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 0025-3227r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved. PII: S0025-3227Ž. 99 00034-1 V.M. DekoÕ et al.rMarine Geology 161() 1999 229±245 231 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 weather- ing. 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 cop- per±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.