Volatile budgets and gold mobilization in metasomatized sub-continental lithospheric mantle

Abstract

Metasomatized sub-continental lithospheric mantle (SCLM) is increasingly proposed to control the formation of giant hydrothermal gold provinces. However, the key budgets for gold and volatile components in the SCLM under gold provinces remain poorly constrained, thus hindering definitive genetic models of gold metallogenesis. The Cenozoic hydrothermal orogenic gold deposits in the Ailaoshan gold belt in SE Tibet that are genetically related to metasomatized SCLM, and adjacent penecontemporaneous lamprophyre dikes that are suggested to have a similar source, provide a rare opportunity to resolve this source budget issue. Integrated data on siderophile elements of the lamprophyre dikes, volatile contents of their melt inclusions, and their whole-rock geochemistry are utilized to constrain the source they share with the gold deposits. The lamprophyre primary magmas are characterized by high H₂O (3.6 wt%), Cl (up to ∼4700 ppm) and S (up to ∼3100 ppm) concentrations, with modeling suggesting that their mantle sources are also significantly enriched in volatiles (H₂O > 1000 ppm, Cl > 60 ppm), overlapping with those of the most volatile-rich magmas and mantle endmembers in the earth. Excluding those dikes that were either affected by auriferous fluids or experienced sulfide separation, the dikes have variable but generally normal Au concentrations (0.10–3.33 ppb), with estimated mantle sources comprising less than 1 ppb Au. This range is lower than or comparable to that of Au contents of global mantle peridotites and the primitive upper mantle, indicating that metasomatism processes did not significantly fertilize the SCLM beneath the gold province with anomalously high Au. Consequently, it appears that high volatile contents of the metasomatized SCLM, which promoted generation of auriferous fluid, ore-metal extraction, subsequent fluid transfer and advection, had a more important, and previously underestimated role in formation of the gold deposits.