deen et al-1994

Economic Geology Vol. 89, 1994, pp. 1924-1938

The Magmatic Hydrothermal System at Julcani, Peru' Evidence from Fluid Inclusions and Hydrogen and Oxygen Isotopes

JEFFREY A. DEIGN, Department of Geological Sciences, University of Colorado-Boulder, Boulder, Colorado 80309

ROBERT O. RYE, U.S. Geological Survey, Box 25046, Mail Stop 963, Denver Federal Center, Denver, Colorado 80225

JAMES L. MUNOZ, AND JOHN W. DREXLt Department of Geological Sciences, University of Colorado-Boulder, Boulder, Colorado 80309

Abstract

The Julcani district consists of at least six mineralized centers in a 16-km 2 late Miocene, dacitic to rhyolitic dome complex that is built on a 3,000-m-thick Paleozoic-Mesozoic clastic sedimentary section. Mineralization occurred within 0.5 m.y. of the youngest dome formation and was prior to emplacement of an anhydrite-bearing dacite dike. Hydrothermal events began with formation of fracture-controlled acid sulfate alteration zones having vuggy silica cores and successive quartz-alunite and quartz-kaolinite envelopes. The acid sulfate alteration in the center of the district was overlapped and followed by the emplacement of radiating swarms of tourmaline-pyrite breccia dikelets cemented by quartz-containing saline fluid inclusions. Several centers of mineralization developed 0.5 to 2 km apart along reacti- vated regional structures after the emplacement of a dacite dike most likely related to an underlying intrusion. The ores consist of Ag-Cu-Pb-W-Bi-Au-bearing mineral assemblages that occur as fracture fillings within the domes and underlying tuffs. Mineralization reached within 200 to 300 m of the paleosurface. The ores are strongly zoned with a general succes- sion of quartz + pyrite + wolfamite-, enargite-, tetrahedrite-, galena-, and barite-dominant assemblages grading outward east and west from the central zone. Late-stage siderite and/or botryoidal pyrite overprint all zones, even at the deepest levels.

Filling temperatures and salinities for inclusion fluids in main-stage minerals range from 250 to 325C and 8 and 19 wt percent NaC1 equiv, respectively. Filling temperatures for inclusions in late-stage siderite range from 170 to 225C and salinities average about 7.5 wt percent NaC1 equiv. Temperatures tend to correlate with salinities and both tend to decrease in the fluids of successively younger and more distal minerals from wolframite to enargite to siderite.

The calculated bDH o and bso H o values of the preore alunite-forming fluids are about -46 _+ 5 and 7 _+ i per mi, respectively. The DHo and lsOao values of inclusion fluids in later main-stage wolframite, enargite, tetrahedrite, and galena and late-stage siderite show a linear trend, ranging from -60 to -130 and 4 to -18 per mil, respectively. Based on the composition of biotite phenocyrsts in glassy volcanic rocks that bracket ore deposition, primary magmatic fluids had Dao and Oao values of about -70 _+ 20 and 9.7 _+ 0.6 per mil, respectively.

The fluids responsible for main-stage mineralization as well as preore acid sulfate and tourmaline breccia fluids were distinctly magmatic. These ore fluids, however, were highly exchanged (similar in isotopic composition to alunite-forming fluids) and were not derived directly from the magma but from the low water-rock environment between the fluid-rich carapace of the magma and the brittle-ductile transition of the overlying rocks. Ore deposition occurred when the exchanged magmatic fluids mixed with meteoric water at higher levels.

In Memoriam

Jeffery A. Deen, a budding economic geologist, died in an automobile accident near Cerro de Pasco, Peru, on July 13, 1994. Jeff was born December 3, 1957, in Oakland, California, to Dr. and Mrs. Robert Deen. He received a B.S. in 1980 from the University of California, Santa Cruz; he obtained both an M.S. (1987) and a Ph.D. (1990) from the University of Colorado, Boulder. Jeff

0361-0128/94/1646/1924-1554.00 1924

MAGMATIC HYDROTHERMAL SYSTEM, JULCANI, PERU 1925

had returned to field-oriented studies, in the country he loved, to work on regional exploration for Rio Tinto Zinc. Jeff will be greatly missed by family and friends.

Introduction

THE Julcani mining district of Peru is located near the crest of the central Andean Cordillera, approxi- mately 200 km southeast of Lima (Fig. 1). Production from the district began in Spanish colonial times when free-milling gold was exploited from near-surface quartz-wolframite veins. Since the 1940s, the district has become well known for production of bis- muth, silver, copper, lead, zinc, and tungsten. At present, the principal production is silver, with by-prod- ucts of copper, gold, lead, and zinc. During the 1980s production of silver ore averaged 700 tons per day with an average grade of 20 oz per ton.

