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Three assemblages of ores have been recognized at the Proterozoic porphyry copper deposit, Malanjkhand, India. They are: (1) primary ore containing chalcopyrite and pyrite; (2) moderately oxidized ore containing chalcocite, covellite, bornite, and other secondary copper sulfides; and (3) intensely oxidized ore containing copper sulphates, carbonates, chlorides, oxides and native copper. The Malanjkhand chalcopyrite alters, by continued oxidation, to the following sequence of minerals: chalcopyrite (CuFeS2)-bornite (Cu5FeS4)-idaite (Cu3FeS4)-covellite (CuS)-yarrowite (Cu9S8)-spionkopite (Cu39S28)-geerite (Cu1.60S)-anilite (Cu1.75S)-djurleite (Cu1.943S)-chalcocite (Cu2S). The alteration beyond chalcocite follows the reverse sequence to covellite, i.e., chalcocite-djurleite-anilite-geerite-spionkopite-yarrowite-covellite. The excess copper from the reverse sequence produces copper sulphates, carbonates, oxides, chlorides and native copper. Additional formation of copper sulphates, carbonates, etc. is due to variable conditions of Eh-pH, mineral solubilities in water, and dissolution of pyrite at relatively high Eh-pH values. These factors give rise to different complex ions such as SO42−, Cu(OH)2, Cu(SO)4, Cu(CO3)22−, and CuCl+. The chalcopyrite in the primary ore tarnishes by the formation of films of FeO(OH), bornite, idaite, covellite, anilite, and chalcocite. The tarnish varies in color from deep yellow, purple yellow, purple, violet, orange pink, blue, peacock blue, and dark grey to black. The alteration of Malanjkhandchalcopyrite has been considered to be due to galvanic interactions. Probable and possible stability relations among various phases are suggested in terms of Eh-Ph and attendant galvanic reactions in oxygenated and carbonated waters percolating through the rock and orebody. Several electrochemical equations are proposed for the reactions and stability fields of the minerals are constructed using available thermodynamic data. |
