HYDROTHERMAL ALTERATION AND TOURMALINE-ALBITE EQUILIBRIA AT THE COXHEATH PORPHYRY CU-MO-AU DEPOSIT, NOVA-SCOTIA

The Coxheath deposit is a zoned Cu-Mo-Au porphyry system related to Ha drynian calc-alkaline volcanic and plutonic rocks of the Avalon Zone i n northeastern Nova Scotia. Alteration minerals are distributed over 3 0 km(2), and cross the contact of a composite porphyritic granitic plu ton and overlying volcanic units. In the core of the pluton, a zone of potassic alteration comprises quartz-feldspar stockwork veinlets that contain varying proportions of chalcopyrite + bornite + molybdenite a nd accessory magnetite + hematite. Hydrothermal titanite, apatite, and actinolite also occur. Peripheral to the potassic zone, propylitic al teration assemblages contain chlorite + epidote + calcite + pyrite in veins or as patchy replacements. Farther outward, volcanic units have been affected by widespread and intense phyllic alteration and sericit ization, and by the formation of quartz stockworks and up to 2-5% diss eminated pyrite. The outermost zones were overprinted by argillic alte ration, characterized by kaolinite, pyrophyllite, and local chalcedoni c quartz. Tourmaline occurs in stockwork veinlets with albite, definin g a restricted zone of sodic alteration that overlaps the central pota ssic zone and inner margin of the propylitic zone. Compositions of the tourmaline tend toward dravite, with significant dravite-povondraite solid solution marked by Fe-Al exchange in the octahedral site. A mino r proportion of the uvite component and charge balance according to st oichiometry indicate Fe2+/(Fe2+ + Fe3+) values of 0.4 to 0.9. Fluid-mi neral equilibria at the hematite-magnetite buffer suggest that tourmal ine precipitation is strongly influenced by cooling, and by the activi ty of boric acid, B(OH)(3). Boric acid is the dominant aqueous species of boron in hydrothermal systems, and is stable across a wide range i n pH encompassing alkaline to acidic mineralizing conditions. As a rel atively non-volatile component, modeling suggests that boric acid beco mes concentrated in the liquid fraction of boiling fluids, contributin g to the precipitation of tourmaline within the residual hydrothermal component, and enhancing the zoning of hydrothermal minerals.