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<category>Original
<title>Disseminated sulphides in basalts from the northern Central Indian Ridge: implications on
late-stage hydrothermal activity
<author>Ranadip Banerjee, Dwijesh Ray
R. Banerjee (corresponding author) (e-mail: [email protected])
Geological Oceanography Division, CSIR-National Institute of Oceanography, Dona Paula, Goa
403004, India
D. Ray
PLANEX, Physical Research Laboratory, Ahmedabad 380009, India
Received: 27 August 2014
Accepted: 15 December 2014
Abstract
This study examined the mineralogy and mineral chemistry of disseminated sulphides (mainly
chalcopyrite-pyrite) in partly altered basalts from the northern Central Indian Ridge, Indian
Ocean in order to understand the role of hydrothermal alterations and infer possible sulphide
formation history. Pyrite and chalcopyrite are dominant sulphide minerals and generally
associated with the oxide phases including magnetite and often ilmenite. Close association of
sulphide and oxide minerals suggests that they are paragenetically related. Sulphides also occur
as late impregnated veins cutting through the basaltic hosts. The chemical compositions of pyrite
(avg. Fe 46.3 wt%, S 53.7 wt%) and chalcopyrite (avg. Cu 34.4 wt%, Fe 30.7 wt%, S 34.7 wt%)
are almost uniform, while the secondary ilmenite often shows MnO enrichment (up to 3.0–
3.4 wt%). The associated altered minerals typically resemble the greenschist facies mineral
assemblages—e.g. chlorite±epidote. Evidence of albitisation and silicification suggests lowtemperature hydrothermal alteration processes. This is supported by the bulk Au content (up to
60 ppb) of host-altered basalts with pyrite mineralisation. Au is usually associated with late-stage
pyrites and thus related with low-temperature hydrothermal activity. Close to the dredge location,
tectonic activity around the Vityaz megamullion might have promoted hydrothermal circulation
1
and subsequent alteration of the mineral constituents in basalts, eventually inducing the formation
of late-stage disseminated sulphide minerals in these rocks.
Electronic supplementary material The online version of this article (doi: 10.1007/s00367-...)
contains supplementary material, which is available to authorized users.
<heading1>Introduction
Since the discovery of the first active submarine massive hydrothermal sulphide vent in the
eastern Pacific in 1979, a growing number of new vent sites have been identified mostly on the
fast-spreading East Pacific Rise (EPR) and slow-spreading Mid-Atlantic Ridge (MAR), and even
on the ultraslow-spreading Southwest Indian Ridge (SWIR) and Gakkel Ridge in the Arctic
(Baker and German 2004). These discoveries along mid-ocean ridges have kindled much interest
primarily on account of the high concentrations of base metals (Cu, Zn) and many noble metals
(Au, Ag, Pd, Pt) at these hydrothermal vents (e.g. Hannington et al. 1986; Pasava et al. 2007). In
addition to basalt-hosted hydrothermal sulphide vents, increasing reports of ultramafic-hosted
seafloor hydrothermal vent sulphides have further helped to refine our knowledge on these
intriguing systems—e.g. Rainbow (German et al. 1996), Ashadze (near 13°N; Beltenev et al.
2003), 13°30′N MAR (Beltenev et al. 2007), Nibelzen (8°18′S; Melchert et al. 2008), 14°45′N
Logatchev (Batuev et al. 1994), Saldanha (36°34′N) and Menez Hom (37°8′N; Marques et al.
2007).
Compared to substantial investigations of hydrothermal seafloor mineralization at spreading
centres of the Pacific and Atlantic oceans dating back to the mid-1960s, systematic exploration
for hydrothermal activity in the Indian Ocean ridge system is relatively recent (see Banerjee and
Ray 2003). The GEMINO (Geothermal Metallogenesis in Indian Ocean) program of 1983–1986
(Herzig and Plueger 1988), and subsequent discoveries of the Sonne field and the MESO zone
(named after the two research vessels RV Meteor and RV Sonne), raised enough interest for later
investigations (Plüger et al. 1990; Halbach et al. 1998; Münch et al. 1999). Both these sites were
identified as fossil hydrothermal vent fields. However, later evidence of hydrochemical
signatures typical for hydrothermal plumes was recorded over several segments on the Central
Indian Ridge (CIR), and the discoveries of the Kairei and Edmond hydrothermal fields on the
CIR were the first direct observations of active hydrothermal discharge and vent biota in the
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Indian Ocean (Gamo et al. 2001; Hashimoto et al. 2001; Van Dover et al. 2001; Wang et al.
