Abstract

The Amba Dongar carbonatite complex is located approximately 400 km northeast of Bombay, India, and consists of a carbonatite ring dike and a number of syenitic intrusions which were emplaced into Late Cretaceous Bagh sandstones and Early to Late Eocene Deccan volcanic rocks. Hydrothermal activity associated with intrusion of the carbonatite was responsible for fenitization of the surrounding sandstones, and deposition of economic quantities of fluorite (11.6 Mt of 30% CaF 2 ). Fluorite mineralization occurs as veins and vug fillings, localized along fractures within the calcite carbonatite, near its contact with the sandstone.Fluid inclusions in fluorite indicate a low temperature-low salinity (<160 degrees C and 0.6-0.3 wt % NaCl equiv) ore fluid, which decreased in temperature and salinity as it evolved. Crushing experiments performed on fluid inclusions in both fluorite and quartz indicate the presence of 0.08 m of dissolved CO 2 , and analyses of leachates and decrepitate residues from fluid inclusions show significant concentrations of Ca, Al, Na, Cl, and S. Oxygen and hydrogen isotope compositions of fluid inclusions suggest that the ore fluid was dominantly meteoric and that it had equilibrated with sedimentary carbonate-bearing rocks. Log f (sub O 2 ) and pH conditions of the fluid, at the time of ore formation, are interpreted to have been >-42 and <3.5, respectively. The presence of Al and S in the fluid, the molar equivalence of Na and Cl, and the positive deviation of 18 O and D from the meteoric water line point to a small contribution from orthomagmatic fluids.A model is proposed in which the intrusion of a carbonatite magma at high crustal levels caused faulting and fracturing of the surrounding country rocks, and was accompanied by the release of orthomagmatic fluids, expressed as extensive K and Na metasomatism of the surrounding sandstones. With the decline of orthomagmatic activity, a meteoric water-dominated hydrothermal system was initiated by the heat of the intrusion. The interaction of Ca-bearing meteoric fluids with the last vestiges of F-bearing orthomagmatic NaCl brines caused deposition of large quantities of fluorite at the site of mixing.

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