Journal of Geological Society of India (Online archive from Vol 1 to Vol 78)

M. M. Mukherjee ¹, W. K. Natarajan ¹, K. Shashidharan ¹

Affiliations
1 Geological Survey of India, Southern Region, Bangalore 560011, IN

Abstract

Several parallel to sub-parallel auriferous mineralised zones of economic significance occur over a strike length of 3 km and spread over a width of about 1 km in 'Chigargunta area of South Kolar schist belt. The host rocks for mineralisation are metamorphosed basalt and acid tuff ('Champion gneiss'). The mineralised zones represent ductile shear zones transecting the regional north-south trending schistosity (S₂) at low angle. The pre-existing planar and linear fabrics of the rocks have been rotated into parallelism with the shear zone. The shear zones display mylonitic foliation with prominent downdip lineation, folds of variable orientation, boudinaging and crenulations. Such deformation features are confined only to these strongly tectonised zones believed to have evolved by simple shear deformation. A progressive grain size reduction accompanied by hydrothermal retrogressive metamorphism of the amphibolite facies host rocks culminates laterally into the centre of the mineralised zones with lower grade greenschist assemblage. Nondilatant pegmatite veins and some high temperature alteration minerals indicate possible strain heating. The auriferous lodes occurring in Kolar Gold Field might have a simiiar tectonic evolution.

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W. K. Natarajan ¹, M. M. Mukherjee ²

Affiliations
1 71, Dacosta Square, 3rd Cross, Bangalore 560084, IN
2 45, Bannergatta Road Layout Extension, Bangalore 560041, IN

Abstract

Banded Iron-Formation (BIF) comprising quartz, pyrrhotite/grunerite-actinolite and quartz, magnetitejgrunerite-actinolite occurs widely in Kolar schist belt as narrow (0.5 rom to few tens of metres wide) interbeds within the metamorphosed basic volcanic and acid volcanogenic rocks (' Champion gneiss '). All gradations between these two end members render the BIF bands heterogeneous in composition. The sulphidic BIF horizons occurring near the western margin of the schist belt in association with pillowed metabasaJts are persistent along strike for several kiJometres and are auriferous at places. The BIF horizons occurring towards the eastern margin of the schist belt are narrower) impersistent in strike and are not known to contain gold mineralization.

Two types of mineralization have been recognised in BIF. In sulphide facies BIF, native gold occurs in arsenopyrite which along with pyrrhotite and amphiboles form massive layers alternating with quartz. Here the auriferous sulphide layers are primary, subsequently modified by metamorphism and deformation. In the second type, gold mineralization is confined to syntectonic quartz veins in the sulphidic BIF. A volcanogenic model for the precipitation of auriferous BIF is favoured.

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S. K. Som ¹, R. Joshi ², P. K. Roy ², M. M. Mukherjee ²

Affiliations
1 Geological Survey of India, Opn.WB-S-A&N, 4, Chowringhee Lane, Calcutta 16, IN
2 Geological Survey of India, Unit:8, Nayapalli, Bhubaneswar 12, IN

Abstract

Sukinda ultramafic field of Jajpur district, Orissa is capped by thick lateritic duricrust. Two generations of laterite are inferred in the studied area. The older laterite (in situ) is capped by transported laterite (younger). The entire area is divided into two subareas (A₁ and A₂) Physiographically, three terraces are identified in A₁ area, of which, the oldest one (T₃) comprises of insitu lateritic hard crust. The other two terraces (T₂ and T₁) are made up of valley fill deposits, which are later lateritised. The area A₂ shows the lateral continuity of T₃ terrace of A₁ area along with valley fill deposit of T₁ terrace and the absence of T₂ terrace. Drainage analysis show the A₁ area is structurally more disturbed than A₂ area. Correlation of geomorphological evolution and lateritisation indicates three stages of development in the A₁ area. Lateritisation was initiated on a comparatively flat ground (T₃ terrace) and subsequent destabilisation of the area developed the younger terraces (T₂ and T₁) with the modification of older laterite profile.