2014). Signatures of hydrothermal plumes have also been detected recently at two locations over
the Carlsberg Ridge (CR), Indian Ocean (Ray et al. 2012), at the northern side of the present
study area. Other recent discoveries are the Dodo and Solitaire hydrothermal fields along the CIR
at 18–20°S (Nakamura et al. 2012), and the first active hydrothermal vent to be reported from the
Southwest Indian Ridge, one of the closest neighbours of the CIR (Tao et al. 2012). Indeed, the
growing numbers of hydrothermal fields identified along slow- and ultraslow-spreading ridges
like the CIR and SWIR, and their association with variegated magmatic, tectonic and alteration
processes are attracting ever more research today.
Occurrences of hydrothermal activity and sulphide mineralisation are relatively poorly known in
the northern Central Indian Ridge (NCIR). Discoveries of megamullions, especially in the
amagmatic spreading segments of the NCIR, are also important in testing hydrothermal
circulation processes within the slowly spreading oceanic crust. The mineral chemistry of
hydrothermally altered gabbros, and the geochemistry of fresh gabbros recovered adjacent to
megamullions and their possible petrogenetic relations with basalts have been explored by Ray et
al. (2009a, 2011). Ray et al. (2011) found that fresh cumulate gabbros and basalts are not cogenetic. The role of hydrothermal circulation in gabbros has also been suggested for high- and
low-temperature alteration processes, but to date sulphides have not been reported for those
altered gabbros (see Ray et al. 2009a).
Within this context, exploring the mineralogy of altered basalts, associated gabbros and
disseminated sulphides in those basalts unarguably provides an excellent opportunity to assess
the complex interplay between oceanic crust fracturing due to tectonically active megamullions
and subsequent hydrothermal alteration, accounting for the formation of disseminated sulphides
at the NCIR. In a novel attempt, this study examines the mineralogy and mineral chemistry of
disseminated sulphides in a suite of partly altered basalts recovered from the northern segment of
the Vityaz transform fault at the NCIR. Moreover, the possible role of low-temperature, late
hydrothermal activity in the formation of these sulphides is discussed.
<heading1>Geological setting
The NCIR (spreading rate ca. 26–38 mm/year; Kamesh Raju et al. 2012) stretches between 5°–
10°S and is one of the more poorly understood mid-ocean ridge systems worldwide, constituting
3
a prominent accreting plate boundary regime of Australia-Africa to the south and India-Africa to
the north. Geophysical studies have revealed the presence of numerous transform faults, nontransform discontinuities, ridge-transform intersection highs and megamullions along different
segments of the NCIR (Drolia et al. 2003; Drolia and DeMets 2005; Kamesh Raju et al. 2012).
The NCIR is characterised by short-ridge segments and long transform geometry (maximum
displacement at the Vema transform fault is ~243 km). The depth of the seafloor including the
axial valley, transform fault, and outer ridge flank of the NCIR varies from 1,500 to 6,500 m.
The adjacent morphology of the present dredge location is a topographic high earlier known as
the Kurchatov seamount (cf. named by Russian scientists aboard the RV Academic Kurchatov).
Drolia and DeMets (2005) successfully mapped this feature more extensively, and named it the
Vityaz megamullion (Fig. 1). More recently, complete multibeam coverage and structural
analyses revealed that the megamullion represents an inner-corner, prominent elongated domal
high with typical ridge-parallel corrugations (Kamesh Raju et al. 2012).
The megamullion rises from the 4,700 m deep rift axis to a depth of 2,300 m over a distance of
about 9 km, with an inward-facing slope of about 20°. It is considered as an active detachment
fault in a relatively magma-starved condition; however, late-stage volcanism is equally often
important in slow-spreading accretionary processes (cf. Morishita et al. 2009). One scenario
could be that the active detachment fault facilitated extension and thereby induced magmatic
intrusion periodically, despite the location being part of magma-poor ridge segments (Ildefonse et
al. 2007). However, asymmetric spreading of this particular ridge segment probably negates this
scenario (Drolia and DeMets 2005; Kamesh Raju et al. 2012). Numerous abyssal hills, transformparallel lineations and disturbed seafloor fabrics are common at the ridge segments north of the
Vityaz transform fault, which displaces the ridge segment more than 100 km (Drolia et al. 2003).
The half spreading rate of this particular segment is 18 mm/year, typical for slow-spreading crust.
<heading1>Materials and methods
Rock samples were collected from onboard the ORV Sagar Kanya during cruise SK195 in 2003.
The samples (DR13, water depth 2,258 m, 5°27′S, 68°32′E, cf. onboard global positioning
system) were taken by means of chain-bag dredges from the southern side of the Vityaz
megamullion, adjacent to the Vityaz transform fault and close to the flank of the Kurchatov
4
seamount (cf. Drolia and DeMets 2005). The dredge recovered mostly altered basalts (70%) and
gabbro (30%). Assemblages of disseminated sulphide minerals were restricted to altered basalts.
