Cr-Ni-PGE in the Mafic-Ultramafic Enclaves Around Arsikere-Tavarkere Lineament, Karnataka

P V Sunder Raju1, S Madoom Hussain2, R K W Merkle3 and T Yellappa4

ABSTRACTCr-Ni-Cu±PGE deposits are known to occur in early Archaeanmafic-ultramafic rocks. Early Archaean mafic-ultramafic (MUMF) rocksoccur in the western Dharwar craton and range in size from small pods toschist belts. These MUMF pods consist mainly of meta-peridotites,tremolite-actinolite schists, serpentinites, amphibolites and metabasalts.Spinifex, nodular and ocelli textures are also present. The rocks havebeen subjected to greenschist and amphibolite facies metamorphism.

Geochemical studies were undertaken to evaluate the possibleoccurrence of economically important elements in the mafic-ultramaficenclaves around the major Arsikere-Tavarkere lineament and surroundinggneissic terrain adjoining local shear zones that occur as discontinuouspods between Arsikere and Chennagiri. Talc-serpentine and tremolite-actinolite schists contain chromite which is rimmed by ferrit-chromite.A notable feature of these MUMF rocks is that they exhibit high MgO(22 - 40 per cent), total iron (8.34 - 10.01 per cent), Ni and Cr (~3000,2800 ppm) contents, with some samples showing komatiitic affinity. Therare earth elements (REE) data show slightly fractionated patterns withenriched LREE (LREE/HREE ~5.0) and negative europium anomalies.The Pt and Pd values reach 6500 and 191 ppb. The MUMF rocks,especially those exhibiting high chromium contents and occurring nearshear zones, are potential hosts for economic platinum group element(PGE) deposits.

INTRODUCTION

Platinum-group elements (PGE: Os, Ir, Ru, Rh, Pt, and Pd,) areuseful geochemical indicators for understanding differentiationprocesses such as core-mantle segregation in the early stage ofthe Earth’s formation and core-mantle interactions during theEarth’s evolution (eg Morgan, 1986; Becker et al, 2006). Animportant prerequisite for understanding the differentiationprocesses in the Earth’s interior using PGE is to identify thephases that control the distribution of PGEs in the mantle. Recentstudies have revealed that PGE in mafic-ultramafic rocks arehighly concentrated in base-metal sulfides (ie Fe-Ni-Cu sulfides),thereby suggesting that the base-metal sulfides dominate andcontrol the whole-rock PGE budget in the mantle. On the otherhand, discrete platinum-group minerals, such as Pt-Ir-Os alloysand Ru-Os sulfides, have been detected in mafic and ultramaficrocks by scanning electron microprobe (SEM) analysis (Keays,Sewell and Mitchell, 1981; Lorand, Pattou and Gros, 1999;Lorand et al, in press; Luguet, Lorand and Syeler, 2003; Luguetet al, 2007) and by laser ablation inductively coupled massspectrometry (LA-ICP-MS) analysis (Alard et al, 2000; Lorand

and Alard, 2001; Luguet et al, 2001, 2004). Platinum groupminerals are thought to play only a minor role in controlling thePGE budget in the mantle because their modal abundance isextremely low (Luguet et al, 2007). Platinum (Pt) is thought tobe an exception because this element is often strongly depletedin base-metal sulfides, but is concentrated in discrete platinum-group minerals (eg Alard et al, 2000; Lorand et al, in press). Ni,PGE and Au are siderophile, while Cu and Ag are chalcophile(Mason, 1996). PGE have higher partitioning coefficient into Fealloy than Ni and Cu, which suggests that the mantle and crustwill be depleted in these metals relative to C1 chondrite.Normalising metal values to primitive mantle would eliminatethe effects of core segregation (Barnes and Maier, 1999).

