The root zones of oceanic hydrothermal systems - Wiley Online Library

JOURNAL OF GEOPHYSICAL

RESEARCH,

VOL. 99, NO. B3, PAGES 4703-4713, MARCH 10, 1994

The root zones of oceanic hydrothermal systems: Constraints from the Samail ophiolite (Oman) Pierre Nehlig Bureau de Recherches G•ologiques et MiniSres, Orleans, France

Thierry Juteau, Valerie Bendel, and Joseph Cotten Groupe de Recherche "Gen•se et Evolution des Domaines Oc•aniques," Universit• de Bretagne Occidentale, Brest, France

Abstract. The Cretaceous Samail ophiolite in Oman exposesan almost complete, 500km-long, along-axissectionof oceanic crust, providing a unique opportunity to study the geometry, physical conditions, and effects of the hydrothermal circulation that fed the volcanic-hostedmassive-sulfidedeposits. These fossil dischargezones are rooted in the sheeted-dikecomplex, down to the transition zone with the plutonic sequence.The sheeted-dikecomplex as a whole was affected by greenschist-faciesmetamorphism (albite, actinolite, chlorite, quartz, sphene). Diabase dikes are commonly altered into three major alteration end-memberstermed spilite, mineralized spilite, and epidosite. These alteration patterns usually follow dikes, thus resultingin a well-marked alongstrike vertical anisotropy. Their vertical distribution is also anisotropic: epidosites occur mainly in the basal sheeted-dikecomplex, and mineralized spilites are usually limited to its top and to the transition zone with the volcanic extrusives. The alongstrike distribution of the different alteration patterns also shows a well-marked correlation with the occurrenceof massive-sulfidedeposits.The sheeted-dikecomplex underlying these depositsis characterized by a sharp increase in the volume of epidosite and mineralized spilite. The stratigraphicposition of the epidosite zones just above the magma chamber and in the lower sheeted-dikecomplex, their attitude parallel to the margins of the dikes, the recorded high trapping temperatures, the high water/rock ratios, their textural reconstitutionand base metal depletion suggestthat they formed in a focused upflow portion of a vertical, along-strikeconvecting, hightemperature (subcritical) hydrothermal system a short time after emplacement of the dikes. The common occurrenceof epidositebands devoid of veins, and the absenceof major fracturing or listric faulting, indicates that formation of epidosite does not first require tectonic extension. Introduction

At present, the technological limitations of deep crustal drilling restrict the direct study of deep-seated conditions and the effects of hydrothermal circulation along oceanic spreading centers. Conversely, when the fossil root zones of hydrothermal systems are exposed by tectonic activity and erosion, the rocks have recorded the time-integrated overprinting of all subsequent processes involved in crustal generation. Despite this complexity, the best opportunity for the three-dimensional study of alteration and fluid circulation in the oceanic crust is in ophiolites on land. The well-preserved Samail ophiolite in the Sultanate of Oman is ideal for such a study, since it allows for observation of both vertical and along-strike sections over long distances.The ophiolite is exposedin a huge thrust sheet of Cretaceousoceanic lithosphere, more than 500 km long, 150 km wide, and between 5 and 15 km thick (Figure 1). Previous studies in various areas of the Samail ophiolite have established the validity of an internal stratigraphy Copyright 1994 by the American Geophysical Union. Paper number 93JB02663. 0148-0227/94/93 JB-02663 $05.00

[Reinhardt, 1969; Glennie et al., 1974; Lippard et al., 1986; Juteau et al., 1988] conforming to the classic Penrose Conference definition. The mantle sequence, composed of harzburgitic tectonite [Nicolas et al., 1988], is overlain by the crustal sequence that is subdivided into (1) a layered magmatic unit; (2) high-level gabbro with minor plagiogranite bodies; (3) a sheeted-dike complex; and (4) an extrusive sequenceof pillow lava and lava flows, interbedded with and overlain by fine-grained pelagic sedimentary rocks [Coleman, 1981; Alabaster, 1982; Juteau et al., 1988]. The study of 25 major prospects and occurrences by Lescuyer et al. [1988] has shown that nearly all massive sulphidedepositswere emplaced during a major break in the Cretaceous volcanic activity, immediately after eruption of basaltic lavas of mid-ocean ridge basalt (MORB) composition (Figures 1 and 2), and that they formed during the main accretion stage that developed the main part of the plutonic sequence, sheeted-dike complex and the first volcanic unit. One sulphide deposit (Aarja, Figure 1) has been shown to be slightly off-axis [Haymon et al., 1989] but still related to the main accretion stage. The Daris prospect (Figure 1) is located in the upper volcanic succession [Lescuyer et al., 1988] and is related to an off axis diapir [Reuber et al., 1991].

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Whereas the transition zone to the overlying lava usually is very sharp and occurs within tens of meters, the lower transition zone is gradational over tens to hundreds of meters. Furthermore, this lower transition zone is characterized by complex intrusive relationships between the dikes, plagiogranite, and high-level gabbro, all of which usually are mutually intrusive [Nehlig et al., 1990]. Most of the sheeted-dike complex has one dominant NNW trend, with only minor crosscutting dikes (less than 5%). However, in the Daris area (Figure 1), two major dike orientations have been mapped: 130ø and 010ø [Nehlig, 1989]. Detailed structural mapping has shown that the complexity of this area could be linked to an off-axis mantle diapir [Nehlig, 1989; Reuber et al., 1991]. Hydrothermal Relationships

Figure 1. Geologic sketch map of the Samail ophiolite, showing the location of the main sulphide occurrences (triangles). All except Daris [Lescuyer et al., 1988] were emplaced during a break in the Cretaceous volcanic activity, immediately after eruption of basaltic pillow lavas of midocean ridge basalt composition. Daris was emplaced in the upper volcanic unit and is probably related to an off-axis diapir [Nehlig, 1989; Reuber et al., 1991].