Mineral chemical analyses were performed using a JEOL JXA-8900R electron probe micro
analyser (EPMA) with five wavelength dispersive spectrometers at the Ocean Research Institute,
Tokyo, Japan (15 kV accelerating voltage, 50 nA specimen current, <10 µm beam diameter).
Data were reduced using the online ZAF correction program. Natural standards (Fe in magnetite,
Cu in pure metal, S in pyrite, Ni in pure metal, Cr in Cr2O3) were employed for instrument
calibration. Glass standard NIST 610 runs performed before, during and after the analyses gave
Cr values of 390, 400 and 405 ppm respectively, which is within the error (4%) of the certified
value (415 ppm).
All trace elements, including rare-earth elements (REEs) of host bulk rocks, were analysed by
means of an inductively coupled plasma mass spectrometer (ICP-MS; model ELAN DRC II,
Perkin Elmer Sciex Instruments, USA) at the National Geophysical Research Institute,
Hyderabad, India, following the methods described in Balaram et al. (1999). The accuracy and
precision of measurements on international rock standards JB-2 (basalt) and MRG-1 (gabbro)
were better than ±5%.
Platinum group elements (PGEs), Ag, Au and total S were analysed in altered basalts and
gabbros, using both Ni and Pb fire assays at the United States Geological Survey (USGS),
California, USA, with an ICP-MS finish (Baedecker 1987). For the Ni-sulphide assays, the
sample powder (~1 g) was mixed with soda ash, borax, silica, sulphur and nickel carbonate, and
fused at 1,200 °C. During fusion, Ni, PGEs and Au sulphides were formed. The mixture was
treated with HCl, whereby the Ni sulphide was dissolved and removed. The remainder was
dissolved in aqua regia in a borosilicate test tube and diluted with nitric acid; that solution was
then analysed by ICP-MS. Pt, Pd and Au were also analysed by lead fire assays; here, the
reported Au values are from that procedure, which is more accurate for Au than are Ni fire
assays. The accuracy and precision of the PGE and Au results reported here for the international
reference standard SRM7b were better than 10%.
<heading1>Results
<heading2>Petrography
5
Typically fresh pillow basalts are not commonly found in the collected suite of samples. Instead,
the dark greyish coloured altered basalts, frequently fractured and often with a greenish tint,
dominate the assemblage. Specks of sulphides are unevenly distributed in these altered basalts as
disseminated grains or fine stringers, often identifiable by the naked eye (Fig. 2a, b). Under the
microscope, interlocking texture of plagioclase feldspar and pyroxene is recognised. Extensive
vein networks of chlorite±epidote±quartz, fractured plagioclase and pyroxene grains are also
present. The texture of these altered basalts is often brecciated. At times the altered plagioclase
laths are found to be associated with pyroxene grains. Chlorite/sericite(?) as a replacement
product occurs along the fractures of the plagioclase or as deformed wavy bands or sometimes
showing weak schistosity. Quartz generally occurs as recrystallised veins closely associated with
albite. In backscattered electron (BSE) images, the alteration of plagioclase to albite is distinctly
visible and clinopyroxenes also show evidence of alteration (Fig. 3a, b). Secondary ilmenites are
also found to occur as thin lamellae (Fig. 3b).
Chalcopyrite and pyrite are the most common sulphide minerals found in the studied basalts.
These opaque minerals occur mostly as anhedral, skeletal aggregates (Fig. 4a, b). The pyrites are
often closely associated with large magnetite grains (Fig. 4c). Chalcopyrite and pyrite locally
show close association (Fig. 4d). Sulphides occur also in the form of fine cross-cutting veinlets
(Fig. 4e, f). The cross-cutting relations of the veinlets indicate that they are of late-stage origin,
postdating the surrounding silicates. Large subhedral magnetites are the major oxide phase,
commonly associated with sulphides (Fig. 4c). The BSE images of anhedral, skeletal aggregates
of pyrite and magnetite indicate their close association (Fig. 5a–c). Small anhedral chalcopyrites
show frequent association as a replacement texture within pyrites (Fig. 5d). Chalcopyrites often
occur as isolated anhedral grains (Fig. 5e).
The mineralogy of a few altered gabbro reveals that angular-shaped, fractured and at times
twined plagioclase is their dominant phenocryst. Relict plagioclase grains commonly exhibit
weak undulose extinction and contain tapering twins. Similar textures of altered gabbros have
been found at an adjacent site on the megamullion (Ray et al. 2009a). Typical high-temperature
mineralogical assemblages commonly found in gabbros (e.g. high Ti-bearing brown hornblende,
Ray et al. 2009a) are totally absent in the presently studied gabbros.
