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: esther11@vt.edu
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
13
1079 Page 2 of 21
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
13
Contrib Mineral Petrol (2014) 168:1079
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
Page 3 of 21 1079
Contrib Mineral Petrol (2014) 168:1079
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 <1 wt%, where the analytical error is
better than 4 % (1σ). Element distribution maps of selected
grains and areas were collected using the EDS (energy dispersive spectrometer) system and were run between 2 and
12 h and at a current of 40–100 nA, depending on run time.
Bulk rock samples were powdered using an alumina
mill and were fluxed into homogeneous glass disks with
ultrapure Li2B4O7 from Spex® (certified ≪1 ppm blank for
all trace elements) at the Petrology Lab at Virginia Tech
for X-ray fluorescence (XRF) and inductively coupled
plasma mass spectrometry (ICP-MS) analyses. Major elements were collected following the methods described in
Mazza et al. (2014). The analytical error for 10 replicates
of BHVO-2g was <1.5 % for all major elements. Trace elements were collected from the same fluxed glasses with an
Agilent 7500ce ICPMS coupled with a Geolas laser ablation system, with a He flow rate of ~1 L/m−5 Hz and an
energy density on sample ~7–10 J/cm2, following the procedures detailed in Mazza et al. (2014). Data were calibrated against USGS standards BHVO-2g, BCR-2g and
BIR-1g, using Si from XRF as an internal standard and the
standard element values reported in Kelley et al. (2003).
The analytical error for 10 replicates of BHVO-2g was
13
1079 Page 4 of 21
<5 % for all elements with the exception of Ni, Cu and Yb
(<7 %).
Results
Mineralogy
The samples studied here are lherzolites, clinopyroxenerich harzburgites and dunites with a degree of serpentinization ranging between 30 and 100 % (Table 1). Hydration led to replacement of olivine by serpentine forming a
typical mesh texture. In all samples, olivine replacement is
more advanced than decomposition of pyroxene, where initial replacement by serpentine typically occurs along fractures within the grains. The serpentinization textures show
a mesh texture to rare ribbon veins and parallel veining,
indicating serpentinization under static conditions. Serpentinization of olivine resulted in the formation of serpentine + brucite veins, with almost pure brucite present in the
center of some veins. Hydration also led to the formation of
minor amounts of chlorite and amphibole.
The opaque mineral associations include pentlandite,
magnetite, awaruite, pyrrhotite, heazlewoodite, violarite
and smythite (Table 1; Fig. 2). Numerous peridotite samples contain copper sulfides (chalcopyrite, Cu-pentlandite,
sugakiite [Cu(Fe,Ni)8S8], samaniite [Cu2(Fe,Ni)7S8], chalcocite, bornite and cubanite; Table 2), native copper and
copper–iron–nickel alloys (Table 1; Fig. 3, 4, 5). Within the
partly serpentinized peridotites, the sulfide minerals occur
mainly in the serpentine groundmass or in serpentine veins,
and only rarely within pyroxene, suggesting that most
sulfides formed as secondary phases. In some extensively
serpentinized samples, sulfides are finely dispersed in the
serpentine groundmass as grains <2 μm and could not be
determined microscopically or by EMP analyses.
Pentlandite is the most abundant sulfide mineral and
was detected in all of the analyzed samples. The most common association is pentlandite + magnetite + awaruite
with traces of pyrrhotite and heazlewoodite. In these samples, magnetite predominantly occurs within the cleavage
planes of the pentlandite and with awaruite either as thin
veins or as thin rims along the pentlandite grains (Fig. 3a,
b). Cobalt-pentlandite was only detected in one sample
(SE10_06; Table 1) that is almost entirely serpentinized,
with Co-pentlandite being intergrown with awaruite. Pyrrhotite occurs in a few samples as inclusions or along fractures within pentlandite, but was only detected in association with Cu-bearing sulfides. Additionally, element
distribution maps suggest the presence of pyrrhotite as a
reaction rim between pentlandite and magnetite (Fig. 4c,
d). Heazlewoodite was observed in two samples (SE10_02,
SE_P4; Table 1): (1) as an intergrowth with a decomposed
13
Contrib Mineral Petrol (2014) 168:1079
pentlandite crystal and native copper, located within an
altered pyroxene, and (2) as intergrowth with Cu-bearing
sulfide and Cu–Fe–Ni alloys within the serpentine mesh
texture.
One completely serpentinized sample (SE_P5; Table 1)
contains the assemblage pentlandite + violarite + smythite + chalcopyrite + magnetite. Violarite and smythite
form a sub-microscopic intergrowth, are often partly
rimmed by magnetite and contain inclusions of pentlandite
(suggesting that violarite and smythite replace pentlandite).
Chalcopyrite forms veins and is occasionally also rimmed
by magnetite.
Cu-bearing sulfide minerals are present in almost all
of the analyzed samples and are located together with the
Cu-bearing metals within serpentine veins or the serpentine mesh texture, and only rarely occur within pyroxene.
Within single-sulfide grains, Cu contents increase from
the edge inward, with native copper forming a dendritic
rim and the Cu-richest sulfur phases forming either along
the edges, within fractures increasingly replacing pentlandite or as grains together with Cu-rich pentlandite. The
most abundant Cu-phases are Cu-pentlandite (Cu content
<3 wt%), sugakiite and samaniite, and the latter two are
the Cu-rich pentlandite varieties (Table 2). In rare Cu-rich
sulfide grains, the Cu-rich pentlandite is accompanied by
cubanite, chalcocite and fine-grained intergrowths of bornite and chalcocite, while native copper forms dendritic
structures into the serpentine groundmass (Fig. 4a, b). In
several samples (SE10_01, SE10_02, SE10_05, SE10_19,
SE_P4; Table 1), Cu–Fe–Ni alloys occur as thin veins
within or along the edge of Cu-bearing sulfides, and in a
few samples, Cu–Fe–Ni alloys occur in grains associated
with awaruite. Occasionally, serpentine adjacent to sulfides
is enriched in Cu, forming bluish halos (under reflected
light) along the sulfide grains and in contact with Cuenriched areas of sulfides (e.g., Fig. 3c). Pentlandite that
is rimmed by magnetite is never intergrown with a Cu-rich
phase.
Magnetite occurs in most samples as intergrowths with
pentlandite (as described above), but also as grains <2 μm
in the center of serpentine veins, as fine-grained veins cutting the mesh texture, and in some entirely serpentinized
samples overgrowing the serpentine mesh texture as large,
euhedral grains (up to 0.3 mm). In these latter samples,
magnetite does not occur within serpentine veins. Additionally, the serpentine in the groundmass has relatively high
Mg# of 93–96 implying that most of the Fe in the system
has partitioned into magnetite.
