Quartz Oxygen Isotope Constraints on the Petrogenesis of Tourmaline‑rich Rocks in the Cornubian Batholith, SW England, with Implications for Sn Mineralization

We investigate the origin and evolution of quartz populations in tourmaline‑bearing rocks of the Cornubian Batholith, SW England, to constrain fluid sources and Sn‑mineralization processes. Our study focuses on massive quartz–tourmaline rocks (MQT) because of their spatial and genetic association with Sn mineralization. MQT occur as small stock‑ to dyke‑like bodies (typically < 300 m) exemplified by Roche Rock and Porth Ledden. Fifteen quartz samples from MQT and comparative quartz from biotite granite, tourmaline granite, pegmatite, tourmaline breccia and veins (Porth Ledden, Porthmeor Cove, Roche Rock, Tresayes, Wheal Remfry) were subject to SEM‑cathodoluminescence and oxygen‑isotope (δ18O) analysis. SEM‑CL reveals multiple quartz generations: primary magmatic quartz (granites, aplites, MQT), pegmatitic quartz, secondary fracture‑fill quartz, oscillatory hydrothermal overgrowths on magmatic cores (typical in MQT), and complexly zoned vein quartz. All δ18O values are relatively high (+ 11.5 to + 27.7‰), mostly clustering between + 12 and + 15‰. Magmatic quartz (granites, aplites) range from + 11.5 to + 14.8‰, overlapping pegmatite quartz, implying incorporation into the granitic melts of high‑δ18O metasedimentary material, consistent with local Devonian metasediments. Hydrothermal quartz spans + 12.7 to + 27.7‰; two anomalously high values (+ 25.6, + 27.7‰) in crystal rims suggest late addition of formation waters. We infer that MQT at Porth Ledden and Roche Rock formed by partial metasomatic replacement of tourmaline granite due to infiltration and entrapment of pneumatolytic, B‑rich fluids in the roof zone. Metasomatism dissolved K‑feldspar, mobilizing K, Rb, Ba, Sr, Cs, Pb and notably Sn, producing cavities later infilled by hydrothermal quartz and tourmaline. A progressive increase in δ18O from magmatic to late hydrothermal quartz suggests fluid cooling and possible mixing with oxidizing formation waters, rather than a component of meteoric-derived waters that contributed to the precipitation of cassiterite.