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New discoveries of massive sulfides on the East Pacific Rise
Marine Geology, 84 (1988) 179 190 Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands
179
NEW DISCOVERIES OF MASSIVE SULFIDES ON THE EAST PACIFIC RISE V. MARCHIG, H. GUNDLACH, G. HOLLER and M. WILKE Federal Institute for Geosciences and Natural Resources, P.O. Box 51 01 53, D.3000 Hannover 51 (F.R.G.) (Received October 10, 1987; revised and accepted April 21, 1988)
Abstract Marchig, V., Gundlach, H., Holler, G. and Wilke, M., 1988. New discoveries of massive sulfides on the East Pacific Rise. In: R. Oberh~insli and P. Stoffers (Editors), Hydrothermal Activity and Metalliferous Sediments on the Ocean Floor. Mar. Geol., 84: 179-190. Newly discovered massive sulfide occurrences were mapped on the East Pacific Rise between 18° and 24°S. Large occurrences of sulfides are restricted to zones in which there is more tectonic strain than usual for sea-floor spreading centers. These sites are also characterized by a relatively shallow bathymetric position and greater coverage with sheet lava than with pillow lava. Several hydrothermal sources with no sulfide precipitate on the sea floor suggest precipitation of sulfide below the sea floor (stockwork mineralization). Animal communities that depend on sulfide oxidation are scarce and small - - at least, that is, the fauna that is large enough to be recognized in underwater photographs. The animal communities around hydrothermal sources which profit from the larger food supply provided by the primary fauna are present in dense populations over relatively large areas. The enrichment of the hydrothermal component in the sediments adjacent to a spreading center is not typical of hydrothermal fields along the East Pacific Rise. It can be concluded from the evaluation of the sediment that the hydrothermal activity increases northwards.
Introduction
Hydrothermal activity at the diverging plate boundary represented by the East Pacific Rise (EPR) was first observed by BostrSm and Peterson (1968) in the form of metallogenous sediments surrounding the EPR. For a long time after this discovery it was thought that metalliferous sediment is the only hydrothermal product at diverging plate boundaries, and that massive sulfides could develop only in extraordinary trap situations, like in the Red Sea. After the discovery of massive sulfides in the Galapagos spreading center (Corliss et al., 1977), a series of discoveries of massive sulfides 0025-3227/88/$03.50
on the EPR followed (Francheteau et al., 1979; Lonsdale et al., 1980; Rise Project Group, 1980; Francheteau and Ballard, 1983; Normark et al., 1983; B~icker et al., 1985). The work of the GEOMETEP (Geothermal Metallogenesis East Pacific) Project began in 1981 and was initially concentrated on the rapidly diverging plate boundaries in the southern part of the EPR. In this study we will discuss some new findings from the GEOMETEP 4 cruise (November, 1985-January, 1986). The cruise investigated relationships between tectonics, volcanism, hydrothermal activity and ore formation along the sea-floor spreading centers in the Eastern Pacific. The investigations began with mapping the sea floor with
© (1988) Elsevier Science Publishers B.V.
180
the multibeam automatic charting system - Seabeam. The spreading center located with Seabeam was then studied by combined TV and color slide photography. Massive sulfides were sampled with a TV-controlled grab. In addition to the investigations within the spreading center, cores were taken from the surrounding sediment with a gravity corer as near to the spreading center as possible. Two new sulfide fields were found and two sulfide discoveries from the previous expedition (Tufar et al., 1984, 1986; Biicker et al., 1985) were confirmed as being hydrothermal fields of significant dimensions. There are widespread occurrences of smaller sulfide deposits between the hydrothermal fields. Hydrothermal sources without sulfide precipitates were also observed. The area of investigation is shown in Fig.1. I ~8°S !
0os!
y
i
I
PLATE
22° S-:
24° S
, ~ ~
I
J
~]~----- -I
EAST[RPLATE
26os 116°W
11z+°W
112°W
Fig.1. Sketch map of the E a s t Pacific Rise in the study area. O c c u r r e n c e s of massive sulfides are m a r k e d w i t h stars, 1.arge ones for extensive fields and smaller ones for less extensive occurrences. The locations of the a r e a s in the detailed m a p s in Figs.5-7 are m a r k e d by arrows.
This paper will concentrate on the hydrothermal activity on the EP Rise including the spreading center at the western boundary of the Easter plate. The activity in the spreading center at the eastern boundary of the Easter plate is examined in another paper (Marchig et al., in prep.) owing to the exceptional situation here of hydrothermal activity on a propagating rift.
Hydrothermal o u t l e t s with f o r m a t i o n o f m a s s i v e sulfides All the sulfide fields are situated in recent spreading centers, which can be recognized due to the presence of fresh glassy lava with no pelagic sediment cover. The hydrothermal vents are located along tectonic fissures and cracks, frequently on dip-slip faults at the margin of the central valley of the spreading center. Sulfides are deposited as massive sulfide in the form of chimneys directly over the vents or as impregnations in the basalt around the vents. The vents are surrounded by an intensely yellow hydrothermal sediment. In areas with recent hydrothermal activity the yellow sediment appears locally between pillows or in folds of platy lava. In older fields the yellow hydrothermal sediment appears in the form of well-developed mounds. Sediment with the same appearance found at the back-arc spreading center in the Lau Basin proved to be Fe silicate (Von Stackelberg et al., 1985). It forms platy aggregates; the platelets of this yellow sediment can be transported several tens of meters from their original positions. At older hydrothermal vents, the mounds of yellow hydrothermal sediment are covered by a dull, dark hydrothermal precipitate, probably MnO 2. In this case the yellow sediment is visible only in patches, where burrowing organisms transported it to the sediment surface. Some primary precipitation of pure Fe hydroxide was also observed in the hydrotherreal fields containing massive sulfides. One of the large massive sulfide chimneys on the EPR at 18.5°S was sampled without causing the destruction of the chimney. It has been
181 described by Marchig et all, 1988. The chimney has mineral assemblages very similar to the ones described in the North Pacific, but the zonation, i.e., the growth history, is distinctly different. In one of the fossil spreading centers parallel to the one which is currently active, we observed a massive sulfide occurrence almost completely covered by pelagic sediments (B.G.R. with R.F., 1986). The fossil structures which used to be active spreading centers and then drifted away by plate movement probably contain large quantities of sulfide. The pelagic sediment that covers the massive sulfide is highly oxidized and isolates the massive sulfide from contact with oxidizing sea water. Pore water in the sediment can also oxidize the massive sulfide but it is not present in sufficient quantities to oxidize more than an insignificant amount of sulfide. The remaining sulfide within the pelagic sediment is then protected from oxidation, as well as from detection.
Animal communities associated with the hydrothermal outlets with formation of massive sulfides Thiel and Tuerkay (1987) divide the animals of hydrothermal biotopes into four categories: (1) Sessile animals whose existence depends directly on hydrothermal discharge sites. An example of this type is Riftia, a large tube worm that lives in symbiosis with sulfideoxidizing bacteria. (2) Sessile animals which are usually, but not always, associated with hydrothermal discharge sites. Examples of this group are Bathymodiolus and Calyptogena, which are usually found in dense populations covering cracks that have residual hydrothermal activity. Serpulidae can also be included in this group. (3) Vagile species that remain very near hydrothermal discharge sites, such as Diplacanthopoma (the "vent fish") or Bythogradiae (a small white crab). (4) Species which occur in the area around
discharge sites, profiting from the good food supply resulting from the high biological activity of the discharge area. Actiniaria (the sea anemone), and Saxipendium, aptly called the "spaghetti worm", are typical of this group. The most frequent type of hydrothermal discharge in the study area is the black smoker, a narrow outlet around which a vent of massive sulfide has formed. The strong jet stream coming out of a black smoker is too hot to support any life. The animals of groups 1 and 2 need H2S in the water for their existence, but on the black smokers they can exist only on the outside of the massive sulfide walls where the diffusing sulfide-containing water has cooled sufficiently to support life. This could be a reason why Riftia, Bathymodiolus, etc. are very rare in the study area (when present, the specimens are very small). In addition to the outside of vent walls, conditions for this group of animals are present only along rare cracks where the hydrothermal discharge still contains H2S, but has cooled sufficiently. The vagile species (group 3), which remain in the vicinity of the vents, are frequent. The populations of the group 4 animals, the ones living in the area around hydrothermal discharge sites, are considerably more dense than those in the other areas studied on the EPR and the Galapagos spreading zone. They also cover much larger areas than in other spreading zones. Typical are dense covers of small actinia (Fig.2) and Saxipendium (Fig.3). Dense populations of Saxipendium and Actiniaria in large areas adjacent to hydrothermally active areas indicate high biological activity directly on the vents, which ensures their food supply. Because we observe only rare biological activity directly on the vents, it is probable that most of this biological activity is that of microorganisms. Hydrothermal o u t l e t s w i t h o u t f o r m a t i o n o f m a s s i v e sulfides Besides the hydrothermal vents associated with the formation of massive sulfide chimneys, the hydrothermal fields also contained
182
Fig.2. An animal community around a hydrothermal outlet, mostly Actiniaria and crustaceans. The photograph was taken in the hydrothermal field at 18°S (exact position: 18°27'S, 113°24'W, water depth 2640 m).
