Native Copper and A-Copper–Zinc in Sediments from the TAG Ž / Hydrothermal Field Mid-Atlantic Ridge, 268N : Nature and Origin

Marine Geology 161Ž. 1999 229±245 www.elsevier.nlrlocatermargeo

Native copper and a-copper±zinc in sediments from the TAG hydrothermal fieldž/ Mid-Atlantic Ridge, 268N : nature and origin V.M. Dekov a,), Z.K. Damyanov b, G.D. Kamenov c, I.K. Bonev d, K.B. Bogdanov e a Department of Geology and Paleontology, UniÕersity of Sofia, 15 Tzar OsÕoboditel BlÕd., 1000 Sofia, Bulgaria b Central Laboratory of Mineralogy and Crystallography, Bulgarian Academy of Sciences, 92 RakoÕski Str., 1000 Sofia, Bulgaria c Department of Geology, Florida International UniÕersity, UniÕersity Park PC 304, Miami, FL 33199, USA d Section of Mineralogy, Geological Institute, Bulgarian Academy of Sciences, bl. 24 Acad. G. BoncheÕ Str., 1113 Sofia, Bulgaria e Department of Mineralogy, Geochemistry and Ore Geology, UniÕersity of Sofia, 15 Tzar OsÕoboditel BlÕd., 1000 Sofia, Bulgaria Received 21 July 1998; accepted 19 March 1999

Abstract

Native copper and a-copper±zinc occur as strands and elongated grainsŽ. up to 300 mm in length within the sediments from the Trans-Atlantic GeotraverseŽ. TAG hydrothermal field Ž Mid-Atlantic Ridge, 268N . . They are remarkably similar in composition and crystal structure to copper and copper±zinc occurrences found in other natural environments. The results of mineralogical studies are discussed in terms of the possible mechanisms of native metal formation in the complex TAG field: with an asymmetric and highly-fractured rift valley, and mature active and relict sulfide mounds. Native copper and a-copper±zinc grains disseminated in the TAG sediments are either inherited from:Ž. 1 primary magmatic or metamorphic crustal source;Ž. 2 hydrothermal deposits; and Ž. 3 the alteration of primary deposits, or formed Ž. 1 authigenically, or Ž. 2 biogenically withinron the sediment cover. Native metallic particles could have been formed as accessory minerals disseminated in the ridge crest basic rocks andror massive sulfide mounds. Degradation of these rocks and mass wasting of the mounds have liberated the metallic grains which have, in turn, been deposited into adjacent sediments. q 1999 Elsevier Science B.V. All rights reserved.

Keywords: native copper; native a-copper±zinc; TAG hydrothermal field; Mid-Atlantic Ridge

1. Introduction diversity of environmentsŽ see Cornwall, 1956; Ram- dohr, 1975, for comprehensive reviews. . The presence of native copper and zinc in any The presence of native copper is less common in natural environment is always somewhat surprising oceanic than in continental environments. It occurs in view of the ease with which they both combine in pillow basalts which are remnants of ancient with other elements, especially sulfur and oxygen. oceanic crustŽ. Nagle et al., 1973 , in veins, vesicles Nevertheless, zero-valent copper and zinc exist in a and groundmass of oceanic basic rocks underlying the sediment coverŽ Von der Borch et al., 1974; Kennett et al., 1975; Ovenshine et al., 1975; Talwani ) Corresponding author. Fax: q359-2-44-64-87; E-mail: et al., 1976; Roberts et al., 1984; Leinen et al., 1986; [email protected] LeHuray, 1989; Puchelt et al., 1996. , in both pelagic

2. Geological setting sites: one, in the vicinity of the active mound; an- other, near the dead MIR moundŽ. Fig. 1B,C . In the spring of 1988, the 15th cruise of RrV The sediment coresŽ. stainless steel gravity tubes Akademik MstislaÕ Keldysh, with two submersibles were sampled in 1±2-cm batches and stored hermeti- Ž.Mir-1 and Mir-2 aboard, took place in the North cally in polyethylene bags prior to analysis. Equal Atlantic at the TAG hydrothermal field. amountsŽ. 100 ml of natural wet sample were washed The TAG hydrothermal field lies in a rift valley with distilled water to extract soluble salts, wet-sieved on the slow-spreading Mid-Atlantic RidgeŽ. MAR through a set of plastic sieves and air dried in a Žhalf rate of accretion 1.1±1.3 cm yry1 , McGregor et controlled laboratory environment Ž.;208C. X al., 1977.Ž between Atlantis 30840 N. and Kane The coarse fractionsŽ. those )0.10 mm were Ž.248N fracture zones Ž SempereÂÂ et al., 1990 .Ž Fig. examined under a binocular stereomicroscope. Cop- 1A. . The rift valley is characterized by an asymmet- per and copper±zinc metallic particles were found in ric structure: the east wall is higher and steeper than only seven samplesŽ nannofossil-foraminiferal oozes the west wallŽ. Rona et al., 1986, 1993a and exposes with dispersed hydrothermal precipitates and basaltic the sheet-dike complex and gabbro layer on its lower clasts; Table 1; Fig. 1C. , although all the other partŽ. Zonenshain et al., 1989 . samples from these sites were thoroughly re-ex- An active sulfide mound is situated east of the amined. Metallic grains were picked out by hand spreading axis on the rift valley floor. It is a large with a steel needle for further mineralogical investi- Ž.200±250 m in diameter; 40±50 m in height , steep- gations. Preliminary diagnostics of the separated sided edifice. Its central part consists of a high-tem- mineral grains were carried by means of semi- perature black smoker system surrounded by a 100 quantitative EDS analyses on their natural faces. m wide platform with a complex of white smokers The micromorphology, size and chemical compo- and an apron of oxidizing sulfide talus and metallif- sition of copper and copper±zinc alloy specimens erous sedimentsŽ. Rona et al., 1986, 1993a,b . were investigated by SEM JEOL Super-probe 733 Two dead sulfide moundsŽ. named Alvin and MIR with a System-5000 ORTEC EDS and SPRINT-III occur on the lower east wall of the rift valley and are program and SEM JSM-35 CF with Tractor North- undergoing extensive erosion and mass-wasting ern-2000 EDS at 20 kV. The following standards ŽLisitsyn et al., 1989; Lisitsyn, 1992; Rona et al., were used: pure synthetic copperŽ. Cu Ka and pure 1993a,b. . The MIR inactive hydrothermal zone found synthetic zincŽ. Zn Ka . The detection limits were in 1988Ž. Lisitsyn et al., 1989 is located between the 0.5 wt.%. Point analyses, onto polished sections, low-temperature zone, some 300 m higher on the were applied to establish the composition of the wallŽ. Rona et al., 1984; Thompson et al., 1985 , and metallic phases; back-scattered electron images the active high-temperature sulfide moundŽ Rona et Ž.BEI , line scannings and X-ray mappingÐto clarify al., 1993a,b.Ž . This relict hydrothermal deposit about the spatial phase distribution. 1 km in diameter. shows various stages of weathering. The thin sediment cover, surrounding the TAG Table 1 a sulfide mounds, is composed of nannofossil-for- Location of the investigated metallic particles aminiferal ooze with frequent intercalations of clastic Gravity Water Core Metallic Sample a sulfide-oxyhydroxide flows and basaltic shard flows core a depth interval particles Ž.mcm Ž . Ž.Metz et al., 1988; Lisitsyn, 1992 . Serpentinized mafic clasts have been dredged from the rift valley 1785 3665 0±2 copper±zinc H103.S2 Ž. 35±37 copper H103.S14 floor Zonenshain et al., 1989 . 52±53 copper H105IV.S45 1891 3528 30±31 copper±zinc H104.S18 31±32 copper±zinc H104.S36 3. Materials and methods 35±36 copper±zinc H105I.S1 40±41 copper±zinc H105I.S61 40±41 copper±zinc H106II.S37 We studied in detail the mineralogy of TAG sedimentsŽ. Damyanov et al., 1998 cored at two a For core locations see Fig. 1. 230 V.M. DekoÕ et al.rMarine Geology 161() 1999 229±245

