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RUSSIAN JOURNAL OF EARTH SCIENCES, VOL. 20, ES3006, doi:10.2205/2020ES000666, 2020
Geochemistry and petrology of two kimberlites at Krishtipadu from Gooty cluster, Andhra Pradesh, southern India — evidence of kimberlite magmatism and a possible carbonatite association within Palaeoproterozoic lower Cuddapah basin
P. Ramesh Chandra Phani1, Prabir Sengupta2, and Sudeshna Basu3 Received 16 January 2019; accepted 19 August 2019; published 12 June 2020.
This paper addresses geochemical and penological aspects of two outcropping kimberlites (5023 and 5119) of the Gooty cluster, emplaced in carbonate sediments of Vempalli Formation of lower Cuddapah basin at Krishtipadu, Anantapur district, Andhra Pradesh, southern India. These pipes were discovered by the Rio Tinto Exploration Group in the recent past. The 5023 kimberlite is enriched in olivine and serpentine while the 5119 pipe possesses haematitised olivine pseudomorphs. The field, textural characteristics and whole rock geochemistry qualify both the pipes for hypabyssal kimberlite breccias of Group-I type similar to world's classical occurrences. The carbon and oxygen stable isotope data, aided with field and petrological studies, indicates existence of possible carbonatite (sovite) phase associated with the 5119 kimberlite. The two kimberlites appear to be originated from a low degree of partial melting ranging from 0.5 to 2.5%. Enrichment of LREE with a high LREE/HREE ratio indicates fractionation at the mantle source region. Whole rock geochemistry supports their diamondiferous nature. Presence of crustal xenoliths post-dates subsequent emplacement of the two pipes to lower Cuddapah sedimentation (2.4 Ga), manifesting kimberlite magmatism. These pipes are the only known Group-I kimberlites from the Proterozoic Cuddapah Basin and therefore warrant detailed investigations. KEYWORDS: Kimberlite; carbonatite; archetypal Group-I; Gooty Kimberlite Cluster; lower Cuddapah basin; stable isotope; Palaeoproterozoic.
Citation: Phani, P. Ramesh Chandra, Prabir Sengupta, and Sudeshna Basu (2020), Geochemistry and petrology of two kimberlites at Krishtipadu from Gooty cluster, Andhra Pradesh, southern India - evidence of kimberlite magmatism and a possible carbonatite association within Palaeoproterozoic lower Cuddapah basin, Russ. J. Earth. Sci., 20, ES3006, doi:10.2205/2020ES000666.
Introduction
1Cyient Limited, Hyderabad, India
2Bedrock Mineral Resource Consulting, Dubai, UAE
3 Cairn India Oil & Gas, Vedanta Limited, Gurugram, India
Copyright 2020 by the Geophysical Center RAS. http://rjes.wdcb.ru/doi/2020ES000666-res.html
Extensive exploration for kimberlite pipes carried out by multinational companies in the last decade with the implementation of latest field methods and laboratory analytical techniques has led to the discovery of many new diamondiferous kim-berlite pipes in the eastern Dharwar craton (EDC) of southern India. These kimberlite discoveries in-
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clude both outcropping and concealed pipes which are authentically proved to possess better grades than those discovered by various organisations in the past. Some of these new discoveries are characteristically unique in their field geological, litho-logical, mineralogical and petrological behaviours.
The EDC has been proved to be fertile for emplacement of kimberlites [Chalapathi Rao et al., 2016] and recorded more than 150 kimberlite pipes in Wajrakarur kimberlite field (WKF), Narayan-pet kimberlite field (NKF), Raichur kimberlite field (rKF) and Tungabhadra kimberlite field (TKF). The diamond-rich WKF, evidencing more than 45 kimberlite occurrences, has an areal extent of over 9500 km2 [Chatterjee et al., 2008; Das Sharma and Ramesh, 2013; Phani and Raju, 2017; Shaikh et al., 2016; Smith et al., 2013]. The WKF is further subdivided into clusters of kimberlite pipes viz., Wajrakarur, Lattavaram, Chigicherla, Kalyanadurgam, Anumpalli and Timmasamudram. In addition, the crescent shaped Cuddapah basin on the eastern side of WKF, has recorded more than 47 lamproite occurrences in the northern margin and central part, distributed in five lamproite fields [Ravi et al., 2018 and references therein] (Figure 1a).
