Contrib Mineral Petrol (2014) 168:1079 DOI 10.1007/s00410-014-1079-2
ORIGINAL PAPER
Hydrothermal processes in partially serpentinized peridotites from Costa Rica: evidence from native copper and complex sulfide assemblages Esther M. Schwarzenbach · Esteban Gazel · Mark J. Caddick
Received: 5 March 2014 / Accepted: 23 October 2014 © Springer-Verlag Berlin Heidelberg 2014
Abstract Native metals and metal alloys are common in serpentinized ultramafic rocks, generally representing the redox and sulfur conditions during serpentinization. Variably serpentinized peridotites from the Santa Elena Ophiolite in Costa Rica contain an unusual assemblage of Cubearing sulfides and native copper. The opaque mineral assemblage consists of pentlandite, magnetite, awaruite, pyrrhotite, heazlewoodite, violarite, smythite and copperbearing sulfides (Cu-pentlandite, sugakiite [Cu(Fe,Ni)8S8], samaniite [Cu2(Fe,Ni)7S8], chalcopyrite, chalcocite, bornite and cubanite), native copper and copper–iron–nickel alloys. Using detailed mineralogical examination, electron microprobe analyses, bulk rock major and trace element geochemistry, and thermodynamic calculations, we discuss two models to explain the formation of the Cu-bearing mineral assemblages: (1) they formed through desulfurization of primary sulfides due to highly reducing and sulfur-depleted conditions during serpentinization or (2) they formed through interaction with a Cu-bearing, higher temperature fluid (350–400 °C) postdating serpentinization, similar to processes in active high-temperature peridotite-hosted hydrothermal systems such as Rainbow and Logatchev. As mass balance calculations cannot entirely explain the extent of the native copper by desulfurization of primary sulfides, we propose that the native copper and Cu sulfides Communicated by O. Müntener. Electronic supplementary material The online version of this article (doi:10.1007/s00410-014-1079-2) contains supplementary material, which is available to authorized users. E. M. Schwarzenbach (*) · E. Gazel · M. J. Caddick Department of Geosciences, Virginia Tech, 4044 Derring Hall, Blacksburg, VA 24061, USA e-mail:
[email protected]
formed by local addition of a hydrothermal fluid that likely interacted with adjacent mafic sequences. We suggest that the peridotites today exposed on Santa Elena preserve the lower section of an ancient hydrothermal system, where conditions were highly reducing and water–rock ratios very low. Thus, the preserved mineral textures and assemblages give a unique insight into hydrothermal processes occurring at depth in peridotite-hosted hydrothermal systems. Keywords Native copper · Sulfides · Peridotite · Serpentinization · Santa Elena Ophiolite
Introduction Serpentinization is a widespread process that is found where ultramafic rocks react with seawater, hydrothermal fluids or metamorphic fluids within subduction zones (e.g., Cannat et al. 1992; Hyndman and Peacock 2003; Mével 2003; Früh-Green et al. 2004; Cannat et al. 2010). During reaction of water with the primary minerals olivine and pyroxene, H2 is formed due to oxidation of Fe2+ to Fe3+ (e.g., Frost 1985; Bach et al. 2006). As a result, highly reducing conditions are produced that are rarely seen in other geological environments. These high H2 conditions allow the stability of native metals, Fe–Ni alloys (e.g., awaruite, taenite) and other rare sulfides such as heazlewoodite or polydymite (Frost 1985; Klein and Bach 2009). Despite this seemingly hostile environment, serpentinization has been shown to provide the necessary energy source for microbial activity and peridotite-hosted hydrothermal systems have been found to host diverse microbial communities (Kelley et al. 2005; Brazelton et al. 2006; Russel et al. 2010; Brazelton et al. 2011), making these environments of great interest for studying processes that link the
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geochemical cycles between the lithosphere, hydrosphere and biosphere (Früh-Green et al. 2004; Schwarzenbach et al. 2012, 2013b). Variably serpentinized peridotites and their sulfide and oxide assemblages have been studied in ultramafic bodies tectonically emplaced on continents (Eckstrand 1975; Garuti et al. 1984; Peretti et al. 1992), along mid-ocean ridges, where detachment faulting causes exposure of ultramafic rocks to seawater inducing extensive serpentinization (Bach et al. 2004; Alt et al. 2007; Delacour et al. 2008a, b), in fossil peridotite-hosted hydrothermal systems (Hopkinson et al. 2004; Schwarzenbach et al. 2013b), within the mantle wedge or the subducting plate (Hyndman and Peacock 2003; Alt and Shanks 2006; Scambelluri and Tonarini 2012) and in cratonic lithospheric mantle xenoliths (Lorand and Gregoire 2006). Typical primary sulfides in peridotites are pentlandite ± pyrrhotite ± chalcopyrite and occur as inclusions within silicates (e.g., Lorand 1989a, b). Only rarely are native metals found in peridotites, while extensive Cu–Fe–Ni sulfides are usually associated with seafloor hydrothermal systems or are exposed on the continent as volcanogenic massive sulfide (VMS) ore deposits. Specifically, Cu-rich sulfide assemblages have been related to hydrothermal leaching of mafic sequences, but native copper has also been related to alteration of primary Cu-bearing sulfide minerals or even of primary mantle origin (e.g., Abrajano and Pasteris 1989; Tsushima et al. 1999). The opaque