Browsing by Author "Troitzsch, U"

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    Analytical techniques for probing small-scale layers that preserve information on gas–solid interactions
    (Mineralogical Society of America, 2018-11-01) Dalby, KN; Berger, JA; Brand, HEA; Cairney, JM; Eder, K; Eggins, SM; Herring, A; Hervig, RL; Kreider, PB; Mernagh, TB; Palm, AB; Renggli, CJ; Troitzsch, U; Yue, L; King, PL
    It has been 23 years (as we type) since Carroll and Holloway published the “Volatiles in Magmas” MSA volume (Carroll and Holloway 1994). The 1994 volume dealt with how to safely sample high-temperature gases and analytical methods for volatiles in glasses, which included secondary ion mass spectroscopy and vibrational spectroscopy. Since that time, some things have changed, and some have remained the same. There is still a disconnect between laboratory models of high-temperature gas–solid processes and field observations (Cashman et al. 2017), but we are starting to close that gap with rapid advances in technology. © 2018 Mineralogical Society of America
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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.
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    Radiocarbon age constraints for a pleistocene–holocene transition rock art style: the northern running figures of the East Alligator River region, western Arnhem Land, Australia
    (Elsevier, 2017-02-01) Jones, T; Levchenko, VA; King, PL; Troitzsch, U; Wesley, D; Williams, AA; Nayingull, A
    In this article we present nine radiocarbon age determinations producing a minimum age and a minimum age range for a regionally distinct rock art style known as the Northern Running Figures from Red Lily Lagoon, western Arnhem Land Australia. These radiocarbon determinations provide age constraints for both Pleistocene and early Holocene rock art in western Arnhem Land. The radiocarbon age determinations are produced from extracting calcium oxalate contained within mineral crusts associated with the rock art. Significantly this study employs a novel separation technique, designed to effectively isolate the oxalate compounds from the mineral crust sample using chemical pre-treatment, and demonstrates significant time offsets between radiocarbon age determinations for the calcium oxalates and other carbon inclusions contained within mineral crusts.© 2016, Elsevier Ltd.