A COMPARISON OF IRON ORE DEPOSITS IN PUERTO RICO: IMPLICATIONS FOR IRON TRANSPORT AND DEPOSITION IN SKARN SYSTEMS

The geologic history of Puerto Rico (PR) as an extinct island arc within the Caribbean plate has resulted in the formation of numerous magmatic-hydrothermal metal deposits, including three iron (Fe) skarns: Tibes, Keystone, and Island Queen. Iron itself is critical for the infrastructure of modern society and will be key for a successful transition toward green energy. Understanding the source, transport, and deposition of Fe in the crust is crucial for finding and securing sufficient resources. The Fe skarns of Puerto Rico are unique natural laboratories for studying the concentration of Fe in magmatic-hydrothermal systems as they are rare unaccreted examples of their ore deposit sub-type. Tibes is located in south-central PR and is associated with the small, dioritic Tibes stock. Keystone and Island Queen are located on the east side of the island near Juncos and are associated with the much larger San Lorenzo batholith.

Recent field mapping and petrography of all three sites reveal both similarities and diverse characteristics under the umbrella of Fe skarns. All three Fe deposits contain magnetite-rich ore bodies and appear to be spatially related to island-scale fault systems. However, the deposits vary in mineralogy, host rock lithology, source pluton size, as well as the abundance, size, and orientations of their ore bodies. Geochemically, the major and trace element compositions of the magnetite ore at each of the three sites are similar, but the stable isotopic (Fe, O) compositions are distinct, ranging from strictly magmatic (Tibes) or meteoric (Island Queen) to mixed fluid (Keystone) signatures.

A global comparison of these deposits to data published on magnetite from other Fe skarns highlights relative consistency in the elemental composition and texture of the ore, but potential distinction in isotopic signatures. Ongoing work explores several hypotheses to explain these differences including accretion, alteration, and contributions from host rocks and/or the source pluton. These data are also used to evaluate the utility of Fe and O isotope analysis of magnetite in skarn systems, with broader implications for the source, transport, deposition, and alteration of ore in Fe skarns.