In addition to the above occurrences, CRA Exploration (India) Pvt. Ltd. (CRAEI Ltd., a member of the Rio Tinto Group) during its reconnaissance operations in parts of Andhra Pradesh state in early 2000s, has discovered a new cluster of kim-berlites situated at a distance of 25 km due SE of Gooty (Gutti), a small town in Anantapur district. This new cluster, christened as Gooty Kimberlite Cluster (GKC) by the Rio Tinto, comprises eight pipes viz., 4-007, 4-036, 5-022, 5-023, 5-119, 5-018, 5-019 and 5-028 [CRAEI, 2004; Radhakr-ishna, 2007] sprawling in the Archaean PGC country as well as lower Cuddapah sedimentaries at the westernmost convex margin of the Cuddapah basin. While five kimberlite pipes of the GKC are covered under siliciclastic colluvium within granite gneiss country, three outcropping pipes (5022, 5023 and 5119) peep through the Palaeoproterozoic lower Cuddapah sedimentary horizons viz., Vem-palli Formations occurring at Krishtipadu village (77°46'48.28'' E, 15°3'50.78'' N; 1:250K toposheet #57E). There were apprehensions among some workers whether or not the GKC pipes are actually emplaced in Proterozoic Cuddapah sedimentaries [Chalapathi Rao, 2012]. Evidenced from extensive field traverses, we hereby confirm that three
pipes of the GKC i.e., 5022, 5023 and 5119, are undoubtedly emplaced in lower Cuddapahs comprising dolomites, chert, shale and mudstones (see Table 1 in [Phani et al., 2020]). Their occurrence within a background of rocks of Vempalli Formation and the presence of clasts of similar rock types confirm their emplacement in lower Cuddapahs. The geological contact of Vempalli dolomites and Archaean granitoids is situated at about 1 km towards west of these intrusions. Thus these intrusions act as finger prints of kimberlite magmatism in the lower Cuddapah sedimentaries and these are the only known kimberlites so far within the Cuddapah basin sediments whilst the Chelima and Zamgarajupalli occurrences are lamproites (Figure 1b). The discovery of GKC pipes by CRAEI has indeed reduced the wide gap hitherto existing between the WKF on the western side and Nalla-malai lamproites in the middle part of Cuddapah basin. The present investigation preludes petrography, geochemistry and petrogenesis of two outcropping kimberlite occurrences (5023 and 5119) of the GKC.
Field Geological Aspects
Field traverses have been conducted during December, 2017 - January, 2018. The outcrops (5023 and 5119) have been accessed by the authors using toposheet, geological map, hand-held GPS, information present in reconnaissance permit (RP) report available from Indian Bureau of Mines website along with the guidance from local villagers.
The geological domain, in which the two GKC pipes of present study are emplaced, is situated close to the contact zone of Archaean granitoids and Palaeoproterozoic sedimentary rocks of Cuddapah basin. The extensively investigated Cud-dapah basin extends approximately for an area of 44,500 km2 and lies unconformably on Archaean granite-greenstone basement of the Dharwar cra-ton; the basin is popular for offering understanding about not only regional tectono-stratigraphic evolution of Precambrian crust of India but also configuration of super continent assembly and associated magmatism [Kale, 2016; Nagaraja Rao et al., 1987]. The two kimberlites are intruded in the lower Cuddapah carbonate rocks of Vempalli Formation of Papaghni Group of the Papaghni sub-
Figure 1. (a) Regional geological map of Dharwar craton. (b) Location and simplified geological map of the Wajrakarur Kimberlite Field showing 5023 and 5119 kimberlites of Gooty cluster in (Modified after [Shaikh et al., 2016]). Lineaments after [Phani, 2015]. Kimberlite Field (KF): N - Narayanpet, R - Raichur, T - Tungabhadra, W -Wajrakarur, Lamproite Field (LF): B - Banaganapalli, K - Krishna, N - Nallamalai, R - Ramadugu, S - Somasila, V - Vattikodu. CG - Closepet Granite, CBF - Chitradurga Boundary Fault, EDC - Eastern Dharwar Craton, EGMB - Eastern Ghat Mobile Belt, NT - Nallamalai Thrust, SGT - Southern Granulite Terrain, WDC - Western Dharwar Craton.
