OXYGEN ISOTOPE SYSTEMATICS OF SKARN GARNET: FLUID FLOW AND ORE FORMING HISTORIES OF THE CORDILLERAN ARC

Oxygen isotopes are a powerful tool useful for determining the origins of fluids driving decarbonation reactions in contact metamorphic skarn deposits. We present results from the largest study of oxygen isotope ratios in skarn minerals to date (20 skarns; 255 analyses), highlighting variations in fluid sources, fluxes and pathways in skarn-forming hydrothermal systems during Jurassic and Cretaceous magmatism of the California magmatic arc. Skarns surveyed represent spatial and temporal cross-sections through these arc systems. We document several low-δ¹⁸O skarns, the significance of which were first recognized by Valley and O’Neil (1982). The regional nature of this study permits assessment of controls of host-rock lithology, magma chemistry and tectonic setting on the attendant ore mineralization in skarns which have seen various classification schemes based on redox state (e.g., Newberry, 1983).

To potentially link fluid composition, major element chemistries, and the presence or absence of tungsten mineralization (as scheelite), we apply a variation of Newberry’s classification to skarns in our study and skarns from literature. This classification is not always directly confirmed by δ¹⁸O values, but reiterates that fluid sources are not strictly limited by tectonic setting or overall redox conditions.

Meteoric fluids are considered more oxidizing than magmatic or metamorphic fluids, and manifest as garnet with strongly negative δ¹⁸O values; whereas, garnets formed by interplay with magmatic fluid will have δ¹⁸O values >6‰. Reducing skarns, the most abundant skarn type, exhibit a range of oxygen isotope compositions (3.3‰ to 9.1‰) suggesting magmatic fluids and fluids produced via devolatilization of host rocks dominate skarn formation. A subset of strongly reduced skarns exhibit δ¹⁸O-values (6.1 to 7.2‰) free of meteoric influence. Though not all oxidized skarns surveyed exhibit low δ¹⁸O values, the only skarns to show heavy influence of meteoric fluids are oxidized. Meteoric-dominated oxidized skarns lack or contain only minor scheelite, and garnet compositions approach end-member andradite. We suggest and continue to investigate whether oxygen isotope compositions of skarn garnet provide a proxy for quality and quantity of scheelite development in oxidized and moderately reduced skarns.