<heading2>Mineral chemistry
6
The mineral chemical compositions of apparently fresh and altered mineral phases (plagioclase
and clinopyroxene) as found in the host basalts are provided in Table 1 of the electronic
supplementary material available online for this article. Fresh plagioclase feldspars are almost
uniform in chemical composition (An50Ab50) and frequently transform to pure albite (Ab90). The
clinopyroxene composition of host basalts varies from Wo37-38 En36-37 Fs26, and this is not a very
common phase found in moderately phyric plagioclase basalts from the NCIR. Altered
clinopyroxenes are generally FeO-rich (24–27 wt%) and MgO-poor (7–9 wt%) compared to their
fresh counterparts.
The composition of chalcopyrite is fairly homogeneous (average Cu ~34 wt%, Fe ~31 wt%, S
~35 wt%), while pyrite shows an average composition of Fe ~46 wt% and S ~54 wt% (Table 1).
The mineralogical compositions of these chalcopyrites and pyrites are plotted in a Cu-Fe-S
ternary diagram in Fig. 6a, together with comparative data from the MESO zone, CIR (extinct
hydrothermal field, Münch et al. 1999). Published data from the MAR and EPR (data sources:
EPR: Fouquet et al. 1993; trans-Atlantic geotraverse hydrothermal field (TAG), MAR: Tivey et
al. 1995) are plotted in separate Cu-Fe-S ternary diagrams in Fig. 6b, c. The sulphides from the
NCIR show essentially no data scatter, contrasting strongly with those from the MESO zone
(Fig. 6a). Chimneys from the MESO zone show high contents of base metals, particularly Cu and
Fe (>40 wt%, Münch et al. 1999). More importantly, NCIR sulphides (both chalcopyrite and
pyrite) fall under similar compositional fields, and are comparable to other well known sulphide
fields from the EPR and MAR (Fig. 6b, c, Table 1).
S, Fe, Cu, Ni and Zn Kα X-ray maps further distinguish the elemental distribution patterns,
especially within the chalcopyrite-pyrite-magnetite association (Figs. 7, 8). It is interesting to
note that only chalcopyrite among the chalcopyrite-pyrite assemblage shows some Zn enrichment
(Figs. 7e, 8e). Pyrite grains are uniformly Zn-poor (Fig. 8e). Zn contents within chalcopyrite
grains reach up to 0.13 wt%. Chalcopyrite with close association to magnetite also shows similar
Zn enrichment (Fig. 7e).
As can be seen in Table 2, pyrite occurs mainly as four types: (1) euhedral grains, (2) skeletal
aggregates, these being the most common, (3) associated with chalcopyrite and (4) associated
with magnetite. Pyrite in association with magnetite generally shows an enrichment in Co (avg.
up to 0.98 wt%). At one spot the highest Co content within pyrite is reached at 1.59 wt%. In
addition, the pyrite with magnetite association displays high Fe contents (range 45.21–
7
47.26 wt%). Skeletal amorphous-type pyrite grains have moderate Co contents (avg. up to
0.66 wt%), whereas euhedral pyrites have the lowest Co contents (avg. up to 0.14 wt%).
Chalcopyrite mainly shows bimodal associations: (1) separate anhedral grains and (2) replaced
grains within pyrite. In anhedral chalcopyrites, Cu is 33.12–34.51 wt%, Fe 28.99–30.91 wt% and
S 34.11–35.39 wt%; when occurring as replaced phase within pyrite, chalcopyrites are
characterised by high S (34.68–36.73 wt%) and Fe (30.37–31.45 wt%) but relatively low Cu
(31.92–33.65 wt%). The Zn content in chalcopyrite is generally low (<1 wt%). FeOt of magnetite
varies from 63 to 66 wt% and, interestingly, ilmenites are uniformly MnO-rich (3–3.4 wt%; see
Table 2 in online supplementary material).
<heading2>Whole-rock geochemistry
Bulk Cu contents of altered basalts generally vary up to 201 ppm, and Zn up to 194 ppm.
Compared to normal (N) MORB (Hofmann 1988), the samples of the present study show
enrichment in fluid mobile elements (Rb, Cs) and also in Ba and U (see Fig. 1 in online
supplementary material). Total REE (ΣREE) of altered basalts vary from 52 to 97 ppm, which is
slightly higher compared to N-MORB (ΣREE ~39 ppm, Sun and McDonough 1989). However,
whole-rock chondrite normalised REEs are typically LREE depleted [(La/Sm)N ~0.23 to 0.61,
N=chondrite normalised], whereas HREEs show a pattern [(Gd/Yb)N ~1.30 to 1.37] similar to
depleted MORB.