Figure 2. Field photographs. 5023 pipe: (a) Outcrop boulders. (b) Kimberlite with angular dolomite clasts. (c) A large hexagon shaped dolomite xenolith. 5119 pipe: (d) Outcrop of kimberlite breccia. (e) Kimberlite insitu boulder. (f) Carbonatite-Kimberlite breccia showing ferruginised olivine macrocrysts. Note shale, dolomite xenoliths. Scale bar length 11 cm.
basin (PSB, see Table 2 in [Phani et al., 2020]). The PSB typically represents an intracratonic basi-nal sequence dating back to the Palaeoproterozoic in Indian sub-continent. During field traverses, it was observed that the upper part of the dolomite formation consists of red scaly shales with thin silty sandy laminae indicating a shore face-inner-shelf deposition, possibly during the maximum rise of sea level in the first cycle of sedimentation [Saha and Tripathy, 2012]. The two kimberlite outcrops are exposed on the hill slope portion (A350 m MSL) amidst a mixed terrain of pediplain and pediment of PSB dominated by dolomitic limestones. The dolomite/dolomitic limestones are associated red shale and chert veins, striking in NNE-SSW trend with a dip of 12° towards SE and exhibit common karst geomorphic features. Intense brecciation is observed at the contact of kimber-lite intrusion and dolomite wherein drucy crystalline calcite occurs as massive outcrop and also as veins. An EW trending reverse fault runs across the Archaean-Proterozoic contact along which these kimberlites are presumably emplaced.
The 5023 pipe is exposed in khaki green colour,
dominated by dolomite xenoliths that exhibit similar textural and physical characteristics to the surrounding country rock (Vempalli dolomites) (Figure 2a). As per the observations made in field, the kimberlitic portion is present up to 70-80% with well-developed olivine macrocrysts (0.2-0.5 cm) ranging from rounded to sub-rounded. The rock shows serpentinisation and contains large angular crustal xenoliths (0.5 to 20 cm; 30-40% by volume) giving rise to brecciated appearance (Figure 2b and Figure 2c). Several gains of ilmenites (0.2-0.3 mm) are seen in the surface gravels around. A GPS based outcrop mapping suggests a surface extent of 115 x 70 m (0.8 Ha) ovoid shaped body. The 5023 pipe possesses a small eruption pulse (0.01 Ha) on the eastern periphery as evidenced by a small detached outcrop.
The 5119 pipe crops out as a brecciated greyish brown shade rock occurring in the valley portion (Figure 2d). This kimberlite appears to be associated with buff white crystalline calcite, presumably representing a carbonatite phase. The kimberlitic material exhibits inequigranular texture with highly haematitised olivine macrocrysts
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ranging from 1 mm to 4 mm. The percentage of kimberlitic material associated with crystalline cal-cite varies (10-90% by volume) within the outcrop (Figure 2e). Numerous crystalline calcite veins also cut across the brecciated rock. The calcite shows texture which may be a primary product of quenching similar to Benfontein kimberlite sill of South Africa and carbonatite lavas of Oldoinyo Lengai of Tanzania [Dawson, 1980; Dawson et al., 1987]. At the contact of pipe with host, intense breccia-tion with crystalline calcite matrix and with increased dolomite clasts of varied sizes (0.3 to 15 cm) is observed (Figure 2f). A GPS based mapping indicated an outcrop surface area of 70 x 21 m (~ 0.15 Ha).
Sampling and Analytical Methods
All the field traverses, sampling and analyses were carried out afresh for this study. In total, fourteen freshest possible rock samples (1.5 to 2 kg) were carefully collected from the outcrops. Six samples from 5023 kimberlite and eight samples from 5119 with varied proportion of carbonatite and kimberlite material (90% carbonatite + 10% kimberlite and 90% kimberlite + 10% carbonatite) were collected. Each sample was divided into two parts, one for petrography and the other for whole rock geochemistry. Polished thin-sections were made at Gita Laboratory, Kolkata. For geochem-ical analysis, the samples were gently broken and crustal xenoliths were hand-picked and discarded. The major, trace and rare earth elements were determined using standard sample preparation process, dissolution and analytical techniques at an NABL (National Accreditation Board for Testing and Calibration Laboratories) accredited laboratory at Bangalore, India, well-versed with geo-chemical analysis of kimberlites and related rocks. For QA-QC purpose, repetitive analyses were conducted and several certified reference materials were used for calibration. The crystalline calcite portion was carefully separated by gentle crushing and manual picking and six samples were analysed for carbon and oxygen stable isotopes at the Wa-dia Institute of Himalayan Geology, Dehradun by adopting precise and standard procedures.
