GARNET EVIDENCE FOR PROGRADE ORE MINERALIZATION IN THE TIBES IRON SKARN, PUERTO RICO

Island-arc magmatic activity followed by ~40 million years of erosion have formed and exposed a wealth of ore deposits across Puerto Rico. While these deposits are economically valuable, they also preserve records of magmatic activity, fluid movement, and metallogenesis within the crust. The ore deposits of Puerto Rico are an ideal natural laboratory to assess the roles of melts and fluids in ore formation as magmatic activity ceased in the Oligocene and extensive uplift and erosion has exposed pristine shallow- to mid-crustal intrusive bodies and associated ores. The Tibes Iron skarn is an ore deposit located in south-central Puerto Rico and is exposed along the Río Portugués. The deposit has three distinct zones: a massive magnetite zone, a hornfels zone, and a marble zone, with abundant garnet across all three zones. U-Pb ages of garnets from the three groups yield three distinct ages: 61.20 ± 0.42 Ma for the garnets associated with magnetite, 58.35 ± 0.19 Ma for the garnets associated with marble, and 56.26 ± 0.23 Ma for the garnets associated with hornfels. Zircon U-Pb ages for the Tibes stock, the intrusive body responsible for the skarn, are 60.5 ± 1.6 Ma. The age of the garnets associated with magnetite are coeval with the age of the intrusive body, suggesting that ore mineralization occurred during the prograde stage of skarn formation, likely during stage two – magmatic fluid exsolution. This is not consistent with the typical skarn mineralization model, however, this is supported by O- and Fe-isotope data from the magnetite, which indicate a higher-temperature magmatic origin for the magnetite. Garnets associated with the marble are younger than the age of the Tibes stock and may have formed late in the prograde phase or in the retrograde phase of skarn formation. The young ages of garnets associated with hornfels suggest that they formed during the retrograde phase of the skarn, while the hornfels itself likely formed during stage 1 of the prograde phase.