Oxygen and hydrogen isotopic studies of ore deposition and metamorphism at the Raul mine, Peru

Geochimy ev Cosmochunico Au11 Vol. 43. pp. 1633 10 1643 Q Pergamon Press Ltd.1979. Printed inGreatBritain

Oxygen and hydrogen isotopic studies of ore deposition and metamorphism at the Raul mine, Peru EDWARDM. RIPLEY Department of Geology, Indiana University, Bloomington, IN 47405, U.S.A. and HIROSHIOHMOT~

Department

(Received 27

of Geosciences, The Pennsylvania State University. University Park, PA 16802. U.S.A. February

1979;

accepted

in revisedform

8

June 1979)

Abstract-The volcano-sedimentary sequence at the Raul mine, central Peru. consists of andesitic volcanics, graywackes, and siltstones, and has been metamorphosed to the upper greenschist-lower amphibolite facies at temperatures of 400-500°C. Isotopic data (0 and H) have been collected from: (a) quartz andmagnetite from stratiform ores, (b) amphiboles from amphibolite units that host stratiform ores, (c) calcite from late veins, (d) detrital quartz from graywackes, and (e) whole rocks. Interunit differences in quartz and magnetite 6”O.values suggest that these minerals have resisted isotopic exchange during metamorphism, and that quartz-magnetite isotopic temperatures (380414°C) represent primary formational temperatures. Calculated 6’*0 values of water in equilibrium with quartz and magnetite range from 9.1 to 12.6R, Amphibole 6’*0 and 6D values show no interunit differences and suggest that the amphiboles have exchanged isotopes with a large metamorphic fluid reservoir. Calculated 6’*0,20 and 6DH,o values range from 8 to 12”, and - 3 to + 42”,, respectively. 6’*OHZo values calculated from #‘O calcite and fluid inclusion filling temperatures range from 7.5 to lo& Water extracted from fluid inclusions in calcite has a 6D value of -2O:‘,,, 6“O values of metamorphosed graywackes and volcanic sediments are not atypical, but andesitic lavas are “O-rich (S-100,) compared to normal andesites. Waters involved in ore deposition, metamorphism, and late vein formation at Raul are all thought to have a common source, principally seawater. The S’*O +o and 6D,,o values could be produced by evaporation of seawater, shale ultrafiltration, and isotopic exchange with host rocks during deep circulation through the volcano-sedimentary pile. A model is proposed whereby coastal ocean water is restricted from the open sea by volcanic island arcs, and subsequently undergoes evaporation. Circulation of this water is initiated by heat a&ted with seafloor volcanism. ‘so-enrichment in andesites may be produced by isotopic exchange with high ‘*O waters at elevated temperatures and sufficiently high water/rock ratios.

INTRODUCFION THE RAUL mine,

owned by the Pativilca Mining Company, Lima, Peru, is located near the coast of central Peru, about 100 km south of Lima (Fig. 1). The deposit is a stratabound, pyritic copper deposit situated within a sequence of submarine volcanic and sedimentary rocks of Cretaceous age. Recent petrologic, mineralogic, and sulfur isotopic studies of the Raul area (RIPLEY and OHMOTO, 1977) suggest that the deposit formed at or near the seawater-seafloor interface from submarine hot springs at temperatures in

The ores and country rocks of the Raul mine area have been metamorphosed to the upper greenschistlower amphibolite facies. Characteristic mineral assemblages include amphibole (actinolite-hornblende). epidote, chlorite, plagioclase, and minor quartz and calcite. It is possible that heated seawater also played a part in the regional metamorphism of the area. Seawater involvement in metamorphism, based on isotopic studies, has been suggested for the Troodos Complex (SPOONERet al., 1974; MAGARITZand TAYLOR, 1974), St. Paul’s rocks near the mid-Atlantic ridge (SHEPPARDand EPSTEIN,1970). ophiolitic rocks from E. Liguria, Italy and Pindos, Greece (SPOONER

a range of 7O-350°C. In particular, sulfur isotope data suggest that most of the sulfur in the ore was derived et al., 19743, and submarine greenstones from the midfrom seawater sulfate. Seawater that circulated deep Atlantic ridge (MUEHLENBACHSand CLAYTON. 1972b). in the volcanic rocks is considered to have been the To date, the importance of seawater in hydrothermal predominant ore fluid. In this regard the Raul deposit metamorphism has not been well documented for bears similarities to a variety of volcanogenic depoareas other than those of oceanic spreading centers. sits, such as the Kuroko ores. the Cyprus ores. the The Raul area has been part of a consuming plate Bathhurst mines of Canada, and the Mt. Lye11 and Roseberry deposits of Tasmania (OHMOTO and RYE, margin system since Jurassic times (JAMES,1971). The purpose of this paper is to present the results of 1979). 1633

1634

E. M.

RIPLEY and

H. OHMOTO .

oxygen and hydrogen isotopic investigations of ore and country rocks from Raul that yield evidence as to the sources of waters involved in ore mineralization and metamorphism.

