19 The Indian Mineralogist, Vol. 44, No. 1, January 2010, pp. 19-44 © 2010 The Mineralogical Society of India, Mysore, ISSN : 0019-5928 PETROMINERALOGY, GEOCHEMISTRY AND GEOCHRONOLOGY OF RADIOACTIVE LAMPROPHYRE DYKES AND ASSOCIATED ERINPURA GRANITES AROUND ANDOR, SIROHI DISTRICT, RAJASTHAN, INDIA P.B. MAITHANI1, RAHUL BANERJEE1*, R. GUJAR2, R. CHOPRA3 AND B.K. PANDEY1 Atomic Minerals Directorate for Exploration and Research AMD Complex, Begumpet, Hyderabad - 500 6291 Central Region, Civil Lines, Nagpur - 440 0012 Northeastern Region, Shillong - 793 011 3 * Email: rahulbnrg@gmail.com ABSTRACT Southern part of Rajasthan, well known for lamprophyre intrusives associated with Ajabgarh supracrustals of South Delhi Fold Belt (SDFB) and alkaline complexes. They display a new assemblage of lamprophyres with Neoproterozoic Erinpura granites near Andor in Sirohi district. These lamprophyres have shown anomalous uranium contents (upto 0.116% U 3O8) along the highly ferruginised and fractured contact zones with Erinpura granites. Petrochemically, they show ultrabasic basanitic composition and sodi-potassic nature similar to alkaline lamprophyres (AL) and can be classified as ‘camptonite’. However, radioactive lamprophyres have significantly low CaO content (~1.4%). They also show significant enrichment of Cu, Ni, Zn, V and Y as compared to non-radioactive lamprophyre. The associated peraluminous high-K calc-alkaline pink granites exhibit features similar to highly evolved S-type granites as evidenced by their highly fractionated and differentiated (DI: 80–91) nature. The Rb–Sr whole-rock isochron data of Andor lamprophyres have yielded an age of 740±64 Ma with initial 87Sr/86Sr ratio of 0.7071±0.0018. These mafic dykes show close spatial and temporal association with felsic magmatism i.e., Erinpura granite plutons of Sirohi and adjoining areas (c. ~740 Ma) and can be inferred that they are coeval to Erinpura granites and probably emplaced during the waning stage of Neoproterozoic tectonomagmatic cycle of SDFB. Presence of discrete uranium minerals (pitchblende and davidite) together with ilmenite and anatase mostly associated with ferruginous matter along the microfractures account for the uraniferous anomalies in lamprophyres, whereas resistate minerals viz., monazite, thorite, zircon and fine opaques associated with fluorite mainly control the radioactive phases in granites. A pronounced and progressive enrichment of uranium has been indicated in both lamprophyres and granites, where post-magmatic events such as hydrothermal activity and alteration phenomenon leading to epigenetic vein type uranium mineralisation. Hence, widespread prevalence of lamprophyres in Sirohi district viz., Isra, Danwa, Pipela, not only confirms alkaline magmatism but also makes them potential targets for exploration considering their close spatial association either with Cu-Zn-Au or U mineralisation. INTRODUCTION Lamprophyre dykes are widespread throughout the world and occur in varied geological setup right from Precambrian to recent, but got very little attention due to its complex and highly altered nature. However, lamprophyre research came into the prominence during last three decades considering their significance in tectono-magmatic evolution studies (Allan and Carmichael, 1984; Leat et al., 1986; Wyman and Kerrich, Ind. Mineral., v.44, no. 1, January 2010 20 LAMPROPHYRE DYKES AND ASSOCIATED ERINPURA GRANITES, RAJASTHAN, INDIA 1989a; Rock, 1991; Shand et al., 1994; Duggan and Jaques, 1996; Madhavan et al., 1998; Kirmani and Fareeduddin, 1998, 2000; Xu et al., 2007) and their close temporal-spatial relationship to gold and diamond mineralisation (Boyle, 1979; Rock and Groves, 1988; Wyman and Kerrich, 1989b; Taylor et al., 1994; Ashley et al., 1994; Huang et al., 2002). In Rajasthan, several lamprophyre dykes are reported from southern part, which are either associated with undersaturated alkaline complexes, viz. Kishangarh and Mundwara areas (Sharma, 1969; Chatterjee, 1974; Rock and Paul, 1989) or occurring as alkaline intrusives in Ajabgarh supracrustals of South Delhi Fold Belt (SDFB) such as Pipela and Danwa lamprophyres (Fareeduddin et al., 1995, 2001). AMD’s integrated efforts during late nineties have resulted in identifying a new association of lamprophyres with Erinpura granites, viz. Isra and Andor lamprophyres (Banerjee, 1996, 1997; Gurjar and Chopra, 1996; Gurjar, 1997; Banerjee et al., 1998; Maithani et al., 1998, 2000, 2008). Both, Andor lamprophyre and Isra lamprophyres, located 35 km apart, are found to be radioactive in nature and hence, gives a new dimension to mineral exploration in Sirohi area i.e., uranium exploration. Mafic dykes, 2.25 km NW of Andor (25°02’17" : 72°51’12"; T.S. No. 45C/16), are found to contain anomalous uranium values upto 0.116% U3O8 and identified as lamprophyre (Bergman, 1987) belonging to the alkaline branch of lamprophyre clan (Rock, 1987, 1991; Woolley et al., 1996). The uranium mineralisation is predominantly recorded along the contact zones of lamprophyres with pink granites showing significant structural control. These dykes might have provided the thermal gradient for the mobilisation of uranium from fertile Erinpura granites. Various field and geochemical features together with Rb–Sr age data have indicated that these lamprophyres are coeval to Erinpura granites. This paper reports first geochronological data of lamprophyres from Sirohi district, Rajasthan and discusses petrogenesis and uranium mineralisation associated with Andor lamprophyres and Erinpura granites. GEOLOGICAL SETTING Regional Geology Regionally, the NNE–SSW trending SDFB, represented by Mesoproterozoic metasediments of Sirohi Group and Delhi Supergroup, occupies the axial zone of Aravalli– Delhi orogenic belt (Volpe and MacDougall, 1990; Deb et al., 2001). This belt is developed on paleorifts within reworked