Recent hydrothermal activity of Norris Geyser Basin-Continued 2 GEOLOGY AND REMARKABLE THERMAL ACTIVITY OF NORRIS GEYSER BASIN, YELLOWSTONE NATIONAL PARK geysers, to extents not known elsewhere in the park and perhaps in the world. Evolution of individual vents seems to be going on continually; many are rapid changes involving only a single vent, but some are widespread changes involving numerous vents crudely aligned to the north and northeast for liz to Zkm, parallel to common fracture

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... These widespread changes seem to be seasonally controlled, being most common in August and September and perhaps related to scarcity of local water on the floors of Porcelain and Back Basins. Hydrothermal mineralogy in core samples from three research holes drilled entirely in Lava Creek Tuff to a maximum depth of -331.6 m permits an interpretation of the hydrothermal alteration history. Primary plagioclase and mafic phenocrysts were completely removed at an early stage of alteration. Fe and Mn were mobilized early, and Fe oxides were pervasively deposited in the tuff. Fractures were filled with cryptocrystalline mixtures of chalcedony, clays, and Fe oxides. In two drill holes, subsequent alteration was superimposed upon the earlier oxidation, resulting in intervals of bleaching of tuff in which the Fe oxide was reduced to sulfide. Fluctuating water compositions are commonly associated with refracturing of the tuff, seen where several stages of alteration are superimposed. A deep, mildly acidic alteration is indicated by bleached tuff and an assemblage of pyrite, chlorite, and mixed-layer illite-montmorillonite in the deepest drill hole. Near-surface acid fluids have bleached the upper levels of tuff in all three drill holes, resulting in deposition of alunite, kaolinite, opal, ~-cristobalite, a-cristobalite, and locally sulfides. Fluids recirculating downward deposited kaolinite, a-cristobalite, and chalcedony in latest fractures. In the present system, self-sealing (mainly by silica-clay mixtures that depend on temperature and water composition) is taking place. Locally, two or more stages of alteration may be distinguished, but no certain criteria were recognized to correlate all alteration stages with differences in contemporaneous surface activity. Most studies of active hydrothermal areas have noted chemical differences in fluids and alteration products but have given little attention to differences and models to explain evolution in types. This report, in contrast, emphasizes the kinds of changes in vents and their changing chemical types of waters and then provides models for explaining these differences. Norris Basin is probably not an independent volcanic-hydrothermal system. The basin and nearby acid-leached areas (from oxidation of I-IzS-enriched vapor) are best considered as parts of the same system, extending from Norris Basin to Roaring Mountain and possibly to Mammoth. If so, are they parts of a single large system centered within the Yellowstone caldera, or are Norris Basin and the nearby 'altered areas both parts of one or more young independent corridor systems confined, at least in the shallow crust, to the Norris-Mammoth Corridor? Tentatively, we favor the latter relation, probably having evolved in the past "-'300,000 years. A model for large, long-lived, volcanic-hydrothermal activity is also suggested, involving all of the crust and upper mantle and using much recent geophysical data bearing on crust-mantle interrelations. Our model for large systems is much superior to previous suggestions for explaining continuing hydrothermal activity over hundreds of thousands of years, but is less attractive for the smaller nonhomogenized volcanic system actually favored here for the Norris-Mammoth Corridor.