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