Fig.3. An animal community around a hydrothermal outlet: Saxipendium worm community (position: 19°22'S, 113°32'W, water depth 2786 m). h y d r o t h e r m a l o u t l e t s t h a t do n o t f o r m m a s s i v e sulfides. In this t y p e of diffuse o u t l e t t h e h y d r o t h e r m a l s o l u t i o n s seep out o v e r l a r g e t a l u s fields. T h e s e h y d r o t h e r m a l o u t l e t s a r e visible by u n d e r w a t e r T V o w i n g to t h e d a r k clouds of t u r b i d w a t e r a s c e n d i n g slowly from the sea floor a n d t h e d e n s e a n i m a l p o p u l a t i o n , w h i c h consists of o n l y h i g h e r a n i m a l s s u c h as fish and c r a b s w h i c h a r e c o n s t a n t l y in r a p i d m o t i o n t h r o u g h t h e clouds of t u r b i d w a t e r (Fig.4). T h e p r e s e n c e of t h e s e a n i m a l s d i r e c t l y w i t h i n the a r e a of a s c e n d i n g h y d r o t h e r m a l s o l u t i o n s h o w s t h a t t h e s e s o l u t i o n s a r e n o t of the h i g h t e m p e r a t u r e w h i c h p r e v a i l s in t h e b l a c k s m o k e r j e t streams.
Fig.4. A diffuse hydrothermal outlet in a tallus field. Finegrained Mn oxide precipitate is visible between basaltic fragments. The animal community consists mostly of fish and crustaceans. The photograph was taken in the hydrothermal field at 21°S (exact position: 21°31'S, 114°17'W, water depth 2790 m).
T h e talus fields on which we observed these h y d r o t h e r m a l sources are covered with grainy, dull b l a c k m a t t e r , obviously p r e c i p i t a t e d from the turbid h y d r o t h e r m a l solution. It is improbable t h a t this p r e c i p i t a t e is composed of sulfides as sulfides on the sea floor u s u a l l y h a v e intense reddish b r o w n p a t c h e s due to oxidation of Fe(II) sulfide to Fe(III) hydroxide. We suggest t h a t this dull b l a c k p r e c i p i t a t e is M n oxide. M a n g a nese oxide p r e c i p i t a t e s from the h y d r o t h e r m a l solution at a l a t e r stage t h a n sulfide. Therefore, these h y d r o t h e r m a l fields p r o b a b l y c o n t a i n not only m a s s i v e sulfides on the sea floor but also sulfide p r e c i p i t a t e s below the sea floor. This suggests a s t o c k w o r k m i n e r a l i z a t i o n in b a s a l t c o m p a r a b l e to t h a t observed in ophiolites, which are believed to be fossil ores of the s a m e genesis as those i n v e s t i g a t e d here. T h e o c c u r r e n c e of sulfides below the sea floor w o u l d also e x p l a i n t h e d e n s e o c c u r r e n c e of h i g h e r a n i m a l s on the sea floor r e p r e s e n t i n g t h e end of a food c h a i n b a s e d on sulfideoxidizing m i c r o o r g a n i s m s . The occurrence of hydrothermally active areas depends on the local tectonic strain
As a r e s u l t of spreading, the E P R is a t e c t o n i c s t r a i n zone. On some s e c t i o n s of t h e
183 spreading center th e r e is evidence for tectonic strain in addition to the basic tectonic strain caused by the spreading. The sulfide fields (Fig.l) are found only in these zones of additional tectonic strain. The field at 23°S is situated on the EPR just s o u t h of a large transform fault (Fig.5). This part of the EPR bends eastwards and the h y d r o t h e r m a l field is located directly on the bend in the axis of the rise. Because it contains well-developed massive sulfide chimneys rising from h y d r o t h e r m a l mounds, this field appears to h a v e been active for a long time. The massive sulfides are partly oxidized and the chimneys broken up. The two o th er fields are present at 18° and 21°S. On the basis of the appearance of the sulfides, these h y d r o t h e r m a l sources have also been active for a long time. A well-developed central valley (Figs.6 and 7) which is honeycombed with fissures and cracks is evidence for the additional tectonic strain in these two fields.
23°26 S
23°28 S
1 <
23°30 S
21°24 S
21°26 S
;< /
/
21°28 S
21°30 S
¢
/
11&°18W
114016 W
Fig.6. Detailed bathymetric map (contour interval 100 m) of the hydrothermal field at 21°S. The occurrences of massive sulfides are marked by stars.
The obvious association of large hydrothermally active areas with the additional tectonic strain is a consequence of the larger am ount of tectonic fissuring in these areas. The numerous fissures serve as channels for hydrot herm al flow, which climaxes in the formation of massive sulfides on and probably also under the sea floor. The occurrence of hydrothermally areas correlates with the coverage spreading zone by sheet lava
active of the
23Q32
.. -. _ _~0o0_:: . . . . .
F
,.,,, 115°36 W
I 115°32 W
115°28 W
Fig.5. Detailed bathymetric map (contour interval 100 m) of the hydrothermal field at 23°S. The occurrences of massive sulfides are marked by stars.
A statistical evaluation of the areas covered with sheet lava instead of pillow lava was made by counting the slides made at u n d e r w a t e r photographic stations and calculating those with sheet lava as a percentage of all slides. As the observation area was covered with about 16,000 slides, we consider the statistical evaluation to be highly significant. Typical sheet and pillow lavas are shown in Figs.8 and 9, respectively. The evaluation shows t h a t at the stations
184
18°2G
18°26
18°28
Fig.8. Typical sheet lava with folded surface -- the socalled "curtain fold" lava. This lava is very fresh, as can be seen by the bright glassy surface. 18°30
18°32
113°26 W
11)°22 W
Fig.7. Detailed bathymetric map (contour interval 100 m) of the hydrothermal field at 18°S. The occurrences of massive sulfides are marked by stars. In addition, the 2650 m contour line is shown to illustrate the morphology more clearly.
with w e a k h y d r o t h e r m a l o c c u r r e n c e s , a n average of 3°//0 of the sea floor in the s p r e a d i n g c e n t e r was covered with sheet lava. At the s t a t i o n s w h e r e we observed widespread massive sulfides, an a v e r a g e of 33% of the sea floor w i t h i n the s p r e a d i n g zone was covered with sheet lava. (Within the s p r e a d i n g c e n t e r of the s t u d y a r e a t h e r e was no u n d e r w a t e r s t a t i o n w h i c h was t o t a l l y devoid of h y d r o t h e r m a l occurrences.) Sheet lava, pillow l a v a and t a l u s all h a v e the same c h e m i c a l c o m p o s i t i o n w i t h i n the r a n g e of differentiation. F u r t h e r , the tholeiitic b a s a l t in pillows indicates slow e x t r u s i o n t h r o u g h narrow conduits, while the same basalt in sheets indicates rapid e x t r u s i o n of large q u a n t i t i e s of lava. The o b s e r v a t i o n t h a t massive sulfides are
Fig.9. Typical pillow lava with a striated surface.
associated with sites w i t h i n the s p r e a d i n g c e n t e r t h a t h a v e a h i g h e r p e r c e n t a g e of sheet lava t h a n pillow lava is in a g r e e m e n t with the observation that hydrothermal activity occurs in a r e a s of e n h a n c e d t e c t o n i c strain. Due to the widespread fissuring, n o t o n l y does i n c r e a s e d h y d r o t h e r m a l c i r c u l a t i o n o c c u r in these areas, but the fissures p r o d u c e more lava t h a n in o t h e r parts of the s p r e a d i n g zone.
The occurrence of hydrothermally active a r e a s c o r r e l a t e s w i t h t h e w a t e r d e p t h at the spreading center The r a n g e of w a t e r depths at the o b s e r v a t i o n s t a t i o n s in the s p r e a d i n g zones is s h o w n in
185 116~W
Latitude 23% i 2500
27OO E 280O
22% i
21% i
20°S i
19°S i
115°W
11L°W
113%V
18% i
H H
i
core
1
length :
- 100 cm
Fi~i~ii~t
> EO - 200 cm
t FZ
OSC
Fig.10. Range of water depth at the photographic stations in the spreading zone plotted against latitude. The latitudes of three large hydrothermal fields (H), as well as the latitudes of the fracture zone (FL0 and overlapping spreading center (OSC) are also shown.
23%
i
2 2~°S
Fig.10. It can be seen that the highest parts of the spreading zone are those that are particularly rich in hydrothermal massive sulfides. Normally, a spreading zone is at greater water depth in fracture zones than in areas between fracture zones. This could account for the hydrothermal fields at 18 ° and 21°S, alt h o u g h there is no fracture zone between them but there is an overlapping spreading center (Wilke and Lonsdale, 1987). However, it does not account for the hydrothermal field at 23.5°S, which, although it is adjacent to a large fracture zone, is at a shallower water depth than other parts of the spreading zone so near a fracture zone.
