MAGMATIC PROCESSES AND SKARN FORMATION - AN EXAMPLE FROM THE EMPIRE CU-ZN SKARN DEPOSIT, CUSTER COUNTY, IDAHO

Discovery of the world class Antamina Cu-Zn skarn deposit in Peru has sparked renewed interest in exploration for large skarn deposits worldwide. However, little is known about skarns that contain both Cu and Zn. Recent study of the Empire Cu-Zn skarn deposit, Custer County, Idaho sheds new light on this deposit class and the importance of high F activities in associated intrusions. The Empire Cu-Zn skarn deposit has 3 unusual features: 1) Zn, which normally is deposited distally at low temperature, is directly associated with Cu 2) endoskarn is more abundant than exoskarn and 3) quartz phenocrysts are anomalously vermicular.

The Cu-Zn skarn mineralization occurs along the northeastern part of the Mackay stock, at the contact between granite porphyry and the White Knob Limestone. Igneous rock alteration involves the following time/space sequence: 1) sericitization and silicification, 2) dark-grey scapolite veinlets with very fine grained wollastonite halos, and 3) larger veins composed mainly of pink-red brown garnet and minor pyroxene. These veins are enveloped by wollastonite and cut or reopen the earlier scapolite veinlets. These early assemblages were then affected by retrograde alteration including idocrase, quartz, calcite, chlorite and, fluorite.

The Cu-Zn skarn at the Empire Mine is closely related to the granite porphyry phase of the Mackay stock. The extremely vermicular texture of the quartz phenocrysts, the high F concentrations in amphibole and biotite, and the widely distributed fluorite in retrograde skarn indicate the F activity of the magma was high. The F content of amphibole and biotite are 1.79-2.96% and 0.33 – 3.80%, respectively.

The documented high F activity at Empire has several important effects including: 1) lowering the solidus temperature of the magma and thus changing the timing, temperature, and duration of hydrothermal fluids. Given the increased solubility of Zn relative to Cu at lower temperatures, this may explain the high Zn/Cu ratios 2) affecting the stability of silicate minerals such as quartz phenocrysts that are corroded by F-rich hydrothermal fluids 3) favoring the formation of endoskarn, in the same sense that carbonate wall rocks are dissolved by weakly to moderately acidic hydrothermal fluids, silicic rocks can be readily attacked by F-rich fluids.