ŽBerger and von Rad, 1972; Hollister et al., 1972; per±zinc alloy in the TAG sediments, andŽ. 2 to Zemmels et al., 1972; Schlanger et al., 1976; Siesser, comment on the genesis of these metallic particles. 1978; Knox, 1985. , and metalliferous sediments ŽJenkyns, 1976; Lazur et al., 1984; Marchig et al., 1986.Ž , and in sea-floor massive sulfides Minniti and Bonavia, 1984; Hannington et al., 1988. . Metallic copper particles suspended in the deep North At- lantic watersŽ. Jedwab, 1979 could presumably have been liberated from outcropping rocks by submarine erosion. More disputable are the occurrences of the copper±zinc alloy. Native copper±zinc was first found in close intergrowths with troilite and feldspars in the lunar regoliths brought by Apollo 11 ŽGay et al., 1970. . Findings of Cu±Zn alloy grains followed in kimberlitesŽ McCallum and Eggler, 1976; Kovalskii, 1985.Ž , basic and ultrabasic rocks Okrugin et al., 1981; Rudashevskii et al., 1987; Glavatskih, 1990; Nishida et al., 1994.Ž , hydrothermal ores Clark and Sillitoe, 1970; Novgorodova et al., 1979; Dom- brovskaya et al., 1984. , volcanic-sedimentary se- quencesŽ. Lazur et al., 1988 and pelagic sediments of the Pacific OceanŽ Shterenberg and Vassileva, 1979; Lazur et al., 1984; Shterenberg and Voronin, 1994. . While examining the sediments from the vicinity of MIR zone, TAG hydrothermal field, for sulfides and oxyhydroxidesŽ. Damyanov et al., 1998 , we found small red and yellow metallic particles in some of the samples. The purpose of our study isŽ. 1 to present and describe the occurrences of native copper and cop-

Fig. 1.Ž. A Index map showing the location of TAG area at the Mid-Atlantic Ridge.Ž. B Bathymetric map Ž isobaths in meters; hatchures point downslope.Ž based on Rona et al., 1993a . of the investigated area showing the positions of sediment sample sta- tionsŽ. solid dots , the active high-temperature sulfide mound Ž. star and the inactive MIR hydrothermal zoneŽ.Ž. dark pattern . C Stratigraphic cross-section of the TAG cores 1785 and 1891 showing the lithology, vertical distribution of the main chemical tracer of hydrothermal activityÐFeŽ. on carbonate-free basis Ž.based on Lisitsyn et al., 1989 , and positions of native copper Ž.hollow arrows and native copper±zinc alloy Ž. black arrows findings.Ž. 1 Nannofossil-foraminiferal and foraminiferal oozes; Ž.2 volcanic glass fragments; Ž. 3 basaltic basement; Ž. 4 disseminated hydrothermal componentsŽ sulfides, Fe- and Mn-oxyhydroxides, quartz.Ž. ; 5 clastic sulfide-oxihydroxide layer; Ž. 6 nontron- itic lenses. 232 V.M. DekoÕ et al.rMarine Geology 161() 1999 229±245

The X-ray diffraction patterns of the metallic copper and six copper±zinc particlesŽ Fig. 1C; Table particles were obtained with a 57.3 mm Gandolfi 1.. camera employing Ni-filtered Cu Ka radiation with- The copper particles are elongated strands up to out internal standardŽ with an operating voltage of 40 200 mm in lengthŽ. Fig. 2A , with a very high kV and a beam current of 19 mA. . metallic luster in freshly polished or cut pieces, The reflectance spectra of Cu±Zn alloy were ob- delicately pink±red in color, but which soon darkens tained by means of computerized Zeiss MPM mi- to `copper-red', isotropic. The particles were almost crophotometer with Hamamatsu R928HA type always covered by black stains of tiny irregular photomultiplier, SiC standard Ž.a 860 and Lambda- crystalsŽ. Fig. 2B containing sulfur Ž EDS data . . scan software. The copper±zinc grains are elongated along one Zeiss mhp-160 microhardness tester was em- of the axesŽ. Fig. 2C with sizes ranging from 50 to ployed for microhardness measurements. 150 mm in one direction to 250±300 mm in the other. The color of the very freshly polished surface is yellowish-white, which darkens to a `gold-yellow' 4. Results if the polished surface is not protected with a coating of immersion oil. It is isotropic with a high metallic Native copper and copper-zinc alloy occurrences luster. Some of the particles were partly covered by in TAG sediments are fairly rare: we found only two black±brown Fe±Mn oxyhydroxidesŽ. Fig. 2D .