Mineral chemistry
The chemical composition of all of the analyzed sulfide
and metal phases is reported in the supplementary material
Original
sample name
W
10°54.325′ 85°47.167′ hz
10°50.524′ 85°47.257′ hz
10°50.436′ 85°45.178′ lz
10°51.882′ 85°40.408′ lz
10°50.142′ 85°47.560′ du
10°50.142′ 85°47.560′ du
SE10_09 SE10_09
SE10_16 SE10_16
SE_010510_3
SE_010510_4
SE_020611_4
SE_050111_6
SE_050611_19 10°54.515′ 85°54.821′ hz
SE_P1
SE_P2
SE_P3
SE_P4
SE_P5
30
100
40–50
100
90–95
85–90
40–50
50–60
30–40
60
90–95
×
×
×
×
×
Co–ptl po
×
hz
×
×
×
×
×
×
×
×
×
×
×
×
×
×
×
sug sam cc
×
×
bn
×
cub sm
vl
cp
×
×
×
×
×
×
×
×
×
×
×
×
×
×
×
mgt aw/Fe–Ni
alloys
×
×
×
×
×
×
×
×
×
×
×
×
Sulfides <2 μm disseminated in the groundmass could not be determined by EMP
×
×
×
×
×
×
×
×
ptl
Opaque mineralogyb
Occurrence of opaque minerals as determined by reflected light microscopy and EMP analyses
b
Lithology: lz = lherzolite; du = dunite; hz = harzburgite
a
SE10_19 SE10_19
SE10_12 SE10_12
10°54.620′ 85°54.397′ hz
10°51.580′ 85°41.088′ hz
10°54.936′ 85°48.608′ lz
10°54.936′ 85°48.608′ du
SE10_06 SE10_06
40–50
30–40
Lithologya Degree of
serpentinization (%)
10°55.582′ 85°51.922′ lz
10°55.582′ 85°51.922′ lz
N
Coordinates
SE10_05 SE10_05
SE10_02 SE10_02
SE10_01 SE10_01
Sample
name
Table 1 Mineralogical description of the Santa Elena peridotites
×
×
×
×
×
Cu–Fe–Ni
alloys
×
×
×
×
×
×
Native Cu
Contrib Mineral Petrol (2014) 168:1079
Page 5 of 21 1079
13
1079 Page 6 of 21
Contrib Mineral Petrol (2014) 168:1079
S
Pentlandite
20
0
100
80
pyrite
40
chalcopyrite
60
heazlewoodite
pentlandite
bornite
chalcocite
80
40
60
pyrrhotite
0
20
10
0
0
20
40
60
80
0
10
Fe
Ni+Cu+Co
awaruite (Ni2 Fe) awaruite (Ni3 Fe)
Fig. 2 Chemical composition of the analyzed sulfides of the studied
peridotites. Filled gray circles indicate elevated Cu contents (>3wt%)
Table 2 Mineral abbreviations and formulas
Mineral name
Abbreviation
Formula
Awaruite
Bornite
Chalcocite
Chalcopyrite
Covellite
Cubanite
Cuprite
Heazlewoodite
Magnetite
Pentlandite
Polydymite
Pyrite
Pyrrhotite
Samaniite
Smythite
Sugakiite
Taenite
aw
bn
cc
ccp
cv
cub
cpr
hz
mgt
ptl
pd
py
po
sam
sm
sug
ta
Ni2Fe to Ni3Fe
Cu5FeS4
Cu2S
CuFeS2
CuS
CuFe2S3
Cu2O
Ni3S2
Fe3O4
(Fe,Ni)9S8
Ni3S4
FeS2
FeS
Cu2(Fe,Ni)7S8
(Fe,Ni)13S16 to (Fe,Ni)9S11
Cu(Fe,Ni)8S8
γ Fe–Ni alloy
Violarite
vl
FeNi2S4
(Table S1) and plotted in Fig. 2. The stoichiometric formula
of the sulfides and metals mentioned in the text is given in
Table 2. Several analyses plot between stoichiometric mineral compositions suggesting that they represent a mixture
of at least two mineral phases that form a sub-microscopic
intergrowth. Zn contents in all analyzed sulfides and alloys
are <0.7wt%.
13
Pentlandite analyses reveal a large range in composition with Fe contents of 10.0–44.2 wt% and Ni contents
of 21.0–52.1 wt%, the stoichiometric composition ranging from (Fe6.1Ni2.9)S8 to (Fe3.6Ni5.4)S8. Highest Fe contents are found in grains that are intergrown with magnetite ± awaruite. In a few samples, two groups can be
characterized as having either low or high Fe contents.
Co contents are generally <1 wt% but can be as high as
9.5 wt%; in one sample, it reaches 22.2 wt%. Element distribution maps of several grains further suggest that Co is
preferentially incorporated into magnetite rather than pentlandite. Cu contents are low (<0.9 wt%) in most pentlandites that have been unaffected by Cu-alteration.
Pyrrhotite
All of the analyzed pyrrhotite has elevated Ni contents
(3.3–6.0 wt%). Many analyses indicate that pyrrhotite
forms a sub-micrometric intergrowth with another phase
(e.g., with pentlandite, Cu-pentlandite or Fe–Ni alloy;
Fig. 2) and are not single mineral grain analyses. Cu contents are <0.7 wt% (except for two analyses), and Co contents are <0.4 wt%.
Heazlewoodite
All heazlewoodite analyses indicate slightly elevated Fe
contents (1.6–4.2 wt%) with a stoichiometric composition of approximately (Ni2.85Fe0.15)S2. Highest Fe contents
are observed in Co-rich heazlewoodite (<3.5–7.5 wt%). All
other heazlewoodite analyses yielded Co contents <0.5 wt%.
Violarite and smythite
Analyses of both violarite and smythite yielded low totals,
while spot analyses with the EDS indicate the presence of
small amounts of oxygen. Smythite has a stoichiometric
composition of (Fe4.3Ni4.7)S11 to (Fe3.0Ni6.0)S11 and Co contents of up to 3.0 wt%. Violarite has a stoichiometric composition of approximately Fe1.0Ni1.9S4, variable Cu contents (up
to 4.4 wt%) and Co contents of <1.3 wt%. The presence of
violarite and smythite together with chalcopyrite in one sample (SE_P5; Table 1) will not be discussed in further detail
as violarite and smythite are most likely the result of late,
low-temperature weathering of pentlandite and pyrrhotite,
respectively (Craig 1971; Furukawa and Barnes 1996). Indication for weathering is also given by the detection of oxygen in several grains, possibly present as a hydrous phase.
Thus, we infer that this sulfide assemblage is unrelated to the
formation of native copper discussed in this manuscript.
Contrib Mineral Petrol (2014) 168:1079
Page 7 of 21 1079
Fig. 3 Reflected light images of sulfides from four different samples.
a Large pentlandite grain intergrown with magnetite and awaruite
(Ni2Fe; sample SE10_19). One EMP analysis suggests the presence
of a Fe–Ni–Cu alloy with strongly varying composition. b Pentlandite with an Fe–Ni alloy forming thin veins in the pentlandite and
magnetite as exsolutions in pentlandite (sample SE10_05). EMP
analyses suggest the presence of rare pyrrhotite as inclusions in pentlandite. c Pentlandite intergrown with Cu-rich pentlandite and native
Cu. Blue areas are serpentine with traces of Cu (sample SE10_02). d
Pentlandite (~100 μm) intergrown with magnetite, and native Cu and
an Fe–Ni alloy along the rim (sample SE10_09)
Fe–Ni alloys
grain (Fig. 4). Sugakiite and samaniite are the Cu-bearing
pentlandite varieties with Cu occupying one and two sites,
respectively, in the crystal structure (Table 2). Stoichiometric compositions are approximately Cu0.9(Fe4.1Ni4.0)S8 to
Cu2.5(Fe4.5Ni2.0)S8. Thus, reaction of Cu with pentlandite
here resulted in a mixing line between Cu-free pentlandite
and samaniite with highest Cu contents of 19.3 wt%, and
Cu variably replacing either Fe or Ni in the crystal structure
(Co <0.9 wt%). Chalcocite is rare and has slightly elevated
Fe and Ni contents (Fe = 1.7–5.1 wt%; Ni = 0.5–2.2 wt%)
suggesting a sub-microscopic intergrowth with bornite
(Fig. 5a). Similarly, a few measurements are situated on a
mixing line between chalcocite and pentlandite. Cubanite
was detected in only one sample and has a stoichiometric
composition of Cu(Fe1.9Ni0.1)S3. Chalcopyrite was only
detected associated with pentlandite, violarite and smythite
Analyzed Fe–Ni alloys have a large range in composition from Ni57Fe43 to Ni75Fe25 (Figs. 2, 5b), and only a
few measurements could be classified as awaruite (Ni2Fe
to Ni3Fe). For the alloys with no significant Cu contents
(<1 wt%), this range corresponds to a Ni content of 49.6–
72.3 wt% and Fe contents of 21.8–39.6 wt%. Co contents
are <2.0 wt%, and S contents are generally <1 wt%.
