Sulfide Mineralization in an Ultramafic-Rock Hosted Seafloor

Home , Rainbow Vent Field, Serpentinite, Ultramafic rock

Marine Geology 245 (2007) 20–39 www.elsevier.com/locate/margeo

Sulfide mineralization in an ultramafic-rock hosted seafloor hydrothermal system: From serpentinization to the formation of Cu–Zn–(Co)-rich massive sulfides ⁎ Ana Filipa A. Marques a,b, , Fernando J.A.S. Barriga a, Steven D. Scott b

a CREMINER (LA/ISR), Universidade de Lisboa, Faculdade de Ciências, Departamento de Geologia, Edif. C6 Piso 4. 1749-016 Campo Grande, Lisboa, Portugal b Scotiabank Marine Geology Research Laboratory, Department of Geology, University of Toronto, 22 Russell St., Toronto, Canada M5S 3B1

Received 12 December 2006; received in revised form 2 May 2007; accepted 12 May 2007

Abstract

The Rainbow vent field is an ultramafic rock-hosted seafloor hydrothermal system located on the Mid-Atlantic ridge issuing high temperature, acidic, metal-rich fluids. Hydrothermal products include Cu–Zn–(Co)-rich massive sulfides with characteristics comparable to those found in mafic volcanic-hosted massive sulfide deposits. Petrography, mineralogy and geochemistry of nonmineralized and mineralized rocks sampled in the Rainbow vent field indicate that serpentinized peridotites host the hydrothermal vent system but serpentinization reactions occurred prior to and independently of the sulfide mineralization event. The onset of sulfide mineralization is reflected by extensive textural and chemical transformations in the serpentine-group minerals that show clear signs of hydrothermal corrosion. Element remobilization is a recurrent process in the Rainbow vent field rocks and, during simple peridotite serpentinization, Ni and Cr present in olivine and pyroxene are incorporated in the pseudomorphic serpentine mesh and bastite, respectively. Ni is later remobilized from pseudomorphic serpentine into the newly formed sulfides as a result of extensive hydrothermal alteration. Bulk-rock geochemistry and correlation coefficients discriminate the different processes: serpentinization, sulfide mineralization and superficial seafloor low-temperature processes related to the circulation of seawater (e.g. carbonatization, sulfide oxidation and B and U uptake). © 2007 Elsevier B.V. All rights reserved.

Keywords: Rainbow vent field; Mid-Atlantic ridge; serpentinite; seafloor hydrothermal systems; volcanic-hosted massive sulfide deposits

1. Introduction form as a result of sub-seafloor heat-driven seawater circulation reacting with crustal or upper mantle rocks Ancient and modern volcanic-hosted massive leaching metals and subsequently precipitating them as sulfide deposits (VMS) are conventionally thought to sulfides when the fluid mixes with cold, metal- depleted, ambient seawater (Scott, 1985; Large, 1992 and references therein; Ohmoto, 1996; Barrie and ⁎ Corresponding author. Present address: Scotiabank Marine Geol- Hannington, 1999). The leaching process can account ogy Research Laboratory, Department of Geology, University of Toronto, 22 Russell St., Toronto, Canada M5S 3B1. Tel.: +1 351 750 for accumulations of metals but mass balance cal- 00 00(26282); fax: +1 351 750 00 64. culation fails to fully explain the formation of some E-mail address: [email protected] (A.F.A. Marques). giant VMS deposits (Yang and Scott, 2003, 2006). The

0025-3227/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.margeo.2007.05.007 A.F.A. Marques et al. / Marine Geology 245 (2007) 20–39 21 presence of ore metal-rich magmatic fluids may play (30° N) and the high-temperature systems of Rainbow an important role on the formation of massive sulfide (36°14′ N) and Logatchev (14°45′ N) (Batuyev et al., deposits contributing large quantities of metals (e.g. 1994; German et al., 1996; Donval et al., 1997; Ni+Cu+Zn+Fe) and volatiles although the magmatic Langmuir et al., 1997; Fouquet and IRIS Scientific signature typically is masked by the large volumes of Party, 2001; Kelley et al., 2001; Barriga et al., 2003). circulating seawater (de Ronde, 1995; Yang and Scott, The high-temperature, ultramafic-hosted Rainbow vent 1996, 2002, 2006). The modern seafloor hydrothermal field produces significant Cu–Zn–(Co) rich massive systems, with their black smokers issuing hot metal- sulfide accumulations with conspicuous chemical and rich vent fluids, hosted in mafic or felsic volcanic textural similarities to those found in modern basalt- rocks, represent modern analogs for the formation of hosted seafloor hydrothermal systems and ancient, on- VMS ore deposits (e.g. Scott, 1985; Bischoff and land, mafic-hosted VMS deposits (Marques et al., Rosenbauer, 1989; Fouquet et al., 1993; Goodfellow 2006). In this paper, mineral chemistry, and major and and Franklin, 1993; Rona and Scott, 1993; Rona et al., trace element geochemistry of nonmineralized and 1993; Seyfried and Ding, 1995; Herzig and Hanning- mineralized rocks sampled from the Rainbow vent ton, 1995; Von Damm, 1995; Fouquet et al., 1996; field have been investigated in order to distinguish Donval et al., 1997; Langmuir et al., 1997; Scott, between peridotite serpentinization processes, ubiqui- 1997; Charlou et al., 2000; Douville et al., 2002; Von tous in exposed mantle outcrops, from later, over- Damm et al., 2003). In particular, sediment-free basalt- printing, localized magmatic/hydrothermal-driven hosted seafloor hydrothermal systems like TAG, sulfide-mineralization. MESO and Snake Pit share characteristics comparable to those of on-land mafic VMS deposits having Cu– 2. Regional setting and hydrothermalism Zn–(Co) sulfide mineralization with high Cu/Zn ratios (e.g. Kase et al., 1990; Rona et al., 1993; Herzig and The Rainbow hydrothermal vent field is an ultra- Hannington, 1995; Tivey et al., 1995; Herzig et al., mafic rock-hosted hydrothermal field located at 36°14′ 1998; Münch et al., 1999; Lawrie and Miller, 2000). N–33°53′ W, south of the Azores archipelago and on The recent discovery of modern seafloor hydrothermal the Mid-Atlantic Ridge at ∼2300 m depth (Fig. 1A). systems hosted in ultramafic rocks, i.e. serpentinized Rainbow's particular geological setting, within an inside peridotites, in slow to ultra-slow spreading ridges corner of a non-transform offset between the AMAR represents a remarkable novelty in the conceptual and South AMAR second order segments (Fig. 1B), models of VMS ore deposit formation. In most cases, plays an important role in the tectonic and magmatic on-land ultramafic-hosted sulfide deposits, if not character of the system (Fouquet et al., 1997; Gràcia clearly of primary magmatic origin, are linked to et al., 2000; Parson et al., 2000). First discovered in serpentinization processes with element remobilization 1994 (German et al., 1996), the Rainbow vent field area of metals (Fe, Ni and Co) from primary silicates (e.g. underwent extensive surveys of its water column olivine) into hydrothermal sulfides if enough H2Sis chemistry and plume paths that revealed the strongest available (e.g. Eastern Metals, Canada — Auclair thermal and chemical output recorded so far at the Mid- et al., 1993; Tsangli, Greece — Economou and Atlantic ridge (German and Parson, 1998; Thurnherr Naldrett, 1984; Hayachine, Japan — Shiga, 1987; and Richards, 2001; Thurnherr et al., 2002). In 1997, the Limassol Forest, Cyprus — Thalhammer et al., 1986; first direct observations of the Rainbow hydrothermal Bou Azzer, Morocco — Wafik et al., 2001). Thus, the field using the submersible Nautile (Barriga et al., 1997; study of modern ultramafic-hosted seafloor hydrother- Fouquet et al., 1997) revealed a vigorously venting mal systems may provide new insights into the hydrothermal system with 10 groups of active massive formation of some ancient VMS spatially related to sulfide chimneys producing high temperature (up to ultramafic rocks such as the Outokumpu Cu–Co–Au 364 °C), acidic (pH=2.8), Cl-, metal-and REE-rich deposits of Finland (Gaál and Parkkinen, 1993; fluids (Donval et al., 1997; Douville et al., 1997, 2002) Loukola-Ruskeeniemi, 1999; Sorjonen-Ward et al., with significant H2 and methane concentrations (Char- 2004)andtheKiddCreekCu–Zn deposit of Canada lou et al., 2002). The Rainbow vent fluids have the (Barrie et al., 1999). highest concentrations of Fe, Mn, Cu, Zn, Co and Ni The five well known ultramafic-hosted seafloor ever reported for Mid-Atlantic ridge hydrothermal vents hydrothermal sites at MAR include the low and in both mafic and ultramafic environments. These moderately low-temperature systems of Saldanha particular properties were also imprinted within the (36°34' N), Menez Hom (37°8′ N) and Lost City surrounding hydrothermal sediments recording important 22 A.F.A. Marques et al. / Marine Geology 245 (2007) 20–39