Fig.ll. Thickness of the sediment cover as indicated by the lengths of the cores obtained. The numbers beside the sampling stations give the core lengths in centimeters. The most dense shading indicates the thickest sediments. 116°W
115~d
114°W
"~iiiiiii~
18oS
113°W
i
" -
hydrothermn[
20°S
. . . .
30-
39%
~v~;~
~0-
~9%
50-
59%
21°S -
Sediments A sediment core was taken on each side of the EPR near each of the TV and photographic stations. For this purpose the ship sailed at right angles to the spreading center until the first sedimentary basin was observed on the Seabeam and the 3.5-kHz echo-sounder records. The locations of the 21 coring sites are shown in F i g s . l l and 12 where the sediment thickness and the amount of the hydrothermal material are also shown. The cores were sampled with a short (3 m)
moffer
19°S
-
-
-
> 60%
22°S .
eT'
2~S
2~°S
Fig.12. Content of hydrothermal matter in the sediment as indicated by the average content of hydrothermal matter in the cores. The numbers beside the sampling stations give the percentage of hydrothermal matter in the sediment (average for the core), The locations of hydrothermal fields densely covered with massive sulfides are marked by thick lines over the spreading zone.
:
186
gravity corer, the length of the cores varying between 44 and 273 cm. In the case of some of these cores, because of damaged corers or basalt fragments in the core catcher, we are certain th at we have sampled the complete sediment profile from sea floor down to the basement. In the case of other short cores we assume th at all of the sediment profile was also cored, as otherwise the corer would have penetrated deeper in this type of sediment, which is very soft with a high pore-water content. Only in the case of cores which are longer than 2 m is there a chance t h a t the sediment cover is thicker t ha n the penetration depth. The map of sediment thickness in F i g . l l was drawn on the basis of these results. The sediment thickness increases from east to west and from south to north. The minimum sediment thickness is observed in the southeastern
Fe,2O3 Mn0 As Cu Ha V Zn P205 SO3 Ba Ni Pb Th Y Rb
parts of the area and the maximum in the northwestern parts. The 21 cores were sampled on average every 15 cm (228 samples) and analysed by X-ray diffraction. The contents of carbonate, silicate and hydrothermal oxides were calculated from these analyses, as described by Marchig and Erzinger (1986). The mean concentrations of the elements on a carbonate-free basis are given in the table adjacent to the correlation matrix in Fig.13. This sediment contains a high proportion of hydrothermal material: it is composed of 50% organogenic calcite (coccolite and foraminifera) 41% hydrothermal oxides and 9% silicate, which is a poorly sorted hyaloclastite which has been weathered to varying degrees. Calculated on carbonate-free basis the sediment contains an average of 83% hydrothermal oxides and 17% hyaloclastites.
(%) 30.70 ±~27 (%) 7rio ~ 1.8~ MnI N \ O positive,highly significant (mglkg) 315 ± 10/~ AslC~Q"'~";~,',~ A negative,highly significant (mglkg} 831 ± 226 CuI O O O \ ' + x (ingle) 60 ± 2~ (mglkg) 752 ± 1(~ V l ~ ~ M ° " (mg/kg) 3/<.7 ± 78 (%) 223 ± 0.96 (%) 1.% ± 0.37 (mg/kg) 16% ± 696 ih 0 .... ~-,7o;. (rng/kg) L,23 ± 103 Ni O O S ~ ~@++0 (mg/kg) 30 ± 32 Pb OOO\" (mg/kg) 37 ± 13 O ! O O O \ %, (rng/kg) 77 ± 17 (mg/kg) 25 ± 8 Rb A O OOOOO~ ~ N~ (%) 3.18 ± 089 o A ± ~ , 1(20 (%) 0.78 ± 0.19 AA A ...... O l O ~ / ~ . HgO (%) 1.82 ± 0,7"/ M g l A A A A A A A A SAd~,A I SiO2 (%) 12.8/+ ± 6,12 S i l A A A A A A A A A A A A I J O \ '°," TiO2 (%) 0.25 ± 0.26 T ~ , ~ A A A A A A A A A I O I O O \ ,%
">~o,~
't
At203 (%) Zr
(mg/kg)
~%,,,,
2.1~.,zo9 A ~ , A A A A A A 51 ± 19 Z~l
SA AAAAAAAA
A
~lFe Mn ~Co Mo V Zn PI S I B ~
AA
'
O I
/QQQ~.°,
)OOO O',,Y
RblNa K I M ~ i
coo%* iO/ol so • ~2 coc~ A AAAOIOOOOOO]A Hy.ko C°/ol 83 * ~S " m 4 O O O O O O O O O O
A, Zd
A AAAAA
x in the sediment
Fig.13. Correlation matrix calculated from 228 sediment analyses calculated on a carbonate-free basis. The statistically highly significant correlation coefficients (99.9% significance level) are plotted as symbols. The table to the left gives the values of the mean concentration and standard deviation for the element or oxide (also on a carbonate-free basis, Hy.ka).
187
The results of the chemical analysis calculated on a carbonate-free basis were used to calculate the correlation matrix in Fig.13. Three different groups of elements can be distinguished in the matrix: (1) Mn and Fe correlate with As, Cu, Mo, V, Zn and P, i.e., these elements mostly occur bound to hydrothermal oxides; (2) Mg, Si and Zr correlate with the hydrothermally nonactive elements Ti and A1, i.e., these elements are bound preferentially to hyaloclastites, and (3) Ba, Ni, Pb, Th, Y and Rb form a group of elements which correlate with each other because they are enriched by different diagenetic processes in the sediment. These three groups of elements correspond to those obtained in previous investigations of the sediments adjacent to the EPR in areas which, owing to their geographic position, contain only traces of continental detritus and biogenic SiO 2 (Marchig and Gundlach, 1982; Marchig et al., 1986). Exceptional behavior was observed only for P, which in this case correlates with the hydrothermal precipitates. Phosphorus is hydrothermally mobile, and apatite belongs to the accessory minerals formed together with hydrothermal ores. Organogenic apatite also becomes enriched in sediment during diagenesis, as residual apatite after dissolution of carbonate. This biogenic apatite is usually predominant over hydrothermal apatite in zones containing a hydrothermal component and a high proportion of organogenic calcite. In such cases, P correlates with diagenetically enriched elements. Distinguishing between hydrothermal and diagenetic apatite is possible owing to differences in their contents of Sc, Y, La and REE's: Diagenetic apatite is enriched in these elements. We have suggested the P2Os/Y ratio as being very suitable for this purpose (Marchig et al., 1982). A plot of Y against P2Os in the carbonate-free component is shown in Fig.14. On the basis of this figure it can be concluded that most of the apatite in this sediment is of hydrothermal origin, i.e., there has been much less diagenesis in this sediment than in sediments containing hydrothermal
°° o o
o o
,./hydr0thermaL
o
apatite
oo
o~e
/~.0-
o
/
o%o
3.0-
oo o oO
o o o
o o
~OOo o oo j o
o~
;-to
°
2.0-
1.0" ~o
. o
"-" bi0genic
Y
apotite
(mg/kg)
Fig.14. Yttrium plotted against P2Os for all the 228 sediment samples calculated on a carbonate-free basis. Linear correlation for pure biogenic apatite and for pure hydrothermal apatite are drawn as shaded areas.
material that were sampled at some distance from the EPR. The objective of the sediment evaluation was the development of an exploration method for finding hydrothermal fields in the spreading zone in which massive sulfides are produced. This method would be based on the composition of the sediments adjacent to the spreading zone. In areas with weak hydrothermal activity, the elevated content of hydrothermal oxides in the sediment is an indication of a hydrothermal field in the spreading zone. This is the case for the sediment surrounding the Easter Plate East spreading zone (Marchig et al., in prep.). The geographic distribution of hydrothermal material (obtained from the average content of hydrothermal matter in the core material) in the study area is shown in Fig.12. Also shown is the location of three large hydrothermal fields containing frequent massive sulfides in the spreading zone.
