What Controls critical metal Sn-W-Ta deposit formation in granite batholiths? Unravelling the magmatic and hydrothermal evolution of the Bushveld Comp

Large granite provinces are associated with the development of critical metal resources such as tin, tungsten, tantalum and lithium. These metals are required for essential new green technologies to push forward into a low-carbon economy.

Granitic melts are transported to mid crustal levels where fractional crystallisation can concentrate metals of interest. Exsolved volatiles from these granitic systems can then form a magmatic-hydrothermal ore deposit which hosts these critical metals. However, while all granitic melts exsolve volatiles, economic ore deposits rarely form from these systems and when they do, they are very short-lived.

There are multiple generations of magmatism within these systems and it not always known which magmatic event is linked to the economic mineralisation. The role that incremental batholith emplacement, the rates of emplacement and the amount of melt present within the magmatic system have on the formation of ore deposits is not known. These are fundamental questions which will lead to better geological models for ore formation.

Typically, magmatic events have been dated via tools such as zircon U-Pb, however now novel high-precision analytical techniques can be used to yield new mineral chronometers. These are tools which will allow the precise dating of hydrothermal events as the ore (cassiterite U-Pb) which precipitated from them will be directly dated. Combining geochronology of the ore-forming event with high-precision temporal records on the host granitic system (through the use of zircon U-Pb via high-precision ID-TIMS) will enable a link of batholith construction to the economic mineralisation.

Zircons from the host granite will also provide a record of melt evolution through the analysis of in-situ tracers such as Lu-Hf isotopes (which are sensitive to source) and trace elements of economic metals. Thus, for the first time linking potential changes in source and metal contents of magmatic events to their eventual mineralization will provide new perspectives on the nature of these complex ore-forming systems.

These analytical techniques will be applied to the A-type granites and associated Sn-deposits of the Lebowa Granite Suite, Bushveld Complex, South Africa and to the Cornubian batholith and associated Sn-province of Cornwall, SW England. These field sites have good exposure which will make sample collection easy and are two unique magmatic systems. The two contrasting systems will allow a comparison focusing on the emplacement history of the granites and how the rates of pluton construction and melt volumes affect ore formation potential.

Analytical work will be carried out by laser ablation techniques at both the BGS and the St Andrews Isotope Geochemistry Laboratories, while the ID-TIMS geochronology on magmatic zircons and hydrothermal cassiterite will take place at the BGS Geochronology and Tracers Facility.