geochemestry alteracion vms

Economic Geology Vol. 79, 1984, pp. 1880-1896

Geochemistry of the Alteration Pipe at the Bruce Cu-Zn Volcanogenic Massive Sulfide Deposit, Arizona*

PETER B. LARSON**

Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, California 91125

Abstract

The Bruce volcanogenic massive sulfide deposit is a conformable lens which lies at an andesite-rhyolite contact within the 1.7- to 1.0-b.y.-old Yavapai Series exposed south of Bagdad, Arizona. The upper part of a funnel-shaped alteration pipe replaces andesite within the stratigraphic footwall of the deposit. This pipe can be divided into three zones based on characteristic mineralogy: a chlorite zone consisting of chlorite (40-90 wt %) and quartz completely replaces the andesite in the central part of the pipe. A sericite zone containing subequal amounts of chlorite, sericite, and quartz is peripheral to the chlorite zone in the upper part of the pipe. The modal abundances of alteration minerals decrease outward through a gradational zone to unaltered andesite. Amphibole and biotite occur as metamorphic minerals; they are most abundant in the gradational zone. Some biotite is texturally and chemically related to the chlorite and sericite and is also probably hydrothermal in origin. All of the rocks within the pipe have been depleted in •80 with whole-rock •i•80 values ranging from 2.4 to 7.6 per mil. A linear relationship exists between the oxygen isotope composition and the quantity of hydrothermal chlorite in the alteration pipe. Both the amount of chlorite and the •80 depletion are functions of the extent to which the rock has reacted with the hydrothermal fluid. The bulk chemistry of the altered rocks, when measured as a function of increasing alteration progress (decreasing •i180), shows strong enrichments in MgO and FeO and depletions in SiO2, K20, Na•O, and CaO. The most altered and most 180-depleted samples are nearly completely altered to chlorite; their bulk chemistry approaches that of the chlorite in the sample. All of the ferromagnesian minerals become more Mg-rich toward the center of the pipe. The Fe/(Fe + Mg) ratio of chlorite ranges from 0.8 in the least altered sample to 0.3 in the center of the chlorite zone. Extrapolating the linear relationship between the amount of chlorite in a sample and its •i•80 value to 100 percent chlorite gives a value of 2.1 for the pure chlorite. At 250 ø and 300øC water in equilibrium with this chlorite would be 1.1 and 2.05 per mil, respectively. These data are comparable to •i180 values of water issuing from hot springs on the East Pacific Rise and are consistent with a slightly 180-shifted seawater as a source for the ore- forming solutions. Prior to entering the alteration pipe, the seawater reacted with volcanic rocks at depth, lowering the pH of the fluid such that chlorite was stable when the fluid entered the pipe. Subsequent formation of chlorite continued to lower the pH and decreased the activity of Mg in the solution, such that the fluid moved into the sericite stability field.

Introduction

THIS study presents mineralogical, chemical, and oxygen isotope analyses of samples collected across the alteration pipe at the Bruce mine, a 1.7-b.y.-old volcanogenic massive sulfide deposit in west-central Arizona. Approximately 2 million tons of ore aver- aging about 12 percent zinc and 4 percent copper were produced from the Bruce deposit up to 1976, when mining ceased. Chemical analyses of rocks and microprobe analyses of individual mineral grains from within the alteration pipe at the Bruce deposit

* Contribution 3903, Publications of the Division of Geological and Planetary Sciences, California Institute of Technology, Pasa- dena, California 91125

** Present address: Department of Geology, Washington State University, Pullman, Washington 99164-2812

were carried out to determine aspects of the composition of the ore-forming solution and the results of the interaction of this solution with the wall rocks.

Oxygen isotope analyses of hydrothermally altered rocks within the pipe were made in order to define the isotopic composition and origin of the ore- forming solutions.

Since the volcanogenic model for certain Canadian Precambrian massive sulfide deposits was developed nearly 20 years ago (Gilmour, 1965; Roscoe, 1965), it has served successfully as a basis for studies of numerous Phanerozoic and Proterozoic deposits throughout the world (Franklin et al., 1981). A number of volcanogenic massive sulfide bodies, similar to the Bruce deposit, have been mined in central Arizona (Donnelley and Hahn, 1981), the largest of which, the United Verde and Verde Extension in

0361-0128/84/366/1880-17$2.50 1880

BRUCE MINE GEOCHEMISTRY 1881

the Jerome area, produced 34 million tons of ore (Anderson and Creasey, 1958; Anderson and Nash, 1972).

Geochemical studies similar to those of the present work have recently been carried out on several of the Archean deposits in Canada (Roberts and Rear- don, 1978; Riverin and Hodgson, 1980). In addition, oxygen isotope data for a number of Canadian Ar- chean massive sulfide alteration zones are now avail-

able, including the Amulet A mine (Beaty and Taylor, 1982), the Kidd Creek mine (Beaty and Taylor, 1980), and the Matagami deposit (Costa et at., 1980). Several Phanerozoic deposits have also been isotopically investigated (e.g., Ohmoto and Rye, 1974; Heaton and Sheppard, 1976; Casey and Tay- lor, 1982). However, detailed correlation between quantitative alteration mineralogy and oxygen isotope variations within the alteration pipes is lacking. An aim of the present study is to fill this gap partially. Both isotopic analyses and the determination of quantitative mineralogy can be used to measure the extent of water-rock interaction within such hydrothermal systems. Thus, by utilizing both techniques, we can define the extent of the reaction of hydrothermal solutions with their country rocks in the massive sulfide environment.

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-r'• SILICIC INTRUSIVE ROCKS /.•, GENERAL STRIKE & DIP

FII GABBRO & ANORTHOSITE [] MINE

F .,'.!• / COPPER KING • HILLSIDE MICA SCHIST 2BRUCE & OLD DICK 3COPPER QUEEN

BUTTE FALLS TUFF • ARIZONA •1-I/1 BRIDLE FORMATION SHOWING RHYOLITE

FIG. 1. Generalized geologic map of the Bagdad area, Yavapai County, Arizona, modified after Anderson et al. (1955).

Geologic Setting of the Bruce Deposit

Regional geology

The Bruce deposit is one of four closely spaced votcanogenic massive sulfide lenses located 4 to 6 km southwest of Bagdad, Arizona (Fig. 1) (Larson, 1976, 1977). The geology of the Bagdad area is described by Anderson et at. (1955).

The oldest rocks in the Bagdad area were initially correlated with the Precambrian Yavapai schist in the Jerome area, Arizona (Butler and Wilson, 1938). These rocks were subsequently included within the newly defined Yavapai Series of the Prescott-Jerome area (Anderson et al., 1955; Anderson and Creasey, 1958). The most recent detailed discussion of the Yavapai Series in central Arizona is that of Anderson et al. (1971). The volcanic stratigraphy they describe cannot be extended into the Bagdad area on the basis of tithotogic or stratigraphic correlation, but if the Yavapai Series is considered a time-stratigraphic designation (Anderson and Silver, 1976), then the inclusion of the Bagdad rocks within the Yavapai Series remains valid based on radiometric dating (Silver, 1968).

