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Geochemical characterization of tungsten-bearing granites from Rajasthan, India
ELSEVIER
JOURNAL OF GEOCNE#IICAl EXPLORAllON Journal of Geochemical
Exploration
60 (1997) 173- 184
Geochemical characterization of tungsten-bearing Rajasthan, India ’ Pankaj K. Srivastava Department
of Geology lJni[,ersit?:
granites from
*, A.K. Sinha
of Rqjusthan.
Jaipur
- 302 004, India
Received 22 May 1995: accepted 9 January 1997
Abstract Tungsten mineralization in Rajasthan (northwestern India) is genetically associated with granites of Late Proterozoic age. These granites are emplaced on the western margin of the Delhi fold belt. In order to investigate possible geochemical parameters for regional exploration and to delineate areas with tungsten potential a comparative study of the geochemical results for the granites associated with tungsten mineralization (Tungsten granites) and those which are not related with any mineralization (Barren granites) in the same or nearby areas from Rajasthan, was undertaken. Tungsten granites are found enriched in SiO,, total alkalis and are depleted in CaO, FeO, TiO, and MgO. The Tungsten granites in general have a higher concentration of W than the Barren granites. They are also enriched in Rb, Li, Sn, Nb, B. Be and are depleted in Ba and Sr and are highly differentiated. Higher values for Rb/Sr, Li/K and Ba/Sr and lower values of K/Rb, Ba/Rb and Mg/Li of Tungsten granites in comparison with Barren granites were found to be the most significant discriminators. A Geochemical Characterization Index (GCI) is proposed using the trace element discriminators for characterizing Tungsten granites and Barren granite. The GCI has a positive value for the Tungsten granites while the Barren granites have negative values. The proposed Geochemical Characterization Index is useful for delineating the granites genetically and spatially associated with tungsten mineralization and hence can be used as an exploration tool. 0 1997 Elsevier Science B.V. Keywords;
Granite; Tungsten;
Geochemical
Characterization
Index: Rajasthan
1. Introduction Tungsten deposits are generally found associated with acid igneous rocks. A number of geochemical
* Corresponding author. Present address: Department of Geology, University of Jammu, 180 004. Jammu, India. Tel.: +9119 I-452-987. ’ Submitted to the 17th IGES; editorial handling by Dr. G.F. Taylor.
studies of granitic rocks associated with various types of mineralization have been undertaken in recent years, to establish criteria useful for finding targets for detailed prospecting (e.g., Flinter et al., 1972: Tauson and Kozolov, 1973; Tischendorf, 1977; Drake, 198 1; Ohlander, 1985). However additional work is required for establishing geochemical characterization of granites related to tungsten mineral-
0375.6742/97/$17.00 0 1997 Elsevier Science B.V. All rights reserved. PII SO375-6742(97)00005 -8
:___.__
IZiillLl~l.
In India the tungsten occurrences are confined mainly to granites of Precambrian to early Palaeo-
P.K. Sriuastaua, .A.K. Sinha/Joumal
174
of Geochemical Exploration 60 (19971 173-184
!__L_l
jiiiii) RI
Dalhl
Super
Arovallt
Group
SuperGroup
Basement
Fig.
1. Simplified
regional
geological
map (after Gupta
et al..
1980).
zoic age and most of them are present in the state of Rajasthan. Bhattacharjee et al. (1993) have suggested a possible tungsten belt running from Balda in Rajasthan to Tosham in Haryana which includes all the tungsten occurrences of Rajasthan (Fig. 11. The geochemical signature of the granites associated with the tungsten (Tungsten granites) and those which are not related to the mineralization (Barren granites) from the same or nearby areas of Rajasthan are compared in order to investigate the possible geochemical parameters for exploration of tungsten prospects. To distinguish the Tungsten granites, only genetic and spatial relationships of the granite with tungsten mineralization is considered. The size/grade of the mineralization is not taken into account for classifying the Tungsten granites. The study may also provide a better understanding of the genesis of tungsten mineralization in the area.
2. Regional
Table 1 Average major element composition of granites associated with tungsten mineralization (WG) and Barren granites (BG) from Rajasthan WG (n = 43)
SiO TiO: AU’, Fe,% Fe0 MnO MgG CaO NazO K2O p205
can be Banded
BG (n = 35)
4
o
x
rr
72.84 0.17 15.68 0.20 0.98 0.04 0.13 0.24 3.12 5.21 0.11
2.53 0.05 2.61 0.15 0.20 0.02 0.04 0.09 0.75 0.80 0.04
68.92 0.56 13.34 2.38 2.93 0.07 0.82 1.98 3.95 3.84 0.16
4.23 0.18 3.34 1.12 1.45 0.02 0.29 0.27 1.52 1.01 0.04
33.70 30.61 25.74 5.82
5.45 3.21 3.50 1.21
23.32 30.08 21.25 10.25
4.60 3.56 1.18 1.80
Norms
Q Or Ab An
geology
The Precambrian geology of Rajasthan grouped into four major litho-units, namely
Gneissic Complex (BGC), Aravalli Super Group, Delhi Super Group and Late Proterozoic ErinpuraMalani igneous suite. The geology of the region was first detailed by Heron (1935) and subsequently modified by Raja Rao (19761, Gupta et al. (1980) and Roy (1988). Various tectonic models have been proposed for the evolution of the area by Sychanthavong and Desai (19771, Sinha-Roy (1988) and Deb and Sarkar (1990). The BGC is considered by many workers as the basement, although mobilization of this basement is suggested to have complicated the basement-cover relations at many places (Naha and Roy, 1983). It comprises granodiorite-gneiss, migmatites and intrusions of gneissic granites with enclaves of amphibolites and mica-schists. The cover rocks comprise the Aravallis and equivalent rocks that occur as extensive outliers (Sinha-Roy, 1988). The Aravalli Sequence has developed in a wedge-shaped area thinning towards the north and widening towards the south. Development of a platform facies along the eastern margin and a deep-water facies at the centre where a tectonic zone containing ultramafic rocks occurs, are conspicuous features of the Aravalli sys-
x =
mean value,
analyses.
(T =
std. deviation,
n =
total number
of
175
P.K. Sriuastaua, .A.K. Sinha/ Journal of Geochemical Exploration 60 (1997) 173-184
.
r
GRANITES
TUNGSTEN
0 BARREN GRANITES
”
,
Fig. 2. Na,O-K,O
diagram
for Tungsten
and Barren granites of
Rajasthan.
tern (Roy, 1988). Many important Pb-Zn deposits are associated with this system. A large phosphorite deposit and low-grade uranium mineralization are also associated with the Aravalli. The Delhi Sequence extends from Gujrat in the south to Haryana in the north. It is narrow near the centre and widens towards both ends (Heron, 1935). The Delhi Super Group exhibits two contrasting and diachronous facies: (1) an older shallow water sedimentary facies with volcanics; and (2) a younger deep-water carbonate facies with volcanics and mafic-ultramafics. Deb and Sarkar (1990) preferred to divide the sequence into two separate tectono-lithologic belts, viz. North Delhi Belt and South Delhi Belt. Some of the important Cu and stratiform ZnPb-Cu deposits are associated with the Delhi Belt. The major tungsten mineralization is associated with the intrusive granites on the western margin of the Delhi Super Group.
3. Acid magmatic
jasthan. These are: (1) 3000-2900 Ma (Untala granite); (2) 2600-2500 Ma (Birach granite); (3) 20001900 Ma (Darwal and Amet granites); (4) 1700- 1500 Ma (granites of Alwar Basins and Ajmer granites); and (5) 850-750 Ma (Erinpura and Malani igneous suite). In addition, Srivastava (1988) recorded a 500-450-Ma thermal event in the region and related it with the emplacement of the Jalore and Siwana granites. The 850-750 Ma time span marks the most extensive acid magmatic activity over a wide area in the southern part of the Aravalli Mountain Range and also to the west of it. From the near contemporaneity of ages of the granites occurring in the main axial zone of the Aravalli-Delhi erogenic fold belt in the form of the Erinpura granite and acid effusives and intrusives in the trans-Avalli Belt as Malani rhyolite and granites, Chowdhary et al. (1984) preferred to designate a common mechanism for this igneous activity. Chattopadhyay et al. (1989) suggested that the granitic activity in the southern part of the Aravalli Mountain Range is a part of an anorogenic magmatism, post-dating the Delhi Orogeny. The W mineralization in Rajasthan is found to be associated with the granites formed during this magmatic event. All granites associated with W mineralization were considered to be part of the Erinpura granites by earlier workers (Heron, 1935; Raja Rao, 1976), but
(CoO+MgOtMnO)
(CoO+MgOtMn0~
events of Rajasthan
An almost continuous record of basic and acid magmatism dating from 3500 Ma to 450 Ma is present in Rajasthan. Chowdhary et al. (1984) have dated five major periods of acid magmatism in Ra-
Fig. 3. SiO,-(CaO + MgO + MnO)-(AI,O, + NazO + K,O) ternary diagram showing composition of Tungsten and Barren granites of Rajasthan. Symbols used are the same as in Fig. 2.