I

GEOLOGIC HISTORY OF THE RAUL AREA General features

Fig. 1. Geologic provinces of central Peru and location of Raul mine (modified after PETERSEN,1965).

The studied area (1 km’) is located in the Coastal Mesozoic Belt of Central Peru, approximately 5 km west of the Coastal Batholith (Fig. 1). The belt is characterized by a thick succession of andesitic volcanic rocks and marine sedimentary rocks of Jurassic and Cretaceous age, and probably represents the filling of a eugeosynclinal trough (COBBINGand PITCHER,1972a). This trough has been suggested to be the inner arc basin of a volcanic island arc system (WILSON et al., 1967). The structure of the belt is relatively simple. Beds strike to the northwest and dip westerly at an average of about 30”. Although block faulting is common, folding is minor. Within the mine area there are five distinct volcano-sedimentary units dominated by andesites (Units III-IV), andesitic pyroclastics (Units I-III-IV), graywackes (Units II-III), siltstones (Unit V), and amphibolites (Units IIIII-V) totalling about 1 km in thickness (Figs 2 and 3). Two types of intrusive rocks, one a dacite-granodiorite probably related to the Coastal Batholith, and the other

LEGEND

Andesitic Graywacke

lavas and

minoted

Fig. 2. Plan geologic map of the -30

and minor

pyroclostics tufts

and stringer

ore

level, Raul mine. Based on mapping by the geologic staff at the Raul mine.

Oxygen and hydrogen isotopic studies

1635

In addition to the distinct styles of rn~n~al~tio~ trace element and sulfur isotopic data indicate that each zone of mineralization formed at different times and from ore fluids that had experienced different physicochemical histories (RIPLEYand OHMOTO, 1977). The layered ores are thought to represent hot spring deposition at the seafloor, whereas fracture filling and disseminated ore may represent sulfide impregnation and open-space filling of volcanic rocks immediately beneath the seafloor. Sulfur isotopic characteristics of the ores also suggest that the major source of sulfur was seawater sulfate that was reduced to sulfide, probably by reaction with ferrous iron, during convective circulation through hot volcanic rocks. Fluid inclusion (quartz with sulfides) and sulfide phase relations (coexisting p~rhotite-p~te~halcop~ite) suggest temperatures for ore mineral~ation of 340”-350°C for Unit II and 70”-350°C for Unit V. Fig. 3. Generalized stratigraphic section at the RauI mine. Unit names in parentheses are those used at the mine. a series of andesite dikes, also occur in the mine vicinity. Within the volcano-sedimentary sequence metamorphic effects in feldspars are displayed by a modification of original An contents and partial replacement by chlorite, epidote, clinozoisite, and calcite. Primary ferromagnesium minerals have been replaced by amphibole, chlorite, and epidote. Only detrital quartz in graywackes and siltstones remains as an undoubted primary mineral. The preservation of primary textures and absence of foliation or schistosity in country rocks aids greatly in identifi#tion of prototypes. These features, together with the lack of macroscopic, tectonic deformational features within the belt suggest that metamorphism was predominantly thermal or hydrothermal in nature, and may have been related to activity of the Coastal Batholith. Most of the andesite flows at Raul are porphyritic, characterized by phenocrysts of altered plagioclase (An N-48) and amphibole (predominantly hornblende), pseudomorphous after pyroxene. Matrix material includes plagioclase miaolites, epidote, hornblende, clinozoisite, chlorite, ilmenite, sphene, and minor quartz. Pyroclastic rocks of the area contain broken crystals of altered plagioclase and rock fragments of andesite dispersed in a groundmass mi~ralo~~Iy similar to that of the flows. Graywackes consist of subroun~d to subangular quartz grains and lesser amounts of highly saussuritized plagiodase grains set in a matrix dominated by blue-green hornblende and chlorite. Minor phases in the graywackes are ahanite, epidote, and scapolite. Siltstones consist of angular to subangular quartz grains, with minor plagioclase crystals set in a very fine matrix composed principally of chlorite. The more tuffaceous units contain broken plagioclase grains set in a chlorite-rich groundmass, with only minor quartz. Amphibolites at the Raul mine consist predominantly of felty to tabular hornblende, accompanied by varying amounts of plagioclase, quartz, chlorite, epidote, allanite, scapolite, ilmenite, calcite, and sphene. The amphibolites are mineralogically nearly. identical to the matrix portions of graywackes, and in several localities grade into layers containing detrital quartz and feldspar fragments. Strata~ound sulfide ores Ore is composed of chalcopyrite-pyrite accumulations that occur as texturally distinct assemblages within each unit at the Raul mine. Sulfides may occur as fine disseminations in lava flows (Unit IV), as fracture fillings or replacements of primary mafic minerals in flows and tuffs (Unit III). or as thin layers alternating with amphibolitic material in graywacke or siltstone units (Units II, V. and locally in III-referred to as manto ore). Magnetite may also occur associated with sulfides in several of the ore mantos.