General comparisons of the sulphide mineralogy and textures observed

in both the East Pacific

Rise and the Samail

ophiolite were made by Ixer et al. [1984], Haymon et al. [1984], and Lescuyer et al. [1988]. They suggested that emission and precipitation of the mineralized fluids on the seafloor occurred in the same way as present-day sulfide deposition on the East Pacific Rise. Furthermore, other striking features that are observed both on the East Pacific Rise and in the Oman mineralization are fossil hydrothermal worm tubes similar to those described

The dikes have undergone a pervasive greenschist-facies hydrothermal metamorphism (albite, actinolite, chlorite, epidote, quartz, sphene) resulting in a greenish, brownish, or reddish color. Diabase dikes are commonly altered into epidosite, spilite, and mineralized spilite. In fact, as we will see later, even what we call diabase dikes are hydrothermally altered dikes. However, because they are the least altered dikes, we chose to keep the term diabaseto describe them.

In the field, diabasedikes are grey-green in color, whereas spilitic dikes are dark green to blue. Mineralized spilites have a reddish color and correspond to quartz- and sulfiderich spilitic rocks crosscut by a dense net of quartz-sulfide veins. Epidosite dikes are light green. All intermediates between diabase and the three alteration types are present. The relative

amounts

and levels

I•1 Volcanic Unit

•] Sheeted dikes

zones that can be traced down into the

sheeted-dike complex [Nehlig, 1989]. These stockwork zones are in turn rooted within epidosite-rich zones at the base of the sheeted-dike complex (Figure 2). The purpose of this paper is to describethe different types of alteration found in the sheeted-dike complex with a special emphasis on epidosite. Structural, petrological, geochemical, and fluid inclusion data are used to constrain a model of hydrothermal circulation within the sheeted-dike complex. Location and detailed description of the samples and data presented here are given by Nehlig [1989] (microfilms are available upon request). Field

of these

around black smokers

[Haymon et al., 1984]. The massive-sulfide deposits are underlain by stockwork veins and alteration

of occurrence

various alteration patterns are not random. Whereas diabase dikes are the most common, the relative volumes of epi-

[] P!agiogranite

Q High levelgabbro D Epidosite "•] Stockwork

Relations

Magmatic Relationships

The sheeted-dike complex has an average thickness of 1500 m [Pallister, 1981; Lippard et al., 1986; Nehlig, 1989] and consists of almost 100% dikes with lava and gabbro screens in the upper and lower transition zones, respectively. The typical width of the dikes is between 0.5 and 2 m and is similar to the width of dikes in other ophiolites (e.g., Troodos) and in Ocean Drilling Program (ODP) hole 504B, 200 km south of the Costa Rica rift [Adamson, 1985].

Figure 2. relative

Schematiclithostratigraphiccolumn showingthe

location

of the

main

units

discussed

in the text.

Epidosite is particularly abundant at the base of the sheeteddike complex and around plagiogranite plutons. It occurs mainly as stripeswithin dikes that are oriented parallel to the dikes.

The transition

zone between

the sheeted-dike

com-

plex and the overlying volcanic unit is usually very rich in stockworks.

NEHLIG

ET AL.: ROOT ZONES OF OCEANIC HYDROTHERMAL

dosite and mineralized spilite increase drastically in zones underlying major sulfide deposits. In addition, the relative amounts of epidosite decrease and mineralized spilite increase upward from the base of the sheeted-dikecomplex (Figure 2). Usually, near the transition to extrusive rocks, the sheeted-dikecomplex is invaded by numerousveins, and dikes are transformed into mineralized spilite. There thus seemsto be a correlation between massive-sulfidedeposits and the presenceof epidositesand mineralized spilites. There is considerablevariation in the type of the alteration pattern from one dike to the next. For example, a dike altered to epidosite may be enclosedby diabase or spilite dikes. This, along with the occurrence of diabase dikes truncatingepidositedikes, indicatesthat the epidositedikes were altered a short time after their emplacement and that there is a close temporal relationshipbetween dike emplace-

SYSTEMS

4705

Mineralogy and Petrography Magmatic Petrography

The dikes in the sheeted-dikecomplex have equigranular medium- to fine-grainedophitic, subophitic,and intergranular textures. They are mostly aphyric to sparselyplagioclase and/or pyroxene phyric rocks. The primary igneousmineralogy preserved in the diabase dikes consists of calcic plagioclase, clinopyroxene, and Fe-Ti oxides. However, most of these minerals show an almost complete replacement by greenschist-facies minerals. Rare core relicts of

clinopyroxene haveaveragecompositions ofwo41_49 En32_50 Fs7_19,and although most plagioclase has been albitized duringthe hydrothermalcirculation, anorthite compositions as high as 48% may be found (Figure 3). Hydrothermal Petrography

ment and alteration.