Cu‑phases
Element mapping of sulfide phases has shown that Cu
replaces pentlandite from the edge inward, forming Cupentlandite, sugakiite and samaniite, locally bornite and
chalcocite and native copper along the edge of the sulfide
13
1079 Page 8 of 21
Contrib Mineral Petrol (2014) 168:1079
Fig.  4 a Sulfide grain under reflected light (sample SE10_19_E). b
Elemental map (individual element maps of Fe, Ni, S and Cu overlain
to reveal color differences) of the grain shown in a. The map reveals a
dendritic rim of native Cu forming around the pentlandite (ptl) grain,
while native Cu also forms in the serpentine groundmass and within
the pentlandite as veins. Darker red is either a fine-grained mixture
between pentlandite + chalcocite (cc) or sugakiite (sug) + samaniite
(sam). c Sulfide grain under reflected light (sample SE10_02_D). d
Elemental map (individual element maps of Fe, Ni, S and Cu overlain
to reveal color differences) of the grain shown in c. The map reveals a
dendritic Cu rim on one side, the presence of samaniite (sam) replacing pentlandite and magnetite exsolution within pentlandite with the
possible presence of pyrrhotite at the contact between pentlandite and
magnetite
in one sample and has low Ni contents (<1.2 wt%) and
no detectable Co or Zn. Native copper generally contains
strongly variable contents of Fe, Ni and S (Fig. 5b), with S
contents up to 10 wt% (Fig. 5a) that may be an analytical
artifact due to the fine-grained mineral structure and small
grain size of the native copper present in these samples.
Cu contents are between 3.9 and 80.6 ppm (Table 3; Fig. 6).
Overall, samples in which no Cu-bearing mineral assemblages could be detected microscopically have Cu contents
<15 ppm. In contrast, samples that contain abundant Cu-bearing sulfides and native copper as determined both by mineralogical observations and by EMP analyses have Cu contents
of 16.8 to 80.6 ppm. In general, the lherzolite samples have
slightly higher Cu contents than harzburgites (Table 3). There
is no distinct relationship between major or trace element
bulk rock composition and Cu content with the exception of
Zn, which shows a linear correlation (r2 = 0.86). Cu and Zn
contents are within the range of partly serpentinized peridotites from other locations, but lower than most serpentinites
from the Logatchev and the Rainbow hydrothermal systems
(Fig. 6a). Compared to a depleted MORB mantle (DMM),
Bulk rock chemistry
We determined bulk rock major and trace element compositions of twelve samples (Table 3; Fig. 6). For many incompatible trace elements, concentrations are at or below detection limits, reflecting their low abundance in the whole rock
peridotites (e.g., most light rare earth elements (LREE) are
below detection limits and are thus not reported in Table 3).
13
Page 9 of 21 1079
Contrib Mineral Petrol (2014) 168:1079
(a)
Discussion
S
20
0
100
60
chalcopyrite
cubanite
sugakiite
60
covellite
chalcocite 40
40
80
bornite
80
samaniite
0
(b)
20
40
60
80
0
10
0
Cu
10
0
20
Fe+Ni
Cu
40
80
20
0
100
60
60
Ni
Mineralogical record of highly reducing conditions
during serpentinization
80
40
0
20
40
60
80
0
Fe
10
0
10
0
20
The presence of native copper in ultramafic sequences has
previously been ascribed to either a magmatic or a metasomatic origin. In the samples from the Santa Elena Ophiolite, the presence of native copper and a diverse sulfide
mineralogy is of particular interest in understanding the
redox as well as the sulfur conditions during the hydrothermal alteration (serpentinization) of these peridotites. Native
copper of primary magmatic origin generally occurs as single crystals or globule inclusions within olivine, magnetite or chromite phenocrysts and is attributed to formation
at very low sulfur fugacities or from sulfur-undersaturated
magmas (Barkov et al. 1998; Zhang et al. 2006). As a secondary origin, many authors have attributed native copper
and Cu–Fe sulfides in serpentinized ultramafic rocks to low
S and O2 fugacities during serpentinization by a strongly
reducing fluid (Eckstrand 1975; Frost 1985; Lorand 1987).
Alternatively, native copper can be related to the addition
of Cu by a hydrothermal fluid, for example in the basement
of a peridotite-hosted hydrothermal field.
In the following, we will first discuss the character and
impact of metasomatizing fluids on the studied peridotite
samples and then discuss two models for the origin of the
native copper and Cu-rich sulfide assemblages in order to
determine the alteration history of the Santa Elena peridotites and to gain insight into the hydrothermal processes
that occur beneath oceanic spreading centers.
Ni
awaruite (Ni2 Fe) awaruite (Ni3 Fe)
Fig.  5 a Composition of the Cu sulfides (includes analyses of
sulfides with Cu contents >1 wt%) in S–Cu–Fe + Ni space. b Fe–Ni–
Cu alloys in the serpentinized peridotites plotted in Cu–Fe–Ni space.
All plotted analyses have S contents of <3 wt%. The analyzed samples plot on a mixing trend between a Fe–Ni end-member that has a
composition similar to awaruite and native Cu
the typically fluid mobile elements Ba (Fig. 6b), K and Pb
show distinct enrichments, with Pb showing up to 100 times
DMM values (Fig. 6c). However, the fluid mobile element
Sr is not enriched in the Santa Elena peridotites compared to
DMM, showing very low concentrations (Fig. 6d). The LREE
are strongly fractionated relative to the HREE (Table 3) as is
expected from depleted upper mantle peridotites (e.g., Salters and Stracke 2004) and did not significantly suffer hightemperature alteration, which typically enriches the LREE
(Boschi et al. 2006b; Paulick et al. 2006).
Serpentinization leads to some of the most reducing conditions in natural systems. During reaction of water with
olivine (Eq. 1a, 1b) and pyroxene (Eq. 2), Fe2+ in the primary mineral phases is oxidized to Fe3+ with water as the
reducing agent forming H2 according to the following set
of reactions (Bach et al. 2006; Beard et al. 2009; Frost et al.
2013; Schwarzenbach et al. 2013a):
Olivine + fluid = Mg-rich serpentine + Fe-rich brucite
(1a)
Mg-rich serpentine + Fe-rich brucite + fluid
= Mg-rich serpentine + Mg-rich brucite
+ magnetite + H2
(1b)
Pyroxene + fluid = serpentine + talc
(2)
In oceanic serpentinites, hydrogen fugacities tend to
decrease with progressive serpentinization as the rock
becomes depleted in olivine, which is typically associated with increased fracturing allowing more seawater to
enter the system, thus, additionally increasing water–rock
13
13
SE10_02
38.01
<l.o.d
0.13
8.24
0.12
44.72
0.14
0.03
<l.o.d
2,070
19.1
13.4
2,680.6
116.9
6.5
45.8
1.64
<l.o.d.
0.33
0.88
2.00
0.07
0.06
<l.o.d.
<l.o.d.
<l.o.d.
<l.o.d.
0.42
1,666
23.4
57.5
2,620.3
101.5
25.1
49.1
1.19
2.04
0.51
0.64
1.66
<l.o.d.
<l.o.d.
0.29
0.08
0.23
0.23
0.21
SE10_06
43.92
0.05
2.65
8.56
0.14
39.92
2.16
0.03
0.02
SE10_05
<l.o.d = below limit of detection
Major elements (in wt%)
43.80
SiO2
0.06
TiO2
3.14
Al2O3
8.92
Fe2O3
MnO
0.14
MgO
38.40
CaO
2.71
0.03
K2O
0.02
P2O5
Trace elements (in ppm)
Ni
1,644
Sc
23.9
V
66.9
Cr
2,500.6
Co
102.7
Cu
32.4
Zn
55.3
Sr
1.04
Y
2.45
Zr
0.35
Sn
<l.o.d.
Ba
1.78
La
<l.o.d.
Ce
0.03
Dy
0.40
Ho
0.09
Er
0.26
Yb
0.31
Pb
0.39
Sample
1818
20.5
33.7
2,474.9
109.3
16.8
47.3
0.21
0.54
<l.o.d.
0.26
1.44
<l.o.d.
<l.o.d.
<l.o.d.
0.02
0.05
0.07
<l.o.d.