176
P.K. Srioastaw
.A.K. Sinha/ Journal of Geochemical Exploration 60 (19971 173-184
F
A iNazO+
K20)
Fig, 4. ACF diagram for the composition
CFeO+MnO+MgO)
of Tungsten and Barren granites of Rajasthan.
recent detailed studies of the areas suggest that these granites are of different ages ranging from the Erinpura to Malani phases. The Balda granite which is related to the W mineralization at Balda is intruded into the Erinpura granite and is therefore younger (Chattopadhyay et al., 1982; Srivastava, 1990). However, Chakranarayana et al. (1986) have correlated the Balda granite with the nearby Belka Pahar granite which is dated as 840 * 50 Ma. Similarly, the granite of the Degana area is considered as equivalent to the Jalor granite of Malani igneous suite (Pandian, 1986). The Sewariya granite which is related to the W mineralization in the Kalni-Kotariya area is a part of the Erinpura granite (Bhattacharjee et al., 1993).
4. Tungsten
occurrences
in Rajasthan
Rajasthan is known to contain a few but well known occurrences of W at Degana in the Naguar district, Balda in the Sirohi district and KalniKotariya in the Naguar and Pali districts. This mineralization occurs along the western fringe of the
Symbols used are the same as in Fig. 2
Middle to Late Proterozoic Delhi fold belt is associated with a SOO-km-long NNE-SSW-trending lineament. A large part of this lineament is unexplored. The W mineralization of Rajasthan is associated with the 850-750 Ma magmatic event which has given rise to the anorogenic granitic intrusions mainly in the western fringe of the Delhi fold belt. The mineralization is found in quartz veins and rarely in pegmatites (often greisenized) in the form of wolframite. However, in Degana the mineralization is also occurs as disseminated in the granite apart from the pockets of wolframite in the quartz veins. Some scheelite-rich skam mineralization is also reported from areas where these granites intruded into the calcareous metasediments. However, the skam mineralization is not of economic significance. Minor sulphide mineralization is also present with the tungsten mineralization.
5. Geochemistry
of granites
The granites chosen for this study represent a variety of both mineralized and barren occurrences.
P.K. Srkastava, .A.K. Sinha/ Journal of Geochemical Exploration 60 (19971 173-184
oL--Ll
3
B
Rb
I
Sr
!
I
Ba
Nb
-
TUNGSTEN
o---a
BARREN
c-s
AVERAGE
I
Zr
I
Zn
ill
GRANITES GRANITES GRANITES
I
Sn
J
W
Fig. 5. Trace element variation diagram for granites of Rapsthan.
The Tungsten granites from the area included in the study differ in character as well as in age. Balda granite is leucocratic and rich in muscovite while Degana and Sewariya granites are two-mica granites with more mafic minerals. The Balda granite is equigranular while the Degana and Sewariya granites are porphyritic with big phenocrysts of K-feldspar. These granites are described in detail by Srivastava (1990); Bhattacharjee et al. (1993) and Chattopadhyay et al. (1994). Similarly the granites included in the Barren granite group represent various areas of Rajasthan and are of Early Precambrian to late Palaeozoic ages. Therefore, all the granites, irrespective of their composition, origin and ages, are divided into two groups: (1) the WG, granites which are associated with tungsten mineralization, i.e. Tungsten granites, and (2) the BG, granites which are not associated with mineralization, i.e. the Barren granites. For convenience these abbreviations will be used in the rest of the text.
The granites included into WG and BG groups from the Balda, Degana and Kalni-Kotariya areas are analysed by XRF and AAS for their major and trace element contents. Geochemical data for granites from other areas are taken from the published literature (Basu, 1982; Gangopadhyay and Lahiri, 1988; Eby and Kochhar, 1990; Rahman and Zainuddin, 1990, 1993; Bhattacharjee et al., 1993; Munshi, 1993). For the purpose of comparison the geochemical data for the entire granite suite are considered.
6. Major element geochemistry The major element composition of the studied granites is given in Table 1. WG are mostly silica-rich with SiO, ranging from 69 to 78%, mostly falling between 71% and 73%. The normative quartz of WG is normally between 30 and 40%, except for Sewariya granite where average CaO is 1.29, whereas
P.K. Sriuastaua, .A.K. Sinha / Journal of Geochemical Exploration 60 (I 997) 173-184
178
Table 2 Trace element concentration (median values) of the granites associated with tungsten mineralization (WG) and Barren granites (BG) from Rajasthan
cu
the Balda granite and Degana granite are very poor in CaO content. They are also depleted in Fe0 and MgO. Ishihara (197 1) suggested that W mineralization in Japan is commonly associated with less oxidised granites but no obvious change in Fe,O,/FeO ratio was observed for WG and BG groups in Rajasthan. However, the K,O/Na,O ratios for the tungsten-bearing granites are restricted to a limited range (1.27-2.76) but are scattered for the Barren granites (Fig. 2). Juniper and Kleeman (1979) used a SiO,-(CaO + Fe0 + MgO)-(Al,O, + Na,O + K,O) diagram to discriminate between Sn granites and Barren granites. This does not hold true in the case of W-bearing granites and an overlap is observed (Fig. 3) between the areas of WG and BG. When the compositions of granites are plotted in a (Na,O + K,O)-CaO-(Fe0 + MnO + MgO) temiary diagram (Fig. 41, a clear demarcation between WG and BG is observed. It suggests that the W granites are enriched in total alkalis and are depleted in Cao, FeO, MnO and MgO. This is in agreement with the results of Liu and Ma (1993).
27 68 69 11 3 74 34 II 33 209 94 507 1263
_
Pb Zn Sn W Nb B Be Li Rb Sr Ba Zr
570 663 57 87 146
K/Rb Rb/Sr Ba/Rb Mg/Li
101.1 11.8 0.2 4.9
31 103 18 43 174 _
150.9 2.2 2.4 345.4
Rb
DIORITES OTZ-DIORITES ---
Sr
Ba Fig. 6. Rb-Ba-Sr ternary diagram for the Tungsten Symbols used are the same as in Fig. 2.
and Barren granites of Rajasthan
(fields are after El Bouseily and El
sofiary,1975).
P.K. SriL)astaua, .A.K. Sinha/ Journal of Geochemical Exploration 60 (1997) 173-184
Barren granites. The Degana granite shows a very high value for W ranging from 300 to 400 ppm which can be attributed to the presence of disseminated wolframite mineralization. The variation diagram (Fig. 5) suggests higher value for Sn, W, Nb, B, Rb. and Be and depletion of Ba and Sr for WG in comparison to Bg. The lower K/Rb ratios along with lower Ti in the WG suggest that they are strongly differentiated products of the parent magma. The same is reflected by a Rb-Ba-Sr ternary diagram (Fig. 61, where the W-mineralized granites fall in the field of strongly differentiated granites. Tischendorf (1977) proposed that K/Rb ratios above 100 with Rb values below 500 ppm and an enrichment of Li, Be, W, Sn, Nb and Ta together with depletion in Ba, Sr and Zr, are some of the characteristics of mineralized granites. Beus and Oyzerman (19651, Sheraton and Black (19731, Lawrence (19741, Groves and McCarthy (1978) and Biste (1982) have used elemental ratios such as K/Rb, Mg/Li and Ba/Sr as one of the criteria in discriminating between mineralized and barren granites. In Rajasthan, the K/Rb, Mg/Li and Rb/Sr
According to Pandian (19861, Srivastava (1990) and Bhattacharjee et al. (1993) the granites included in the WG group are of the S-type category of Chappel and White (1974). But no systematic difference could be observed between BG and WG for the molar ratio of Al,O,/(Na,O + K,O + Al,O,) which is one of the criteria in distinguishing S- and I-type granites.