Fig. 2. Scanning electron micrographs of:Ž. A lamella-shaped copper particle Ž H105IV.S45 .Ž. ; B sulfur-containing isometric-grained aggregates on the surface of the copper particle shown atŽ.Ž A close-up of the central part .Ž. ; C copper±zinc grain Ž H104.S18 .Ž general view.Ž ; D . ferromanganese microcrust Ž arrow . on the surface of a copper±zinc grain Ž H104.S18, lower part . . Scale bars equal 100 mmC, Ž. 10 mmŽ. A,D and 1 mmB. Ž. V.M. DekoÕ et al.rMarine Geology 161() 1999 229±245 233

The reflectance spectra curve of Cu±Zn alloy from different natural materials, while the most used Ž.Fig. 3 resembles more that of gold with the compo- synthetic copper±zinc alloys have different XRD sition Au0.80 Ag 0.20 than that of copperŽ QDF II cards patternsŽ. Table 3 . a140 and a82, respectively; Criddle and Stanley, The investigated copper particles represent ex- 1986. . tremely pure CuŽ. 98.8±99.8 wt.% with minor Microhardness measurements on the copper±zinc amounts of Sn and SŽ. 0.n wt.% . For comparison, alloyŽ Vickers microhardness VHN, 20 g load, speci- published data on the chemical composition of native men H104.S18. are in the 79.8±84.1 kgrmm2 range copper from other oceanic environments have been Žmean of 81.6 kgrmm2 . by 10 indentations with a compiledŽ. Table 4 . These data show that the trace perfect to concave shape. For comparison, the micro- elements are present in natural copper from the hardness of copper±zinc from the quartz veins of the oceanic sediments in extremely low concentrations. South Ural, Russia, is 110 kgrmm2 Ž Novgorodova, The investigated copper±zinc alloy contains about 1980. . The standard range of synthetic copper is, 80% Cu and 20% ZnŽ. Table 5 . Only one of the s r 2 respectively, VHN100 79±99 kg mmŽ QDF II microprobe analyses revealed traces of FeŽ 0.04 card a 82; Criddle and Stanley, 1986. . wt.%.Ž. . On the basis of the crystal structure Table 3 The extreme thinness of the copper lamellae and and chemical composition of this alloy, we can their scratched surface after repolishing prevented conclude that it is a-Cu±Zn, or Zn-bearing copper the execution of accurate microhardness and re- Ž.Hansen and Anderko, 1958 . Copper±zinc particles flectance spectra measurements. from the TAG hydrothermal field contain less Zn The X-ray diffraction studies proved the exam- than all the other native Cu±Zn alloys from various ined particles to be Cu( and Cu±Zn alloyŽ Tables 2 natural environments excluding those findings at the and 3. . The sharp and clear reflections on the X-ray Dulcinea copper deposit, ChileŽ Clark and Sillitoe, diffraction patterns show that the particles are well 1970.Ž Table 5 . . The native Cu±Zn particles found crystallized. Both minerals are cubic. The XRD re- in sandstones and clay shales of the Devonian vol- sults for copperŽ. Table 2 and copper±zinc Ž. Table 3 canic-sedimentary formation from the Middle Ural, from TAG sediments agree well with those reported RussiaŽ. Lazur et al., 1988 have the closest chemical compositions to our data which implies formation in a similar deep-sea setting near the active magmatic zones. Back-scattered electron imagesŽ.Ž BEI Fig. 4A,D . , and X-ray mappingŽ. Fig. 4B,C and line-scanning Ž.Fig. 4E,F of copper±zinc particles at characteristic Cu Kaa and Zn K lines show a regular spatial distribution of elements and homogeneity of the alloy.

5. Discussion In view of the complexity of the TAG hydrothermal field, comments on the origin of metallic Cu and a-Cu±Zn in the TAG sediments must take all possible contributions into account. For example, all the described metallic occurrences in the TAG sediments were found in sediment layers older than 8 kyrŽ. Fig. 1C . This implies that they were formed in pre-technogenic times. A num- Fig. 3. Reflectance spectra forŽ. 1 copper±zinc alloy Ž H104.S18 . , ber of undoubted native copper grains have already Ž.2 gold, and Ž. 3 copper. been found in the TAG depositsŽ Hannington et al., 234 V.M. DekoÕ et al.rMarine Geology 161() 1999 229±245 A hkl Ž. l Id 4-836 r I a A hkl b , Id A hkl Id Debye-Scherrer Debye-Scherrer A hkl ba a , Id Vassileva, 1979 Voronin, 1994 Ê ÊÊÊ Ê A hkl Ž. Ž. Ž. Ž. Ž. N Pacific Ocean 8 aa a ±± ±± ± ± ± ± 1 ± 2 2.02 ± 1.90 ± ± ± ± ± ± ± ± ± ± ± ± 2 ± ± 2.006 ± 2 ± 1.412 ± ± ± ± ± ± ± ± ± ± ± Id MAR 26 H105IV.S45 1051610 2.96 1 2.32 2.08 ±9 ± 1116 1.817 ±3 10 ±7 1.275 2004 ± 1.222 2.103 ± 1.088 220 65 OB-254-1 1.044 ±2 ± 0.909 311 8 ±5 1.82 0.831 222 ±2 ± 0.830 400 8 10 1.28 0.810 331 ± ± 6 0.808 ± ± ± ± 2.09 1.10 ± ± ± ± 1.05 6 420 ± ± ± ± ± ± 5 ± ± ± ± 1.81 ± ± 10 8 ± ± 1.28 ± ± ± ± 4 ± ± 2.092 1.09 3 ± ± ± ± 1.05 8 ± ± ± ± ± ± 2.312 ± 8 ± ± 1.818 ± 100 ± ± ± ± 8 ± 1.283 ± ± 5 ± 2.088 ± 1 ± ± 1.089 ± ± ± 46 1.044 111 ± ± 1.154 ± ± ± 20 ± ± ± 1.808 ± ± ± ± 17 ± ± 1.278 ± 200 5 ± ± ± ± 1.090 ± 220 ± ± ± 1.043 311 ± 3 ± 222 ± ± ± 9 ± 0.903 ± 8 ± 400 0.829 ± 0.808 ± 331 420 ± a Ê kV 40 ± ± ± ± Ž. mA 19 ± ± ± ± Ž. Ž. 0 Intensities estimated visually. Analytical conditions I Results Table 2 X-ray diffraction data for native andReference synthetic copper Present studyFilterU Shterenberg and Ni Shterenberg and Okrugin et al., 1981 ± JCPDS ±a Aa ± 3.609 ± ± ± ± 3.615 Object and areaof investigation Pelagic sediments,Sample TAG hydrothermal field,Radiation NE Pacific Ocean Pelagic sediments,Camera Cu K Clarion fault zone, Pelagic st.System sediments, 674, Platform, Russia 57.3 mm Gandolfi Basic rocks, Siberian Cubic Fe K 55.3 mm Copper URS-55 synthetic 57.3 mm Cubic Fe K 57.3 mm Cubic Fe K ± Cubic ± Cubic V.M. DekoÕ et al.rMarine Geology 161() 1999 229±245 235