Bulk Au contents in NCIR rocks (Table 3) are noticeably higher than the crustal average
(1.8 ppb, McLennan 2001), ranging between 2 and 61 ppb. More importantly, the partially
altered basalts show considerably higher Au contents (up to 61 ppb, sample DR13/A) compared
to the associated gabbros (up to 27 ppb). The reported highest Au contents in altered basalts are
up to 30 times higher than the average crustal value (see Table 3).
<heading1>Discussion
The alteration of plagioclase to albite suggests that NCIR basalts have experienced lowtemperature (greenschist facies) alteration processes (Seyfried et al. 1988). Fresh plagioclase
feldspars are almost uniform in composition (An50Ab50) and frequently transform to pure albite
(Ab90) due to hydrothermal alteration or metasomatism (Marks et al. 2010). This is in contrast to
earlier reported plagioclase compositions from fresh CR and NCIR basalts, which are generally
8
more calcic (phenocryst ~An60-90, groundmass ~An35-79; Iyer and Banerjee 1993; Ray et al.
2009b). Secondary silica (cf. silicification) is also found to occur as an accessory phase during
greenschist facies metamorphism (Seyfried et al. 1988). In addition, close associations of albite,
chlorite and actinolite further characterise interactions of the host rock with a fluid having high
Na/Mg, which is typical for greenschist facies alteration (Seyfried et al. 1988). Results of
microprobe analyses show that, during the albitisation process, plagioclase feldspars loose both
FeOt and MgO (see Table 1 in electronic supplementary material). By contrast, alteration of
clinopyroxenes is likely to resemble amphibole, and mostly involves gain of FeOt and Al2O3 and
loss of CaO.
Disseminated pyrite-chalcopyrite associations within the altered basalts of the NCIR could be
primarily linked to fracturing of basaltic host rocks due to tectonic events, followed by migration
of metal-rich hydrothermal fluid and subsequent precipitation. Lack of mineral zonation and the
homogeneous mineral composition of the studied sulphide phases suggest a relatively short
period of transport and subsequent precipitation of mineralising fluid (Hannington and Scott
1988, 1989). The mineral paragenesis of the studied sulphide (chalcopyrite-pyrite) and oxide
(magnetite) phases suggests the following sequence of mineral formation:
magnetite>pyrite>chalcopyrite. The close association amongst sulphide-oxide minerals further
suggests that they are paragenetically related. The occurrences of sulphides as small veins or
veinlets (Fig. 4f) further imply transport of metal-rich fluids through small veinlets, whereas
disseminated sulphides are mostly coeval with the alteration process because few sulphides occur
within late-stage veinlets. MnO-rich (3–3.4 wt%) ilmenite probably refers to a secondary origin
with formation due to interaction with MnO-rich hydrothermal fluid (see Table 2 in online
supplementary material).
In the present study, assemblages of sulphide minerals lack signatures of typical high-temperature
chalcopyrite-type composition and textures that generally form during massive sulphide
mineralisation (Murphy and Meyer 1998). Instead, a late-stage, low-temperature hydrothermal
activity appears more conducive for the formation of these disseminated sulphides. It is therefore
suggested that late-stage hydrothermal fluids, characterised by decreasing temperature (<250 °C),
facilitated precipitation of chalcopyrite and pyrite (cf. Fouquet et al. 2010). The Co contents of a
few pyrite grains often exceed 1.59 wt%, which is relatively high compared to that reported for
sulphide from the MESO zone, CIR (up to 1,700 ppm, Halbach et al. 1998). However, EPMA
9
studies show that Fe of pyrite sometimes replaces Co. According to Hekinian and Fouquet
(1985), partial enrichment of Co can often be attributed to leaching of Fe from Co-bearing pyrite,
thereby increasing the residual Co. The high Co contents of the studied pyrites also suggest that
pyrite might have formed at a late stage (Münch et al. 1999). The absence of Mo and Se in the
sulphide minerals further rules out the possibility of involvement of high-temperature fluid,
which is common for black smoker vent fluids (Petersen et al. 2009; Fouquet et al. 2010).
Enrichment of Zn within chalcopyrite as compared to associated pyrite and/or magnetite suggests
that remobilization occurred at lower temperature (Figs. 7e, 8e; Fouquet et al. 2010).
<heading2>Importance of high bulk Au content
High bulk Au contents of sulphide-bearing host rocks (~60 ppb) suggest that gold levels in Cuand Fe-rich sulphides may be due to inheritance from low-temperature, gold-favourable mineral
association (Hannington and Scott 1988). Hannington and Scott (1988) further explained that the
ability to form bisulphide and bisulphate complexes within the pyrite stability field prompted Au
to co-precipitate at lower temperatures within late-stage pyrite.
