Fluid flow during low-[delta]¹⁸O skarn formation : insights from Empire Mountain, Mineral King, Sierra Nevada
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A two-dimensional numerical simulation of oxygen isotope transport in the shallow crust has been developed to examine the mechanisms of fluid flow during the formation of skarns. The Empire Mountain skarn in Mineral King, Sierra Nevada -- the motivation of this study -- has recently been identified as a low-δ¹⁸O skarn and interpreted as indicating the presence surface fluids during the onset of its formation (D'Errico et al., 2012). Rapid and sporadic changes in δ¹⁸O values within a single garnet have been interpreted as alternation between meteoric and magmatic fluid during garnet growth. D'Errico et al. (2012) conclude that multiple events of hydrofracturing of existing rock created transient, high-permeability conduits in the shallow crust, providing a low-resistance pathway for surface fluids to reach skarn-forming depths (~2-5 km). However, hydrofracturing typically occurs during retrograde metamorphic conditions which conflicts with the hypothesized prograde hydrofracturing during the incipient formation of the Empire Mountain skarn. Given this discrepancy, the objectives of this study are twofold: to explore mechanisms that can explain (1) the existence of meteoric δ¹⁸O signatures at skarn-forming depths and (2) the rapid and sporadic changes in fluid δ¹⁸O values throughout skarn formation. Numerical simulations reveal three ways in which a meteoric fluid signature can exist at skarn-forming depths during the onset of skarn formation: (1) convection-driven drawdown of surface waters in uniform, high permeability country rock; (2) existence of a large permeability contrast in the surrounding rock (e.g., extensional faults) that entrains surface water to depth; and (3) existence of pore fluids in isotopic equilibrium with ¹⁸O-depleted minerals prior to the magmatic intrusion. Possibilities (1) and (2) are difficult to substantiate given limited knowledge of the once overlying stratigraphy whereas the third provides the simplest explanation. Large fluctuations in fluid oxygen isotope compositions are observed in all three scenarios. In our model, the largest fluctuations in δ¹⁸O values occur while garnet is thermodynamically stable, in areas nearest to the magmatic body, and downstream relative to the topography-driven flow field. However, low δ¹⁸O and δ¹³C values of carbonates at Empire Mountain indicate infiltration-driven fluid flow during metamorphism, likely caused by the ductile collapse of carbonate pore space followed by brecciation. Ultimately, even though there exist scenarios that demonstrably show the occurrence of low-δ¹⁸O skarns, future simulations must include rock deformation (i.e., ductile closure of pore space and hydrofracturing) and mineral reactions to gain further insight to how low-δ¹⁸O skarns incorporate fluids with meteoric oxygen isotope signatures.