In contrast to the underlying plutonic units, the sheeteddike complex is characterized by pervasive alteration. The most common alteration paragenesis of the dikes is albite + containsmallmillimeterto centimeter-sized patchesof actinolite + chlorite + epidote + quartz + prehnite + epidosite. Epidosite appearsalso, as previously noted in the sphene + chalcopyrite + pyrite ___calcite. The magmatic Troodos ophiolite by Richardson et al. [1987], as narrow plagioclaseis almost entirely albitized, while the clinopybands, a few centimeters thick, subparallel to the dikes roxenes are uralitized or chloritized. Chlorite, epidote, and margins, and alternating with spilitic bands. This suggests prehnite are found as aggregatesbetween the plagioclase that water-rock ratios and fluid permeability varied widely, laths, and epidote is found as grains replacingfeldspar. The even within individual dikes, such that equilibrium was not iron-titaniumoxidesare commonlyaltered to sphene.Traces of pyrite and chalcopyrite occur in veins and within the attained even during retrograde off-axis alteration. Epidotization of the dikes is sometimes obviously con- dikes, in close association with chloritic patches. Calcite trolled by fracturing and developsas a front moving outward generally was the last phase to crystallize and appears from veins. However, the bandsof epidositedescribedin the mainly in microfractures. Although most dikes (diabase previous paragraph are generally not related to a central spilite - mineralized spilite) preserve their magmatic intervein. This implies that fracturingis not requiredfor epidosite granular texture, the epidosite alteration (quartz-epidote) is texturally distinct in that the relict igneous textures have to develop. been obliterated by a granoblastic secondarytexture. While diabase dikes are characterized by relict igneous minerals(clinopyroxeneand calcic plagioclase)and are only Vein Geometry and Density The whole sheeted-dike complex is crosscut by a dense slightly altered (actinolite, albite), spilites are marked by a net of epidote-quartz-sulfideveins. Whereas veins from the pervasive chloritization and albitization, the mineralized spilites by an increase of quartz versus plagioclase and baseof the sheeted-dikecomplexmainly containepidoteand abundantdisseminatedpyrite, and epidosite by a complete quartz, those from the top are marked by an increase in recrystallization into epidote and quartz. sulfides. In the transition zone between sheeted-dike comFigure 3 illustrates the chemical compositions of the plex and extrusive rocks, quartz/sulfide veins invade the secondary minerals within diabase and the three alteration dikes and form stockworks. Surroundingthese veins, the types. Diabase may preserve calcic plagioclaseand igneous diabaseis pervasively altered into mineralized spilite. pyroxene. The average plagioclase composition, including The epidote-quartz-sulfide veins are, in more than 90% of both low-temperature sodic grains and high-temperature the cases,parallel to the dike margins. However, some dikes relict igneousgrains, is An•3. The pistacite content of the exhibit a polygonal fracture pattern with the veins being epidotesis between 14 and 32%, and the amphibolesplot in oblique or perpendicular to the dike margins. Such vein the actinolite to hornblende fields of Leake's [1978] classifisystemsaffect only the last dikes emplacedand are restricted cation. Chlorite plots mainly in the pychnochlorite field of to thesedikes. Thus the hydrothermalvein systemwithin the Hey's [1954] classification diagram. sheeted-dikecomplex can be consideredto be mainly alongThe altered spilite, mineralized spilite, and epidosite The alteration pattern may also vary within dikes. For example, within the epidositezone, diabasedikes commonly

strike and vertical.

Nehlig et al. [1990] have shown that most veins (70%) have widths less than 2 mm, that there is no clear relationship between width and stratigraphiclevel within the sheeted-dike complex, and that the vein density increasesdrastically from the high-level gabbros toward the sheeted-dike complex. Similarly, the transition of the sheeted-dike complex to the volcanic unit is marked by an increase in the density of quartz-sulfideveins, while epidote-quartzveins are almost

absent

at this level.

rocks, in contrast to diabase, contain no relict igneous minerals and are composed of albite, pychnochloriteripidolite-diabantite, and actinolite to actinolitic hornblende. The pistacite content of epidote is between 12 and 30%. The only differences between the four lithologies are that the plagioclase compositions for diabase are distinctly more anorthitic and the amphiboles are more hornblende-rich than for the different alteration types and that the chlorites for the mineralized spilites have lower Fe/Fe+Mg ratios. There are thus no major differences in mineral composition between

4706

NEHLIG

ET AL.' ROOT ZONES OF OCEANIC

HYDROTHERMAL

SYSTEMS

.

ß

.

,

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AI IV

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NEHLIG

Table

1.

ET AL.: ROOT ZONES

OF OCEANIC

HYDROTHERMAL

SYSTEMS

4707

RepresentativeAnalysesof Diabase, Epidosite, Mineralized Spilite, and Spilite Mineralized

Diabase

Epidosite

PN 1

PN 9

50.50 1.14

52.00 1.60

52.20 1.06

49.65 0:79

Fe203

9.78

12.34

10.69

MnO

0.16

0.21

MgO

6.96

5.36 6.82

9.56

5.14 0.08 0.15

3.51 0.09 0.1

SiO2 TiO2

AI203

CaO

Na20 K20 P205

14.95

11.13

3.03 0.14 0.12

14.09

PN 427

PN 4

Spilite

PN 863

PN 128

Spilite

DI-EP

PN 805

9-4

20.18

38.80 0.83

24.40

55.00 1.30

53.45 1.33

- 3.09 -0.81

6.50

9.77

10.33

10.02

-5.95

0.13

0.11

0.09

0.24

0.10

-0.10

6.37

4.66

0.12

6.52

5.17

-3.73

0.54

3.35

7.66

4.06 0.07 0.15

7.03 0.06 0.30

- 1.53 -0.07 0.15 0.17

15.00

14.68

3.66 0.01 0.30

23.77

0.03 0.02 0.25

14.30

16.03

PF

1.34

1.97

1.02

2.17

2.35

5.77

3.16

H20

0.10

0.07

0.08

0.07

0.07

0.62

0.26

Total Li Rb Sr Ba V Cr Co Ni Cu Zn

99.35 2 3 173 30 262 196 33 62 6 38

99.83 2 3 96 10 309 41 37 28 29 46

99.81 2 3 137 35 280 28 44 23 9 22

99.78 1 2 280 20 105 36 14 17 49 18

100.46

98.90

2 1 690 109 459 25 16 2 31 16

7 3 110 44 207 2 23 4 322 1465

5.81

100.26 2 1 127 10 226 18 28 10 8 21

The DI-EP columngivesthe major elementchemicalfluxes(in gramsper 100g) involved in the isovolumicalterationof a diabase(PN 9, core of the dike, see Figure 6) to an epidosite(PN 9, margin of the dike). Location and descriptionof the samples are given by Nehlig [1989].

the different rock types, and they are only characterized by various amounts of the minerals with all possible intermedi-

a decreasein Si, Fe, Na, Mg, Ti, V, Zn, Cr, Co, and Ni and an increase in Ca, A1, Ba, and Sr.

ates.

