Decoupled geochemical behavior of As and Cu in hydrothermal systems

Cu-rich and As-rich growth zones in pyrite provide new insights into the composition of late-stage magmatic fluids and their host hydrothermal ore deposits. The pyrite is from the Pueblo Viejo (Dominican Republic) and Yanacocha (Peru) high-sulfidation gold-silver deposits, which are thought to form from hydrothermal systems that interacted with magmatic vapor plumes. Electron microprobe analysis, secondary ion mass spectrometry, and elemental maps show that pyrite, the most common sulfide mineral in both deposits, contains three different types of growth zones: (1) As-rich zones that are enriched in Au, Ag, Sb, Te, and Pb, (2) Cu-rich zones with significantly lower concentrations of these elements, and (3) barren pyrite zones with no other elements. These zones are interpreted to result from mixing between the pyrite-forming fluid and vapors that invaded the main hydrothermal system episodically. Comparison to experimental studies of elemental partitioning and analyses of fumaroles and fluid inclusions from magmatic-hydrothermal systems suggests that the As-rich vapor formed at high and possibly magmatic temperatures, whereas the Cu-rich vapor formed at lower temperatures, possibly during migration of the original magmatic vapor. The presence of finely spaced multiple growth zones in pyrite suggests that the composition of at least high-sulfidation hydrothermal systems can be affected intermittently and repetitively by vapors, probably from underlying magmas.