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Ore Geology Reviews
Fluid inclusion characteristics of the Uti gold deposit,
Hutti-Maski greenstone belt, southern India
Biswajit Mishraa,*, Nabarun Palb, Amit Basu Sarbadhikaria
aDepartment of Geology and Geophysics, Indian Institute of Technology, Kharagpur 721302, IndiabReliance Industries Ltd., E & P (Petroleum) DAKC, Petroleum House, Koparkhairane, Mumbai 400709, India
Received 15 April 2003; accepted 23 October 2004
Available online 18 January 2005
Abstract
The Uti gold deposit occurs within amphibolites of the late Archaean Hutti-Maski greenstone belt of southern India. Gold
mineralization is associated with intense silicification, biotite-actinolite- and sulfide-rich alteration zones in between small
anastomozing shear zones. Characteristic biotite-K-feldspar alteration and arsenopyrite thermometry constrain the mineralizing
event to above 400 8C, which is concordant with evidence of dynamic recrystallization in mineralized quartz veins. Shearing,
concomitant alteration and Au-mineralization took place at post-peak metamorphic conditions. A post-mineralization, sub-
greenschist facies assemblage of prehnite/pumpellyite and hydrogarnet later imprinted the rocks. Fluid inclusion micro-
thermometric studies on mineralized and barren quartz veins reveal similar fluid chemistry, P–T and, surprisingly, a complete
absence of carbonic inclusions. Fluid inclusion textures and the shape of Th histograms point towards moderate reequilibration
during a phase of isothermal decompression. The microthermometric data, coupled with stability relations of the hydrous Ca–Al
silicates, indicate post-mineralization isobaric cooling followed by a near-isothermal decompression. The observed
temperature–salinity variation is schematically explained by simple cooling of a low salinity heated meteoric fluid, followed
by isothermal mixing with a high salinity granitic fluid. However, fluid mixing was later than, and unrelated to mineralization.
On the other hand, formation of auriferous sulfide-bearing quartz veins at Uti was essentially due to remobilization and
attendant fluid–rock interaction involving suitable host rocks that contained primary gold.
D 2005 Elsevier B.V. All rights reserved.
Keywords: Gold mineralization; Fluid inclusions; Hydrothermal alteration; Ca–Al silicates; Hutti-Maski
1. Introduction
The late Archaean Hutti-Maski schist belt, situated
in the Raichur District of Karnataka, southern India,
0169-1368/$ - see front matter D 2005 Elsevier B.V. All rights reserved.
doi:10.1016/j.oregeorev.2004.10.002
* Corresponding author.
E-mail address: [email protected] (B. Mishra).
hosts a number of small gold deposits along with Hutti,
the largest, currently operational gold mine in India.
The structural setting, mineralization and fluid compo-
sitions of these Au-occurrences exhibit many similar-
ities to typical greenstone-hosted mesothermal lode-
gold deposits, or dorogenic goldT deposits (Naganna,
1987; Pal and Mishra, 2002). The Uti gold mine is one
26 (2005) 1–16
100
DOLERITE DYKE
Au BEARING LODE
PEGMATITE ACID VOLCANIC ROCK
QMS
GRANITOID
INDEXCGA
FGA
GA LODE NUMBER
FOLIATION
FAULT
0 200 400 m
N
Calcutta •
Mumbai
• Hyderabad
•
HMSB
Bay of Bengal
Ara
bian
Sea
200 Km
V V V V V V
V
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- - - - - - - - - - - - - - - -
--------------------------------
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- - - -
--------
- - - -- - - - -
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+ + + ++ + +
+
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+
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75°85
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5
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HuttiWandali
Maski
76°40 /
70°E
16°N
14°N
12°N
10°N
80°E 90°E
76°50 /
16°0 /
16°10 /
16°20 /
NGanjali
1D
1C
1A
1
5A
5B
6B
6A
7
89
PP
FF
1B
1A
Buddini
Yelagatti
Hira-Buddini
Uti
Kavit
10 km
alPeninsular Gneissic ComplexYounger GranitoidsQuartz-gold-sulfide lodesHutti Group of rocks
Bangalore
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2
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Fig. 1. Location and simplified geological maps of the Hutti-Maski greenstone belt and the geological map of Uti (modified from the original
map prepared by the Geological Survey of India and the Hutti Gold Mines Ltd.).
B. Mishra et al. / Ore Geology Reviews 26 (2005) 1–162
B. Mishra et al. / Ore Geology Reviews 26 (2005) 1–16 3
of these smaller occurrences and lies near the northern
tip of the schist belt, about 20 km NE of Hutti (Fig. 1).
Although, the mineralization at Hutti has drawn the
attention of several workers (Anantha Iyer and Vasu-
dev, 1979; Safonov et al., 1980; Biswas et al., 1985;
Naganna, 1987; Saha and Venkatesh, 2001; Pal and
Mishra, 2002; Pandalai et al., 2003; Kolb et al., 2004),
very little effort has been made to evaluate the geology
and gold mineralization in the Uti area.
Late Archaean orogenic gold deposits generally
display many common characteristics (e.g., Kerrich,
1989; Groves et al., 1998) and CO2-rich fluid
inclusions are invariable found associated with them
(e.g., Robert and Kelly, 1987; Guha et al., 1991;
Groves, 1993, Mavrogenes et al., 1995; Mikucki,
1998, Dugdale and Hagemann, 2001). The association
of CO2-bearing fluid inclusions in gold-bearing quartz
(Fcarbonate) veins is so strong that some authors
proposed the compositional characteristics of fluid
inclusions as an exploration tool to distinguish
auriferous and barren quartz veins (e.g., Klemd et
al., 1993; Mavrogenes et al., 1995; Klemd and Ott,
1997). Preliminary observation of fluid inclusions
from Hutti, Hira-Buddini and Wandali mines in the
Hutti-Maski schist belt also confirmed the presence of
carbonic inclusions in the auriferous veins in these
deposits (Mandal, 1999; Sarbadhikari, 2001; Pal and
Mishra, 2002). In contrast, the complete absence of
carbonic inclusions from the quartz veins from Uti
(Sarbadhikari, 2001) poses some interesting questions
on the conviction of a direct association of gold-
bearing quartz veins and carbonic inclusions. A
detailed investigation of fluid inclusions from the
different quartz veins in Uti was therefore carried out.