Fig. 4.Ž. A Back-scattered electron image of a copper±zinc particle Ž H104.S18 .Ž. . B Electron microprobe Cu Ka X-ray mapping of the same area as shown inŽ.Ž. A . C Electron microprobe Zn Ka X-ray mapping of the same area as shown in Ž.Ž. A . D Back-scattered electron image,Ž. E Cu Kaa X-ray line scanning, and Ž. F Zn K X-ray line scanning of a copper±zinc particle Ž H104.S18 . . Scale bars equal 10 mm Ž.A±F .

1988. . The composition and crystal structure of the differ from those of the most used anthropogenic reported copper and copper±zinc particles are close alloysŽ. Tables 2±5 . All the tools used in the sample to those of previously proved native occurrences and treatment were plastic or made of stainless steel and 236 V.M. DekoÕ et al.rMarine Geology 161() 1999 229±245 A hkl ±± ± ±± ± Id 3 1.207 ± 2 2.358 ± 10 2.090 ± b b b b b A hkl ba , Id A hkl ± ± ± 3± 2.026 ± 200 ± 1 1.171 222 ± ± ± 8 2.338 111 Id 3 1.204 ± 1 1.227 311 1 1.476 ± 1 1.433 220 b b b b b A hkl ba , 1 1.175 222 Id ) 2 2.046 200 1 1.227 311 1 1.440 220 3 2.360 111 b b b b b A hkl ba , Id 0.5 2.040 200 1979Debye-Scherrer Debye-Scherrer 1987 1984 Debye-Scherrer N Russia mountain, Russia Russia 8 ÊÊÊÊÊÊ A hkl Ž. Ž. Ž. Ž. Ž. Ž. aa a ± ± ± 0.5 1.179 222 ± ± ± ± ± ± ± ± ± 6 2.087 ± ± ± ± ± ± ± ±±±±±±±±±±±±±±±±±± ±±±±±±±±±±±±±±±±±± ±±±±±±±±±±±±±±±±±±± ±± ± ±±±±±±±±±±±±±±±±±±± 1±±±±±±±±±±±±±±±±±± ± 1.442 1 220 1.229 311 ±±±±±±±±±±±±±±±±±± ±±±±±±±±±±±±±±±±±± ±±±±±±±±±±±±±±±±±± ±±±±±±±±±±±±±±±±±± ±±±±±±±±±±±±±±±±±± ±±±±±±±±±±±±±±±±±± ±±±±±±±±±±±±±±±±±± ± ± ± 7 2.358 111 Id 10 2.113 111 10 2.133 111 10 2.138 111 10 2.137 111 10 2.114 111 10 2.140 ± 7 1.824 2005 7 1.2917 1.851 220 200 1.0993 10 4 1.309 311 1.05514 1.853 10 220 222 0.9123 1.113 200 3 0.839 7 400 0.817 6 311 1.303 331 1.066 ± 420 4 1.854 ± 220 222 ± ± 1.113 200 3 ± 3 ± 7 311 1.311 1.065 ± 4 1.838 ± 220 222 ± ± 1.118 200 3 2 ± ± 2 311 ± 1.302 1.068 ± 3 1.848 ± 220 222 ± 1.109 ± 3 1 ± 311 ± ± 1.316 1.016 ± 3 ± ± ± 222 ± 1.115 1 ± ± ± 1.032 ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± field, MAR 26 H104.S18 ± OB-523-1 Sample 3 Sample 773 ± a Ê kV 40 ± ± ± ± 40 Ž. mA 19 ± ± ± ± 19 Ž. Ž. 0 Analytical conditions I Results Table 3 X-ray diffraction data for native andReference synthetic copper±zinc alloys Object and areaof Present investigation study Pelagic sediments, TAG hydrothermal Gold-bearingRadiation quartz veins, Novgorodova South etFilter Ural, al.,U Basic rocks, Siberian Okrugin et al., Platform, Cu 1981 Russia K Ultrabasic rocks Rudashevskii et Ni al., Koryakskoe Gibsit-alunit ores, Dombrovskaya et al., Glavatskih, Basalts, 1990 Kamchatka, Fe K West Pribaikalie, ± Russia Fe K ± ± ± Fe Ka A ± Cu K 3.65 Ni 3.68 3.69 ± 3.674 ± Sample CameraSystem 57.3 mm Gandolfi Cubic 57.3 mm Cubic 57.3 mm ± Cubic Cubic 57.3 mm Cubic 57.3 mm Gandolfi Cubic V.M. DekoÕ et al.rMarine Geology 161() 1999 229±245 237 x A hkl 0.61 0.39 Id 19-179 Cu Zn O Ž. r I b a A hkl Id 25-322 1 1.361 205 10 1.335 002 1 1.382 211 ± ± ± r I a - - A hkl aaa Id 26-571 r I a A hkl Id 2-1231 r I a ÊÊ Ê ÊÊ A hkl Ž. Ž. Ž. Ž. Ž. 58 11 1 11 Id 25-1228 -Cu Zn Cu±Zn Zhangengite Cu±Zn Cu±Zn r ± ±±±± ±±±±±± ±± ±±± ±±±± ± ±± ±± ± 20 ± ± ± ± 1.203 ± 5 ± 211 1 ± 1 ± 1.042 5 0.983 3 220 0.889 ± 300 0.851 ± 311 0.788 10 222 ± ± 321 ± ± 0.989 ± ± ± 119 ± ± ± ± ± ± 9 ± ± ± ± ± 1.064 ± ± ± 220 ± ± ± ± 5 ± ± ± ± ± 1.002 ± ± ± 222 ± ± ± ± ± ± ± ± ± ±±± ±± ± ± ± ±± ± ±±±± ±±±±±± ±± ±±± ± ±± ± ± ±± 6 ± ± ±± ± ± ± 2.95 ± ± ± ± ± 100 ± ±± ± 100 ± ± ± ± ± 40 ±±±± ±± ±±± ± 3.45 ± ± ± ± 3.33 ± ± 101 ± 003 ± ± ± ± ± ± ± ± ± 60 ± ± ± ± ± ± ± ± 2.16 ± ± ± ± ± ± 110 ± ± ± ± ± ± ± ± ± ± 10 ± ± ± ± ± ± 100 1.66 ± ± 15 2.12 ± ± 006 ± ± ± ± 110 2.65 100 ± ± 20 ± 001 ± 2.134 2.23 ± ± 200 020 ± 35 ± 5 1.966 120 1.588 121 I 24 1.253 550 1.091 ± 741 ± ± ± ± ± 20 10 1.225 214 1.113 ± 009 5 ± 1.111 ± 009 5 65 1.289 1.118 012 040 2 1.306 631 2 1.319 210 20 1.306 ± g Ž. Ž. Ž. Ž. Ž. 75 2.55 2.36 222 321100 ± ± 2.0885 ±3 330 ±3 1.889 100 1.808 ±6 332 1.737 2.08 ± 422 ± ± 510 1.476 ± 110 ± 1 600 20 ± ± ± 15 ± 1.702 2.08 ± ± 111 1.474 ± 104 ± 10 200 40 17 ± 10 30 ± 1.74 1.86 2.07 ± 1.82 1.489 202 ± ± 104 024 113 ± 60 ± ± 35 ± ± ± ± 2.03 1.833 ± ± ± 111 200 ± ± ± ± ± ± ± ± 20 5 ± ± ± 1.908 1.545 ± 210 220 ± Ž. aa Ê kV ± ± ± ± 40 Ž. mA ± ± ± ± 19 Ž. Ž. 0 Intensities estimated visually. Analytical conditions I Results Object and area of investigationSample synthetic synthetic synthetic synthetic synthetic Table 3 continued Reference JCPDSFilterU JCPDS ± JCPDS ± JCPDS Ni JCPDS ±a Aa Ni 8.860 2.948 4.318 4.256 ± CameraSystem ± Cubic Cubic ± Hexagonal ± Hexagonal ± Orthorhombic ± Radiation ± ± Cu K Cu K Cu K 238 V.M. DekoÕ et al.rMarine Geology 161() 1999 229±245