188 In every pair of cores taken (one each side of the EPR at the TV stations), the one to the west has a higher content of hydrothermal matter and is longer. This can be explained by assuming a westward abyssal current that transports large amounts of hydrothermal discharge material but does not influence carbonate precipitation. It is also observed that the northern part of the area has a higher average content of hydrothermal matter in the sediment than is present in the southern part. The zones of elevated concentrations of hydrothermal matter in the sediment are not adjacent to the zones of frequent occurrence of massive sulfides in the spreading center. This shows that the absolute content of hydrothermal matter in an abyssal sediment is a suitable indicator for exploration for massive sulfide fields only in areas of weak hydrothermal activity; it does not provide useful results on the southern part of the EPR, where high rates of hydrothermal precipitation are everywhere present. The elevated content of hydrothermal matter in the sediments in the northwestern part of the area corresponds to the relatively large thicknesses of the sediments in the area. In Fig.15, core length is plotted for 21 cores against the average content of hydrothermal matter in those cores. There is a statistically significant positive correlation between these two parameters. The areas with thicker sediment cover correlate with the areas with higher hydrothermal input, both of them increasing towards the north. This is suggestive of increasing hydrothermal activity from south to north. This assumption can be made only on the basis of the analyses of the adjacent sediment and cannot be confirmed by observation of massive sulfides within the spreading zone. Conclusions
The highest spreading rate observed for a diverging plate boundary was determined for the spreading zone of the East Pacific Rise between 18° and 24°S. An exception to this high
r = 078 n 21
300
200 aa
100 -
,/,/ 20
..... 40 hydrofherma{
60 maffer
(%)
Fig.15. A v e r a g e c o n t e n t of h y d r o t h e r m a l m a t t e r in the core v e r s u s the l e n g t h of the core. Two linear c o r r e l a t i o n s were calculated, y = a + b x and x = a + b y . Core 160 w a s treated as a n o m a l o u s and w a s not t a k e n into consideration.
spreading rate is the part of the East Pacific Rise south of a large fracture zone forming the northern boundary of Easter plate: The Easter microplate divides the diverging plate boundary of the East Pacific Rise into two branches, thus halving the spreading rate. There is lateral displacement at one place along the spreading zone, at a large fracture zone (the north fracture zone of the Easter plate), and one large overlapping spreading center at 20.5°S. Otherwise, the course of the East Pacific Rise is relatively smooth, with only small cross fractures and overlapping spreading centers, as is typical for spreading centers with high spreading rates. Hydrothermal activity seems to be exceptionally high also. All of the seventeen photographic stations within the active spreading zone (two stations were in fossil spreading zones) over a distance of 138 km revealed some indications of hydrothermal activity. The typical hydrothermal outlet in this area is a black smoker, a narrow conduit with a massive sulfide chimney. Areal outlets, from which hydrothermal solutions slowly seeped
189
from talus fields, were also observed. We conclude t h a t the hydrothermal solutions of this type are residual solutions remaining after formation of stockwork mineralization below the sea floor. The cracks containing the residual hydrothermal solutions overgrown with shells or worms are seldom. Four steps of hydrothermal activity were observed: the first one producing massive sulfides, the second one the yellow Fe silicate sediment, the third one a dull black precipitate, which we assume to be Mn oxide, and the fourth one a mixture of Fe and Mn hydroxides which precipitates outside of the spreading center. Three large hydrothermal fields densely covered with massive sulfide chimneys were observed. All of them are associated with the area in which there is some additional tectonic strain. In the southernmost of these hydrothermal fields, the additional tectonic strain is demonstrated by the bend in the spreading zone; in the two more northern hydrothermal fields this additional strain is shown by a welldeveloped central valley t h a t is highly fractured. All three of these hydrothermal fields are on the higher parts of the spreading zone and have a greater coverage by sheet lava t h a n by pillow lava. The elevated position, as well as the occurrence of sheet lava, can also be explained by additional tectonic strain. The sediment near the spreading zone has a high proportion of hydrothermal matter: an average of 41% of the sediment and 83% of the carbonate-free fraction is of hydrothermal origin. The thickness of the sediment cover, as well as the hydrothermal component, increase northwards; from this we conclude t h a t the hydrothermal input in the ocean also increases from south to north. This contradicts the discovery of large hydrothermal fields not only at 18.5°S with the highest hydrothermal input in the surrounding sediment, but also at 21.5 ° and 23.5°S, by which position the hydrothermal input in the surrounding sediment is distinctly lower t h a n in the northernmost position. It shows that at
very hydrothermally active areas the input of hydrothermal precipitate in pelagic sediment does not assist in locating massive sulfide concentrations within the spreading zone.
References B~icker, H., Lange, J. and Marchig, V., 1985. Hydrothermal activity and sulphide formation in axial valleys of the East Pacific Rise crest between 18 and 22°S. Earth Planet. Sci. Lett., 72: 9-22. BostrSm, K. and Peterson, M.N.A., 1968. The origin of aluminium-poor ferromanganoan sediments in areas of high heat flow on the East Pacific Rise. Mar. Geol., 7: 427-447. Bundesanstalt fiir Geowissenschaften und Rohstoffe with Reedereigemeinschaft Forschungsschiffahrt (B.G.R. with R.F.), 1986. Fahrtbericht SO 40, GEOMETEP 4. Bundesanst. Geowiss. Rohst. with Reedereigem. Forschungsschiffahrt, Hannover, BMFT Proj. 03R3260. Corliss, J.B. and Ballard, R.D., 1977. Oasis of life in the cold abyss. J. Natl. Geogr. Soc., 152: 441-453. Francheteau, J. and Ballard, R.D., 1983. The East Pacific Rise near 21°N, 13°N and 20°S: inferences for alongstrike variability of axial processes of the mid-ocean ridge. Earth Planet. Sci. Lett., 64: 93-116. Francheteau, J., Needham, H., Choukroune, P., Siguret, M., Ballard, R., Fox, P., Normark, W., Carranza, A., Cordoba, D., Guerrero, V., Rangin, C., Bougalt, H., Cambon, P. and Hekinian, R., 1979. Massive deep-sea sulfide ore deposits discovered by submersible on the East Pacific Ridge: Project RITA, 21°N. Nature, 277: 523-528. Lonsdale, P.F., Bischoff, J.L., Burns, V.M., Kastner, M. and Sweeney, R.E., 1980. A high temperature hydrothermal deposit in the seabed at the Gulf of California spreading center. Earth Planet. Sci. Lett., 49: 8-20. Marchig, V. and Gundlach, H., 1982. Iron-rich metalliferous sediments on the East Pacific Rise: Prototype of undifferentiated metalliferous sediments on divergent plate boundaries. Earth Planet. Sci. Lett., 58: 361-382. Marchig, V., Gundlach, H., MSller, P. and Schley, F., 1982. Some geochemical indicators for discrimination between diagenetic and hydrothermal metalliferous sediments. Mar. Geol., 50: 241-256. Marchig, V., Erzinger, J. and Heinze, P.M., 1986. Sediment in the Black Smoker Area of the East Pacific Rise (18.5°S). Earth Planet. Sci. Lett., 79: 93-106. Marchig, V. and Erzinger, J., 1986. Chemical composition of Pacific sediments near 20°S - - changes with distance from the East Pacific Rise. In: H. Leinen and D.K. Read, Initial Reports of the Deep Sea Drilling Project, Vol.92. U.S. Gov. Print Off., Washington, D.C., pp.371-381. Marchig, V., RSsch, H., Lalou, C., Brichet, E. and Oudin, E., 1988. Mineralogical zonation and radiochronological relations in a large sulfide chimney from the East Pacific Rise at 18.5°S. In: T.J. Barret and J.S. Fox (Editors),
190 Recent Hydrothermal Mineralisation at Seafloor Spreading Centres. Can. Mineral., in press. Marchig, V., Wilke, M. and Holler, G., in prep. Spreading Center at Easter Plate East: Tectonic and Hydrothermal Activity and Surrounding Sediment. (Critical Reviews in Marine Science.) CRC Press, Baton Rouge, La. Normark, W.R., Morton, J.L., Koski, R.A., Clague, D.A. and Delaney, J.R., 1983. Active hydrothermal vents and sulfide deposits on the southern Juan de Fuca Ridge. Geology, 11: 158-163. Rise Project Group, 1980. East Pacific Rise: hot springs and geophysical experiments. Science, 207:1421 1433. Thiel, H. and Tuerkay, M., 1987. Biologische Beobachtungen aus Hydrothermal Aktiven Bereichen des Siidost Pazifik. Bericht fiber die biologischen Ergebnisse GEOMETEP. In: Abschlussbericht SO 40 der 4 Fahrt mit F.S. SONNE im Rahmen des GEOMETEP-Programms.
Bundesanst. Geowiss. Rohst. with Reedereigem. Forschungsschiffahrt, Hannover and Bremen, BMFT Proj. 03R3620, pp.179-209. Tufar, W., Gundlach, H. and Marchig, V., 1984. Zur Erzparagenese rezenter Sulfid-Vorkommen aus dem sfidlichen Pazifik. Mitt. Osterr. Geol. Ges.,74: 185-245. Tufar, W., Gundlach, H. and Marchig, V., 1986. Ore paragenesis of recent sulfide formations from the East Pacific Rise. Monogr. Ser. Miner. Deposits, 25: 75-93. Von Stackelberg, U. et al., 1985. Hydrothermal sulfide deposits in back-arc spreading centers in the Southwest Pacific. Bundesanst. Geowiss. Rohst. Circ., 2: 3-14. Wilke, M. and Lonsdale, P., 1987. GEOMETEP 4 bathymetry report. In: Abschlussbericht SO 40 - - der 4 Fahrt mit F.S. SONNE im Rahmen des GEOMETEP-Programms. Bundesanst. Geowiss. Rohst. with Reedereigem. Forschungsschiffahrt, Hannover and Bremen, BMFT Proj. 03R3620, pp.13 43.
