Using microanalysis of minerals to track geochemical processes during metamorphism: examples from the Mary Kathleen fold belt, Queensland, and the Eastern Mt. Lofty Ranges, South Australia

Hammerli, Johannes (2014) Using microanalysis of minerals to track geochemical processes during metamorphism: examples from the Mary Kathleen fold belt, Queensland, and the Eastern Mt. Lofty Ranges, South Australia. PhD thesis, James Cook University.

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Abstract

Understanding the behaviour of major and trace elements during metamorphism is fundamental for our understanding of the geochemical evolution of the Earth's crust and the formation of orogenic orebodies. Furthermore, it is essential to know how key elements and radiogenic isotopes behave in metamorphic/hydrothermal systems in order to apply them meaningfully to solve important questions in geosciences. Metamorphic/hydrothermal reactions are most evidently preserved at the mineral scale, so in situ microanalytical techniques are best suited for tracing the record of metamorphism or hydrothermal alteration. In this thesis, I outline new analytical developments for in situ analysis of halogens in minerals and fluid samples, and of Sm-Nd isotopes in REE-rich minerals. These techniques, in conjunction with comprehensive bulk rock and mineral geochemistry and element distribution analysis, are then applied to well-characterised metamorphic rocks from the Adelaide Fold Belt and Mt Isa Inlier. Although fluid is an essential ingredient for mass transport during metamorphism, it is often difficult to identify the source of metamorphic/hydrothermal fluids. Traditionally, fluid inclusions have been used to gain insights into the source and composition of fluids. Until very recently, quantification of key elements such as bromine and chlorine in fluid inclusions relied almost solely on bulk rock analyses techniques (i.e., crush-leach). These methods do not allow distinction between different fluid inclusion generations that might hold crucial information on the evolution of a hydrothermal system and associated mineralization. The development of in situ LA-ICP-MS analysis of chlorine and bromine in fluid inclusions now allows for the targeting of individual fluid inclusions of a specific fluid type in a mineral. In this thesis these techniques were further tested and refined, and applied for the first time to a range of natural scapolite group minerals, minerals assumed to reflect the Cl/Br content of the coexisting hydrothermal fluids. The results show that fluid sources can be identified with a ~ 25 μm resolution in Cl and Br bearing minerals. This technique was applied on scapolite minerals from skarns, regional metamorphic rocks and a mineralized shear-zone of the Mary Kathleen Fold Belt in the Mt. Isa inlier. While scapolite minerals in skarns contain Cl/Br ratios typically associated with granitic fluids, metamorphic scapolite indicates that fluids were dominantly derived from basinal brines formed from sub-aerial evaporation of seawater beyond the point of halite saturation. This bittern fluid infiltrated the underlying sedimentary sequences prior to regional metamorphism. Zoned scapolite in the mineralized shear-zone records three discrete pulses of magmatic and metamorphic fluid, and it is suggested that fluid mixing may have assisted mineralization along and around this shear-zone.

To investigate element mobility during metamorphism, I studied the Eastern Mt. Lofty Ranges in South Australia. Metamorphic rocks of the Mt. Lofty Ranges have a relatively simple metamorphic history, and metamorphic gradients and widespread up-temperature fluid flow has been documented previously. This allows monitoring of mineral and bulk rock compositional changes (or lack thereof) during metamorphism across a regional metamorphic gradient from ~350–400 ˚C to migmatite grade (~ 650–700 ˚C) at ~0.3–0.5 GPa, in a confined framework. The results show that, despite widespread up-temperature fluid flow, major elements and most trace elements are isochemical during metamorphism. These elements are effectively redistributed into newly formed major minerals or accessory phases. Monazite or allanite and xenotime control the whole rock concentration of REE whereas apatite and titanite are minor REEs hosts. The only non-volatile mobile elements are Zn, Pb, Cs and As whose concentrations decreased with increasing metamorphic grade. The Zn and Pb depletion was progressive with increasing temperature in staurolite-absent psammo-pelites, with losses of ~ 75% of the original Zn and ~ 50 % of the original Pb from the rocks from high-grade metamorphic zones. Microanalysis showed that biotite is a key mineral for Zn sequestration by concentrating >80 % of the Zn in the bulk rock. Zinc and Pb likely partitioned into a Cl-rich hydrothermal/metamorphic fluid that led to the observed depletion of Pb and Zn in the bulk rock. Simple mass balance calculations show that ~27 Mt of Zn and ~2.7 Mt of Pb were mobilized during prograde metamorphism, which is comparable to the amounts of base metals found in world class Pb-Zn deposits. Hence, prograde metamorphism of sedimentary rock packages is a viable base metal source for the formation of some Pb-Zn deposits, provided that the metamorphic fluid contains sufficient Cl to effectively mobilise metals from the metamorphic system into ore-forming environments. The observed As loss is consistent with the recrystallization of As-bearing pyrite to As-poor pyrrhotite, confirming previous studies. Cesium depletion in migmatites can be explained by the incompatibly of Cs in micas in high-grade metamorphic rocks. Significant element mobility during metamorphism is likely only achieved under conditions with high fluid flux.

