The role of phosphorous on the immiscibility process in andesites

Abstract

Kiruna-type deposits represent an important source of iron ores made of massive magnetite bodies containing disseminated interstitial apatite. The El Laco deposit (Chile) is one of the largest Kiruna-type deposits. Their origin is still misunderstood and debated today, hypotheses are based on a magmatic origin involving an immiscibility process in an andesitic melt leading to the formation of an unusual Fe-P-rich melt, or on a hydrothermal origin. Our assumption regards the possibility to promote immiscibility by increasing the phosphorous content in a silicate melt of andesitic composition. We postulate that this kind of phenomenon could happen in realistic and natural conditions. Massive magnetite ores found at El Laco and in Kiruna-type deposits would crystallize from this Fe-P-rich melt. Several experiments were performed at conditions that could be representative of the crust (5 kbar – ca. 1000 °C – 4 wt% H2O) by means of a piston-cylinder apparatus. Our results indicate that no immiscibility is observed in any sample despite the addition of phosphorous in various forms. This lack of immiscibility has not a single causative factor but it may be due to a combination of parameters interacting together. These parameters are the chemistry of the parental melt, and the relatively-high pressure that narrows the two-liquid field, raises the water saturation point, prevents the P2O5 saturation and thus inhibits the immiscibility development. We measure the phosphorous saturation at 1.37 wt% in the andesitic melt. Regarding the genesis of Kiruna-type deposit, we can suppose that the experimental parameters should not be fully representative of crust conditions beneath the El Laco volcano.