179
NEW DISCOVERIES OF MASSIVE SULFIDES ON THE EAST PACIFIC RISE V. MARCHIG, H. GUNDLACH, G. HOLLER and M. WILKE Federal Institute for Geosciences and Natural Resources, P.O. Box 51 01 53, D.3000 Hannover 51 (F.R.G.) (Received October 10, 1987; revised and accepted April 21, 1988)
Abstract Marchig, V., Gundlach, H., Holler, G. and Wilke, M., 1988. New discoveries of massive sulfides on the East Pacific Rise. In: R. Oberh~insli and P. Stoffers (Editors), Hydrothermal Activity and Metalliferous Sediments on the Ocean Floor. Mar. Geol., 84: 179-190. Newly discovered massive sulfide occurrences were mapped on the East Pacific Rise between 18° and 24°S. Large occurrences of sulfides are restricted to zones in which there is more tectonic strain than usual for sea-floor spreading centers. These sites are also characterized by a relatively shallow bathymetric position and greater coverage with sheet lava than with pillow lava. Several hydrothermal sources with no sulfide precipitate on the sea floor suggest precipitation of sulfide below the sea floor (stockwork mineralization). Animal communities that depend on sulfide oxidation are scarce and small - - at least, that is, the fauna that is large enough to be recognized in underwater photographs. The animal communities around hydrothermal sources which profit from the larger food supply provided by the primary fauna are present in dense populations over relatively large areas. The enrichment of the hydrothermal component in the sediments adjacent to a spreading center is not typical of hydrothermal fields along the East Pacific Rise. It can be concluded from the evaluation of the sediment that the hydrothermal activity increases northwards.
Introduction
Hydrothermal activity at the diverging plate boundary represented by the East Pacific Rise (EPR) was first observed by BostrSm and Peterson (1968) in the form of metallogenous sediments surrounding the EPR. For a long time after this discovery it was thought that metalliferous sediment is the only hydrothermal product at diverging plate boundaries, and that massive sulfides could develop only in extraordinary trap situations, like in the Red Sea. After the discovery of massive sulfides in the Galapagos spreading center (Corliss et al., 1977), a series of discoveries of massive sulfides 0025-3227/88/$03.50
on the EPR followed (Francheteau et al., 1979; Lonsdale et al., 1980; Rise Project Group, 1980; Francheteau and Ballard, 1983; Normark et al., 1983; B~icker et al., 1985). The work of the GEOMETEP (Geothermal Metallogenesis East Pacific) Project began in 1981 and was initially concentrated on the rapidly diverging plate boundaries in the southern part of the EPR. In this study we will discuss some new findings from the GEOMETEP 4 cruise (November, 1985-January, 1986). The cruise investigated relationships between tectonics, volcanism, hydrothermal activity and ore formation along the sea-floor spreading centers in the Eastern Pacific. The investigations began with mapping the sea floor with
© (1988) Elsevier Science Publishers B.V.
180
the multibeam automatic charting system - Seabeam. The spreading center located with Seabeam was then studied by combined TV and color slide photography. Massive sulfides were sampled with a TV-controlled grab. In addition to the investigations within the spreading center, cores were taken from the surrounding sediment with a gravity corer as near to the spreading center as possible. Two new sulfide fields were found and two sulfide discoveries from the previous expedition (Tufar et al., 1984, 1986; Biicker et al., 1985) were confirmed as being hydrothermal fields of significant dimensions. There are widespread occurrences of smaller sulfide deposits between the hydrothermal fields. Hydrothermal sources without sulfide precipitates were also observed. The area of investigation is shown in Fig.1. I ~8°S !
0os!
y
i
I
PLATE
22° S-:
24° S
, ~ ~
I
J
~]~----- -I
EAST[RPLATE
26os 116°W
11z+°W
112°W
Fig.1. Sketch map of the E a s t Pacific Rise in the study area. O c c u r r e n c e s of massive sulfides are m a r k e d w i t h stars, 1.arge ones for extensive fields and smaller ones for less extensive occurrences. The locations of the a r e a s in the detailed m a p s in Figs.5-7 are m a r k e d by arrows.
This paper will concentrate on the hydrothermal activity on the EP Rise including the spreading center at the western boundary of the Easter plate. The activity in the spreading center at the eastern boundary of the Easter plate is examined in another paper (Marchig et al., in prep.) owing to the exceptional situation here of hydrothermal activity on a propagating rift.
Hydrothermal o u t l e t s with f o r m a t i o n o f m a s s i v e sulfides All the sulfide fields are situated in recent spreading centers, which can be recognized due to the presence of fresh glassy lava with no pelagic sediment cover. The hydrothermal vents are located along tectonic fissures and cracks, frequently on dip-slip faults at the margin of the central valley of the spreading center. Sulfides are deposited as massive sulfide in the form of chimneys directly over the vents or as impregnations in the basalt around the vents. The vents are surrounded by an intensely yellow hydrothermal sediment. In areas with recent hydrothermal activity the yellow sediment appears locally between pillows or in folds of platy lava. In older fields the yellow hydrothermal sediment appears in the form of well-developed mounds. Sediment with the same appearance found at the back-arc spreading center in the Lau Basin proved to be Fe silicate (Von Stackelberg et al., 1985). It forms platy aggregates; the platelets of this yellow sediment can be transported several tens of meters from their original positions. At older hydrothermal vents, the mounds of yellow hydrothermal sediment are covered by a dull, dark hydrothermal precipitate, probably MnO 2. In this case the yellow sediment is visible only in patches, where burrowing organisms transported it to the sediment surface. Some primary precipitation of pure Fe hydroxide was also observed in the hydrotherreal fields containing massive sulfides. One of the large massive sulfide chimneys on the EPR at 18.5°S was sampled without causing the destruction of the chimney. It has been
181 described by Marchig et all, 1988. The chimney has mineral assemblages very similar to the ones described in the North Pacific, but the zonation, i.e., the growth history, is distinctly different. In one of the fossil spreading centers parallel to the one which is currently active, we observed a massive sulfide occurrence almost completely covered by pelagic sediments (B.G.R. with R.F., 1986). The fossil structures which used to be active spreading centers and then drifted away by plate movement probably contain large quantities of sulfide. The pelagic sediment that covers the massive sulfide is highly oxidized and isolates the massive sulfide from contact with oxidizing sea water. Pore water in the sediment can also oxidize the massive sulfide but it is not present in sufficient quantities to oxidize more than an insignificant amount of sulfide. The remaining sulfide within the pelagic sediment is then protected from oxidation, as well as from detection.
Animal communities associated with the hydrothermal outlets with formation of massive sulfides Thiel and Tuerkay (1987) divide the animals of hydrothermal biotopes into four categories: (1) Sessile animals whose existence depends directly on hydrothermal discharge sites. An example of this type is Riftia, a large tube worm that lives in symbiosis with sulfideoxidizing bacteria. (2) Sessile animals which are usually, but not always, associated with hydrothermal discharge sites. Examples of this group are Bathymodiolus and Calyptogena, which are usually found in dense populations covering cracks that have residual hydrothermal activity. Serpulidae can also be included in this group. (3) Vagile species that remain very near hydrothermal discharge sites, such as Diplacanthopoma (the "vent fish") or Bythogradiae (a small white crab). (4) Species which occur in the area around
discharge sites, profiting from the good food supply resulting from the high biological activity of the discharge area. Actiniaria (the sea anemone), and Saxipendium, aptly called the "spaghetti worm", are typical of this group. The most frequent type of hydrothermal discharge in the study area is the black smoker, a narrow outlet around which a vent of massive sulfide has formed. The strong jet stream coming out of a black smoker is too hot to support any life. The animals of groups 1 and 2 need H2S in the water for their existence, but on the black smokers they can exist only on the outside of the massive sulfide walls where the diffusing sulfide-containing water has cooled sufficiently to support life. This could be a reason why Riftia, Bathymodiolus, etc. are very rare in the study area (when present, the specimens are very small). In addition to the outside of vent walls, conditions for this group of animals are present only along rare cracks where the hydrothermal discharge still contains H2S, but has cooled sufficiently. The vagile species (group 3), which remain in the vicinity of the vents, are frequent. The populations of the group 4 animals, the ones living in the area around hydrothermal discharge sites, are considerably more dense than those in the other areas studied on the EPR and the Galapagos spreading zone. They also cover much larger areas than in other spreading zones. Typical are dense covers of small actinia (Fig.2) and Saxipendium (Fig.3). Dense populations of Saxipendium and Actiniaria in large areas adjacent to hydrothermally active areas indicate high biological activity directly on the vents, which ensures their food supply. Because we observe only rare biological activity directly on the vents, it is probable that most of this biological activity is that of microorganisms. Hydrothermal o u t l e t s w i t h o u t f o r m a t i o n o f m a s s i v e sulfides Besides the hydrothermal vents associated with the formation of massive sulfide chimneys, the hydrothermal fields also contained
182
Fig.2. An animal community around a hydrothermal outlet, mostly Actiniaria and crustaceans. The photograph was taken in the hydrothermal field at 18°S (exact position: 18°27'S, 113°24'W, water depth 2640 m).