Fig. 1. Location of the Rainbow vent field; (A) Study area within the mid-Atlantic ridge south of the Azores archipelago; (B) three-dimensional representation of the AMAR and South AMAR segments linked by a non-transform offset (NTO) in which the Rainbow vent field is located; bathymetric data and software from IFREMER (©ADELIE). hydrothermal input of Fe, Mn, Cu and REE (Cave et al., samples were selected to produce polished thin sections 2002; Chavagnac et al., 2005; Dias and Barriga, 2006). (Table 1). Textural classification of serpentine-group minerals followed the classification defined by Wicks 3. Sampling and Analytical Work and Plant (1979), Wicks and Whittaker (1977) and Wicks et al. (1977). Modal analysis was applied to 6 Samples were either dredged or collected using a representative serpentinite samples and 5 representative remotely operated vehicle (ROV- Victor 6000) during stockwork samples using 5X objective lens and a the IRIS and SEAHMA1 cruises (Fig. 2) onboard the minimum count of 2000 points per thin section. Olivine research vessel L'Atalante from the French Research (n=3), clinopyroxene (n=1), Cr-spinel (n=63) and Institute for the Exploitation of the Sea (IFREMER) serpentine-group minerals (n=19), where n represents (Fouquet and IRIS Scientific Party, 2001; Barriga et al., the number of analysis, were analyzed with a CAMECA 2003). One hundred and twenty-eight samples were SX50 electron microprobe (EPMA) in the Department studied macroscopically of which 71 representative of Geology at the University of Toronto. The silicate A.F.A. Marques et al. / Marine Geology 245 (2007) 20–39 23

Fig. 2. Location of samples collected during SEAHMA cruise ROV dive (SH1-PL5-###) and IRIS (IR-DR-##) and SEAHMA dredges (SH1-DR3). Dredge IR-DR-09 located NE out of the representation area; bathymetric data and software from IFREMER (©ADELIE). routine had 15 KeV accelerating voltage, 15 nA beam current and beam size of 1 μm. The low EPMA totals current and beam size of 3 μm whereas the Cr-spinel observed in the serpentine-group minerals, especially routine had 20 KeV accelerating voltage, 30 nA beam from the sulfide-bearing serpentinites, likely result from

Table 1 Sample location chart with (n) that indicates the number of samples studied followed by the rock type; serpentinites (SRP), stockwork (ST), semi- massive sulfides (SMS), Cu-rich massive sulfides (Cu-MS), steatites (STEA), others (O) and brecia of rodingitic material (Brc) Dredge Location n SRP ST STEA SMS Cu-MS MS-ch O Latitude Longitude Cruise Start Finish Start Finish IR-DR-01 Rainbow 11 5 4 – 2 –––36.32 36.27 −33.9 −33.75 IRIS (2001) IR-DR-02 Rainbow 38 19 5 – 5 – 6 3 36.27 36.23 −33.9 −33.75 IRIS (2001) IR-DR-03 Rainbow 22 3 4 2 3 6 2 2 36.23 36.19 −33.9 −33.75 IRIS (2001) IR-DR-09 Rainbow 18 17 –– – – – 1 36.15 36.15 −33.5 −33.5 IRIS (2001) SHI-DR03 Rainbow 31 1 8 6 3 – 5 36.23 36.2 −33.91 −33.89 SEAHMA (2002) Dive IR-88-01 Rainbow X 36.23 −33.91 IRIS (2001) IR-89-01 Rainbow X 36.22 −33.90 IRIS (2001) IR-96-03 Rainbow X 36.22 −33.90 IRIS (2001) SH1-PL05-96 Rainbow Brc 36.23 −33.91 SEAHMA (2002) SH1-PL05-98 Rainbow X 36.23 −33.91 SEAHMA (2002) SH1-PL05-99 Rainbow X 36.23 −33.91 SEAHMA (2002) SH1-PL05-100 Rainbow X 36.23 −33.91 SEAHMA (2002) SH1-PL05-101 Rainbow X 36.23 −33.90 SEAHMA (2002) 24 A.F.A. Marques et al. / Marine Geology 245 (2007) 20–39 hydrothermal corrosion processes as discussed further one specific sulfide assemblage that was analyzed. Po is below. Serpentine-group minerals were also observed pyrrhotite, Py is pyrite and Sph is sphalerite. under a scanning electron microscope (SEM), a JEOL JSM-840 with secondary electrons (SE) and back- FePoþISS ¼ 1:741564 Stotal 0:8788113 Cutotal ð1Þ scattered electrons (BSE) in the Department of Geology ¼ : þ : ð Þ at the University of Toronto. This equipment had FePyþISS 0 870782 Stotal 0 43940635 Cutotal 2 additional features of an energy dispersive analytical ¼ : ð Þ system (EDS), PGT Image and image output to a FeISS 0 878812 Cutotal 3 Polaroid camera. X-ray diffraction was carried out on 9 Fe þ þ ¼ 1:741564 S selected powdered bulk samples in the Department of Po Sph ISS total 0:854328 Zn Chemistry at the University of Lisbon using a Philips total 0:878776Cu ð4Þ PW 1710 diffractometer equipped with a PW1820/00 total μ goniometer and the following settings: Cu 40 kV, 40 A FePyþSphþISS ¼ 0:870782 Stotal þ 0:439 Cutotal Θ – and 2 3 70°. Software employed was PC-APD 3.6 0:345Zntotal ð5Þ from Philips Scientific coupled with Identify software (© Carlos Carvalho) for spectra analysis and mineral The remaining iron, implicitly distributed in silicates, identification. Twenty-three representative samples oxides and carbonates results from the difference were selected for bulk-rock chemical analyses at between total Fe (Fetot; Table 4) and Fe in the sulfides Activation Labs, Canada using the 4Lithoresearch (Eqs. (1)–(5)). For most of the semi-massive to massive Quant analytical package for major and trace elements. sulfide samples, the application of the whole-rock data The procedure for rock pulverization was as follows: (1) in the above equations was not successful; calculated selected samples were diamond-sawn into small slices Fesulfides was, in most cases, higher than the analyzed (b1.5 cm thick); (2) slices were then ground with a Fetot. One possible explanation lies again in inaccurate S diamond wheel in order to eliminate brass contamina- values that strongly influence these results and the tion; (3) samples were carefully washed with distilled disparity of sulfur abundances observed between the two water and dried at 25 °C; (4) dried slices were methods leads, in both cases, to unreasonable contrasting fragmented with a hammer enclosed within thick contents of Fesulfides. Another factor is the presence of paper envelopes to prevent contamination from the abundant magnetite or Cr-spinels that may not be com- hammer; and (5) small fragments were then powdered in pletely dissolved during the digestion, leading to low Fe an agate ring mill. Major element geochemistry, espe- totals. cially in the semi-massive and massive sulfides had analytical totals below the expected 100%. This resulted 4. Petrography and mineral chemistry primarily from incomplete volatilization of sulfur during the analytical procedures. Two different approaches Fig. 3 depicts rocks sampled in the Rainbow vent were attempted in order to determine sulfur contents, field illustrating the evolution of the hydrothermal ICP (b20 wt.% S) and LECO (sulfide-rich samples) system. Non-mineralized pseudomorphic serpentinites with significantly different results. Since Fe, Cu and Zn (Fig. 3A) gradually become mineralized and comprise are significant components of the massive sulfide non-pseudomorphic featureless serpentine with sulfides samples an approach using a norm-like calculation that occur disseminated or in veinlets – the stockwork– adapted from Relvas (2000) was attempted in order to (Fig. 3B,D). Poorly mineralized stockwork serpentinite determine the total iron accommodated within the has been locally replaced by talc resulting in steatites sulfides. This procedure assumed that all Cu resides in with rare magnetite, relic Cr-spinel and disseminated chalcopyrite, all Zn is in sphalerite and Fe is distributed sulfides (Fig. 3C). Semi-massive sulfides result from in stoichiometric chalcopyrite, sphalerite (Fe: Zn gradual pervasive replacement of the non-pseudomor- proportion determined using EPMA data), pyrite, phic serpentine in the stockwork by sulfides (Fig. 3E) pyrrhotite, oxides, silicates and carbonates. The only and, as a result of incomplete replacement, one can still sulfur-bearing phases considered are pyrite, pyrrhotite, observe host rock relics of extremely altered serpenti- chalcopyrite (in the form of Intermediate Solid Solution nite. Cu-rich massive sulfides are almost exclusively (ISS) with chalcopyrite (Ccp) exsolution referred to as composed of non-porous aggregates of ISS/Ccp with ISS/Ccp) and sphalerite. Below are listed the derived signs of recrystallization and derive from the replace- equations that relate S, Cu and Zn total concentrations ment of pyrite-rich semi-massive sulfides (Fig. 3F). with Fe within the sulfides. Each Eqs. (1)–(5) represents Massive sulfide chimneys are porous with concentric A.F.A. Marques et al. / Marine Geology 245 (2007) 20–39 25

