Volatile budgets and evolution in porphyry-related magma systems, determined using apatite

Abstract

Volatile-bearing minerals, such as apatite, Ca5(PO4)3(OH,F,Cl), can record changes in dissolved magmatic volatile species during differentiation and, unlike melt inclusions, are sensitive to the presence of an exsolved fluid phase. Populations of apatite crystals from an individual sample can therefore be used to define the progressive volatile evolution of melt ± fluid during magma differentiation. Despite the importance of fluid chemistry in mineralisation processes, this approach remains relatively underdeveloped for porphyry mineralisation scenarios. Here, we present a model, including a standalone MATLAB app, for melt + apatite ± fluid fractionation that incorporates non-ideal, temperature-dependent KDs for OH-Cl-F exchange and permits an analysis of uncertainty arising from non-unique parameter combinations. We apply the model to apatite from the Fe-Cu-Au Corrocohuayco porphyry-skarn system and analyse differences in volatile saturation state and fluid salinity between different units. We find that there is little difference in the overall fluid salinity, and thus the fluid copper loads, but that the more primitive unit (i.e. the gabbrodiorites) reached fluid saturation much later (after around 50% crystallisation) than the more evolved units, implying that the melt volatile concentration recorded by the apatites in the gabbrodiorites is not representative of the initial magma volatile budget. This work demonstrates that apatite can be a good alternative means of reconstructing the evolving magmatic fluid salinity within mineralising systems, and linking this to the trace metal content of the melt.