P-T conditions qf metamorphism Regional stratigraphy indicates that the maximum depth of burial in the Raul area was iess than 5 km (COJMNG and PITCH&J& 1971a; JAMES, 1971). Assuming a rock density of 2.6 g/cm3, this depth corresponds to - 1.3 kb. The pressure due to overlying seawater is thought to have been less than 200 bars. Therefore, load pressures less ‘than -- 1.5 kb are thought to have existed during metamorphism at Raul. The predominance of hornblende over actinolite and the occurrence of Ca-piagioclase at Raul suggest that metamorphism was of upper greenschist to lower amphibolite facies (LIOU et al., 1974). According to Liou es at. an upper limit for the greenschist assemblage ~bit~hlorite-actinolite-epidote may be set at approximately 475°C at 2 kb Pn,o. The lower limit of the amphibolite assemblage plagioclase-hornblende occurs at 550°C at the same pressure condition. Where Pa,, < P,,,, these temperature limits may be lowered. The distribution of sulfur isotopes between coexisting pyrite and chalcopyrite proved unsuccessful as a geothermometer, as fractionation varied widely (in some instances 6% cpY> 634Si,,). The A values suggest that although sulfur isotopic homogenization was not completely established, partial isotopic exchange between metamorphic fluids and primary sulfide minerals did occur. At Raul a temperature of metamorphism somewhere between 300 and 500°C is considered reasonable, based on the occurrence of actinolite, hornblende, Ca-plagioclase, chlorite, and epidote. SPOONERand FYFE (1973) and HART (1973) suggest from empirical evidence that actinolite forms in subseafloor systems at temperatures in excess of 300°C. Actinolite was produced in basalt-seawater experiments (Mom, 1976) only at temperatures greater than 400°C and at pressures higher than 500 bars. Lute stage vein mineralization In the central and northern sectors of Unit III a series of post-metamorphic veins containing calcite, sphalerite. galena, and minor chalcopyrite occur near contacts with the dacite intrusive. Fluid inclusion homogenization temperatures are 270 + 30°C for calcite and 150 & 10°C for sphalerite. In addition to being mineralogically distinct, the veins also exhibit sulfur isotope values which are very different from those of stratabound sulfides. Thermal history of the Rauf area From the temperature data previously summarized a generalized thermal history of the Raul area may be developed (Fig 4). Manto deposition in Unit II. and probably in Unit HI. occurred at temperatures of 340-350°C. Manto deposition in Unit V occurred at a maximum temperature of 350°C but may bave been lower. Metamorphism occurred at temperatures from 300 to 500°C. Peak metamorphism may have been related to intrusion of the Coastal Batholith. Geologic evidence indicates that calcite-

E. M.

1636

RIPLEY

and H.

~HMOT~

600

,500

r,

400

PC

I

300

Ca -plag.+ Hornblende t Actinolite

P Cow

’ Ftuta lnclueions in Quartz

TPy-PoCp 1

i

:P I

corr.

FluId

I

Inclus

nr 9 ; P Cow,

200

_

100 b-P0

0

Metamcrphlsm

Vein CatCIte

VClfl Sphalerits

Fig. 4. Summary of temperature variation during ore deposition, metamorphism, and late vein formation at Raul. sphalerite veins are younger than stratabound pyritechalcopyrite mineralization and the peak of metamorphism. Calcite deposition occurred at temperatures from 240 to 300°C whereas sphalerite precipitated at much lower temperatures in the range 140 to 160°C.

ANALYTICAL

TECHNIQUES

The following samples were analyzed for oxygen isotopic composition: (1) 24 samples of magnetite from mantos in Units II, III, and V, (2) 5 samples of quartz from magnetite-bearing mantos in Unit II, (3) 2 quartz separates from a graywacke horizon in Unit II, (4) 15 amphibole separates from amphibolites, (5) 2 calcite samples from post-metamorphic veins, and (6) 11 whole rocks. Samples analyzed for hydrogen isotope composition include: (1) 15 amphibole separates, (2) 11 whole rocks, and (3) 1 sample of water extracted from fluid inclusions in vein calcite. Hornblende and quartz were concentrated for isotopic analysis by standard heavy liquid and magnetic techniques. All samples were ground to - 100 + 200 mesh prior to analysis. Quartz separates were treated with 30% hydrofluorosilicic acid to remove feldspar. The fine-grained nature of much of the magnetite at Raul necessitated a variety of methods in magnetite separation. Initial separation was accomplished by repeated centrifuging in methelyne iodide (S.G. = 3.30), or by drilling magnetite-rich areas with a dental drill. The resulting concentrate was then poured through a tilted inverted Y-shaped Pyrex tube filled with acetone, and a magnet held near the upper arm of the Y. Final separation was accomplished electromagnetically by use of a vibrating flat card (HERRICK, 1973). Sample purity was generally greater than 987;. Oxygen for 6lsO analyses of magnetite, quartz, amphiboles, and whole rocks was liberated by reaction with BrF5 as described by CLAYTONand MAYEDA (1963). Oxygen was converted to CO2 for isotopic analyses by reaction with carbon produced by heating a disc-shaped graphite electrode. CO1 from calcite was liberated by reaction with H,POI at 25°C for a l-day period (MCCREA, 1950). Water was released from amphiboles and whole rocks by heating until fused in an induction furnace. Hydrogen for analysis was produced by passing the HZ0 over hot uranium metal in the manner described by FRIEDMAN(1953). Values are reported as per mil deviations from SMOW; oxygen analyses are generally reproducible to +O.l”&, whereas the analytical precision for hydrogen analyses is usually f2”/,.