Fig. 3. Photographs of representative samples from the Rainbow vent field. (A) non-mineralized serpentinite with visible pseudomorphic mesh and bastite textures; (B) poorly mineralized stockwork serpentinite; (C) steatite after poorly mineralized stockwork; (D) more evolved mineralized stockwork serpentinite; (E) semi-massive sulfide rock with serpentinite relicts and extensive seafloor oxidation; (F) Cu-rich semi-massive sulfide rock; (G) Zn-rich massive sulfide chimney; (scale bar=1 cm). mineralogical zonation composed of interlocking arrays semblages (Fig. 3G). A detailed description of the min- of sphalerite-rich with minor amounts of pyrite and eralogy and textures of the sulfide-bearing rocks from chalcopyrite in the lower temperature assemblages and the Rainbow vent field was produced by Marques et al. pyrrhotite and ISS/Ccp in the higher temperature as- (2006). 26

Table 2 EPMA analysis of primary minerals olivine (Ol) and clinopyroxene (Cpx), pseudomorphic serpentine-group minerals (mesh, bastite — Bast and exsolution lamellae in bastite — EL) and non- pseudomorphic (Np) serpentine-group minerals including fibrous (Np-F) and interpenetrating γ-serpentine (Np-γ) Mineral IRDR09T IRDR1A5 IRDR09T IRDR02B IRDR02C01 IRDR3F IRDR02E01 Texture Ol Ol Ol Srp Srp Srp Srp Mesh Cpx Srp Srp Srp Srp Srp Srp Srp Srp Srp Srp Srp Srp Np Srp Srp Srp # Mesh Mesh Mesh Bast Bast Bast Bast Bast EL EL EL Np Np Np Np Np-F NP-γ 328 329 330 331 332 333 49 335 31 32 29 30 35 36 37 2 3 4 5 34 22 20 21

SiO2 41.01 41.15 41.24 42.34 41.13 40.91 38.79 53.46 39.56 42.97 37.85 35.12 37.69 33.53 35.38 39.07 31.38 40.25 32.77 34.07 31.87 41.94 35.86 wt.% TiO2 0.07 – 0.05 0.06 0.03 –– 0.16 0.01 – 0.06 0.07 – 0.01 – 0.00 0.02 0.00 0.01 0.02 0.01 0.02 0.00 wt.% ...Mruse l aieGooy25(07 20 (2007) 245 Geology Marine / al. et Marques A.F.A. Al2O3 – 0.01 0.01 0.85 2.24 1.58 0.16 3.33 0.83 0.47 2.78 5.61 2.82 1.16 1.26 0.37 0.25 0.29 0.26 0.19 0.28 0.92 0.22 wt.% FeO 11.45 11.66 11.56 5.25 5.81 5.76 2.33 4.00 2.53 2.56 5.49 7.07 5.17 2.63 2.24 1.26 5.63 4.76 5.09 3.82 3.58 4.07 2.82 wt.% MnO 0.23 0.16 0.16 0.10 0.01 0.08 0.03 0.09 0.04 0.03 0.06 0.06 0.10 0.07 0.05 0.02 0.08 0.15 0.08 0.06 0.04 0.02 0.01 wt.% MgO 49.33 49.31 49.01 39.35 37.25 38.42 36.82 19.82 37.11 38.57 37.23 34.83 36.22 31.56 32.11 37.51 23.67 34.47 25.44 28.97 27.95 36.70 29.70 wt.% CaO – 0.16 0.14 0.10 0.11 0.13 0.03 19.74 0.04 – 0.01 0.01 0.01 0.03 0.02 0.01 0.01 0.03 0.02 0.11 0.02 0.01 0.06 wt.% Na2O ––––––0.05 0.27 0.01 – 0.02 0.00 0.03 0.02 – 0.02 0.13 0.09 0.09 0.15 0.13 0.00 0.05 wt.% K2O – 0.07 – 0.01 ––0.02 0.04 –––0.03 0.02 ––0.00 0.03 0.01 0.01 0.02 0.01 0.00 0.00 wt.% NiO 0.26 0.21 0.26 0.34 0.30 0.29 0.00 0.09 –––––––––––– –– – wt.% Cr2O3 0.06 0.02 ––0.06 0.00 0.04 1.22 0.72 0.54 0.72 1.04 0.60 1.18 1.45 0.34 0.00 0.02 0.07 0.02 0.03 0.03 0.01 wt.% – Total 102.4 102.7 102.4 88.41 86.94 87.17 78.26 102.2 80.86 85.14 84.21 83.84 82.65 70.19 72.5 78.61 61.19 80.07 63.83 67.43 63.93 83.72 68.74 39 Si (IV) 0.990 0.991 0.995 1.979 1.957 1.946 2.014 1.935 1.993 2.045 1.869 1.764 1.891 1.958 1.989 2.01 2.12 2.06 2.11 2.07 2.04 2.04 2.11 Al (IV) ––– 0.021 0.043 0.054 – 0.065 0.007 – 0.131 0.236 0.109 0.042 0.011 ––––– –– – Al (VI) 0.025 0.083 0.035 0.010 0.006 0.042 0.026 0.031 0.095 0.058 0.037 0.072 0.02 0.02 0.02 0.02 0.01 0.02 0.05 0.02 Mg 1.775 1.770 0.763 2.741 2.642 2.725 2.849 1.070 2.786 2.737 2.742 2.607 2.709 2.747 2.692 2.87 2.39 2.63 2.45 2.62 2.67 2.66 2.61 Fe2+ 0.231 0.235 0.233 0.205 0.31 0.229 0.101 1.121 0.107 0.102 0.227 0.297 0.217 0.129 0.105 0.05 0.32 0.20 0.27 0.19 0.19 0.17 0.14 Mn 0.005 0.003 0.003 0.004 – 0.003 – 0.003 –––––––––0.01 –– –– – Ti 0.001 – 0.001– 0.002 0.001 –– 0.004 –––––––––––– –– – Cr 0.001 –– – 0.002 –– 0.017 0.029 0.020 0.028 0.041 0.024 0.055 0.064 0.01 –––– –– – Ni 0.005 0.004 0.005 0.013 0.012 0.011 – 0.003 ––––––– Ca – 0.004 0.004 0.005 0.006 0.006 – 0.766 –––––––––––0.01 –– – K – 0.002 0.000 0.001 – – – ––––––––––––– –– – Na ––––––0.005 0.010 ––––––––0.02 0.01 0.01 0.02 0.02 – 0.01 Total 3.01 3.01 3.00 3.00 2.98 3.01 2.96 4.00 2.97 2.89 3.03 3.05 3.02 2.97 2.93 2.97 2.75 2.87 2.76 2.86 2.91 2.89 2.77 A.F.A. Marques et al. / Marine Geology 245 (2007) 20–39 27

