Abstract

High-salinity fluid inclusions from many porphyry copper and skarn deposits fall on linear trends in the H 2 O-NaCl-KCl system, called the halite trend, and they have been interpreted to result from separation of KCl-bearing halite before entrapment of the fluid in the inclusion. In addition to Na, K, and Cl, halite trend inclusions can also contain abundant Fe, Ca, and S, all of which could have an effect on the orientation of the trend in the H 2 O-NaCl-KCl system, making halite subtraction a less likely explanation for the observed compositional trend. We have attempted to evaluate this possibility by analysis of inclusion daughter minerals, decrepitates, and leachates, as well as phase volume measurements for daughter minerals, in fluid inclusions from the Granisle porphyry copper deposit. These observations show that FeCl 2 composes 31 to 45 wt percent of the inclusion and that the resulting halite trend inclusion salinities range from 70 to 84 wt percent of NaCl + KCl + FeCl 2 . CaCl 2 and MnCl 2 are minor components. The liquid contains smaller but significant amounts of sulfur, probably as HSO (super -) 4 and its complexes, as well as CO 2 , CO, and CH 4 in minor amouns. Trends established by these data point toward NaCl in multicomponent space, thus supporting halite (or halite solid solution) subtraction over the alternative mechanisms proposed to explain the halite trend.

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