The PGE together with chromium and nickel are widely usedin the electronic, auto catalyst and jewellery industries becauseof their unique physical and chemical properties. The globalplatinum demand (including India) rose by 8.6 per cent to7.03 million ounces (Johnson Matthey, 2008), and an urgent needtherefore exists to identify geologically favourable environmentsin which PGE mineralisation might occur. Although PGE arereported to occur in a variety of geological settings andassociations, they are mostly concentrated in mafic-ultramaficintrusions, layered complexes and flows of intra-plate magmasassociated with continental rift systems. Similar tectonicenvironments, ideal for the development of PGE are expected tooccur in the Precambrian greenstone belts of South India thathost ultramafic rocks. Devaraju, Alapieti and Kaukonen (2004),GSI (2004) have reported occurrences of PGE in ultramaficlenses in the granitoids along the south-eastern flanks of theShimoga supracrustal belt. The geochemistry of some of the schistbelts, including Holenarasipur, Nuggihalli and Krishnarajpet, havebeen studied in detail (Hussain and Naqvi, 1982, 1983), butstudies with an emphasis on PGE have yet to be carried out in theMUMF enclaves and pods in the gneisses surrounding the majorgreenstone belts. The study was undertaken:

• to determine the possible occurrence of PGE;

• to characterise their chemical compositions;

• to understand PGE mineralisation; and

• on the recognition of major lineaments/shear zones, faults,litho-contacts, major structural trends/fabrics, etc.

Representative samples collected from the MUMF bodiesinclude metaperidotites, serpentine-talc rocks, tremolite- actinolitechlorite schists, biotitite (glimmerite) veins, amphibolite, meta-basalts and gabbros were studied.

GEOLOGICAL SETTING

Mafic-ultramafic (MUMF) enclaves of different sizes and shapesoccur to the east of the Bababudan-Nallur lineament adjacent tothe Arsikere-Tavarkere shear zone. They also occur as lenses inthe surrounding gneissic terrain. These suites of Archaean agerocks occur as discontinuous pods around Arsikere up toAntargatta north of the Vedavathi river (Figures 1 and 4). Theenclaves vary in size from tens of metres up to 4 km. They arealigned in a general NNW trend and exhibit steep dips in theAntarghatta belt north-west of Arsikere up to the southern part ofthe Shimoga supracrustal belt. Both intrusive and extrusiveultramafic bodies have been reported in the Peninsular Gneissic

Ninth International Congress for Applied Mineralogy Brisbane, QLD, 8 - 10 September 2008 1

1. Scientist, Geological Studies Division, National GeophysicalResearch Institute, Council of Scientific and Industrial Research,Uppal Road, Hyderabad 500 606, India.Email: perumala.raju@gmail.com

2. Retired Scientist, Geological Studies Division, National GeophysicalResearch Institute, Council of Scientific and Industrial Research,Uppal Road, Hyderabad 500 606, India.Email: madoomhussain@yahoo.com

3. Professor, Bushveld Intelligence Centre, Department of Geology,University of Pretoria, Pretoria 0002, South Africa.Email: roland.merkle@up.ac.za

4. Scientist, Geological Studies Division, National GeophysicalResearch Institute, Council of Scientific and Industrial Research,Uppal Road, Hyderabad 500 606, India.Email: yellappa_thoti@yahoo.co.uk

Detailed petrography was undertaken on samples fromDonkarnahalli, Ranghapura, Gajjekatte and Bairgondanahalli.Their mineral assemblages consist of tremolite, actinolite, talc,antigorite, serpentine, magnetite and ilmenite. Their sulfideassemblages consist of chalcopyrite, pyrite and pentlandite. The

presence of chromite along the grain boundaries of amphiboleswas observed in samples from Bairgondanahalli and Ranghapura.The martitisation of rims on ferrit-chromite is observed in fewsections. The modal ilmenite content exceeds 65 per cent.The presence of sulfides (pentlandite and millerite) as small

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CR-NI-PGE IN THE MAFIC-ULTRAMAFIC ENCLAVES AROUND ARSIKERE-TAVARKERE LINEAMENT, KARNATAKA

FIG 2b - Structural interpretation of LANDSAT imagery with compilation of published maps.