Table 3 bow rocks so that there are common occurrences of relict Average EPMA compositions of Cr-spinel (Cr-sp) and ferritchromit Cr-spinel in sulfide-bearing serpentinites. Relict Cr- (Fe-chr) from early and evolved stockwork (IR-DR-02-D; IR-DR-02-E, respectively) steatite (SH1-PL5-100R) and semi-massive sulfides spinel has a translucent reddish-brown color with shapes (IR-DR-01-G02; SH1-DR3-2-3) varying from euhedral to anhedral (skeletal, graphic, and rounded). Hydrothermal alteration of spinel is ubiqui- IR-DR- IR-DR- SH1-1PL5- IR-DR- SH1- IR-DR- 02-D 02-E 100R 01-G-02 DR3-2-3 01-02 tous and all Cr-spinels exhibit alteration halos along fractures and rims expressed as a highly reflecting Cr-Sp Cr-Sp Cr-Sp Cr-Sp Cr-Sp Fe-Chr opaque material, occasionally “ferritchromit”. Other, n=6 n=9 n=22 n=11 n=15 n=2 less abundant, alteration products are magnetite and SiO2 0.25 0.03 0.07 0.06 0.04 0.055 lesser chlorite and other phyllosilicates that rim the MgO 7.98 13.50 12.22 14.18 14.01 2.355 spinel grains. Sixty-five EPMA determinations of relict Al O 18.46 27.70 23.98 24.83 30.97 3.38 2 3 Cr-spinel found in representative samples of the stock- V2O3 0.13 0.15 0.17 0.17 0.15 0.095 MnO 0.68 0.20 0.23 0.20 0.20 4.53 work, steatite and semi-massive sulfides are in Table 3. FeO 25.20 15.50 18.11 15.36 15.63 46.12 The composition of the Cr-spinel grains varies widely, NiO 0.10 0.08 0.06 0.11 0.08 0.03 even within the same rock sample. Cr2O3 abundances TiO2 0.04 0.02 0.04 0.11 0.03 0.16 range from 33.82 to 46.60 wt.% (Cr# from 40.79 to Cr2O3 43.44 39.63 42.47 42.98 36.05 34.83 ZnO 0.27 0.22 0.19 0.12 0.25 1.045 64.76%), with Al2O3 varying from 32.93 to 16.60 wt.%, CaO 0.01 0.02 0.01 0.02 0.01 0.035 MgO from 17.40 to 14.87 wt.% (Mg# from 37.99% to CoO 0.03 0.01 0.01 – 0.02 0 67.20%) and FeO (as total Fe) from 26.34 to 15.00 wt.%. Total 96.59 97.05 97.56 98.14 97.45 92.635 Trace amounts of Si (max. 1.23 wt.% SiO2), V (max. – Si 0.06 0.00 0.01 0.01 0.01 0.23 wt.% V O ), Mn (max. 4.9 wt.% MnO), Ni (max. Mg 3.16 4.95 4.56 5.18 5.03 1.065 2 3 Al 5.77 8.02 7.07 7.17 8.80 1.21 0.21 wt.% NiO), Ti (max. 0.21 wt.% TiO2), Zn (max. V 0.03 0.03 0.03 0.03 0.03 0.025 1.11 wt.% ZnO in the rim) and Co (max. 0.05 wt.% CoO) Mn 0.15 0.04 0.05 0.04 0.04 1.175 were also detected. Cr-spinel from a poorly mineralized Fe 2+ 4.52 2.95 3.33 2.74 2.86 5.63 sample of the stockwork (IR-DR-02-D) has Mg/Feb1 Fe 3+ 1.07 0.23 0.47 0.41 0.29 6.19 and Al/Crb1(Fig. 4)). All other studied Cr-spinels Ni 0.02 0.02 0.01 0.02 0.02 0.005 N Ti 0.01 0.00 0.01 0.02 0.01 0.035 have Mg/Fe 1 in which two groups, stockwork (IR-DR- Cr 9.11 7.70 8.41 8.33 6.87 8.43 02-E) and semi-massive sulfides (SH1-DR3-2-3) have Zn 0.05 0.04 0.04 0.02 0.04 0.24 Al/CrN1 and the other two, steatite (SH1-PL5-100R) Ca ––– – – 0.005 and semi-massive sulfides (IR-DR01-G-02), have Al/ ––– – – Co 0 Crb1(Fig. 4). O323232323232

4.1. Harzburgites and primary mineral phases (olivine, pyroxene and Cr-spinel)

Poorly serpentinized harzburgites are rare with equigranular texture predominantly composed of olivine with lesser amounts of orthopyroxene and clinopyroxene, pseudomorphic serpentine (mesh) partially repla- cing olivine and accessory Cr-spinel. EPMA data on olivine, clinopyroxene and serpentine-group minerals are listed in Table 2. Olivine (Fo88) relicts are optically homogeneous, colorless, subhedral and contain small amounts of Cr (∼0.03 wt.% Cr2O3) and significant Ni (0.24 wt.% NiO). The rare relicts of pyroxenes are subhedral to euhedral and are associated with Cr- spinel grains. One clinopyroxene mineral [(Wo38En54 Fs ) (Ca (Mg Fe )Si Al O )] contains 6 0.77 1.07 0.12 1.94 0.06 6 Fig. 4. Cr/(Cr+Al) vs. Fe2+ /(Fe2+ +Mg) of Cr-spinel from early and high contents of Cr (1.22 wt.% Cr2O3), Ti (0.16 wt.% evolved stockwork (IR-DR-02-D; IR-DR-02-E, respectively) steatite TiO2) and Ni (0.09 wt.% NiO). Cr-spinel endured (SH1-PL5-100R) and semi-massive sulfides (IR-DR-01-G02; SH1- serpentinization and sulfide mineralization in the Rain- DR3-2-3) after Kimball (1990). 28 A.F.A. Marques et al. / Marine Geology 245 (2007) 20–39

Fig. 5. SEM microphotographs taken from non-mineralized serpentinites, stockwork serpentinites, and semi-massive sulfides and from the Rainbow vent field; (A) serpentine plates in bastite with scarce magnetite grains and (B) non-segmented vein filled by γ-serpentine fibbers normal to the vein walls; (C) stockwork serpentinite with corrosion fronts in altered non-pseudomorphic featureless serpentine in contact with sulfides; (D) semi-massive sulfide rock with highly corroded/altered serpentine-group minerals within pyrite and pyrrhotite. (py) pyrite; (po) pyrrhotite; (alt–srp) altered non-pseudomorphic serpentine.