Low Th as well as U contents of these basalts suggest their alteration under reducing conditions
at relatively low temperatures (<250 °C), which differs considerably from oxidatively altered
rocks of ODP Leg 504B (Bach et al. 2003) and seawater-altered basalts (Verma 1992). Also
noteworthy are the Au values reaching ppm level in mid-ocean ridge polymetallic sulphides and
sulphide-bearing rocks, mostly associated with late-stage low-temperature (<300 °C) venting
(Hannington et al. 1986, 1991; Harvey-Kelly et al. 1988). Au mineralisation within polymetallic
massive sulphides have been found at mid-ocean ridges, as well as in fore- and back-arc
environments (cf. Herzig and Hannington 2000). High Au contents are mostly well discussed for
several back-arc spreading centres (e.g. the Lau, Manus and North Fiji basins, and the Okinawa
and Mariana troughs). Sulphides with Au are not uncommon from the CIR, and the Au-bearing
pyrites reported from the MESO zone of the southern CIR have attracted much interest (Halbach
et al. 1998). Halbach et al. (1998) further explained that the decomposition of Au complexes
[Au(HS)2-] favours fixation of Au in pyrite and promotes co-precipitation of Au at lower
temperatures together with later-stage pyrite. Thus, the formation of sulphides as fine stringers
and small irregular patches adjacent to many fractures within the altered NCIR basalts is
considered to have resulted from the circulation of metal-rich fluids through fractured oceanic
10
crust, and possibly the disseminated pyrites are the host phase for Au. Further high-resolution
micro-analytical data are needed to test this hypothesis.
<heading2>Geodynamic implications
Recent work has revealed that the nature of the oceanic crust, especially at slow-spreading ridges
like the MAR, is not uniform and therefore crustal accretion and alteration processes play a major
role in temporal and spatial variations in tectonic and magmatic processes (Escartín et al. 2008).
This is reflected in the higher degree of ridge segmentation, the presence of different types of
discontinuities and also the exposure of variegated rock types. The geological settings of the
present study area (adjacent to the Kurchatov seamount and the Vityaz megamullion) have
possibly played a crucial role to induce the alteration process of basalts. The occurrences of
gabbro further suggest that the removal of upper basaltic crust and exposure of gabbro occurred
exclusively due to the effect of active low-angle detachment faulting (Drolia and DeMets 2005).
Adjacent ridge segments of the Vityaz megamullion show evidence of asymmetric spreading and
extensive tectonic extension, and therefore could have facilitated the exhumation process
(Kamesh Raju et al. 2012). This would also favour the alteration of basalts by interaction of lowtemperature hydrothermal fluid, consistent with their petrographic characteristics, viz. albitisation
of plagioclase feldspar, alteration of pyroxenes and formation of hydrothermal sulphides.
Predominance of broken mineral grains, wavy schistosity and brecciated texture in altered basalts
suggest the effects of possible tectonic activity related to the nearby tectonically active zone with
low-angle detachment faults. Local tectonic activity also might have promoted the formation of
fissures and cracks in these rock suits, and subsequently facilitated the transport of metal-rich
fluids at a later stage. Oceanic core complexes with large offsets are typical features of slowspreading crust like the NCIR, and not only can facilitate exhumation of lower crustal rocks but
also could be instrumental for structural control of developing hydrothermal systems. This
scenario accommodates most seafloor hydrothermal sulphide deposits (Ashadze, Logatchev,
Semyenov) associated with uplifted lower crustal rocks at different MAR segments with an
asymmetric mode of accretion (Escartín et al. 2008). Moreover, detachment faults could be
conduits for the circulation of seawater and discharge of hydrothermal fluids (Tivey et al. 2003;
McCaig et al. 2007).
11
<heading1>Conclusions
The findings of this study of disseminated sulphides within altered basalts from an off-axis site
adjacent to the Vityaz megamullion along the NCIR lead to the following conclusions:
1.
Pyrite and chalcopyrite are the predominant sulphide phase, while magnetite and often
ilmenite are the main oxide phases. Pyrite and chalcopyrite are chemically homogenous, whereas
ilmenite often shows MnO enrichment.
2.
The mineralogy of the host samples (partly altered basalts) with alteration assemblage of
chlorite±epidote typically refers to greenschist facies of metamorphism. Evidences of albitisation
and silicification indicate their formation due to low-temperature hydrothermal alteration
processes.
3.
Higher Au contents (as compared to typical oceanic crustal values) in the host basalts
with pyrite mineralisation indicate that Au is very possibly associated with late-stage pyrites and
therefore genetically related with low-temperature hydrothermal activity.
