Sir: In his commentary on the Wilson et al. (2003) paper on the Ridgeway alkalic porphyry Au-Cu deposit, Smith (2005) highlights some of the difficulties in distinguishing between zones of propylitic alteration that characterize the distal portions of porphyry (and other) hydrothermal systems and mineralogically similar assemblages related to regional low-grade burial metamorphism. This problem has long been recognized, and explorers and researchers of porphyry deposits have to contend with it on a daily basis. The problem has recently been reviewed by Gifkins et al. (2005), who provide criteria to help discriminate between hydrothermal and metamorphic mineral assemblages in...

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