42.12
0.01
1.11
8.40
0.13
42.25
1.03
0.03
<l.o.d
SE10_09
1,726
20.9
47.5
2,249.6
105.3
20.1
46.3
0.18
1.48
0.29
1.05
1.46
<l.o.d.
<l.o.d.
0.23
0.06
0.18
0.22
0.32
43.36
0.04
1.99
8.69
0.14
40.80
1.71
0.03
<l.o.d
SE10_12
1,710
22.1
50.6
2,267.1
102.5
80.8
68.7
0.32
1.84
0.29
0.33
1.91
<l.o.d.
<l.o.d.
0.32
0.07
0.20
0.23
17.66
42.43
0.04
2.04
8.48
0.13
39.58
1.86
0.02
0.01
SE10_16
2,673
24.6
9.3
2,424.7
120.4
29.4
46.3
2.07
<l.o.d.
<l.o.d.
0.33
4.69
<l.o.d.
<l.o.d.
<l.o.d.
<l.o.d.
<l.o.d.
<l.o.d.
1.43
<l.o.d.
39.59
<l.o.d
0.29
7.28
0.11
46.91
0.09
0.03
<l.o.d
SE_P1
1,598
21.1
60.2
2,344.5
97.5
26.3
46.9
0.31
2.23
0.54
0.37
1.60
<l.o.d.
0.03
0.35
0.09
0.28
0.30
43.44
0.05
2.50
8.36
0.13
38.78
2.21
0.03
0.01
SE10_19
Table 3 Major (in wt%) and trace element compositions (in ppm) of the Santa Elena peridotites
1.50
2,326
23.1
11.4
3,045.7
123.1
68.2
55.2
1.42
<l.o.d.
<l.o.d.
0.28
5.04
<l.o.d.
<l.o.d.
<l.o.d.
<l.o.d.
<l.o.d.
<l.o.d.
39.63
<l.o.d
0.45
7.62
0.11
46.60
0.04
0.03
<l.o.d
SE_P2
<l.o.d.
2,382
24.2
14.0
3,347.6
126.1
3.9
42.1
1.09
0.09
0.21
0.91
1.88
0.05
0.06
<l.o.d.
<l.o.d.
<l.o.d.
<l.o.d.
42.68
<l.o.d
0.40
8.79
0.11
40.55
0.02
0.03
<l.o.d
SE_P3
0.28
1,833
23.6
35.9
2,958.2
110.4
9.2
51.3
0.83
0.50
0.17
0.31
1.58
<l.o.d.
<l.o.d.
<l.o.d.
0.02
0.06
0.07
42.82
0.02
1.21
8.67
0.14
42.90
0.82
0.03
<l.o.d
SE_P4
0.18
2,017
22.7
37.0
3,121.5
117.1
24.2
49.5
1.43
0.11
<l.o.d.
0.32
1.49
<l.o.d.
<l.o.d.
<l.o.d.
<l.o.d.
<l.o.d.
<l.o.d.
43.05
0.01
0.72
8.89
0.13
40.27
0.05
0.03
0.01
SE_P5
0.16–0.21
6–9
<2.2
<0.5
<1.4
<2.2
3.8–5.4
2.2–2.9
0.06–0.09
0.08–0.11
0.12–0.22
0.18–0.26
0.02–0.04
0.02–0.05
0.02
0.09–0.16
0.01–0.03
0.02–0.07
0.04–0.09
<0.01
<0.01
<0.01
<0.01
<0.01
<0.01
<0.01
<0.01
<0.01
Range of detection limit
1079 Page 10 of 21
Contrib Mineral Petrol (2014) 168:1079
Page 11 of 21 1079
Contrib Mineral Petrol (2014) 168:1079
(a)
(c)
1000
1000
100
100
Cu
10000
Cu
10000
Santa Elena
Logatchev serpentinites
Rainbow serpentinites
Mariana forearc
15° 20’ fracture zone
Atlantis Massif
DMM
10
1
0.1
10
100
1000
Zn
10
1
0.1
0.001
10000
0.1
1
10
100
1000
100
1000
10000
Pb
(d)
(b)
10000
1000
1000
100
100
Cu
10000
Cu
0.01
10
10
1
1
0.1
0.01
0.1
1
Ba
10
100
1000
0.1
0.01
0.1
1
10
Sr
Fig. 6 Bulk rock compositions of the Santa Elena peridotites: Cu
variations shown against a Zn, b Ba, c Pb and d Sr contents, and
compared with ultramafic rocks from other locations. Additional data
are serpentinites from the basement of the Logatchev hydrothermal
field (Augustin et al. 2012), the basement of the Rainbow hydrothermal field (Marques et al. 2007), the Mid-Atlantic Ridge 15°20′N fracture zone (ODP Leg 209) (Paulick et al. 2006; Kodolanyi et al. 2012),
the Atlantis Massif (Delacour et al. 2008c) and the Mariana fore arc
conical seamounts (ODP Leg 125) (Savov et al. 2005; Kodolanyi
et al. 2012). This shows that the Santa Elena peridotites have Cu, Zn
and Ba contents within the range of ultramafic rocks from similar settings and are at the high end of Pb content and low end of Sr content
of such rocks (see text for “Discussion”). DMM (depleted MORB
mantle) after Salters and Stracke (2004)
ratios and oxygen fugacities (Frost 1985; Alt and Shanks
1998; Delacour et al. 2008c; Schwarzenbach et al. 2012).
Oxygen fugacity is effectively fixed until olivine is no
longer in contact with fluid (Frost 1985; Schwarzenbach
et al. 2013a). As the opaque mineralogy is a function of
hydrogen/oxygen and sulfur fugacities (Eckstrand 1975;
Frost 1985; Klein and Bach 2009), entirely serpentinized
peridotites usually preserve an opaque mineral assemblage consisting of high-sulfur assemblages (e.g., pyrite,
vaesite) and magnetite and/or hematite (Delacour et al.
2008b; Schwarzenbach et al. 2012). In contrast, high H2
fugacities typical for the initial stages of serpentinization allow the stability of native metals and Fe–Ni alloys
(Frost 1985; Alt and Shanks 1998). Accordingly, many
peridotite-hosted hydrothermal systems record a typical
redox gradient with highly reducing conditions during
initial serpentinization trending to more oxidizing conditions during late stages of serpentinization (Alt and
Shanks 1998; Delacour et al. 2008b; Schwarzenbach et al.
2012).
In the Santa Elena peridotites, strongly reducing conditions during serpentinization are recorded by the presence of awaruite and rare heazlewoodite. The assemblage
pentlandite + awaruite + magnetite is formed as the result
of the destabilization of pentlandite and is commonly
observed in serpentinized peridotites (Eckstrand 1975; Peretti et al. 1992; Klein and Bach 2009). This desulfurization
of pentlandite can be described by the reaction (Eq. 3, after
Klein and Bach 2009):
Ni4.5 Fe4.5 S8 + 4H2 + 4H2 O = 1.5Ni3 Fe + Fe3 O4 + 8H2 S
(3)
13
1079 Page 12 of 21
S
(b)
SE10_19
SE10_12
SE10_01
Co9S8
100
600°C
0
(a)
Contrib Mineral Petrol (2014) 168:1079
500°C
20
400°C
80
300°C
60
FeS2
NiS2
FeNi
F
eN 2S4
Ni3S4
40
NiS
(Fe,Ni)
(Fe
(F
F Ni)
i))9S8
Ni3S2
80
Msss
Ms
60
Fe7S8
FeS
40
200°C
20
40
Fe9S8
0
0
60
Ni
80
FeNi3±x
0
10
Fe
10
0
20
Ni9S8
Fig.  7 a Mineral stabilities in the ternary diagram Fe–Ni–S with
coexisting pentlandite and awaruite compositions from three different
samples. Phase relations are for 250 °C, redrawn from Craig (1973).
b Pentlandite analyses in the ternary system Fe9S8–Ni9S8–Co9S8,
with temperature-dependent stability fields for the formation of pentlandite (after Kaneda et al. 1986). Pentlandite occupies the widest
range in composition at 500 °C, separating into two stability fields at
temperatures <200 °C
Additionally, the presence of awaruite in almost all of
the samples, even including the entirely serpentinized peridotites, suggests low water–rock ratios. Assuming serpentinization temperatures of <250 °C (as will be discussed in
the next section), stabilization of awaruite requires water–
rock ratios of <1 during water–rock interaction (approximated after calculations by Klein et al. 2009). Such low
water–rock ratios could have maintained prolonged highly
reducing conditions even in almost entirely serpentinized
samples during alteration up to the complete replacement
of the primary minerals by serpentine in some samples.
olivine is strongly serpentinized. This relationship suggests
serpentinization temperatures of <250 °C.
