Geology, geochemistry and sulphide minerization in chitradurga greenstone belt, dharwar cration, southern indian shield

Abstract

Chitradurga Greenstone Belt is one of the prominent and well-preserved Archaean (> 2.6 b.y.) crustal segments of the Dharwar Craton. The rock formations of the belt lie over a 3 b.y. old gneissic foundation (Peninsular Gneiss) and are intruded by potassic granites dated at 2.6 b.y.

The present study represents an integrated approach to understand the stages in the development of bedded pyrite and basemetal sulphide deposits. Accordingly, the regional stratigraphy has been worked out to determine the stratigraphic position of the volcanic pile hosting the sulphide deposits and also to fix the stratigraphic position of the sulphide deposits within the volcanic pile.

The western margin of the belt is dominated by a 1000 m thick succession of sediments, chiefly of terrigenous clastics. Pyritiferous quartz-pebble conglomerate forms the lowermost unit and it lies unconformably over the 3 b.y. old migmatitic Peninsular Gneiss. It is succeeded by current-bedded orthoquartzite which is interstratified with amygdaloidal metabasalt. Polymict Talya conglomerate succeeds the orthoquartzite–basalt sequence. The conglomerate is overlain by limestone and banded manganiferous and ferruginous quartzite.

The interior of the belt is occupied by a 5500 m thick succession of volcanic rocks which form a pile known as the Jogimardi volcanic pile. The volcanic pile is composed of 60% lava flows and 40% volcaniclastic rocks and bedded cherts. The lava suite is 5000 m thick and consists of 85% pillowed, massive, amygdaloidal basalts and only 3% rhyolite. Basalts occupy the lower stratigraphic levels. Variolite lava and rhyolite occur interstratified with volcaniclastics overlying the basalt flows. Three widespread sulphide-facies cherty iron formations mark periods of quiescence, of which the first one occurs as an interflow formation and the other two occur within volcaniclastics. Amygdaloidal basalt, which forms the lowermost flows of the lava suite, is in direct contact with Peninsular Gneiss.

Volcaniclastics are well bedded and are produced as a result of shallow subaqueous, partly subaerial pyroclastic activity. The pyroclastic volcanism was episodic in nature and the related volcaniclastic sedimentation was cyclic. The base of each cycle is marked by agglomerate/tuffaceous agglomerate, which are followed upward by chert. Thick-bedded sulphide-facies cherty iron formations mark the closing of a megacycle, which comprises three to four cycles of volcaniclastic and chemical sedimentation. Two megacycles have been recognised in the upper stratigraphic levels of the Jogimardi volcanic pile. Waterlain tuffs commonly show graded bedding, slump structures and scour-and-fill structures.

Metabasalts and variolite lava exhibit well-preserved quench plagioclase textures which are comparable with those of submarine basalts from the Atlantic Sea and Archaean quench-textured tholeiites from the Abitibi volcanic belt, Canada. Submarine hydrothermal alteration due to basalt–seawater interaction is thought to have produced greenschist mineral assemblages in Jogimardi volcanics.

The Jogimardi volcanic pile has been affected by two phases of deformation, of which the main first phase has folded the entire volcanic pile into a steep (55°) SSE-plunging anticline. The anticline is flanked on either side by regional synclines which are filled chiefly by greywacke and shale. Two prominent N–S trending synclines are recognised. The eastern syncline is overlain by banded ferruginous jasper and shale.

Geochemical characteristics indicate that the basalts are of low-K tholeiitic type. REE patterns of the basalts overlap modern mid-ocean ridge and low-K tholeiitic basalts and are strikingly similar to those of the Archaean Midland greenstone belt. The basalts may have been generated by equilibrium melting of eclogite or garnet-bearing mantle source regions. The geochemical features are comparable to low-K tholeiitic basalts. The overall geological features of the belt such as marginal sediments and abundant greywacke clearly justify such a comparison. Yet certain geochemical features such as lower MgO and higher Al?O? do not strictly fit into any of the modern tectonic environments.

The following sulphide deposits are recognised:

i) Stratiform basemetal massive sulphide type (A-type) ii) Bedded pyrite deposits (S-type) iii) Quartz-chalcopyrite-pyrite veins and massive sulphide veins localised along shear zones (C-type) (presently mined in the Ingladhal copper mines) iv) Pb-Cu-As sulphide-bearing druzy quartz veins localised along tensional fractures (D-type)

From A- to D-type, the different types are genetically related and trace an evolutionary pattern of mineralisation. The stratiform sulphide bodies possess tuff inclusions and laminations. The mafic tuffs hosting the sulphide bodies possess finely disseminated sulphides. Major element composition, REE pattern and ratios of relatively immobile elements of the host mafic tuffs are comparable to the average composition of the immediately underlying low-K tholeiitic basalt. However, trace elements (Cu, Pb, Zn, Co, Ni, Sb and Li) show a great increase (up to 270%) in their abundance relative to the average metabasalt. The stratiform basemetal sulphides and sulphidic cherts also show immobile element ratios comparable to those of the underlying basalt.

Mass balance calculations carried out in order to quantify the relative proportions of volcaniclastic/chemical and sediment components in stratiform basemetal sulphide bodies show that they represent mixtures of (i) fine-grained mafic volcanic tuffs (about 30%) and (ii) chemical precipitates (sulphides) (about 70%). The sulphides presumably were precipitated from hydrothermal solutions derived from seawater–basalt interaction. Sulphur isotope studies suggest that sulphur is from seawater and that the metal-bearing ore fluids were generated by convective circulation of seawater within the volcanic pile.

The distribution of trace elements Cu, Zn, Pb, Co and Ni in the stratiform sulphides indicates: (i) a submarine volcanic association of global distribution; (ii) the deposits belong to the Cu-Zn-pyrite type formed under near-proximal facies comparable to the Gersvik type of Caledonian deposits, Norway; and (iii) the metal contents bear a consanguineous relationship with volcanic activity.

The structurally controlled vein-type deposits (C- and D-types) represent products of regeneration of stratiform basemetal sulphides during shearing and through metamorphogenic hydrothermal fluids under the influence of the intrusive Chitradurga granite. The transitional geochemical characteristics of the basalts are thought to have governed the Cu-Zn-Pb type of the stratiform massive basemetal sulphide deposits of the Ingladhal sulphide zone.