COMBINING OPTICAL PROPERTIES, MINERAL CHEMISTRY, MICROTEXTURES, AND FIELD RELATIONS OF TOURMALINE TO TRACK THE MAGMATIC-HYDROTHERMAL TRANSITION

Tourmaline from the St Byron lobe of the Land’s End granite shows a great variety in color and mineral chemistry. By combining optical properties, major and trace element chemistry and field relations, late-magmatic tourmaline can be distinguished from closed and open system hydrothermal tourmaline. In some localities a transition from disseminated, brown magmatic tourmaline to hydrothermal, blue tourmaline can be directly observed. In other localities, an external, or at least less local, fluid precipitated colorless-green pleochroic tourmaline. The Sn-mineralization in the area is unequivocally related to this type of tourmaline. The data set includes 680 LA-ICP-MS and 900 EPMA spot analyses of 16 samples from six localities along the SW coast of the Land’s End area. The compositional evolution of the major elements of the three main tourmaline types is well established; brown tourmaline plot in the middle of the schorl field, blue tourmaline are more Fe-rich, and green tourmaline plot in the dravite field. Some trace elements show a correlation with color and plot as distinct groups in binary plots. This is particularly evident in the Co vs Ni and Zn plots, where blue (~0.2-2 µg/g Co, ~150-300 µg/g Zn, ~0.5-2 µg/g Ni), brown (~10-40 µg/g Co, ~250-350 µg/g Zn, ~1-40 µg/g Ni) and green (~2-20 µg/g Co, ~25-60 µg/g Zn, ~30-100 µg/g Ni) tourmaline form separate clusters. Other general trends including groups based on color are the positive correlations between Sn and Sr and Y. In both cases, all three elements increase from brown to blue to green. Brown and blue tourmaline may also be distinguished in the Sc vs V and Nb vs Ta plots. Growth zoning is commonly observed, typically in the form of a brown core overgrown by a section of oscillatory zoned brown-blue tourmaline, a blue zone, and in some samples an oscillatory zoned pale blue-dark blue zone. This is interpreted to represent the transition from a melt-dominated to a fluid-dominated stage without a significant influence of external fluids. The green tourmaline may crosscut and overprint former tourmaline generations, and is often observed together with quartz as mineral replacements. This is interpreted to be the result of subsolidus fluid-rock interaction.