Abstract

The Granisle and Bell porphyry copper deposits, in north central British Columbia, share numerous similarities but differ in that ore at Granisle is associated with potassic alteration, whereas ore at Bell is associated with an overprint of sericitic alteration. Fluid inclusions in the deposits can be classified as a vapor-rich type A and liquid-rich types B(liquid + vapor); C(liquid + vapor + halite); D(liquid + vapor + halite + sylvite); E(liquid + vapor + halite + K-Fe-Cl phase); F(liquid + vapor + halite + sylvite + K-Fe-Cl phase); and G(liquid + vapor + halite + at least one unidentified phase). Type A inclusions constitute between 25 and 90 percent of the inclusions at Granisle and a relatively constant 70 percent at Bell. Of the liquid-rich inclusions, types D and F dominate at Granisle and types C and E dominate at Bell. Type B inclusions are rare in both deposits. About 90 percent of the liquid-rich inclusions at Granisle homogenize by vapor disappearance; the dominant types D and F inclusions exhibit a bimodal distribution of homogenization temperatures with maxima at 500 degrees and 900 degrees C. About 75 percent of the inclusions at Bell homogenize by halite disappearance after disappearance of the vapor, with the dominant type C inclusions homogenizing largely between 400 degrees and 600 degrees C. Types D and F inclusions in both deposits fall along the same linear compositional trend in the NaCl-KCl-H 2 O system regardless of their temperature or conditions of homogenization.Our interpretation of these observations in the context of phase equilibria in the NaCl-KCl-H 2 O system and the Na/K/Ca geothermometer indicates that:1. Potassic alteration and ore at Granisle formed at temperatures ranging from 400 degrees to possibly 800 degrees C from solutions with salinities of up to 70 percent and K/Na atomic ratios of over 0.2. These solutions were probably derived directly from a magma and changed composition by crystallization of halite but not by boiling or mixing with meteoric water.2. Sericitic alteration and ore at Bell formed at temperatures of 400 degrees to 600 degrees C from solutions with K/Na atomic ratios of 0.2 or less and salinities similar to those at Granisle. These solutions probably resulted from equilibration of meteoric water with the host intrusions at about 500 degrees C and enhancement of salinity by boiling.3. Most crystallization at Granisle took place during periods of high pressure whereas that at Bell occurred during periods of lowered pressure.Comparison of our results with fluid inclusion data on other porphyry copper deposits suggests that deposits containing undisturbed potassic zones are similar to Granisle and deposits with sericitic and phyllic overprints are similar to Bell.

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