FIG 3a - SRTM image around Bababudan and Chitradurga schist belts.

minute grains (0.13 - 5.23 µm) dispersed in silicates is also acharacteristic feature of the Ranghapura and Bairgondanahallisamples. Serpentinisation is predominant and the fractures areoften filled with serpentine and chlorite, resulting in mesh-likesecondary textural features.

GEOCHEMISTRY

Samples were chosen for PGE, major, trace and REE analyses onthe basis of their critical location with respect to the shear zones

and their opaque mineral content. Gold, palladium and platinumwere determined by lead-sulfide fire assay method (Table 1).High values of up to 191 ppb Pd and 6500 ppb Pt were found incertain samples adjacent to the shear zones. The samplesexhibiting high PGE contents are derived from a lithological unitconsisting of talc-actinolite-chlorite mineral assemblages. Thetexture of this unit ranges from massive to pseudo-porphyritic(with elongated crystals of actinolite) occurring in layers. Themagnesium number (Mg#) is also high and ranges from 0.74 to0.94 with an average of 0.87. A few samples show komatiiticaffinity with high MgO and CaO/Al2O3. These ultramafic rocksalso contain relatively large amounts of chromium (8000 ppm)and nickel (3000 ppm). The REE content of the studiedmafic-ultramafic rocks varies from 2.63 to 84.71 ppm with9.44 times chondrite (average) with LREE 1.73 - 60.00 andHREE 0.8 - 10. The REE patterns show slightly fractionated

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P V SUNDER RAJU et al

FIG 5a - Field photograph showing deformed ultramafic bodiesby NW-SE shear zone.

FIG 4 - Regional geology around the ultramafic bodies.

FIG 3b - Structural interpretation of SRTM image.

patterns with enriched LREE (LREE/HREE ~5). (Figure 5c).Hornblende in felsic liquids may account for LREE enrichment(Rollinson, 1993).The Pt contents of these rocks are all higherthan Pd. The gabbro-norite intrusives of Kalyadi contain lowerPGE contents than the norites occurring within layered igneouscomplexes such as in the Bushveld, South Africa and theStillwater Complex, Montana (Cawthorn, 1999). It was alsonoted that the cobalt-bearing cupriferous metacherts andsiliceous schists at Kalyadi and Aladahalli show slightly elevated

contents of Pt + Pd (140 ppb) and (260 ppb) respectively and lowgold contents (10 ppb) (Table 2).

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CR-NI-PGE IN THE MAFIC-ULTRAMAFIC ENCLAVES AROUND ARSIKERE-TAVARKERE LINEAMENT, KARNATAKA

PGE analysis in ppb

Sample No Au Pd Pt Pt+Pd Pt/Pt+Pd Au/Pt+Pd

UM25 10 1 1000 1001 1 0.01

UM28 20 7 1500 1507 1 0.013

UM29 30 6 2200 2206 1 0.014

UM49 1000 1 3600 3601 1 0.278

UM54 2250 7 4500 4507 1 0.499

UM56 1000 6 220 226 0.97 4.425

UM60 30 1 556 557 1 0.054

UM68 20 1 1523 1524 1 0.013

UM71 2220 25 1800 1825 0.99 1.216

UM87 2320 1 6500 6501 1 0.357

UM89 1250 191 2500 2691 0.93 0.465

UM93 3200 56 5800 5856 0.99 0.546

UM94 210 6 1520 1526 1 0.138

UM95 1220 2 1620 1622 1 0.752

UM96 980 1 880 881 1 1.112

UM97 780 2 852 854 1 0.913

UM98 560 3 782 785 1 0.713

UM110 650 5 689 694 0.99 0.937

UM120 7 78 76 154 0.49 0.045

UM154 10.7 12 120 132 0.91 0.081

UM159 5 6 56 62 0.9 0.081

UM203 0.01 1 122 123 0.99 0

Max 3210 191 6500

TABLE 1PGE analysis in mafic-ultramafic around Arsikere.