Geochemical

Geochemistry Chemical Compositions

Fluxes

Geochemical fluxes in the different rock types have been calculated using the Gresens [1967] method assuming that the alteration process is isovolumetric. This hypothesis is supported by detailed petrographic observations of thin sectionsthat do not show a volume changeduring alteration. To estimate the geochemical fluxes, a mineralogical approach and a whole rock approach were used. The mineralogicalapproach (Table 2) showsthat all major oxides can be lost or gained during the various alterations.

A total of 85 samples, mainly from the central part of the ophiolite (around Daris, Figure 1) but also from the northern and southernmassifs, was analyzed for both major and trace elements; the differences between diabase, epidosite, spilite, and mineralized spilite, so obvious in the field and in thin section, are equally clear in their bulk chemical compositions. Representative major and trace element compositions Only H:O showsa positiveflux from the hydrothermalfluid for the different rock types are given in Table 1 and shown into the rock. However, it is noteworthy that during the graphically in Figure 4. When compared to the diabase, the alteration of a parent rock (e.g., 47% clinopyroxene, 49% epidositesare depletedin silica, magnesium,and sodiumand plagioclase,and 4% titanomagnetite)into the least altered enriched in aluminum and calcium. In the spilites, sodium rocks termed diabase (actinolite, albite, sphene), the main and water are enriched and calcium is leached compared to fluxes are a gain in Si and Na and a loss of Ti, A1, Mn, Mg, the diabase. In the mineralized spilites, most major elements and Ca. If the parent rock is altered into a spilite (albite, are diluted by quartz and sulfide precipitation, except for chlorite, sphene),then Fe, A1, and Na are gainedby the rock MgO and alkalis that are enriched relative to the other rocks, while Si, Ti, Mn, and Ca are lost. During formation of an on the basis of oxide ratios. epidosite(epidote, quartz, chlorite), Na is lost, while Fe and Copper and zinc are highly depleted in all rocks except in Ca are gained.Thus dependingon the relative abundanceof the mineralized spilitesfrom the stockwork zones, which are the different alteration types, positive or negative fluxes are possiblefor the sheeted-dikecomplex taken as a whole. characterized by sulfide precipitation (Figure 5). One of the main problemsin calculatingthe masstransfer Four samplestaken within a singledike show a continuous chemical variation from diabase to epidosite (Figure 6) with between rocks of ophiolites and seawater is the difficulty in

4708

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ROOT ZONES

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HYDROTHERMAL

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MgO CHLORITE

PHIBOLES

o

o o

o

o o

o

EPIDOTE

ALBITE CaO

Na20+K20

Figure 4. MgO-Na20+K20-CaO ternary diagramfor diabase(open circles), epidosite(solid circles), spilite(solid squares),and mineralizedspilites(opensquares)samples.Samplesfrom the samedike are linked by a line. Also indicated(soliddiamonds)are the chemicalcompositionsof the major hydrothermal minerals. The two stars marked a and b correspondto the protoliths chosenfor the geochemicalfluxes calculations(a is PROT1, least altered samples;b is PROT2, MORB composition).

140 120 lOO

[] Diabase

80

ß Epidosite

Zn (ppm)

ß

o Spilite

60

c Min. Spilite

40

20

0

20

40

60

80

100

120

140

Cu (ppm)

Figure 5. Cu-Zn diagramfor dike samplesof diabase,epidosite,spilite,and mineralizedspilite(in part off diagram). Solid square indicatesthe mean Cu-Zn content of MOR basalts [Bougault and Hekinian, 1974; Hubberten et al., 1983].

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ET AL.:

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reliably reconstructing the original rock composition. As a result, the choice of a protolith is highly speculative. We have chosen two different protoliths (Table 3): the first corresponds to the mean composition of glasses and least altered basalts from Deep Sea Drilling Project (DSDP) leg 82 (Mid-Atlantic Ridge between 30ø and 40øN) [Viereck et al., 1989], and the secondcorrespondsto the mean composition of the five least altered (lowest loss on ignition) diabase samples from the Samail ophiolite sheeted-dike complex, recalculated to an anhydrous composition. The composition of the chosen protoliths is given in Table 3, along with the chemical fluxes involved during formation of an epidosite (fluxes averaged for 21 samples). Table 3 shows that formation of an epidosite is associatedwith loss of Si, Mg, and Na

and a gain of A1, Fe, Ca, and H20. The precipitation of abundant epidote (quartz and sulfides

54

5O

HYDROTHERMAL

SYSTEMS

4709

EPIDOSITE

iPNq

DIABASE

PN 7

PN 8

PN 9

2O

15

in veins) requires a source for Si, Ca, A1, and Fe. All these elements are provided by the hydrothermal reactions in the host rocks: silica by uralitization, chloritization, and epidotization, calcium by uralitization, albitization, and chloritization, A1 by chloritization, and Fe by alteration of ilmenite to sphene. Compared to the typical Cu and Zn content of MOR basalts (71 ppm Cu and 73 ppm Zn [Bougault and Hekinian, 1974; Hubberten et al., 1983]), all rock types except the mineralized spilites are depleted in Zn and Cu: Diabase has 38 ppm Cu and 49 ppm Zn on average for 40 samples, epidosite has 12 ppm Cu and 31 ppm Zn on average for 21 samples, spilite has 26 ppm Cu and 36 ppm Zn on average for 10 samples, and the mineralized spilites have 450 ppm Cu

CaO

and309ppmZn onaverage forninesamples.' Microthermometric

Constraints

A microthermometric study was carried out on primary fluid inclusions trapped in three samples of quartz and epidote from epidote/quartz veins. The measurements were carried out with a Chaixmeca heating and freezing stage [Poty et al., 1976] and a U.S. Geological Survey (USGS) stage [Roeder, 1984]. The reproducibility of the measurements is estimated at -+2øCfor temperatures around 400øC and _+0.2øC below

