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

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Abstract
The Pliocene El Laco deposits of magnetite-apatite iron ore of the Kiruna type, situated on the flanks of a volcano in northern Chile, can morphologically and structurally be described as extrusive and intrusive magmatic orebodies with hydrothermal overprinting. Fluid inclusions in pyroxene and apatite record different stages in the transition from a late-magmatic remnant fluid, probably exsolved during crystallization of an ore magma, to hydrothermal 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 dike-vein system at Cristales Grandes were formed at >800°C. Pyroxene-magnetite veins in the Laco Sur ore indicate crystallization in fissures during degassing of a flow emplaced at still higher temperatures. Melt inclusions 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 depth and pressure, the water content of the exsolving fluids increased. Apatite crystals from pyroclastic ore at Laco Sur trapped a homogeneous high-salinity aqueous fluid (40 eq. wt.% NaCl) at a pressure of approximately 1 kbar. Apatite-hosted fluid inclusions from an ore dike at Rodados Negros have 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 400–500°C. The early apatite in San Vicente Bajo is overgrown by a second generation precipitated at 170–180°C from two coexisting immiscible aqueous fluids with salinities of 4.2–14.8 and 29–42 eq. wt.% NaCl, respectively. At Cristales Grandes apatite was deposited from low- to moderately saline aqueous solutions (0–20.3 eq. wt.% NaCl) at temperatures that fluctuated around the boiling point (150 ± 10°C) to cooler conditions.

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