75-34 Timing and Evolution of Ore Forming Fluids in the Keystone Skarn, Puerto Rico

Session: Mineralogy, Geochemistry, Petrology, and Volcanology Student Session (Posters)

Presenting Author:

Authors:

Abstract:

The Keystone iron skarn, located near Juncos in eastern Puerto Rico, formed through metasomatic processes triggered by the intrusion of the Late Cretaceous San Lorenzo batholith into calc-volcaniclastic host rocks. The skarn comprises two main mineralized lenses: an upper, high-grade iron ore body and a lower garnet-bearing zone composed of magnetite, hematite, garnet, epidote, and quartz. Garnet crystals are primarily grandite, a grossular-andradite solid solution, and occur in veins commonly associated with magnetite.

This study investigates the timing, structural characteristics, and fluid evolution associated with garnet formation at Keystone. Petrographic observations are used to classify the garnets into three distinct groups based on mineralogical associations: a garnet-magnetite group, a garnet-calcite group, and a garnet-alteration group. These observations are paired with major and trace element geochemistry, rare earth element (REE) analysis, and U-Pb geochronology to reconstruct the history of the skarn and better understand fluid-rock interactions and garnet growth conditions.

Findings from Keystone are compared with those from the Tibes skarn in southern Puerto Rico, where U-Pb dating of garnet reveals a younger and more prolonged mineralization event (~60 to ~55 Ma), likely linked to Eocene tectonism and intrusion of the Tibes stock. Preliminary U-Pb data from Keystone samples yield ages ranging from ~70 to ~80 Ma. These results suggest contrasting mineralization histories: garnet at Tibes appears to be contemporaneous with the associated pluton, indicating a dominantly magmatic fluid origin, whereas garnet at Keystone postdates the San Lorenzo batholith, implying significant rework by meteoric hydrothermal fluids. The introduction of meteoric fluids was also supported by previous studies of Fe and O isotopic signatures of magnetite, which indicated the involvement of multiple fluid sources during the later stages of formation.

This work provides new insights into skarn formation processes and the broader magmatic and tectonic evolution of the Puerto Rico island arc setting.

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

© Copyright 2025 The Geological Society of America (GSA), all rights reserved.