but genetically important sulfides. Secondly, the oxychloride atacamite represents at least locally a primary phase precipitated from a cupric chloride transport fluid. There are strong sedimentological and diagenetic controls on mineralization, both of which are in accord with a genetic model of detrital copper- bearing minerals derived from a volcanogenic crys- talline source area to the east of the San Pedro basin.

These minerals experienced the same sedimentary processes as other detrital phases deposited as part of the playa detritus of the Artolla Member. This model is further reinforced by lead isotope data.

During later diagenesis of the sedimentary pile, chloride-rich ground water, derived through related diagenesis of the abundant evaporites in the succes- sion leached copper from these disseminated, uneco- nomic copper-bearing detrital minerals and redepos- ited the metal in the Artolla sands where physico- chemical conditions must have been favorable for the

continued passage and concentration of these brines. The result of the above processes was the formation of high-grade deposits. The stability relations of ata- camite (Fig. 9) indicate that deposition is enhanced by medium to low pH values, low temperatures (<50øC), and high Eh. Given a high chloride ion ac- tivity it is possible that this mineral could represent a primary ore mineral (Flint, 1985b).

The fact that the ore deposits at San Bartolo contain little obvious syngenetic organic material is at variance with many documented red-bed deposits where such material provided a localized reducing environment, promoting sulfide 'precipitation (Gustafson and Wil- liams, 1981). The deposit shares many common fea- tures with some sandstone uranium deposits where a reducing environment was produced through the in- troduction of epigenetic, dissolved humates. In this model the epigenetically introduced organic matter would act as a substrate for sulfate-reducing bacteria. These organisms can survive in lithified sediments at temperatures of over 90øC (B. Spiro, pers. commun., 1985), which are similar to temperatures indicated by the diagenetic mineral assemblages present in the Artolla host rocks. The evaporite sulfate could have provided a sulfur source for the subsequent biogenic reduction to sulfide, which would then combine with the copper ions in the intrastratal chloride solutions to form the observed copper-bearing sulfides.

The paragenetic relationship among other phases is more difficult to explain, particularly the along- strike passage from matrix oxide and native metal into sulfides. Whereas much of the cuprite was originally native copper, it is difficult to accept on the rare ev- idence of sulfides overgrown by cuprite that all the mineralization was originally sulfide. Furthermore, if the deposit was originally sulfide then relict sulfides should be present in fresh underground material from

ORE GENESIS CONTROLS, SAN BARTOLO DEPOSIT 777

the Artolla mine, especially as these minerals are found in weathered outcrop in the Palicaye area. Copper sulfides are preserved in other deposits sit- uated in similar arid, highly oxidizing environments (Woodward et al., 1974; Ripley et al., 1980).

Differing intrastratal conditions along strike pos- sibly resulted in the precipitation of native copper at Artolla and sulfides at Palicaye. These differences could have been in Eh-pH relationships with lower pH values favoring sulfide precipitation. At constant temperature and Eh conditions, the precipitation of native copper depends mainly on pH (Fig. 9). An added factor could have been local variations in the

supply of sulfur. A rise in Eh can also promote the preferential precipitation of native copper.

The fracture-filling style of mineralization at the head of Quebrada Palicaye clearly represents a sep- arate phase of mineralization. Its discordant nature indicates that this event postdates the matrix miner- alization. The paucity of gangue minerals, simple ore mineralogy, and lead and sulfur isotope signatures ar- gue against a separate hydrothermal event. This min- eralization is confined to an area of structural com-

plexity in vertically bedded sandstones near the junc- tion with the underlying Palicaye Member. This later folding and related thrusting produced the brecciation and possibly caused a remobilization of preexisting matrix ores, resulting in the described suite of sulfides and native copper. Structural remobilization of ores has been documented in several deposits including the geologically similar Corocoro native copper dis- trict where high-grade pods of sulfides are found in the limbs of later folds (Ljunggren and Meyer, 1964).

It is concluded that the San Bartolo copper deposits represent a possible subclass of true red-bed copper occurrences, which have characteristics of the more important sediment-hosted, stratiform deposits (where ores are associated with but are not hosted by the red beds) and sandstone uranium orebodies, in which metallogenesis is commonly linked to the pres- ence of a mobile reductant. This study highlights the necessity of an integrated sedimentological and geo- chemical approach to sediment-hosted copper de- posits. Of particular importance is the need for careful study of host-rock diagenesis which may allow more precise constraints to be placed on timing and con- ditions of mineralization.

