Johannes Hammerli

The Role of Halogens During Regional and Contact Metamorphism

Halogens are important elements for a range of geological processes during metamorphism from stabilizing mineral phases to being important ligands for mass transfer. Halogens are highly incompatible in most minerals, which makes it difficult to unravel their presence in the past. Minerals useful for understanding halogen behaviour during metamorphism include: scapolite, apatite, titanite, biotite, and amphibole. However, their ability to incorporate halogens depends on parameters such as bulk rock composition, fluid properties, and water-rock ratios. Comprehensive studies of halogens in regional metamorphic rocks and minerals, such as the Clearwater Region, Idaho, USA or the Mary Kathleen Fold Belt, Mt Isa Inlier, Australia, show that halogen contents are highly variable on a bulk rock- and rock layer-scale, reflecting protolith variations. Where low fluid-rock ratios occurred during regional metamorphism, pre-exisiting variations in halogen compositions and ratios across individual layers were not eliminated, resulting in large differences between halogen concentrations on a mineral- and rock-layer scale. Research on F and Cl in apatite in siliceous marbles from five classic aureoles highlights the use of this mineral regarding rock or fluid buffering, and in establishing fluid sources. Chlorine enrichment in biotite and amphibole, associated with regional albitization observed in Cloncurry, Australia or the Bamble Sector Norway, demonstrate advection of saline fluids during albitization and K-feldspar metasomatism that occur in association with regional mineralization. Chlorine-bearing fluids are capable of mobilizing large amounts of metals during large-scale metamorphism on a regional, whole rock, and mineral scale. Consequently, fluid flow could be an essential prerequisite to actively discharge metals from the metamorphic rocks. Recent analytical advancements allow for more routine analyses of halogen contents in minerals and fluid inclusions. For instance, in situ LA-ICP-MS analyses of Cl and Br allow for the reconstruction of the interaction of halogen-bearing fluids with crustal rocks in complex geological settings that have undergone multiple hydrothermal events. In such cases, scapolite can be used as an archive for fluid properties during metamorphism. For example, within the Mount Isa Inlier, it was found that the fluids, which interacted with calc-silicates in the Mary Kathleen Fold Belt, were of bittern brine derivation contrasting with the Cloncurry Region, where the fluids show evidence of dissolved halite. Magmatic fluid interaction with calc-silicate rocks was found to be localized.

Pb and Zn Behaviour during LP/HT Metamorphism: Implications for Pb‐Zn Ore Genesis

Studies on zinc and lead mobility during regional metamorphism are rare and contentious (e.g., Haack et al., 1984; Pitcairn et al., 2006) as we currently lack information on controlling factors for Zn and Pb enrichment or depletion during regional metamorphism. Better understanding of Pb and Zn behaviour will help to shed light on the long-standing discussion on the original source of base metals that feed Pb-Zn ore systems. In this study, we systematically studied Zn and Pb behaviour on a whole-rock and mineral scale during prograde metamorphism using a set of well-characterised psammopelite samples. The combination of bulk–rock and mineral geochemistry with Zn isotope data allows a comprehensive understanding of Pb and Zn migration in metamorphic systems. The study site is the Eastern Mount Lofty Ranges, South Australia, metamorphosed during the Delamerian orogeny at ~ 500 Ma (e.g. Hammerli et al., 2014; Fig. 1). Metamorphic conditions range from ~350 ̊C to the onset of partial melting in the presence of excess aqueous fluid at ~ 650–700 ̊C (3 to 5 kbar). Stable isotope studies indicate widespread up-temperature fluid flow during metamorphism, which may have triggered significant element mobility (e.g., Oliver et al., 1998).