Anderson et at. (1955) have mapped three units within the Yavapai Series in the Bagdad area (Fig. 1); from oldest to youngest they are: the Bridle Formation, which hosts the Bruce and other massive sulfides; the Butte Falls tuff; and the Hillside mica schist. The Bridle Formation comprises mafic to

silicic metavolcanic rocks and interbedded tuffaceous

sedimentary rocks. Local pillow structures in basalts and relict bedding and channeling in the sedimentary rocks indicate a subaqueous environment and can be used to determine stratigraphic facing. South of Bagdad the Bridle Formation crops out in two sub- parallel belts lying 5 km apart that strike northeast to north-northeast and dip steeply to the northwest. The western belt, in which the massive sulfide deposits are found, is overturned. The two belts may represent the limbs of an overturned, northeast- trending syncline (Anderson et at., 1955). The Butte Falls tuff and Hillside mica schist are composed of siliceous schist, slate, and impure quartzite. Rhyotite horizons are found within the Butte Falls tuff, and Silver (1968) reports a U-Pb age of 1,760 m.y. for zircons from one such horizon.

In the Bagdad area the Yavapai Series has been intruded by other Precambrian rocks of diverse composition (Anderson et at., 1955). A layered gabbro-anorthosite complex intrudes the Bridle For- mation 5 km west of Bagdad. A porphyritic alaskite intrudes the layered complex and the Bridle For- mation. Younger Precambrian diabase is locally present. The Lawler Peak granite (1,375 m.y. old, Silver, 1968) occurs as a circular pluton 1 km northeast of Bagdad. Late Cretaceous and Tertiary stocks and dikes are also present. Actinotite, chlorite, epidote, muscovite, quartz, and pyrite are wide- spread in the Precambrian rocks. These minerals

1882 PETER B. LARSON

were developed during regional greenschist metamorphism (Anderson et al., 1955). Alternatively, they may have resulted from sea-floor hydrothermal alteration. Anderson et al. (1955) report the common occurrence of massive quartz and epidote pods within the western belt of the Bridle Formation.

Geology of the Bruce deposit

The Bruce massive sulfide deposit is a lenticular, concordant body consisting of 90 percent sulfide minerals (Fig. 2). It can be classified as a Cu-Zn massive sulfide based on the scheme of Franklin et

at. (1981). The lens is oval with maximum dimensions of 175 m along strike and 450 m downdip and has a maximum thickness of about 10 m near its center.

The long axis of the deposit rakes 70 ø southwest. The lens lies at the stratigraphic contact between an older andesitc and the overlying Dick rhyolite (strike N 40 ø E, dip 75 ø NW). Pillow structures in basalts in the western belt of the Bridle Formation show that the rocks are overturned and the Dick

rhyolite is the footwall of the deposit in the mine. The lens thins toward its margins and pinches out in a banded quartz-sericite-pyrite schist which lies along the andesite-rhyolite contact in the mine area.

The schist contains thin bands of pyrite, parallel to the contacts of the schist, which merge into the

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•.•; v F'•l DIABASE DIKE u, DICK RHYOLITE

" • U• ANDESITE, SHOWING LIMITS OF STRONG ALTERATION

FIC. 2. Geologic cross section of the 1400 through 2000 crosscuts of the Bruce mine, facing northeast. The geology has been projected to a vertical plane which trends N 55 ø W and passes through the shaft. Sample intervals along the upper three crosscuts are also shown. (Modified after unpublished mapping by Robert Clayton.)

massive sulfide lens. Identical relations were ob-

served by the author during underground tours of massive sulfide deposits in the Noranda area, Quebec. This similarity between the Bruce deposit and the Canadian deposits suggests that the quartzosericite- pyrite schist at the Bruce deposit is an exhalative unit.

Two similar but smaller deposits, the Copper King and the Copper Queen, lie stratigraphically below and above, respectively, the Bruce deposit (Fig. 1). A fourth deposit, the Old Dick, lies imme- diately adjacent to the Bruce deposit on the same stratigraphic horizon. The lower edge of the Old Dick is 60 m updip from the upper edge of the Bruce. The Old Dick lens is exposed as an oxidized gossan on the surface. Baker and Clayton (1968) describe the geology and history of the Copper Queen and Old Dick deposits, and Anderson et al. (1955) briefly report the geology and history of the Copper King and Old Dick deposits. All of these deposits are morphologically, mineralogically, and texturally similar.

The Bruce massive sulfide lens is strikingly mineralogically banded. This banding is concordant with the orientation of the lens and results from variations

in the relative proportion of pyrite to sphalerite on a scale of approximately i cm. Pyrite is generally euhedral and occurs in a matrix of anhedral inter-

locking sphalerite. Chalcopyrite and minor arseno- pyrite and galena occur with the sphalerite, along with traces of pyrrhotite, cubanitc, and mackinawite as exsolution laminae and fine fracture fillings in chalcopyrite. Galena from the Bruce lens yields a model Pb isotopic age of about 1,740 m.y. (Clayton and Baker, 1973), identical to the age of the Yavapai schist within analytical limits (Anderson and Silver, 1976).

The alteration associated with the Bruce massive sulfide lens occurs in the footwall andesitc as a

funnel-shaped zone with the mouth of the funnel in contact with the massive sulfide lens (Fig. 2). The alteration is well exposed in the workings of the mine for approximately 100 m away from the ore- body. Drill intersections indicate that the zone ex- tends for at least 400 m away from the lens. The alteration zone grades into the massive sulfide through a meter-wide zone consisting of more than 50 percent fragments of altered andesitc in a string- erlike matrix of sulfides. This constitutes the bulk of

the stringer ore zone at the Bruce deposit. Scattered patches of alteration are also present in the Dick rhyolite stratigraphically above the lens.

Originally, studies of the massive sulfides near Bagdad concluded that these deposits were Laramide or younger (Anderson et al., 1955; Baker and Clay- ton, 1968). Anderson (1968b, 1969) was the first to define the volcanogenic nature of the deposits, based


on: (1) the association of the Bagdad deposits with characteristic volcanic host rocks; (2) the form, mineralogy, and concordance of the massive sulfide lens and its association with an exhalative facies; (3) the similarity in age of the host rocks and the sulfide deposit; and (4) the mineralogy and morphology of the alteration pipe. All of these features conform to the volcanogenic model (Sangster, 1972; Spence and de Rosen-Spence, 1975; Franklin et al., 1981).