Archaean granite-greenstone-granulite basement terrain (Banded Gneissic Complex; BGC) and show indications of resurgent tectonics in which dilatational and compressional regimes alternated in time (Sinha-Roy et al. 1995). The SDFB is flanked towards southeast by Paleoproterozoic rocks of Aravalli Supergroup while vast expense of Neoproterozoic (720–835 Ma; Crawford, 1970; Choudhary et al. 1984) plutono-volcanic felsic magmatism i.e., Erinpura and Malani igneous suites successively mark the northwestern flank (Coulson, 1933; Heron, 1935, 1953; Roy, 1988; Chore, 1990; Gupta et al., 1997). The general tectono-stratigraphic succession (after Roy and Jakhar, 2002; GSI, 2004; Ramakrishnan and Vaidyanadhan, 2008) is given in Table 1. Ind. Mineral., v.44, no. 1, January 2010 P.B. MAITHANI AND OTHERS 21 Table 1: Generalised tectono-stratigraphic succession of the study area (Modified after Roy and Jakhar, 2002; GSI, 2004; Ramakrishnan and Vaidyanadhan, 2008) Supergroup Group Malani Igneous Suite Broad Lithology Age Dyke Swarms, Granites, Neoproterozoic Felsic and Mafic Volcanics (750 – 720 Ma) Mafic and Ultramafic intrusions in SDFB and Erinpura Granite Erinpura Granite Grey and Pink Granites, Neoproterozoic Granitic Gneisses (835 – 720 Ma) Sindreth Mafic and Felsic Volcanics, Neoproterozoic Conglomerate, Quartzite (850 – 800 Ma) Sirohi Carbonaceous Phyllite, Quartzite, Schists, Marble Ajabgarh/Kumbalgarh Arkose, Marble, Metabasalt, Mesoproterozoic SDFB Metapelite, Metagreywacke, (1100 – 900 Ma) Delhi Alwar/ Gogunda Carbonaceous Phyllite, NDFB Schists, Ophiolite Suite, Mesoproterozoic Rayanhalla Conglomerate, Quartzite (1600 –1450 Ma) ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ Unconformity ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ Upper Aravalli Ultramafic flows, Arenites, Metabasalt, Stromatolitic Aravalli Middle Aravalli Dolostone, Carbonates, Palaeoproterozoic Phyllite, Greywacke, (2200–1800 Ma) Lower Aravalli Conglomerate, Quartzite ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ Unconformity ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ Mewar Gneiss Gneisses, Granitoids, Archaean Amphibolites, Quartzites (3300–3000 Ma) Local Geology The area north of Sirohi exposes Sirohi Group of rocks represented by phyllite/ carbonaceous phyllite, quartz mica schist with intercalated quartzite, dolomitic marble and calc-silicate bands. They occur in a long elongated trough and exhibit complex structural pattern. These are traversed by massive emplacements of Erinpura plutonic and Malani volcanic suites (Fig. 1a). Erinpura plutonic suite, representing a significant magmatic event at the culmination of Delhi orogeny in this sector, consists of granites, granitic gneisses and syenites while rhyolites and porphyritic rhyolites mark the Malani volcanic suite. Andor and adjoining areas are largely covered by soil/alluvium with isolated outcrops of granite and rhyolites. The Erinpura plutonic suite is mainly represented by grey and pink granites in the study area. These are medium to coarse grained and at places show porphyritic nature due to the presence of large phenocrysts of K-feldspar. The granites are seen traversed by a number of NW–SE, NE–SW and nearly E–W trending mafic and felsic dykes, quartz and aplite veins and occasionally pegmatites. Some of the mafic dykes emplaced within pink granites, located at 2.25 km NW of Andor, are identified as lamprophyres. These NW–SE and nearly N–S dykes are delineated close to the northern Ind. Mineral., v.44, no. 1, January 2010 22 LAMPROPHYRE DYKES AND ASSOCIATED ERINPURA GRANITES, RAJASTHAN, INDIA extremity of 30km long NNE–SSW trending Ker–Palri lineament considered to be post Erinpura fault (Sharma, 1991). These lamprophyres are characterized by dark brown/ brownish grey to greyish black coloured, hard and compact central portion and reddish brown coloured, highly fractured, granulated and ferruginised peripheral portion i.e., the contact zone with granites. PETROGRAPHY Lamprophyres Lamprophyre bodies at Andor can be classified into two distinct zones i.e., chilled margin and central porphyritic zone, based on variations in grain size, colour, texture and Fig.1a. Geological map of Andor and adjoining areas, Sirohi district, Rajasthan showing radioactivity anomalies b. Detailed geological map showing radioactive (RA) and non-radioactive (NRA) lamprophyres associated with Erinpura granites Ind. Mineral., v.44, no. 1, January 2010 P.B. MAITHANI AND OTHERS 23 mineralogy. The chilled margin is a melanocratic to mesocratic cryptocrystalline layer characterized by microporphyritic texture with a few phenocrysts/microlaths of plagioclase set in an almost aphanatic matrix. This zone is probably affected by dynamic metamorphism resulting in highly fractured and granulated constituents. Besides, abundant greenish yellow to deep red coloured ferruginous material has infiltrated along the fractures and practically masking the mineralogy. The transition from chilled margin zones to central porphyritic zone is gradual with progressive increase in size and quantity of phenocrysts. The central zone is characterized by melanocratic fine grained rock exhibiting porphyritic, glomeroporphyritic and subophitic textures. They predominantly contain euhedral to subhedral phenocrysts of plagioclase (labradorite and andesine), titanaugite, hornblende (barkevikite), biotite, titanomagnetite, a few olivine and nepheline grains in a groundmass composed of feldspars, pyroxene, chlorite, calcite, Ti rich iron oxides and interstitial glassy material, and resemble closely to ‘camptonite’ of alkaline lamprophyre (AL) clan. Elongated laths of plagioclase are generally oriented in an irregular fabric giving rise to the subophitic texture by the presence of their off-sets in the pyroxene grains. Another conspicuous feature is the occurrence of coarse and tabular