Acknowledgments

I would like to thank Harry Clemmey and Peter Turner for help during fieldwork. Graham Walls of Alfred H. Knight International Ltd., Antofagasta, kindly undertook assay work on some samples. Thanks are due to Gordon Stanley and the staff of the Pacific Steam Navigation Company in Liverpool and Anto- fagasta for valued logistical help. The staff of the Sta- ble Isotope Unit, British Geological Survey, are thanked for training in and help with sulfur isotope

analyses. Lead isotope work was carried out at Leeds with the help and advice of Bob Cliff and Jon David- son. The original manuscript was improved greatly by the critical comments of the reviewers and the editorial board of Economic Geology. This paper was written during tenure of a Natural Environment Re- search Council research studentship.

November 29, 1984; June 4, 1985

REFERENCES

Adams, S. S., Curtis, H. S., Hafen, P. L., and Salek-Nejad, H., 1978, Interpretation of postdepositional processes related to the formation and destruction of the Jackpile-Paguate uranium deposit, northwest New Mexico: ECON. GEOL., v. 73, p. 1635- 1654.

Aoyaki, K., and Asakawa, T., 1984, Paleotemperature analysis by authigenic minerals and its application to petroleum exploration: Am. Assoc. Petroleum Geologists Bull., v. 68, p. 902-913.

Barriero, B. A., and Clark, A. H., 1984, Lead isotopic evidence for evolutionary changes in magma-crust interaction, central Andes, southern Peru: Earth Planet Sci. Letters, v. 69, p. 430- 442.

Binda, P. L., 1975, Detrital bornite grains in the late Precambrian B graywacke of Mufulira, Zambia: Mineralium Deposita, v. 10, p. 101-107.

Bjorlykke, A., and Thorpe, R. I., 1982, The source of lead in the Osen sandstone lead deposit on the Baltic shield, Norway: ECON. GEOL., v. 77, p. 430-440.

Brown, A. C., 1971, Zoning in the White Pine copper deposit, Ontonogan County, Michigan: ECON. GEOL., v. 66, p. 543- 573.

-- 1981, Stratiform copper deposits and pene-exhalative en- vironments labs. I: Geol. Soc. America Abstracts with Programs, v. 13, p. 418.

-- 1984, Alternative sources of metals for stratiform copper deposits: Precambrian Research, v. 25, p. 61-74.

Bruggen, J., 1934, Grundzuge der Geologie und logerstatten- kunde Chiles: Mathematisch-Naturwissenschaftlichen Klasse de

Heidelberger Akademie der Wissenschaften, 362 p. Coira, B., Davidson, J., Mpodozis, C., and Ramos, V., 1982, Tec-

tonic and magmatic evolution of the Andes of northern Argen- tina and Chile: Earth-Sci. Rev., v. 18, p. 303-332.

Coleman, M. L., 1980, Corrections for mass spectrometric analysis of SO•: London, Inst. Geol. Sci. Isotope Geology Unit, unpub. stable isotope report 45, 22 p.

Coleman, M. L., and More, M.P., 1978, A method for the direct reduction of sulfates to SO• for isotopic analysis: Analytica Chemica, v. 50, p. 1594-1595.

Davidson, C. F., 1965, A possible mode of origin of strata-bound copper ores: ECON. GEOL., v. 60, p. 942-954.

Dingman, R. J., 1965, Pliocene age of the ash-flow deposits of the San Pedro area, Chile: U.S. Geol. Survey Prof. Paper 525C, p. 63-67.

-- 1967, Geology and groundwater resources of the Salar de Atacama, Antofagasta Province, Chile: U.S. Geol. Survey Bull. 1219, 49 p.

Doe, B. R., and Delevaux, M. H., 1972, Source of lead in southeast Missouri galena ores: ECON. GEOL., v. 67, p. 409-425.

Doe, B. R., and Stacey, J. S., 1974, The application of lead isotopes to the problems of ore genesis and ore prospect evaluation: A review: ECON. GEOL., v. 69, p. 757-776.