Previous studies of the Julcani district have con- centrated on the geology of the district, mineralogy of the veins and associated alterated rocks, elemental zoning in the veins, and mineral chemistry of the igneous rocks (Goodell, 1970; Goodell and Petersen, 1974; Petersen et al., 1977; Scherkenbach, 1978; Wilson, 1979; Shelnutt, 1980; Drexler, 1982; Bene- vides, 1983; Scherkenbach and Noble, 1984; Drexler and Munoz, 1985; Shelnutt and Noble, 1985). Field and radiometric dating studies at Julcani have shown that economic mineralization was related to a series of hydrothermal events that occurred during the emplacement of a dacitic igneous complex over a span of less than 0.5 m.y. (Noble and Silberman, 1984). This

well-defined sequence of igneous and hydrothermal events provides an exceptional opportunity to study high-level magmatic fluid evolution leading to the formation of a major epithermal ore deposit, a topic explored in this paper.

Fluid inclusion and hydrogen and oxygen isotope studies of the vein material and cogenetic igneous rocks have permitted reconstruction of the magmatic hydrothermal fluid evolution in the district. Our data suggest that not only the preore acid sulfate and tourmaline breccia fluids but also the fluids responsible for main-stage mineralization were distinctly magmatic in origin. These magmatic fluids, however, were highly exchanged; they were not derived directly from a magma but from a low water-rock environment between the fluid-rich carapace of the magma and the brittle-ductile transition zone of the overlying rocks. Ore deposition apparently occurred when the exchanged magmatic fluids mixed with meteoric water at high levels.

Geologic Background A Miocene dacite-rhyodacite volcanic complex

comprises the host rocks for vein mineralization in the Julcani district (Fig. 2). This volcanic complex is underlain by a 3,000-m-thick Paleozoic to Mesozoic miogeoclinal sedimentary sequence consisting of

Guy ( Surinam

CC rench Guiana

Chile

Uruguoy

0 500 Km

FIG. 1. Location map of the Julcani mining district, Peru.

1926 DEEN ET AL.

7450 '

Jp

PAL

6 Kg

Pm

j-lp //

j-p JULCANI

HERMINIA

p

PAL J'p Jp

QIc

Landslide debris, colluvium, and alluvium Q- Andesire and basaltic andesire

Rocks of Julcani volcanic center Volcanic domes and associated lavas Pyroclastic rocks, breccia flows, etc. ...

Rumichaca Group Chulec Formation Goyllarisquizga Group Pucara Group Mitu Group Ambo Group or Excelsior Group PJ-J

FIC. 2. Regional geologic map of the Julcani mining district (from Petersen et al., 1977).

tightly folded shale and phyllite with subordinate quartzite, arenites, lutites, and conglomeratic red beds. Upper Triassic to Cretaceous limestones and sandstones unconformably overlie the miogeoclinal sedimentary sequence. The Miocene volcanic and hydrothermal centers at Julcani are related spatially to the trends of the Lircay (northwest-southeast) and Tucclla (north-south) faults (Petersen et al., 1977; Shelnutt and Noble, 1985). Most major vein sets of the district coincide with the Lircay fault trend.

Volcanic and hydrothermal stages Five distinct volcanic and four interspersed hydro-

thermal stages have been identified in the Julcani district (Fig. 3). Their chronology is based on the work of Petersen et al. (1977) and Noble and Silberman (1984).

Volcanic activity (stage I) began with the eruption of dacitic pyroclastics, breccias, and base surges from a central vent near the center of the district, at 10.1

Ma. This stage was followed by the emplacement of numerous, dacite-rhyolite domes and flows, which produced the 16-km 2 dome field (stage II) that hosts the Julcani ores. Following this stage, activity was related to preore acid sulfate alteration and, subsequently, partly to intrusion of quartz-pyrite-tourmaline breccia dikes near the center of the district. De- position of main-stage hydrothermal minerals in the veins, which followed the emplacement of the Tenta- dora dike (stage III), was interrupted at 9.7 Ma by emplacement of the anhydrite-bearing Bulolo dike (stage IV). Deposition of late-stage hydrothermal min- erMs in the veins and later collapse of the system followed. The final igneous event at the Julcani complex occurred at 7.0 Ma, with emplacement of solitary rhyolite domes (stage V) on the northern edge of the district.

The location of major mineralized veins and outline of preore altered rocks in the district are shown in Figure 4. The following description of the hydrothermal events and their districtwide time-space relationships are based on field and laboratory observa- tions by Goodell (1970), Noble (1978), Scherken- bach (1978), Shelnutt (1980), Benavides (1983), and Deen (1990).

Fro. 3. Generalized geologic map of the Julcani mining district showing igneous stages and outline of individual domes (modified from Petersen et al., 1977). The central vent for the pyroelastic eruptions coincides with the large dome in the center of the map (note that not all domes are labeled). Note the dome alignment is in a north-south direction and also coincident with the A-A' section. The A-A' section is shown in Figure 5. Dashed rectangle = enlarged area of Figure 4, MzPz = Mesozoic-Paleozoic sedimentary rocks. The white area surrounding the domes consists of short flows and antobrecciated volcanic material.


MzPz

Acid sulfate

Limit of breccia dikes

FIC. 4. Enlarged area of the Julcani mining district outlined in Figure 3 showing pre- and postore dikes, outlines of area of preore radial breccia dikes and acid sulfate alteration, and location of major deposits. Symbols same as in Figure 3.

Preore acid sulfate alteration The first recognized hydrothermal event in the dis-

trict resulted in acid sulfate alteration covering an area of about 2 km 2 in the central portion of the district (Fig. 4). This alteration is predominantly con- fined to major veins, faults, fractures, and arcuate zones within domes and flows. In the last case, the alteration followed dome foliation features related to eruptive emplacement.

