Gorda Ridge pp 51-75 | Cite as

Multistage Hydrothermal Systems in the Blanco Fracture Zone

  • Roger Hart
  • Jochen Hoefs
  • Douglas Pyle
Conference paper


The deeper portions of paleohydrothermal systems are exposed in the Blanco Fracture Zone. Multistage hydrothermal alteration, brecciation, and sulfide deposition occurred over changing regimens of temperature and fluid chemistry. Oxygen isotope analyses and secondary mineral phases indicate that metagabbros formed at temperatures in excess of 500°C. Basalt and diabase breccias formed at temperatures between 350°C and 500°C. Quartzchert-cemented breccias formed at temperatures below 200°C. Vein fillings of carbonate, prehnite, and zeolites formed at lower temperatures. The 87Sr/86Sr ratio of the fluids and the water/rock (W/R) ratio did not change appreciably during the hydrothermal events in the Blanco Fracture Zone. The temperature changes may have been the result of tectonic movement of the crust relative to heat sources. In the case of chert-cemented breccias the change of temperature rapidly followed brecciation and could not have been the result of either tectonic movement or transportation within the hydrothermal system. We propose that adiabadic cooling followed brecciation. The data suggest that the temperature and pressure of seawater increased as it circulated at depth in the Blanco Fracture Zone. The increase of temperature resulted in a decrease in the solubility of Mg and Cr. These elements were stripped from seawater in exchange for Si, Ca, and probably Al, which were leached from the crust and dissolved in the fluids at high temperatures. Cooling of the crust by hydrothermal solutions resulted in contraction and brecciation. The hydrothermal fluids expanded into the cracks in the crust and cooled adiabatically. The drop of temperature of the fluids caused a decrease in the solubility of Si, Ca, and Al. These elements precipitated in secondary minerals within the crack system. Ti was also mobilized locally within the crack system during this stage. The secondary minerals filled the cracks and curtailed hydrothermal circulation. This caused the crust to be reheated by equilibrating with the regional thermal gradient. The overall process was initiated again. These multiple episodes are now recorded in the hydrothermal phases. Sulfide deposition occurred in at least three stages: (1) disseminated pyrrhotite and minor pen-landite precipitated in the gabbros; (2) pyrite with minor chalcopyrite and sphalerite formed in veins in brecciated basalts; and (3) sphalerite, bornite, chalcopyrite, and galena formed in late stage drusy quartz.


Fracture Zone Hydrothermal Fluid Hydrothermal Alteration Massive Sulfide Deposit Econ Geol 
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© Springer-Verlag New York Inc. 1990

Authors and Affiliations

  • Roger Hart
  • Jochen Hoefs
  • Douglas Pyle

There are no affiliations available

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