Water was extracted from fluid inclusions in calcite by crushing the sample in an evacuated stainless steel tube as described by ROEDDER et al. (1963) and RYE (1966). Water was collected and converted to hydrogen as described above. 6isO and 6D values of water in equilibrium with specific minerals were calculated using ore formation or metamorphic temperature estimations and mineral-water fractionation factors. The oxygen fractionation factor for hornblende-water is an approximation based on analyses of igneous and metamorphic rocks analyzed by TAYLOR (1%8), Srrmsr and TAYLOR (1969ab), and TAYLOR and EPSTE~J(1962). Magnetite-water, quartz-water, and quartzmagnetite fractions were taken from BECKER and CLAYTON (1976). Calcite-water fractionation factors were those determined by O’N~ILL et al. (1969). The hydrogen isotopic fractionation factor for amphibole-water is based on the work of SUZUOKIand Epsrm~ (1976), and takes into account the composition (Mg, Fe, Al) of the amphibole. Chemical compositions of amphiboles were determined using an ETEC Autoprobe. Chemical formulas are given in Table 1. It should be noted that amphibole compositions are variable, probably representing primary variations in premetamorphic volcanic or volcanic-related rock types.

ANALYTICAL

RESULTS

Oxygen and hydrogen isotopic data from whole rocks, mineral separates, and fluid inclusions are presented in Tables 2-5, and displayed in Figs 5 and 6. 6isO values of magnetite range from 1.1 to 4.6”~. Magnetite from mantos in Units II and III show similar isotope values averaging 3.6”&,,whereas magnetite from Unit V is lighter, averaging 2.69&,.Variation within a single unit is small. generally less than 1X&,. Quartz occurring with magnetite in the Unit II mantos is characterized by 6’sO values of 14.3 to 15.3%,,. These values are different from those of detrital quartz in graywackes which has a S’s0 value of 12.19&,.Amphiboles possess a fairly restricted oxygen isotopic composition with values ranging between 6.8 and 10.39/,. Only three values are less than 8.0”,. Consistent interunit Si*O variations do not exist. Whole rock St*0 values range from 7.8 to 10.8’&, with average values as follows: andesites- +97&, graywacke- + 10.6”/, tuffaceous siltstone- +8.37&. Vein calcite 6tsO values are f 14.2 and + 14.0”/,. 6D values of amphiboles from the Raul area range from -21 to -6O’&. Because amphibole is the dominant hydrous mineral in the analyzed whole rocks their 6D values

1637

Oxygen and hydrogen isotopic studies

Table I. Chemical formulas for amphiboles from amphi~lite units at the Raul mine

are not significantly different from those of amphiboles, ranging from -30 to -49yw DI!XXJSSION isotopic

temperature

6l sO oj water

from

quartz-magnetite

pairs

and

in the ore fluid

Temperatures indicated by the oxygen isotopic fractionation factors for coexisting ~~etit~u~~

pairs of Unit II (10.7 to 11.7~~) are between 380 and 414°C. Filling temperatures of fluid inclusions in quartz are 350 f 10°C. The filling temperatures probably require little, if any, pressure correction if ores were deposited on the seafloor (RIPLEYand Omom, 1977). The maximum pressure correction based on recon-

struction of the maximum depth of burial in the Raui area is _ 100°C. In light of analytical uncertainties

Table 2. 5leO of quartz and magnetite together with calculated S”OH* Unit :: II :: II III

III III III V V V V V V V V V V V V V V II II II II II II

Sample No.