Mineral assemblages in rocks affected by the acid sulfate alteration are similar to but less well developed than those at Summitville, Colorado (Stoffre- gen, 1987; Rye et al., 1990). The vertical extent of acid sulfate alteration approaches 600 m with zones

widening away from structures toward the surface (Deen, 1990). A typical high-level acid sulfate alteration zone displays a vuggy silica core of extreme acid leaching surrounded by an alunite + quartz + pyrite and kaolinite + quartz + pyrite envelope that grades outward into propylitically altered rocks. Preore tourmaline q- quartz q- pyrite breccia dikes

Discontinuous, tourmaline-quartz-pyrite-bearing breccia dikes generally cut the acid sulfate alteration assemblages, although in places mutual crosscutting relationships are exhibited (Deen, 1990). The high- est concentration of breccia dikes, covering an area of about 3 km , is observed in the vicinity of the Ten- tadora mine, coincident with the postulated vent for the stage I pyroclastic eruptions in the district (Peter- sen et al., 1977; Shelnutt and Noble, 1985).

Alteration associated with breccia dike emplacement involved intense silicification and formation of pyrite and tourmaline. Later, sericite and carbonate alteration associated with main-stage mineralization affected some portions of the breccia dikes (Bena- vides, 1983; Shelnutt and Noble, 1985). Main- and late-stage vein assemblages

Seven mines (Mimosa, Estela, Tentadora, Lucrecia, Herminia, Nueva Herminia, and Manto), hosted al- most entirely within stage II dacitic domes (Fig. 4), have produced Ag-Cu-Pb-W-Bi-Au-bearing ore from fracture-filling veins. A full description and detailed vein mineralogy are given by Benavides (1983).

Although differences in mineral assemblages occur between the various mines, a generalized time-space mineral sequence can be identified for the district (Fig. 5). The ores are strongly zoned with a succes- sion of quartz q- pyrite q- wolframite-, enargite-, tetrahedrite-, galena-, and barite-dominant assemblages grading outward east and west from the central zone at the Tentadora and Estela mines. Overall, there is a mushroom-shaped distribution of mineral zones (Fig. 5). Presumably, this zoning reflects in part the lateral

A A

FIc. 5. Cross section of the Julcani mining district at the time of ore deposition showing projected location of orebodies and districtwide mineral zoning. Geologic units are the same as in Figure 3. Shaded patterns = stoped areas of mines discussed in the text. Abbreviations: En = enargite, Gal = galena, Py = pyrite, Qtz = quartz, Tet = tetrahedrite, Wo = wolframite. Gold is in the quartz + pyrite + wolframite zone. Most of the silver in the district is mined from the tetrahedrite zone.

1928 DEENETAL.

movement of fluids in the district, as suggested by the elemental zoning studies of Goodell (1970) and Good- ell and Petersen (1974). Their data indicated that a large portion of the ore fluids in the district origi- nated in the Tentadora-Estela region; these flowed laterally through the Lucrecia and Hermina mine areas a distance of 2.5 kin. Deen (1990) suggests that mineralization at the Mimosa mine area, in the north- east part of the district, was probably associated with a separate hydrothermal center.

The outer mineral assemblages were precipitated on inner assemblages in the later mineral stages throughout the district. The late-stage mineral assemblages are similar to the distal main-stage assemblages, with the addition ofbotryoidal pyrite and barite that are present throughout the district. Wall-rock alteration associated with mineralization

The districtwide alteration associated with mineralization (distinct from the central acid sulfate alteration) around the veins at Julcani resulted in a sequence of prograding mineral assemblages similar to the classic butte-type alteration pattern. Presumably, as each stage of alteration in vein wall rock developed, earliei' formed zones were displaced outward (Meyer and Hemley, 1967). The alteration mineral assemblages are zoned around the district center-- the Tentadora and Estela mines--where advanced argillic assemblages are well developed. At the Mi- mosa and Nueva Herminia mine areas, sericite is the principal alteration mineral. At the outer limits of the district propylitic assemblages are dominant in the wall rock.

Fluid Inclusion Studies

Results of fluid inclusion studies of main- and late- stage minerals

The following minerals contained primary fluid inclusions that were suitable for study: quartz, apatite, and wolframite in proximal (or early) zones, enargite (or luzonite) and quartz in intermediate zones; and siderite in distal (or late-stage) zones. Fluid inclusion data are summarized in Table 1 and plotted in Fig- ures 6 and 7. The majority of primary inclusions in main-stage minerals at Julcani can be classified as liq- uid-rich inclusions with a vapor bubble (Roedder, 1984). Halite and sylvite daughter minerals have not been observed, but in some cases small greenish bire- fringent prisms (barite?) are present in fluid inclusions in quartz.

In the center of the district, homogenization temperatures and salinities of primary inclusions in early wolframite from Tentadora cluster around 320 _+ 5C and 15 wt percent NaC1 equiv, respectively. In- clusions in cores of coexisting quartz crystals have filling temperatures averaging about 335C; salini-

ties of such inclusions were unmeasurable because of their small size. Homogenization temperatures and salinities of primary inclusions in early apatite from the Estela mine average about 300C and 14 wt percent NaC1 equiv, respectively.