7. Trace element geochemistry The trace element composition of the two groups of granites shows a large variation because they were collected from a large area and represent different types of granitoids. Therefore the trace element composition for the WG and BG groups is reported in the form of median values rather than average values (Table 2). Tungsten granites generally have a higher concentration of W than Barren granites. Within the mineralized granites, the Balda and Sewariya granites have 2 to 5 times higher concentration of W than the
400
179
-
0
I
.
\
l .
?
/
.
!
i Oi OS
. . l.
l
*a l.
..--L-i
\ I
/ IO
I.0
I 100
Rb/Sr
Fig. 7. Rb/Sr
vs K/Rb
plot for the Tungsten granites and Barren granites of Rajasthan.
Symbols used are the same as in Fig. 2
180
P.K. Sricastaca,
.A.K. Sinha/Joumul
qf Geochemical
ratios successfully discriminate the W-bearing granites from the Barren granites. When the K/Rb ratio is plotted against Rb/Sr ratios (Fig. 7) the WG group forms a distinct field with lower K/Rb ratios and higher Rb/Sr ratio than the BG group. Student’s t-test was applied with a level of confidence of 0.95 to determine whether there are any significant changes between the two groups. As the t-test requires normally distributed populations, the calculations are based on log-transformed concentrations. It suggests that WG is enriched in alkalis (Na,O + K,O), Rb, Nb, Li, W, Sn and are depleted in CaO, MgO, FeO, Ba and Sr. The resulting elemental ratios are efficient in discriminating Wbearing granites from Barren granites.
8. Discussion To discriminate the ore potentiality of granites, the geochemical results must be combined with the geological data.Schuling (1967) has argued that the Sn deposits of the world were restricted to well-defined ‘tin provinces’ and suggested an inhomogeneous distribution of the metal in the crust and also that the crust may be the main source of tin. Wright and McCurry (1973) and Kovalenko and Kovalenko (1984) have proposed that a geochemical culmination may exist in the deep crust or upper mantle. The importance of the composition of the source rocks in the formulation of W-bearing granites has been emphasised by Hesp (1971) and Flinter et al. (1972). However, Sheraton and Black (1973) and Hosking (1976) observed that not all the granitic rocks or even fractionated granites within a Sn-W province are associated with mineralization. But it is apparent that a combination of factors including metal-rich source rocks, well-advanced magmatic differentiation and presence of volatiles are necessary if economic deposits of W are to be formed. Ayres et al. (1982), Imeokperia (1983) and Liu and Ma (1993) observed that the small granite bodies intruding metasediments served as the most productive granite in a multi-stage intrusive terrain whereas the large batholiths are barren. Ohlander (1985) found that the larger granitic batholiths in Sweden are in situ and the small granitic bodies, which are related with MO mineralization, are of subcrustal origin and
Exploration 60 (1997) 173-184
the ore fluid is evolved during the movement of intrusive magma. It is also observed in Rajasthan where the granitic intrusions at Balda and Degana have an outcrop area of less than 5 km*. This is in agreement with Tischendorf (1977) and Stemprok (1979) who suggested that the mineralized granites form late erogenic plutons, which are often small and of shallow emplacement. Tauson and Kozolov (1973) have suggested that trace element concentrations and elemental ratios indicate the source of a rock, its mode of origin and also the evolution of the parent magma during its crystallization. Smirnov (1968) used metallogenic analyses to show that ore deposits of W, Sn, Li, Nb and Ta were related to ‘palingenic magma’ of crustal origin. From the major and trace element studies of the granites it may be suggested that WG are derived from metasedimentary source material. Liu and Ma (1993) suggested that the granitization and anatexis of W-bearing formations are probably important in the genesis of ore-generating granites. They also suggested that during multi-stage remelting-intrusion-differentiation processes, W would be mobilized, migrating upwards and concentrating in favourable structures at the top of the late small granite stocks. Orlova et al. (1987) and Stemprok (1990) have also proved experimentally, that a large amount of W can be dissolved in the anatectic melt produced from sediments which contain grains of W minerals, particularly scheelite. The fact that all the Tungsten granites in Rajasthan are of the S-type, i.e. formed by partial melting of crustal rocks, suggests that these granites might be products of the multistage melting including the anatexis of the geosynclinal sediments of the Delhi Super Group which might be enriched in W. However, this simplified conclusion needs further investigation. The W was mobilised and migrated upward and concentrated in structurally favourable localities such as shear zones and fractures. Greisenization is considered by many workers to be directly related to the action of volatile-rich hydrothermal solutions on the host rocks. Gundsambu (1974) has suggested that the W mineralization associated with acidic hydrothermal solution have commonly resulted in greisenization of the host granite. Greisenization is the common hydrothermal alteration present within the granites of WG in Rajasthan.
P.K. Srkastaua, .A.K. Sinha / Journal of Geochemical Exploration 60 (1997) 173-184
Deaano Granite .
I
I
0
BARREN
GRANITE
.
TUNGSTEN
Bolda
GRANITE South Elizabeth Moreebo South
Erinpura Average China Jalors
Granite
Sordenion
Sewar~a
South
Creek
Granite
Chmo
181
(India)
Granite (IndIal . W Field (China) .
(Australta)
(Ausfroliol
Grznlte
Granit:
(Italy)
(India)
Granite (Indlo) 0 Granite 0
Granite 0 Granite 0
Avg
(Chino)
(lnd~o)
Bundelkhand
Granite (Ind~o) 0 Grovite (Austroital 0 Granite (IndIal
Dumbano AharRIver D~doGronlte
cAustral:l 0
L -6
-5
1
1
_I -4
-3
-2
-I
0
I
2
3
I 4
I
1
5
6
Fig. 8. Geochemical Characterization Index for different Tungsten granites and Barren granites. (Data computed from Turekian and Wedepohl. 1961; Sheraton and Black, 1973; Biste, 1982; Eby and Kochhar, 1990; Rahman and Zainuddin. 1990: Liu and Ma. 1993; Bhattacharjee et al.. 1993; Rahman and Zainuddin. 1993; and these authors’ work.)
It may have resulted from the volatile-rich hydrothermal ore fluids which were derived from the parent magma during the late stages of cooling. Srivastava and Sinha (1995) emphasized the use of greisenization and quartz-muscovite-fluorite + tourmaline alteration as a guide for mineral exploration for W deposits. Flinter et al. (1972) and Juniper and Kleeman (1979) suggested the usefulness of major element chemistry in discriminating mineralized granites from the Barren granites. Stemprok and Skorv (1974), Srivastava (1990) and Liu and Ma (1993) suggested that the granites spatially and genetically associated with the Sn, W and MO mineralization are characterized by the relative enrichment of SiO, and depletion of some major element oxides such as CaO, MgO and FeO. But Drake (1981) and Ohlander (1985) reported that the major elements are not useful in discriminating mineralized granites from Barren granites. Keith et al. (1989) and Newberry et al. (1990) reported a similar geochemistry for the W-related and W-barren granites and concluded that the exploration should focus on mineralogical, textu-
ral and geological features indicative of highly fractionated magmas in deeper environments rather than the geochemical features of granitoids. The present study, however, upholds the observation of Stemprok and Skorv (1974) and Liu and Ma (1993). The granites related to W mineralization in South China are SiO,-rich, KzO + Na,O > 7.5, K,O > Na,O and contents of Ca, Mg, Fe, Ti are relatively lower than in average granites (Liu and Ma, 1993). The W granites of Rajasthan show the same pattern. Biste (1980) presented a ternary diagram based on compositional data of Rb-Ba-Sr from a number of granites and found that the mineralized types fall in a particular field which is similar to the highly differentiated field of Fig. 6. Extending his studies to the granites of South Sardinia, Biste (1982) found noticeable differences in the trace element composition of metallogenetic and non-metallogenetic granites. In the mineralized granites he has noted relatively higher concentrations of F, Rb, Li and Sn with increased Rb/Sr, Li/K and Ba/Sr ratios and comparatively depleted concentrations of Sr, Ba and Zr and lower values of K/Rb. Ba/Rb and Mg/Li. Groves and
182
P.K. Sriuastaua, .A.K. Sinha / Journal of Geochemical Exploration 60 11997) 173% 184
McCarthy (1978) have also emphasized the efficacy of concentration ranges of Sr and Ba as a possible tool for evaluating the economic potential of granitoid rocks and reaffirmed the usefulness of the Ba/Rb ratio to identify Sn and W granitoids. Tischendorf (1977) observed that Ba, Sr and Zr are typically depleted, whereas Be, Li, Rb, Sn, W and Nb are expected to be enriched in the W- and Sn-bearing granitoids. The present study confirms that the low values of K/Rb ratio and high values of Rb/Sr ratio would suggest W-bearing granite. It may be inferred from this study in Rajasthan that the distinction between Tungsten granites and Barren granites within any particular region apparently can be made on the basis of one or more of the elemental ratios, viz. K/Rb, Rb/Sr, Ba/Rb, Mg/Li. Very high ratios of Rb/Sr and very low ratios of K/Rb, Mg/Li and Ba/Rb are indicative of W-Sn mineralization. From these relations there seems to be a reasonable expectation that WG could be discriminated on the basis of combining the variations in K, Mg, Rb, Sr, Ba and Li through a multiplicative ratio technique. Using these trace element discriminators, a Geochemical Characterization Index (GCI) Rb3 X Li X lo4 is proposed as: GCI = log,, MgXKXBaXSr for characterizing W-bearing granites from the Barren granites. In this calculation all the values are in ppm. It is found that the Geochemical Characterization Index for the W-bearing granites has values which are orders of magnitude higher than for Barren granites. A positive value for GCI will represent W-bearing granites while a negative number will represent Barren granites. The index has been positively tested (Fig. 8) for many of the granites from India and the rest of the world.