xenocrystic plagioclase with corroded margins forming glomeroporphyritic segregates. At places ocellar texture is also recorded by the presence of ocelli of ~1 mm diameter composed with zeolites (analcime) and globules of brownish amphibole. Bladed ilmenite, skeletal and octahedral titanomagnetite, euhedral pyrite, chalcopyrite and anatase are identified as major ore minerals in these lamprophyres. Besides, in one sample an anhedral grain of sphalerite (370m) is seen surrounded by chlorite and charged with fine granules of chalcopyrite. The lamprophyres have shown moderate to high degree of mineralogical alterations such as intense sericitisation and saussuritisation along the cleavage planes of plagioclase laths, moderate degree of chloritisation and uralitisation of pyroxenes etc. A lot of pale green pleochroic chlorite also occurs as the groundmass together with relict pyroxene and biotite grains signifying the alteration phenomenon. Alteration of minor and ore minerals are also common viz., serpentinisation of olivine, alteration of nepheline to zeolites, titanomagnetite and ilmenite to anatase and hydrated iron oxides, pyrite to goethite, and replacement of chalcopyrite by covellite on the grain boundaries. Some of the thin sections contain network of Ti rich iron oxide veinlets indicating effects of ferruginisation. Hence, presence of sericite, chlorite, serpentine, calcite, zeolites etc. are indicative of their induction through hydrothermal alterations during post-magmatic stage. As far as radioactive mineral is concerned, the CN-85 film autoradiographic studies indicated scattered alpha tracks corresponding to ferruginous matter. However, the track density is comparatively higher over some completely opaque patches as observed along the fine veinlets suspected to be containing pitchblende (Fig. 2 a, b). Besides, X-ray mineralogical analyses have identified davidite, anatase and ilmenite as radioactive minerals in these lamprophyres. Ind. Mineral., v.44, no. 1, January 2010 24 LAMPROPHYRE DYKES AND ASSOCIATED ERINPURA GRANITES, RAJASTHAN, INDIA Granites The granites are medium to coarse grained and show typical hypidiomorphic granular texture. However, in graphic granites intergrowth between quartz and alkali feldspar is also noticed at many places. The essential minerals consist of quartz, alkali feldspar and plagioclase while biotite, muscovite, chlorite, sericite, monazite, zircon, sphene, allanite and fluorite are the accessory minerals. Other ore and opaque minerals include pyrite, chalcopyrite, magnetite, hematite, anatase and goethite. Modal composition of these granites shows 32.5–38.8% quartz, 32.5–50.4% alkali feldspar, 12.1–24.8% plagioclase, 2.8–4% biotite and 0.3–2.1% resistate and opaque minerals. Quartz shows anhedral shape with minor strain effects, and at places slightly fractured in nature. Alkali feldspar is mainly represented by orthoclase perthite, which occur in subhedral form and show mild sericitisation along the cleavage traces giving rise to hazy appearance. Besides, flame and vein perthites are seen following the cleavage planes and occasionally dominating over the K-feldspars. Plagioclase is albite-oligoclase type, subhedral, moderately sericitised and partly epidotised in nature. At places, pre-blastic sericitised plagioclase patches are enclosed in K-feldspar megablasts. Biotite is the most common accessory constituent and contains inclusions of radioactive minerals viz., thorite, zircon and sphene. They are seen replaced by fluorite and muscovite (Fig. 2 c, d) signifying potash metasomatic alterations. Fig. 2a & b. Photomicrograph showing disseminated pitchblende along microfractures in Andor lamprophyres and corresponding alpha tracks . c. Association of biotite, secondary muscovite and fluorite in granites suggest metasomatic alterations. d. Formation of secondary muscovite in granite due to potash metasomatic replacement in biotite. Ind. Mineral., v.44, no. 1, January 2010 P.B. MAITHANI AND OTHERS 25 RADIOACTIVITY Significant uranium anomalies have been recorded in two lamprophyre dykes (35– 135m x 1–5m) within medium grained pink coloured Erinpura granites exposed at about 2.25 km NW of Andor (Fig 1a, b). The dykes are highly fractured, altered and ferruginised at their contact with granites. High order radioactivity is mostly recorded at lamprophyre– granite contact zones and associated with ferruginous matter. Physical assay results of radioactive lamprophyre samples have shown values upto 0.116%U3O8 with practically no thorium while non-radioactive lamprophyre has analysed upto 5ppm U. Chemical analysis of two radioactive lamprophyre samples have indicated 0.45 to 0.10% U3O8(total) and 0.0183 to 0.0433% U3O8(leachable) indicating comparatively low leachability. Presence of discrete uranium minerals pitchblende and davidite associated with ferruginous matter along the microfractures account for the uraniferous anomalies in Andor lamprophyre. Mixed uranium–thorium anomalous values have also been recorded in Erinpura granites in Andor and adjoining areas showing an average assay value of 26ppm U3O8 and 74ppm ThO2 (n=24) with U:Th ratio varying from 0.92 to 13.64. This signifies the labile nature of uranium in the system and therefore, it could be a good source for uranium in lamprophyres. The radioactive phases in pink granites are mainly controlled by resistate minerals viz., monazite, thorite, zircon and fine opaques associated with fluorite. Sample assay results of different lithounits are given in Table 2. Table 2 : Radiometric assay data of Andor and adjoining area eU3O8 (ppm) Range Average 20 - 70 45.42 Range Average 200 – 820 561.67 U3O8 ( ˜ /γ) ThO2 (ppm) (ppm) Granite (n = 24) 5 – 100 24 – 140 26.21 74.25 Lamprophyre (n = 6) 290 – 1160 2.5 – 10 860 5 %K Th/U 2.5 – 8.8 3.93 0.92 – 13.64 4.10 0.06 – 3.3 1.88 ~0.0 – 0.04 0.01 SAMPLING AND ANALYTICAL