4.
The asymmetrically slowly spreading, less magmatically robust ridge segments of the
NCIR and the presence of the off-axis active low-angle detachment fault (Vityaz megamullion)
are further conducive for low-temperature hydrothermal alteration of the ocean crust and
formation of sulphide minerals.
These new findings suggest that off-axis locations may be equally favourable to host sulphide
minerals, and parallel investigations of the rift valley and off-axis areas are necessary in the
search for new seafloor hydrothermal sulphide sites in and along the NCIR. Occurrences of active
hydrothermal vent sites at the NCIR are as yet unknown. The present findings are the first
dealing with disseminated sulphides in basalts from the northern Central Indian Ridge, and could
help in future searches for new hydrothermal sulphide-bearing zones or possible active vent sites
along this ridge system and adjoining areas.
Acknowledgements
We are grateful to the Director, CSIR-National Institute of Oceanography, India for permission to
publish this work. Ship time was provided by the Ministry of Earth Sciences (MoES, New Delhi).
This research was partly funded by ONR grant no. 0014-97-1-0925 and the CSIR Network
Programme (COR0006). D.R. is grateful to the CSIR, New Delhi for financial support in the
form of a Senior Research Fellowship. R.B. is thankful to INSA, New Delhi and JSPS, Tokyo for
12
a fellowship to visit ORI, Tokyo for analytical work. We appreciate the help and support of the
captain, crew members and all other colleagues during sampling operations onboard the ORV
Sagar Kanya, cruise SK195. J.R. Hein is especially thanked for his aid in analyses of noble
metals including Au at the USGS, Menlo Park, California. Valuable comments from S. Misra,
two anonymous reviewers and the editors are gratefully acknowledged. This is NIO contribution
no. ##.
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Fig. 1 Location map showing a the Indian Ocean Ridge systems and b the dredge location
(DR13) near the Vityaz transform fault, northern Central Indian Ridge. CR Carlsberg Ridge, CIR
Central Indian Ridge, RTJ Rodriguez Triple Junction, SWIR Southwest Indian Ridge, SEIR
Southeast Indian Ridge
Fig. 2 Unevenly distributed disseminated sulphide grains in two hand specimens of altered basalt
from the NCIR
Fig. 3 a, b BSE images showing the alteration of plagioclase (Plag) to albite (Alb) and pyroxene
(Px) to altered pyroxenes (Alt Px). Anhedral chalcopyrite (Cpy), magnetites (Mgt) and ilmenite
(Ilm) are also seen as associated phases
Fig. 4 a, b Anhedral skeletal aggregates of chalcopyrite (Cpy) and pyrite (Py). c Pyrite closely
associated with large subhedral magnetite (Mgt) grains. Magnetite is the only associated oxide
phase. d Closely associated chalcopyrite and pyrite. e, f Fine veinlets of pyrite
Fig. 5 a–c BSE images of pyrite (Py) occurring as euhedral cubic-shaped or as skeletal
amorphous aggregates. Magnetite (Mgt) is the common associated mineral. d Small anhedral
18
chalcopyrites (Cpy) associated as a replacement texture within pyrites. e Chalcopyrite occurring
as isolated anhedral grains. f Sulphide minerals as veinlet within a silicate matrix
Fig. 6 a Cu-Fe-S ternary diagram of chalcopyrite (Cpy) and pyrite (Py) in northern CIR rocks
(NCIR), compared with sulphides from the MESO zone, Central Indian Ridge (extinct
hydrothermal field; Münch et al. 1999). b, c Corresponding data for chalcopyrite and pyrite in
sulphides from the East Pacific Rise (EPR, Fouquet et al. 1993) and the trans-Atlantic
geotraverse hydrothermal field, Mid-Atlantic Ridge (TAG-MAR, Tivey et al. 1995)
Fig. 7 a–c BSE and S+Fe distribution images with evidence of chalcopyrite (Cpy) replacing a
portion of a large pyrite (Py) grain (upper right-hand corner). d, e Corresponding images of the
replacement feature at variable light intensities for Cu (d) and Zn (e). Cu shows higher light
intensity only at the corner of the chalcopyrite grain (red versus orange, d). Only chalcopyrite
shows some Zn enrichment (as detected by EPMA, up to 0.09 wt%); pyrite is uniformly Zn-poor
Fig. 8 BSE and elemental distribution images (S, Cu, Fe, Zn) revealing chalcopyrite (Cpy) in
close association with magnetite (Mgt). Again, chalcopyrite shows Zn enrichment (as detected by
EPMA, up to 0.13 wt%)
Table 1 Comparison of mineral chemical compositions of presently studied sulphide minerals
with those reported for other mid-oceanic ridge sites by Fouquet et al. (1993), Tivey et al. (1995),
Halbach et al. (1998), and Münch et al. (1999). Cpy Chalcopyrite, Py pyrite
End members (wt%)
Cu
Fe
Zn
S
Sulphide
minerals
Present study (slow spreading), Northern Central Indian Ridge (NCIR)
34.37
30.67
n.d.