9. Conclusions A work plan for future exploration is now possible by the synthesis of available field and laboratory data and the ore potential of the granitoids can be assessed by major and trace element geochemistry. From the present study it may be concluded that: (1) The W-bearing granites are enriched in (Na,O + K,O), Rb, Li, Sn, W, Nb and are depleted in CaO, MgO, FeO, Sr and Ba.
(2) The W-bearing granites are the product of highly differentiated magma. (3) The elemental ratios K/Rb, Mg/Li, Ba/Rb and Rb/Sr can be successfully used to discriminate between W-bearing and Barren granites. (4) Positive GCI values, i.e. log,,(Rb3 X Li X 104/Mg X K X Ba X Sr) for any granite will suggest its W potentiality in an area. (5) The W gr am‘tes of Rajasthan are the result of multi-stage melting and anatexis of metasediments of the Delhi Super Group.
Acknowledgements The financial assistance to first author (PKS) from Council of Scientific and Industrial Research and Department of Science and Technology, New Delhi is sincerely acknowledged. The manuscript has been significantly improved by critical reviews of Prof. Peter Pollard and Dr. Graham Taylor.
References Ayres, L.D., Averill, S.A. and Wolfe, W.J., 1982. An Archean molybdenite occurrence of possible porphyry type at Setting Net Lake. northwestern Ontario, Canada. Econ. Geol.. 77: 1105-01119. Basu, S.K.. 1982. Phases of acid igneous activities in Malani suite of rocks around Jhunjhunu, Rajasthan. Rec. Geol. Surv. hid., 127: 89-94. Beus. A.A. and Oyzerman, M.T., 1965. Distribution of Rb in igneous rocks and correlation between Rb and K. Geochemistry, 2: 986-988. Bhattachatjee, J., Fareeduddin, and Jai,, S.S., 1993. Tectonic setting, petrochemistry and tungsten metallogeny of Sewariya granite in south Delhi fold belt, Rajasthan. J. Geol. Sot. Ind., 42: 3-16. Biste, M., 1980. Die Anwendung geochemischer Indicatoren of die Zin-Hoffigkeit herzynischer Granite in Siid Sordinanean. Berl. Geowiss. Abh., A/18: I-107. of South Sordinanean Granite Biste, M., 1982. Geochemistry compared to their tin potential. In: A.M. Evans (Editor), Mineralization Associated with Acid Magmatism. John Wiley and Sons Ltd., New York, pp. 37-49. Chakranarayana, A.B.. Powar, K.B., Pandey, K. and Gopalan, K., 1986. Rb/Sr dating of Belka Pahar Granite, Sirohi district, Rajasthan. J. Geol. Sot. Ind.. 28: 325-327.
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Chappel, B.W. and White, A.J.R., 1974. Two contrasting granitic types. Pac. Geol., 8: 173-190. Chattopadhyay, B., Mukhopadhyay, A.K., Singhai, R.K., Bhattacharjee, J.R. and Hore, R.K., 1982. Post Erinpura acid magmatism in Sirohi and its bearing on the tungsten mineralization Proc. Symp. Metallogeny of the Precambrian, IGCP. 91: 115-132. Chattopadhyay, B., Mukhopadhyay, A.K. and Bhattacharjee, J.. 1989. The wolframogene Balda pluton in the envelope of Proterozoic Erinpura rapakivi granitic complex, India. (Abstr.). Geol. Surv. Finl., Spec. Pap., 8: 26. Chattopadhyay, B., Chattopadhyay, S. and Bapna. VS., 1994. Geology and geochemistry of Degana Pluton - a Proterozoic rapakivi granite in Rajasthan, India. J. Mineral. Petrol.. 50: 69-82. Chowdhary, A.K., Gopalan, K. and Shastry, C.A., 1984. Present status of geochronology of the Precambrian rocks of Rajasthan. Tectonophysics, 105: 131-140. Deb, M. and Sarkar, S.C., 1990. Proterozoic tectonic evolution in Aravalli-Delhi erogenic complex, northwestern India. Precambrian Res., 46: 115-137. Drake, K.M., 1981. Geochemistry of tin, tungsten and molybdenum in Swedish Proterozoic granitoids; its potential use in regional mineral exploration. Ph.D. Thesis, Medd. Stockholm Univ. Geol. Inst., 249: l-106. Eby, G.N. and Kochhar, N., 1990. Geochemistry and petrogenesis of Malani igneous suit, north Peninsular India. J. Geol. Sot. Ind., 36: 109-130. El Bouseily, A.M. and El Sokkary, A.A., 1975. The relation between Rb. Ba and Sr in granitic rocks. Chem. Geol., 16: 207-219. Flinter, B.H., Hesp, W.R. and Rigby, D., 1972. Selected geochemical and petrological features of granitoids of New England Complex, Australia and their relation to Sn. W, MO, and Cu mineralization. Econ. Geol., 67: 1241-1262. Gangopadhyay, P.K. and Lahiri, A., 1988. Anjana granite and associated rocks of Deogarh, Udaipur district. Rajasthan. Mem. Geol. Sot. Ind., 7: 102-106. Groves, D.I. and McCarthy, T.S., 1978. Fractional crystallization and origin of tin deposits in granitoids. Mineral. Deposita. 13:
11-26. Gundsambu, T., 1974. Genetic association of Sn and W deposits of Mongolia with granitoid magmatism and concentration of ore element. In: M. Stemprok (Editor), Mineralization Associated with Acid Magmatism. Ustredni Ustiv. Geol. Prague, pp. 79-89. Gupta, S.N., Arora, Y.K., Mathur, R.K., Iqbaluddin, Prasad, B., Shahi, T.N. and Sharma, S.B., 1980. Lithostratigraphic map of Aravalli region. Southern Rajasthan and northeastern Gujrat. Geol. Surv. Ind. Pub]., Hyderabad. Heron, A.M., 1935. Synopsis of Pre Vindhyan Geoloy of RaJputana. Mem. Geol. Surv. Ind.. 79, 389 pp. Hesp, W.R., 1971. Correlation between tin content of granitic rocks and their chemical and mineralogical composition. Geochemical Exploration. Can. Inst. Min. Metal., Spec. Vol., 11: 341-353. Hosking, K.J.F., 1976. The search for tungsten deposit. Geol. Sot. Malaysia Bull., 5: l-70.