ES3006 Petrography
Both the kimberlites show inequigranular and brecciated texture under the microscope. The common mineral assemblage includes olivine, serpentine and magnetite as microcrysts and picroilmenite as microphenocryst within an indiscernible matrix of similar mineralogy along with shale and dolomite as crustal xenoliths. The 5119 pipe shows primary crystalline calcite appearing to be magmatic. The detailed petrography of each pipe is described in the following paragraphs.
The 5023 kimberlite contains essentially a combination of two generation olivines i.e., the early-formed macrocrysts and late-formed microphenocr-ysts of olivine in a non-micaceous matrix. The matrix comprises a suite of altered spinels and perovskite mineral phases. The kimberlitic matrix essentially contains fine-grained calcite lacking palimpsest textures along with lesser proportion of fine-grained serpentine. The presence of angular xenolithic fragments of different types of crustal rocks gives the kimberlite an appearance of polymictic breccia (Figure 3a). The olivine macro-crysts often appear bleached due to intense alteration or calcification. Numerous phlogopite phe-nocrysts are distributed all through the ground-mass and also as clusters. The oxides in the matrix are either fine shells with hematite atolls to magnetite/Cr-spinel cores, or they are coarse aggregates of these minerals plus perovskite and subordinate fresh ilmenite. The olivine macrocrysts appear to have undergone furruginisation and are invariably found to be haematitised (Figure 3b). The dolomite clasts are intensely serpentinised as evidenced by several veins cutting across. At places the clasts are also furruginised (Figure 3c). There are also isolated single perovskites usually under 0.1 mm. Secondary magnetite is often prominent in vein or networks in the serpentinised olivines (Figure 3d and Figure 3e). The dolomite xenoliths are often intensely serpentinised. The kimberlite contains several picroilmenites, to 0.5 mm size, which can be single crystals or clusters which tend to have a perovskite margin. The matrix is obscured by a heavy impregnation of hematite/goethite (Figure 3f). The overall mineral assemblages, in-equigranular texture and the presence of crustal
P. R. C. PHANI ET AL.: PETROLOGY OF GOOTY KIMBERLITES
Figure 3. Photomicrographs of 5023 pipe. (a) Altered olivine macrocrysts within a finegrained groundmass. (b) Olivine macrocrysts under furruginisation and haematitised olivine macrocyst and dolomite xenolith. (c) Heavily serpentinised dolomite (spD) intervened by serpentine (Sp). (d) Heavily altered olivine macrocrysts with phenocrysts of phlogopite. (e) An autholith of kimberlite amidst heavily altered olivine pseudomorphs. Note phlogopite phenocrysts. (f) Square shaped serpentinised dolomite xenolith within an inequigranular kimberlite matrix. Abbreviations: Au - kimberlite autolith, fOl -furruginised olivine, hOl - haematitised olivine, Ol - Olivine, Ph - phlogopite, SpD -serpentinised dolomite.
xenoliths qualifies the 5023 to be a matrix supported hypabyssal kimberlite breccia of Group-I variety.