4.2. Serpentinization and sulfide mineralization dominant serpentine-group mineral in vein-filling fibrous γ-serpentine is clinochrysotile (2Mcl). SEM photomicro- 4.2.1. Textures in serpentine-group minerals graphs are in good agreement with the petrographic Serpentine pseudomorphic textures are predominant in textural observations (Fig. 5A, B). With sulfide mineral- the non-mineralized serpentinites in which mesh cells are ization and formation of stockwork in serpentinite, the the most common texture followed by bastites (after pyroxene) and hourglass serpentine. Mesh cells are non- deformed with colorless rims of α-serpentine and mesh centers filled by isotropic serpentine and, rarely, relict olivine. Bastites composed of γ-serpentine and magnetite contain relicts of pyroxene. XRD patterns identify these pseudomorphic domains mainly as lizardite (1T Al, 6T) associated with magnetite. Non-pseudomorphic textures are rare but, where present, are interlocking and clearly overprint the pseudomorphic serpentine-group minerals. Two types of transitional textures are observed: α- serpentine ribbon veins, which occur sporadically, and isotropic serpentine with dispersed magnetite mineraliz- ing the mesh net. Isotropic serpentine overprints pseudomorphic serpentine. Late en-echelon sigmoidal tension Fig. 6. Scanning electron microphotograph (from EPMA) exhibiting cracks, filled by γ-serpentine fibers oriented normal to Cl clusters entrapped within non-pseudomorphic featureless serpentine the vein walls, are ubiquitous in all serpentinites. The (NP-srp) in stockwork serpentinite. A.F.A. Marques et al. / Marine Geology 245 (2007) 20–39 29 characteristic pseudomorphic serpentine textures are and discussed by Marques et al. (2006). Bulk-rock gradually overprinted by non-pseudomorphic serpentine geochemistry of the serpentinite is strongly affected by and only a few relict hourglass textures and bastites can sulfide mineralization and seafloor oxidation and still be distinguished. Although interlocking and inter- carbonatization. Silica in serpentinites decreases with penetrating γ-serpentine occur, the most common non- increasing sulfide mineralization and similar patterns pseudomorphic serpentine texture in the stockwork is occur with MgO and Al2O3. An exception occurs in the massive, featureless, colorless and nearly isotropic. Ni- talc-rich steatites that are Si-rich with lower MgO and rich pyrite, the first sulfide to occur is followed by Cu–Fe Al2O3 contents. Aluminum abundances are mineral- sulfides (intermediate solid solution with exsolved dependent since Al is accommodated in minor amounts chalcopyrite — ISS/Ccp) occasionally pyrrhotite and in serpentine-group minerals and in accessory minerals rare sphalerite. A second-generation coarse-grained like chlorite and Cr-spinel. CaO abundances are almost magnetite (II) often replaces pyrite. XRD patterns indicate exclusively related to the presence of late-stage that the non-pseudomorphic featureless serpentine is carbonates that occur in all rock types. Co and Ni are clinochrysotile (2Mcl) coupled with magnetite. The within expected values (81–184 ppm Co; 1430– continuous sulfide mineralization, through pervasive 1950 ppm Ni) and prevail in the poorly mineralized replacement and hydrothermal corrosion, promotes the samples from the stockwork (e.g. IR-DR-02-D and IR- destruction of serpentine-group minerals, visible as DR-03-A). With increasing sulfide mineralization there corrosion fronts in the stockwork (Fig. 5C) and semi- is a slight decrease in Ni and increase in Co (88– massive sulfides (Fig. 5D). 360 ppm Co; 206–1715 ppm Ni), which become more accentuated in the semi-massive sulfides (1680– 4.2.2. Mineral chemistry of serpentine-group minerals 3120 ppm Co; 104–448 ppm Ni) and massive sulfides Only 11 EPMA determinations of serpentine-group (4770–6300 ppm Co; below 1 ppm Ni). Cu abundances minerals from non-mineralized serpentinites and 8 from that are relatively low in the serpentinites, stockwork and the stockwork were considered since oxide weight steatite increase sharply in the semi-massive and massive percentages were consistently low, commonly below sulfides (ave. 15 wt.% Cu and 28 wt.% Cu, respectively). 60 wt.% (Table 2) even considering the presence of Zn concentrations are low except in the massive sulfide other elements such as Cl and S (Fig. 6). Low totals were chimneys as these are sphalerite-rich compared to the by far more significant in the serpentine-group minerals other sulfide-bearing rocks (Marques et al., 2006). from the stockwork which may be explained by the Serpentinites contain up to 66 ppm B, decreasing with extensive hydrothermal corrosion combined with high the increasing extent of hydrothermal sulfide mineraliza- contents of H2O. Mesh serpentine has low Al and Cr and tion. An exception is one B-rich semi-massive sulfide high Ni (up to 0.34 wt.% NiO) and Ca (up to 0.13 wt.%) sample (59 ppm B, SH1-DR3-2-1) that exhibits extreme matching the precursor olivine composition. Exsolution seafloor oxidation. U is found in significant amounts in lamellae in bastite are Al-rich (5.61 wt.% Al2O3) and serpentinites (max. 1.67 ppm U), increasing in the stock- Cr-rich (1.45 wt.% Cr2O3) relating to the primary work serpentinites (max. 6.57 ppm U) and reaching the pyroxene compositions. In the pseudomorphic serpen- highest amounts in the steatites (max. 18.1 ppm U) and tine from non-mineralized serpentinites, Si correlates oxidized semi-massive sulfides (max. 37.7 ppm U). positively with Mg and negatively with Fe2+. The op- Correlation coefficients of particular elements were plot- posite occurs in the non-pseudomorphic serpentine from ted against all other elements to produce a correlation the stockwork. pattern (Fig. 7). Elements having similar correlation pat- On the other hand, Si correlates inversely with Al terns, hence similar correlation coefficients with all other (reflecting the Al for Si substitution in the tetrahedral site) elements, likely share a common origin or similarity of in pseudomorphic serpentine contrasting with the con- secondary processes. Correlation coefficient patterns are stant Al values in non-pseudomorphic serpentine likely in agreement with petrographic observations and mineral because the extent of Al substitution for Si is greater in chemistry discriminating between (1) primary assem- lizardite than in chrysotile (Caruso and Chernosky, 1979; blages (peridotite– serpentinite) with significant SiO2– O'Hanley, 1996). MgO–Cr2O3–Ni and Al2O3–V–Sc that have increasing positive affinity with HREE and inverse affinity with 5. Bulk-rock geochemistry elements related to the sulfide mineralization (Fig. 7A); (2) sulfide mineralization characterized by the presence of Bulk-rock results are listed in Table 4. Rare-earth Co–Au–Cu (Fig. 7A) or Zn–As–Ag–Cd–Sb rich assem- elements and Nd isotope composition were published blages (Fig. 7B); and (3) late superficial carbonatization 30 A.F.A. Marques et al. / Marine Geology 245 (2007) 20–39

Table 4 Bulk-rock geochemistry of serpentinites (SRP), stockwork (ST), semi-massive sulfides (SMS), Cu-rich massive sulfides (Cu-MS), steatites (STEA); (n.a.) not analyzed Type SRP SRP SRP SRP SRP SRP SRP ST ST ST ST ST STEA IRDR IRDR IRDR IRDR IRDR IRDR IRDR IRDR IRDR IRDR IRDR SH1D 1A5 2B 2B4 9A5 9B3 9P 3S 2D 3A 3F 2C R3-9-4 Sample SiO2 % 39.08 37.47 35.68 35.18 38.53 36.43 32.77 39.47 33.34 24.48 31.51 18.70

Al2O3 % 1.33 0.49 0.58 0.44 0.72 0.62 0.66 0.55 0.48 0.28 0.41 0.18 Fe2O3 % 7.99 7.42 6.63 7.89 6.57 6.94 6.59 9.08 20.67 27.27 12.98 22.90 MnO % 0.046 0.05 0.076 0.218 0.063 0.067 0.634 0.087 0.087 0.158 0.116 0.240 MgO % 36.36 39.24 39.63 37.19 38.82 37.15 34.24 37.52 30.25 23.24 29.21 20.33 CaO % 1.91 0.02 0.02 2.31 0.09 2.05 5.92 0.08 0.14 4.18 0.89 1.47

Na2O % 0.22 0.12 0.1 0.2 0.14 0.15 .011 0.12 0.43 0.57 0.52 0.48 K2O % 0.02 0.04 0.07 0.04 0.03 0.05 b0.01 0.08 0.05 0.09 0.03 b0.01 TiO2 % 0.026 b0.001 0.005 0.005 0.005 0.004 b0.001 0.004 b0.001 b0.001 b0.001 b0.001 P2O5 % 0.03 0.01 n.d. 0.01 0.02 0.01 0.02 n.d. n.d. 0.01 b0.01 b0.01 LOI % 12.98 13.43 15.81 15.53 13.65 15.08 18.96 12.75 13.27 15.69 15.39 23.80 Total % 99.99 98.29 98.50 99.01 98.64 98.55 99.79 99.85 98.72 95.97 99.15 98.50

