Reevaluating Fluid Sources During Skarn Formation: An Assessment of the Empire Mountain Skarn, Sierra Nevada, USA

Abstract

Oxygen isotope analyses of skarn minerals have long been used to fingerprint the variable fluid sources involved in skarn formation. The Empire Mountain skarn of the Sierra Nevada (California, USA) batholith is identified as a low-δ18O skarn and is thought to form due to surface fluid involvement that was enhanced by fractures that formed during host rock brecciation. Although geochemically well characterized, the Empire Mountain skarn is less understood in terms of its hydrodynamic history. In this study, we develop a two-dimensional model of oxygen isotope transport during high-temperature fluid-rock interactions to assess the mechanisms by which low-δ18O garnets could form exclusive of brecciation. Highlighting regions nearest to the intrusion that could form garnet, we make three primary observations: (1) the oxygen isotope composition of the fluids, and not temperature, dominantly controls δ18Ogarnet values; (2) >6‰ increases in δ18Ogarnet, from negative to positive values, are observed over the maximum time frame of garnet thermodynamic stability; and (3) incremental emplacement of the intrusion can produce oscillations in δ18Ogarnet values. Without invoking brecciation, we find that low-δ18O garnets can form without the presence of surface fluids; they instead source 18O-depleted pore fluids from adjacent units. Further, surface fluids that do not equilibrate with the surrounding rock at depth become low-δ18O fluid sources during later stages of pluton emplacement. This study underscores that pore fluids at depth, regardless of their equilibrium state, can act as dormant low-δ18O fluid sources and may be responsible for low-δ18O-valued garnets.