Temperatures can also be estimated from Fe–Ni–S phase
assemblages (Fig. 7a), using, for example, the composition of co-existing pentlandite and awaruite, or Ni contents
in pyrrhotite (Craig 1973; Misra and Fleet 1973). In three
samples, coexisting pentlandite and awaruite suggest temperatures of around 250 °C (Fig. 7a; Craig 1973; Vaughan
and Craig 1997). However, several pentlandite + awaruite
assemblages suggest higher formation temperatures of
300–400 °C (after Craig 1973; Vaughan and Craig 1997).
Similarly, elevated Ni contents (3.3–6.0 wt%) in pyrrhotite detected in a few samples suggest temperatures
≥400 °C, where complete pyrrhotite–millerite monosulfide
solid solution occurs (Craig 1973; Misra and Fleet 1973).
Another temperature proxy is the pentlandite composition in the ternary system Fe9S8–Ni9S8–Co9S8 suggested
by Kaneda et al. (1986) (Fig. 7b). Most of the analyzed
pentlandite plots within the entire field that is stable down
to temperatures of <200 °C. However, pentlandite compositions in one sample preserve formation temperatures
of >300 °C and possibly even >600 °C (Fig. 7b). In summary, these temperature proxies suggest multiple phases of
alteration with temperatures around 200 °C likely reflecting
the main stage of serpentinization and a higher temperature
fluid influx event at 350–400 °C, which we infer to postdate
serpentinization (see below). We infer that even higher temperatures (>400 °C) are associated with primary pentlandite and pyrrhotite still partially preserved in the samples.
Proxies for alteration temperatures
Serpentinization of ultramafic rocks in oceanic settings
generally takes place between 150 and 500 °C. Temperatures are largely controlled by the tectonic setting and the
presence or absence of a magmatic heat source, and may
vary over the time span of serpentinization (e.g., Cannat
et al. 1992; Früh-Green et al. 1996; Agrinier and Cannat
1997; Schwarzenbach et al. 2013b). The upper temperature limit of mineral alteration is controlled by the stability
of olivine and pyroxene. In the presence of a fluid and at
elevated SiO2 activities, pyroxene decomposition is faster
at >350–400 °C than decomposition of olivine, while at
<250 °C pyroxene is more stable and serpentinization of
olivine is faster than breakdown of pyroxene (Martin and
Fyfe 1970; Bach et al. 2004; Frost and Beard 2007). In the
Santa Elena peridotites, pyroxene is almost intact, while
13
Contrib Mineral Petrol (2014) 168:1079
Influence of changing redox conditions and the formation
of native copper
Cu-bearing sulfide mineral assemblages and native copper have been described in several ultramafic sequences,
revealing both similarities and differences to the observations presented in this study. In the Zambales ophiolite
(Philippines), Abrajano and Pasteris (1989) describe a primary magmatic assemblage consisting of pyrrhotite, pentlandite, chalcopyrite, magnetite and a secondary assemblage consisting of chalcocite, cubanite, digenite, bornite
and idaite together with native copper. They attribute the
secondary assemblage to re-equilibration and alteration of
Cu-bearing sulfides as a result of reducing conditions during serpentinization. Similarly, Lorand (1987) suggest that
native copper in lherzolites and harzburgites from the Bay
of Islands Ophiolite (Newfoundland) formed as an alteration product and through breakdown of primary sulfides
due to interaction with strongly reducing fluids during
serpentinization. The peridotites from the Bay of Islands
Ophiolite preserve similar textures to those observed in the
Santa Elena peridotites with native copper or awaruite rims
around pentlandite. However, native copper is sometimes
located within serpentine veinlets and Lorand (1987) did
not report the occurrence of samaniite, sugakiite and submicrometric intergrowths of various sulfide minerals. Cubearing mineral assemblages have also been described in
plagioclase lherzolites from the Horoman peridotite complex (Japan; Kitakaze 2008; Kitakaze et al. 2011), where
samaniite and sugakiite were described in detail for the first
time. In these peridotites, native copper is attributed to a
magmatic origin with formation through crystallization of
an immiscible metallic liquid. The primary copper was subsequently partly oxidized and formed a secondary Cu-rich
mineral assemblage (Ikehata and Hirata 2012).
Most of the sulfides observed in the Santa Elena peridotites are located within serpentine veins and suggest
a secondary formation for the sulfide and metal mineral
assemblages. Only a few pentlandite grains located within
ortho- and clinopyroxene suggest a primary, magmatic
origin, even though several of these sulfide grains were
affected by the metasomatizing fluid while the pyroxene
was partly serpentinized. As discussed above, the most
common association of pentlandite + awaruite + magnetite can be attributed to formation during serpentinization.
As proposed for the Zambales and Bays of Islands ophiolite, highly reducing conditions caused destabilization of
pentlandite and the re-equilibration of the primary sulfide
assemblage (Klein and Bach 2009). The mineralogical
observations and the element distribution maps suggest that
the Cu-bearing mineral assemblages formed as a secondary
feature after pentlandite re-equilibration. Evidence thereof
is that awaruite is partly replaced by Cu–Fe–Ni alloys and
Page 13 of 21 1079
that pentlandite–awaruite intergrowths that are rimmed by
magnetite, which likely formed as a result of desulfurization of pentlandite, are never intergrown with Cu-bearing
phases. Hence, the Cu-bearing phases most likely formed
as fluid conditions changed (chemically and possibly also
to higher temperatures) subsequent to the main stage of serpentinization. This locally overprinted the opaque mineral
assemblages to form the Cu-bearing mineral assemblages.
Evidence that several different fluids interacted with the
peridotites is suggested by the preservation of indicators of
variable alteration temperatures, as described above.
Stability of native copper as a function of oxygen
and sulfur fugacity
To test for the influence of changing temperature and oxygen and sulfur fugacities on the sulfide assemblages, we
calculated equilibrium mineral stability fields in the system
MgO, SiO2, Fe, S, Cu, H and O at 0.5 kbars and temperatures of 200 and 350 °C (Fig. 8). Gibbs free energy minimization with Perple_X (Connolly 2005) used thermodynamic end-member data for silicate, oxide and sulfide
phases from supcrt92 (Johnson et al. 1992 and references
therein). Olivine and orthopyroxene were modeled as
binary Fe–Mg solutions, and the primary bulk rock composition was modeled as a system comprising of 66.6 mol%
olivine and 33.3 mol% orthopyroxene, representing
approximately the composition of the studied harzburgites
(Table 3). Copper was added to form native copper and Cu
sulfides, and Fe was added as a component to permit stability of iron sulfides, native metal, FeO in silicates and Fe2O3
in oxides. Consideration of MgO (rather than Mg) assumes
that all Mg is divalent, unlike the case for Fe. We did not
include Ni and, therefore, pentlandite, due to the lack of
thermodynamic data for Cu-bearing pentlandite, sugakiite
and samaniite, which are the most abundant Cu-bearing
sulfide phases in the studied samples, and the lack of wellconstrained mixing models for Cu in pentlandite. We thus
assume that the stability fields of the Cu-bearing phases in
the calculations can be used to approximate the changing
conditions observed in the samples and that the addition of
Ni will have limited effect on the Fe–Cu assemblages.