34.69
Cpy
n.d.
46.3
n.d.
53.69
Py
Central Indian Ridge (intermediate spreading), Indian Ocean
MESO zone
SONNE field
36.80
23.44
n.d
39.76
Cpy
3.07
38.59
n.d.
58.34
Py
31.36
20.3
n.d.
48.1
Cpy
3.76
39.62
n.d.
56.62
Py
32.6
4.02
32.4
Py
Other slow-spreading settings
Galapagos
4.48
19
Snake pit
12.42
35.47
7
30.88
Py+Cpy
TAG
33.52
30.50
n.d.
34.80
Cpy
0.32
46.55
n.d.
53.34
Py
S Explorer Axial
3.23
25.78
4.85
28.3
Py
Seamount
0.4
4.95
18.31
18.8
Py
S Juan de Fuca
0.16
19.79
36.72
39.27
Py
East Pacific Rise, 13°N
7.83
25.95
8.17
35.12
Py
East Pacific Rise, 11°N
1.92
22.39
28
35.7
Py
East Pacific Rise, 21°N
0.58
12.44
19.76
31.41
Py
East Pacific Rise, 17°26′S
1.25
36.25
5.55
41
Py
East Pacific Rise 21°50′S
2.39
28.59
21.74
40.74
Py
Fast-spreading settings
Table 2 Chemical composition (weight%, range and average) of pyrite and chalcopyrite mineral
grains of various morphologies from the NCIR (n sample size)
Pyrite
Element Euhedral
(cubic) (n=4)
S
Chalcopyrite
Skeletal
With magnetite With
(n=5)
(n=17)
Isolated
chalcopyrite anhedral
(n=12)
(n=23)
53.71–54.17
51.76–54.61
52.95–
34.11–35.39
34.68–
(52.91)
(53.91)
(53.61)
54.65
(34.73)
36.73
(35.34)
44.85–45.29
45.59–46.02
45.21–47.26
44.94–
28.99–30.91
30.37–
(45.01)
(45.87)
(46.16)
46.58
(30.06)
31.45
(45.96)
Cu
(n=4)
52.21–53.47
(53.95)
Fe
With pyrite
n.a.
n.a.
n.a.
n.a.
(30.87)
33.12–34.51
31.92–
(33.81)
33.65
(32.78)
Zn
n.a.
n.a.
n.a.
n.a.
Up to 0.14
Up to 0.09
(0.03)
Co
0.06–0.14
0.09–0.66
0.08–0.98
0.08–1.59
0.03–0.07
0.05–0.07
20
(0.09)
(0.66)
(0.22)
(0.28)
(0.05)
(0.06)
Table 3 Contents of Au and other precious metals in NCIR basalts, compared with average
MORB values from Fryer and Greenhough (1992)a and Rehkamper et al. (1999)b, and crustal
values from McLennan (2001)c
Element
Au
Ru
Rh
Pd
(ppb)
(ppb)
(ppb) (ppb)
Re
Os
Ir
Pt
Ag
S
(ppb
(ppb)
(ppb)
(ppb)
(pp
(wt%)
)
Detection
m)
1
1
1
1
1
3
0.1
1
0.3
0.01
DR13/18
2
<1
1
<1
<1
<3
<0.1
<1
<0.3 <0.01
DR13/A′
61
<1
<1
<1
<1
<3
<0.1
<1
<0.3 0.07
DR13/6
44
<1
<1
<1
<1
<3
<0.1
<1
<0.3 0.01
DR13/B′
27
<1
<1
<1
<1
<3
<0.1
<1
<0.3 0.08
DR13
31
<1
1
<1
<1
<3
<0.1
4
<0.3 <0.01
NCIR
6–27
3–5
1–2
1–6
<1
4–5
2.4–
4–7
<0.3 0.01–
limit /
sample no.
gabbros
Average Leg
3.6
3.2
0.22
0.31
8.1
n.a.
-
MORBa,b
0.69
0.03
n.a.
0.473
n.a.
Upper
1.8
n.a.
n.a
0.5
0.4
0.11
0.05
7.3
n.a.
n.a.
≤0.013 0.01
0.277
n.a.
n.a.
0.05
n.a.
0.05 n.a.
115 MORBa
0.02
continental
crustc
21