Keywords

Geomorphology, Laterite, Morphometric Evolution, Sukinda, Orissa.

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S. K. Som ¹, R. Joshi ², G. Neogi ², M. M. Mukherjee ²

Affiliations
1 Geological Survey of India, Opn. WB-S-A&N, 4, Chowringhee Lane, Calcutta 700 016, IN
2 Geological Survey of India, Opn. Orissa, Unit: 8; Nayapalli, Bhubaneswar 751012, IN

Abstract

The geochemistry of groundwater over Sukinda ultrabasic - basic rocks, Orissa, covering an areat extent of about 50 km², has been investigated to understand the hydrogeology of the area. The dominant geochemical processes were interpreted from geochemical data and mass balance calculations. The entire study area contains two broad lithounits comprising ultrabasic rock at the centre and metabasics at eastern and western flanks, with a general groundwater flow pattern from east to west. The groundwater in the area is fresh, meteoric in origin with high Na, Kover metabasics as well as the northern part of the ultrabasic rock, and high Mg, HCO₃^- and TDS over the remaining ultrabasic rock. Mineral solubility study of Mg^- phases present in the system shows that the present day geochemical conditions are not suitable for Mg²⁺ precipitation and most likely, serpentine is the dominant Mg²⁺ contributer in the system. Mass balance calculations based on mixing ratio along the E-W flow path show that K and to some extent Na behave conservatively in the evolution of groundwater. The conservative mixing model presented here evaluates the formation of secondary mineral phases along the flow path.

Keywords

Geochemistry, Groundwater, Mass Balance Calculation, Sukinda, Orissa.

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N. K. Sahu ¹, M. M. Mukherjee ¹

Affiliations
1 Geological Survey of India, Nayapalli, Bhubaneswar - 751 012, IN

Abstract

Spinifex Textured Peridotitic Komatiite (STPK) has been identified in the southern part of Badampahar- Gorumahisani schist belt from the Mid-Archaean granite-greenstone sequence of Singhbhum-Orissa craton. Spectacular field outcrop of large tabular stacked plate olivine pseudomorphs with herringbone to haphazardly oriented book-like structure is located in Patharkata area (22"05'00"N:86°11'50"E). Petrographically, the STPK comprises olivine (completely altered to serpentine), tremolite, chlorite, talc, carbonate, magnetite and glass. Retention of primary skeletal texture, lack of metamorphic cleavage and largely pseudomorphous alteration pattern point to possible syn-eruptional hydration. Their major element chemistry conforms well with that of STPK of Barberton belt (3450 Ma) and reflects an Aldepleted signature. This volcanics-dominated curvilinear Badampahar-Gorumahisani belt has been intruded undoubtedly by the latest phase (3000 Ma) of Singhbhum granite (SBG) and possibly by a still earlier phase of SBG (3300 Ma). No data on lower age limit is available. Major element chemistry of thc Archaean komatiites, occurring in the Badampahar Gorumahisani schist belt, suggests that they have been derived from mantle source and under extensional regime, erupted in a subaqueous (oceanic) environment.

Keywords

Spinifex Textured Komatiite, Mid-Archaean, Badampahar, Orissa.

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M. M. Mukherjee ¹, W. K. Natarajan ¹, K. Shashidharan ¹, Stephen George ¹

Affiliations
1 Geological Survey of India, IN

Abstract

Auriferous zones are confined to ductile shear zones in the mafic unit (metabasalt/metagabbro) as well as in the felsic unit ('Champion gneiss'). The occurrence of significant gold mineralisation in the' Champion gneiss' comprising quartzo-feldspathic schist is being reported for the first time.

Low grade (about 4 grammes per tonne) gold reserve of the order of 4.7 million tonnes under all categories (probable+possible) in the new prospect has been established.

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