As a function of oxygen and sulfur fugacities, native
copper has a relatively large stability field at both 200 and
350 °C (shaded area in Fig. 8a, b). At very reducing conditions and low sulfur fugacities, the stability field of native
copper overlaps with the stability field of native iron and
at fairly oxidizing conditions it partly overlaps with hematite (Fig. 8). This implies that changing oxygen fugacity has a limited effect on the stability of native copper if
sulfur fugacities are low. In contrast, the sulfur fugacity of
the fluid has a significant effect on the Cu-bearing mineral
assemblages. At low fO2, an increase in fS2 results in the
13
1079 Page 14 of 21
Contrib Mineral Petrol (2014) 168:1079
+H2O
+H2
2
Brc Cu Atg Mag
1
Ol Cu Atg Mag
-50
Ol Cu Tcl
Ol Opx Cu
-55
Ol Opx Cu Fe
-60
Brc Cv Atg Hem
Brc Atg
Mag Cc
-25
-20
4
5
8
Ol Tcl
Ccp Po
Ol
Opx Ol Opx
Bn Ccp Po
Po
-15
6
2O
+H S
2
7 +H
10 13
12
9
11
-10
log f S2
3 4 5
6
7 8 9
Ol Cu Tcl
-35
Ol Opx Cu Tcl
Ol Opx Cu
-40
-45
-50
-5
Ol Cu Tcl Atg
-55
-60
1: Ol Atg Mag Bn
2: Ol Atg Mag Ccp
3: Ol Tcl Cc
4: Ol Tcl Bn
Ol Opx Cu Fe
5: Ol Tcl Ccp
6: Ol Tcl Ccp Po
7: Ol Opx Cc
8: Ol Opx Bn
9: Ol Opx Ccp
-25
-20
-15
Ol Opx Ccp Po
-45
3
Ol Atg
Mag Cc 1 2
Ol Cu Atg Mag
+H2O
+H2
Ol Atg
Cc Hem
Ol Opx Bn Po
-40
-30
Brc Atg Cc Hem
Brc Cu
Atg Hem
Ol Cpr Atg Hem
Ol Cu Atg Hem
-25
log f O2
log f O2
-35
350 ˚C
-20
Ol Opx Cv Py
-30
Brc Cpr
Atg Hem
Ol Opx Py Bn
-25
(b)
1: Ol Atg Mag Bn
2: Brc Atg Mag Bn
3: Brc Atg Bn Hem
4: Brc Atg Mag Ccp
5: Brc Atg Ccp Po
6: Brc Py Atg Ccp
7: Brc Py Atg Bn
8: Ol Atg Ccp Po
9: Ol Py Atg Ccp
10: Ol Py Atg Bn
11: Ol Py Tcl Ccp
12: Ol Py Tcl Bn
13: Ol Cv Py Tcl
+H2
+H2 S
-20
Ol Opx Cu Po
(a) 200 ˚C
-10
-5
log f S2
Fig. 8 Stabilities of mineral assemblages in the system MgO,
SiO2, Fe, S, Cu, H and O calculated by Gibbs free energy minimization. Stability fields are calculated at 0.5 kbars and temperatures
of a 200 °C, representing the main stage of serpentinization, and b
350 °C, representing conditions during a higher temperature postserpentinization event. The shaded areas represent the stability field of
native copper. Colored fields represent the volatile phase (H2, H2S or
H2O) in equilibrium with the assemblage
formation of bornite followed by formation of chalcopyrite. At high fO2, an increase in fS2 results in the formation
of chalcocite before forming bornite, chalcopyrite, then
covalite and higher sulfur phases (Fig. 8a, b). Accordingly,
if primary chalcopyrite is present, a decrease in fS2 should
first result in the formation of bornite and/or chalcocite
and then native copper. Importantly, higher temperatures
significantly increase the stability field of native copper to
both higher oxygen and sulfur fugacities. Thus, a fluid with
a given composition could produce chalcopyrite and pyrrhotite at 200 °C while producing native copper at 350 °C
(Fig. 8a, b). Additionally, H2S and H2O are favored at low
temperatures.
In summary, the calculations show that decomposition
of chalcopyrite caused by decreasing fO2 and fS2 results
in the formation of bornite, chalcocite and, at lowest fS2,
native copper. Conversely, they show that Cu-bearing
sulfides can form by combined oxidation and sulfidation
of primary native copper. However, since all of the studied assemblages point to highly reducing conditions, we
exclude the possibility that the Cu-bearing sulfides in the
Santa Elena peridotites formed through late stage coupled oxidation and sulfidation of primary native copper,
as for example suggested for Cu-bearing assemblages in
the Horoman Complex (Ikehata and Hirata 2012). Thus,
we infer that the observed Cu-bearing assemblages may
have formed due to interaction with sulfur-depleted fluids
during serpentinization at <250 °C. This may have caused
primary sulfides to (partly) breakdown, releasing Cu into
the fluid and forming a new mineral assemblage, e.g., Cubearing pentlandite, sugakiite, samaniite, rare bornite and
native copper. Note that none of the samples that contain
native copper preserve any chalcopyrite (chalcopyrite
was only detected in one sample in association with violarite and smythite, which is inferred to have formed as a
result of late, low-temperature weathering), suggesting
that all initial chalcopyrite was replaced during desulfurization. An alternative hypothesis requires incorporation of
Cu into pentlandite and subsequent release of this Cu during pentlandite breakdown. In the next section, we further
test whether decomposition of these primary sulfides due
to highly reducing and sulfur-depleted serpentinizing conditions could have resulted in the formation of the relatively large amounts of native copper observed in the thin
sections.
13
A mass balance test of copper exsolution from primary
sulfides
A mass balance calculation based on element distribution maps of three representative sulfide grains allows us
to evaluate whether breakdown of the Cu component of
either (1) pentlandite or (2) pentlandite + chalcopyrite,
due to thermodynamic instability during highly reducing
Contrib Mineral Petrol (2014) 168:1079
and sulfur-depleted conditions, formed the native copper
observed in the samples. EMP point analyses indicate that
the Cu rims revealed by element mapping are relatively
pure Cu with generally <1 wt% of Fe, Ni and S, while
highly variable Cu contents are present within the outline
of the original pentlandite (e.g., Fig. 4b, d). We estimated
total Cu contents using the area of the Cu rim on the element maps of three different sulfide grains. In model 1,
we compare this value with the amount of Cu present in
the primary pentlandite (area calculated using the outline of the entire pentlandite without the Cu rim)—Cu
contents in pentlandite are generally low and <0.4 wt%
(Puchelt et al. 1996; Luguet et al. 2003; Schwarzenbach
et al. 2012). In model 2, we compare the calculated value
with the amount of Cu present in primary pentlandite and
variable amounts of primary chalcopyrite, which is often
found as an accessory phase in peridotites (Alt et al.
2007; Delacour et al. 2008b; Schwarzenbach et al. 2012).
For both calculations, we use the following assumptions:
(1) The relative amount of native copper formed upon
breakdown of the primary sulfides is equal in two and
three dimensions (i.e., the area of the element maps can
be used as a proxy for the total volume), (2) the area of
the primary sulfides is approximated by the contour of
the mapped sulfide and assumes that no fractures were
present in the primary grain, (3) the area calculated as
native copper contains no traces of any other elements
(e.g., Fe, Ni, S) and (4) all of the Cu originally present
in the primary sulfides is expelled and forms now native
copper. Additionally, we assume for model 1 that the primary pentlandite had a composition of (Fe5.7Ni3.2Cu0.1)
S8, which corresponds to 0.83 wt% Cu (0.44 vol% Cu)
and is assumed to be a maximum value of Cu in pentlandite. For model 2, we assume that the primary sulfide
was comprised of pentlandite and between 2 and 25 vol%
chalcopyrite.
