Timescales of magmatic-hydrothermal tin mineralisation in the Cornubian Orefield, SW England

Abstract

Tin has been classed as a critical metal for over a decade, with demand rising due to its role in the green energy transition. Mined predominately as cassiterite (SnO₂), primary tin deposits form from magmatic-hydrothermal processes commonly associated with late-stage, evolved granitic melts. These systems develop over 10s Myrs from multiple discrete magmatic and hydrothermal events, but the timing of Sn-mineralisation within the long emplacement history of granite batholiths remain poorly constrained, largely due to limitations in traditional geochronological methods. This thesis applies U-Pb geochronological techniques - isotope dilution thermal ionisation mass spectrometry (ID-TIMS) and laser ablation inductively coupled mass spectrometry (LA-ICP-MS) - to date cassiterite to produce an updated mineralisation geochronological framework across the Cornubian Orefield, SW England.

The U-Pb systematics of nine cassiterite reference materials were further characterised by ID-TIMS. Three materials produced repeatable lower intercept ages and were used as primary and secondary reference materials within a matrix-matched LA-ICP-MS approach.

Twenty-eight cassiterite samples from across the Cornubian Orefield were analysed by LA-ICP-MS. The cassiterite dates show that tin mineralisation spanned the entire construction history of the Cornubian Batholith between ca. 293 - 275 Ma, with a period of high tin deposition at <282 Ma. The contemporaneous nature of cassiterite with host magmatism (within ±4 Ma) suggests multiple parental magmatic sources such that tin formed in association with both the lower and higher temperature dehydration melting of muscovite and biotite mica.

High-precision ID-TIMS dating of cassiterite and magmatic zircon from the South Crofty Mine revealed: 1) host magmatism was generally unrelated to cassiterite-bearing lodes; 2) lode formation commenced ~4 Myrs post host granite emplacement; 3) lode formation occurred during two distinct mineralising events at ca. 288 Ma and 280 Ma. Thus, both regionally and within a single mined deposit, cassiterite-bearing lodes have multiple parental magmas, likely facilitated by the continuous regional extensional and prolonged magmatism.

Date of Award	2 Dec 2025
Original language	English
Awarding Institution	University of St Andrews
Supervisor	Nicholas Gardiner (Supervisor) & Simon Tapster (Supervisor)