GSA Connects 2021 in Portland, Oregon

Paper No. 229-3
Presentation Time: 2:05 PM


BAREFOOT, Marisa1, BILENKER, Laura1, HUDGINS, Tom2 and GIOVANNETTI-NAZARIO, David2, (1)Department of Geosciences, Auburn University, Auburn, AL 36849, (2)Department of Geology, University of Puerto Rico, Mayaguez, Mayaguez, PR 00681

Puerto Rico’s dynamic geologic history has resulted in a wealth of ore deposits across the island. Due to their variable geologic origins and unusual compositions, ore deposits are not only potential economic resources, they are valuable records of magmatic activity, metal transport, and fluid movement within the crust. The Tibes deposit, near Ponce, is currently classified as an iron skarn, which typically forms when an intrusive body comes into contact with a carbonate host rock. Iron is used to make steel, a vital commodity in green energy technologies like wind turbines. Understanding the processes that form ore deposits is key for finding new resources. Contrary to its classification, field observations of Tibes are inconsistent with that of a typical skarn. Massive magnetite bodies appear linear and have an orientation related to the local faulting, indicating potential structural control on the deposit. By analyzing the geochemical signature of Tibes magnetite, we will better understand how it formed and reevaluate its classification. Trace elements in magnetite allow us to identify its origin, while stable iron and oxygen isotope ratios reflect the source of those elements, depositional processes, and potential alteration. Petrographic observations such as mineral textures and their associations allow us to constrain the sequence of mineralization. Preliminary geochemical data support the hypothesis that Tibes is not a skarn. Trace element data of individual magnetite grains reveal higher concentrations of fluid immobile elements (e.g. Ti) than is expected for a skarn. Instead, concentrations of Ti, V, Ca, Al, Mn align with those observed in porphyry and iron oxide-apatite (IOA) deposits. Measurements of the isotopic composition of Tibes magnetite reveal δ56Fe values within the range of 0.21-0.39 per mil and δ18O values between 3.35-4.80 per mil. These values are also consistent with data for magmatic-hydrothermal ore systems like porphyries and IOAs rather than skarns. By combining field and petrographic observations with additional geochemical data for samples throughout the deposit, we are developing a genetic model for Tibes and reevaluating its current classification. A reclassification of Tibes would significantly impact our understanding of Puerto Rico’s geologic and tectonic history.