The alkalic Au-Cu porphyry deposits of NSW, Australia: Evidence for coexisting melt and hydrothermal fluids in comb quartz layers

摘要

Alternating bands of comb quartz and quartz monzonite porphyry occur in some of the syn- to late-mineral intrusive phases associated with the Goonumbla and Cadia alkalic porphyry gold-copper deposits. The Goonumbla comb quartz layers are weakly mineralized and contain minor bornite with coexisting silicate melt and fluid inclusions, indicating that quartz layers were precipitated by hydrothermal fluids exsolved from and contained within the crystallizing monzonite melt. The chalcopyrite-magnetite-raineralized comb quartz layers from Cadia do not contain silicate melt inclusions, even though individual layers are intergrown with megacryst-bearing monzonite porphyry. While comb quartz textures are common in granitic intrusions associated with porphyry molybdenum deposits, the examples from Cadia and Goonumbla indicate that alkalic monzonite melts in shallow crustal settings are also capable of producing such features.