DEVELOPING A GENETIC MODEL FOR THE TIBES IRON DEPOSIT, PUERTO RICO

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 useful records of magmatic activity, metal transport, and fluid movement within the crust. Characterizing these deposits is key for understanding Puerto Rico’s complicated and debated geologic history. The Tibes deposit is currently classified as an iron skarn, which forms when an intrusive body comes into contact with a limestone host rock. Contrary to its classification, preliminary field observations are inconsistent with that of a typical skarn. Massive magnetite bodies appear linear and have an orientation related to the island’s fault system, indicating potential structural control on the ore deposit. By analyzing the geochemical signature of Tibes, 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 and reveal potential alteration or mixing of geologic fluids. Mineral textures and associations will help identify the processes that were at play and constrain the sequence of events. Preliminary geochemical data support the hypothesis that Tibes is not a skarn. Trace element data obtained by electron microprobe analysis of individual magnetite grains reveal higher concentrations of fluid immobile elements, such as Ti, than would be expected for a skarn. Concentrations of Ti, V, Ca, Al, Mn align with those observed in porphyry and iron oxide-apatite (IOA) deposits. Measurements of the iron isotope composition of Tibes magnetite are within the range of 0.21-0.39 per mil (9 analyses from 3 samples: ratio of 56 to 54 relative to IRMM14). These preliminary values are also consistent with data for higher temperature magmatic systems like porphyries and IOAs rather than skarns. By combining field and petrographic observations with additional geochemical data for samples throughout the deposit, we will develop a genetic model for Tibes and revise its current classification. The potential reclassification of the Tibes deposit would have significant impacts on our understanding of Puerto Rico’s geologic and tectonic history.