Anhydrite-bearing andesite and dacite as a source for sulfur in magmatic-hydrothermal mineral deposits

Magmatic anhydrite from andesites and dacites occurs as inclusions in high- and low-aluminum amphibole and pyroxene and indicates that sulfate-saturated magmas spanned a period of six million years at Yanacocha, Peru. Magmatic anhydrite from Yanacocha and other sites is characterized by light rare earth element - enriched patterns and elevated strontium contents distinct from magmatic-hydrothermal anhydrite. Petrologic arguments suggest that the hydrous and oxidized Yanacocha magmas contained more than ∼1000 ppm sulfur both dissolved in the melt and as a separate sulfate phase, which is sufficient to provide all the sulfur for the genetically related giant sulfur-rich Yanacocha epithermal gold deposits. High-aluminum amphiboles contain unusual anhydrite with wormy and amoeboidal textures, which are tentatively interpreted to represent trapping of an immiscible CaSO4-water melt together with sulfur-rich apatite at a temperature of ∼950 °C and a water pressure >3 kbar: Such unusually sulfate-rich magmas may be required to produce sulfur-rich magmatic-hydrothermal mineral deposits. © 2008 The Geological Society of America.