Impact-induced hydrothermal activity within the Haughton impact structure,arctic Canada: Generation of a transient, warm, wet oasis

Field studies and analytical scanning electron microscopy indicate that a h ydrothermal system was created by the interaction of water with hot, impact -generated rocks following formation of the 24 km diameter, 23 Ma Haughton impact structure. Hydrothermal alteration is recognized in two settings: wi thin polymict impact breccias overlying the central portion of the structur e, and within localized pipes in impact-generated concentric fault systems. The intra-breccia alteration comprises three varieties of cavity and fract ure filling: (a) sulfide with carbonate, (b) sulfate, and (c) carbonate. Th ese are accompanied by subordinate celestite, barite, fluorite, quartz and marcasite. Selenite is also developed, particularly in the lower levels of the impact breccia sheet. The fault-related hydrothermal alteration occurs in 1-7 m diameter subvertical pipes that are exposed for lengths of up 20 m . The pipes are defined by a monomict quartz-carbonate breccia showing pron ounced Fe-hydroxide alteration. Associated sulfides include marcasite, pyri te and chalcopyrite. We propose three distinct stages in the evolution of t he hydrothermal system: (1) Early Stage (> 200 degreesC), with the precipit ation of quartz (vapor phase dominated); (2) Main Stage (200-100 degreesC), with the development of a two-phase (vapor plus liquid) zone, leading to c alcite, celestite, barite, marcasite and fluorite precipitation; and (3) La te Stage (< 100 degreesC), with selenite and fibroferrite development throu gh liquid phase-dominated precipitation. We estimate that it took several t ens of thousands of years to cool below 50 degreesC following impact. Durin g this time, Haughton supported a 14 km diameter crater lake and subsurface water system, providing a warmer, wetter niche relative to the surrounding terrain. The results reveal how understanding the internal structure of im pact craters is necessary in order to determine their plumbing and cooling systems.