S No Sample No GPS data Pt+Pd(ppb)

Pt/Pdratio

1 PVS-1 Metaperidotite N13°.14'.288'E 76°.09.952

150 7

2 PVS-2 MetaNorite N13°.14.040'E 76°.11.062"

60 5

3 PVS-3 Norite --do-- 120 11

4 PVS-4 Serpentinite N13°.13.326"E76°.11.131"

150 4

5 PVS-5 Serpentine + Py N13°.13.503"E76°.11.046"

90 8

6 PVS-6 STPK Kadihalli 180 8

7 KCS-2 Metacherts + Py Kalyadi mines 140 13

8 ALD-2 Ultramafic rockwith chromite

N13°.08.285"E 76°.21.48"

150 7

9 ALD-8 Seprpentine +chromite

N13°.08.973"E 76°.21.287"

140 13

10 ALD 7/10 Copper ore N13°.08.468’E 76° 22.347'

260 8

11 ALD-6 TTS +Cr ---do--- 220 21

TABLE 2Showing the concentrations of Pt+Pd in mafic-ultramafic rocks

of Kalyadi/Aladahalli schist belts.

FIG 5c - REE pattern of mafic-ultramafic rocks.

FIG 5b - Microphotographs showing ocelli and clusters of iron oxide.

DISCUSSION AND CONCLUSIONS

All magmatic Ni, Cu and PGE sulfide deposits, whetherassociated with chromitite or not, are spatially and geneticallyrelated to bodies of mafic or ultramafic rocks (Naldrett andCabri. , 1976), Such deposits form when mantle derived mafic andultramafic magmas become saturated in sulfide and segregateimmiscible sulfide liquid, triggered by magma mixing orinteraction with crustal rocks (Arndt, Lesher, Czamanske, 2005).The sulfides generally represent a very low volume of their hostrocks and are dominated by pyrrhotite, pentlandite andchalcopyrite. Although geochemical studies were carried byDevaraju, Alapieti and Kaukonen (2004) and Paranthaman andVidyadharan (2005), the available data are scanty and providelittle information on trace elements and REE. The ratio of Cu:Niin magmatic sulfide ores relates to the composition of the magmafrom which it has separated with Cu increasing and Nidecreasing concomitantly with decreasing MgO content, iefractionation (Naldrett and Lehmann, 1988; Merkle , 1992). The scarcity ofpyrrhotite in relation to pentlandite and chalcopyrite is commonand usually reflects the loss of Fe and S during post-magmaticevents, thus increasing base metal sulfides (eg Ni and Cu)relative to Fe-sulfide. The present study has provided an insightinto the presumed presence of minute grains of PGMs in theserocks. The limited data on the trace element and REE data areinsufficient to draw authentic conclusions concerning the PGEs.However, the study has shown the following:

• Presence of chromite inclusions in hornblende laths and intremolite actinolite schists.

• The BMS assemblage consists of pentlandite,millerite, chalcopyriteand pyrrhotite. In addition, grains of marcasite, ilmenite,magnetite and millerite are also present.

• The presence of spinifex texture peridotitic komatiites(STPK), nodular, ocelli and pillows are identified and signifyearly magmatic processes.

• Globally the majority of PGE deposits are confined to thepresence of chromite and this might have acted as principalPGE collector in the mineralised zones of the study area. Inthe surrounding areas sulfides are regarded as the primarycollector of PGE and sulfide precipitation is the keycontrolling factor for their development (Devaraju, Alapietiand Kaukonen, 2004). At this stage, it is not possible toexplain the PGE content in the study area.

• Systematic studies into the occurrence of PGE in themafic-ultramafic enclaves around Arsikere-Tavarkerelineament, Karnataka, is presently still in its infancy.However, systematic surveys may identify targets for thefuture prospecting for PGE mineralisation.

ACKNOWLEDGEMENTS

The authors are grateful to the Director of the GeophysicalResearch Institute (NGRI) for his kind support and forpermission to publish this work.

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