The Formation of Iron Ore at the Keystone Skarn, Puerto Rico

Abstract

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 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 is hosted in volcaniclastic rocks and dominantly made up of magnetite in the form of two subparallel ridges. The ore bodies potentially extend to another hilltop ~0.52km away. A zone of alteration runs perpendicular to the ridges. The trace element (Ti, Al, V, Ca, Mn) concentrations in the magnetite are consistent with global skarns and indicate a hydrothermal origin rather than crystallization from magma. The δ56Fe (-0.12‰ to 0.21‰) and δ18O (0.10‰ to 2.00‰) values of Keystone magnetite are consistent with global accreted and unaccreted Fe skarns. The Fe and O isotopic signatures indicate multiple fluid sources but point to initial magmatic-hydrothermal fluids and small proportions of meteoric influx in later stages. Oscillatory zonation within the magnetite indicates changing formation conditions related to additional fluid input, evolving source pluton, or reactions between the fluid and host rock. Truncations of the oscillatory zoning indicate episodic dissolution and reprecipitation processes. Mineralogical and textural evidence imply that both redox dependent and independent reactions altered the magnetite to martite. Overall, a genetic model of the Keystone skarn is proposed as three main stages: 1) intrusion of an Fe-rich pluton, 2) magmatic-hydrothermal fluid release and metal deposition, 3) introduction of late Si-Al-Ca-rich fluids and meteoric water. This new characterization of Keystone and detailed analysis of its ore will inform the people of Puerto Rico in mineral resource assessment and exploration. This contribution is also applicable to global exploration for Fe skarns during a key time for securing Fe as a commodity.