Entwistle, L. P., and Gouin, L. O., 1955, The chalcocite-ore de- posits at Corocoro, Bolivia: ECON. GEOL., v. 50, p. 555-570.

Eugster, H. P., 1985, Oil shales, evaporites and ore deposits: Geochim. et Cosmochim. Acta, v. 49, p. 619-635.

Farrar, E., Clark, A. H., Haynes, S. T., and Quirt, G. S., 1970,

7 7 8 $. FLINT

K-Ar evidence for the post-Paleozoic migration of granitic in- trusions foci in the Andes of northern Chile: Earth Planet. Sci.

Letters, v. 10, p. 60-66. Flint, S., 1985a, Alluvial fan and playa sedimentation in an Andean

arid, closed basin: The Pacencia Group (mid Tertiary) of north- ern Chile: Geol. Soc. London Jour., v. 142, p. 533-546.

-- 1985b, On the paragenetic position of atacamite in low temperature red bed copper deposits of northern Chile: Zen- tralblatt Geologie Palaontologie (in press).

-- 1985c, The sedimentology, diagenesis and copper miner- alisation of continental sediments in the central Andes: Unpub. Ph.D. thesis, Univ. Leeds, 403 p.

Flint, S., Clemmey, H., and Turner, P., 1986, Conglomerate- hosted copper mineralization in Andean unroofing molasse: The Coloso Formation (lower Cretaceous) of northern Chile: Geol. Mag., (in press).

Goble, R. J., and Smith, D. G. W., 1973, Electron microprobe investigation of copper sulfides in the Precambrian Lewis Series ofS. W. Alberta, Canada: Canadian Mineralogist, v. 12, p. 95- 103.

Granger, H. C., Santos, E. S., Dean, B. G., and Moore, F. G., 1961, Sandstone-type uranium deposits at Ambrosia Lake, New Mexico--an interim report: ECON. GEOL., v. 56, p. 1179-1209.

Guest, J. E., 1969, Upper Tertiary ignimbrites in the Andean cor- dillera of part of the Antofagasta province, northern Chile: Geol. Soc. America Bull., v. 80, p. 337-345.

Gustafson, L. B., and Williams, N., 1981, Sediment-hosted strat- iform deposits of copper, lead, and zinc: ECON. GEOL. 75TH ANNIV. VOL., p. 179-213.

Harrington, H., 1961, Geology of parts of Antofagasta and Atacama provinces, north Chile: Am. Assoc. Petroleum Geologists Bull., v. 45, p. 169-197.

Hollingsworth, S. E., and Rutland, R. W. R., 1968, Studies of Andean uplift Part 1--Post Cretaceous evolution of the San Bartolo area, north Chile: Geol. Jour., v. 6, p. 49-62.

Holmes, I., Chambers, A.D., Ixer, R. A., Turner, P., and Vaughan, D. J., 1983, Diagenetic processes and the mineralization in the Triassic of central England: Mineralium Deposita, v. 18, p. 365- 377.

Ixer, R. A., and Vaughan, D. J., 1982, The primary ore mineralogy of the Alderley Edge deposit, Cheshire: Mineralog. Mag., v. 46, p. 485-492.

James, D. E., 1971, Plate tectonic model for the evolution of the central Andes: Geol. Soc. America Bull., v. 82, p. 3325-3346.

Ljunggren, P., and Meyer, H. C., 1964, The copper mineralization in the Corocoro basin, Bolivia: ECON. GEOL., v. 59, p. 110- 125.

McCauley, J. F., 1961, Uranium in Pennsylvania: Pennsylvania Geol. Survey 4th Circ. Bull. M43, 71 p.

McNutt, R. H., Clark, A. H., and Zentilli, M., 1979, Lead isotopic compositions of Andean igneous rocks, latitudes 26 ø to 29 ø south: Petrologic and metallogenic implications: E½ON. GEOL., v. 74, p. 827-837.

Moench, R. H., and Schlee, J. S., 1967, Geology and uranium deposits of the Laguna district, New Mexico: U.S. Geol. Survey Prof. Paper 519, 117 p.

Munoz, J., 1975, On stratiform copper deposits of Chile: Geol. Soc. Belgique Annales, v. 98, p. 17-21.

Newberg, D. W., 1967, Geochemical implications of chrysocolla- bearing gravels: ECON. GEOL., v. 62, p. 932-956.