Mineral*

J-34

Mt Mt Mt Mt Mt Mt Mt Mt Mt

204 J-10

J-51 J-49 J-9 R-48 R-45 R-43 R-44 CH-41 R-11 CH-27 CH-4t R-26 CH-36 R-27 CH-37 CH-38 CH-30 CH-15 CH-12 CH-2 R-32 J-34 204 J-10 J-51 J-49 J-32

Mt Mt Mt Mt Mt Mt Mt Mt Mt Mt Mt Mt Mt Mt Mt 2:: ::: 2:

~‘*o(%r+l

PO&

3.3 3.0 4.2 3.5 3.4 4.2 4.0 3.1 3.7 3.5 3.0 1.1 2.5 2.8 3.0 2.7 2.0 4.6 2.5 3.0 2.5 2.6 3.0 2.0 14.3 14.7 15.3 14.3 14.6 12.1

(detrital)

II

A-28

values

12.2 (de%al)

* Minerals: Mt, magnetite; Qtz, quartz. t Calculated at T = 400°C for Units II and III and 350°C for Unit V.

10.8 10.5 11.7 11.0 10.9 11.7 11.5 10.6 11.2 11.0 11.0 9.1 10.5 10.8 11.0 10.7 10.0 12.6 10.5 11.0 10.5 10.6 11.0 10.0 10.5 10.9 11.5 10.5 10.8 -

1638

E. M. RIPLEYand H. OHMOTO Table 3. 6”‘O and SD of amphiboles

Unit

Sample No.

II II

J-10 J-45 J-53 J-54 J-60 J-64 A-40 A-31

II II II II II II III III V V V V V

together with calculated bl*OnXO and 6D,,, values

10.5-11.3 10.8-t 1.6 11.1-11.9 11.5-12.3 9.4-10.2 11.5-12.3 9.7-10.5 10.5-l 1.3 10.1-10.9 8.1-8.9 11.2-12.0 11.0-11.8 9.1-9.9 8.2-9.0

9.3 9.6 9.9 10.3 a.2 10.3 8.5 9.3 8.9 6.9 10.0 9.8 8.0 7.0 6.8

z R-31 R-36 R-38

8.0-8.8

43-33 38-26 30-18 29-17 28-16 37-25 15-3 46-34 14-2 9-3 46-34 l-11 37-25 18-6 42-30

-33 -41 -25 -21 -51 -48 -49 -37 -33. -37 -30 -60 -38 -37 -24

* 6’80n20 and SDn,o calculated for a temperature range of 400”~500°C.

and the possible differences in mineral-water fractionation factors for salt-rich and salt-poor solutions (see TRUESDELL, 1974) the temperature discrepancies are considered small. The problem is that it is not clear if the temperatures represent original depositional temperatures, peak metamorphic temperatures, or something in between. The difficulty in interpreting the temperature information is also reflected in establishing whether or not calculated 6’*0 values of water in equilibrium with quartz and magnetite represent ore forming or metamorphic fluids. 6180 values of water in equilibrium with quartz and magnetite at temperatures from 380 to 410°C range from 9.1 to 12.6”/,. The consistency of the A values for coexisting quartz and magnetite

suggests that these minerals have remained close to isotopic equilibrium with one another. This may be a consequence of quartz and magnetite retaining their primary isotopic composition or re-equilibrating during, or more likely after, peak metamorphism. In the of ore-forming fluids would second case the P*O be impossible to calc$ye. Although both quartz and magnetite are fairly resistant to isotopic exchange after formation (CLAYTONet al., 1968), many authors (e.g. DAHL, 1978; DEINES,1977; JAMB and CLAY-IQN, 1972) have indicated that re-equilibration between quartz and magnetite may occur after the peak of a metamorphic event (quartz-magnetite temperatures may reflect lower than peak metamorphic temperatures). In the Raul area however, magnetite from

Table 4. 6i*O of calcite together with calculated 6i80n10 values and a 6D value from water extracted from fluid inclusions

Unit III III

Sample No. 198 210

14.0 14.2

7.5-10.4 7.3-10.2

-20

* Calculated for a temperature range of 250”-350°C. Table 5. 6”‘O and 6D of whole rocks PO Unit I II II III III III III III IV IV V V

Sample No. 205 A-28 J-32 M-40 C-38

c-34 c-22 c-20 c-15 P-9 CH-16 CH-26

Rock tw Andesite Tuff Graywacke Graywacke Andesite Tuff Andesite Andesite Andesite Andesite Andesite Andesite Tuffaceous Siltstone Tuffaceous Siltstone

(“3 8.7 10.5 10.8 8.6 8.6 9.7 10.6 8.1 10.7 10.5 7.8 8.8

-41 -47 -49 -47 -35 -33 -45 -44 -33 -41 -33 -30

1639

Oxygen and hydrogen isotopic studies

* l% :

0

1

2

3

4

6

6

7

6

6

,D

0

12

13

W

16

i6

d’6Ol%)

of minerals and whole rocks in Units I-V, Raul mine. Symbols: closed circle = magnetite, closed triangle = amphibole, open triangle = calcite, open

Fig. 5. Variation in the oxygen isotopic composition

circle = quartz in manto, open square = detrital quartz, plus sign = whole rock.

