GENESIS OF THE CARBONATITE-HOSTED FLUORITE DEPOSIT AT AMBA DONGAR, INDIA - EVIDENCE FROM FLUID INCLUSIONS, STABLE ISOTOPES, AND WHOLE ROCK-MINERAL GEOCHEMISTRY

The Amba Dongar carbonatite complex is located approximately 400 km no rtheast of Bombay, India, and consists of a carbonatite ring dike and a number of syenitic intrusions which were emplaced into Late Cretaceo us Bagh sandstones and Early to Late Eocene Deccan volcanic rocks. Hyd rothermal activity associated with intrusion of the carbonatite was re sponsible for fenitization of the surrounding sandstones, and depositi on of economic quantities of fluorite (11.6 Mt of 30% CaF2). Fluorite mineralization occurs as veins and vug fillings, localized along fract ures within the calcite carbonatite, near its contact with the sandsto ne. Fluid inclusions in fluorite indicate a low temperature-low salini ty (<160 degrees C and 0.6-0.3 wt % NaCl equiv ore fluid, which decrea sed in temperature and salinity as it evolved. Crushing experiments pe rformed on fluid inclusions in both fluorite and quartz indicate the p resence of 0.08 m of dissolved CO2, and analyses of leachates and decr epitate residues from fluid inclusions show significant concentrations of Ca, Al, Na, CI, and S. Oxygen and hydrogen isotope compositions of fluid inclusions suggest that the ore fluid was dominantly meteoric a nd that it had equilibrated with sedimentary carbonate-bearing rocks. Log f(O2), and pH conditions of the fluid, at the time of ore formatio n, are interpreted to have been >-42 and <3.5, respectively. The prese nce of Al and S in the fluid, the molar equivalence of Na and Cl, and the positive deviation of O-18 and D from the meteoric water line poin t to a small contribution from orthomagmatic fluids. A model is propos ed in which the intrusion of a carbonatite magma at high crustal level s 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 th e decline of orthomagmatic activity, a meteoric water-dominated hydrot hermal system was initiated by the heat of the intrusion. The interact ion of Ca-bearing meteoric fluids with the last vestiges of F-bearing orthomagmatic NaCl brines caused deposition of large quantities of flu orite at the site of mixing.