The deposit occurs in alkalic and transitional basalts of upper Paleozoic age formed in an oceanic island tectonic environment. Two major lensoid massive sulfide bodies composed mainly of pyrite and chalcopyrite are underlain by chemically precipitated massive siliceous, and talcose rocks. The composition and temperature of the hydrothermal solutions have been constrained by a consideration of observed mineral associations and relative stabilities of talc, pyrrhotite, pyrite, chalcopyrite, and magnetite. Based on these calculations, areas of talc deposition at Chu Chua are interpreted to represent the vents of the hydrothermal solutions on the sea floor. Owing to cooling and mixing of the talc-depositing solutions with normal seawater, silica and the sulfides also were precipitated. Lead isotope compositions of pyrite and chalcopyrite in the deposit are similar to those of the host basalts, suggesting that the basalts were most likely the source of metals in the hydrothermal solutions. It is proposed that chemically precipitated siliceous and talc rocks may be used as exploration guides for massive sulfides associated with basaltic rocks, particularly in an oceanic island tectonic setting.--Modified journal abstract.

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