From the results obtained, an attempt has been made
to constrain the P–T of gold mineralization and
possible fluid evolutionary path. In addition, since
no description of this deposit is available in the
literature, a short account of mineralization and
hydrothermal alteration patterns are also included.
2. Geological setting
2.1. Hutti-Maski schist belt
The Hutti-Maski schist belt, which hosts the Uti
gold mine, is a volcanic-dominated late Archaean
greenstone belt in the Eastern Dharwar Craton of
southern India. It occurs as a hook-shaped 100 km
long belt with a general N–S trend within the
surrounding Dharwar Batholith (Chadwick et al.,
2000). Pillow-bearing basaltic rocks dominate and
are followed in abundance by acid to intermediate
volcanics and quartz/rhyolite porphyry. The inner
margin of the schist belt is made up of metavolcanics
with intercalations of silicic material, represented by
quartzite, quartz-sericite and biotite schist. A narrow
band of polymict conglomerates, containing grano-
dioritic clasts, interbedded with greywackes in the
northeastern part of the belt, indicates that the schist
belt overlay an older gneissic complex (Naganna,
1987). Younger intrusive granitoids, namely the
Kavital and Yelagatti, surround the schist belt, and
are mainly exposed in the northern and northwestern
parts (Fig. 1). Most of the greenstones are metamor-
phosed under upper greenschist to amphibolite facies
conditions (Biswas, 1990; Mishra and Pal, 2001; Pal,
2003) though there is a lack of detailed work. The
regional structural pattern, according to Roy (1979,
1991), can be summarized as three successive phases
of folding, where upright large-scale folds (F2)
trending NW–SE were superimposed on the F1isoclinal folds (NNW–SSE trending antiforms and
synforms). The F2 folds have given the hook-shaped
pattern of the schist belt. The F3 folds are recognized
by the presence of puckering, minor folding and local
fracture slips parallel to the F2 axial plane. The schist
belt has also been affected by a number of late faults
and fracture planes. Granodiorite clasts in the con-
glomerate yield a SHRIMP weighted mean207Pb/206Pb zircon age of 2576F12 Ma (Vasudev et
al., 2000), indicating that the development of the basin
in the Hutti belt began very late in the Archean history
of this part of the Dharwar Craton.
Naganna (1987), Curtis and Radhakrishna (1995)
and Saha and Venkatesh (2001) worked on various
aspects of gold mineralization at the Hutti Mine. The
mineralization is hosted by laminated (fault-fill)
quartz-veins and associated alteration halos along
steeply dipping shear zones within the amphibolites.
Fluid inclusion studies suggest that low salinity (3.9 to
13.5 wt.% NaCl equiv.) H2O–CO2 rich fluid were
responsible for gold-rich laminated quartz vein
formation in the Hutti deposit (Pal and Mishra,
2002). According to these authors, an original H2O–
Fig. 2. (a) Development of shear foliation within fine-grained
amphibolites and (b) detached quartz veins within amphibolite, eas
B. Mishra et al. / Ore Geology Reviews 26 (2005) 1–164
CO2–CH4–(+NaCl) fluid split into aqueous (H2O–
NaCl) and carbonic (CO2FCH4) fluids. Gold was
dominantly transported as bisulfide complexes. The
P–T, estimated from intersecting isochores of co-
existing and coeval carbonic and aqueous inclusions,
furnish mineralization conditions in the range of 1.0 to
1.7 kbar and 280 to 320 8C, on the retrograde
metamorphic path (Pal and Mishra, 2002; Pal, 2003).
2.2. Uti deposit
The Uti block is located in the northern part of the
Hutti-Maski schist belt. The area consists of coarse-
and fine-grained amphibolites, metamorphosed felsic
volcanics and garnetiferous amphibolite as the major
lithologies, and subordinate amounts of quartz mica
schists (Fig. 1). Dolerite dykes, granite, pegmatite and
quartz veins intrude these rocks. The gold lodes are
mostly confined to the metabasic units. According to
Biswas (1990), the coarse-grained amphibolite is a
metamorphosed, pre-tectonic, sill-like intrusive of
meta-gabbro within the fine-grained amphibolite.
Pillow structures are recognized in the fine-grained
amphibolites, but due to intense deformation, it is
impossible to deduce the direction of younging.
The Uti deposit is closely associated with small
NNE–SSW trending shear zones, which are best
observed in fine-grained amphibolites and the meta-
sedimentary patches. Because of their small size, these
shear zones are not mappable. However, they are
easily recognized in outcrop, by development of shear
foliation (with sinistral sense, Fig. 2a) and sheared-off
fragments of quartz veins (Fig. 2b). These highly
deformed zones extend from 50 to 700 m along strike
with varying width of 2 to 10 m. Although the
mineralized shear zones are discontinuous in nature,
they more or less maintain parallelism amongst
themselves. This shear foliation representing D1
deformation event in Uti has a steep dip (z808) andshallow lineations on the foliation plane. A second
shear-zone foliation can also be recognized here
having very similar orientations (making only a small
angle with the previous) but showing very steep or
near vertical lineations. This represents D2 deforma-
tion and the sense of shear is reverse. Broad open
folds recognizable by gentle warping of the foliation
and the dolerite dyke represents the F3 folding event
(D3). The overall orientation of this F3 axial plane is
t
NE–SW. The local tectonic trend of the area is cut
across by small E–W trending post-tectonic faults, a
mappable one is shown near lodes 6 and 7 (Fig. 1).
