75-35 Insights into Skarn Genesis from Garnet Oxygen Isotopes at the Tibes Deposit, Puerto Rico

Session: Mineralogy, Geochemistry, Petrology, and Volcanology Student Session (Posters)

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Abstract:

Garnet crystals are key recorders of geochemical processes involved in magmatic-hydrothermal activity, which help to develop the formation story of a skarn. Puerto Rico is an island arc of approximately 120 to 45 Ma (Jolly et al., 1998) and is part of the most extensively preserved oceanic arc sequences in the world. The preservation of this island arc, the exposure of its deeper parts by ~45 Ma of erosion, and its lack of accretion onto a continent make it an ideal location to study magmatic plumbing systems, magmatic hydrothermal fluids, and metal transport. Located in the southern region of Puerto Rico, in the municipality of Ponce, is the Tibes calcic iron skarn, one of three characterized skarns in Puerto Rico. The Tibes stock, the intrusion responsible for skarn formation, has a zircon U-Pb age of 60.5 ± 1.6 Ma (Perez, 2008). Previous work identified and dated three mineralogically distinct garnet populations in the skarn surrounding the Tibes stock: garnet-magnetite (60.28±0.56 Ma), garnet-marble (58.31±0.19 Ma), and garnet-hornfels (55.6±0.39 Ma) (Giovanetti-Nazario, 2022).

The detailed characterization of Tibes by previous research indicates a magmatic-hydrothermal source for the ore-forming fluids, inconsistent with the general skarn model of ore deposition during the infiltration of meteoric fluids. Abundant garnet and excellent exposure provide the opportunity to determine when/if this skarn transitions from a magmatic-hydrothermal system to a meteoric-hydrothermal system. By using the δ¹⁸O values of garnets alongside pre-existing age data, we can determine changes in the Tibes skarn's hydrothermal system. The δ¹⁸O values of bulk garnets from six samples range narrowly from 6.15 - 7.56‰ across all three garnet populations, which is consistent with a magmatic-hydrothermal fluid source. This aligns with previous analyses of magnetite ore by Barefoot (2021), consistent with a magmatic source of the ore fluid in Tibes with minimal meteoric input or post-depositional alteration, but extends the magmatic hydrothermal history into younger, ore-barren rocks. Planned intragrain analysis of O isotopes by secondary ion mass spectrometry may shed light on fluid changes during crystal growth.

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

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