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Imeokperia, E.G., 1983. Geochemical aspects of evolution and mineralization of AFU younger granite complex (northern Nigeria). Chem. Geol., 40: 71-82. Ishihara, S., 1971. Model and chemical composition of granitic rocks related to the major molybdenum and tungsten deposits in the inner zone of southwest Japan. J. Geol. Sot. Jpn., 77: 44-452. Juniper, D.N. and Kleeman, J.D., 1979. Geochemical characterization of some tin-mineralizing granites of New South Wales. J. Geochem. Explor., 11: 321-333. Keith, S.B., van Middelaar, W.T., Clarke, A.H. and Hodgson, C.J. 1989. Granitoid textures, compositions and volatile fugacities associated with the formation of tungsten dominated skarn deposits. Rev. Econ. Geol., 4: 235-250. Kovalenko, V.I. and Kovalenko, N.I., 1984. Problems of origin, ore bearing and evolution of rare metal granitoids. Phys. Earth Planet. Inter., 35: 229-251. Lawrence, G., 1974. The use of Rb/Sr ratios as guide to mineralization in the Galway granite, Ireland. In: Geochemical Exploration, 1974. Inst. Min. Metal]., London, pp. 353-370. Liu, Y. and Ma, D., 1993. Vein type tungsten deposits of China and adjoining regions. Ore Geol. Rev., 8: 233-246. Munshi, R.L., 1993. Petrological status of hornblende quartzite and associated rocks of Basantgarh area, Sirohi district, Rajasthan. J. Geol. Sot. Ind., 42: 61-66. Naha, K. and Roy, A.B., 1983. The problem of the Precambrian basement in Rajasthan, Western India. Precambrian Res., 19:217-223. Newberry, R.J., Burns, L.E., Swanson, S.E. and Smith, T.E., 1990. Comparative petrologic evolution of the Sn and W granites of Fairbanks Circle area, interior Alaska. Geol. Sot. Am. Prof. Pap., 246: 121-142. Ohlander, B., 1985. Geochemical characteristics of granites associated with Proterozoic molybdenite mineralization in northern Sweden. Chem. Geol., 51: 247-263. Orlova, G.P.. Lapin, A.A. and Ryabchikova, I.D., 1987. Experimental study of equilibria in the system Scheelite-GraniteFluid under parameters of hyperbysal magmatism. Geol. Rud. Mestorosh., 4: 107-l 10. Pandian, MS., 1986. Metallogenic investigations of Degana Tungsten mining district, Rajasthan. Ph.D. Thesis. Indian School of Mines, Dhanbad, 106 pp. (unpubl.) Rahman, A. and Zainuddin, SM., 1990. Geochemistry and genesis of Ahar River Granite, Rajasthan. J. Geol. Sot. Ind., 36: 202-210. Rahman, A. and Zainuddin, S.M., 1993. Bundelkhand Granites: an example of collision-related Precambrian Magmatism and its relevance to the evolution of central Indian Shield. J. Geol., 101: 413-419. Raja Rao, C.S., 1976. Precambrian sequence of Rajasthan. Misc. Pub]. Geol. Surv. lnd., 23: 497-516. Roy. A.B., 1988. Stratigraphic and tectonic framework of Aravalli Mountain Range. Mem. Geol. Sot. Ind.. 7: 3-32. Schuling, B.D., 1967. Tin belts on the continents round the Atlantic Ocean. Econ. Geol., 62: 540-550. Sheraton, W. and Black, L.P., 1973. Geochemistry of mineralized granitic rocks of northern Queensland. J. Geochem. Explor., 2: 33 l-348.
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Sinha-Roy, S., 1988. Proterozoic Wilson cycle in Rajasthan. Mem. Geol. Sot. Ind., 7: 95-108. Smimov, V.I., 1968. Source of ore orming material. Econ. Geol., 63: 380-389. Srivastava, P.K., 1990. Tungsten metallisation associated with acid magmatism in Sirohi, Rajasthan. Ph. D. Thesis (unpubl.). Indian School of Mines, pp. l-125. Srivastava, P.K. and Sinha, A.K., 1995. Mineral assemblages and wall rock alterations as exploration guides for tungsten: a study from western India. Proc. Recent Res. Geol. W. India. Baroda, pp. 95-102. Srivastava, R.K., 1988. Magmatism in Aravalli mountain range and its environs. Mem. Geol. Sot. Ind., 7: 77-84. Stemprok, M., 1990. Solubility of tin, tungsten and molybdenum oxides in felsic magmas. Miner. Deposita, 25: 205-212. Stemprok, M., 1979. Mineralized granites and their origin. Episodes, 3: 20-24. Stemprok, M. and Skorv, M.I., 1974. Composition of Sn bearing
granites from the Krusne Hory metallogenic province of Czechoslovakia. Sbor. Geol. Ved. Lozisk. Geol., 16: 7-87. Sychanthavong. S.P. and Desai, S.D., 1977. Proto-plate tectonics controlling the Precambrian deformations and metallogenic epochs in Northwestern India. Mineral. Sci. Eng., 9: 218-236. Tauson, L.V. and Kozolov, V.D.. 1973. Distribution functions and ratios of trace element concentrations as estimates of the ore bearing potential of granites. In: Geochemical Exploration, Inst. Min. Metall., London, pp. 37-44. Tischendorf, G., 1977. Geochemical and petrographic criteria of silicic magmatic rocks associated with rare metal mineralization. In: M. Stemprok (Editor), Metallization Associated with Acid Magmatism. Ustredni Ustiv. Geol. Prague, 2: 41-96. Turekian, K.K. and Wedepohl, K.H., 1961. Distribution of the elements in some major units of earth crust. Bull. Geol. Sot. Am., 72: 175-192. Wright. J.B. and McCurry, P.. 1973. Magmas, mineralization and sea floor spreading. Geol. Rundsch., 62: I I6- 125.
JOURNAL OF GEOCNE#IICAl EXPLORAllON Journal of Geochemical
Exploration
60 (1997) 173- 184
Geochemical characterization of tungsten-bearing Rajasthan, India ’ Pankaj K. Srivastava Department
of Geology lJni[,ersit?:
granites from
*, A.K. Sinha
of Rqjusthan.
Jaipur
- 302 004, India
Received 22 May 1995: accepted 9 January 1997
Abstract Tungsten mineralization in Rajasthan (northwestern India) is genetically associated with granites of Late Proterozoic age. These granites are emplaced on the western margin of the Delhi fold belt. In order to investigate possible geochemical parameters for regional exploration and to delineate areas with tungsten potential a comparative study of the geochemical results for the granites associated with tungsten mineralization (Tungsten granites) and those which are not related with any mineralization (Barren granites) in the same or nearby areas from Rajasthan, was undertaken. Tungsten granites are found enriched in SiO,, total alkalis and are depleted in CaO, FeO, TiO, and MgO. The Tungsten granites in general have a higher concentration of W than the Barren granites. They are also enriched in Rb, Li, Sn, Nb, B. Be and are depleted in Ba and Sr and are highly differentiated. Higher values for Rb/Sr, Li/K and Ba/Sr and lower values of K/Rb, Ba/Rb and Mg/Li of Tungsten granites in comparison with Barren granites were found to be the most significant discriminators. A Geochemical Characterization Index (GCI) is proposed using the trace element discriminators for characterizing Tungsten granites and Barren granite. The GCI has a positive value for the Tungsten granites while the Barren granites have negative values. The proposed Geochemical Characterization Index is useful for delineating the granites genetically and spatially associated with tungsten mineralization and hence can be used as an exploration tool. 0 1997 Elsevier Science B.V. Keywords;
Granite; Tungsten;
Geochemical
Characterization
Index: Rajasthan
1. Introduction Tungsten deposits are generally found associated with acid igneous rocks. A number of geochemical
* Corresponding author. Present address: Department of Geology, University of Jammu, 180 004. Jammu, India. Tel.: +9119 I-452-987. ’ Submitted to the 17th IGES; editorial handling by Dr. G.F. Taylor.
studies of granitic rocks associated with various types of mineralization have been undertaken in recent years, to establish criteria useful for finding targets for detailed prospecting (e.g., Flinter et al., 1972: Tauson and Kozolov, 1973; Tischendorf, 1977; Drake, 198 1; Ohlander, 1985). However additional work is required for establishing geochemical characterization of granites related to tungsten mineral-
0375.6742/97/$17.00 0 1997 Elsevier Science B.V. All rights reserved. PII SO375-6742(97)00005 -8
:___.__
IZiillLl~l.
