55. HYDROTHERMAL ACTIVITY IN THE GUAYMAS BASIN, GULF OF CALIFORNIA: A SYNTHESIS1

Observations of the physical properties of sediments, the chemistry of interstitial waters and solid phases, the min-eralogy of the sediments, and the isotope geochemistry of interstitial waters and sediments have led to the conclusion that two types of hydrothermal systems are operative in the Guaymas Basin of the Gulf of California: (1) hydrothermal activity associated with relatively shallow basaltic sill intrusions in highly porous sediments—generally of relatively short duration and associated with temperatures <200°C—and (2) hydrothermal activity caused by large magmatic in-trusions at greater depths. This latter activity involves recharge of the hydrothermal system by basin bottom waters; fractures in low-porosity sediments and faults in the sea floor serve as conduits for fluids heated up to temperatures in excess of 300°C. This type of hydrothermal activity lasts long enough to lead to substantial alteration of sediments into a well-developed greenschist facies: chlorite-quartz-albite-sphene-epidote-Fe-sulfides. The mechanisms of the hy-drothermal systems are discussed in detail and are consistent with the extensive heat flow data obtained in this area