Abstract

Although magmatic-hydrothermal ore deposits may contain as much as 1 billion tonnes (Gt) of sulfur as hydrothermal sulfides and sulfates, whether the genetically related magmas are particularly sulfur rich remains an open question. In oxidized melts, the presence of magmatic anhydrite would provide some evidence of this, but its preservation potential in the rock record is low, owing to the high solubility of sulfate in low-temperature aqueous fluids. In this study we have examined granitic and porphyry samples from porphyry copper ore deposits using quantitative evaluation of materials by scanning electron microscopy (QEMSCAN) and scanning electron microscopy-energy dispersive spectrometry (SEM-EDS) to identify small and sparse inclusions of anhydrite preserved in a range of igneous minerals including apatite, amphibole, plagioclase, quartz, and titanite. In total, anhydrite inclusions were present in 11 different samples from six sulfur-rich magmatic-hydrothermal ore deposits. Electron microprobe analysis (EMPA) of anhydrite inclusions suggests that primary magmatic anhydrite typically has elevated Sr (>2,500 ppm) and P (>500 ppm) compared to hydrothermal anhydrite; however, in many cases this is obscured by postentrapment exchange between the inclusion and host mineral or hydrothermal alteration of host minerals. Where the composition of inclusions is inconclusive, we have used cathodoluminescence imagery of zoning patterns in host minerals to help distinguish between igneous and hydrothermal origins. This study provides evidence that anhydrite saturation may be a common feature of arc magmas, particularly those associated with ore deposits, even though little evidence of magmatic anhydrite is preserved. Widespread anhydrite saturation has significant implications for any consideration of magmatic sulfur budgets, particularly in low-temperature, evolved magmas, which have little capacity to carry sulfur species in the melt.

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