The Dick rhyolite remains an enigmatic unit. Anderson et al. (1955) and Baker and Clayton (1968) interpret the rhyolite to be intrusive. However, the overall stratiform and conformable nature of the

rhyolite and the control that the andesite-rhyolite contact exerts over the position of the massive sulfide lens and its associated exhalative horizon imply a geologic environment similar to that associated with many other massive sulfide deposits (Sangster, 1972; Franklin et al., 1981). The stratigraphic sequence at the horizon of the ore lens (rhyolite over andesite) is unusual but not unique among Precambrian deposits (Sangster, 1972). The Dick rhyolite might therefore be considered an extrusive part of the Bridle Formation volcanic pile.

Mineralogic zoning in the alteration pipe General statement: The mineralogy of the altera-

tion zone was determined by petrographic examination of samples collected in the underground workings of the mine (Fig. 2). Crosscuts have been driven from the shaft to the massive sulfide lens in the stratigraphic footwall of the deposit. These crosscuts provide excellent exposures of the unaltered and altered andesite. Three of the crosscuts, those on the 1400, 1550, and 1700 levels (the levels are spaced vertically 150 feet apart and are numbered based on level depth beneath the shaft collar), were selected for detailed study. The 1400 crosscut lies in unaltered or weakly altered andesite, except near the ore lens where it has been altered to quartz, chlorite, sericite, and pyrite. The 1550 crosscut traverses from moderately altered andesite near the shaft through chloritic alteration into sericitic alteration near the ore lens. The 1700 crosscut lies completely in chloritized andesite. The sampling on the 1400, 1550, and 1700 crosscuts provides a cross section through approximately the upper half of the alteration zone in a plane perpendicular to the plane of the lens.

The 1850 and 2000 crosscuts were only examined superficially. Below the 1700 level, the 1850 crosscut lies completely in chloritized andesite. The 2000 crosscut traverses from unaltered andesite at the shaft through a thin sericitized zone into chloritized andesite near the lens.

The mineralogy of the alteration zone is charac- terized by simple assemblages of major phases,

which grade outward into relatively unaltered andesite. Three mineralogically distinct zones can be defined: a central chlorite zone, a gradational chlorite-amphibole-biotite zone marginal to the chlorite zone, and an upper marginal sericite zone (see Fig. 5). Quartz and pyrite are ubiquitous. The present mineralogy of the alteration zone reflects both chemical and mineralogic changes due to the massive sulfide system and mineralogic changes due to posthydrothermal metamorphism.

Chlorite zone: The chlorite zone occurs in the

central part of the pipe. The andesite in this zone is entirely converted to alteration phases. Chlorite generally makes up more than half the volume of the rock and typically occurs as massive foliated to felty laths with subordinate intergrown quartz. The quartz is locally concentrated into 1-cm-thick, irregular veinlike zones. Untwinned sodic plagioclase, euhedral twinned epidote rods, euhedral magnetite, and needles of rutile are ubiquitous accessory phases. Thin veinlets of calcite are also usually present. Pyrite is found throughout the zone and grades from a few pertent to as much as 15 percent of the rocks within 10 m of the sulfide lens. Chalcopyrite and sphalerite are minor phases close to the lens, where the pyrite increases to greater than background concentrations. On the 1700 level, in the chlorite zone 50 m from the lens, a 1-cm-thick clot of biotite and actinolite with a rim of anhedral quartz grains was noted.

Gradational zone: This zone marks the gradational transition from the chlorite zone to the unaltered

andesite. Amphibole, quartz, and biotite are the predominant phases. The amphibole is acicular near the chlorite zone and tabular in the outer and upper part of the gradational zone. The acicular and tabular amphiboles are referred to as actinolite and hornblende, respectively, in the following discussion. A more rigorous nomenclature is discussed below. Both textural varieties coexist in the central part of the 1550 crosscut without apparent reaction textures between them. The amphiboles are euhedral, non- foliated, and penetrate or replace all other phases including quartz, chlorite, and biotite, and thus, texturally the amphiboles appear to be the last major phase to have formed.

The amphibole minerals probably formed as contact metamorphic phases related to the intrusion of the footwall diabase dike (Fig. 2) or a similar, nearby, intrusive rock. Quartz and chlorite are found in irregular veinlike zones and may coexist with calcite. Chlorite is common as selvages to these veins, together with biotite which occurs in discrete grains or intergrown with the chlorite along cleavage planes. Away from the veins sodic plagioclase, and minor magnetite, sphene, and pyrite are found in addition to all the phases described above. The


quantity of alteration phases in the rock decreases gradationally away from the chlorite zone. Sericite was not observed in the gradational zone.

Sericite zone: This zone stratigraphically underlies the massive sulfide lens on the perimeter of the alteration pipe but does not persist with depth. Quartz, chlorite, pyrite, calcite, and sericite are ubiquitous. Chlorite and sericite are commonly intergrown along cleavage planes. Sodic plagioclase, epidote, magnetite, sphene, and biotite are wide- spread accessories. Biotite occurs as uncommon grains adjacent to porous openings such as relict amygdules or veinlets, or as grains lining pyrite sites. The biotite and chlorite are often intergrown along cleavage planes. The spatial relationship between the chlorite and sericite zones is unclear as

the underground exposure is of limited extent. In the 2000 crosscut a thin sericitic zone separates the chlorite zone from unaltered andesite, but examination of the 1400 and 1550 crosscuts suggests that the gradational zone lies between the chlorite and sericite zones. Nowhere in these upper levels is the chlorite zone seen to grade directly into the sericite zone, although such a contact is not precluded based on the mapping.

Unaltered andesite: Andesite collected around

the shaft on the 4100 level is relatively unaltered and resembles surface andesite samples. The andesite comprises a fine-grained groundmass of sodic plagioclase and minor quartz. Biotite, chlorite, hornblende,

pyrite, and magnetite occur as scattered secondary grains. Relict amygdules are filled with variable mixtures of quartz, calcite, biotite, chlorite, and muscovite.

The unaltered footwall rock composition (U.A. in Table 1) suggests that the prealteration footwall rock may have been more siliceous than typical andesite, but postemplacement alteration may have affected the whole-rock chemistry of the samples from which the unaltered andesite composition was calculated. Therefore, the classification of these rocks as andesite by previous workers (Anderson et al., 1955; Baker and Clayton, 1968) is uncritically ac- cepted for use here, although a rigorous classification based on rock chemistry is warranted.

Summary: Alteration pipes associated with Cu- Zn-type massive sulfides tend to have chloritic cores and sericitic halos (Franklin et al., 1981), and the Bruce deposit conforms to this model with some variations. Texturally, the biotite in the gradational zone is closer to that in the alteration assemblage than that in the metamorphic assemblage. For example, the biotite is concentrated, with or without sericite, along thin selvages adjacent to porous zones such as fractures and amygdules. Also, the biotite is clearly cut by euhedral actinolite, which is probably a product of contact metamorphism. Moreover, the sericite-zone assemblage has not been metamorphosed to a biotite-bearing gradational-zone assemblage. A review of the literature suggests that biotite

TABLE 1. Bulk Chemical Analyses and Densities of the Bruce Mine Samples

140-1 140-2 140-3 155-1 155-2 155-3 155-4 170-1 170-4 170-9 170-10 170-11 U.A?