Geological Survey of India (GSI) and Hutti Gold
Mines Ltd. (HGML) have delineated 17 mineralized
zones (lodes) over a strike length of about 3.5 km,
with individual strike lengths of about 50 to 700 m
and widths from 0.3 to 24 m (Curtis and Radhak-
rishna, 1995). Amongst these, only lode 4 is currently
productive; lode 2 and lode 3 were abandoned in the
recent past; all the other lodes carry evidence of
ancient workings. Two shafts were sunk on lode 3 to a
depth of 60 m and limited drive development was
done on two levels (30 and 60 m vertical depth),
which revealed mineralization over a strike length of
72 m with an average width of 2.8 m and an average
grade of 2.15 g/t Au. On the other hand, out of total
700 m strike length, 300 m from the northern end
(Fig. 1) along the strike length of lode 4 is found
suitable for opencast mining up to a depth of 60 m. A
total reserve of 6.51 Mt of ore is estimated up to 60 m
of lode 9. The pencil tip and hammer point due north.
B. Mishra et al. / Ore Geology Reviews 26 (2005) 1–16 5
depth at an average grade of 2.64 g/t Au (Sangurmath,
2001). Daily production is about 400 tonnes with an
average grade of 2.5 g/t Au.
3. Methodology
All the samples used in this study were collected
from the open pits. Electron probe microanalysis of
selected minerals (both silicates and sulfides) from
polished-thin sections from the outer and inner
alteration zones from the Uti deposit were carried
out using a wavelength-dispersive JEOL Super probe
(JXA-8900R) at the Institut fur Mineralogie und
Lagerst7ttenlehre, RWTH Aachen, Germany.
Microthermometric data have been obtained in this
study from vein quartz samples, from both from the
mineralized and unmineralized parts of the Uti shear
zone. For each sample, microthermometric analyses
were carried out on more than one small chip broken
from the double-polished wafers. Heating-freezing
runs were conducted on a Fluid Inc. adapted USGS
gas flow heating-freezing system at the Indian
Institute of Technology, Kharagpur. A detailed
description of the instrument and microthermometric
procedures is given in Mishra and Panigrahi (1999).
Reduction of microthermometric data was carried out
with the FLINCOR program (Brown, 1989), using the
equation of state of Zhang and Frantz (1987) for the
H2O–NaCl system.
4. Host rock and gold mineralization
The unaltered metamorphic assemblage of the host
amphibolites constitutes hornblende (calcic amphib-
oles of composition varying from ferro-hornblende to
ferro-tschermakite), plagioclase (oligoclase to ande-
sine), quartz and ilmenite. Detailed petrographic and
phase petrologic studies were carried out in the
vicinity of lode 4, over which HGML operates the
open cast mine in Uti. Here, the mineralized portion is
entirely confined within coarse-grained amphibolite.
A region of variable thickness (few centimeters to a
few meters) adjacent to the gold lode constitutes the
outer alteration zone. The transformation from unal-
tered rocks to the outer alteration zone is marked by
the presence of grunerite in addition to the Ca-
amphiboles, skeletal ilmenite, biotite and a large
increase in the proportion of quartz. The quartz- and
plagioclase-rich groundmass in these rocks occasion-
ally shows static recrystallization. Clusters of biotite
grew at the expanse of metamorphic hornblende and
generally occur together with skeletal ilmenite.
Ilmenite is sometimes partially replaced by pyrite.
Microprobe analytical data of pertinent silicate mine-
rals are furnished in Table 1.
Intense silicification, presence of thin quartz veins
(ranging in thickness from a few millimeters to a few
centimeters), large increase of sulfides and presence of
gold characterize the lode zone or the inner alteration
zone. Other than sulfides, these inner zones mostly
contain quartz, K-feldspar, plagioclase, biotite, actino-
lite and titanite. Within the unsheared lenses, biotite
and actinolite occur as rosettes (Fig. 3a), possibly
implying their formation under open hydrothermal
conditions.
Ore microscopic study reveals that sulfides, where
pyrite is the most dominant phase, occur as patchy
disseminations within the altered amphibolite, with-
out any preferred orientation. Other ore minerals
present, listed in the order of decreasing abundance,
are arsenopyrite, pyrrhotite, lfllingite, chalcopyrite
and sphalerite. The textural relationship amongst
lfllingite, arsenopyrite and pyrrhotite is particularly
interesting as their mutual disposition indicates the
possible timing of mineralization with respect to
metamorphism (Barnicoat et al., 1991). In Uti, all
three minerals occur together and in there lfllingiteis always associated with arsenopyrite and is
invariably rimmed by the later phase (Fig. 3b).
Pyrrhotite is nowhere observed in contact with
lfllingite. This specific texture is suggestive of
development during retrograde metamorphism where
lfllingite reacts with pyrrhotite to produce arseno-
pyrite rims on lfllingite and a more sulfur-deficient
pyrrhotite (Neumayr et al., 1993). Rapid equilibra-
tion during cooling has obliterated peak metamorphic
textures and retrograde textures as the one described
here are commonly observed. Experimental work by
Tomkins and Mavrogenes (2001) has shown that
textural observation, combined with analyses of gold
distribution, can help to constrain the timing of gold
