PRELIMINARY INSIGHTS INTO IRON TRANSPORT AND DEPOSITION AT THE ISLAND QUEEN SKARN, PUERTO RICO

Puerto Rico hosts Fe skarns that are a result of magmatic intrusion, isochemical alteration of carbonate-rich host rock, and later introduction of meteoric fluids. This research uses geochemical analysis to characterize the Island Queen skarn with a focus on determining the origin of the Fe ore, the evolution of fluid flow, and the extent of fluid-rock interaction. Fe skarns form in island arc settings that are often later accreted. This makes Puerto Rico an exceptional natural laboratory to conduct Fe skarn research, as it is an extinct, unaccreted island arc and has an active mining ban, leaving the skarn outcrop available for study. The purpose of this project is not to influence the mining policies in place, but instead to take advantage of the opportunity to study how Fe is transported and deposited within Earth’s crust.

Fifteen samples were collected so far from the Island Queen skarn and thin sections were made for each. The samples include iron oxide, primarily as magnetite, as well as limestone host rock with varying degrees of alteration. Using a petrographic microscope, the textures and mineralogy of the ore samples were observed using both transmitted and reflected light. With the use of electron microprobe analysis, the mineral textures and relationships were observed at a finer scale. Any compositional zoning preserved in the garnets and magnetite, or lack thereof, was used to understand the evolution of the fluid composition and flow. The combination of trace element concentrations and stable isotope ratios in magnetite can determine the origin of ore as magmatic fluid, meteoric fluid, or a combination of both. In situ trace element analysis was performed to measure the concentrations of Ti, V, Ca, Al, and Mn in magnetite. A preliminary set of 11 samples of Island Queen magnetite were analyzed for their Fe isotope compositions (+/-2SD) relative to IRMM-14, which ranged from -0.41‰ +/- 0.04 to -0.01‰ +/- 0.05, except for one sample at +0.11‰ +/- 0.04. These values indicate a possible meteoric origin for the fluid that transported Fe at Island Queen, but O isotope ratios measured in the same samples will provide additional context. The results from these preliminary samples will be used to guide future fieldwork and geochemical analysis.