SiO2 64.3 57.7 54.2 33.7 37.8 52.1 56.4 30.1 25.8 27.5 34.1 52.0 62.5 A12Oa 14.2 14.9 16.9 18.6 15.2 15.5 15.1 12.4 20.6 21.9 18.5 14.4 13.5 Fe2Oa 2.46 2.79 3.66 3.93 2.50 2.09 2.66 9.14 5.15 3.86 2.80 2.17 FeO 2 5.48 8.20 7.11 13.9 8.89 9.18 9.14 12.6 14.1 13.6 12.4 8.39 8.462 MgO 1.00 1.94 3.66 13.9 9.90 6.86 3.88 10.9 17.4 19.7 17.9 10.2 1.66 CaO 5.67 6.97 5.65 3.27 3.34 5.57 5.14 3.69 1.04 0.77 0.76 3.44 3.46 Na20 2.95 2.79 2.39 0.39 1.55 2.13 2.44 0.03 0.14 0.21 0.11 1.32 3.28 K20 2.39 2.77 2.60 0.45 0.46 0.16 1.98 0.10 0.07 0.04 0.12 0.60 0.85 MnO 0.16 0.17 0.14 0.16 0.11 0.12 0.16 0.32 0.39 0.46 0.25 0.10

TiO2 0.92 1.58 1.60 1.61 1.55 1.57 1.42 1.05 1.33 0.72 0.74 1.23 0.84 S 0.25 0.19 0.81 1.62 0.98 0.00 0.05 7.21 1.74 1.15 0.60 0.13 CO2 1.15 0.50 0.45 0.86 0.08 0.17 0.19 1.32 0.16 0.10 0.05 0.00 Cu a 43 66 96 751 50 51 46 1.51% 0.20% 183 320 42 42 Zn • 261 259 935 0.18% 290 372 279 2.17% 648 445 713 226 Pb • 40 44 67 87 91 43 48 92 52 63 238 48

Total 100.93 100.50 99.17 90.95 82.36 95.45 98.56 95.24 88.12 90.01 88.33 93.85 94.55

MgO/MgO + FeO 0.154 0.191 0.340 0.500 0.527 0.428 0.298 0.464 0.552 0.592 0.591 0.549

t•(g/cm s) 2.63 2.87 2.80 2.83 2.73 2.74 2.70 2.95 2.73 2.80 2.73 2.63

Sample locations are shown in Figure 2. Note that the "B" prefix for each sample number has been omitted • U.A. denotes unaltered andesRe; this partial analyses is an average of a number of analyses from the Bruce mine area (D. Douglas,

1976, pers. commun.) 2 Total iron reported as FeO a Cu, Pb, and Zn reported as ppm, unless otherwise noted; all other analyses are in weight percent


is not uncommonly associated with metamorphosed massive sulfide alteration pipes in the Canadian Shield (e.g., Spence and de Rosen-Spence, 1975; Roberts and Reardon, 1978; Riverin and Hodgson, 1980; Franklin et al., 1981; Hall, 1982), although most authors interpret biotite textures to represent metamorphism and not hydrothermal alteration (e.g., Riverin and Hodgson, 1980; Hall, 1982).

Geochemistry of the Alteration Zone

Sampling procedures

Chip samples weighing 1.5 to 2.5 kg were collected along 10- to 15-m-long intervals in each of the 1400, 1550, and 1700 levels, together with hand samples collected near the center of each interval. Samples are numbered in the sequence of collection. Sample numbers are prefixed by a B and an abbreviated level designation. Thus, sample B- 140-1 was the first sample collected on the 1400 level. The chip samples were analyzed for their bulk chemistry and their oxygen isotope composition. Thin sections for petrographic examination and pol- ished microprobe mounts were cut from the hand samples. Analyses of the chip samples represent the chemistry of the entire interval. The mineralogy and mineralogical compositions of the centrally located hand samples are assumed to be representative of

ß the entire interval. Inasmuch as the whole-rock compositions, solid solution mineral compositions, and mineral abundances are all broadly gradational from one interval to the next, this assumption is probably valid.

Analytical techniques

Bulk chemistry of the channel samples (Table 1) was determined by Georesearch Labs, Salt Lake City. SiOg compositions were measured colorimet- rically. FeO concentrations were determined through titration, and FegO3 concentrations were assumed to be the difference between the titrimetric FeO and total iron as determined by atomic absorption. All other components except S and COg were determined by atomic absorption. S and COg concentrations were measured at the University of Arizona using an induction furnace. S was measured using a digital titrator. COg was absorbed by Askerite, a COg absorbant compound, which was weighed before and after each run to determine the mass of COg generated during oxidation of the sample. Density measurements were made on finely powdered samples using a pycnometer. The density data are re- peatable within 0.01 g/cm 3.

The electron microprobe at the University of Arizona was used to determine mineral compositions (Table 2). An accelerating voltage of 15 kV and a beam current of 0.5 •A were used for all analyses.

Each component was counted for 20 seconds. Inten- sity data were reduced to concentrations using the method of Bence and Albee (1969, see also Albee and Ray, 1970). All iron was assumed to be divalent except in epidote, where all iron was assumed to be trivalent. The difference between the sum of the weight percent oxides for a mineral and 100 percent by weight was assumed to be the concentration of water in the mineral. Isotopic analyses (see Table 4) were performed at Caltech using the fluorine extraction technique, essentially that as described by Taylor and Epstein (1962).

Metasomatism and bulk chemistry

The chemical compositions and densities of the samples are shown in Table 1, together with the average composition of ten samples of unaltered Bridle Formation andesite collected along strike approximately i km northeast of the Bruce mine (Dean Douglas, 1976, unpub. data). This average unaltered andesite composition is nearly identical to that of sample B-140ol, which was collected near the shaft on the 1400 crosscut and is the least

altered underground sample. B-140-1 contains slightly higher SiOg, AlgO3, and KgO, which might indicate the development of hydrothermal mica. Many of the samples total less than 100 weight percent oxides as a result of large amounts of water (not analytically determined) bound in hydrous minerals, particularly chlorite.

In order to compare accurately the gains and losses from one altered rock to the next, volume changes associated with alteration or compositional changes relative to an immobile component must be known. Usually comparisons are made assuming constancy of volume. The preservation of unde- formed textures and structures in altered rocks as-

sociated with massive sulfide deposits at Mattagami Lake, Quebec (Roberts and Reardon, 1978), and the Millenbach mine, Quebec (Riverin and Hodgson, 1980), has led those authors to conclude that volume has been preserved in the alteration zones associated with those deposits. Such an assumption also appears to be valid for the Bruce alteration zone.