mineralization relative to metamorphism. Microprobe
analyses of pertinent sulfide phases from Uti (Table
2) reveal that both arsenopyrite and pyrrhotite
Table 1
Selected EPMA data of biotite, amphibole, pumpellyite, prehnite and hydrogarnet from Uti
Biotite Amphibole Pumpellyite Prehnite Hydrogarnet
U15-1 U15-2 U56-1 U15-1 U77-1 U02/1 U56-1 U56-2 U15-1 U56-1 U15-1 U56-1
SiO2 33.87 32.39 34.85 43.99 45.51 51.38 34.86 34.88 42.32 41.22 34.57 35.33
TiO2 2.58 2.40 1.26 0.35 0.71 0.00 0.16 0.06 0.19 0.23 0.43 0.49
Al2O3 16.63 16.69 18.61 11.50 8.51 0.49 18.25 16.89 23.14 23.61 6.69 9.25
Fe2O3 16.08 18.47 3.62 4.57 20.03 17.06
FeO 22.51 23.74 23.28 19.08 22.22 34.02
MnO 0.07 0.05 0.09 0.27 0.25 0.52 0.01 0.02 0.01 0.02 0.05 0.03
MgO 9.29 10.41 7.69 8.06 7.60 9.75 1.71 1.51 0.37 1.04 0.05 0.39
CaO 0.17 0.16 0.06 11.83 10.99 0.62 21.70 21.79 25.98 24.21 34.02 33.54
Na2O 0.13 0.09 0.10 1.03 1.28 0.07 0.00 0.03 0.01 0.04
K2O 8.36 5.68 9.07 0.50 0.25 0.00 0.02 0.01 0.02 0.21
H2Ocalc 4.34 4.85
Total 93.61 91.61 95.01 96.61 97.32 96.85 92.79 93.66 100.00 100.00 95.84 96.09
Si 5.35 5.19 5.42 6.69 6.96 8.00 5.84 5.84 5.88 5.76 2.81 2.85
Ti 0.31 0.29 0.15 0.04 0.08 0.00 0.02 0.01 0.02 0.02 0.03 0.03
Al(IV) 2.65 2.81 2.58 1.31 1.04 0.00 2.12 2.24
Al(VI) 0.44 0.35 0.83 0.75 0.49 0.09 1.67 1.65
Altot 3.09 3.16 3.41 2.06 1.53 0.09 3.61 3.34 3.79 3.89 0.64 0.88
Fe3+ 0.08 0.00 0.00 2.02 2.33 0.38 0.48 1.33 1.09
Fe2+ 2.97 3.18 3.03 2.34 2.84 4.43 0.03 0.06
Mn 0.01 0.01 0.01 0.03 0.03 0.07 0.00 0.00 0.00 0.00 0.00 0.00
Mg 2.19 2.49 1.78 1.83 1.73 2.26 0.43 0.38 0.08 0.22 0.01 0.05
Ca 0.03 0.03 0.01 1.93 1.80 0.10 3.90 3.91 3.87 3.63 2.96 2.89
Na 0.04 0.03 0.03 0.30 0.38 0.02
K 1.68 1.16 1.80 0.10 0.05 0.00
OHtot 0.16 0.16 0.19 0.15
Total 15.67 15.54 15.64 15.40 15.40 14.97 15.98 15.97 17.81 17.89 8.00 8.00
Ferro-Hbl Ferro-Hbl Grn 6.31 5.13 Hgrt
67.93 56.10 And
0.18 1.56 Py
0.96 1.90 Alm
0.10 0.06 Sp
24.51 35.25 Gr
Hbl: hornblende, Grn: grunerite, Hgrt: hydrogarnet, And: andradite, Py: pyrope, Alm: almandine, Sp: spessartine, Gr: grossular. Formulae for
pumpellyite and prehnite are calculated on basis of 24.5 and 22 oxygen atoms, respectively. All iron assumed to be Fe3+ and water content is
calculated by the difference from 100%. Hydrogarnet formula is calculated on the basis of five cations. For garnet end member calculation, only
hydrogrossular component was considered, without any hydroandradite.
B. Mishra et al. / Ore Geology Reviews 26 (2005) 1–166
sometimes contain invisible gold. Such a situation
arises when gold and sulfides were introduced under
post-peak metamorphic conditions (Tomkins and
Mavrogenes, 2001). Arsenopyrite thermometry is
undertaken in the fS2-buffered assemblage arseno-
pyrite+lfllingite+pyrrhotite (Fig. 3b). Several arsen-
opyrite grains from two different samples, having
suitable textures, were analyzed by the electron
microprobe and the temperatures calculated follow-
ing phase relation in the Fe–As–S system (Kretsch-
mar and Scott, 1976). Though the calculated
temperature and fS2 vary from 357 to 466 8C and
�8.66 to �11 log units, respectively (Table 2), most
of them cluster between 420 and 460 8C with the
average being 426 8C. However, these temperatures
may perhaps be considered as minimum estimates,
considering the retrograde formation of arsenopyrite.
It was observed that the biotites in both the inner
and outer alteration zones are partially replaced by
secondary Ca–Al silicates like prehnite, pumpellyite
and hydrogarnet. Prehnite always occurs as elongate
lenses in biotite, parallel to the cleavage together with
micron-sized hydrogarnet (Fig. 3c). Fe-rich pumpelly-
ite also occurs similarly within the biotites from the
Act
Act
Plag
50 µm
a
Pr
Hgrt
Bt
50 m
Pr
Bt
Pr
c
Pm
Kfs
Hgrt
Bt
30 m d
Po Lo
Asp
100 mµ b
Fig. 3. Photomicrograph (a) and back-scattered electron images (b–d) of pertinent silicate and sulfide assemblages from the Uti region. (a)
Actonolite (Act) rosettes from the inner alteration zone (crossed polars); (b) arsenopyrite (Asp)–lfllingite (Lo)–pyrrhotite (Po) association wherelfllingite is rimmed by arsenopyrite; (c) Thin lenses of prehnite (Pr) and small hydrogarnet (Hgrt) flakes hosted in unaltered biotite (Bt) along
the biotite cleavage. (d) Lens-shaped pumpellyite (Pm) within biotite from the garnetiferous amphibolites.
B. Mishra et al. / Ore Geology Reviews 26 (2005) 1–16 7
garnetiferous amphibolites (Fig. 3d). Lattice defects in
the biotite structure causes incipient leaching of K+
ions and concomitant attraction of H+ ions onto the
negatively charged biotite cleavage, that induces
crystallization of Ca–Al silicates, carbonates or K-
feldspar (Boles and Johnson, 1983; Nijland et al.,
Table 2
EPMA data of selected sulfides and arsenides from Uti
Sample no. U6-1 U6-2 U6-3 U6-4 U6-5 U6-6 U6
Po Po Asp Lo Asp Asp As
S 39.62 38.55 18.94 2.75 18.76 18.74 1
As 0.04 0.09 47.06 68.73 47.36 48.06 4
Fe 59.73 59.45 32.97 27.73 32.85 32.88 3
Co 0.09 0.07 0.07 0.10 0.07 0.05
Ni 0.00 0.00 0.00 0.00 0.00 0.00
Cu 0.03 0.00 0.00 0.00 0.05 0.09
Ag 0.00 0.04 0.01 0.00 0.00 0.00
Au 0.00 0.01 0.00 0.00 0.00 0.00
Total 99.51 98.20 99.04 99.32 99.08 99.82 9
At.% As in Asp 34.70 34.97 35.31 3
T (8C) 391 411 438 42
log fS2 �10.99 �10.2 �9.5 �Temperature and log fS2 are calculated from the buffered assemblage of l
method of Kretschmar and Scott (1976).
