A FIELD AND GEOCHEMICAL CHARACTERIZATION OF ORE AT THE KEYSTONE IRON SKARN, PUERTO RICO

Iron (Fe) is essential to modern life, primarily for its use in steel. Understanding the processes that transport and deposit Fe within the crust is crucial to finding and evaluating Fe deposits for economic potential. The complex tectonic and magmatic history of Puerto Rico as an extinct and unaccreted island arc was conducive to the formation of many ore deposits, but they are generally understudied. In this investigation, field observations, petrography, and geochemical analysis of Fe ore from the Keystone skarn provide records of fluid movement and metal transport in an unaccreted Fe skarn. Keystone contains two sub-parallel ore bodies that are dominated by magnetite with some hematite. Throughout the ore, there are quartz-filled vugs, fine-grained garnet, and minor epidote. In situ measurements of elemental concentrations can be used to determine the composition of and chemical changes within the Fe ore and stable isotope ratios of O and Fe in magnetite can be used to determine the source of Fe and the extent of weathering. Additionally, paired O and Fe isotope data will contribute to a growing body of literature investigating the sources of magnetite ore. Initial measurements of Ti, Al, V, Ca, Mn in Keystone ore are consistent with a non-magmatic fluid precipitating magnetite. On the other hand, preliminary O isotope data are consistent with global values from magnetite formed from magmatic systems. The differing interpretations of elemental and O isotope data for the source of Keystone Fe ore detail the complexity of the formation of the Keystone skarn and the need for additional field and geochemical analyses. Puerto Rico is also home to the Tibes (south central) and Island Queen (eastern, near Keystone) Fe skarns, and future study includes a detailed comparison of these deposits with Keystone. Preliminary magnetite O isotope compositions show three distinct ranges; Keystone and Tibes data indicate a magmatic O source and Island Queen results are consistent with meteoric fluids. Continued work includes petrography, elemental, and isotopic analyses. The unaccreted nature of Puerto Rico’s Fe skarns makes them excellent natural laboratories; a detailed characterization of them will expand our understanding of Fe source, transport, and deposition in skarn deposits globally.