Compositions of representative samples of each of three alteration subzones and of the least altered

andesite, in g per 1,000 cm 3, are plotted against alteration grade (Fig. 3). Similar plots for alteration pipes at the Mattagami Lake deposit (Roberts and Reardon, 1978) and the Millenbach mine (Riverin and Hodgson, 1980) have recently been published. The chlorite zone is considered to be the most

altered zone at the Bruce deposit, and B-170-9 was chosen to represent this zone as it contains the highest chlorite content of any sample. The sericite zone and gradational zone (B-140o3 and B-155-3 in


+1 +1 +1 +1 +1 +1 +1

+1 +1 +1 +1 +l +l +1

+1 +l +1 +1 +1 +1 +1

+1 +l +1 +1 +l +l +1


3000

2000

•000

500

200

•oo

• 30

ff 2o

•o

AI203 MgO FeO

SiO 2

CoO

TiO 2 MnO

Na20

co 2

K20

I I I

B-140-1 B-NO-:5 B-155-3 B-170 -9 FRESHEST SERICITE GRADATIONAL CHLORITE

ZONE ZONE ZONE

•80:+Z6 +6.0 +5.6 +2.4 > INCREASING ALTERATION

FIG. 3. Progressive changes in bulk chemistry as a function of the increasing alteration grade in four representative samples from the Bruce alteration pipe.

Fig. 3) are both marginal to the chlorite zone and it is not clear which, if either, represents the next "most altered" rock. Significant chemical variations as a function of alteration intensity as defined by Figure 3 are:

1. Both FeO and MgO increase gradationally as the rock becomes progressively altered. MgO shows a more dramatic increase than FeO or any other component. It makes up 19.7 percent by weight of the rock in the most altered sample.

2. AI•O3 and TiO•, which are both often considered immobile in the hydrothermal environment associated with massive sulfides, show small variations with increasing intensity of alteration.

3. Silica remains relatively constant through the sericite and gradational zones but drops significantly in the chlorite zone.

4. CaO, Na•O, and K•O are all extensively leached from the chlorite zone; K•O drops to about 0.04 wt percent, Na•O and CaO make up less than I wt percent of the most altered rock.

The extreme changes in the concentrations of components in the chlorite zone can be attributed

to the stability of chlorite in this zone and a large water/rock ratio integrated over the duration of the hydrothermal system. The whole-rock chemistry of sample B-170-9 (Table 1) is largely determined by the composition of the dominant phase in that sample, chlorite (Table 2). In order to convert the andesitc in this zone to alteration phases totally and to leach such large amounts of CaO, Na•O, and K•O, a significant volume of hydrothermal solution must have flowed through and reacted with the andesitc. The mineralogy and chemistry of the most highly altered rocks within the pipe must have been controlled by the fluid composition, similar to relationships observed by Riverin and Hodgson (1980) at the Millenbach mine, where the footwall rocks consist of both an andesitc and a rhyolite: at Millen- bach, the bulk chemistry of the most intensely altered zones that transect both lithologies is about the same and is controlled by the mineralogy of the alteration facies and not by the initial rock composition. Also, the Mattagami Lake deposit contains a striking example where the most altered rocks consist of a talc-actinolite schist; in this deposit Roberts and Reardon (1978) have concluded that the solution composition controlled the formation of these alteration products under conditions of high permeability and high water flux.

Mineral compositions

Chlorite: Chlorite compositions (Table 2) are plotted (Fig. 4) on the chlorite grid of Foster (1962) together with published chlorite analyses from Ca- nadian Archcan deposits (Franklin et al., 1975; Roberts and Reardon, 1978), the United Verde mine at Jerome, Arizona (Nash, 1973), stockwork chlorites

1.0 I •

0.8-

0.6

0.4

0.2

BRUCE

DEPOSIT-•, UNITED VERDE DEPOSIT

MATTA B I DEPOSIT'""%

/• MATTAGAMI

• \ /•,•,•,•,•,•,•,•,•X: BAsARLTs

I I I I [ 02•.0 2.2 2.4 2.6 2.8 3.0 3.2

Si FORMULA POSITIONS

FIG. 4. Compositions of chlorite from the Bruce mine alteration pipe. For comparison, published analyses of chlorites from a number of other massive sulfides and from altered oceanic crust are also shown. Data sources are listed in the text.


from the Cyprus deposits (Heaton and Sheppard, 1976), average chlorites from the West Shasta district, California (Reed, 1977), and chlorite from altered basalts dredged along the Mid-Atlantic Ridge (Humphris and Thompson, 1978). Chlorites produced as a result of seawater-basalt interaction in oceanic crust lie within a restricted range of Si concentration. Precambrian examples span a wider range of Si concentrations which overlaps the younger chlorites at one extreme. Both groups exhibit a similar range of Fe/(Fe + Mg) ratios.

Within the Bruce alteration zone the chlorite is highest in Mg in the most altered rocks and lowest in Mg in those least altered. The Fe/(Fe + Mg) ratio is intermediate between these two extremes for chlorite from the sericite and gradational zones, and the ratio increases systematically away from the center of the alteration pipe. At the Mattagami Lake deposit, Roberts and Reardon (1978) found no correlation between this ratio in chlorite and the degree of alteration within the vitroclastic tuff, although chlorites occurring with talc in the most altered rocks exhibited a very narrow range of Fe/(Fe + Mg) = 0.43 to 0.49. The alteration chlorite at the Amulet

Upper A deposit does exhibit an increase in Mg with an increase in alteration intensity (Hall, 1982).

Micas: Sericite from the sericite zone and biotite from all three zones have been analyzed (Table 2). Based on the classification proposed by Deer et al. (1962), the trioctahedral mica in sample B-170-1 is phlogopite and those in samples B-140-1, B-140-3, and B-155-3 are biotite. The Fe/(Fe + Mg) ratios for biotite and chlorite coexisting in the samples where biotite is hydrothermal are nearly identical (chlorite/biotite = 1.00 to 1.07), whereas a distinctly different ratio is obtained in the chlorite and biotite of B-170-1, where the biotite is probably contact metamorphic in origin (chlorite/biotite = 1.86). The Fe/(Fe 4- Mg) ratio for coexisting chlorite, biotite, and sericite in sample B-140-3 is identical for each mineral at 0.56.

Amphiboles: The amphiboles show a wide range in compositions (Table 2). Using the nomenclature proposed by Hawthorne (1983) the amphiboles in the samples are: tabular ferrotschermakitic hornblende in sample B-140-2, tabular magnesiohorn- blende in sample B.-155-2, both tabular magnesio- hornblende and acicular actinolitic hornblende in sample B-155-3, and acicular actinolite in sample B- 170-11. The Fe/(Fe + Mg) ratio in the amphiboles increases away from the chlorite zone. In the sample in which they coexist (B-155-3), the actinolite is more magnesian than the hornblende. In all cases the amphiboles have a lower Fe/(Fe + Mg) ratio than chlorites in the same sample.

Epidote: Epidote occurs as a minor phase in samples from the chlorite zone. Limited partial

microprobe analyses suggest that the epidote composition is close to Ca2FeA12SiaO•2(OH), pure end- member epidote.