contained no metallic copper and zinc. There is no evidence for contamination such as the presence of stainless steel flakes. From a thermodynamic and chemical point of view, the genesis of Cu( and Cu±Zn is not forbidden in this environmentŽ Garrels and Christ, 1965. . We therefore conclude that the investigated metallic particles are native. The TAG area is far from the adjacent continents Ž.Fig. 1A and any river flow would not be able to carry metallic particles with relatively high specific gravity away from the near-shore zones. Moreover, it is well known that more than 90% of the riverine suspended matter settles at the searriver zone of mixing. Onshore erosion and mechanical transport of these particles to the TAG site by sea currents may therefore be discarded as a possible mechanism of origin as well. On the other hand, transport by the winds in an aerosol state is probable. The central

Atlantic Ocean Deep, Pacific Ocean north Atlantic receives the highest aerosol flux to the ocean from the northeastern trades blowing from the SaharaŽ. Lisitsyn, 1972 . The winds are capable of entraining any particle of a similar size and carrying it long distances. However, the extensive abrasion of the particles in the aerosol clouds, river streams or during shoreline erosion would smooth the soft metallic grains. The studied copper and copper±zinc grains are not rounded and abraded, as would be the case if this had been caused by any long-acting Plateau, South Pacific Ocean 364, Angola Basin,Cocos±Nazca rift, Hess transporting agent. A terrigenous nature of the studied particles is therefore unlikely. The hypotheses for the origin of the native copper and a-copper±zinc grains disseminated in the TAG sediments are: inheritance from a primary source, or formation withinron the sediment cover. Different primary sources are considered below.

5.1. Formation in the oceanic crust, peculiarly the lower leÕels of the crust N DSDP site 317A, Manihiki DSDP site site 895, 8 5.1.1. During the magmatic crystallization.

Ž. Primary or `magmatic' copper and a-copper±zinc could be formed from an immiscible metal liquid or by reduction and crystallization from a silicate melt. Native metal particles in the mafic sequences of : H105IV.S45 H103.S14 ± ± 6C