1994). Crystallization of lens shaped pumpellyite and
K-feldspar selectively within biotite flakes (Fig. 3d)
thus appear to be the result of dcatalytic effectT withinbiotite during late stage fluid activity. The alteration
event that resulted in the formation of these phases
was pervasive in the entire Uti region and occurred at
-7 U7-1 U7-2 U7-3 U7-4 U7-5 U7-6 U7-7
p Lo Lo Py Py Asp Asp Lo
8.81 3.02 2.80 54.25 53.66 18.67 18.35 2.77
7.99 69.55 70.15 0.03 0.15 47.80 48.15 68.46
3.04 28.01 27.87 46.75 46.63 33.04 32.67 27.70
0.07 0.04 0.07 0.05 0.03 0.06 0.11 0.08
0.01 0.00 0.00 0.00 0.00 0.00 0.04 0.00
0.00 0.02 0.00 0.00 0.00 0.00 0.00 0.00
0.00 0.00 0.00 0.04 0.00 0.09 0.09 0.01
0.05 0.01 0.00 0.08 0.07 0.00 0.00 0.00
9.97 100.6 100.9 101.2 100.5 99.66 99.42 99.02
5.19 35.17 35.63
8 427 464
9.77 �9.73 �8.66
fllingite (Lo)+arsenopyrite (Asp) and pyrrhotite (Po), following the
B. Mishra et al. / Ore Geology Reviews 26 (2005) 1–168
considerably lower P–T conditions compared to those
which resulted in gold mineralization. This aspect is
further discussed in a later section on fluid evolution.
5. Fluid inclusion studies
5.1. Sample description and fluid inclusion
petrography
Doubly polished wafers suitable for fluid inclusion
studies were prepared from quartz veins and veinlets
present both within the mineralized coarse-grained
200 µm
20 µm
12 µm
a
c d
e
Fig. 4. Photomicrographs of mineralized quartz vein and different inclu
recrystallized grains surrounding porphyroclasts in mineralized quartz v
aqueous biphase (type-Ia) inclusions, (c) type-Ib aqueous inclusions, (d)
inclusions and (f) hook-shaped inclusion (left) surrounded by smaller inclu
See text for discussion.
amphibolites (lode 4) and from the garnetiferous
amphibolites (and metasedimentary intercalations) in
the east, away from the mineralized zones (Fig. 1).
The quartz within these veins shows variable degrees
of undulose and patchy extinction and evidence of
dynamic recrystallization, especially along the grain
boundaries of large porphyroclasts. The presence of
sutured grain boundaries with recrystallized bulges
and sub-grain rotation exhibiting core-mantle struc-
ture (Fig. 4a) suggest dominance of the bulging
recrystallization mechanism (Drury et al., 1985).
These microtextures generally indicate dynamic
recrystallization of quartz under greenschist facies
20 µm
15 µm
14 µmb
f
sion types from Uti. (a) Core-mantle structure illustrated by small
ein. Note the strain-wavy extinction in quartz grains; (b) primary
primary polyphase inclusion, (e) trail-bound secondary polyphase
sions. All photographs were taken at ambient laboratory temperature.
B. Mishra et al. / Ore Geology Reviews 26 (2005) 1–16 9
conditions (400 to 500 8C) by localized grain
boundary migration, which affects only the boundary
region of the porphyroclasts (Drury and Urai, 1990;
Stipp et al., 2002a,b; Piazolo et al., 2002). The veins
and veinlets from mineralized and barren rock types
show similar microtextures and thus must have been
deformed under identical P–T regimes. Out of 40
wafers on which petrographic studies were conducted,
9 were chosen for microthermometric experiments.
This choice was primarily based on the clarity under
microscope, relatively larger size and number of the
inclusions contained, and representation from both
mineralized and unmineralized rock types. In the
present study, inclusions randomly distributed in three
dimensions or aligned parallel to crystal faces (or
grain boundaries) in host quartz are regarded as
primary and those which are confined to a trail-bound
healed fracture planes that terminate internally at grain
boundaries are considered pseudo-secondary. Inclu-
sion dtrailsT occurring as planar/curvi-planar groups
and askew to crystal boundary are secondary. The
different types of inclusions, present in each of the
wafers studied, are as follows:
(1) Type-Ia: These are aqueous biphase primary
inclusions (Fig. 4b), and by far the dominant of
all types of inclusions, being abundant in most of
the samples. The inclusion size varies from 2 to
20 Am. These inclusions are of various shapes
and sizes with a clear rounded vapor bubble
floating in the liquid at room temperature (25
8C). There is, however, a distinct sub-type of theaqueous biphase inclusions present in Uti, which
can be classified as type-1b. These occur in large
numbers and in distinctive clusters in particular
planes, which is not confined, to any single large
grain (Fig. 4c). At room temperature, these
inclusions contain no vapor bubbles and appear
to be monophase, but on extreme cooling
beyond �100 8C, followed by slow warming
the vapor bubble appears. These inclusions are
generally small (2 to 4 Am) and circular to
elliptical in shape. However, within these small
regular inclusions, a few large (8 to 12 Am) ones
are also present, which are invariably irregular in
shape and show necking effect.
(2) Type-II: These are aqueous polyphase inclusions
with an aqueous liquid, a rounded vapor bubble
and a halite crystal. The halite crystals show
characteristic cubic outline. Size of the crystals
and bubbles vary in the range 2.5 to 6 Am (in
terms of the largest edge) and 2.5 to 5 Am(diameter), respectively. These occur as primary
(Fig. 4d) as well as secondary (Fig. 4e)
inclusions.