In India the tungsten occurrences are confined mainly to granites of Precambrian to early Palaeo-
P.K. Sriuastaua, .A.K. Sinha/Joumal
174
of Geochemical Exploration 60 (19971 173-184
!__L_l
jiiiii) RI
Dalhl
Super
Arovallt
Group
SuperGroup
Basement
Fig.
1. Simplified
regional
geological
map (after Gupta
et al..
1980).
zoic age and most of them are present in the state of Rajasthan. Bhattacharjee et al. (1993) have suggested a possible tungsten belt running from Balda in Rajasthan to Tosham in Haryana which includes all the tungsten occurrences of Rajasthan (Fig. 11. The geochemical signature of the granites associated with the tungsten (Tungsten granites) and those which are not related to the mineralization (Barren granites) from the same or nearby areas of Rajasthan are compared in order to investigate the possible geochemical parameters for exploration of tungsten prospects. To distinguish the Tungsten granites, only genetic and spatial relationships of the granite with tungsten mineralization is considered. The size/grade of the mineralization is not taken into account for classifying the Tungsten granites. The study may also provide a better understanding of the genesis of tungsten mineralization in the area.
2. Regional
Table 1 Average major element composition of granites associated with tungsten mineralization (WG) and Barren granites (BG) from Rajasthan WG (n = 43)
SiO TiO: AU’, Fe,% Fe0 MnO MgG CaO NazO K2O p205
can be Banded
BG (n = 35)
4
o
x
rr
72.84 0.17 15.68 0.20 0.98 0.04 0.13 0.24 3.12 5.21 0.11
2.53 0.05 2.61 0.15 0.20 0.02 0.04 0.09 0.75 0.80 0.04
68.92 0.56 13.34 2.38 2.93 0.07 0.82 1.98 3.95 3.84 0.16
4.23 0.18 3.34 1.12 1.45 0.02 0.29 0.27 1.52 1.01 0.04
33.70 30.61 25.74 5.82
5.45 3.21 3.50 1.21
23.32 30.08 21.25 10.25
4.60 3.56 1.18 1.80
Norms
Q Or Ab An
geology
The Precambrian geology of Rajasthan grouped into four major litho-units, namely
Gneissic Complex (BGC), Aravalli Super Group, Delhi Super Group and Late Proterozoic ErinpuraMalani igneous suite. The geology of the region was first detailed by Heron (1935) and subsequently modified by Raja Rao (19761, Gupta et al. (1980) and Roy (1988). Various tectonic models have been proposed for the evolution of the area by Sychanthavong and Desai (19771, Sinha-Roy (1988) and Deb and Sarkar (1990). The BGC is considered by many workers as the basement, although mobilization of this basement is suggested to have complicated the basement-cover relations at many places (Naha and Roy, 1983). It comprises granodiorite-gneiss, migmatites and intrusions of gneissic granites with enclaves of amphibolites and mica-schists. The cover rocks comprise the Aravallis and equivalent rocks that occur as extensive outliers (Sinha-Roy, 1988). The Aravalli Sequence has developed in a wedge-shaped area thinning towards the north and widening towards the south. Development of a platform facies along the eastern margin and a deep-water facies at the centre where a tectonic zone containing ultramafic rocks occurs, are conspicuous features of the Aravalli sys-
x =
mean value,
analyses.
(T =
std. deviation,
n =
total number
of
175
P.K. Sriuastaua, .A.K. Sinha/ Journal of Geochemical Exploration 60 (1997) 173-184
.
r
GRANITES
TUNGSTEN
0 BARREN GRANITES
”
,
Fig. 2. Na,O-K,O
diagram
for Tungsten
and Barren granites of
Rajasthan.
tern (Roy, 1988). Many important Pb-Zn deposits are associated with this system. A large phosphorite deposit and low-grade uranium mineralization are also associated with the Aravalli. The Delhi Sequence extends from Gujrat in the south to Haryana in the north. It is narrow near the centre and widens towards both ends (Heron, 1935). The Delhi Super Group exhibits two contrasting and diachronous facies: (1) an older shallow water sedimentary facies with volcanics; and (2) a younger deep-water carbonate facies with volcanics and mafic-ultramafics. Deb and Sarkar (1990) preferred to divide the sequence into two separate tectono-lithologic belts, viz. North Delhi Belt and South Delhi Belt. Some of the important Cu and stratiform ZnPb-Cu deposits are associated with the Delhi Belt. The major tungsten mineralization is associated with the intrusive granites on the western margin of the Delhi Super Group.
3. Acid magmatic
jasthan. These are: (1) 3000-2900 Ma (Untala granite); (2) 2600-2500 Ma (Birach granite); (3) 20001900 Ma (Darwal and Amet granites); (4) 1700- 1500 Ma (granites of Alwar Basins and Ajmer granites); and (5) 850-750 Ma (Erinpura and Malani igneous suite). In addition, Srivastava (1988) recorded a 500-450-Ma thermal event in the region and related it with the emplacement of the Jalore and Siwana granites. The 850-750 Ma time span marks the most extensive acid magmatic activity over a wide area in the southern part of the Aravalli Mountain Range and also to the west of it. From the near contemporaneity of ages of the granites occurring in the main axial zone of the Aravalli-Delhi erogenic fold belt in the form of the Erinpura granite and acid effusives and intrusives in the trans-Avalli Belt as Malani rhyolite and granites, Chowdhary et al. (1984) preferred to designate a common mechanism for this igneous activity. Chattopadhyay et al. (1989) suggested that the granitic activity in the southern part of the Aravalli Mountain Range is a part of an anorogenic magmatism, post-dating the Delhi Orogeny. The W mineralization in Rajasthan is found to be associated with the granites formed during this magmatic event. All granites associated with W mineralization were considered to be part of the Erinpura granites by earlier workers (Heron, 1935; Raja Rao, 1976), but
(CoO+MgOtMnO)
(CoO+MgOtMn0~
events of Rajasthan
An almost continuous record of basic and acid magmatism dating from 3500 Ma to 450 Ma is present in Rajasthan. Chowdhary et al. (1984) have dated five major periods of acid magmatism in Ra-
Fig. 3. SiO,-(CaO + MgO + MnO)-(AI,O, + NazO + K,O) ternary diagram showing composition of Tungsten and Barren granites of Rajasthan. Symbols used are the same as in Fig. 2.
176
P.K. Srioastaw
.A.K. Sinha/ Journal of Geochemical Exploration 60 (19971 173-184
F
A iNazO+
K20)
Fig, 4. ACF diagram for the composition
CFeO+MnO+MgO)
of Tungsten and Barren granites of Rajasthan.
recent detailed studies of the areas suggest that these granites are of different ages ranging from the Erinpura to Malani phases. The Balda granite which is related to the W mineralization at Balda is intruded into the Erinpura granite and is therefore younger (Chattopadhyay et al., 1982; Srivastava, 1990). However, Chakranarayana et al. (1986) have correlated the Balda granite with the nearby Belka Pahar granite which is dated as 840 * 50 Ma. Similarly, the granite of the Degana area is considered as equivalent to the Jalor granite of Malani igneous suite (Pandian, 1986). The Sewariya granite which is related to the W mineralization in the Kalni-Kotariya area is a part of the Erinpura granite (Bhattacharjee et al., 1993).
4. Tungsten
occurrences
in Rajasthan
Rajasthan is known to contain a few but well known occurrences of W at Degana in the Naguar district, Balda in the Sirohi district and KalniKotariya in the Naguar and Pali districts. This mineralization occurs along the western fringe of the
Symbols used are the same as in Fig. 2
Middle to Late Proterozoic Delhi fold belt is associated with a SOO-km-long NNE-SSW-trending lineament. A large part of this lineament is unexplored. The W mineralization of Rajasthan is associated with the 850-750 Ma magmatic event which has given rise to the anorogenic granitic intrusions mainly in the western fringe of the Delhi fold belt. The mineralization is found in quartz veins and rarely in pegmatites (often greisenized) in the form of wolframite. However, in Degana the mineralization is also occurs as disseminated in the granite apart from the pockets of wolframite in the quartz veins. Some scheelite-rich skam mineralization is also reported from areas where these granites intruded into the calcareous metasediments. However, the skam mineralization is not of economic significance. Minor sulphide mineralization is also present with the tungsten mineralization.
5. Geochemistry
of granites
The granites chosen for this study represent a variety of both mineralized and barren occurrences.
