Compositionally distinct, saline hydrothermal solutions, Naica Mine, Chihuahua, Mexico

Naica is a chimney-manto, limestone-replacement, skarn-sulfide deposit typical of those that have yielded much of the silver, lead, and zinc production of the western hemisphere. Fluid inclusions in fluorite deposited throughout late skarn and most sulfide mineralization at Naica provide the first direct observations of main-stage fluids in these deposits. The inclusions can be divided into three compositionally distinct groups: (A) liquid + vapor (119 degrees -379 degrees C homogenization temperatures), (B) liquid + vapor + halite (237 degrees -- 369 degrees C; salinities of 31-43 equiv. wt % NaCl containing less than 12% KCl); (C) liquid + vapor + halite + sylvite (277 degrees - 490 degrees C; 52-63 equiv. wt % NaCl containing 22-31% KCl) with some high temperature inclusions (565 degrees -684 degrees C; approximately 40 equiv. wt % NaCl with approximately 25% KCl). Types A, low-salinity B, and high-temperature C inclusions homogenize by vapor disappearance. High-salinity type B and most type C inclusions homogenize by halite disappearance. Evidence for boiling is seen in type A and low-salinity type B inclusions.Types A, B, and C inclusion solutions occupy relatively small, compositionally distinct fields in the NaCl-KCl-H 2 O system. High-salinity type B and most type C solutions were apparently saturated with halite. The composition of high-temperature, unsaturated type C solutions could have been controlled by equilibration with granitic rock and that of saturated, low-temperature solutions could have been controlled by halite subtraction. Wide and completely overlapping spatial distribution of all three inclusion types, combined with the fact that all three solutions are fully miscible, requires that the three compositionally distinct solutions were present in the deposit at different times. The most likely order of appearance of the solutions was C, then B, and finally A with pressures considerably above lithostatic during C solution activity, lower but still exceeding lithostatic during early B activity, and hydrostatic during late B and throughout A time.