a the lower oceanic crust may have crystallized as discrete crystals directly from an immiscible metallic liquid separated at the early stages of mafic magma Chemical composition in wt.% of native copper from different natural objects Table 4 Reference:Object and areaof investigation: Present study Pelagic sediments, TAG MAR hydrothermal 26 field,Sample Phase: Volcaniclastic sediments,CuSnSi Nannofossil ooze,Ca copper Troctolite, DSDP TiAl 98.8Mg 0.3Fe 99.0Ni ± Jenkyns, 1976 99.3 ± ± ±±±±±±±Co copper ±±±±±±±Mn ± 99.1 ± ±S 0.2 ± ± 99.0 ± ± ± ± 99.8 ± ± ± 99.5 Siesser, 1978 ± ± ± ± ± ± copper ± ± ± 0.4 ± ± Puchelt ± ± ± et ± al., ± 1996 ± ± 0.2 ± ± ± ± ± ± 0.1 5.6 ± ± 0.1 ± ± 0.2 ± ± ± ± ± 99.22 0.4 copper ± ± ± ± ± ± ± ± 0.03 81.7 copper ± ± 0.03 0.01 0.01 0.01 0.01 ± 0.06 0.64 ± ± 0.01 ± ± ± 6.7 6.6 ± V.M. DekoÕ et al.rMarine Geology 161() 1999 229±245 239 - b - a -CuZn a Ž. - 0.071.24 ± ± ± ± a - b - - b a - b - a - b - a - a - a - a - ±±±± ± ±±±± ± ±±±± ±±±±± ± ±±±±± ± ±±±±± ± ± ± ± ± ± ± ± ± ± ± CuZn CuZn CuZn CuZn CuZn CuZn CuZn CuZn 1 2 F-3 DJ-5-1 OB-242-3 OB-523-1 BB-8-1 a 111121234± -CuZn ± Sillitoe,1970 1970 et al., 1979 Chile Russia a ±±±±±±±±±±± ±±±±±±±±±±± -±±±± b - a - b - CuZn CuZn CuZn CuZn CuZn CuZn CuZn CuZn CuZn CuZn a N copper regoliths, quartz veins, 8 ±±±±±±± ± Ž. -CuZn -CuZn Russia733a 41-7 41-25 4 Ural, Russia Izu-Bonin arc et al., 1984 1987 1988 data from their Fig. 5 H104.S18 ± ± a a a CuZnFePbSn 64.02Mn 30.85 3.19 ± 63.09 ± 54.02 37.28 0.04 61.14 44.95 ± 52.97 37.96 54.7 45.24 ± 46.7 ± 60.9 ± ± 38.4 56.4 ± ± 21.6 ± 55.8 0.12 27.2 79.52 ± ± ± ± 78.61 20.97 52.6 20.64 ± ± 47.4 52.8 ± 0.04 47.2 60.3 0.05 ± ± 39.6 60.7 0.04 39.5 ± ± 58.5±86.7 0.03 ± 41.5±13.3 21.8 2.22 ± 16.4 0.85 ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± NiCo0.08±±±±±±±±±±±±±±± 3.43 ± ± ± ± Pb±±±±±±±± ± Sn±±±±±±±± 0.3±0.5 CuZnFe 79.81 80.14Mn±±±±±±±± 20.14 ± 78.63Ni±±±±±±±± 19.86 0.04 78.84 21.37Co±±±±±±±± ± 79.90 21.16 ± 80.01 20.16 78.37 19.97 90.5 21.36 9.5 55±70 45±30 66.87 67.09 32.66 61.88 33.42 62.20 36.10 62.31 36.71 36.71 60.72 33.17 62.99 37.17 58.54 41.29 Chemical composition in wt.% of native copper±zinc alloys from different natural objects Table 5 Reference Present studyObject andarea of Pelagic sediments, TAG hydrothermal field,Sample MAR 26 Dulcinea Lunar Gold-bearing Basic rocks, Siberian Platform, Russia Clark and Gay et al., Novgorodova Okrugin et al.,Reference 1981 DombrovskayaObject and Kovalskii, 1985area of Gibsit±alunitinvestigation Pribaikalie, Kimberlite pipes, kimberlite ores, WestPhase Ultrabasic rocks, Rudashevskii et breccia, al., Yakutiya, Russia Koryakskoe Volcanic- Lazur et al., Glavatskih, 1990 Basalts, Kamchatka, mountain, Russia sedimentary Anorthite Russia megacrysts, Nishida et al., 1994 deposits, Middle olivine-basaltic lava, Hachijojima island, investigation deposits, LunaSample South Ural, Phase 240 V.M. DekoÕ et al.rMarine Geology 161() 1999 229±245

2q 4q 2q 3q evolution at high p, T, and low fugacities of sulfur of ilmeniteŽ Fe Ti O324.Ž , chromite Fe Cr O. , w 2q 2q 4q x and oxygenŽ. e.g., Oleynikov, 1981 . The initial tem- ulvospinel FeŽ Fe Ti. O4 etc.Ž all presented in peratures of copper and a-copper±zinc crystalliza- the ridge crest rocks. and reduction of some ele- tion in the present case are about 10838C and 10008C, ments to zero-valent state. respectivelyŽ. Fig. 5 . Below the 10838C±9808C curve, the metallic melt passes entirely into the solid 5.1.2. During eÕolution of the oceanic crust: serpen- state. Such temperatures are characteristic for mag- tinization matic systems only. Under special conditions during serpentinization Formation of native metal particles in a silicate when the oxygen supply is limited, removal of S matrix may accompany the crystallization of the from primary sulfides disseminated in the basic and major mineral phases under low fO22 and fSŽ e.g., ultrabasic rocks may facilitate the formation of na- Reid et al., 1970. . It is possible that the mid-ocean tive metalsŽ. Ramdohr, 1967 . Primary copper sul- ridge basicrultrabasic magmatic system, or at least fides in the basic lithologies exposed on the lower discrete zones of it, remained essentially closed and east wall of the TAG valley might be replaced by that an exchange of valence electrons by the metal native copperŽ. e.g., Puchelt et al., 1996 , probably elementsŽ Fe3q , Ti3q , Cr2q etc.. took place during during bedrock serpentinization. Findings of serpen- cooling and crystallization of magmas. These elec- tinized rock clasts on the rift valley floor support the tron exchange reactions would allow the formation idea of `metamorphic' origin of the native copper

