ARTICLE D.V. Subba Rao á S.M. Naqvi Geological setting, mineralogy, geochemistry and genesis of the Middle Archaean Kalyadi copper deposit, western Dharwar craton, southern India Received: 8 September 1995 / Accepted: 18 November 1996 Abstract The Kalyadi polymetallic copper deposit oc- curs within the Middle Archaean (³3.0 Ga), medium- grade Kalyadi schist belt which consists predominantly of ultrama®c-ma®c schists interbedded with chemogenic chert, detrital high Al-Mg schists and siliceous schists. This sedimentary exhalative type (SEDEX type) ore- body is the only copper deposit hosted in cherts in the western Dharwar craton. The Kalyadi supracrustal rocks are intruded by tonalite-trondhjemitic gneisses (ca. 3.0 Ga) and granite (ca. 2.6 Ga). The Kalyadi copper deposit is polygenetic in nature. The primary ores represented by disseminations of pyrite  linneite and chalcopyrite  magnetite essentially along the bedding lamination of the metachert are referred to as the metamorphosed chert-sulphide rhythmites of a pri- mary stratiform type. The ore is of low-grade and re- cords imprints of at least two events of deformation. Pyrite is characterised by high-Co values (262± 4524 ppm) and high±Co/Ni ratios (3.0±19.7). Rare earth element patterns of the primary ores and the host metacherts are identical, characterised by La enrich- ment, absence of Eu anomalies and ¯at to depleted HREE patterns with d 34 S= )0.8&. The secondary (remobilised) ores are structurally controlled occurring as veins and stringers discordant to the bedding lami- nation or schistosity. The constituent ores are chalco- pyrite-pyrite-pyrrhotite with minor pentlandite. These sulphides with low-Co/Ni ratios (0.87±1.80), have either a strong positive or negative Eu anomaly and show slight HREE enrichment. The d 34 S value ranges from +2.64 to )4.29&. It is interpreted that the primary stratiform ores and the cherts were derived from volca- nogenic hydrothermal ¯uids as syngenetic/chemical deposits in a deep sea environment. The secondary epi- genetic mineralisation is related to subsequent mig- matisation, deformational events and granitic activity. Introduction Archaean schist belts (greenstone belts) in the Karnat- aka nucleus of the Dharwar craton, southern India have been classi®ed into ``older'' (3.4±3.0 Ga) and ``younger'' (3.0±2.6 Ga) schist belts (Radhakrishna and Naqvi 1986). These belts host fourteen well-known sulphide occurrences ranging in age from 3.4±2.0 Ga (Vasudev 1983). It is believed that the Early-Middle Archaean supracrustal belts throughout the world are devoid of economically workable syngenetic/stratiform copper sulphide deposits (Vasudev 1983; Radhakrishna 1984; Sarkar 1988; Groves and Barley 1994). However, the stratiform copper deposit in the Kalyadi schist belt is an exception. Unlike most massive sulphide deposits in Archaean greenstone belts which show a close spatial relationship with volcanic rocks, the Kalyadi copper deposit is sediment-hosted, similar to some rare deposits like the Sullivan in Canada. These deposits are com- monly referred to as the ``SEDEX'' (sedimentary ex- halative) deposits. The Kalyadi copper deposit is of low- grade type (0.7% cut o grade for Cu) with mineable reserves of 1.04 million tonnes. The ore-body is rich in pyrite and chalcopyrite in this older (Middle Archaean) Kalyadi greenstone belt, in contrast to the younger (2.6 Ga) sulphide deposits of the craton which, in ad- dition to copper, have subordinate Zn, Pb, Ag and Au mineralisation, for instance, the basemetal deposits of Chitradurga greenstone belt (Late Archaean). The present investigation is aimed at understanding the early Precambrian sedimentation, the nature of the associated metallogeny, and the genesis of the Kalyadi copper deposit. Mineralium Deposita (1997) 32: 230±242 Ó Springer-Verlag 1997 Editorial handling: P.G. Eriksson D.V. Subba Rao á S.M. Naqvi (&) National Geophysical Research Institute, Uppal Road, Hyderabad-500007, India