PEAK TEMPERATURES DETERMINED BY TIQ, ZIR AND TOURMALINE SECTOR ZONING: A CASE STUDY FROM THE WHEAL BUSY MINE, CORNWALL, U.K.

Temperature (T) conditions of a tourmaline-rich sample from Wheal Busy, a historic copper, tin, and arsenic mine in Cornwall, U.K., provide important insights into the evolution of this economically significant area. The application of three distinct and complementary single-mineral geothermometers allows for a more robust determination the T conditions of this sample. The sample is composed of golden-brown pleochroic tourmaline (tur) (90%), parallel linear bands of fine rutile (5%) and zircon (2%), quartz (3%), and traces (<1%) of ilmenite, xenotime, monazite, and bismuthinite. Tur grains are 0.05-0.3 mm with many crystals displaying oscillatory and sector zoning. Rutile grains are 0.005-0.02 mm and contain inclusions of zircon. Minor quartz grains are interstitial. Chemical and textural characterization of tur, rutile and quartz was carried out through EPMA, BSE, and panchromatic CL analysis. The presence of rutile and zircon suggests the system is Ti- and Zr-saturated, an important precondition for the application of two of the geothermometers.

The tur grains, oriented parallel to the c axis, were analyzed using corresponding oscillatory bands on the a and c⁺ sectors to determine the tur T. Both sectors are fluor-schorl, schorl, foitite, and fluor-uvite, but show distinct differences in element partitioning between the sectors. Following the calibration of van Hinsberg and Schumacher (2007), tur yields a T of 550-610 °C. Stated uncertainty is ± 100 °C at 650°C; ± 10°C at 350°C. Rutile contains minor Zr, Nb, Fe²⁺, and Sn. Using the Zr-in-rutile calibration of Kohn (2020) with an estimated pressure of 3 kbar, rutile grains yield a T of 442-601 ± 10-15 °C. Ti in quartz is zoned with cores possessing a brighter signal than the rims, suggesting higher Ti cores. Additionally, the cores contain fractures filled with low intensity quartz. Using the Ti in quartz calibration developed by Wark and Watson (2006), the cores yield a T of 547-577 ± 2-5 °C, while the rims and infilled fractures lack Ti. This suggests partial replacement of quartz at lower T. These three mineral geothermometers yield similar temperatures within errors of 570 ± 50 °C. These values are considered to represent the peak T conditions for this tourmaline-rich sample.