THE SATURATION OF ANDESITIC MAGMA BY COPPER-IRON SULFIDE MINERALS AND LIQUIDS AS A FUNCTION OF OXYGEN FUGACITY, SULFUR FUGACITY, AND IRON TO COPPER RATIO OF THE MELT

The importance of sulfide minerals and liquids in modeling the budgets of economically important metals in mafic magmas has long been recognized in the context of magmatic sulfide deposits, and in highly evolved magmas associated with porphyry and related deposits. A more complete picture of the effect of sulfide separation on the metal budgets of arc magmas throughout their evolution and passage through the crust will require knowledge of the evolution of the sulfide phases themselves. Further, these sulfide phases, and their interactions with silicate melts, need to be understood as a function of bulk composition, fO₂, and fS₂. Toward this end, experiments have been performed at 1000°C and 150 MPa to evaluate the conditions necessary for saturation of an andesitic magma with respect to pyrrhotite (po) and a Cu-Fe sulfide liquid (CFSL). The experiments in this study were conducted in rapid-quench MHC cold-seal vessels in Cu-Au alloy capsules, and have been chosen to fill a gap in data applicable to magmas of intermediate compositions. fO₂ was controlled by Co-CoO or Ni-NiO buffers, and fS₂ was calculated from po composition. Run products were analyzed by using EPMA and LA-ICP-MS. Fe/Cu ratio of silicate melts and coexisting sulfide phases were compared at known fO₂ and fS₂ to develop a method for determining the identity (composition) of the saturating sulfide phase.

The expected sequence of sulfide stability with increasing Cu in an andesitic magma under these conditions is: po, po + CFSL, CFSL, CFSL + bornite (bn), and bn. In these preliminary experiments po + CFSL co-stability (and therefore the Fe/Cu boundary of po stability) occurs at Fe/Cu ratios in the silicate melt ranging from 1100 to 600, with higher values being attained in higher fO₂ and fS₂ experiments. Fe/Cu ratios in po ranged from 21 to 14. These correlate to Cu concentrations in po of 2.8±0.6 to 4.2±0.2 wt. %, which are lower than initially expected possibly due to the effect of oxygen on the stability of the sulfide liquid. The composition of the CFSL in the experiments was ~ 20% Cu, 43% Fe, 32% S, with Fe/Cu ratios ranging from 2.2 to 1.8. Ultimately, the identity (composition) of sulfides separated from magma will influence the availability of chalcophile elements to ore-forming processes through the effects of differences in D^po/melt and D^CFSL/melt for those elements.