title: Constraining ore genesis and palaeoclimate change using (U-Th)/He/Ne dating and stable isotope geochemistry
Creators: name: Currie, David
Publisher: name: University of Glasgow
Year: 2024

description: Providing absolute geochronological constraints on events in Earth history is of fundamental importance to modern geology. In ore geology, understanding the absolute timing of ore-forming processes provides insight into the tectonic and/or climatic events that drove their formation and can inform ore exploration models. Ore deposits are typically dated by U-Pb geochronology of various U and Th – bearing accessory minerals related to the ore system, like zircon, apatite, and monazite. However, many ore deposits do not contain these accessory minerals thus, it is necessary to continue with the development of new analytical techniques to date ore mineralisation.

In this PhD I have developed the (U-Th)/He and Ne geochronology techniques for Fe-oxide/oxyhydroxides, and applied them to two distinct ore systems:

The Attepe iron deposits of Central Anatolian Plateau, Turkey, provided the opportunity to develop (U-Th)/He dating to deeply weathered supergene ore mineralisation. This has been used to unravel the timing of palaeoclimate change on the Anatolian plateau.

In the second study I have revisited the paragenesis of the Leadhills-Wanlockhead Pb-Zn deposit in the Southern Uplands of Scotland, identifying previously unrecorded hematite mineralisation. Careful sample selection was followed by hematite (U-Th)/Ne dating, while He and S stable isotope analysis of sulfides were used to determine the source of ore fluids and metals.

Established Fe-oxide-oxyhydroxide (U-Th)/He techniques determine U, Th and He on the same mineral fragment. However, volatilisation of U and Th during the heating required for He extraction is common and leads to erroneously old He ages and over-dispersed age populations. The method developed in this thesis completely avoids the volatilisation issue by determining the He content of minerals separately from U and Th in multiple aliquots of several mg of each sample. A repeatable and time efficient sample preparation, petrography, chemistry, laser heating, and gas analysis schedule was established. New He age determinations of the Attepe iron deposits, central Turkey, have been used to determine climatic history of the Anatolian plateau. The data reveal pervasive supergene mineralisation and He loss-corrected (U-Th)/He ages of between 5.18 and 0.95 Ma. The presence of supergene iron oxy-hydroxides throughout the ore deposits suggests that the Plio-Pleistocene climate was hotter and more humid than today. The latest goethite precipitation (0.95 Ma) constrains the onset of aridification across the region to sometime in the last million years.

Hematite (U-Th)/Ne dating relies on the production of nucleogenic 21Ne from the reaction 18O(α,n)21Ne. To resolve the concentration of radiogenic 21Ne from the atmosphere-derived Ne trapped in minerals, a Thermo Fisher ARGUS VI mass spectrometer and an automated ultra-high vacuum gas purification and separation system was developed specifically for Ne isotope analysis. Sample preparation, petrography and chemistry work was similar to the (U-Th)/He analysis. In addition, I developed a repeatable and time efficient laser heating and gas analysis schedule for multi-aliquot (U-Th)/Ne analysis. Developments mean less sample material is required compared to previous studies and it negates the use of flux-melting based extraction of Ne which can result in damage to hardware.

Hematite is widespread in the Leadhills-Wanlockhead Orefield (LWO) and post-dates early (Caledonian) quartz veining and pre-dates the main phase base metal mineralisation that has previously been shown to be Carboniferous in age. New hematite (U-Th)/Ne ages reveal three hematite precipitation phases at around 230 Ma, 200 Ma, and 155 Ma. Sulfur isotope composition of ore sulfides revealed a homogenous signal and an equilibrium precipitation temperature of 170 to 210°C. Sulfur and metals are suggested to be sourced from the underlying basement. Helium isotope composition of ore sulfides reveals a crustal source of helium was the source of heat in the ore-forming process at LWO. The highest 3He/4He recorded is &lt; 1% of typical mantle signature, implying that mantle-derived heat from deep magmatic bodies was not the driver of the ore fluids. The timing of Pb-Zn mineralisation across the LWO is suggested to be linked to a wider western European series of tectonic events associated with the breakup of Pangaea.