0øC. Salinities

of unsaturated

300 t

2001

••

100 ß

50

Sr

-

solutions

were obtained using the equations of Potter et al. [ 1977] and have an error of -+0.2 wt % NaC1 eq. The main results are presented in Table 4 and Figure 7. The salinities of all fluid-inclusionsrange between 2.7 and 13.1, with an average salinity of 4.5 wt % NaC1 eq. The average salinitiesin each sampleare slightly higher than that of seawater (3.2 wt % NaC1 eq). The homogenization temperatures of all measured fluid inclusions are between 247ø and 402øC (average 346øC). Nehlig [1989] has shown that the fluid pressure within the sheeted-dike complex is close to the calculated hydrostatic pressure. The hydrostatic pressureswere calculated using an

•= 40

Cr

Co

30 20

10

Ba

Figure 6. Major (oxides) and trace element variations in four samplesfrom a single dike (lower sheeted-dike complex from Wadi Fizh). From the epidosite margin (PN 4) to the diabase core (PN 9) through the intermediate samples PN 7

averagecrustaldensityof 2.80g/cm3, a seawater densityof 1 g/cm3, andan averagehydrothermal fluiddensityof 0.9 g/cm3.Theoceandepthof 2300mwassuggested by Spooner and PN 8, the SiO2, Fe203, MgO, Na20, TiO2, Ni, Co, Cr, and Bray [1985], who found fluid inclusion evidence for boiling in a massive-sulfide deposit (Lasail) from the northern Samail ophiolite at pressures between 200 and 250 bars corresponding to ocean depths of 2000 to 2500 m. The hydrostatic pressure correction (Table 4) for all mean homogenization temperatures within all samples is around

Zn, and V increaseprogressively,whereas A1203, CaO, Ba, and Sr decrease. Copper decreasesfrom the core of the dike toward its epidotized margins, where it increases; this sharp increase may be linked to the occurrence of a small mineralized quartz/epidote/sulfidevein along the dike margin. The major element oxides have been normalized to 100 without the ignition loss.

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Table 2. Major Chemical Fluxes Involved in the Alteration of Clinopyroxene (CPX) Into Actinolite (AMPH), Plagioclase(AN 60) Into Albite (ALB), PlagioclaseInto Epidote (EP), Amphibole Into Chlorite (CHL), and Ilmenite (ILM) Into Sphene (SPH) CPX

SiO2 TiO2 A1203 FeO MnO

AN 60

ILM

AMPH

CHL

ALB

EPID

SPH

CPX AMPH

AN 60 ALB

AN 60 EP

51.80 0.59 2.78

53.10 0.00 29.80

0.10 45.50 0.30

50.33 0.64 4.28

29.02 65.90 0.03 0.00 18.37 21.50

38.07 0.15 23.96

32.40 33.40 2.80

-6.28 -0.01 1.09

11.34 0.00 -8.78

-5.44 0.19 0.19

7.97 0.27

0.00 0.00

51.70 3.00

14.98 0.32

21.11 0.44

0.00 0.00

11.03 0.20

2.30 0.10

5.58 0.02

0.00 0.00

13.81 0.25

MgO

15.72

0.00

0.10

13.95

17.54

0.00

0.20

0.70

-3.10

CaO

20.68

12.50

0.30

10.81

0.26

1.70

22.89

26.60

-10.90

Na20

0.22

4.66

0.00

0.81

0.05

10.80

0.05

0.30

D

3.45

2.70

4.70

3.12

2.82

2.64

3.38

3.50

0.00 -10.84

0.51

0.25 16.15

5.96

-4.54

AMPH CHL

-24.10 -0.61 12.32 4.10 0.08

1.90 -10.57

-0.76

ILM SPH

24.03 -20.63 1.79 -49.99 -2.98

0.42 12.06

0.22

Fluxes are given in grams per 100 g. the mineral chemical compositionsare mean compositions measured in the sheeted-dike complex. The fluxes are calculated for isovolumic transformations. The calculations show that all oxides can be gained or lost in the sheeted-dike complex, dependingon the relative volumes of parental minerals involved in the transformations.

25øC. This provides a mean trapping temperature of 339øC for the fluids trapped in the quartz from the quartz-epidote vein (sample OM321 from the top of the sheeted-dikecomplex underlying the Bayda massive-sulfide deposit). The fluid-inclusions within epidote samples (PN 804 and 791) from the base of the sheeteddike complex of Wadi Haymiliyah (10 km west of Daris, Figure 1) gave much higher mean trapping temperatures of 390øCand 405øC. The latter values are very close to the critical curve (Figure 7).

Epidotized Root Zones of the Hydrothermal Ore-Forming Circulation The stratigraphic position of the epidosite at the base of the sheeted-dike complex, in the roots of the stockworks, and directly overlying the fossil magma chamber suggests that it is the root zone of the hydrothermal circulation where downflowing hydrothermal fluids were heated by an immediately underlying magma chamber and reacted with the rocks. Unlike the other rocks of the sheeted-dikecomplex, the epidositesno longer have magmatic textures and show a significantreductionin the number of phasespresent(quartz and epidote). These zones have experienced high water-rock ratios. In addition, field studies have shown that within the epidosite-rich zones, epidosite stripes are interlayered with spilite stripes. They are oriented parallel to the dike margins and are truncated by later dikes, suggestingthat they formed early during the period of dike injection. Whole rock geochemicalfluxes indicate that comparedto the presumed protoliths, the epidosites are depleted in Si, Mg, and Na and are enriched in A1, Fe, Ca, and H20. Schiffman et al. [1987] showed that the almost total Mg depletion in epidositesfrom the Troodos ophiolite requires very high water/rock ratios (around 500), assumingthat the fluidshad Mg contentsof 2 mmol/kg (similar to thoseventing at modern high-temperature hot springs [Shanks and Seyfried, 1987]). Calculationsfor the Samail ophiolite provide similar water/rock ratios [Nehlig, 1989]. However, it mustbe emphasized that these high water-rock ratios presumably have only a local significance,most of the Mg being only locally redistributedfrom epidosite bands to spilite bands. Several workers have documented whole rock, base metal

depletions in epidosites (from the Troodos ophiolite: Schiffman et al. [1987], Richardson et al. [1987], and Schiffman et al. [1990]; and from the Josephine ophiolite: Harper et al. [1988] and Alexander et al. [1993]) and have suggestedthat these rocks represent a source area for metals in volcanic-hosted massive-sulfidedeposits. Although this depletionhas also been observedin the Samail ophiolite (our study), it must be stressedthat with the exception of the mineralized spilites, the spilite and diabase dikes are almost as depleted in copper and zinc as the epidosites and might represent additional metal sources. Along with the changes in major element composition, leachinghas removed 70% copper and 60% zinc on average within the sheeted-dike complex, not taking into account stockworks and associatedmineralized spilites. To supply