CO2 % 0.07 b0.05 0.29 3.75 0.18 1.91 7.5 0.07 0.22 4.3 0.95 4.20 S % 0.04 0.05 0.13 0.04 0.06 0.04 0.04 0.12 1 8.35 1.260 6.4 S⁎ % n.a. n.a. n.a. n.a. n.a. n.a. n.a. n.a. n.a. n.a. 0.91 4.52 Cl % 0.25 0.23 0.21 0.21 0.16 0.2 0.21 0.13 0.33 0.6 0.64 0.65 Ba ppm 2 2 1 3 b1 b182b142b1 Sr ppm 8 2 1 6 2 423 149 6 5 825 118 9 Y ppm 1 b1 b11b1 b1 b11b1 b1 b1 b1 Sc ppm 8.1 6.3 6.6 5.2 6.8 6.5 7.3 8.4 6.7 3.5 6.4 2.9 Zr ppm b1548b1 b1 b13b1411 Be ppm b1 b1 b1 b1 b1 b1 b1 b1 b1 b1 b1 b1 V ppm 55 19 22 22 29 20 27 28 13 15 6 11 Au ppb b2 b2 b24b2 b2 4 315384875 As ppm 7.9 1.4 1.6 2.6 5.4 1.3 2.4 1.5 0.9 4.5 1 1 Br ppm 18.9 25.2 39.2 27.4 28 26.7 30.5 15 38.1 38.6 23 28 Co ppm 184 91 87 81 83 98 130 99 56 88 69 178 Cr ppm 2700 1730 2120 1830 2480 2110 2380 2800 2850 1650 1740 1060 Ir ppb b5 b5 b5 b5 b5 b5 b5 b5 b5 b5 b5 b5 Sb ppm 0.6 b0.2 b0.2 b0.2 b0.2 b0.2 0.3 b0.2 b0.2 b0.2 0.4 b0.1 Se ppm b3 b3 b3 b3 b3 b3 b3 b3 b34b3 b3 Ag ppm b0.5 b0.5 b0.5 b0.5 b0.5 b0.5 b0.5 b0.5 b0.5 b0.5 0.5 1.5 Cd ppm 2.8 4.6 Cu ppm 542 b10 b10 b10 b10 b10 123 16 84 652 269 5033 Ni ppm 1730 1780 1630 1430 1430 1950 1820 1620 877 1130 1715 700 Pb ppm b5 b5 b5 b5 b5 b5 b5 b5 b5 b585 Zn ppm 530 35 b30 98 44 37 380 105 212 811 331 2052 Bi ppm b0.1 b0.1 b0.1 b0.1 b0.1 b0.1 b0.1 b0.1 b0.1 b0.1 19 10 B ppm 65 57 66 34 57 64 49 26 19 42 25 14 Hg ppb 5 b5 Ga ppm 1 b1 b1 b11b1 b1 b1 b1 b1 b1 b1 Ge ppm 1 0.9 0.7 0.8 0.6 0.9 b0.5 0.6 1 0.5 1.0 1.4 Rb ppm b1 b1 b1 b1 b1 b1 b1 b1 b11b12 Y ppm 1.5 b0.5 b0.5 b0.5 b0.5 b0.5 b0.5 0.6 b0.5 b0.5 b0.5 b0.5 Nb ppm b0.2 b0.2 b0.2 b0.2 b0.2 b0.2 b0.2 b0.2 b0.2 b0.2 0.6 0.8 Mo ppm b2 b2 b2 b2 b2 b253b22b24 In ppm b0.1 b0.1 b0.1 b0.1 b0.1 b0.1 b0.1 b0.1 b0.1 b0.1 b0.1 b0.1 Sn ppm b1 b1 b1 b1 b1 b1 b1 b1 b1 b1 −1 b1 Cs ppm b0.1 b0.1 b0.1 b0.1 b0.1 b0.1 b0.1 b0.1 b0.1 b0.1 b0.1 b0.1 Hf ppm b0.1 b0.1 b0.1 b0.1 b0.1 b0.1 b0.1 b0.1 b0.1 b0.1 0.4 b0.1 Ta ppm b0.01 b0.01 b0.01 b0.01 b0.01 b0.01 b0.01 b0.01 b0.01 b0.01 b0.01 b0.01 W ppm b0.5 b0.5 b0.5 b0.5 b0.5 b0.5 b0.5 b0.5 b0.5 b0.5 0.6 0.8 Tl ppm 0.06 b0.05 b0.05 b0.05 b0.05 b0.05 0.14 b0.05 b0.05 b0.05 b0.05 b0.05 Th ppm 0.1 b0.05 b0.05 b0.05 b0.05 b0.05 b0.05 b0.05 b0.05 b0.05 b0.05 b0.05 U ppm 0.8 0.74 b0.10 0.6 0.36 0.63 1.67 5.77 3.81 0.88 1.95 6.57 SST* — determined ST by LECO; Cu SMS and Zn — elements SMS in italic areSMS in %. SMS MS Cu MS Ch STEA STEA STEA SH1D SH1D SH1D SH1D IRDR IRDR SH1D IR96-3 SH1D IRDR IRDR R3-9-5 R3-9-9 R3-2-3 R3-2-1 01C1 01G2 R3-1-2 R3-8-3 3E 3W Sample A.F.A. Marques et al. / Marine Geology 245 (2007) 20–39 31

Table 4 (continued) ST ST SMS SMS SMS SMS MS Cu MS Ch STEA STEA STEA SH1D SH1D SH1D SH1D IRDR IRDR SH1D IR96-3 SH1D IRDR IRDR R3-9-5 R3-9-9 R3-2-3 R3-2-1 01C1 01G2 R3-1-2 R3-8-3 3E 3W Sample 37.64 27.38 0.82 10.25 2.19 0.84 0.41 b0.01 52.54 58.12 57.14 0.23 0.18 0.10 0.03 0.03 0.05 0.06 b0.01 0.14 0.22 0.18 7.80 17.37 61.37 48.23 15.06 26.21 32.40 50.98 7.43 9.58 10.41 0.061 0.037 0.009 0.055 0.033 0.047 0.001 b0.001 0.008 0.009 0.011 33.52 21.74 0.42 4.17 1.53 0.74 0.02 b0.01 24.53 26.41 26.55 0.03 3.63 b0.01 0.02 0.01 0.01 0.01 b0.01 b0.01 b0.01 b0.01 0.14 0.40 0.05 0.15 0.10 0.05 0.08 0.02 0.16 0.21 0.23 b0.01 b0.01 0.03 0.05 b0.01 b0.01 b0.01 0.02 b0.01 0.01 b0.01 b0.001 0.002 b0.001 b0.001 b0.001 b0.001 b0.001 b0.001 b0.001 b0.001 b0.001 b0.01 0.01 b0.01 0.01 b0.01 b0.01 b0.01 b0.01 b0.01 n.d. n.d. 10.88 21.35 24.93 19.06 16.26 16.71 15.06 14.24 4.62 4.28 4.53 99.94 98.69 93.57 91.09 53.10 66.48 55.39 71.82 99.02 98.84 99.05 b0.05 3.30 0.11 0.33 b0.05 0.06 b0.05 b0.05 b0.05 b0.05 b0.05 10.5 9.35 42.6 19.6 29.5 32.7 37.5 37.8 1.27 0.52 0.31 0.81 6.59 22.73 13.18 16.00 17.85 17.91 14.70 1.01 n.a. n.a. 0.18 0.63 0.12 0.29 0.24 0.12 0.18 0.04 0.17 0.11 0.11 4 9 8 72 4 3 15 9 b12b1 5 564 1 7 1 b12b1 b1 b1 b1 b1 b1 b1 b1 b1 b1 b1 b1 b1 b1 b1 1.5 3.1 0.9 0.7 0.3 0.2 b0.1 b0.1 1.9 2 2 b1134 4 3 2 2 513b1 b1 b1 b1 b1 b1 b1 b1 b1 b1 b1 b1 b59b5 b5 b5 b5 b5 b5 b59b5 14 118 4450 1900 12100 1530 6300 5770 13 9 11 1.3 2.8 1.1 6.8 b0.5 1 7.9 680 0.6 0.7 n.d. 7.6 25.3 10.2 13 12.6 10.3 10 6.3 7.4 17 23.6 121 360 1680 1730 2810 3120 4770 6300 336 223 189 b5 1010 1240 7 244 204 b5 b5 671 785 495 b5 b5 b5 b5 b5 b5 b5 b5 b5 b5 b5 0.1 0.9 b0.1 0.5 b0.1 b0.1 b0.1 17.7 0.1 b0.2 b0.2 b35b3 b3 134 139 42 b3 b3 b3 b3 b0.3 0.8 2.0 3.4 6.6 9.7 5.0 46.2 b0.3 b0.5 b0.5 1.6 1.4 2.7 4.1 5.0 2.7 2.3 76.5 1.3 248 3644 2500 2.98 19.78 22.18 27.98 8.35 582 129 61 206 909 309 104 439 448 b1 b1 577 778 602 9 b3 b3 b3 6 109 40 65.0 4 b5 b5 48 132 124 1152 1757 394 672 5.122 51 536 78 20 13 b2 b2 b2778.7 15 b0.1 b0.1 12 13 2 59 b2 b2 b2 b2 b2 b26 b55b5 39 5 10 93 145 b5 b1214b1 b1 224211 1.4 0.8 0.5 0.6 b0.5 b0.5 0.5 10.7 1.8 0.9 1.3 b1 b1 b12b1 b1 b1 b1 b1 b1 b1 b0.5 b0.5 b0.5 1.3 b0.5 b0.5 b0.5 b0.5 b0.5 b0.5 b0.5 0.8 0.9 0.4 0.4 0.3 0.3 0.5 0.3 0.3 b0.2 b0.2 34 6 9 53 19 18 16 12 3 10 3 0.3 0.2 b0.1 0.9 4.8 17.8 1.2 1.4 b0.1 b0.1 b0.1 b1 b122b16631b1 b1 b1 b0.1 b0.1 b0.1 b0.1 b0.1 b0.1 b0.1 b0.1 b0.1 b0.1 b0.1 b0.1 0.2 b0.1 b0.1 b0.1 b0.1 b0.1 b0.1 b0.1 b0.1 b0.1 b0.01 b0.01 b0.01 b0.01 0.02 b0.01 0.07 b0.01 b0.01 b0.01 b0.01 b0.5 b0.5 b0.5 1.0 b0.5 0.5 0.6 1.6 b0.5 b0.5 b0.5 b0.05 0.12 b0.05 b0.05 b0.05 b0.05 b0.05 b0.05 b0.05 0.1 b0.05 b0.05 b0.05 b0.05 b0.05 b0.05 b0.05 b0.05 b0.05 b0.05 b0.05 b0.05 0.59 0.81 0.61 37.7 1.25 0.77 0.12 0.21 1.01 18.1 3.42 32 A.F.A. Marques et al. / Marine Geology 245 (2007) 20–39

