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

Wheeler, J.¹, Henley, R. W.², Gardner, J.¹, Mernagh, T.², Leys, C.³, Troitzsch, U.², Bevitt, J.⁴, Brink, F.², Knuefing, L.², Limaye, A.² Turner, M.² & Zhang, Y.²

¹ Department of Earth, Ocean, and Ecological Sciences, University of Liverpool, Liverpool, UK

 ² Australian National University, Canberra

³ P.T. Freeport Indonesia, Papua

⁴ Australian Nuclear Science and Technology Organisation, NSW 2234, Australia

Porphyry copper deposits are not formed just by crystallisation of ores from Cu-bearing hydrothermal fluids; metasomatism can be involved. We present a metamorphic point of view of the Grasberg porphyry deposit in Papua, which is hosted by intensely altered calc-alkaline plutonic rocks characterised by albite and anhydrite. 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]. We split a complex set of reactions into conceptual “building blocks”: these did not happen in a particular order but help to explain our observations.

• 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. [2].
• 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.

Partly because of these reactions the potassic zone contain more K-feldspar than the protoliths. This is not due to the introduction of magma-derived K by metasomatism; a large chemical dataset shows that unaltered and altered rocks have similar major element bulk compositions. The “HSC Chemistry” package which includes thermodynamics of gases with varied chemistry has been used for preliminary models of reaction.

Electron Backscatter Diffraction work on the new albite shows it is replacing plagioclase inheriting the crystallographic orientation. This resembles microstructures in “coupled dissolution precipitation” reactions [3] though we are not implying the reaction mechanisms are necessarily the same. The thermodynamics and kinetics of the plagioclase breakdown will affect the overall amount of copper ore formed.

[1] Henley et al. JVGR (2022) 432: 107710.

[2] Henley et al. Nat Geosci (2015) 8: 210.

[3] Gardner et al. Lithos (2021) 396-397.