In order to understand crustal evolutionary processes and crustal fractionation via for example melt production in migmatitic systems equivalent to the high-grade zone of the Eastern Mt. Lofty Ranges, geochemists widely rely on radiogenic isotopes. However recent claims of Nd and Sr isotope disequilibrium during anatexis question the reliability radiogenic isotopes. Microanalysis of REE-rich accessory minerals was used to investigate Nd isotope equilibration during metamorphism in order to assess to potential of disequilibrium situations during high-grade metamorphism. The results are used to demonstrate that apatite retains an original, probably detrital, highly variable Nd isotopic signature until at least 500 ˚C, before being isotopically homogenized. In contrast, allanite and titanite are equilibrated at temperatures as low as 350–400 ˚C. REE-rich accessory minerals in high-grade rocks (~600 ˚C) show very similar initial Nd isotope values at the time of metamorphism. I conclude that Nd isotope disequilibrium between crustal melts and metasedimentary sources is unlikely. Furthermore, in situ microanalysis of radiogenic isotopes can help to identify external melt components in migmatites that would not be resolvable by conventional bulk rock analysis.

Item ID: 39966
Item Type: Thesis (PhD)
Keywords: accessory minerals; anataxis; biotite; bromine; bromine and chlorine; chlorine; fluid flow; fluid inclusions; fluid sources; geochemistry; geology; halogens; hydrothermal fluids; igneous; LA-ICP-MS; laser ablation MC-ICP-MS; Mary Kathleen fold belt; metal transport; metamorphic; metamorphism; mineralogical chemistry; minerals; Mount Lofty Ranges; MVT; Nd isotope equilibration; Pb-Zn deposits; Queensland; rocks; scapolite; scapolite
Additional Information:

Publications arising from this thesis are available from the Related URLs field. The publications are:

Chapter 1: Hammerli, Johannes, Rusk, Brian, Spandler, Carl, Emsbo, Poul, and Oliver, Nicholas H.S. (2013) In situ quantification of Br and Cl in minerals and fluid inclusions by LA-ICP-MS: a powerful tool to identify fluid sources. Chemical Geology, 337-338. pp. 75-87. Item availability may be restricted.

Chapter 2: Hammerli, J., Spandler, C., Oliver, N.H.S., and Rusk, B. (2014) Cl/Br of scapolite as a fluid tracer in the earth's crust: insights into fluid sources in the Mary Kathleen Fold Belt, Mt. Isa Inlier, Australia. Journal of Metamorphic Geology, 32 (1). pp. 93-112. Item availability may be restricted.

Chapter 3: Hammerli, J., Kemp, A.I.S., and Spandler, C. (2014) Neodymium isotope equilibration during crustal metamorphism revealed by in situ microanalysis of REE-rich accessory minerals. Earth and Planetary Science Letters, 392. pp. 133-142. Item availability may be restricted.

Chapter 5: Hammerli, Johannes, Spandler, Carl, Oliver, Nicholas H.S., Sossi, Paolo, and Dipple, Gregory M. (2015) Zn and Pb mobility during metamorphism of sedimentary rocks and potential implications for some base metal deposits. Mineralium Deposita, 50 (6). pp. 657-664.

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Date Deposited: 01 Dec 2015 03:12
FoR Codes: 04 EARTH SCIENCES > 0402 Geochemistry > 040203 Isotope Geochemistry @ 33%
04 EARTH SCIENCES > 0403 Geology > 040303 Geochronology @ 33%
04 EARTH SCIENCES > 0403 Geology > 040304 Igneous and Metamorphic Petrology @ 34%
SEO Codes: 97 EXPANDING KNOWLEDGE > 970104 Expanding Knowledge in the Earth Sciences @ 50%
84 MINERAL RESOURCES (excl. Energy Resources) > 8401 Mineral Exploration > 840108 Zinc Ore Exploration @ 50%
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