The Proterozoic Kayad Zn-Pb deposit in Ajmer, Rajasthan is located within the Aravalli-Delhi Fold Belt in western India. Mineralization of sphalerite and galena, commonly associated with chalcopyrite and pyrrhotite, is hosted primarily by graphitic quartz mica schist (QMS) and subordinately by calc-silicate, quartzite and pegmatite. In QMS, both massive ore, evidently formed by remobilization, and laminated ore are present. Calc-silicate and quartzite contain disseminated sulfides while pegmatites contain sulfide veins that cause massive mineralization. Mineral replacement textures such as replacement of albite and muscovite by K-feldspars and biotite by chlorite suggest that Fe-Cu sulfides, represented by chalcopyrite and pyrrhotite, formed during potassic and acidic alteration. Trace element characterization demonstrates that between co-existing chalcopyrite and pyrrhotite, Ag, Zn, Sn, In, Cd, Ga are strongly portioned in chalcopyrite whereas Co and Ni are partitioned in pyrrhotite. High concentration of Ag (up to ~9000 ppm) in chalcopyrite adds to economic potential of the deposit. Barring the exception of massive ore in QMS, the trace element compositions of chalcopyrite and pyrrhotite are host-rock-independent suggesting profound control of the parental hydrothermal fluid on their composition. In laminated QMS, pegmatite and quartzite, higher Co, Cd, Mn and In in sphalerite and higher Ag, Sb, Bi, Se in galena compared to chalcopyrite suggest that these phases co-crystallized during their formation. In contrast in remobilized massive ore in QMS, higher Co, Cd and Mn in sphalerite over chalcopyrite and higher Ga, Sn and In in chalcopyrite over sphalerite and galena possibly indicate co-crystallization followed by recrystallization. Furthermore, constant yet distinct Cd:Zn ratios of chalcopyrite and co-existing sphalerite may indicate involvement of two different fluids. The observed trace element characteristics can be best explained by co-crystallization of chalcopyrite and sphalerite by fluid-mixing and subsequent recrystallization leading to the formation of part of the massive ore.

Present work deals with paleogeographic reconstruction through sequence modelling, facies and micro-facies analysis and bio-geochemical investigations of late Archaean Banded Iron Formations (BIFs) from Kushtagi-Hungund Schist Belt (KHSB) of Eastern Dharwar Craton (EDC), South Indian Shield (SIS). This comparatively less metamorphosed schist belt of EDC is correlated to the Bababudan Group of Dharwar Supergroup, Western Dharwar Craton (WDC). The lower and upper age constraints have been established from dating of 3.4-3.1 Ga old underlying Sargur Group and TTG gneisses, and 2.5 Ga old younger granites, intruding the KHSB metasediments, respectively. Further, metavolcanics intercalating with the metasediments of KHSB have been dated at 2750-2670 Ma. In this work, special emphasis has been given to the shaley-BIFs of this schist belt which is comprised of thin (few millimeters to a maximum of 1.2-1.5 cm thick) alternating units of iron rich shale, chert and hematite. Facies analysis and sequence model reveal deep water, offshore paleogeography, where proximal outer shelf is dominated by shaley siliciclastics and distal outer shelf and further deep oceanic succession is occupied by chert-hematite-dominated chemogenic sediment suits. Micro-facies analysis of the shaley-BIFs elucidated the interaction between chemogenic and deep water siliciclastic and volcaniclastic shaley sediments within micro domains. Primary chert layers contain several permineralized structures associated with carbonaceous matters. Recent RAMAN spectrometric analysis (compared to previous data in provided figure) and Carbon-isotopic values (d13C values ranging from -22.08 to -30.84 ‰ -VPDB, n= 12) of corresponding Total Organic Carbon (TOC) (ranging from 0.03 to 0.14 %) from cherts and shales indicate preservation of Archean biogenic remnants. Recent elemental (major and trace) and oxygen isotopic data, associated with the micro-facies systematics of shale-chert-iron oxide units, have been compared to previously published geochemical data sets derived from KHSB and other BIFs of SIS to provide important clues and new insights regarding late Archean ocean water chemistry, redox state, paleoclimate and control of tectonics and provenance on sedimentation pattern, prior to Great Oxygenation Event (GOE).