Fig.3. An animal community around a hydrothermal outlet: Saxipendium worm community (position: 19°22'S, 113°32'W, water depth 2786 m). h y d r o t h e r m a l o u t l e t s t h a t do n o t f o r m m a s s i v e sulfides. In this t y p e of diffuse o u t l e t t h e h y d r o t h e r m a l s o l u t i o n s seep out o v e r l a r g e t a l u s fields. T h e s e h y d r o t h e r m a l o u t l e t s a r e visible by u n d e r w a t e r T V o w i n g to t h e d a r k clouds of t u r b i d w a t e r a s c e n d i n g slowly from the sea floor a n d t h e d e n s e a n i m a l p o p u l a t i o n , w h i c h consists of o n l y h i g h e r a n i m a l s s u c h as fish and c r a b s w h i c h a r e c o n s t a n t l y in r a p i d m o t i o n t h r o u g h t h e clouds of t u r b i d w a t e r (Fig.4). T h e p r e s e n c e of t h e s e a n i m a l s d i r e c t l y w i t h i n the a r e a of a s c e n d i n g h y d r o t h e r m a l s o l u t i o n s h o w s t h a t t h e s e s o l u t i o n s a r e n o t of the h i g h t e m p e r a t u r e w h i c h p r e v a i l s in t h e b l a c k s m o k e r j e t streams.
Fig.4. A diffuse hydrothermal outlet in a tallus field. Finegrained Mn oxide precipitate is visible between basaltic fragments. The animal community consists mostly of fish and crustaceans. The photograph was taken in the hydrothermal field at 21°S (exact position: 21°31'S, 114°17'W, water depth 2790 m).
T h e talus fields on which we observed these h y d r o t h e r m a l sources are covered with grainy, dull b l a c k m a t t e r , obviously p r e c i p i t a t e d from the turbid h y d r o t h e r m a l solution. It is improbable t h a t this p r e c i p i t a t e is composed of sulfides as sulfides on the sea floor u s u a l l y h a v e intense reddish b r o w n p a t c h e s due to oxidation of Fe(II) sulfide to Fe(III) hydroxide. We suggest t h a t this dull b l a c k p r e c i p i t a t e is M n oxide. M a n g a nese oxide p r e c i p i t a t e s from the h y d r o t h e r m a l solution at a l a t e r stage t h a n sulfide. Therefore, these h y d r o t h e r m a l fields p r o b a b l y c o n t a i n not only m a s s i v e sulfides on the sea floor but also sulfide p r e c i p i t a t e s below the sea floor. This suggests a s t o c k w o r k m i n e r a l i z a t i o n in b a s a l t c o m p a r a b l e to t h a t observed in ophiolites, which are believed to be fossil ores of the s a m e genesis as those i n v e s t i g a t e d here. T h e o c c u r r e n c e of sulfides below the sea floor w o u l d also e x p l a i n t h e d e n s e o c c u r r e n c e of h i g h e r a n i m a l s on the sea floor r e p r e s e n t i n g t h e end of a food c h a i n b a s e d on sulfideoxidizing m i c r o o r g a n i s m s . The occurrence of hydrothermally active areas depends on the local tectonic strain
As a r e s u l t of spreading, the E P R is a t e c t o n i c s t r a i n zone. On some s e c t i o n s of t h e
183 spreading center th e r e is evidence for tectonic strain in addition to the basic tectonic strain caused by the spreading. The sulfide fields (Fig.l) are found only in these zones of additional tectonic strain. The field at 23°S is situated on the EPR just s o u t h of a large transform fault (Fig.5). This part of the EPR bends eastwards and the h y d r o t h e r m a l field is located directly on the bend in the axis of the rise. Because it contains well-developed massive sulfide chimneys rising from h y d r o t h e r m a l mounds, this field appears to h a v e been active for a long time. The massive sulfides are partly oxidized and the chimneys broken up. The two o th er fields are present at 18° and 21°S. On the basis of the appearance of the sulfides, these h y d r o t h e r m a l sources have also been active for a long time. A well-developed central valley (Figs.6 and 7) which is honeycombed with fissures and cracks is evidence for the additional tectonic strain in these two fields.
23°26 S
23°28 S
1 <
23°30 S
21°24 S
21°26 S
;< /
/
21°28 S
21°30 S
¢
/
11&°18W
114016 W
Fig.6. Detailed bathymetric map (contour interval 100 m) of the hydrothermal field at 21°S. The occurrences of massive sulfides are marked by stars.
The obvious association of large hydrothermally active areas with the additional tectonic strain is a consequence of the larger am ount of tectonic fissuring in these areas. The numerous fissures serve as channels for hydrot herm al flow, which climaxes in the formation of massive sulfides on and probably also under the sea floor. The occurrence of hydrothermally areas correlates with the coverage spreading zone by sheet lava
active of the
23Q32
.. -. _ _~0o0_:: . . . . .
F
,.,,, 115°36 W
I 115°32 W
115°28 W
Fig.5. Detailed bathymetric map (contour interval 100 m) of the hydrothermal field at 23°S. The occurrences of massive sulfides are marked by stars.
A statistical evaluation of the areas covered with sheet lava instead of pillow lava was made by counting the slides made at u n d e r w a t e r photographic stations and calculating those with sheet lava as a percentage of all slides. As the observation area was covered with about 16,000 slides, we consider the statistical evaluation to be highly significant. Typical sheet and pillow lavas are shown in Figs.8 and 9, respectively. The evaluation shows t h a t at the stations
184
18°2G
18°26
18°28
Fig.8. Typical sheet lava with folded surface -- the socalled "curtain fold" lava. This lava is very fresh, as can be seen by the bright glassy surface. 18°30
18°32
113°26 W
11)°22 W
Fig.7. Detailed bathymetric map (contour interval 100 m) of the hydrothermal field at 18°S. The occurrences of massive sulfides are marked by stars. In addition, the 2650 m contour line is shown to illustrate the morphology more clearly.
with w e a k h y d r o t h e r m a l o c c u r r e n c e s , a n average of 3°//0 of the sea floor in the s p r e a d i n g c e n t e r was covered with sheet lava. At the s t a t i o n s w h e r e we observed widespread massive sulfides, an a v e r a g e of 33% of the sea floor w i t h i n the s p r e a d i n g zone was covered with sheet lava. (Within the s p r e a d i n g c e n t e r of the s t u d y a r e a t h e r e was no u n d e r w a t e r s t a t i o n w h i c h was t o t a l l y devoid of h y d r o t h e r m a l occurrences.) Sheet lava, pillow l a v a and t a l u s all h a v e the same c h e m i c a l c o m p o s i t i o n w i t h i n the r a n g e of differentiation. F u r t h e r , the tholeiitic b a s a l t in pillows indicates slow e x t r u s i o n t h r o u g h narrow conduits, while the same basalt in sheets indicates rapid e x t r u s i o n of large q u a n t i t i e s of lava. The o b s e r v a t i o n t h a t massive sulfides are
Fig.9. Typical pillow lava with a striated surface.
associated with sites w i t h i n the s p r e a d i n g c e n t e r t h a t h a v e a h i g h e r p e r c e n t a g e of sheet lava t h a n pillow lava is in a g r e e m e n t with the observation that hydrothermal activity occurs in a r e a s of e n h a n c e d t e c t o n i c strain. Due to the widespread fissuring, n o t o n l y does i n c r e a s e d h y d r o t h e r m a l c i r c u l a t i o n o c c u r in these areas, but the fissures p r o d u c e more lava t h a n in o t h e r parts of the s p r e a d i n g zone.
The occurrence of hydrothermally active a r e a s c o r r e l a t e s w i t h t h e w a t e r d e p t h at the spreading center The r a n g e of w a t e r depths at the o b s e r v a t i o n s t a t i o n s in the s p r e a d i n g zones is s h o w n in
185 116~W
Latitude 23% i 2500
27OO E 280O
22% i
21% i
20°S i
19°S i
115°W
11L°W
113%V
18% i
H H
i
core
1
length :
- 100 cm
Fi~i~ii~t
> EO - 200 cm
t FZ
OSC
Fig.10. Range of water depth at the photographic stations in the spreading zone plotted against latitude. The latitudes of three large hydrothermal fields (H), as well as the latitudes of the fracture zone (FL0 and overlapping spreading center (OSC) are also shown.
23%
i
2 2~°S
Fig.10. It can be seen that the highest parts of the spreading zone are those that are particularly rich in hydrothermal massive sulfides. Normally, a spreading zone is at greater water depth in fracture zones than in areas between fracture zones. This could account for the hydrothermal fields at 18 ° and 21°S, alt h o u g h there is no fracture zone between them but there is an overlapping spreading center (Wilke and Lonsdale, 1987). However, it does not account for the hydrothermal field at 23.5°S, which, although it is adjacent to a large fracture zone, is at a shallower water depth than other parts of the spreading zone so near a fracture zone.