5.2. Microthermometry
The temperature of final ice melting (Tm) of type-
1a inclusions from samples from the lode zone varies
from �19.5 8C to 0 8C, with a relative clustering of
data between �4 8C to 0 8C. These inclusions
always homogenize in liquid state and the temper-
ature of liquid–vapor homogenization (Th) varies
from 96 to 397 8C with a broad clustering between
180 and 280 8C; the mean and median of the
frequency distribution being 225.5 and 226.8 8C,respectively. The inclusions are generally of low
salinity (0 to 22.01 wt.% NaCl equiv.), with a mean
salinity value of 5.8 wt.% NaCl equiv. The density
varies from 0.6 to 1.07 g cm�3 with the average
being 0.874 g cm�3. It is important to note that type-
1a aqueous inclusions from quartz veins in unmin-
eralized rocks show very similar microthermometric
data. Th values of these inclusions vary from 98 to
350 8C with a clustering between 180 and 220 8C,and the mean and median being 210 and 205 8C,respectively. The salinity and density vary from
0.497 to 17.86 wt.% NaCl equiv. and from 0.632 to
1.065 g cm�3, with the averages being 3.34 wt.%
NaCl and 0.888 g cm�3, respectively. Hence, it is
very clear that there is no difference in gross fluid
chemistry and Th of primary aqueous biphase (type-
Ia) inclusions present in the mineralized and the
unmineralized zones. Attempts to see sample-wise
variation in pertinent microthermometric data, in
both the broad sample types, i.e., mineralized and
barren, did not reveal any conclusive indication
pertaining to relatively less or more deformed
(reequilibrated) nature of the host quartz. On the
other hand, if we assemble all the data, the overall
shape and the distribution pattern of the Th and
salinity histograms remains same as the earlier ones.
In the combined Th histogram (Fig. 5a), although the
range of data is 300 (96 to 397 8C), there is a strong
clustering of Th values between 180 and 280 8C
0
10
20
30
40
50
60
70 n =276
50 100 150 200 250 300 350 400
2
4
6
8
10
12 n= 23
Ts,NaCl(oC)
100 150 200 250 300 350 400
10
20
30
40
50 n= 77
Ts,NaCl(oC) secondary
80 120 160 200 240 280 320 360
2
4
6
8
10
n= 23
Th(oC)
80 120 160 200 240 280 320 360 400 0 2 4 6 8 10 12 14 16 18 20 22 240
10
20
30
40
50
60n =276
Th(all data)
a b
c d
e f
50 100 150 200 250 300 350 400
5
10
15
20
25
30
35 n= 77
Th (oC) secondary
Fre
quen
cyF
requ
ency
Fre
quen
cy
Fre
quen
cyF
requ
ency
Fre
quen
cy
Salinity (Wt% NaCl equiv.)
primary
primary
Fig. 5. (a, b) Th and salinity histograms of aqueous biphase (type-Ia) inclusions from quartz veins in both mineralized and unmineralized
regions. (c, d) Ts,NaCl histogram from primary and secondary halite-bearing polyphase inclusions respectively. (e, f) Th histograms of the
polyphase inclusions.
B. Mishra et al. / Ore Geology Reviews 26 (2005) 1–1610
where the mean and median Th are 220.2 and 215.15
8C, respectively. Salinity varies from 0 to 22 wt.%
NaCl equiv. (Fig. 5b), while more than 75% of the
data are below 8 with the average being 5.64 wt.%
NaCl equiv.
The shape and the statistical parameters of the
homogenization temperature frequency histogram of
the biphase aqueous inclusions are reviewed in detail
to see if we can extract more information on the P–
T of fluid evolution. The Th histogram is unimodal
and skewed to higher temperatures, has a low
standard deviation but a high range and the mean,
median and mode are all nearly the same. These
features are characteristic of fluid inclusion reequili-
bration resulting from plastic deformation (see Fig.
4f) under conditions of high internal overpressure
(Vityk and Bodnar, 1998). Inclusions are subjected
to high internal overpressures, if for example
trapping is followed by uplift of the rocks at nearly
isothermal conditions. Such reequilibration is gen-
erally accompanied by a decrease in density of the
original fluid. Isochores representing the minimum
Th represents conditions close to the initial entrap-
ment, whereas those from the maximum Th con-
B. Mishra et al. / Ore Geology Reviews 26 (2005) 1–16 11
ditions represent the final reequilibration conditions.
Isochores drawn from the mean and median Th of
the histogram represent the internal overpressure that
can be maintained over geologic time by the
majority of reequilibrated inclusions (see Vityk and
Bodnar, 1998).
Type-1b inclusions, which are present in almost all
the samples, showed explosion textures, which are
unmistakable evidence of high internal overpressure.
Frequently large, hook like or annular inclusions (Fig.
4f) and large leaked out inclusions occurring within
planes of tiny inclusions, termed as decrepitation
clusters, are observed (Fig. 4c). These textures are
similar to those described by Sterner and Bodnar
(1989) and Vityk and Bodnar (1995) for inclusions
reequilibrated under conditions of isothermal decom-
pression. These inclusions are extremely metastable,
which hindered systematic microthermometry in
them. Nevertheless, it was observed that they have a
Th range of 80 to 130 8C with majority of data
clustering around 100 to 110 8C. This implies that
these inclusions were trapped at high P–T conditions,
pertaining to higher initial densities. During subse-
quent uplift, the majority of these inclusions could not
survive the high internal overpressure and have thus
leaked or burst generating the striking flower shaped
inclusions (Fig. 4c). Accordingly, these inclusions are
not considered for further interpretation of micro-
thermometric data.
The characteristics of the type-II halite-bearing
polyphase inclusions, which occur as primary as well
as trail-bound secondary inclusions in both sample
types, are identical to type-Ia inclusions. Hence, data
from mineralized and unmineralized regions are
clubbed together and discussed as a single group.