Unit V is isotopically different from magnetite in Units II and III (similar to sulfur isotopic and trace element differences). In addition detrital quartz separated from graywacke layers that are in direct contact with the manto amphibolite are isotopically very different than quartz in the mantos. If complete isotopic exchange occurred between magnetite and a large metamorphic &rid reservoir one would expect to find magnetite of similar isotopic composition. The same would be true of quartz from different units. It appears that either fluid-rock communication was extremely restricted+ or that quartz and magnetite resisted isotopic exchange during metamorphism. In view of. the isotopic information regarding amphiboles (below) and the submarine geologic environment that existed in the Raul area, it is suggested that a large fluid reservoir was available, principally seawater. The favored hypothesis, therefore, is that differences in 6lsO values of magnetite from Units II-III and V represent primary compositional vari-

Unit V

A

unit IV

:+t

+

A

+

+ ++ l +:

Unit III

unit II

Unit

I

kvTzFEz

-70

_)

,lO

dD(%)

Fig. 6. Variation in the hydrogen isotopic composition of amphiboles (closed triangles) and whole rocks (plus signs) in Units 1-V. Raul mine.

ations, and quad-ma~etite A values reflect inherited prototype values. The calculated 61sOu,o values are thought to represent the isotopic composition Of water in equilibrium with quartz and magnetite at the time of initial mineral fractionation. 6D and b’s0 o~a~h~o~es

and metamorphic water

The oxygen and hydrogen isotopic composition of fluids in equilibrium with amphiboles at 400~-5oo”C are calculated as: 6”0,,, = 10.0 t_ 2:& and SD,, = 17.5 & 28.5X. In order to evaluate the HrO values it is first necessary to briefly discuss the petrogenesis of the amphiboies in the vicinity of the Raul mine. There are at least two different origins for amphiboles present in the Raul area. Amphibolites occur in Units II, III, and V only within sedimentary sequences and are mineralogically identical to the matrix portion of graywackes. Within Unit II amphibolites grade into areas rich in detrital quartz and feldspar. The matrix material of graywacke horizons was most likely clay minerals (e.g. montmorillonite, chloritic clay) that was later metamorphosed to chlorite, and then to amphibole. The amphibolites apparently represent a hiatus in quartz-feldspar deposition, or rapid a~um~ation of clay minerals. In this situation it is important to note that the amphiboles formed by dehydration of prototype minerals. Amphiboles in the tuffs and flows probably formed by a different mechanism. Textural evidence suggests that primary pyroxene has been replaced by amphibole. In this case amphibole formation wouid be related to a hydration process. The simiiarity in @‘O and 6D values for amphiboles of different origins suggests that the amphiboles in the Raul area exchanged isotopes with a large fluid reservoir. The 6rsO and 6D values of the amphiboles would depend on the initial 6’sO and AD values of the fluid, T, and water/rock atomic ratios (for a

E. M. RIPLEYand H. OHMOTO

1640

&“O (9bo) Fig. 7. Isotopic composition of fluids involved in ore deposition, metamorphism, and vein deposition in the Raul area. Possible trends of seawater ‘sO-exchange with country rocks, evaporation, and shale ultrafiltration are shown. The meteoric water line is from CRAG (1961). detailed discussion of 0 and H isotopic systematics during rock-water interaction see WENNERand TAYLOR (1973) and ONTO and RYE (1974)). The fact that aI80 and 6D of amphibol$from different localities at Raul are very similar, suggests that the water/ rock ratio during exchange was very large (> LOO?). The isotopic com~sition of the water invotved in exchange with the amphibole would be essentially the same before and after exchange (i.e. CANTOinitial H,O = al80 finai H20). 6lsO of water in equilibrium with calcite and 6D of water in calcite Juid inclusions The P80 value of water in equilibrium with vein calcite from Unit III at 270 jr 30°C is 8.8 f 1.5%. One sample of Hz0 extracted from fluid inclusions in vein calcite had a 6D value of -2OY/,.

Source and e~~urion of ore-formic

and ~ta~rphic

&ids 6180 and 6D values of water in equilibrium with quartz and magnetite (primary ore fluid), amphiboles (metamorphic fluid), and calcite (post-metamorphic fluid) are compared in Fig. 7. The similarity in isotopic composition of the fluids suggests that they may have had a common origin. A logical source for this fluid is seawater because: (1) neither magmatic nor meteoric water is likely to evolve into water with such high al80 and 6D values [see discussion on the evo-

lutionary path for magmatic and meteoric water during interaction with country rocks presented in OHMOTOand RYE (197411. Only the values for watei in equilibrium with calcite are at all suggestive that mixing of two Auids (one characterized by high 6D, the other much lower) could have occurred. (2) Sulfur isotopic data suggests that the majority of the sulfur in the ore-forming fluid was derived from seawater sulfate. However, the observed &I80 and 6D values of the ore-forming and metamorphic fluids are not the same as those of normal seawater. If the fluids were of seawater origin they must have undergone extensive evolutionary changes. One method by which both 6D and S180 could be increased is if the water were derived from evaporating seawater. CRAIG et al. (1963) have shown that on a 61Xt80 plot evaporation of natural water bodies proceeds along a slope of from +4 to + 6 relative to their input waters. Experiments conducted by CRAIG et al. (1963) and theoretical considerations by CRAIG and GORDON (1965) indicate that an isolated water body comes to an isotopic stationary state, due to molecular exchange with the atmosphere, befoie the entire body has evaporated away. Their studies indicate that with a relative humidity of 750/, 6D values of the water in an evaporating, isolated body may increase as much as 40’6, and require evaporation of only about 20”/, of the water mass. 6”O values at the same conditions may increase approxi-

