Geologic History and Timing of Mineralization at the Haile Gold Mine, South Carolina

摘要

New constraints on the geologic history and timing of mineralization at the Haile gold deposit in South Carolina are provided by radiometric dating of igneous and metamorphic samples using Re-Os, U-Pb, and ~(40)Ar/~(39)Ar methods. Seven Re-Os measurements were made on impure mineral separates of fine-grained molybdenite containing variable quantities of pyrite, pyrrhotite, muscovite, and illite. Four measurements produced an iso-chron at 548.7 ± 2.0 Ma (MSWD = 1.04). An additional age of 541 + 2 Ma suggests that molybdenite may have been produced over a ~8 Ma timespan (541-549 Ma), but the preferred interpretation is that molybdenite formed primarily at -549 Ma. Broadly correlative Mo and Au abundances in drill samples, and the common presence of molybdenite in Au-bearing zones, indicate that Au mineralization at Haile probably also occurred at -549 Ma. Contrasting Re-Os ages produced at 564 ± 2 and 529 ± 2 Ma are interpreted to be inaccurate, due to the effects of impure mineral separates, and possibly resulting from loss of radiogenic Os from fine-grained molybdenite to other sulfide minerals. Some mobilization and loss of Re, possibly under oxidizing conditions during late Paleozoic metamorphism, may explain two relatively old Re-Os ages determined here and by Stein et al. (1997). Laser ablation dating of zircons from a metavolcanic sample at Haile yielded a weighted mean age of 560 ± 8 Ma. This age is similar to the published time window of 551 to 555 Ma for magmatism at Haile, based on U-Pb zircon dating by ion probe (Ayuso et al., 2005). Similar timing of magmatism and Au mineralization indicates that the hydrothermal systems at Haile were likely driven by magmatism and were therefore not the product of collision, orogeny, and metamorphism. These results confirm that magmatism and mineralization at Haile were active at a time when the Carolina terrane was still in a peri-Gondwana location, well prior to its accretion to eastern Laurasia. Ages of some detrital zircons imply an anomalously young depositional age for the metasedimentary unit that hosts the Haile orebodies (<540 Ma). These ages conflict with an older age of 560 ± 8 Ma determined for the overlying metavolcanic unit. This conflict is interpreted to indicate that folding has overturned the geologic units at Haile. A possible Devonian-age metamorphic event at -390 Ma is documented by ~(40)Ar-~(39)Ar dating of porphyroblastic biotite collected from mineralized rock at Haile. Zircon ages from the Dutchman Creek Gabbro (weighted mean of 314 ± 2 Ma) and 40Ar-39Ar ages on biotite separates from dikes that crosscut the Haile property (311 ± 3, 311 ± 2 Ma) document a pulse of mafic and alkaline magmatism in the late Carboniferous. These late-stage metamorphic and magmatic events appear to be unrelated to mineralization at Haile.