Potassium silicate alteration in porphyry copper‑gold deposits: a case study at the giant maar-diatreme hosted Grasberg deposit, Indonesia

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dc.contributor.authorHenley, RWen_AU
dc.contributor.authorMernagh, TPen_AU
dc.contributor.authorLeys, Cen_AU
dc.contributor.authorTroitzsch, Uen_AU
dc.contributor.authorBevitt, JJen_AU
dc.contributor.authorBrink, Fen_AU
dc.contributor.authorGardner, Jen_AU
dc.contributor.authorKnuefing, Len_AU
dc.contributor.authorWheeler, Jen_AU
dc.contributor.authorLimaye, Aen_AU
dc.contributor.authorTurner, Men_AU
dc.contributor.authorZhang, Yen_AU
dc.date.accessioned2024-12-05T21:03:05Zen_AU
dc.date.available2024-12-05T21:03:05Zen_AU
dc.date.issued2022-12en_AU
dc.date.statistics2024-09-13en_AU
dc.description.abstractPotassium silicate alteration is a hallmark of the porphyry copper deposits that supply two thirds of the world's annual copper demand. These deposits formed in the cores of calc-alkaline to alkaline volcanic systems from the flux of magmatic gas that transported copper and other metals from source to the surface. The giant 3.2 Ma Grasberg Cu[sbnd]Au deposit formed within a maar-diatreme complex following a resurgence in magmatism. The defined resources of this deposit occur from a few hundred metres depth to 1.7 km below the paleosurface which, uniquely in this deposit, is partially preserved as a section of maar tuffs. Potassium silicate alteration has commonly been interpreted as being the result of the addition of potassium to the porphyry copper host rocks via pervasive interaction with a large volume of a potassium-rich brine that is commonly presumed to be of magmatic origin. However, the data reported here show that alteration at the deposit scale is essentially isochemical with respect to the major rock-forming components and that only sulphur and the economic metals (Cu, Mo, Au, etc.) are added by flux of reactive magmatic gas containing SO2 and HCl from source intrusions at depth. Silicate solubilities are very low so that only a minor fraction of the total alkalis in the host rock are extracted by alteration reactions and then discharged at the paleo-surface. Reaction of the magmatic gas phase with plagioclase results in the coupled deposition of anhydrite (CaSO4) and disproportionation of SO2 to release H2S. The in-situ release of H2S immediately scavenges Cu and other chalcophile metals from the continuing magmatic gas flux to form the Cu-, Fe- and other sulphides that, in sufficient concentration, make up the economic reserve available to mining. The sequestration of Ca into anhydrite, along with deposition of silica into early quartz veins, increases the concentration of the other major components (K2O, Na2O, MgO, etc) in the remaining silicate assemblage within the porous host rock. The result is the development of intermingled potassium-enriched silicate and sulphur-rich (anhydrite-sulphide) sub-assemblages that constitute the mineralised phyllic or potassic alteration zones. These crystallise according to their pressure and depth into alteration assemblages dominated by potassic phyllosilicates, quartz and pyrite in the phyllic alteration zone, and alkali feldspar and phlogopitic-biotite plus minor andalusite and corundum in the central potassic zone. Dissolution and recrystallisation of primary magmatic biotite in the host rock releases K as well as Fe, the latter (along with amphibole and feldspar) providing iron for the formation of chalcopyrite, bornite and pyrite. The in-situ release of H2S through anhydrite formation, immediately scavenges Cu and other chalcophile metals from the continuing magmatic gas flux to form the Cu[sbnd]Fe[sbnd] sulphides that, in sufficient concentration, make up the economic reserve available to mining. Understanding of the alteration processes during porphyry copper formation also provides insights into gas-solid reactions processes inside active magmatic arc volcanoes but the magnitude of copper mineralisation is dependent on the original metal content of the source of the magmatic gas phase. © 2022 Elsevier B.V.en_AU
dc.identifier.articlenumber107710en_AU
dc.identifier.citationHenley, R. W., Mernagh, T., Leys, C., Troitzsch, U., Bevitt, J., Brink, F., Gardner, J., Knuefing, L., Wheeler, J., Limaye, A., Turner, M., & Zhang, Y. (2022). Potassium silicate alteration in porphyry copper‑gold deposits: a case study at the giant maar-diatreme hosted Grasberg deposit, Indonesia. Journal of Volcanology and Geothermal Research, 432, 107710. doi:10.1016/j.jvolgeores.2022.107710en_AU
dc.identifier.issn0377-0273en_AU
dc.identifier.journaltitleJournal of Volcanology and Geothermal Researchen_AU
dc.identifier.urihttp://dx.doi.org/10.1016/j.jvolgeores.2022.107710en_AU
dc.identifier.urihttps://apo.ansto.gov.au/handle/10238/15777en_AU
dc.identifier.volume432en_AU
dc.languageEnglishen_AU
dc.language.isoenen_AU
dc.publisherElsevieren_AU
dc.subjectCopperen_AU
dc.subjectFeldsparsen_AU
dc.subjectPorosityen_AU
dc.subjectDepositsen_AU
dc.subjectPotassium silicatesen_AU
dc.subjectVolcanic rocksen_AU
dc.subjectPotassiumen_AU
dc.subjectEnrichmenten_AU
dc.subjectIndonesiaen_AU
dc.subjectDataen_AU
dc.titlePotassium silicate alteration in porphyry copper‑gold deposits: a case study at the giant maar-diatreme hosted Grasberg deposit, Indonesiaen_AU
dc.typeJournal Articleen_AU
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