PROCEDURES Lamprophyre and granite samples for isotopic and chemical analyses were collected from Andor and adjoining areas showing radioactivity anomalies. All precautions were taken to collect relatively fresh samples by discarding the weathered portions. Seven samples each from lamprophyres and granites were selected for Rb–Sr whole rock isotopic studies whereas five radioactive and one non-radioactive lamprophyre samples and eleven granite samples were subjected to major and trace element analyses. Rb–Sr isotopic analysis was conducted on a VG354 mass spectrometer while major and trace elements analyses of six lamprophyre and three granite samples were performed on a Philips X’unique II– WDXRFS (Wavelength Dispersive X-Ray Fluorescence Spectroscopy) controlled by computer with X-40 and Super Q software. In addition, eight granite samples were also analysed for their major element geochemistry by conventional wet chemical methods and atomic absorption spectrometry. Ind. Mineral., v.44, no. 1, January 2010 26 LAMPROPHYRE DYKES AND ASSOCIATED ERINPURA GRANITES, RAJASTHAN, INDIA GEOCHRONOLOGY Abundances of Rb, Sr and isotopic composition of Sr for lamprophyre and granite samples of Andor area are given in Table 3 and 4. Various observations related to Rb– Sr isochron ages and initial 87Sr/86Sr ratios of these rocks are discussed below. Table 3 : Whole rock Rb–Sr data of Andor lamprophyre 87 87 S. No. Rb (ppm) Sr (ppm) Rb/86Sr Sr/86Sr ADR/2 111 54.9 5.89 0.7698 ADR/1 120 118 2.95 0.7389 ADR/6 151 243 1.80 0.7261 ADR/7 159 244 1.89 0.7259 ADR/5 137 249 1.60 0.7240 ADR/3 182 200 2.65 0.7192 ADR/4 101 279 1.04 0.7187 Errors (2σ) 2% 1% 2% 0.05% Age : 740±64 Ma; Initial 87Sr/86Sr : 0.7071±0.018; MSWD : 6.9 Table 4 : Whole rock Rb–Sr data of Erinpura granite of Andor area S. No. EG/2 EG/7 EG/6 EG/5 EG/1 EG/3 EG/4 Errors (2σ) Rb (ppm) Sr (ppm) 720 650 665 634 611 696 670 2% 13.3 14.2 14.4 14.2 14.2 18.0 20.2 1% 87 Rb/86Sr 185.5 151.5 152.2 146.6 141.2 123.2 105.3 2% 87 Sr/86Sr 2.5809 2.1836 2.1255 2.1007 2.0937 1.7659 1.7007 0.05% Model Age (with I.R. 0.7) 711 Ma 686 Ma 655 Ma 669 Ma 692 Ma 607 Ma 667 Ma The isotopic data plots of lamprophyre samples in the conventional Rb–Sr evolution diagram (Fig. 3) have yielded an isochron age of 740±64 Ma with an initial 87Sr/86Sr ratio of 0.7071±0.0018 with a MSWD of 6.9. However, the sample point (ADR/3) deviates from this linear array and hence, excluded. Such deviation is possibly due to the partial open system behaviour, where Rb is possibly added during K-metasomatism as evident from petromineralogical studies. The initial 87Sr/86Sr ratio of 0.7071 is slightly higher than the contemporary depleted mantle reservoir and suggests some crustal contamination during the ascent of the mafic magma. Field disposition indicates that these are probably coeval to Erinpura granites. The 87Rb/86Sr ratios of granite samples of Andor area are abnormally high (105 – 185) as compared to normal granites. Regression of all these points together yields a linear array with an intercept corresponding to probable initial 87Sr/ 86Sr ratio of 0.4399±0.353 (diagram not shown), which is an improbable value considering the primordial 87Sr/86Sr ratio BABI (Basaltic Achondrite Best Initial), i.e. 0.69897. This abnormal disposition points towards the possibility of either loss of radiogenic 87Sr from the granite during post-emplacement thermal event or the addition of Rb by post-magmatic processes. Field and petrological studies have indicated signatures of K-metasomatism Ind. Mineral., v.44, no. 1, January 2010 P.B. MAITHANI AND OTHERS 27 Fig. 3. Rb–Sr isochron plot for the Andor lamprophyres. and hence, possibilities of introduction of Rb during this process by the influence of Kand volatile-rich fluids are stronger. This is also evident from very low abundances of Sr (14.2–20.2 ppm) inspite of presence of 12–24.8% plagioclase. The apparent age calculation of these granite samples by assuming initial 87Sr/86Sr ratio of 0.70 has indicated 607–711 Ma (Table 4). Erinpura granite plutons of Sirohi and adjoining areas have yielded an isochron age of ca.~740 Ma (Crawford, 1970; Sarkar, 1980; Sarkar et al., 1992; Pandey et al., 1995, Chabria et al., 1997) and hence, the granitic rocks of the present study area may also be considered of the same emplacement age. GEOCHEMISTRY AND PETROGENESIS The major element data, CIPW norms and other chemical parameters as well as corresponding trace element contents of lamprophyres and granites are given in Table 5 and 6, respectively. Major Elements Major element data indicates that Andor lamprophyres are silica-undersaturated and have moderate to high alumina and high to very high total iron, titanium and volatiles contents. The radioactive and non-radioactive lamprophyres display marked difference in their chemical compositions. This is evident from slightly low SiO2 (36.41–41.61%; except one sample), CaO (1.05–1.76%), Na2O (0.73–2.57%) and K2O (1.25–2.93%), and high Al2O3 (15.04–24.14%), FeOt (18.54–28.15%), and TiO2 (3.15–5.78%) contents in radioactive lamprophyres as compared to that of non-radioactive lamprophyre (SiO2: 45.78%; Al2O3: 16.59%; FeOt: 14.82%; TiO2: 2.05%; CaO: 9.03%; Na2O: 3.02%; K2O: 4.58%). Andor lamprophyres are also characterized by moderately low MgO content (5.60–6.38%; except two samples) with corresponding MG# ranging from 32.35 to 40.29, which can be attributed to the presence of high iron content. Besides, significantly low Ind. Mineral., v.44, no. 1, January 2010 28 LAMPROPHYRE DYKES AND ASSOCIATED ERINPURA GRANITES, RAJASTHAN, INDIA Table 5 : Major oxide and Trace element data of Lamprophyres of Andor area Major oxides in wt.