Oxygen and hydrogen isotopic studies mately 79,. Although these results cannot be strictly applied to conditions at Rat& they do indicate that evaporation of seawater is a plausible method of producing the obs&ved isotopic composition of the fluids involved in ore deposition and metamorphism. it should be noted that although the chemical composition of fluids involved in rne~rno~h~rn could not be determined, fluids responsible for ore deposition at Raul are thought to have been quite saline based on analyses of fluid inclusions in massive sulfides (RIPLEY and OHMOTO, 1977; RIPLEY, 1976). Evaporation of seawater could, in part, explain the high salt content of the Auids involved in ore formation. Another method by which 6D and 61sO values of seawater could be increased is by shale (clay membrane) ultrafiltration (GRAF et al., 1965; COPLEN and HANSHAW, 1973). In this process water circulating through shale or clay units undergoes extensive fractionation. The residual water is enriched in both 6D and a’*0 over the starting water and ultrafiltrate. The isotopic composition of the residual fluid falls along a line of slope -3.1 relative to the input water. Shale ultrafiltration is also effective in increasing the salinity of the residual water. The &I),,, values of met~orphic fluids at Raul are most easily explained as originating from evaporating seawater and/or residual seawater trapped during a shale ultrafiltration process. Isotopic exchange with country rocks is also suggested, principally because the observed magnitude of the oxygen isotope shift (6’*OuIo - + lo?_ for ore, met~o~hic, and post-metamorphic fluids) is larger than that expected from evaporation or ultrafiltration alone. At temperatures on the order of 4OO”-500°C it would be very unlikely for circulating water to resist isotopic exchange with country rocks. The negative 6D,, values calculated from some of the ~phi~les and determined for water from fluid inclusions in calcite may also be explained in part by exchange reactions with country rocks having lower SD values. It is possible that waters responsible for calcite vein formation also contained a magmatic and/or meteoric water component. “0

enrichment in andesites

Although metamorphosed graywackes and volcanic sediments at Raul possess 6i*O values which fall within the common range for such metamorphic rocks FAYLOR, 1974), andesites are slightly isO enriched compared to typical andesites (S’*O 67%). Taylor (1974) states that metasedimentary rocks commonly retain their metamorphic SrsO values. If this is the case in the Raul area, it is possible that the meta-andesite 6180 values may also be representative of those of the pre-metamorphic lavas. High 6i80 lavas at Raul could have been produced by partial melting of ‘*O-rich sediments and metasediments within or near a subduction zone. TAYLOR and TURI (1976) have recently reported high-“*0 lavas in Italy

1641

produced by a~i~lation or partial melting of i80-rich sedimentary rocks. However, the lack of ‘*O-enriched andesites from arc-trench associations throughout the world suggests that partial melting of sediments in and above a subduction zone does not necessarily produce “O-rich lavas. A more likely and consistent model centers on seafloor metamorphism and isotopic exchange between the lavas and circulating, modified seawater. Calculations indicate that 6’*0 enrichments of 2yW could have been produced in the andesites by exchange with evolved seawater having a 6’sO value of -7‘$,,. It is therefore concluded that at Raul andesite SisO values of 8.1-10.?ya,, could have been caused by interaction with “O-rich waters at temperatures on the order of 400°C. This interpretation assumes that isotopic values Were ‘frozen in’ near peak metamorphic temperatures, in equilibrium with modified seawater. R~quilibration of andesite and water 18O/16O ratios during retrograde cooling would also lead to invalues, due principally to more creased ~i*Oandcsitc positive mineral-water fractionation factors at low temperatures. S’ 80,,d,si,c values greater than + 10.7:~~ have not been determined for Raul samples, and therefore low temperature reequilibration is not considered to have been an important process governing oxygen isotopic compositions. Although relatively low temperature (I 200°C) hydrothermal alteration and/or weathering can also produce ‘*O enrichments in volcanic rocks (GARLICK and DYMOND, 1970; MUEHLENBACHS and CLAYTON,1972a), these processes are discounted at Rauf based on mineral assemblages and textural relations.

