LARGE HYDROTHERMAL EXPLOSIONS ON THE YELLOWSTONE PLATEAU: IMPLICATIONS FOR MINERALIZED BRECCIA DEPOSITS

In Yellowstone, hot spring systems consistently have shown the presence of magmatic CO₂, H₂S, and He, based on isotope characteristics. Here, at least 20 large (>100-2800 meters in diameter) hydrothermal explosion craters have developed over the past 16,000 years, recurring approximately once every 700 years. Being spatially within the 640-ka Yellowstone caldera and along the active Mammoth-Norris tectonic corridor, the hydrothermal systems clearly fueled by magmatic heat in the Yellowstone caldera. The scale of the individual hydrothermal events dwarfs similar features in geothermal areas elsewhere in the world. Our studies of large hydrothermal explosion events here indicate: 1) none are directly associated with eruptive volcanic or shallow intrusive events; 2) several historical explosions have been triggered by seismic events; 3) extensively altered lithic clasts and co-mingled matrix material that form hydrothermal explosion deposits indicate that explosions occur in areas previously subjected to intense hydrothermal processes; 4) many lithic clasts in explosion breccia deposits preserve evidence of repeated fracturing and vein-filling; and 5) areal dimensions of many large hydrothermal explosion craters are similar to those of its active geyser basins and thermal areas. Most explosion craters in Yellowstone continue to host active hydrothermal systems contributing to additional post-explosion mineralization.

Recent studies have shown the connection between magmatism and hydrothermal processes in epithermal systems to be critical to understanding processes leading to mineralization. Hydrothermal explosion breccia is an important host for epithermal precious metal deposits; however, the role of magmatic volatiles and causes of pre- and post-explosion mineralization is poorly known and the foci of new research. Ore-bearing hydrothermal explosion deposits present along the margins of the Snake River Plain in older part of the Yellowstone hotspot track contain high grade Au mineralization present at relatively shallow depths. These deposits overlie calderas associated with the mantle-plume-driven Yellowstone hotspot, as also is the case on the Yellowstone Plateau.