CLAY MINERALOGY OF ACTIVE HYDROTHERMAL CHIMNEYS AND AN ASSOCIATED MOUND, MIDDLE VALLEY, NORTHERN JUAN-DE-FUCA RIDGE

Active hydrothermal sites were sampled during 1990 in a series of ALVI N dives in the Middle Valley area of the northern Juan de Fuca Ridge. This study focused on the hydrothermal clay minerals precipitated with in active anhydrite-rich and sulfide-bearing chimneys and an active hy drothermal mound at the Area of Active Venting (AAV) and Bent Hill sit es. Based on XRD, SEM, ATEM and electron-microprobe results, the domin ant clay mineral in the chimneys is saponite. In the AAV, Fe-rich sapo nite, Fe-rich smectite and chlorite also were observed in trace to min or amounts. Smectite occurs only as a mixed-layer component, with eith er illite or chlorite at Bent Hill. The Central Site hydrothermal moun d in the AAV contains lizardite, saponite and possibly a mixed-layer m ineral of these two phases. The clay minerals were directly precipitat ed in the active vents and hydrothermal mound by interaction of cold M g-rich seawater and moderately hot (< 276-degrees-C) hydrothermal flui ds with a low f(O2) and Si activity.