CONCLUSION”’ MODEL FOR HYDROTHERMAL ORE DEPOSITION AND

~~AMORPHISM THE RAUL

fN

AREA

Hydrogen and oxygen isotope studies suggest that a large portion of water involved in stratabound ore deposition, metamorphism, and post metamorphic calcite-sphalerite vein formation in the area of the Raul mine originated from seawater that had been modified by evaporation and/or a shale membrane process. The formation of a restricted basin can be envisioned by examining the regional setting that may have existed in the Raul area at the time of ore deposition and metamorphism {Fig. 8). At the present time subduction of the Nazca Piate is occurring off the coast of Peru and Chile. According to JAMES(1971X subduction has been in progress since Jurassic times. and initiated with the development of a volcanic arc off the continental shelf. The Coastal Mesozoic Belt’ may represent a portion of the volcanic and sedimentary material which accumulated between the arc and the continent. Stratigraphic evidence indicates that the andesitic and related rocks of the Coastal Mesozoic Belt were deposited on pre-existing sialic crust

E. M. RIPLEYand H. OHMOTU

1642

FJ &j f?J

Sssslb

aad &nd&t*s

I-fJ

Volc~nicl~rlic S*dimMlS

ea

&Ce*niQ CruM

@

CQrrtin*nt& CtW

NIIC~ PI8te

AntWir~n

P1stff

Fig. 8. Schematic diagram illustrating hypothetical geologic environment during ore deposition and metamorphism at the Raul mine. Restriction of seawater between the continent and volcanic arc allows evaporation and changes in the oxygen and hydrogen isotopic composition OFseawater. Heat associated with sub-seaBoor volcanism causes convective circulation of seawater through the volcano-sedimentary pile.

and P~TCWER,1972b). It is possible that Iocal, restricted basins formed along the trough, bounded by the continent on the inner side’and the volcanic arc on the ocean side. Modern examples of small ocean basins trapped between island arcs and continents are the Sea of Okhotsk and the south Fiji Basin (MENAXD, t%7; ICARIO, 1970; DEWEY and BIRD, 1970). The volcanic arc aeed not have been above sea level, but only function as a sill, restricting free flow from the basin into the open sea. The isotopic composition of seawater could then have changed due to evaporation. The sedimentary sequence in the coastal Mesozoic Belt includes shale units that could have been important for isotope fractionation due to shale u~t~I~a~on~ Sulfur isotope studies and 6’sO values of quartz and magnetite suggest that hydrothermal ore deposition was caused by reaction of heated rocks and circulating, modified seawater. Heat was suppEed by sub-seai?oor volcanic activity, and possibly by underlying intrusjves related to development of the Coastal Batholith. Metamorphism may have been occurring at depth in the vol~nu-~diment~y pi& at the same time as ore deposition was occurring near the seafloor-seawater interface. in any case oxygen and hydrogen isotopic composition of amphiboles and whole rocks in the Raui area suggest that the metamorphic rocks have ~ui~ibrat~ with a large fluid reservoir composed principally of modified seawater. Although calcite-sphalerite vein deposition occurred after peak metamorphic temperatures were achieved, isotopic corn~s~tio~ of calcite and water from &id inclusions suggest that the veins also formed in large part from circulating evolved seawater.

(COBBING

~c~now~~~~~~i~-S~a~ thanks is due Mr Luis Hochschild, owner of the Pativilca Mining Company, who pro-

vided financial assistance for field and analytical study of the Raui mine. Engs.JOSED~vzscovt. manager of the Raul

mine,

ALFONZO

%RREDE,

T-zo

R.&v&&

and

Mrovu.

~ORDOZO

provided geologic assistance at the mine and access to company maps and records. We wish to express appreciation to Dr ULRICH PETERSEN of Harvard University who tirst introduced us to the geotogic problems at the Raul mine. Kind acknowied~ement is due Drs A. W. ROSE,C. W. BURNHAM, and H. 1. BARNES of The Penasylvania State University, and Dr R. 0. RYE of the United States Geological Survey for useful suggestions for the improvement of this manuscript. This research &as in part supported by N&onal Science Foundation grants GA-31901 and EAR 76-03724 to H. OHMOTo, aa; an Indiana University Faculty Fellowship to E. M. RIPLEY. REFERENCFS BECKXX. R. W. and CLAYTONR. N. (1976) Oxygen isotope study of a Precambrian banded iron formation, Wamersefy Range, Western Australia. Genchim. Coslnochirn. Aera 40, If53-li65. CLAYTON R. N. and MAYET)~T. K. (1963) The use of bromine penttiuoride in the extraction of oxygen from oxides and silicates for isotopic analysis. Geochim, Cosnurchim. Acra 27, 43-52. CLAYTONR. N.. MUFFLERL. J. P. and WHY D. E. {1%8f Oxygen isotope study of caltite and silicates of the River Range No. 1 well, Salton Sea geothermal field, California. Am. J. Sci. 266. 9611-979. COBSING E. 3. and PITCHER W. S. (1972a) The Coastat Batholith of Central Peru. J. Ceol. Sot. Land. 128, 421460.

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