Fig. 7. Correlation patterns representing correlation coefficients of selected elements; (A) SiO2 (similar plots are obtained for MgO, Cr and Ni), Al2O3 (similar plots are obtained for V, Sc) and Co (similar plots are obtained for Cu and Au) ; (B) B, CaO (similar plots are obtained for CO2 and Sr) and Zn (similar plots are obtained for As, Ag, Cd, Sb). A.F.A. Marques et al. / Marine Geology 245 (2007) 20–39 33

affecting all rock types represented by CaO–Sr–CO2 exhibit serpentine-group minerals with contrasting (Fig. 7B). Co and Ni, although ultramafic related, show a textures and chemistry that overprint pseudomorphic strikingly contrasting behavior (Fig. 7A). Although both serpentine of the non-mineralized serpentinites. This represent the sulfide mineralization event, Co–Au–Cu textural transformation in the serpentinites is consistent patterns show no affinity towards Zn–As–Ag–Cd–Sb. with bulk geochemical variation as seen by the decrease The latter are concentrated preferentially in the massive in primary elements (SiO2, MgO, Al2O3 and Ni) and sulfide chimneys as opposed to the former that precipitate increase in Fe2O3 (as Fetotal), Cu, Zn and Co reflecting in the sub-seafloor stockwork and semi and massive the onset of sulfide-mineralization. sulfides hence representing different conditions. 6.2. Element remobilization 6. Discussion Element remobilization during serpentinization of The serpentinization of peridotite rocks in the Rain- peridotite occurs when elements like Fe, Co and Ni are bow ridge area is a ubiquitous process as reflected by the released from primary silicates and are incorporated in widespread occurrence of serpentinites observed during the newly formed serpentine and magnetite or, in the submersible dives and in dredge hauls. The hydration of presence of aqueous H2S, precipitate as Ni–Co–Fe sul- olivine during serpentinization produces excess iron that fides (Gülaçar and Delaloye, 1976; Shiga, 1987; Alt and is accommodated in magnetite releasing H2 that, com- Shanks, 2003). In the Rainbow vent field, the break- bined with dissolved CO2, produces abiogenic hydro- down of Ni-bearing olivine produced Ni-bearing serpen- carbon gas like CH4 (Janecky and Seyfried, 1986; tine mesh whereas the hydration of Cr and Al-bearing Shiga, 1987; Berndt et al., 1996; Allen and Seyfried, pyroxene resulted in Al and Cr-rich serpentine bastite. 2004). This large-scale interaction between seawater Bastite is the only form of pseudomorphic serpentine and mantle rocks is evidenced by the hydrothermal input that prevails in the sulfide-bearing serpentinites, even 3 into the water column of H2 and CH4 plus Mn and δ He when other pseudomorphic textures (mesh and hour- producing particularly high plume anomalies in the glass) have been overprinted. Some authors (e.g., Rainbow ridge area (e.g. Charlou et al., 1993; German O'Hanley, 1996) argue that it is the higher Al and Cr et al., 1996; Bougault et al., 1998). Enhanced seawater in lizardite after enstatite that stabilize bastite to a greater circulation may induce other low-temperature processes extent when compared to lizardite after olivine. No such as B uptake during retrograde serpentinization obvious link was found between the extent of sulfide decreasing with increasing temperature and ceasing at mineralization and the degree of alteration of the Cr- around 300 °C (Bonatti, 1976; Seyfried and Dibble, spinel. Instead, there are other concurrent factors that 1980; Sanford, 1981; Bonatti et al., 1984) and U seem to enhance the extent of hydrothermal alteration in enrichment due to low-temperature seafloor oxidation of Cr-spinel (e.g., fractures, grain size and shape). The sulfides that fix seawater U (German et al., 1993; Mills absence of sulfides related to serpentinization reactions et al., 1993). The observed values for B and U reflects suggests low aS2 during serpentinization. progressively increasing temperatures with strong During the early stages of sulfide mineralization, seawater influence in the serpentinites and low temper- hydrothermal Ni-rich sulfides (e.g., pyrite in stockwork; ature seawater oxidation of sulfides exposed at the Marques et al., 2006) precipitate in veins within a non- seafloor. pseudomorphic (Ni-free) serpentine. Thus, Ni and most likely Co, released from the pseudomorphic serpentine 6.1. Serpentinization prior to sulfide mineralization while experiencing textural and chemical modifications, were redistributed into the newly formed sulfides, Serpentinites from the Rainbow area are non- explaining why the bulk-rock Ni and Co contents in mineralized, lizardite-dominated with pseudomorphic the serpentinite and early stockwork remained constant. textures and resulted from low-temperature retrograde Co and Ni contents in the stockwork strongly suggest serpentinization under static conditions (Costa, 2005; that serpentinites represent the primordial Ni and Co Marques et al., 2006). Petrographic observations source during the early stages of hydrothermal sulfide indicate that serpentinites host the sulfide mineralization mineralization, whereas the extreme Co enrichment but low temperature serpentinization preceded the onset towards the upper parts of the hydrothermal system of sulfide mineralization and is not genetically linked to (semi-massive sulfides, Cu-rich massive sulfides and the event. Sulfide-bearing serpentinites (stockwork and massive sulfide chimneys) are likely from an external semi-massive sulfides) are restricted to the vent area and source. Exothermic serpentinization reactions alone 34

Table 5 Comparison of characteristics from selected ultramafic and mafic-hosted modern seafloor hydrothermal systems, located at the Mid-Atlantic Ridge (MAR), Galapagos Spreading Center (GSC), Juan de Fuca (JF), East Pacific Rise (EPR), Middle Valley (MV) and Central Indian Ridge (CIR) Site Location Setting Depth Host Rocks Assemblage Bulk Sulfide Geochemistry Vent Age (yr) Vent Temp.

Cu % Zn % Co ppm Ni ppm Au ppm Pb ppm Ag ppm 20 (2007) 245 Geology Marine / al. et Marques A.F.A. Rainbow 36°14′N MAR 2300 Serpentinite Po+ISS/Ccp+Sp 8.4 5.1 6300 b1 5.8 65 46.2 ~30 000 365 °C Cu-MS 28 0.1 4770 b1 6.3 40 5 SMS (n=4) 11.3 0.09 2335 325 2.3 28.75 5.4 Logatchev 14°45′ N MAR 2970 Serpentinite Logatchev 1 28 2.86 409 149 9.61 3900 353 °C Logatchev 2 14.7 25.4 500 20 23.8 Lost City 30° N MAR 800 Serpentinite Cc+Arg+Brc ∼30000 40–75 °C Saldanha MAR Serpentinite b10 °C TAG 26° N MAR 3650 Basalt sulfide chimney 13.4 0.6 531 48 0.48 300– Py-Ccp 12.8 1.4 75 50 1.43 sulfide mound 2.4 0.4 234 b10 0.5 59 9 Snake Pit 23 °N MAR 3530 Basalt massive sulfide 12.4 7 228 21 2.2 665 111 341 °C Galapagos GSC Basalt massive sulfide 4.48 4.02 2.88 16 0.3 420 46 Juan de Fuca 44° N JF Pacific Basalt massive sulfide 0.16 36.7 11 8 0.1 2600 178 EPR 13° N EPR Basalt 7.83 8.17 968 b20 0.26 500 49 EPR 11° N EPR Basalt 1.92 28 16 – 0.154 670 38 EPR EPR EPR Basalt 0.58 19.8 14 3 0.15 2100 98