Primary and secondary polyphase inclusions show
variations in halite dissolution temperature (Ts,NaCl)
in the range of 91 to 364 8C and 135 to 386 8C,respectively. Nineteen out of 23 values of Ts,NaCl of
primary saline inclusions are within the range of 200
to 350 8C (Fig. 5c), which is in contrast to a
prominent peak of 50 inclusions (out of 77) between
200 and 250 8C (Fig. 5d) for the Ts,NaCl of secondary
halite-bearing inclusions. There is no discernible
clustering of liquid–vapor homogenization temper-
atures (Th) of primary type-II inclusions in their
distribution within 96 to 322 8C (Fig. 5e). On the
contrary, a prominent clustering for Th of secondary
polyphase inclusions, for which 37 out of 77 data
lies in the range of 150 to 200 8C (Fig. 5f) within
their distribution of 123 to 378 8C. Ranges in Ts,NaCland Th of primary and secondary halite-bearing
inclusions on the first look do not show any dif-
ference. But for both the variables the higher values
are apparently for secondary inclusions (Ts,NaCl=386
8C, Th=378 8C). Such near-similarity in both the
variables (Th and Ts,NaCl) of the primary and
secondary inclusions indicates crack formation (and
healing), very soon after the formation of quartz
veins.
5.3. Thermobarometry
For the application of pressure correction to the
recorded homogenization temperatures, estimation of
entrapment pressure is necessary. Roedder and Bodnar
(1980) and Shepherd et al. (1985) outlined four
principal methods of pressure estimation. These are:
(i) vapor pressure of the fluid at Th, (ii) fluid isochores
used in conjunction with independent geothermom-
eters, (iii) intersecting fluid isochores for coeval and
cogenetic fluids, and (iv) dissolution of daughter
minerals (halite), especially in the case where
Ts,NaClNTh. The minimum pressure that a liquid can
sustain is equal to its vapor pressure at any given
temperature and composition. Our fluid inclusion
study did not reveal any evidence of boiling. Hence,
the first method can not be adopted. In the absence of
any independent geothermometers for the quartz
veins, the second method also can not be utilized.
Again in the absence of CO2 inclusions in the Uti
samples, P–T estimation procedure is confined to the
dissolution of the halite-bearing inclusions, where
Ts,NaClNTh, following the procedure of Bodnar (1994).
For the purpose of isochore construction, the gross
chemistry of aqueous inclusions are modeled in the
H2O–NaCl system. The halite liquidi at different
halite dissolution temperatures are considered pres-
sure independent. This is a reasonable approximation
at temperatures below 500 8C (cf. Gunter et al., 1983).
A few P–T intersections of isochores of halite-bearing
inclusions and the corresponding halite liquidi at the
temperature of halite dissolution from mineralized
quartz veins, as shown in Fig. 6 are 0.93 kbar/251 8C(I1), 2.14 kbar/249 8C (I2), 2.24 kbar/218 8C (I3) and
2.56 kbar/305 8C (I4). It must be noted that the
100 150 200 250 300 350 400
1.146
1.197
1.187
1.192
I2I3
I1
I1 = 0.93 kb/251oCI2 = 2.14 kb/249oCI3 = 2.24 kb/218oCI4 = 2.56 kb/305
oC I4
1
2
3
4I = 0.93 kbar/251 C1
O
I = 2.14 kbar/249 C2O
I = 2.24 kbar/218 C3O
I = 2.56 kbar/305 C4O
Temperature (°C)
Pre
ssu
re (
kbar
)
Fig. 6. Estimation of P–T by the method of halite dissolution.
Intersections of inclusion isochors with the corresponding halite
liquidi at the temperature of halite dissolution for four primary
inclusions are shown.
B. Mishra et al. / Ore Geology Reviews 26 (2005) 1–1612
pressure obtained by this method pertains to minimum
trapping conditions.
6. Fluid evolution and discussion
The fluid evolution diagram for Uti represented by
the Th–salinity plot (Fig. 7a) shows a two-stage
evolution. The first stage relates to simple cooling
from little above 400 8C of a low saline (c3.0 wt.%
NaCl equiv.) fluid (Fl1), followed by near-isothermal
(c200 8C) mixing with a high saline fluid (Fl2). Such
a picture emerges out of 276 number of primary
0 5 10 15 20 25 30 35 40 45
50
100
150
200
250
300
350
400
450
500
T h(o C
)
Salinity (wt% NaCl equiv.)
T(o C
)
Fl1
Fl 2
a
Fig. 7. (a) Th–salinity plot. Primary aqueous biphase and polyphase i
(Ts,NaClNTh: apex up and Ts,NaClbTh: apex down), respectively. Fl1 and FI2curve. (b) Th–Tm plot after Dubessy et al. (2002). See text for discussion
aqueous biphase (type-IA) inclusions. The remaining
23 constitute polyphase inclusions that surround the
halite saturation curve (Fig. 7a). While the low saline,
high temperature fluid could perhaps be of heated
meteoric water parentage, a granite-derived fluid
qualifies for the high saline component in Fig. 7a.
Dubessy et al. (2002) have recently provided theoret-
ical Th–Tm diagrams for different kinds of fluid
mixing, by carrying out numerical modeling of
mixing in the H2O–NaCl system. Careful observation
of the pattern generated from the Th–Tm diagram (Fig.
7b) reveals a spread of data points parallel to the
compositional axis (Tm), which is an indication of
isothermal mixing of two different fluids of contrast-
ing salinities. On the other hand, the granitic fluid
(Fl2) must have evolved from a temperature in excess
of 305 8C, as obtained from the highest temperature
by halite liquidus thermobarometry (Fig. 6). The
diagram (Fig. 7b) also indicates pressure variations
during trapping, such that the normal fluid regime was
lithostatic and fluid mixing was associated with
decrease in fluid pressure (Dubessy et al., 2002).
Having said this, it is obligatory to state here that such
fluid evolution model is schematic to a large extent,
primarily because of the reequilibrated nature of the
inclusions involved.
As mentioned before, alteration minerals at Uti are
partially replaced by secondary hydrous Ca–Al
silicates. The fluid phase responsible for stability of
secondary Ca–Al silicates must have been relatively
low or devoid in CO2, so as to form silicates rather
-24 -20 -16 -12 -8 -4 0
50
100
150
200
250
300
350
400
450
500
h
Tm(oC)
b
nclusions are represented by unfilled squares and filled triangles
stand for fluids 1 and 2, respectively. HSC stands for halite saturation
.
