Abstract

The origin of hypogene alteration and mineralization features in the Escondida porphyry Cu deposit resulted from intense overprinting related to three main hydrothermal stages. The beginning of each stage is recorded by deposition of bright quartz crystals on vein walls that precipitated from high-temperature fuids. In the deepest zones of the deposit, the earliest stage started with exsolution of intermediate-density fuids, which transported high concentrations of Cu, whereas Mo was not detected. However, in the shallow zones, the early stage began with depressurization and unmixing of intermediate-density fuids, which generated both a hypersaline and vapour-rich fuid phase, producing an important geochemical segregation between both phases. The transitional stage also started with circulation of intermediate-density fuids but never experienced unmixing. These fuids transported the highest Mo concentrations in the deposit; however, Cu displays lower concentra- tions relative to intermediate-density fuids from the early stage. The beginning of the late stage was also associated with intermediate-density fuids; however, Cu and Mo were below the detection limits in most of the analysed fuids. During the evolution of the three stages, the fuids experienced gradual cooling, which promoted the precipitation of euhedral and zoned quartz crystals that overgrew the early high-luminescence quartz generations. Sometimes, sulfde minerals display euhedral crystal boundaries with zoned quartz, suggesting that mineralization started during these phases. The latest events detected in each main stage are linked to cooler and low-salinity fuids, from which dark quartz and hypogene sulfdes precipitated along microfractures and interstitial spaces developed in the earlier quartz generations.