XY Fig. 5. Diagram of equilibrium of copper±zinc systemŽ. after Hansen and Anderko, 1958 . X X sinvestigated a-Cu±Zn; DCDsa-Cu±Zn from Dulcinea copper depositsŽ. Clark and Sillitoe, 1970 ; MUsa-Cu±Zn from Middle Ural volcanic-sedimentary deposits Ž Lazur et al., 1988.Ž. ; Otherssdata for all other Cu±Zn occurrences found in the literature for references see Table 3 . V.M. DekoÕ et al.rMarine Geology 161() 1999 229±245 241 found in surrounding sediments. Whether the serpen- development of a three-stage alteration sequence tinization processes might also be responsible for the Ž.Humphris et al., 1998 . Early high-temperature wa- formation of the a-copper±zinc is uncertain. ter±rock reactions ŽTs;3008C; waterrrocks; 300. resulted in the first stage of alteration of the 5.2. Formation related to hydrothermal processes at fresh basalt to a chlorite"quartz"pyrite assem- the ridge crest() high temperature processes blage. Copper and zinc were leached from the rock, with almost all of the copper being lost. During the 5.2.1. By direct precipitation from hydrothermal so- second stage this assemblage was replaced by parag- lutions onite"quartz"pyrite as a result of reactions with a Reduced sulfur in ridge crest hydrothermal solu- hydrothermal fluid enriched in alkalis. This resulted tions plays an important role in the transport and in additional uptake of Cu and Zn. The final stage deposition of metal speciesŽ Von Damm et al., comprised further silicification of the paragonite-rich 1985a,b; Von Damm and Bischoff, 1987. . However, assemblage. Altered rocks showed enrichment in the sulfur activity of submarine hydrothermal circu- copper and zinc. lation cell would provide a sink for the chalcophiles Geochemical dataŽ. Humphris et al., 1998 suggest Ž.Cu, Zn as sulfides rather than as metals in the that almost all of the copper is removed from the zero-valent state. Experiments and thermodynamic basalts during hydrothermal alteration within the calculations have shown that the crystallization of stockwork zone. As we have no data available for copper in a zero-valent state from sulfur-bearing the copper content in the TAG hydrothermal fluids, water solutions is possible only in high-alkaline solu- values for other seafloor hydrothermal vents are tionsŽ. fairly rare in the nature and at Eh values typically fairly lowŽ. Von Damm, 1995 implying that favorable for reducing reactionsŽ Kolonin and Ptitsin, most of the copper leached from the basalts has been 1974. . However, formation of Zn( from hydrother- reprecipitated in the stockwork and sulfide mound mal solutions is not likely because of thermodynamic Ž.Humphris et al., 1998 . restrictions: native zinc is stable at high partial pres- We suppose that these hydrothermal alteration sures of hydrogen which are realized below the processes might lead to the formation of metallic stability line of water. The chemistry of the hy- copper and zinc-bearing copper in the basaltic layer drothermal fluids ejected from the TAG hot springs of the oceanic crust not only directly in the upflow Ž.Campbell et al., 1988 does not support a direct zone, but in the surrounding country rock undergoing precipitation of copper and copper±zinc from the hydrothermal alteration as well. hydrothermal solutions. 5.3. Formation by marine alteration( low tempera- 5.2.2. As secondary phases in basalts from the wa- ture processes) ter±rock interaction Occurrences of native copper associated with sul- 5.3.1. Physical degradation of the sulfide mounds fides and sulfates in veins within the basaltic base- and their halmyrolysis ment drilled at many DSDP sitesŽ Von der Borch et Primary sulfides of the TAG active and relict al., 1974; Kennett et al., 1975; Leinen et al., 1986. sulfide mounds include pyrite, marcasite, chalcopy- suggest that its formation may result from hydrother- rite, sphalerite and borniteŽ. Rona et al., 1993a,b . mal water±rock interactions rather than direct pre- Exposed surfaces of sulfide mounds undergo rapid cipitation from a metal-bearing fluid. The native oxidation owing to the infiltration of cold, oxy- copper, sulfides and sulfates in these veins are ac- genated and slightly alkaline seawater into the porous companied by secondary minerals such as smectite, sulfide body. Seawater penetrates into cracks and celadonite, talc and chlorite. along grain boundaries in the sulfides and reacts with Studies of the mineralogy and geochemistry of the the minerals to produce strongly acid pore fluids. An hydrothermally altered basalts drilled at the TAG acidic environment in the sulfide mound is generated active mound and underlying stockworkŽ Leg 158, by a series of sulfide-oxidation reactionsŽ Nordstrom, Ocean Drilling Program. have resulted in the 1982. . Dissolution of the primary and secondary 242 V.M. DekoÕ et al.rMarine Geology 161() 1999 229±245 metal sulfides in the acidic, oxidizing solutions re- from the active mound and sulfide mound secondary lease Fe, Cu and ZnŽ. Thornber, 1985 . Under these pore waters. Agents that have been suggested to conditions, Cuq and Cu2q are carried in seawater accomplish the metallic copper precipitation are: or- solutions dominantly as cuprous chloride complexes ganic matterŽ. e.g., Lovering, 1927 , and calcite, y 2y 2q ŽCuCl23 and CuCl. and Cu ions, respectively prehnite, or zeolites, in association with chloride Ž.Rose, 1976 . At pH-values -6 where Cuq and solutionsŽ. e.g., Cornwall, 1956 . Low-temperature Cu2q can exist with Fe2q in solution, Cuq will be Ž.F2008C diffuse fluid flow and sulfide body sec- able to disproportionate to give native CuŽ Thornber, ondary pore fluids could seep through the sulfide 1985. : mound and surrounding sediment cover, react with the organic matter of the buried dead hydrothermal 2Cuq™Cu2qqCu( fauna and precipitate metallic copper. Cornwall Ž.1956 has reported studies in which native copper Cu( might also precipitate from solution at pH)6 was precipitated from chloride solutions in the pres- and Eh-0Ž. Hannington, 1993, his fig. 7 . ence of calcite at temperatures of 2008±2508C. It Zn is a mobile element in acid solutionsŽ. pH-6 seems possible that low-temperature diffuse fluids where Zn2q is usually supplied through the weather- mixed with the secondary cupriferous chloride solu- ing of sphalerite ZnS. Clark and SillitoeŽ. 1970 tions penetrated through the calcareous oozes and attributed the genesis of native a-Cu±Zn in associa- precipitated metallic copper. tion with native Cu to the supergene alteration pro- ()II Biogenic origin. Long ago LoveringŽ. 1927 cesses of sphaleriteŽ. ZnS ±cuprian sphalerite was able to produce metallic copper experimentally

ŽŽCu,Zn . S . ±djurleite Ž Cu34 ZnS . assemblage. We from copper solutions by using swamp bacteria. Two have no data on the presence ofŽ. Cu,Zn S and means of reduction of cupriferous solutions to metal-