B. Mishra et al. / Ore Geology Reviews 26 (2005) 1–16 13
than carbonates (Robinson and Bevins, 1999). By
calculations in the CMASH–CO2 system, Digel and
Ghent (1994) have shown that the upper limit of the
stable prehnite+pumpellyi te assemblage is
XCO2c0.002 at 1.5 kbar and 250 8C. Hydrogarnetsgenerally form within a temperature range of 300 to
400 8C, being mostly independent of pressure.
According to Liou et al. (1983), the minimum
temperature of hydrogarnet formation is 340 8C,which decreases slightly with increasing fO2. Prehnite
in metabasites generally have an upper P–T stability
of 400 8C and 3 kbar. Pumpellyite, on the other hand,
is stable from 1 to 8 kbar and between 150 and 350 8C(Liou et al., 1985). The P–T stability of prehnite-
pumpellyite facies have been specified at 200 to 300
8C and 2 to 3 kbar by Frey et al. (1991) by the
calculation of average activity values in the
NCMASH system. The pertinent portion pertaining
to the stability of prehnite-pumpellyite assemblage is
reproduced in Fig. 8. Two representative isochores
covering the median Th values of the type-Ia
inclusions are also drawn, whose intersection with
the average temperature range (420 to 460 8C),obtained from arsenopyrite thermometry, furnishes
the P–T box pertaining to mineralization and alter-
100 200 300 400 500 600
8
7
6
5
4
3
2
1
Temperature (°C)
Pre
ssu
re(k
bar
) 0.901
1.065
Hgrt
Pm
Pr
Mineralization +inner alteration
Pm+Pr
Fig. 8. P–T diagram showing the overall fluid evolution path at Uti.
The isochors marked with densities of 0.901 and 1.065 g cm�3 are
for the representative inclusions which show median Th in Fig. 5a.
The vertical shaded area encompasses the maximum stability limits
of hydrogarnet (Hgrt). The triangles represent the corresponding
intersection points (I1 through I4) in Fig. 6. See text for discussion.
ation (Fig. 8). The above temperature range also
compares reasonably well with the same inferred from
the deformation microtextures in the mineralized
quartz veins, as was mentioned before. Then the two
near-isobaric cooling paths are drawn to explain
stabilization of either prehnite or pumpellyite, along
with hydrogarnet.
The formation of the Ca–Al silicates within the
biotites is thus definitely a consequence of fluid-
driven retrograde reactions at temperatures b400 8Cand pressures b3 kbar. In the coarse-grained amphib-
olites, the assemblage prehnite+hydrogarnet is com-
mon, whereas the garnetiferous amphibolites show the
assemblage pumpellyite+hydrogarnet. For the former
assemblage, a temperature range of 350 to 400 8C is
necessary, while a little lower temperature (300 to 350
8C) is sufficient for the latter. The final phase of fluidevolution was an isothermal decompression event, as
evident from the fluid inclusion textures, observed in
type-Ib inclusions.
7. Summary
Gold mineralization in Uti is structurally controlled
along small shear zones and is characterized by
intense silicification together with the presence of
biotite, K-feldspar, actinolite, titanite, pyrite, arsen-
opyrite, pyrrhotite, lfllingite, chalcopyrite and spha-
lerite. Mineralization was post-peak metamorphic in
nature. The rocks underwent a post-mineralization
initial isobaric cooling leading to stabilization of a
sub-greenschist facies alteration assemblage of pum-
pellyite/prehnite (+hydrogarnet), followed by an iso-
thermal decompression event. Stabilization of phases
like prehnite and pumpellyite requires an H2O-rich
fluid with very low CO2 (XCO2b0.002) (Digel and
Ghent, 1994), which also points towards the fact that
the later fluid activity in Uti was of extremely low
CO2 content. Such an alteration event was followed
by near-isothermal decompression, the last in the fluid
evolutionary history of Uti.
Several authors (e.g., Mikucki, 1998; Dugdale and
Hagemann, 2001) have stressed that the ore fluid
composition in different greenstone-hosted Au-depos-
its (from sub-greenschist to upper-amphibolite facies
metamorphic conditions) is generally neutral to
weakly alkaline, of low salinity (V3 wt.% NaCl
B. Mishra et al. / Ore Geology Reviews 26 (2005) 1–1614
equiv.) and carbonic, with XCO2=0.05 to 0.25. In this
context, the complete absence of carbonic inclusions
at Uti is somewhat enigmatic and all the more
surprising, as CO2-bearing inclusions are indeed
present in other deposits in the same Hutti-Maski
schist belt (Pal and Mishra, 2002). A plausible
explanation can be inferred from the nature of the
auriferous lodes and the location of Uti; i.e., the
proximity to the Yellagatti and Kavital granitoids (Fig.
1). The lodes in Uti are localized in small shear zones
whose formation post-dated peak-metamorphism and
possibly synchronous with granite emplacement.
Since the shear zones are post-metamorphic, the
(aqueous-carbonic) metamorphogenic fluid, although
it might have been released by metamorphic devolati-
lization reactions, was lost because of lack of suitable
structures necessary for fluid channelling and focus-
ing. As mentioned before, there is no difference in the
fluid inclusion characteristics of the mineralized and
barren quartz veins. Hence, mineralization and char-
acteristic hydrothermal alteration were both primarily
a product of remobilization due to the interaction of
heated meteoric water (?) with appropriate host rock,
where gold was already available. Although the
meteoric fluid possibly mixed with a high saline
granitic fluid, fluid mixing was of no ore genetic
significance. This is because such mixing was clearly
post-mineralization and at a lower temperature.
Acknowledgements
This study has been financially supported by a
grant to BM from the Department of Science and
Technology, Government of India, under the area of
bDeep Continental StudiesQ (ESS/16/116/98). The
authors acknowledge the help and cooperation
extended by Dr. M.L. Patil and Dr. P. Sangurmath,
Hutti Gold Mines, during the fieldwork. Special
thanks are due to Prof. F.M. Meyer for providing
the necessary facilities for the second author to carry
out microprobe analyses at the IML, RWTH Aachen
under a DST-DAAD collaborative project. Construc-
tive comments from John Ridley and an anonymous
reviewer, along with editorial suggestions from Nigel
Cook, helped the authors to revise the manuscript.
However, the authors own the responsibility for the
interpretation presented in the paper.
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