COMPARISON OF GARNET CHEMISTRY AND MINERALIZATION AT THE KEYSTONE AND TIBES IRON SKARNS, PUERTO RICO

Puerto Rico, an archipelago in the extinct Greater Antilles Arc, is home to many ore deposits, including iron skarns. Two of the iron deposits that were historically prospected or mined provide an opportunity to compare skarn processes and timing across the main island. This study focuses on the Keystone deposit in eastern Puerto Rico along the Great Northern Puerto Rico Fault Zone, and the Tibes iron skarn in south-central Puerto Rico along the Great Southern Puerto Rico Fault Zone.

The Keystone skarn contains high-grade magnetite ore formed from metasomatism caused by the ca. 75 Ma intrusion of the San Lorenzo Batholith into volcaniclastic host rock. The ore body forms two main lenses; the lower lens is garnetiferous and the upper lens has high-grade iron ore. Mineralogically, the skarn consists of massive magnetite intergrown with martite, specular hematite, garnet and epidote. Garnet crystals are relatively small and often form thin veins within the iron oxide ore. A garnet U-Pb age places Keystone garnet mineralization at 70.34 ± 2.91 Ma.

The Tibes iron skarn contains about 16 high-grade magnetite ore bodies that formed when the Tibes stock intruded carbonate-bearing sedimentary rock at ~60 Ma. Garnet is found in three populations, each with crystals of different sizes and abundance. These populations are categorized as the garnet-magnetite population, garnet-marble population, and garnet-hornfels population. Preliminary studies determined that the garnets are grandite in composition. U-Pb ages for Tibes garnets fall into three ranges based on their population: 55.6±0.39 Ma for garnet-hornfels, 58.31±0.19 Ma for garnet-marble, and 60.28±0.56 Ma for garnet-magnetite.

We will present a detailed geochemical and petrographic comparison of garnet mineralization from the Keystone and Tibes skarns. This information sheds light on differences in processes and timing of iron skarn formation from similar locations during the same regional tectonic events.