Plagioclase: Based on Si/A1 ratios, plagioclase analyzed in two samples is nearly identical in composition, An42 in B-170-9 and Ana8 in B-170-10. Mea- surements of extinction angles in twinned plagioclase from B-140-1 and B-140-2 show these plagioclases to range from An•6 to An4o. In the chlorite zone the plagioclase is relict magmatic phenocrystic plagioclase and the grains are corroded and penetrated by chlorite laths.

Distribution of phases within the alteration zone

The modal abundances of minerals comprising assemblages in rocks at the Bruce mine (Table 3) have been calculated from the chemical analyses using the least squares regression procedure of Villas and Norton (1977). In this procedure, a set of equations relates the mass concentration of each component in a rock to the relationship between the mass abundances of the mineral phases and the concentrations of the components in these phases. In matrix notation this set of equations can be written

mC = X, (1)

where C is the chemical composition of the mineral phases, X is the composition of the rock, and m is the mineralogical composition of the sample. Such calculations have been conducted by computer for each sample interval in the Bruce alteration zone. The bulk chemistry of each interval (Table 1) was input as X in equation (1), and the microprobe analyses (Table 2) or the stoichiometric compositions of chemically unvariable phases (such as quartz) were input as C in equation (1), allowing the calculation of m, the mineralogy of the sample. For each calculation only those phases identified in hand sample or thin section are included.

In Table 3, concentrations for phases with standard deviations less than 10 percent are considered ac- ceptable. The inclusion of minor phases in the calculations causes large deviations and seriously affects the results for the major phases, as the linear approximation includes the compositions of all phases in the best fit. For this reason several minor phases were excluded from the calculations in some of the samples. The standard deviations for several of the minerals in sample B-155-3 are high, which is probably the result of the inclusion of several accessory mineral phases in the calculations. In some samples the initial computed mineral abundances totaled as low as 90 percent because the calculations were performed using anhydrous compositions. The abundances of hydrous minerals were recalculated by hand to include the hydrous component. Sphalerite


listed in Table 3 was calculated by hand assuming that all Zn in the samples was included in pure sphalerite.

The distribution and concentration of mineral

phases in the samples are schematically shown together with the general alteration patterns in Figure 5. Chlorite in the chlorite zone ranges from 41 to 93 percent, with the bulk of the remainder of the rock consisting of quartz (1-27%). Chlorite decreases gradationally out to the least altered andesitc, where it composes only 1 percent of the rock. The sample from the sericite zone, B-140-3, comprises 23 percent chlorite with 21 percent sericite and 33 percent relict plagioclase. Biotite is most abundant in the outer part of the gradational zone where it ranges up to 24 percent of the rock. Amphiboles are most abundant in the inner part of the gradational zone where they compose up to 16 percent of the rock.

Oxygen isotopic compositions of the altered rock Whole-rock oxygen isotope compositions of the

samples have been determined (Table 4). The altered rocks range in $•80 from 2.4 to 7.6, with the lightest oxygen occurring in the most altered rocks. Typically, unaltered andesites fall within the range from 5.5 to 7.5 per mil (Taylor, 1968); thus, relative to "normal" unaltered andesites, all samples within the altered zone at the Bruce mine are depleted in •80 as a result of exchange and reaction with the hydrothermal fluids. Oxygen isotope data are available for a number of massive sulfide deposits, including the Amulet A deposit, Quebec (Beaty and Taylor, 1982), the Kidd Creek mine, Ontario (Beaty and Taylor, 1980), the Cyprus deposits (Heaton and Sheppard, 1976), the Raul mine in Peru (Ripley and Ohmoto, 1979), the deposits at Ducktown in Tennessee (Addy and Ypma, 1977), the Japanese Kuroko deposits (Ohmoto and Rye, 1974; Hattori and Sakai, 1979; Green et al., 1980), the West Shasta district, Cali- fornia (Casey and Taylor, 1982), and the Matagami deposit, Quebec (Costa et al., 1980). The oxygen isotope compositions of altered rocks in most of these deposits were compared by Beaty and Taylor (1982). Their figure 8 is reproduced here (Fig. 6) together with data from the Bruce deposit. The depletion in the Bruce altered rocks is similar to those recorded at the Fukazawa (Kuroko), Ducktown, Amulet A, and Cyprus deposits. Similar •sO depletions in altered rocks associated with massive sulfides were observed in the West Shasta district.

The amount of chlorite produced during the alteration process (Table 4) correlates directly with the oxygen isotope composition of the whole rock (Fig. 7). A least squares fit to these data defines a linear relationship between the isotopic composition as the independent variable and the weight percent chlorite developed in the altered rock. The corre-


FIG. 5. A schematic representation of the distribution of alteration and metamorphic phases in the Bruce alteration pipe, based on the concentrations in weight percent of the phases in the samples as listed in Table 3.

lation coefficient between these data sets is -0.98. Extrapolating this linear relationship to zero percent chlorite defines the oxygen isotope composition of unaltered footwall rock end member at the Bruce mine to be 7.4 per mil. This lies within the range of average unaltered andesite (Taylor, 1968). The quantity of chlorite produced in the rock during alteration, ignoring the development of all other phases, and the ]sO depletion in the rock as a result of interaction with the hydrothermal fluids, both measure the progress of the reaction between the fluids and the rocks. The two procedures give nearly identical results. This correlation is independent of the Fe/(Fe + Mg) ratio of the chlorite. Extrapolating this line to 100 percent chlorite defines the 6]$0 of the chlorite to be 2.1. The 61sO of the alteration chlorite at the Amulet A mine is 1.6 (Beaty and Taylor, 1982). Two chlorite analyses of 1.5 and 2.0 have been reported for the Shasta King deposit in the West Shasta district (Casey and Taylor, 1982). Average 61sO values of chlorite from the Ducktown deposits range from 2.2 to 3.1 (Addy and Ypma, 1977). At 250 ø and 300øC, water in equilibrium with the end-member Bruce chlorite would have a 61sO of 1.1 and 2.1, respectively, based on the chlorite-water fractionation curve of Wenner and Taylor (1971). These water compositions are nearly

T,•BLE 4. Whole-Rock Oxygen Isotope Analyses

$•s0 (per mil, SMOW)

B-140-1 7.6 B-140-2 7.2 B-140-3 6.0 B-155-1 3.7 B-155-2 5.0 B-155-3 5.6 B-155-4 6.9 B-170-1 4.0 B-170-4 3.3 B-170-9 2.4 B-170-10 3.4 B-170oll 4.9

SCHEMATIC B'SO TRAVERSES THROUGH MASSIVE SULFIDE ALTERATION ZONES

• OUCKTOWN AM• E T

k c•P.us • BRUCE

i I

15

COUNTRY L COUNTRY ROCKS ROCKS

FIG. 6. Comparison of whole-rock •]sO changes in the Bruce alteration pipe to variations in a number of other massive sulfide alteration zones. The original diagram is from Beaty and Taylor (1982), with data sources listed in the text.

identical to the calculated ore fluids at the Amulet

A mine (Beaty and Taylor, 1982), the Cyprus deposits (Heaton and Sheppard, 1976), the Kuroko deposits (Ohmoto and Rye, 1974; Hattori and Sakai, 1979), and the West Shasta district (Casey and Taylor, 1982), and are consistent with seawater convectively driven through a submarine volcanic pile as a source of the ore-forming solutions for the Bruce mine. The calculated/i]sO value of 1 to 2 for

the fluid is very similar to the slightly ]SO-shifted value of 1.6 (H. Craig, pers. commun.) observed in the present-day, 350øC seawater-hydrothermal system at 21øN on the East Pacific Rise, which is actively depositing sulfides on the ocean floor.