P.K. Srkastava, .A.K. Sinha/ Journal of Geochemical Exploration 60 (19971 173-184
oL--Ll
3
B
Rb
I
Sr
!
I
Ba
Nb
-
TUNGSTEN
o---a
BARREN
c-s
AVERAGE
I
Zr
I
Zn
ill
GRANITES GRANITES GRANITES
I
Sn
J
W
Fig. 5. Trace element variation diagram for granites of Rapsthan.
The Tungsten granites from the area included in the study differ in character as well as in age. Balda granite is leucocratic and rich in muscovite while Degana and Sewariya granites are two-mica granites with more mafic minerals. The Balda granite is equigranular while the Degana and Sewariya granites are porphyritic with big phenocrysts of K-feldspar. These granites are described in detail by Srivastava (1990); Bhattacharjee et al. (1993) and Chattopadhyay et al. (1994). Similarly the granites included in the Barren granite group represent various areas of Rajasthan and are of Early Precambrian to late Palaeozoic ages. Therefore, all the granites, irrespective of their composition, origin and ages, are divided into two groups: (1) the WG, granites which are associated with tungsten mineralization, i.e. Tungsten granites, and (2) the BG, granites which are not associated with mineralization, i.e. the Barren granites. For convenience these abbreviations will be used in the rest of the text.
The granites included into WG and BG groups from the Balda, Degana and Kalni-Kotariya areas are analysed by XRF and AAS for their major and trace element contents. Geochemical data for granites from other areas are taken from the published literature (Basu, 1982; Gangopadhyay and Lahiri, 1988; Eby and Kochhar, 1990; Rahman and Zainuddin, 1990, 1993; Bhattacharjee et al., 1993; Munshi, 1993). For the purpose of comparison the geochemical data for the entire granite suite are considered.
6. Major element geochemistry The major element composition of the studied granites is given in Table 1. WG are mostly silica-rich with SiO, ranging from 69 to 78%, mostly falling between 71% and 73%. The normative quartz of WG is normally between 30 and 40%, except for Sewariya granite where average CaO is 1.29, whereas
P.K. Sriuastaua, .A.K. Sinha / Journal of Geochemical Exploration 60 (I 997) 173-184
178
Table 2 Trace element concentration (median values) of the granites associated with tungsten mineralization (WG) and Barren granites (BG) from Rajasthan
cu
the Balda granite and Degana granite are very poor in CaO content. They are also depleted in Fe0 and MgO. Ishihara (197 1) suggested that W mineralization in Japan is commonly associated with less oxidised granites but no obvious change in Fe,O,/FeO ratio was observed for WG and BG groups in Rajasthan. However, the K,O/Na,O ratios for the tungsten-bearing granites are restricted to a limited range (1.27-2.76) but are scattered for the Barren granites (Fig. 2). Juniper and Kleeman (1979) used a SiO,-(CaO + Fe0 + MgO)-(Al,O, + Na,O + K,O) diagram to discriminate between Sn granites and Barren granites. This does not hold true in the case of W-bearing granites and an overlap is observed (Fig. 3) between the areas of WG and BG. When the compositions of granites are plotted in a (Na,O + K,O)-CaO-(Fe0 + MnO + MgO) temiary diagram (Fig. 41, a clear demarcation between WG and BG is observed. It suggests that the W granites are enriched in total alkalis and are depleted in Cao, FeO, MnO and MgO. This is in agreement with the results of Liu and Ma (1993).
27 68 69 11 3 74 34 II 33 209 94 507 1263
_
Pb Zn Sn W Nb B Be Li Rb Sr Ba Zr
570 663 57 87 146
K/Rb Rb/Sr Ba/Rb Mg/Li
101.1 11.8 0.2 4.9
31 103 18 43 174 _
150.9 2.2 2.4 345.4
Rb
DIORITES OTZ-DIORITES ---
Sr
Ba Fig. 6. Rb-Ba-Sr ternary diagram for the Tungsten Symbols used are the same as in Fig. 2.
and Barren granites of Rajasthan
(fields are after El Bouseily and El
sofiary,1975).
P.K. SriL)astaua, .A.K. Sinha/ Journal of Geochemical Exploration 60 (1997) 173-184
Barren granites. The Degana granite shows a very high value for W ranging from 300 to 400 ppm which can be attributed to the presence of disseminated wolframite mineralization. The variation diagram (Fig. 5) suggests higher value for Sn, W, Nb, B, Rb. and Be and depletion of Ba and Sr for WG in comparison to Bg. The lower K/Rb ratios along with lower Ti in the WG suggest that they are strongly differentiated products of the parent magma. The same is reflected by a Rb-Ba-Sr ternary diagram (Fig. 61, where the W-mineralized granites fall in the field of strongly differentiated granites. Tischendorf (1977) proposed that K/Rb ratios above 100 with Rb values below 500 ppm and an enrichment of Li, Be, W, Sn, Nb and Ta together with depletion in Ba, Sr and Zr, are some of the characteristics of mineralized granites. Beus and Oyzerman (19651, Sheraton and Black (19731, Lawrence (19741, Groves and McCarthy (1978) and Biste (1982) have used elemental ratios such as K/Rb, Mg/Li and Ba/Sr as one of the criteria in discriminating between mineralized and barren granites. In Rajasthan, the K/Rb, Mg/Li and Rb/Sr
According to Pandian (19861, Srivastava (1990) and Bhattacharjee et al. (1993) the granites included in the WG group are of the S-type category of Chappel and White (1974). But no systematic difference could be observed between BG and WG for the molar ratio of Al,O,/(Na,O + K,O + Al,O,) which is one of the criteria in distinguishing S- and I-type granites.
7. Trace element geochemistry The trace element composition of the two groups of granites shows a large variation because they were collected from a large area and represent different types of granitoids. Therefore the trace element composition for the WG and BG groups is reported in the form of median values rather than average values (Table 2). Tungsten granites generally have a higher concentration of W than Barren granites. Within the mineralized granites, the Balda and Sewariya granites have 2 to 5 times higher concentration of W than the
400
179
-
0
I
.
\
l .
?
/
.
!
i Oi OS
. . l.
l
*a l.
..--L-i
\ I
/ IO
I.0
I 100
Rb/Sr
Fig. 7. Rb/Sr
vs K/Rb
plot for the Tungsten granites and Barren granites of Rajasthan.
Symbols used are the same as in Fig. 2
180
P.K. Sricastaca,
.A.K. Sinha/Joumul
qf Geochemical
ratios successfully discriminate the W-bearing granites from the Barren granites. When the K/Rb ratio is plotted against Rb/Sr ratios (Fig. 7) the WG group forms a distinct field with lower K/Rb ratios and higher Rb/Sr ratio than the BG group. Student’s t-test was applied with a level of confidence of 0.95 to determine whether there are any significant changes between the two groups. As the t-test requires normally distributed populations, the calculations are based on log-transformed concentrations. It suggests that WG is enriched in alkalis (Na,O + K,O), Rb, Nb, Li, W, Sn and are depleted in CaO, MgO, FeO, Ba and Sr. The resulting elemental ratios are efficient in discriminating Wbearing granites from Barren granites.
