Pluton‐Wallrock Interaction of the Empire Quartz Diorite, Southern Sierra Nevada: Implications for Skarn Formation in the Empire Mountain Pendant

At Empire Mountain, in the south‐central Sierra Nevada, intrusion of the Cretaceous Empire quartz diorite caused the complete conversion of former calcareous wallrock to a skarn. We present a model of the genesis of the Empire skarn system. Only the most proximal zone of garnet\u3eclinopyroxene exoskarn is preserved in the pendant, but excellent three‐dimensional exposure permits the characterization of the magma‐wallrock interaction. The granoblastic andraditic cores observed in the garnetite skarns are evidence of the early crystallization of anhydrous calc‐silicate mineralization during prograde metamorphism, while the more grossular‐rich rims and veins most likely filled in during a second period of garnet growth. Tabular zones of abundant hydrous skarn minerals (epidiote and actinolite), brecciated garnetite and vug‐filling, large, euhedral garnets (up to 15 cm in diameter) in late stage quartz represent localized retrograde mineralization along faults, joints or intrusive contacts. Low δ18O values of Empire Mountain skarn [δ18O (garnet) = –1.8 to 2.9‰; δ18O (quartz) = 5 to 7.6‰; δ18O (epidote) = 2.8‰] indicate a strong meteoric component was present throughout both prograde and retrograde skarn mineralization. In addition, ion microprobe Ti‐in‐zircon analyses suggest a high crystallization temperature (850 ‐ 930°C) of zircon. These data support the model that the magma was hot and of low volatile content, and therefore able to ascend adiabatically to shallow and relatively cool crustal levels, permitting the unusually early initiation of brittle deformation and involvement of meteoric fluids throughout the evolution of the hydrothermal system. The result of these processes is a large garnetite skarn pendant at Empire Mountain, the first low‐δ18O skarn locality reported in the western North America