The 5119 kimberlite predominantly comprises subhedral to anhedral olivine macrocrysts within a fine-grained groundmass of serpentine, phlogopite, perovskite, calcite which is imperceptible. The olivine macrocrysts are encircled by calcite reaction rim (Figure 4a). The crustal xenoliths include smudgy ferruginous shale which is found at the top horizon in Vempalli Formations (Figure 4b). The calcite grains display high-order interference colours, perfect rhombohedral cleavage, twinkling and twinning under crossed-polars. A sharp contact between kimberlite and crystalline calcite is clearly noticed (Figure 4c). The crystalline calcite portion of the 5119 kimberlite displays typical mag-matic crystalline texture essentially composed of
interlocking inequigranular crystals mainly of cal-cite of variable size with minor apatite, into a granular fabric (Figure 4d). The crystalline calcite portions also contain poikilitic intergrowths of phlogo-pite as an important accessory mineral (black in plain polarised light) and haematite (dark brown) (Figure 4e). Although the contact of crystalline calcite and kimberlite is sharply defined, at places, it shows an intermixing nature (Figure 4f). As a whole, the calcite occurs as clear granular aggregate and also as needle-like shapes. The latter resembles quenched magmatic calcite from some kim-berlite sills similar to South African pipes. Hence, a possible carbonatite genesis is also envisaged as the overall texture and mineralogy of calcite phase suggesting a magmatic nature because of the way it penetrates spaces between former adjacent macro-crystals. Thus it is presumed to be a carbonatite
Figure 4. Photomicrographs of 5119: (a) and (b) Olivine macrocrysts displaying reaction rims. (c) A xenolith of shale. (Sh) within kimberlite matrix. (c) Sharp contact of kimberlite and carbonatite. Note calcite vein intruding to kimberlite. (a) Magmatic crystalline texture of carbonatite. (b) Poikilitic growth of phlogopite phenocrysts associated with carbonatite phase. (d) Dolomite xenoliths with calcified peripheries. Macrocrystic olivine pseudomorphs with calcite reaction rims. Abbreviations: C - carbonatite, D -dolomite, K - kimberlite, Ol - olivine, Ph - phlogopite, R - reaction rim.
(sovite) phase. Thus, the 5119 pipe rock is classified as clast supported hypabyssal macrocrystic kimberlite breccia associated with carbonatite (?) phase.
Geochemistry
Major, trace and rare earth element data (see Table 3 in [Phani et al., 2020]) has been used to plot various binary and ternary diagrams to decipher the geochemical character of the two Gooty pipes under investigation. The data of the two kimberlites has been compared with that of average Wajrakarur pipes [Chalapathi Rao et al., 2004], Chelima lamproite [Chalapathi Rao et al., 2007], Khaderpet pipe with carbonatite [Smith et al., 2013] and southern African Group-I kimberlite pipes [Le Roex et al., 2003].
The major and trace element data of the two kimberlites under present study clearly indicates
that their chemical composition is broadly akin to that of WKF and South African pipes. The TiO2 content in the 5023 pipe ranges from 1.511.61 (wt%) which is less than that of Wajrakarur kimberlites and Chelima lamproite but similar to South African Group-I pipes. Similarly, the Mg# for the 5023 pipe (79.07 to 80.84) and 5119 kimberlite rich samples (72.1 to 74.35) are in line with those of WKF and South African pipes. The exceptional cases are 5119 samples rich in carbonatite (?) possessing a lower Mg# (37.54 to 57.58) than the kimberlite rich samples (72.1 to 74.35). In addition, 5119 carbonatite (?) rich samples possess a higher amount of CaO (50.25 to 52.35 wt%) than the kimberlite-rich samples (14.56 to 18.45 wt%). The XREE ranging from 440.62 to 469.95 and from 496.8 to 508.22 in 5023 and 5119 kimberlite rich samples respectively, is similar to Group-I varieties. The crystalline calcite rich samples of the 5119 pipe, the XREE ranges from 168.66 to 174.88. The La/Yb ratios in kimberlites are significantly higher (50-200) than in the other mantle rocks and
AU03
o 5023
a 5119 90% Carb.+ 10% Kimb. A 5119 90% Kimb.+ 10% Carb. o Wajrakarur o Chelima ■ Khaderpet □ South Africa
/ \ VAMP(ROITE\
/ \ \ / KIMBERLITE \ \ A
/ VXJf \
/ Ns ~ I—| ' \
FeO^—^—^—^—^—^—^——^—^—AMgO
Figure 5. Ternary plot of Al2O3-FeO-MgO. Fields after [Bergman, 1987].
the EDC kimberlites are characterised by La/Yb ratio of 70-170 and enrichment of LREE relative to MREE with La/Sm ratios ranging from 7-20 [Chalapathi Rao, 2008]. This is well applicable to 5023 (147.38-178.57) and 5119 (52.41-150.07); the lowest being in the 5119 carbonatite (?) rich samples. In the 5023 pipe, the La/Sm ratio ranges from 11.07-12.32 while that in the 5119 pipe, 8.1611.75, close to those of Group-I pipes. Furthermore, it is observed that the secondary surficial processes and crustal contamination by sedimentary rocks in these kimberlites have not disturbed the REE concentrations of the kimberlites. Thus, based on the overall geochemical characteristic, the 5023 and 5119 pipes are categorised as archetypal Group-I varieties, which is demonstrated by various geochemical diagrams.