8. Discussion To discriminate the ore potentiality of granites, the geochemical results must be combined with the geological data.Schuling (1967) has argued that the Sn deposits of the world were restricted to well-defined ‘tin provinces’ and suggested an inhomogeneous distribution of the metal in the crust and also that the crust may be the main source of tin. Wright and McCurry (1973) and Kovalenko and Kovalenko (1984) have proposed that a geochemical culmination may exist in the deep crust or upper mantle. The importance of the composition of the source rocks in the formulation of W-bearing granites has been emphasised by Hesp (1971) and Flinter et al. (1972). However, Sheraton and Black (1973) and Hosking (1976) observed that not all the granitic rocks or even fractionated granites within a Sn-W province are associated with mineralization. But it is apparent that a combination of factors including metal-rich source rocks, well-advanced magmatic differentiation and presence of volatiles are necessary if economic deposits of W are to be formed. Ayres et al. (1982), Imeokperia (1983) and Liu and Ma (1993) observed that the small granite bodies intruding metasediments served as the most productive granite in a multi-stage intrusive terrain whereas the large batholiths are barren. Ohlander (1985) found that the larger granitic batholiths in Sweden are in situ and the small granitic bodies, which are related with MO mineralization, are of subcrustal origin and
Exploration 60 (1997) 173-184
the ore fluid is evolved during the movement of intrusive magma. It is also observed in Rajasthan where the granitic intrusions at Balda and Degana have an outcrop area of less than 5 km*. This is in agreement with Tischendorf (1977) and Stemprok (1979) who suggested that the mineralized granites form late erogenic plutons, which are often small and of shallow emplacement. Tauson and Kozolov (1973) have suggested that trace element concentrations and elemental ratios indicate the source of a rock, its mode of origin and also the evolution of the parent magma during its crystallization. Smirnov (1968) used metallogenic analyses to show that ore deposits of W, Sn, Li, Nb and Ta were related to ‘palingenic magma’ of crustal origin. From the major and trace element studies of the granites it may be suggested that WG are derived from metasedimentary source material. Liu and Ma (1993) suggested that the granitization and anatexis of W-bearing formations are probably important in the genesis of ore-generating granites. They also suggested that during multi-stage remelting-intrusion-differentiation processes, W would be mobilized, migrating upwards and concentrating in favourable structures at the top of the late small granite stocks. Orlova et al. (1987) and Stemprok (1990) have also proved experimentally, that a large amount of W can be dissolved in the anatectic melt produced from sediments which contain grains of W minerals, particularly scheelite. The fact that all the Tungsten granites in Rajasthan are of the S-type, i.e. formed by partial melting of crustal rocks, suggests that these granites might be products of the multistage melting including the anatexis of the geosynclinal sediments of the Delhi Super Group which might be enriched in W. However, this simplified conclusion needs further investigation. The W was mobilised and migrated upward and concentrated in structurally favourable localities such as shear zones and fractures. Greisenization is considered by many workers to be directly related to the action of volatile-rich hydrothermal solutions on the host rocks. Gundsambu (1974) has suggested that the W mineralization associated with acidic hydrothermal solution have commonly resulted in greisenization of the host granite. Greisenization is the common hydrothermal alteration present within the granites of WG in Rajasthan.
P.K. Srkastaua, .A.K. Sinha / Journal of Geochemical Exploration 60 (1997) 173-184
Deaano Granite .
I
I
0
BARREN
GRANITE
.
TUNGSTEN
Bolda
GRANITE South Elizabeth Moreebo South
Erinpura Average China Jalors
Granite
Sordenion
Sewar~a
South
Creek
Granite
Chmo
181
(India)
Granite (IndIal . W Field (China) .
(Australta)
(Ausfroliol
Grznlte
Granit:
(Italy)
(India)
Granite (Indlo) 0 Granite 0
Granite 0 Granite 0
Avg
(Chino)
(lnd~o)
Bundelkhand
Granite (Ind~o) 0 Grovite (Austroital 0 Granite (IndIal
Dumbano AharRIver D~doGronlte
cAustral:l 0
L -6
-5
1
1
_I -4
-3
-2
-I
0
I
2
3
I 4
I
1
5
6
Fig. 8. Geochemical Characterization Index for different Tungsten granites and Barren granites. (Data computed from Turekian and Wedepohl. 1961; Sheraton and Black, 1973; Biste, 1982; Eby and Kochhar, 1990; Rahman and Zainuddin. 1990: Liu and Ma. 1993; Bhattacharjee et al.. 1993; Rahman and Zainuddin. 1993; and these authors’ work.)
It may have resulted from the volatile-rich hydrothermal ore fluids which were derived from the parent magma during the late stages of cooling. Srivastava and Sinha (1995) emphasized the use of greisenization and quartz-muscovite-fluorite + tourmaline alteration as a guide for mineral exploration for W deposits. Flinter et al. (1972) and Juniper and Kleeman (1979) suggested the usefulness of major element chemistry in discriminating mineralized granites from the Barren granites. Stemprok and Skorv (1974), Srivastava (1990) and Liu and Ma (1993) suggested that the granites spatially and genetically associated with the Sn, W and MO mineralization are characterized by the relative enrichment of SiO, and depletion of some major element oxides such as CaO, MgO and FeO. But Drake (1981) and Ohlander (1985) reported that the major elements are not useful in discriminating mineralized granites from Barren granites. Keith et al. (1989) and Newberry et al. (1990) reported a similar geochemistry for the W-related and W-barren granites and concluded that the exploration should focus on mineralogical, textu-
ral and geological features indicative of highly fractionated magmas in deeper environments rather than the geochemical features of granitoids. The present study, however, upholds the observation of Stemprok and Skorv (1974) and Liu and Ma (1993). The granites related to W mineralization in South China are SiO,-rich, KzO + Na,O > 7.5, K,O > Na,O and contents of Ca, Mg, Fe, Ti are relatively lower than in average granites (Liu and Ma, 1993). The W granites of Rajasthan show the same pattern. Biste (1980) presented a ternary diagram based on compositional data of Rb-Ba-Sr from a number of granites and found that the mineralized types fall in a particular field which is similar to the highly differentiated field of Fig. 6. Extending his studies to the granites of South Sardinia, Biste (1982) found noticeable differences in the trace element composition of metallogenetic and non-metallogenetic granites. In the mineralized granites he has noted relatively higher concentrations of F, Rb, Li and Sn with increased Rb/Sr, Li/K and Ba/Sr ratios and comparatively depleted concentrations of Sr, Ba and Zr and lower values of K/Rb. Ba/Rb and Mg/Li. Groves and
182
P.K. Sriuastaua, .A.K. Sinha / Journal of Geochemical Exploration 60 11997) 173% 184
McCarthy (1978) have also emphasized the efficacy of concentration ranges of Sr and Ba as a possible tool for evaluating the economic potential of granitoid rocks and reaffirmed the usefulness of the Ba/Rb ratio to identify Sn and W granitoids. Tischendorf (1977) observed that Ba, Sr and Zr are typically depleted, whereas Be, Li, Rb, Sn, W and Nb are expected to be enriched in the W- and Sn-bearing granitoids. The present study confirms that the low values of K/Rb ratio and high values of Rb/Sr ratio would suggest W-bearing granite. It may be inferred from this study in Rajasthan that the distinction between Tungsten granites and Barren granites within any particular region apparently can be made on the basis of one or more of the elemental ratios, viz. K/Rb, Rb/Sr, Ba/Rb, Mg/Li. Very high ratios of Rb/Sr and very low ratios of K/Rb, Mg/Li and Ba/Rb are indicative of W-Sn mineralization. From these relations there seems to be a reasonable expectation that WG could be discriminated on the basis of combining the variations in K, Mg, Rb, Sr, Ba and Li through a multiplicative ratio technique. Using these trace element discriminators, a Geochemical Characterization Index (GCI) Rb3 X Li X lo4 is proposed as: GCI = log,, MgXKXBaXSr for characterizing W-bearing granites from the Barren granites. In this calculation all the values are in ppm. It is found that the Geochemical Characterization Index for the W-bearing granites has values which are orders of magnitude higher than for Barren granites. A positive value for GCI will represent W-bearing granites while a negative number will represent Barren granites. The index has been positively tested (Fig. 8) for many of the granites from India and the rest of the world.
9. Conclusions A work plan for future exploration is now possible by the synthesis of available field and laboratory data and the ore potential of the granitoids can be assessed by major and trace element geochemistry. From the present study it may be concluded that: (1) The W-bearing granites are enriched in (Na,O + K,O), Rb, Li, Sn, W, Nb and are depleted in CaO, MgO, FeO, Sr and Ba.
(2) The W-bearing granites are the product of highly differentiated magma. (3) The elemental ratios K/Rb, Mg/Li, Ba/Rb and Rb/Sr can be successfully used to discriminate between W-bearing and Barren granites. (4) Positive GCI values, i.e. log,,(Rb3 X Li X 104/Mg X K X Ba X Sr) for any granite will suggest its W potentiality in an area. (5) The W gr am‘tes of Rajasthan are the result of multi-stage melting and anatexis of metasediments of the Delhi Super Group.
Acknowledgements The financial assistance to first author (PKS) from Council of Scientific and Industrial Research and Department of Science and Technology, New Delhi is sincerely acknowledged. The manuscript has been significantly improved by critical reviews of Prof. Peter Pollard and Dr. Graham Taylor.
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