A Petrographic and Geochemical Characterization of Magnetite Ore at the Island Queen Skarn, Puerto Rico

Abstract

Skarns classically form as the result of a multi-stage process including magmatic intrusion, alteration of carbonate-rich host rock, metasomatism, and later introduction of meteoric fluids. These processes transfer and concentrate metals like iron (Fe) into economic deposits. Puerto Rico serves as a natural laboratory to characterize mass transfer across Fe skarns. It is an extinct, unaccreted island arc and has a mining ban, leaving the outcrops available for study with minimal destruction. This research takes advantage of this opportunity to study Fe transport and deposition through fieldwork, petrography, and geochemical analyses of the Island Queen skarn. At Island Queen, magnetite ore partially replaces limestone lenses hosted within volcaniclastics. Seventeen samples were analyzed including Fe oxide ore, limestone host rock with varying degrees of replacement, and weakly magnetic volcaniclastics. Petrography revealed predominantly magnetite, frequently with martite rims and evidence of dissolution-reprecipitation, and minor hematite, garnet, quartz, and chlorite. Elemental concentrations were measured in the ore, and electron images and X-ray maps revealed zoning in the magnetite that reflects changes in the ore-forming fluid. The Fe and oxygen (O) isotope compositions of the magnetite and host rocks indicate a dominant meteoric component in the fluid(s) that transported Fe and/or later altered the magnetite.