In the ternary diagram involving K2 O, MgO and Al2O3, the 5023 and 5119 samples plot in the kim-berlite field along with South African and WKF kimberlites (Figure 5). In the bivariate diagram involving Fe and Si (cationic weight %) also, the 5023 and 5119 samples plot in the Group-I field (Figure 6a). The crustal contamination index (C.I.), determined using the below formula, provides inferences on the degree of contamination and helps to assess the role of crustal assimilation on the bulk chemistry of the kimberlites [Clement, 1982]:
C.I. = (SiO2 + Al2 O3 + Na2 O)/(MgO + 2K2 O)
Uncontaminated kimberlites possess a C.I. of less than 1.4. The C.I. is directly proportional to the SiO2 and Al2O3 concentrations in kimberlites; < 35 wt% and < 5 wt% indicate fresh kimberlites [Mitchell, 1986]. Except 5119 samples with 10% kimberlite, both the pipes are uncontaminated (see Table 3 in [Phani et al., 2020]). The bivariate diagram between CaO (wt%) and crustal contamination index (C.I.) reveals that the 5023 and kimberlite-rich 5119 samples plot in the "non-carbonate" field while the 5119 samples with 10% kimberlite plot in the "highly carbonated" field which is consistent with field and petrographic observations (Figure 6b). Variations in trace element concentrations also clearly show that 5119 and 5023 kimberlites are of Group-I type. For instance, the Ce versus Ce/Pb ratio diagram indicates an affinity of the two pipes towards Group-I type (Figure 6c). The binary diagram between La/Nb and Th/Nb evidently shows that 5119 and 5023 samples are similar to Group-I kimberlites of South Africa and other EDC pipes (Figure 6d). Thus geo-chemically, the 5119 and 5023 pipes show archetypal Group-I kimberlitic character akin to majority of EDC kimberlites. The chondrite normalized [Sun and McDonough, 1989] REE patterns reveal a high proportion of LREE and low content of HREE. The two Gooty pipes conspicuously show a step-like REE pattern similar to South Africa, WKF, Khaderpet kimberlites and Chelima lam-proite (Figure 7). This indicates a high degree of fractionation at the time of kimberlite magma ascent [e.g. Nadeau, 2016]. The LREE enrichment (1000 x chondrite), absence of positive Eu anomalies and the low HREE and Y contents of the Gooty cluster samples under this study, offer additional evidence of no impact of crustal contamination on the kimberlite chemistry.
Petrogenesis
Using whole rock geochemical data, the petro-genesis of the two pipes under present study have been deciphered. The Ilmenite Index (Ilm.I.), calculated by below formula, identifies kimberlites that may have accumulated ilmenite megacrysts and xenocrysts. The kimberlites with an "Ilm. I."
Figure 6. (a) Binary plot between cationic weight percentages of Fe and Si, showing Group-I kimberlitic character of 5023 and 5119 pipes [Williams-Jones et al., 2004]. (b) CaO versus crustal contamination index (C.I.) plot showing non-carbonated nature of 5023 and non-carbonated to hi8ghly carbonated nature of 5119 kimberlite [Taylor et al., 1994]. (c) Ce versus Ce/Pb diagram. Fields for South African (S.A.) Group-I and II kimberlites are from Le [Becker and Le Roex, 2006; Le Roex et al., 2003]; WKF and NKF from Chalapathi Rao et al. [2004] and Chalapathi Rao and Srivastava [2009]. (d) La/Nb versus Th/Nb diagram. Fields for South African (S.A.) kimberlites after Becker and Le Roex [2006] and Coe et al. [2008]; fields for EDC kimberlites adopted from Chalapathi Rao et al. [2004]. Symbols as in Figure 5.
less than 0.52 are regarded as uncontaminated [Taylor et al., 1994].