Table 3. Major Chemical Fluxes Involved in the Alteration of Two Different Protoliths (PROT1 and PROT2) Into Epidosite (EPID 1 and EPID2, Respectively) PROT1

SiO2 TiO2 A1203 F220 3

PROT2

52.94 1.69 14.72 10.89

50.26 1.10 15.80 10.34

MnO

0.11

0.17

MgO

4.69

7.84

CaO

8.41

Na20 K20 Ignition loss Density

4.82 0.08 1.24 2.82

12.48

2.13 0.24

EPID 1

-2.23 -0.21 3.36 0.28 0.03

-1.91 7.64

-3.55 -0.05

EPID2

- 1.42 0.65 2.49 0.78 -0.04

-5.51 2.79

-0.83 0.00

2.90

Fluxes are in grams per 100 g. PROT2 is the average composition of DSDP leg 82 glassesand least altered basalts [Viereck et al., 1989], while PROT1 is the average composition of the five least altered samples from the sheeted-dike complex (analyses of Nehlig [1989] and Bendel [1989]). The isovolumetric fluxes EPID1 and EPID2 are averaged for 21 epidosite samples.A negative flux indicates that the element is substracted

from the rock.

NEHLIG

Table

ET AL.' ROOT ZONES

OF OCEANIC

HYDROTHERMAL

SYSTEMS

4711

Description of the SamplesSelectedfor Fluid Inclusion Studies and Principal Microthermometric

4.

Results

Homogenization

Samples OM 321 PN 804 PN 791

Host Mineral

Lithology

Depth, m

HP, bars

N

INTTH, øC

Q E E

Q/E/S vein Q/E vein Q/E vein

760 1300 1400

290 320 330

40 28 23

247-376 345-384 372-402

Fusion

Mean

N

INTSal, wt % NaC1 eq

Mean, wt % NaC1 eq

314 365 380

17 6 19

4.2-13.1 3.1-4.9 2.7-4.0

5.9 4.0 3.4

Host mineralsare Q, quartz; E, epidote;Lithology is petrologyof the studiedsamples;depth is subbasementdepth within the crust; HP is calculatedhydrostaticpressure;homogenization:N is numberof measurements;INTTh is homogenization temperature intervals; mean is mean homogenizationtemperature; fusion: N is number of measurements;INTSal is variations of the fluid inclusion salinities; mean is mean salinities.

fashion. As a result, the resistance to flow is minimum, and

the copper precipitated in the Lasail massive-sulfidedeposit

(2 x 105t Cu [Alabasteret al., 1980])andassuming a 100% abundant dissolution of the hostrock might generate the high efficiency in the precipitation of copper into the massive

sulfides, this requiresthe alterationof 4000 x 106 t or a volume of 1.4 km 3 of sheeted dikes. Fluid inclusion measurementswithin quartz/epidote veins from the base of the sheeted-dike complex indicate that the epidosite assemblagewas formed by interaction with fluids with a salinity equivalent to that of seawater at temperatures between 370øC and 430øC (average 390øC), similar to those inferred for the root zones of black smoker type oceanic circulation [Campbell et al., 1988;Bischoff and Pitzer, 1989]. These conditions place the hydrothermal fluids close to the critical point where the transport properties of seawater evolve exponentially [Norton, 1984]: as the hydrothermal fluids approach the critical point, the coefficient of thermal expansionincreasesto reach a maximum, while the viscosity decreasesand is close to its minimum. This might explain the vein-absent bands of epidosite so common in the dikes by allowing major fluid circulation in an unfractured lowpermeability rock. In addition, quartz solubility in seawater reaches a maximum at temperatures below the two-phase boundary at these hydrostatic pressures [Bischoff and Pitzer, 1985]. Other minerals might behave in a similar

local permeabilities required to explain the high fluid/rock ratios necessary to generate the epidosite. Two different interpretations of the relation of epidosite formation to intrusive and tectonic activity were suggested in studies of epidosites in the Solea graben area of the Troodos ophiolite [Richardson et al., 1987; Bettison-Varga et al., 1992]. Schiffman et al. [1987] and later on BettisonVarga et al. [1992] suggested that significant extensional deformation followed by intrusion of gabbro is required to produce massive epidosite zones. In contrast, Richardson et al. [1987] suggestedthat epidosites represent the hydrothermal reaction zones, in which the descending cold seawater became heated by the underlying magma chamber and reacted with the surrounding rock. As shown by BettisonVarga et al. [1992], the distinction between these two models

is critical

to models

of oceanic

Wt % NaCI .......

ß

0M321

ß

PN791

ß

PN804 Seawater

Critical

200

300

crustal

evolution

because the first implies that development of black smoker type hydrothermal systems follows significant tectonic extension and may be episodic, whereas the latter model is not. Detailed mapping and petrologic studies in the Samail ophiolite have shown that there is considerablevariation in the type of alteration from one dike to the next and that as a

400

sal.

curve

500

Trapping temperature (øC)

Figure 7. Salinity (weight percent NaC1 eq) versus homogenizationtemperature (degrees Celsius) diagramfor epidote-hostedfluid inclusionsin samplesPN 791, PN 804, and OM 321. The seawater salinity (3.2 wt % NaC1 eq) is indicated with a dashed line; the dotted line correspondsto the critical curve of seawater.

