Magnetite geochemistry and mineralogy of iron skarn in the southern Wag Water Belt, Jamaica: Implications for ore genesis

The Mavis Bank iron skarn mineralisation is located within a Paleocene to Eocene rift system typified by terrestrial and marine deposits, and extensive lava flows and subvolcanic intrusions of adakitic composition. Iron occurs primarily as massive, lenticular magnetite bodies at the contact between the intrusions and calcic Fe skarn (metasomatised limestone) which grades into a low-grade marble with disseminated magnetite. Magnetite is also disseminated in shales and lavas located within the main mineralised zones. Skarn mineralogy is poorly developed in both the endoskarn and the exoskarn with only minor occurrence of diopside. Retrograde and alteration minerals, such as epidote, chlorite, and hematite, are more common. Sulphides are rare, highly altered, and dominated by pyrite, pyrrhotite, and minor chalcopyrite. Petrographic analysis, including micro-XRF-EDS mapping of polished thin sections, shows that massive magnetite and calcic Fe skarns have minor disseminated Ti and S phases along with trace amounts of Cu and Co. Scanning electron microscope-backscattered electron (SEM-BSE) imaging of the exoskarn highlights the extent of retrograde alteration and later supergene weathering, and the presence of dissolution and reprecipitation features. The in situ trace element composition of magnetite in massive magnetite and calcic Fe skarn was determined using LA-ICP-MS. The results, when plotted on multi-element and discriminant diagrams, including Ni/(Cr + Mn) vs Ti + V, Ca + Al + Mn vs Ti + V, Ti vs V, and Ti vs Ni/Cr, are generally consistent with the geochemical signatures of skarn deposits worldwide. The mineralogy and trace element signatures suggest that the skarn deposit was formed at relatively shallow levels by low temperature hydrothermal fluids with high fO2 and fS. In addition, δ18O values of 6.6 and 7.1 ‰ obtained from massive magnetite are typical of infiltrative skarns and are consistent with a mixture of magmatic and meteoric water, and limestone oxygen isotope signatures. •Prograde skarn mineralogy is poorly developed.•Magnetite shows dissolution and reprecipitation, and extensive alteration.•Massive magnetite has low concentrations of compatible elements such as Ti, and V.•The skarn mineralisation was formed by low temperature hydrothermal fluids.•The δ18O value of magnetite reflects fluid mixing and wall-rock interaction.