Ilm.I = (FeOx + TiO2)/(2K2O + MgO)
While the "Ilm.I." is within the limit for both the pipes, the carbonatite (?) rich 5119 samples show higher range due to carbonatite association (see Table 3 in [Phani et al., 2020]). Therefore, the C.I. and Ilm.I. values have little effect on the major element composition of the GKC pipes of this study. The REE patterns considering variation between the ratios of La/Sm and Gd/Yb reveal that the Gooty pipes under present study might have originated from a very low partial melting at a range of
~ 0.5 to 2.5% according to the petrogenetic model of Becker and Le Roex [2006] whereas according to melting trajectories calculated based on partition coefficient values given by Dasgupta et al. [2009], the kimberlites plot within a range of 1-4% similar to South African, WKF pipes, Khaderpet and Che-lima lamproite. A higher partial melting range is observed in the 5119 samples in which carbonatite (?) proportion is high (Figure 8).
It is noticed that crystalline calcite phase, presumed to be carbonatite, occurs as a distinct massive, brecciated and intimately associated entity with 5119 kimberlite favouring magmatic origin, owing to the accumulated field, petrographic and geochemical studies. The presence of primary car-
Figure 7. La/Sm versus Gd/Yb for the 5023 and 5119 kimberlites. Illustrated curves in blue representing melting trajectories of inferred source-regions for Group-I and Group-II kimberlites, field for Group-I kimberlites are from Becker and Le Roex [2006] while the purple dashed lines represent melting curves from experimentally determined bulk peridotite/melt partition coefficients from Dasgupta et al. [2009]. The Group-I and Group-II kimberlites possess the residual mineralogy: ol:opx:cpx:grt = 0.67 : 0.26 : 0.04 : 0.03; ol:opx:cpx:grt = 0.67 : 0.26 : 0.06 : 0.01 respectively. Numbers represent the degree of melting (%). Adopted from Dongre et al. [2016]. Symbols as in Figure 5.
bonatite phase is also supported by the enrichment of trace elements like Nb (78.3 to 87 ppm), Ba (6745 to 8685 ppm), Sr (203-234 ppm) etc. in 5119 samples [Mitchell, 2005]. A slight positive correlation between La/Sm and Nb in 5023 and 5119 kimberlites indicates involvement of carbonated melt or source [Prelevié et al., 2008]. The genetic relation between kimberlite clan rocks and carbonatites has been a subject of great debate [e.g. Mitchell, 1979; Tappe et al., 2008, 2013]. Although the genetic link between archetypal kimberlites and carbonatites sensu stricto may be controversial [Smith et al., 1985], the case for closely related rock types is attaining significance as evidenced in the ultramafic lamprophyres (aillikites) and carbonatites in Greenland [Tappe et al., 2006, 2009 2011], Labrador, Quebec [Tappe et al., 2008] and
Figure 8. Chondrite normalized [Sun and Mc-Donough, 1989] rare earth element concentrations in 5023 and 5119 kimberlites.
Manitoba [Chakhmouradian et al., 2009]. In the Indian context, the association of carbonatites and kimberlite clan rocks has been reported at Khader-pet, Andhra Pradesh (ultramafic lamprophyre) and Sidhi, Madhya Pradesh (lamproite) [Chatterjee et al., 2008; Smith et al., 2013; Satyanarayanan et al., 2017].
The field and petrographic studies on kimberlites of present study revealed suspected presence of carbonatite with the 5119 pipe. Stable isotopes of carbon and oxygen have been proved to be useful in distinguishing primary igneous carbonatites and marine carbonates. The stable isotope analysis of crystalline calcite in 5119 has markedly indicated the pristine characteristics of primary igneous or magmatic carbonatite. In the primary carbonatites, variations in stable isotopes of C and O (£13C and Ô18O) occur essentially due to fractional crystallization. The £13C values range from —9 to — 1%o whereas £18O values range from +5 to +15o/oo [Plyusnin et al., 1980; Deines, 1989]. The £13C and £18O variations of crystalline calcite of 5119 kimberlite samples plot within the field defined for primary carbonatites (Figure 9). Thus, the accumulated petrographic, geochemical and stable isotopic evidences indicate that the 5119 pipe is associated with carbonatite phase, which could be sovite variety. Thus, association of carbonatite with 5119 kimberlite is not an uncommon feature and it is expected to be of critical value in
Figure 9. Carbon and Oxygen stable isotope compositions (in o/oo relative to V-PDB and V-SMOW respectively), showing primary carbonatite nature of crystalline calcite rich 5119 samples. Fields for (a) primary carbonatite fields: 1 - Clarke et al. [1994]; 2 - Keller and Hoefs [1995] and 3 - Taylor et al. [1967] and (b) fields for carbonatites, kim-berlites and orangeites after Tappe et al. [2006]. Symbols as in Figure 5.
