Fluid Inclusion Studies in Fluorite at Milpo Mine (Atacocha District), Central Peru

Abstract

Fluid inclusions in fluorite from Milpo mine (Atacocha district) Central Peru provide interesting observations on the hydrothermal phase in these deposits. The inclusions can be divided into four compositionally distinct types. Type I (liquid + vapor + halite ± sylvite) shows homogenization temperatures between 415° and 345 °C and salinities of 35–59 wt.% NaCl equiv. The evolution of this type takes place in the succession as follows: I_a → I_b → I_c. All inclusions for type I_a homogenize by halite disappearance. Type II (liquid + vapor) homogenizes to vapor in the same temperature range as type I_a. This fact suggests boiling conditions. Type III (liquid + vapor + halite) homogenizes between 190° and 350 °C and contains 30–37.5 wt.% NaCl. Type IV homogenizes between 160° and 300 °C and has low salinities of 6–18 wt.% NaCl. Type I inclusions occupy a compositionally distinct field in the NaCl-KCl-H₂O system and correspond to a fluid A of magmatic origin. The inclusions of type III and type IV occupy a very small field along the NaCl-H₂O boundary and represent a fluid B which initially had low salinity. The fluid A has gross densities of 1.10–1.05 g/cm³ and the fluid B densities of 0.8–0.93 g/cm³. Mixing phenomena of these fluids with different densities occur. The approximated pressure at the time of trapping is low from 140 to 200 bars.