% Sam. No. LM-1$ LM-2 LM-3 LM-4 LM-5 LM-6 AL* CM* 45.78 38.49 36.41 41.61 50.58 40.62 42.5 43.30 SiO2 TiO 2 2.05 5.64 3.15 5.78 4.88 5.10 2.9 2.9 Al 2O 3 16.59 24.14 23.88 15.04 17.52 20.76 13.7 14.2 Fe 2O 3 12.0 11.7 FeOt 14.82 20.54 28.15 23.78 18.54 21.03 MnO 0.28 0.28 0.20 0.23 0.22 0.23 0.2 0.19 MgO 5.61 0.99 0.05 6.38 5.60 6.05 7.1 6.4 CaO 9.03 1.33 1.76 1.41 1.27 1.05 10.3 9.9 Na 2O 3.02 2.13 0.87 0.90 0.73 2.57 3.0 3.0 4.58 2.93 2.05 2.84 1.25 2.26 2.0 2.1 K 2O P2O 5 0.47 0.37 0.16 0.28 0.25 0.21 0.74 0.7 H 2O 3.1 3.1 Total 102.23 96.84 96.68 98.25 100.84 99.88 Trace Elements (in ppm) Ba 363 436 287 197 63 78 Rb 76 271 282 238 91 203 Sr 334 45 41 51 63 56 Y 47 142 71 240 137 157 Zr 412 335 170 749 636 703 Nb 12 48 28 39 21 43 V 364 545 426 612 547 603 Cr 91 1823 Co 38 69 49 54 Ni 41 19 289 15 23 51 Cu 14 72 27 <5 <5 342 Zn 63 643 47 271 325 776 Ce 219 89 86 125 99 143 Pb <5 255 33 212 123 226 Ratios A/NK 1.67 3.62 6.54 3.30 6.86 3.11 A/CNK 0.63 2.65 3.49 2.11 3.60 2.42 K2O/Na2O 1.52 1.38 2.36 3.16 1.71 0.88 Mg # 40.29 7.91 0.32 32.35 35.00 33.90 CIPW Norms (Calculated on anhydrous basis and Fe as FeOt) Qtz 19.17 Or 14.04 17.32 12.11 16.78 7.39 13.36 Ab 18.02 7.36 7.62 6.18 21.75 An 18.18 4.18 7.69 5.17 4.67 3.84 Hy 19.85 24.67 41.58 42.56 40.34 15.45 Ol 21.12 5.15 4.17 5.90 0.00 22.64 Ne 13.84 Lu 10.21 Ilm 3.90 10.72 5.99 10.98 9.27 9.69 Ap 1.11 0.88 0.38 0.66 0.59 0.50 * Average values of Alkaline Lamprophyres (AL; n=854) and Camptonite (CM; n=456) after Rock (1991) $ LM-1: Non-radioactive lamprophyre; LM-2 to LM-6: Radioactive lamprophyre Ind. Mineral., v.44, no. 1, January 2010 P.B. MAITHANI AND OTHERS 29 Table 6 : Major oxide and Trace element data of Granites of Andor area Major oxides in wt.% Sam. No. GR-1 GR-2 GR-3 GR-4 GR-5 GR-6 GR-7 GR-8 GR-9 GR-10 GR-11 SiO2 77.37 77.61 77.19 76.56 73.73 74.12 75.74 73.86 68.81 71.03 72.30 TiO 2 0.17 0.17 0.12 0.17 0.24 0.10 0.18 0.19 0.50 0.16 0.22 12.53 13.05 11.89 11.76 12.96 12.59 11.82 12.40 14.46 13.88 13.53 Al 2O 3 Fe 2O 3 0.11 0.57 0.42 0.51 0.6 2.07 0.6 0.85 FeO 2.53 1.63 1.72 1.61 1.5 1.6 1.72 1.64 0.7 1.5 1.62 MnO 0.03 0.03 0.03 0.02 0.02 0.02 0.02 0.01 0.04 0.02 0.04 MgO 0.53 0.46 0.15 0.22 0.18 0.42 0.20 0.21 0.64 0.20 0.14 CaO 0.30 0.23 0.83 0.55 0.20 0.85 0.83 0.65 1.63 0.75 0.51 Na 2O 1.24 1.19 2.24 1.42 2.10 2.28 2.02 2.06 1.98 2.96 1.89 K 2O 4.28 4.88 5.54 3.97 4.82 4.66 4.82 4.17 4.71 4.42 4.68 P 2O 5 <0.01 <0.01 <.01 0.24 0.23 0.24 0.21 0.20 0.40 0.17 0.20 1.40 1.30 1.50 0.80 1.60 1.90 1.80 1.91 H 2O + H 2O 0.40 0.20 0.60 0.20 0.40 0.41 0.60 0.60 Total 98.99 99.26 99.72 98.43 98.05 99.4 99.07 97.99 98.25 98.09 98.49 Trace Elements (in ppm) Ba 82 85 58 Rb 468 658 673 Sr 41 37 25 Y 73 78 90 Zr 141 184 142 Nb 26 41 22 V 27 16 <5 Cr 48 Co 24 23 Ni <5 <5 47 Cu 27 11 <5 Zn 89 12 36 Ce 32 <5 118 Pb 72 40 42 Ratios A/NK 1.88 1.80 1.23 1.77 1.49 1.43 1.38 1.57 1.73 1.44 1.66 A/CNK 1.74 1.70 1.06 1.54 1.43 1.22 1.18 1.36 1.28 1.26 1.49 K2O/Na2O 3.45 4.10 2.47 2.80 2.30 2.04 2.39 2.02 2.38 1.49 2.48 Mg # 27.19 33.47 13.45 20.10 14.32 28.70 14.77 15.28 29.70 15.46 9.76 CIPW Norms (Calculated on anhydrous basis and Fe as FeOt) Qtz 50.35 49.59 39.60 52.28 42.80 41.25 44.06 44.78 34.82 35.47 42.66 Or 25.29 28.84 32.74 23.46 28.48 27.54 28.48 24.64 27.83 26.12 27.66 Ab 10.49 10.07 18.95 12.02 17.77 19.29 17.09 17.43 16.75 25.05 15.99 An 1.45 1.11 4.08 1.16 0.00 2.65 2.75 1.92 5.47 2.61 1.22 Hy 5.74 3.91 3.39 0.55 0.45 1.05 0.50 0.52 4.07 0.50 0.35 Mt 1.20 1.14 1.20 1.42 1.01 1.54 2.23 Ilm 0.32 0.32 0.23 0.27 0.46 0.19 0.34 0.36 0.95 0.30 0.42 Hm 1.61 0.67 0.81 0.89 0.66 0.00 0.44 0.09 Ap 0.01 0.01 0.01 0.57 0.36 0.57 0.50 0.47 0.95 0.40 0.47 D.I. 86.13 88.50 91.29 87.75 89.05 88.08 89.64 86.85 79.41 86.64 86.31 Sample Nos. GR-1 to GR-3 analysed by XRF method; GR-4 to GR-11 by wet chemical method Ind. Mineral., v.44, no. 1, January 2010 30 LAMPROPHYRE DYKES AND ASSOCIATED ERINPURA GRANITES, RAJASTHAN, INDIA CaO content (~1.4%) in radioactive lamprophyres as compared to the average alkaline lamprophyres (AL: ~10%) is commensurate with the comparatively high iron (~22%) and alumina (~20%) contents. Though these lamprophyres display sodi-potassic nature but the dominance of potash over sodium is apparent from moderately high K2O/Na2O ratio (0.88–3.16). The enrichment of potash is probably due to the post-magmatic metasomatic alterations which are in accordance with the petrographic signatures. In K2O–MgO–Al2O3 plot (Fig. 4) of Bergman (1987), mafic rocks of Andor area are classified as lamprophyres substantiating the petrographic observations. Further discrimination of these lamprophyres based on (Na2O+K2O) versus SiO2 diagram shows their affinity towards alkaline lamprophyre (AL) branch (Fig. 5) as defined by Rock (1987). This is also apparent from the major element chemistry of Andor lamprophyres which resembles to a large extent with the average chemistry of AL belonging to Paleoproterozoic to Recent age and collected from varied tectono-magmatic association worldwide (Rock, 1991; Le Maitre, 2002) except the higher K2O/ Na2O ratio i.e., >1. This chemical dissimilarity is perhaps due to the introduction of potash in the system during hydrothermal alterations at a later stage. In SiO2 vs. MgO plot (not shown), these AL are falling in the field of Camptonite. This is further supported by the overall chemical composition displaying typical basanitic norms i.e., ab+or+an+ne+di+ol normative mineral assemblage. The total absence of normative quartz and predominance of plagioclase over alkali feldspar together with presence of little amount of feldspathoids also supports their classification as camptonite. The petromineralogical features such as dominance of plagioclase over alkali feldspar phenocrysts and feldspars over feldspathoids in groundmass corroborates the above observation. Fig. 4. K2O – MgO – Al2O3 plot showing Classification of Lamprophyric Rocks (Fields after Bergman, 1987) Ind. Mineral., v.44, no. 1, January 2010 Fig. 5. (K2O+Na2O) vs. SiO2 diagram for Andor lamprophyres. UML – Ultramafic Lamprophyre; AL – Alkaline Lamprophyre; LL – Lamproite; CAL – Calc-alkaline Lamprophyre (Fields after Rock, 1987). Symbols same as in Fig. 4. P.B. MAITHANI AND OTHERS 31 In total alkali–silica (TAS) diagram of Le Bas et al. (1986) with different boundary lines and coordinates as given by Rickwood (1989), Andor lamprophyres show scatter in the basanite field and close to its boundary with foidites, and belong to the alkaline series based on the alkaline–tholeiitic divide (Fig. 6). This is in accordance with the above discussed normative composition classifying Andor lamprophyres as camptonite. The radioactive lamprophyres are found to be oversaturated in respect of alumina as indicated by very high ASI (Alumina Saturation Index) i.e., molar A/CNK and A/NK varying from 2.11 to 3.60 and 3.11 to 6.86, respectively. The non-radioactive lamprophyre show characteristic metaluminous nature with molar A/CNK and A/NK values of 0.63 and 1.67, respectively (Table 5). Fig. 6. Total alkali–silica diagram (Le Bas et al., 1986) with alkaline and subalkaline subdivision boundaries (compiled by Rickwood, 1989) for rocks of Andor area. ( ) Lamprophyre (+) Granite As far as major element data of associated pink granite is concerned, all samples show normal SiO2 (68.81–77.61%) and Al2O3 (11.76–14.46%) contents. They are rich in K2O (3.97–5.54%) and Na2O (1.19–2.96%), and low in CaO (0.20–1.63%), MgO (0.15– 0.64%) and MnO (0.01–0.04%) contents (Table 6). Predominance of alkali content over calcium is shown by high alkali-lime ratio (4.10–34.60) in these granites. In TAS diagram, they plot distinctly in granite field (Fig. 6) and belong to subalkaline suite. These subalkalic rocks are further categorized as belonging to high–K calc-alkaline suite in SiO2-–K2-O diagram (Fig. 7a) of Peccerillo and Taylor (1976) and Le Maitre et al. (1989). The high-K character is also concordant with their high Al2O3 content (>11%) and supported by noticeable abundance of potash feldspars. The pink granites of Andor area have indicated Al saturated nature and display a dominant peraluminous behaviour with A/CNK (molar Al2O3/[CaO+Na2O+K2O]; Clarke, 1981) values ranging from 1.06 to 1.74. This is confirmed by sample plots in alumina saturation diagram (A/CNK vs. A/NK; Shand, 1943) where clustering is well within the peraluminous field (Fig. 7b). The alumina saturation index Ind. Mineral., v.44, no. 1, January 2010 32 LAMPROPHYRE DYKES AND ASSOCIATED ERINPURA GRANITES, RAJASTHAN, INDIA (ASI: Al/[Ca–1.67P+Na+K] ; Zen, 1986) was also widely used by Chappell and White (1974, 1992), Chappell (1999) and other workers to discriminate I-type (A/CNK <1.1) and S-type (A/CNK >1.1) granites indicating probable magma sources. Accordingly, almost all the samples of study area straddle in the field of S-type granite (Fig. 7b). These features together with high differentiation index (79.41–91.29) point towards derivation of magma from crustal source for these peraluminous calc-alkaline granitoids with relatively high alkali content. Fig. 7 a. K2O vs. SiO2 diagram (after Peccerillo and Taylor, 1976) illustrating the high-K calc-alkaline affinity of granites and shoshonitic affinity of Andor lamprophyres. Subdivision (broken line) after Le Maitre et al. (1989) and Rickwood (1989) (nomenclature in parentheses). b. Shand’s molar parameters A/NK = Al2O3/(Na2O+K2O) vs. A/CNK = Al2O3/ (CaO+Na2O+K2O) of Andor rocks (fields after Maniar and Piccoli, 1989). Dashed line represents boundary between I- and S- type granites (Chappell and White, 1992). Symbols same as in Fig. 6. Trace Elements Overall trace element distribution pattern of Andor lamprophyre indicates moderate to high abundance of Y (47–240ppm), Zr (170–749ppm), Ce (86–219ppm), V (364– 612ppm), and Co (38–69ppm) while Ni (15–51ppm; except one sample) and Cu (<5– 27ppm; except two samples) show low concentration. It is pertinent to note that trace element pattern of radioactive lamprophyres show 2–5 times enrichment in Co, Ni, Zn, Rb, Y, Nb, Pb and Sn, and depletion in Sr, Sc and Ce as compared to non-radioactive samples. The LILE contents of radioactive lamprophyres viz., Sr (41–63ppm) and Ba (63–436ppm) are much lower than those of AL (Sr: 990ppm; Ba: 930ppm; Rock, 1991) while Rb content (91–282ppm) is much higher than AL (50ppm). Probably this variation is the resultant of late stage intense hydrothermal alterations. The radioactive lamprophyres are enriched in some HFSE such as U, Zr and Y approaching to the values of radioactive/ rare metal bearing pegmatite, which probably signifies that either the parental magma source is enriched lithospheric mantle or extraction of these elements from granitoid wall rocks while emplacement and ascent of lamprophyric melt along the crustal fractures. Ind. Mineral., v.44, no. 1, January 2010 P.B. MAITHANI AND OTHERS 33 The analytical results of granites have indicated thorium dominated radioelemental abundance pattern (Th : 21–123 ppm (av. 65 ppm); U : 4–85 ppm (av. 22 ppm)). The granitoids are also marginally enriched in other HFSE viz., Ce (upto 118 ppm), Zr (upto 184 ppm), Y (upto 90 ppm), Nb (upto 41 ppm) and Pb (upto 72 ppm) and transition elements (Cr: 48 ppm; Zn: 12–89 ppm; Ni: <5–47 ppm and Sn 6–64 ppm) as compared to low Ca granites (Turekian and Wedephol, 1961). It is interesting to note that these trace elements show comparatively higher concentration in the contact zone portion of granite with lamprophyres than the samples away from this zone. Among the LILE, Rb show high abundance (468–673 ppm) while Sr (25–41 ppm) and Ba (58–85 ppm) show strong depletion. Predominance of Rb over Sr is quite apparent and indicating an evolved crustal source. In the Rb–Ba–Sr diagram (not shown) all samples fall in highly differentiated granite field. Tectonic implications Tectonomagmatic discrimination based on major and relatively immobile high field strength (HFS) elements have been attempted to know the geodynamic