Fluid inclusions in enargite from intermediate zones at the Herminia mine were examined by means of an infrared microscope (Campbell et al., 1984a; Campbell and Robinson-Cook, 1987). Rare, nega- tive-crystal inclusions were present, but most were irregular planar inclusions that appear to have been trapped on the ( 010) or ( 011 ) crystal faces and may be secondary. Homogenization temperatures from both types of inclusions in enargite mostly range from 240 to 280C, with some as low as 200C. Salinities range from 8 to 19 wt percent NaC1 equiv. Interme- diate-stage quartz from the Tentadora mine varies from well-terminated crystals to polyhedral aggre- gates to chalcedonic bands. Homogenization temperatures of inclusions from intermediate- and late-stage quartz ranged from 245 to about 300C; salinities of inclusions in quartz range from 2.1 to 11.5 wt percent NaC1 equiv.

Filling temperatures and salinities for the inclusion fluids in late-stage siderite from the Herminia mine varied from 178 to 222C and from 7.2 to 7.7 wt percent NaC1 equiv, respectively.

A histogram (Fig. 6) indicates that the salinities for the inclusions in proximal (earlier) minerals are generally higher than those in distal (later) minerals. This trend holds even for minerals within a given mineral zone. The enargite samples cover the entire enargite zone of the Herminia mine. Innermost enargites near the center of the district have generally higher salinities (10-19 wt % NaC1 equiv) than their distal coun- terparts (8-13 wt % NaC1 equiv). Interpretation of fluid inclusion data

Paleogeographic reconstructions (Noble, 1978) based on morphological features of volcanic domes suggest that the present-day surface is only 200 to 300 m below the surface that existed during mineralization. If one assumes 250 m of cover above the present surface at Tentadora, fluids present in inclusions on the 420-m mine level were trapped 620 m below the original surface. If fractures were open to the surface, the 620-m depth corresponds to a hydrostatic pressure of 61 bars. Under these conditions, an aqueous fluid with 13 wt percent salinity will boil at ,280C (Haas, 1976).

At times local pressure in the hydrothermal system appears to have exceeded hydrostatic pressure. Fluid inclusions in quartz and wolframite from the 250-m level at the Tentendora mine (with 17 wt % NaC1 equiv) homogenized upon heating between 330 _ 10C, which is higher than boiling temperatures pre- dicted for hydrostatic conditions. At Tentadora the

MAGMATIC HYDROTHERMAL SYSTEM, JULCANI, PERU 199,9

T,BL 1. Heating and Freezing Determinations from Primary Fluid Inclusions

Vein Sample location Mineral Chip T T Salinity 4 Hermina 270 V.4NW Quartz 3 186

186 192 192 194

Hermina 270 V.4NW Quartz 2 194 2OO 212 214

Hermina 270 V.4NW Quartz 1 225 230 230

** -1.6 2.7 ** -2.8 4.6 ** -2.9 4.8

Herminia 420 Docentia R.444 Quartz i 228 236 248

Herminia 420 Docentia R.444 Quartz 2A 204 -3.1 5.1 230 -4.0 6.4 237 -4.1 6.6

Herminia 420 Docentia R.444 Quartz 2B 267 294 -4.0 6.4

Herminia 420 Docentia R.444 Quartz 3A 216 216 220 -4.5 7.2 221

Herminia 420 Docentia R.444 Quartz 3B 221 221 235 267 28O

Tentadora 300 V.28 Cumbre 4 Quartz i 260 261 262 276

Tentadora 300 V.28 Cumbre 4 Quartz 2 278 28o 283 284 285

Tentadora 300 V.28 Cumbre 4 Quartz 3 258 262 265 -5.5 8.5 266

Tentadora 300 V.28 Cumbre 4 Quartz 4 266 273 281

** -4.2 6.7 Tentadora 300 V.28 Cumbre 4 Quartz 5 239 -1.1 1.9

239 241 244 245

** -2.1 3.5 Tentadora 300 V.28 Cumbre 4 Quartz 6 245

246 247 252 254 262

Tentadora 300 V.28 Cumbre 4 Quartz 7 272 -7.3 10.8 274 274 282 -7.3 10.8

1930 DEENETAL.

TMI 1. (Cont.)

Vein Sample location Mineral Chip T[ T Salinity 4 Tentadora 300 V.28 Cumbre 4

Mimosa

Mimosa

N. Herminia

N. Herminia

V.7 1/2 San Demietrio 7692

V.7 1/2 San Demietrio 7692

730 V. Bianca G. 70E S

730 V. Bianca G. 70E

Estela 490 Rampa 318

Herminia 460 V. Docentia R. 462

Herminia 460 V. 2NW T.325-24




Quartz 8 252 253 256 267

** -5.5 8.5 Quartz i 266

275 276 276

Quartz 2 221 224 228 228 238 241

Siderite i 204 2O7 214 218 218 219 221 235

Siderite 1 218 218 219 219 220 220 222 223

Apatite i 300 -10.0 14.0 302 303 303 303 303 304 305 303

Siderite 1 179 -4.5 7.2 197 -4.9 7.7 215 220

** -4.5 7.2 ** -4.7 7.4

Quartz 1 240 243 -2.7 4.5 247 249 -2.3 3.9

Quartz 2 219 220 224 -7.6 11.2 228 236

Quartz 3 215 226 229 231

Quartz 4 189 190 194 196 199

MAGMATIC HYDROTHERMAL SYSTEM, JULCANI, PERU

TABr 1. (Cont.)

