Petrochronology of hydrothermal rutile in mineralized porphyry Cu systems
Michaël Schirra, Oscar Laurent
Abstract
Hydrothermally formed rutile is a common mineral in porphyry Cu (-Au-Mo) deposits and has the potential to provide versatile information about the physical-chemical conditions and timing of mineralization. In this study, we investigated the trace element composition and U-Pb dates of hydrothermal rutile by LA-ICP-MS together with its petrographic context, in four well-studied porphyry systems: Batu Hijau (Indonesia), Bingham Canyon (USA), El Salvador (Chile) and Northparkes (Australia). A similar sequence of rutile generations occupying comparable textural positions was found to be common to all investigated deposits. We particularly focused on the most common rutile generation that is formed by the breakdown of biotite and temporally closely associated with mineralization as demonstrated by intergrowth relationships between individual, blocky rutile grains and Cu-Fe sulfides as well as cathodoluminescence imaging of mineralized quartz veins. Compositionally, rutile shows large trace element variations within and among the porphyry deposits, indicating that the compositions of both the local host rock and the fluid phase influence that of hydrothermal rutile. In comparison with non-hydrothermal rutile of various petrogenetic origins, rutile from porphyry Cu deposits show characteristic differences regarding their Sn, W, Si and Fe concentrations, indicating that rutile might be used to fingerprint porphyry-style mineralization. Due to its temporal link to Cu mineralization, texturally controlled in-situ U-Pb dating of blocky rutile grains by LA-ICP-MS provides the possibility to directly determine the absolute timing of Cu mineralization. Despite variable incorporation of initial Pb in the investigated hydrothermal rutile, we obtained robust lower intercept U-Pb dates that overlap within uncertainty with previously obtained mineralization ages for all four deposits, including those based on high-precision (ID-TIMS) zircon dating of bracketing porphyry intrusions. This demonstrates that rutile can be used to accurately predict the age of porphyry Cu deposit formation and with a precision better than 1.5% relative for deposits older than ca. 10 million years.