mineralogy in serpentinized peridotites records the hydrogen/oxygen and sulfur fugacities during serpentinization reactions (Frost 1985; Klein and Bach 2009). While initial serpentinization allows the stability of native metals and metal alloys, completely serpentinized peridotites typically preserve high-sulfur assemblages and magnetite or hematite (Eckstrand 1975; Alt and Shanks 1998; Delacour et al. 2008a; Schwarzenbach et al. 2012). Thus, the study of the opaque mineral assemblages is key to understanding the evolution of the hydrogen and sulfur fugacities during the serpentinization process. Additionally, serpentinites play an important role in many global geochemical cycles and control the transport of various species (e.g., H2O, sulfur) into the mantle (e.g., Scambelluri et al. 1995; Ulmer and Trommsdorff 1995; Scambelluri and Tonarini 2012; Alt et al. 2013). Revealing the processes that accompany serpentinization is therefore required to completely characterize the geochemical cycling between Earth’s surface and Earth’s mantle. On the Santa Elena peninsula in Costa Rica, variably serpentinized peridotites crop out together with layered and pegmatitic gabbros and are intruded by mafic dikes (Gazel et al. 2006). Due to the low degree of serpentinization and low degree of weathering in most samples, different stages of serpentinization and various sulfide textures are well preserved. This makes the Santa Elena peridotites ideal
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to study the successive processes that are associated with the hydration of peridotites. Moreover, the discovery of the presence of native copper together with a very diverse sulfide mineralogy in the samples from the Santa Elena Ophiolite is of particular interest in understanding both the redox and the sulfur conditions during serpentinization, and the possible interaction with high-temperature hydrothermal fluids (>350 °C) pre- or postdating lower temperature serpentinization (~200–250 °C). Here, we present data on the opaque mineralogy, sulfide and metal mineral chemistry and bulk rock chemistry of the Santa Elena peridotites with the goal of evaluating the source and speciation of the copper-bearing assemblages and to give insights into the hydrothermal evolution of these peridotites.
Geological setting and sample selection The Santa Elena Ophiolite is located on the west coast of Costa Rica and comprises an area of 250 km2 of mafic and ultramafic lithologies (Fig. 1; Gazel et al. 2006). Geotectonically, Costa Rica is today situated on the triple junction of the Cocos, Caribbean and Nazca Plates. Along the Middle American Trench, the Cocos Plate is being subducted underneath the Caribbean Plate, resulting in an active volcanic front, while along the pacific side of Costa Rica several oceanic complexes have been accreted onto the Caribbean Plate (Hauff et al. 2000; Denyer and Gazel 2009; Herzberg and Gazel 2009 and references therein). The Santa Elena peridotites have generally been correlated with peridotites cropping out along the Costa Rica–Nicaragua border, suggesting an E-W fossil suture zone between different tectonic blocks (Tournon et al. 1995). The Santa Elena Ophiolite is locally covered by reef limestones of Campanian age, suggesting that the Santa Elena Peninsula was emplaced during the Upper Cretaceous with a peridotitic complex at the hanging-wall and an igneous-sedimentary complex at the footwall—the Santa Rosa Accretionary complex (Baumgartner and Denyer 2006; Denyer and Gazel 2009). The Santa Elena Nappe (Fig. 1) contains variably serpentinized peridotites, dunites and locally layered gabbros. Various generations of pegmatitic gabbros and diabase dikes cut the peridotites. Some of these dikes do not preserve chilled margins, suggesting that they were emplaced into a hot mantle host preceding serpentinization (Gazel et al. 2006). A secondary mineralogy in the mafic lithologies composed of albite + epidote + actinolite + chlorite has been ascribed to ocean floor metasomatism (Gazel et al. 2006). The samples studied here were collected at various locations within the Santa Elena Ophiolite (Fig. 1) and include boulders within streams that were collected due to great preservation conditions. The samples include
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85°60’
85°55’
85°45’
85°50’
85°40’
N
Pacific Ocean
SE_P5
SE_P9
SE10_01, 02
SE10_05, 06
10°55’
SE10_09
SE_P7
SE_P3
Potrero Grande tectonic window
SE_P8
Playa Santa Rosa
SE10_19 SE_P4
Islas Murciélago
SE10_12
SE10_16
SE_P1, P2
Punta El Respingue
Costa Rica
10°50’
SE_P10
Layered gabbros (124 Ma)
Santa Elena Nappe
Santa Rosa Accretionary Complex
Pillow and massive basalts (109 Ma)
Dike swarm
Dolerite dikes
Faults
5 km
Santa Elena thrust
Fig. 1 Geological map of the Santa Elena Ophiolite in Costa Rica with the location of the analyzed peridotite samples (after Gazel et al. 2006)
lherzolites, harzburgites and dunites with variable degrees of serpentinization.
Methods The mineralogy and petrology of the peridotites were initially studied in thin section with transmitted and reflected light microscopy. The mineral chemistry of the sulfides was determined on a Cameca SX-50 electron microprobe (EMP) at 15 kV acceleration potential, 20 nA current and 1 μm beam size, using natural and synthetic mineral standards. Relative analytical error is better than 1 % (1σ) except for element contents
