Abstract

The formation of phosphorus–iron oxide (P–Fe) immiscible melts and their possible connection to the genesis of Kiruna-type and Nelsonite deposits was experimentally investigated by adding phosphoric acid (H3PO4), water, and sulfur, to andesite at 100–450 MPa, 500–900 °C, at the NiNiO and magnetite-hematite fO2 buffers using internally heated gas vessels. The addition of up to 8.02 wt% of H3PO4 to the andesite causes crystallization of apatite. At higher concentrations of H3PO4 whitlockite crystallizes, and at concentrations above ~ 11.4% H3PO4 (at 800 °C, 385 MPa) an immiscible P–Fe melt forms. Adding sulfur at low fO2 (NiNiO) causes an additional immiscible Fe–S melt to form. Increasing the fO2 to the hematite-magnetite buffer causes the sulfur-rich melt to shift in composition to a Ca–S–O melt, and the coexisting P-Fe melt to incorporate large amounts of SO4. Immiscible P-Fe melts can form at temperatures above 1100 °C down to 600 °C (at 400 MPa). Mass balance calculations show that some experimentally produced P-Fe rich immiscible liquids may result in mineral assemblages similar to those found at some Kiruna-type deposits, such as actinolite-rich dikes, and apatite-rich veins. Depending on the geological conditions and the composition the fractionation of a P-Fe melt may result in the formation of nelsonites at high pressures, high temperatures, and low fO2 or Kiruna-type deposits at lower temperatures and higher fO2.