Browsing by Author "Gardner, J"

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    Inelastic neutron scattering as a means for determining the magnetic exchange interactions in the frustrated quantum spin chain, Linarite
    (Australian Institute of Physics, 2021-02-02) Rule, KC; Willenberg, B; Mole, RA; Wolter-Giraud, A; Tennant, A; Ehlers, G; Studer, AJ; Gardner, J; Suellow, S
    One of the simplest models exhibiting one dimensional (1D) frustrated quantum interactions is the so called J1-J2 model. In this model competing ferromagnetic nearest-neighbour interactions (J1 > 0) and antiferromagnetic next-nearest-neighbours (J2 < 0) can give rise to novel phenomena such as multiferroicity for spiral spin states. Linarite, PbCuSO4(OH)2 is a natural mineral ideally suited to the study of frustration in J1-J2 systems due to an accessible saturation field and the availability of large single crystals well suited to neutron investigations. In this material the Cu2+ ions form spin S = 1/2 chains along the b direction with dominant nearest-neighbour FM interactions and a weaker next-nearest-neighbour AFM coupling, resulting in a magnetically frustrated topology. We present a neutron scattering study of linarite revealing a helical magnetic ground state structure with an incommensurate propagation vector of (0, 0.186, ½) below TN = 2.8K in zero magnetic field. From detailed measurements in magnetic fields up to 12 T (B || b), a very rich magnetic phase diagram will be presented. In particular we will present new inelastic neutron scattering data and compare this with theoretical modelling of the spin Hamiltonian. These theoretical calculations imply that linarite possesses an xyz exchange anisotropy. Our data establish linarite as a model compound of the frustrated one-dimensional spin chain, with ferromagnetic nearest-neighbour and antiferromagnetic next-nearest-neighbour interactions.
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    Potassium silicate alteration in porphyry copper‑gold deposits: a case study at the giant maar-diatreme hosted Grasberg deposit, Indonesia
    (Elsevier, 2022-12) Henley, RW; Mernagh, TP; Leys, C; Troitzsch, U; Bevitt, JJ; Brink, F; Gardner, J; Knuefing, L; Wheeler, J; Limaye, A; Turner, M; Zhang, Y
    Potassium 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.