1931

Vein 1 Sample location Mineral Chip T T Salinity 4

Tentadora

Tentadora

Tentadora

Esteal 530

Tentadora

250 V. Del Medio G. 324E




Tentadora 420 Docentia T470

Tentadora 420 R500

Tentadora 630 Docentia 245W

Quartz i 323 328

Quartz 2 335 335 336 337 338 338 338 339

Quartz 3 297 299 -4.7 7.4 299

** -4.6 7.3 Wolframite 1 .... 7.3 10.9

.... 7.5 11.1

.... 6.7 10.1 287 -8.5 12.3 305 -8.4 12.2

Wolframit e i 318 - 13.2 17.2 322 -12.3 16.3 317 -12.2 16.2 318 -11.5 15.5 321 -9.2 13.1 319 -11.8 15.8 317 -11.8 15.8

* -11.5 15.5 320 -12.6 16.6

Enargite i 201' -6.6 10.0 236 -7.5 11.1 277 -7.0 10.5 243 254 252

** -9.5 13.4 240* -15.4 19.1 251 -15.0 18.8 249 -15.5 19.2 253 -9.7 13.7 254 -9.8 13.8 245 -8.6 12.4

Enargite 1 273 -9.8 13.8 202 -9.2 13.0

** -9.5 13.4 201 -9.8 13.7

Enargite i 245 -7.6 11.2 ** -7.3 10.9

297 -7.7 11.4 ** -5.2 8.1 ** -7.0 10.5

294 -7.6 11.2 247

For vein locations see Peterson et al. (1977) or contact author 2 Temperature (C) ofhomogenization; * = vapor bubble did not return after homogenization, Th noted are for bubble disappearance;

* = not measured; *** = decrepitated a Temperature (C) of freezing 4 Wt percent NaCI equiv as calculated using the equation of Potter et al. (1977)

1932 DEENETAL.

5 10 15 20

wt% equivalent NaCI

I Early Wolframite from inner zones at Tentadora and Estela Enargite from inner zones at Tentadera Enargite from outer zones at Tentadora

Mid to late stage quartz from Tentadora and Herminia Late siderite from outer zones from N. Herminia and Herminia

FIG. 6. Histogram of salinity of primary fluid inclusions in time and space at Julcani. Paragenetically early and proximal fluids are more saline than later, more distal fluids.

large quantities of fine-grained hydrothermal quartz appear to have created a hydrologic seal that could support pressures in excess of hydrostatic. Hydrother- mal shattering of this quartz seal apparently was not common because very little vein breccia is observed.

Fluid inclusion salinities tend to decrease in successively younger and more distal minerals from wol- framitc to enargite to quartz to siderite (Fig. 6). Al- though there is considerable scatter in the data, temperatures also tend to decrease with salinities of the fluids (Fig. 7). This presumed time-space decrease in temperature and salinity of the fluids is consistent with dilution of saline ore fluids with cooler, dilute meteoric waters.

Stable Isotope Studies &D and dSO data of igneous minerals

To minimize the possibility of posteruptive isotopic exchange, measurements were made only on phenocrysts taken from glassy (presumably unaltered) rocks. The Fe-Ti oxide compositions determined by Drexler and Munoz (1985) were used to estimate an average eruptive temperature (860C) for the parent magmas. The stable isotope data are summarized in Tables 2 and 3 and Figures 8 to 10. Figure 8 summa- rizes the bD and bx80 data from biotite phenocrysts in stages I to IV volcanic rocks.

From these data, the isotopic composition of H20 in equilibrium with biotite at 860C was calculated using the biotite-H20 deuterium fractionation curves

20

10

0

15O

o o

[] Wolframite

Enargite Quartz ' ' i .... m .... 1

200 250 300 350

T h C

0 Siderite

FIG. 7. Salinity vs. homogenization temperatures of primary fluid inclusions from the Julcani district. The low homogenization temperatures from a few fluid inclusions in enargite are believed to be due to necking.

of Suzoki and Epstein (1976) and the biotite-HO ]80 fractionation curve of Bottinga and Javoy (1973). The calculated bDu2 o and b18Ou2 o values of these fluids, shown by the hachured box in Figure 8, range from -53 to -89 per mil and 9.6 to 10.1 per mil, respectively. Calculations of b18Oao values in equilibrium with coexisting feldspar and quartz phenocrysts gave similar results (Deen, 1990). Because rocks of igneous stages I and IV bracket ore deposition at Jul- cani, the calculated values (Fig. 8) define the isotopic composition of the fluids that could have exsolved from the hottest magmas during main-stage mineralization. As will be shown subsequently, fluids that exsolved from lower temperature magmas or exchanged with crystalline wall rock had different compositions.