Ocola, L. G., and Meyer, R. P., 1973, Crustal structure from the Pacific basin to the Brazilian shield between 12 ø and 30 ø south

latitude: Geol. Soc. America Bull., v. 84, p. 3387-3404.

Peters, E., 1977, Direct leaching of sulfides: Chemistry and ap- plications: Metallurg. Trans., v. 7b, p. 505-517.

Pilger, Jr., R. H., 1984, Cenozoic plate kinematics, subduction and magmatism: South American Andes: Geol. Soc. London Jour., v. 141, p. 793-802.

Potter, R. W., 1977, An electrochemical investigation of the sys- tem copper-sulfur: ECON. GEOL., v. 72, p. 1524-1542.

Renfro, A. R., 1974, Genesis of the evaporite-associated stratiform metalliferous deposits--a sabkha process: E½ON. GEOL., v. 69, p. 33-45.

Rickard, D. T., 1971, The chemistry of copper in natural aqueous solutions: Stockholm Contr. Geology, v. 23, 64 p.

Ripley, E. M., Lambert, M. W., and Berendsen, P., 1980, Min- eralogy and paragenesis of red-bed copper mineralization in the lower Permian of south central Kansas: E½ON. GEOL., v. 75, p. 722-729.

Robinson, B. W., and Kusakabe, M., 1975, Quantitative prepa- ration of SO2 for 34S/32S analysis from sulphides by combustion with cuprous oxide: Analytica Chemica, v. 47, p. 1179-1181.

Rose, A. W., 1976, The effect of cuprous chloride complexes in the origin of red-bed copper and related deposits: E½ON. GEOL., v. 71, p. 1036-1048.

Roseboom Jr., E. H., 1966, An investigation of the system Cu-S and some natural copper sulfides between 25øC and 700øC: ECON. GEOL., v. 61, p. 641-672.

Ruxton, P., 1981, The sedimentology and diagenesis of copper bearing rocks of the southern margin of the Damaran orogenic belt, Namibia and Botswana: Unpub. Ph.D. thesis, Univ. Leeds, 241 p.

Rye, R. O., and Ohmoto, H., 1974, Sulfur and carbon isotopes and ore genesis: A review: ECON. GEOL., v. 69, p. 826-842.

Sangster, D. F., 1976, Sulphur and lead isotopes in strata-bound deposits, in Wolf, K. H., ed. Handbook of stratiform and strata- bound ore deposits: Amsterdam, Elsevier, v. 2, p. 219-266.

Sawkins, F. J., 1984, Metal deposits in relation to plate tectonics: Berlin, Springer-Verlag, 325 p.

Sillitoe, R. H., and Clark, A. H., 1969, Copper and copper-iron sulfides as the initial products of supergene oxidation, Copiapo mining district, northern Chile: Am. Mineralogist, v. 54, p. 1684-1710.

Sillitoe, R. H., and Hart, S. R., 1984, Lead-isotopic signatures of porphyry copper deposits in oceanic and continental settings, Colombian Andes: Geochim. et. Cosmochim. Acta, v. 48, p. 2135-2142.

Stauder, W., 1975, Subduction of the Nazca plate under Peru as evidenced by focal mechanisms and by seismicity: Jour. Geo- phys. Research, v. 80, p. 1053-1064.

Travisany, V., 1978, Geologia del distrito cuprifero San Bartolo: Unpub. thesis, Santiago de Chile, Inst. Inv. Geologia, 69 p.

Turner, P., 1980, Continental red beds: Developments in Sedi- mentology, no. 29. 562 p.

Turner-Peterson, C. E., 1985, Lacustrine humate model for pri- mary uranium ore deposits, Grants uranium district, New Mex- ico: Am. Assoc. Petroleum Geologists Bull., v. 69, p. 1999- 2020.

Walker, T. R., Waugh, B., and Crone, A. J., 1978, Diagenesis in first-cycle desert alluvium of Cenozoic age, south-western United States and northeastern Mexico: Geol. Soc. America

Bull., v. 89, p. 19-32. Woodward, L. A., Kaufman, W. H., Schumacher, O. L., and Tal-

bott, L. W., 1974, Strata-bound copper deposits in Triassic sandstones of Sierra Nacimiento, New Mexico: ECON. GEOL., v. 69, p. 108-120.