Evolution of the Water-Rock System Metasomatic interaction between wall rocks and

hydrothermal solutions results in the mutual ex-

0% Chlorile = 7.35%ø 6 - e•,,,,,,,.,• 8180 = -0.053 Ch10rile +7.35

2 18 0 o o B 0 tlOOYo Ch10rite=2.1Yoo -

0 20 40 60 80 100

CHLORITE (Wt. %)

FIG. 7. Correlation between whole-rock oxygen isotope composition and the weight percent chlorite in the samples,


change of a number of components. The degree to which this exchange occurs is a function of, among other variables, the compositions of the fluid phase and of the minerals stable in a particular environment, and also the time-integrated volume of fluid that has reacted with the rock. The types of chemical and isotopic analyses described above lead to an estimation of aspects of the composition of the liquid phase involved in the reactions and to a clearer understanding of its role in the ore-forming process.

The hydrothermal alteration associated with the Bruce deposit was produced by the reaction of the ore-forming solution with the Bridle Formation wall rocks during upward discharge of the fluid into the overlying ocean. Stable isotope studies of the altered rocks from the Bruce and other volcanogenic deposits indicate that the initial source of the fluid was

seawater. Recharge of fresh seawater into this hydrothermal regime occurs at some distance from the discharge sites.

Early evolution of the fluid

Prior to entering the alteration pipe beneath the sulfide lens, the fluid must have flowed through and reacted with a significant volume of rock. Such zones of altered rock have been called lower semi-

conformable alteration zones (Franklin et at., 1981). The fluid which is upfiowing through the alteration pipe is not fresh seawater but has evolved somewhat. The $•so values of the fluid in equilibrium with chlorite in the Bruce pipe at 250øC is 1.1. The initial unaltered Precambrian seawater entering into the hydrothermal system probably had a •i•sO close to 0 per mil (Beaty and Taylor, 1982).

The concentrations of elements in the hydrothermal solution also shift during reactions in the lower semiconformable zone, and these changes exert an important control over the stability of alteration phases in the alteration pipe. The compositional changes in the fluid are balanced by complementary changes in the rocks with which they are reacting. Such changes have been documented in several massive sulfide terranes by analyses of these altered rocks (Anderson, 1968a; Descarreaux, 1973; MacGeehan, 1978; Gibson, 1979; MacGeehan and MacLean, 1980; Franklin et at., 1981). One such palcogeothermal system, that associated with the Archcan Garon Lake deposit, Quebec, has been documented in detail (MacGeehan, 1978; Mac- Geehan and MacLean, 1980). Tholeiitic basalts for about 1 km beneath the Garon Lake deposit were extensively altered, and quartz and epidote were extensively precipitated along fractures. Geochemical changes in the basalt include addition of H20 and SiO2 and depletion of MgO. K20, which was initially low in the rocks, remains low. Comformable pods of quartz and epidote within the western belt of the

Bridle Formation south of Bagdad (Anderson et al., 1955) suggest that a similar lower semiconformable alteration zone may have been associated with the Bruce deposit. Although the composition of the hydrothermal fluid cannot be exactly defined, changes in certain components within the fluid during lower semiconformable alteration can be quali- tatively assessed. The precipitation of epidote increases the hydrogen ion concentration in the fluid according to

2Ca +2 + Fe +a + 2A1 +a + 3SiO•aq + 7H•O

-- Ca•FeAI•SiaO•2(OH) + 13H +. (2)

Similar reactions can be written for other minerals

produced during lower semiconformable alteration. Generally, these reactions lower the pH. In addition, replacement of any feldspar by epidote, when balanced by conservation of aluminum, also decreases pH.

Fluid-rock reactions in the alteration pipe

The stability of phases of interest in the alteration pipe has been plotted as a function of the activity ratios of Mg +a and K + and H + at 250øC (Fig. 8), together with the composition of modern seawater at 250øC (M. Reed, 1982, oral commun.). The oxygen isotope composition of seawater has appar- ently remained constant throughout at least the last 3 billion years as a result of the buffering control exerted by water convectively circulating through and reacting with submarine volcanic rocks (Taylor, 1977; Gregory and Taylor, 1981; Beaty and Taylor, 1982). Such a mechanism could also have maintained relatively constant magnesium and potassium levels in the ocean. The simplified system MgO q- KaO +AI•Oa + SiO• + H•O was used to model the alteration and contains all the product phases of interest. Thermodynamic data for the minerals was taken from Helgeson et al. (1978), and data for the solution and aqueous species were taken from Hetgeson and Kirkham (1974) and Helgeson et al. (1981). Based on microprobe data for chlorite and biotite in the alteration pipe the activities for clinochlore and phlogopite were set at 0.6 for the purpose of constructing Figure 8.

Exact mixing models that would allow the calculation of activities of clinochlore and phlogopite in the chlorite and biotite, respectively, as functions of mole fractions of the end-member compositions are not well known. The activities are therefore

assumed to be equal to the mole fraction of these end members for typical mineral compositions of the altered rocks. The model presented below is not exact and only defines generalized chemical interaction between the fluid and rocks, and the activity


8 • SEAWATER'ncr• eose (]MD++ T: 250øC • • increase OK+ MgO+ K20 +AI203 • / •'decreose pH +SiO 2 + H20

CLINOCHLORE • •" L•ME•CON F0R MA BLE 0 PHLOGOP'TE: 0'6 7 / • ALTERATION 0CLiNOCHLORE: 0.6

• ALTERATION i • PHLOGOPITE

• KAOLINITE m m m (m , , ,

2 • 4 5

10g (0,+/0,+) FIG. 8. Stability relations in the system MgO + K•O + Al•O3

+ SiO• + H•O at 250øC. Activities of phlogopite and clinochlore •e set at 0.6. Thermodynamic data •e from Helgeson et al. (1978), Helgeson and Kirkham (1974), and Helgeson et al. (1981). •e activity ratios for modern seawater at 250øC •e also plotted (M. Reed, 1982, oral commun.). Schematic trajec- tories of the fluid composition away from seawater are shown for independently increasing the activities of Mg +• or K + or decreasing pH. A possible fluid path for lower semiconformable •teration, which significantly decre,es pH •d slightly incre,es the activity of Mg +•, is shown. This alteration shi•s the fluid composition into the clinochlore field, such that clinochlore is stable when the fluid enters the alteration pipe. Subsequent production of chlorite continues to decrede pH but consumes Mg +• ions, eventually shifting the fluid composition to stability with sericite. If the activity of water is not less than 1, the sericite field may not be intersected.

relations in the chlorite and biotite used in this model are thus considered valid.