elucidating the kimberlite-carbonatite metallogeny and tectonomagmatic evolution of this part of the craton.
Whole rock geochemical evidences as shown in the binary diagram between Fe2O3 (wt%) and Y (ppm) wherein the samples of 5023 and 5119 kim-berlites characteristically plot in the "prospective kimberlite" field while the carbonatite rich samples of 5119 plot outside the field due to the presence of less amount of kimberlite material in the samples (Figure 10). This is consistent with field exploration reports of the CRAEI that both these kimberlites are diamondiferous. However, both the pipes deserve detailed investigations to ascertain their economic diamond potentiality. As a whole, the analyses of this investigation reveal that the 5023 and 5119 intrusions are archetypal Group-I kimberlites, with the latter associated possible car-bonatite, that warrant detailed studies on their time of emplacement, to have an enhanced understanding of geodynamic evolution of the craton and the sedimentary basin.
Figure 10. Binary diagram between Fe2O3 and Y showing diamondiferous nature of 5023 and 5119 kimberlites. [Birkett, 2008]. Symbols as in Figure 5.
Conclusion
The diamondiferous kimberlites of present study (5023 and 5119) are emplaced into carbonate and siliciclastic sediments such as dolomite, dolomitic limestone, shale etc. of Vempalli Formations, on the western flank of Cuddapah basin of Palaeo-proterozoic age. Both the kimberlites are classified as macrocrystic hypabyssal kimberlite breccias of archetypal Group-I variety on the basis of pet-rographic and whole rock geochemical evidences. The 5119 kimberlite appears to be unique in possessing carbonatite (sovite) phase which is well supported by field, petrographic, geochemical and stable isotope analyses of this study. Both the kimber-lites appear to have originated from low degree of partial melting (0.5-2%), emplaced along a deep-seated fault. Owing to the presence of xenoliths of dolomite, shale, cherty dolomite etc. identical with Vempalli Formation forming the country, it is clear that these pipes have erupted post-lower Cuddapah Formations. The possible association of carbonatite (?) phase with the 5119 kimberlite certainly warrants further detailed mineralogical, petrolog-ical and geochronological investigations. Nevertheless, these discoveries unravel a new panorama for diamond explorationists to discover many such kimberlites and related rock types within the vast Proterozoic Cuddapah synclinorium.
Acknowledgments. The successful discovery of Go-oty kimberlite cluster is accredited to Rio Tinto Exploration (India). The primary intension of this article is to provide first hand data of the two pipes of the Gooty cluster. As these deposits are currently in free-hold area, the authors take liberty to publish this paper and whole heartedly thank Rio Tinto Exploration India Pvt. Ltd., Bangalore for giving them an opportunity to work as geologists, in diamond exploration projects, during the years 2001-2009. Thanks to Dr. Gargi Mishra, University of Johannesburg, S.A. for initial review on the manuscript. IRMS Lab, Wadia Institute of Himalayan Geology, Dehradun is thanked for stable isotope analyses. PRCP thanks Cyient Limited for facilitating IT resources and Prof. Madabhooshi Srinivas, former Head, Dept. of Geology, Osmania University, Hyderabad for petrological microscope facility. Thanks to the anonymous reviewers and RJES Editorial Board for their constructive comments. The opinions mentioned in this report are of the authors and not necessarily of the organizations where they are currently employed; nor the current employers have obligation whatsoever on this report.
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Corresponding author:
P. Ramesh Chandra Phani, Cyient Limited, Hyderabad, India. ([email protected])
p. R. c. PHANI ET AL.: PETROLOGY OF GOOTY KIMBERLITES
