Genesis of porphyry copper deposits: key roles for plagioclase and anhydrite in metasomatism

Genesis of porphyry copper deposits: key roles for plagioclase and anhydrite in metasomatism
Wheeler, J., Henley, R. W., Gardner, J., Mernagh, T., Leys, C., Troitzsch, U., Bevitt, J., Brink, F., Knuefing, L., , Limaye, A., Turner, M. & Zhang, Y.
Porphyry copper deposits form by metasomatic alteration of rocks by Cu-bearing hydrothermal fluids derived from crystallizing magmas, but these processes remain unclear in detail. The Grasberg porphyry deposit in Papua is one of the largest in the world and is hosted by intensely altered calc-alkaline plutonic rocks that contain more K-Feldspar than their protoliths. This was thought to be due to the introduction of magma-derived K by metasomatism, but a district-scale chemical dataset shows that unaltered and altered rocks have similar major element bulk compositions. Instead we propose that plagioclase reacts with magmatic SO2 to form anhydrite and albite, and this releases H2S that plays a major role in Cu ore formation [1]. Sequestration of Ca into anhydrite, along with deposition of silica into quartz veins, increased the relative concentration of other major components such as K2O in the remaining silicate alteration assemblage. To document the metasomatic processes, we split a complex set of reactions into some simpler conceptual “building blocks”, not implying that the reactions happened in such a time order. We note that volcanic gases are dominated by molecular not ionic species [2].
• SO2 (in the volcanic gas) reacts with water to becomes H2S and H2SO4.
• H2SO4 reacts with Ca from plagioclase to form anhydrite (in veins) c.f. [3]
• Albite is left over
• H2S reacts with Cu (in the volcanic gas) to form Cu minerals
• Fe from biotite similarly reacts to form CuFe minerals
• K-feldspar is left over.
Leaving aside kinetics, even modelling the equilibria in such gas-solid reactions is complicated. The thermodynamic packages usually used in metamorphism cannot deal with SO2-HCl-Cu bearing volcanic gases. We have experimented with the “HSC Chemistry” package which includes thermodynamics of gases with varied chemistry but is limited in terms of solid solution that can be modelled [4].
Preliminary EBSD work on the new albite shows it is replacing plagioclase directly and inherits the crystallographic orientation. This resembles microstructures in “coupled dissolution precipitation” reactions [5] though we are not implying the reaction mechanisms are necessarily the same. In any case the thermodynamics and kinetics of the plagioclase breakdown will affect the overall amount of copper ore so the reaction is of economic as well as general interest.