TaSLz 2. The b]SOso, and bD Values orAlunite from the Acid Sulfate Alteration Zones

Vein Location 1808o 4 (%o) bD (%o) Herminia 390 V 14-198 8410 16.9

420 R.340 V. 14-198 #1 16.9 420 R.340 V. 14-198 #5 15.6 -50 420 V. 2NW G 348 17.1 -46 420 V. 2NW. T34W -47 420 V. DOC CR 666 G465 14.4 -47 500 Esperanza 14.9 -56

Lucrecia 300 Bypass 908 16.0 -59 JAD-49 14.6 JAD-50 15.0 JAD-51 14.4 -51

For description of techniques, see Wasserman et al. (1992)


TABs 3. Hydrogen and Oxygen Isotope Data of Fluid Inclusions

Vein Location Mineral tSDHa O (%0) 18OltaO (%0) Estela 520 V. Wo -60 1.4

520 V. Cpy -88 -4.1 530 V. Cpy-Tet -64 0.6 490 V. Py -75 -1.3 490 V. Apa -95

Herminia 360 V2NW Rampa En -72 -1.7 390 V.DOC G.362 (8676) Bar -122 390 V.DOC G.362 (8676) En -93 390 V.DOC G.539 En -63 0.9 390 V.DOC G.539 En -66 420 V.DOC R.500 En-Tet -98 420 V.DOC R.500 T470-20 Bar -122 -16.4 420 V.DOC R.500 T470-20 Bar -127 420 V.T470 En-Tet -63 1.3 420 V2 G340 W Tet -65 1.0 460 V.2NW G351 NW Sid -87 -5.5 460 V.462 T 591-462 Sid -110 500 V.462 T13-14 Bar -136 -18.8 500 V.462 T13-14 Bar -131 500 V.462 T14-13 Bot. Py -129 500 V.462 T14-13 Bot. Py -126 -18.4 500 V.462 T14-13 Tet -67 500 V2NW G397-W Tet -69 -1.1 630 V.DOC 245-30W En-Tet -79 270 T.014-24 (8693) Stib -88 4.9

Mimosa 400 V.HADA T26 Bot. Py -110 -17.8 430 V.NELY G. 708 Tet -82 430 V.NELY T708 Sid -104 460 V.PORV R.611 G.611E Gal -62 3.7 490 V. PORV R.625E Sid -100 530 V.CAST.669 T. 669 Tet -62 570 V.PORV R.605 Cpy -70 1.4 570 V.PORV R620 Sid -129 -17.8 570 V.PORV R605 Tet -70 -0.3 570 V.A G.588 Tet-Qtz -68 3.0 Unknown Bot. Py -126 -18.4

Tentadora 250 V.Del Medio Late Sphal -90 -8.1 250 V.Del Medio Wo -70 -1.5 250 V.Del Medio Wo -67 420 V. Mont Py -112 -8.7 J-78 Wo-Py-Qtz -77

Mineral abbreviations: Apa = apatite, Bar -- barite, Bot. -- botryoidal, Cpy -- chalcopyrite, En -- enargite, = quartz, Sid = siderite, Sphal -- sphalerite, Stib -- stibnite, Tet = tetrahedrite, Wo -- wolframite

Techniques used are summarized in Richardson et al. (1988) Gal -- galena, Py -- pyrite, Qtz

bD and b180 data of alunite in acid sulfate alteration assemblages

The D and 18Oso 4 values (Fig. 8, Table 2) of the magmatic hydrothermal alunites show a narrow range in both values (-46 to -59 and 14.4-17.1%0, respectively). The S isotope data for alunite (Deen, 1990) are typical ofmagmatic hydrothermal environ- ments in which sulfate is derived from the dispropor- tionation of SO2 (Rye et al., 1992; Rye, 1993). Tem- peratures calculated from the sulfur isotope fractionation of six individual pyrite-alunite pairs by means of the equation of Ohmoto and Rye (1979) average 248 _ 15C (Deen 1990). On the basis of these temperatures and the fractionation factors of Stoffregren

et al. (1994) for 180 and deuterium between alu- nitelso4 / and water, the calculated Dio and 18f 'HO values for fluids in equilibrium with the alunites are -41 to -52 per mil and 6.2 to 8.9 per mil, respectively (Fig. 8). It is noteworthy that the compositions of the alunite fluids were more depleted in 180 but enriched in D relative to the primary magmatic fluids (rig. 8). bDno and is Ont values for main- and late-stage fluids

The bDio and b sr values of inclusion fluids in X-HgO main- and late-stage minerals from the Estela, Mi- mosa, Tentadora, and Herminia mines are summa-

1934 DEEN ET AL.

O dl Alunite Y/ I o -40 ' II Biotite / r- _-- I -40

o '"'"' I -leo -120 -120

-10] I -160 -160

-15 -10 -5 0 5 10 15 20

518OH20 FIG. 8. The D and 80 values from biotites in glassy rocks

from different stages of igneous rocks, and the calculated D.2 o and sO.2 o values of fluid derived from their magmas at 860C. The eruptive temperature was derived from the Fe-Ti oxide geo- thermometer. Also shown are the D and 8Oso 4 values of alunite and the calculated Do and 80o values of their fluids at 250C. The 250C temperature was derived from sulfur isotope fractionation of coexisting pyrite alunite pairs (Deen, 1990). For Figures 8-10: MWL = meteoric water line, PMW = projected magmatic water.