Fig.ll. Thickness of the sediment cover as indicated by the lengths of the cores obtained. The numbers beside the sampling stations give the core lengths in centimeters. The most dense shading indicates the thickest sediments. 116°W
115~d
114°W
"~iiiiiii~
18oS
113°W
i
" -
hydrothermn[
20°S
. . . .
30-
39%
~v~;~
~0-
~9%
50-
59%
21°S -
Sediments A sediment core was taken on each side of the EPR near each of the TV and photographic stations. For this purpose the ship sailed at right angles to the spreading center until the first sedimentary basin was observed on the Seabeam and the 3.5-kHz echo-sounder records. The locations of the 21 coring sites are shown in F i g s . l l and 12 where the sediment thickness and the amount of the hydrothermal material are also shown. The cores were sampled with a short (3 m)
moffer
19°S
-
-
-
> 60%
22°S .
eT'
2~S
2~°S
Fig.12. Content of hydrothermal matter in the sediment as indicated by the average content of hydrothermal matter in the cores. The numbers beside the sampling stations give the percentage of hydrothermal matter in the sediment (average for the core), The locations of hydrothermal fields densely covered with massive sulfides are marked by thick lines over the spreading zone.
:
186
gravity corer, the length of the cores varying between 44 and 273 cm. In the case of some of these cores, because of damaged corers or basalt fragments in the core catcher, we are certain th at we have sampled the complete sediment profile from sea floor down to the basement. In the case of other short cores we assume th at all of the sediment profile was also cored, as otherwise the corer would have penetrated deeper in this type of sediment, which is very soft with a high pore-water content. Only in the case of cores which are longer than 2 m is there a chance t h a t the sediment cover is thicker t ha n the penetration depth. The map of sediment thickness in F i g . l l was drawn on the basis of these results. The sediment thickness increases from east to west and from south to north. The minimum sediment thickness is observed in the southeastern
Fe,2O3 Mn0 As Cu Ha V Zn P205 SO3 Ba Ni Pb Th Y Rb
parts of the area and the maximum in the northwestern parts. The 21 cores were sampled on average every 15 cm (228 samples) and analysed by X-ray diffraction. The contents of carbonate, silicate and hydrothermal oxides were calculated from these analyses, as described by Marchig and Erzinger (1986). The mean concentrations of the elements on a carbonate-free basis are given in the table adjacent to the correlation matrix in Fig.13. This sediment contains a high proportion of hydrothermal material: it is composed of 50% organogenic calcite (coccolite and foraminifera) 41% hydrothermal oxides and 9% silicate, which is a poorly sorted hyaloclastite which has been weathered to varying degrees. Calculated on carbonate-free basis the sediment contains an average of 83% hydrothermal oxides and 17% hyaloclastites.
(%) 30.70 ±~27 (%) 7rio ~ 1.8~ MnI N \ O positive,highly significant (mglkg) 315 ± 10/~ AslC~Q"'~";~,',~ A negative,highly significant (mglkg} 831 ± 226 CuI O O O \ ' + x (ingle) 60 ± 2~ (mglkg) 752 ± 1(~ V l ~ ~ M ° " (mg/kg) 3/<.7 ± 78 (%) 223 ± 0.96 (%) 1.% ± 0.37 (mg/kg) 16% ± 696 ih 0 .... ~-,7o;. (rng/kg) L,23 ± 103 Ni O O S ~ ~@++0 (mg/kg) 30 ± 32 Pb OOO\" (mg/kg) 37 ± 13 O ! O O O \ %, (rng/kg) 77 ± 17 (mg/kg) 25 ± 8 Rb A O OOOOO~ ~ N~ (%) 3.18 ± 089 o A ± ~ , 1(20 (%) 0.78 ± 0.19 AA A ...... O l O ~ / ~ . HgO (%) 1.82 ± 0,7"/ M g l A A A A A A A A SAd~,A I SiO2 (%) 12.8/+ ± 6,12 S i l A A A A A A A A A A A A I J O \ '°," TiO2 (%) 0.25 ± 0.26 T ~ , ~ A A A A A A A A A I O I O O \ ,%
">~o,~
't
At203 (%) Zr
(mg/kg)
~%,,,,
2.1~.,zo9 A ~ , A A A A A A 51 ± 19 Z~l
SA AAAAAAAA
A
~lFe Mn ~Co Mo V Zn PI S I B ~
AA
'
O I
/QQQ~.°,
)OOO O',,Y
RblNa K I M ~ i
coo%* iO/ol so • ~2 coc~ A AAAOIOOOOOO]A Hy.ko C°/ol 83 * ~S " m 4 O O O O O O O O O O
A, Zd
A AAAAA
x in the sediment
Fig.13. Correlation matrix calculated from 228 sediment analyses calculated on a carbonate-free basis. The statistically highly significant correlation coefficients (99.9% significance level) are plotted as symbols. The table to the left gives the values of the mean concentration and standard deviation for the element or oxide (also on a carbonate-free basis, Hy.ka).
187
The results of the chemical analysis calculated on a carbonate-free basis were used to calculate the correlation matrix in Fig.13. Three different groups of elements can be distinguished in the matrix: (1) Mn and Fe correlate with As, Cu, Mo, V, Zn and P, i.e., these elements mostly occur bound to hydrothermal oxides; (2) Mg, Si and Zr correlate with the hydrothermally nonactive elements Ti and A1, i.e., these elements are bound preferentially to hyaloclastites, and (3) Ba, Ni, Pb, Th, Y and Rb form a group of elements which correlate with each other because they are enriched by different diagenetic processes in the sediment. These three groups of elements correspond to those obtained in previous investigations of the sediments adjacent to the EPR in areas which, owing to their geographic position, contain only traces of continental detritus and biogenic SiO 2 (Marchig and Gundlach, 1982; Marchig et al., 1986). Exceptional behavior was observed only for P, which in this case correlates with the hydrothermal precipitates. Phosphorus is hydrothermally mobile, and apatite belongs to the accessory minerals formed together with hydrothermal ores. Organogenic apatite also becomes enriched in sediment during diagenesis, as residual apatite after dissolution of carbonate. This biogenic apatite is usually predominant over hydrothermal apatite in zones containing a hydrothermal component and a high proportion of organogenic calcite. In such cases, P correlates with diagenetically enriched elements. Distinguishing between hydrothermal and diagenetic apatite is possible owing to differences in their contents of Sc, Y, La and REE's: Diagenetic apatite is enriched in these elements. We have suggested the P2Os/Y ratio as being very suitable for this purpose (Marchig et al., 1982). A plot of Y against P2Os in the carbonate-free component is shown in Fig.14. On the basis of this figure it can be concluded that most of the apatite in this sediment is of hydrothermal origin, i.e., there has been much less diagenesis in this sediment than in sediments containing hydrothermal
°° o o
o o
,./hydr0thermaL
o
apatite
oo
o~e
/~.0-
o
/
o%o
3.0-
oo o oO
o o o
o o
~OOo o oo j o
o~
;-to
°
2.0-
1.0" ~o
. o
"-" bi0genic
Y
apotite
(mg/kg)
Fig.14. Yttrium plotted against P2Os for all the 228 sediment samples calculated on a carbonate-free basis. Linear correlation for pure biogenic apatite and for pure hydrothermal apatite are drawn as shaded areas.
material that were sampled at some distance from the EPR. The objective of the sediment evaluation was the development of an exploration method for finding hydrothermal fields in the spreading zone in which massive sulfides are produced. This method would be based on the composition of the sediments adjacent to the spreading zone. In areas with weak hydrothermal activity, the elevated content of hydrothermal oxides in the sediment is an indication of a hydrothermal field in the spreading zone. This is the case for the sediment surrounding the Easter Plate East spreading zone (Marchig et al., in prep.). The geographic distribution of hydrothermal material (obtained from the average content of hydrothermal matter in the core material) in the study area is shown in Fig.12. Also shown is the location of three large hydrothermal fields containing frequent massive sulfides in the spreading zone.
