MODELING FLUID FLOW AND STABLE ISOTOPE TRANSPORT DURING SKARN FORMATION: INSIGHTS FROM EMPIRE MOUNTAIN, MINERAL KING PENDANT, SIERRA NEVADA

Fluid flow during skarn formation is responsible for massive fluxes of CO₂and the deposition of ore metals. Commonly, it is thought that most skarns form from an initial pulse of magmatic fluid and later incorporate surface-derived meteoric fluids and host rock-derived metamorphic fluids (e.g., Meinert et al., 2005). However, geochemical observations at Empire Mountain in the Sierra Nevada Batholith suggest the opposite trend. The oxygen isotope composition of skarn garnets reveals meteoric water input at the onset of skarn formation with magmatic fluids entering the system later (D’Errico et al., 2012). Using numerical modeling and geochemical analysis of zoned skarn minerals (O isotopes, trace elements), a preliminary thermodynamic and isotopic history of the Empire Mountain skarn is constructed. This study tests the hypothesis that a major brecciation event enhanced fluid flow from the surface to greater depths wherein the skarn formed. The primary objective is to determine whether or not other mechanisms aside from brecciation can cause this shift in isotopic composition.

Preliminary finite volume models of topography-driven flow and stable isotope transport show that pre-existing permeability heterogeneity within crust enhances flow in high-permeability layers, especially along high angle normal faults. However, faulting alone cannot induce the large fluctuations in fluid δ¹⁸O values recorded by skarn garnets. Future simulations will include transient adjustments of permeability in response to fluid overpressure during pluton emplacement in order to simulate brecciation. Furthermore, we hope to integrate constraints from geochemical observations into the model in order to determine the timing and magnitude of the fluid fluxes. Future geochronological measurements and trace element analyses of zoned garnet will help quantify garnet growth rates and thus fluid fluxes throughout skarn formation.