Based on the above-mentioned assumptions, we calculate for model 1 that the fraction of Cu originally present
in the pentlandite was 10–15 times lower than the volume
of native copper now observed in the thin sections. For
model 2, we calculate that the fraction of Cu originally
present in the primary phases was 6–10 times (with 2 %
of the sulfide being chalcopyrite), 3–6 times (with 5 %
ccp) and <2 times (with 25 % ccp) lower than the volume of native copper now observed. Note that chalcopyrite is typically only present in trace amounts in peridotites and is best represented by assuming 2 vol% (or less)
of chalcopyrite. Thus, for both models, the calculations
suggest either that the initial peridotite was unusually Cu
rich or that an external source of additional Cu is necessary to produce the native copper abundance observed
here. However, reconsidering the above-listed assumptions, several additional points need to be considered:
Page 15 of 21 1079
(1) during desulfurization, not all of the Cu is expelled
from the pentlandite, with up to 0.3 wt% of Cu still present in the analyzed pentlandite. This suggests that the
amount of native copper produced should be even smaller
if all Cu originated from sulfides. (2) No chalcopyrite was
found in association with native copper, samaniite or sugakiite. As significant amounts of primary pentlandite are
still preserved in the samples, we would expect at least
trace amounts of primary chalcopyrite to also still be present if native copper formed as a result of breakdown of
primary chalcopyrite. (3) On the other hand, given that
there are also variable amounts of Fe, Ni and S in the Cu
rims (especially along the edges), the above calculation
may lead to an overestimation of the calculated area for
native copper. Assuming that only 50 vol% of the calculated area is pure Cu, decomposition of 90 vol% pentlandite and 10 vol% chalcopyrite could possibly have
produced the observed native copper rims. (4) Cu may
be mobile in the fluid and could have locally been accumulated around sulfide grains. The three calculated areas
would therefore not represent the Cu expelled from just
the adjacent pentlandite, but the amount expelled from
several pentlandite grains. In summary, both points (1)
and (2) strongly support that an external source for Cu
was necessary, while points (3) and (4) relax this assertion
and suggest that native copper could have formed through
breakdown of the primary sulfides if chalcopyrite was
present and/or if Cu was mobile on a thin section scale.
Evidence for an external source for copper
in the serpentinites
The average upper mantle contains little Cu (~25–30 ppm
Cu; Sun 1982; Salters and Stracke 2004) with Cu contents reported for abyssal peridotites generally below 30–
50 ppm (Niu 2004; Savov et al. 2005; Paulick et al. 2006;
Delacour et al. 2008c; Zeng et al. 2012) and between 3
and 40 ppm in orogenic peridotites (Garuti et al. 1984;
Lorand 1989a and references therein). Overall, Cu contents should be lower in harzburgites than in lherzolites as
Cu is incompatible during partial melting (Lorand 1989a).
Cu contents of the Santa Elena peridotites are between 3.9
and 80.6 ppm and are within the range of variably serpentinized peridotites from the Mid-Atlantic Ridge 15°20′N
fracture zone (ODP Leg 209), the Mariana forearc conical seamounts (ODP Leg 125) and the Atlantis Massif
(IODP Leg 304/305) (Fig. 6). The Cu contents of several
harzburgites are similar to those of the lherzolite samples,
while some of the dunites have higher Cu contents than
the harzburgites. Together this suggests that the Cu contents are not simply related to the amount of melt that may
have been extracted and that they may represent secondary
enrichment.
13
1079 Page 16 of 21
Similarities to ultramafic rocks affected by Black Smoker
type fluids
Cu-rich sulfide deposits are typically found in correlation
with heat-driven seawater circulation reacting with crustal to upper mantle rocks, which are both the source of the
heat and the metals that eventually form the ore deposits (e.g., von Damm 1990; German et al. 1993; Rona and
Scott 1993; Fouquet et al. 1996; Candela 2003; German
and von Damm 2003). Volcanogenic massive sulfide ore
deposits (VMS deposits) are generally considered to be
their equivalent on the continent and are characterized by
high Cu and variably elevated Zn, Pb, Ag and Au contents
(e.g., Candela 2003). Active seafloor hydrothermal systems
vent hot (300 to >400 °C), acidic and metal-rich fluids and
are abundant along ocean ridges; well-studied examples
include TAG, Broken Spur and Lucky Strike (German et al.
1993; Langmuir et al. 1997; German and von Damm 2003).
These are hosted by mafic rocks and comprise a complex
system of hydrothermal fluid circulation in the subsurface
that results in the formation of the massif sulfide deposits. Several hydrothermal systems have been discovered
that are hosted by ultramafic rocks, but vent acidic, metalrich fluids at temperatures >350 °C, while the ultramafic
basement is undergoing serpentinization (Fouquet et al.
1997; Charlou et al. 2002; Douville et al. 2002; Allen and
Seyfried 2004; Schmidt et al. 2007). The Rainbow and
Logatchev hydrothermal systems are two examples of
such ultramafic-hosted hydrothermal systems and are both
located along the Mid-Atlantic Ridge (Fouquet et al. 1997;
Petersen et al. 2009). In both systems, it has been suggested that the heat is supplied by gabbroic intrusions in
the footwall of the hydrothermal system, where the hydrothermal fluids leach metals in the mafic rocks (Allen and
Seyfried 2004; Petersen et al. 2009), while the serpentinites
preserve a low-temperature seawater signature (<250 °C).
The systems typically comprise a range from stockwork
serpentinites with disseminated sulfides or sulfides in veinlets to massive Cu sulfides, suggesting that in both hydrothermal systems, Cu is locally added by hydrothermal fluids (leached at high temperatures from mafic sequences)
resulting in a heterogeneous element distribution. The
ultramafic basement of the Logatchev and Rainbow fields
has highly variable Cu contents of 3 ppm to 2.692 wt%
for serpentinites at Logatchev (Augustin et al. 2008) and
<10 ppm to 542 ppm in serpentinites and up to 5,033 ppm
in the stockwork at Rainbow (Marques et al. 2007). In both
cases, Cu/Zn ratios are typically <1 (Marques et al. 2007;
Augustin et al. 2012).
In the Santa Elena peridotites, Cu-rich sulfides and
native copper occur in most samples (see Table 1). Cu contents of the bulk rocks are generally lower in samples with
little Cu detected in thin section (<15 ppm), while samples
13
Contrib Mineral Petrol (2014) 168:1079
with significant amounts of microscopically obvious Cu
sulfides contain up to 81 ppm Cu. Cu/Zn ratios are below
1.2 and average 0.5, similar to the ratios of the Logatchev
and Rainbow serpentinites. Most sulfides and all of the Curich sulfides are located within serpentine and clearly indicate a secondary origin, suggesting as described above that
the Cu-bearing mineral assemblage formed after the main
stage of serpentinization. Even though Cu contents are relatively low compared to the basement rocks from Logatchev
and Rainbow, the peridotites from Santa Elena could represent sections of a similar hydrothermal system, where
Cu-bearing, high-temperature hydrothermal fluids, possibly derived from adjacent mafic lithologies, could have
locally interacted with the peridotites after serpentinization.
Similarly, Marques et al. (2007) describe that localized
magmatic/hydrothermal sulfide mineralization postdate
serpentinization in the basement of the Rainbow hydrothermal system, while the basement rocks preserve typical serpentine mesh and hour glass textures. The studies of
Logatchev and Rainbow infer that the main stage of serpentinization of the ultramafic basement takes place through
interaction with seawater, without extensive sulfide mineralization. Subsequent interaction with high-temperature,
acidic hydrothermal fluids occurs primarily in a localized
upflow zone, forming stockwork and massif sulfide deposits (Marques et al. 2006, 2007; Augustin et al. 2012). Away
from the upflow zone interaction with high-temperature fluids probably only occurs locally, resulting in heterogeneous
metal enrichments. Accordingly, the peridotites from Santa
Elena may have only locally been affected by high-temperature, Cu-bearing hydrothermal fluids, which are likely
responsible for the formation of the higher temperature signatures (350–400 °C) detected in a few samples.