rized in Figure 9 and Table 3. The majority of the analyses are from inclusion fluids in sulfides that are not susceptible to 80 exchange with the host mineral. Recent work (R.R. Seal and R.O. Rye, unpub. data) has shown that 80 exchange between fluid inclusion waters and host is generally not significant in quartz samples younger than 20 Ma. It is on this basis that the sOno values of quartz fluids at Julcani can also be determined directly from analyses of inclusion fluids.

i", Pyrite /[3 Wolframite -20 1 B Enargite I O Tetrahedrite -40 /[: :> Chalcopyrite I V Galena ' I>- Sphalerite ' ...l,0,.,t -60 I A Stibnite 14 Siderite I O Barite o -leo

-12o -14o -160 -2 -15 -10 -5 0 5 10 15

18OH20 zc. 9. Du,o and 8Ou, o values of i,clusio, fluids from Jul-

cai mai- ad !ate-stage vei minerals. The data from all of the mies form a linear trend extending oE o the meteoric water lie as indicated by the values [or fluids in late-stage minerals. Symbols: solid = Hermia, faded = Tentadora and stela, open = Mimosa.

b Pyrite ' . X, .. 400oc r Wolframite / ./ ..lu.n,[e "00C [] Enargite / ''/ fluds %1_ Tetrahedrite / .. 600o0 Chalcopyrite 001 '?---o Galena / .... 1/7ou

Stibnite I/ ( Sderite r o O Barite I ; ::....

-2( -15 -10 -5 0 5 10 15 18OH20

FIG. 10. The $D o and $80 o values of main- and late-stage ore fluids and preore alunite fluids. Also shown is the isotopic exchange curve for meteoric water at 400C with vying water/rock ratios, as well as the exchange trajectory for magmatic water with wall rocks at decreasing temperatures of 860 to 400C. The exchange curve for meteoric water and the exchange trajectory for magmatic water were calculated as in Ohmore and Rye (1974) for felsic rocks having compositions compatible with that of the biotites in the glassy rocks at Julcani.

The range in Dn o and 18On o values for main- stage inclusion fluid is from -11 to -60 and -8.7 to 3.7 per mil, respectively. There is considerable overlap of isotopic values in fluids from different mineral phases. Fluids from minerals of the Mimosa mine appear to have somewhat larger lsOno values compared with fluids from appropriate minerals from other mines.

Unlike the isotopic fluid compositions that were calculated from measurements on biotite and alunite, the main-stage and late-stage values are direct determinations of the isotopic composition of the water in the ore fluids. The only uncertainty about these data is the extent to which secondary and pseudosecon- dary inclusions are present in some of the samples, and consequently, the extent to which the data are for a single generation as opposed to multiple genera- tions of fluid inclusion populations.

The Do and lsOno values from fluids extracted from late-stage barite, siderite, and botryoidal pyrite fall on or near the meteoric water line over ranges of -110 to -138 per mil and -16.4 to -18.8 per mil, respectively. These values are close to those determined for present-day mine waters (Deem 1990). The compositions of the main- and late-stage fluids form a continuous field extending from the meteoric water line toward the composition of magmatic fluids.

Discussion

The details of how magmatic fluids evolve into ore- forming fluids in high-level veins depend on many


factors such as the water and volatile contents of the magmas, the rate at which the magmas ascend and cool, the physical and chemical characteristics of the surrounding rocks, the rate of new magma generation, and the nature and magnitude of internal and external stresses. General principles governing magmatic fluid evolution in the epithermal environment have been summarized by Rye (1993). The stable isotope and fluid inclusion data on the preore and ore fluids, and the well-dated sequence of shallow igneous and magmatic hydrothermal events provide firm constraints on the fluid evolution at Julcani.

Isotopic exchange of meteoric water with igneous rock8

Meteoric water can approach the isotopic composition of magmatic water by isotope exchange with high-temperature igneous rocks under conditions of very low water/rock ratios (Ohmoto and Rye, 1974; Campbell et al., 1984b). In Figure 10 a water-rock exchange curve calculated for meteoric water in equilibrium at various water/rock ratios with Julcani dacites at 400C is compared with the data on the isotopic composition of the fluids. The equation used to generate the curve and the limits of its application are discussed by Ohmoto and Rye (1974). The exchange curve shows the resultant isotopic compositions for water equilibrating in a one-pass closed system calculated as a function of differing water/rock ratios and using initial isotopic compositions: Di-o = -130 per mil, lsOi_ o -- -17.5 per mil for watermd

- .'? xs, 8 5 er mll for rock t$Droek = --NZ per mn, o X_roek = . p ' . As shown in Figure 10, only very low effective

water/rock ratios (

1936 DEENETAL.

bly more accurately reflect the composition of magmatic water in equilibrium with rock prior to the emplacement of the breccia dikes. Although the ore fluids were of magmatic origin, exchange with deep crystalline rocks in a low water-rock environment apparently controlled their isotopic compositions. Mixing of exchanged magmatic and meteoric water

It is apparent from Figure 10 that the isotopic compositions of both meteoric water and exchanged magmatic water at Julcani are well defined. Because the measured isotopic compositions of the ore-forming fluids fall between these two extremes on a well-defined linear trend, the simplest interpretation of the data is that much of mineralization was the result of the mixing of meteoric waters with exchanged magmatic fluid. The fluid inclusion evidence that paragenetically early minerals trapped hotter and more saline fluids than their later and more distal counter- parts strongly supports this conclusion.