4712

NEHLIG

ET AL.'

ROOT

ZONES

OF OCEANIC

result there must be a close temporal relationship between dike emplacement and its transformation into epidosite. This, along with the absenceof major detachmentzones and no evidence of tectonic extension, implies that the formation of epidosite does not depend upon prior deformation.

HYDROTHERMAL

SYSTEMS

Bettison-Varga, L., R. J. Varga, and P. Schiffman, Relation between ore-forming hydrothermal systems and extensional deformation in the Solea graben spreadingcenter, Troodos ophiolite, Cyprus, Geology, 20, 987-990, 1992. Bischoff, J. L., and K. S. Pitzer, Phase relations and adiabats in boiling seafloor geothermal systems, Earth Planet. Sci. Lett., 75, 327-338, 1985.

Conclusion We have shown that the alteration

within the sheeted-dike

complex has the following characteristics. The sheeted-dike complex shows four major lithologies termed diabase, spilite, epidosite, and mineralized spilite. Diabase rocks are by far the most common and are altered, having an albiteactinolite (+chlorite+sphene) hydrothermal paragenesis along with magmatic Ca-plagioclase and clinopyroxene relicts. Spilite has an albite-chlorite (+actinolite+ sphene) hydrothermal paragenesis,mineralized spilites exhibit high quartz/albite and chlorite/actinolite ratios and are crosscut by very abundanttiny quartz-sulfideveins, and epidositeis characterizedby an epidote-quartz paragenesis.The volume of mineralized spilite and epidosite increasesdrastically in zones underlying massive-sulfidedeposits. Whole rock chemical flux calculations for epidosite indicate a loss of Si, Mg, and Na and a gain of Al, Fe, Ca, and H20. The epidosite and spilites are stronglydepletedin Cu and Zn and may have provided the metals precipitated in the massive-sulfidedeposits. Microthermometric fluid inclusion measurementsprovide trapping temperatures at the base of the sheeted-dike complex up to 430øC, with salinities slightly higher than that of seawater.

tems, and alteration zones, indicates that the fluids must

have mainly convected in along-strike vertical planes. The alteration

levels or listtic faults within

the sheeted-dikecomplex implies that formation of epidosite does not depend upon prior deformation. Acknowledgments. Financial support from the Bureau de Recherches G6ologiques and MiniSres and from the PNEHO is acknowledged, as are facilities from the Oman Ministry of Petroleum and Minerals represented by M. M. Kassim. The paper was edited by H. M. Kluywer. We thank R. Koski, D. Naidoo, and J. Pallister whose reviews improved the manuscript significantly.

References Adamson, A. C., Basement lithostratigraphy, Deep Sea Dtilling Project, hole 504B, Initial Rep. Deep Sea Drill. Proj., 83,121-127, 1985.

Alabaster, T., The interrelationship between volcanic and hydrothermal processesin the Oman ophiolite, Ph.D. thesis, 408 pp., Open Univ., Milton Keynes, England, 1982. Alabaster, T., J. A. Pearce, D. I. J. Mallick, and I. M. E1 Boushi, The volcanic stratigraphyand location of massive-sulphidedeposits in the Oman ophiolite, in Ophiolites, Proceedings of the International Ophiolite Symposium, edited by A. Panayotou, pp. 751-757, Cyprus Geological Survey Department, Nicosia, 1980. Alexander, R. J., G. D. Harper, and J. R. Bowman, Oceanicfaulting and fault-controlled subseafloor hydrothermal alteration in the sheeted dyke complex of the Josephine Ophiolite, J. Geophys. Res., 98, 9731-9759, 1993.

Bendel, V., L'hydrothermalisme oc6anique dans le complexe filonien du massif de Haylayn (Ophiolite de Seamil, Oman): Mineralogie, microthermom6trie des inclusions fluides, bilans chimiques des 6changes, Master thesis, 78 pp., Brest Univ., Brest, France, 1989.

1988.

Haymon, R. M., R. A. Koski, and C. Sinclair, Fossils of hydrothermal vent

worms

discovered

in Cretaceous

sulfide

ores of the

Samail Ophiolite, Oman, Science, 223, 1407-1409, 1984. Haymon, R., R. A. Koski, and M. J. Abrams, Hydrothermal dischargezones beneath massive sulfide depositsmapped in the Oman ophiolite, Geology, 17, 531-535, 1989. Hey, M. H., A new review of the chlorites, Min. Mag., 30,277-292, 1954.

The vertical and along-strike geometry of the vein sys-

absence of horizontal

Bischoff, J. L., and K. S. Pitzer, Liquid-vapor relations for the system NaC1-H20: Summary for the PTX surface from 300 to 500øC. Am. J. Sci., 289, 217-248, 1989. Bougault, H., and R. H6kinian, Rift valley in the Atlantic ocean near 36ø50'N: Petrology and geochemistry of basaltic rocks, Earth Planet Sci. Lett., 24, 249-261, 1974. Campbell, A. C., M. R. Palmer, T. S. Klinkhammer, J. M. Edmond, J. R. Lawrence, J. Casey, G. Thompson, S. Humphties, P. Rona, and J. A. Karson, Chemistry of hot springs on the Mid-Atlantic Ridge, Nature, 335, 514-519, 1988. Coleman, R. G., Tectonic setting for ophiolite obduction in Oman, J. Geophys. Res., 86, 2497-2508, 1981. Glennie, K. W., M. G. A. Boeuf, M. W. Hugues-Clark, M. Moody-Stuart, W. F. H. Pilaar, and B. M. Reinhardt, Geology of the Oman Mountains. Kon. Ned. Geol. Mijnbouwk. Genoot. Verh., 31,423 p., 1974. Gresens, R. L., Composition-volume relationships of metasomatism, Chem. Geol., 2, 47-65, 1967. Harper, G. D., J. R. Bowman, and R. Kuhns, Field, chemical and isotopic study of subseafloormetamorphism in the Josephine Ophiolite, California-Oregon, J. Geophys. Res., 93, 4625-4656,

Hubberten, H. W., R. Emmermann, and H. Puchelt, Geochemistry of basalts from Costa Rica Rift sites 504 and 505 (Deep Sea Drilling Project legs69 and 70), Initial Rep. Deep Sea Drill. Proj., 69, 791-804, 1983.