Cu34 ZnS in the TAG deposits but their marine lic copper were believed to be possible:Ž. 1 the waste formation on primary Cu and Zn hydrothermal sul- products resulting from the metabolism of the bacte- fides seems possible. Copper and zinc could there- ria might reduce the copper compound to native fore be released from primary sulfides during marine copper;Ž. 2 the copper compound might be con- low-temperature alteration of the TAG sulfide sumed by the organisms and broken down by their mounds by ambient seawater and migrate chemically metabolism, and native copper thus precipitated. outward through the gossans and reprecipitate in a Submarine hydrothermal fields are typically zero-valent state as Cu( and a-Cu±Zn within sec- colonised by chemolithoautotrophic metal- and sul- ondary sulfides. phur-oxidizing bacteria. Zierenberg and Schiffman It is possible that the copper and a-copper±zinc Ž.1990 have observed bacterial filaments replaced by have been formed authigenically or biogenically Ag- and Cu-sulphide and sulphosalt minerals at the withinron the sediment cover, as discussed below. northern Gorda Ridge hydrothermal field. They con- ()I Authigenic origin ( neoformation during sedi- sider that bacterially mediated processes can selec- mentation or by early diagenetic processes) in sedi- tively precipitate Ag, Cu, As and S. Live bacterial ments. The possibility of syn- or early diagenetic mats were observed at the active hydrothermal mound precipitation of metallic copper in the TAG nanno- on the studied spreading centreŽ. Lisitsyn, 1992 . The fossil-foraminiferal oozes is more difficult to assess. examined metalliferous sediments contain fossilised The TAG copper is associated with minerals exhibit- bacterial filaments encapsulated in Fe-oxyhydroxides ing signs of secondary dissolution, recrystallization Ž.Damyanov et al., 1998 . Do hydrothermal vent bac- and reprecipitationŽ. Damyanov et al., 1998 and its terial mats influence Cu( and a-Cu±Zn precipitation diagenetic mechanism of formation cannot be ruled in the TAG hydrothermal field? out. Syn- and diagenetic origin of copper in pelagic Several investigations have shown that relatively oozes requires a source of the metal and a precipitat- large quantities of metallic cations are complexed by ing agent. bacteria. Colloidal Au(, accumulated at discrete sites Two possible metal sources exist in the TAG in the bacterial cell wall, has been observedŽ Be- sediment cover: hydrothermal solutions discharging veridge and Murray, 1976. . Experiments show that V.M. DekoÕ et al.rMarine Geology 161() 1999 229±245 243 bacterial cells are capable of removing metals from Acknowledgements solutions by concentration and precipitation on and within cell wallsŽ. Mullen et al., 1989 . On the other We are indebted to the captain, crew and research hand, the bacterial mats may influence physico- staff of the RrV Akademik MstislaÕ Keldysh as well chemical parameters of a microenvironment and as the pilots of the Mir submersible team for their favour deposition of some mineral speciesŽ e.g., support in obtaining samples from the TAG field. sulphides, sulphosalts, oxides. from hydrothermal The authors thank V.M. Kuptsov and V.V. Serova fluidsŽ Zierenberg and Schiffman, 1990; Pracejus Žboth from the Institute of Oceanology, Russian and Halbach, 1996. . Academy of Sciences. for radiocarbon age dating We consider that a bacterially mediated origin for and preparing lithologic descriptions, respectively. Cu( and a-Cu±Zn seems possible in the TAG field V. MarchigŽ Bundesanstalt furÈ Geowissenschaften inhabited by dense vent biota. und Rohstoffe, Germany.Ž , J. Hein USGS, USA . and H. StoynovŽ. University of British Columbia, Canada provided much valuable literature. We are also grate- ful to J. HoltŽ. Sheffield, UK for improving our 6. Conclusions English. The manuscript benefited from the sugges- tions kindly made by D. StubenÈÈŽ Universitat Karl- sruhe, Germany.Ž , A.M. Karpoff EOST, CGS, This paper describes mineralogical features of France.Ž and G.P. Glasby Tokyo University, Japan . . native copper and a-copper±zinc particles found in The financial support from the Bulgarian National the sediments of the TAG hydrothermal field. Equiv- Science FoundationŽ. grant NZ-420 is gratefully ac- alent genetic pathways are possible to explain the knowledged. V.M. Dekov was supported in part occurrence of these native metals in the TAG field through a post-doctoral fellowship from the German sediments. There is probably not a single simple Service for Academic ExchangeŽ. DAAD . process responsible for the formation of these minerals in the studied environment. The available data are compatible with five mechanisms of formation of native copper and a-copper±zinc in the TAG field: References Ž.i formation in the oceanic crust Ž during the magmatic crystallization andror during evolution of the Ž..Ž. Berger, W.H., von Rad, U., 1972. Cretaceous and Cenozoic crust serpentinization ; ii formation related to sediments from the Atlantic Ocean. Init. Rep. Deep-Sea Drill. high-temperature hydrothermal processes at the ridge Proj. 14, 787±954. crestŽ.Ž. seawater±rock interaction ; iii formation dur- Beveridge, T.J., Murray, R.G.E., 1976. Uptake and retention of ing the halmyrolysis of sulfide mounds;Ž. iv authi- metals by cell walls of Bacillus subtilis. J. Bacteriol. 127, genic formation in sediments;Ž. v biogenic forma- 1502±1518. Campbell, A.C., Palmer, M.R., Klinkhammer, G.P., Bowers, T.S., tion. If the native metallic particles have been formed Edmond, J.M., Lawrence, J.R., Casey, J.F., Thompson, G., as accessory phases disseminated in the ridge crest Humphris, S., Rona, P., Karson, J.A., 1988. Chemistry of hot basic rocks andror massive sulfide mounds, the springs on the Mid-Atlantic Ridge. Nature 335, 514±519. degradation of these rocks and mounds has liberated Clark, A.N., Sillitoe, R.H., 1970. Native zinc and a-Cu,Zn from the metallic grains which have, in turn, been de- mina Dulcinea de Llampos, Copiapo,Â Chile. Am. Mineral. 55, 1019±1021. posited into adjacent sediments. Cornwall, H.R., 1956. A summary of ideas on the origin of native The TAG field with its more fractured nature than copper deposits. Econ. Geol. 51, 615±631. the fast-spreading centers and mature sulfide mounds Criddle, A.J., Stanley, C.J., 1986. The quantitative data file for ore with extensive gossan's halmyrolysis is the best area minerals. Edition British MuseumŽ. Natural History , 420 pp. for studying the processes of native metal formation. Damyanov, Z.K., Dekov, V.M., Lisitsyn, A.P., Bogdanov, Y.A., Aidanliiski, G., Dimov, V.I., 1998. Mineralogical features of It is hoped that the hypotheses presented will stimu- the near sulfide mound sediments: MIR zone, TAG hydrother- late further ideas on the genesis of metallic occur- mal fieldŽ. Mid-Atlantic Ridge, 268N . N. Jb. Miner. Abh. 174, rences in similar environments. 43±78. 244 V.M. DekoÕ et al.rMarine Geology 161() 1999 229±245

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