Bent Hill MV Basalt/Sediment Po+Wt+ISS 1.2 11.2 2600 265 °C – MESO Zone 23°25′ S 2865 Basalt Py+Ccp 29.4 0.5 583.7 127.8 0.7 300 55.3 b50 000 39 Py+Marc 6.2 0.8 1089.6 70.4 0.6 500 22.4 Data sources: Rainbow — Douville et al. (2002) and Marques et al. (2006); Logatchev — Lein et al. (2001) and Rouxel et al. (2004); Lost City — Kelley et al. (2001); Saldanha — Barriga et al. (2003);TAG— Rona et al. (1993), Herzig et al. (1998), Lawrie and Miller (2000) and Douville et al. (2002); Snake Pit — Fouquet et al. (1993); Galapagos — Hannington (1989) in Fouquet et al. (1993); Juan de Fuca — Bischoff et al. (1983) in Fouquet et al. (1993); EPR — Bischoff et al. (1983) in Kase et al. (1990); Bent Hill — Goodfellow and Franklin (1993) and MESO — Münch et al. (1999). Table 6 Major and trace metal abundances in end-member vent fluids from the Mid-Atlantic Ridge, after Douville et al., 2002

Location Rainbow Logatchev Lost City Menez Lucky Broken TAG Snake Pit Exp. 20 (2007) 245 Geology Marine / al. et Marques A.F.A. Gwen Strike Spur Host rock Seawater 36°14′ N 14°45′ N 30° N 37°50′ N 37°17′ N 29° N 26° N 23° N Seawater Ultramafic Ultramafic Ultramafic Basalt Basalt Basalt Basalt Basalt Basalt T (°C) – 365 353 40–75 284–271 185–324 360 363 341 400 pH 7.8 2.8 3.3 9–9.8 4.4–4.5 3.4–5.5 – 3.1 3.7 3.2 MgmM53000 00000 Ca mM 10.2 67 28 23–31 32–33 31–42 12 28 11 22.4 Na mM 464 553 438 482–497 313 344 420 550 515 474 Cl mM 546 750 515 546–549 380 400 413–554 650 550 581 H2S mM 0 1 0.8 0.064 b1.5 0.6–3.4 – 4-Mar 6 Fe mM 0.0045 24000 2500 – b2–18 70–920 1970 5170 2400 3975 Mn mM 0.0013 2250 330 – 59–68 77–450 254 710 400 1005 Co mM b213 b2 – b2 b2 – b2 b2 Ni mM b23 b2 – b2 b2 – b2 b2 Cu mM 0.0033 140 27 – b2 b2–30 43 130 35 31 Zn mM 0.028 160 29 – b2 b2–40 72 83 53 214 Ag mM 0.023 47 11 – 4.3–17 4.7–25 – 51 31 Cd nM 0.7 130 63 – b9–12 18–79 – 66 440 –

Pb nM 0.013 148 86 – 21–56 35–130 – 110 265 39 REE pM 10 16800 3600 – 160–350 190–2550 900 1800 4300 Data ⁎ ⁎ ⁎ ⁎⁎⁎ ⁎ ⁎⁎⁎ ⁎ ⁎ ⁎⁎⁎⁎ Data from (⁎) Douville et al. (2002); (⁎⁎) Kelley et al. (2001) and Allen and Seyfried (2004),(⁎⁎⁎) James et al. (1995) and (⁎⁎⁎⁎) experimental results from seawater-basalt interactions at 400 °C, Seewald and Seyfried (1990). 35 36 A.F.A. Marques et al. / Marine Geology 245 (2007) 20–39 cannot account for the sulfide accumulation and metal- ate–brucite chimneys, low temperature venting (∼40– rich high-temperature fluids issuing from the Rainbow 75 °C) and high pH fluids (9–10) (Kelley et al., 2001; vent field requiring the presence of a magmatic heat Früh-Green et al., 2003; Ludwig et al., 2006). The source at depth (e.g. Cave et al., 2002; Lowell and Rona, higher temperature ultramafic-rock hosted systems, 2002; Mével, 2003; Allen and Seyfried, 2004; Marques Rainbow and Logatchev, exhibit mixed features be- et al., 2006). tween sediment-starved basalt-hosted systems with high bulk-rock Cu/Zn ratios, high Cu and Co and low Pb and 6.3. Basalt-and Peridotite-hosted seafloor hydrothermal Ni (Table 5), and ultramafic-hosted systems with high systems methane anomalies and ubiquitous serpentinization reactions within the host rocks (Batuyev et al., 1994; The structure of modern basalt-hosted seafloor Bogdanov et al., 1995; Donval et al., 1997; Mozgova hydrothermal deposits (e.g. Koski et al., 1984; Fouquet et al., 1999; Marques et al., 2006). et al., 1993; Rona et al., 1993; Humphris, 1995; Herzig and Hannington, 1995; Tivey et al., 1995; Fouquet et al., 7. Concluding remarks 1996; Goodfellow et al., 1997; Scott, 1997) resembles the sulfide-bearing rocks of the Rainbow vent field with The Rainbow vent field is a product of multiple four main types of mineralization: (1) stockwork events as seen in the mineralogy and geochemistry of (sulfide stringer zone), (2) semi-massive sulfides the sampled rocks. Seawater circulation in exposed (replacement sulfides), (3) Cu-rich massive sulfides mantle rocks around the Rainbow ridge area induced (zone-refined) and (4) massive sulfide chimneys. The low temperature serpentinization reactions. The pres- presence, in the Rainbow vent field, of replacement ence of an intruded magmatic body at depth was likely fronts between the sulfides and host rocks and the the motor that induced the onset of restricted high presence of host rock-relicts within the semi-massive temperature hydrothermal circulation with pervasive sulfides is diagnostic of sub-seafloor replacement as in sulfide mineralization that caused significant textural VMS deposits (Doyle and Allen, 2003). In particular, and chemical changes in the serpentine-group minerals massive sulfides in the Rainbow vent field show a Cu– and relict Cr-spinels. The ultramafic signature, inherited Zn–(Co) type mineralization with high Cu/Zn ratios, by the host rocks at Rainbow vent field, is masked by chemical zonation and stockwork veining comparable to the intense hydrothermal activity that confers a mafic Cu–Zn mafic VMS deposits on land (Sangster and character to the sulfide mineralization. Nevertheless, Scott, 1976; Fouquet et al., 1988; Hannington et al., some ultramafic characteristics can prevail such as the 1991; Fouquet et al., 1993; Hannington et al., 1995; presence of Ni-rich sulfides in the stockwork and high Humphris, 1995; Tivey et al., 1995; Hannington et al., methane anomalies. The structure, mineralogy and bulk- 1997; Doyle and Allen, 2003; Gemmell and Large, rock chemistry of the massive sulfides found in the 2004). This zonation reflects hydrothermal reworking Rainbow vent field resemble modern and ancient basalt- by zone refining processes that seem to be of extreme hosted hydrothermal deposits while the host rock importance in the Rainbow Cu-rich massive sulfides alteration, apart from the active area, reflects simple with high contents of Au. On the other hand, Zn-rich serpentinization processes. assemblages that predominate in the sulfide chimneys from the Rainbow vent field are accompanied by As, Sb, Acknowledgments Pb, Cs, Ag, Au and Co similar to several known basalt- hosted seafloor hydrothermal systems (e.g. Zierenberg Fundação para a Ciência e Tecnologia (FCT) provided et al., 1984; Fouquet et al., 1988; Hannington and Scott, funds through the project SEAHMA (FFCUL/FCT- 1989; Hannington et al., 1991; Tivey et al., 1995). PDCTM/P/MAR/15281/1999) and a PhD scholarship Tables 5 and 6 compare geochemical characteristics of (SFRH/BD/2978/2000) given to A.F.A. Marques. Scott’s some ultramafic and mafic-hosted seafloor hydrother- research is funded by the Natural Sciences and Engineer- mal systems from different seafloor spreading centers. ing Research Council of Canada. A.F.A. Marques also Menez Hom and Saldanha exhibit high CH4 anomalies benefited from a student research grant from the Society (Fouquet and IRIS Scientific Party, 2001) but venting is of Economic Geologists Foundation. We thank Yves diffuse and not exceeding 10 °C (Fouquet and IRIS Fouquet from IFREMER for scientific support at sea, Scientific Party, 2001; Barriga et al., 2003). The Lost Claudio Cermignani (University of Toronto) for technical City field records 30 000 years of hydrothermal activity and analytical support with the EPMA and SEM, and driven by serpentinization reactions producing carbon- Álvaro Pinto for help with the SEM. A.F.A. Marques et al. / Marine Geology 245 (2007) 20–39 37

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