188 In every pair of cores taken (one each side of the EPR at the TV stations), the one to the west has a higher content of hydrothermal matter and is longer. This can be explained by assuming a westward abyssal current that transports large amounts of hydrothermal discharge material but does not influence carbonate precipitation. It is also observed that the northern part of the area has a higher average content of hydrothermal matter in the sediment than is present in the southern part. The zones of elevated concentrations of hydrothermal matter in the sediment are not adjacent to the zones of frequent occurrence of massive sulfides in the spreading center. This shows that the absolute content of hydrothermal matter in an abyssal sediment is a suitable indicator for exploration for massive sulfide fields only in areas of weak hydrothermal activity; it does not provide useful results on the southern part of the EPR, where high rates of hydrothermal precipitation are everywhere present. The elevated content of hydrothermal matter in the sediments in the northwestern part of the area corresponds to the relatively large thicknesses of the sediments in the area. In Fig.15, core length is plotted for 21 cores against the average content of hydrothermal matter in those cores. There is a statistically significant positive correlation between these two parameters. The areas with thicker sediment cover correlate with the areas with higher hydrothermal input, both of them increasing towards the north. This is suggestive of increasing hydrothermal activity from south to north. This assumption can be made only on the basis of the analyses of the adjacent sediment and cannot be confirmed by observation of massive sulfides within the spreading zone. Conclusions
The highest spreading rate observed for a diverging plate boundary was determined for the spreading zone of the East Pacific Rise between 18° and 24°S. An exception to this high
r = 078 n 21
300
200 aa
100 -
,/,/ 20
..... 40 hydrofherma{
60 maffer
(%)
Fig.15. A v e r a g e c o n t e n t of h y d r o t h e r m a l m a t t e r in the core v e r s u s the l e n g t h of the core. Two linear c o r r e l a t i o n s were calculated, y = a + b x and x = a + b y . Core 160 w a s treated as a n o m a l o u s and w a s not t a k e n into consideration.
spreading rate is the part of the East Pacific Rise south of a large fracture zone forming the northern boundary of Easter plate: The Easter microplate divides the diverging plate boundary of the East Pacific Rise into two branches, thus halving the spreading rate. There is lateral displacement at one place along the spreading zone, at a large fracture zone (the north fracture zone of the Easter plate), and one large overlapping spreading center at 20.5°S. Otherwise, the course of the East Pacific Rise is relatively smooth, with only small cross fractures and overlapping spreading centers, as is typical for spreading centers with high spreading rates. Hydrothermal activity seems to be exceptionally high also. All of the seventeen photographic stations within the active spreading zone (two stations were in fossil spreading zones) over a distance of 138 km revealed some indications of hydrothermal activity. The typical hydrothermal outlet in this area is a black smoker, a narrow conduit with a massive sulfide chimney. Areal outlets, from which hydrothermal solutions slowly seeped
189
from talus fields, were also observed. We conclude t h a t the hydrothermal solutions of this type are residual solutions remaining after formation of stockwork mineralization below the sea floor. The cracks containing the residual hydrothermal solutions overgrown with shells or worms are seldom. Four steps of hydrothermal activity were observed: the first one producing massive sulfides, the second one the yellow Fe silicate sediment, the third one a dull black precipitate, which we assume to be Mn oxide, and the fourth one a mixture of Fe and Mn hydroxides which precipitates outside of the spreading center. Three large hydrothermal fields densely covered with massive sulfide chimneys were observed. All of them are associated with the area in which there is some additional tectonic strain. In the southernmost of these hydrothermal fields, the additional tectonic strain is demonstrated by the bend in the spreading zone; in the two more northern hydrothermal fields this additional strain is shown by a welldeveloped central valley t h a t is highly fractured. All three of these hydrothermal fields are on the higher parts of the spreading zone and have a greater coverage by sheet lava t h a n by pillow lava. The elevated position, as well as the occurrence of sheet lava, can also be explained by additional tectonic strain. The sediment near the spreading zone has a high proportion of hydrothermal matter: an average of 41% of the sediment and 83% of the carbonate-free fraction is of hydrothermal origin. The thickness of the sediment cover, as well as the hydrothermal component, increase northwards; from this we conclude t h a t the hydrothermal input in the ocean also increases from south to north. This contradicts the discovery of large hydrothermal fields not only at 18.5°S with the highest hydrothermal input in the surrounding sediment, but also at 21.5 ° and 23.5°S, by which position the hydrothermal input in the surrounding sediment is distinctly lower t h a n in the northernmost position. It shows that at
very hydrothermally active areas the input of hydrothermal precipitate in pelagic sediment does not assist in locating massive sulfide concentrations within the spreading zone.
References B~icker, H., Lange, J. and Marchig, V., 1985. Hydrothermal activity and sulphide formation in axial valleys of the East Pacific Rise crest between 18 and 22°S. Earth Planet. Sci. Lett., 72: 9-22. BostrSm, K. and Peterson, M.N.A., 1968. The origin of aluminium-poor ferromanganoan sediments in areas of high heat flow on the East Pacific Rise. Mar. Geol., 7: 427-447. Bundesanstalt fiir Geowissenschaften und Rohstoffe with Reedereigemeinschaft Forschungsschiffahrt (B.G.R. with R.F.), 1986. Fahrtbericht SO 40, GEOMETEP 4. Bundesanst. Geowiss. Rohst. with Reedereigem. Forschungsschiffahrt, Hannover, BMFT Proj. 03R3260. Corliss, J.B. and Ballard, R.D., 1977. Oasis of life in the cold abyss. J. Natl. Geogr. Soc., 152: 441-453. Francheteau, J. and Ballard, R.D., 1983. The East Pacific Rise near 21°N, 13°N and 20°S: inferences for alongstrike variability of axial processes of the mid-ocean ridge. Earth Planet. Sci. Lett., 64: 93-116. Francheteau, J., Needham, H., Choukroune, P., Siguret, M., Ballard, R., Fox, P., Normark, W., Carranza, A., Cordoba, D., Guerrero, V., Rangin, C., Bougalt, H., Cambon, P. and Hekinian, R., 1979. Massive deep-sea sulfide ore deposits discovered by submersible on the East Pacific Ridge: Project RITA, 21°N. Nature, 277: 523-528. Lonsdale, P.F., Bischoff, J.L., Burns, V.M., Kastner, M. and Sweeney, R.E., 1980. A high temperature hydrothermal deposit in the seabed at the Gulf of California spreading center. Earth Planet. Sci. Lett., 49: 8-20. Marchig, V. and Gundlach, H., 1982. Iron-rich metalliferous sediments on the East Pacific Rise: Prototype of undifferentiated metalliferous sediments on divergent plate boundaries. Earth Planet. Sci. Lett., 58: 361-382. Marchig, V., Gundlach, H., MSller, P. and Schley, F., 1982. Some geochemical indicators for discrimination between diagenetic and hydrothermal metalliferous sediments. Mar. Geol., 50: 241-256. Marchig, V., Erzinger, J. and Heinze, P.M., 1986. Sediment in the Black Smoker Area of the East Pacific Rise (18.5°S). Earth Planet. Sci. Lett., 79: 93-106. Marchig, V. and Erzinger, J., 1986. Chemical composition of Pacific sediments near 20°S - - changes with distance from the East Pacific Rise. In: H. Leinen and D.K. Read, Initial Reports of the Deep Sea Drilling Project, Vol.92. U.S. Gov. Print Off., Washington, D.C., pp.371-381. Marchig, V., RSsch, H., Lalou, C., Brichet, E. and Oudin, E., 1988. Mineralogical zonation and radiochronological relations in a large sulfide chimney from the East Pacific Rise at 18.5°S. In: T.J. Barret and J.S. Fox (Editors),
190 Recent Hydrothermal Mineralisation at Seafloor Spreading Centres. Can. Mineral., in press. Marchig, V., Wilke, M. and Holler, G., in prep. Spreading Center at Easter Plate East: Tectonic and Hydrothermal Activity and Surrounding Sediment. (Critical Reviews in Marine Science.) CRC Press, Baton Rouge, La. Normark, W.R., Morton, J.L., Koski, R.A., Clague, D.A. and Delaney, J.R., 1983. Active hydrothermal vents and sulfide deposits on the southern Juan de Fuca Ridge. Geology, 11: 158-163. Rise Project Group, 1980. East Pacific Rise: hot springs and geophysical experiments. Science, 207:1421 1433. Thiel, H. and Tuerkay, M., 1987. Biologische Beobachtungen aus Hydrothermal Aktiven Bereichen des Siidost Pazifik. Bericht fiber die biologischen Ergebnisse GEOMETEP. In: Abschlussbericht SO 40 der 4 Fahrt mit F.S. SONNE im Rahmen des GEOMETEP-Programms.
Bundesanst. Geowiss. Rohst. with Reedereigem. Forschungsschiffahrt, Hannover and Bremen, BMFT Proj. 03R3620, pp.179-209. Tufar, W., Gundlach, H. and Marchig, V., 1984. Zur Erzparagenese rezenter Sulfid-Vorkommen aus dem sfidlichen Pazifik. Mitt. Osterr. Geol. Ges.,74: 185-245. Tufar, W., Gundlach, H. and Marchig, V., 1986. Ore paragenesis of recent sulfide formations from the East Pacific Rise. Monogr. Ser. Miner. Deposits, 25: 75-93. Von Stackelberg, U. et al., 1985. Hydrothermal sulfide deposits in back-arc spreading centers in the Southwest Pacific. Bundesanst. Geowiss. Rohst. Circ., 2: 3-14. Wilke, M. and Lonsdale, P., 1987. GEOMETEP 4 bathymetry report. In: Abschlussbericht SO 40 - - der 4 Fahrt mit F.S. SONNE im Rahmen des GEOMETEP-Programms. Bundesanst. Geowiss. Rohst. with Reedereigem. Forschungsschiffahrt, Hannover and Bremen, BMFT Proj. 03R3620, pp.13 43.