The Evolution of Magmatic Ore Fluids at Julcani It is now possible to postulate a sequence of

igneous and hydrothermal events leading to mineralization at Julcani. The magmas that intruded below Julcani contained 4 to 6 wt percent H20 (Drexler, 1982), and they probably intersected the melt-vapor saturation curve at about 6 to 7 km (Whitney, 1988). The H20-saturated magmas continued to exsolve fluids as crystallization progressed and the magmas continued to rise until they reached the final level of emplacement about 4 km below the palcosurface. The exsolved magmatic fluids most likely accumu- lated in apophyses to form a fluid-saturated carapace (Burnham, 1979) at the top of the intrusion. Fluids such as these are known to be rich in metals sufficient to form ore deposits (Holland, 1972) and are believed to be the primary fluids responsible for porphyry-type mineralization (Henley and McNabb, 1978; Burnham, 1979).

At about the 400C isotherm in an epithermal environment, the rocks surrounding the fluid-saturated carapace of the intrusion exhibited ductile or quasiplastic behavior (Fournier, 1987, 1989, 1992). This quasiplastic behavior of the rocks between the brittle-ductile transition in the rocks and the fluid-rich carapace of the crystallizing magma would have inhib- ited the escape of fluid from the carapace of the magma. In such a quasiplastic zone, water/rock ratios are low, fluid transfer takes place primarily through diffusion, and fluid residence time can be significant. Under such conditions, isotopic exchange of the magmatic waters with the surrounding rocks (primarily coeval igneous rocks) at submagmatic temperatures would have been likely. Above the brittle-ductile transition, near the 400C isotherm, the fluids would have encountered less than lithostatic fluid pressures

(Fournier, 1987, 1992). Thus, fluids that crossed the brittle-ductile transition into this hydrostatic pressure zone most likely would have boiled, forming a two-phase plume of steam at upper levels and brine at lower levels (Henley and McNabb, 1978; Four- nier, 1987; Giggenbach, 1987; Rye, 1993). Residual fluids that remained in rocks below the brittle-ductile transition would have increased in salinity as the vapor was removed, and they would have ascended rapidly to higher levels with a sudden drop in pressure on the system.

K-Ar dating (Noble and Silberman, 1984) suggests that emplacement of the Tentadora dike, acid sulfate alteration, and all of the hydrothermal events occurred in a relatively short time span. The first hydrothermal fluid at Julcani, producing the magmatic hydrothermal acid sulfate alteration assemblages, was probably a magmatic fluid vapor derived from the upper levels of the plume that formed above the brittle-ductile transition. The very dense saline fluids ac- companying the quartz-tourmaline-pyrite breccia dikes may have been a product of a phase in the evolution of the magma or they may have been a residual hypersaline brine which formed below the brittle- ductile transition as an overlying vapor plume developed. These dense fluids, which were emplaced ex- plosively, required a large pressure drop in the rocks to allow them to rise to high levels. The rupture of the system may have occurred in response to seismic activity along the Lircay fault, the major structural feature in the Julcani district.

The Tentadora dike was emplaced shortly after the breccia dikes in an orientation different from the structures hosting the other igneous and hydrothermal elements of the district. It is most likely that the dike was emplaced under a stress field controlled by intrusion of magma, rather than by the regional stress field. Internal stresses related to the cooling and crystallization of the magma would have caused the preexisting vein structures to open and the position of the brittle-ductile transition in the rocks to fluctu- ate. With each fluctuation or drop in the level of the brittle-ductile transition, saline ore fluids from the lithostatic environment would have ascended rapidly to high levels. As the magmatic hydrothermal fluids rose to the levels of the veins, they would have boiled, displaced, and eventually mixed with meteoric waters in the porous volcanic rocks perched on the relatively impermeable sedimentary rocks, leading to the precipitation of vein minerals and formation of the concentric mineral zones observed in the district. At times the fluids in the center of the district at Tentadora were probably above hydrostatic pressures, very near the palcosurface in response to the sealing of the open spaces in the veins by quartz deposition. This intermittent seal also probably helped constrain fluid flow laterally away from the


district center. When the magmatic hydrothermal system at Julcani collapsed, the precipitation of main- stage distal mineral assemblages migrated toward the center of the district. As meteoric water invaded the veins, late-stage gangue minerals were precipitated over earlier main-stage mineral zones.

Acknowledgments Drexler and Munoz gratefully acknowledge the fi-

nancial support by National Science Foundation grant EAR 85-17610, as well as logistic support from many individuals at CIA Minas Buenaventura. Phil Goodell supplied samples for study. A special thanks to Mike Wasserman at the U.S. Geological Survey stable isotope laboratory, Denver, for analytical support.

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deen et al-1994

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