setting at the time of felsic and mafic magma evolution in Andor area. On the geotectonic discrimination diagram of (Batchelor and Bowden, 1985) based on multicationic factors R1 (4Si – 11[Na+K] – 2[Fe+Ti]) and R2 (6Ca+2Mg+Al) as proposed by De La Roche et al. (1980), the felsic granitoid samples show scatter between post-orogenic to syn-collisional fields (Fig. 8a), which is in accordance with the higher K, Na and total Fe contents as compared to that of Ca and Mg contents. In contrast, lamprophyres show clustering of sample points around late-orogenic field due to variable but higher CaO, MgO and Al2O3 contents as compared to K2O, Na2O and FeOt contents, which is in accordance with their mafic nature. The continental to spreading centre island geotectonic position of these lamprophyres with basanitic composition are distinctly shown by sample plots in Mgo–Feot–Al2O3 ternary diagram (Pearce et al., 1977; Fig. 8b). Ti, Zr, Y, Nb and Th based different discrimination diagrams point towards prevalence of a within plate (WP) to mid-oceanic ridge basalt (MORB) tectonic regime for Andor lamprophyres similar to the inferences drawn based on major element discrimination. This is evident from Zr–Th–Nb discrimination diagram (Wood, 1980), where these lamprophyres exhibit strong affinity to MORB suite (Fig. 8c) while in Ti/100–Zr–Y.3 ternary diagram (Pearce and Cann, 1973) they straddle between MORB and calc-alkaline basalt (CAB) tectono-magmatic association (Fig. 8d). Further, the study of the ratio Zr/Y relative to the index of fractionation Zr (Pearce and Norry, 1979) have shown scattered sample plots adjacent to the boundaries of WP basalt and MORB fields (Fig. 8e). Besides, the sample plots in widely used Y, Nb and Rb concentrations based tectonic discrimination diagram (Pearce et al., 1984) has indicated WP geodynamic setting for lamprophyres whereas associated granites fall in the syn-collisional field (Fig. 8f). These features of Andor lamprophyres can be explained by the magmatism at divergent or passive continental margins, a typical condition for generation of alkaline lamprophyres (Rock, 1991). Ind. Mineral., v.44, no. 1, January 2010 34 LAMPROPHYRE DYKES AND ASSOCIATED ERINPURA GRANITES, RAJASTHAN, INDIA Fig. 8. Tectonic discrimination diagrams for Andor lamprophyres and associated granites. Symbols same as in Fig. 6. a. R1-R2 multicationic plot (fields after Batchelor and Bowden,1985) b. MgO–FeO(t) –Al2O3 diagram (after Pearce et al., 1977) c. Zr –Th–Nb discrimination (fields after Wood, 1980) d. Ti–Zr–Y discrimination diagram (Fields after Pearce and Cann,1973) e. Zr/Y–Zr discrimination diagram (Fields after Pearce and Norry,1979) f. Rb–(Y+Nb) discriminant diagram (fields after Pearce et al.,1984) Ind. Mineral., v.44, no. 1, January 2010 P.B. MAITHANI AND OTHERS 35 DISCUSSIONS AND CONCLUSION The southern part of Rajasthan, especially Sirohi district, exposes ideal setup and congenial conditions for growth of different mineralisation. This is apparent from the fact that during the waning stages of Mesoproterozoic SDFB, the area was subjected to widespread emplacement of Neoproterozoic plutono-volcanic sequences and intense structural deformations. This intrusive igneous activity is associated with various mineralisations such as base metal, tungsten, gold etc. In this scenario, lamprophyres associated with different complexes have also played significant role in mineralisation, which is evident from the close spatial and temporal association of Pipela and Danva alkaline lamprophyres with base metal and gold mineralisation (Fareeduddin et al., 1995, 2001). Though lamprophyres are well known worldwide for their association with gold, base metal, tin and tungsten mineralisation, but uranium mineralisation associated with calc-alkaline and alkaline lamprophyres is also not uncommon. Such lamprophyres are reported from Keewatin, Canada (LeCheminant et al., 1987), Massif Central, France (Chalier and Sabourdy, 1987), Sesio-Lanzo, Italy (Ulmer et al., 1983) and Hopi Buttes, USA (Foland et al., 1980). Similarly, uranium mineralisation has also been reported in lamprophyres associated with highly evolved, S-type Erinpura granites at Isra and Andor in Sirohi district, Rajasthan (Banerjee et al., 1998; Maithani et al., 1998, 2000, 2008), and confirms the widespread presence of alkaline magmatism in the SDFB and Erinpura granites. Neoproterozoic plutono-volcanic sequence of Sirohi area is profusely traversed by mafic and felsic dykes, aplite and quartzo-feldspathic veins. The Andor lamprophyres represent the mafic member of these dyke suites, and mostly restricted to Erinpura granite country. This spatial and temporal association of Andor lamprophyres with Erinpura granites probably signifies their comagmatic nature. Besides, Rb–Sr whole rock age of 740 Ma has further confirmed their coeval nature as Erinpura granites in Sirohi and adjoining areas have also dated c.~740 Ma. In addition, the Andor lamprophyres show a prominent NW–SE trend similar to that of Isra, Pipela and Danva lamprophyres. These features together with non-association lamprophyre dykes with Malani suite, suggest widespread prevalence of alkaline magmatism at the culmination of Delhi orogeny (1000– 800 Ma; Volpe and McDougal, 1990) and prior to initiation of Malani volcanism (~750Ma). Andor lamprophyres are of alkaline type and fall in the category of lamprophyres occurring within Precambrian granitic terrain. Bulk rock chemistry shows silica-poor, and Al2O3 enriched nature together with comparatively higher concentration of TiO2 and FeOt similar to camptonite of AL clan, and plot in the field of basanite belonging to alkaline suite. However, some crustal signature vis-à-vis metasomatic/hydrothermal alterations can be deduced from significantly lower content of CaO and MgO (only in two samples), which can be attributed to corresponding