Ixer, R. A., T. Alabaster, and J. A. Pearce, Ore petrography and geochemistry of massive-sulphide deposits within the Samail ophiolite, Oman, Trans. Inst. Min. Metall., Sect. B, 93, 114-124, 1984.

Juteau, T., M. Beurrier, R. Dahl, and P. Nehlig, Segmentationat a fossil spreadingaxis: the plutonic sequenceof the Wadi Haymiliyah area (Haylayn Block, Samail Nappe, Oman), Tectonophysics, 151, 167-197, 1988.

Leake, E., Nomenclature of amphiboles, Bull. Mineral., 101,453467, 1978. Lescuyer, J. L., E. Oudin, and M. Beurrier, Review of the different types of mineralization related to the Oman ophiolitic volcanism, in 7th IAGODD SymposiumLullea 1986 (Sweden), pp. 489-500, Schweitzerbart'sche, Stuttgart, Germany, 1988. Lippard, S. J., A. W. Shelton, and I. G. Gass (Eds.), The Ophiolite

of Northern Oman, Mem. Geol. $oc. •London 11, 178 pp., Blackwell Scientific, Boston, Mass., 1986. Nehlig, P., Etude d'un syst•me hydrothermal oc6anique fossile: L'ophiolite de Samail (Oman), Ph.D. thesis, 600 pp., Univ. Bretagne Occidentale, Brest, France, 1989. Nehlig, P., P. Watremez, and T. Juteau, Geometry of the ridge axis hydrothermal circulation systemsin the oceanic crust: The example of the Oman ophiolite, in Ophiolites, Oceanic Crustal Analogues, edited by J. Malpas, E. Moores, A. Panayotou, and C. Xenophontos, pp. 415-433, Cyprus Geological Survey Department, Nicosia, 1990. Nicolas, A., G. Ceuleneer, F. Boudier, and M. Misseri, Structural mapping in the Oman ophiolites- Mantle diapitism along an oceanic ridge, Tectonophysics, 151, 27-56, 1988. Norton, D. L., Theory of hydrothermal systems,Annu. Rev. Earth Planet. $ci., 12, 155-177, 1984.

Pallister, J. S., Structure of the sheeted dyke complex of the Samall ophiolite near Ibra, J. Geophys. Res., 86, 2661-2672, 1981. Potter, R. W., II, R. S. Babcock, and D. L. Brown, The volumetric properties of aqueous sodium chloride solutions from 0 to 500øC

NEHLIG

ET AL.:

ROOT

ZONES

OF OCEANIC

and pressures up to 2000 bars based on a regression of the available literature data, U.S. Geol. Surv. Bull., 1421-C, 36 pp., 1977.

Poty, B., J. Leroy, and L. Jachimowicz, Un nouvel appareil pour la roesure des temp6ratures sous le microscope: L'installation de microthermom6trie

Chaix Meca. Bull. Soc. Fr. Mineral.

Cristal-

!ogr., 99, 182-186, 1976. Reinhardt, B. M., On the genesisand emplacement of ophiolites in the Oman mountains geosyncline, Schweiz. Mineral. Petrogr. Mitt., 49, 1-30, 1969.

Reuber, I., P. Nehlig, and T. Juteau, Evidence of a fossil off-axis mantle diapir, deduced from crustal structures in the Haylayn Block (Samail Nappe, Oman), J. Geodyn., 13, 253-278, 1991. Richardson, C. J., J. R. Cann, H. G. Richards, and J. G. Cowan, Metal depleted root zones of the Troodos ore-forming hydrothermal systems, Cyprus, Earth Planet. Sci. Lett., 84,243-253, 1987. Roeder, E., Fluid-Inclusions, Rev. Mineral., 12,644 pp., 1984. Schiffman, P., B. M. Smith, R. J. Varga, and E. M. Moores, Geometry, conditions and timing of off-axis hydrothermal metamorphism and ore-deposition in the Solea graben, Nature, 325,

HYDROTHERMAL

SYSTEMS

by J. Malpas, E. Moores, A. Panayotou, and C. Xenophontos, pp. 673-683, Cyprus Geological Survey Department, Nicosia, 1990. Shanks, W. C., and W. E. Seyfried, Stable isotope studies of vent fluids and chimney minerals, southern Juan de Fuca Ridge: Metasomatism and seawater sulphate reduction, J. Geophys. Res., 11, 11,387-11,399, 1987. Spooner, E. T. C., and C. J. Bray, Fluid-inclusion evidence for boiling at 370øC in the stockwork of the Lasail ophiolitic hydrothermal massive-sulfide deposit, Oman (abstract), Eos Trans. AGU, 66,724-725, 1985. Viereck, L. G., M. F. J. Flower, J. Hertogen, H. U. Schmincke, and G. A. Jenner, The genesisand significanceof N-Morb sub-types, Contrib. Mineral. Petrol., 102, 112-126, 1989. V. Bendel, J. Cotten, and T. Juteau, GDR "Genb•se et Evolution des Domaines Oc6aniques," UBO, 6, Av. Le Gorgeu, 29287 Brest Cedex, France.

P. Nehlig, BRGM/SGN/GEO,

B.P. 6009, 45060 Orleans Cedex,

France.

423--425, 1987.

Schiffman, P., L. A. Bettison, and B. M. Smith, Mineralogy and geochemistry of epidosites from the Solea graben, Troodos ophiolite, Cyprus, in Ophiolites, Oceanic Crustal Analogues, edited

4713

(Received March 12, 1993; revised August 5, 1993; accepted September 16, 1993.)