Evidence from trace elements
Evidence for interaction with high-temperature hydrothermal fluids is often preserved in trace element compositions,
e.g., in flat REE patterns or elevated concentrations of fluid
mobile elements (Niu 2004; Boschi et al. 2006a; Paulick
et al. 2006). Serpentinites from mid-ocean ridge environments at which substantial melt was generated characteristically preserve strongly depleted trace element concentrations. As a result of subsequent serpentinization, hydrated
peridotites show relative enrichments in the fluid mobile
elements (Cl, B, Rb, U, Pb, Sb, Sr and Li), even though
concentrations are usually below primitive mantle values
(Niu 2004; Kodolanyi et al. 2012; Deschamps et al. 2013).
The Santa Elena peridotites generally have very low incompatible trace element concentrations, with most of the analyzed REEs at or below the detection limits (Table 3).
However, our data suggest that the LREEs are particularly
depleted compared to the HREEs (Ce/Yb ~0.1), indicating
Contrib Mineral Petrol (2014) 168:1079
a lack of the flat REE pattern (Ce/Yb >1) suggested previously as evidence for interaction of peridotites with substantial amounts of hydrothermal fluids (e.g., Boschi et al.
2006a). The trace element compositions indicate enrichments in Ba, K and Pb (Fig. 6) compared to DMM (after
Salters and Stracke 2004). In contrast, Sr concentrations
are very low compared to most mid-ocean ridge serpentinites (Fig. 6d: e.g., from the Atlantis Massif, the 15°20′N
FZ; Paulick et al. 2006; Delacour et al. 2008c). The low Sr
concentrations support the observations discussed above
that water–rock ratios were low and that seawater introduction was limited.
Analyzed Pb concentrations are particularly high
(Fig. 6c), with the unusual enrichment of Pb most likely
reflecting either the abundance of sulfide phases (in which
Pb, being a chalcophile element, is preferentially incorporated) or resulting from high-temperature hydrothermal
fluids (Staudigel 2003). Furthermore, Ba is also slightly
enriched compared to DMM, especially in two samples
(Fig. 6b). Ba is relatively immobile during serpentinization, but highly mobile during hydrothermal alteration, e.g.,
during massive sulfide mineralization (Niu 2004). Accordingly, high-temperature vent fluids such as at Rainbow,
Broken Spur, TAG and Lucky Strike typically have strongly
elevated Ba contents (von Damm 1990; James et al. 1995;
Douville et al. 2002). Compared with other partly serpentinized peridotites, the Santa Elena samples have Ba contents
similar to those from Logatchev and most samples from the
15°20′N fracture zone (Fig. 6b). Importantly, some serpentinites from the 15°20′N FZ preserve high alteration temperatures (>350 °C; Alt et al. 2007), which are generally
associated with elevated Ba contents (Fig. 6c; Paulick et al.
2006; Kodolanyi et al. 2012). The relatively small amount
of Cu-bearing phases in the Santa Elena peridotites implies
that interaction with high-temperature fluids was significantly less extensive than at Logatchev and Rainbow, or
in silica metasomatized serpentinites from the 15°20′N FZ
or the Atlantis Massif. This possibly also explains the low
REE concentrations. Additionally, substantial alteration of
REE compositions during hydrothermal alteration requires
high (>102) water–rock ratios (Bau 1991). Thus, the impact
of the high-temperature fluid on the bulk rock trace element composition of the Santa Elena peridotites was probably only minor, but the variably elevated Cu, Zn, Ba and
Pb contents record evidence of this limited hydrothermal
activity.
Summary and concluding remarks
The peridotites of the Santa Elena Ophiolite contain mineral assemblages that are characteristic for serpentinization of ultramafic rocks at highly reducing conditions,
Page 17 of 21 1079
temperatures <250 °C and very low water–rock ratios. As
discussed, two models may explain the formation of Cubearing sulfides and native copper: (1) the Cu-bearing
phases formed through breakdown of primary sulfides
due to highly reducing and sulfur-depleted serpentinization conditions at approximately 200–250 °C, or (2) they
formed through introduction of a Cu-bearing, higher temperature fluid (350–400 °C) postdating serpentinization,
similar to processes today occurring at the Logatchev
and Rainbow hydrothermal fields. Model 1 does not well
explain observations in thin sections, as more native copper
is present than even in the most conservative mass balance
calculations performed here. Unless there was significant
chalcopyrite present as a primary sulfide (which all had to
be replaced during desulfurization) or Cu was soluble during desulfurization, it is unlikely that the studied Cu-bearing phases simply formed by breakdown of the primary
sulfides during serpentinization. Furthermore, simple desulfurization of primary sulfides cannot explain the higher
temperature signatures preserved in awaruite–pentlandite
pairs, unless serpentinization started at higher temperatures
(350–400 °C) than predicted by the silicate mineralogy.
However, the calculations performed in this study support
mineralogical observations from other settings (e.g., the
Zambales ophiolite) in which reducing and sulfur-depleted
serpentinizing fluids resulted in breakdown of primary
sulfides, forming bornite, chalcocite and native copper.
Consequently, we favor the second model and suggest
that Cu-bearing sulfides and native copper in the Santa
Elena peridotites were produced primarily through interaction with a high-temperature fluid after serpentinization. Even though Cu contents are significantly lower than
in typical black smoker type systems, they are within the
range of partly serpentinized peridotites from Rainbow and
Logatchev. The similarities to these systems, coupled with
elevated Cu, Zn, Ba and Pb contents, suggest that the Santa
Elena peridotites experienced similar processes to the basement of the Rainbow or Logatchev hydrothermal systems,
but at a less extensive scale. This is consistent with the
low degrees of serpentinization in most samples, the lack
of pronounced Sr enrichments and the fact that low water–
rock ratios (<1) resulted in minor modification of bulk rock
trace element geochemistry (e.g., the LREE) by the fluids.
Accordingly, we suggest that the primary minerals dominate the bulk rock geochemistry.
The Cu in these peridotites could have been sourced
from layered gabbros and diabase dikes today exposed on
the Santa Elena peninsula. No extensive Cu–Fe–Ni sulfide
deposits (such as stockwork or massif sulfide deposits) are
preserved on the Santa Elena peninsula, but the upper sections of a possible hydrothermal system may have been
removed during tectonic emplacement, erosion and/or
weathering. Further evidence that the studied peridotites
13
1079 Page 18 of 21
preserve the lower section of a hydrothermal system is
also given by the unusually low water–rock ratios and
strongly reducing conditions, reflecting strongly limited
seawater introduction. Additionally, direct exposure of the
peridotites to seawater would most likely have resulted in
the formation of carbonate veins, as observed in seawaterexposed peridotites (Bach et al. 2011; Schwarzenbach
et al. 2013b), but which are absent throughout the studied samples. This suggests that the peridotites were not
directly exposed to seawater, but were serpentinized at
depth, presumably in a Cretaceous hydrothermal system
along an oceanic spreading center during the opening of
the Caribbean Sea that separated North and South America
(Pindell et al. 2006). We therefore suggest that the structures and mineral assemblages of the Santa Elena peridotites provide a unique insight into the hydrothermal evolution of peridotite-hosted hydrothermal systems, revealing
the processes that occur within the deeper sections of the
oceanic lithosphere.
Acknowledgments We thank J. Beard for motivating discussions
and J. Snow for providing additional samples. S. Mazza, W. Whalen
and H. Brooks helped with sample preparation and analytical work, L.
Fedele and R. Tracy helped with EMP analyses. The authors acknowledge the valuable cooperation of the Area de Concervacion Guanacaste, especially R. Blanco Segura (Research Program Coordinator)
and M. M. Chavarría (Biodiversity especialities). Field assistance and
participation by P. Madrigal, J. Calvo, M. Loocke and S. Wright was
fundamental for field expeditions. Logistics and intellectual collaboration with P. Denyer (Central American School of Geology, UCR)
was key for this project. We also thank O. Müntener, R. Frost and an
anonymous reviewer for helpful comments that greatly improved the
manuscript. This project was supported by the National Science Foundation award No. EAR-1019327 to Gazel. E.S. and M.C. gratefully
acknowledge support from Virginia Tech Department of Geosciences.
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