The activity of water was probably less than one. Anderson and Nash (1972) studied fluid inclusions in quartz beneath the United Verde massive sulfide, at Jerome, Arizona, and concluded that the ore fluid, contained an appreciable amount of CO2. The inclusions contained from 6 to 15 mole percent CO2. The United Verde mine is a massive sulfide deposit that is genetically contemporaneous with the Bruce deposit. Several daughter minerals were identified in the inclusions, including halite and a carbonate mineral. The activity of water in the hydrothermal fluid was certainly less than i and several possible values for the H20 activity were used in the con- struction of Figure 8.

Modern seawater plots within the stability field of phlogopite in Figure 8. This relationship is valid

for temperatures down to at least 25øC. The effect of decreasing the pH and slightly increasing the activity of Mg +• in the fluid, accomplished during lower semiconformable alteration, would be to shift the fluid composition into the stability field of clinochlore. Clinochlore production could then proceed according to the reaction

5Mg +2 + 2AI +3 + 3SiO•aq q- 12H20

= MgsAl•.Si30]o(OH)s + 16H +, (3)

which consumes magnesium ions while continuing to produce hydrogen ions. Reaction of the feldspars to produce clinochlore, when balanced on aluminum, also consumes Mg +• and lowers pH. Experimental and theoretical studies verify that such a drop in pH occurs during rock-seawater interaction in suboceanic hydrothermal systems (Mottl and Sey- fried, 1980; Seyfried and Bischoff, 1981; Reed, 1983). Eventually this could shift the fluid composition such that sericite also becomes a stable phase (Fig. 8). Lowering the activity of water below 1 can significantly expand the sericite field. If the activity of water is maintained near 1, it is conceivable that the fluid composition would never establish equilibrium with sericite but could with kaolinite. Chlorite is the solid phase that is in equilibrium with the hydrothermal fluid as the fluid enters the alteration pipe. The degree to which a rock has interacted with this fluid can be measured either by quantity of chlorite produced in the rock or by the overall •80 depletion of the rock (Fig. 7).

Alteration model

A schematic alteration model has been developed based on the distribution and stability of phases identified within the alteration zone (Fig. 9). An initial amount of fluid enters the pipe and reacts with the andesite to produce a small amount of chlorite. Continued fluid flow through the central part of the pipe eventually produces a massive Mg- rich chlorite core. As chlorite is produced, the composition of the fluid shifts such that sericite also becomes stable, and this fluid now flowing through the upper portion of the alteration pipe reacts with the rock to make the sericite zone. Once the central

part of the pipe has been totally altered, the fluid flowing through this chlorite zone no longer has a tendency to shift to sericite stability as no new chlorite is produced. Thus no sericite is found in the upper portion of the chlorite zone; any sericite that existed there would have reacted back to chlo-

rite. Mixing between the hydrothermal fluid and fresh seawater along the edge of the pipe could shift the fluid composition into the biotite field and produce minor amounts of biotite in the outer gradational zone. Biotite may possibly be stabilized by


SEAFLOOR HOTSPRINGS MASSIVE SULFIDE pRECIPITATES

GRADATIONAL ZONE•TE • • •e ••/ / STABILITY, INTERMEDIATE

EVOLVED SEAWATER, WATER/ROCK RATIO,

• ST•ILI•

fLOl0 iNPUT EVOL•D SEAWATER, CHLORITE ST•, HI• WA•R/ROCK •TIO, STRONG DEPLETION IN H•VY OXYGEN

•G. 9. Schematic model for the development of the alteration pipe at the Bruce deposit. Arrows represent schematic fluid flow paths.

such mixing during retrograde hydrothermal activity as the system collapses and would overprint prograde chlorite on the edge of the pipe.

Summary and Conclusions

Bulk chemical and microprobe analyses have been used to define effects of water-rock interaction in the 1.7-b.y.-old Bruce volcanogenic massive sulfide deposit. The principal conclusions of this work are:

1. The alteration pipe beneath the deposit comprises an inner chlorite zone and an upper marginal sericite zone. Both of these zones grade outward through a gradational zone in which the quantity of alteration products decreases gradationally to unaltered andesitc. All compositionally variable phases, as well as the bulk-rock system, become more Mg rich toward the center of the pipe.

2. The quantity of chlorite (volumetrically the dominant alteration mineral) and the oxygen isotope compositions of all the samples are related by a well-defined linear correlation, and both of these alteration effects can be used as a measure of the degree to which a sample has reacted with the hydrothermal fluid. The most altered and most 180- depleted samples collected in the pipe are nearly pure chlorite, and their bulk chemistry approaches that of the chlorite in the sample. This suggests that the fluid chemistry controlled the composition and mineralogy of the alteration zone in those areas in which a large volume of fluid reacted with the rock.

3. Biotite occurs as both a metamorphic mineral (with amphibole) or as an alteration mineral (with chlorite and sericite).

4. The development of the alteration assemblages can be modeled using seawater as a hydrothermal fluid. Stable isotope studies of altered rocks at the Bruce and other similar deposits are compatible with the seawater model. Semiconformable alteration at depth reduces the pH of the heated seawater such that chlorite is stable when the fluid enters the

alteration pipe. Progressive chlorite production in the pipe subsequently shifts the fluid composition such that sericite becomes stable. The stabilization of sericite is enhanced by water activities less than unity. The sericite zone in the upper part of the pipe is therefore produced by fluids that have already been depositing chlorite. In the central part of the pipe, where the rock has been totally altered to a chlorite-dominant assemblage, an upper sericite zone would be absent or destroyed, because the fluid could not shift into the sericite stability field. Minor hydrothermal biotite could be produced near the margins of the pipe due to mixing between the hydrothermal fluid and fresh seawater.

Acknowledgments

Microprobe and whole-rock data presented in this paper were generated as part of an M.S. thesis at the University of Arizona. Spencer Titley, who served as thesis advisor, is gratefully acknowledged, together with Denis Norton and Timothy Loomis. The early work was supported in part by the Cyprus Mines Corporation. The Department of Geosciences at the University of Arizona provided microprobe time. Support at Caltech has been provided by NSF grant EAR 78-16874. Hugh Taylor, whose encouragement induced me to write this paper, is gratefully acknowledged. Thanks go to H. Taylor, T. Bowers, and J. Edmond for critical reviews of early versions of this manuscript. All interpretations are, however, solely the responsibility of the author.

May 6, 1983; June 6, 1984

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