higher iron content while enrichment of Al2O3 probably indicates slightly differentiated nature. This is further strengthening by their close association with Erinpura granite pluton as well as substantial alteration of granites along the contact zones. The trace element distribution pattern also point towards the strong alkaline nature and their generation in a divergent tectono-magmatic setup close to the continental margins as evident from their distribution in MORB and near WPB fields in various tectonic discrimination diagrams. There are several hypotheses Ind. Mineral., v.44, no. 1, January 2010 36 LAMPROPHYRE DYKES AND ASSOCIATED ERINPURA GRANITES, RAJASTHAN, INDIA regarding generation of such alkaline lamprophyres viz., by fractionation of alkaline basalt or similar magma, by fractionation of olivine and titanomagnetite of Kimberlitic magma etc. (Carmichael et al., 1974; Rock, 1991). But these hypotheses do not satisfy the association of lamprophyres with granite. Besides, Philpotts (1976) has observed that the rocks formed by fractional crystallisation should have low TiO2 whereas those derived from liquid immiscibility have high TiO2- content of several percent as is the case with Andor lamprophyres. Hence, the model of magmatic crystallisation involving two immiscible liquids, as suggested by Philpotts (1976) and Eby (1980) for the origin of alkaline rocks appears to be more logical in present context. The contamination of a primary ultramafic/mafic magma with granitic crust/coeval felsic magma is also corroborated by the association of felsic intrusives with lamprophyres, a part of mafic suite including other mafic and ultramafic rocks. Both, the mafic and felsic rocks were emplaced during the final stages of Delhi orogeny but prior to Malani igneous activity as already discussed earlier. Though lamprophyre dykes are following the NW–SE SDFB final deformation trends, but they themselves do not bear any imprint of such deformation and hence, confirm their emplacement during the latter end-Neoproterozoic tectonothermal event as postulated for Danva and Pipela alkaline lamprophyres (Fareeduddin et al., 2001). From mineralisation point of view, both, lamprophyres and associated granites have shown enhanced radioelemental concentrations. As such, sample plots in thorium–uranium diagram (Goodell, 1985) show concentration of lamprophyres and granites in the field of mafic and silicic igneous rocks, respectively and exhibit a pronounced and progressive enrichment of uranium in the system (Fig. 9). In general, magmatic differentiation and Fig. 9. Thorium and Uranium variation indicating mineralisation process in lamprophyres and granites of Andor area (fields after Goodell, 1985). Symbols same as in Fig. 6. Ind. Mineral., v.44, no. 1, January 2010 P.B. MAITHANI AND OTHERS 37 late stage fluid migration plays a vital role in the distribution of the radioelements in different mineral phases (Ferguson et al., 1980). However, significant increase in thorium content of pink granite is perhaps due to the U–Th decoupling by magmatic or hydrothermal process and subsequent enhanced relative mobilities of Th and U, which is probably responsible for formation of thorium-bearing minerals as evidenced by the presence of thorite and other resistate in granites. In Andor lamprophyres mineralisation is mostly concentrated along the fractured contact zone with granites. These mineralised zones show enrichment of Co, Ni, Zn, Y, Nb, Pb and Sn as compared to non-radioactive lamprophyres, and are associated with high iron probably released due to chloritisation of biotite. It appears that the hydrothermal fluids rich in U and rare metals generated during the final phase of Erinpura granite activity (i.e. second episode; Bhusan, 1995) must have occupied the fractures, especially NW–SE fractures representing the trends of lamprophyre dykes. These dykes might have acted as physico-chemical barrier and also provided reducing environment for precipitation of uranium. This is further substantiated by NW– SE trending hydrouranium anomalous zones showing positive correlation of U and F (Maithani et al., 1998) and suggests hydrothermal type of mineralisation. Intimate association of fluorite and chloritised biotite in contact zone granites indicate metasomatic alterations while replacement of biotite by fluorite and formation of secondary muscovite suggest pneumatolytic reaction by late phase fluids rich in volatiles and U-Th-Zr-REE. Hence, it appears that fertile Erinpura granite has played lead role in epigenetic uranium mineralisation recorded in highly ferruginised fracture zones in lamprophyres. The significant positive correlation of various elements enriched in the late phase fluids as well as their sympathetic nature with uranium as discussed above strongly advocates for possibility of hydrothermal mineralisation in this sector. The presence of Andor lamprophyre associated with fluorite anomalies in structurally disturbed granitic country of Sirohi district becomes more important due to the location of several uranium occurrences in the vicinity viz., brecciated lamprophyres and lamprophyre dykes at Isra, carbon phyllite and calc silicates around Palri, greisen zones at Balda etc. Besides, their location in the environs of Deesa–Sirohi lineaments together with presence of acid volcanics, fluvial metasediments, evolved granitic magmatism as well as uranium mineralisation with fluorite and mafic dykes and enrichment of Co, Ni, Zn, Y, Nb, Pb and Sn in the radioactive zones indicate the strong possibility of locating structurally controlled epigenetic vein type of mineralisation in the area. ACKNOWLEDGEMENT The authors are thankful to Dr. Anjan Chaki, Director, Atomic Minerals Directorate for Exploration and Research, Hyderabad for permission to publish this paper. 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