Fluid inclusions in magnetite-apatite ore from a cooling magmatic system at El Laco, Chile

The Pliocene El Laco deposits of magnetite-apatite iron ore of the Kiruna t ype, situated on the flanks of a volcano in northern Chile, can morphologic ally and structurally be described as extrusive and intrusive magmatic oreb odies with hydrothermal overprinting. Fluid inclusions in pyroxene and apat ite record different stages in the transition from a late-magmatic remnant fluid, probably exsolved during crystallization of an ore magma, to hydroth ermal fluids of successively lower temperature and salinity. Hydrous saline Na-K chloride melt inclusions with anhydrite daughter crystals in pyroxene intergrown with magnetite from ore lava at Laco Sur and ore breccia in a d ike-vein system at Cristales Grandes were formed at >800 degrees C. Pyroxen e-magnetite veins in the Laco Sur ore indicate crystallization in fissures during degassing of a flow emplaced at still higher temperatures. Melt incl usions like those in pyroxene also occur in an early generation of apatite from San Vicente Bajo, a subvolcanic orebody. During continuous crystallization of the iron oxide magmas, at decreasing d epth and pressure, the water content of the exsolving fluids increased. Apa tite crystals from pyroclastic ore at Lace Sur trapped a homogeneous high-s alinity aqueous fluid (40 eq. wt.% NaCl) at a pressure of approximately 1 k bar. Apatite-hosted fluid inclusions from an ore dike at Rodados Negros hav e a salinity of up to 60 eq, wt.% NaCl and show evidence of abrupt pressure release. Minimum temperature for the crystallization of this apatite is 40 0-500 degrees C. The early apatite in San Vicente Bajo is overgrown by a se cond generation precipitated at 170-180 degrees C from two coexisting immis cible aqueous fluids with salinities of 4.2-14.8 and 29-42 eq. wt.% NaCl, r espectively. At Cristales Grandes apatite was deposited from low- to modera tely saline aqueous solutions (0-20.3 eq. wt.% NaCl) at temperatures that f luctuated around the boiling point (150+/-10 degrees C) to cooler condition s.