Fe solubilities in very saline hydrothermal fluids; their relation to zoning in some ore deposits

The Fe concentrations in fluids present in very saline fluid inclusions from the King Island W skarn, hole 16, F-Sn-W deposit and the Mary Kathleen U-rare earth element skarn have been determined using the phase deposit volume method. Fe-bearing daughter minerals include magnetite, pyrite, amarantite (Fe (super +3) (SO ₄ )(OH) . - 3H ₂ O), and hydrated ferrous chloride. The fluid inclusion liquid compositions are inferred from a combined analysis of first melting temperatures (= eutectic temperatures of the salt system), daughter crystal identification by scanning electron microscopy in conjunction with an energy dispersive X-ray analyzer, and relevant published experimental salt solubilities. Maximum Fe solubilities show an exponential increase to 9 wt percent Fe with increasing total anion content of the fluid, especially in the 20 to 30 wt percent interval. Fe solubility is mainly dependent on total chloride contents, with fluoride contributing very little to increasing the Fe solubility. Temperature, f (sub O ₂ ) , and f (sub S ₂ ) appear to have a minor influence on the total Fe solubilities.Some of the samples contain solid Fe-bearing crystals which coexisted with the fluid inclusions and probably buffered Fe at the appropriate a _Fe values during genesis, but the Fe contents of most fluids were probably undersaturated with respect to Fe-bearing minerals. Over the range for which the total anion content of the fluid is 25 to 35 wt percent, the apparent maximum Fe content of fluids tends to decrease in the following order when the Fe-bearing daughter crystals are present: magnetite > pyrite > hydrated ferrous chloride [raquo] amarantite, although anomalies to this trend occur.The Fe solubility data are used to explain some Fe mineral overprints (stages II and III) of skarns, the magnetite-Fe chlorite-cassiterite veins in tin granites, and the zonal pattern present in porphyry copper systems. The Fe data suggest that the highly saline, Fe-bearing solutions present in the cores of porphyry systems and other near-contact deposits are not a result of increased salinity of hydrothermal fluids due to boiling of meteoric solutions but are inherent from an orthomagmatic source.